A significant number of human immunodeficiency virus epitope-specific cytotoxic T lymphocytes detected by tetramer binding do not produce gamma interferon

Evaluation of cytomegalovirus-specific T-cell reconstitution in patients after various allogeneic haematopoietic stem cell transplantation using interferon-gamma-enzyme-linked immunospot and human leucocyte antigen tetramer assays with an immunodominant T-cell epitope

Cytomegalovirus (CMV) infection is a major complication for patients who received allogeneic haematopoietic stem cell transplantation (HSCT). Accurate monitoring of CMV-specific T-cell reconstitution is required for appropriate decision on treatment, such as anti-viral drugs, which have adverse effects. Although human leucocyte antigen (HLA) tetramer and interferon-gamma-enzyme-linked immunospot (IFN-gamma-ELISPOT) assays have been used to measure CMV-specific T cells, detailed comparison of these assays and kinetics of anti-CMV T-cell reconstitution between reduced-intensity transplantation (RIST) and conventional HSCT has not yet been performed. In this study, we performed prospective comparative monitoring of CMV-specific T cells using HLA tetramer and IFN-gamma-ELISPOT assays with a single immunodominant CMV(495) peptide in 28 HLA-A*0201 and 9 HLA-A*0206 patients after various allogeneic HSCTs. The IFN-gamma-ELISPOT assay was more sensitive for evaluation of functional T cells than the HLA tetramer assay, and CMV-specific T cells were reconstituted earlier in patients who received RIST without anti-thymocyte globulin (ATG) than those receiving RIST with ATG or conventional HSCT. The threshold level for protection from CMV reactivation was estimated as over 1 x 10(6) cells/l peripheral blood with the IFN-gamma-ELISPOT assay. These results demonstrate that the IFN-gamma-ELISPOT assay with CMV(495) provides more accurate evaluation on CMV immunity in HLA-A*0201 and -A*0206 patients, and may be useful for determining timing of various treatments.

Role of thymic output in regulating CD8 T-cell homeostasis during acute and chronic viral infection

Although it is well documented that CD8 T cells play a critical role in controlling chronic viral infections, the mechanisms underlying the regulation of CD8 T-cell responses are not well understood. Using the mouse model of an acute and chronic lymphocytic choriomeningitis virus (LCMV) infection, we have examined the relative importance of peripheral T cells and thymic emigrants in the elicitation and maintenance of CD8 T-cell responses. Virus-specific CD8 T-cell responses were compared between mice that were either sham thymectomized or thymectomized (Thx) at approximately 6 weeks of age. In an acute LCMV infection, thymic deficiency did not affect either the primary expansion of CD8 T cells or the proliferative renewal and maintenance of virus-specific lymphoid and nonlymphoid memory CD8 T cells. Following a chronic LCMV infection, in Thx mice, although the initial expansion of CD8 T cells was normal, the contraction phase of the CD8 T-cell response was exaggerated, which led to a transient but striking CD8 T-cell deficit on day 30 postinfection. However, the virus-specific CD8 T-cell response in Thx mice rebounded quickly and was maintained at normal levels thereafter, which indicated that the peripheral T-cell repertoire is quite robust and capable of sustaining an effective CD8 T-cell response in the absence of thymic output during a chronic LCMV infection. Taken together, these findings should further our understanding of the regulation of CD8 T-cell homeostasis in acute and chronic viral infections and might have implications in the development of immunotherapy.

Toward a detailed characterization of feline immunodeficiency virus-specific T cell immune responses and mediated immune disorders

Infection of domestic cats with feline immunodeficiency virus (FIV) is associated with the development of an acquired immunodeficiency syndrome (AIDS). The pathogenesis of FIV is not fully understood but it has been reported that the immune system is progressively impaired during disease progression. As a result, anti-FIV specific immune response will usually not clear the virus and the acute stage is followed by a chronic asymptomatic phase. The overall objective of this study was to characterized FIV-induced immune cellular responses and -mediated immune disorder following the first weeks post-infection. Using both cytokine ELISpot and intracellular staining assays, FIV-specific T cells were monitored at 6, 9 and 12 weeks post-infection. We demonstrated that both IFNgamma(+) and, CD4 and CD8 TNFalpha(+) T cells specifically respond to FIV antigens. These responses were found to reach a peak at 9 weeks post-infection. It was further shown that the TNFalpha(+)CD8(+) responding T cells were contained within a CD8beta(low)CD62L(-) T cell subpopulation, expanded in FIV-infected cats. This T cell subpopulation which present features of activated CD8 T cells was further shown to be susceptible to spontaneous apoptosis following a short-term in vitro culture. Moreover, it was observed that cell death by apoptosis of this T cell subset was increased following FIV antigen-recognition. Therefore, FIV might alter immune homeostasis in inducing chronic activation of TNFalpha(+)CD8(+) T cells which eventually will die following antigen contact while deleting CD4(+) T cells. Interestingly, this study confirmed the strong similarity between FIV and HIV pathogenesis.

Therapeutic HIV vaccines: An update

The goal of therapeutic immunization in HIV infection is to induce strong and sustained immune responses against HIV. Several arguments suggest that the immune system may contain HIV replication in the long term before declining. The potency on new antiviral regimens in clinic has provided an opportunity to improve the control of the virus replication by the immune system. In clinical terms, this might allow patients to delay or to stop antiviral therapy and to minimize the duration of the exposition to antiviral drugs.

Not All Tetramer Binding CD8+ T Cells Can Produce Cytokines and Chemokines Involved in the Effector Functions of Virus-Specific CD8+ T Lymphocytes in HIV-1 Infected Children

In the pediatric human immunodeficiency virus type-1 (HIV-1) infection, the presence of cytotoxic T lymphocytes (CTL) is associated with a slow progression to AIDS. The secretion of cytokines by CTLs may be critical in the control of viral infection. We used the combination of cell surface and intracellular staining to study the functionality of tetramer binding CD8+ T cells recognizing two HIV-1 immunodominant epitopes, in peripheral blood mononuclear cells from HIV-1-infected children. A fraction of tetramer positive CD8+ T cells produce cytokines (IFN-gamma, TNF-alpha) or chemokines (CCL4, CCL5) after ex vivo stimulation with the cognate peptide. There was a negative correlation between the plasma viral load and the percentage of CD8+ Tetramer Gag+ T cells secreting IFN-gamma. This is the first report in the context of pediatric HIV-1 infection showing that only a fraction of HIV-1-specific CD8+ T cells have the capacity to produce cytokines and chemokines implicated in their antiviral functions.

Genetic manipulation of telomerase in HIV-specific CD8+ T cells: enhanced antiviral functions accompany the increased proliferative potential and telomere length stabilization

A large proportion of the CD8(+) T cell pool in persons chronically infected with HIV consists of cells that show features of replicative senescence, an end stage characterized by irreversible cell cycle arrest, multiple genetic and functional changes, and shortened telomeres. The objective of our research was to determine whether constitutive expression of the gene for the human telomerase (hTERT) can prevent senescence-induced impairments in human virus-specific CD8(+) T cells, particularly in the context of HIV-1 disease. Our results indicate that hTERT-expressing HIV-specific CD8(+) lymphocytes show both an enhanced and sustained capacity to inhibit HIV-1 replication in in vitro coculture experiments, as well as prolonged ability to produce IFN-gamma and TNF-alpha in response to stimulation with HIV-1-derived peptides, as compared with vector-transduced controls. Loss of CD28 expression, the signature change of replicative senescence in cell culture, was retarded in those CD8(+) T cell cultures that had high levels of CD28 at the time of hTERT transduction. These findings suggest that telomere shortening may be the primary driving force behind several aspects of CD8(+) T cell dysfunction associated with replicative senescence. We also demonstrate reduced accumulation of the p16(INK4a) and p21(WAF1) cell cycle inhibitors in hTERT-transduced lymphocytes, providing a possible mechanism by which stable hTERT expression is able to circumvent the senescence barrier in CD8(+) T cells. Given the key role of CD8(+) T cell function in controlling a variety of acute and latent viral infections, approaches to retard the functional decrements associated with replicative senescence may lead to novel types of immunotherapy.

Therapeutic dendritic-cell vaccine for chronic HIV-1 infection

We present the results of a preliminary investigation of the efficacy of a therapeutic dendritic cell (DC)-based vaccine for HIV-1. We immunized 18 chronically HIV-1-infected and currently untreated individuals showing stable viral loads for at least 6 months with autologous monocyte-derived DCs loaded with autologous aldrithiol-2-inactivated HIV-1. Plasma viral load levels were decreased by 80% (median) over the first 112 d following immunization. Prolonged suppression of viral load of more than 90% was seen in 8 individuals for at least 1 year. The suppression of viral load was positively correlated with HIV-1-specific interleukin-2 or interferon-gamma-expressing CD4(+) T cells and with HIV-1 gag-specific perforin-expressing CD8(+) effector cells, suggesting that a robust virus-specific CD4(+) T-helper type 1 (T(H)1) response is required for inducing and maintaining virus-specific CD8(+) effectors to contain HIV-1 in vivo. The results suggest that inactivated whole virus-pulsed DC vaccines could be a promising strategy for treating people with chronic HIV-1 infection.

Identification of seventeen new simian immunodeficiency virus-derived CD8+ T cell epitopes restricted by the high frequency molecule, Mamu-A*02, and potential escape from CTL recognition

MHC class I-restricted CD8+ T cells play an important role in controlling HIV and SIV replication. In SIV-infected Indian rhesus macaques (Macaca mulatta), comprehensive CD8+ T cell epitope identification has only been undertaken for two alleles, Mamu-A*01 and Mamu-B*17. As a result, these two molecules account for virtually all known MHC class I-restricted SIV-derived CD8+ T cell epitopes. SIV pathogenesis research and vaccine testing have intensified the demand for epitopes restricted by additional MHC class I alleles due to the shortage of Mamu-A*01+ animals. Mamu-A*02 is a high frequency allele present in over 20% of macaques. In this study, we characterized the peptide binding of Mamu-A*02 using a panel of single amino acid substitution analogues and a library of 497 unrelated peptides. Of 230 SIVmac239 peptides that fit the Mamu-A*02 peptide-binding motif, 75 peptides bound Mamu-A*02 with IC50 values of < or = 500 nM. We assessed the antigenicity of these 75 peptides using an IFN-gamma ELISPOT assay with freshly isolated PBMC from eight Mamu-A*02+ SIV-infected macaques and identified 17 new epitopes for Mamu-A*02. The synthesis of five Mamu-A*02 tetramers demonstrated the discrepancy between tetramer binding and IFN-gamma secretion by SIV-specific CD8+ T cells during chronic SIV infection. Bulk sequencing determined that 2 of the 17 epitopes accumulated amino acid replacements in SIV-infected macaques by the chronic phase of infection, suggestive of CD8+ T cell escape in vivo. This work enhances the use of the SIV-infected macaque model for HIV and increases our understanding of the breadth of CD8+ T cell responses in SIV infection.

Evaluating the cytotoxicity of innate immune effector cells using the GrB ELISPOT assay

Background

This study assessed the Granzyme B (GrB) ELISPOT as a viable alternative to the ⁵¹Cr-release assay for measuring cytotoxic activity of innate immune effector cells. We strategically selected the GrB ELISPOT assay because GrB is a hallmark effector molecule of cell-mediated destruction of target cells.

Methods

We optimized the GrB ELISPOT assay using the human-derived TALL-104 cytotoxic cell line as effectors against K562 target cells. Titration studies were performed to assess whether the ELISPOT assay could accurately enumerate the number of GrB-secreting effector cells. TALL-104 were treated with various secretion inhibitors and utilized in the GrB ELISPOT to determine if GrB measured in the ELISPOT was due to degranulation of effector cells. Additionally, CD107a expression on effector cells after effector-target interaction was utilized to further confirm the mechanism of GrB release by TALL-104 and lymphokine-activated killer (LAK) cells. Direct comparisons between the GrB ELISPOT, the IFN-γ ELISPOT and the standard ⁵¹Cr-release assays were made using human LAK cells.

Results

Titration studies demonstrated a strong correlation between the number of TALL-104 and LAK effector cells and the number of GrB spots per well. GrB secretion was detectable within 10 min of effector-target contact with optimal secretion observed at 3–4 h; in contrast, optimal IFN-γ secretion was not observed until 24 h. The protein secretion inhibitor, brefeldin A, did not inhibit the release of GrB but did abrogate IFN-γ production by TALL-104 cells. GrB secretion was abrogated by BAPTA-AM (1,2-bis-(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid tetra(acetoxymethyl) ester), which sequesters intracellular Ca²⁺, thereby preventing degranulation. The number of effector cells expressing the degranulation associated glycoprotein CD107a increased after interaction with target cells and correlated with the stimulated release of GrB measured in the ELISPOT assay.

Conclusions

Because of its high sensitivity and ability to estimate cytotoxic effector cell frequency, the GrB ELISPOT assay is a viable alternative to the ⁵¹Cr-release assay to measure MHC non-restricted cytotoxic activity of innate immune cells. Compared to the IFN-γ ELISPOT assay, the GrB ELISPOT may be a more direct measure of cytotoxic cell activity. Because GrB is one of the primary effector molecules in natural killer (NK) cell-mediated killing, detection and enumeration of GrB secreting effector cells can provide valuable insight with regards to innate immunological responses.

Boosting T cell-mediated immunity to tyrosinase by vaccinia virus-transduced, CD34(+)-derived dendritic cell vaccination: a phase I trial in metastatic melanoma

PURPOSE

Six American Joint Committee on Cancer stage IV melanoma patients were enrolled into a Phase I study of vaccination with autologous CD34(+)-derived dendritic cells transduced with a modified vaccinia Ankara virus encoding human tyrosinase gene (MVA-hTyr).

EXPERIMENTAL DESIGN

Patients received a first intravenous injection of 1 x 10(8) MVA-hTyr-transduced dendritic cells, followed by three s.c. injections at a 14-day interval.

RESULTS

Treatment was well tolerated, except for low-grade fever (three of six patients), mild erythema at injection site (five of six), and vitiligo (two of six). A partial response, involving shrinkage of an s.c. nodule, later surgically removed, was observed in 1 patient, who then remained disease-free (>850 days). By human lymphocyte antigen tetramer analysis, significant and often long-lasting increases in frequency of T cells directed to tyrosinase(368-376) but not to gp100(209-217) were documented in periphery of 4 of 5 HLA-A*0201+ patients, a few days after vaccine administration. In addition, maturation phenotype of tyrosinase-specific T cell shifted toward the T effector memory/T terminally differentiate stages (CCR7(-)CD45RA(-/+)) in synchrony with the T-cell frequency peaks. By enzyme-linked immunospot in peripheral blood of five HLA-A*0201+ patients, we found that the vaccine could induce interferon gamma-releasing effector cells directed to HLA-A*0201/tyrosinase(368-376) and to vaccinia virus HLA-A*0201/H3L(184-192) epitopes. Moreover, an interferon gamma response after vaccination was elicited even against the HLA-DRB1-1501/tyrosinase(386-406) epitope in one out of two HLA-A* DRB1-01501+ patients.

CONCLUSIONS

These results indicate that vaccination with MVA-hTyr-transduced dendritic cells is well tolerated, can possibly produce clinical responses, and activates tyrosinase- and vaccinia virus-specific T cells in vivo. These data suggest a broad utility of the MVA vector for targeting tumor-associated antigens to dendritic cells for tumor immunotherapy.

Increased expression of the natural killer cell inhibitory receptor CD85j/ILT2 on antigen-specific effector CD8 T cells and its impact on CD8 T-cell function

We investigated whether inhibitory natural killer cell receptor (iNKR) expression contributes to impaired antigen-specific cytotoxicity and interferon-gamma (IFN-gamma) production by CD8 T cells during chronic infection. iNKR immunoglobulin-like transcript-2 (ILT2/CD85j) is expressed on 40-55% of cytomegalovirus (CMV)-, Epstein-Barr virus (EBV)- and human immunodeficiency virus (HIV)-specific CD8 T cells in both healthy and HIV-infected donors. Other iNKRs (CD158a, b1, e1/e2, k, CD94/NKG2A) are expressed on only a small minority of CD8 T cells and are not preferentially expressed on tetramer-staining virus-specific cells. In normal donors, ILT2 is expressed largely on perforin(+) CD27(-) effector cells. However, in HIV-infected donors, only a third of ILT2(+) cells are also perforin(+). In both normal and HIV-infected donors, ILT2(+) cells are prone to spontaneous apoptosis. Therefore, ILT2 is normally expressed during effector cytotoxic T-lymphocyte (CTL) differentiation, but can also be expressed when effector maturation is incomplete, as in HIV infection. The effect of ILT2 on CD8 cell function was assessed by preincubating effector cells with ILT2 antibody. While blocking ILT2 engagement has no appreciable effect on cytotoxicity, it increases antiviral IFN-gamma production by approximately threefold in both normal and HIV-infected donors. Thus, ILT2 expression, increased on antiviral CD8 cells in chronic infection, may interfere with protective CD8 T-cell function by suppressing IFN-gamma production.

Dissection of antiviral and immune regulatory functions of tumor necrosis factor receptors in a chronic lymphocytic choriomeningitis virus infection

The effector function of CD8 T cells is mediated via cell-mediated cytotoxicity and production of cytokines like gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha). While the roles of perforin-dependent cytotoxicity, IFN-gamma, and TNF-alpha in controlling acute viral infections are well studied, their relative importance in defense against chronic viral infections is not well understood. Using mice deficient for TNF receptor (TNFR) I and/or II, we show that TNF-TNFR interactions have a dual role in mediating viral clearance and downregulating CD8 and CD4 T-cell responses during a chronic lymphocytic choriomeningitis virus (LCMV) infection. While wild-type (+/+) and TNFR II-deficient (p75(-/-)) mice cleared LCMV from the liver and lung, mice deficient in TNFR I (p55(-/-)) or both TNFR I and TNFR II (double knockout [DKO]) exhibited impaired viral clearance. The inability of p55(-/-) and DKO mice to clear LCMV was not a sequel to either suboptimal activation of virus-specific CD8 or CD4 T cells or impairment in trafficking of LCMV-specific CD8 T cells to the liver and lung. In fact, the expansion of LCMV-specific CD8 and CD4 T cells was significantly higher in DKO mice compared to that in +/+, p55(-/-), and p75(-/-) mice. TNFR deficiency did not preclude the physical deletion of CD8 T cells specific for nucleoprotein 396 to 404 but delayed the contraction of CD8 T-cell responses to the epitopes GP33-41 and GP276-285 in the viral glycoprotein. The antibody response to LCMV was not significantly altered by TNFR deficiency. Taken together, these findings have implications in development of immunotherapy in chronic viral infections of humans.

Differential tissue-specific regulation of antiviral CD8+ T-cell immune responses during chronic viral infection

The hallmarks of the immune response to viral infections are the expansion of antigen-specific CD8(+) cytotoxic T lymphocytes (CTLs) after they encounter antigen-presenting cells in the lymphoid tissues and their subsequent redistribution to nonlymphoid tissues to deal with the pathogen. Control mechanisms exist within CTL activation pathways to prevent inappropriate CTL responses against disseminating infections with a broad distribution of pathogen in host tissues. This is demonstrated during overwhelming infection with the noncytolytic murine lymphocytic choriomeningitis virus, in which clonal exhaustion (anergy and/or deletion) of CTLs prevents immune-mediated pathology but allows persistence of the virus. The mechanism by which the immune system determines whether or not to mount a full response to such infections is unknown. Here we present data showing that the initial encounter of specific CTLs with infected cells in lymphoid tissues is critical for this decision. Whether the course of the viral infection is acute or persistent for life primarily depends on the degree and kinetics of CTL exhaustion in infected lymphoid tissues. Virus-driven CTL expansion in lymphoid tissues resulted in the migration of large quantities of CTLs to nonlymphoid tissues, where they persisted at stable levels. Surprisingly, although virus-specific CTLs were rapidly clonally exhausted in lymphoid tissues under conditions of chronic infection, a substantial number of them migrated to nonlymphoid tissues, where they retained an effector phenotype for a long time. However, these cells were unable to control the infection and progressively lost their antiviral capacities (cytotoxicity and cytokine secretion) in a hierarchical manner before their eventual physical elimination. These results illustrate the differential tissue-specific regulation of antiviral T-cell responses during chronic infections and may help us to understand the dynamic relationship between antigen and T-cell populations in many persistent infections in humans.

HIV-1-specific cytotoxicity is preferentially mediated by a subset of CD8(+) T cells producing both interferon-gamma and tumor necrosis factor-alpha

CD8(+) T cells play a crucial role in the control of viral infections by direct elimination of infected cells and secretion of a number of soluble factors. Recent data suggest that HIV-1-specific CD8(+) T cell subsets may differ in their ability to exert these effector functions. Here, we directly compared the cytokine secretion patterns and cytotoxic capacity of HIV-1-specific CD8(+) T cells, using a flow-cytometric cytotoxicity assay based on caspase-3 activation in dying target cells. These experiments revealed considerable intraindividual and interindividual differences among epitope-specific T-cell effector functions: while the frequency of HIV-1-specific CD8(+) T cells secreting interferon-gamma but no tumor necrosis factor-alpha (TNF-alpha) following antigenic stimulation was only weakly correlated to their cytotoxic activity (R = 0.05, P =.57), a subset of CD8(+) T cells secreting both inter-feron-gamma and TNF-alpha was substantially more strongly associated with cytotoxicity (R = 0.67, P <.001). This subset of CD8(+) T cells also exhibited stronger intracellular perforin expression and more pronounced direct ex vivo HIV-1-specific cytoxicity than CD8(+) T cells secreting solely interferon-gamma following sorting of these subpopulations according to their cytokine profile. These results suggest that HIV-1-specific cytotoxicity of CD8(+) T cells is preferentially mediated by a subset of CD8(+) T cells secreting both interferon-gamma and TNF-alpha.

Functional simian immunodeficiency virus Gag-specific CD8+ intraepithelial lymphocytes in the mucosae of SIVmac251- or simian-human immunodeficiency virus KU2-infected macaques

The vaginal and rectal mucosae are the first line of cellular immune defense to sexually transmitted human immunodeficiency virus type 1 (HIV-1) entry. Thus, intraepithelial lymphocytes (IELs) may be important in the immune response to HIV infection. Here we investigated whether functional IELs in mucosal compartments could be visualized by direct staining with a tetrameric complex specific for the simian immunodeficiency virus (SIV) immunodominant Gag epitope in either separated IEL cells or tissues of macaques infected with SIVmac251. Of the 15 Mamu-A*01-positive macaques studied here, eight were chronically infected with either SIVmac251 or simian-human immunodeficiency virus (SHIV) KU2 and the remaining seven were exposed mucosally to SIVmac251 and sacrificed within 48 h to assess the local immune response. Gag-specific CD8+ T-cells were found in separated IELs from the rectum, colon, jejunum, and vagina of most infected animals. Direct staining of tetramers also revealed their presence in intact tissue. These Gag-specific IELs expressed the activation marker CD69 and produced IFN-gamma, suggesting an active immune response in this locale.

A method to increase tetramer staining efficiency of CD8+ T cells with MHC-peptide complexes: therapeutic applications in monitoring cytotoxic T lymphocyte activity during hepatitis B and C treatment

The development of peptide-MHC tetrameric complexes heralds a new era in the study of antigen-specific T cells and their role in viral infections. However, the frequencies of tetramer-staining CD8+ T cells in fresh peripheral blood mononuclear cells (PBMCs) are usually below 1% in patients with chronic hepatitis B and C viruses (HBV and HCV) as well as human immunodeficiency virus (HIV) infections, which makes difficult the comparison and sequential evaluation of different individuals. Thus, the development of a method to enumerate efficiently antigen-specific CD8+ T cells will be clinically beneficial in monitoring the antiviral cellular immunity during therapy. We report here a modified CRI-p culture method (cytotoxic T lymphocyte response index of the epitope-peptide method), using a panel of peptides to stimulate PBMCs in bulk culture. The modified CRI-p cultured cells were, in turn, subjected to fluorescence-activated cell sorter (FACS) analysis, tetramer staining or T-cell functional assays to quantify the antiviral immunity of HLA-A2 (+) HBV and HCV patients receiving antiviral therapies. The results obtained showed that patients with a sustained response had a significantly higher increase in the frequencies of tetramer staining of virus-specific CD8+ T cells than did nonresponders. This method permits semi-quantitative determination of the relative strength of CTL activity against a panel of peptides and provides a large number of cells for FACS analysis from a single blood sampling. Significantly, it achieves high frequencies of tetramer staining of CD8+ T cells allowing the data of different individuals to be easily compared and sequentially evaluated. The mechanisms involved in this method are discussed.

Optimization of recombinant vaccinia-based ELISPOT assay

The ELISPOT assay has been considered as one of the most sensitive assays to measure antigen-specific CD8 T cells in vitro. Recently, recombinant vaccinia was successfully used to express internally processed target antigens in host cells in direct ex-vivo ELISPOT assays. However, the background in these assays was relatively elevated, and the risk of killing effector T cells was high. Therefore, we examined in this study an alternative approach where the replication of recombinant vaccinia virus was inhibited by the usage of Cidofovir in vitro. Our data indicate that recombinant vaccinia-infected target cells treated with Cidofovir retained their functional activity and present internally processed antigens more efficiently to T cells than non-treated ones. We also identify the optimum doses of Cidofovir to be in the range of 0.75-0.075 microg/ml. Thus, Cidofovir treatment of the target cells prior to antigen stimulation could be a useful methodology to increase the sensitivity of the ELISPOT assay.

Immunogenicity of multiple gene and clade human immunodeficiency virus type 1 DNA vaccines

The ability to elicit an immune response to a spectrum of human immunodeficiency virus type 1 (HIV-1) gene products from divergent strains is a desirable feature of an AIDS vaccine. In this study, we examined combinations of plasmids expressing multiple HIV-1 genes from different clades for their ability to elicit humoral and cellular immune responses in mice. Immunization with a modified Env, gp145DeltaCFI, in combination with a Gag-Pol-Nef fusion protein plasmid elicited similar CD4(+) and CD8(+) cellular responses to immunization with either vector alone. Further, when mice were immunized with a mixture of Env from three clades, A, B, and C, together with Gag-Pol-Nef, the overall potency and balance of CD4(+)- and CD8(+)-T-cell responses to all viral antigens were similar, with only minor differences noted. In addition, plasmid mixtures elicited antibody responses comparable to those from individual inoculations. These findings suggest that a multigene and multiclade vaccine, including components from A, B, and C Env and Gag-Pol-Nef, can broaden antiviral immune responses without immune interference. Such combinations of immunogens may help to address concerns about viral genetic diversity for a prospective HIV-1 vaccine.

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.

Phenotype and Functional Characteristics of HIV-Specific Cytotoxic CD8+ T Cells in Chronically Infected Patients

BACKGROUND

Cytotoxic CD8+ T cells (CTLs) are critical for the control of viral infections. Although these cells can be recognized in most HIV-infected individuals, they fail to successfully control HIV replication. Distinct functional defects seem to limit their efficacy in HIV infection, although they have been not fully elucidated.

PATIENTS AND METHODS

Blood lymphocytes collected from 61 HLA-A0201+, untreated, chronically HIV-infected individuals were examined for the presence of CTLs against epitopes from HIV Gag and Pol proteins, using tetrameric complexes. Several functional aspects of these cells were further analyzed (immunophenotype; ability to produce interferon-gamma (IFN-gamma) in response to the specific peptide; proliferative capacity; and cytolytic activity). Lymphoproliferative responses of these cells confronting different stimulus were also evaluated. A longitudinal analysis was carried out in a subgroup who underwent antiretroviral therapy and were followed for 6 months.

RESULTS

CD8+ T cells staining with the tetramer complexes (Tet+) were detected in 44% of patients, with TetGag+ cells being more frequently detected and at higher levels than TetPol+ cells. Most Tet+ cells expressed a memory phenotype, showed an impaired ability to produce IFN-gamma when stimulated with the cognate peptide, and showed a very low expansion when cultured in the presence of the peptide. There was a negative correlation between the proportion of Tet+ cells producing IFN-gamma and plasma HIV-RNA. Although Tet+ cells diminished in most individuals after beginning antiretroviral therapy, some patients showed de novo appearance of Tet+ cells.

CONCLUSIONS

Most Tet+ cells in chronic HIV-infected individuals express a memory phenotype and show an impaired production of IFN-gamma and a lower proliferative response to specific HIV antigens. Interestingly, some individuals under successful antiretroviral therapy may show de novo appearance of specific CTLs. The implications of these findings are relevant for a better understanding of virus-host interactions.

Sensitive and viable identification of antigen-specific CD8+ T cells by a flow cytometric assay for degranulation

Flow cytometric detection of antigen-specific CD8+ T cells has previously been limited to MHC-class I tetramer staining or intracellular cytokine production, neither of which measure the cytolytic potential of these cells. Here we present a novel technique to enumerate antigen-specific CD8+ T cells using a marker expressed on the cell surface following activation induced degranulation, a necessary precursor of cytolysis. This assay measures the exposure of CD107a and b, present in the membrane of cytotoxic granules, onto the cell surface as a result of degranulation. Acquisition of cell surface CD107a and b is associated with loss of intracellular perforin and is inhibited by colchicine, indicating that exposure of CD107a and b to the cell surface is dependent on degranulation. CD107a and b are expressed on the cell surface of CD8+ T cells following activation with cognate peptide, concordant with production of intracellular IFNgamma. Finally, CD107-expressing CD8+ T cells are shown to mediate cytolytic activity in an antigen-specific manner. Measurement of CD107a and b expression can also be combined with MHC-class I tetramer labeling and intracellular cytokine staining to provide a more complete assessment of the functionality of CD8+T cells expressing cognate T cell receptors (TCR).

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.

Effector cell mediated cytotoxicity measured by intracellular Granzyme B release in HIV infected subjects

CD8+ cytotoxic T lymphocyte (CTL) activity is currently believed to be one of the key immunologic mechanisms responsible for the prevention or attenuation of HIV-1 infection. The induction of CD8+ T cell activation may also result in the production of soluble or non-classical lytic factors that are associated with protection from infection or slower disease progression. Traditionally, CD8+ CTL responses have been measured by the classic chromium release assay, monitoring the ability of T cells (Effector cells) to lyse radiolabelled HLA - matched "target cells" that express the appropriate antigen-MHC complex. This method is not only labor intensive, semi quantitative assay at best, but also needs fresh, non-cryopreserved cells. Recently, cytokine specific ELISPOT assays or tetrameric MHC-I/ peptide complexes have utilized to directly quantitate circulating CD8+ effector cells, and these assays are more sensitive, quantitative and reproducible than the traditional CTL lysis assay and can also be performed on cryopreserved cells. Although these are reproducible assays for the assessment of soluble antiviral activity secreted by activated T cell populations they can be extremely expensive to perform. We have used FACS Analysis to measure Granzyme B release as a function of cell mediated cytotoxicity. This method helps quantitate the CTL activity and also identifies the phenotype of the cells elucidating this immune response. The method described not only monitors immunological response but also is also simple to perform, precise and extremely time efficient and is ideal for screening a large number of samples.

Cross-reactive CD8+ T cell epitopes identified in US adolescent minorities

Vaccines designed to bring forth CD8+ T cell responses in different racial and ethnic groups will require inclusion of T cell epitopes presented by various MHC class I molecules. This study was designed to identify new CD8+ T cell epitopes in HIV-infected African American and Hispanic youth as well as to determine the frequency of responses to both novel and previously described HIV-1 epitopes in a cohort of racially and ethnically diverse individuals. We found 8 MHC class I-restricted CD8+ T cell epitopes that had not been previously described, another 8 epitopes that were restricted by class I alleles not previously associated with these epitopes, and 8 additional epitopes that have been described previously. In a larger cohort, we demonstrated that 11 (69%) of these 16 newly described immunogens were recognized by individuals of different race or ethnicity. Most HIV-1-specific CD8+ T cell epitopes identified were either novel or restricted by alternative MHC class I alleles. Frequent recognition of several of these CTL epitopes in persons of diverse racial backgrounds bodes well for the development of a broadly reactive HIV-1 vaccine.

Prior DNA immunization enhances immune response to dominant and subdominant viral epitopes induced by a fowlpox-based SIVmac vaccine in long-term slow-progressor macaques infected with SIVmac251

A therapeutic vaccine for individuals infected with HIV-1 and treated with antiretroviral therapy (ART) should be able to replenish virus-specific CD4+ T-cells and broaden the virus-specific CD8+ T-cell response in order to maintain CD8+ T-cell function and minimize viral immune escape after ART cessation. Because a combination of DNA and recombinant poxvirus vaccine modalities induces high levels of virus-specific CD4+ T-cell response and broadens the cytolytic activity in naive macaques, we investigated whether the same results could be obtained in SIVmac251-infected macaques. The macaques studied here were long-term nonprogressors that naturally contained viremia but were nevertheless treated with a combination of antiviral drugs to assess more carefully the effect of vaccination in the context of ART. The combination of a DNA expressing the gag and pol genes (DNA-SIV-gp) of SIVmac239 followed by a recombinant fowlpox expressing the same SIVmac genes (FP-SIV-gp) was significantly more immunogenic than two immunizations of FP-SIV-gp in SIVmac251-infected macaques treated with ART. The DNA/FP combination significantly expanded and broadened Gag-specific T-cell responses measured by tetramer staining, ELISPOT, and intracellular cytokine staining and measurement of ex vivo cytolytic function. Importantly, the combination of these vaccine modalities also induced a sizeable expansion in most macaques of Gag-specific CD8-(CD4+) T-cells able to produce TNF-alpha. Hopefully, this modality of vaccine combination may be useful in the clinical management of HIV-1-infected individuals.

Heterogeneity of the simian immunodeficiency virus (SIV) specific CD8(+) T-cell response in mucosal tissues during SIV primary infection

Virus-specific CD8(+) T cells play an important role in controlling viral replication during acute primary infection. At this early stage, mucosal tissues represent a major site of viral replication. Therefore, the presence of functional virus-specific CD8(+) effector T cells in the mucosa during primary infection is a key issue in the pathogenesis of infection. In order to evaluate the extent of this response, six rhesus macaques were infected with simian immunodeficiency virus (SIV)mac251 and sacrificed on day 28 following infection. The functional activity of SIV-effector CD8(+) T cells was evaluated by means of a gamma-IFN ELISpot assay with autologous cells expressing SIV env, gag, pol and nef genes as antigen-presenting cells. This evaluation was performed on PBMCs, spleen, peripheral lymph node, gut-associated lymph node and lamina propria lymphocytes isolated from different mucosal sites. In parallel, the cell-associated viral load was quantified in all these tissues. Five macaques had gamma-IFN SIV-specific CD8(+) T cells in PBMCs and/or lymph nodes. However, in these macaques, these CD8(+) T cells were only present in seven mucosal sites out of 24 tested (the lamina propria lymphocytes of the duodenum, jejunum, ileum and colon were evaluated separately for each animal), whereas they were detected in all corresponding gut-associated lymph nodes. In addition, the mean frequency of SIV-specific gamma-IFN-secreting CD8(+) T cells was 117 +/- 228 per 10(6) cells in the lamina propria vs. 958 +/- 1184 in gut associated lymph nodes (P = 0.001). No overall correlation was observed between the CD8(+) T-cell activity and the viral load: among the 17 mucosal sites in which the virus was isolated, no specific activity was detected in 13 sites. In conclusion, these data indicate that the frequencies of SIV-specific gamma-IFN-secreting CD8(+) T cells are low in the mucosa during early primary infection. This may be of importance with regard to the intense viral replication observed in the mucosa at this stage.

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.

T cell responses to viral infections: lessons from lymphocytic choriomeningitis virus

The elaboration of a successful immune response is critical for the clearance of viral infections. CD8 T cells can directly kill virus-infected cells and also produce cytokines that modulate virus replication. Thus, the failure to induce or sustain these responses can profoundly impact the outcome of infections. Lymphocytic choriomeningitis virus (LCMV) infection of mice has proven to be one of the most informative experimental systems for examining antiviral T cell responses. In recent years, the application of newly developed approaches to analyze these responses has revealed that acute infections induce remarkably high levels of antiviral T cells. By contrast, protracted or chronic infections are associated with both the functional impairment and deletion of virus-specific CD8 T cells. This article discusses some of our findings using LCMV infection of mice as well as their relevance to other infections of animals and humans.

Determinants of HIV-specific CD8 T-cell responses in HIV-infected pediatric patients and enhancement of HIV-gag-specific responses with exogenous IL-15

Cellular immune responses play a central role in controlling HIV-1 infection. HIV-specific IFN-gamma production by CD8 T cells was evaluated in 17 HLA-A2+ HIV-infected pediatric patients (age range 1 month to 16 years) in an ELISPOT assay. Most patients (15/17) exhibited responses to HIV-gag, followed by responses to envelope gp120, gp41, and V3 loop. Only 7 patients responded to all four antigenic peptides. Treatment-related immune reconstitution of CD4 T cells was associated with increase in gag-specific responses, but these declined with prolonged viral suppression. Exogenous IL-15 resulted in augmentation of HIV-gag-specific response in 71% of patients, while IL-2 and IL-7 had variable effects, augmenting responses in 25% patients. Thus, HIV-specific CD8 T-cell responses are dependent on both CD4 T-cell help and antigenic stimulation. The cytokine IL-15 may be a useful modality as adjunctive therapy to augment HIV-specific memory CD8 T cells.

Viral persistence alters CD8 T-cell immunodominance and tissue distribution and results in distinct stages of functional impairment

Chronic viral infections often result in ineffective CD8 T-cell responses due to functional exhaustion or physical deletion of virus-specific T cells. However, how persisting virus impacts various CD8 T-cell effector functions and influences other aspects of CD8 T-cell dynamics, such as immunodominance and tissue distribution, remains largely unknown. Using different strains of lymphocytic choriomeningitis virus (LCMV), we compared responses to the same CD8 T-cell epitopes during acute or chronic infection. Persistent infection led to a disruption of the normal immunodominance hierarchy of CD8 T-cell responses seen following acute infection and dramatically altered the tissue distribution of LCMV-specific CD8 T cells in lymphoid and nonlymphoid tissues. Most importantly, CD8 T-cell functional impairment occurred in a hierarchical fashion in chronically infected mice. Production of interleukin 2 and the ability to lyse target cells in vitro were the first functions compromised, followed by the ability to make tumor necrosis factor alpha, while gamma interferon production was most resistant to functional exhaustion. Antigen appeared to be the driving force for this loss of function, since a strong correlation existed between the viral load and the level of exhaustion. Further, epitopes presented at higher levels in vivo resulted in physical deletion, while those presented at lower levels induced functional exhaustion. A model is proposed in which antigen levels drive the hierarchical loss of different CD8 T-cell effector functions during chronic infection, leading to distinct stages of functional impairment and eventually to physical deletion of virus-specific T cells. These results have implications for the study of human chronic infections, where similar T-cell deletion and functional dysregulation has been observed.

A simian replication-defective adenoviral recombinant vaccine to HIV-1 gag

In animal models, E1-deleted human adenoviral recombinants of the serotype 5 (AdHu5) have shown high efficacy as vaccine carriers for different Ags including those of HIV-1. Humans are infected by common serotypes of human adenovirus such as AdHu5 early in life and a significant percentage has high levels of neutralizing Abs to these serotypes, which will very likely impair the efficacy of recombinant vaccines based on the homologous virus. To circumvent this problem, a novel replication-defective adenoviral vaccine carrier based on an E1-deleted recombinant of the chimpanzee adenovirus 68 (AdC68) was developed. An AdC68 construct expressing a codon-optimized, truncated form of gag of HIV-1 induces CD8(+) T cells to gag in mice which at the height of the immune response encompass nearly 20% of the entire splenic CD8(+) T cell population. The vaccine-induced immune response provides protection to challenge with a vaccinia gag recombinant virus. Induction of transgene-specific CD8(+) T cells and protection against viral challenge elicited by the AdC68 vaccines is not strongly inhibited in animals preimmune to AdHu5 virus. However, the response elicited by the AdHu5 vaccine is greatly attenuated in AdHu5 preimmune animals.

A systematic comparison of methods to measure HIV-1 specific CD8 T cells

Several methods are now available to evaluate the frequencies of virus-specific CD8 T cells but require a systematic comparison to help at choosing the best strategy for evaluation. First, we compared the ELISpot-IFNgamma assay, intracellular IFNgamma staining and HLA class I tetramer-binding assay to quantify the HIV-specific CD8 T cells. Second, we determined the frequency of recognition of HIV antigens and evaluated whether the mode of antigen presentation might influence the results: We compared HIV antigen presentation in the same ELISpot-IFNgamma assays by using recombinant vaccinia viruses (rVVs) encoding for HIV-LAI Gag, Pol, Env, Nef, Tat and Vif proteins, or a panel of 49 synthetic 8-11 amino acid length peptides tested either individually or pooled. Third, we compared the antigens recognized by memory CTL analysis using chromium release assay (CRA) on CTL lines and by effector CD8 cell analysis using ELISpot assay. Our results show that: (1) Flow cytometry and ELISpot assay measuring IFNgamma production give the same frequency of HIV-specific CD8 T cells; (2) tetramer-binding assay detects more HIV-specific CD8 T cells than other methods; (3) pools of optimal peptides and sum frequencies of individual optimal peptides give similar results in ELISpot assay; (4) ELISpot assays using peptides are more sensitive than those using rVV; and (5) CRA and ELISpot assay when using rVV provide a comparable profile of HIV antigen recognition by memory CTLs (CRA) and effector CTLs (ELISpot) in two thirds cases. These results have important implications for the choice of immunological methods to evaluate CD8 T cells responses to vaccines.

Emergence of cytotoxic T lymphocyte escape mutants following antiretroviral treatment suspension in rhesus macaques infected with SIVmac251

Structured treatment interruption (STI) of antiretroviral drugs has been proposed as an alternative approach for managing patients infected with HIV-1. While STI is thought to spare drug-related side effects and enhance the HIV-1-specific immune response, the long-lasting clinical benefit of this approach remains uncertain, particularly in patients with long-standing HIV-1 infection. Here, we investigated the basis of unabated virological replication following different STI regimens in rhesus macaques that expressed the MHC class I Mamu-A*01 molecule treated during acute and long-standing infection with SIVmac251. An amino acid change at the anchor residue within the immunodominant Mamu-A*01-restricted Gag(181-189) CM9 epitope (T --> A) in one of six macaques with acute SIVmac251 infection and in three of four macaques with long-standing SIVmac251 infection (T --> A; T --> S; S --> C) was found in the majority of plasma virus. These amino acid changes have been shown to severely decrease binding of the corresponding peptides to the Mamu-A*01 molecule and, in the case of the T --> A change, escape from CTL. In one macaque with long-standing SIVmac251 infection, a mutation emerged that conferred resistance to one of the antiretroviral drugs (PMPA) as well. These results provide insights into the mechanism underlying the limited capacity of repeated interruption of antiretroviral therapy as an approach to restrain viral replication. In addition, these data also suggest that interruption of therapy may be less effective in chronic infection because of preexisting immune escape and that immune escape is a risk of interruption of therapy.

Association between virus-specific T-cell responses and plasma viral load in human immunodeficiency virus type 1 subtype C infection

Virus-specific T-cell immune responses are important in restraint of human immunodeficiency virus type 1 (HIV-1) replication and control of disease. Plasma viral load is a key determinant of disease progression and infectiousness in HIV infection. Although HIV-1 subtype C (HIV-1C) is the predominant virus in the AIDS epidemic worldwide, the relationship between HIV-1C-specific T-cell immune responses and plasma viral load has not been elucidated. In the present study we address (i) the association between the level of plasma viral load and virus-specific immune responses to different HIV-1C proteins and their subregions and (ii) the specifics of correlation between plasma viral load and T-cell responses within the major histocompatibility complex (MHC) class I HLA supertypes. Virus-specific immune responses in the natural course of HIV-1C infection were analyzed in the gamma interferon (IFN-gamma)-enzyme-linked immunospot assay by using synthetic overlapping peptides corresponding to the HIV-1C consensus sequence. For Gag p24, a correlation was seen between better T-cell responses and lower plasma viral load. For Nef, an opposite trend was observed where a higher T-cell response was more likely to be associated with a higher viral load. At the level of the HLA supertypes, a lower viral load was associated with higher T-cell responses to Gag p24 within the HLA A2, A24, B27, and B58 supertypes, in contrast to the absence of such a correlation within the HLA B44 supertype. The present study demonstrated differential correlations (or trends to correlation) in various HIV-1C proteins, suggesting (i) an important role of the HIV-1C Gag p24-specific immune responses in control of viremia and (ii) more rapid viral escape from immune responses to Nef with no restraint of plasma viral load. Correlations between the level of IFN-gamma-secreting T cells and viral load within the MHC class I HLA supertypes should be considered in HIV vaccine design and efficacy trials.

Ablation of CD8 and CD4 T cell responses by high viral loads

To evaluate the impact of sustained viral loads on anti-viral T cell responses we compared responses that cleared acute lymphocytic choriomeningitis virus infection with those that were elicited but could not resolve chronic infection. During acute infection, as replicating virus was cleared, CD8 T cell responses were down-regulated, and a pool of resting memory cells developed. In chronically infected hosts, the failure to control the infection was associated with pronounced and prolonged activation of virus-specific CD8 T cells. Nevertheless, there was a progressive diminution of their effector activities as their capacity to produce first IL-2, then TNF-alpha, and finally IFN-gamma was lost. Chronic lymphocytic choriomeningitis virus infection was also associated with differential contraction of certain CD8 T cell responses, resulting in altered immunodominance. However, this altered immunodominance was not due to selective expansion of T cells expressing particular TCR Vbeta segments during chronic infection. High viral loads were not only associated with the ablation of CD8 T cell responses, but also with impaired production of IL-2 by virus-specific CD4 T cells. Taken together, our data show that sustained exposure to high viral loads results in the progressive functional inactivation of virus-specific T cell responses, which may further promote virus persistence.

Immune intervention strategies for HIV-1 infection of humans in the SIV macaque model

Studies in the SIVmac macaque model have demonstrated that the extent of virus-specific CD4+ and CD8+ T-cell responses induced by vaccination prior to virus-challenge exposure correlate with viremia containment following establishment of infection. These findings led to the hypothesis that active immunization with vaccines able to induce virus-specific T-cell responses following the establishment of infection could also ameliorate the virological outcome. Here, we will review the relative effect of ART and vaccination during primary SIVmac infection of macaques.

Presence of HIV-1 Gag-specific IFN-gamma+IL-2+ and CD28+IL-2+ CD4 T cell responses is associated with nonprogression in HIV-1 infection

HIV immunity is likely CD4 T cell dependent. HIV-specific CD4 T cell proliferative responses are reported to correlate inversely with virus load and directly with specific CD8 responses. However, the phenotype and cytokine profile of specific CD4 T cells that correlate with disease is unknown. We compared the number/function of Gag p24-specific CD4 T cells in 17 HIV-infected long-term nonprogressors (LTNPs) infected for a median of 14.6 years with those of 16 slow progressors (SPs), also HIV infected for a median of 14 years but whose CD4 count had declined to <500 cells/ micro l. Compared with SPs, LTNPs had higher numbers of specific CD4s that were double positive for IFN-gamma and IL-2 as well as CD28 and IL-2. However, CD4 T cells that produced IL-2 alone (IL-2(+)IFN-gamma(-)) or IFN-gamma alone (IFN-gamma(+)IL-2(-)) did not differ between LTNPs and SPs. The decrease in p24-specific CD28(+)IL-2(+) cells with a concomitant increase of p24-specific CD28(-)IL-2(+) cells occurred before those specific for a non-HIV Ag, CMV. p24-specific CD28(-)IL-2(+) cells were evident in LTNPs and SPs, whereas the CMV-specific CD28(-)IL-2(+) response was confined to SPs. The difference between LTNPs and SPs in the Gag p24 IFN-gamma(+)IL-2(+) response was maintained when responses to total Gag (p17 plus p24) were measured. The percentage and absolute number of Gag-specific IFN-gamma(+)IL-2(+) but not of IFN-gamma(+)IL-2(-) CD4s correlated inversely with virus load. The Gag-specific IFN-gamma(+)IL-2(+) CD4 response also correlated positively with the percentage of Gag-specific IFN-gamma(+) CD8 T cells in these subjects. Accumulation of specific CD28(-)IL-2(+) helpers and loss of IFN-gamma(+)IL-2(+) CD4 T cells may compromise specific CD8 responses and, in turn, immunity to HIV.

Frequencies of ex vivo-activated human immunodeficiency virus type 1-specific gamma-interferon-producing CD8+ T cells in infected children correlate positively with plasma viral load

HIV-specific CD8+ T cells are critical in controlling human immunodeficiency virus (HIV) replication. We present the evaluation of a gamma-interferon (IFN-gamma)-based enzyme linked immunospot (ELISPOT) assay for the quantification of HIV-specific CD8+ T cells from HIV-infected children. We studied 20 HLA-A*0201-positive HIV-infected children. The IFN-gamma production in response to stimulation with two HLA-A*0201-restricted immunodominant CD8 epitopes (SLYNTVATL [SL9] in Gag and ILKEPVHGV [IV9] in Pol) was tested using the ELISPOT assay. The results were compared to labeling with the corresponding tetramers. Among the 20 children, 18 had detectable responses against the SL9 and/or the IV9 epitope using the ELISPOT assay (medians, 351 and 134 spot-forming cells/10(6) peripheral blood mononuclear cells, respectively). Comparison of results from the tetramer and ELISPOT assays suggests that only a fraction of HIV-specific CD8+ T cells were able to produce IFN-gamma. Most importantly, we found that the frequencies of IFN-gamma-producing CD8+ T cells were positively correlated with the viral load whereas the frequencies of tetramer-binding CD8+ T cells were not. The high sensitivity of the ELISPOT assay and the fact that this functional assay provided information different from that of tetramer labeling support its use for measurement of HIV-specific CD8+ T cells. In conclusion, our results show that the ex vivo-activated IFN-gamma-producing HIV-specific CD8+ T-cell subset is dependent upon continuous antigenic stimulation.

The functional CD8 T cell response to HIV becomes type-specific in progressive disease

High levels of HIV-specific CD8 T cells are demonstrable throughout HIV disease using laboratory assays that measure responses to consensus epitopes. In acute infection, the dynamics of the antiviral CD8 T cell response correlate well with the decline in viremia. However in chronic infection, although responses are detected against a broader spectrum of epitopes, virus-specific CD8 T cells are apparently unable to control viral replication. To investigate whether CD8 T cells responding to consensus epitopes may have lost their in vivo relevance in the chronic phase because of viral evolution driven by immune pressure, we compared the CD8 T cell response to CD4 T cell targets infected with either lab-adapted HIV(IIIB) or the patient's own virus. The magnitude of the IFN-gamma response declined with disease progression, especially to autologous virus. T cell receptor (TCR) clonotypes of HIV(IIIB) and autologous virus-responding cells were determined by sequencing TCR beta chain variable (TCRBV) genes. In two of three asymptomatic donors, the dominant clonotypes overlapped, whereas in five symptomatic patients, the TCR clonotypes responding to HIV(IIIB) virus were completely different from those responding to autologous virus. Moreover, in cytolytic assays, T cell lines derived from IFN-gamma(+) cells responding to lab-adapted or autologous virus cross-recognized target cells infected with either virus in asymptomatic subjects with shared TCR clonotypes but not in progressors with differing clonotypes. Therefore, in advanced-stage patients, viral-specific CD8 T cells recognizing consensus epitopes persist from an earlier response but no longer effectively recognize autologous virus.

Functional discrepancies in HIV-specific CD8+ T-lymphocyte populations are related to plasma virus load

The potent ability of current antiretroviral drug regimens to control human immunodeficiency Virus-1 (HIV-1) replication, in conjunction with the clinical practice of structured therapeutic interruptions, provides a system in which virus levels are manipulated during a persistent infection in humans. Here, we exploit this system to examine the impact of variable plasma virus load (pVL) on the functionality of HIV-specific CD8+ T-lymphocyte populations. Using both ELISpot methodology and intracellular cytokine staining for interferon (IFN)-gamma to assess functional status, together with fluorochrome-labeled peptide-major histocompatibility complex (pMHC) class I tetramer analysis to detect the physical presence of CD8+ T lymphocytes expressing cognate T-cell receptors (TCRs), we observed that the proportion of HIV-specific CD8+ T lymphocytes capable of mounting an effector response to antigen challenge directly ex vivo is related to the kinetics of virus exposure. Specifically, (a) after prolonged suppression of pVL with antiretroviral therapy (ART), physical and functional measures of HIV-specific CD8+ T-lymphocyte frequencies approximated; and (b) the percentage of functionally responsive cells in the HIV-specific CD8+ T lymphocyte populations declined substantially when therapy was discontinued and pVL recrudesced in the same patients. These results corroborate and extend observations in animal models that describe nonresponsive CD8+ T lymphocytes in the presence of high levels of antigen load and have implications for the interpretation of quantitative data generated by methods that rely on functional readouts.

Vaccination of macaques with long-standing SIVmac251 infection lowers the viral set point after cessation of antiretroviral therapy

A cohort of rhesus macaques with long-standing SIVmac251 infection (> or =5 mo) was treated with continuous antiretroviral therapy (ART). A group of eight macaques was vaccinated with or without simultaneous administration of low dose IL-2 with the highly attenuated poxvirus vector (NYVAC) vaccine candidate expressing the SIVmac structural gag-pol-env (gpe) genes and a novel chimeric fusion protein derived from the rev-tat-nef (rtn) regulatory genes. Control groups consisted of mock-vaccinated macaques or animals treated only with IL-2. Vaccination significantly expanded both virus-specific CD4(+) and CD8(+) T cell responses, and IL-2 further increased the vaccine-induced response to an immunodominant Gag epitope. Following antiretroviral treatment interruption, the viral set point was significantly lower in vaccinated than in control macaques for at least 4 consecutive mo, and viral containment was inversely correlated with vaccine-induced, virus-specific CD4(+) and CD8(+) T cell responses. These data provide the proof of concept that therapeutic vaccination before cessation of ART may be a feasible approach in the clinical management of HIV-1 infection.

Containment of simian immunodeficiency virus infection in vaccinated macaques: correlation with the magnitude of virus-specific pre- and postchallenge CD4+ and CD8+ T cell responses

Macaques infected with the SIV strain SIVmac251 develop a disease closely resembling human AIDS characterized by high viremia, progressive loss of CD4(+) T cells, occurrence of opportunistic infection, cachexia, and lymphomas. We report in this study that vaccination with the genetically attenuated poxvirus vector expressing the structural Ags of SIVmac (NYVAC-SIV-gag, pol, env) in combination with priming with DNA-SIV-gag, env resulted in significant suppression of viremia within 2 mo after mucosal exposure to the highly pathogenic SIVmac251 in the majority of vaccinated macaques. The control of viremia in these macaques was long lasting and inversely correlated to the level of both pre- and postchallenge Gag-specific lymphoproliferative responses, as well as to the level of total SIV-specific CD4(+) T lymphocyte responses at the peak of acute viremia as detected by intracellular cytokine-staining assay. Viremia containment also correlated with the frequency of the immunodominant Gag(181-189)CM9 epitope-specific CD8(+) T cells present before the challenge or expanded during acute infection. These data indicate, for the first time, the importance of vaccine-induced CD4(+) Th cell responses as an immune correlate of viremia containment. The results presented in this work also further demonstrate the potential of a DNA-prime/attenuated poxvirus-boost vaccine regimen in an animal model that well mirrors human AIDS.

Differences between T cell epitopes recognized after immunization and after infection

Evidence suggests that cellular immune responses play a crucial role in the control of HIV and SIV replication in infected individuals. Several vaccine strategies have therefore targeted these CD8(+) and CD4(+) responses. Whether vaccination induces the same repertoire of responses seen after infection is, however, a key unanswered question in HIV vaccine development. We therefore compared the epitope specificity induced by vaccination to that present postchallenge in the peripheral blood. Intracellular cytokine staining of PBMC stimulated with overlapping 15/20-mer peptides spanning the proteins of SIV were measured after DNA/modified vaccinia Ankara vaccination of eight rhesus macaques. Lymphocytes from 8 animals recognized a total of 39 CD8 epitopes and 41 CD4 epitopes encoded by the vaccine. T cell responses were again monitored after challenge with SIVmac239 to investigate the evolution of these responses. Only 57% of all CD8(+) T cell responses and 19% of all CD4(+) T cell responses present after vaccination were recalled after infection as measured in the peripheral blood. Interestingly, 29 new CD8 epitopes and 5 new CD4 epitopes were recognized by PBMC in the acute phase. These new epitopes were not detected after vaccination, and only some of them were maintained in the chronic phase (33% of CD8 and no CD4 responses). Additionally, 24 new CD8 epitopes and 7 new CD4 epitopes were recognized by PBMC in the chronic phase of infection. The repertoire of the immune response detected in the peripheral blood after immunization substantially differed from the immune response detected in the peripheral blood after infection.

Immune recovery in HIV disease: role of the thymus and T cell expansion in immune reconstitution strategies

While the progressive depletion of CD4(+) T cells is the hallmark of the impact of HIV on the immune system, considerable data also point to the loss of T cell function. The question is: Can the immune system recover from this insult and what are the therapeutic strategies available to us to mediate this immune recovery? This review will focus on our current knowledge of immune recovery following treatment with highly active antiretroviral therapy (HAART). Enhancement of thymic function in generating de novo T cell synthesis post-HAART has also emerged as a viable immune recovery strategy. Advances in molecular (T cell receptor excision circle assay) and conventional (computed tomography scans of the thymus) approaches to evaluate the role of the thymus in immune recovery as well as potential agents that might enhance thymic output (interleukin-7, IL-7) will contribute greatly to the assessment of the success of these approaches as immune recovery strategies. In this review, we will integrate this new information in the context of the current strategies for HIV therapy leading to long-term immune reconstitution.

Tetramer binding and secretion of interferon-gamma in response to antigenic stimulation are compatible with a range of affinities of MHC:TCR interaction and distinct programs of cytotoxic T-lymphocyte activation

Tetramer staining and detection of IFN-gamma secretion in response to specific stimulation are widely used to quantify and isolate specific T-cells. However, it remains unclear how these assays reflect different functional outcomes of T-cell triggering with MHC:peptide ligands. An immunogenic EBV-derived A11-restricted CTL peptide epitope and its partially agonistic analogue trigger different programs of activation induced cell death (AICD) in specific CTLs. In this study we analysed a panel of CTL clones, bulk CTL cultures and PBMCs isolated from HLA A11-positive EBV-infected individuals for their ability to bind tetrameric complexes assembled with either of the two peptides and correlated tetramer binding with the activity of the peptides in functional assays. This analysis demonstrates that specific tetramer staining and secretion of IFN-gamma are compatible with at least two activation programs in CTLs. One of these programs corresponds to full-scale CTL activation and death of a proportion of activated T-cells in a Fas-dependent manner. In contrast, the alternative program is characterized by selective induction of IFN-gamma and TNF-alpha, absence of proliferative response and Fas-independent cell death. These findings may have important implications for the evaluation of data obtained with MHC:peptide tetramers and IFN-gamma secretion assays, especially in experimental systems with extensive antigenic variability.

Cross-clade T lymphocyte-mediated immunity to HIV type 1: implications for vaccine design and immunodetection assays

Identifying immunodominant regions of HIV-1 that are recognized by CD8(+) T lymphocytes in infected individuals may be important for the design and evaluation of candidate HIV-1 vaccines, particularly for developing countries. In this study, cryopreserved peripheral blood mononuclear cells (PBMCs) from 15 chronically HIV-1-infected U.S. volunteers were screened for HIV-1 Gag-specific T lymphocyte interferon gamma production in an enzyme-linked immunospot (ELISpot) assay matrix format, using overlapping HIV-1 subtype A, B, and C Gag peptide pools. In the initial matrix screen, responses to HIV-1 subtype B Gag were detected in 11 of 15 (73%) of seropositive individuals and in none of 4 HIV-1-seronegative controls. There were differences in both the breadth and magnitudes of the responses observed in the matrix assay. Responses varied in breadth, ranging from broad responses (more than four peptides) of moderate magnitude (<100 spot-forming cells [SFCs]/10(5) PBMCs) to narrowly focused (two or fewer peptides), more potent responses (>150 SFC/10(5) PBMCs). Responses to A, B, and C clade peptides of HIV-1 Gag revealed that all responders to subtype B peptides were also found to recognize corresponding peptides from at least one of the other clades. The ability to recognize cross-clade peptides with one or two amino acid substitutions relative to the B clade peptide was both peptide and patient dependent. Overall, our results show that the ELISpot matrix algorithm described here may be an efficient approach for characterizing cross-clade CD8(+) T cell responses in either seropositive individuals or in seronegative HIV-1 vaccine recipients.

Characterization of cytotoxic function of CMV-pp65-specific CD8+ T-lymphocytes identified by HLA tetramers in recipients and donors of stem-cell transplants

BACKGROUND

Human cytomegalovirus (CMV) is a ubiquitous herpesvirus that is an important complication of bone marrow and allogeneic stem-cell transplant (HSCT). CD8 T-lymphocytes have an important role in immunity against CMV, but correlation between antigen-specific subpopulations of these cells and protection are still unclear.

METHODS

Flow analysis with fluorescently-conjugated human leukocyte antigen (HLA) class I tetramers (Tet) was used to investigate levels of CMV-specific CD8 T-lymphocytes in peripheral blood monocyte cells (PBMC) samples from HSCT donors and recipients and their ability to produce interferon (IFN)-gamma on stimulation with either CMV antigenic peptide or nonspecific mitogenic stimulation. Chromium release assays were used to evaluate ex vivo CMV-specific cytotoxicity associated with the PBMC samples.

RESULTS

Use of Tet in conjunction with fluorescently conjugated anti-T-cell receptor (TCR) beta-chain variable (Vbeta) monoclonal antibodies indicated that the Vbeta repertoires associated with Tet cells seen in two HSCT recipients were similar to the Vbeta repertoires of the Tet cells in their HSCT donors. Significant ex vivo cytotoxicity against peptide-loaded targets was measured from several recipient samples after transplant. However, PBMC from the HSCT donors, even when containing populations of CMV-specific Tet cells capable of secreting IFN-gamma in response to peptide stimulation, possessed no ex vivo CMV-specific cytotoxicity.

CONCLUSIONS

We hypothesize that in the setting of the reconstituting immune system of HSCT recipients, CMV reactivation may stimulate a functional change in CMV-specific CD8 T-lymphocytes, rendering them able to directly lyse target cells presenting CMV antigens without in vitro stimulation. These findings have important implications for development of vaccines designed to induce protective cellular immunity to CMV in transplant recipients.