Pathogenic and apathogenic courses of SIV infection are associated with distinct and characteristic regulatory patterns of G1/S and G2/M cell cycle checkpoints in CD4+ T cells

Dysregulation of both the cell cycle within the CD4(+) T cells and T cell responses is characteristic for pathogenic HIV infection in humans and experimental SIV infection in rhesus macaques. However, SIV infection in sooty mangabeys does not lead to either an AIDS-like disease or such CD4(+) T cell dysregulation. A previous study has highlighted a potential role for cell cycle regulatory proteins in these distinct clinical outcomes. This study was performed to characterize the effect of SIV infection on the expression of cell cycle-related molecules in CD4(+) T cells of rhesus macaques and sooty mangabeys in attempts to define activation-induced gene expression patterns associated with disease resistance or susceptibility. First, T cell receptor (TCR)-mediated cell activation induced gene expression profiles that were unique to CD4(+) T cells from SIV-naive sooty mangabeys and rhesus macaques. More importantly, distinct and reproducible gene expression patterns were detected in CD4(+) T cells as a result of in vivo SIV infection in animals from each of the two species. In addition, SIV infection in both species showed significant differential effects on TCR activation-induced expression with a reproducible alteration of 10 genes highlighted by discordant effects on expression of Cyclin D3, Cyclin B, and RAD17. Therefore SIV infection in rhesus macaques and sooty mangabeys exhibits distinct and reproducible effects on cell cycle regulation in CD4(+) T cells during T cell activation that may be the basis for disease susceptibility vs. resistance in these two species, respectively.     

Central memory CD4 T cells are the predominant cell subset resistant to anergy in SIV disease resistant sooty mangabeys

OBJECTIVE: HIV infection in humans and experimental SIV infection in rhesus macaques leads to the loss of recall antigen responses, immunological anergy in CD4 T cells and AIDS. In contrast, natural infection in sooty mangabeys with SIV does not lead to disease despite high viral loads accompanied by resistance of their CD4 T cells to undergo immunological anergy. The objective of the study was to further elucidate the mechanisms that contribute to anergy resistance in CD4 T cells from sooty mangabeys and identify whether the anergy resistance is a function of a specific subset or the entire cell population. MATERIALS AND METHODS: Susceptibility or resistance to anergy was analyzed in antigen specific and defined CD4 T cell subsets from peripheral blood of sooty mangabeys and rhesus macaques by using an in vitro anergy inducing protocol; expression of the anergy associated gene GRAIL was measured. RESULTS: Antigen specific CD4 T cells from SIV disease resistant sooty mangabey are relatively resistant to the development of anergy. This resistance is associated with a lack of an upregulation of GRAIL. Conversely, rhesus macaque CD4 T cells are sensitive to anergy induction associated with upregulation of GRAIL. Furthermore the anergy resistant phenotype of sooty mangabey CD4 T cells predominantly resides in central memory cells defined either as CD4+CD45RA-CD62L+ or CD4+CD28+CD95+CCR7+. CONCLUSIONS: The maintenance of recall responses in sooty mangabeys is associated with the resistance of central memory CD4 T cells to the induction of anergy which may represent an important mechanism underlying SIV disease resistance in this species.     

Paucity of CD4+CCR5+ T cells is a typical feature of natural SIV hosts

In contrast to lentiviral infections of humans and macaques, simian immunodeficiency virus (SIV) infection of natural hosts is nonpathogenic despite high levels of viral replication. However, the mechanisms underlying this absence of disease are unknown. Here we report that natural hosts for SIV infection express remarkably low levels of CCR5 on CD4+ T cells isolated from blood, lymph nodes, and mucosal tissues. Given that this immunologic feature is found in 5 different species of natural SIV hosts (sooty mangabeys, African green monkeys, mandrills, sun-tailed monkeys, and chimpanzees) but is absent in 5 nonnatural/recent hosts (humans, rhesus, pigtail, cynomolgus macaques, and baboons), it may represent a key feature of the coevolution between the virus and its natural hosts that led to a nonpathogenic infection. Beneficial effects of low CCR5 expression on CD4+ T cells may include the reduction of target cells for viral replication and a decreased homing of activated CD4+ T cells to inflamed tissue.     

Bone marrow-based homeostatic proliferation of mature T cells in nonhuman primates: implications for AIDS pathogenesis

Bone marrow (BM) is the key hematopoietic organ in mammals and is involved in the homeostatic proliferation of memory CD8(+) T cells. Here we expanded on our previous observation that BM is a preferential site for T-cell proliferation in simian immunodeficiency virus (SIV)-infected sooty mangabeys (SMs) that do not progress to AIDS despite high viremia. We found high levels of mature T-cell proliferation, involving both naive and memory cells, in healthy SMs and rhesus macaques (RMs). In addition, we observed in both species that lineage-specific, BM-based T-cell proliferation follows antibody-mediated in vivo CD4(+) or CD8(+) T-cell depletion, thus indicating a role for the BM in maintaining T-cell homeostasis under depleting circumstances. We also observed that, in SIV-infected SMs, but not RMs, the level of proliferation of BM-based CD4(+) T cells is higher than that of circulating CD4(+) T cells. Interestingly, limited BM-based CD4(+) T-cell proliferation was found in SIV-infected SMs with low CD4(+) T-cell counts, suggesting a regenerative failure in these animals. Collectively, these results indicate that BM is involved in maintaining T-cell homeostasis in primates and suggest a role for BM-based CD4(+) T-cell proliferation in determining the benign nature of natural SIV infection of SMs.     

Differential Th17 CD4 T-cell depletion in pathogenic and nonpathogenic lentiviral infections

Acute HIV infection is characterized by massive loss of CD4 T cells from the gastrointestinal (GI) tract. Th17 cells are critical in the defense against microbes, particularly at mucosal surfaces. Here we analyzed Th17 cells in the blood, GI tract, and broncheoalveolar lavage of HIV-infected and uninfected humans, and SIV-infected and uninfected sooty mangabeys. We found that (1) human Th17 cells are specific for extracellular bacterial and fungal antigens, but not common viral antigens; (2) Th17 cells are infected by HIV in vivo, but not preferentially so; (3) CD4 T cells in blood of HIV-infected patients are skewed away from a Th17 phenotype toward a Th1 phenotype with cellular maturation; (4) there is significant loss of Th17 cells in the GI tract of HIV-infected patients; (5) Th17 cells are not preferentially lost from the broncheoalveolar lavage of HIV-infected patients; and (6) SIV-infected sooty mangabeys maintain healthy frequencies of Th17 cells in the blood and GI tract. These observations further elucidate the immunodeficiency of HIV disease and may provide a mechanistic basis for the mucosal barrier breakdown that characterizes HIV infection. Finally, these data may help account for the nonprogressive nature of nonpathogenic SIV infection in sooty mangabeys.     

Treatment of SIV-infected sooty mangabeys with a type-I IFN agonist results in decreased virus replication without inducing hyperimmune activation

A key feature differentiating nonpathogenic SIV infection of sooty mangabeys (SMs) from pathogenic HIV/SIV infections is the rapid resolution of type I IFN (IFN-I) responses and IFN-stimulated gene expression during the acute-to-chronic phase transition and the establishment of an immune quiescent state that persists throughout the chronic infection. We hypothesized that low levels of IFN-I signaling may help to prevent chronic immune activation and disease progression in SIV-infected SMs. To assess the effects of IFN-I signaling in this setting, in the present study, we administered recombinant rhesus macaque IFNα2-IgFc (rmIFNα2) to 8 naturally SIV-infected SMs weekly for 16 weeks. Gene-expression profiling revealed a strong up-regulation of IFN-stimulated genes in the blood of treated animals, confirming the reagent's bioactivity. Interestingly, we observed an approximately 1-log decrease in viral load that persisted through day 35 of treatment. Flow cytometric analysis of lymphocytes in the blood, lymph nodes, and rectal biopsies did not reveal a significant decline of CD4(+) T cells, a robust increase in lymphocyte activation, or change in the level of SIV-specific CD8(+) T cells. The results of the present study indicate that administration of type I IFNs in SIV-infected SMs induces a significant anti-viral effect that is not associated with a detectable increase in chronic immune activation.     

Requirements for simian immunodeficiency virus antigen-specific in vitro proliferation of T cells from infected rhesus macaques and sooty mangabeys

The measurement of cell-mediated immunity against the etiologic agent of human AIDS (HIV) in the non-human primate model of AIDS (simian immunodeficiency virus, SIV) has been difficult. In general, culture of peripheral blood mononuclear cells from HIV-1- and SIV-infected humans and monkeys, respectively, with purified inactivated HIV and SIV virus preparations has given inconsistent or negative proliferative responses. However, we describe herein an assay which consists of coculturing monocytes that have been pulsed with inactivated SIVsmm with nylon-wool-purified autologous T cells, leading to antigen-specific T-cell proliferation. The proliferative response, which predominantly occurs in CD4+ T cells, is major histocompatibility complex (MHC) class II-restricted and requires antigen processing. This assay will greatly facilitate the identification of the immunodominant epitopes recognized by T cells in sooty mangabeys, which are naturally infected but remain clinically asymptomatic, and in rhesus macaques, in which experimental infection leads to clinical symptomatology similar to human AIDS, eventually resulting in death.     

Vaginal CD4+ T cells express high levels of CCR5 and are rapidly depleted in simian immunodeficiency virus infection

Worldwide, the majority of human immunodeficiency virus-1 cases occur through heterosexual transmission, yet little is known regarding the phenotype of CD4(+) T cells in the vaginal mucosa. In the present study, lymphocytes were compared from the lymph nodes, blood, and vagina from uninfected and simian immunodeficiency virus (SIV)-infected macaques. In mature female macaques, 54%-67% of the vaginal CD4(+) T cells expressed C chemokine receptor 5 (CCR5), whereas 84%-99% coexpressed CXC chemokine receptor 4. In contrast, only 4.4%-14.8% of peripheral blood and 2.4%-13% of lymph-node CD4(+) T cells coexpressed CCR5. Moreover, CCR5 mean channel fluorescence was significantly higher on CD4 cells from the vagina, compared with those from blood. In macaques intravenously infected with SIV, rapid depletion of CD4(+) T cells was observed in the vagina, particularly among the CCR5(+)CD4(+) subset. This demonstrates that large numbers of CD4(+) T cells expressing high levels of CCR5 reside within the vagina and that these cells are preferentially targeted for elimination by SIV infection.     

Experimental Pneumocystis carinii pneumonia in simian immunodeficiency virus-infected rhesus macaques

To establish experimental Pneumocystis carinii infection in simian immunodeficiency virus (SIV)-infected macaques as a model of acquired immunodeficiency syndrome (AIDS)-associated P. carinii pneumonia (PCP), SIV-infected macaques were inoculated intrabronchially with macaque-derived P. carinii, and P. carinii-specific polymerase chain reaction (PCR) and flow cytometric analysis of bronchoalveolar lavage fluid were done biweekly for up to 44 weeks after inoculation. All inoculated animals had a P. carinii-specific PCR product after infection. CD8(+) T cells in lung lavage samples from SIV- and P. carinii-coinfected animals increased to >90% of total CD3(+) cells, a pattern associated with naturally acquired P. carinii infection. Progression of disease also was correlated with increased neutrophil infiltration to the lungs. The animals had a protracted period of asymptomatic colonization with P. carinii before progression to PCP. The development of a model of PCP in SIV-infected rhesus macaques provides the means to study AIDS-associated PCP.     

Plasmacytoid dendritic cells are recruited to the colorectum and contribute to immune activation during pathogenic SIV infection in rhesus macaques

In SIV/HIV infection, the gastrointestinal tissue dominates as an important site because of the impact of massive mucosal CD4 depletion and immune activation-induced tissue pathology. Unlike AIDS-susceptible rhesus macaques, natural hosts do not progress to AIDS and resolve immune activation earlier. Here, we examine the role of dendritic cells (DCs) in mediating immune activation and disease progression. We demonstrate that plasmacytoid DCs (pDCs) in the blood up-regulate β7-integrin and are rapidly recruited to the colorectum after a pathogenic SIV infection in rhesus macaques. These pDCs were capable of producing proinflammatory cytokines and primed a T cytotoxic 1 response in vitro. Consistent with the up-regulation of β7-integrin on pDCs, in vivo blockade of α4β7-integrin dampened pDC recruitment to the colorectum and resulted in reduced immune activation. The up-regulation of β7-integrin expression on pDCs in the blood also was observed in HIV-infected humans but not in chronically SIV-infected sooty mangabeys that show low levels of immune activation. Our results uncover a new mechanism by which pDCs influence immune activation in colorectal tissue after pathogenic immunodeficiency virus infections.     

Elevated interleukin-7 levels not sufficient to maintain T-cell homeostasis during simian immunodeficiency virus-induced disease progression

Elevated levels of interleukin 7 (IL-7) have been correlated with various T-cell depletion conditions, including HIV infection, and suggested as an indicator of HIV disease progression (AIDS and death). Here, the assessment of pathogenic simian immunodeficiency virus (SIVmac239) infection in rhesus macaques demonstrated a clear association between a significant elevation in IL-7 levels and disease progression. In 5 macaques that progressed to simian AIDS and death, elevated IL-7 levels were unable to restore T-cell homeostasis. In contrast, increased IL-7 levels were followed by relatively high and stable T-cell numbers in the SIV-infected macaques with a slow-progressing phenotype. Further, studies in sooty mangabeys that do not progress to simian AIDS and that maintain stable T-cell numbers despite high levels of viral replication support the importance of IL-7 and T-cell homeostasis in disease progression. These data suggest that during pathogenic SIV infection with high viral replication, elevated IL-7 levels are unable to recover T-cell homeostasis, thereby leading to disease progression. The utility of IL-7 as a potential immunotherapeutic agent to improve HIV/SIV-related T-cell depletion may therefore depend on controlling the pathogenic effects of viral replication prior to the administration of IL-7.     

Rapid mucosal CD4(+) T-cell depletion and enteropathy in simian immunodeficiency virus-infected rhesus macaques

BACKGROUND & AIMS: Human immunodeficiency virus (HIV) infection leads to severe immunologic and functional disturbances in the intestinal tract in late stages of the disease. Information on mucosal pathology directly after infection is limited. We characterized this early phase in rhesus macaques infected with simian immunodeficiency virus (SIV). Methods: Eight rhesus macaques were infected with SIV. Upper endoscopy was performed at defined times before and after infection. Viral load, percentage of CD4(+) T cells, villus height, crypt depth, and Ki-67-positive crypt cells were analyzed in duodenal biopsy specimens. Serum beta-carotene and vitamin D levels were assessed. RESULTS: A rapid increase of duodenal SIV core protein (p27) concentration and an almost complete loss of intestinal CD4(+) T cells was found within 2 weeks after infection. A decrease of villus height was observed, and the percentage of Ki-67-positive (proliferating) crypt cells increased. Serum concentrations of vitamin D decreased in 6 of 8 animals, and beta-carotene concentrations decreased in 3 of 8 animals after infection. CONCLUSIONS: Mucosal SIV replication and intestinal CD4(+) T cell depletion are early events in SIV-infected rhesus macaques. The structural changes of the mucosa strongly support the concept of HIV/SIV-induced enteropathy. In contrast to late-stage human HIV infection, early small intestinal villous atrophy in SIV infection is associated with crypt hyperplasia.     

Ethanol Suppression of the Functional State of Polymorphonuclear Leukocytes Obtained From Uninfected and Simian Immunodeficiency Virus Infected Rhesus Macaques

BACKGROUND: Human immunodeficiency virus (HIV) infection and alcohol abuse frequently coexist in the host and are known to suppress individually the host response to a variety of opportunistic infections. METHODS: This study examined the effects of in vitro ethanol exposure on several functions of polymorphonuclear leukocytes (PMNs) that were obtained from uninfected and simian immunodeficiency virus (SIV)-infected rhesus macaques, at the asymptomatic and terminal stages of infection. RESULTS: The PMNs obtained from rhesus macaques at both the asymptomatic and terminal stage of SIV disease had elevated phagocytic activity and increased CD11b expression compared with PMNs from uninfected animals. In vitro 100 mM ethanol suppressed phagocytosis and CD11b adhesion molecule expression by PMNs, regardless of the stage of SIV infection. Treatment of PMNs with granulocyte colony-stimulating factor (G-CSF) attenuated the inhibitory effect seen with prior ethanol exposure. CONCLUSIONS: These data demonstrate that the functional state of PMNs from uninfected as well as SIV-infected rhesus macaques is impaired by direct exposure to intoxicating concentrations of ethanol and that this effect can be attenuated by G-CSF. If alcohol intoxication similarly suppressed PMN function in vivo, it would further increase susceptibility of these hosts to secondary infections. Furthermore, G-CSF may be useful in overcoming the suppressive effects of ethanol on PMN function in such patients.     

The AIDS resistance of naturally SIV-infected sooty mangabeys is independent of cellular immunity to the virus

In contrast to human immunodeficiency virus (HIV)-infected humans, natural hosts for simian immunodeficiency virus (SIV) very rarely progress to acquired immunodeficiency syndrome (AIDS). While the mechanisms underlying this disease resistance are still poorly understood, a consistent feature of natural SIV infection is the absence of the generalized immune activation associated with HIV infection. To investigate the immunologic mechanisms underlying the absence of AIDS in SIV-infected sooty mangabeys (SMs), a natural host species, we performed a detailed analysis of the SIV-specific cellular immune responses in 110 SIV-infected SMs. We found that while SIV-specific T-cell responses are detectable in the majority of animals, their magnitude and breadth are, in fact, lower than what has been described in HIV-infected humans, both in terms of cytokine production (ie, IFN-gamma, TNF-alpha, and IL-2) and degranulation (ie, CD107a expression). Of importance, SIV-specific T-cell responses were similarly low when either SIVmac239-derived peptides or autologous SIVsmm peptides were used as stimuli. No correlation was found between SIV-specific T-cell responses and either viral load or CD4+ T-cell count, or between these responses and markers of T-cell activation and proliferation. These findings indicate that the absence of AIDS in naturally SIV-infected sooty mangabeys is independent of a strong cellular immune response to the virus.     

Simian immunodeficiency virus-specific T-cell-mediated proliferative response of infected rhesus macaques

While cell-mediated immunity is known to play an important role in controlling viral infections, its role in human and experimental animal models of human AIDS has not been established. To address this issue, four juvenile rhesus macaques were infected with simian immunodeficiency virus SIVMAC. Freshly isolated peripheral blood mononuclear cells from these SIVMAC-infected macaques and four uninfected control macaques were assessed for T-cell proliferative activity to SIV, monthly, for 10 consecutive months. T cells from SIV-infected monkeys failed to proliferate in response to SIV added directly to the culture. However, when SIV was processed by autologous antigen-presenting cells prior to culture with purified T cells, proliferative responses were uniformly demonstrated in SIV-infected monkeys, but not in uninfected controls. Proliferation in response to heat-inactivated SIV was mediated by CD4+ T cells and was shown to be MHC class II-restricted. However, the proliferative response to infectious SIV was mediated by both CD4+ and CD8+ T cells and was MHC class-restricted. As disease progressed, a decline in the T-cell proliferative response was observed.     

Timely triggering of homeostatic mechanisms involved in the regulation of T-cell levels in SIVsm-infected sooty mangabeys

Sooty mangabeys, the natural host of simian immunodeficiency virus (SIVsm), generally avoid progressive depletion of CD4+ T cells and opportunistic infections associated with infection of humans (HIV) and macaques (SIVmac). The means by which the SIVsm-infected mangabeys maintain CD4+ T-cell levels despite high rates of viral replication is unknown. One cytokine that has a key role in the regulation of T-cell levels is interleukin-7 (IL-7). Here, the longitudinal assessment of 6 SIVsm-infected mangabeys identified an early increase in plasma IL-7 levels at weeks 1 to 5 after infection. This IL-7 increase correlated with an early decline in CD4+ T-cell levels (decline of 492-1171 cells/microL) accompanying acute viremia. Elevated IL-7 levels were followed by increased T-cell proliferation (Ki67) and maintenance of lower but stable (more than 500 cells/microL) CD4+ T-cell levels in each mangabey through 37 weeks of infection. These data contrast with our earlier studies in SIVmac-infected macaques, in which the IL-7 increase was delayed until 20 to 40 weeks after infection, just before the onset of simian AIDS. Taken together, these data suggest that timely triggering of IL-7 is important for stabilizing healthy T-cell levels in mangabeys and that timely administration of exogenous IL-7 may show benefit during pathogenic SIVmac and HIV infection.     

Systemic and Intestinal Viral Reservoirs in CD4+ T Cell Subsets in Primary SIV Infection

The HIV reservoir size in target CD4+ T cells during primary infection remains unknown. Here, we sorted peripheral and intestinal CD4+ T cells and quantified the levels of cell-associated SIV RNA and DNA in rhesus macaques within days of SIVmac251 inoculation. As a major target cell of HIV/SIV, CD4+ T cells in both tissues contained a large amount of SIV RNA and DNA at day 8–13 post-SIV infection, in which productive SIV RNA highly correlated with the levels of cell-associated SIV DNA. Memory CD4+ T cells had much higher viral RNA and DNA than naïve subsets, yet memory CD4+ T cells co-expressing CCR5 had no significant reservoir size compared with those that were CCR5-negative in blood and intestine. Collectively, memory CD4+ T cells appear to be the major targets for primary infection, and viral reservoirs are equally distributed in systemic and lymphoid compartments in acutely SIV-infected macaques.     

Simian immunodeficiency virus-induced lymphatic tissue fibrosis is mediated by transforming growth factor beta 1-positive regulatory T cells and begins in early infection

In human immunodeficiency virus (HIV) infection, collagen deposition and fibrosis within the T cell zone disrupt the lymphatic tissue architecture, contributing to depletion of CD4(+) T cells and limiting immune reconstitution. We used relevant animal and in vitro models to investigate the kinetics and possible underlying mechanism(s) of this process. In the lymphatic tissue of simian immunodeficiency virus (SIV)-infected rhesus macaques, we observed parallel increases in immune activation, transforming growth factor (TGF) beta 1-positive regulatory T (T(reg)) cells, and collagen type I deposition by 7 days after inoculation, consistent with the hypothesis that early immune activation elicits a countering T(reg) cell response associated with TGF beta 1 expression and collagen deposition. In support of this hypothesis and the possible role of fibrosis in viral pathogenesis, we show (1) spatial colocalization and temporal concordance in levels of TGF beta 1(+) T(reg) cells and collagen deposition; (2) TGF beta 1(+) inducible T(reg) cell stimulation of primary lymphatic tissue fibroblasts to produce collagen type I in vitro; and (3) high levels of immune activation, TGF beta 1(+) T(reg) cells, and collagen deposition in pathogenic SIV infection of macaques, in contrast to apathogenic SIV infection in sooty mangabeys in which levels of immune activation, TGF beta 1(+) T(reg) cells, and collagen deposition were low. We thus conclude that the response of TGF beta 1(+) T(reg) cells to immune activation in early SIV/HIV infection is a double-edged sword: TGF beta 1(+) T(reg) cells normally have a positive effect by limiting immunopathological and autoreactive immune responses, but they also have a negative effect by dampening the antiviral immune response and, as we show here, causing deleterious effects on CD4(+) T cell homeostasis by inducing collagen deposition in lymphatic tissues.     

Alterations in T lymphocyte profiles of bronchoalveolar lavage fluid from SIV- and Pneumocystis carinii-coinfected rhesus macaques

The goal of this study was to examine SIV- and Pneumocystis carinii-coinfected rhesus macaques as a model of P. carinii infection in HIV-seropositive humans. The influence of P. carinii infection on the cellular composition of bronchoalveolar lavage (BAL) fluid from SIV-infected and normal rhesus macaques was examined by flow cytometric analysis and polymerase chain reaction (PCR). BAL fluid from SIV- and P. carinii coinfected macaques showed a substantial T lymphocyte influx composed of more than 90% CD8+ T cells. These results are in contrast to BAL fluid from SIV-infected macaques with no detectable P. carinii-specific PCR product, where CD4+ T cells were present in significant numbers and the CD8+ T cell population was less than 70% of total CD3+ lymphocytes. We observed no significant differences in peripheral blood CD4+ or CD8+ T cell levels in the SIV-infected animals, regardless of P. carinii status, indicating that the CD8+ T cell infiltration in the lungs of the P. carinii-positive animals was likely the result of P. carinii infection. These results demonstrate that although peripheral blood CD4+ T cell levels are predictive of susceptibility to P. carinii infection in this model, the levels are not reflective of the T cell profile in the lung during SIV and P. carinii coinfection. The SIV- and P. carinii-coinfected macaques showed a spectrum of lung disease severity that was histologically similar to human P. carinii pneumonia (PCP). Interestingly, even mild P. carinii infection was sufficient to alter the normal CD4+/CD8+ T cell profiles in the lungs of SIV-infected rhesus macaques. These results are similar to immunologic findings in human AIDS-associated PCP and support the usefulness of this model in the study of immune responses to P. carinii.     

Cryptosporidiosis in rhesus macaques challenged during acute and chronic phases of SIV infection

The intestinal immune dysfunction due to loss of mucosal and peripheral CD4(+) T cells in individuals with HIV/AIDS is presumably responsible for the establishment of persistent cryptosporidiosis. Simian immunodeficiency virus (SIV)-infected macaques were used to investigate the phase/timing in SIV infection, which permits a self-limiting Cryptosporidium parvum infection to become persistent in immunodeficient hosts because of significant mucosal immune defects. Two groups of SIV-infected macaques were challenged with C. parvum; one was challenged during the acute SIV infection phase (2 weeks post-SIV infection) and the second was challenged during the chronic SIV phase (CD4 counts 200-500 cells/μl of blood). Samples (fecal, blood, biopsy, and necropsy) were collected at different time points after infection to correlate the progression of disease with the immune status of the animals. All seven SIV-infected macaques challenged during the acute phase of SIV infection became persistently infected and excreted oocysts for 1-4 months. However, four of the six in the chronic SIV phase became infected with cryptosporidiosis, of which one survived 2 weeks and one became naturally infected. Sequential analysis of CD4(+) in blood and intestines of coinfected macaques exhibited pronounced losses of CD4 T cells during the first 2 weeks after SIV infection, followed by transient rebound of CD4 T cells in the gut after C. parvum infection, and then a gradual loss over subsequent months. Persistent cryptosporidiosis was more consistently induced during the acute SIV phase indicating that profound viral damage to gut lymphoid tissue during the acute phase was more conducive, compared with the chronic phase, to establishing persistent cryptosporidiosis than low circulating CD4 T cells.