Differential cell tropism of feline immunodeficiency virus molecular clones in vivo

Independent studies have demonstrated different cell tropisms for molecular clones of feline immunodeficiency virus (FIV). In this report, we examined three clones, FIV-pF34, FIV-14, and FIV-pPPR, for replication in Crandell feline kidney (CrFK) cells, feline peripheral blood mononuclear cells (PBMC), and feline macrophage cultures. Importantly, cell tropism for these three clones was also examined in vivo. FIV-pF34 replication was efficient in CrFK cells but severely restricted in PBMC, whereas replication of FIV-pPPR was vigorous in PBMC but severely restricted in CrFK cells. FIV-14 replication was productive in both CrFK cells and PBMC. Interestingly, all three molecular clones replicated with similar efficiencies in primary feline monocyte-derived macrophages. In vivo, FIV-pF34 proved least efficient for establishing persistent infection, and proviral DNA when detectable, was localized predominately to nonlymphoid cell populations (macrophages). FIV-pPPR proved most efficient for induction of a persistent viremia in vivo, and proviral DNA was localized predominately in CD4(+) and CD8(+) lymphocyte subsets. FIV-14 inoculation of cats resulted in an infection characterized by seroconversion and localization of proviral DNA in CD4(+) lymphocytes only. Results of this study on diverse FIV molecular clones revealed that in vitro replication efficiency of an FIV isolate in PBMC directly correlated with replication efficiency in vivo, whereas proficiency for replication in macrophages in vitro was not predictive for replication potential in vivo. Also, infection of both CD4(+) and CD8(+) lymphocyte subsets was associated with higher virus load in vivo. Results of the studies on these three FIV clones, which exhibited differential cell tropism, indicated a correlation between in vitro and in vivo cell tropism and virus replication.     

Transmigration of macrophages across the choroid plexus epithelium in response to the feline immunodeficiency virus

Although lentiviruses such as human, feline and simian immunodeficiency viruses (HIV, FIV, SIV) rapidly gain access to cerebrospinal fluid (CSF), the mechanisms that control this entry are not well understood. One possibility is that the virus may be carried into the brain by immune cells that traffic across the blood–CSF barrier in the choroid plexus. Since few studies have directly examined macrophage trafficking across the blood–CSF barrier, we established transwell and explant cultures of feline choroid plexus epithelium and measured trafficking in the presence or absence of FIV. Macrophages in co-culture with the epithelium showed significant proliferation and robust trafficking that was dependent on the presence of epithelium. Macrophage migration to the apical surface of the epithelium was particularly robust in the choroid plexus explants where 3-fold increases were seen over the first 24 h. Addition of FIV to the cultures greatly increased the number of surface macrophages without influencing replication. The epithelium in the transwell cultures was also permissive to PBMC trafficking, which increased from 17 to 26% of total cells after exposure to FIV. Thus, the choroid plexus epithelium supports trafficking of both macrophages and PBMCs. FIV significantly enhanced translocation of macrophages and T cells indicating that the choroid plexus epithelium is likely to be an active site of immune cell trafficking in response to infection.     

Anti-feline immunodeficiency virus (FIV) soluble factor(s) produced from antigen-stimulated feline CD8(+) T lymphocytes suppresses FIV replication

Feline immunodeficiency virus (FIV) causes AIDS-like symptoms in infected cats. Concanavalin A (ConA)-stimulated peripheral blood mononuclear cells (PBMC) from chronically FIV strain PPR-infected cats readily expressed FIV. In contrast, when PBMC from these animals were stimulated with irradiated, autologous antigen-presenting cells (APC), at least a 10-fold drop in viral production was observed. In addition to FIV-specific cytotoxic T lymphocytes, anti-FIV activity was demonstrated in the cell-free supernatants of effector T lymphocytes stimulated with APC. The FIV-suppressive activity was induced from APC-stimulated PBMC of either FIV-infected or uninfected cats but not from ConA-stimulated PBMC. Suppression of FIV strain PPR replication was observed for both autologous and heterologous feline PBMC, was dose dependent, and demonstrated cross-reactivity and cell specificity. It was also demonstrated that the anti-FIV activity originated from CD8(+) T lymphocytes and was mediated by a noncytolytic mechanism.     

Evidence for CD8+ antiviral activity in cats infected with feline immunodeficiency virus.

Human immunodeficiency virus (HIV) causes a long, asymptomatic infection characterized by normal to elevated numbers of circulating CD8+ cells and a progressive decline in CD4+ cells. It has been speculated that HIV-specific antiviral activity driven by CD8+ T cells may control viral replication during this period and maintain the clinically asymptomatic stage of disease. The disease induced in cats by feline immunodeficiency virus (FIV) is similar to HIV in that it is characterized by a long asymptomatic stage with a progressive decline in CD4+ cells, culminating in AIDS. In the present study, we demonstrate that FIV is more readily isolated from CD8+ T-cell-depleted peripheral blood mononuclear cells (PBMC) of FIV-infected cats than from unfractionated PBMC cultures. In addition, CD8+ T cells isolated from FIV-positive cats demonstrating anti-FIV activity in PBMC cultures inhibit FIV infection of FCD4E cells in vitro. Anti-FIV activity is not found in FIV- negative cats and is not characteristic of cats acutely infected with FIV but is present in the majority of chronically infected, clinically asymptomatic and symptomatic cats. Decreases in plasma and cell-associated viremia during the acute-stage FIV infection appears to precede the appearance of CD8+ anti-FIV cells in the circulation. In summary, this study demonstrates a population(s) of CD8+ T cells in chronically FIV-infected cats capable of suppressing FIV replication in cultured PBMC. The significance of anti-FIV CD8+ cells in the immunopathogenesis of the infection and disease progression has yet to be determined.     

Decline in CD4+ cell numbers in cats with naturally acquired feline immunodeficiency virus infection.

T-cell subsets were studied by fluorescence-activated cell sorter analysis in 57 feline immunodeficiency virus (FIV)-seropositive cats with naturally acquired FIV infection to see whether CD4(+)-CD8+ alterations were comparable to those observed in human immunodeficiency virus-infected patients. CD4+ values were decreased and CD8+ values were increased. The CD4+/CD8+ ratio was reduced to 1.6, compared with 3.3 in 33 FIV-seronegative control cats. Variance analysis of data showed a significant influence of FIV seropositivity, sex, and spaying of female cats on CD4+ values. CD8+ values were significantly influenced by FIV seropositivity, age, and breed. These findings indicate a similarity between FIV and human immunodeficiency virus infections, as far as alterations of T-cell subsets are concerned.     

Feline immunodeficiency virus infection phenotypically and functionally activates immunosuppressive CD4+CD25+ T regulatory cells

Disease progression of feline immunodeficiency virus (FIV) infection is characterized by up-regulation of B7.1 and B7.2 costimulatory molecules and their ligand CTLA4 on CD4(+) and CD8(+) T cells. The CD4(+)CTLA4(+)B7(+) phenotype described in FIV(+) cats is reminiscent of CD4(+)CD25(+)CTLA4(+) cells, a phenotype described for immunosuppressive T regulatory (Treg) cells. In the present study, we describe the phenotypic and functional characteristics of CD4(+)CD25(+) T cells in PBMC and lymph nodes (LN) of FIV(+) and control cats. Similar to Treg cells, feline CD4(+)CD25(+) but not CD4(+)CD25(-) T cells directly isolated from LN of FIV(+) cats do not produce IL-2 and fail to proliferate in response to mitogen stimulation. Unstimulated CD4(+)CD25(+) T cells from FIV(+) cats significantly suppress the proliferative response and the IL-2 production of Con A-stimulated autologous CD4(+)CD25(-) T cells compared with unstimulated CD4(+)CD25(+) T cells from FIV(-) cats. Flow-cytometric analysis confirmed the apparent activation phenotype of the CD4(+)CD25(+) cells in LN of chronically FIV(+) cats, because these cells showed significant up-regulation of expression of costimulatory molecules B7.1, B7.2, and CTLA4. These FIV-activated, anergic, immunosuppressive CD25(+)CTLA4(+)B7(+)CD4(+) Treg-like cells may contribute to the progressive loss of T cell immune function that is characteristic of FIV infection.     

Altered cell tropism and cytopathicity of feline immunodeficiency viruses in two different feline CD4-positive, CD8-negative cell lines.

Two interleukin-2-dependent feline CD4-positive and CD8-negative cell lines, MYA-1 and the newly established FeL-039, were used as host cells for infection with feline immunodeficiency virus (FIV). All FIV strains used, the Petaluma strain and several new isolates, were highly cytopathic to MYA-1. In contrast, the kinetics of FIV replication in FeL-039 differed greatly depending on the strain tested, i.e., noninfectious strain, highly cytopathic strain, and less cytopathic strain producing a persistent state for a long period. It appears, therefore, that cell tropism for FIV differed with each FIV strain tested even in T-cell lines showing similar cell surface phenotypes. Cytopathicity of FIV is evidently due to both the FIV strain and the host T cell.     

Immunologic abnormalities in pathogen-free cats experimentally infected with feline immunodeficiency virus.

Blood mononuclear cells from 47 cats experimentally infected with feline immunodeficiency virus (FIV) were examined by using monoclonal antibodies directed against feline CD4 and CD8 homologs, a pan-T-cell antigen, and cell surface immunoglobulin. Significant inversion of the CD4+/CD8+ T-cell ratio was observed only in cats that were infected for 18 months or more. This inversion was associated with a decrease in the absolute numbers of CD4+ T cells and a concomitant increase in CD8+ cells. However, the total numbers of circulating T and B cells were not significantly reduced. Cats infected with FIV for 24 to 28 months also had significantly elevated levels of serum immunoglobulin G (IgG), but normal levels of IgA and IgM. The long-term decline in CD4+ T cells and hypergammaglobulinemia observed in FIV-infected cats resemble the abnormalities occurring in humans after human immunodeficiency virus infection.     

Establishment of a Feline T-Lymphoblastoid Cell Line Susceptible to Feline Immunodeficiency Viruses

A feline T-lymphoblastoid cell line susceptible to feline immunodeficiency viruses (FIV) was established and designated as Yu-1 cells. Yu-1 cells continued to grow over one year with more than 100 successive passages in the presence of human recombinant interleukin-2. Surface antigens of Yu-1 cells were feline Pan-T+, CD4+, and CD8. Susceptibility of Yu-1 cells to FIV strains were higher than that of the primary culture of the feline peripheral blood mononuclear cells, indicating that this cell line should be useful for isolation, propagation, and neutralization test of FIV.     

Ability of CD8(+) T cell anti-feline immunodeficiency virus activity correlated with peripheral CD4(+) T cell counts and plasma viremia

In the host defense mechanism against feline immunodeficiency virus (FIV) infection, CD8(+) T cells specifically attack virus-infected cells and suppress the replication of the virus in a non-cytolytic manner by secreting soluble factors. In this study, we measured CD8(+) T cell anti-FIV activity in 30 FIV-infected cats. We investigated its relationship with the number of peripheral blood lymphocytes, particularly the CD4(+) T cell and CD8(+) T cell counts, and the relationship between anti-FIV activity and the number of T cells of CD8alpha(+)beta(lo) and CD8alpha(+)beta(-) phenotypes. A clearly significant correlation was observed between anti-FIV activity and the number of CD4(+) T cells. A weaker anti-FIV activity was associated with a greater decrease in the number of CD4(+) T cells. However, there was no significant correlation between anti-FIV activity and the number of B or CD8(+) T cells. Compared with SPF cats, FIV-infected cats had significantly higher CD8alpha(+)beta(lo) T cell and CD8alpha(+)beta(-) T cell counts, but, no significant correlation was observed between these cell counts and anti-FIV activity. This anti-FIV activity significantly correlated with plasma viremia, which was detected in cats with a weak anti-FIV activity. These results suggest that the anti-FIV activity of CD8(+) T cells plays an important role in plasma viremia and the maintenance of CD4(+) T cells in the body. It is unlikely that CD8alpha(+)beta(lo) or CD8alpha(+)beta(-) T cells appearing after FIV infection represent a phenotype of CD8(+) cells with anti-FIV activity.     

Probing the interaction between feline immunodeficiency virus and CD134 by using the novel monoclonal antibody 7D6 and the CD134 (Ox40) ligand

The feline immunodeficiency virus (FIV) targets activated CD4-positive helper T cells preferentially, inducing an AIDS-like immunodeficiency in its natural host species, the domestic cat. The primary receptor for FIV is CD134, a member of the tumor necrosis factor receptor superfamily, and all primary viral strains tested to date use CD134 for infection. We examined the expression of CD134 in the cat using a novel anti-feline CD134 monoclonal antibody (MAb), 7D6, and showed that as in rats and humans, CD134 expression is restricted tightly to CD4+, and not CD8+, T cells, consistent with the selective targeting of these cells by FIV. However, FIV is also macrophage tropic, and in chronic infection the viral tropism broadens to include B cells and CD8+ T cells. Using 7D6, we revealed CD134 expression on a B220-positive (B-cell) population and on cultured macrophages but not peripheral blood monocytes. Moreover, macrophage CD134 expression and FIV infection were enhanced by activation in response to bacterial lipopolysaccharide. Consistent with CD134 expression on human and murine T cells, feline CD134 was abundant on mitogen-stimulated CD4+ T cells, with weaker expression on CD8+ T cells, concordant with the expansion of FIV into CD8+ T cells with progression of the infection. The interaction between FIV and CD134 was probed using MAb 7D6 and soluble CD134 ligand (CD134L), revealing strain-specific differences in sensitivity to both 7D6 and CD134L. Infection with isolates such as PPR and B2542 was inhibited well by both 7D6 and CD134L, suggesting a lower affinity of interaction. In contrast, GL8, CPG, and NCSU were relatively refractory to inhibition by both 7D6 and CD134L and, accordingly, may have a higher-affinity interaction with CD134, permitting infection of cells where CD134 levels are limiting.     

Antibodies against the CD44 p80, lymphocyte homing receptor molecule augment human peripheral blood T cell activation

The CD44 inhibitor Lutheran [In(Lu)]-related p80 molecule has recently been shown to be identical to the Hermes-1 lymphocyte homing receptor and to the human Pgp-1 molecule. We have determined the effect of addition of CD44 antibodies to in vitro activation assays of PBMC. CD44 antibodies did not induce PBMC proliferation alone, but markedly enhanced PBMC proliferation induced by a mitogenic CD2 antibody pair or by CD3 antibody. CD44 antibody addition had no effect upon PBMC activation induced by PHA or tetanus toxoid. CD44 antibody enhancement of CD2 antibody-induced T cell activation was specific for mature T cells as thymocytes could not be activated in the presence of combinations of CD2 and CD44 antibodies. CD44 antibody enhancement of CD2-mediated T cell triggering occurred if CD44 antibody was placed either on monocytes or on T cells. In experiments with purified monocyte and T cell suspensions, CD44 antibodies A3D8 and A1G3 augmented CD2-mediated T cell activation by three mechanisms. First, CD44 antibody binding to monocytes induced monocyte IL-1 release, second, CD44 antibodies enhanced the adhesion of T cells and monocytes in CD2 antibody-stimulated cultures, and third, CD44 antibodies augmented T cell IL-2 production in response to CD2 antibodies. Thus, ligand binding to CD44 molecules on T cells and monocytes may regulate numerous events on both cell types that are important for T cell activation. Given that recent data suggest that the CD44 molecule may bind to specific ligands on endothelial cells (vascular addressin) and within the extracellular matrix (collagen, fibronectin), these data raise the possibility that binding of T cells to endothelial cells or extracellular matrix proteins may induce or up-regulate T cell activation in inflammatory sites.     

Detection of Hepatitis B Virus (HBV) Genomes and HBV Drug Resistant Variants by Deep Sequencing Analysis of HBV Genomes in Immune Cell Subsets of HBV Mono-Infected and/or Human Immunodeficiency Virus Type-1 (HIV-1) and HBV Co-Infected Individuals

The hepatitis B virus (HBV) and the human immunodeficiency virus type 1 (HIV-1) can infect cells of the lymphatic system. It is unknown whether HIV-1 co-infection impacts infection of peripheral blood mononuclear cell (PBMC) subsets by the HBV. Aims To compare the detection of HBV genomes and HBV sequences in unsorted PBMCs and subsets (i.e., CD4+ T, CD8+ T, CD14+ monocytes, CD19+ B, CD56+ NK cells) in HBV mono-infected vs. HBV/HIV-1 co-infected individuals. Methods Total PBMC and subsets isolated from 14 HBV mono-infected (4/14 before and after anti-HBV therapy) and 6 HBV/HIV-1 co-infected individuals (5/6 consistently on dual active anti-HBV/HIV therapy) were tested for HBV genomes, including replication indicative HBV covalently closed circular (ccc)-DNA, by nested PCR/nucleic hybridization and/or quantitative PCR. In CD4+, and/or CD56+ subsets from two HBV monoinfected cases, the HBV polymerase/overlapping surface region was analyzed by next generation sequencing. Results All analyzed whole PBMC from HBV monoinfected and HBV/HIV coinfected individuals were HBV genome positive. Similarly, HBV DNA was detected in all target PBMC subsets regardless of antiviral therapy, but was absent from the CD4+ T cell subset from all HBV/HIV-1 positive cases (P<0.04). In the CD4+ and CD56+ subset of 2 HBV monoinfected cases on tenofovir therapy, mutations at residues associated with drug resistance and/or immune escape (i.e., G145R) were detected in a minor percentage of the population. Summary HBV genomes and drug resistant variants were detectable in PBMC subsets from HBV mono-infected individuals. The HBV replicates in PBMC subsets of HBV/HIV-1 patients except the CD4+ T cell subpopulation.     

Dynamics of viral and proviral loads of feline immunodeficiency virus within the feline central nervous system during the acute phase following intravenous infection

Animal models of human immunodeficiency virus 1, such as feline immunodeficiency virus (FIV), provide the opportunities to dissect the mechanisms of early interactions of the virus with the central nervous system (CNS). The aims of the present study were to evaluate viral loads within CNS, cerebrospinal fluid (CSF), ocular fluid, and the plasma of cats in the first 23 weeks after intravenous inoculation with FIV(GL8). Proviral loads were also determined within peripheral blood mononuclear cells (PBMCs) and brain tissue. In this acute phase of infection, virus entered the brain in the majority of animals. Virus distribution was initially in a random fashion, with more diffuse brain involvement as infection progressed. Virus in the CSF was predictive of brain parenchymal infection. While the peak of virus production in blood coincided with proliferation within brain, more sustained production appeared to continue in brain tissue. In contrast, proviral loads in the brain decreased to undetectable levels in the presence of a strengthening PBMC load. A final observation in this study was that there was no direct correlation between viral loads in regions of brain or ocular tissue and the presence of histopathology.     

Mapping the domains of CD134 as a functional receptor for feline immunodeficiency virus

The feline homologue of CD134 is the primary binding receptor for feline immunodeficiency virus (FIV), targeting the virus preferentially to activated CD4+ helper T cells. However, strains of FIV differ in utilization of CD134; the prototypic strain PPR requires a minimal determinant in the first cysteine-rich domain (CRD1) of feline CD134 to confer near-optimal receptor function, while strains such as GL8 require additional determinants in the CD134 CRD2. We map this determinant to a loop in CRD2 governing the interaction between the receptor and its ligand; the amino acid substitutions S78N-S79Y-K80E restored full viral receptor activity to the CDR2 of human CD134 in the context of feline CD134, with tyrosine-79 appearing to be the critical residue for restoration of receptor function.     

Differential utilization of CD134 as a functional receptor by diverse strains of feline immunodeficiency virus

The feline homologue of CD134 (fCD134) is the primary binding receptor for feline immunodeficiency virus (FIV), targeting the virus preferentially to activated CD4+ helper T cells. However, with disease progression, the cell tropism of FIV broadens such that B cells and monocytes/macrophages become significant reservoirs of proviral DNA, suggesting that receptor utilization may alter with disease progression. We examined the receptor utilization of diverse strains of FIV and found that all strains tested utilized CD134 as the primary receptor. Using chimeric feline x human CD134 receptors, the primary determinant of receptor function was mapped to the first cysteine-rich domain (CRD1) of fCD134. For the PPR and B2542 strains, the replacement of CDR1 of fCD134 (amino acids 1 to 64) with human CD134 (hCD134) alone was sufficient to confer nearly optimal receptor function. However, evidence of differential utilization of CD134 was revealed, since strains GL8, CPGammer (CPG41), TM2, 0827, and NCSU1 required determinants in the region spanning amino acids 65 to 85, indicating that these strains may require a more stringent interaction for infection to proceed.     

A role for the neuronal protein collapsin response mediator protein 2 in T lymphocyte polarization and migration

The semaphorin-signaling transducer collapsin response mediator protein 2 (CRMP2) has been identified in the nervous system where it mediates Sema3A-induced growth cone navigation. In the present study, we provide first evidence that CRMP2 is present in the immune system and plays a critical role in T lymphocyte function. CRMP2 redistribution at the uropod in polarized T cells, a structural support of lymphocyte motility, suggests that it may regulate T cell migration. This was evidenced in primary T cells by small-interfering RNA-mediated CRMP2 gene silencing and blocking Ab, as well as CRMP2 overexpression in Jurkat T cells tested in a chemokine- and semaphorin-mediated transmigration assay. Expression analysis in PBMC from healthy donors showed that CRMP2 is enhanced in cell subsets bearing the activation markers CD69+ and HLA-DR+. Heightened expression in T lymphocytes of patients suffering from neuroinflammatory disease with enhanced T cell-transmigrating activity points to a role for CRMP2 in pathogenesis. The elucidation of the signals and mechanisms that control this pathway will lead to a better understanding of T cell trafficking in physiological and pathological situations.     

HIV-1 Tat induces monocyte chemoattractant protein-1-mediated monocyte transmigration across a model of the human blood-brain barrier and up-regulates CCR5 expression on human monocytes.

AIDS dementia is characterized by neuronal loss in association with synaptic damage. A central predictor for clinical onset of these symptoms is the infiltration of monocytes and macrophages into CNS parenchyma. Chronic HIV-1 infection of monocytes also allows these cells to serve as reservoirs for persistent viral infection. Using a coculture of endothelial cells and astrocytes that models several aspects of the human blood-brain barrier, we examined the mechanism whereby the HIV-derived factor Tat may facilitate monocyte transmigration. We demonstrate that treatment of cocultures on the astrocyte side with HIV-1 Tat induced significant monocyte chemoattractant protein (MCP)-1 protein. Astrocytes, but not endothelial cells, were the source of this MCP-1 expression. Supernatants from Tat-treated cocultures induced significant monocyte transmigration, which was detected by 2.5 h after the addition of PBMC. Pretreatment of the supernatants from Tat-stimulated cocultures with an Ab to MCP-1 completely blocked monocyte transmigration. Flow cytometric analysis of Tat-stimulated PBMC demonstrated that Tat up-regulated expression of the chemokine receptor, CCR5, on monocytes in a time-dependent manner. Taken together, our data indicate that HIV-1 Tat may facilitate the recruitment of monocytes into the CNS by inducing MCP-1 expression in astrocytes. These recruited monocytes may contribute to the pathogenesis of HIV-1-associated AIDS encephalitis and dementia.     

T-lymphocyte profiles in FIV-infected wild lions and pumas reveal CD4 depletion

Feline immunodeficiency virus (FIV) is a lentivirus related to human immunodeficiency virus (HIV) that causes feline AIDS in the domestic cat (Felis catus). Serological surveys indicate that at least 25 other species of cat possess antibodies that cross-react with domestic cat FIV. Most infected nondomestic cat species are without major symptoms of disease. Long-term studies of FIV genome variation and pathogenesis reveal patterns consistent with coadaptation of virus and host in free-ranging FIV-Ple-infected African lions (Panthera leo) and FIV-Pco-infected pumas (Puma concolor) populations. This report examined correlates of immunodeficiency in wild and captive lions and pumas by quantifying CD5(+), CD4(+), and CD8(+) T-cell subsets. Free-ranging FIV-Ple-infected lions had immunofluorescence flow cytometry (IFC) profiles marked by a dramatic decline in CD4(+) subsets, a reduction of the CD4(+)/CD8(+) ratio, reduction of CD8(+)beta(high) cells, and expansion of the CD8(+)beta(low) subset relative to uninfected lions. An overall significant depletion in CD5(+) T-cells in seropositive lions was linked with a compensatory increase in total CD5(-) lymphocytes. The IFC profiles were altered significantly in 50% of the seropositive individuals examined. The FIV-Pco-infected pumas had a more generalized response of lymphopenia expressed as a significant decline in total lymphocytes, CD5(+) T-cells, and CD5(-) lymphocytes as well as a significant reduction in CD4(+) T-cells. Like lions, seropositive pumas had a significant decline in CD8(+)beta(high) cells but differed by not having compensatory expansion of CD8(+)beta(low) cells relative to controls. Results from FIV-infected lions and pumas parallel human and Asian monkey CD4(+) diminution in HIV and SIV infection, respectively, and suggest there may be unrecognized immunological consequences of FIV infection in these two species of large cats.