HIV-specific CD8+ T cell proliferation is coupled to perforin expression and is maintained in nonprogressors

IL-2 Production by Virus- and Tumor-Specific Human CD8 T Cells Is Determined by Their Fine Specificity

Memory CD8 T cells mediate rapid and effective immune responses against previously encountered Ags. However, these cells display considerable phenotypic and functional heterogeneity. In an effort to identify parameters that correlate with immune protection, we compared cell surface markers, proliferation, and cytokine production of distinct virus- and tumor-specific human CD8 populations. Phenotypic analysis of epitope-specific CD8 T cells showed that Ag specificity is associated with distinct CCR7/CD45RA expression profiles, suggesting that Ag recognition drives the expression of these molecules on effector/memory T cells. Moreover, the majority of central memory T cells (CD45RAlowCCR7dull) secreting cytokines in response to an EBV epitope produces both IL-2 and IFN-gamma, whereas effector memory CD8 cells (CD45RAdullCCR7-) found in EBV, CMV, or Melan-A memory pools are mostly composed of cells secreting exclusively IFN-gamma. However, these various subsets, including Melan-A-specific effector memory cells differentiated in cancer patients, display similar Ag-driven proliferation in vitro. Our findings show for the first time that human epitope-specific CD8 memory pools differ in IL-2 production after antigenic stimulation, although they display similar intrinsic proliferation capacity. These results provide new insights in the characterization of human virus- and tumor-specific CD8 lymphocytes.

Antibody-targeted MHC complex–directed expansion of HIV-1– and KSHV-specific CD8+ lymphocytes: a new approach to therapeutic vaccination

The ability of therapeutic vaccines to generate large numbers of CD8+ T lymphocytes that have specificity for HIV-1 or other virally infected cells has enormous potential clinical value. However, approaches to produce cytotoxic T lymphocytes (CTLs) in vivo via vaccine technology have thus far been disappointing and the ex vivo production of cells for adoptive transfer is labor intensive and expensive. We describe the results of a 2-step antibody-targeting system for the production of CD8+ T lymphocytes specific for HIV-1 and Kaposi sarcoma–associated herpesvirus (KSHV), suitable for use in vivo. In 8 consecutive human leukocyte antigen–A2 (HLA-A2)–positive HIV-1–infected individuals with Kaposi sarcoma, 2 cycles of this system resulted in more than 1 Log increases of specific anti-HIV and anti-KSHV CD8+ lymphocytes. These expanded cells have an effector phenotype that includes the ability to produce interferon-γ and CD45Ra+/CD69+ staining. We have shown that antibody-targeted B cells can function as effective antigen-presenting molecules and lead to sustained specific T-lymphocyte expansion from peripheral blood mononuclear cells (PBMCs) of immunosuppressed individuals. This approach, which offers an easy and effective protocol for the amplification of specific antiviral and antitumor CTLs, may offer significant advances for in vivo T-cell immunotherapeutic protocols.

Current advances in HIV vaccines

Development of a safe and preventive HIV-1 vaccine is a high priority. Recent advances in HIV vaccine development include an improved understanding of HIV envelope structure, development of techniques that enable a detailed analysis of vaccine-induced immune responses in humans, expansion of the pipeline of promising candidate vaccines, and completion of the first vaccine efficacy trials. A common feature of several preventive vaccine strategies in early clinical trials is their ability to attenuate clinical disease rather than completely prevent HIV infection in nonhuman primates. One or more candidate vaccines will likely advance into efficacy trials within the next few years, while efforts to identify new designs that induce broadly neutralizing antibodies continue with incremental success.

Persistent recognition of autologous virus by high-avidity CD8 T cells in chronic, progressive human immunodeficiency virus type 1 infection

CD8 T-cell responses are thought to be crucial for control of viremia in human immunodeficiency virus (HIV) infection but ultimately fail to control viremia in most infected persons. Studies in acute infection have demonstrated strong CD8-mediated selection pressure and evolution of mutations conferring escape from recognition, but the ability of CD8 T-cell responses that persist in late-stage infection to recognize viruses present in vivo has not been determined. Therefore, we studied 24 subjects with advanced HIV disease (median viral load = 142,000 copies/ml; median CD4 count = 71/ micro l) and determined HIV-1-specific CD8 T-cell responses to all expressed viral proteins using overlapping peptides by gamma interferon Elispot assay. Chronic-stage virus was sequenced to evaluate autologous sequences within Gag epitopes, and functional avidity of detected responses was determined. In these subjects, the median number of epitopic regions targeted was 13 (range, 2 to 39) and the median cumulative magnitude of CD8 T-cell responses was 5,760 spot-forming cells/10(6) peripheral blood mononuclear cells (range, 185 to 24,700). On average six (range, one to 8) proteins were targeted. For 89% of evaluated CD8 T-cell responses, the autologous viral sequence was predicted to be well recognized by these responses and the majority of analyzed optimal epitopes were recognized with medium to high functional avidity by the contemporary CD8 T cells. Withdrawal of antigen by highly active antiretroviral therapy led to a significant decline both in breadth (P = 0.032) and magnitude (P = 0.0098) of these CD8 T-cell responses, providing further evidence that these responses had been driven by recognition of autologous virus. These results indicate that strong, broadly directed, and high-avidity gamma-interferon-positive CD8 T-cells directed at autologous virus persist in late disease stages, and the absence of mutations within viral epitopes indicates a lack of strong selection pressure mediated by these responses. These data imply functional impairment of CD8 T-cell responses in late-stage infection that may not be reflected by gamma interferon-based screening techniques.

Antibody-Dependent Cellular Cytotoxicity via Humoral Immune Epitope of Nef Protein Expressed on Cell Surface

Antibodies against various proteins of HIV type 1 (HIV-1) can be detected in HIV-1-infected individuals. We previously reported that the level of Ab response against one Nef epitope is correlated with HIV-1 disease progression. To elucidate the mechanism for this correlation, we examined Ab-dependent cellular cytotoxicity (ADCC) against target cells expressing Nef. We observed efficient cytotoxicity against Nef-expressing target cells in the presence of patient plasma and PBMCs. This ADCC activity was correlated with the dilution of plasma from HIV-1-infected patients. Addition of a specific synthetic peptide (peptide 31:FLKEKGGLE) corresponding to the Nef epitope reduced cell lysis to approximately 50%. These results suggest that PBMCs of HIV-1-infected patients may exert ADCC via anti-Nef Abs in the patients' own plasma and serve as a mechanism used by the immune system to regulate HIV-1 replication.

Immune activation and CD8+ T-cell differentiation towards senescence in HIV-1 infection

Progress in the fight against the HIV/AIDS epidemic is hindered by our failure to elucidate the precise reasons for the onset of immunodeficiency in HIV-1 infection. Increasing evidence suggests that elevated immune activation is associated with poor outcome in HIV-1 pathogenesis. However, the basis of this association remains unclear. Through ex vivo analysis of virus-specific CD8(+) T-cells and the use of an in vitro model of naïve CD8(+) T-cell priming, we show that the activation level and the differentiation state of T-cells are closely related. Acute HIV-1 infection induces massive activation of CD8(+) T-cells, affecting many cell populations, not only those specific for HIV-1, which results in further differentiation of these cells. HIV disease progression correlates with increased proportions of highly differentiated CD8(+) T-cells, which exhibit characteristics of replicative senescence and probably indicate a decline in T-cell competence of the infected person. The differentiation of CD8(+) and CD4(+) T-cells towards a state of replicative senescence is a natural process. It can be driven by excessive levels of immune stimulation. This may be part of the mechanism through which HIV-1-mediated immune activation exhausts the capacity of the immune system.

CD4 responses to conserved HIV-1 T helper epitopes show both negative and positive associations with virus load in chronically infected subjects

Characterization of immune responses to immunodominant CD4 epitopes in HIV-1 that are associated with control of HIV infection could be used to strengthen the efficacy of polyepitope HIV vaccines. We measured both the proliferative and the CD4 interferon (IFN)-gamma and interleukin (IL)-2 cytokine responses specific for 11 previously identified HIV-1 T helper epitopes in 10 HIV-infected non-progressors (LTNPs) (infected for a median of 15 years with a stable CD4 count of >500 cells x 10(6)/l), and seven slow progressors (SPs) (infected for a median of 15 years with a CD4 count that had declined to <500 cells x 10(6)/l). Both groups were antiretroviral treatment-naive at the time of evaluation. The median virus load of SP group was higher than that of the LTNP group (P = 0.0002). The CD4 response to a peptide pool representing all potential CD4 Gag epitopes and to Gag p24 protein was also studied. Compared to SPs, LTNPs had higher numbers of Gag-specific IFN-gamma+IL-2+ CD4s (P = 0.0059). The Gag-specific cytokine and proliferative responses correlated inversely with virus load (P = 0.03 and 0.0002, respectively), highlighting the potential importance of this response in immunity to HIV. A direct correlation was noted between proliferation and the Gag-specific IL-2 (P = 0.0053) rather than IFN-gamma response (P = 0.1336), demonstrating that the proliferation assay reflected the IL-2 rather than the IFN-gamma secreting capacity of CD4 cells. Several subjects with diverse class II DRB1 alleles responded, confirming the 11 selected peptides to be both antigenic and conserved. CD4 cytokine responses to one Gag and two conserved Pol peptides correlated negatively with virus load. The cytokine response to two additional Pol peptides correlated positively with virus load. The data indicate that there is not an absolute correlation between the CD4 immune response to conserved and broadly antigenic helper T cell epitopes in HIV non-progression.

MHC class Ia-restricted memory T cells inhibit expansion of a nonprotective MHC class Ib (H2-M3)-restricted memory response

Listeria monocytogenes infection generates major histocompatibility complex (MHC) class Ia-restricted and MHC class Ib-(H2-M3)-restricted effector and memory CD8+ T cells. However, only MHC class Ia-restricted memory cells expand after rechallenge, and it is unknown if MHC class Ib-restricted memory CD8+ T cells generated by vaccination are protective. We show here that H2-M3-restricted memory CD8+ T cells were capable of secondary expansion but, in contrast to primary H2-M3-restricted effector cells, failed to provide protective immunity. In lm-immune mice, MHC class Ia-restricted memory CD8+ T cells prevented the expansion of H2-M3-restricted memory T cell populations by limiting dendritic cell antigen presentation. Thus, protective immunity by H2-M3-restricted T cells is limited to primary infection, indicating that memory MHC class Ia-restricted T cells prevent nonessential immune responses during secondary infection.

Mycobacterium tuberculosisRecall Antigens Suppress HIV-1 Replication in Anergic Donor Cells via CD8+T Cell Expansion and Increased IL-10 Levels

Mycobacterium tuberculosis (MTb) is the leading cause of death in the setting of AIDS. MTb enhances the pathogenicity and accelerates the course of HIV disease and, furthermore, infection with HIV-1 increases the risk of reactivation or reinfection with MTb. In this study, we show that host-specific recall responses to one pathogen, MTb, has a direct effect upon the regulation of a second pathogen, HIV-1. Using cells from immunocompetent former tuberculosis (TB) patients who displayed either a persistently positive (responsive) or negative (anergic), delayed-type hypersensitivity (DTH) reaction to intradermal injection of purified protein derivative (PPD), we investigated the effect of recall Ags to MTb upon the replication of HIV-1 primary isolates in vitro. We show that HIV-1 replication of a T cell-tropic isolate was significantly impaired in MTb-stimulated PBMC from PPD-anergic donors. Furthermore, these donors displayed a significant increase in CD8(+) T cells and IL-10 levels and lower levels of IL-2 and TNF-alpha relative to PPD-responsive donors in response to PPD stimulation. Strikingly, CD8(+) T cell depletion and blocking of IL-10 significantly increased HIV-1 replication in these PPD-anergic donors, indicating that an immunosuppressive response to MTb recall Ags inhibits HIV-1 replication in PPD-anergic individuals. Therefore, immunotherapeutic approaches aimed at recapitulating Ag-specific MTb anergy in vivo could result in novel and effective approaches to inhibit HIV-1 disease progression in MTb/HIV-1 coinfection.

Effector HIV-specific cytotoxic T-lymphocyte activity in long-term nonprogressors: associations with viral replication and progression

Ex vivo effector cytotoxic T-lymphocyte (CTL) activity was assessed in 27 members of the Australian Long-Term Nonprogressor cohort and correlated with genetic, virological, and immunological markers. The 27 individuals were antiretroviral naive with CD4(+) T-cell counts of >500 cells/ microl for more than 8 years after human immunodeficiency virus type 1 (HIV-1) infection. Effector CTL activity was determined using a standard ex vivo chromium release assay. Individuals with CTL activity (HIV-1 env(IIIB) or pol or gag) were then compared to those without CTL activity in relation to plasma HIV-1 RNA, ICD p24 antigen, beta(2)-microglobulin, CD4 and CD8 T-cell counts, CCR5 and CCR2b genotypes, and progression to CD4 <500 cells/microl or commencement of antiretroviral treatment. Of the 27 individuals examined, 19 had no detectable effector CTL activity. The eight individuals with detectable CTL activity had significantly higher plasma levels of HIV-1 RNA (P = 0.014), immune complex dissociated p24 antigen (P = 0.006), and beta(2)-microglobulin (P = 0.009). There was increased risk of progression within 4 years of study entry in individuals with detectable effector CTL activity, higher plasma levels of HIV-1 RNA, higher beta(2)-microglobulin levels, and higher immune complex dissociated p24 antigen levels at enrollment (P = 0.017, P = 0.004, P = 0.027, P = 0.008 respectively). Multivariate analysis demonstrated viral load remained the strongest predictor of disease progression within this group (P = 0.017). There were no significant associations between CTL response and chemokine receptor genotype. These findings demonstrate the importance of HIV replication in generating an effector CTL response and show that effector CTL activity may be an early predictor of progression in people with long-term asymptomatic HIV infection.

Avances en la inmunopatología de la infección por el VIH

The progressive and irreversible destruction of the immune system represents the hallmark of HIV infection. Even though this process is directly related to the infection of CD4 lymphocytes, the destruction of this lymphocyte population observed along infection is not solely due to a direct cytopathic effect elicited by HIV replication. AIDS immune pathogenesis is an extremely complex phenomenon involving different mechanisms which are not fully understood yet. In this article the mechanisms involved in HIV infection and destruction of target cells and the immune response elicited by HIV are analysed. Finally, viral escape mechanisms to host immune response are considered.

Enhanced T-Cell Maturation, Differentiation and Function in HIV-1-Infected Individuals after Growth Hormone and Highly Active Antiretroviral Therapy

Background

Strong virus-specific helper and cytotoxic T-cell responses correlate with non-progression during HIV-1 infection. Administration of antiretroviral therapy (ART) during the chronic phases of HIV-1 infection fails to restore these responses in most patients.

Design and methods

We assessed the changes in immune function of 12 HIV-1-positive individuals treated with ART for over 4 years, who received 4 mg/day of recombinant human growth hormone (rhGH) for 12 weeks and were then randomized into groups receiving either placebo, twice weekly or alternate day dosing of rhGH. Peripheral blood was drawn for phenotypic analysis and functional assays at time points 0, 12 and 24 weeks.

Results

At week 12, we observed significant increases in naive CD4 T cells ( P<0.01) and effector CD8 T cells based on CD45RA and CCR7 expression ( P<0.02). In addition, we observed a rise in HIV-1 antigen-specific CD4 ( P<0.005) and CD8 ( P<0.05) T-cell responses. Twelve weeks post-randomization into placebo, alternate day or twice weekly dosing (24 weeks post-baseline), the phenotype and function of the virus-specific effector CD8 T cells seen at week 12 was maintained in most patients regardless of randomization arm and despite the disappearance of HIV-1-specific CD4 T-cell responses.

Conclusions

Concomitant administration of rhGH at 4 mg/day with highly active ART appears to partially reverse some of the defects exerted on the immune system by HIV-1. This combination may represent a valuable immunotherapeutic intervention aiding in the treatment of chronic HIV-1 infection.

Therapeutic vaccines against infectious diseases

Therapeutic vaccines against chronic infectious diseases aim at eliciting broad humoral and cellular immune responses against multiple target antigens. Importantly, the development of such vaccines will help to establish surrogate markers of protection in humans and thus will augment the subsequent development of efficient prophylactic vaccines. A combination of synthetic small-molecule drugs and immunotherapeutics is likely to represent a powerful means of controlling chronic infections in the future. Challenges faced in developing therapeutic vaccines include the following: first, overcoming the potential impairment of immune responses due to established infection; second, optimizing schedules of vaccine administration in combination with standard of care chemotherapy; and third, defining what biological and immunological read-outs should be used to infer vaccine efficacy.

Therapeutic vaccination with MVA-HIV-1 nef elicits Nef-specific T-helper cell responses in chronically HIV-1 infected individuals

Vaccination is currently considered as an additional therapeutic approach to stimulate HIV-specific immune response in subjects that could not naturally control HIV. Ten chronically HIV infected individuals have been vaccinated with a modified vaccinia Ankara (MVA)-HIV-1(LAI)-nef vector in order to assess safety and immunogenicity. No significant adverse effects were observed during the course of vaccination indicating for the first time that the highly attenuated vaccinia-virus vector MVA is safe in HIV-1 infected individuals. We observed a CD4 T-cell response to Nef in the majority of vaccinated chronically HIV infected individuals. In two subjects CD4 T-cell response was directed to previously unidentified Nef epitopes. The strong Nef-specific CD4 T-cell response elicited by MVA-nef vaccination provides a rationale for immunotherapeutic interventions in HIV infected individuals with suppressed CD4 T-cell responses. Moreover, the CD4 T-cell response elicited was comparable with that usually detected in long-term non-progressor (LTNP) suggesting an improvement in the immunological status of the vaccinated subjects. Furthermore, the new putative CD4 epitopes described here hold promise as important tools for epitope-based vaccination.

Active-specific immunization against melanoma: Is the problem at the receiving end?

The recent progress in tumor immunology is a striking example of the successful application of modern biotechnology to understand the complex phenomenon of immune-mediated cancer rejection. Tumor antigens were identified and successfully utilized in active immunization trials to induce tumor antigen-specific T cells. This achievement has left, however, clinicians and researchers perplexed by the paradoxical observation that immunization-induced T cells can recognize tumor cells in standard assays but cannot induce tumor regression. A closer look at T cell physiology and tumor biology suggests that this observation is not so surprising. Here, we argue that successful immunization is one of several steps required for tumor clearance while more needs to be understood about how T cells localize and are effective within a tumor microenvironment impervious to the execution of their effector function.

Inhibition of HIV-1 infection by monoclonal antibodies to carbohydrates of Schistosoma mansoni

Patients infected with HIV-1 develop a potent humoral immune response against the virus, but HIV-1 primary isolates are remarkably resistant to neutralizing antibodies. Considering that the envelope glycoprotein of HIV-1 (gp120/41) is heavily glycosylated, we investigated whether anti-carbohydrate antibodies could inhibit HIV-1 infection in vitro. We studied the neutralizing activity of three monoclonal antibodies (mAbs) raised to carbohydrates of Schistosoma mansoni, against seven primary isolates of HIV-1. Assays were performed infecting peripheral blood mononuclear cells from normal donors with viral isolates previously treated with mAbs. Viral strains used were tropic for the coreceptors CCR5, CXCR4, and dual-tropic ones. We found that the anti-glycan mAbs vigorously inhibited HIV-1 infection, regardless of the preferential coreceptor usage of the isolate, in a dose-response manner. Importantly, five isolates were resistant to neutralization by two HIV-1 antibody-positive human sera endowed with potent anti-HIV-1 inhibitory activity. Our findings suggest that carbohydrates of the HIV-1 viral envelope may be a target of an effective humoral immune response elicited by vaccination.

Discordant expression of perforin and granzyme A in total and HIV-specific CD8 T lymphocytes of HIV infected children and adolescents

OBJECTIVE

Perforin and granzyme are cytotoxic effector molecules that are believed to play essential roles in cytotoxic T cell (CTL) activity. We tested the hypothesis that dysregulation of these effector molecules contributes to defects of CD8 antiviral immune responses in pediatric subjects in chronic stages of perinatal HIV infection.

DESIGN/METHOD

Studies of CD8 T cells were conducted in 33 treatment experienced HIV+ patients (median age, 10.6 years) and in 14 age-matched healthy controls. CD8 T cells specific for HIV Gag and Pol peptides were identified in HLA-A2+ patients by tetramer binding assays. HIV-specific and total CD8 T cells were examined for perforin, granzyme and expression of CD27, a marker that is lost in terminally differentiated cells.

RESULTS

Three populations of CD8 T cells were identified: granzyme+ perforin+; granzyme+ perforin- and cells negative for both perforin and granzyme. In HIV infected patients, granzyme+ cells were increased in total CD8 T cells (39% versus 13% in controls) and were highest in HIV Gag-specific CD8 cells (42%). Perforin+ CD8 T cells were approximately fivefold fewer than granzyme+ CD8 T cells and were enriched in CD27 negative cells. Most HIV-specific CD8 cells were CD27+. Granzyme expression in CD8 T cells correlated negatively with CD4 percentage and positively with virus load.

CONCLUSION

A disproportionate and generalized increase in CD27+, granzyme+, CD8 T cells is a hallmark of established pediatric HIV infection. These findings support the concept of skewed maturation, with failure of CD8 T cells to mature into perforin-enriched, CD27-negative, effector cells.

Evidence for Gag p24-specific CD4 T cells with reduced susceptibility to R5 HIV-1 infection in a UK cohort of HIV-exposed-seronegative subjects

AIM

To characterize HIV-1 Gag p24-specific CD4 cell responses in HIV-exposed-seronegative (ES) individuals.

METHODOLOGY

Twelve ES individuals, of diverse ethnicity and wild type for the CCR5 Delta-32 mutation, were identified. Controls were HIV-negative blood donors. Gag p24-specific and total Vbeta+ CD4 cells that expressed MIP-1beta, IFN-gamma and IL-2 were enumerated by intracytoplasmic cytokine staining. beta-Chemokine expression was correlated with susceptibility to R5 HIV-1 infection, as measured by polymerase chain reaction for integrated HIV-1 and by p24 enzyme-linked immunosorbent assay.

RESULTS

Similar numbers of mitogen-stimulated and Vbeta+ MIP-1beta+, IFN-gamma+ and IL-2+ T cells were found in ES and HIV-negative control subjects. However, all ES subjects tested had an HIV Gag p24-specific MIP-1beta+, IFN-gamma+ and IL-2+ CD4 T-cell response that was rare in controls. p24-Specific cells of all ES but no control subjects could be expanded by in-vitro Ag/IL-2 stimulation, and when re-stimulated with an overlapping peptide series showed evidence of a broad CD4 cell memory response directed against multiple regions of Gag p24. Mitogen-stimulated ES CD4 cells were as susceptible to HIV infection as those from control subjects, but p24-specific IFN-gamma+ CD4 cells of six out of seven ES subjects tested were less susceptible to R5 HIV-1 infection than the counterpart fraction depleted of p24-specific IFN-gamma+ cells. The addition of blocking anti-beta-chemokine antibodies did not promote R5 HIV-1 infection of p24-specific IFN-gamma+ cells.

CONCLUSION

Specific CD4 cell immunity, characterized by a broadly directed memory Gag-p24 CD4 cell response and reduced susceptibility of specific CD4 cells to R5 HIV-1 infection, is a likely correlate of non-transmission.

Chronic active Epstein-Barr virus infection associated with mutations in perforin that impair its maturation

Chronic active Epstein-Barr virus infection (CAEBV) is a rare disease in which previously healthy persons develop severe, life-threatening illness. Mutations in the perforin gene have been found in familial hemophagocytic lymphohistiocytosis, which shares some features with CAEBV. We studied a patient who died at age 18, 10 years after the onset of CAEBV. The patient had high titers of antibodies to EBV, EBV RNA in lymph nodes, T-cell lymphoproliferative disease, and hemophagocytic lymphohistiocytosis. DNA sequencing showed novel mutations in both alleles of the perforin gene that resulted in amino acid changes in the protein. The quantity of the native form of perforin from the patient's stimulated peripheral blood mononuclear cells (PBMCs) was extremely low and immunoblotting showed accumulation of an uncleaved precursor form of perforin. Stimulated PBMCs from the patient were defective for Fas-independent cytotoxicity. These data imply that mutations in this patient resulted in reduced perforin-mediated cytotoxicity by his lymphocytes. This is the first case in which perforin mutations have been shown to result in accumulation of the uncleaved, immature form of perforin. Mutations in the perforin gene are associated with some cases of CAEBV with hemophagocytic lymphohistiocytosis.

Diminished proliferation of human immunodeficiency virus-specific CD4+ T cells is associated with diminished interleukin-2 (IL-2) production and is recovered by exogenous IL-2

Virus-specific CD4(+) T-cell function is thought to play a central role in induction and maintenance of effective CD8(+) T-cell responses in experimental animals or humans. However, the reasons that diminished proliferation of human immunodeficiency virus (HIV)-specific CD4(+) T cells is observed in the majority of infected patients and the role of these diminished responses in the loss of control of replication during the chronic phase of HIV infection remain incompletely understood. In a cohort of 15 patients that were selected for particularly strong HIV-specific CD4(+) T-cell responses, the effects of viremia on these responses were explored. Restriction of HIV replication was not observed during one to eight interruptions of antiretroviral therapy in the majority of patients (12 of 15). In each case, proliferative responses to HIV antigens were rapidly inhibited during viremia. The frequencies of cells that produce IFN-gamma in response to Gag, Pol, and Nef peptide pools were maintained during an interruption of therapy. In a subset of patients with elevated frequencies of interleukin-2 (IL-2)-producing cells, IL-2 production in response to HIV antigens was diminished during viremia. Addition of exogenous IL-2 was sufficient to rescue in vitro proliferation of DR0101 class II Gag or Pol tetramer(+) or total-Gag-specific CD4(+) T cells. These observations suggest that, during viremia, diminished in vitro proliferation of HIV-specific CD4(+) T cells is likely related to diminished IL-2 production. These results also suggest that relatively high frequencies of HIV-specific CD4(+) T cells persist in the peripheral blood during viremia, are not replicatively senescent, and proliferate when IL-2 is provided exogenously.

Measuring T cell-mediated cytotoxicity using fluorogenic caspase substrates

Cytotoxic T lymphocytes (CTLs) play a major role in the immune response against viruses and other intracellular pathogens. In addition, CTLs are implicated in the control of tumor cells in certain settings. Accurate measures of CTL function are of critical importance to study the pathogenesis of infectious diseases and to evaluate the efficacy of new vaccines and immunotherapies. To this end, we have recently developed a flow cytometry-based CTL (FCC) assay that measures the CTL-induced caspase activation within target cells using cell permeable fluorogenic caspase substrates. This novel assay reliably detects, by flow cytometry or fluorescence/confocal microscopy, antigen-specific CTLs in a wide variety of human and murine systems, and is safer and more informative than the standard 51Cr-release assay. In addition, the flow cytometric CTL (FCC) assay provides an alternative method that is often more sensitive and physiologically informative when compared to previously described FCC assays, as it measures a biological indicator of apoptosis within the target cell. The FCC assay may thus represent a useful tool to further understand the molecular and cellular mechanisms that underlie CTL-mediated killing during tumorigenesis or following infection with viruses or other intracellular pathogens.

Yeast-derived human immunodeficiency virus type 1 p55(gag) virus-like particles activate dendritic cells (DCs) and induce perforin expression in Gag-specific CD8(+) T cells by cross-presentation of DCs

To evaluate the immunogenicity of human immunodeficiency virus (HIV) type 1 p55(gag) virus-like particles (VLPs) released by budding from yeast spheroplasts, we have analyzed the effects of yeast VLPs on monocyte-derived dendritic cells (DCs). Yeast VLPs were efficiently incorporated into DCs via both macropinocytosis and endocytosis mediated by mannose-recognizing receptors, but not the mannose receptor. The uptake of yeast VLPs induced DC maturation and enhanced cytokine production, notably, interleukin-12 p70. We showed that yeast membrane components may contribute to DC maturation partly through Toll-like receptor 2 signaling. Thus, Gag particles encapsulated by yeast membrane may have an advantage in stimulating Gag-specific immune responses. We found that yeast VLPs, but not the control yeast membrane fraction, were able to activate both CD4(+) and CD8(+) T cells of HIV-infected individuals. We tested the effect of cross-presentation of VLP by DCs in two subjects recruited into a long-term nonprogressor-slow progressor cohort. When yeast VLP-loaded DCs of these patients were cocultured with peripheral blood mononuclear cells for 7 days, approximately one-third of the Gag-specific CD8(+) T cells were activated and became perforin positive. However, some of the Gag-specific CD8(+) T cells appeared to be lost during in vitro culture, especially in a patient with a high virus load. Our results suggest that DCs loaded with yeast VLPs can activate Gag-specific memory CD8(+) T cells to become effector cells in chronically HIV-infected individuals, but there still remain unresponsive Gag-specific T-cell populations in these patients.

Turnover of lymphocytes and conceptual paradigms in HIV infection

Prevailing views concerning the pathogenic mechanisms of AIDS have shifted from models that focus primarily on direct HIV-mediated killing of CD4+ T cells to models that emphasize the pathogenic role of generalized immune system activation. The observation that increases in T cell turnover seen in HIV-infected individuals primarily reflect increased proliferation of effector-memory T cells supports the concept that chronic immune activation plays a prominent, if not predominant, role in the pathogenesis of AIDS.

Similarities and differences in CD4+ and CD8+ effector and memory T cell generation

Naive CD4+ and CD8+ T cells undergo unique developmental programs after activation, resulting in the generation of effector and long-lived memory T cells. Recent evidence indicates that both cell-intrinsic and cell-extrinsic factors regulate memory T cell differentiation. This review compares and contrasts how naive CD4+ and CD8+ T cells make the transition to effector and/or memory cells and discusses the implications of these findings for vaccine development.

Quantitative studies of CD8+ T-cell responses during microbial infection

MHC class I tetramer staining, intracellular cytokine staining and ELISPOT assays have made it possible to quantify CD8(+) T-cell responses precisely during and following viral and bacterial infection. Although these quantitative methods are by now familiar and trusted components of the immunologist's toolbox, their application to models of microbial infection continues to provide surprising insights into mammalian adaptive immunity. In the past year there have been many exciting new findings on CD8(+) T-cell priming, expansion and memory formation in response to microbial infection.

Major histocompatibility complex class I alleles associated with slow simian immunodeficiency virus disease progression bind epitopes recognized by dominant acute-phase cytotoxic-T-lymphocyte responses

Certain major histocompatibility complex class I (MHC-I) alleles are associated with delayed disease progression in individuals infected with human immunodeficiency virus (HIV) and in macaques infected with simian immunodeficiency virus (SIV). However, little is known about the influence of these MHC alleles on acute-phase cellular immune responses. Here we follow 51 animals infected with SIV(mac)239 and demonstrate a dramatic association between Mamu-A*01 and -B*17 expression and slowed disease progression. We show that the dominant acute-phase cytotoxic T lymphocyte (CTL) responses in animals expressing these alleles are largely directed against two epitopes restricted by Mamu-A*01 and one epitope restricted by Mamu-B*17. One Mamu-A*01-restricted response (Tat(28-35)SL8) and the Mamu-B*17-restricted response (Nef(165-173)IW9) typically select for viral escape variants in early SIV(mac)239 infection. Interestingly, animals expressing Mamu-A*1 and -B*17 have less variation in the Tat(28-35)SL8 epitope during chronic infection than animals that express only Mamu-A*01. Our results show that MHC-I alleles that are associated with slow progression to AIDS bind epitopes recognized by dominant CTL responses during acute infection and underscore the importance of understanding CTL responses during primary HIV infection.

Immunopathogenesis and immunotherapy in AIDS virus infections

The heterogeneity of HIV and the different human leukocyte antigen (HLA) backgrounds of infected individuals have posed challenges to understanding the pathogenesis of HIV infection. But continuing advances in our knowledge of the role of immune responses in controlling HIV viremia should help to define goals for immune-based therapies and vaccine strategies against AIDS.

Specific nature of cellular immune responses elicited by chimpanzees against HIV-1

Recent epidemiologic and phylogenetic analyses suggest that in the human population human immunodeficiency virus (HIV-1) is a relatively new pathogen that arose by zoonotic transmission from chimpanzees. In humans the morbidity and mortality figures due to HIV infection are extremely high. In a very small percentage of the human population, however, individuals have been identified who were infected for more than 20 years and have no evidence of disease progression. In contrast to most infected humans, almost all chimpanzees appear to be resistant to the pathologic effects caused by lentiviruses such as HIV-1. Here we review the characteristics of the HIV-1-specific cell-mediated immune responses mounted by chimpanzees, and we postulate the mechanisms that have evolved that facilitate their resistance to acquired immunodeficiency syndrome.

Parasitic infection and the polarized Th2 immune response can alter a vaccine-induced immune response

The AIDS epidemic in the Developing World represents a major global crisis. It is imperative that we develop an effective vaccine. Vaccines are economically the most efficient means of controlling viral infections. However, the development of a vaccine against HIV-1 has been a formidable task, and in developing countries chronic parasitic infection adds another level of complexity to AIDS vaccine development. Helminthic and protozoan infections, common in developing countries, can result in a constant state of immune activation that is characterized by a dominant Th2 type of cytokine profile, high IgE levels, and eosinophilia. Such an immune profile may have an adverse impact on the efficacy of vaccines, in particular, an HIV-1 vaccine. Indeed, the CD8 cellular immune response and the corresponding Th1 type cytokines that enhance the CD8 cellular immune response are important for clearing many viral infections. It is believed that an antigen specific CD8 cellular immune response will be an important component of an HIV-1 vaccine.

The differential ability of HLA B*5701+ long-term nonprogressors and progressors to restrict human immunodeficiency virus replication is not caused by loss of recognition of autologous viral gag sequences

Although the HLA B(*)5701 class I allele is highly overrepresented among human immunodeficiency virus (HIV)-infected long-term nonprogressors (LTNPs), it is also present at the expected frequency (11%) in patients with progressive HIV infection. Whether B57(+) progressors lack restriction of viral replication because of escape from recognition of highly immunodominant B57-restricted gag epitopes by CD8(+) T cells remains unknown. In this report, we investigate the association between restriction of virus replication and recognition of autologous virus sequences in 27 B(*)57(+) patients (10 LTNPs and 17 progressors). Amplification and direct sequencing of single molecules of viral cDNA or proviral DNA revealed low frequencies of genetic variations in these regions of gag. Furthermore, CD8(+) T-cell recognition of autologous viral variants was preserved in most cases. In two patients, responses to autologous viral variants were not demonstrable at one epitope. By using a novel technique to isolate primary CD4(+) T cells expressing autologous viral gene products, it was found that 1 to 13% of CD8(+) T cells were able to respond to these cells by gamma interferon production. In conclusion, escape-conferring mutations occur infrequently within immunodominant B57-restricted gag epitopes and are not the primary mechanism of virus evasion from immune control in B(*)5701(+) HIV-infected patients. Qualitative features of the virus-specific CD8(+) T-cell response not measured by current assays remain the most likely determinants of the differential abilities of HLA B(*)5701(+) LTNPs and progressors to restrict virus replication.

Apoptosis as an HIV strategy to escape immune attack

Viruses have evolved numerous mechanisms to evade the host immune system and one of the strategies developed by HIV is to activate apoptotic programmes that destroy immune effectors. Not only does the HIV genome encode pro-apoptotic proteins, which kill both infected and uninfected lymphocytes through either members of the tumour-necrosis factor family or the mitochondrial pathway, but it also creates a state of chronic immune activation that is responsible for the exacerbation of physiological mechanisms of clonal deletion. This review discusses the molecular mechanisms by which HIV manipulates the apoptotic machinery to its advantage, assesses the functional consequences of this process and evaluates how new therapeutics might counteract this strategy.

Multiparameter precursor analysis of T-cell responses to antigen

Triggering of the T-cell receptor by cognate antigen induces a variety of cellular events leading to cell proliferation and differentiation. While the plasticity and diversity of T-cell responses have been recognized for a long time, few quantitative studies have been conducted to measure what proportion of specific T cells will enter a given differentiation program after antigen stimulation. In the present study, we analyzed human T cells cultured with influenza-peptide-loaded dendritic cells. We compared three individual methods for assaying the frequency of antigen-specific T cells: ELISPOT, tetramer-binding, and proliferation. The three methods yielded similar but not identical results. In order to study these differences at the single cell level, we developed a multiparameter flow cytometric method, which allows simultaneous analysis of antigen-specific tetramer binding, T-cell proliferation, and cytokine production. Based on these data, we used flow precursor frequency analysis to calculate the proportion of eight different precursor subsets in the original, resting population. We conclude that approximately half of the cells that bound specific tetramers actually proliferated and synthesized IFNgamma in response to antigen. In addition, similar numbers of cells that did not bind tetramer proliferated (but did not synthesize IFNgamma). The method allows for an estimate of the precursor frequency of each functional subset within the initial population. It could be applied to additional markers of function and differentiation, combining all parameters into a description of the complex response potential of a T-cell pool.

Differential impairment of lytic and cytokine functions in senescent human immunodeficiency virus type 1-specific cytotoxic T lymphocytes

Telomere length is abnormally short in the CD8(+) T-cell compartment of human immunodeficiency virus type 1 (HIV-1)-infected persons, likely because of chronic cell turnover. Although clonal exhaustion of CD8(+) cytotoxic T lymphocytes (CTL) has been proposed as a mechanism for loss of antigen-specific responses, the functional consequences of exhaustion are poorly understood. Here we used telomerase transduction to evaluate the impact of senescence on CTL effector functions. Constitutive expression of telomerase in an HIV-1-specific CTL clone results in enhanced proliferative capacity, in agreement with prior studies of other human cell types. Whereas the CTL remain phenotypically normal in terms of antigenic specificity and requirements for proliferation, their cytolytic and antiviral capabilities are superior to those of control CTL. In contrast, their ability to produce gamma interferon and RANTES is essentially unchanged. The selective enhancement of cytolytic function in memory CTL by ectopic telomerase expression implies that loss of this function (but not cytokine production) is a specific consequence of replicative senescence. These data suggest a unifying mechanism for the in vivo observations that telomere lengths are shortened in the CD8(+) cells of HIV-1-infected persons and that HIV-1-specific CTL are deficient in perforin. Telomerase transduction could therefore be a tool with which to explore a potential therapeutic approach to an important pathophysiologic process of immune dysfunction in chronic viral infection.

Nonpathogenic SIV infection of sooty mangabeys is characterized by limited bystander immunopathology despite chronic high-level viremia

HIV-infected humans and SIV-infected rhesus macaques who remain healthy despite long-term infection exhibit exceptionally low levels of virus replication and active antiviral cellular immune responses. In contrast, sooty mangabey monkeys that represent natural hosts for SIV infection do not develop AIDS despite high levels of virus replication and limited antiviral CD8(+) T cell responses. We report here that SIV-infected mangabeys maintain preserved T lymphocyte populations and regenerative capacity and manifest far lower levels of aberrant immune activation and apoptosis than are seen in pathogenic SIV and HIV infections. These data suggest that direct consequences of virus replication alone cannot account for progressive CD4(+) T cell depletion leading to AIDS. Rather, attenuated immune activation enables SIV-infected mangabeys to avoid the bystander damage seen in pathogenic infections and protects them from developing AIDS.

Lineage relationship and protective immunity of memory CD8 T cell subsets

Memory CD8 T cells can be divided into two subsets, central (T(CM)) and effector (T(EM)), but their lineage relationships and their ability to persist and confer protective immunity are not well understood. Our results show that T(CM) have a greater capacity than T(EM) to persist in vivo and are more efficient in mediating protective immunity because of their increased proliferative potential. We also demonstrate that, following antigen clearance, T(EM) convert to T(CM) and that the duration of this differentiation is programmed within the first week after immunization. We propose that T(CM) and T(EM) do not necessarily represent distinct subsets, but are part of a continuum in a linear naive --> effector --> T(EM) --> T(CM) differentiation pathway.

Will we be able to 'spot' an effective HIV-1 vaccine?

Many HIV-1 vaccine efforts have heavily emphasized class I-restricted CD8(+) cytotoxic T lymphocytes (CTLs) as a potentially important arm of immunity. Commonly used CTL assays describe only specificity and frequency, and not antiviral effects. However, increasing evidence indicates that CTL antiviral function is determined by the complex interplay of multiple virologic and cellular factors, and thus these measurements are an incomplete reflection of CTL efficacy. Our current understanding of the factors determining HIV-1-specific CTL antiviral efficacy is inadequate to interpret fully the significance of detected CTLs in vaccine and pathogenesis studies. To assess CTLs as a protective immune response, it will be crucial to elucidate these mechanisms and/or devise new assays that directly reflect the interaction of CTLs and HIV-1.

New insights into the immunology and evolution of HIV

Fewer than one million HIV infected individuals are currently receiving anti-retroviral therapy. The limitations of such treatment have underscored the need to develop more effective strategies to control the spread and pathogenesis of HIV. Typically, naturally occurring protective immune responses provide the for such development. It is now clear however that HIV can utilise the milieu of an activated immune system to its own replicative advantage. Mobilisation of the immune response, intended to thwart the virus, may instead fuel its dissemination, 'immune escape' and spread. The immense genetic variation of HIV contributes to lack of immune control and the development of progressive disease in the majority of infected, untreated individuals. Further delineation of the intimate interactions between the HIV and the immune system will be critical and recent advances in this direction are discussed.