In ~50% of HIV patients, a “switch” to a dominant CXCR4-tropic phenotype occurs, corresponding to an accelerated rate of CD4+
T cell loss and a shift to TH
2 immunity (1
). Although the precise mechanisms have not been elucidated, the CCR5-to-CXCR4 tropic shift in coreceptor usage among emerging variants is influenced by HAART, antiviral immune responses and the dynamic interactions between viral quasispecies and their various target cell populations within viral habitats and reservoirs (27
). Our findings that IgE-Fcε
RI interactions up-regulate functional expression of CXCR4 and thus expand the window of susceptibility of prMCs to both X4 and R5X4 viruses (1
), and that this expanded range of virus susceptibility occurs during a well-defined period of early MC development (2
), are significant because they provide new insights into how IgE-mediated allergy or helminthic coinfection may facilitate the establishment of a MC reservoir of persistent X4 virus infection. This mechanism may influence a movement to a predominant X4 virus phenotype and the progression of HIV/AIDS in infected individuals.
We have reported that mature tissue MCs can archive latent, inducible infectious HIV in vivo (10
). Data presented in this study now show that IgE-Fcε
RI binding, even in the absence of cross-linking Ag, and at IgE concentrations as low as 100 ng/ml, can expand the presence of both X4- and R5X4-HIV in populations of prMCs that supply the developing MC reservoir of persistent infection ( and ). Although the data obtained with IgE in the absence of Ag would support a role for monomeric IgE in the regulation of specific MC responses as has been previously reported (31
), we cannot rule out the possibility that the responses were due to dimers or small oligomers of IgE forming over the duration of the experiments. Regardless, these responses were observed under conditions, which were not sufficient to induce significant MC degranulation ().
Our data also revealed that, in the presence of streptavidin- or superallergen-induced coaggregation, up-regulation of CXCR4 expression occurs much more rapidly and suggest that maximum levels of virus susceptibility can be achieved with submaximal induction of CXCR4 mRNA expression (compare , , and ). These findings indicate that IgE-Fcε
RI aggregation can promote the inductive signaling pathway leading to functional expression of CXCR4 on prMCs. Furthermore, the IgE effect is specifically mediated through an Fcε
RI-dependent pathway. Omalizumab, which effectively inhibits binding of IgE to Fcε
RI (the predominant IgE receptor expressed throughout MC ontogeny) (32
), specifically uncoupled and completely abolished IgE-dependent enhancement of prMC CXCR4 expression and susceptibility to X4-HIV (). The broader implications of these findings are that pre-existing comorbid conditions such as atopic disease or helminthic infections with elevated IgE levels may significantly contribute to the establishment of X4 and R5X4 virus within the MC reservoir during HIV infection.
Our observation that 3- to 5-wk-old prMCs, but not mature (8-wk-old or more) MCs ( and ), were susceptible to X4 virus infection in the presence of IgE or cross-linked IgE has at least two important consequences. First, transmission of X4 virus to prMCs occurs not only in the presence of receptor-bound IgE but also predominates in those tissue compartments that support the growth and maintenance of the HIV-susceptible Fcε
prMC phenotype. MCs with this progenitor phenotype are found in the circulation (33
). However, whether tissue MCs with a similar IgE-sensitive progenitor phenotype are present within specialized tissue compartments (e.g., the gastrointestinal tract) remains to be determined. Secondly, up-regulation of the functional cell surface expression CXCR4 in CD34+
progenitor cells is mediated by a cAMP-dependent pathway (34
). Up-regulation of cell surface expression of CXCR4 on lymphocytes by cAMP (35
) has not only been shown to enhance chemotaxis in response to SDF-1, but also susceptibility to infection with HIV (37
). Ongoing studies in our laboratory with LAD cells (12
), which emulate in many ways the IgE inducible expression of CXCR4 on primary 3- to 5-wk-old prMCs, support the involvement of cAMP in the inducible expression of CXCR4 on these prMCs (our unpublished observation). Therefore, our findings that IgE-mediated regulation of CXCR4 on MCs recedes as they mature ( and ) suggest that signaling pathways mediated by IgE-Fcε
RI interactions may also coordinately change and evolve as prMCs mature. A broader implication of these findings is that progenitors of potentially other, yet to be described, cell lineages may also become transiently susceptible to HIV infection during unique stages of development and then covertly maintain latent persistent infection in a similar fashion as they mature into otherwise HIV infection-resistant cell phenotypes.
These results also underscore the general theme and overall significance of the relationship between MC maturational or developmental stages and susceptibility to both R5- and (now) X4-HIV infection. The dynamic constitutive (IgE-independent) expression of CXCR5 and corresponding relative susceptibility to infection with R5 virus varies with the developmental stage of prMCs derived from either fetal (2
) or adult CD34+
-derived committed progenitors ( and ). As shown in , constitutive expression of CXCR5 is maximal at 2–3 wk of culture and then begins a dramatic decline. These observations may explain differences in maximum levels of R5 SS cDNA obtained from prMCs exposed to virus at wk 4–5 () and wk 3–4 (). MCs appear unique among non-T cell lineages in that their transient susceptibility to infection to both R5 and X4 viruses is related to biological and IgE-independent and – dependent physiologic characteristics at different stages of development. Thus, during ongoing HIV infection, conditions that are associated with MC mobilization, recruitment and survival, such as IgE-mediated allergy, schistosomiasis, or other helminth infections (31
), may establish a reservoir of latently infected “tissue” MCs harboring infectious R5-, X4-, and R5X4-HIV within diverse tissue compartments.
Many questions remain regarding the clinical significance and the role of IgE in the pathogenesis of HIV infection in the presence of coinfection, especially in the gut. Adult female S. mansoni
residing in the intestinal vasculature release eggs that potently induce IgE and Th2 immune responses (39
). The shift toward a Th2 immune environment limits host antiviral immune responses and promotes survival of MCs (40
). One prediction based on our findings that IgE-Fcε
RI interactions enhance expression of CXCR4 and susceptibility to X4 and R5/X4-tropic HIV/SIV ( and ) is that increased levels of IgE and prMCs during allergy or a helminth infection would expand the presence of archived X4 and R5X4 viruses in the MC reservoir and thus proportionately increase the opportunity for X4 and R5X4 viruses to persist and eventually emerge as viral fitness landscapes change during disease progression. To our knowledge, no formal studies correlating IgE levels and CCR5-to-CXCR4 tropism shifts during HIV or SIV infection have been conducted. However, several new reports of clinical findings indicate that effective HAART, which suppresses viral replication and evolution, creates an environment necessary for the emergence of CXCR4-tropic variants archived in cellular reservoirs, and are thus consistent with this prediction from our model (27
). Acute S. mansoni
infection also increases susceptibility to systemic SHIV Clade C infection in rhesus macaques after mucosal virus exposure (25
). The IgE-SmEA-mediated enhancement of rhesus macaque prMC susceptibility to X4-tropic SHIV33a () makes it feasible to study the consequences of schistosomiasis coinfection and SHIV disease progression in this nonhuman primate model of AIDS.
Many questions also remain regarding the relative clinical and pathological significance of the MC lineage as a target of HIV infection. Knowledge of the scope and diversity of susceptible target cell populations that comprise different reservoirs of persistent HIV infection during HAART now includes many non-T cell lineages, such as macrophages, dendritic cells (DCs), and others (41
). MCs represent a new lineage of target cells that share many important similarities with macrophages and DCs in terms of their ability to serve as agents of disseminated HIV infection (4
). All three of these cell lineages are primarily susceptible to R5-HIV and only to a lesser extent X4-HIV (4
). HIV-infected MCs, macrophages, and DCs are all capable of interacting with and spreading HIV infection to susceptible populations of T cells (4
). Like macrophages, MCs are widely distributed among various tissues, including the intestinal lamina propria and gut mucosal tissues, which are now recognized as the major anatomical sites for HIV transmission and replication during acute infection (5
). MCs, like DCs, can be found in regional lymph nodes and secondary lymphoid organs and can interact with and induce clonal expansion of Ag-specific susceptible T cells (46
). However, although macrophages and DCs, as well as many progenitor cells, constitutively express CXCR4, they remain refractory or only marginally susceptible to infection with X4-HIV (43
). Therefore, unlike MCs, their potential role in X4 virus infection, expansion, and viral persistence is most likely limited.
Human MCs also differ from macrophages and DCs in their ability to harbor latent inducible infectious HIV proviral DNA (4
). Reactivation of viral replication in latently infected mature MCs does not affect MC viability and can be achieved through multiple activation pathways, including signaling through TLR2, TLR4, TLR9, and IgE cross-linking (4
). Also, because latency is only established after infected prMCs differentiate into HIV-infection resistant mature forms, clonally conserved virus can become archived in the MC reservoir (4
The significance of this observation is becoming increasingly apparent with reports of two important findings from studies of persistent HIV infection during antiretroviral therapy. First, in some patients on long-term effective HAART, the source of residual viremia from invariant clones not found in circulating T cells may be a monocyte-macrophage lineage progenitor capable of producing virus that can infect CD4+
T cells (48
). It is unlikely that macrophages are the source because viral evolution is accelerated in infected tissue macrophages relative to T cells (49
). Second, there is new evidence that X4-HIV is archived during periods of productive infection and is able to persist even after long-term administration of effective HAART (27
). How this action happens and the extents to which MCs contribute to X4 virus persistence during HAART have not been elucidated.
Our data suggest that MCs are the primary, if not exclusive, Fcε
RI-expressing cell lineage that may serve as an inducible target for X4-HIV infection. Although other cell lineages, such as eosinophils, do express mRNA and protein for Fcε
RI, evidence from both flow microfluorometry and functional assays indicate that Fcε
RI is not expressed in any significant levels on the cell surface; thus limiting IgE-Fcε
RI-induced responses by eosinophils (33
). Mature eosinophils do express low levels of CD4 and CXCR4 and are moderately susceptible to X4, but not R5, tropic HIV (52
). However, unlike MCs, they succumb to viral-mediated apoptosis and necrosis and are thereby unlikely to serve as reservoirs of latent infection. Furthermore, unlike MCs, eosinophils develop and mature in the bone marrow from CD34+
progenitors and are released to the peripheral blood as mature cells. Because of their relatively short half-life (7–14 days) compared with tissue MCs (months) and the susceptibility of eosinophils to viral cytotoxicity (unlike MCs), they would only present a minor contribution, if any, to persistent retroviral infection.
Many important details regarding the clinical significance and basic pathogenic mechanisms of the role of IgE in the establishment of a MC reservoir of persistent X4-HIV infection in vivo must be elucidated. For instance, the kinetics and conditions for establishing a MC reservoir in vivo during atopy or helminthic parasitic coinfection, as well as its significance to HIV disease progression, especially during HAART, will have to be addressed with the proper animal models. Our findings, however, make the design and implementation of these studies conceivable. The results of IgE-SmEA-mediated enhancement of rhesus macaque prMC susceptibility to CXCR4-tropic SHIV33a
() provide the opportunity to address these questions using the established rhesus macaque models of schistosomiasis/SIV coinfection (40