Animal models of AIDS

Animal models of AIDS are essential for understanding the pathogenesis of retrovirus-induced immune deficiency and encephalopathy and for development and testing of new therapies and vaccines. AIDS and related disorders are etiologically linked to members of the lentivirus subfamily of retroviruses; these lymphocytopathic lentiviruses are designated human immuno-deficiency virus type 1 (HIV-1) and human immuno-deficiency virus type 2 (HIV-2). The only animals susceptible to experimental HIV-1 infection are the chimpanzee, gibbon ape, and rabbit but AIDS-like disease has not yet been reported in these species. Macaques can be persistently infected with some strains of HIV-2 but no AIDS-like disease has resulted. It is not yet clear how suitable HIV-infected SCID-hu mice will be as a model for AIDS. Several subfamilies of naturally occurring cytopathic retroviruses cause immune suppression, including fatal immunodeficiency syndromes in chickens, mice, cats, and monkeys. Domestic cats suffer immunosuppression from both an onco-virus, feline leukemia virus, and a member of the lentivirus subfamily, feline immunodeficiency virus (FIV). Asian macaques are susceptible to fatal simian AIDS from a type D retrovirus, indigenous in macaques, and from a lentivirus, simian immunodeficiency virus (SIV), which is indigenous to healthy African monkeys. SIV is the animal lentivirus most closely related to HIV. Of these animal models, the lentivirus infections of cats (FIV) and macaques (SIV) appear to bear the closest similarity in their pathogenesis to HIV infection and AIDS. This review will summarize these various animal model systems for AIDS and illustrate their usefulness for antiviral therapy and vaccinology.     

Apoptosis in SIV infection

Pathogenic human immunodeficiency virus (HIV)/Simian immunodeficiency virus (SIV) infection is associated with increased T-cell apoptosis. In marked contrast to HIV infection in humans and SIV infection in macaques, the SIV infection of natural host species is typically nonpathogenic despite high levels of viral replication. In these nonpathogenic primate models, no observation of T-cell apoptosis was observed, suggesting that either SIV is less capable of directly inducing apoptosis in natural hosts (likely as a result of coevolution/coadaptation with the host) or, alternatively, that the indirect T-cell apoptosis plays the key role in determining the HIV-associated T-cell depletion and progression to acquired immune deficiency syndrome (AIDS). Understanding the molecular and cellular mechanisms responsible for the disease-free equilibrium in natural hosts for SIV infection, including those determining the absence of high levels of T-cell apoptosis, is likely to provide important clues regarding the mechanisms of AIDS pathogenesis in humans.     

HIV/AIDS: in search of an animal model

AIDS is among the most devastating diseases of our time, claiming the lives of approximately 3 million people per year. The primary cause of AIDS, human immunodeficiency virus type 1 (HIV-1), is a pathogen that is highly specific for humans and generally does not infect or cause disease in other species. This property complicates the generation of animal models that are urgently needed to test new antiretroviral therapies and vaccines. The most practical animal models developed to date consist of infection of rhesus macaques with a simian immunodeficiency virus (SIV) or chimeric HIV/SIV viruses. Although these models are useful for particular applications, the fact that SIV is a distinct virus compared with HIV-1 represents a significant limitation to their use. Here, we discuss the uses and limitations of existing models and recent advances that might lead to better animal models for HIV/AIDS.     

The role of the chemokine receptor CXCR4 in infection with feline immunodeficiency virus.

Infection with feline immunodeficiency virus (FIV) leads to the development of a disease state similar to AIDS in man. Recent studies have identified the chemokine receptor CXCR4 as the major receptor for cell culture-adapted strains of FIV, suggesting that FIV and human immunodeficiency virus (HIV) share a common mechanism of infection involving an interaction between the virus and a member of the seven transmembrane domain superfamily of molecules. This article reviews the evidence for the involvement of chemokine receptors in FIV infection and contrasts these findings with similar studies on the primate lentiviruses HIV and SIV (simian immunodeficiency virus).     

Dynamics of simian immunodeficiency virus SIVmac239 infection in pigtail macaques

Pigtail macaques (PTM) are an excellent model for HIV research; however, the dynamics of simian immunodeficiency virus (SIV) SIVmac239 infection in PTM have not been fully evaluated. We studied nine PTM prior to infection, during acute and chronic SIVmac239 infections, until progression to AIDS. We found PTM manifest clinical AIDS more rapidly than rhesus macaques (RM), as AIDS-defining events occurred at an average of 42.17 weeks after infection in PTM compared to 69.56 weeks in RM (P = 0.0018). However, increased SIV progression was not associated with increased viremia, as both peak and set-point plasma viremias were similar between PTM and RM (P = 0.7953 and P = 0.1006, respectively). Moreover, this increased disease progression was not associated with rapid CD4(+) T cell depletion, as CD4(+) T cell decline resembled other SIV/human immunodeficiency virus (HIV) models. Since immune activation is the best predictor of disease progression during HIV infection, we analyzed immune activation by turnover of T cells by BrdU decay and Ki67 expression. We found increased levels of turnover prior to SIV infection of PTM compared to that observed with RM, which may contribute to their increased disease progression rate. These data evaluate the kinetics of SIVmac239-induced disease progression and highlight PTM as a model for HIV infection and the importance of immune activation in SIV disease progression.     

Simian immunodeficiency virus rapidly penetrates the cervicovaginal mucosa after intravaginal inoculation and infects intraepithelial dendritic cells

Despite recent insights into mucosal human immunodeficiency virus (HIV) transmission, the route used by primate lentiviruses to traverse the stratified squamous epithelium of mucosal surfaces remains undefined. To determine if dendritic cells (DC) are used by primate lentiviruses to traverse the epithelial barrier of the genital tract, rhesus macaques were intravaginally exposed to cell-free simian immunodeficiency virus SIVmac251. We examined formalin-fixed tissues and HLA-DR(+)-enriched cell suspensions to identify the cells containing SIV RNA in the genital tract and draining lymph nodes within the first 24 h of infection. Using SIV-specific fluorescent in situ hybridization combined with immunofluorescent antibody labeling of lineage-specific cell markers, numerous SIV RNA(+) DC were documented in cell suspensions from the vaginal epithelium 18 h after vaginal inoculation. In addition, we determined the minimum time that the SIV inoculum must remain in contact with the genital mucosa for the virus to move from the vaginal lumen into the mucosa. We now show that SIV enters the vaginal mucosa within 60 min of intravaginal exposure, infecting primarily intraepithelial DC and that SIV-infected cells are located in draining lymph nodes within 18 h of intravaginal SIV exposure. The speed with which primate lentiviruses penetrate mucosal surfaces, infect DC, and disseminate to draining lymph nodes poses a serious challenge to HIV vaccine development.     

Primate models for human immunodeficiency virus infection. Evolution of receptor use during pathogenesis

Animal models greatly facilitate understanding of transmission, pathogenesis and immune responses in HIV and SIV infection and provide models for studies on the effect of candidate drugs or vaccines. However, there are several aspects that one should consider when drawing conclusions from results obtained from animal models. First, the genetic relationship of primate lentiviruses cannot be disregarded because it is known that HIV-1 is more closely related to SIV of chimpanzee origin (SIVcpz) than to SIV from sooty mangabey (SIVsm) origin. Nevertheless, SIVsm and SIVmac are the ones most often used as model systems. Second, there are differences in the biological properties, like CXCR4 use and CD4-independent coreceptor use, of HIV and SIV. These differences might be relevant in virus transmission, pathogenesis and in evoking immune responses. Third, in vivo and in vitro selection may influence the results. Neutralizing antibodies may play a role in selection of variant viruses since neutralization sensitive, CD4-independent SIVsm variants seemed to be suppressed in animals that mounted a neutralizing antibody response. It is tempting to speculate that neutralizing antibodies shape the SIV/HIV infection by selecting variants with a more "closed" envelope conformation with consequences for both receptor binding and neutralization sensitivity. The SIV/monkey model, although it has important advantages, may not answer all questions asked about HIV-1 infection in human.     

Characterization and role of lentivirus-associated host proteins

Enveloped viruses obtain their envelopes during the process of budding from infected cells. During this process, however, these viruses acquire parts of the host cell membranes and host cell-derived proteins as integral parts of their mature envelopes. These host-derived components of viral envelopes may subsequently exhibit various effects on the life cycle of the virus; virus cell interactions, especially host response to virus-incorporated self-proteins; and the pathogenesis of the disease induced by these viruses. Although it was known for some time that various viruses incorporate host cell-derived proteins, the issue of the role of these proteins has received increased attention, specifically in connection with human immunodeficiency virus (HIV) infection and development of acquired immunodeficiency syndrome (AIDS) in humans. The aim of this review is to summarize our current knowledge of the analysis and role of host-derived proteins associated with enveloped viruses, with emphasis on the potential role of these proteins in the pathogenesis of AIDS. Clearly, differences in the clinical outcome of those nonhuman primates infected with simian immunodeficiency virus (SIV) that are disease resistant compared with SIV-infected species that are disease susceptible provide a unique opportunity to determine whether differences in the incorporation of distinct sets of host proteins play a role with distinct clinical outcomes.     

Macrophage-derived simian immunodeficiency virus exhibits enhanced infectivity by comparison with T-cell-derived virus

Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infect and productively replicate in macrophages and T lymphocytes. Here, we show that SIV virions derived from macrophages have higher levels of infectivity than those derived from T cells. The lower infectivity of T-cell-derived viruses is influenced by the quantity or type of mannose residues on the virion. Our results demonstrate that the cellular origin of a virus is a major factor in viral infectivity. Cell-type-specific factors in viral infectivity, and organ-specific or disease stage-specific differences in cellular derivation of virions, can be critical in the pathogenesis of HIV and AIDS.     

Immunodeficiency in the absence of high viral load in pig-tailed macaques infected with Simian immunodeficiency virus SIVsun or SIVlhoest

Simian immunodeficiency virus (SIV) is known to result in an asymptomatic infection of its natural African monkey host. However, some SIV strains are capable of inducing AIDS-like symptoms and death upon experimental infection of Asian macaques. To further investigate the virulence of natural SIV isolates from African monkeys, pig-tailed (PT) macaques were inoculated intravenously with either of two recently discovered novel lentiviruses, SIVlhoest and SIVsun. Both viruses were apparently apathogenic in their natural hosts but caused immunodeficiency in PT macaques. Infection was characterized by a progressive loss of CD4(+) lymphocytes in the peripheral blood and lymph nodes, generalized lymphoid depletion, a wasting syndrome, and opportunistic infections, such as Mycobacterium avium or Pneumocystis carinii infections. However, unlike SIVsm/mac infection of macaques, SIVlhoest and SIVsun infections in PT macaques were not accompanied by high viral loads during the chronic disease stage. In addition, no significant correlation between the viral load at set point (12 weeks postinfection) and survival could be found. Five out of eight SIVlhoest-infected and three out of four SIVsun-infected macaques succumbed to AIDS during the first 5 years of infection. Thus, the survival of SIVsun- and SIVlhoest-infected animals was significantly longer than that of SIVagm- or SIVsm-infected macaques. All PT macaques maintained strong SIV antibody responses despite progression to SIV-induced AIDS. The development of immunodeficiency in the face of low viremia suggests that SIVlhoest and SIVsun infections of macaques may model unique aspects of the pathogenesis of human immunodeficiency virus infection in humans.     

Immunological memory and acquired immunodeficiency syndrome pathogenesis

Infection with the human immunodeficiency virus results in profound perturbations in immunological memory, ultimately resulting in increased susceptibility to opportunistic infections and acquired immunodeficiency syndrome (AIDS). We have used rhesus macaques infected with the simian immunodeficiency virus (SIV) as a model to understand better the effects of AIDS virus infection on immunological memory. Acute infection with SIV resulted in significant deficits in CD4+ helper responses to cytomegalovirus (CMV) as well as CMV-specific cytotoxic T-lymphocyte and neutralizing antibody responses. Reactivation of CMV was associated with high levels of SIV replication and suppression of both T-helper and cytotoxic responses to CMV. We have also studied the effects of SIV infection on T-cell turnover in non-human primates. T-cell turnover was evaluated using the nucleoside analogue bromodeoxyuridine (BrdU) in combination with five-colour flow cytometric analysis. T cells in normal animals turned over at relatively rapid rates, with memory cells turning over more quickly than naive cells. In SIV-infected animals, the labelling and elimination rates of both CD4+ and CD8+ BrdU-labelled cells were increased by two- to threefold compared with normal controls. Further analysis of immunological memory in non-human primates should offer the opportunity to extend immunological insights from murine models to the pathogenesis and prevention of AIDS.     

Derivation and characterization of a highly pathogenic isolate of human immunodeficiency virus type 2 that causes rapid CD4+ cell depletion in Macaca nemestrina

With few exceptions, humans are the only species known to develop acquired immunodeficiency syndrome (AIDS) after human immunodeficiency virus (HIV) infection. We report here that an isolate of HIV type 2, EHO, readily established persistent infection in 100% of Macaca nemestrina in three consecutive transmission studies. Of the eight infected animals, five showed persistently high virus load and six developed AIDS-like diseases or CD4+ cell depletion within 4 years of infection. The pathology and clinical signs closely parallel those of HIV-1 infection of humans, including lymphadenopathy, anemia, CD4+ cell depletion, and opportunistic infections. A cell-free virus stock was established from the lymph nodes of an animal that developed AIDS-like diseases. This virus, HIV-2/287, was highly pathogenic in M. nemestrina, causing CD4+ cell depletion within 2-8 weeks postinfection. While both HIV-2 EHO and HIV-2/287 use predominantly CXCR4, the latter shows greatly enhanced replicative capacity in macaque peripheral blood mononuclear cells (PBMCs). The establishment of a human immunodeficiency virus that causes rapid and reproducible CD4 cell depletion in macaques could facilitate the study of HIV pathogenesis and the development of effective vaccines and therapy against AIDS.     

The comparison of genetic variation in the envelope protein between various immunodeficiency viruses and equine infectious anemia virus

The envelope protein (Env) of lentiviruses such as HIV, SIV, FIV and EIAV is larger than that of other retroviruses. The Chinese EIAV attenuated vaccine is based on Env and has helped to successfully control this virus, demonstrating that envelope is crucial for vaccine. We compared Env variation of the four kinds of lentiviruses. Phylogenetic analysis showed that the evolutionary relationship of Env between HIV and SIV was the closest and they appeared to descend from a common ancestor, and the relationship of HIV and EIAV was the furthest. EIAV had the shortest Env length and the least number of potential N-linked glycosylation sites (PNGS) as well as glycosylation density compared to various immunodeficiency viruses. However, HIV had the longest Env length and the most PNGS. Moreover, the alignment of HIV and SIV showed that PNGS were primarily distributed within extracellular membrane protein gp120 rather than transmembrane gp41. It implies that the size difference among these viruses is associated with a lentivirus specific function and also the diversity of env. There are low levels of modification of glycosylation sites of Env and selection of optimal protective epitopes might be useful for development of an effective vaccine against HIV/AIDS.     

In vivo natural killer cell depletion during primary simian immunodeficiency virus infection in rhesus monkeys

The contribution of natural killer (NK) cells to the immune containment of human immunodeficiency virus infection remains undefined. To directly assess the role of NK cells in an AIDS animal model, we depleted rhesus monkeys of >88% of CD3(-) CD16(+) CD159a(+) NK cells at the time of primary simian immunodeficiency virus (SIV) infection by using anti-CD16 antibody. During the first 11 days following SIV inoculation, when NK cell depletion was most profound, a trend toward higher levels of SIV replication was noted in NK cell-depleted monkeys compared to those in control monkeys. However, this treatment did not result in significant changes in the overall levels or kinetics of plasma viral RNA or affect the SIV-induced central memory CD4(+) T-lymphocyte loss. These findings are consistent with a limited role for cytotoxic CD16(+) NK cells in the control of primary SIV viremia.     

Propagation and dissemination of infection after vaginal transmission of simian immunodeficiency virus

In the current global AIDS pandemic, more than half of new human immunodeficiency virus type 1 (HIV-1) infections are acquired by women through intravaginal HIV exposure. For this study, we explored pathogenesis issues relevant to the development of effective vaccines to prevent infection by this route, using an animal model in which female rhesus macaques were exposed intravaginally to a high dose of simian immunodeficiency virus (SIV). We examined in detail the events that transpire from hours to a few days after intravaginal SIV exposure through week 4 to provide a framework for understanding the propagation, dissemination, and establishment of infection in lymphatic tissues (LTs) during the acute stage of infection. We show that the mucosal barrier greatly limits the infection of cervicovaginal tissues, and thus the initial founder populations of infected cells are small. While there was evidence of rapid dissemination to distal sites, we also show that continuous seeding from an expanding source of production at the portal of entry is likely critical for the later establishment of a productive infection throughout the systemic LTs. The initially small founder populations and dependence on continuous seeding to establish a productive infection in systemic LTs define a small window of maximum vulnerability for the virus in which there is an opportunity for the host, vaccines, or other interventions to prevent or control infection.     

Derivation and characterization of a highly pathogenic isolate of human immunodeficiency virus type 2 that causes rapid CD4+ cell depletion in Macaca nemestrina

With few exceptions, humans are the only species known to develop acquired immunodeficiency syndrome (AIDS) after human immunodeficiency virus (HIV) infection. We report here that an isolate of HIV type 2, EHO, readily established persistent infection in 100% of Macaca nemestrina in three consecutive transmission studies. Of the eight infected animals, five showed persistently high virus load and six developed AIDS-like diseases or CD4⁺ cell depletion within 4 years of infection. The pathology and clinical signs closely parallel those of HIV-1 infection of humans, including lymphadenopathy, anemia, CD4⁺ cell depletion, and opportunistic infections. A cell-free virus stock was established from the lymph nodes of an animal that developed AIDS-like diseases. This virus, HIV-2/287, was highly pathogenic in M. nemestrina, causing CD4⁺ cell depletion within 2–8 weeks post-infection. While both HIV-2 EHO and HIV-2/287 use predominantly CXCR4, the latter shows greatly enhanced replicative capacity in macaque peripheral blood mononuclear cells (PBMCs). The establishment of a human immunodeficiency virus that causes rapid and reproducible CD4⁺ cell depletion in macaques could facilitate the study of HIV pathogenesis and the development of effective vaccines and therapy against AIDS.     

SIV-induced activation of the blood-brain barrier requires cell-associated virus and is not restricted to endothelial cell activation

It has never been determined if activation of the blood-brain barrier (BBB) during simian immunodeficiency virus/human immunodeficiency virus (SIV/HIV) infection is a function of high levels of circulating virus or if the virus has to be within a cell capable of crossing the BBB to activate it. In vitro models of the BBB are becoming recognized as an acceptable method for determining the cellular events associated with HIV neuroinvasion. Cell free virus (when added in the physiologically relevant lumen) although capable of activating the endothelial cells of our in vitro BBB did not activate astrocytes beneath. SIVmac251-infected CEMx174 cells, however, were capable of activating both components of the BBB model. Here we demonstrate that an in vitro model of the BBB can be activated in a physiologically relevant manner, that SIV requires to be cell-associated and that endothelial cells of the BBB are not the only components that are activated during SIV neuroinvasion.     

Progress and challenges in therapies for AIDS in nonhuman primate models

Efforts to develop animal models for human immunodeficiency virus type-1 (HIV-1) vaccine testing have focused on lentivirus infection of nonhuman primates. A long-term goal of this primate research is to utilize the models to understand the mechanisms of pathogenesis leading to AIDS. Because the time to disease is compressed relative to HIV infection in humans, therapeutic strategies and compounds can be tested in nonhuman primate models in a shorter time frame and under more controlled conditions than are possible in many clinical studies. Recent interventive studies in primates using antiviral drugs or passive immune globulin (IgG) have demonstrated that multiple log reductions in plasma virus can be achieved and sustained, with accompanying health benefits. Information gained about timing and dosage may be of utility in designing clinical studies. The development of reliable and predictable animal models for effective therapies and vaccines against AIDS remains a critical priority for primate research.     

Making the animal model for AIDS research more precise: the impact of major histocompatibility complex (MHC) genes on pathogenesis and disease progression in SIV-infected monkeys

Experimentally infected rhesus monkeys serve as an indispensable animal model to assess the pathogenesis, to validate therapy approaches and to develop vaccination strategies against viral diseases such as AIDS threatening the human population. Upon infection with simian immunodeficiency virus (SIV), a retrovirus closely related to the human immunodeficiency virus (HIV), macaques develop clinical manifestations similar to those of HIV-infected humans. As in humans, the disease course is variable. Polymorphic genes of the major histocompatibility complex (MHC) are required for the initiation and regulation of a specific immune response and represent a major host factor accounting for the differential outcome of infection. During the last few years, our understanding of the structure and function of the rhesus macaque MHC has increased substantially. Functional studies have led to the identification of specific SIV and HIV peptide epitopes presented by rhesus macaque MHC molecules. The subsequent development of MHC class I tetramers has allowed further insight into the cellular immune response following SIV-infection. Detailed studies demonstrated that viral escape mutants are generated during the acute and chronic phase of infection and explain why control of viral replication ultimately fails. Furthermore, particular MHC haplotypes which influence disease progression have been discovered. Thus, MHC-typing can have a prognostic potential. The further elucidation of the rhesus macaque MHC and the search for other relevant genes will remain an important task for future research and will stimulate all immunologically-related investigations in macaques.