Limited ability of humoral immune responses in control of viremia during infection with SIVsmmD215 strain

Making a Monkey out of Human Immunodeficiency Virus/Simian Immunodeficiency Virus Pathogenesis: Immune Cell Depletion Experiments as a Tool to Understand the Immune Correlates of Protection and Pathogenicity in HIV Infection

Understanding the underlying mechanisms of HIV pathogenesis is critical for designing successful HIV vaccines and cure strategies. However, achieving this goal is complicated by the virus's direct interactions with immune cells, the induction of persistent reservoirs in the immune system cells, and multiple strategies developed by the virus for immune evasion. Meanwhile, HIV and SIV infections induce a pandysfunction of the immune cell populations, making it difficult to untangle the various concurrent mechanisms of HIV pathogenesis. Over the years, one of the most successful approaches for dissecting the immune correlates of protection in HIV/SIV infection has been the in vivo depletion of various immune cell populations and assessment of the impact of these depletions on the outcome of infection in non-human primate models. Here, we present a detailed analysis of the strategies and results of manipulating SIV pathogenesis through in vivo depletions of key immune cells populations. Although each of these methods has its limitations, they have all contributed to our understanding of key pathogenic pathways in HIV/SIV infection.

Assessment of anti-CD20 antibody pre-treatment for augmentation of CAR-T cell therapy in SIV-infected rhesus macaques

During chronic HIV and SIV infections, the majority of viral replication occurs within lymphoid follicles. In a pilot study, infusion of SIV-specific CD4-MBL-CAR-T cells expressing the follicular homing receptor, CXCR5, led to follicular localization of the cells and a reduction in SIV viral loads in rhesus macaques. However, the CAR-T cells failed to persist. We hypothesized that temporary disruption of follicles would create space for CAR-T cell engraftment and lead to increased abundance and persistence of CAR-T cells. In this study we treated SIV-infected rhesus macaques with CAR-T cells and preconditioned one set with anti-CD20 antibody to disrupt the follicles. We evaluated CAR-T cell abundance and persistence in four groups of SIVmac239-infected and ART-suppressed animals: untreated, CAR-T cell treated, CD20 depleted, and CD20 depleted/CAR-T cell treated. In the depletion study, anti-CD20 was infused one week prior to CAR-T infusion and cessation of ART. Anti-CD20 antibody treatment led to temporary depletion of CD20+ cells in blood and partial depletion in lymph nodes. In this dose escalation study, there was no impact of CAR-T cell infusion on SIV viral load. However, in both the depleted and non-depleted animals, CAR-T cells accumulated in and around lymphoid follicles and were Ki67+. CAR-T cells increased in number in follicles from 2 to 6 days post-treatment, with a median 15.2-fold increase in follicular CAR-T cell numbers in depleted/CAR-T treated animals compared to an 8.1-fold increase in non-depleted CAR-T treated animals. The increase in CAR T cells in depleted animals was associated with a prolonged elevation of serum IL-6 levels and a rapid loss of detectable CAR-T cells. Taken together, these data suggest that CAR-T cells likely expanded to a greater extent in depleted/CAR-T cell treated animals. Further studies are needed to elucidate mechanisms mediating the rapid loss of CAR-T cells and to evaluate strategies to improve engraftment and persistence of HIV-specific CAR-T cells. The potential for an inflammatory cytokine response appears to be enhanced with anti-CD20 antibody treatment and future studies may require CRS control strategies. These studies provide important insights into cellular immunotherapy and suggest future studies for improved outcomes.

Anti-viral efficacy of a next-generation CD4-binding site bNAb in SHIV-infected animals in the absence of anti-drug antibody responses

Broadly neutralizing antibodies (bNAbs) against HIV-1 are promising immunotherapeutic agents for treatment of HIV-1 infection. bNAbs can be administered to SHIV-infected rhesus macaques to assess their anti-viral efficacy; however, their delivery into macaques often leads to rapid formation of anti-drug antibody (ADA) responses limiting such assessment. Here, we depleted B cells in five SHIV-infected rhesus macaques by pretreatment with a depleting anti-CD20 antibody prior to bNAb infusions to reduce ADA. Peripheral B cells were depleted following anti-CD20 infusions and remained depleted for at least 9 weeks after the 1^st anti-CD20 infusion. Plasma viremia dropped by more than 100-fold in viremic animals after the initial bNAb treatment. No significant humoral ADA responses were detected for as long as B cells remained depleted. Our results indicate that transient B cell depletion successfully inhibited emergence of ADA and improved the assessment of anti-viral efficacy of a bNAb in a SHIV-infected rhesus macaque model.

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Highly potent CD4bs bNAb reduces viremia up to 4 log₁₀ in SHIV-infected animals

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Sustained B cell depletion prevents development of ADA responses

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Lack of ADA enables multiple bNAb infusions over 12 weeks

The COVID misinfodemic: not new, never more lethal

'Infodemia' is a portmanteau between 'information' and 'epidemics', referring to wide and rapid accumulation and dissemination of information, misinformation, and disinformation about a given subject, such as a disease. As facts, rumors and fears mix and disperse, the misinfodemic creates loud background noise, preventing the general public from discerning between accurate and false information. We compared and contrasted key elements of the AIDS and COVID-19 misinfodemics, to identify common features, and, based on experience with the AIDS pandemic, recommend actions to control and reverse the SARS-CoV-2 misinfodemic that contributed to erode the trust between the public and scientists and governments and has created barriers to control of COVID-19. As pandemics emerge and evolve, providing robust responses to future misinfodemics must be a priority for society and public health.

Therapeutic Vaccines for the Treatment of HIV

Despite the success of anti-retroviral therapy (ART) in transforming HIV into a manageable disease, it has become evident that long-term ART will not eliminate the HIV reservoir and cure the infection. Alternative strategies to eradicate HIV infection, or at least induce a state of viral control and drug-free remission are therefore needed. Therapeutic vaccination aims to induce or enhance immunity to alter the course of a disease. In this review we provide an overview of the current state of therapeutic HIV vaccine research and summarize the obstacles that the field faces while highlighting potential ways forward for a strategy to cure HIV infection.

HIV-1-Specific Chimeric Antigen Receptor T Cells Fail To Recognize and Eliminate the Follicular Dendritic Cell HIV Reservoir In Vitro

The major obstacle to a cure for HIV infection is the persistence of replication-competent viral reservoirs during antiretroviral therapy. HIV-specific chimeric antigen receptor (CAR) T cells have been developed to target latently infected CD4⁺ T cells that express virus either spontaneously or after intentional latency reversal. Whether HIV-specific CAR-T cells can recognize and eliminate the follicular dendritic cell (FDC) reservoir of HIV-bound immune complexes (ICs) is unknown. We created HIV-specific CAR-T cells using human peripheral blood mononuclear cells (PBMCs) and a CAR construct that enables the expression of CD4 (domains 1 and 2) and the carbohydrate recognition domain of mannose binding lectin (MBL) to target native HIV Env (CD4-MBL CAR). We assessed CAR-T cell cytotoxicity using a carboxyfluorescein succinimidyl ester (CFSE) release assay and evaluated CAR-T cell activation through interferon gamma (IFN-γ) production and CD107a membrane accumulation by flow cytometry. CD4-MBL CAR-T cells displayed potent lytic and functional responses to Env-expressing cell lines and HIV-infected CD4⁺ T cells but were ineffective at targeting FDC bearing HIV-ICs. CD4-MBL CAR-T cells were unresponsive to cell-free HIV or concentrated, immobilized HIV-ICs in cell-free experiments. Blocking intercellular adhesion molecule-1 (ICAM-1) inhibited the cytolytic response of CD4-MBL CAR-T cells to Env-expressing cell lines and HIV-infected CD4⁺ T cells, suggesting that factors such as adhesion molecules are necessary for the stabilization of the CAR-Env interaction to elicit a cytotoxic response. Thus, CD4-MBL CAR-T cells are unable to eliminate the FDC-associated HIV reservoir, and alternative strategies to eradicate this reservoir must be sought.IMPORTANCE Efforts to cure HIV infection have focused primarily on the elimination of latently infected CD4⁺ T cells. Few studies have addressed the unique reservoir of infectious HIV that exists on follicular dendritic cells (FDCs), persists in vivo during antiretroviral therapy, and likely contributes to viral rebound upon cessation of antiretroviral therapy. We assessed the efficacy of a novel HIV-specific chimeric antigen receptor (CAR) T cell to target both HIV-infected CD4⁺ T cells and the FDC reservoir in vitro Although CAR-T cells eliminated CD4⁺ T cells that express HIV, they did not respond to or eliminate FDC bound to HIV. These findings reveal a fundamental limitation to CAR-T cell therapy to eradicate HIV.

Expression of CD20 after viral reactivation renders HIV-reservoir cells susceptible to Rituximab

The identification of exclusive markers to target HIV-reservoir cells will represent a significant advance in the search for therapies to cure HIV. Here, we identify the B lymphocyte antigen CD20 as a marker for HIV-infected cells in vitro and in vivo. The CD20 molecule is dimly expressed in a subpopulation of CD4-positive (CD4⁺) T lymphocytes from blood, with high levels of cell activation and heterogeneous memory phenotypes. In lymph node samples from infected patients, CD20 is present in productively HIV-infected cells, and ex vivo viral infection selectively upregulates the expression of CD20 during early infection. In samples from patients on antiretroviral therapy (ART) this subpopulation is significantly enriched in HIV transcripts, and the anti-CD20 monoclonal antibody Rituximab induces cell killing, which reduces the pool of HIV-expressing cells when combined with latency reversal agents. We provide a tool for targeting this active HIV-reservoir after viral reactivation in patients while on ART.

Here, the authors identify B lymphocyte antigen CD20 as a marker for HIV-infected T cells and provide evidence for the potential use of anti-CD20 antibodies in combination with latency reversing agents for depletion of viral reactivated CD4 T cells in patients on antiretroviral therapy.

The B-Cell Follicle in HIV Infection: Barrier to a Cure

The majority of HIV replication occurs in secondary lymphoid organs (SLOs) such as the spleen, lymph nodes, and gut-associated lymphoid tissue. Within SLOs, HIV RNA⁺ cells are concentrated in the B-cell follicle during chronic untreated infection, and emerging data suggest that they are a major source of replication in treated disease as well. The concentration of HIV RNA⁺ cells in the B-cell follicle is mediated by several factors. Follicular CD4⁺ T-cell subsets including T-follicular helper cells and T-follicular regulatory cells are significantly more permissive to HIV than extrafollicular subsets. The B cell follicle also contains a large reservoir of extracellular HIV virions, which accumulate on the surface of follicular dendritic cells (FDCs) in germinal centers. FDC-bound HIV virions remain infectious even in the presence of neutralizing antibodies and can persist for months or even years. Moreover, the B-cell follicle is semi-immune privileged from CTL control. Frequencies of HIV- and SIV-specific CTL are lower in B-cell follicles compared to extrafollicular regions as the majority of CTL do not express the follicular homing receptor CXCR5. Additionally, CTL in the B-cell follicle may be less functional than extrafollicular CTL as many exhibit the recently described CD8 T follicular regulatory phenotype. Other factors may also contribute to the follicular concentration of HIV RNA⁺ cells. Notably, the contribution of NK cells and γδ T cells to control and/or persistence of HIV RNA⁺ cells in secondary lymphoid tissue remains poorly characterized. As HIV research moves increasingly toward the development of cure strategies, a greater understanding of the barriers to control of HIV infection in B-cell follicles is critical. Although no strategy has as of yet proven to be effective, a range of novel therapies to address these barriers are currently being investigated including genetically engineered CTL or chimeric antigen receptor T cells that express the follicular homing molecule CXCR5, treatment with IL-15 or an IL-15 superagonist, use of bispecific antibodies to harness the killing power of the follicular CD8⁺ T cell population, and disruption of the follicle through treatments such as rituximab.

T-bet-expressing B cells during HIV and HCV infections

T-bet-expressing B cells, first identified as perpetuators of autoimmunity, were recently shown to be critical for murine antiviral responses. While their role in human viral infections remains unclear, B cells expressing T-bet or demonstrating a related phenotype have been described in individuals chronically infected with HIV or HCV, suggesting these cells represent a component of human antiviral responses. In this review, we discuss the induction of T-bet in B cells following both HIV and HCV infections, the factors driving T-bet⁺ B cell expansions, T-bet's relationship to atypical memory B cells, and the consequences of T-bet induction. We propose potential antiviral roles for T-bet⁺ B cells and discuss whether this population poses any utility to the HIV and HCV immune responses.

T-bet+ B cells are induced by human viral infections and dominate the HIV gp140 response

Humoral immunity is critical for viral control, but the identity and mechanisms regulating human antiviral B cells are unclear. Here, we characterized human B cells expressing T-bet and analyzed their dynamics during viral infections. T-bet+ B cells demonstrated an activated phenotype, a distinct transcriptional profile, and were enriched for expression of the antiviral immunoglobulin isotypes IgG1 and IgG3. T-bet+ B cells expanded following yellow fever virus and vaccinia virus vaccinations and also during early acute HIV infection. Viremic HIV-infected individuals maintained a large T-bet+ B cell population during chronic infection that was associated with increased serum and cell-associated IgG1 and IgG3 expression. The HIV gp140-specific B cell response was dominated by T-bet-expressing memory B cells, and we observed a concomitant biasing of gp140-specific serum immunoglobulin to the IgG1 isotype. These findings suggest that T-bet induction promotes antiviral immunoglobulin isotype switching and development of a distinct T-bet+ B cell subset that is maintained by viremia and coordinates the HIV Env-specific humoral response.

The HIV-1 transmission bottleneck

It is well established that most new systemic infections of HIV-1 can be traced back to one or a limited number of founder viruses. Usually, these founders are more closely related to minor HIV-1 populations in the blood of the presumed donor than to more abundant lineages. This has led to the widely accepted idea that transmission selects for viral characteristics that facilitate crossing the mucosal barrier of the recipient’s genital tract, although the specific selective forces or advantages are not completely defined. However, there are other steps along the way to becoming a founder virus at which selection may occur. These steps include the transition from the donor’s general circulation to the genital tract compartment, survival within the transmission fluid, and establishment of a nascent stable local infection in the recipient’s genital tract. Finally, there is the possibility that important narrowing events may also occur during establishment of systemic infection. This is suggested by the surprising observation that the number of founder viruses detected after transmission in intravenous drug users is also limited. Although some of these steps may be heavily selective, others may result mostly in a stochastic narrowing of the available founder pool. Collectively, they shape the initial infection in each recipient.

Chimeric Trojan Protein Insertion in Lentiviral Membranes Makes Lentiviruses Susceptible to Neutralization by Anti-Tetanus Serum Antibodies

This study describes the initial testing of a novel strategy for neutralization of lentiviruses using the fundamental biology of enveloped viruses' assembly and budding. In the field of gene therapy, viral vector surface proteins have been manipulated in order to redirect host cell specificity by alteration of pseudo-types. This study tested whether known viral pseudo-typing proteins or surface proteins known to be recruited to the human immunodeficiency virus (HIV) envelope could be engineered to carry neutralizing epitopes from another microorganism onto the lentiviral surface. The results identify ICAM1 as a novel vehicle for lentiviral pseudo-typing. Importantly, the study shows that in a model lentiviral system, ICAM1 can be engineered in chimeric form to result in expression of a fragment of the tetanus toxoid on the viral membrane and that these viruses can then be neutralized by human serum antibodies protective against tetanus. This raises the possibility of delivering chimeric antigens as a gene therapy in HIV-infected patients.

Animal Models for HIV Cure Research

The HIV-1/AIDS pandemic continues to spread unabated worldwide, and no vaccine exists within our grasp. Effective antiretroviral therapy (ART) has been developed, but ART cannot clear the virus from the infected patient. A cure for HIV-1 is badly needed to stop both the spread of the virus in human populations and disease progression in infected individuals. A safe and effective cure strategy for human immunodeficiency virus (HIV) infection will require multiple tools, and appropriate animal models are tools that are central to cure research. An ideal animal model should recapitulate the essential aspects of HIV pathogenesis and associated immune responses, while permitting invasive studies, thus allowing a thorough evaluation of strategies aimed at reducing the size of the reservoir (functional cure) or eliminating the reservoir altogether (sterilizing cure). Since there is no perfect animal model for cure research, multiple models have been tailored and tested to address specific quintessential questions of virus persistence and eradication. The development of new non-human primate and mouse models, along with a certain interest in the feline model, has the potential to fuel cure research. In this review, we highlight the major animal models currently utilized for cure research and the contributions of each model to this goal.

Enhanced HIV-1 immunotherapy by commonly arising antibodies that target virus escape variants

Antibody-mediated immunotherapy is effective in humanized mice when combinations of broadly neutralizing antibodies (bNAbs) are used that target nonoverlapping sites on the human immunodeficiency virus type 1 (HIV-1) envelope. In contrast, single bNAbs can control simian-human immunodeficiency virus (SHIV) infection in immune-competent macaques, suggesting that the host immune response might also contribute to the control of viremia. Here, we investigate how the autologous antibody response in intact hosts can contribute to the success of immunotherapy. We find that frequently arising antibodies that normally fail to control HIV-1 infection can synergize with passively administered bNAbs by preventing the emergence of bNAb viral escape variants.

Mucosal simian immunodeficiency virus transmission in African green monkeys: susceptibility to infection is proportional to target cell availability at mucosal sites

African green monkeys (AGMs) are naturally infected with a simian immunodeficiency virus (SIVagm) that is nonpathogenic in its host. Although SIVagm is common and widespread, little is known about the mechanisms that govern its transmission. Since the earliest virus-host interactions may provide key insights into the nonpathogenic phenotype of SIVagm, we developed a mucosal transmission model for this virus. Using plasma from an acutely infected AGM as the virus inoculum, we exposed adult and juvenile AGMs, as well as pigtailed macaques (PTMs) as a nonnatural host control, by mucosal routes to increasing titers of virus and compared the doses needed to establish a productive infection. Four juvenile and four adult AGMs as well as two PTMs were intrarectally (IR) exposed, while two additional adult female AGMs were intravaginally (IVAG) exposed. No animal became infected following exposure to 10(5) RNA copies. Both PTMs but none of the AGMs became infected following exposure to 10(6) RNA copies. Finally, all adult AGMs and two of the four juvenile AGMs became infected following exposure to 10(7) RNA copies, acquiring either one (2 IR infected juveniles, 1 IR infected adult, 2 IVAG infected adults) or two (3 IR infected adults) transmitted founder viruses. These results were consistent with immunophenotypic data, which revealed a significant correlation between the percentage of CD4(+) T cells expressing CCR5 in the mucosa and the susceptibility to infection, in terms of both the viral dose and the numbers of transmitted founder viruses. Moreover, studies of uninfected AGMs showed that the fraction of CCR5-expressing CD4(+) T cells increased significantly with age. These results indicate that (i) AGMs are readily infected with SIVagm by both intrarectal and intravaginal routes, (ii) susceptibility to infection is proportional to the number of available CCR5(+) CD4(+) target cells in the mucosa, and (iii) the paucity of CCR5(+) CD4(+) target cells in infant and juvenile AGMs may explain the near absence of vertical transmission.

Functional cure of SIVagm infection in rhesus macaques results in complete recovery of CD4+ T cells and is reverted by CD8+ cell depletion

Understanding the mechanism of infection control in elite controllers (EC) may shed light on the correlates of control of disease progression in HIV infection. However, limitations have prevented a clear understanding of the mechanisms of elite controlled infection, as these studies can only be performed at randomly selected late time points in infection, after control is achieved, and the access to tissues is limited. We report that SIVagm infection is elite-controlled in rhesus macaques (RMs) and therefore can be used as an animal model for EC HIV infection. A robust acute infection, with high levels of viral replication and dramatic mucosal CD4⁺ T cell depletion, similar to pathogenic HIV-1/SIV infections of humans and RMs, was followed by complete and durable control of SIVagm replication, defined as: undetectable VLs in blood and tissues beginning 72 to 90 days postinoculation (pi) and continuing at least 4 years; seroreversion; progressive recovery of mucosal CD4⁺ T cells, with complete recovery by 4 years pi; normal levels of T cell immune activation, proliferation, and apoptosis; and no disease progression. This “functional cure” of SIVagm infection in RMs could be reverted after 4 years of control of infection by depleting CD8 cells, which resulted in transient rebounds of VLs, thus suggesting that control may be at least in part immune mediated. Viral control was independent of MHC, partial APOBEC restriction was not involved in SIVagm control in RMs and Trim5 genotypes did not impact viral replication. This new animal model of EC lentiviral infection, in which complete control can be predicted in all cases, permits research on the early events of infection in blood and tissues, before the defining characteristics of EC are evident and when host factors are actively driving the infection towards the EC status.

A small proportion of HIV-infected patients control viral replication and disease progression in the absence of any antiretroviral treatment. Understanding the mechanisms of viral control in these elite controllers may help to identify new therapeutic approaches in order to control HIV infection. However, elite controllers are identified AFTER control is established, therefore it is difficult to identify the virus and host factors that drive the infection to the controlled status. We identified an animal model (the rhesus macaque infection with SIVagm) in which, after massive acute viral replication and CD4+ T cell depletion, SIV infection is controlled in 100% of cases during chronic infection. This “functional cure” of SIVagm infection in rhesus macaques results in a complete immune restoration after four years and can be reverted by depleting the cellular immune responses in vivo. An animal model of elite controlled lentiviral infection in which complete control can be predicted in all cases permits research on the early events of infection when host factors are actively driving the infection towards the controlled status to understand the pathogenesis of HIV/SIV infections and design of new approaches for controlling HIV infection.

Memory B cells and CD8⁺ lymphocytes do not control seasonal influenza A virus replication after homologous re-challenge of rhesus macaques

This study sought to define the role of memory lymphocytes in the protection from homologous influenza A virus re-challenge in rhesus macaques. Depleting monoclonal antibodies (mAb) were administered to the animals prior to their second experimental inoculation with a human seasonal influenza A virus strain. Treatment with either anti-CD8α or anti-CD20 mAbs prior to re-challenge had minimal effect on influenza A virus replication. Thus, in non-human primates with pre-existing anti-influenza A antibodies, memory B cells and CD8α⁺ T cells do not contribute to the control of virus replication after re-challenge with a homologous strain of influenza A virus.

Immunodeficiency lentiviral infections in natural and non-natural hosts

The host immune system is profoundly affected during the acute phase of progressive immunodeficiency lentiviral infections. Studies of these alterations have been quite restricted in humans because of the limited availability of samples from acutely HIV-infected persons. Therefore, numerous studies have turned attention to nonhuman primate models. Specifically, SIV-infected rhesus macaques (RMs) have been informative for understanding the pathogenesis of HIV infection in humans. Indeed, advantages of the nonhuman primate model include the ability to study the very early events after infection and the ability to retrieve copious amounts of tissues. In addition, nonhuman primates allow for comparative studies between non-natural and natural hosts for SIV, in which SIV infection results in progression, or not, to AIDS, respectively. Although SIV infection of RM is the best model for HIV infection, the immunologic and/or virologic phenomena in SIV-infected RM do not always reflect those seen in HIV-infected humans. Here virologic and immunologic aspects of acute HIV infection of humans and SIV infection of Asian and African nonhuman primates are discussed and compared in relation to how these aspects relate to disease progression.

Vesicular stomatitis virus-simian retrovirus type 2 vaccine protects macaques from detectable infection and B-cell destruction

Natural infection with simian retrovirus (SRV) has long been recognized in rhesus macaques (RMs) and may result in an AIDS-like disease. Importantly, SRV infections persist as a problem in recently imported macaques. Therefore, there is a clear need to control SRV spread in macaque colonies. We developed a recombinant vesicular stomatitis virus (VSV)-SRV vaccine consisting of replication-competent hybrid VSVs that express SRV gag and env in separate vectors. The goal of this study was to assess the immunogenicity and protective efficacy of the VSV-SRV serotype 2 vaccine prime-boost approach in RMs. The VSV-SRV vector (expressing either SRV gag or env) vaccines were intranasally administered in 4 RMs, followed by a boost 1 month after the first vaccination. Four RMs served as controls and received the VSV vector alone. Two months after the boost, all animals were intravenously challenged with SRV-2 and monitored for 90 days. After the SRV-2 challenge, all four controls became infected, and viral loads (VLs) ranged from 10(6) to 10(8) SRV RNA copies/ml of plasma. Two animals in the control group developed simian AIDS within 7 to 8 weeks postinfection and were euthanized. Anemia and weight loss were observed in the remaining controls. During acute infection, severe B-cell depletion and no significant changes in T-cell population were observed in the control group. Control RMs with greater preservation of B cells and lower VLs survived longer. SRV-2 was undetectable in vaccinated animals, which remained healthy, with no clinical or biological signs of infection and preservation of B cells. Our study showed that the VSV-SRV vaccine is a strong approach for preventing clinically relevant type D retrovirus infection and disease in RMs, with protection of 4/4 RMs from SRV infection and prevention of B-cell destruction. B-cell protection was the strongest correlate of the long-term survival of all vaccinated and control RMs.

Quantification of the relative importance of CTL, B cell, NK cell, and target cell limitation in the control of primary SIV-infection

CD8+ cytotoxic T lymphocytes (CTLs), natural killer (NK) cells, B cells and target cell limitation have all been suggested to play a role in the control of SIV and HIV-1 infection. However, previous research typically studied each population in isolation leaving the magnitude, relative importance and in vivo relevance of each effect unclear. Here we quantify the relative importance of CTLs, NK cells, B cells and target cell limitation in controlling acute SIV infection in rhesus macaques. Using three different methods, we find that the availability of target cells and CD8+ T cells are important predictors of viral load dynamics. If CTL are assumed to mediate this anti-viral effect via a lytic mechanism then we estimate that CTL killing is responsible for approximately 40% of productively infected cell death, the remaining cell death being attributable to intrinsic, immune (CD8+ T cell, NK cell, B cell) -independent mechanisms. Furthermore, we find that NK cells have little impact on the death rate of infected CD4+ cells and that their net impact is to increase viral load. We hypothesize that NK cells play a detrimental role in SIV infection, possibly by increasing T cell activation.

Pattern of SIVagm infection in patas monkeys suggests that host adaptation to simian immunodeficiency virus infection may result in resistance to infection and virus extinction

Patas monkeys were not reported to carry species‐specific simian immunodeficiency virus (SIV), but cross‐species transmission of SIVagm to patas monkeys occurred in the wild. We report that patas monkeys share immunophenotypic features with natural hosts of SIV; that is, low levels of CD4+ T cells and low CCR5 expression on CD4+ T cells. In 1 patas monkey with undetectable levels of CD4+ T cells, experimental exposure to SIVagm did not result in infection. The other experimentally infected patas monkeys showed an infection pattern similar to SIV infection in natural hosts. Thus, down‐regulation of CD4 and CCR5 expression on CD4+ T cells may effectively control human immunodeficiency virus acquisition and result in SIV extinction.

Where the wild things are: pathogenesis of SIV infection in African nonhuman primate hosts

African nonhuman primates that are natural hosts of simian immunodeficiency virus (SIV) are generally spared from disease progression. Pathogenic and nonpathogenic SIV infections share some major features: high viral replication, massive acute depletion of mucosal CD4(+) T cells, and partial control of the virus by both adaptive and innate immune responses. A key distinction of natural SIV infections is rapid and active control of immune activation and apoptosis of T cells that contributes to the integrity of mucosal barrier and lack of microbial translocation. This allows partial recovery of CD4(+) T cells and preservation of the function of other immune cell subsets. A better understanding of the mechanisms underlying the lack of disease in natural hosts for SIV infection will likely provide important clues as to the therapy of HIV-1 infection.

Seronegative HIV-1 infection: a review of the literature

HIV-1-specific antibodies can be detected in HIV-1-positive patients within weeks of primary infection. Rare cases have been reported of patients who are persistently seronegative despite evidence of HIV-1 infection. We present a retrospective review of the clinical, virologic and immunologic characteristics of 25 persistently seronegative patients whose cases have been published to date and postulate a biologic mechanism for this phenomenon.

Neutralizing antibodies in SIV control: co-impact with T cells

Human immunodeficiency virus type 1 (HIV-1) and pathogenic simian immunodeficiency virus (SIV)-infected naïve hosts experience a characteristic absence of early and potent virus-specific neutralizing antibody (NAb) responses preceding establishment of persistent infection. Yet conversely, we have recently shown that NAbs passively immunized in rhesus macaques at early post-SIV challenge are capable of playing a critical role in non-sterile viremia control with implications of antibody-enhanced antigen presentation. In a current follow-up study we have further reported that NAbs mediate rapid elicitation of polyfunctional virus-specific CD4+ T-cells in vivo. The NAb-immunized macaques mounting these responses exhibited sustained viremia control for over 1 year, accompanied with robust anti-SIV cellular immunity. Perspectives obtained from the results are discussed.

The role of antibodies in HIV vaccines

Licensed vaccines against viral diseases generate antibodies that neutralize the infecting virus and protect against infection or disease. Similarly, an effective vaccine against HIV-1 will likely need to induce antibodies that prevent initial infection of host cells or that limit early events of viral dissemination. Such antibodies must target the surface envelope glycoproteins of HIV-1, which are highly variable in sequence and structure. The first subunit vaccines to enter clinical trails were safe and immunogenic but unable to elicit antibodies that neutralized most circulating strains of HIV-1. However, potent virus neutralizing antibodies (NAbs) can develop during the course of HIV-1 infection, and this is the type of antibody response that researchers seek to generate with a vaccine. Thus, current vaccine design efforts have focused on a more detailed understanding of these broadly neutralizing antibodies and their epitopes to inform the design of improved vaccines.

Experimental depletion of CD8+ cells in acutely SIVagm-infected African Green Monkeys results in increased viral replication

Background

In vivo CD8⁺ cell depletions in pathogenic SIV infections identified a key role for cellular immunity in controlling viral load (VL) and disease progression. However, similar studies gave discordant results in chronically-infected SMs, leading some authors to propose that in natural hosts, SIV replication is independent of cellular immunity. To assess the role of cellular immune responses in the control of SIV replication in natural hosts, we investigated the impact of CD8⁺ cell depletion during acute SIV infection in AGMs.

Results

Nine AGMs were infected with SIVagm.sab and were followed up to day 225 p.i. Four were intravenously infused with the cM-T807 antibody on days 0 (50 mg/kg), 6, and 13 (10 mg/kg, respectively) post infection (p.i.). CD8⁺ cells were depleted for up to 28 days p.i. in peripheral blood and LNs in all treated AGMs. Partial CD8⁺ T cell depletion occurred in the intestine. SIVagm VLs peaked at similar levels in both groups (10⁷-10⁸ RNA copies/ml). However, while VLs were controlled in undepleted AGMs, reaching set-point levels (10⁴-10⁵ RNA copies/ml) by day 28 p.i., high VLs (>10⁶ RNA copies/ml) were maintained by day 21 p.i. in CD8-depleted AGMs. By day 42 p.i., VLs were comparable between the two groups. The levels of immune activation and proliferation remained elevated up to day 72 p.i. in CD8-depleted AGMs and returned to preinfection levels in controls by day 28 p.i. None of the CD8-depleted animals progressed to AIDS.

Conclusion

CD8⁺ cells are responsible for a partial control of postacute viral replication in SIVagm.sab-infected AGMs. In contrast to macaques, the SIVagm-infected AGMs are able to control viral replication after recovery of the CD8⁺ T cells and avoid disease progression.

Suppression of adaptive immune responses during primary SIV infection of sabaeus African green monkeys delays partial containment of viremia but does not induce disease

One of the most puzzling observations in HIV research is the lack of pathogenicity in most nonhuman primate species that are natural hosts of simian immunodeficiency virus (SIV) infection. Despite this, natural hosts experience a level of viremia similar to humans infected with HIV or macaques infected with SIV. To determine the role of adaptive immune responses in viral containment and lack of disease, we delayed the generation of cellular and humoral immune responses by administering anti-CD8- and anti-CD20 lymphocyte-depleting antibodies to sabaeus African green monkeys (Chlorocebus sabaeus) before challenge with SIV(sab9315BR). In vivo lymphocyte depletion during primary infection resulted in a brief elevation of viremia but not in disease. Based on the magnitude and timing of SIV-specific CD8(+) T-cell responses in the lymphocyte-depleted animals, CD8(+) T-cell responses appear to contribute to viral containment in natural hosts. We found no evidence for a contribution of humoral immune responses in viral containment. These studies indicate that natural hosts have developed mechanisms in addition to classic adaptive immune responses to cope with this lentiviral infection. Thus, adaptive immune responses in natural hosts appear to be less critical for viral containment than in HIV infection.

Alcohol's role in HIV transmission and disease progression

Alcohol use has negative effects on HIV disease progression through several mechanisms, including transmission, viral replication, host immunity, and treatment efficacy. Research with animal models has explored the effect of alcohol intake on several aspects of simian immunodeficiency virus (SIV) disease progression. Data suggest that the increased SIV levels observed in alcohol-consuming animals may represent an increase in virus production as opposed to a decrease in host defense. Results also suggest that changes in nutritional balance and metabolism, as a possible consequence of a proinflammatory state, together with increased virus production in animals consuming alcohol, accelerate SIV and possibly HIV disease progression. Further studies using the animal model are necessary.

Effect of B-cell depletion on viral replication and clinical outcome of simian immunodeficiency virus infection in a natural host

Simian immunodeficiency virus (SIV)-infected African nonhuman primates do not progress to AIDS in spite of high and persistent viral loads (VLs). Some authors consider the high viral replication observed in chronic natural SIV infections to be due to lower anti-SIV antibody titers than those in rhesus macaques, suggesting a role of antibodies in controlling viral replication. We therefore investigated the impact of antibody responses on the outcome of acute and chronic SIVagm replication in African green monkeys (AGMs). Nine AGMs were infected with SIVagm.sab. Four AGMs were infused with 50 mg/kg of body weight anti-CD20 (rituximab; a gift from Genentech) every 21 days, starting from day -7 postinfection up to 184 days. The remaining AGMs were used as controls and received SIVagm only. Rituximab-treated AGMs were successfully depleted of CD20 cells in peripheral blood, lymph nodes (LNs), and intestine, as shown by the dynamics of CD20+ and CD79a+ cells. There was no significant difference in VLs between CD20-depleted AGMs and control monkeys: peak VLs ranged from 10(7) to 10(8) copies/ml; set-point values were 10(4) to 10(5) SIV RNA copies/ml. Levels of acute mucosal CD4+ T-cell depletion were similar for treated and nontreated animals. SIVagm seroconversion was delayed for the CD20-depleted AGMs compared to results for the controls. There was a significant difference in both the timing and magnitude of neutralizing antibody responses for CD20-depleted AGMs compared to results for controls. CD20 depletion significantly altered the histological structure of the germinal centers in the LNs and Peyer's patches. Our results, although obtained with a limited number of animals, suggest that humoral immune responses play only a minor role in the control of SIV viral replication during acute and chronic SIV infection in natural hosts.

B cells in HIV infection and disease

In recent years, intense research efforts have been dedicated to elucidating the pathogenic mechanisms of HIV-associated disease progression. In addition to the progressive depletion and dysfunction of CD4(+) T cells, HIV infection also leads to extensive defects in the humoral arm of the immune system. The lack of immune control of the virus in almost all infected individuals is a great impediment to the treatment of HIV-associated disease and to the development of a successful HIV vaccine. This Review focuses on advances in our understanding of the mechanisms of B-cell dysfunction in HIV-associated disease and discusses similarities with other diseases that are associated with B-cell dysfunction.