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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Future Virol. Author manuscript; available in PMC 2011 May 1.
Published in final edited form as:
Future Virol. 2010 July; 5(4): 435–451.
doi:  10.2217/fvl.10.34
PMCID: PMC2949073

Variation in the biological properties of HIV-1 R5 envelopes: implications of envelope structure, transmission and pathogenesis


HIV-1 R5 viruses predominantly use CCR5 as a coreceptor to infect CD4+ T cells and macrophages. While R5 viruses generally infect CD4+ T cells, research over the past few years has demonstrated that they vary extensively in their capacity to infect macrophages. Thus, R5 variants that are highly macrophage tropic have been detected in late disease and are prominent in brain tissue of subjects with neurological complications. Other R5 variants that are less sensitive to CCR5 antagonists and use CCR5 differently have also been identified in late disease. These latter variants have faster replication kinetics and may contribute to CD4 T-cell depletion. In addition, R5 viruses are highly variable in many other properties, including sensitivity to neutralizing antibodies and inhibitors that block HIV-1 entry into cells. Here, we review what is currently known about how HIV-1 R5 viruses vary in cell tropism and other properties, and discuss the implications of this variation on transmission, pathogenesis, therapy and vaccines.

Keywords: CCR5, CD4, CD4+ T cell, envelope, HIV-1, macrophage, macrophage tropism, neuroAIDS, R5, receptor, tropism

HIV-1 was first described in 1983 [1] and CD4 identified as its receptor in 1984 [2,3]. It was quickly apparent that CD4 alone was not sufficient for infection and that a putative second determinant was required for HIV-1 entry. The identification of CXCR4 in 1996 by Ed Berger’s group as the coreceptor for T-tropic HIV-1 strains [4] thus heralded a new era for HIV-1 receptor and cell tropism research. Well before 1996, it was clear that there were two types of HIV-1 that infected primary T cells or macrophages with different efficiencies. The first type became known as T-tropic or syncytium-inducing, owing to their capacity to infect and induce syncytia in T-cell lines as well as in primary T-cell cultures. The second group was termed nonsyncytium-inducing, macrophage tropic or M tropic, owing to their inability to induce syncytia in T-cell lines or peripheral blood mononuclear cells (PBMCs) and their capacity to infect primary macrophage cultures [57]. While CXCR4 was the coreceptor for T-tropic, syncytium-inducing strains, CCR5 was identified as the coreceptor for M-tropic, nonsyncytium-inducing strains [810] and HIV-1 viruses are now termed R5, X4 and R5×4, depending on their use of the two coreceptors [11]. R5 viruses efficiently infect primary CD4+ memory T cells that express CCR5. For many, so-called R5 tropism became synonymous with macrophage tropism or M tropism. However, extensive research from our group and others has shown that R5 envelopes confer widely divergent abilities to infect primary macrophages, along with diverse sensitivities to entry inhibitors and neutralizing antibodies (nAbs) [1216]. Here, we review the current understanding of the cell tropisms of HIV-1 R5 viruses and discuss their implications for transmission, pathogenesis and therapy.

Variation in macrophage tropism of HIV-1 R5 viruses

In 1987, Koyanagi et al. described JR-FL and JR-CSF as macrophage-tropic and non-macro-phage-tropic R5 isolates from brain tissue and cerebrospinal fluid (CSF) [17]. We also reported that primary R5 viral isolates varied in their capacity to infect primary macrophages by at least 1000-fold [18] and differed in their capacity to exploit low CCR5 levels for infection [19]. More recently, Gorry et al. described an R5 isolate from brain tissue that was highly tropic and fusigenic for primary macrophages [16]. We and others have confirmed Gorry’s observations and demonstrated that R5 envelopes derived by PCR directly from brain tissue are frequently highly tropic for macrophages [1214]. By contrast, R5 envelopes from blood and immune tissue frequently infected primary macrophages very inefficiently [14]. Overall, different studies have revealed a wide spectrum of macrophage infectivity conferred by R5 envelopes, which covered 3–4 orders of magnitude [12,1416,20,21] (Figure 1). Similarly, others have reported that R5 virus isolates from the blood of adult and pediatric AIDS subjects conferred an enhanced macro-phage tropism compared with isolates from earlier stages of disease [2224]. These reports suggest that HIV-1 variants with an increased capacity to infect macrophages may evolve or become more prevalent late in disease.

Figure 1
Macrophage infectivity of HIV-1 R5 envelopes amplified from different tissues

There are a number of points to make regarding the aforementioned studies. First, our recent work has focused on evaluating the phenotypes of envelopes as opposed to full-length clones or viral isolates. Although it is likely that the envelope predominantly controls macrophage infectivity it should be remembered that other HIV-1 genes or variation in long terminal repeat promoter sequences might also have affects. Second, while we have PCR-amplified macrophage-tropic envelopes from brain tissue and at a lower frequency from blood, semen and immune tissues, it is not known which cell type(s) supported their replication in vivo. It seems very likely that highly macrophage-tropic envelopes from brain tissue were derived from brain macrophages or microglial cells. However, the origin of macrophage-tropic R5 envelopes present in plasma, PBMCs, semen or immune tissue in these studies is less clear. When tested, highly macrophage-tropic R5 viruses have generally infected primary CD4+ T cells with at least similar efficiencies as non-macrophage-tropic viruses [14]. Therefore, we do not know whether macrophage-tropic envs evolved by adapting for replication in macrophages or whether other selective pressures in vivo (e.g., nAbs) played a role. For example, low levels of nAbs in the brain may allow envelopes with a more open conformation, higher CD4 affinity and increased macrophage tropism to evolve. This subject will be discussed in more detail later.

Determinants of R5 macrophage tropism & effects on envelope structure

The capacity of R5 envelopes to confer macrophage infection correlated with their ability to exploit low levels of cell surface CD4 for infection [12,14,21]. In addition, we noted that macrophage infectivity correlated with sensitivity to reagents that blocked glycoprotein (gp)120–CD4 interactions [13], including soluble CD4 and an anti-CD4 monoclonal antibody (mAb; Q4120), as well as BMS-378806, a small molecule that targets a hydrophobic cavity on gp120 close to the CD4 binding site (CD4bs) [25]. There was also a strong trend in our studies and a significant correlation in a study by Dunfee et al. between macrophage infectivity and increasing sensitivity to the CD4bs mAb, b12 [13,26], which was consistent with an increased exposure of the CD4bs and the b12 epitope. Together, these different studies point to changes in the affinity of the envelope for CD4 that result (at least in part) from enhanced exposure of the CD4bs and proximal epitopes. The increase in env–CD4 affinity, thus, explains the capacity of macrophage-tropic env proteins to exploit low levels of cell surface CD4 on macrophages for infection [2729].

Consistent with this interpretation, determinants of R5 macrophage tropism were mapped to gp120 residues within or proximal to the CD4bs. Thus, Dunfee et al. described a polymorphism in the C2 part of the CD4bs that contained an asparagine at residue 283 (N283) (Figure 2). N283 was highly represented in R5 envelopes from brain tissue of subjects with HIV-associated dementia (HAD) and conferred enhanced macrophage infectivity in env mutants [30]. Non-HAD subjects predominantly carried I283 or T283. In Dunfee’s study, N283 was structurally modeled as conferring a tighter gp120–CD4 interaction by facilitating the formation of a hydrogen bond with Q40 on CD4. We also demonstrated a profound influence of N283 on macrophage infectivity [31]. However, we identified many env proteins where the presence or absence of N283 did not correlate with macrophage infectivity [14,31]. In our studies, we identified further determinants on the variable flanks of the CD4 binding loop (Figure 2) that influenced macrophage infectivity [31]. Residues on the N-terminal flank of the loop were adjacent to CD4 contact residues and probably affect the exposure of this site on the trimeric envelope (Figure 2). In addition, Sterjovski reported that a potential glycosylation site (N362) on the same flank increased the fusigenicity of envelopes but did not examine macrophage infectivity [32]. Consistent with these observations, a recent study by Wu et al. indicated that the variable N-terminal flank of the CD4-binding loop plays a major role in resistance to neutralization by the CD4bs mAb, b12 [33].

Figure 2
HIV-1 glycoprotein 120 residues involved in CD4-binding, exposure of the CD4 binding site and macrophage tropism

In our study, determinants in the V3 loop (residues 308 and 332) also contributed to full macrophage tropism and to modulate sensitivity to entry inhibitors that target CD4–gp120 interactions. Substitutions at these sites were previously found in the brain of HAD subjects [34] and in CSF-derived env [35,36]. The location of the V3 loop on the ligand-free trimer is not known but presumably must sit close enough to the CD4bs to influence its exposure as suggested in an early study by Wyatt et al. [37].

Finally, the N-glycan at N386 at the N-terminal end of the V4 loop is also proximal to the CD4-binding loop (Figure 2). The presence or absence of this glycan has been demonstrated to affect macrophage tropism [31,38] and sensitivity to CD4bs mAb b12 [26,39,40], indicating that it acts to protect CD4 contact residues (most likely those in the proximal CD4 binding loop). Our recent work indicates that determinants on the N-terminal flank of the CD4-binding loop act by shifting the orientation of proximal glycans potentially protecting or exposing loop residues that contact CD4 [DUENAS-DECAMP M, UNPUBLISHED OBSERVATION.].

The effect of R5 macrophage tropism on other envelope properties

As discussed, variation in macrophage tropism of R5 envelopes results from changes in residues that alter gp120–CD4 interactions. This directly affects the sensitivity of envelopes to reagents that block gp120–CD4 interactions, including soluble CD4 and an anti-CD4 mAb (Q4120) [13]. Variation in macrophage tropism also directly impacts on the sensitivity to nAbs that target the CD4bs, including b12, consistent with an increased exposure of this site and over-lapping neutralization epitopes [13,26]. In addition, we also noted a trend toward decreased sensitivity to the glycan-specific mAb, 2G12 [13], probably caused by (at least in part) the loss of critical glycan sites that may contribute to the exposure of the CD4bs in macrophage-tropic env proteins. Nevertheless, the focus of these determinants on env–CD4 interactions was further highlighted by the lack of correlation between macrophage tropism and sensitivity to reagents that targeted other envelope sites or functions and blocked other stages of entry. These included inhibitors of env–CCR5 interactions and gp41 conformational changes [13]. Finally, there was no evidence for increased exposure of CD4i epitopes, as indicated by the lack of sensitivity to 17b, a mAb that recognizes the conserved part of the coreceptor binding site [13]. Together, these observations point to a focused re-architecturing of the residues proximal to the CD4bs that affect the exposure of CD4 contact residues without dramatically affecting other envelopes sites or properties.

Whether macrophage tropism affects sensitivity to nAbs in HIV-1+ human sera is under study. If modulation of macrophage tropism is relatively restricted to the CD4bs, there may only be limited effects on sensitivity to nAbs that target other sites on the envelope. It might be expected that the more macrophage-tropic envelopes will have enhanced sensitivity to CD4bs nAbs in human sera. However, such nAbs are relatively infrequent among HIV-1+ sera, while the presence of nAbs directed to other conserved neutralization targets may obscure their effect [4144].

Other envelope properties that affect receptor use & tropism

While our group has focused on macrophage tropism and CD4 use of R5 envelopes, others have investigated variation in the use of CCR5. Several studies have indicated that CCR5 use varied amongst different HIV-1 strains and evolved during the course of infection [45,46]. Altered use of CCR5 was revealed by changes in the capacity of R5 viral isolates to infect indicator cells expressing chimeric CCR5/CXCR4 receptors, with late disease isolates able to use a broader range of chimeras compared with early isolates [4749]. These changes were observed in the absence of a switch to CXCR4 use. Repits et al. also described equivalent R5 variants from late-stage disease with an increased positive charge on the surface of gp120 [45,46] and fewer potential N-linked glycosylation sites [50]. The increased charge was focused in the variable loops (but not V3) and, again, was not associated with a switch to CXCR4 use. Such variants carried an enhanced replicative capacity and were more resistant to entry inhibitors including RANTES/CCL5 and CCR5 antagonists compared with viruses from earlier disease stages [45,46,51,52]. The increased positive charge of gp120 enhances the attachment of virions to cell surfaces that are negatively charged [46]. However, the precise mechanisms that affect CCR5 use are less clear, although changes in how gp120 interacts with the second extracellular loop were proposed [48].

For the envelopes that we studied, overall gp120 charge also correlated with sensitivity to CCR5 antagonists, but not with sensitivity to other entry inhibitors (e.g., soluble CD4 and T20). However, neither gp120 charge nor sensitivity to CCR5 antagonists correlated with macrophage infectivity or with sensitivity to inhibitors of gp120–CD4 interactions. Thus, the evolution of gp120 charge in R5 isolates is independent and apparently unrelated to the variation in macrophage tropism described here.

There are also viral isolates and envelopes that confer use of low CCR5 levels for infection. We and others have reported that a subset of highly macrophage-tropic R5 isolates or envelopes from the brain that can exploit low CD4 levels for infection, can also infect cells via low levels of CCR5 [12,16]. Nonetheless, we have also noted that a fraction of non-macrophage-tropic env proteins can also use low CCR5 levels for infection [PETERS PJ, UNPUBLISHED OBSERVATION]. The mechanisms and env determinants that confer low CCR5 use are not currently known. Low CCR5 use may enable such variants to target cells that express low amounts of CCR5, including dendritic cell (DC) lineage cells and particular CD4 T-cell populations (e.g., central memory cells) [53].

The possible implications of these different R5 envelope properties for transmission and pathogenesis are discussed later.

Environmental pressures in vivo that select for different R5 envelope tropisms

The selective pressures that modulate the properties of R5 envelopes in vivo are poorly understood. The simple view would be that macrophage-tropic variants have adapted for replication in macrophages while non-macrophage-tropic variants have been selected for T-cell replication. However, R5 viruses do not readily segregate into macrophage-tropic and non-macrophage-tropic groups. Instead there is a spectrum in the extent that different R5 viruses or envelopes confer macrophage infection (Figure 1). Moreover, all R5 envelopes that we tested conferred infection of primary phytohemagglutinin/IL-2 stimulated CD4+ T cells or PBMCs [14]. Nevertheless, highly macrophage-tropic variants in the brain have probably adapted for efficient infection of macrophages and microglial cells present there. However, if all R5 variants can infect T cells anyway, what then selects for non-macrophage-tropic variants that interact less efficiently with CD4? It is likely that nAbs select for envelopes that have evolved to protect critical functional sites (e.g., the CD4bs). Such variants may be compromised in their capacity to bind CD4 but will not be as severely affected during infection of CD4+ T cells that express high levels of CD4. By contrast, the brain is protected by the blood–brain barrier, which usually excludes antibodies [5456]. Replication in this environment may select for envelopes with a more open conformation that can interact efficiently with CD4 and infect macrophages or microglia that carry low levels of CD4. This scenario is supported by the increased sensitivity of highly macrophage-tropic brain-derived env proteins to neutralization by the CD4bs mAb, b12 [13,26]. On the other hand, non-macrophage-tropic env proteins have been detected early in infection when nAbs are likely to be low or absent [57,58]. Thus, during this early stage of replication there would not be a selection pressure imposed by nAbs to prevent virus env proteins from evolving a more open conformation and allowing an efficient interaction with CD4. Thus, the selective pressures that prevent these early variants from evolving a more open envelope consistent with a macrophage-tropic phenotype are not understood.

Do different HIV-1 clades confer distinct or unique R5 envelope properties?

HIV-1 is highly variable and has been categorized into different groups and subtypes or clades. HIV-1 groups M, N and O represent three separate zoonotic transfers from chimpanzees or gorillas [59,60]. Group M has spread pandemically and has been further divided into subtypes or clades along with several circulating recombinant forms. The vast majority of research on HIV receptor use and tropism has been performed using clade B viral isolates or molecularly cloned viruses or env genes. Although it is clear that R5 envelopes from other clades or circulating recombinant forms confer similar properties to clade B, some important distinctions are emerging. It is well established that variants using CXCR4 evolve less frequently in clade C infections compared with clade B [6165], while in clade D, such variants may be even more frequent [66]. Clade C envelopes also carry a relatively conserved variable V3 loop (compared with clade B) that frequently lacks the conserved N-linked glycosylation site at the N-terminus, and which is usually present in other clades [67]. Clade C R5 isolates were also reported to infect CD34+ progenitor cells much more efficiently than clade B viruses, and were associated with anemia in Africa [68]. The extent to which HIV-1 infects CD34+ progenitors has become controversial, with some suggesting limited infection for clade B viruses [68,69], despite detection of both CD4 and CCR5 [68,70]. However, a recent study has demonstrated infection of CD34+ progenitor cells in vitro using a reporter pseudovirus carrying an R5×4 clade B envelope. The same study also showed infection in vivo in (presumably) clade B subjects and the establishment of a latent viral reservoir in such cells [71]. Thus, the extent that clade B, C and other non-B clades infect CD34+ progenitor cells requires further evaluation. Nonetheless, the distinct properties of clade C envelopes that are emerging may have been factors that contributed to the rapid spread of this clade, which is the most prevalent worldwide [72].

Whether different HIV-1 clades differ in their replicative fitness is currently unclear. Such differences (if they exist) could potentially have a profound effect on their pathogenic potential and rate of spread within a population. Thus, a recent study indicated that African clade C isolates were less fit for replication in PBMCs compared with other group M clades (including A, B and D), but competed well in cervical, penile and rectal explant cultures, consistent with efficient transmission [73]. However, another study reported that Indian clade C viruses conferred a greater replicative capacity in PBMCs compared with clade A [74]. It was also reported that heterosexually transmitted clade A [75] and C viruses [76] carried gp120s with a shorter sequence length, fewer glycosylation sites and increased neutralization sensitivity. Such envelope characteristics may confer an advantage for transmission and seemed to contrast with heterosexual [75] or male-to-male [77] clade B transmission, where such env proteins were not detected. These observations imply that the env genes of different clades selected at transmission may have distinct properties or that the ‘transmission efficiency’ of env genes can be conferred by different gp120 modifications in different clades. However, more recent data regarding mainly male-to-male transmission of clade B viruses indicate a weak relationship between transmitted viruses and shorter gp120s with fewer glycosylation sites [78].

Interestingly, the vast majority of clade B R5 envelopes that have been amplified directly from patient material without culture can use CCR3 as well as CCR5 on CD4+ indicator cells in vitro [12,79]. The use of CCR3 is intriguing and it is possible that there are important CCR3+ cell types in vivo that are targeted by HIV-1. For example, CD4+ Th2 T cells express CCR3 [80], while there is evidence that CCR3 may be an important coreceptor for the infection of microglial cells in the brain [81,82]. Nonetheless, infection of PBMCs in vitro by R5R3 envelopes is usually blocked entirely by CCR5 antagonists and a role for CCR3 in vivo has not yet been established [83]. Interestingly, not all R5 envelopes from different clades exhibit the R5R3 phenotype. Thus, clades A and C R5 envelopes frequently use formyl peptide receptor-like (FPRL)1 as a coreceptor in addition to CCR5, while clade D R5 envelopes use neither CCR3 nor FPRL1 [84]. Similar to CCR3, the significance of FPRL1 use in vivo remains to be proven. If CCR3 and FPRL1 are not important coreceptors in vivo, why do some R5 envelopes carry the capacity to use them? One possible explanation is that these coreceptors share some structural determinant with CCR5, which is critical for coreceptor function [85].

As more research is performed on non-clade B envelopes, other differences in properties are likely to be identified. Nonetheless, it is likely that the majority of data obtained for clade B R5 envelopes will be highly relevant for other clades.

Does variation in R5 envelope properties affect transmission?

The discovery of a polymorphism in the CCR5 coding region revealed that HIV-1 R5 strains are predominantly transmitted. Thus, individuals homozygous for a mutant form of CCR5 with a 32 bp deletion (Δ32 CCR5) do not express a functional CCR5 receptor and are substantially protected from infection via sexual transmission [86,87]. Other studies on homozygous Δ32/Δ32 CCR5 subjects have also indicated protection via other routes of infection (e.g., via blood contact [88] or mother-to-child transmission [89]). The protection from HIV-1 infection conferred by the Δ32/Δ32 CCR5 genotype indicates that viruses using CXCR4 or other coreceptors rarely transmit. CXCR4-using strains have less opportunity for transmission since R5 strains predominate in most infected individuals until late in disease. Several studies also indicate that R5 viruses predominate in semen, despite the fact that CXCR4-using variants can sometimes be detected [9093]. Extensive stromal cell-derived factor-1 expression by mucosal epithelia may also act as a barrier to X4 viruses [94]. Interestingly, all CXCR4-using viruses, including R5×4 viruses that can use CCR5, are selected against at transmission. However, these dual-tropic R5×4 strainsare frequently sensitive to inhibition by CCR5 chemokines when CXCR4 is absent. Such sensitivity may be sufficient for β-chemokines to prevent R5×4 transmitting via a CCR5 route. These, along with and a series of other potential advantages for R5 viruses [95], may combine to restrict transmission of CXCR4-using viruses.

Whether the different R5 env tropisms described previously affect HIV transmission has not yet been fully addressed. Variants that can exploit low CD4 or CCR5 for infection may have an advantage if cellular targets expressing low amounts of these receptors are critical for transmission. This paradigm will apply for any of the routes of transmissions. Several reviews have already extensively covered mechanisms of HIV-1 transmission [9699]. This article will mainly focus on heterosexual male-to-female transmission. Sexual transmission of R5 viruses is very inefficient, with a transmission rate of one in 1000 exposures frequently cited for both male-to-female and female-to-male transmission [100104]. High viral loads in the acute phase and in late disease, the presence of sexually transmitted infections and lack of male circumcision are factors that increase transmission to rates as high as one in 10–100 exposures [104]. The presence of a tight bottleneck at some stage in trans mission is supported by the fact that usually only a single variant is transmitted sexually. For example, Abrahams et al. recently reported that in 171 clade B and C transmission events, a single variant was transmitted on 78% of occasions [105]. However, recent studies have shown more frequent infection by more than one, and sometimes by several, strains in men who have sex with men and intravenous drug users [106,107].

The bottleneck is almost certainly in the recipient rather than the donor since viral RNA or DNA is readily detected in semen [90,108] and vaginal fluids of many infected male and female subjects, respectively [108115]. In addition, we have observed a variety of R5 phenotypes in semen [14], while this has not been extensively investigated for vaginal fluids. Therefore, it seems probable that the very different properties of R5 envelopes that impact on the use of CD4 and CCR5 and affect replicative capacity will have an impact on the ability of HIV-1 to overcome the bottleneck and establish infection.

Cell types important for transmission

The cell types that are first targeted by HIV-1 following transmission have not been unambiguously defined (reviewed in [98,99]). For sexual transmission, HIV must penetrate either the stratified epithelia of the vagina, ectocervix or the penis, or, alternatively, the single columnar epithelial cell layers of the endocervix or the rectum. For transmission across mucosa that are protected by a stratified epithelium, there is evidence for infection of Langerhan’s cells (LCs), CD4+ T cells and macrophages, as well as capture and transfer of virions by LCs to T cells (see reviews [96,98]). LCs penetrate the cell layers of the stratified epithelium and extend dendrites through the extracellular spaces. They are the closest potentially susceptible cells to the surface of the vagina or penis [98,116,117]. LCs could become infected or capture virions via cell surface langerin (a C-type lectin) or other mechanisms [118]. Following maturation and migration into the stroma or to the regional lymph nodes, LCs may then transfer virions to susceptible T cells while presenting antigen via immunological synapses [96]. Although this is a compelling hypothesis, recent studies have suggested that capture of virions by langerin on immature LCs results in their degradation rather than infection or transinfection of T cells [119]. Nonetheless, activated LCs were reported to be sensitive to infection or able to confer transinfection [96,120], indicating that the LC activation state may be critical in determining whether transmission occurs or fails. In addition, it has been difficult to clearly show productive infection of cervico-vaginal LCs following infection of ectocervical explant cultures [118]. By contrast, LCs purified from skin or present in abraded skin explants are permissive for HIV-1 R5 viruses in vitro [121], and LCs in the skin of infected subjects are clearly infected with HIV-1 [122,123]. Together, these observations indicate that at least some LC populations are permissive to HIV-1. In the simian immunodeficiency virus macaque (SIVmac) model, LCs in the genital mucosa of macaques were reported to be infected within 24 h of exposure to SIVmac [124]. Overall, the role of LCs in transmission remains unclear and requires further evaluation.

CD4+ memory T cells that localize in the lamina propria at sites of transmission are likely to play a major role at some stage during transmission. Under conditions of immune activation (e.g., sexually transmitted infections), CD4+ memory T cells can penetrate the epithelial layers close to the surface and become targets for infection in cultures of intact vaginal epithelial tissue [97,118,125]. In this situation, such cells could be the first infected, resulting in a localized focus of replication and dispersal of virus to sites in the underlying stroma. Gupta et al. provided evidence that CD4+ T cells were initial targets for infection in cervical explant cultures when the barrier was maintained [126], while others have found infected T cells within 24 h of infection of nonbarrier cervical explant cultures [127,128].

Macrophages and DC-SIGN+ DC cells in the submucosa below the stratified epithelium are probably not close enough to the surface of the stratified epithelium to be the first cells infected during transmission via the vagina/ectocervix or penis. Nonetheless, these cell types may play other roles in establishing and spreading the infection in a new host. Since macrophages are long lived, they could help establish a sustained infection. Greenhead used ectocervical or vaginal explants that were infected with-out a barrier and reported that the majority of infected cells were CD68+ macrophages [127]. Macrophages also became infected in a similar study of ectocervical explant infection by Cummins et al. [129].

The DC-SIGN+ DCs may act to spread an initial focus of HIV infection from sites of transmission to regional lymph nodes [99]. DCs support transinfection of CD4+ T cells by virions captured via glycosaminoglycans or other mechanisms (e.g., DC-SIGN) [130133]. Myeloid DCs appear to carry an early postentry restriction to HIV but may still support a low-level productive infection [133]. Regardless, HIV-1 envelopes that confer more efficient entry into such cells will have a better chance of overcoming this restriction to set-up a productive infection.

Langerhan’s cells are present in mucosa that have stratified epithelia on their surfaces, but are largely absent at mucosal sites with a single-cell barrier [134]. Such mucosa include the endocervix and rectum, which are protected by a single layer of columnar epithelial cells [134]. Submucosal macrophages and DCs may play a more significant role in transmission across mucosa at these sites, although macrophages in the intestine were reported to be resistant to HIV-1 infection [135,136].

Receptor levels & transmission

The amount of CD4 and CCR5 on the surface of cells that are potential targets during transmission is likely to have a profound effect on whether they can be infected or not. While macrophages express low levels of CD4, LCs carry low amounts of both CD4 and CCR5. If infection of these cell types is required for transmission, then virus strains that can exploit these low receptor levels will be required. However, examination of the tropism properties of HIV-1 viruses and their envelopes amplified from the plasma of acutely infected individuals has not yet indicated a role for macrophages. Thus, so-called ‘founder’ viruses did not efficiently infect macro phages [137]. These viruses were derived from consensus sequences of entire genomes amplified by PCR shortly after transmission and are predicted to represent those of the transmitted founder virus [137]. Similarly, clade B envelopes from the acute or early stages of infection were demonstrated to confer variable and, at best, modest levels of infectivity for macrophages [57]. These latter env proteins were not ‘corrected’ to consensus founder sequences and it is not known whether they accurately represent the viral strains that were transmitted. Nonetheless, these studies strongly indicate that macrophage tropism is not important for HIV-1 transmission. Since DCs, including LCs, express low levels of CD4 and CCR5, it is likely that these observations also rule out both cell types as important targets for infection during transmission, although expression of surface molecules (e.g., DC-SIGN) that efficiently capture virions could potentially compensate for low receptor levels. Nonetheless, these observations point towards CD4+ T cells as the critical cell type targeted during transmission. The bottleneck could then be explained by the requirement of the transmitting virus (or infected donor cell) to find a susceptible CD4+ T cell for infection. However, whether such T cells are the first cell contacted by the transmitting virus or LCs (that are not infected) play a role in transferring virus to T cells remains to be determined. It is also possible that transmitting viruses carry other unique properties (e.g., enhanced CCR5 use or an increase in replicative capacity) that may increase the likelihood of transmission.

Influence of R5 envelope variation on pathogenesis

While R5 viruses are predominantly transmitted, CXCR4-using (X4 and R5×4) variants can be isolated from at least 50% of AIDS patients in clade B infections and confer a more rapid loss of CD4+ T cells and faster disease progression [5,51,138140]. CXCR4 is more widely expressed on different CD4+ T cell populations (compared with CCR5) and CXCR4-using viruses have a broader T-cell tropism [141,142]. Nevertheless, CD4 depletion and AIDS occur in patients from whom only CCR5-using viruses can be isolated [65,143]. This is particularly apparent in clade C infections, where CXCR4-using variants are detected in far fewer individuals [6165] and AIDS presumably occurs in the absence of CXCR4-using variants, caused directly by CCR5-using R5 viruses. The role of the switch from CCR5 to CXCR4 use for disease progression has been covered by previous reviews [144147] and will not be addressed in detail here.

HIV-1 infection and destruction of the CD4+ T-helper population is the primary cause of the resulting immunodeficiency. In the first few weeks of infection, HIV-1 R5 viruses decimate the CCR5 CD4+ memory cell population present at the mucosa, including in the intestine [148151]. Whether R5 viruses with distinct envelope properties impact on this early loss of the memory cell population or on subsequent systemic pathogenic outcomes is unclear.

R5 variants with an enhanced macrophage tropism [2224] or alternatively with altered CCR5 use [45,46,51,52] have been detected in late disease and may instigate the late decline of CD4+ T cells in the absence of CXCR4-using variants or represent a stage in R5 envelope evolution prior to the emergence of CXCR4-using variants. Enhanced R5 macrophage tropism is associated with an increased affinity of gp120 for CD4, which may increase the efficiency of entry for all CD4+ cell types, including T cells. R5 variants with an increased gp120 charge and altered CCR5 use are associated with faster replication kinetics [45,46] and it is easy to envisage how the emergence of such variants could precipitate a faster decline in CD4+ T cells during late disease. However, this is not proven.

Furthermore, it is not known whether R5 variants detected in the blood along with an enhanced macrophage tropism are predominantly replicating in CD4+ T cells, or whether they have been transported to the blood from tissues where macrophages are major reservoirs of viral infection (e.g., the brain). For example, as demonstrated for SIV, when CD4+ T cells are depleted in late disease, macrophage-tropic R5 variants derived from macrophages in immune tissue may predominate in the blood [152].

It is also worth considering the CD4+ central memory T cell (Tcm) population. Such cells express substantially lower levels of CCR5 compared with effector memory cells and survive the acute phase of infection [153,154]. It has been suggested that the preservation of this cell population allows for sufficient CD4+ effector memory cells to be generated during the asymptomatic phase of disease following the decimation of effector memory cells at mucosal sites early in infection [53,153]. Nevertheless, disease progression is associated with the eventual loss of the Tcm population [53,153] and it is tempting to speculate that their loss is connected to the emergence of putative R5 variants capable of efficiently infecting cells via low levels of CCR5. Consistent with this possibility, Groot et al. reported that Tcms were less susceptible to R5 HIV-1 compared with effector memory cells [155], while Heeregrave et al. reported that HIV-1 sequences recovered from Tcms were compartmentalized compared with those from naive or effector cells [156].

Highly macrophage-tropic R5 viruses play a clearer role in neuropathogenesis (see reviews [157159]). In the absence of therapy, up to approximately 30% of AIDS patients suffer neurological complications, including HAD [157]. The brain is colonized by HIV-1 early in infection [160,161], but it is then difficult to detect virus there during the asymptomatic phase of infection [162165]. As disease progresses, the CD16+ pool of monocytes detected in blood become increasingly activated and expanded [166,167], and this may predispose monocytes for traffic through the blood–brain barrier [168]. Chemokines (e.g., MCP-1), produced by perivascular macrophages in the brain, also act to attract blood monocytes, further exacerbating the situation [169]. Some of the CD16+ monocytes recruited through the blood–brain barrier are likely to be infected and will take HIV-1 in with them [170,171]. This scenario is consistent with the highly macrophage-tropic viruses or envelopes detected in brain tissue of AIDS subjects [12,14,16,21]. As previously discussed, perivascular macrophages and microglia are the main targets for infection [157]. However, there is also evidence that HIV-1 infects astrocytes in the brain in vivo [172183], although this has been controversial [184]. Churchill et al. used laser capture microscopy to isolate immunostained astrocytes and multinucleated giant cells (MNGCs; fused macrophages) before PCR-amplifying V3 loop sequences. Although only two subjects were analyzed, it was striking that V3 sequences from astrocytes were distinct from those derived from proximal MNGCs [175]. This observation is consistent with infection of astrocytes by distinct HIV-1 variants rather than their contamination with fragments of infected MNGCs during their preparation for laser capture microscopy [184]. Astrocyte infection is curious since these cells do not express CD4, although CCR5 expression has been detected in situ in rhesus macaques [185]. The mechanism of HIV entry into astrocytes is therefore unclear, although cell surface receptors, including the mannose receptor, galactocerebroside and undefined receptors, have been demonstrated to bind gp120 and virions [186188]. Whether any of these receptors can explain astrocyte infection by substituting for CD4 so that CCR5 can act as a coreceptor is not known.

Highly macrophage-tropic R5 variants present in the brain probably play a major role in instigating the pathological processes that result in dementia and other neurological dysfunctions. Replication of HIV-1 R5 viruses in the brain could also result in the emergence of variants that are resistant to highly active antiretroviral therapy drugs that penetrate the blood–brain barrier inefficiently. The precise mechanisms that result in neuropathogenesis remain unclear. HIV-1 does not usually infect neurons, despite the fact that there is frequently substantial loss of such cells [157,189]. Neuropathology is closely associated with the accumulation of activated monocyte-derived macrophages in the perivascular regions and generalized immune activation in the brain [157]. A proportion of the macrophages are infected with HIV-1 and some fuse with uninfected macrophages to form MNGCs, which are a hallmark of dementia [157]. Infected and uninfected macrophages that are activated produce an array of cellular and viral proteins. These proteins are believed to further activate bystander cells, including astrocytes, and thus contribute to general immune activation in the brain [157]. Such products may also cause apoptosis of neurons and astrocytes, which is associated with dementia [157].

Although it is clear that R5 viruses present in the brain are highly macrophage tropic, it is not known whether neurovirulent variants are associated with neuropathogenesis. In the SIV/rhesus macaque model, macrophage-tropic variants were described that differed drastically in neurovirulence outcomes and may be a precedent for HIV-1 in the brain [190,191]. How the enhanced macrophage tropism that we and others have described relates to neurovirulence is also unknown. This is because the viral determinants of neurovirulence are not known and may include nonenvelope genes (e.g., nef) [190]. Many studies have reported that the HIV-1 envelope has various toxic affects on different brain cells in vitro in many different assays [157]. However, it remains unclear how these in vitro assays of toxicity relate to the neuropathogenic events in vivo. Currently, there are no suitable animal models to ascertain whether a particular HIV-1 isolate or envelope can cause or even influence neuropathogenesis.

Does variation in R5 virus tropism & receptor use affect therapy with CCR5 antagonists?

Maraviroc, a CCR5 antagonist, is a small organic molecule (molecular weight: 513.67) that is licensed for use in therapy throughout the developed world. In vitro selection experiments have indicated that R5 variants found in the asymptomatic phase will be highly sensitive and unlikely to readily evolve escape variants [192]. Maraviroc-resistant viruses selected in vitro had adapted to use CCR5 bound by the drug rather than switching to use CXCR4 [192,193]. In vitro escape was not straightforward, requiring multiple passages in increasing concentration of maraviroc to generate the set of mutations required for resistance [192,194]. Currently, maraviroc is not being used as a first line of defense owing to the superior effectiveness and pharmacological profile of other drugs. Nevertheless, an important niche has been found for maraviroc as a salvage or change therapy for individuals who develop resistance variants or cannot tolerate first-line therapies [195]. However, this means maraviroc is used at later stages of disease compared with the first-line drugs and there are obvious issues with this strategy. CXCR4-using variants are more likely to emerge at later stages of infection and their presence would probably render treatment ineffective. It is also possible that CCR5 blockade may actually select for the more virulent CXCR4-using variants if they are present at low levels, as has been suggested in some of the maraviroc trials [196,197]. This possibility means that subjects who are candidates for maraviroc therapy need to be screened for the presence of CXCR4-using variants before treatment can begin [198,199]. This process is expensive and may further weigh therapeutic choices against maraviroc [196]. Finally, the detection of R5 variants with decreased sensitivity to CCR5 antagonists late in disease [45,46,51,192] is also a consideration for the use of maraviroc at this stage, as they may represent viral strains that have a head start in evolving resistance. Nevertheless, maraviroc and other candidate CCR5 antagonists are potent inhibitors of CCR5-mediated entry. Such reagents will be used in combination with other HIV inhibitors, which will contribute to the prevention of viral replication and greatly reduce the likelihood of the emergence of resistant variants that retain CCR5 use or have switched to CXCR4.


R5 viruses are predominantly transmitted and cause AIDS even in the absence of CXCR4-using variants. Over 25 years after the identification of HIV-1, we are still discovering the extent of variability in the biological properties conferred by the envelopes of R5 viruses. R5 envelopes confer variable extents of macrophage infectivity, vary in their use of receptors CD4 and CCR5 and in sensitivity to inhibitors, and antibodies that block viral entry and viral entry kinetics. Appreciating the extent of the variation in these different properties is crucial for fully understanding how HIV-1 transmits and for the optimal design of vaccines that will target transmissible HIV-1 variants. In addition, highly macrophage-tropic R5 variants predominate in the brain tissue of subjects with HAD, although it is unclear how these variants confer neurovirulence. Finally, HIV-1 replication in brain tissue may result in the emergence of variants that are resistant to highly active antiretroviral therapy drugs that only inefficiently penetrate the blood–brain barrier.

Future perspective

The effects of HIV-1 R5 variability on transmission and pathogenesis are still poorly understood and will be the focus of future studies. Better appreciation of the environmental pressures in vivo that select for or against R5 envelopes with distinct properties will provide insights into the phenotypes and envelope changes that are associated with AIDS and neuropathogenesis, and will aid the design of novel approaches for intervention. Understanding the full characteristics of transmitted non-macrophage-tropic R5 viruses will facilitate the generation of vaccines and microbicides that target the most vulnerable stages of viral entry and envelope sites.

Executive summary

Variation in macrophage tropism of HIV-1 R5 viruses

  • [filled square] Macrophage tropism of R5 viruses varies greatly and increases in later stages of disease.
  • [filled square] R5 macrophage tropism is associated with enhanced interactions with CD4 and variation in residues within or proximal to the CD4 binding site.
  • [filled square] Highly macrophage-tropic R5 viruses are present in the brain and associated with neuropathogenesis.

Evolution in CCR5 use

  • [filled square] As disease progresses, R5 envelopes also evolve altered use of CCR5, an increase in envelope positive charge and reduced sensitivity to CCR5 chemokines.
  • [filled square] Altered use of CCR5 appears unrelated to changes in macrophage tropism.

Cross-clade variation of R5 viruses

  • [filled square] Clade C R5 viruses may infect CD34+ progenitor cells more efficiently than clade B.

Role of R5 virus variation in transmission

  • [filled square] Transmission is usually conferred by R5 viruses.
  • [filled square] Transmitted viruses appear non-macrophage tropic, indicating that CD4+ T cells are probably the first targets for infection.

Role of R5 virus variation in pathogenesis

  • [filled square] R5 virus variants that confer enhanced interactions with CD4 and/or CCR5 become more prevalent late in disease and may cause the depletion of CD4+ T cells in the absence of CXCR4-using variants.
  • [filled square] Macrophage-tropic R5 viruses cause neuropathogenesis via infection of brain macrophages and microglia.
  • [filled square] Replication of R5 viruses in the brain results in the upregulation of factors toxic to neurons and astrocytes, and leads to the activated state of astrocytic cells.

Considerations for therapy with CCR5 antagonists

  • [filled square] The use of CCR5 antagonists in therapy may select for CXCR4-using variants or resistant R5 variants that can exploit drug-occupied CCR5.
  • [filled square] However, the use of CCR5 antagonists with other highly active antiretroviral therapy drugs will greatly reduce the chances of resistance developing.


The authors’ research on the properties of HIV-1 R5 viruses is currently sponsored by NIH grants HD049273, R01 MH64408 and P01 AI082274, and is supported by the University of Massachusetts Medical School Center for AIDS Research.


Financial & competing interests disclosure

The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.


Papers of special note have been highlighted as:

[filled square][filled square] of considerable interest

1. Barre Sinoussi F, Chermann JC, Rey F, et al. Isolation of a T-lymphotropic retrovirus from a patient at risk for acquired immune deficiency syndrome (AIDS) Science. 1983;220:868–871. [PubMed]
2. Dalgleish AG, Beverley PC, Clapham PR, Crawford DH, Greaves MF, Weiss RA. The CD4 (T4) antigen is an essential component of the receptor for the AIDS retrovirus. Nature. 1984;312:763–767. [PubMed]
3. Klatzmann D, Champagne E, Chamaret S, et al. T-lymphocyte T4 molecule behaves as the receptor for human retrovirus LAV. Nature. 1984;312:767–768. [PubMed]
4. Feng Y, Broder CC, Kennedy PE, Berger EA. HIV-1 entry cofactor: functional cDNA cloning of a seven-transmembrane, G protein-coupled receptor. Science. 1996;272:872–877. [PubMed]
5. Asjo B, Morfeldt Manson L, Albert J, et al. Replicative capacity of human immunodeficiency virus from patients with varying severity of HIV infection. Lancet. 1986;2:660–662. [PubMed]
6. Tersmette M, de Goede RE, Al BJ, et al. Differential syncytium-inducing capacity of human immunodeficiency virus isolates: frequent detection of syncytium-inducing isolates in patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex. J. Virol. 1988;62:2026–2032. [PMC free article] [PubMed]
7. Gartner S, Markovits P, Markovitz DM, Kaplan MH, Gallo RC, Popovic M. The role of mononuclear phagocytes in HTLV-III/LAV infection. Science. 1986;233:215–219. [PubMed]
8. Deng H, Liu R, Ellmeier W, et al. Identification of a major co-receptor for primary isolates of HIV-1. Nature. 1996;381:661–666. [PubMed]
9. Dragic T, Litwin V, Allaway GP, et al. HIV-1 entry into CD4+ cells is mediated by the chemokine receptor CC-CKR-5. Nature. 1996;381:667–673. [PubMed]
10. Alkhatib G, Combadiere C, Broder CC, et al. CC CKR5: a RANTES, MIP-1α, MIP-1β receptor as a fusion cofactor for macrophage-tropic HIV-1. Science. 1996;272:1955–1958. [PubMed]
11. Berger EA, Doms RW, Fenyo E-M, et al. A new classification for HIV-1. Nature. 1998;391:240. [PubMed]
12. Peters PJ, Bhattacharya J, Hibbitts S, et al. Biological analysis of human immunodeficiency virus type 1 R5 envelopes amplified from brain and lymph node tissues of AIDS patients with neuropathology reveals two distinct tropism phenotypes and identifies envelopes in the brain that confer an enhanced tropism and fusigenicity for macrophages. J. Virol. 2004;78:6915–6926. [PubMed][filled square][filled square] Demonstrates that R5 envelopes from brain and lymph node tissue differed remarkably in their capacities to use low levels of CD4 for entry and infection of primary macrophages by at least 1000-fold
13. Peters PJ, Duenas-Decamp MJ, Sullivan WM, et al. Variation in HIV-1 R5 macrophage-tropism correlates with sensitivity to reagents that block envelope: CD4 interactions but not with sensitivity to other entry inhibitors. Retrovirology. 2008;5:5. [PubMed][filled square][filled square] Demonstrates that R5 macrophage tropism correlates with sensitivity to reagents that block envelope-CD4 interactions and confirms that this tropism is directly related to variation in this early entry event
14. Peters PJ, Sullivan WM, Duenas-Decamp MJ, et al. Non-macrophage-tropic human immunodeficiency virus type 1 R5 envelopes predominate in blood, lymph nodes, and semen: implications for transmission and pathogenesis. J. Virol. 2006;80:6324–6332. [PubMed][filled square][filled square] Demonstrates that non-macrophage-tropic R5 envelopes are highly prevalent outside the brain, including immune tissue
15. Gorry PR, Bristol G, Zack JA, et al. Macrophage tropism of human immunodeficiency virus type 1 isolates from brain and lymphoid tissues predicts neurotropism independent of coreceptor specificity. J. Virol. 2001;75:10073–10089. [PubMed][filled square][filled square] Demonstrates that macrophage tropism was a better predictor of neurotropic viruses than the coreceptors they used for entry
16. Gorry PR, Taylor J, Holm GH, et al. Increased CCR5 affinity and reduced CCR5/CD4 dependence of a neurovirulent primary human immunodeficiency virus type 1 isolate. J. Virol. 2002;76:6277–6292. [PubMed][filled square][filled square] First study to demonstrate that a brain HIV-1 isolate could use low levels of CD4 for entry
17. Koyanagi Y, Miles S, Mitsuyasu RT, Merrill JE, Vinters HV, Chen IS. Dual infection of the central nervous system by AIDS viruses with distinct cellular tropisms. Science. 1987;236:819–822. [PubMed][filled square][filled square] Seminal study that showed for the first time that there are profound differences in the tropism of HIV-1 R5 isolates
18. Simmons G, Wilkinson D, Reeves JD, et al. Primary, syncytium-inducing human immunodeficiency virus type 1 isolates are dual-tropic and most can use either Lestr or CCR5 as coreceptors for virus entry. J. Virol. 1996;70:8355–8360. [PMC free article] [PubMed]
19. Dejucq N, Simmons G, Clapham PR. Expanded tropism of primary human immunodeficiency virus type 1 R5 strains to CD4+ T-cell lines determined by the capacity to exploit low concentrations of CCR5. J. Virol. 1999;73:7842–7847. [PubMed][filled square][filled square] Subset of R5 isolates were shown for the first time to be able to exploit low levels of CCR5 for infection. Such variants may be able to target distinct CD4+ T-cell populations (e.g., central memory T cells) that express low CCR5. Infection and depletion of such cells may be critical events in the development of AIDS
20. Martin J, LaBranche CC, Gonzalez-Scarano F. Differential CD4/CCR5 utilization, gp120 conformation, and neutralization sensitivity between envelopes from a microglia-adapted human immunodeficiency virus type 1 and its parental isolate. J. Virol. 2001;75:3568–3580. [PMC free article] [PubMed]
21. Thomas ER, Dunfee RL, Stanton J, et al. Macrophage entry mediated by HIV Envs from brain and lymphoid tissues is determined by the capacity to use low CD4 levels and overall efficiency of fusion. Virology. 2007;360:105–119. [PubMed][filled square][filled square] Demonstrates that R5 envelopes confered a spectrum of macrophage tropism rather than distinct macrophage-tropic and non-macrophage-tropic phenotypes. Nevertheless, macrophage tropism still varied over several orders of magnitude
22. Gray L, Sterjovski J, Churchill M, et al. Uncoupling coreceptor usage of human immunodeficiency virus type 1 (HIV-1) from macrophage tropism reveals biological properties of CCR5-restricted HIV-1 isolates from patients with acquired immunodeficiency syndrome. Virology. 2005;337:384–398. [PubMed]
23. Li S, Juarez J, Alali M, et al. Persistent CCR5 utilization and enhanced macrophage tropism by primary blood human immunodeficiency virus type 1 isolates from advanced stages of disease and comparison to tissue-derived isolates. J. Virol. 1999;73:9741–9755. [PMC free article] [PubMed]
24. Tuttle DL, Anders CB, Aquino-De Jesus MJ, et al. Increased replication of non-syncytium-inducing HIV type 1 isolates in monocyte-derived macrophages is linked to advanced disease in infected children. AIDS Res. Hum. Retroviruses. 2002;18:353–362. [PubMed][filled square][filled square] References [2224] demonstrate that R5 iolates from late disease frequently confer increased macrophage infectivity. Could suggest that enhanced macrophage tropism confers an increased virulence for CD4+ T cells and contribute to disease progression
25. Ho HT, Fan L, Nowicka-Sans B, et al. Envelope conformational changes induced by human immunodeficiency virus type 1 attachment inhibitors prevent CD4 binding and downstream entry events. J. Virol. 2006;80:4017–4025. [PMC free article] [PubMed]
26. Dunfee RL, Thomas ER, Gabuzda D. Enhanced macrophage tropism of HIV in brain and lymphoid tissues is associated with sensitivity to the broadly neutralizing CD4 binding site antibody b12. Retrovirology. 2009;6:69. [PubMed][filled square][filled square] Enhanced R5 macrophage tropism was demonstrated to correlate with increased b12 sensitivity, indicating an increased exposure of the CD4 binding site
27. Bannert N, Schenten D, Craig S, Sodroski J. The level of CD4 expression limits infection of primary rhesus monkey macrophages by a T-tropic simian immunodeficiency virus and macrophagetropic human immunodeficiency viruses. J. Virol. 2000;74:10984–10993. [PMC free article] [PubMed]
28. Lee B, Sharron M, Montaner LJ, Weissman D, Doms RW. Quantification of CD4, CCR5, and CXCR4 levels on lymphocyte subsets, dendritic cells, and differentially conditioned monocyte-derived macrophages. Proc. Natl Acad. Sci. USA. 1999;96:5215–5220. [PubMed]
29. Mori K, Rosenzweig M, Desrosiers RC. Mechanisms for adaptation of simian immunodeficiency virus to replication in alveolar macrophages. J. Virol. 2000;74:10852–10859. [PMC free article] [PubMed]
30. Dunfee RL, Thomas ER, Gorry PR, et al. The HIV Env variant N283 enhances macrophage tropism and is associated with brain infection and dementia. Proc. Natl Acad. Sci. USA. 2006;103:15160–15165. [PubMed][filled square][filled square] First to report on envelope determinants of R5 macrophage tropism. The presence of an asparagine at residue 283 (N283) in the CD4 binding site was shown to confer increased macrophage infection and to be associated with envelopes in the brain of subjects with HIV-associated dementia
31. Duenas-Decamp MJ, Peters PJ, Burton D, Clapham PR. Determinants flanking the CD4 binding loop modulate macrophage tropism of human immunodeficiency virus type 1 R5 envelopes. J. Virol. 2009;83:2575–2583. [PubMed][filled square][filled square] Mapped further determinants of R5 macrophage tropism and demonstrated that residues immediately adjacent to CD4 contact residues in the CD4-binding loop were involved
32. Sterjovski J, Churchill MJ, Ellett A, et al. Asn362 in gp120 contributes to enhanced fusogenicity by CCR5-restricted HIV-1 envelope glycoprotein variants from patients with AIDS. Retrovirology. 2007;4:89. [PMC free article] [PubMed]
33. Wu X, Zhou T, O’Dell S, Wyatt RT, Kwong PD, Mascola JR. Mechanism of HIV-1 resistance to monoclonal antibody b12 that effectively targets the site of CD4 attachment. J. Virol. 2009;83:10892–10907. [PMC free article] [PubMed]
34. Power C, McArthur JC, Johnson RT, et al. Distinct HIV-1 env sequences are associated with neurotropism and neurovirulence. Curr. Top. Microbiol. Immunol. 1995;202:89–104. [PubMed]
35. Pillai SK, Pond SL, Liu Y, et al. Genetic attributes of cerebrospinal fluid-derived HIV-1 env. Brain. 2006;129:1872–1883. [PubMed]
36. Strain MC, Letendre S, Pillai SK, et al. Genetic composition of human immunodeficiency virus type 1 in cerebrospinal fluid and blood without treatment and during failing antiretroviral therapy. J. Virol. 2005;79:1772–1788. [PMC free article] [PubMed]
37. Wyatt R, Thali M, Tilley S, et al. Relationship of the human immunodeficiency virus type 1 gp120 third variable loop to a component of the CD4 binding site in the fourth conserved region. J. Virol. 1992;66:6997–7004. [PMC free article] [PubMed]
38. Dunfee RL, Thomas ER, Wang J, Kunstman K, Wolinsky SM, Gabuzda D. Loss of the N-linked glycosylation site at position 386 in the HIV envelope V4 region enhances macrophage tropism and is associated with dementia. Virology. 2007;367:222–234. [PMC free article] [PubMed]
39. Duenas-Decamp MJ, Peters P, Burton D, Clapham PR. Natural resistance of human immunodeficiency virus type 1 to the CD4bs antibody b12 conferred by a glycan and an arginine residue close to the CD4 binding loop. J. Virol. 2008;82:5807–5814. [PMC free article] [PubMed]
40. Sanders RW, van Anken E, Nabatov AA, et al. The carbohydrate at asparagine 386 on HIV-1 gp120 is not essential for protein folding and function but is involved in immune evasion. Retrovirology. 2008;5:10. [PMC free article] [PubMed]
41. Walker LM, Phogat SK, Chan-Hui PY, et al. Broad and potent neutralizing antibodies from an African donor reveal a new HIV-1 vaccine target. Science. 2009;326(5950):285–289. [PMC free article] [PubMed]
42. Sather DN, Armann J, Ching LK, et al. Factors associated with the development of cross-reactive neutralizing antibodies during HIV-1 infection. J. Virol. 2009;83:4713–4715.
43. Li Y, Svehla K, Louder MK, et al. Analysis of the neutralization specificities in polyclonal sera derived from human immunodeficiency virus type-1 infected individuals. J. Virol. 2008;83:1045–1059. [PMC free article] [PubMed]
44. Binley JM, Lybarger EA, Crooks ET, et al. Profling the specificity of neutralizing antibodies in a large panel of plasmas from patients chronically infected with human immunodeficiency virus type 1 subtypes B and C. J. Virol. 2008;82:11651–11668. [PMC free article] [PubMed]
45. Repits J, Oberg M, Esbjornsson J, et al. Selection of human immunodeficiency virus type 1 R5 variants with augmented replicative capacity and reduced sensitivity to entry inhibitors during severe immunodeficiency. J. Gen. Virol. 2005;86:2859–2869. [PubMed][filled square][filled square] HIV-1 R5 variants with enhanced replicative capacity and reduced sensitivity to CCR5 ligands are shown to emerge in severe immunodeficiency. These variants may contribute to the depletion of CD4+ T cells in the absence of CXCR4-using variants
46. Repits J, Sterjovski J, Badia-Martinez D, et al. Primary HIV-1 R5 isolates from end-stage disease display enhanced viral fitness in parallel with increased gp120 net charge. Virology. 2008;379:125–134. [PubMed]
47. Karlsson I, Antonsson L, Shi Y, et al. HIV biological variability unveiled: frequent isolations and chimeric receptors reveal unprecedented variation of coreceptor use. AIDS. 2003;17:2561–2569. [PubMed]
48. Karlsson I, Antonsson L, Shi Y, et al. Coevolution of RANTES sensitivity and mode of CCR5 receptor use by human immunodeficiency virus type 1 of the R5 phenotype. J. Virol. 2004;78:11807–11815. [PMC free article] [PubMed]
49. Cavarelli M, Karlsson I, Zanchetta M, et al. HIV-1 with multiple CCR5/CXCR4 chimeric receptor use is predictive of immunological failure in infected children. PLoS ONE. 2008;3:e3292. [PMC free article] [PubMed]
50. Borggren M, Repits J, Kuylenstierna C, et al. Evolution of DC-SIGN use revealed by fitness studies of R5 HIV-1 variants emerging during AIDS progression. Retrovirology. 2008;5:28. [PMC free article] [PubMed]
51. Scarlatti G, Tresoldi E, Bjorndal A, et al. In vivo evolution of HIV-1 co-receptor usage and sensitivity to chemokine-mediated suppression. Nat. Med. 1997;3:1259–1265. [PubMed][filled square][filled square] Provides an early indication that R5 variants in late disease have evolved decreased sensitivity to CCR5 chemokines, even without switching to CXCR4
52. Jansson M, Backstrom E, Bjorndal A, et al. Coreceptor usage and RANTES sensitivity of non-syncytium-inducing HIV-1 isolates obtained from patients with AIDS. J. Hum. Virol. 1999;2:325–338. [PubMed]
53. Okoye A, Meier-Schellersheim M, Brenchley JM, et al. Progressive CD4+ central memory T cell decline results in CD4+ effector memory insufficiency and overt disease in chronic SIV infection. J. Exp. Med. 2007;204:2171–2185. [PMC free article] [PubMed]
54. Bullard DE, Bourdon M, Bigner DD. Comparison of various methods for delivering radiolabeled monoclonal antibody to normal rat brain. J. Neurosurg. 1984;61:901–911. [PubMed]
55. Kuang F, Wang BR, Zhang P, et al. Extravasation of blood-borne immunoglobulin G through blood-brain barrier during adrenaline-induced transient hypertension in the rat. Int. J. Neurosci. 2004;114:575–591. [PubMed]
56. Triguero D, Buciak JB, Yang J, Pardridge WM. Blood-brain barrier transport of cationized immunoglobulin G: enhanced delivery compared with native protein. Proc. Natl Acad. Sci. USA. 1989;86:4761–4765. [PubMed]
57. Isaacman-Beck J, Hermann EA, Yi Y, et al. Heterosexual transmission of human immunodeficiency virus type 1 subtype C: macrophage tropism, alternative coreceptor use, and the molecular anatomy of CCR5 utilization. J. Virol. 2009;83:8208–8220. [PubMed][filled square][filled square] Demonstrates that R5 envelopes from the acute and near-acute stage of infection varied in their capacity to infect primary macrophages. However, overall macrophage infection was, at best, only modest
58. Richards KH, Asa-Chapman MM, McKnight A, Clapham PR. Modulation of HIV-1 macrophage-tropisism among R5 envelopes occurs before detection of neutralizing antibodies. Retrovirology. 2010;7:48. [PMC free article] [PubMed]
59. Keele BF, Van Heuverswyn F, Li Y, et al. Chimpanzee reservoirs of pandemic and nonpandemic HIV-1. Science. 2006;313:523–526. [PMC free article] [PubMed]
60. Van Heuverswyn F, Li Y, Neel C, et al. Human immunodeficiency viruses: SIV infection in wild gorillas. Nature. 2006;444:164. [PubMed]
61. Peeters M, Vincent R, Perret JL, et al. Evidence for differences in MT2 cell tropism according to genetic subtypes of HIV-1: syncytium-inducing variants seem rare among subtype C HIV-1 viruses. J. Acquir. Immune Defc. Syndr. Hum. Retrovirol. 1999;20:115–121. [PubMed]
62. Abebe A, Demissie D, Goudsmit J, et al. HIV-1 subtype C syncytium- and non-syncytium-inducing phenotypes and coreceptor usage among Ethiopian patients with AIDS. AIDS. 1999;13:1305–1311. [PubMed]
63. Bjorndal A, Sonnerborg A, Tscherning C, Albert J, Fenyo EM. Phenotypic characteristics of human immunodeficiency virus type 1 subtype C isolates of Ethiopian AIDS patients. AIDS Res. Hum. Retroviruses. 1999;15:647–653. [PubMed]
64. Morris L, Cilliers T, Bredell H, Phoswa M, Martin DJ. CCR5 is the major coreceptor used by HIV-1 subtype C isolates from patients with active tuberculosis. AIDS Res. Hum. Retroviruses. 2001;17:697–701. [PubMed]
65. Cecilia D, Kulkarni SS, Tripathy SP, Gangakhedkar RR, Paranjape RS, Gadkari DA. Absence of coreceptor switch with disease progression in human immunodeficiency virus infections in India. Virology. 2000;271:253–258. [PubMed]
66. Huang W, Eshleman SH, Toma J, et al. Coreceptor tropism in human immunodeficiency virus type 1 subtype D: high prevalence of CXCR4 tropism and heterogeneous composition of viral populations. J. Virol. 2007;81:7885–7893. [PMC free article] [PubMed]
67. Spira S, Wainberg MA, Loemba H, Turner D, Brenner BG. Impact of clade diversity on HIV-1 virulence, antiretroviral drug sensitivity and drug resistance. J. Antimicrob. Chemother. 2003;51:229–240. [PubMed]
68. Redd AD, Avalos A, Essex M. Infection of hematopoietic progenitor cells by HIV-1 subtype C, and its association with anemia in southern Africa. Blood. 2007;110:3143–3149. [PubMed]
69. Davis BR, Schwartz DH, Marx JC, et al. Absent or rare human immunodeficiency virus infection of bone marrow stem/progenitor cells in vivo. J. Virol. 1991;65:1985–1990. [PMC free article] [PubMed]
70. Shen H, Cheng T, Preffer FI, et al. Intrinsic human immunodeficiency virus type 1 resistance of hematopoietic stem cells despite coreceptor expression. J. Virol. 1999;73:728–737. [PMC free article] [PubMed]
71. Carter CC, Onafuwa-Nuga A, McNamara LA, et al. HIV-1 infects multipotent progenitor cells causing cell death and establishing latent cellular reservoirs. Nat. Med. 2010;16:446–451. [PMC free article] [PubMed]
72. Shankarappa R, Chatterjee R, Learn GH, et al. Human immunodeficiency virus type 1 env sequences from Calcutta in eastern India: identification of features that distinguish subtype C sequences in India from other subtype C sequences. J. Virol. 2001;75:10479–10487. [PMC free article] [PubMed]
73. Abraha A, Nankya IL, Gibson R, et al. CCR5- and CXCR4-tropic subtype C human immunodeficiency virus type 1 isolates have a lower level of pathogenic fitness than other dominant group M subtypes: implications for the epidemic. J. Virol. 2009;83:5592–5605. [PMC free article] [PubMed]
74. Rodriguez MA, Ding M, Ratner D, et al. High replication fitness and transmission efficiency of HIV-1 subtype C from India: implications for subtype C predominance. Virology. 2009;385:416–424. [PubMed]
75. Chohan B, Lang D, Sagar M, et al. Selection for human immunodeficiency virus type 1 envelope glycosylation variants with shorter V1-V2 loop sequences occurs during transmission of certain genetic subtypes and may impact viral RNA levels. J. Virol. 2005;79:6528–6531. [PMC free article] [PubMed]
76. Derdeyn CA, Decker JM, Bibollet-Ruche F, et al. Envelope-constrained neutralization-sensitive HIV-1 after heterosexual transmission. Science. 2004;303:2019–2022. [PubMed]
77. Frost SD, Liu Y, Pond SL, et al. Characterization of human immunodeficiency virus type 1 (HIV-1) envelope variation and neutralizing antibody responses during transmission of HIV-1 subtype B. J. Virol. 2005;79:6523–6527. [PMC free article] [PubMed]
78. Liu Y, Curlin ME, Diem K, et al. Env length and N-linked glycosylation following transmission of human immunodeficiency virus type 1 subtype B viruses. Virology. 2008;374:229–233. [PMC free article] [PubMed]
79. Aasa-Chapman MM, Aubin K, Williams I, McKnight A. Primary CCR5 only using HIV-1 isolates does not accurately represent the in vivo replicating quasi-species. Virology. 2006;351:489–496. [PubMed]
80. Sallusto F, Mackay CR, Lanzavecchia A. Selective expression of the eotaxin receptor CCR3 by human T helper 2 cells. Science. 1997;277:2005–2007. [PubMed]
81. He J, Chen Y, Farzan M, et al. CCR3 and CCR5 are co-receptors for HIV-1 infection of microglia. Nature. 1997;385:645–649. [PubMed]
82. Agrawal L, Maxwell CR, Peters PJ, et al. Complexity in human immunodeficiency virus type 1 (HIV-1) co-receptor usage: roles of CCR3 and CCR5 in HIV-1 infection of monocyte-derived macrophages and brain microglia. J. Gen. Virol. 2009;90:710–722. [PubMed]
83. Willey SJ, Reeves JD, Hudson R, et al. Identification of a subset of human immunodeficiency virus type 1 (HIV-1), HIV-2, and simian immunodeficiency virus strains able to exploit an alternative coreceptor on untransformed human brain and lymphoid cells. J. Virol. 2003;77:6138–6152. [PMC free article] [PubMed]
84. Nedellec R, Coetzer M, Shimizu N, et al. Virus entry via the alternative coreceptors CCR3 and FPRL1 differs by human immunodeficiency virus type 1 subtype. J. Virol. 2009;83:8353–8363. [PMC free article] [PubMed]
85. Gray L, Churchill MJ, Keane N, et al. Genetic and functional analysis of R5×4 human immunodeficiency virus type 1 envelope glycoproteins derived from two individuals homozygous for the CCR5δ32 allele. J. Virol. 2006;80:3684–3691. [PMC free article] [PubMed]
86. Carrington M, Dean M, Martin MP, O’Brien SJ. Genetics of HIV-1 infection: chemokine receptor CCR5 polymorphism and its consequences. Hum. Mol. Genet. 1999;8:1939–1945. [PubMed]
87. Dean M, Carrington M, Winkler C, et al. Genetic restriction of HIV-1 infection and progression to AIDS by a deletion allele of the CKR5 structural gene. Hemophilia Growth and Development Study, Multicenter AIDS Cohort Study, Multicenter Hemophilia Cohort Study, San Francisco City Cohort, ALIVE Study. Science. 1996;273:1856–1862. [PubMed]
88. Wilkinson DA, Operskalski EA, Busch MP, Mosley JW, Koup RA. A 32-bp deletion within the CCR5 locus protects against transmission of parenterally acquired human immunodeficiency virus but does not affect progression to AIDS-defining illness. J. Infect. Dis. 1998;178:1163–1166. [PubMed]
89. Philpott S, Burger H, Charbonneau T, et al. CCR5 genotype and resistance to vertical transmission of HIV-1. J. Acquir. Immune Defc. Syndr. 1999;21:189–193. [PubMed]
90. Curran R, Ball JK. Concordance between semen-derived HIV-1 proviral DNA and viral RNA hypervariable region 3 (V3) envelope sequences in cases where semen populations are distinct from those present in blood. J. Med. Virol. 2002;67:9–19. [PubMed]
91. Paranjpe S, Craigo J, Patterson B, et al. Subcompartmentalization of HIV-1 quasispecies between seminal cells and seminal plasma indicates their origin in distinct genital tissues. AIDS Res. Hum. Retroviruses. 2002;18:1271–1280. [PubMed]
92. Pillai SK, Good B, Pond SK, et al. Semen-specific genetic characteristics of human immunodeficiency virus type 1 env. J. Virol. 2005;79:1734–1742. [PMC free article] [PubMed]
93. Lawrence P, Berlier W, Delezay O, et al. Construction and tropism characterisation of recombinant viruses exhibiting HIV-1 env gene from seminal strains. Virology. 2009;386:373–379. [PubMed]
94. Agace WW, Amara A, Roberts AI, et al. Constitutive expression of stromal derived factor-1 by mucosal epithelia and its role in HIV transmission and propagation. Curr. Biol. 2000;10:325–328. [PubMed]
95. Margolis L, Shattock R. Selective transmission of CCR5-utilizing HIV-1: the ‘gatekeeper’ problem resolved? Nat. Rev. Microbiol. 2006;4:312–317. [PubMed]
96. de Jong MA, Geijtenbeek TB. Human immunodeficiency virus-1 acquisition in genital mucosa: Langerhans cells as key-players. J. Intern. Med. 2009;265:18–28. [PubMed]
97. Hladik F, Hope TJ. HIV infection of the genital mucosa in women. Curr. HIV/AIDS Rep. 2009;6:20–28. [PubMed]
98. Hladik F, McElrath MJ. Setting the stage: host invasion by HIV. Nat. Rev. Immunol. 2008;8:447–457. [PMC free article] [PubMed]
99. Wu L. Biology of HIV mucosal transmission. Curr. Opin. HIV AIDS. 2008;3:534–540. [PMC free article] [PubMed]
100. Wawer MJ, Gray RH, Sewankambo NK, et al. Rates of HIV-1 transmission per coital act, by stage of HIV-1 infection, in Rakai, Uganda. J. Infect. Dis. 2005;191:1403–1409. [PubMed]
101. Shiboski SC, Padian NS. Epidemiologic evidence for time variation in HIV infectivity. J. Acquir. Immune Defc. Syndr. Hum. Retrovirol. 1998;19:527–535. [PubMed]
102. Leynaert B, Downs AM, de Vincenzi I. Heterosexual transmission of human immunodeficiency virus: variability of infectivity throughout the course of infection. European Study Group on Heterosexual Transmission of HIV. Am. J. Epidemiol. 1998;148:88–96. [PubMed]
103. Saracco A, Veglia F, Lazzarin A. Risk of HIV-1 transmission in heterosexual stable and random couples. The Italian Partner Study. J. Biol. Regul. Homeost. Agents. 1997;11:3–6. [PubMed]
104. Powers KA, Poole C, Pettifor AE, Cohen MS. Rethinking the heterosexual infectivity of HIV-1: a systematic review and meta-ana-lysis. Lancet Infect. Dis. 2008;8:553–563. [PMC free article] [PubMed]
105. Abrahams MR, Anderson JA, Giorgi EE, et al. Quantitating the multiplicity of infection with human immunodeficiency virus type 1 subtype C reveals a non-poisson distribution of transmitted variants. J. Virol. 2009;83:3556–3567. [PMC free article] [PubMed]
106. Bar KJ, Li H, Chamberland A, et al. Wide variation in the multiplicity of HIV-1 infection among injection drug users. J. Virol. 2010;84(12):6241–6247. [PMC free article] [PubMed]
107. Li H, Bar KJ, Wang S, Decker JM, et al. High multiplicity infection by HIV-1 in men who have sex with men. PLoS Pathog. 2010;6:e1000890. [PMC free article] [PubMed]
108. Craigo JK, Gupta P. HIV-1 in genital compartments: vexing viral reservoirs. Curr. Opin. HIV AIDS. 2006;1:97–102. [PubMed]
109. Benki S, McClelland RS, Emery S, et al. Quantification of genital human immunodeficiency virus type 1 (HIV-1) DNA in specimens from women with low plasma HIV-1 RNA levels typical of HIV-1 nontransmitters. J. Clin. Microbiol. 2006;44:4357–4362. [PMC free article] [PubMed]
110. De Pasquale MP, Leigh Brown AJ, Uvin SC, et al. Differences in HIV-1 pol sequences from female genital tract and blood during antiretroviral therapy. J. Acquir. Immune Defc. Syndr. 2003;34:37–44. [PubMed]
111. Kovacs A, Chan LS, Chen ZC, et al. HIV-1 RNA in plasma and genital tract secretions in women infected with HIV-1. J. Acquir. Immune Defc. Syndr. 1999;22:124–131. [PubMed]
112. Sutthent R, Sumrangsurp K, Wirachsilp P, et al. Diversity of HIV-1 subtype E in semen and cervicovaginal secretion. J. Hum. Virol. 2001;4:260–268. [PubMed]
113. Tirado G, Jove G, Kumar R, et al. Differential virus evolution in blood and genital tract of HIV-infected females: evidence for the involvement of drug and non-drug resistance-associated mutations. Virology. 2004;324:577–586. [PubMed]
114. Tirado G, Jove G, Reyes E, et al. Differential evolution of cell-associated virus in blood and genital tract of HIV-infected females undergoing HAART. Virology. 2005;334:299–305. [PubMed]
115. Zhu T, Wang N, Carr A, et al. Genetic characterization of human immunodeficiency virus type 1 in blood and genital secretions: evidence for viral compartmentalization and selection during sexual transmission. J. Virol. 1996;70:3098–3107. [PMC free article] [PubMed]
116. Nishibu A, Ward BR, Jester JV, Ploegh HL, Boes M, Takashima A. Behavioral responses of epidermal Langerhans cells in situ to local pathological stimuli. J. Invest. Dermatol. 2006;126:787–796. [PubMed]
117. Miller CJ, McChesney M, Moore PF. Langerhans cells, macrophages and lymphocyte subsets in the cervix and vagina of rhesus macaques. Lab. Invest. 1992;67:628–634. [PubMed]
118. Hladik F, Sakchalathorn P, Ballweber L, et al. Initial events in establishing vaginal entry and infection by human immunodeficiency virus type-1. Immunity. 2007;26:257–270. [PMC free article] [PubMed]
119. de Witte L, Nabatov A, Pion M, et al. Langerin is a natural barrier to HIV-1 transmission by Langerhans cells. Nat. Med. 2007;13:367–371. [PubMed]
120. Fahrbach KM, Barry SM, Ayehunie S, Lamore S, Klausner M, Hope TJ. Activated CD34-derived Langerhans cells mediate transinfection with human immunodeficiency virus. J. Virol. 2007;81:6858–6868. [PMC free article] [PubMed]
121. Zaitseva M, Blauvelt A, Lee S, et al. Expression and function of CCR5 and CXCR4 on human Langerhans cells and macrophages: implications for HIV primary infection. Nat. Med. 1997;3:1369–1375. [PubMed]
122. Kawamura T, Cohen SS, Borris DL, et al. Candidate microbicides block HIV-1 infection of human immature Langerhans cells within epithelial tissue explants. J. Exp. Med. 2000;192:1491–1500. [PMC free article] [PubMed]
123. Tschachler E, Groh V, Popovic M, et al. Epidermal Langerhans cells - a target for HTLV-III/LAV infection. J. Invest. Dermatol. 1987;88:233–237. [PubMed]
124. Hu J, Gardner MB, Miller CJ. Simian immunodeficiency virus rapidly penetrates the cervicovaginal mucosa after intravaginal inoculation and infects intraepithelial dendritic cells. J. Virol. 2000;74:6087–6095. [PMC free article] [PubMed]
125. Johansson EL, Rudin A, Wassen L, Holmgren J. Distribution of lymphocytes and adhesion molecules in human cervix and vagina. Immunology. 1999;96:272–277. [PubMed]
126. Gupta P, Collins KB, Ratner D, et al. Memory CD4+ T cells are the earliest detectable human immunodeficiency virus type 1 (HIV-1)-infected cells in the female genital mucosal tissue during HIV-1 transmission in an organ culture system. J. Virol. 2002;76:9868–9876. [PMC free article] [PubMed]
127. Greenhead P, Hayes P, Watts PS, Laing KG, Griffin GE, Shattock RJ. Parameters of human immunodeficiency virus infection of human cervical tissue and inhibition by vaginal virucides. J. Virol. 2000;74:5577–5586. [PMC free article] [PubMed]
128. Hu Q, Frank I, Williams V, et al. Blockade of attachment and fusion receptors inhibits HIV-1 infection of human cervical tissue. J. Exp. Med. 2004;99:1065–1075. [PMC free article] [PubMed]
129. Cummins JE, Jr, Guarner J, Flowers L, et al. Preclinical testing of candidate topical microbicides for anti-human immunodeficiency virus type 1 activity and tissue toxicity in a human cervical explant culture. Antimicrob. Agents Chemother. 2007;51:1770–1779. [PMC free article] [PubMed]
130. Gummuluru S, Rogel M, Stamatatos L, Emerman M. Binding of human immunodeficiency virus type 1 to immature dendritic cells can occur independently of DC-SIGN and mannose binding C-type lectin receptors via a cholesterol-dependent pathway. J. Virol. 2003;77:12865–12874. [PMC free article] [PubMed]
131. Hatch SC, Archer J, Gummuluru S. Glycosphingolipid composition of human immunodeficiency virus type 1 (HIV-1) particles is a crucial determinant for dendritic cell-mediated HIV-1 trans-infection. J. Virol. 2009;83:3496–3506. [PMC free article] [PubMed]
132. Izquierdo-Useros N, Naranjo-Gomez M, Archer J, et al. Capture and transfer of HIV-1 particles by mature dendritic cells converges with the exosome-dissemination pathway. Blood. 2009;113:2732–2741. [PubMed]
133. Wu L, KewalRamani VN. Dendritic-cell interactions with HIV: infection and viral dissemination. Nat. Rev. Immunol. 2006;6:859–868. [PMC free article] [PubMed]
134. Iwasaki A. Mucosal dendritic cells. Annu. Rev. Immunol. 2007;25:381–418. [PubMed]
135. Meng G, Sellers MT, Mosteller-Barnum M, Rogers TS, Shaw GM, Smith PD. Lamina propria lymphocytes, not macrophages, express CCR5 and CXCR4 and are the likely target cell for human immunodeficiency virus type 1 in the intestinal mucosa. J. Infect. Dis. 2000;182:785–791. [PubMed]
136. Shen R, Richter HE, Clements RH, et al. Macrophages in vaginal but not intestinal mucosa are monocyte-like and permissive to human immunodeficiency virus type 1 infection. J. Virol. 2009;83:3258–3267. [PMC free article] [PubMed]
137. Salazar-Gonzalez JF, Salazar MG, Keele BF, et al. Genetic identity, biological phenotype, and evolutionary pathways of transmitted/founder viruses in acute and early HIV-1 infection. J. Exp. Med. 2009;206:1273–1289. [PMC free article] [PubMed]
138. Connor RI, Ho DD. Human immunodeficiency virus type 1 variants with increased replicative capacity develop during the asymptomatic stage before disease progression. J. Virol. 1994;68:4400–4408. [PMC free article] [PubMed]
139. Tersmette M, Lange JM, de Goede RE, et al. Association between biological properties of human immunodeficiency virus variants and risk for AIDS and AIDS mortality. Lancet. 1989;1:983–985. [PubMed]
140. Shankarappa R, Margolick JB, Gange SJ, et al. Consistent viral evolutionary changes associated with the progression of human immunodeficiency virus type 1 infection. J. Virol. 1999;73:10489–10502. [PMC free article] [PubMed]
141. Blaak H, van’t Wout AB, Brouwer M, Hooibrink B, Hovenkamp E, Schuitemaker H. In vivo HIV-1 infection of CD45RA+CD4+ T cells is established primarily by syncytium-inducing variants and correlates with the rate of CD4+ T cell decline. Proc. Natl Acad. Sci. USA. 2000;97:1269–1274. [PubMed]
142. Ostrowski MA, Chun TW, Justement SJ, et al. Both memory and CD45RA+/CD62L+ naive CD4+ T cells are infected in human immunodeficiency virus type 1-infected individuals. J. Virol. 1999;73:6430–6435. [PMC free article] [PubMed]
143. de Roda Husman AM, van Rij RP, Blaak H, Broersen S, Schuitemaker H. Adaptation to promiscuous usage of chemokine receptors is not a prerequisite for human immunodeficiency virus type 1 disease progression. J. Infect. Dis. 1999;180:1106–1115. [PubMed]
144. Cavarelli M, Scarlatti G. Phenotype variation in human immunodeficiency virus type 1 transmission and disease progression. Dis. Markers. 2009;27:121–136. [PubMed]
145. Hartley O, Klasse PJ, Sattentau QJ, Moore JP. V3: HIV’s switch-hitter. AIDS Res. Hum. Retroviruses. 2005;21:171–189. [PubMed]
146. Mosier DE. How HIV changes its tropism: evolution and adaptation? Curr. Opin. HIV AIDS. 2009;4:125–130. [PMC free article] [PubMed]
147. Regoes RR, Bonhoeffer S. The HIV coreceptor switch: a population dynamical perspective. Trends Microbiol. 2005;13:269–277. [PubMed]
148. Mattapallil JJ, Roederer M. Acute HIV infection: it takes more than guts. Curr. Opin. HIV AIDS. 2006;1:10–15. [PubMed]
149. Brenchley JM, Schacker TW, Ruff LE, et al. CD4+ T cell depletion during all stages of HIV disease occurs predominantly in the gastrointestinal tract. J. Exp. Med. 2004;200:749–759. [PMC free article] [PubMed]
150. Veazey RS, DeMaria M, Chalifoux LV, et al. Gastrointestinal tract as a major site of CD4+ T cell depletion and viral replication in SIV infection. Science. 1998;280:427–431. [PubMed]
151. Lim SG, Condez A, Lee CA, Johnson MA, Elia C, Poulter LW. Loss of mucosal CD4 lymphocytes is an early feature of HIV infection. Clin. Exp. Immunol. 1993;92:448–454. [PubMed]
152. Igarashi T, Imamichi H, Brown CR, Hirsch VM, Martin MA. The emergence and characterization of macrophage-tropic SIV/HIV chimeric viruses (SHIVs) present in CD4+ T cell-depleted rhesus monkeys. J. Leukoc. Biol. 2003;74:772–780. [PubMed]
153. Grossman Z, Meier-Schellersheim M, Paul WE, Picker LJ. Pathogenesis of HIV infection: what the virus spares is as important as what it destroys. Nat. Med. 2006;12:289–295. [PubMed]
154. Picker LJ, Hagen SI, Lum R, et al. Insufficient production and tissue delivery of CD4+ memory T cells in rapidly progressive simian immunodeficiency virus infection. J. Exp. Med. 2004;200:1299–1314. [PMC free article] [PubMed]
155. Groot F, van Capel TM, Schuitemaker J, Berkhout B, de Jong EC. Differential susceptibility of naive, central memory and effector memory T cells to dendritic cell-mediated HIV-1 transmission. Retrovirology. 2006;3:52. [PMC free article] [PubMed]
156. Heeregrave EJ, Geels MJ, Brenchley JM, et al. Lack of in vivo compartmentalization among HIV-1 infected naive and memory CD4+ T cell subsets. Virology. 2009;393:24–32. [PMC free article] [PubMed]
157. Gonzalez-Scarano F, Martin-Garcia J. The neuropathogenesis of AIDS. Nat. Rev. Immunol. 2005;5:69–81. [PubMed]
158. Irish BP, Khan ZK, Jain P, et al. Molecular mechanisms of neurodegenerative diseases induced by human retroviruses: a review. Am J. Infect. Dis. 2009;5:231–258. [PMC free article] [PubMed]
159. Yadav A, Collman RG. CNS inflammation and macrophage/microglial biology associated with HIV-1 infection. J. Neuroimmune Pharmacol. 2009;4:430–447. [PubMed]
160. An SF, Scaravilli F. Early HIV-1 infection of the central nervous system. Arch. Anat. Cytol. Pathol. 1997;45:94–105. [PubMed]
161. Davis LE, Hjelle BL, Miller VE, et al. Early viral brain invasion in iatrogenic human immunodeficiency virus infection. Neurology. 1992;42:1736–1739. [PubMed]
162. Bell JE, Busuttil A, Ironside JW, et al. Human immunodeficiency virus and the brain: investigation of virus load and neuropathologic changes in pre-AIDS subjects. J. Infect. Dis. 1993;168:818–824. [PubMed]
163. Donaldson YK, Bell JE, Ironside JW, et al. Redistribution of HIV outside the lymphoid system with onset of AIDS. Lancet. 1994;343:383–385. [PubMed]
164. Gosztonyi G, Artigas J, Lamperth L, Webster HD. Human immunodeficiency virus (HIV) distribution in HIV encephalitis: study of 19 cases with combined use of in situ hybridization and immunocytochemistry. J. Neuropathol. Exp. Neurol. 1994;53:521–534. [PubMed]
165. Teo I, Veryard C, Barnes H, et al. Circular forms of unintegrated human immunodeficiency virus type 1 DNA and high levels of viral protein expression: association with dementia and multinucleated giant cells in the brains of patients with AIDS. J. Virol. 1997;71:2928–2933. [PMC free article] [PubMed]
166. Thieblemont N, Weiss L, Sadeghi HM, Estcourt C, Haeffner-Cavaillon N. CD14lowCD16high: a cytokine-producing monocyte subset which expands during human immunodeficiency virus infection. Eur. J. Immunol. 1995;25:3418–3424. [PubMed]
167. Pulliam L, Gascon R, Stubblebine M, McGuire D, McGrath MS. Unique monocyte subset in patients with AIDS dementia. Lancet. 1997;349:692–695. [PubMed]
168. Ancuta P, Wang J, Gabuzda D. CD16+ monocytes produce IL-6, CCL2, and matrix metalloproteinase-9 upon interaction with CX3CL1-expressing endothelial cells. J. Leukoc. Biol. 2006;80:1156–1164. [PubMed]
169. Gonzalez E, Rovin BH, Sen L, et al. HIV-1 infection and AIDS dementia are influenced by a mutant MCP-1 allele linked to increased monocyte infiltration of tissues and MCP-1 levels. Proc. Natl Acad. Sci. USA. 2002;99:13795–13800. [PubMed]
170. Gartner S. HIV infection and dementia. Science. 2000;287:602–604. [PubMed]
171. Clay CC, Rodrigues DS, Ho YS, et al. Neuroinvasion of fluorescein-positive monocytes in acute simian immunodeficiency virus infection. J. Virol. 2007;81:12040–12048. [PMC free article] [PubMed]
172. An SF, Groves M, Giometto B, Beckett AA, Scaravilli F. Detection and localisation of HIV-1 DNA and RNA in fixed adult AIDS brain by polymerase chain reaction/in situ hybridisation technique. Acta Neuropathol. 1999;98:481–487. [PubMed]
173. Bell JE. The neuropathology of adult HIV infection. Rev. Neurol. 1998;154:816–829. [PubMed]
174. Blumberg BM, Gelbard HA, Epstein LG. HIV-1 infection of the developing nervous system: central role of astrocytes in pathogenesis. Virus Res. 1994;32:253–267. [PubMed]
175. Churchill MJ, Gorry PR, Cowley D, et al. Use of laser capture microdissection to detect integrated HIV-1 DNA in macrophages and astrocytes from autopsy brain tissues. J. Neurovirol. 2006;12:146–152. [PubMed]
176. Churchill MJ, Wesselingh SL, Cowley D, et al. Extensive astrocyte infection is prominent in human immunodeficiency virus-associated dementia. Ann. Neurol. 2009;66:253–258. [PubMed]
177. Kolson DL, Lavi E, Gonzalez-Scarano F. The effects of human immunodeficiency virus in the central nervous system. Adv. Virus Res. 1998;50:1–47. [PubMed]
178. Nuovo GJ, Gallery F, MacConnell P, Braun A. In situ detection of polymerase chain reaction-amplified HIV-1 nucleic acids and tumor necrosis factor-α RNA in the central nervous system. Am. J. Pathol. 1994;144:659–666. [PubMed]
179. Ranki A, Nyberg M, Ovod V, et al. Abundant expression of HIV Nef and Rev proteins in brain astrocytes in vivo is associated with dementia. AIDS. 1995;9:1001–1008. [PubMed]
180. Thompson KA, Churchill MJ, Gorry PR, et al. Astrocyte specifc viral strains in HIV dementia. Ann. Neurol. 2004;56:873–877. [PubMed]
181. Tornatore C, Chandra R, Berger JR, Major EO. HIV-1 infection of subcortical astrocytes in the pediatric central nervous system. Neurology. 1994;44:481–487. [PubMed]
182. Torres-Munoz J, Stockton P, Tacoronte N, Roberts B, Maronpot RR, Petito CK. Detection of HIV-1 gene sequences in hippocampal neurons isolated from postmortem AIDS brains by laser capture microdissection. J. Neuropathol. Exp. Neurol. 2001;60:885–892. [PubMed]
183. Trillo-Pazos G, Diamanturos A, Rislove L, et al. Detection of HIV-1 DNA in microglia/macrophages, astrocytes and neurons isolated from brain tissue with HIV-1 encephalitis by laser capture microdissection. Brain Pathol. 2003;13:144–154. [PubMed]
184. Bissel SJ, Wiley CA. Human immunodeficiency virus infection of the brain: pitfalls in evaluating infected/affected cell populations. Brain Pathol. 2004;14:97–108. [PMC free article] [PubMed]
185. Westmoreland SV, Alvarez X, deBakker C, et al. Developmental expression patterns of CCR5 and CXCR4 in the rhesus macaque brain. J. Neuroimmunol. 2002;122:146–158. [PubMed]
186. Ma M, Geiger JD, Nath A. Characterization of a novel binding site for the human immunodeficiency virus type 1 envelope protein gp120 on human fetal astrocytes. J. Virol. 1994;68:6824–6828. [PMC free article] [PubMed]
187. Harouse JM, Bhat S, Spitalnik SL, et al. Inhibition of entry of HIV-1 in neural cell lines by antibodies against galactosyl ceramide. Science. 1991;253:320–323. [PubMed]
188. Liu Y, Liu H, Kim BO, et al. CD4-independent infection of astrocytes by human immunodeficiency virus type 1: requirement for the human mannose receptor. J. Virol. 2004;78:4120–4133. [PMC free article] [PubMed]
189. Everall IP, Luthert PJ, Lantos PL. Neuronal loss in the frontal cortex in HIV infection. Lancet. 1991;337:1119–1121. [PubMed]
190. Mankowski JL, Flaherty MT, Spelman JP, et al. Pathogenesis of simian immunodeficiency virus encephalitis: viral determinants of neurovirulence. J. Virol. 1997;71:6055–6060. [PMC free article] [PubMed]
191. Babas T, Dewitt JB, Mankowski JL, Tarwater PM, Clements JE, Zink MC. Progressive selection for neurovirulent genotypes in the brain of SIV-infected macaques. AIDS. 2006;20:197–205. [PubMed]
192. Moore JP, Kuritzkes DR. A piece de resistance: how HIV-1 escapes small molecule CCR5 inhibitors. Curr. Opin. HIV AIDS. 2009;4:118–124. [PMC free article] [PubMed]
193. Westby M, Smith-Burchnell C, Mori J, et al. Reduced maximal inhibition in phenotypic susceptibility assays indicates that viral strains resistant to the CCR5 antagonist maraviroc utilize inhibitor-bound receptor for entry. J. Virol. 2007;81:2359–2371. [PMC free article] [PubMed]
194. Melbey T, Westby M. Inhibitors of viral entry. Handb. Exp. Pharmacol. 2009;189:177–202. [PubMed]
195. Westby M, van der Ryst E. CCR5 antagonists: host-targeted antiviral agents for the treatment of HIV infection, 4 years on. Antivir. Chem. Chemother. 2010;20:179–192. [PubMed]
196. Ray N. Maraviroc in the treatment of HIV infection. Drug Des. Devel. Ther. 2009;2:151–161. [PMC free article] [PubMed]
197. Westby M, Lewis M, Whitcomb J, et al. Emergence of CXCR4-using human immunodeficiency virus type 1 (HIV-1) variants in a minority of HIV-1-infected patients following treatment with the CCR5 antagonist maraviroc is from a pretreatment CXCR4-using virus reservoir. J. Virol. 2006;80:4909–4920. [PMC free article] [PubMed]
198. Braun P, Wiesmann F. Phenotypic assays for the determination of coreceptor tropism in HIV-1 infected individuals. Eur. J. Med. Res. 2007;12:463–472. [PubMed]
199. Low AJ, Swenson LC, Harrigan PR. HIV coreceptor phenotyping in the clinical setting. AIDS Rev. 2008;10:143–151. [PubMed]