Estimating the effectiveness of simian immunodeficiency virus-specific CD8+ T cells from the dynamics of viral immune escape

Antiviral CD8(+) T cells are thought to play a significant role in limiting the viremia of human and simian immunodeficiency virus (HIV and SIV, respectively) infections. However, it has not been possible to measure the in vivo effectiveness of cytotoxic T cells (CTLs), and hence their contribution to the death rate of CD4(+) T cells is unknown. Here, we estimated the ability of a prototypic antigen-specific CTL response against a well-characterized epitope to recognize and kill infected target cells by monitoring the immunodominant Mamu-A*01-restricted Tat SL8 epitope for escape from Tat-specific CTLs in SIVmac239-infected macaques. Fitting a mathematical model that incorporates the temporal kinetics of specific CTLs to the frequency of Tat SL8 escape mutants during acute SIV infection allowed us to estimate the in vivo killing rate constant per Tat SL8-specific CTL. Using this unique data set, we show that at least during acute SIV infection, certain antiviral CD8(+) T cells can have a significant impact on shortening the longevity of infected CD4(+) T cells and hence on suppressing virus replication. Unfortunately, due to viral escape from immune pressure and a dependency of the effectiveness of antiviral CD8(+) T-cell responses on the availability of sufficient CD4(+) T cells, the impressive early potency of the CTL response may wane in the transition to the chronic stage of the infection.     

GagCM9-specific CD8+ T cells expressing limited public TCR clonotypes do not suppress SIV replication in vivo

Several lines of evidence suggest that HIV/SIV-specific CD8(+) T cells play a critical role in the control of viral replication. Recently we observed high levels of viremia in Indian rhesus macaques vaccinated with a segment of SIVmac239 Gag (Gag(45-269)) that were subsequently infected with SIVsmE660. These seven Mamu-A*01(+) animals developed CD8(+) T cell responses against an immunodominant epitope in Gag, GagCM9, yet failed to control virus replication. We carried out a series of immunological and virological assays to understand why these Gag-specific CD8(+) T cells could not control virus replication in vivo. GagCM9-specific CD8(+) T cells from all of the animals were multifunctional and were found in the colonic mucosa. Additionally, GagCM9-specific CD8(+) T cells accessed B cell follicles, the primary residence of SIV-infected cells in lymph nodes, with effector to target ratios between 20-250 GagCM9-specific CD8(+) T cells per SIV-producing cell. Interestingly, vaccinated animals had few public TCR clonotypes within the GagCM9-specific CD8(+) T cell population pre- and post-infection. The number of public TCR clonotypes expressed by GagCM9-specific CD8(+) T cells post-infection significantly inversely correlated with chronic phase viral load. It is possible that these seven animals failed to control viral replication because of the narrow TCR repertoire expressed by the GagCM9-specific CD8(+) T cell population elicited by vaccination and infection.     

De novo generation of escape variant-specific CD8+ T-cell responses following cytotoxic T-lymphocyte escape in chronic human immunodeficiency virus type 1 infection

Human immunodeficiency virus type 1 (HIV-1) evades CD8(+) T-cell responses through mutations within targeted epitopes, but little is known regarding its ability to generate de novo CD8(+) T-cell responses to such mutants. Here we examined gamma interferon-positive, HIV-1-specific CD8(+) T-cell responses and autologous viral sequences in an HIV-1-infected individual for more than 6 years following acute infection. Fourteen optimal HIV-1 T-cell epitopes were targeted by CD8(+) T cells, four of which underwent mutation associated with dramatic loss of the original CD8(+) response. However, following the G(357)S escape in the HLA-A11-restricted Gag(349-359) epitope and the decline of wild-type-specific CD8(+) T-cell responses, a novel CD8(+) T-cell response equal in magnitude to the original response was generated against the variant epitope. CD8(+) T cells targeting the variant epitope did not exhibit cross-reactivity against the wild-type epitope but rather utilized a distinct T-cell receptor Vbeta repertoire. Additional studies of chronically HIV-1-infected individuals expressing HLA-A11 demonstrated that the majority of the subjects targeted the G(357)S escape variant of the Gag(349-359) epitope, while the wild-type consensus sequence was significantly less frequently recognized. These data demonstrate that de novo responses against escape variants of CD8(+) T-cell epitopes can be generated in chronic HIV-1 infection and provide the rationale for developing vaccines to induce CD8(+) T-cell responses directed against both the wild-type and variant forms of CD8 epitopes to prevent the emergence of cytotoxic T-lymphocyte escape variants.     

HIV-1-specific interleukin-21+ CD4+ T cell responses contribute to durable viral control through the modulation of HIV-specific CD8+ T cell function

Functional defects in cytotoxic CD8(+) T cell responses arise in chronic human viral infections, but the mechanisms involved are not well understood. In mice, CD4 cell-mediated interleukin-21 (IL-21) production is necessary for the maintenance of CD8(+) T cell function and control of persistent viral infections. To investigate the potential role of IL-21 in a chronic human viral infection, we studied the rare subset of HIV-1 controllers, who are able to spontaneously control HIV-1 replication without treatment. HIV-specific triggering of IL-21 by CD4(+) T cells was significantly enriched in these persons (P = 0.0007), while isolated loss of IL-21-secreting CD4(+) T cells was characteristic for subjects with persistent viremia and progressive disease. IL-21 responses were mediated by recognition of discrete epitopes largely in the Gag protein, and expansion of IL-21(+) CD4(+) T cells in acute infection resulted in lower viral set points (P = 0.002). Moreover, IL-21 production by CD4(+) T cells of HIV controllers enhanced perforin production by HIV-1-specific CD8(+) T cells from chronic progressors even in late stages of disease, and HIV-1-specific effector CD8(+) T cells showed an enhanced ability to efficiently inhibit viral replication in vitro after IL-21 binding. These data suggest that HIV-1-specific IL-21(+) CD4(+) T cell responses might contribute to the control of viral replication in humans and are likely to be of great importance for vaccine design.     

Polyomavirus-infected dendritic cells induce antiviral CD8(+) T lymphocytes

CD8(+) T cells are critical for the clearance of acute polyomavirus infection and the prevention of polyomavirus-induced tumors, but the antigen-presenting cell(s) involved in generating polyomavirus-specific CD8(+) T cells have not been defined. We investigated whether dendritic cells and macrophages are permissive for polyomavirus infection and examined their potential for inducing antiviral CD8(+) T cells. Although dendritic cells and macrophages both supported productive polyomavirus infection, dendritic cells were markedly more efficient at presenting the immunodominant viral epitope to CD8(+) T cells. Additionally, infected dendritic cells, but not infected macrophages, primed anti-polyomavirus CD8(+) T cells in vivo. Treatment with Flt3 ligand, a hematopoietic growth factor that dramatically expands the number of dendritic cells, markedly enhanced the magnitude of virus-specific CD8(+) T-cell responses during acute infection and the pool of memory anti-polyomavirus CD8(+) T cells. These findings suggest that virus-infected dendritic cells induce polyomavirus-specific CD8(+) T cells in vivo and raise the potential for their use as cellular adjuvants to promote CD8(+) T cell surveillance against polyomavirus-induced tumors.     

Gag-specific CD8+ T lymphocytes recognize infected cells before AIDS-virus integration and viral protein expression

CD8(+) T cells are a key focus of vaccine development efforts for HIV. However, there is no clear consensus as to which of the nine HIV proteins should be used for vaccination. The early proteins Tat, Rev, and Nef may be better CD8(+) T cell targets than the late-expressed structural proteins Gag, Pol, and Env. In this study, we show that Gag-specific CD8(+) T cells recognize infected CD4(+) T lymphocytes as early as 2 h postinfection, before proviral DNA integration, viral protein synthesis, and Nef-mediated MHC class I down-regulation. Additionally, the number of Gag epitopes recognized by CD8(+) T cells was significantly associated with lower viremia (p = 0.0017) in SIV-infected rhesus macaques. These results suggest that HIV vaccines should focus CD8(+) T cell responses on Gag.     

Regulation of CD8+ T cells undergoing primary and secondary responses to infection in the same host

Naive Ag-specific CD8(+) T cells expand, contract, and become memory cells after infection and/or vaccination. Memory CD8(+) T cells provide faster, more effective secondary responses against repeated exposure to the same pathogen. Using an adoptive transfer system with low numbers of trackable nontransgenic memory CD8(+) T cells, we showed that secondary responses can be comprised of both primary (naive) and secondary (memory) CD8(+) T cells after bacterial (Listeria monocytogenes) and/or viral (lymphocytic choriomeningitis virus) infections. The level of memory CD8(+) T cells present at the time of infection inversely correlated with the magnitude of primary CD8(+) T cell responses against the same epitope but directly correlated with the level of protection against infection. However, similar numbers of Ag-specific CD8(+) T cells were found 8 days postinfection no matter how many memory cells were present at the time of infection. Rapid contraction of primary CD8(+) T cell responses was not influenced by the presence of memory CD8(+) T cells. However, contraction of secondary CD8(+) T cell responses was markedly prolonged compared with primary responses in the same host mice. This situation occurred in response to lymphocytic choriomeningitis virus or L. monocytogenes infection and for CD8(+) T cell responses against multiple epitopes. The delayed contraction of secondary CD8(+) T cells was also observed after immunization with peptide-coated dendritic cells. Together, the results show that the level of memory CD8(+) T cells influences protective immunity and activation of naive precursors specific for the same epitope but has little impact on the magnitude or program of the CD8(+) T cell response.     

Whole Genome Deep Sequencing of HIV-1 Reveals the Impact of Early Minor Variants Upon Immune Recognition During Acute Infection

Deep sequencing technologies have the potential to transform the study of highly variable viral pathogens by providing a rapid and cost-effective approach to sensitively characterize rapidly evolving viral quasispecies. Here, we report on a high-throughput whole HIV-1 genome deep sequencing platform that combines 454 pyrosequencing with novel assembly and variant detection algorithms. In one subject we combined these genetic data with detailed immunological analyses to comprehensively evaluate viral evolution and immune escape during the acute phase of HIV-1 infection. The majority of early, low frequency mutations represented viral adaptation to host CD8+ T cell responses, evidence of strong immune selection pressure occurring during the early decline from peak viremia. CD8+ T cell responses capable of recognizing these low frequency escape variants coincided with the selection and evolution of more effective secondary HLA-anchor escape mutations. Frequent, and in some cases rapid, reversion of transmitted mutations was also observed across the viral genome. When located within restricted CD8 epitopes these low frequency reverting mutations were sufficient to prime de novo responses to these epitopes, again illustrating the capacity of the immune response to recognize and respond to low frequency variants. More importantly, rapid viral escape from the most immunodominant CD8+ T cell responses coincided with plateauing of the initial viral load decline in this subject, suggestive of a potential link between maintenance of effective, dominant CD8 responses and the degree of early viremia reduction. We conclude that the early control of HIV-1 replication by immunodominant CD8+ T cell responses may be substantially influenced by rapid, low frequency viral adaptations not detected by conventional sequencing approaches, which warrants further investigation. These data support the critical need for vaccine-induced CD8+ T cell responses to target more highly constrained regions of the virus in order to ensure the maintenance of immunodominant CD8 responses and the sustained decline of early viremia.     

Differential evolution and stability of epitope-specific CD8(+) T cell responses in EBV infection

Murine models of lymphocytic choriomeningitis virus infection suggest that the memory CD8(+) T cell repertoire is reflective of the CD8(+) T cell repertoire generated during acute infection. Less is known regarding the evolution of CD8(+) T cell repertoires during human viral infections. We therefore examined epitope-specific CD8(+) T cell responses in a large cohort of individuals with acute through latent Epstein-Barr virus infection. Using 16 of 20 published EBV epitopes restricted by HLA-A2, HLA-A3 or HLA-B7, we showed that lytic cycle-specific CD8(+) T cell responses predominated during acute EBV infection. However, whereas HLA-A2(+)-restricted BMLF-1-specific CD8(+) T cell responses were maintained through latency, HLA-A2(+)- and HLA-B7(+)-restricted BZLF-1, as well as HLA-A3(+)-restricted BRLF-1 CD8(+) T cell responses, were generated but not readily maintained. Analyses of CD8(+) T cell responses to EBV latent cycle Ags showed delayed detection and lower frequencies of latent epitope-specific CD8(+) T cell responses during acute EBV infection, with maintenance of these responses 1 yr post-EBV infection. Early BMLF-1 and EBNA-3A epitope-specific CD8(+) T cell frequencies did not correlate with their frequencies at 1 yr postinfection. Interestingly, populations of EBV-specific CD8(+) T cells were stable during 20 mo in our long term EBV-seropositive populations, suggesting homeostasis between virus and the host immune system. This study demonstrates that CD8(+) T cell repertoires generated during persistent viral infections are not simply reflective of the initial pool of CD8(+) T cells and provides evidence that the generation of CD8(+) T cell responses to a persistent infection is a dynamic process.     

Timing and magnitude of type I interferon responses by distinct sensors impact CD8 T cell exhaustion and chronic viral infection

Type I interferon (IFN-I) promotes antiviral CD8(+)T cell responses, but the contribution of different IFN-I sources and signaling pathways are ill defined. While plasmacytoid dendritic cells (pDCs) produce IFN-I upon TLR stimulation, IFN-I is induced in most cells by helicases like MDA5. Using acute and chronic lymphocytic choriomeningitis virus (LCMV) infection models, we determined that pDCs transiently produce IFN-I that minimally impacts CD8(+)T cell responses and viral persistence. Rather, MDA5 is the key sensor that induces IFN-I required for CD8(+)T cell responses. In the absence of MDA5, CD8(+)T cell responses to acute infection rely on CD4(+)T cell help, and loss of both CD4(+)T cells and MDA5 results in CD8(+)T cell exhaustion and persistent infection. Chronic LCMV infection rapidly attenuates IFN-I responses, but early administration of exogenous IFN-I rescues CD8(+)T cells, promoting viral clearance. Thus, effective antiviral CD8(+)T cell responses depend on the timing and magnitude of IFN-I production.     

Viremia control despite escape from a rapid and potent autologous neutralizing antibody response after therapy cessation in an HIV-1-infected individual

The neutralizing Ab response after primary HIV-1 infection is delayed relative to the virus-specific CD8(+) T cell response and the initial decline in plasma viremia. Because nearly all HIV-1 infections result in AIDS, it would be instructive to study cases where neutralizing Ab production commenced sooner. This was done in subject AC10, an individual treated during early infection and in whom a rapid autologous neutralizing Ab response was detected after therapy cessation as rebound viremia declined and remained below 1000 RNA copies/ml of blood for over 2.5 years. This subject's Abs were capable of reducing the infectivity of his rebound virus by >4 logs in vitro at a time when rebound viremia was down-regulated and virus-specific CD8(+) T cells were minimal, suggesting that neutralizing Abs played an important role in the early control of viremia. The rebound virus did not exhibit an unusual phenotype that might explain its high sensitivity to neutralization by autologous sera. Neutralization escape occurred within 75 days and was proceeded by neutralizing Ab production to the escape variant and subsequent escape. Notably, escape was not associated with a significant rise in plasma viremia, perhaps due to increasing CD8(+) T cell responses. Sequence analysis of gp160 revealed a growing number of mutations over time, suggesting ongoing viral evolution in the face of potent antiviral immune responses. We postulate that an early effective neutralizing Ab response can provide long-term clinical benefits despite neutralization escape.     

Decay kinetics of human immunodeficiency virus-specific CD8+ T cells in peripheral blood after initiation of highly active antiretroviral therapy.

We measured the longitudinal responses to 95 HLA class I-restricted human immunodeficiency virus (HIV) epitopes and an immunodominant HLA A2-restricted cytomegalovirus (CMV) epitope in eight treatment-naive HIV-infected individuals, using intracellular cytokine staining. Patients were treated with highly active antiretroviral therapy (HAART) for a median of 78 weeks (range, 34 to 121 weeks). Seven of eight patients maintained an undetectable viral load for the duration of therapy. A rapid decline in HIV-specific CD8(+) T-cell response was observed at initiation of therapy. After an undetectable viral load was achieved, a slower decrease in HIV-specific CD8(+) T-cell response was observed that was well described by first-order kinetics. The median half-life for the rate of decay was 38.8 (20.3 to 68.0) weeks when data were expressed as percentage of peripheral CD8(+) T cells. In most cases, data were similar when expressed as the number of responding CD8(+) T cells per microliter of blood. In subjects who responded to more than one HIV epitope, rates of decline in response to the different epitopes were similar and varied by a factor of 2.2 or less. Discontinuation of treatment resulted in a rapid increase in HIV-specific CD8(+) T cells. Responses to CMV increased 1.6- and 2.8-fold within 16 weeks of initiation of HAART in two of three patients with a measurable CMV response. These data suggest that HAART quickly starts to restore CD8(+) T-cell responses to other chronic viral infections and leads to a slow decrease in HIV-specific CD8(+) T-cell response in HIV-infected patients. The slow decrease in the rate of CD8(+) T-cell response and rapid increase in response to recurrent viral replication suggest that the decrease in CD8(+) T-cell response observed represents a normal memory response to withdrawal of antigen.     

Human cytomegalovirus proteins pp65 and immediate early protein 1 are common targets for CD8+ T cell responses in children with congenital or postnatal human cytomegalovirus infection

Recombinant modified vaccinia Ankara- and peptide-based IFN-gamma ELISPOT assays were used to detect and measure human CMV (HCMV)-specific CD8(+) T cell responses to the pp65 (UL83) and immediate early protein 1 (IE1; UL123) gene products in 16 HCMV-infected infants and children. Age at study ranged from birth to 2 years. HCMV-specific CD8(+) T cells were detected in 14 (88%) of 16 children at frequencies ranging from 60 to >2000 spots/million PBMC. Responses were detected as early as 1 day of age in infants with documented congenital infection. Nine children responded to both pp65 and IE1, whereas responses to pp65 or IE1 alone were detected in three and two children, respectively. Regardless of the specificity of initial responses, IE1-specific responses predominated by 1 year of age. Changes in HCMV epitopes targeted by the CD8(+) T cell responses were observed over time; epitopes commonly recognized by HLA-A2(+) adults with latent HCMV infection did not fully account for responses detected in early childhood. Finally, the detection of HCMV-specific CD8(+) T cell responses was temporally associated with a decrease in peripheral blood HCMV load. Taken altogether, these data demonstrate that the fetus and young infant can generate virus-specific CD8(+) T cell responses. Changes observed in the protein and epitope-specificity of HCMV-specific CD8(+) T cells over time are consistent with those observed after other primary viral infections. The temporal association between the detection of HCMV-specific CD8(+) T cell responses and the reduction in blood HCMV load supports the importance of CD8(+) T cells in controlling primary HCMV viremia.     

Effect of chronic viral infection on epitope selection, cytokine production, and surface phenotype of CD8 T cells and the role of IFN-gamma receptor in immune regulation

Regulation of CD8 T cell responses in chronic viral infections is not well understood. In this study, we have compared the CD8 T cell responses to immunodominant and subdominant epitopes during an acute and a chronic lymphocytic choriomeningitis virus (LCMV) infection in mice. The epitope hierarchy of the primary CD8 T cell response was similar in acute and chronic LCMV infections. However, strikingly, the epitope hierarchy of the primary CD8 T cell response was conserved in the T cell memory only in an acute but not in a chronic LCMV infection. Interestingly, in an acute infection, increasing the viral dose caused significant changes in the epitope hierarchy of the LCMV-specific memory CD8 T cell pool, with no effect on the primary CD8 T cell response. Functional and phenotypic analyses revealed that exposure of CD8 T cells to extended periods of antigenic stimulation could lead to long-term defects in cytokine production and alteration in expression of cell surface L-selectin (CD62L). Whereas expression of CD44 was minimally altered, a greater proportion of LCMV-specific memory CD8 T cells were CD62L(low) in mice that have recovered from a chronic LCMV infection, compared with acutely infected mice. Mechanistic studies showed that IFN-gammaR deficiency altered the epitope hierarchy of the pool of LCMV-specific memory CD8 T cells without significantly affecting the immunodominance of the primary CD8 T cell response in an acute infection. Taken together, these findings should further our understanding about the regulation of T cell responses in human chronic viral infections.     

Two Overlapping Subdominant Epitopes Identified by DNA Immunization Induce Protective CD8+ T-Cell Populations with Differing Cytolytic Activities

Subdominant CD8(+) T-cell responses contribute to control of several viral infections and to vaccine-induced immunity. Here, using the lymphocytic choriomeningitis virus model, we demonstrate that subdominant epitopes can be more reliably identified by DNA immunization than by other methods, permitting the identification, in the virus nucleoprotein, of two overlapping subdominant epitopes: one presented by L(d) and the other presented by K(d). This subdominant sequence confers immunity as effective as that induced by the dominant epitope, against which >90% of the antiviral CD8(+) T cells are normally directed. We compare the kinetics of the dominant and subdominant responses after vaccination with those following subsequent viral infection. The dominant CD8(+) response expands more rapidly than the subdominant responses, but after virus infection is cleared, mice which had been immunized with the "dominant" vaccine have a pool of memory T cells focused almost entirely upon the dominant epitope. In contrast, after virus infection, mice which had been immunized with the "subdominant" vaccine retain both dominant and subdominant memory cells. During the acute phase of the immune response, the acquisition of cytokine responsiveness by subdominant CD8(+) T cells precedes their development of lytic activity. Furthermore, in both dominant and subdominant populations, lytic activity declines more rapidly than cytokine responsiveness. Thus, the lysis(low)-cytokine(competent) phenotype associated with most memory CD8(+) T cells appears to develop soon after antigen clearance. Finally, lytic activity differs among CD8(+) T-cell populations with different epitope specificities, suggesting that vaccines can be designed to selectively induce CD8(+) T cells with distinct functional attributes.     

Contribution of CD8+ T cells to containment of viral replication and emergence of mutations in Mamu-A*01-restricted epitopes in Simian immunodeficiency virus-infected rhesus monkeys

Here, we investigated the containment of virus replication in simian immunodeficiency virus (SIV) infection by CD8(+) lymphocytes. Escape mutations in Mamu-A*01 epitopes appeared first in SIV Tat TL8 and then in SIV Gag p11C. The appearance of escape mutations in SIV Gag p11C was coincident with compensatory changes outside of the epitope. Eliminating CD8(+) lymphocytes from rhesus monkeys during primary infection resulted in more rapid disease progression that was associated with preservation of canonical epitopes. These results confirm the importance of cytotoxic T cells in controlling viremia and the constraint on epitope sequences that require compensatory changes to go to fixation.     

HIV-specific CD8+ T cell function in children with vertically acquired HIV-1 infection is critically influenced by age and the state of the CD4+ T cell compartment

The immunology of vertical HIV transmission differs from that of adult infection in that the immune system of the infant is not fully matured, and the factors that influence the functionality of CD8(+) T cell responses against HIV in children remain largely undefined. We have investigated CD8(+) T cell responses in 65 pediatric subjects with vertically acquired HIV-1 infection. Vigorous, broad, and Ag dose-driven CD8(+) T cell responses against HIV Ags were frequently observed in children who were older than 3 years of age and maintained CD4(+) T cell counts >400 cells/ micro l. In contrast, younger age or a CD4(+) T cell count <400 cells/ micro l was associated with poor CD8(+) T cell responses and high HIV loads. Furthermore, subjects with a severely depleted and phenotypically altered CD4(+) T cell compartment had circulating Gag-specific CD8(+) T cells with impaired IFN-gamma production. When viral load was not suppressed by antiviral treatment, subjects that fell below the putative age and CD4(+) T cell count thresholds had significantly reduced CD8(+) T cell responses and significantly higher viral loads. Thus, the data suggest that fully effective HIV-specific CD8(+) T cell responses take years to develop despite an abundance of Ag in early life, and responses are further severely impaired, independent of age, in children who have a depleted or skewed CD4(+) T cell compartment. The results are discussed in relation to differences between the neonatal and adult immune systems in the ability to respond to HIV infection.     

Distinct kinetics of Gag-specific CD4+ and CD8+ T cell responses during acute HIV-1 infection

HIV infection is characterized by a gradual deterioration of immune function, mainly in the CD4 compartment. To better understand the dynamics of HIV-specific T cells, we analyzed the kinetics and polyfunctional profiles of Gag-specific CD4(+) and CD8(+) T cell responses in 12 subtype C-infected individuals with different disease-progression profiles, ranging from acute to chronic HIV infection. The frequencies of Gag-responsive CD4(+) and CD8(+) T cells showed distinct temporal kinetics. The peak frequency of Gag-responsive IFN-γ(+)CD4(+) T cells was observed at a median of 28 d (interquartile range: 21-81 d) post-Fiebig I/II staging, whereas Gag-specific IFN-γ(+)CD8(+) T cell responses peaked at a median of 253 d (interquartile range: 136-401 d) and showed a significant biphasic expansion. The proportion of TNF-α-expressing cells within the IFN-γ(+)CD4(+) T cell population increased (p = 0.001) over time, whereas TNF-α-expressing cells within IFN-γ(+)CD8(+) T cells declined (p = 0.005). Both Gag-responsive CD4(+) and CD8(+) T cells showed decreased Ki67 expression within the first 120 d post-Fiebig I/II staging. Prior to the disappearance of Gag-responsive Ki67(+)CD4(+) T cells, these cells positively correlated (p = 0.00038) with viremia, indicating that early Gag-responsive CD4 events are shaped by viral burden. No such associations were observed in the Gag-specific CD8(+) T cell compartment. Overall, these observations indicated that circulating Gag-responsive CD4(+) and CD8(+) T cell frequencies and functions are not synchronous, and properties change rapidly at different tempos during early HIV infection.     

CD4+CD8+ T cells represent a significant portion of the anti-HIV T cell response to acute HIV infection

Previous studies have revealed that HIV-infected individuals possess circulating CD4(+)CD8(+) double-positive (DP) T cells specific for HIV Ags. In the present study, we analyzed the proliferation and functional profile of circulating DP T cells from 30 acutely HIV-infected individuals and 10 chronically HIV-infected viral controllers. The acutely infected group had DP T cells that showed more proliferative capability and multifunctionality than did both their CD4(+) and CD8(+) T cells. DP T cells were found to exhibit greater proliferation and higher multifunctionality compared with CD4 T cells in the viral controller group. The DP T cell response represented 16% of the total anti-HIV proliferative response and >70% of the anti-HIV multifunctional response in the acutely infected subjects. Proliferating DP T cells of the acutely infected subjects responded to all HIV Ag pools with equal magnitude. Conversely, the multifunctional response was focused on the pool representing Nef, Rev, Tat, VPR, and VPU. Meanwhile, the controllers' DP T cells focused on Gag and the Nef, Rev, Tat, VPR, and VPU pool for both their proliferative and multifunctional responses. Finally, we show that the presence of proliferating DP T cells following all HIV Ag stimulations is well correlated with proliferating CD4 T cells whereas multifunctionality appears to be largely independent of multifunctionality in other T cell compartments. Therefore, DP T cells represent a highly reactive cell population during acute HIV infection, which responds independently from the traditional T cell compartments.     

Patterns of CD8+ immunodominance may influence the ability of Mamu-B*08-positive macaques to naturally control simian immunodeficiency virus SIVmac239 replication

Certain major histocompatibility complex (MHC) class I alleles are strongly associated with control of human immunodeficiency virus and simian immunodeficiency virus (SIV). CD8(+) T cells specific for epitopes restricted by these molecules may be particularly effective. Understanding how CD8(+) T cells contribute to control of viral replication should yield important insights for vaccine design. We have recently identified an Indian rhesus macaque MHC class I allele, Mamu-B*08, associated with elite control and low plasma viremia after infection with the pathogenic isolate SIVmac239. Here, we infected four Mamu-B*08-positive macaques with SIVmac239 to investigate why some of these macaques control viral replication. Three of the four macaques controlled SIVmac239 replication with plasma virus concentrations below 20,000 viral RNA copies/ml at 20 weeks postinfection; two of four macaques were elite controllers (ECs). Interestingly, two of the four macaques preserved their CD4(+) memory T lymphocytes during peak viremia, and all four recovered their CD4(+) memory T lymphocytes in the chronic phase of infection. Mamu-B*08-restricted CD8(+) T-cell responses dominated the acute phase and accounted for 23.3% to 59.6% of the total SIV-specific immune responses. Additionally, the ECs mounted strong and broad CD8(+) T-cell responses against several epitopes in Vif and Nef. Mamu-B*08-specific CD8(+) T cells accounted for the majority of mutations in the virus at 18 weeks postinfection. Interestingly, patterns of viral variation in Nef differed between the ECs and the other two macaques. Natural containment of AIDS virus replication in Mamu-B*08-positive macaques may, therefore, be related to a combination of immunodominance and viral escape from CD8(+) T-cell responses.