Simian immunodeficiency virus infection of monkeys as a model system for the study of AIDS pathogenesis, treatment, and prevention

Evaluating the antibody response elicited by diverse HIV envelope immunogens in the African green monkey (Vervet) model

African Green (Vervet) monkeys have been extensively studied to understand the pathogenesis of infectious diseases. Using vervet monkeys as pre-clinical models may be an attractive option for low-resourced areas as they are found abundantly and their maintenance is more cost-effective than bigger primates such as rhesus macaques. We assessed the feasibility of using vervet monkeys as animal models to examine the immunogenicity of HIV envelope trimer immunogens in pre-clinical testing. Three groups of vervet monkeys were subcutaneously immunized with either the BG505 SOSIP.664 trimer, a novel subtype C SOSIP.664 trimer, CAP255, or a combination of BG505, CAP255 and CAP256.SU SOSIP.664 trimers. All groups of vervet monkeys developed robust binding antibodies by the second immunization with the peak antibody response occurring after the third immunization. Similar to binding, antibody dependent cellular phagocytosis was also observed in all the monkeys. While all animals developed potent, heterologous Tier 1 neutralizing antibody responses, autologous neutralization was limited with only half of the animals in each group developing responses to their vaccine-matched pseudovirus. These data suggest that the vervet monkey model may yield distinct antibody responses compared to other models. Further study is required to further determine the utility of this model in HIV immunization studies.

Single-Cell Profiling of Latently SIV-Infected CD4+ T Cells Directly Ex Vivo to Reveal Host Factors Supporting Reservoir Persistence

HIV-1 cure strategies aiming to eliminate persistent infected cell reservoirs are hampered by a poor understanding of cells harboring viral DNA in vivo. We describe a novel method to identify, enumerate, and characterize in detail individual cells infected in vivo using a combination of single-cell multiplexed assays for integrated proviral DNA, quantitative viral and host gene expression, and quantitative surface protein expression without any in vitro manipulation. Latently infected CD4+ T cells, defined as harboring integrated provirus in the absence of spliced viral mRNA, were identified from macaque lymph nodes during acute, chronic, and combination antiretroviral therapy (cART)-suppressed simian immunodeficiency virus (SIV) infection. Latently infected CD4+ T cells were most abundant during acute SIV (~8% of memory CD4+ T cells) and persisted in chronic and cART-suppressed infection. Productively infected cells actively transcribing viral mRNA, by contrast, were much more labile and declined substantially between acute and chronic or cART-suppressed infection. Expression of most surface proteins and host genes was similar between latently infected cells and uninfected cells. Elevated FLIP mRNA and surface CD3 expression among latently infected cells suggest increased survival potential and capacity to respond to T cell receptor stimulation. These findings point to a large pool of latently infected CD4+ T cells established very early in acute infection and upregulated host factors that may facilitate their persistence in vivo, both of which pose potential challenges to eliminating HIV-1 reservoirs. IMPORTANCE Effective combination antiretroviral therapy controls HIV-1 infection but fails to eliminate latent viral reservoirs that give rise to viremia upon treatment interruption. Strategies to eradicate latently infected cells require a better understanding of their biology and distinguishing features to promote their elimination. Tools for studying these cells from patients are currently limited. Here, we developed a single-cell method to identify cells latently infected in vivo and to characterize these cells for expression of surface proteins and host genes without in vitro manipulation, capturing their in vivo state from SIV-infected macaques. Host factors involved in cell survival and proliferation were upregulated in latently infected cells, which were abundant in the earliest stages of acute infection. These studies provide insight into the basic biology of latently infected cells as well as potential mechanisms underlying the persistence of HIV-1/SIV reservoirs to inform development of novel HIV-1 cure strategies.

HIV Reservoirs: Modeling, Quantification, and Approaches to a Cure

Biomedical research in animal models depends heavily on nonhuman primates (NHP) (Phillips et al., Am J Primatol 76(9):801-827, 2014). In their physiology, neurobiology, and, most importantly, their susceptibility to infectious diseases and subsequent immune responses, NHPs have many parallels with humans (Rhesus Macaque Genome Sequencing and Analysis Consortium et al., Science 316(5822):222-234, 2007). Different species of NHPs have served as important animal models for numerous infectious diseases spanning a wide range of pathogens (Gardner and Luciw, ILAR J 49(2):220-255, 2008). As a result of recognizing their utility in HIV research, NHPs have contributed to groundbreaking studies of disease pathogenesis, vaccination, and curative research (London et al., Lancet 2(8355):869-873, 1983; Henrickson et al., Lancet 1 (8321):388-390, 1983). Many African NHPs are considered natural hosts for SIV in which SIV infection is usually nonprogressive and does not cause acquired immunodeficiency syndrome (AIDS) (Chahroudi et al., Science 335(6073):1188-1193, 2012; Taaffe et al., J Virol 84(11):5476-5484, 2010). However, cross-species transmission of SIV strains to other NHPs or to humans (nonnatural hosts) leads to progressive disease and AIDS (Paiardini et al., Annu Rev Med 60:485-495, 2009). In particular, SIV infection of Asian rhesus macaques recapitulates many features of HIV infection in humans and therefore has become a widely used approach for contemporary HIV research into virus persistence and cure strategies (Gardner and Luciw, FASEB J 3(14):2593-2606, 1989). There are multiple factors that should be considered in HIV/SIV studies using NHPs including the particular monkey species and geographic background, age and sex, certain genetic properties, virus strain, route and dose of infection, interventional treatments, and prespecified study outcomes. Here, we discuss consideration of these factors to address specific questions in HIV cure research.

Interests of the Non-Human Primate Models for HIV Cure Research

Non-human primate (NHP) models are important for vaccine development and also contribute to HIV cure research. Although none of the animal models are perfect, NHPs enable the exploration of important questions about tissue viral reservoirs and the development of intervention strategies. In this review, we describe recent advances in the use of these models for HIV cure research and highlight the progress that has been made as well as limitations using these models. The main NHP models used are (i) the macaque, in which simian immunodeficiency virus (SIVmac) infection displays similar replication profiles as to HIV in humans, and (ii) the macaque infected by a recombinant virus (SHIV) consisting of SIVmac expressing the HIV envelope gene serving for studies analyzing the impact of anti-HIV Env broadly neutralizing antibodies. Lessons for HIV cure that can be learned from studying the natural host of SIV are also presented here. An overview of the most promising and less well explored HIV cure strategies tested in NHP models will be given.

Residual Proviral Reservoirs: A High Risk for HIV Persistence and Driving Forces for Viral Rebound after Analytical Treatment Interruption

Antiretroviral therapy (ART) has dramatically suppressed human immunodeficiency virus (HIV) replication and become undetectable viremia. However, a small number of residual replication-competent HIV proviruses can still persist in a latent state even with lifelong ART, fueling viral rebound in HIV-infected patient subjects after treatment interruption. Therefore, the proviral reservoirs distributed in tissues in the body represent a major obstacle to a cure for HIV infection. Given unavailable HIV vaccine and a failure to eradicate HIV proviral reservoirs by current treatment, it is crucial to develop new therapeutic strategies to eliminate proviral reservoirs for ART-free HIV remission (functional cure), including a sterilizing cure (eradication of HIV reservoirs). This review highlights recent advances in the establishment and persistence of HIV proviral reservoirs, their detection, and potential eradication strategies.

Simian-Human Immunodeficiency Virus SHIV.C.CH505 Persistence in ART-Suppressed Infant Macaques Is Characterized by Elevated SHIV RNA in the Gut and a High Abundance of Intact SHIV DNA in Naive CD4+ T Cells

Mother-to-child transmission of human immunodeficiency virus type 1 (HIV-1) continues to cause new pediatric cases of infection through breastfeeding, a setting where it is not always possible to initiate early antiretroviral therapy (ART). Without novel interventions that do not rely on daily ART, HIV-1-infected children face lifelong medications to control infection. A detailed analysis of virus persistence following breast milk transmission of HIV-1 and ART has not been performed. Here, we used infant rhesus macaques orally infected with simian/human immunodeficiency virus (SHIV) (SHIV.C.CH505) to identify cellular and anatomical sites of virus persistence under ART. Viral DNA was detected at similar levels in blood and tissue CD4+ T cells after a year on ART, with virus in blood and lymphoid organs confirmed to be replication competent. Viral RNA/DNA ratios were elevated in rectal CD4+ T cells compared to those of other sites (P ≤ 0.0001), suggesting that the gastrointestinal tract is an active site of virus transcription during ART-mediated suppression of viremia. SHIV.C.CH505 DNA was detected in multiple CD4+ T cell subsets, including cells with a naive phenotype (CD45RA+ CCR7+ CD95-). While the frequency of naive cells harboring intact provirus was lower than in memory cells, the high abundance of naive cells in the infant CD4+ T cell pool made them a substantial source of persistent viral DNA (approximately 50% of the total CD4+ T cell reservoir), with an estimated 1:2 ratio of intact provirus to total viral DNA. This viral reservoir profile broadens our understanding of virus persistence in a relevant infant macaque model and provides insight into targets for cure-directed approaches in the pediatric population.IMPORTANCE Uncovering the sanctuaries of the long-lived HIV-1 reservoir is crucial to develop cure strategies. Pediatric immunity is distinct from that of adults, which may alter where the reservoir is established in infancy. Thus, it is important to utilize pediatric models to inform cure-directed approaches for HIV-1-infected children. We used an infant rhesus macaque model of HIV-1 infection via breastfeeding to identify key sites of viral persistence under antiretroviral therapy (ART). The gastrointestinal tract was found to be a site for low-level viral transcription during ART. We also show that naive CD4+ T cells harbored intact provirus and were a major contributor to blood and lymphoid reservoir size. This is particularly striking, as memory CD4+ T cells are generally regarded as the main source of latent HIV/simian immunodeficiency virus (SIV) infection of adult humans and rhesus macaques. Our findings highlight unique features of reservoir composition in pediatric infection that should be considered for eradication efforts.

From Rhesus macaque to human: structural evolutionary pathways for immunoglobulin G subclasses

The Old World monkey, Rhesus macaque (Macaca mulatta, Mm), is frequently used as a primate model organism in the study of human disease and to test new vaccines/antibody treatments despite diverging before chimpanzees and orangutans. Mm and humans share 93% genome identity with substantial differences in the genes of the adaptive immune system that lead to different functional IgG subclass characteristics, Fcγ receptors expressed on innate immune cells, and biological interactions. These differences put limitations on Mm use as a primary animal model in the study of human disease and to test new vaccines/antibody treatments. Here, we comprehensively analyzed molecular properties of the Fc domain of the four IgG subclasses of Rhesus macaque to describe potential mechanisms for their interactions with effector cell Fc receptors. Our studies revealed less diversity in the overall structure among the Mm IgG Fc, with MmIgG1 Fc being the most structurally like human IgG3, although its CH2 loops and N297 glycan mobility are comparable to human IgG1. Furthermore, the Fcs of Mm IgG3 and 4 lack the structural properties typical for their human orthologues that determine IgG3's reduced interaction with the neonatal receptor and IgG4's ability for Fab-arm exchange and its weaker Fcγ receptor interactions. Taken together, our data indicate that MmIgG1-4 are less structurally divergent than the human IgGs, with only MmIgG1 matching the molecular properties of human IgG1 and 3, the most active IgGs in terms of Fcγ receptor binding and Fc-mediated functions. PDB accession numbers for deposited structures are 6D4E, 6D4I, 6D4M, and 6D4N for MmIgG1 Fc, MmIgG2 Fc, MmIgG3 Fc, and MmIgG4 Fc, respectively.

Combined single-cell quantitation of host and SIV genes and proteins ex vivo reveals host-pathogen interactions in individual cells

CD4 T cells harboring HIV-1/SIV represent a formidable hurdle to eradicating infection, and yet their detailed phenotype remains unknown. Here we integrate two single-cell technologies, flow cytometry and highly multiplexed quantitative RT-PCR, to characterize SIV-infected CD4 T cells directly ex vivo. Within individual cells, we correlate the cellular phenotype, in terms of host protein and RNA expression, with stages of the viral life cycle defined by combinatorial expression of viral RNAs. Spliced RNA⁺ infected cells display multiple memory and activation phenotypes, indicating virus production by diverse CD4 T cell subsets. In most (but not all) cells, progressive infection accompanies post-transcriptional downregulation of CD4 protein, while surface MHC class I is largely retained. Interferon-stimulated genes were also commonly upregulated. Thus, we demonstrate that combined quantitation of transcriptional and post-transcriptional regulation at the single-cell level informs in vivo mechanisms of viral replication and immune evasion.

HIV-1, and its simian counterpart, SIV, infect and kill CD4 T cells, resulting in their massive depletion that ultimately leads to AIDS in the absence of antiretroviral therapy. With effective therapy, these cells are largely preserved, but a subset harbors latent virus that can persist for decades and reemerge upon therapy interruption, preventing HIV-1 cure. To prevent or eliminate productive cellular infection, there is tremendous demand to identify host factors expressed by these cells in vivo, which may serve as unique biomarkers or drug targets. Here we provide the first detailed combined transcriptomic and protein expression profile of SIV-infected cells directly ex vivo using novel single-cell technologies. Our survey of activation markers, interferon-stimulated genes, and viral restriction factors identified multiple host genes differentially expressed by SIV-infected cells and will inform future therapeutic strategies.

Nonhuman Primate Models for Studies of AIDS Virus Persistence During Suppressive Combination Antiretroviral Therapy

Nonhuman primate (NHP) models of AIDS represent a potentially powerful component of the effort to understand in vivo sources of AIDS virus that persist in the setting of suppressive combination antiretroviral therapy (cART) and to develop and evaluate novel strategies for more definitive treatment of HIV infection (i.e., viral eradication "cure", or sustained off-cART remission). Multiple different NHP models are available, each characterized by a particular NHP species, infecting virus, and cART regimen, and each with a distinct capacity to recapitulate different aspects of HIV infection. Given these different biological characteristics, and their associated strengths and limitations, different models may be preferred to address different questions pertaining to virus persistence and cure research, or to evaluate different candidate intervention approaches. Recent developments in improved cART regimens for use in NHPs, new viruses, a wider array of sensitive virologic assay approaches, and a better understanding of pathogenesis should allow even greater contributions from NHP models to this important area of HIV research in the future.

Preclinical Toxicology of Vaccines1

Immunity to targeted infectious diseases may be conferred or enhanced by vaccines, which are manufactured from recombinant forms as well as inactivated or attenuated organisms. These vaccines have to meet requirements for safety, quality, and efficacy. In addition to antigenic components, various adjuvants may be included in vaccines to evoke an effective immune response. To ensure the safety of new vaccines, preclinical toxicology studies are conducted prior to the initiation of, and concurrently with, clinical studies. There are five different types of preclinical toxicology study in the evaluation of vaccine safety: single and/or repeat dose, reproductive and developmental, mutagenicity, carcinogenicity, and safety pharmacology. If any adverse effects are observed in the course of these studies, they should be fully evaluated and a final safety decision made accordingly. Successful preclinical toxicology studies depend on multiple factors including using the appropriate study designs, using the right animal model, and evoking an effective immune response. Additional in vivo and in vitro assays that establish the identity, purity, safety, and potency of the vaccine play a significant role in assessing critical characteristics of vaccine safety.

Nonhuman primate models for the evaluation of HIV-1 preventive vaccine strategies: model parameter considerations and consequences

PURPOSE OF REVIEW

Nonhuman primate (NHP) models of AIDS are powerful systems for evaluating HIV vaccine approaches in vivo. Authentic features of HIV-1 transmission, dissemination, target cell tropism, and pathogenesis, and aspects of anti-HIV-1 immune responses, can be recapitulated in NHPs provided the appropriate, specific model parameters are considered. Here, we discuss key model parameter options and their implications for HIV-1 vaccine evaluation.

RECENT FINDINGS

With the availability of several different NHP host species/subspecies, different challenge viruses and challenge stock production methods, and various challenge routes and schemata, multiple NHP models of AIDS exist for HIV vaccine evaluation. The recent development of multiple new challenge viruses, including chimeric simian-human immunodeficiency viruses and simian immunodeficiency virus clones, improved characterization of challenge stocks and production methods, and increased insight into specific challenge parameters have resulted in an increase in the number of available models and a better understanding of the implications of specific study design choices.

SUMMARY

Recent progress and technical developments promise new insights into basic disease mechanisms and improved models for better preclinical evaluation of interventions to prevent HIV transmission.

Nonhuman Primates and Humanized Mice for Studies of HIV-1 Integrase Inhibitors: A Review

Since the discovery of the first inhibitors of HIV replication, drug resistance has been a major problem in HIV therapy due in part to the high mutation rate of HIV. Therefore, the development of a predictive animal model is important to identify impending resistance mutations and to possibly inform treatment decisions. Significant advances have been made possible through use of nonhuman primates infected by SIV, SHIV, and simian-tropic HIV-1 (stHIV-1), and use of humanized mouse models of HIV-1 infections. In this review, we describe some of the findings from animal models used for the preclinical testing of integrase strand transfer inhibitors. These models have led to important findings about the potential role of integrase strand transfer inhibitors in both the prevention and treatment of HIV-1 infection.

Generation and characterization of a SIVmac239 clone corrected at four suboptimal nucleotides

BACKGROUND

SIVmac239 is a commonly used virus in non-human primate models of HIV transmission and pathogenesis. Previous studies identified four suboptimal nucleotides in the SIVmac239 genome, which putatively inhibit its replicative capacity. Since all four suboptimal changes revert to the optimal nucleotide consensus sequence during viral replication in vitro and in vivo, we sought to eliminate the variability of generating these mutations de novo and increase the overall consistency of viral replication by introducing the optimal nucleotides directly to the infectious molecular clone.

RESULTS

Using site directed mutagenesis of the full-length/nef-open SIVmac239 clone, we reverted all four nucleotides to the consensus/optimal base to generate SIVmac239Opt and subsequently tested its infectivity and replicative capacity in vitro and in vivo. In primary and cell line cultures, we observed that the optimized virus displayed consistent modest but not statistically significant increases in replicative kinetics compared to wild type. In vivo, SIVmac239Opt replicated to high peak titers with an average of 1.2 × 10(8) viral RNA copies/ml at day 12 following intrarectal challenge, reaching set-point viremia of 1.2 × 10(6) viral RNA copies/ml by day 28. Although the peak and set point viremia means were not statistically different from the original "wild type" SIVmac239, viral load variation at set point was greater for SIVmac239WT compared to SIVmac239Opt (p = 0.0015) demonstrating a greater consistency of the optimized virus. Synonymous mutations were added to the integrase gene of SIVmac239Opt to generate a molecular tag consisting of ten genetically distinguishable viral variants referred to as SIVmac239OptX (Del Prete et al., J Virol. doi: 10.1128/JVI.01026-14 , 2014). Replication dynamics in vitro of these optimized clones were not statistically different from the parental clones. Interestingly, the consistently observed rapid reversion of the primer binding site suboptimal nucleotide is not due to viral RT error but is changed post-integration of a mismatched base via host proofreading mechanisms.

CONCLUSIONS

Overall, our results demonstrate that SIVmac239Opt is a functional alternative to parental SIVmac239 with marginally faster replication dynamics and with increased replication uniformity providing a more consistent and reproducible infection model in nonhuman primates.

Correlation of central memory CD4+ T-Cell decrease in the peripheral blood with disease progression in SIVmac251-infected Chinese rhesus macaques

BACKGROUND

Correlation of CD4(+) Tcm cells in the peripheral blood to disease progression in SIVmac251 infection was examined in Chinese rhesus macaques.

METHODS

Plasma viral RNA loads were measured by a quantitative real-time reverse transcription-PCR (qRT-PCR) assay for SIV gag. Disease progression was determined based on time of survival. Phenotyping of CD4(+) T-cell subsets in the peripheral blood was longitudinally performed by flow cytometry.

RESULTS

Although CD4(+) T-cell decrease and low CD4(+)/CD8(+) T-cell ratio in the peripheral blood after SIVmac251 infection did not correlate with disease progression, CD4(+) Tcm cell decrease was observed to be correlated to disease progression in the SIVmac251-infected Chinese rhesus macaques.

CONCLUSIONS

Our findings suggest that CD4(+) Tcm cell decrease could be used as a predictive marker for defining the pathogenesis of the SIV disease and consequently HIV/SIV vaccine efficacy in Chinese rhesus macaques.

The importance of monocytes and macrophages in HIV pathogenesis, treatment, and cure

Monocytes and macrophages play critical roles in HIV transmission, viral spread early in infection, and as a reservoir of virus throughout infection. There has been a recent resurgence of interest in the biology of monocyte subsets and macrophages and their role in HIV pathogenesis, partly fuelled by efforts to understand difficulties in achieving HIV eradication. This article examines the importance of monocyte subsets and tissue macrophages in HIV pathogenesis. Additionally, we will review the role of monocytes and macrophages in the development of serious non-AIDS events including cardiovascular disease and neurocognitive impairment, their significance in viral persistence, and how these cells represent an important obstacle to achieving HIV eradication.

Molecularly tagged simian immunodeficiency virus SIVmac239 synthetic swarm for tracking independent infection events

Following mucosal human immunodeficiency virus type 1 transmission, systemic infection is established by one or only a few viral variants. Modeling single-variant, mucosal transmission in nonhuman primates using limiting-dose inoculations with a diverse simian immunodeficiency virus isolate stock may increase variability between animals since individual variants within the stock may have substantial functional differences. To decrease variability between animals while retaining the ability to enumerate transmitted/founder variants by sequence analysis, we modified the SIVmac239 clone to generate 10 unique clones that differ by two or three synonymous mutations (molecular tags). Transfection- and infection-derived virus stocks containing all 10 variants showed limited phenotypic differences in 9 of the 10 clones. Twenty-nine rhesus macaques were challenged intrarectally or intravenously with either a single dose or repeated, limiting doses of either stock. The proportion of each variant within each inoculum and in plasma from infected animals was determined by using a novel real-time single-genome amplification assay. Each animal was infected with one to five variants, the number correlating with the dose. Longitudinal sequence analysis revealed that the molecular tags are highly stable with no reversion to the parental sequence detected in >2 years of follow-up. Overall, the viral stocks are functional and mucosally transmissible and the number of variants is conveniently discernible by sequence analysis of a small amplicon. This approach should be useful for tracking individual infection events in preclinical vaccine evaluations, long-term viral reservoir establishment/clearance research, and transmission/early-event studies. Importance: Human immunodeficiency virus type 1 transmission is established by one or only a few viral variants. Modeling of limited variant transmission in nonhuman primates with a diverse simian immunodeficiency virus isolate stock may increase the variability between animals because of functional differences in the individual variants within the stock. To decrease such variability while retaining the ability to distinguish and enumerate transmitted/founder variants by sequence analysis, we generated a viral stock with 10 sequence-identifiable but otherwise genetically identical variants. This virus was characterized in vitro and in vivo and shown to allow discrimination of distinct transmission events. This approach provides a novel nonhuman primate challenge system for the study of viral transmission, evaluation of vaccines and other prevention approaches, and characterization of viral reservoirs and strategies to target them.

Four decades of leading-edge research in the reproductive and developmental sciences: the Infant Primate Research Laboratory at the University of Washington National Primate Research Center

The Infant Primate Research Laboratory (IPRL) was established in 1970 at the University of Washington as a visionary project of Dr. Gene (Jim) P. Sackett. Supported by a collaboration between the Washington National Primate Research Center and the Center on Human Development and Disability, the IPRL operates under the principle that learning more about the causes of abnormal development in macaque monkeys will provide important insights into the origins and treatment of childhood neurodevelopmental disabilities. Over the past 40 years, a broad range of research projects have been conducted at the IPRL. Some have described the expression of normative behaviors in nursery-reared macaques while others have focused on important biomedical themes in child health and development. This article details the unique scientific history of the IPRL and the contributions produced by research conducted in the laboratory. Past and present investigations have explored the topics of early rearing effects, low-birth-weight, prematurity, birth injury, epilepsy, prenatal neurotoxicant exposure, viral infection (pediatric HIV), diarrheal disease, vaccine safety, and assisted reproductive technologies. Data from these studies have helped advance our understanding of both risk and resiliency in primate development. New directions of research at the IPRL include the production of transgenic primate models using our embryonic stem cell-based technology to better understand and treat heritable forms of human intellectual disabilities such as fragile X.

TRIM5 alpha drives SIVsmm evolution in rhesus macaques

The antagonistic interaction with host restriction proteins is a major driver of evolutionary change for viruses. We previously reported that polymorphisms of the TRIM5α B30.2/SPRY domain impacted the level of SIVsmm viremia in rhesus macaques. Viremia in macaques homozygous for the non-restrictive TRIM5α allele TRIM5^Q was significantly higher than in macaques expressing two restrictive TRIM5alpha alleles TRIM5^TFP/TFP or TRIM5^Cyp/TFP. Using this model, we observed that despite an early impact on viremia, SIVsmm overcame TRIM5α restriction at later stages of infection and that increasing viremia was associated with specific amino acid substitutions in capsid. Two amino acid substitutions (P37S and R98S) in the capsid region were associated with escape from TRIM5^TFP restriction and substitutions in the CypA binding-loop (GPLPA87-91) in capsid were associated with escape from TRIM5^Cyp. Introduction of these mutations into the original SIVsmE543 clone not only resulted in escape from TRIM5α restriction in vitro but the P37S and R98S substitutions improved virus fitness in macaques with homozygous restrictive TRIM^TFP alleles in vivo. Similar substitutions were observed in other SIVsmm strains following transmission and passage in macaques, collectively providing direct evidence that TRIM5α exerts selective pressure on the cross-species transmission of SIV in primates.

Human immunodeficiency virus (HIV) resulted from the transmission of simian immunodeficiency viruses (SIV) from nonhuman primates followed by adaptation and expansion as a pandemic in humans. This required the virus to overcome a variety of intrinsic host restriction factors in humans in order to replicate efficiently. Similarly, SIV encounters restriction factors upon cross-species transmission between nonhuman primates, specifically from a natural host species such as sooty mangabey monkeys to rhesus macaques. Previously we observed significant differences in the levels of virus replication of SIV among rhesus macaques due to subtle differences in one of these restriction factors, TRIM5 among individual macaques. Although a restrictive version of TRIM5 resulted in lower viremia, we also observed that the virus spontaneously mutated in the viral capsid gene and that these mutations were associated with escape from TRIM5 restriction. In the present study, we found that introduction of these escape mutations into the parental virus confers resistance to TRIM5 both in tissue culture and in macaques. These studies provide direct evidence that TRIM5 is a critical factor influencing the cross-species transmission of SIV in primates.

Efficient mucosal transmissibility but limited pathogenicity of R5 SHIV SF162P3N in Chinese-origin rhesus macaques

BACKGROUND

Infection of rhesus macaques (RMs) of Indian origin with simian immunodeficiency virus or simian-HIV (SHIV) provided powerful tools to study HIV-1 transmission and disease and for testing the efficacy of novel drugs, vaccines, and prevention strategies. In developing alternative nonhuman primate AIDS models for the CCR5 (R5)-tropic SHIVSF162P3N, we characterized virus transmission and infection in Chinese-origin RMs.

METHODS

Virologic, immunologic, and pathogenic evaluations of R5 SHIVSF162P3N infection in Chinese RMs challenged intrarectally (ir) or intravaginally were performed and compared with those previously observed in Indian-origin rhesus exposed to the same inoculum dose and via similar route.

RESULTS

R5 SHIVSF162P3N transmits efficiently across mucosal surfaces in Chinese RMs. The magnitude and kinetics of early virus dissemination after ir inoculation in the Chinese macaques were similar to those observed in Indian rhesus, but a trend toward increased SHIVSF162P3N vaginal infectivity and rapid virus spread was seen in the Chinese macaques compared with the Indian-origin animals. Once infected, however, set point viremia in the ir- and intravaginal-infected Chinese rhesus was significantly lower and the animals survived longer compared with infected Indian rhesus.

CONCLUSIONS

The R5 SHIVSF162P3N/Chinese RM infection model is suitable for studies of mucosal HIV-1 transmission and protection, but the high frequency of spontaneous control of chronic viremia and reduced virulence with SHIVSF162P3N in this macaque subspecies may limit its utility in studying HIV-1 pathogenesis and in evaluating vaccines and antiretrovirals that rely on reduction in chronic viral load or AIDS development as an experimental end point.

Comparative characterization of transfection- and infection-derived simian immunodeficiency virus challenge stocks for in vivo nonhuman primate studies

Simian immunodeficiency virus (SIV) stocks for in vivo nonhuman primate models of AIDS are typically generated by transfection of 293T cells with molecularly cloned viral genomes or by expansion in productively infected T cells. Although titers of stocks are determined for infectivity in vitro prior to in vivo inoculation, virus production methods may differentially affect stock features that are not routinely analyzed but may impact in vivo infectivity, mucosal transmissibility, and early infection events. We performed a detailed analysis of nine SIV stocks, comprising five infection-derived SIVmac251 viral swarm stocks and paired infection- and transfected-293T-cell-derived stocks of both SIVmac239 and SIVmac766. Representative stocks were evaluated for (i) virus content, (ii) infectious titer, (iii) sequence diversity and polymorphism frequency by single-genome amplification and 454 pyrosequencing, (iv) virion-associated Env content, and (v) cytokine and chemokine content by 36-plex Luminex analysis. Regardless of production method, all stocks had comparable particle/infectivity ratios, with the transfected-293T stocks possessing the highest overall virus content and infectivity titers despite containing markedly lower levels of virion-associated Env than infection-derived viruses. Transfected-293T stocks also contained fewer and lower levels of cytokines and chemokines than infection-derived stocks, which had elevated levels of multiple analytes, with substantial variability among stocks. Sequencing of the infection-derived SIVmac251 stocks revealed variable levels of viral diversity between stocks, with evidence of stock-specific selection and expansion of unique viral lineages. These analyses suggest that there may be underappreciated features of SIV in vivo challenge stocks with the potential to impact early infection events, which may merit consideration when selecting virus stocks for in vivo studies.

Heterogeneity in neutralization sensitivities of viruses comprising the simian immunodeficiency virus SIVsmE660 isolate and vaccine challenge stock

The sooty mangabey-derived simian immunodeficiency virus (SIV) strain E660 (SIVsmE660) is a genetically heterogeneous, pathogenic isolate that is commonly used as a vaccine challenge strain in the nonhuman primate (NHP) model of human immunodeficiency virus type 1 (HIV-1) infection. Though it is often employed to assess antibody-based vaccine strategies, its sensitivity to antibody-mediated neutralization has not been well characterized. Here, we utilize single-genome sequencing and infectivity assays to analyze the neutralization sensitivity of the uncloned SIVsmE660 isolate, individual viruses comprising the isolate, and transmitted/founder (T/F) viruses arising from low-dose mucosal inoculation of macaques with the isolate. We found that the SIVsmE660 isolate overall was highly sensitive to neutralization by SIV-infected macaque plasma samples (50% inhibitory concentration [IC50] < 10(-5)) and monoclonal antibodies targeting V3 (IC50 < 0.01 μg/ml), CD4-induced (IC50 < 0.1 μg/ml), CD4 binding site (IC50 ~ 1 μg/ml), and V4 (IC50, ~5 μg/ml) epitopes. In comparison, SIVmac251 and SIVmac239 were highly resistant to neutralization by these same antibodies. Differences in neutralization sensitivity between SIVsmE660 and SIVmac251/239 were not dependent on the cell type in which virus was produced or tested. These findings indicate that in comparison to SIVmac251/239 and primary HIV-1 viruses, SIVsmE660 generally exhibits substantially less masking of antigenically conserved Env epitopes. Interestingly, we identified a minor population of viruses (~10%) in both the SIVsmE660 isolate and T/F viruses arising from it that were substantially more resistant (>1,000-fold) to antibody neutralization and another fraction (~20%) that was intermediate in neutralization resistance. These findings may explain the variable natural history and variable protection afforded by heterologous Env-based vaccines in rhesus macaques challenged by high-dose versus low-dose SIVsmE660 inoculation regimens.

Recent developments in human immunodeficiency virus-1 latency research

Almost 30 years after its initial discovery, infection with the human immunodeficiency virus-1 (HIV-1) remains incurable and the virus persists due to reservoirs of latently infected CD4(+) memory T-cells and sanctuary sites within the infected individual where drug penetration is poor. Reactivating latent viruses has been a key strategy to completely eliminate the virus from the host, but many difficulties and unanswered questions remain. In this review, the latest developments in HIV-persistence and latency research are presented.

Lessons in nonhuman primate models for AIDS vaccine research: from minefields to milestones

Nonhuman primate (NHP) disease models for AIDS have made important contributions to the search for effective vaccines for AIDS. Viral diversity, persistence, capacity for immune evasion, and safety considerations have limited development of conventional approaches using killed or attenuated vaccines, necessitating the development of novel approaches. Here we highlight the knowledge gained and lessons learned in testing vaccine concepts in different virus/NHP host combinations.

The non-human primate model of tuberculosis

Non-human primates (NHPs) are used to model human disease owing to their remarkably similar genomes, physiology, and immune systems. Recently, there has been an increased interest in modeling tuberculosis (TB) in NHPs. Macaques are susceptible to infection with different strains of Mycobacterium tuberculosis (Mtb), producing the full spectrum of disease conditions, including latent infection, chronic progressive infection, and acute TB, depending on the route and dose of infection. Clearly, NHPs are an excellent model of human TB. While the initial aim of the NHP model was to allow preclinical testing of candidate vaccines and drugs, it is now also being used to study pathogenesis and immune correlates of protection. Recent advances in this field are discussed in this review. Key questions such as the effect of hypoxia on the biology of Mtb and the basis of reactivation of latent TB can now be investigated through the use of this model.

Differential innate immune responses to low or high dose oral SIV challenge in Rhesus macaques

Mucosal transmission of HIV predominately occurs during sexual intercourse or breast-feeding and generally results in a successful infection from just one or few founder virions. Here we assessed the impact of viral inoculum size on both viral and immune events within two groups of Rhesus macaques that were non-traumatically, orally inoculated with either multiple low (1000 to 4000 TCID(50)) or high (100,000 TCID(50)) doses of SIV. In agreement with previous studies, more diverse SIV variants were observed in macaques following infection with high dose oral SIV compared to a low dose challenge. In peripheral blood cells, the immune gene transcript levels of CXCL9, IFNγ, TNFα and IL10 remained similar to uninfected macaques. In contrast, OAS and CXCL10 were upregulated following SIV infection in both the high and low dosed macaques, with a more rapid kinetics (detectable by 7 days) following the high SIV dose challenge. In peripheral lymph nodes, an increase in CXCL10 was observed irrespective of viral dose while CXCL9 and OAS were differentially regulated in the two SIV dosed groups. Magnetic bead sorting of CD3+, CD14+ and CD3- /CD14- cells from peripheral blood identified the increase in OAS expression primarily within CD14+ monocytes, whereas the CXCL10 expression was primarily in CD3+ T cells. These findings provide insights into the impact of SIV challenge dose on viral and innate immune factors, which has the potential to inform future SIV/HIV vaccine efficacy trials in which vaccinated hosts have the potential to be infected with a range of viral challenge doses.

Three-dimensional structures of soluble CD4-bound states of trimeric simian immunodeficiency virus envelope glycoproteins determined by using cryo-electron tomography

The trimeric envelope glycoprotein (Env) spikes displayed on the surfaces of simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1) virions are composed of three heterodimers of the viral glycoproteins gp120 and gp41. Although binding of gp120 to cell surface CD4 and a chemokine receptor is known to elicit conformational changes in gp120 and gp41, changes in quaternary structure of the trimer have only recently been elucidated. For the HIV-1 BaL isolate, CD4 attachment results in a striking rearrangement of the trimer from a "closed" to an "open" conformation. The effect of CD4 on SIV trimers, however, has not been described. Using cryo-electron tomography, we have now determined molecular architectures of the soluble CD4 (sCD4)-bound states of SIV Env trimers for three different strains (SIVmneE11S, SIVmac239, and SIV CP-MAC). In marked contrast to HIV-1 BaL, SIVmneE11S and SIVmac239 Env showed only minor conformational changes following sCD4 binding. In SIV CP-MAC, where trimeric Env displays a constitutively "open" conformation similar to that seen for HIV-1 BaL Env in the sCD4-complexed state, we show that there are no significant further changes in conformation upon the binding of either sCD4 or 7D3 antibody. The density maps also show that 7D3 and 17b antibodies target epitopes on gp120 that are on opposites sides of the coreceptor binding site. These results provide new insights into the structural diversity of SIV Env and show that there are strain-dependent variations in the orientation of sCD4 bound to trimeric SIV Env.

Dissecting the role of dendritic cells in simian immunodeficiency virus infection and AIDS

Human immunodeficiency virus (HIV) infection is associated with the loss of the two principal types of dendritic cell (DC), myeloid DC (mDC) and plasmacytoid DC (pDC), but the mechanism of this loss and its relationship to AIDS pathogenesis remain ill-defined. The nonhuman primate is a powerful model to dissect this response for several reasons. Both DC subsets have been well characterized in nonhuman primates and shown to have strikingly similar phenotypic and functional characteristics to their counterparts in the human. Moreover, decline of mDC and pDC occurs in rhesus macaques with end-stage simian immunodeficiency virus (SIV) infection, the model of HIV infection in humans. In this brief review, we discuss what is known about DC subsets in pathogenic and nonpathogenic nonhuman primate models of HIV infection and highlight the advances and controversies that currently exist in the field.

New paradigms for HIV/AIDS vaccine development

HIV-1 and its simian counterpart SIV have been exquisitely tailored by evolution to evade host immunity. By virtue of specific adaptations that thwart individual innate or adaptive immune mechanisms, and an overall replication strategy that provides for rapid establishment of a large, systemic viral population, capable of dynamic adaptation to almost all immune selection pressures, these viruses, once established, almost invariably stay one step ahead of the host's immune system, and in the vast majority of infected individuals, replicate indefinitely. Although many vaccine approaches tested to date have been able to enhance the magnitude of the immune responses to HIV/SIV infection, most of these responses, whether cellular or humoral, have largely failed to be both effectively antiviral and targeted to prevent the emergence of fully functional escape variants. Recent advances, however, have provided strong evidence that the initial stages of infection following mucosal transmission of these viruses are more vulnerable to immune intervention, and have led to the development of vaccine strategies that elicit responses able to effectively intervene in these early stages of infection, either preventing acquisition of infection or establishing early, stringent, and durable control. Here, we place HIV/AIDS vaccine development in the context of the basic immunobiology of HIV and SIV, review the evidence for their vulnerability to immune responses immediately after mucosal transmission, and discuss how this newly recognized vulnerability might be exploited for the development of an effective HIV/AIDS vaccine.

Isolation of potent neutralizing monoclonal antibodies from an SIV-Infected rhesus macaque by phage display

The humoral immune response is a mechanism that potently suppresses or prevents viral infections. However, genetic diversity and resistance to antibody-mediated neutralization are serious obstacles in controlling HIV-1 infection. In this study, we isolated monoclonal antibodies from an SIV-infected macaque by using the phage display method to characterize antibodies in SIV infection. Variable regions of immunoglobulin genes were amplified by rhesus macaque-specific primers and inserted into the phagemid pComb3X, which produced the Fab fragment. Antibodies against SIV proteins were selected by biopanning using an SIV protein-coated 96-well plate. A total of 20 Fab clones obtained included 14 clones directed to gp41, four clones to gp120, and two clones to p27. The anti-gp120 Fab clones completely neutralized the homologous neutralization-sensitive SIVsmH635FC and the genetically divergent SIVmac316, and showed at least 50% inhibition against the neutralization-resistant strain, SIVsmE543-3. Competition ELISA revealed that these anti-gp120 Fab clones recognize the same epitope on gp120 including the V3 loop. Identification of antibodies with potent neutralizing activity will help to elucidate the mechanisms for inducing broadly neutralizing antibodies.

Coreceptor use in nonhuman primate models of HIV infection

SIV or SHIV infection of nonhuman primates (NHP) has been used to investigate the impact of coreceptor usage on the composition and dynamics of the CD4+ T cell compartment, mechanisms of disease induction and development of clinical syndrome. As the entire course of infection can be followed, with frequent access to tissue compartments, infection of rhesus macaques with CCR5-tropic SHIVs further allows for study of HIV-1 coreceptor switch after intravenous and mucosal inoculation, with longitudinal and systemic analysis to determine the timing, anatomical sites and cause for the change in envelope glycoprotein and coreceptor preference. Here, we review our current understanding of coreceptor use in NHPs and their impact on the pathobiological characteristics of the infection, and discuss recent advances in NHP studies to uncover the underlying selective pressures for the change in coreceptor preference in vivo.

Overview of known non-human primate pathogens with potential to affect colonies used for toxicity testing

The increased demand for non-human primates (NHPs) in biomedical research has resulted in alternative sources of animals being used, which has allowed for importation of animals with varying background incidences of bacterial, viral, parasitic, and fungal pathogens. This can be of minimal consequence when animals from different sources are kept isolated. However, when NHPs from different sources with varying incidences of primary and opportunistic pathogens are mixed, there can be a rapid spread of these pathogens and an increase in the seroconversion of susceptible animals. If this process occurs during the conduct of a study, interpretation of that study can be confounded. Furthermore, NHPs imported from areas enzootic for pathogens such as Plasmodium or with high incidences of human diseases such as measles and tuberculosis can introduce diseases that can be a threat to colony health, have zoonotic risk, and can severely impact study outcome. Thus, knowledge of the common primary and opportunistic NHP infections, as well as reemerging pathogens, enables the toxicologist to use information on disease status for pre-study animal selection and intelligent study design. This is particularly important when immunomodulatory compounds are being investigated. Moreover, the toxicologic pathologist well versed in the common spontaneous infections, opportunistic pathogens, and background lesions in NHPs is able to assess possible drug-related effects in drug safety studies. This review identifies the common primary and opportunistic pathogens, as well as newly emerging infections of NHPs, that can directly or indirectly affect colony health and the interpretation of drug safety studies.

Viral sanctuaries during highly active antiretroviral therapy in a nonhuman primate model for AIDS

Highly active antiretroviral therapy (HAART) enables long-term suppression of plasma HIV-1 loads in infected persons, but low-level virus persists and rebounds following cessation of therapy. During HAART, this virus resides in latently infected cells, such as resting CD4(+) T cells, and in other cell types that may support residual virus replication. Therapeutic eradication will require elimination of virus from all reservoirs. We report here a comprehensive analysis of these reservoirs in fluids, cells, and tissues in a rhesus macaque model that mimics HAART in HIV-infected humans. This nonhuman primate model uses RT-SHIV, a chimera of simian immunodeficiency virus containing the HIV-1 reverse transcriptase (RT). Methods were developed for extraction, preamplification, and real-time PCR analyses of viral DNA (vDNA) and viral RNA (vRNA) in tissues from RT-SHIV-infected macaques. These methods were used to identify viral reservoirs in RT-SHIV-infected macaques treated with a potent HAART regimen consisting of efavirenz, emtricitabine, and tenofovir. Plasma virus loads at necropsy ranged from 11 to 28 copies of vRNA per ml. Viral RNA and DNA were detected during HAART, in tissues from numerous anatomical locations. Additional analysis provided evidence for full-length viral RNA in tissues of animals with virus suppressed by HAART. The highest levels of vDNA and vRNA in HAART-treated macaques were in lymphoid tissues, particularly the spleen, lymph nodes, and gastrointestinal tract tissues. This study is the first comprehensive analysis of the tissue and organ distribution of a primate AIDS virus during HAART. These data demonstrate widespread persistence of residual virus in tissues during HAART.

Pathogenic mechanisms in simian immunodeficiency virus infection

PURPOSE OF REVIEW

Recent work in pathogenic simian immunodeficiency (SIV) infection of Asian macaques and in natural, nonpathogenic SIV infections of African nonhuman primate species has demonstrated that persistent activation has profound effects on CD4+ memory T-cell proliferation, differentiation and survival. Disease progression in pathogenic infection has been closely linked to these dynamics, reflecting a complex interplay of virus-mediated killing, the effects of systemic activation and host regenerative mechanisms. We review these recent advances.

RECENT FINDINGS

Massive depletion of CD4+ effector-memory T cells invariably occurs during acute CCR5-tropic SIV infection, but is initially stabilized by new production of these cells from spared central memory precursors above the threshold required to maintain clinical immune competence. In pathogenic (but not natural, apathogenic) infections, a persistent state of immune activation, characterized by multiple, recurrent bursts of lymphocyte proliferation, differentiation, migration, death and functional modification of 'resting' cells, is associated with progressive depletion of central memory CD4+ T cells, and ultimately, a collapse of effector site CD4+ memory populations that is closely associated with overt immune deficiency.

SUMMARY

The importance of maintaining the regenerative capacity of the central-memory compartment of CD4+ T cells is increasingly evident. Defining the physiologic and molecular mechanisms responsible for instability of the CD4+ central-memory T cell pool could enable new immunotherapeutic interventions.

Improved survival in rhesus macaques immunized with modified vaccinia virus Ankara recombinants expressing simian immunodeficiency virus envelope correlates with reduction in memory CD4+ T-cell loss and higher titers of neutralizing antibody

Previous studies demonstrated that immunization of macaques with simian immunodeficiency virus (SIV) Gag-Pol and Env recombinants of the attenuated poxvirus modified vaccinia virus Ankara (MVA) provided protection from high viremia and AIDS following challenge with a pathogenic strain of SIV. Although all animals became infected, plasma viremia was significantly reduced in animals that received the MVA-SIV recombinant vaccines compared with animals that received nonrecombinant MVA. Most importantly, the reduction in viremia resulted in a significant increase in median and cumulative survival. Continued analysis of these animals over the subsequent 9 years has shown that they maintain a survival advantage, although all but two of the macaques have progressed to AIDS. Importantly, improved survival correlated with preservation of memory CD4(+) T cells in the peripheral blood. The greatest survival advantage was observed in macaques immunized with regimens containing SIV Env, and the titer of neutralizing antibodies to the challenge virus prior to or shortly following challenge correlated with preservation of CD4(+) T cells. These data are consistent with a role for neutralizing antibodies in nonsterilizing protection from high viremia and associated memory CD4(+) T-cell loss.

Antiviral therapy during primary simian immunodeficiency virus infection fails to prevent acute loss of CD4+ T cells in gut mucosa but enhances their rapid restoration through central memory T cells

Gut-associated lymphoid tissue (GALT) is an early target of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) and a site for severe CD4+ T-cell depletion. Although antiretroviral therapy (ART) is effective in suppressing HIV replication and restoring CD4+ T cells in peripheral blood, restoration in GALT is delayed. The role of restored CD4+ T-cell help in GALT during ART and its impact on antiviral CD8+ T-cell responses have not been investigated. Using the SIV model, we investigated gut CD4+ T-cell restoration in infected macaques, initiating ART during either the primary stage (1 week postinfection), prior to acute CD4+ cell loss (PSI), or during the chronic stage at 10 weeks postinfection (CSI). ART led to viral suppression in GALT and peripheral blood mononuclear cells of PSI and CSI animals at comparable levels. CSI animals had incomplete CD4+ T-cell restoration in GALT. In PSI animals, ART did not prevent acute CD4+ T-cell loss by 2 weeks postinfection in GALT but supported rapid and complete CD4+ T-cell restoration thereafter. This correlated with an accumulation of central memory CD4+ T cells and better suppression of inflammation. Restoration of CD4+ T cells in GALT correlated with qualitative changes in SIV gag-specific CD8+ T-cell responses, with a dominance of interleukin-2-producing responses in PSI animals, while both CSI macaques and untreated SIV-infected controls were dominated by gamma interferon responses. Thus, central memory CD4+ T-cell levels and qualitative antiviral CD8+ T-cell responses, independent of viral suppression, were the immune correlates of gut mucosal immune restoration during ART.

Progressive CD4+ central memory T cell decline results in CD4+ effector memory insufficiency and overt disease in chronic SIV infection

Primary simian immunodeficiency virus (SIV) infections of rhesus macaques result in the dramatic depletion of CD4⁺ CCR5⁺ effector–memory T (T_EM) cells from extra-lymphoid effector sites, but in most infections, an increased rate of CD4⁺ memory T cell proliferation appears to prevent collapse of effector site CD4⁺ T_EM cell populations and acute-phase AIDS. Eventually, persistent SIV replication results in chronic-phase AIDS, but the responsible mechanisms remain controversial. Here, we demonstrate that in the chronic phase of progressive SIV infection, effector site CD4⁺ T_EM cell populations manifest a slow, continuous decline, and that the degree of this depletion remains a highly significant correlate of late-onset AIDS. We further show that due to persistent immune activation, effector site CD4⁺ T_EM cells are predominantly short-lived, and that their homeostasis is strikingly dependent on the production of new CD4⁺ T_EM cells from central–memory T (T_CM) cell precursors. The instability of effector site CD4⁺ T_EM cell populations over time was not explained by increasing destruction of these cells, but rather was attributable to progressive reduction in their production, secondary to decreasing numbers of CCR5⁻ CD4⁺ T_CM cells. These data suggest that although CD4⁺ T_EM cell depletion is a proximate mechanism of immunodeficiency, the tempo of this depletion and the timing of disease onset are largely determined by destruction, failing production, and gradual decline of CD4⁺ T_CM cells.

Molecular typing of major histocompatibility complex class I alleles in the Indian rhesus macaque which restrict SIV CD8+ T cell epitopes

The utility of the rhesus macaque as an animal model in both HIV vaccine development and pathogenesis studies necessitates the development of accurate and efficient major histocompatibility complex (MHC) genotyping technologies. In this paper, we describe the development and application of allele-specific polymerase chain reaction (PCR) amplification for the simultaneous detection of eight MHC class I alleles from the rhesus macaque (Macaca mulatta) of Indian descent. These alleles were selected, as they have been implicated in the restriction of CD8(+) T cell epitopes of simian immunodeficiency virus (SIV). Molecular typing of Mamu-A 01, Mamu-A 02, Mamu-A 08, Mamu-A 11, Mamu-B 01, Mamu-B 03, Mamu-B 04, and Mamu-B 17 was conducted in a high throughput fashion using genomic DNA. Our amplification strategy included a conserved internal control target to minimize false negative results and can be completed in less than 5 h. We have genotyped over 4,000 animals to establish allele frequencies from colonies all over the western hemisphere. The ability to identify MHC-defined rhesus macaques will greatly enhance investigation of the immune responses, which are responsible for the control of viral replication. Furthermore, application of this technically simple and accurate typing method should facilitate selection, utilization, and breeding of rhesus macaques for AIDS virus pathogenesis and vaccine studies.

IgG Fc receptor III homologues in nonhuman primate species: genetic characterization and ligand interactions

Ig Fc receptors bind to immune complexes through interactions with the Fc regions of specific Ab subclasses to initiate or inhibit the defense mechanisms of the leukocytes on which they are expressed. The mechanism of action of IgG-based therapeutic molecules, which are routinely evaluated in nonhuman primate models, involves binding to the low-affinity FcRIII (CD16). The premise that IgG/CD16 interactions in nonhuman primates mimic those present in humans has not been evaluated. Therefore, we have identified and characterized CD16 and associated TCR zeta-chain homologues in rhesus macaques, cynomolgus macaques, baboons, and sooty mangabeys. Similar to humans, CD16 expression was detected on a lymphocyte subpopulation, on monocytes, and on neutrophils of sooty mangabeys. However, CD16 was detected only on a lymphocyte subpopulation and on monocytes in macaques and baboons. A nonhuman primate rCD16 generated in HeLa cells interacted with human IgG1 and IgG2. By contrast, human CD16 binds to IgG1 and IgG3. As shown for humans, the mAb 3G8 was able to block IgG binding to nonhuman primate CD16 and inhibition of nonhuman primate CD16 N-glycosylation enhanced IgG binding. Clearly, differences in interaction with IgG subclasses and in cell-type expression should be considered when using these models for in vivo evaluation of therapeutic Abs.

No vaccine against HIV yet--are we not perfectly equipped?

Enormous effort has been devoted to the development of a vaccine against human immunodeficiency virus (HIV). But it is proving to be an unprecedented challenge to create an effective vaccine mainly due to the high genetic variability of the virus and the necessity of cytotoxic T lymphocytes (CTL) for containing the infection. Currently pursued vaccine strategies appear to induce CTL in nonhuman primate models but in the early clinical trials, these strategies fail to fully control the viral infection. New strategies that can cover the vast genetic diversity of HIV are needed for the development of a potent vaccine.

Aerosol measles vaccination in macaques: preclinical studies of immune responses and safety

The comparative efficacy and safety of measles vaccination via the aerosol route versus subcutaneous injection has not been fully resolved. We vaccinated cynomolgus monkeys (Macaca fascicularis) with the live-attenuated Edmonston-Zagreb measles virus (MV) vaccine and compared different routes of administration in the immunocompetent and the immunocompromised host. Immunogenicity and protective efficacy of aerosol vaccination using devices similar to those previously used in humans were comparable to those in animals vaccinated by injection. No evidence for a safety hazard associated with the route of vaccination was detected. The results of this study support further clinical evaluation of aerosol vaccination for measles.

Current status of gene therapy strategies to treat HIV/AIDS

Progress in developing effective gene transfer approaches to treat HIV-1 infection has been steady. Many different transgenes have been reported to inhibit HIV-1 in vitro. However, effective translation of such results to clinical practice, or even to animal models of AIDS, has been challenging. Among the reasons for this failure are uncertainty as to the most effective cell population(s) to target, the diffuseness of these target cells in the body, and ineffective or insufficiently durable gene delivery. Better understanding of the HIV-1 replicative cycle, host factors involved in HIV-1 infection, vector biology and application, transgene technology, animal models, and clinical study design have all contributed vastly to planning current and future strategies for application of gene therapeutic approaches to the treatment of AIDS. This review focuses on the newest developments in these areas and provides a strong basis for renewed optimism that gene therapy will have an important role to play in treating people infected with HIV-1.

Simian immunodeficiency virus integration preference is similar to that of human immunodeficiency virus type 1

Simian immunodeficiency virus (SIV) is a useful model for studying human immunodeficiency virus (HIV) pathogenesis and vaccine efficacy. As with all other retroviruses, integration is a necessary step in the replication cycle of SIV. The location of the retrovirus integration site is known to impact on viral gene expression, establishment of viral latency, and other aspects of the replication cycle of a retrovirus. In this study, 148 SIV provirus integration sites were sequenced and mapped in the human genome. Our analysis showed that SIV integration, like that of HIV type 1 (HIV-1), exhibited a strong preference for actively transcribed regions in the genome (A. R. Schroder et al., Cell 110:521-529, 2002) and no preference for the CpG islands or transcription start sites, in contrast to observations for murine leukemia virus (X. Wu et al., Science 300:1749-1751, 2003). The parallel integration target site preferences of SIV and HIV-1 suggest that these lentiviruses may share similar mechanisms for target site selection and that SIV serves as an accurate model of HIV-1 with respect to integration.

Non-human primate models for AIDS vaccine research

Since the discovery of simian immunodeficiency viruses (SIV) causing AIDS-like diseases in Asian macaques, non-human primates (NHP) have played an important role in AIDS vaccine research. A multitude of vaccines and immunization approaches have been evaluated, including live attenuated viruses, DNA vaccines, viral and bacterial vectors, subunit proteins, and combinations thereof. Depending on the particular vaccine and model used, varying degrees of protection have been achieved, including prevention of infection, reduction of viral load, and amelioration of disease. In a few instances, potential safety concerns and vaccine-enhanced pathogenicity have also been noted. In the past decade, sophisticated methodologies have been developed to define the mechanisms of protective immunity. However, a clear road map for HIV vaccine development has yet to emerge. This is in part because of the intrinsic nature of the surrogate model and in part because of the improbability of any single model to fully capture the complex interactions of natural HIV infection in humans. The lack of standardization, the limited models available, and the incomplete understanding of the immunobiology of NHP contribute to the difficulty to extrapolate findings from such models to HIV vaccine development. Until efficacy data become available from studies of parallel vaccine concepts in humans and macaques, the predictive value of any NHP model remains unknown. Towards this end, greater appreciation of the utility and limitations of the NHP model and further developments to better mimic HIV infection in humans will likely help inform future AIDS vaccine efforts.

Impact of bone marrow hematopoiesis failure on T-cell generation during pathogenic simian immunodeficiency virus infection in macaques

Experimental infection of macaques with pathogenic strains of simian immunodeficiency virus (SIV) represents one of the most relevant animal models for studying HIV pathogenesis. In this study, we demonstrated a significant decrease in the generation of CD4+ T cells from bone marrow (BM) CD34+ progenitors in macaques infected with SIVmac251. This decrease appears to result from changes in the clonogenic potential of BM progenitors of both the myeloid and lymphoid lineages. We also demonstrated a significant decrease in the numbers of the most immature long-term culture-initiating cells (LTC-ICs). Hematopoietic failure occurred as early as primary infection, in the absence of CD34+ BM cell infection and was not related to plasma viral load. No major change was observed in the phenotype of BM CD34+ cells from infected macaques, including apoptosis markers such as annexin V staining and BcL-2 expression, but a significantly higher that normal proportion of cells were in the G0/G1 phase. This is the first demonstration that failure of BM hematopoiesis results in impaired T-cell production, which may contribute to the disruption of T-lymphocyte homeostasis characteristic of pathogenic lentiviral infections in primates.

Insufficient production and tissue delivery of CD4+ memory T cells in rapidly progressive simian immunodeficiency virus infection

The mechanisms linking human immunodeficiency virus replication to the progressive immunodeficiency of acquired immune deficiency syndrome are controversial, particularly the relative contribution of CD4⁺ T cell destruction. Here, we used the simian immunodeficiency virus (SIV) model to investigate the relationship between systemic CD4⁺ T cell dynamics and rapid disease progression. Of 18 rhesus macaques (RMs) infected with CCR5-tropic SIVmac239 (n = 14) or CXCR4-tropic SIVmac155T3 (n = 4), 4 of the former group manifested end-stage SIV disease by 200 d after infection. In SIVmac155T3 infections, naive CD4⁺ T cells were dramatically depleted, but this population was spared by SIVmac239, even in rapid progressors. In contrast, all SIVmac239-infected RMs demonstrated substantial systemic depletion of CD4⁺ memory T cells by day 28 after infection. Surprisingly, the extent of CD4⁺ memory T cell depletion was not, by itself, a strong predictor of rapid progression. However, in all RMs destined for stable infection, this depletion was countered by a striking increase in production of short-lived CD4⁺ memory T cells, many of which rapidly migrated to tissue. In all rapid progressors (P < 0.0001), production of these cells initiated but failed by day 42 of infection, and tissue delivery of new CD4⁺ memory T cells ceased. Thus, although profound depletion of tissue CD4⁺ memory T cells appeared to be a prerequisite for early pathogenesis, it was the inability to respond to this depletion with sustained production of tissue-homing CD4⁺ memory T cells that best distinguished rapid progressors, suggesting that mechanisms of the CD4⁺ memory T cell generation play a crucial role in maintaining immune homeostasis in stable SIV infection.