Preston A. Marx

Fc receptor but not complement binding is important in antibody protection against HIV

Most successful vaccines elicit neutralizing antibodies and this property is a high priority when developing an HIV vaccine. Indeed, passively administered neutralizing antibodies have been shown to protect against HIV challenge in some of the best available animal models. For example, antibodies given intravenously can protect macaques against intravenous or mucosal SHIV (an HIV/SIV chimaera) challenge and topically applied antibodies can protect macaques against vaginal SHIV challenge. However, the mechanism(s) by which neutralizing antibodies afford protection against HIV is not understood and, in particular, the role of antibody Fc-mediated effector functions is unclear. Here we report that there is a dramatic decrease in the ability of a broadly neutralizing antibody to protect macaques against SHIV challenge when Fc receptor and complement-binding activities are engineered out of the antibody. No loss of antibody protective activity is associated with the elimination of complement binding alone. Our in vivo results are consistent with in vitro assays indicating that interaction of Fc-receptor-bearing effector cells with antibody-complexed infected cells is important in reducing virus yield from infected cells. Overall, the data suggest the potential importance of activity against both infected cells and free virus for effective protection against HIV.

Prevention of virus transmission to macaque monkeys by a vaginally applied monoclonal antibody to HIV-1 gp120

A topical microbicide reduces the probability of virus transmission when applied to the vagina or rectum of a person at risk of sexually acquiring HIV-1 infection. An effective microbicide could significantly reduce the global spread of HIV-1, particularly if women were able to use it covertly to protect themselves. A microbicide could target the incoming virus and either permanently inactivate it or reduce its infectivity, or it could block receptors on susceptible cells near the sites of transmission. We describe here how vaginal administration of the broadly neutralizing human monoclonal antibody b12 can protect macaques from simian-human immunodeficiency virus (SHIV) infection through the vagina. Only 3 of 12 animals receiving 5 mg b12 vaginally in either saline or a gel and then challenged vaginally (up to 2 h later) with SHIV-162P4 became infected. In contrast, infection occurred in 12 of 13 animals given various control agents under similar conditions. Lower amounts of b12 were less effective, suggesting that protection was dose dependent. These observations support the concept that viral entry inhibitors can help prevent the sexual transmission of HIV-1 to humans.

Effective, low-titer antibody protection against low-dose repeated mucosal SHIV challenge in macaques

Neutralizing antibodies are thought to be crucial for HIV vaccine protection, but studies in animal models suggest that high antibody concentrations are required. This is a major potential hurdle for vaccine design. However, these studies typically apply a large virus inoculum to ensure infection in control animals in single-challenge experiments. In contrast, most human infection via sexual encounter probably involves repeated exposures to much lower doses of virus. Therefore, animal studies may have provided an overestimate of the levels of antibodies required for protection in humans. We investigated whether plasma concentrations of antibody corresponding to relatively modest neutralization titers in vitro could protect macaques from repeated intravaginal exposure to low doses of a simian immunodeficiency virus–HIV chimera (SHIV) that uses the CC chemokine receptor 5 (CCR5) co-receptor. An effector function–deficient variant of the neutralizing antibody was also included. The results show that a substantially larger number of challenges is required to infect macaques treated with neutralizing antibody than control antibody–treated macaques, and support the notion that effector function may contribute to antibody protection. Overall, the results imply that lower amounts of antibody than previously considered protective may provide benefit in the context of typical human exposure to HIV-1.

An Effective AIDS Vaccine Based on Live Attenuated Vesicular Stomatitis Virus Recombinants

We developed an AIDS vaccine based on attenuated VSV vectors expressing env and gag genes and tested it in rhesus monkeys. Boosting was accomplished using vectors with glycoproteins from different VSV serotypes. Animals were challenged with a pathogenic AIDS virus (SHIV89.6P). Control monkeys showed a severe loss of CD4+ T cells and high viral loads, and 7/8 progressed to AIDS with an average time of 148 days. All seven vaccinees were initially infected with SHIV89.6P but have remained healthy for up to 14 months after challenge with low or undetectable viral loads. Protection from AIDS was highly significant (p = 0.001). VSV vectors are promising candidates for human AIDS vaccine trials because they propagate to high titers and can be delivered without injection.

Protection of macaques from vaginal SHIV challenge by vaginally delivered inhibitors of virus-cell fusion

Human immunodeficiency virus type 1 (HIV-1) continues to spread, principally by heterosexual sex, but no vaccine is available. Hence, alternative prevention methods are needed to supplement educational and behavioural-modification programmes. One such approach is a vaginal microbicide: the application of inhibitory compounds before intercourse. Here, we have evaluated the microbicide concept using the rhesus macaque ‘high dose’ vaginal transmission model with a CCR5-receptor-using simian–human immunodeficiency virus (SHIV-162P3) and three compounds that inhibit different stages of the virus–cell attachment and entry process. These compounds are BMS-378806, a small molecule that binds the viral gp120 glycoprotein and prevents its attachment to the CD4 and CCR5 receptors, CMPD167, a small molecule that binds to CCR5 to inhibit gp120 association, and C52L, a bacterially expressed peptide inhibitor of gp41-mediated fusion. In vitro, all three compounds inhibit infection of T cells and cervical tissue explants, and C52L acts synergistically with CMPD167 or BMS-378806 to inhibit infection of cell lines. In vivo, significant protection was achieved using each compound alone and in combinations. CMPD167 and BMS-378806 were protective even when applied 6 h before challenge.

Acute loss of intestinal CD4+ T cells is not predictive of simian immunodeficiency virus virulence.

The predictive value of acute gut-associated lymphoid tissue (GALT) CD4+ T cell depletion in lentiviral infections was assessed by comparing three animal models illustrative of the outcomes of SIV infection: pathogenic infection (SIVsmm infection of rhesus macaques (Rh)), persistent nonprogressive infection (SIVagm infection of African green monkeys (AGM)), and transient, controlled infection (SIVagm infection of Rh). Massive acute depletion of GALT CD4+ T cells was a common feature of acute SIV infection in all three models. The outcome of this mucosal CD4+ T cell depletion, however, differed substantially between the three models: in SIVsmm-infected Rh, the acute GALT CD4+ T cell depletion was persistent and continued with disease progression; in SIVagm, intestinal CD4+ T cells were partially restored during chronic infection in the context of normal levels of apoptosis and immune activation and absence of damage to the mucosal immunologic barrier; in SIVagm-infected Rh, complete control of viral replication resulted in restoration of the mucosal barrier and immune restoration. Therefore, our data support a revised paradigm wherein severe GALT CD4+ T cell depletion during acute pathogenic HIV and SIV infections of humans and Rh is necessary but neither sufficient nor predictive of disease progression, with levels of immune activation, proliferation and apoptosis being key factors involved in determining progression to AIDS.

Sivmac pathogenesis in rhesus macaques of Chinese and Indian origin compared with primary Hiv infections in humans

ObjectiveTo develop a SIV–rhesus macaque (Rh) model of AIDS that more closely approximates HIV pathogenesis in humans. DesignThe pathogenesis of SIV was compared in two different types of Rh, the Chinese (Ch) and Indian (Ind) subspecies. MethodsCh Rh and Ind Rh origin were identified genetically and infected with the SIVmac239 molecular clone. Plasma viral loads, depletion of intestinal lymphocytes with memory phenotype, humoral immune responses and CD4/CD8 cell ratios were compared during acute and steady-state periods of infection. ResultsPlasma viral loads from 7 days after infection through 240 days were significantly lower in Rh of Ch origin compared with Ind Rh. Viral loads in Ch Rh were closer to viral loads observed in untreated humans infected with HIV-1. Depletion of intestinal effector cells was less evident in SIV-infected Ch Rh compared with Ind Rh. An index of intestinal pathogenesis was devised that closely paralleled viral load and severity of infection. There were no rapid progressors to AIDS among 10 Ch Rh. In contrast, three of four Ind Rh progressed rapidly to AIDS. ConclusionsCompared with Ind Rh, SIVmac pathogenesis in Ch Rh was closer to HIV-1 infections in untreated adult humans. The differences were statistically significant. The Ch Rh subspecies is a suitable AIDS model and may have advantages over the rapid and highly pathogenic Ind Rh model. Moreover, Ind Rh supplies are limited and use of Ch Rh provides a new resource.

Normal T-Cell Turnover in Sooty Mangabeys Harboring Active Simian Immunodeficiency Virus Infection

ABSTRACT Sooty mangabeys naturally infected with simian immunodeficiency virus (SIV) remain healthy though they harbor viral loads comparable to those in rhesus macaques that progress to AIDS. To assess the immunologic basis of disease resistance in mangabeys, we compared the effect of SIV infection on T-cell regeneration in both monkey species. Measurement of the proliferation marker Ki-67 by flow cytometry showed that mangabeys harbored proliferating T cells at a level of 3 to 4% in peripheral blood irrespective of their infection status. In contrast, rhesus macaques demonstrated a naturally high fraction of proliferating T cells (7%) that increased two- to threefold following SIV infection. Ki-67+ T cells were predominantly CD45RA−, indicating increased proliferation of memory cells in macaques. Quantitation of an episomal DNA product of T-cell receptor α rearrangement (termed α1 circle) showed that the concentration of recent thymic emigrants in blood decreased with age over a 2-log unit range in both monkey species, consistent with age-related thymic involution. SIV infection caused a limited decrease of α1 circle numbers in mangabeys as well as in macaques. Dilution of α1 circles by T-cell proliferation likely contributed to this decrease, since α1 circle numbers and Ki-67+ fractions correlated negatively. These findings are compatible with immune exhaustion mediated by abnormal T-cell proliferation, rather than with early thymic failure, in SIV-infected macaques. Normal T-cell turnover in SIV-infected mangabeys provides an explanation for the long-term maintenance of a functional immune system in these hosts.

TRIM5 Suppresses Cross-Species Transmission of a Primate Immunodeficiency Virus and Selects for Emergence of Resistant Variants in the New Species

Cross-species transmission of simian immunodeficiency virus from sooty mangabeys (SIVsm) into rhesus macaques, and subsequent emergence of pathogenic SIVmac, required adaptation to overcome restriction encoded by the macaque TRIM5 gene.

Island biogeography reveals the deep history of SIV

Separation of the island of Bioko from West Africa about 10,000 years ago dates the origins of simian immunodeficiency virus. Simian immunodeficiency virus (SIV) lineages have been identified that are endemic to Bioko Island. The time the island formed offers a geological time scale calibration point for dating the most recent common ancestor of SIV. The Bioko viruses cover the whole range of SIV genetic diversity, and each Bioko SIV clade is most closely related to viruses circulating in hosts of the same genus on the African mainland rather than to SIVs of other Bioko species. Our phylogeographic approach establishes that SIV is ancient and at least 32,000 years old. Our conservative calibration point and analyses of gene sequence saturation and dating bias suggest it may be much older.