Pathology of simian immunodeficiency virus induced disease

Unravelling the triad of neuroinvasion, neurodissemination, and neuroinflammation of human immunodeficiency virus type 1 in the central nervous system

Since the identification of human immunodeficiency virus type 1 (HIV-1) in 1983, many improvements have been made to control viral replication in the peripheral blood and to treat opportunistic infections. This has increased life expectancy but also the incidence of age-related central nervous system (CNS) disorders and HIV-associated neurodegeneration/neurocognitive impairment and depression collectively referred to as HIV-associated neurocognitive disorders (HAND). HAND encompasses a spectrum of different clinical presentations ranging from milder forms such as asymptomatic neurocognitive impairment or mild neurocognitive disorder to a severe HIV-associated dementia (HAD). Although control of viral replication and suppression of plasma viral load with combination antiretroviral therapy has reduced the incidence of HAD, it has not reversed milder forms of HAND. The objective of this review, is to describe the mechanisms by which HIV-1 invades and disseminates in the CNS, a crucial event leading to HAND. The review will present the evidence that underlies the relationship between HIV infection and HAND. Additionally, recent findings explaining the role of neuroinflammation in the pathogenesis of HAND will be discussed, along with prospects for treatment and control.

Antiretroviral therapy restores the homeostatic state of microglia in SIV-infected rhesus macaques

Microglia and macrophages are essential for homeostatic maintenance and innate immune response in the brain. They are the first line of defense against infections such as HIV/SIV in the brain. However, they are susceptible to infection and function as viral reservoirs even under effective viral suppression. While current antiretroviral regimens successfully suppress viremia and improve quality of life and lifespan, neurologic complications persist and are in part attributed to activated microglia. We sought to test the hypothesis that brain microglia return to a more homeostatic-like state when viremia is suppressed by combination antiretroviral therapy. Using the SIV-rhesus macaque model, we combined single-cell RNA sequencing, bioinformatics, and pathway analysis to compare gene expression profiles of brain myeloid cells under 4 conditions: uninfected, SIV infected, SIV infected with cART suppression, and SIV encephalitis (SIVE). Our study reveals greater myeloid diversity and an elevated proinflammatory state are associated with untreated SIV infection compared with uninfected animals. The development of encephalitis and suppression of viremia both reduced myeloid diversity. However, they had converse effects on the activation state of microglia and inflammation. Notably, suggestive of a restoration of a homeostatic state in microglia, gene expression and activation of pathways related to inflammation and immune response in cART-suppressed monkeys were most similar to that in uninfected monkeys. Untreated SIV infection shared characteristics, especially in brain macrophages to SIVE, with SIVE showing dramatic inflammation. In support of our hypothesis, our study demonstrates that cART indeed restores this key component of the brain's homeostatic state. Summary: ScRNA-seq of rhesus monkey microglia reveals clusters of cells in activated states in the setting of SIV infection, which is primarily reversed by suppressing viremia with combination antiretroviral therapy.

α4-Integrin Antibody Treatment Blocks Monocyte/Macrophage Traffic to, Vascular Cell Adhesion Molecule-1 Expression in, and Pathology of the Dorsal Root Ganglia in an SIV Macaque Model of HIV-Peripheral Neuropathy

Traffic of activated monocytes into the dorsal root ganglia (DRG) is critical for pathology in HIV peripheral neuropathy. We have shown that accumulation of recently recruited (bromodeoxyuridine(+) MAC387(+)) monocytes is associated with severe DRG pathology and loss of intraepidermal nerve fibers in SIV-infected macaques. Herein, we blocked leukocyte traffic by treating animals with natalizumab, which binds to α4-integrins. SIV-infected CD8-depleted macaques treated with natalizumab either early (the day of infection) or late (28 days after infection) were compared with untreated SIV-infected animals sacrificed at similar times. Histopathology showed diminished DRG pathology with natalizumab treatment, including decreased inflammation, neuronophagia, and Nageotte nodules. Natalizumab treatment resulted in a decrease in the number of bromodeoxyuridine(+) (early), MAC387(+) (late), CD68(+) (early and late), and SIVp28(+) (late) macrophages in DRG tissues. The number of CD3(+) T lymphocytes in DRGs was not affected by natalizumab treatment. Vascular cell adhesion molecule 1, an adhesion molecule that mediates leukocyte traffic, was diminished in DRGs of all natalizumab-treated animals. These data show that blocking monocyte, but not T lymphocyte, traffic to the DRG results in decreased inflammation and pathology, supporting a role for monocyte traffic and activation in HIV peripheral neuropathy.

Monocyte Traffic, Dorsal Root Ganglion Histopathology, and Loss of Intraepidermal Nerve Fiber Density in SIV Peripheral Neuropathy

HIV-associated sensory neuropathy remains the most common neurological complication of HIV infection and is characterized by dorsal root ganglion (DRG) inflammation and intraepidermal nerve fiber density (IENFD) loss. Chronic peripheral immune cell activation and accumulation may cause damage to the DRG, but has not been fully investigated yet. By using an SIV-infected, CD8-lymphocyte-depleted rhesus macaque model, we defined immune cells surrounding DRG neurons and their role in DRG pathology, measured cell traffic from the bone marrow to the DRGs using 5-bromo-2-deoxyuridine (BrdU) pulse, and serially measured IENFD. We found an increase in CD68(+) and CD163(+) macrophages in DRGs of SIV-infected animals. MAC387(+) recently recruited monocytes/macrophages were increased, along with BrdU(+) cells, in the DRGs of SIV-infected macaques. We demonstrated that 78.1% of all BrdU(+) cells in DRGs were also MAC387(+). The number of BrdU(+) monocytes correlated with severe DRG histopathology, which included neuronophagia, neuronal loss, and Nageotte nodules. These data demonstrate that newly recruited MAC387(+)BrdU(+) macrophages may play a significant role in DRG pathogenesis. IENFD decreased early (day 21), consistent with the development of sensory neuropathy in SIV-infected macaques. Decreased IENFD was associated with elevated BrdU(+) cells in the DRG. These data suggest that increased recruitment of macrophages to DRG is associated with severe DRG histopathology and IENFD loss.

Spatiotemporal dynamics of simian immunodeficiency virus brain infection in CD8+ lymphocyte-depleted rhesus macaques with neuroAIDS

Despite the success of combined antiretroviral therapy in controlling viral replication in human immunodeficiency virus (HIV)-infected individuals, HIV-associated neurocognitive disorders, commonly referred to as neuroAIDS, remain a frequent and poorly understood complication. Infection of CD8(+) lymphocyte-depleted rhesus macaques with the SIVmac251 viral swarm is a well-established rapid disease model of neuroAIDS that has provided critical insight into HIV-1-associated neurocognitive disorder onset and progression. However, no studies so far have characterized in depth the relationship between intra-host viral evolution and pathogenesis in this model. Simian immunodeficiency virus (SIV) env gp120 sequences were obtained from six infected animals. Sequences were sampled longitudinally from several lymphoid and non-lymphoid tissues, including individual lobes within the brain at necropsy, for four macaques; two animals were sacrificed at 21 days post-infection (p.i.) to evaluate early viral seeding of the brain. Bayesian phylodynamic and phylogeographic analyses of the sequence data were used to ascertain viral population dynamics and gene flow between peripheral and brain tissues, respectively. A steady increase in viral effective population size, with a peak occurring at ~50-80 days p.i., was observed across all longitudinally monitored macaques. Phylogeographic analysis indicated continual viral seeding of the brain from several peripheral tissues throughout infection, with the last migration event before terminal illness occurring in all macaques from cells within the bone marrow. The results strongly supported the role of infected bone marrow cells in HIV/SIV neuropathogenesis. In addition, our work demonstrated the applicability of Bayesian phylogeography to intra-host studies in order to assess the interplay between viral evolution and pathogenesis.

HIV pathogenesis: the host

Human immunodeficiency virus (HIV) pathogenesis has proven to be quite complex and dynamic with most of the critical events (e.g., transmission, CD4(+) T-cell destruction) occurring in mucosal tissues. In addition, although the resulting disease can progress over years, it is clear that many critical events happen within the first few weeks of infection when most patients are unaware that they are infected. These events occur predominantly in tissues other than the peripheral blood, particularly the gastrointestinal tract, where massive depletion of CD4(+) T cells occurs long before adverse consequences of HIV infection are otherwise apparent. Profound insights into these early events have been gained through the use of nonhuman primate models, which offer the opportunity to examine the early stages of infection with the simian immunodeficiency virus (SIV), a close relative of HIV that induces an indistinguishable clinical picture from AIDS in Asian primate species, but importantly, fails to cause disease in its natural African hosts, such as sooty mangabeys and African green monkeys. This article draws from data derived from both human and nonhuman primate studies.

S100β as a novel and accessible indicator for the presence of monocyte-driven encephalitis in AIDS

AIMS

The pathogenesis of human/simian immunodeficiency virus encephalitis (HIVE/SIVE) remains incompletely understood, but is associated with alterations in the blood-brain barrier. At present, it is not possible to easily determine if an individual has HIVE/SIVE before post mortem examination.

METHODS

We have examined serum levels of the astroglial protein S100β in SIV-infected macaques and show that it can be used to determine which animals have SIVE. We also checked for correlations with inflammatory markers such as CCL2/MCP-1, IL-6 and C-reactive protein.

RESULTS

We found that increased S100β protein in serum correlated with decreased expression of the tight junction protein zonula occludens-1 on brain microvessels. Furthermore, the decrease in zonula occludens-1 expression was spatially related to SIVE lesions and perivascular deposition of plasma fibrinogen. There was no correlation between encephalitis and plasma levels of IL-6, MCP-1/CCL2 or C-reactive protein.

CONCLUSIONS

Together, these data indicate that SIVE lesions are associated with vascular leakage that can be determined by S100β protein in the periphery. The ability to simply monitor the presence of SIVE will greatly facilitate studies of the neuropathogenesis of AIDS.

Dorsal root ganglia damage in SIV-infected rhesus macaques: an animal model of HIV-induced sensory neuropathy

HIV-associated sensory neuropathy (HIV-SN) is currently the most common neurological complication of chronic HIV infection and continues to substantially affect patient quality of life. Mechanisms underlying the neuronal damage and loss observed in sensory ganglia of HIV-infected individuals have not been sufficiently studied. The present study aimed to develop and characterize a model of HIV-SN using SIV-infected CD8 T-lymphocyte-depleted rhesus macaques (Macaca mulatta). Uninfected controls (n = 5), SIV-infected CD8-depleted (n = 4), and SIV-infected non-CD8-depleted (n = 6) animals were used. Of the six non-CD8-depleted animals, three were conventional progressors (progressing to AIDS >1 year after infection) and three were rapid progressors (AIDS within 6 months). Dorsal root ganglia (DRG) were examined for histological hallmarks of HIV-SN, including satellitosis, presence of Nageotte nodules, and neuronophagia, as well as increased numbers of CD68(+) macrophages and abundant viral replication. In contrast to non-CD8-depleted animals, which had mild to moderate DRG pathology, the CD8-depleted SIV-infected animals had moderate to severe DRG damage, with increased numbers of CD68(+) satellite cells. Additionally, there was marked active viral replication in the affected DRG. These findings confirm that many features of HIV-SN can be recapitulated in the CD8-depleted SIV-infected rhesus macaque model within a short time frame and illustrate the importance of this model for study of sensory neuropathy.

Efficient transmission and persistence of low-frequency SIVmac251 variants in CD8-depleted rhesus macaques with different neuropathology

Infection of CD8-depleted rhesus macaques with the genetically heterogeneous simian immunodeficiency virus (SIV)mac251 viral swarm provides a rapid-disease model for simian acquired immune deficiency syndrome and SIV-encephalitis (SIVE). The objective was to evaluate how the diversity of the swarm influences the initial seeding of the infection that may potentially affect disease progression. Plasma, lymphoid and non-lymphoid (brain and lung) tissues were collected from two infected macaques euthanized at 21 days post-infection (p.i.), as well as longitudinal specimens and post-mortem tissues from four macaques followed throughout the infection. About 1300 gp120 viral sequences were obtained from the infecting SIVmac251 swarm and the macaques longitudinal and post-mortem samples. Phylogenetic and amino acid signature pattern analyses were carried out to assess frequency, transmission dynamics and persistence of specific viral clusters. Although no significant reduction in viral heterogeneity was found early in infection (21 days p.i.), transmission and replication of SIV variants was not entirely random. In particular, two distinct motifs under-represented (<4 %) in the infecting swarm were found at high frequencies (up to 14 %) in all six macaques as early as 21 days p.i. Moreover, a macrophage tropic variant not detected in the viral swarm (<0.3 %) was present at high frequency (29-100 %) in sequences derived from the brain of two macaques with meningitis or severe SIVE. This study demonstrates the highly efficient transmission and persistence in vivo of multiple low frequency SIVmac251 founder variants, characterized by specific gp120 motifs that may be linked to pathogenesis in the rapid-disease model of neuroAIDS.

Systemic spironucleosis in 2 immunodeficient rhesus macaques (Macaca mulatta)

Spironucleus spp are parasites of fish and terrestrial vertebrates, including mice and turkeys, that rarely cause extraintestinal disease. Two rhesus macaques (Macaca mulatta) were experimentally inoculated with simian immunodeficiency virus mac251. Both progressed to simian acquired immune deficiency syndrome within 1 year of inoculation and developed systemic protozoal infections in addition to common opportunistic infections, including rhesus cytomegalovirus, rhesus lymphocryptovirus, and rhesus adenovirus. In the first case, the protozoa were associated with colitis, multifocal abdominal abscessation, and lymphadenitis. In the second case, they were one of a number of organisms associated with extensive pyogranulomatous pneumonia and colitis. Ultrastructural, molecular, and phylogenetic analysis revealed the causative organism to be a species of Spironucleus closely related to Spironucleus meleagridis of turkeys. This report is the first of extraintestinal infection with Spironucleus sp in higher mammals and expands the list of opportunistic infections found in immunocompromised rhesus macaques.

CD163, a marker of perivascular macrophages, is up-regulated by microglia in simian immunodeficiency virus encephalitis after haptoglobin-hemoglobin complex stimulation and is suggestive of breakdown of the blood-brain barrier

Macrophages and microglia are the major cell types infected by human immunodeficiency virus and simian immunodeficiency virus (SIV) in the central nervous system. Microglia are likely infected in vivo, but evidence of widespread productive infection (ie, presence of viral RNA and protein) is lacking. This conclusion is controversial because, unlike lymphocytes, macrophages and microglia cannot be discreetly immunophenotyped. Of particular interest in the search for additional monocyte/macrophage-lineage cell markers is CD163; this receptor for haptoglobin-hemoglobin (Hp-Hb) complex, which forms in plasma following erythrolysis, is expressed exclusively on cells of monocyte/macrophage lineage. We examined CD163 expression in vitro and in vivo by multiple techniques and at varying times after SIV infection in macaques with or without encephalitis. In normal and acutely SIV-infected animals, and in SIV-infected animals without encephalitis, CD163 expression was detected in cells of monocyte/macrophage lineage, including perivascular macrophages, but not in parenchymal microglia. However, in chronically infected animals with encephalitis, CD163 expression was detected in activated microglia surrounding SIV encephalitis lesions in the presence of Hp-Hb complex, suggesting leakage of the blood-brain barrier. CD163 expression was also induced on microglia in vitro after stimulation with Hp-Hb complex. We conclude that CD163 is a selective marker of perivascular macrophages in normal macaques and during the early phases of SIV infection; however, later in infection in animals with encephalitis, CD163 is also expressed by microglia, which are probably activated as a result of vascular compromise.

Current concepts in AIDS pathogenesis: insights from the SIV/macaque model

The pathogenesis of AIDS has proven to be quite complex and dynamic, with most of the critical events (e.g., transmission, CD4(+) T cell destruction) occurring in tissues that are not easily accessible for analysis. In addition, although the disease can progress over years, many critical events happen within the first few weeks of infection, when most patients are unaware that they are infected. The nonhuman primate model of AIDS has been used extensively to fill these gaps in our understanding of AIDS pathogenesis.

Expression of CD8alpha identifies a distinct subset of effector memory CD4+ T lymphocytes

Circulating CD4+ CD8+ T lymphocytes have been described in the peripheral blood of humans and several animal species. However, the origin and functional properties of these cells remain poorly understood. In the present study, we evaluated the frequency, phenotype and function of peripheral CD4+ CD8+ T cells in rhesus macaques. Two distinct populations of CD4+ CD8+ T cells were identified: the dominant one was CD4hi CD8lo and expressed the CD8alphaalpha homodimer, while the minor population was CD4lo CD8hi and expressed the CD8alphabeta heterodimer. The majority of CD4hi CD8alphalo T cells exhibited an activated effector/memory phenotype (CCR5lo CD7- CD28- HLA-DR+) and expressed relatively high levels of granzyme B. Intracellular cytokine staining assays demonstrated that the frequency of cytomegalovirus-specific T cells was enriched five-fold in CD4hi CD8alphalo T cells compared to single-positive CD4+ T cells, whereas no consistent enrichment was observed for simian immunodeficiency virus (SIV)-specific T cells. Cross-sectional studies of SIV-infected animals demonstrated that the frequency of CD4hi CD8alphalo T cells was lower in wild-type SIV-infected animals compared to uninfected controls, although prospective studies of SIV-infected animals demonstrated depletion of CD4hi CD8alphalo lymphocytes only in a subset of animals. Taken together, these data suggest that CD4+ T cells expressing CD8alpha represent an effector/memory subset of CD4+ T cells and that this cell population can be depleted during the course of SIV infection.

Systemic arteriopathy in SIV-infected rhesus macaques (Macaca mulatta)

BACKGROUND

Severe disseminated vasculopathy was observed in two simian immunodeficiency virus (SIV)-infected rhesus macaques (Macaca mulatta). These animals developed clinical signs of AIDS, including lymphadenopathy, weight loss, diarrhea and collapse.

RESULTS AND DISCUSSION

Grossly, both animals showed emaciation, lymphadenopathy, vegetations on the mitral valve, renal infarcts and a dilated intestine; one animal had multifocal hemorrhages in multiple organs. Histologically, both cases had disseminated arteriopathy characterized by intimal thickening and fibrosis with varying degrees of vasculitis. The lesion was prominent in the kidney, intestine, pancreas, liver, heart, lymph nodes, spleen and testis. Occasional venules had intimal thickening. Both cases had cytomegalovirus (CMV) infection with intranuclear inclusions, CMV antigen and nucleic acid; some inclusions were observed in endothelial cells within some of the vascular lesions in one of the two. These data suggest that CMV caused the unusual lesions.

A glial endogenous cannabinoid system is upregulated in the brains of macaques with simian immunodeficiency virus-induced encephalitis

Recent evidence supports the notion that the endocannabinoid system may play a crucial role in neuroinflammation. We explored the changes that some elements of this system exhibit in a macaque model of encephalitis induced by simian immunodeficiency virus. Our results show that profound alterations in the distribution of specific components of the endocannabinoid system occur as a consequence of the viral infection of the brain. Specifically, expression of cannabinoid receptors of the CB2 subtype was induced in the brains of infected animals, mainly in perivascular macrophages, microglial nodules, and T-lymphocytes, most likely of the CD8 subtype. In addition, the endogenous cannabinoid-degrading enzyme fatty acid amide hydrolase was overexpressed in perivascular astrocytes as well as in astrocytic processes reaching cellular infiltrates. Finally, the pattern of CB1 receptor expression was not modified in the brains of infected animals compared with that in control animals. These results resemble previous data obtained in Alzheimer's disease human tissue samples and suggest that the endocannabinoid system may participate in the development of human immunodeficiency virus-induced encephalitis, because activation of CB2 receptors expressed by immune cells is likely to reduce their antiviral response and thus could favor the CNS entry of infected monocytes.

Magnetic resonance spectroscopy reveals that activated monocytes contribute to neuronal injury in SIV neuroAIDS

Difficulties in understanding the mechanisms of HIV neuropathogenesis include the inability to study dynamic processes of infection, cumulative effects of the virus, and contributing host immune responses. We used H magnetic resonance spectroscopy and studied monocyte activation and progression of CNS neuronal injury in a CD8 lymphocyte depletion model of neuroAIDS in SIV-infected rhesus macaque monkeys. We found early, consistent neuronal injury coincident with viremia and SIV infection/activation of monocyte subsets and sought to define the role of plasma virus and monocytes in contributing to CNS disease. Antiretroviral therapy with essentially non-CNS-penetrating agents resulted in slightly decreased levels of plasma virus, a significant reduction in the number of activated and infected monocytes, and rapid, near-complete reversal of neuronal injury. Robust macrophage accumulation and productive virus replication were found in brains of infected and CD8 lymphocyte-depleted animals, but no detectable virus and few scattered infiltrating macrophages were observed in CD8 lymphocyte-depleted animals compared with animals not receiving antiretroviruses that were sacrificed at the same time after infection. These results underscore the role of activated monocytes and monocyte infection outside of the brain in driving CNS disease.

Cell tropism of simian immunodeficiency virus in culture is not predictive of in vivo tropism or pathogenesis

SIVmac239/316 is a molecular clone derived from SIVmac239 that differs from the parental virus by nine amino acids in env. This virus, unlike the parental SIVmac239, is able to replicate well in alveolar macrophages in culture. We have not however, observed macrophage-associated inflammatory disease in any animal infected with SIVmac239/316. Therefore, we sought to examine the cell tropism of this virus in vivo in multiple tissues using in situ hybridization combined with immunohistochemistry and multilabel confocal microscopy for viral nucleic acid and multiple cell-type-specific markers for macrophages and T lymphocytes. Tissues examined included brain, heart, lung, lymph nodes, spleen, thymus, and small and large intestine. Matched tissues from macaques infected with the parental SIVmac239 and uninfected macaques were also examined. Many infected cells were detected in the tissues of animals infected with SIVmac239 and SIVmac239/316 although there appeared to be fewer positive cells in animals infected with SIVmac239/316. Surprisingly, in light of the cell culture observations, nearly every simian immunodeficiency virus-infected cell in animals inoculated with SIVmac239/316 was a T lymphocyte rather than a macrophage. This was true both during early infection (first 2 months) and in terminal disease. In contrast, as previously described, SIVmac239 was found in both T cells and macrophages in tissues as early as 21 days after infection. These studies indicate that during both acute and chronic SIVmac239/316 infection T lymphocytes rather than macrophages are the principal targets in vivo. These data combined with the absence of macrophage-associated lesions in SIVmac239/316-infected animals indicate that in vitro cell tropism is not predictive of in vivo tropism or disease pathogenesis.

Identification of T lymphocytes in simian immunodeficiency virus encephalitis: distribution of CD8+ T cells in association with central nervous system vessels and virus

T lymphocytes are found within brains infected with human immunodeficiency virus (HIV) or simian immunodeficiency virus (SIV) where they are a minor, but consistently identified, population. However, little analysis of their phenotypes has been done, and questions concerning whether or not they are viral antigen specific has not been thoroughly examined. We investigated the central nervous system (CNS) of SIV-infected rhesus macaques to identify T-lymphocyte subsets in relation to virus-infected cells and brain microvessels. We have found that a sensitive antigen-retrieval technique greatly enhanced immunohistochemical detection of CD4+ and CD8+ T lymphocytes in control studies. In encephalitic brains of SIV-infected monkeys with acquired immunodeficiency syndrome (AIDS), we found a significant accumulation of CD8+ T lymphocytes but little-to-no accumulation of CD4+ T lymphocytes. CD4+ cells, when detected, were mostly monocyte/macrophages closely associated with CNS vessels. Using a combination of in situ hybridization for SIV RNA, and immunohistochemistry for CD8+ T lymphocytes and/or Glut-1 for endothelial cells on brain microvessels, we found CD8+ T lymphocytes with an angiocentric distribution often adjacent to virus-infected cells. In the CNS of animals with SIV encephalitis, there was a trend of CD8+ T lymphocytes that were not directly juxtaposed with CNS vessels. These data suggest that in brains of SIV-infected monkeys and HIV-infected humans, CD8+ T lymphocytes traffic to and are retained in the CNS in an angiocentric and possibly antigen-specific manner.

Central nervous system damage, monocytes and macrophages, and neurological disorders in AIDS

This review focuses on the role of the extended macrophage/monocyte family in the central nervous system during HIV or SIV infection. The accumulated data, buttressed by recent experimental results, suggest that these cells play a central, pathogenic role in retroviral-associated CNS disease. While the immune system is able to combat the underlying retroviral infection, the accumulation and widespread activation of macrophages, microglia, and perivascular cells in the CNS are held in check. However, with the collapse of the immune system and the disappearance of the CD4(+) T cell population, productive infection reemerges, especially in CNS macrophages. These cells, as well as noninfected macrophages, are stimulated to high levels of activation. When members of this cell group become highly activated, they elaborate a wide spectrum of deleterious substances into the neural parenchyma. In the final phases of HIV or SIV infection, this chronic, widespread, and dramatic level of macrophage/monocyte/microglial activation constitutes a self-sustaining state of macrophage dysregulation, which results in pathological alterations and the emergence of various neurological problems.

Proliferating cellular nuclear antigen expression as a marker of perivascular macrophages in simian immunodeficiency virus encephalitis

Brain perivascular macrophages are a major target of simian immunodeficiency virus (SIV) infection in rhesus macaques and HIV infection in humans. Perivascular macrophages are distinct from parenchymal microglia in their location, morphology, expression of myeloid markers, and turnover in the CNS. In contrast to parenchymal microglia, perivascular macrophages are continuously repopulated by blood monocytes, which undergo maturation to macrophages on entering the central nervous system (CNS). We studied differences in monocyte/macrophages in vivo that might account for preferential infection of perivascular macrophages by SIV. In situ hybridization for SIV and proliferating cellular nuclear antigen (PCNA) immunohistochemistry demonstrated that SIV-infected and PCNA-positive cells were predominantly found in perivascular cuffs of viremic animals and in histopathological lesions that characterize SIV encephalitis (SIVE) in animals with AIDS. Multilabel techniques including double-label immunohistochemistry and combined in situ hybridization and immunofluorescence confocal microscopy revealed numerous infected perivascular macrophages that were PCNA-positive. Outside the CNS, SIV-infected, PCNA-expressing macrophage subpopulations were found in the small intestine and lung of animals with AIDS. While PCNA is used as a marker of cell proliferation it is also strongly expressed in non-dividing cells undergoing DNA synthesis and repair. Therefore, more specific markers for cell proliferation including Ki-67, topoisomerase IIalpha, and bromodeoxyuridine (BrdU) incorporation were used which indicated that PCNA-positive cells within SIVE lesions were not proliferating. These observations are consistent with perivascular macrophages as terminally differentiated, non-dividing cells and underscores biological differences that could potentially define mechanisms of preferential, productive infection of perivascular macrophages in the rhesus macaque model of neuroAIDS. These studies suggest that within CNS and non-CNS tissues there exist subpopulations of macrophages that are SIV-infected and express PCNA.

Simian immunodeficiency virus encephalitis: analysis of envelope sequences from individual brain multinucleated giant cells and tissue samples

Simian immunodeficiency virus (SIV)-infected macaques develop an encephalitis (SIVE) that is pathologically virtually indistinguishable from that associated with HIV infection, with multinucleated giant cells (MNGCs) being the principal histopathological manifestation. To dissect SIV variants responsible for MNGC development, we examined the relationships between env sequences transcribed in individual MNGCs and those from genomic DNA of brain and spleen tissues. The brain-specific variant found in all brain clones was dominant among the clones from MNGCs, suggesting a role in the formation of giant cells. Furthermore, two additional minor groups of sequences were present in MNGCs. One group consisted of sequences closely related to those from spleen, indicating recent and probably multiple episodes of neuroinvasion. The second group represented clones similar or identical to the initial inoculum. The survival of archival sequences and their activation presumably by the fusion of productively and quiescently infected macrophages/microglia identify the central nervous system as a possible anatomical reservoir for latent infection.

Cervicovaginal lamina propria lymphocytes: phenotypic characterization and their importance in cytotoxic T-lymphocyte responses to simian immunodeficiency virus SIVmac251

Most human immunodeficiency virus (HIV) type 1 infections occur by the mucosal route. Thus, it is important to assess the immune responses to HIV in the vaginal, cervical, and rectal compartments. Here we quantitated the virus-specific CD8+ T-cell response and characterized the phenotype of lymphocytes in the genital tracts of naive macaques, macaques acutely or chronically infected with simian immunodeficiency virus SIVmac251, and macaques chronically infected with chimeric simian/human immunodeficiency virus SHIV(KU2.) Vaginal biopsy samples or samples obtained at the time of euthanasia were used in this analysis. The percentage of Gag-specific, tetramer-positive T cells was as high as 13 to 14% of the CD3+ CD8+ T-cell population in the vaginal and cervical laminae propriae of both SIVmac251 and SHIV(KU2) chronically infected macaques. In most cases, the frequency of this response in the cervicovaginal compartment far exceeded the frequency in the blood or the draining iliac lymph node. Vaginal laminae propriae of naive macaques contained 55 to 65% CD3+ CD8+ cells and 28 to 34% CD3+ CD4+ cells, while the majority of intraepithelial cells were CD8+ T cells (75 to 85%). For the same cells, the surface expression of CD62L was low whereas that of alphaEbeta7 was high. No difference in the expression of CD45RA on CD8+ T cells was observed in the chronic stage of SIVmac251 infection. Although no decrease in the percentage of CD4+ cells in the genital tract was observed within the first 12 days of infection, by 6 weeks from SIVmac251 infection and thereafter the percentage of CD4+ T cells was decreased in the laminae propriae of the vagina and cervix. Expression of CD45RA did not differ in naive and acutely SIVmac251 infected macaques. Information on the quality and quantity of local immune responses may help in the design of vaccine strategies aimed at containing viral replication at the site of viral encounter.

Differences in time of virus appearance in the blood and virus-specific immune responses in intravenous and intrarectal primary SIVmac251 infection of rhesus macaques; a pilot study

BACKGROUND

HIV-I can be transmitted by intravenous inoculation of contaminated blood or blood product or sexually through mucosal surfaces. Here we performed a pilot study in the SIVmac251 macaque model to address whether the route of viral entry influences the kinetics of the appearance and the size of virus-specific immune in different tissue compartments.

METHODS

For this purpose, of 2 genetically defined Mamu-A*01-positive macaques, 1 was exposed intravenously and the other intrarectally to the same SIVmac251 viral stock and virus-specific CD8+ T-cells were measured within the first 12 days of infection in the blood and at day 12 in several tissues following euthanasia.

RESULTS

Virus-specific CD8+ T-cell responses to Gag, Env, and particularly Tat appeared earlier in the blood of the animal exposed by the mucosal route than in the animal exposed intravenously. The magnitude of these virus-specific responses was consistently higher in the systemic tissues and GALT of the macaque exposed by the intravenous route, suggesting a higher viral burden in the tissues as reflected by the faster appearance of virus in plasma. Differences in the ability of the virus-specific CD8+ T-cells to respond in vitro to specific peptide stimulation were also observed and the greatest proliferative ability was found in the GALT of the animal infected by the intrarectal route.

CONCLUSIONS

These data may suggest that the natural mucosal barrier may delay viral spreading. The consequences of this observation, if confirmed in studies with a larger number of animals, may have implications in vaccine development.

High levels of viral replication during primary simian immunodeficiency virus SIVagm infection are rapidly and strongly controlled in African green monkeys

In contrast to pathogenic human immunodeficiency virus and simian immunodeficiency virus (SIV) infections, chronic SIVagm infections in African green monkeys (AGMs) are characterized by persistently low peripheral and tissue viral loads that correlate with the lack of disease observed in these animals. We report here data on the dynamics of acute SIVagm infection in AGMs that exhibit remarkable similarities with viral replication patterns observed in peripheral blood during the first 2 weeks of pathogenic SIVmac infections. Plasma viremia was evident at day 3 postinfection (p.i.) in AGMs, and rapid viral replication led by days 7 to 10 to peak viremias characterized by high levels of antigenemia (1.2 to 5 ng of p27/ml of plasma), peripheral DNA viral load (10(4) to 10(5) DNA copies/10(6) peripheral blood mononuclear cells [PBMC]), and plasma RNA viral load (2 x 10(6) to 2 x 10(8) RNA copies/ml). The lymph node (LN) RNA and DNA viral load patterns were similar to those in blood, with peaks observed between day 7 and day 14. These values in LNs (ranging from 3 x 10(5) to 3 x 10(6) RNA copies/10(6) LN cell [LNC] and 10(3) to 10(4) DNA copies/10(6) LNC) were at no time point higher than those observed in the blood. Both in LNs and in blood, rapid and significant decreases were observed in all infected animals after this peak of viral replication. Within 3 to 4 weeks p. i., antigenemia was no longer detectable and peripheral viral loads decreased to values similar to those characteristic of the chronic phase of infection (10(2) to 10(3) DNA copies/10(6) PBMC and 2 x 10(3) to 2 x 10(5) RNA copies/ml of plasma). In LNs, viral loads declined to 5 x 10(1) to 10(3) DNA copies and 10(4) to 3 x 10(5) RNA copies per 10(6) LNC at day 28 p.i. and continued to decrease until day 84 p.i. (<10 to 3 x 10(4) RNA copies/10(6) LNC). Despite extensive viremia during primary infection, neither follicular hyperplasia nor CD8(+) cell infiltration into LN germinal centers was detected. Altogether, these results indicate that the nonpathogenic outcome of SIVagm infection in its natural host is associated with a rapidly induced control of viral replication in response to SIVagm infection, rather than with a poorly replicating virus or a constitutive host genetic resistance to virus replication.

Antiviral treatment normalizes neurophysiological but not movement abnormalities in simian immunodeficiency virus-infected monkeys

Simian immunodeficiency virus (SIV) infection of rhesus monkeys provides an excellent model of the central nervous system (CNS) consequences of HIV infection. To discern the relationship between viral load and abnormalities induced in the CNS by the virus, we infected animals with SIV and later instituted antiviral treatment to lower peripheral viral load. Measurement of sensory-evoked potentials, assessing CNS neuronal circuitry, revealed delayed latencies after infection that could be reversed by lowering viral load. Cessation of treatment led to the reappearance of these abnormalities. In contrast, the decline in general motor activity induced by SIV infection was unaffected by antiviral treatment. An acute increase in the level of the chemokine monocyte chemoattractant protein-1 (MCP-1) was found in the cerebrospinal fluid (CSF) relative to plasma in the infected animals at the peak of acute viremia, likely contributing to an early influx of immune cells into the CNS. Examination of the brains of the infected animals after return of the electrophysiological abnormalities revealed diverse viral and inflammatory findings. Although some of the physiological abnormalities resulting from SIV infection can be at least temporarily reversed by lowering viral load, the viral-host interactions initiated by infection may result in long-lasting changes in CNS-mediated functions.

Simian immunodeficiency virus (SIV)-specific CTL are present in large numbers in livers of SIV-infected rhesus monkeys

The immunopathogenesis of AIDS-associated hepatitis was explored in the SIV/rhesus monkey model. The livers of SIV-infected monkeys showed a mild hepatitis, with a predominantly CD8+ T lymphocyte infiltration in the periportal fields and sinusoids. These liver-associated CD8+ T cells were comprised of a high percentage of SIV-specific CTL as defined by MHC class I/Gag peptide tetramer binding and Gag peptide epitope-specific lytic activity. There was insufficient viral replication in these livers to account for attracting this large number of functional virus-specific CTL to the liver. There was also no evidence that the predominant population of CTL were functionally end-stage cells trapped in the liver and destined to undergo apoptotic cell death in that organ. Interestingly, we noted that liver tetramer-binding cells showed an increased expression of CD62L, an adhesion molecule usually only rarely expressed on tetramer-binding cells. This observation suggests that the expression of specific adhesion molecules by CTL might facilitate the capture of these cells in the liver. These results demonstrate that functional SIV-specific CD8+ T cells are present in large numbers in the liver of chronically SIV-infected monkeys. Thus, the liver may be a trap for virus-specific cytotoxic T cells.

Antiretroviral therapy during primary immunodeficiency virus infection can induce persistent suppression of virus load and protection from heterologous challenge in rhesus macaques

A limited period of chemotherapy during primary immunodeficiency virus infection might provide a long-term clinical benefit even if treatment is initiated at a time point when virus is already detectable in plasma. To evaluate this strategy, we infected rhesus macaques with the pathogenic simian/human immunodeficiency virus RT-SHIV and treated them with the antiretroviral drug (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) for 8 weeks starting 7 or 14 days postinfection. PMPA treatment suppressed viral replication efficiently in all of the monkeys. After chemotherapy ended, virus replication rebounded and viral RNA in plasma reached levels comparable to that of the controls in four of the six monkeys. However, in the other two animals, virus loads peaked only moderately after withdrawal of the drug and then declined to low or even undetectable levels. These low levels of viremia remained stable for at least 31 weeks after cessation of therapy. At this time point, these two monkeys were challenged with SIV(8980) to evaluate whether the host responses which were able to keep RT-SHIV replication under control were also sufficient to protect against infection with a highly pathogenic heterologous virus. Both monkeys proved to be protected against the heterologous virus. In one of the two animals, low levels of SIV(8980) replication were detected. Thus, by chemotherapy during the acute phase of pathogenic virus replication, we could achieve not only persistent virus load suppression in two out of six monkeys but also protection from subsequent heterologous challenge. By this chemotherapeutic attenuation, the replication kinetics of attenuated viruses could be mimicked and a vaccination effect similar to that induced by live attenuated simian immunodeficiency virus vaccines was achieved.

Species-specific, postentry barriers to primate immunodeficiency virus infection

By using replication-defective vectors derived from human immunodeficiency virus type 1 (HIV-1), simian immunodeficiency virus (SIV(mac)), and murine leukemia virus (MuLV), all of which were pseudotyped with the vesicular stomatitis virus (VSV) G glycoprotein, the efficiency of postentry, early infection events was examined in target cells of several mammalian species. Titers of HIV-1 vectors were significantly lower than those of SIV(mac) and MuLV vectors in most cell lines and primary cells from Old World monkeys. By contrast, most New World monkey cells exhibited much lower titers for the SIV(mac) vector compared with those of the HIV-1 vector. Prosimian cells were resistant to both HIV-1 and SIV(mac) vectors, although the MuLV vector was able to infect these cells. Cells from other mammalian species were roughly equivalent in susceptibility to the three vectors, with the exception of rabbit cells, which were specifically resistant to the HIV-1 vector. The level of HIV-1 vector expression was very low in transduced cells of rodent, rabbit, cow, and pig origin. Early postentry restriction of primate immunodeficiency virus infection exhibits patterns largely coincident with species borders and applies to diverse cell types within an individual host, suggesting the involvement of species-specific, widely expressed cellular factors.

Neuropathogenesis of simian immunodeficiency virus in neonatal rhesus macaques

Neonatal human immunodeficiency virus (HIV) infection usually occurs intrapartum or postpartum and results in a higher incidence of neurological dysfunction than is seen in adults. To explore the neuropathogenesis of neonatal HIV infection, we infected neonatal macaques with simian immunodeficiency virus (SIV) and followed the course of infection focusing on early time points. Infected neonates had decreased brain growth and mild histological changes in brain that resembled those seen in pediatric AIDS, including perivascular infiltrates of mononuclear cells, mineralization of vessels in the basal ganglia, and gliosis. The perivascular lesions and gliosis were associated with the presence of occasional infected cells that required in situ hybridization with radiolabeled riboprobes for detection. Using this technique, SIV-infected cells were detected in the brain parenchyma within 7 days of infection. These findings were confirmed by nested PCR for SIVgag DNA in brain and RT-PCR for viral RNA in cerebrospinal fluid. Together, these techniques revealed SIV infection of the CNS in 12 of 13 neonates infected with SIVmac239, 3 of 3 infected with SIVmac251, and 2 of 2 infected with SIVmac239/316. The prevalence of CNS infection was indistinguishable from that of older animals infected with the same dose and stock of virus, but neonates appeared to have fewer infected cells in the CNS and detecting them required more sensitive techniques. This observation was true regardless of inoculum and despite the fact that neonates had equal or greater viral loads in the periphery compared with older animals. These data suggest that maturation-dependent host factors have a major impact on the neuropathogenesis of pediatric AIDS.

Cytotoxic T lymphocytes specific for the simian immunodeficiency virus

A non-human primate model for acquired immunodeficiency syndrome (AIDS), the simian immunodeficiency virus (SIV)-infected rhesus monkey, was used to explore the role of the AIDS virus-specific cytotoxic T-lymphocyte (CTL) response in disease pathogenesis. This CTL response was measured using the major histocompatibility complex (MHC) class I/peptide tetramer technology. Large numbers of tetramer-binding CD8+ T lymphocytes were demonstrable not only in the peripheral blood, but in lymph nodes and even in semen of chronically SIV-infected monkeys. The central role of these effector T lymphocytes in containing SIV spread during primary infection was demonstrated by showing that early SIV clearance during primary infection correlated with the emergence of the tetramer binding CD8+ T lymphocytes and that in vivo depletion of CD8+ lymphocytes eliminated the ability of the infected monkeys to contain SIV replication. These observations suggest that an effective AIDS vaccine should elicit a potent virus-specific CTL response. In fact, a live, recombinant SIV vaccine constructed using the attenuated pox virus vector modified vaccinia Ankara (MVA) elicited a high-frequency CTL response, comparable in magnitude to that elicited by SIV infection itself. This suggests that vaccine modalities such as MVA may prove useful in creating an effective human immunodeficiency virus (HIV) vaccine. These studies also indicate the power of both the SIV/macaque model and MHC class I/peptide tetramers for assessing AIDS vaccine strategies.

A nonhuman primate model for the selective elimination of CD8+ lymphocytes using a mouse-human chimeric monoclonal antibody

Nonhuman primates provide valuable animal models for human diseases. However, studies assessing the role of cell-mediated immune responses have been difficult to perform in nonhuman primates. We have shown that CD8+ lymphocyte-mediated immunity in rhesus monkeys can be selectively eliminated using the mouse-human chimeric anti-CD8 monoclonal antibody cM-T807. In vitro, this antibody completely blocked antigen-specific expansion of cytotoxic T cells and decreased major histocompatibility complex class I-restricted, antigen-specific lysis of target cells but did not mediate complement-dependent cell lysis. In vivo administration of cM-T807 in rhesus monkeys resulted in near total depletion of CD8+ T cells from the blood and lymph nodes for up to 6 weeks. This depletion was not solely complement-dependent and persisted longer in adults than in juveniles. Preservation of B cell and CD4+ T cell function in monkeys depleted of CD8+ lymphocytes was demonstrated by their ability to develop humoral immune responses to the administered chimeric monoclonal antibody. Furthermore, during CD8+ lymphocyte depletion, monkeys developed delayed-type hypersensitivity reactions comprised only of CD4+ T cells but not CD8+ T cells. This CD8+ lymphocyte depletion model should prove useful in defining the role of cell-mediated immune responses in controlling infectious diseases in nonhuman primates.

Simian immunodeficiency virus as a model of human HIV disease

Because of strong clinical, pathological, virological and immunological analogies with HIV infection of humans, infection of macaques with SIV provides a valuable model for exploring crucial issues related to both the pathogenesis and prevention of HIV infection. The model has offered a unique setting for the preclinical evaluation of drugs, vaccines and gene-therapies against HIV, and has helped to identify many virus and host determinants of lentiviral disease. For instance, the importance of an intact nef gene for efficient lentivirus replication and disease induction, and the protective ability of live attenuated, nef-deleted viruses have been first demonstrated in macaques using molecular clones of SIV. More recently, the development of chimeric HIV-SIV vectors able to establish infection and induce disease in macaques has provided new opportunities for the evaluation of vaccination strategies based upon HIV antigens. The aim of this review is to describe the natural course of SIV infection in macaques and to outline how this model has contributed to our understanding of the complex interaction between lentiviruses and host immune system.

Cytotoxic T-lymphocyte responses to cytomegalovirus in normal and simian immunodeficiency virus-infected rhesus macaques

Disseminated cytomegalovirus (CMV) infection is a frequent occurrence in human immunodeficiency virus-infected humans and in simian immunodeficiency virus (SIV)-infected rhesus macaques. Rhesus macaques are a suitable animal model with which to study in vivo interactions between CMV and AIDS-associated retroviruses. Since cytotoxic T lymphocytes (CTL) play a major role in control of viral infections, we have characterized CMV-specific CTL responses in SIV-infected and uninfected rhesus macaques. Autologous fibroblasts infected with rhesus CMV were used to stimulate freshly isolated peripheral blood mononuclear cells from CMV-seropositive animals. Following in vitro stimulation, specific CTL activity against CMV-infected autologous fibroblasts was detected in CMV-seropositive but not in CMV-seronegative normal macaques. CMV-specific CTL activity comparable to that in normal animals was also detected in two CMV-seropositive macaques infected with a live attenuated SIV strain (SIVdelta3) and in two of three macaques infected with pathogenic SIV strains. The CMV-specific CTL response was class I major histocompatibility complex restricted and mediated by CD8+ cells. An early CMV protein(s) was the dominant target recognized by bulk CTL, although the pattern of CTL recognition of CMV proteins varied among animals. Analysis of CMV-specific CTL responses in macaques should serve as a valuable model for CMV immunopathogenesis and will facilitate prospective in vivo studies of immune interactions between CMV and SIV in AIDS.

A chimeric simian/human immunodeficiency virus expressing a primary patient human immunodeficiency virus type 1 isolate env causes an AIDS-like disease after in vivo passage in rhesus monkeys

The utility of the simian immunodeficiency virus of macaques (SIVmac) model of AIDS has been limited by the genetic divergence of the envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1) and the SIVs. To develop a better AIDS animal model, we have been exploring the infection of rhesus monkeys with chimeric simian/human immunodeficiency viruses (SHIVs) composed of SIVmac239 expressing HIV-1 env and the associated auxiliary HIV-1 genes tat, vpu, and rev. SHIV-89.6, constructed with the HIV-1 env of a cytopathic, macrophage-tropic clone of a patient isolate of HIV-1 (89.6), was previously shown to replicate to a high degree in monkeys during primary infection. However, pathogenic consequences of chronic infection were not evident. We now show that after two serial in vivo passages by intravenous blood inoculation of naive rhesus monkeys, this SHIV (SHIV-89.6P) induced CD4 lymphopenia and an AIDS-like disease with wasting and opportunistic infections. Genetic and serologic evaluation indicated that the reisolated SHIV-89.6P expressed envelope glycoproteins that resembled those of HIV-1. When inoculated into naive rhesus monkeys, SHIV-89.6P caused persistent infection and CD4 lymphopenia. This chimeric virus expressing patient isolate HIV-1 envelope glycoproteins will be valuable as a challenge virus for evaluating HIV-1 envelope-based vaccines and for exploring the genetic determinants of HIV-1 pathogenicity.

vpr deletion mutant of simian immunodeficiency virus induces AIDS in rhesus monkeys

In previous experiments, animals infected with SIVmac239 containing a point mutation in the vpr and nef genes developed AIDS-like symptoms after early reversion of the vpr and nef genes. Here we show that two animals in which the nef gene but not the vpr gene had reverted in the first few months did not develop disease during a 3-year observation period even after reversion to a functional vpr gene 70 weeks postinfection. To study the influence of a stable vpr mutation on virus load and pathogenesis, a 43-bp deletion was introduced into the vpr gene of SIVmac239on, a nef-open mutant of SIVmac239. Four rhesus monkeys were inoculated with the vpr deletion mutant (SIV delta vpr), and two control animals were infected with SIVmac239on. Both control animals had persistent antigenemia, high cell-associated virus loads, and elevated neopterin levels. They had to be euthanized 20 and 30 weeks postinfection because of AIDS-related symptoms. However, all four rhesus monkeys inoculated with SIV delta vpr showed only transiently detectable antigenemia. The cell-associated virus loads were high in three of the four animals. Two animals with AIDS-like symptoms had to be euthanized 71 and 73 weeks postinfection. The two remaining monkeys infected with SIV delta vpr were still alive 105 weeks postinfection. In contrast to the SIVmac239on-infected animals, SIV delta vpr-infected animals had strong humoral immune responses and intermittent cellular immune responses to SIV antigens. Our data show that a functional vpr gene is not necessary for pathogenesis. However, vpr-deficient SIVmac239 variants might be slightly attenuated, allowing some animals to resist progression to disease for an extended period of time.

Progression to AIDS in the absence of a gene for vpr or vpx

Rhesus monkeys (Macaca mulatta) were experimentally infected with strains of simian immunodeficiency virus (SIV) derived from SIVmac239 lacking vpr, vpx, or both vpr and vpx genes. These auxiliary genes are not required for virus replication in cultured cells but are consistently conserved within the SIVmac/human immunodeficiency virus type 2/SIVsm group of primate lentiviruses. All four rhesus monkeys infected with the vpr deletion mutant showed an early spike in plasma antigenemia, maintained high virus burdens, exhibited declines in CD4+ lymphocyte concentrations, and had significant changes in lymph node morphology, and two have died to date with AIDS. The behavior of the vpr deletion mutant was indistinguishable from that of the parental, wild-type virus. Rhesus monkeys infected with the vpx deletion mutant showed lower levels of plasma antigenemia, lower virus burdens, and delayed declines in CD4+ lymphocyte concentrations but nonetheless progressed with AIDS to a terminal stage. The vpr+vpx double mutant was severely attenuated, with much lower virus burdens and no evidence of disease progression. These and other results indicate that vpr provides only a slight facilitating advantage for wild-type SIVmac replication in vivo. Thus, progression to AIDS and death can occur in the absence of a gene for vpr or vpx.