Suppression of simian immunodeficiency virus replication in vitro by CD8+ lymphocytes

The AIDS-like disease in rhesus monkeys induced by the simian immunodeficiency virus (SIV) has been used as a model to explore the nature of the T lymphocyte response after infection with viruses of the human immunodeficiency virus family. Activated CD8+ lymphocytes are present in increased numbers in the paracortex of lymph nodes of SIV-infected rhesus monkeys with a lymphadenopathy syndrome. We demonstrate that SIV is more readily isolated from CD8+ lymphocyte-depleted PBL of SIV-infected animals than from their unfractionated PBL. Rather than reflecting the fact that the CD8+ lymphocyte-depleted cell populations are simply enriched for CD4+ lymphocytes, this indicates that CD8+ cells themselves are critical in this regulatory interaction. In fact, CD8+ lymphocytes from SIV-infected but not uninfected rhesus monkeys can block SIV replication in vitro in PBL populations. A T lymphocyte population that blocks replication of viruses of the HIV family may contribute to containing the progression of AIDS.     

The antiviral efficacy of simian immunodeficiency virus-specific CD8+ T cells is unrelated to epitope specificity and is abrogated by viral escape

CD8(+) T lymphocytes appear to play a role in controlling human immunodeficiency virus (HIV) replication, yet routine immunological assays do not measure the antiviral efficacy of these cells. Furthermore, it has been suggested that CD8+ T cells that recognize epitopes derived from proteins expressed early in the viral replication cycle can be highly efficient. We used a functional in vitro assay to assess the abilities of different epitope-specific CD8+ T-cell lines to control simian immunodeficiency virus (SIV) replication. We compared the antiviral efficacies of 26 epitope-specific CD8+ T-cell lines directed against seven SIV epitopes in Tat, Nef, Gag, Env, and Vif that were restricted by either Mamu-A*01 or Mamu-A*02. Suppression of SIV replication varied depending on the epitope specificities of the CD8+ T cells and was unrelated to whether the targeted epitope was derived from an early or late viral protein. Tat(28-35)SL8- and Gag(181-189)CM9-specific CD8+ T-cell lines were consistently superior at suppressing viral replication compared to the other five SIV-specific CD8+ T-cell lines. We also investigated the impact of viral escape on antiviral efficacy by determining if Tat(28-35)SL8- and Gag(181-189)CM9-specific CD8+ T-cell lines could suppress the replication of an escaped virus. Viral escape abrogated the abilities of Tat(28-35)SL8- and Gag(181-189)CM9-specific CD8+ T cells to control viral replication. However, gamma interferon (IFN-gamma) enzyme-linked immunospot and IFN-gamma/tumor necrosis factor alpha intracellular-cytokine-staining assays detected cross-reactive immune responses against the Gag escape variant. Understanding antiviral efficacy and epitope variability, therefore, will be important in selecting candidate epitopes for an HIV vaccine.     

The hope for an HIV vaccine based on induction of CD8+ T lymphocytes--a review

The only long-term and cost-effective solution to the human immunodeficiency virus (HIV) epidemic in the developing world is a vaccine that prevents individuals from becoming infected or, once infected, from passing the virus on to others. There is currently little hope for an AIDS vaccine. Conventional attempts to induce protective antibody and CD8(+) lymphocyte responses against HIV and simian immunodeficiency virus (SIV) have failed. The enormous diversity of the virus has only recently been appreciated by vaccinologists, and our assays to determine CD8(+) lymphocyte antiviral efficacy are inadequate. The central hypothesis of a CTL-based vaccine is that particularly effective CD8(+) lymphocytes directed against at least five epitopes that are derived from regions under functional and structural constraints will control replication of pathogenic SIV. This would be somewhat analogous to control of virus replication by triple drug therapy or neutralizing antibodies.     

CD8+ T cells from HIV-1-infected individuals inhibit acute infection by human and primate immunodeficiency viruses.

T lymphocytes expressing the CD8 surface antigen block HIV replication in CD4+ peripheral blood cells from HIV-infected individuals. We report here that CD4+ cells from HIV seronegative donors, when infected in vitro with HIV, also do not replicate virus when cocultured with CD8+ T cells from HIV-infected individuals. CD8+ cells from HIV-uninfected donors did not show this effect on virus replication. HLA-restriction of the antiviral response was not observed, and virus-containing cells were not eliminated from culture. The antiviral activity was broadly cross-reactive, as CD8+ cells from individuals infected only with HIV-1 suppressed the replication of diverse strains of HIV-1 and HIV-2, as well as the simian immunodeficiency virus. This ability of CD8+ cells to control HIV replication could play an important role in the maintenance of an asymptomatic state in HIV-infected individuals.     

CD8+ lymphocytes suppress HIV production by autologous CD4+ cells without eliminating the infected cells from culture

Studies of separated peripheral blood mononuclear cell subsets have indicated that the CD8+ lymphocyte is the primary cell type responsible for suppressing human immunodeficiency virus (HIV) replication by infected CD4+ cells. The effect of this antiviral activity is dose-dependent and does not involve killing of the infected cell. These observations indicate that this response is distinct from the anti-HIV cytotoxic mechanisms also described for human CD8+ cells.     

Predominant involvement of CD8+CD28- lymphocytes in human immunodeficiency virus-specific cytotoxic activity.

Distinct functional CD8+ T-cell populations have been observed during human immunodeficiency virus (HIV) infection. One of these functions is the inhibition of viral replication by a noncytotoxic mechanism, which was shown to be mediated by the CD8+CD28+ subpopulation. On the other hand, CD8+ T cells exert an HIV-specific cytotoxic activity. The present study shows that CD8+CD28- lymphocytes display this HIV-specific cytotoxic activity, which is detectable immediately after the cells are purified from peripheral blood. The CD28- population is also able to proliferate and to retain its cytotoxic activity after in vitro restimulation with autologous blast cells. Finally, HIV-specific cytotoxic T cells can be obtained in vitro from the CD8+CD28+ population.     

CD8+ T lymphocyte control of HIV replication in cultured CD4+ cells varies among infected individuals

Production of the human immunodeficiency virus (HIV) by cultured peripheral blood mononuclear cells (PMC) from many seropositive individuals is inhibited by the presence of CD8+ T lymphocytes. In a study of 10 subjects, high levels of virus replication could be detected in cultures of purified CD4+ cells, but not in unseparated PMC. Addition of highly purified, autologous CD8+ cells to the enriched CD4+ cells resulted in a dose-dependent inhibition of HIV growth and revealed that for some individuals, even low numbers of CD8+ cells can prevent replication of the virus. The data also indicated that culturing enriched CD4+ cells could greatly enhance detection of infectious virus in blood specimens and demonstrated that the CD4+ molecule is expressed on infected T cells isolated directly from the peripheral blood.     

Sustained dysfunction of antiviral CD8+ T lymphocytes after infection with hepatitis C virus

Hepatitis C virus (HCV) sets up persistent infection in the majority of those exposed. It is likely that, as with other persistent viral infections, the efficacy of T-lymphocyte responses influences long-term outcome. However, little is known about the functional capacity of HCV-specific T-lymphocyte responses induced after acute infection. We investigated this by using major histocompatibility complex class I-peptide tetrameric complexes (tetramers), which allow direct detection of specific CD8+ T lymphocytes ex vivo, independently of function. Here we show that, early after infection, virus-specific CD8+ T lymphocytes detected with a panel of four such tetramers are abnormal in terms of their synthesis of antiviral cytokines and lytic activity. Furthermore, this phenotype is commonly maintained long term, since large sustained populations of HCV-specific CD8+ T lymphocytes were identified, which consistently had very poor antiviral cytokine responses as measured in vitro. Overall, HCV-specific CD8+ T lymphocytes show reduced synthesis of tumor necrosis factor alpha (TNF-alpha) and gamma interferon (IFN-gamma) after stimulation with either mitogens or peptides, compared to responses to Epstein-Barr virus and/or cytomegalovirus. This behavior of antiviral CD8+ T lymphocytes induced after HCV infection may contribute to viral persistence through failure to effectively suppress viral replication.     

Inhibition of human immunodeficiency virus replication in acutely infected CD4+ cells by CD8+ cells involves a noncytotoxic mechanism.

The mechanism by which CD8+ T cells from human immunodeficiency virus (HIV)-infected individuals suppress HIV replication in acutely infected CD4+ T cells was investigated. Cytotoxicity was not involved, as the antiviral activity of the CD8+ cells did not correlate with the ability to lyse HIV-infected or uninfected CD4+ T cells. In addition, the frequency of HIV-infected CD4+ cells increased during coculture with CD8+ T cells even in the absence of detectable levels of virus replication. Moreover, separation of the CD4+ and CD8+ cells by a 0.4-micron-pore-size filter delayed HIV replication, indicating a role, at least in part, for a soluble factor. However, cell contact was required for optimal antiviral activity. These results extend further the observation on the mechanism of antiviral HIV activity by CD8+ cells from infected individuals. They support the conclusion that CD8+ cells can play a major role in preventing development of disease in HIV-infected individuals.     

Trafficking of human immunodeficiency virus type 1-specific CD8+ T cells to gut-associated lymphoid tissue during chronic infection

Gut-associated lymphoid tissue (GALT) is a significant but understudied lymphoid organ, harboring a majority of the body's total lymphocyte population. GALT is also an important portal of entry for human immunodeficiency virus (HIV), a major site of viral replication and CD4(+) T-cell depletion, and a frequent site of AIDS-related opportunistic infections and neoplasms. However, little is known about HIV-specific cell-mediated immune responses in GALT. Using lymphocytes isolated from rectal biopsies, we have determined the frequency and phenotype of HIV-specific CD8(+) T cells in human GALT. GALT CD8(+) T cells were predominantly CD45RO(+) and expressed CXCR4 and CCR5. In 10 clinically stable, chronically infected individuals, the frequency of HIV Gag (SL9)-specific CD8(+) T cells was increased in GALT relative to peripheral blood mononuclear cells by up to 4.6-fold, while that of cytomegalovirus (CMV)-specific CD8(+) T cells was significantly reduced (P = 0.012). Both HIV- and CMV-specific CD8(+) T cells in GALT expressed CCR5, but only HIV-specific CD8(+) T cells expressed alpha E beta 7 integrin, suggesting that mucosal priming may account for their retention in GALT. Chronically infected individuals exhibited striking depletion of GALT CD4(+) T cells expressing CXCR4, CCR5, and alpha E beta 7 integrin, but CD4(+)/CD8(+) T-cell ratios in blood and GALT were similar. The percentage of GALT CD8(+) T cells expressing alpha E beta 7 was significantly decreased in infected individuals, suggesting that HIV infection may perturb lymphocyte retention in GALT. These studies demonstrate the feasibility of using tetramers to assess HIV-specific T cells in GALT and reveal that GALT is the site of an active CD8(+) T-cell response during chronic infection.     

Selective depletion of high-avidity human immunodeficiency virus type 1 (HIV-1)-specific CD8+ T cells after early HIV-1 infection

Human immunodeficiency virus type 1 (HIV-1)-specific CD8+ T cells in early infection are associated with the dramatic decline of peak viremia, whereas their antiviral activity in chronic infection is less apparent. The functional properties accounting for the antiviral activity of HIV-1-specific CD8+ T cells during early infection are unclear. Using cytokine secretion and tetramer decay assays, we demonstrated in intraindividual comparisons that the functional avidity of HIV-1-specific CD8+ T cells was consistently higher in early infection than in chronic infection in the presence of high-level viral replication. This change of HIV-1-specific CD8+ T-cell avidity between early and chronic infections was linked to a substantial switch in the clonotypic composition of epitope-specific CD8+ T cells, resulting from the preferential loss of high-avidity CD8+ T-cell clones. In contrast, the maintenance of the initially recruited clonotypic pattern of HIV-1-specific CD8+ T cells was associated with low-level set point HIV-1 viremia. These data suggest that high-avidity HIV-1-specific CD8+ T-cell clones are recruited during early infection but are subsequently lost in the presence of persistent high-level viral replication.     

Infection of the CD45RA+ (naive) subset of peripheral CD8+ lymphocytes by human immunodeficiency virus type 1 in vivo

To investigate the mechanism and functional significance of infection of CD8+ lymphocytes by human immunodeficiency virus type 1 (HIV-1) in vivo, we determined frequencies of infection, proviral conformation, and genetic relationships between HIV-1 variants infecting naive (CD45RA+) and memory (CD45RO+) peripheral blood CD4+ and CD8+ lymphocytes. Infection of CD3+ CD8+ CD45RA+ cells was detected in 9 of 16 study subjects at frequencies ranging from 30 to 1,400 proviral copies/10(6) cells, more frequently than CD3+ CD8+ lymphocytes expressing the RO isoform of CD45 (n = 2, 70 and 260 copies /10(6) cells). In agreement with previous studies, there was no evidence for a similar preferential infection of CD4+ naive lymphocytes. Proviral sequences in both CD4+ and CD8+ lymphocyte subsets were complete, as assessed by quantitation using primers from the long terminal repeat region spanning the tRNA primer binding site. In six of the seven study subjects investigated, variants infecting CD8+ lymphocytes were partially or completely genetically distinct in the V3 region from those recovered from CD4+ lymphocytes and showed a greater degree of compartmentalization than observed between naive and memory subsets of CD4+ lymphocytes. In two study subjects, arginine substitutions at position 306, associated with use of the chemokine coreceptor CXCR4, were preferentially found in CD4 lymphocytes. These population differences may have originated through different times of infection rather than necessarily indicating a difference in their biological properties. The preferential distribution of HIV-1 in naive CD8+ lymphocytes indeed suggests that infection occurred early in T-lymphocyte ontogeny, such as during maturation in the thymus. Destruction of cells destined to become CD8+ lymphocytes may be a major factor in the decline in CD8+ lymphocyte frequencies and function on disease progression and may contribute directly to the observed immunodeficiency in 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.     

Clonal focusing of epitope-specific CD8+ T lymphocytes in rhesus monkeys following vaccination and simian-human immunodeficiency virus challenge

To afford the greatest possible immune protection, candidate human immunodeficiency virus (HIV) vaccines must generate diverse and long-lasting CD8⁺ T lymphocyte responses. In the present study, we evaluate T-cell receptor Vβ (variable region beta) gene usage and a CDR3 (complementarity-determining region 3) sequence to assess the clonality of epitope-specific CD8⁺ T lymphocytes generated in rhesus monkeys following vaccination and simian-human immunodeficiency virus (SHIV) challenge. We found that vaccine-elicited epitope-specific CD8⁺ T lymphocytes have a clonal diversity comparable to those cells generated in response to SHIV infection. Moreover, we show that the clonal diversity of vaccine-elicited CD8⁺ T-lymphocyte responses is dictated by the epitope sequence and is not affected by the mode of antigen delivery to the immune system. Clonal CD8⁺ T-lymphocyte populations persisted following boosting with different vectors, and these clonal cell populations could be detected for as long as 4 years after SHIV challenge. Finally, we show that the breadth of these epitope-specific T lymphocytes transiently focuses in response to intense SHIV replication. These observations demonstrate the importance of the initial immune response to SHIV, induced by vaccination or generated during primary infection, in determining the clonal diversity of cell-mediated immune responses and highlight the focusing of this clonal diversity in the setting of high viral loads. Circumventing this restricted CD8⁺ T-lymphocyte clonal diversity may present a significant challenge in the development of an effective HIV vaccine strategy.     

Simian immunodeficiency virus-specific CD8+ lymphocyte response in acutely infected rhesus monkeys.

To assess the possible role of cytotoxic T lymphocytes (CTLs) in containing the spread of human immunodeficiency virus in acutely infected individuals, the temporal evolution of the virus-specific CD8+ lymphocyte response was defined in simian immunodeficiency virus of macaques (SIVmac)-infected rhesus monkeys. A brief period of SIVmac plasma antigenemia was seen 9 to 16 days following intravenous infection with SIVmac, ending as the absolute number of CD8+ peripheral blood lymphocytes (PBLs) increased. In a prospective assessment of the ability of CD8+ lymphocytes of these monkeys to suppress SIVmac replication in autologous PBLs, inhibitory activity was detected as early as 4 days, with a more pronounced effect 12 to 16 days following infection. SIVmac Gag- and Nef-specific CD8+ effector cell activities were demonstrable in PBLs of animals by 2 weeks following virus inoculation. In fact, SIVmac-specific CTL precursors were documented in the PBLs of rhesus monkeys 4 to 6 days after SIVmac infection. These studies indicate that AIDS virus-specific CD8+ CTLs are present in PBLs within days of infection and may play an important role in containing the early spread of virus.     

Preservation of functional virus-specific memory CD8+ T lymphocytes in vaccinated, simian human immunodeficiency virus-infected rhesus monkeys

Functional impairment of virus-specific memory CD8(+) T lymphocytes has been associated with clinical disease progression following HIV, SIV, and simian human immunodeficiency virus infection. These lymphocytes have a reduced capacity to produce antiviral cytokines and mediators involved in the lysis of virally infected cells. In the present study, we used polychromatic flow cytometry to assess the frequency and functional capacity of central memory (CD28(+)CD95(+)) and effector memory (CD28(-)CD95(+)) subpopulations of Gag-specific CD8(+) T cells in SIV/simian human immunodeficiency virus-infected rhesus monkeys. The aim of this study was to determine whether Ag-specific, memory CD8(+) T cell function could be preserved in infected monkeys that had been immunized before infection with a vaccine regimen consisting of a plasmid DNA prime followed by a recombinant viral vector boost. We observed that vaccination was associated with the preservation of Gag-specific central memory CD8(+) T cells that were functionally capable of producing IFN-gamma, and effector memory CD8(+) T cells that were capable of producing granzyme B following viral Ag exposure.     

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.