To characterize the correlates of protection from systemic infection in a vaccinated rhesus macaque (RM), RAt-9, which had been challenged sequentially with two related clade C simian-human immunodeficiency viruses (SHIV-Cs) yet remained aviremic for >5 years despite indirect evidence of cryptic infection.
To measure long-term anti-SHIV-C immunity, host genetics and gene-expression patterns for protective correlates.
Long-term immune reactivity was evaluated and identification of virus in RAt-9 was attempted by RT-PCR analysis of concentrated plasma and blood transfer to CD8+ cell-depleted infant macaques. Full MHC genotyping of RAt-9, TRIM5α and KIR3DL allelic expression analysis of PBMC, and microarray gene expression analysis were performed.
All attempts to detect/isolate virus, including blood transfer to CD8+ cell-depleted infant RM, were negative, and the animal maintained normal levels of memory CD4+ T cells in both peripheral blood and gut tissues. However, RAt-9 maintained high levels of anti-SHIV-C humoral and cellular immunity, including reactivity to non-vaccine neoantigens (Nef and Rev), up to 63 months post-initial challenge, suggesting chronic sub-threshold infection. RAt-9 expressed the Mamu A*001 allele but was B*008−B*017−, had a B13 serotype, and had increased expression of killer-cell immunoglobulin-like receptors (KIRs) previously linked to favorable outcomes of lentiviral infection. Elements of the gene expression profiling coincided with genotyping results. RAt-9 also displayed CD8+ cell noncytotoxic antiviral response (CNAR) activity.
RAt-9 is the first example of a virus-exposed, persistently aviremic animal that has maintained long-term, high-level cellular and humoral antiviral immunity in the absence of an identifiable cryptic reservoir.
AIDS; vaccine; SHIV; clade C; long-term immunity; prevention of viremia
The vast diversity of HIV-1 infections has greatly impeded the development of a successful HIV-1/AIDS vaccine. Previous vaccine work has demonstrated limited levels of protection against SHIV/SIV infection, but protection was observed only when the challenge virus was directly matched to the vaccine strain. As it is likely impossible to directly match the vaccine strain to all infecting strains in nature, it is necessary to develop an HIV-1 vaccine that can protect against a heterologous viral challenge. In this study we investigated the ability of polyvalent and consensus vaccines to protect against a heterologous clade B challenge. Rhesus macaques were vaccinated with ConB or PolyB virus-like particle vaccines. All vaccines were highly immunogenic with high titers of antibody found in all vaccinated groups against SIV Gag. Antibody responses were also observed against a diverse panel of clade B envelopes. Following vaccination nonhuman primates (NHPs) were challenged via the vaginal route with SHIVSF162p4. The PolyB vaccine induced a 66.7% reduction in the rate of infection as well as causing a two log reduction in viral burden if infection was not blocked. ConB vaccination had no effect on either the infection rate or viral burden. These results indicate that a polyvalent clade-matched vaccine is better able to protect against a heterologous challenge as compared to a consensus vaccine.
We used the simian immunodeficiency virus mac251 (SIVmac251) macaque model to study the effect of the dose of mucosal exposure on vaccine efficacy. We immunized macaques with a DNA prime followed by SIV gp120 protein immunization with ALVAC-SIV and gp120 in alum, and we challenged them with SIVmac251 at either a single high dose or at two repeated low-dose exposures to a 10-fold-lower dose. Infection was neither prevented nor modified following a single high-dose challenge of the immunized macaques. However, two exposures to a 10-fold-lower dose resulted in protection from SIVmac251 acquisition in 3 out of 12 macaques. The remaining animals that were infected had a modulated pathogenesis, significant downregulation of interferon responsive genes, and upregulation of genes involved in B- and T-cell responses. Thus, the choice of the experimental model greatly influences the vaccine efficacy of vaccines for human immunodeficiency virus (HIV).
The role of antibodies directed against the hyper variable envelope region V1 of human immunodeficiency virus type 1 (HIV-1), has not been thoroughly studied. We show that a vaccine able to elicit strain-specific non-neutralizing antibodies to this region of gp120 is associated with control of highly pathogenic chimeric SHIV89.6P replication in rhesus macaques. The vaccinated animal that had the highest titers of antibodies to the amino terminus portion of V1, prior to challenge, had secondary antibody responses that mediated cell killing by antibody-dependent cellular cytotoxicity (ADCC), as early as two weeks after infection and inhibited viral replication by antibody-dependent cell-mediated virus inhibition (ADCVI), by four weeks after infection. There was a significant inverse correlation between virus level and binding antibody titers to the envelope protein, (R = -0.83, p 0.015), and ADCVI (R = -0.84 p=0.044). Genotyping of plasma virus demonstrated in vivo selection of three SHIV89.6P variants with changes in potential N-linked glycosylation sites in V1. We found a significant inverse correlation between virus levels and titers of antibodies that mediated ADCVI against all the identified V1 virus variants. A significant inverse correlation was also found between neutralizing antibody titers to SHIV89.6 and virus levels (R = -0.72 p =0.0050). However, passive inoculation of purified immunoglobulin from animal M316, the macaque that best controlled virus, to a naïve macaque, resulted in a low serum neutralizing antibodies and low ADCVI activity that failed to protect from SHIV89.6P challenge. Collectively, while our data suggest that anti-envelope antibodies with neutralizing and non-neutralizing FcγR-dependent activities may be important in the control of SHIV replication, they also demonstrate that low levels of these antibodies alone are not sufficient to protect from infection.
Salicylidene acylhydrazide compounds have been shown to inhibit bacterial pathogens, including Chlamydia and Neisseria gonorrhoeae. If such compounds could also target HIV-1, their potential use as topical microbicides to prevent sexually transmitted infections would be considerable. We determined the in vitro anti-HIV-1 activity, cytotoxicity and mechanism of action of several salicylidene acylhydrazides.
Inhibitory activity was assessed using TZMbl cells and primary peripheral blood mononuclear cells (PBMCs) as targets for HIV-1 infection. Anti-viral activity was measured against cell-free and cell-associated virus and in vaginal fluid and semen simulants. Since the anti-bacterial activity of salicylidene acylhydrazides is reversible by Fe2+, we determined whether Fe2+ and other cations could reverse the anti-HIV-1 activity of the compounds. We also employed real-time PCR to determine the stage affected in the HIV-1 replication cycle.
We identified four compounds with 50% HIV-1 inhibitory concentrations of 1 to 7 μM. In vitro toxicity varied but was generally limited. Activity was similar against three R5 clade B primary isolates and whether targets for virus replication were TZMbl cells or PBMCs. Compounds inhibited cell-free and cell-associated virus and were active in vaginal fluid and semen simulants. Fe2+, but not other cations, reversed the anti-HIV-1 effect. Finally, inhibitory effect of the compounds occurred at a post-integration step.
We identified salicylidene acylhydrazides with in vitro anti-HIV-1 activity in the μM range. The activity of these compounds against other sexually transmitted pathogens makes them potential candidates to formulate for use as a broad-spectrum topical genital microbicide.
Salicylidene acylhydrazides; HIV; microbicide; iron chelation
Preclinical studies of HIV-1 vaccine candidates have typically shown post-infection virologic control, but protection against acquisition of infection has previously only been reported against neutralization-sensitive virus challenges1–3. Here we demonstrate vaccine protection against acquisition of fully heterologous, neutralization-resistant virus challenges in rhesus monkeys. Adenovirus/poxvirus and adenovirus/adenovirus vector-based vaccines expressing SIVsmE543 Gag, Pol, and Env antigens resulted in a ≥80% reduction in the per-exposure probability of infection4,5 against repetitive, intrarectal SIVmac251 challenges in rhesus monkeys. Protection against acquisition of infection exhibited distinct immunologic correlates as compared with post-infection virologic control and required the inclusion of Env in the vaccine regimen. These data demonstrate the first proof-of-concept that optimized HIV-1 vaccine candidates can block acquisition of stringent, heterologous, neutralization-resistant virus challenges in rhesus monkeys.
Maternal HIV-1-specific antibodies are efficiently transferred to newborns; their role in disease control is unknown. We administered non-sterilizing levels of neutralizing IgG, including the human neutralizing monoclonal IgG1b12, to six newborn macaques before oral challenge with SHIVSF612P3. All rapidly developed neutralizing antibodies and had significantly reduced plasma viremia for 6 months. These studies support the use of neutralizing antibodies in enhancing B cell responses and viral control in perinatal settings.
Purpose of review
We summarize current information on Fc receptor-mediated anti-viral activities of antibodies. These activites include FcγR-mediated inhibtion and neutralization of HIV on antigen presenting cells (APCs), antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent cell-mediated virus inhibition (ADCVI).
An FcγR-mediated mechanism that results in augmented neutralization and may render non-neutralizing antibodies inhibitory has been demonstrated in APC. ADCC antibody activity correlates inversely with HIV disease progression in humans, and higher vaccine-induced ADCC antibody responses are associated with lower acute SIV viremia levels in macaques. Following vaccination with rgp120, ADCVI antibody levels are higher among those with a lower rate of sexually acquired HIV infection. Non-neutralizing SIV immune serum that prevents infection of newborn macaques after oral challenge has potent ADCVI antibody activity. Abrogating the ability of the Fc segment of the broadly neutralizing monoclonal antibody IgG1b12 to bind to FcγRs and to mediate ADCVI substantially reduces IgG1b12’s protective effect in a SHIV vaginal challenge model.
Fc-FcγR interactions play a critical role in the biological function of antibody and are likely to be instrumental in preventing or modulating lentiviral infection. Exploiting antibody responses that depend on Fc-FcγR interactions may help widen the breadth and increase the potency of vaccine-induced antibody. Although the importance of generating optimal Fab-antigen interactions cannot be overestimated, improving Fc-FcγR interactions through adjuvants or other strategies provides another option for improving HIV vaccines and immunotherapies.
Antibody-dependent cellular cytotoxicity (ADCC); antibody-dependent cell-mediated virus inhibition (ADCVI); neutralization; Fcγ receptor (FcγR); HIV
The membrane-proximal external region (MPER) of HIV-1, located at the C terminus of the gp41 ectodomain, is conserved and crucial for viral fusion. Three broadly neutralizing monoclonal antibodies (bnMAbs), 2F5, 4E10, and Z13e1, are directed against linear epitopes mapped to the MPER, making this conserved region an important potential vaccine target. However, no MPER antibodies have been definitively shown to provide protection against HIV challenge. Here, we show that both MAbs 2F5 and 4E10 can provide complete protection against mucosal simian-human immunodeficiency virus (SHIV) challenge in macaques. MAb 2F5 or 4E10 was administered intravenously at 50 mg/kg to groups of six male Indian rhesus macaques 1 day prior to and again 1 day following intrarectal challenge with SHIVBa-L. In both groups, five out of six animals showed complete protection and sterilizing immunity, while for one animal in each group a low level of viral replication following challenge could not be ruled out. The study confirms the protective potential of 2F5 and 4E10 and supports emphasis on HIV immunogen design based on the MPER region of gp41.
Rare serotype and chimeric recombinant adenovirus (rAd) vectors that evade anti-Ad5 immunity are currently being evaluated as potential vaccine vectors for human immunodeficiency virus type 1 and other pathogens. We have recently reported that a heterologous rAd prime-boost regimen expressing simian immunodeficiency virus (SIV) Gag afforded durable partial immune control of an SIV challenge in rhesus monkeys. However, single-shot immunization may ultimately be preferable for global vaccine delivery. We therefore evaluated the immunogenicity and protective efficacy of a single immunization of chimeric rAd5 hexon hypervariable region 48 (rAd5HVR48) vectors expressing SIV Gag, Pol, Nef, and Env against a homologous SIV challenge in rhesus monkeys. Inclusion of Env resulted in improved control of peak and set point SIV RNA levels following challenge. In contrast, DNA vaccine priming did not further improve the protective efficacy of rAd5HVR48 vectors in this system.
A recombinant adenovirus serotype 5 (rAd5) vector-based vaccine for HIV-1 has recently failed in a phase 2b efficacy study in humans1, 2. Consistent with these results, preclinical studies have demonstrated that rAd5 vectors expressing SIV Gag failed to reduce peak or setpoint viral loads following SIV challenge of rhesus monkeys that lacked the protective MHC class I allele Mamu-A*013. Here we show that an improved T cell-based vaccine regimen utilizing two serologically distinct adenovirus vectors afforded substantially improved protective efficacy in this stringent challenge model. In particular, a heterologous rAd26 prime, rAd5 boost vaccine regimen expressing SIV Gag elicited cellular immune responses with augmented magnitude, breadth, and polyfunctionality as compared with the homologous rAd5 regimen. Following SIVmac251 challenge, monkeys vaccinated with the heterologous rAd26/rAd5 regimen exhibited a 1.4 log reduction of peak and a 2.4 log reduction of setpoint viral loads as well as decreased AIDS-related mortality as compared with control animals. These data demonstrate that durable partial immune control of a pathogenic SIV challenge for over 500 days can be achieved by a T cell-based vaccine in Mamu-A*01-negative rhesus monkeys in the absence of a homologous Env antigen. These findings have important implications for the development of next generation T cell-based vaccine candidates for HIV-1.
Developing an immunogen that elicits broadly neutralizing antibodies (bNAbs) is an elusive but important goal of HIV vaccine research, especially after the recent failure of the leading T cell based HIV vaccine in human efficacy trials. Even if such an immunogen can be developed, most animal model studies indicate that high serum neutralizing concentrations of bNAbs are required to provide significant benefit in typical protection experiments. One possible exception is provided by the anti-glycan bNAb 2G12, which has been reported to protect macaques against CXCR4-using SHIV challenge at relatively low serum neutralizing titers. Here, we investigated the ability of 2G12 administered intravenously (i.v.) to protect against vaginal challenge of rhesus macaques with the CCR5-using SHIVSF162P3. The results show that, at 2G12 serum neutralizing titers of the order of 1∶1 (IC90), 3/5 antibody-treated animals were protected with sterilizing immunity, i.e. no detectable virus replication following challenge; one animal showed a delayed and lowered primary viremia and the other animal showed a course of infection similar to 4 control animals. This result contrasts strongly with the typically high titers observed for protection by other neutralizing antibodies, including the bNAb b12. We compared b12 and 2G12 for characteristics that might explain the differences in protective ability relative to neutralizing activity. We found no evidence to suggest that 2G12 transudation to the vaginal surface was significantly superior to b12. We also observed that the ability of 2G12 to inhibit virus replication in target cells through antibody-mediated effector cell activity in vitro was equivalent or inferior to b12. The results raise the possibility that some epitopes on HIV may be better vaccine targets than others and support targeting the glycan shield of the envelope.
An effective HIV vaccine should elicit broadly neutralizing antibodies, i.e. antibodies that neutralize a wide spectrum of different HIVs in vitro. A number of human monoclonal antibodies have been isolated with broad neutralization and shown to protect macaques against vaginal HIV challenge. Protection is generally correlated with neutralization and requires relatively high antibody concentrations that may be difficult to achieve by vaccination. Here, we show that one monoclonal antibody (2G12) is unusually potent in protection relative to its neutralizing ability as hinted at by earlier data. Further studies eliminate an unusual ability of 2G12 to be transported to the vagina (site of infection) as a possible explanation for our observations. Although the precise mechanism is unclear, the studies have important implications for HIV vaccine design in general by suggesting that some vaccine targets on HIV may be better than others and, specifically, by suggesting that the sugar coat of HIV may be a particularly rewarding target if appropriate immunogens can be designed.
Lentiviruses such as human and simian immunodeficiency viruses (HIV and SIV) undergo continual evolution in the host. Previous studies showed that the late-stage variants of SIV that evolve in one host replicate to significantly higher levels when transmitted to a new host. However, it is unknown whether HIVs or SIVs that have higher replication fitness are more genetically stable upon transmission to a new host. To begin to address this, we analyzed the envelope sequence variation of viruses that evolved in animals infected with variants of SIVMne that had been cloned from an index animal at different stages of infection.
We found that there was more evolution of envelope sequences from animals infected with the late-stage, highly replicating variants than in animals infected with the early-stage, lower replicating variant, despite the fact that the late virus had already diversified considerably from the early virus in the first host, prior to transmission. Many of the changes led to the addition or shift in potential-glycosylation sites-, and surprisingly, these changes emerged in some cases prior to the detection of neutralizing antibody responses, suggesting that other selection mechanisms may be important in driving virus evolution. Interestingly, these changes occurred after the development of antibody whose anti-viral function is dependent on Fc-Fcγ receptor interactions.
SIV variants that had achieved high replication fitness and escape from neutralizing antibodies in one host continued to evolve upon transmission to a new host. Selection for viral variants with glycosylation and other envelope changes may have been driven by both neutralizing and Fcγ receptor-mediated antibody activities.
Although antibodies can prevent or modulate lentivirus infections in nonhuman primates, the biological functions of antibody responsible for such effects are not known. We sought to determine the role of antibody-dependent cell-mediated virus inhibition (ADCVI), an antibody function that inhibits virus yield from infected cells in the presence of Fc receptor-bearing effector cells, in preventing or controlling SIVmac251 infection in rhesus macaques (Macaca mulatta). Using CEMx174 cells infected with simian immunodeficiency virus mac251 (SIVmac251), both polyclonal and monoclonal anti-SIV antibodies were capable of potent virus inhibition in the presence of human peripheral blood mononuclear cell (PBMC) effector cells. In the absence of effector cells, virus inhibition was generally very poor. PBMCs from healthy rhesus macaques were also capable of mediating virus inhibition either against SIVmac251-infected CEMx174 cells or against infected, autologous rhesus target cells. We identified both CD14+ cells and, to a lesser extent, CD8+ cells as the effector cell population in the rhesus PBMCs. Finally, pooled, nonneutralizing SIV-antibody-positive serum, shown in a previous study to prevent infection of neonatal macaques after oral SIVmac251 challenge, had potent virus-inhibitory activity in the presence of effector cells; intact immunoglobulin G, rather than F(ab′)2, was required for such activity. This is the first demonstration of both humoral and cellular ADCVI functions in the macaque-SIV model. ADCVI activity in nonneutralizing serum that prevents SIV infection suggests that ADCVI may be a protective immune function. Finally, our data underscore the potential importance of Fc-Fc receptor interactions in mediating biological activities of antibody.
The mechanisms by which human immunodeficiency virus type 1 (HIV-1) crosses mucosal surfaces to establish infection are unknown. Acidic genital secretions of HIV-1-infected women contain HIV-1 likely coated by antibody. We found that the combination of acidic pH and Env-specific IgG, including that from cervicovaginal and seminal fluids of HIV-1-infected individuals, augmented transcytosis across epithelial cells as much as 20-fold compared with Env-specific IgG at neutral pH or non-specific IgG at either pH. Enhanced transcytosis was observed with clinical HIV-1 isolates, including transmitted/founder strains, and was eliminated in Fc neonatal receptor (FcRn)-knockdown epithelial cells. Non-neutralizing antibodies allowed similar or less transcytosis than neutralizing antibodies. However, the ratio of total:infectious virus was higher for neutralizing antibodies, indicating that they allowed transcytosis while blocking infectivity of transcytosed virus. Immunocytochemistry revealed abundant FcRn expression in columnar epithelia lining the human endocervix and penile urethra. Acidity and Env-specific IgG enhance transcytosis of virus across epithelial cells via FcRn and could facilitate translocation of virus to susceptible target cells following sexual exposure.
HIV-1 causes a sexually transmitted disease. However, the mechanisms employed by the virus to cross genital tract tissue and establish infection are uncertain. Since cervicovaginal fluid is acidic and HIV-1 in cervicovaginal fluid is likely coated with antibodies, we explored the effect of low pH and HIV-1-specific antibodies on transcytosis, the movement of HIV-1 across tight-junctioned epithelial cells. We found that the combination of HIV-1-specific antibodies and low pH enhanced transcytosis as much as 20-fold. Virus that underwent transcytosis under these conditions was infectious, and infectivity was highly influenced by whether or not the antibody neutralized the virus. We observed enhanced transcytosis using antibody from cervicovaginal and seminal fluids and using transmitted/founder strains of HIV-1. We also found that the enhanced transcytosis was due to the Fc neonatal receptor (FcRn), which binds immune complexes at acidic pH and releases them at neutral pH. Finally, staining of human tissue revealed abundant FcRn expression on columnar epithelial cells of penile urethra and endocervix. Our findings reveal a novel mechanism wherein HIV-1 may facilitate its own transmission by usurping the antibody response directed against itself. These results have important implications for HIV vaccine development and for understanding the earliest events in HIV transmission.
Antibodies can prevent lentivirus infections in animals and may play a role in controlling viral burden in established infection. In preventing and particularly in controlling infection, antibodies likely function in the presence of large quantities of virus. In this study, we explored the mechanisms by which antibodies neutralize large inocula of human immunodeficiency virus type 1 (HIV-1) on different target cells. Immunoglobulin G (IgG) from HIV-infected patients was tested for neutralizing activity against primary R5 strains of HIV-1 at inocula ranging from 100 to 20,000 50% tissue culture infective doses. At all virus inocula, inhibition by antibody was enhanced when target cells for virus growth were monocyte-depleted, peripheral blood mononuclear cells (PBMCs) rather than CD4+ lymphocytes. However, enhanced inhibition on PBMCs was greatest with larger amounts of virus. Depleting PBMCs of natural killer (NK) cells, which express Fc receptors for IgG (FcγRs), abrogated the enhanced antibody inhibition, whereas adding NK cells to CD4+ lymphocytes restored inhibition. There was no enhanced inhibition on PBMCs when F(ab′)2 was used. Further experiments demonstrated that the release of β-chemokines, most likely through FcγR triggering of NK cells, contributed modestly to the antiviral activity of antibody on PBMCs and that antibody-coated virus adsorbed to uninfected cells provided a target for NK cell-mediated inhibition of HIV-1. These results indicate that Fc-FcγR interactions enhance the ability of antibody to neutralize HIV-1. Since FcγR-bearing cells are always present in vivo, FcγR-mediated antibody function may play a role in the ability of antibody to control lentivirus infection.
The partial control of viremia during acute human immunodeficiency virus type 1 (HIV-1) infection is accompanied by an HIV-1-specific cytotoxic T-lymphocyte (CTL) response and an absent or infrequent neutralizing antibody response. The control of HIV-1 viremia has thus been attributed primarily, if not exclusively, to CTL activity. In this study, the role of antibody in controlling viremia was investigated by measuring the ability of plasma or immunoglobulin G from acutely infected patients to inhibit primary strains of HIV-1 in the presence of natural-killer (NK) effector cells. Antibody that inhibits virus when combined with effector cells was present in the majority of patients within days or weeks after onset of symptoms of acute infection. Furthermore, the magnitude of this effector cell-mediated antiviral antibody response was inversely associated with plasma viremia level, and both autologous and heterologous HIV-1 strains were inhibited. Finally, antibody from acutely infected patients likely reduced HIV-1 yield in vitro both by mediating effector cell lysis of target cells expressing HIV-1 glycoproteins and by augmenting the release of β-chemokines from NK cells. HIV-1-specific antibody may be an important contributor to the early control of HIV viremia.
Laboratory strains of measles viruses (MV), such as Edmonston and Halle, use the complement regulatory protein CD46 as a cell surface receptor. The receptor usage of clinical isolates of MV, however, remains unclear. Receptor usage by primary patient isolates of MV was compared to isolates that had been passaged on a variety of tissue culture cell lines. All of the isolates could infect cells in a CD46-dependent manner, but their tropism was restricted according to cell type (e.g., lymphocytes versus fibroblasts). The results indicate that patient isolates that have not been adapted to tissue culture cell lines use CD46 as a receptor. In addition, passaging primary MV patient isolates in B95-8 cells selected variants that had alternate receptor usage compared to the original isolate. Thus, changes in receptor usage by MV are dependent upon the cell type used for isolation. Furthermore, our results confirm the relevance of the CD46 receptor to natural measles infection.
In some countries, excessive non-measles-related mortality has been observed among female recipients of high-titer measles vaccines. We determined if differences in the immune response to measles vaccines underlie the excessive female mortality by measuring the measles virus (MV)-specific antibody-dependent cellular cytotoxicity (ADCC) antibody response in 65 3-year-old Gambian children immunized with Edmonston-Zagreb medium-titer (EZ) or Schwarz standard vaccines during infancy. Among the 20 females and 22 males with undetectable anti-MV antibodies at the time of immunization, females had significantly lower ADCC than males (median cytotoxicities of 1/100 serum dilutions = 8.4 and 12%, respectively; P = 0.04). This sex-associated difference was present only among the six female and seven male recipients of EZ vaccine (median cytotoxicities = 5.1 and 19.0%, respectively; P = 0.02). There were no significant sex-associated differences in neutralizing antibody activity. Decreased ADCC antibody activity may contribute to the lower survival rate observed in females receiving high-titer measles vaccination.
Simian-human immunodeficiency virus (SHIV) models for human immunodeficiency virus (HIV) infection have been widely used in passive studies with HIV neutralizing antibodies (NAbs) to test for protection against infection. However, because SHIV-infected adult macaques often rapidly control plasma viremia and any resulting pathogenesis is minor, the model has been unsuitable for studying the impact of antibodies on pathogenesis in infected animals. We found that SHIVSF162P3 infection in 1-month-old rhesus macaques not only results in high persistent plasma viremia but also leads to very rapid disease progression within 12 to 16 weeks. In this model, passive transfer of high doses of neutralizing IgG (SHIVIG) prevents infection. Here, we show that at lower doses, SHIVIG reduces both plasma and peripheral blood mononuclear cell (PBMC)-associated viremia and mitigates pathogenesis in infected animals. Moreover, production of endogenous NAbs correlated with lower set-point viremia and 100% survival of infected animals. New SHIV models are needed to investigate whether passively transferred antibodies or antibodies elicited by vaccination that fall short of providing sterilizing immunity impact disease progression or influence immune responses. The 1-month-old rhesus macaque SHIV model of infection provides a new tool to investigate the effects of antibodies on viral replication and clearance, mechanisms of B cell maintenance, and the induction of adaptive immunity in disease progression.
Primary isolates of HIV-1 resist neutralization by most antibodies to the CD4 binding site (CD4bs) on gp120 due to occlusion of this site on the trimeric spike. We describe 1F7, a human CD4bs monoclonal antibody that was found to be exceptionally potent against the HIV-1 primary isolate JR-FL. However, 1F7 failed to neutralize a patient-matched primary isolate, JR-CSF even though the two isolates differ by <10% in gp120 at the protein level. In an HIV-1 cross clade panel (n = 157), 1F7 exhibited moderate breadth, but occasionally achieved considerable potency. In binding experiments using monomeric gp120s of select resistant isolates and domain-swap chimeras between JR-FL and JR-CSF, recognition by 1F7 was limited by sequence polymorphisms involving at least the C2 region of Env. Putative N-linked glycosylation site (PNGS) mutations, notably at position 197, allowed 1F7 to neutralize JR-CSF potently without improving binding to the cognate, monomeric gp120. In contrast, flow cytometry experiments using the same PNGS mutants revealed that 1F7 binding is enhanced on cognate trimeric Env. BN-PAGE mobility shift experiments revealed that 1F7 is sensitive to the diagnostic mutation D368R in the CD4 binding loop of gp120. Our data on 1F7 reinforce how exquisitely targeted CD4bs antibodies must be to achieve cross neutralization of two closely related primary isolates. High-resolution analyses of trimeric Env that show the orientation of glycans and polymorphic elements of the CD4bs that affect binding to antibodies like 1F7 are desirable to understand how to promote immunogenicity of more conserved elements of the CD4bs.
Eliciting neutralizing antibodies is thought to be a key activity of a vaccine against human immunodeficiency virus (HIV). However, a number of studies have suggested that in addition to neutralization, interaction of IgG with Fc gamma receptors (FcγR) may play an important role in antibody-mediated protection. We have previously obtained evidence that the protective activity of the broadly neutralizing human IgG1 anti-HIV monoclonal antibody (MAb) b12 in macaques is diminished in the absence of FcγR binding capacity. To investigate antibody-dependent cellular cytotoxicity (ADCC) as a contributor to FcγR-associated protection, we developed a nonfucosylated variant of b12 (NFb12). We showed that, compared to fully fucosylated (referred to as wild-type in the text) b12, NFb12 had higher affinity for human and rhesus macaque FcγRIIIa and was more efficient in inhibiting viral replication and more effective in killing HIV-infected cells in an ADCC assay. Despite these more potent in vitro antiviral activities, NFb12 did not enhance protection in vivo against repeated low-dose vaginal challenge in the simian-human immunodeficiency virus (SHIV)/macaque model compared to wild-type b12. No difference in protection, viral load, or infection susceptibility was observed between animals given NFb12 and those given fully fucosylated b12, indicating that FcγR-mediated activities distinct from FcγRIIIa-mediated ADCC may be important in the observed protection against SHIV challenge.
We reported previously that while prolonged tenofovir monotherapy of macaques infected with virulent simian immunodeficiency virus (SIV) resulted invariably in the emergence of viral mutants with reduced in vitro drug susceptibility and a K65R mutation in reverse transcriptase, some animals controlled virus replication for years. Transient CD8+ cell depletion or short-term tenofovir interruption within 1 to 5 years of treatment demonstrated that a combination of CD8+ cell-mediated immune responses and continued tenofovir therapy was required for sustained suppression of viremia. We report here follow-up data on 5 such animals that received tenofovir for 8 to 14 years.
Although one animal had a gradual increase in viremia from 3 years onwards, the other 4 tenofovir-treated animals maintained undetectable viremia with occasional viral blips (≤ 300 RNA copies/ml plasma). When tenofovir was withdrawn after 8 to 10 years from three animals with undetectable viremia, the pattern of occasional episodes of low viremia (≤ 3600 RNA/ml plasma) continued throughout the 10-month follow-up period. These animals had low virus levels in lymphoid tissues, and evidence of multiple SIV-specific immune responses.
Under certain conditions (i.e., prolonged antiviral therapy initiated early after infection; viral mutants with reduced drug susceptibility) a virus-host balance characterized by strong immunologic control of virus replication can be achieved. Although further research is needed to translate these findings into clinical applications, these observations provide hope for a functional cure of HIV infection via immunotherapeutic strategies that boost antiviral immunity and reduce the need for continuous antiretroviral therapy.
Tenofovir; PMPA; SIV; Functional cure; Antiretroviral; HIV