Primary simian immunodeficiency virus SIVmnd-2 infection in mandrills (Mandrillus sphinx)

Mandrills are the only nonhuman primate (NHP) naturally infected by two types of simian immunodeficiency virus (SIV): SIVmnd-1 and SIVmnd-2. We have already reported that the high SIVmnd-1 replication during primary infection contrasts with only transient changes in CD4+ and CD8+ cell counts. Since early virus-host interactions predict viral control and disease progression in human immunodeficiency virus-infected patients, we investigated the dynamics of SIVmnd-2 primary infection in mandrills to examine the impact on immune effectors in blood and lymph nodes (LNs). To avoid in vitro strain selection, all mandrills in this study received plasma from SIVmnd-2-infected mandrills. SIVmnd-2 plasma viremia peaked at 10(7) to 10(8) RNA copies/ml between days 7 and 10. This peak was followed in all four monkeys by a decline in virus replication, with a set point level of 10(5) to 10(6) RNA copies/ml at day 42 postinfection (p.i.). Viral DNA load in PBMC and LNs also peaked between days 7 and 10 (10(5) to 10(6) DNA copies/10(6) cells) and stabilized at 10(3) to 10(4) DNA copies/10(6) cells during the chronic phase. Anti-SIVmnd-2 antibodies were detected starting from days 28 to 32. A transitory decline of CD3+ CD4+ cells in the LNs occurred in animals with high peak VLs. CD4+ and CD8+ T-cell activation in blood and LNs was noted between days 5 and 17 p.i., surrounding the peak of viral replication. This was most significant in the LNs. Activation markers then returned to preinfection values despite continuous and active viral replication during the chronic infection. The dynamics of SIVmnd-2 infection in mandrills showed a pattern similar to that of SIVmnd-1 infection. This might be a general feature of nonpathogenic SIV natural African NHP models.     

Role of CD8(+) lymphocytes in control of simian immunodeficiency virus infection and resistance to rechallenge after transient early antiretroviral treatment

Transient antiretroviral treatment with tenofovir, (R)-9-(2-phosphonylmethoxypropyl)adenine, begun shortly after inoculation of rhesus macaques with the highly pathogenic simian immunodeficiency virus (SIV) isolate SIVsmE660, facilitated the development of SIV-specific lymphoproliferative responses and sustained effective control of the infection following drug discontinuation. Animals that controlled plasma viremia following transient postinoculation treatment showed substantial resistance to subsequent intravenous rechallenge with homologous (SIVsmE660) and highly heterologous (SIVmac239) SIV isolates, up to more than 1 year later, despite the absence of measurable neutralizing antibody. In some instances, resistance to rechallenge was observed despite the absence of detectable SIV-specific binding antibody and in the face of SIV lymphoproliferative responses that were low or undetectable at the time of challenge. In vivo monoclonal antibody depletion experiments demonstrated a critical role for CD8(+) lymphocytes in the control of viral replication; plasma viremia rose by as much as five log units after depletion of CD8(+) cells and returned to predepletion levels (as low as <100 copy Eq/ml) as circulating CD8(+) cells were restored. The extent of host control of replication of highly pathogenic SIV strains and the level of resistance to heterologous rechallenge achieved following transient postinoculation treatment compared favorably to the results seen after SIVsmE660 and SIVmac239 challenge with many vaccine strategies. This impressive control of viral replication was observed despite comparatively modest measured immune responses, less than those often achieved with vaccination regimens. The results help establish the underlying feasibility of efforts to develop vaccines for the prevention of AIDS, although the exact nature of the protective host responses involved remains to be elucidated.     

Distinct cycling CD4(+)- and CD8(+)-T-cell profiles during the asymptomatic phase of simian immunodeficiency virus SIVmac251 infection in rhesus macaques

Elevated CD4 T-cell turnover may lead to the exhaustion of the immune system during human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) infections. However, this hypothesis remains controversial. Most studies of this subject have concerned the blood, and information about the lymph nodes is rare and controversial. We used Ki67 expression to measure cycling T cells in the blood and lymph nodes of uninfected macaques and of macaques infected with a pathogenic SIVmac251 strain or with a nonpathogenic SIVmac251Deltanef clone. During the asymptomatic phase of infection, the number of cycling CD8(+) T cells progressively increased (two- to eightfold) both in the blood and in the lymph nodes of macaques infected with SIVmac251. This increase was correlated with viral replication and the progression to AIDS. In contrast, no increases in the numbers of cycling CD4(+) T cells were found in the blood or lymph nodes of macaques infected with the pathogenic SIVmac251 strain in comparison with SIVmac251Deltanef-infected or healthy macaques during this chronic phase. However, the lymph nodes of pre-AIDS stage SIVmac251-infected macaques contained more cycling CD4(+) T cells (low baseline CD4(+)-T-cell counts in the blood). Taken together, these results show that the profiles of CD4(+)- and CD8(+)-T-cell dynamics are distinct both in the lymph nodes and blood and suggest that higher CD4(+)-T-cell proliferation at the onset of AIDS may lead to the exhaustion of the immune system.     

CD8+ T cell dynamics during primary simian immunodeficiency virus infection in macaques: relationship of effector cell differentiation with the extent of viral replication

Immunological and virological events that occur during the earliest stages of HIV-1 infection are now considered to have a major impact on subsequent disease progression. We observed changes in the frequencies of CD8(bright) T cells expressing different chemokine receptors in the peripheral blood and lymph nodes of rhesus macaques during the acute phase of the pathogenic SIVmac251 infection; the frequency of CD8(bright) T cells expressing CXCR4 decreased, while the frequency of those expressing CCR5 increased. These reciprocal changes in chemokine receptor expression were associated with changes in the proportion of cycling (Ki67(+)) CD8(bright) T cells, and with the pattern of CD8(bright) T cell differentiation as defined by expression of CCR7 and CD45RA. In contrast, during the primary phase of the attenuated SIVmac251Deltanef infection, no major change was observed. Whereas during the acute phase of the infection with pathogenic SIV (2 wk postinfection) no correlate of disease protection was identified, once the viral load set points were established (2 mo postinfection), we found that the levels of cycling and of CCR5- and CXCR4-positive CD8(bright) T cells were correlated with the extent of viral replication and therefore with SIV-infection outcome. Our data reveal that, during primary SIV infection, despite intense CD8 T cell activation and an increase in CCR5 expression, which are considered as essential for optimal effector function of CD8(+) T cells, these changes are associated with a poor prognosis for disease progression to AIDS.     

High levels of human immunodeficiency virus infection of CD8 lymphocytes expressing CD4 in vivo

Human immunodeficiency virus (HIV)-infected CD8 lymphocytes have been reported in vivo, but the mechanism of infection remains unclear. Experiments using the thy/hu mouse model support export of intrathymically infected CD8 precursors, while recent in vitro data suggest that mature CD8 lymphocytes upregulate CD4 upon activation (generating a CD8bright CD4dim phenotype) and are susceptible to HIV infection. To determine whether these mechanisms operate in vivo and to assess their relative importance in the generation of circulating HIV-infected CD8 lymphocytes, we quantified HIV long terminal repeat (LTR) DNA in CD8+ CD4- and CD8bright CD4dim lymphocytes isolated from HIV-infected individuals by fluorescence-activated cell sorting. HIV infection of CD8 lymphocytes was demonstrated in 17 of 19 subjects, with a significant inverse relationship between level of infection and CD4 lymphocyte count (R = -0.73; P < 0.001). The level of HIV infection of CD8bright CD4dim lymphocytes was significantly higher (median, 1,730 HIV LTR copies/10(6) cells; n = 9) than that of CD8+ CD4- lymphocytes (undetectable in seven of nine individuals; P < 0.01) and approached that of CD4 lymphocytes from the same individuals (median, 3,660 HIV LTR copies/10(6) cells). CD8bright CD4dim lymphocytes represented 0.8 to 3.3% of total CD8 lymphocytes and were most prevalent in the memory subset. Thus, HIV-infected CD8 lymphocytes commonly circulate in HIV-infected individuals and are generated through infection of activated CD8 lymphocytes rather than through export of intrathymically infected precursors. The high level of infection of CD8bright CD4dim lymphocytes could have a direct role in the decline in CD8 lymphocyte function that accompanies HIV disease progression.     

Conditional CD8+ T cell escape during acute simian immunodeficiency virus infection

CD8⁺ T cell responses rapidly select viral variants during acute human immunodeficiency virus (HIV)/simian immunodeficiency virus (SIV) infection. We used pyrosequencing to examine variation within three SIV-derived epitopes (Gag_386-394GW9, Nef_103-111RM9, and Rev_59-68SP10) targeted by immunodominant CD8⁺ T cell responses in acutely infected Mauritian cynomolgus macaques. In animals recognizing all three epitopes, variation within Rev_59-68SP10 was associated with delayed accumulation of variants in Gag_386-394GW9 but had no effect on variation within Nef_103-111RM9. This demonstrates that the entire T cell repertoire, rather than a single T cell population, influences the timing of immune escape, thereby providing the first example of conditional CD8⁺ T cell escape in HIV/SIV infection.     

Highly activated CD8(+) T cells in the brain correlate with early central nervous system dysfunction in simian immunodeficiency virus infection.

One of the consequences of HIV infection is damage to the CNS. To characterize the virologic, immunologic, and functional factors involved in HIV-induced CNS disease, we analyzed the viral loads and T cell infiltrates in the brains of SIV-infected rhesus monkeys whose CNS function (sensory evoked potential) was impaired. Following infection, CNS evoked potentials were abnormal, indicating early CNS disease. Upon autopsy at 11 wk post-SIV inoculation, the brains of infected animals contained over 5-fold more CD8(+) T cells than did uninfected controls. In both infected and uninfected groups, these CD8(+) T cells presented distinct levels of activation markers (CD11a and CD95) at different sites: brain > CSF > spleen = blood > lymph nodes. The CD8(+) cells obtained from the brains of infected monkeys expressed mRNA for cytolytic and proinflammatory molecules, such as granzymes A and B, perforin, and IFN-gamma. Therefore, the neurological dysfunctions correlated with increased numbers of CD8(+) T cells of an activated phenotype in the brain, suggesting that virus-host interactions contributed to the related CNS functional defects.     

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.     

The majority of freshly sorted simian immunodeficiency virus (SIV)-specific CD8(+) T cells cannot suppress viral replication in SIV-infected macrophages

Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) primarily infect activated CD4(+) T cells but can infect macrophages. Surprisingly, ex vivo tetramer-sorted SIV-specific CD8(+) T cells that eliminated and suppressed viral replication in SIV-infected CD4(+) T cells failed to do so in SIV-infected macrophages. It is possible, therefore, that while AIDS virus-infected macrophages constitute only a small percentage of all virus-infected cells, they may be relatively resistant to CD8(+) T cell-mediated lysis and continue to produce virus over long periods of time.     

In vivo T-lymphocyte activation and transient reduction of viral replication in macaques infected with simian immunodeficiency virus

While it is well established that cellular activation can increase human immunodeficiency virus (HIV) replication in T lymphocytes, it is also clear that both activated CD8+ and CD4+ T lymphocytes mediate anti-HIV activity. To assess the relative importance of these contrary effects on HIV replication in vivo, we evaluated the consequences of Mycobacterium bovis BCG and staphylococcal enterotoxin B (SEB) inoculation in vivo in rhesus monkeys chronically infected with simian immunodeficiency virus of macaques (SIVmac). BCG inoculation induced as much as a 2.5-log reduction of plasma and intracellular SIV RNA in SIVmac-infected monkeys. This down-regulation of virus replication persisted as long as 4 weeks after BCG inoculation. Similarly, SEB injection resulted in up to a 3-log decrease in plasma and intracellular SIV RNA in SIVmac-infected macaques. Interestingly, the short-term reduction of viremia in these monkeys correlated with the peak in vivo production of SEB- and BCG-induced cytokine responses. However, no long-term clinical benefit was observed in the SIVmac-infected macaques. These studies provide in vivo evidence that potent T-cell stimulation driven by antigens other than the virus itself can, under some circumstances, mediate short-term reduction of viremia in AIDS virus-infected individuals.     

Intrinsic susceptibility of rhesus macaque peripheral CD4(+) T cells to simian immunodeficiency virus in vitro is predictive of in vivo viral replication

Previous studies with simian immunodeficiency virus (SIV) infection of rhesus macaques suggested that the intrinsic susceptibility of peripheral blood mononuclear cells (PBMC) to infection with SIV in vitro was predictive of relative viremia after SIV challenge. The present study was conducted to evaluate this parameter in a well-characterized cohort of six rhesus macaques selected for marked differences in susceptibility to SIV infection in vitro. Rank order relative susceptibility of PBMC to SIVsmE543-3-infection in vitro was maintained over a 1-year period of evaluation. Differential susceptibility of different donors was maintained in CD8(+) T-cell-depleted PBMC, macrophages, and CD4(+) T-cell lines derived by transformation of PBMC with herpesvirus saimiri, suggesting that this phenomenon is an intrinsic property of CD4(+) target cells. Following intravenous infection of these macaques with SIVsmE543-3, we observed a wide range in plasma viremia which followed the same rank order as the relative susceptibility established by in vitro studies. A significant correlation was observed between plasma viremia at 2 and 8 weeks postinoculation and in vitro susceptibility (P < 0.05). The observation that the two most susceptible macaques were seropositive for simian T-lymphotropic virus type 1 may suggests a role for this viral infection in enhancing susceptibility to SIV infection in vitro and in vivo. In summary, intrinsic susceptibility of CD4(+) target cells appears to be an important factor influencing early virus replication patterns in vivo that should be considered in the design and interpretation of vaccine studies using the SIV/macaque model.     

The disruption of macaque CD4+ T-cell repertoires during the early simian immunodeficiency virus infection

T-cell receptor (TCR) complementarily determining region 3 (CDR3) spetratyping analysis was employed to assess the ability of an AIDS virus to disrupt CD4 + T-cell repertoires during the primary infection. Rhesus and pig-tailed macaques infected with simian immunodeficiency virus (SIV)mac 251 and SIVsmmFGb, respectively, were evaluated. Following SIV infection, the macaques exhibited an apparent decline of CD4 + peripheral blood lymphocyte (PBL) counts, which was associated with a change in CDR3 profiles from multiple-length distribution to one- or two-length dominance in the selected TCR Vbeta-expressing CD4 + PBL subpopulations. Molecular analysis of the perturbed cell subpopulations suggested that the CD4 + T cells bearing the dominant CDR3 length were clonally expanded. These results indicate that SIV infection can induce a disruption of macaque CD4 + T-cell repertoires during the primary infection. The finding in this study, therefore, suggests that the virus-induced clonal dominance can contribute to the disruption of CD4 + T-cell repertoires.     

Spatial alterations between CD4(+) T follicular helper, B, and CD8(+) T cells during simian immunodeficiency virus infection: T/B cell homeostasis, activation, and potential mechanism for viral escape

HIV/SIV infections induce chronic immune activation with remodeling of lymphoid architecture and hypergammaglobulinemia, although the mechanisms leading to such symptoms remain to be fully elucidated. Moreover, lymph nodes have been highlighted as a predilection site for SIV escape in vivo. Following 20 rhesus macaques infected with SIVmac239 as they progress from pre-infection to acute and chronic infection, we document for the first time, to our knowledge, the local dynamics of T follicular helper (T(FH)) cells and B cells in situ. Progression of SIV infection was accompanied by increased numbers of well-delineated follicles containing germinal centers (GCs) and T(FH) cells with a progressive increase in the density of programmed death-1 (PD-1) expression in lymph nodes. The rise in PD-1(+) T(FH) cells was followed by a substantial accumulation of Ki67(+) B cells within GCs. However, unlike in blood, major increases in the frequency of CD27(+) memory B cells were observed in lymph nodes, indicating increased turnover of these cells, correlated with increases in total and SIV specific Ab levels. Of importance, compared with T cell zones, GCs seemed to exclude CD8(+) T cells while harboring increasing numbers of CD4(+) T cells, many of which are positive for SIVgag, providing an environment particularly beneficial for virus replication and reservoirs. Our data highlight for the first time, to our knowledge, important spatial interactions of GC cell subsets during SIV infection, the capacity of lymphoid tissues to maintain stable relative levels of circulating B cell subsets, and a potential mechanism for viral reservoirs within GCs during SIV infection.     

Immune activation driven by CTLA-4 blockade augments viral replication at mucosal sites in simian immunodeficiency virus infection

The importance of chronic immune activation in progression to AIDS has been inferred by correlative studies in HIV-infected individuals and in nonhuman primate models of SIV infection. Using the SIV(mac251) macaque model, we directly address the impact of immune activation by inhibiting CTLA-4, an immunoregulatory molecule expressed on activated T cells and a subset of regulatory T cells. We found that CTLA-4 blockade significantly increased T cell activation and viral replication in primary SIV(mac251) infection, particularly at mucosal sites, and increased IDO expression and activity. Accordingly, protracted treatment with anti-CTLA-4 Ab of macaques chronically infected with SIV(mac251) decreased responsiveness to antiretroviral therapy and abrogated the ability of therapeutic T cell vaccines to decrease viral set point. These data provide the first direct evidence that immune activation drives viral replication, and suggest caution in the use of therapeutic approaches for HIV infection in vivo that increase CD4(+) T cell proliferation.     

Correlates of preserved CD4(+) T cell homeostasis during natural, nonpathogenic simian immunodeficiency virus infection of sooty mangabeys: implications for AIDS pathogenesis

In contrast to HIV-infected humans, naturally SIV-infected sooty mangabeys (SMs) very rarely progress to AIDS. Although the mechanisms underlying this disease resistance are unknown, a consistent feature of natural SIV infection is the absence of the generalized immune activation associated with HIV infection. To define the correlates of preserved CD4(+) T cell counts in SMs, we conducted a cross-sectional immunological study of 110 naturally SIV-infected SMs. The nonpathogenic nature of the infection was confirmed by an average CD4(+) T cell count of 1,076 +/- 589/mm(3) despite chronic infection with a highly replicating virus. No correlation was found between CD4(+) T cell counts and either age (used as a surrogate marker for length of infection) or viremia. The strongest correlates of preserved CD4(+) T cell counts were a low percentage of circulating effector T cells (CD28(-)CD95(+) and/or IL-7R/CD127(-)) and a high percentage of CD4(+)CD25(+) T cells. These findings support the hypothesis that the level of immune activation is a key determinant of CD4(+) T cell counts in SIV-infected SMs. Interestingly, we identified 14 animals with CD4(+) T cell counts of <500/mm(3), of which two show severe and persistent CD4(+) T cell depletion (<50/mm(3)). Thus, significant CD4(+) T cell depletion does occasionally follow SIV infection of SMs even in the context of generally low levels of immune activation, lending support to the hypothesis of multifactorial control of CD4(+) T cell homeostasis in this model of infection. The absence of AIDS in these "CD4(low)" naturally SIV-infected SMs defines a protective role of the reduced immune activation even in the context of a significant CD4(+) T cell depletion.     

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.     

Analysis of total human immunodeficiency virus (HIV)-specific CD4(+) and CD8(+) T-cell responses: relationship to viral load in untreated HIV infection

Human immunodeficiency virus (HIV)-specific T-cell responses are thought to play a key role in viral load decline during primary infection and in determining the subsequent viral load set point. The requirements for this effect are unknown, partly because comprehensive analysis of total HIV-specific CD4(+) and CD8(+) T-cell responses to all HIV-encoded epitopes has not been accomplished. To assess these responses, we used cytokine flow cytometry and overlapping peptide pools encompassing all products of the HIV-1 genome to study total HIV-specific T-cell responses in 23 highly active antiretroviral therapy na?ve HIV-infected patients. HIV-specific CD8(+) T-cell responses were detectable in all patients, ranging between 1.6 and 18.4% of total CD8(+) T cells. HIV-specific CD4(+) T-cell responses were present in 21 of 23 patients, although the responses were lower (0.2 to 2.94%). Contrary to previous reports, a positive correlation was identified between the plasma viral load and the total HIV-, Env-, and Nef-specific CD8(+) T-cell frequency. No correlation was found either between viral load and total or Gag-specific CD4(+) T-cell response or between the frequency of HIV-specific CD4(+) and CD8(+) T cells. These results suggest that overall frequencies of HIV-specific T cells are not the sole determinant of immune-mediated protection in HIV-infection.     

The extent of early viral replication is a critical determinant of the natural history of simian immunodeficiency virus infection.

Different patterns of viral replication correlate with the natural history of disease progression in humans and macaques infected with human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV), respectively. However, the viral and host factors influencing these patterns of viral replication in vivo are poorly understood. We intensively studied viral replication in macaques receiving identical inocula of SIV. Marked differences in viral replication patterns were apparent within the first week following inoculation, a time prior to the development of measurable specific immune effector responses to viral antigens. Plasma viral RNA levels measured on day 7 postinoculation correlated with levels measured in the postacute phase of infection. Differences in the susceptibility of host cells from different animals to in vitro SIV infection correlated with the permissiveness of the animals for early in vivo viral replication and hence with the postacute set point level of plasma viremia. These results suggest that host factors that exert their effects prior to full development of specific immune responses are critical in establishing the in vivo viral replication pattern and associated clinical course in subjects infected with SIV and, by extension, with HIV-1.     

Visualization and characterization of respiratory syncytial virus F-specific CD8(+) T cells during experimental virus infection

CTL play a major role in the clearance of respiratory syncytial virus (RSV) during experimental pulmonary infection. The fusion (F) glycoprotein of RSV is a protective Ag that elicits CTL and Ab response against RSV infection in BALB/c mice. We used the strategy of screening a panel of overlapping synthetic peptides corresponding to the RSV F protein and identified an immunodominant H-2K(d)-restricted epitope (F(85-93); KYKNAVTEL) recognized by CD8(+) T cells from BALB/c mice. We enumerated the F-specific CD8(+) T cell response in the lungs of infected mice by flow cytometry using tetramer staining and intracellular cytokine synthesis. During primary infection, F(85-93)-specific effector CD8(+) T cells constitute approximately 4.8% of pulmonary CD8(+) T cells at the peak of the primary response (day 8), whereas matrix 2-specific CD8(+) T cells constituted approximately 50% of the responding CD8(+) T cell population in the lungs. When RSV F-immune mice undergo a challenge RSV infection, the F-specific CD8(+) T cell response is accelerated and dominates, whereas the primary response to the matrix 2 epitope in the lungs is reduced by approximately 20-fold. In addition, we found that activated F-specific effector CD8(+) T cells isolated from the lungs of RSV-infected mice exhibited a lower than expected frequency of IFN-gamma-producing CD8(+) T cells and were significantly impaired in ex vivo cytolytic activity compared with competent F-specific effector CD8(+) T cells generated in vitro. The significance of these results for the regulation of the CD8(+) T cell response to RSV is discussed.     

Dynamics of CCR5 expression by CD4(+) T cells in lymphoid tissues during simian immunodeficiency virus infection

Early viral replication and profound CD4(+) T-cell depletion occur preferentially in intestinal tissues of macaques infected with simian immunodeficiency virus (SIV). Here we show that a much higher percentage of CD4(+) T cells in the intestine express CCR5 compared with those found in the peripheral blood, spleen, or lymph nodes. In addition, the selectivity and extent of the CD4(+) T-cell loss in SIV infection may depend upon these cells coexpressing CCR5 and having a "memory" phenotype (CD45RA(-)). Following intravenous infection with SIVmac251, memory CD4(+) CCR5(+) T cells were selectively eliminated within 14 days in all major lymphoid tissues (intestine, spleen, and lymph nodes). However, the effect on CD4(+) T-cell numbers was most profound in the intestine, where cells of this phenotype predominate. The CD4(+) T cells that remain after 14 days of infection lacked CCR5 and/or were naive (CD45RA(+)). Furthermore, when animals in the terminal stages of SIV infection (with AIDS) were examined, virtually no CCR5-expressing CD4(+) T cells were found in lymphoid tissues, and all of the remaining CD4(+) T cells were naive and coexpressed CXCR4. These findings suggest that chemokine receptor usage determines which cells are targeted for SIV infection and elimination in vivo.