Incomplete CD8(+) T lymphocyte differentiation as a mechanism for subdominant cytotoxic T lymphocyte responses to a viral antigen

Potential killers exposed: tracking endogenous influenza-specific CD8+ T cells

Current influenza A virus (IAV) vaccines stimulate antibody responses that are directed against variable regions of the virus, and are therefore ineffective against divergent strains. As CD8⁺ T cells target the highly conserved, internal IAV proteins, they have the potential to increase heterosubtypic immunity. Early T‐cell priming events influence lasting memory, which is required for long‐term protection. However, the early responding, IAV‐specific cells are difficult to monitor because of their low frequencies. Here, we tracked the dissemination of endogenous IAV‐specific CD8⁺ T cells during the initial phases of the immune response following IAV infection. We exposed a significant population of recently activated, CD25⁺CD43⁺ IAV‐specific T cells that were not detected by tetramer staining. By tracking this population, we found that initial T‐cell priming occurred in the mediastinal lymph nodes, which gave rise to the most expansive IAV‐specific CD8⁺ T‐cell population. Subsequently, IAV‐specific CD8⁺ T cells dispersed to the bronchoalveolar lavage and blood, followed by spleen and liver, and finally to the lung. These data provide important insight into the priming and tissue dispersion of an endogenous CD8⁺ T‐cell response. Importantly, the CD25⁺CD43⁺ phenotype identifies an inclusive population of early responding CD8⁺ T cells, which may provide insight into TCR repertoire selection and expansion. A better understanding of this response is critical for designing improved vaccines that target CD8⁺ T cells.

Novel approaches in polyepitope T-cell vaccine development against HIV-1

RV144 clinical trial was modestly effective in preventing HIV infection. New alternative approaches are needed to design improved HIV-1 vaccines and their delivery strategies. One of these approaches is construction of synthetic polyepitope HIV-1 immunogen using protective T- and B-cell epitopes that can induce broadly neutralizing antibodies and responses of cytotoxic (CD8(+) CTL) and helpers (CD4(+) Th) T-lymphocytes. This approach seems to be promising for designing of new generation of vaccines against HIV-1, enables in theory to cope with HIV-1 antigenic variability, focuses immune responses on protective determinants and enables to exclude from the vaccine compound that can induce autoantibodies or antibodies enhancing HIV-1 infectivity. Herein, the authors will focus on construction and rational design of polyepitope T-cell HIV-1 immunogens and their delivery, including: advantages and disadvantages of existing T-cell epitope prediction methods; features of organization of polyepitope immunogens, which can generate high-level CD8(+) and CD4(+) T-lymphocyte responses; the strategies to optimize efficient processing, presentation and immunogenicity of polyepitope constructs; original software to design polyepitope immunogens; and delivery vectors as well as mucosal strategies of vaccination. This new knowledge may bring us a one step closer to developing an effective T-cell vaccine against HIV-1, other chronic viral infections and cancer.

Fetal-specific CD8+ cytotoxic T cell responses develop during normal human pregnancy and exhibit broad functional capacity

Tolerance of the semiallogeneic fetus presents a significant challenge to the maternal immune system during human pregnancy. T cells with specificity for fetal epitopes have been detected in women with a history of previous pregnancy, but it has been thought that such fetal-specific cells were generally deleted during pregnancy as a mechanism to maintain maternal tolerance of the fetus. We used MHC-peptide dextramer multimers containing an immunodominant peptide derived from HY to identify fetal-specific T cells in women who were pregnant with a male fetus. Fetal-specific CD8(+) T lymphocytes were observed in half of all pregnancies and often became detectable from the first trimester. The fetal-specific immune response increased during pregnancy and persisted in the postnatal period. Fetal-specific cells demonstrated an effector memory phenotype and were broadly functional. They retained their ability to proliferate, secrete IFN-γ, and lyse target cells following recognition of naturally processed peptide on male cells. These data show that the development of a fetal-specific adaptive cellular immune response is a normal consequence of human pregnancy and that unlike reports from some murine models, fetal-specific T cells are not deleted during human pregnancy. This has broad implications for study of the natural physiology of pregnancy and for the understanding of pregnancy-related complications.

Review on the relationship between human polyomaviruses-associated tumors and host immune system

The polyomaviruses are small DNA viruses that can establish latency in the human host. The name polyomavirus is derived from the Greek roots poly-, which means "many," and -oma, which means "tumours." These viruses were originally isolated in mouse (mPyV) and in monkey (SV40). In 1971, the first human polyomaviruses BK and JC were isolated and subsequently demonstrated to be ubiquitous in the human population. To date, at least nine members of the Polyomaviridae family have been identified, some of them playing an etiological role in malignancies in immunosuppressed patients. Here, we describe the biology of human polyomaviruses, their nonmalignant and malignant potentials ability, and their relationship with the host immune response.

Persistent viral infection in humans can drive high frequency low-affinity T-cell expansions

CD8 T cells that recognize cytomegalovirus (CMV) -encoded peptides can be readily detected by staining with human leucocyte antigen (HLA) -peptide tetramers. These cells are invariably highly differentiated effector memory cells with high avidity T-cell receptors (TCR). In this report we demonstrate an HLA-A*0201 restricted CMV-specific CD8 T-cell response (designated YVL) that represents several percent of the CD8 T-cell subset, yet fails to bind tetrameric major histocompatibility complex (MHC) ligands. However, these tetramer-negative cells are both phenotypically and functionally similar to other CMV-specific CD8 T cells. YVL peptide-specific CD8 T-cell clones were generated and found to be of high avidity in both cytotoxicity and interferon-γ (IFN-γ) assays, and comparable with other CMV peptide-specific CD8 T-cell clones. However, under conditions of CD8 blockade, the response was almost nullified even at very high ligand concentrations. This was also the case in IFN-γ experiments using peripheral blood mononuclear cells stimulated with peptide ex vivo. In contrast, all other CMV specificities (tetramer-positive) displayed minimal or only partial CD8 dependence. This suggests that YVL-specific responses depict a low-affinity TCR-MHC-peptide interaction, that is compensated by substantial CD8 involvement for functional purposes, yet cannot engage multivalent soluble ligands for ex vivo analysis. It is interesting that such a phenomenon is apparent in the face of a persistent virus infection such as CMV, where the responding cells represent an immunodominant response in that individual and may present a highly differentiated effector phenotype.

Different vaccine vectors delivering the same antigen elicit CD8+ T cell responses with distinct clonotype and epitope specificity

Prime-boost immunization with gene-based vectors has been developed to generate more effective vaccines for AIDS, malaria, and tuberculosis. Although these vectors elicit potent T cell responses, the mechanisms by which they stimulate immunity are not well understood. In this study, we show that immunization by a single gene product, HIV-1 envelope, with alternative vector combinations elicits CD8(+) cells with different fine specificities and kinetics of mobilization. Vaccine-induced CD8(+) T cells recognized overlapping third V region loop peptides. Unexpectedly, two anchor variants bound H-2D(d) better than the native sequences, and clones with distinct specificities were elicited by alternative vectors. X-ray crystallography revealed major differences in solvent exposure of MHC-bound peptide epitopes, suggesting that processed HIV-1 envelope gave rise to MHC-I/peptide conformations recognized by distinct CD8(+) T cell populations. These findings suggest that different gene-based vectors generate peptides with alternative conformations within MHC-I that elicit distinct T cell responses after vaccination.

MHC-peptide tetramers to visualize antigen-specific T cells

Mature T lymphocytes of the CD8 or CD4 classes bear alphabeta T cell receptors (TCR) that are specific for a molecular complex consisting of a major histocompatibility complex class I or II (MHC class I or II) molecule bound to a unique self or foreign peptide. Until recently, methods for monitoring the T cell immune response to a viral or tumor antigen were restricted primarily to functional assays based on limiting dilution analysis, because the lack of specific molecular reagents to identify clonal T cells obviated approaches to identify and enumerate specific T cells. Development of efficient methods to express and refold MHC class I molecules with synthetic peptides coincided with identification of specific protein sequences that provide the substrate for enzymatic biotinylation. This combination has led to the development of a straightforward method for generating synthetic TCR ligands, making them tetravalent to provide increased avidity, and labeling them through a streptavidin moiety with useful fluorescent tags such as fluorescein or phycoerythrin. This unit describes the preparation of MHC class I/peptide tetramers in detail, including bacterial expression and refolding of the MHC class I light chain, beta2-microglobulin (beta2m), as well as the formation of a complex consisting of the MHC class I heavy chain of interest, beta2m, and a chosen peptide.

Detuning CD8 T cells: down-regulation of CD8 expression, tetramer binding, and response during CTL activation

CD8 is critical for T cell recognition of peptide/class I major histocompatability complex ligands, yet is down-regulated during activation of CD8 T cells. We report that loss of CD8 expression early during in vivo responses to vaccinia virus or Listeria monocytogenes (LM) correlates with decreased T cell staining with specific class I/peptide tetramers and reduced CD8 T cell sensitivity for antigen. Loss of CD8 cell surface expression occurs despite sustained mRNA expression, and CD8 levels return to normal levels during differentiation of memory cells, indicating a transient effect. We determined that during response to LM, CD8 down-regulation is regulated by T cell reactivity to type I interferon (IFN-I) because CD8 loss was averted on IFN-I receptor-deficient T cells. IFN-I alone was not sufficient to drive CD8 down-regulation, however, as antigen was also required for CD8 loss. These results suggest that CD8 effector T cell differentiation involves a transient down-regulation of antigen sensitivity (CTL "detuning"), via reduced CD8 expression, a feature that may focus the effector response on target cells expressing high levels of antigen (e.g., infected cells), while limiting collateral damage to bystander cells.

Mapping and characterization of the primary and anamnestic H-2(d)-restricted cytotoxic T-lymphocyte response in mice against human metapneumovirus

Cytotoxic T lymphocytes (CTLs) are important for the control of virus replication during respiratory infections. For human metapneumovirus (hMPV), an H-2(d)-restricted CTL epitope in the M2-2 protein has been described. In this study, we screened the hMPV F, G, N, M, M2-1, and M2-2 proteins using three independent algorithms to predict H-2(d) CTL epitopes in BALB/c mice. A dominant epitope (GYIDDNQSI) in positions 81 to 89 of the antitermination factor M2-1 and a subdominant epitope (SPKAGLLSL) in N(307-315) were detected during the anti-hMPV CTL response. Passive transfer of CD8(+) T-cell lines against M2-1(81-89) and N(307-315) protected Rag1(-/-) mice against hMPV challenge. Interestingly, diversification of CTL targets to include multiple epitopes was observed after repetitive infections. A subdominant response against the previously described M2-2 epitope was detected after the third infection. An understanding of the CTL response against hMPV is important for developing preventive and therapeutic strategies against the virus.

STAT5-mediated signals sustain a TCR-initiated gene expression program toward differentiation of CD8 T cell effectors

Poorly functional effector CD8 T cells are generated in some pathological situations, including responses to weakly antigenic tumors. To identify the molecular bases for such defective differentiation, we monitored gene expression in naive monoclonal CD8 T cells during responses to TCR ligands of different affinity. We further evaluated whether responses to weak Ags may be improved by addition of cytokines. Transient gene expression was observed for a cluster of genes in response to the weak TCR agonist. Strikingly, gene expression was stabilized by low dose IL-2. This IL-2-sustained gene cluster encoded notably transcripts for CD25, cytolytic effector molecules (granzyme B) and TNF-R family costimulatory molecules (glucocorticoid-induced TNF-R (GITR), OX40, and 4-1BB). IL-2-enhanced surface expression or function was also demonstrated in vivo for these genes. A constitutive active form of STAT5 mimicked the IL-2 effect by sustaining transcripts for the same gene cluster. Consistent with this, under conditions of low avidity TCR engagement and IL-2 treatment, endogenous STAT5 binding to 4-1BB and granzyme B promoters was demonstrated by chromatin immunoprecipitation. This study highlights those genes for which IL-2, via STAT5 activation, acts as a stabilizer of gene regulation initiated by TCR signals, contributing to the development of a complete CD8 T cell effector program.

Loss of CD8 and TCR binding to Class I MHC ligands following T cell activation

The capacity of T cells to bind peptide/MHC ligands changes with T cell development and differentiation. Here we study changes in peptide/MHC multimer binding following T cell activation. Surprisingly, T cell activation caused a marked reduction in specific peptide/MHC Class I multimer binding, which was distinct from transient TCR down-regulation, and was especially dramatic for engagement with low-affinity peptide/MHC ligands. Direct CD8-Class I interactions were also profoundly and rapidly impaired following T cell stimulation, even though surface CD8alpha and CD8beta levels were unchanged after activation, suggesting that decreased CD8 co-receptor binding contributes to this effect. Finally, we show that enzymatic desialylation restores much of the multimer binding on activated T cells, suggesting that altered glycosylation may inhibit TCR/CD8 binding to peptide/MHC ligands. These radical changes in activated T cells' ability to perceive peptide/MHC ligands may contribute to selective outgrowth of clones with high affinity for the stimulatory ligand.

Transient loss of MHC class I tetramer binding after CD8+ T cell activation reflects altered T cell effector function

Engagement of the Ag receptor on naive CD8+ T cells by specific peptide-MHC complex triggers their activation/expansion/differentiation into effector CTL. The frequency of Ag-specific CD8+ T cells can normally be determined by the binding of specific peptide-MHC tetramer complexes to TCR. In this study we demonstrate that, shortly after Ag activation, CD8+ T cells transiently lose the capacity to efficiently bind peptide-MHC tetramer complexes. This transient loss of tetramer binding, which occurs in response to naturally processed viral peptide during infection in vitro and in vivo, is associated with reduced signaling through the TCR and altered/diminished effector activity. This change in tetramer binding/effector response is likewise associated with a change in cell surface TCR organization. These and related results suggest that early during CD8+ T cell activation, there is a temporary alteration in both cell surface Ag receptor display and functional activity that is associated with a transient loss of cognate tetramer binding.

Cytotoxic capacity of hepatitis C virus (HCV)--specific lymphocytes after in vitro immunization with HCV-derived lipopeptides

BACKGROUND

Hepatitis C virus (HCV)-derived lipopeptides can induce epitope-specific immune responses in lymphocytes from HCV-naive individuals. We analyzed whether such T cells generated by in vitro immunization with HCV core-derived lipopeptides exert HCV-specific cytolytic activity.

METHODS

Using a sensitive flow cytometric cytotoxicity assay we characterized HCV-specific cytotoxicity in T cells generated in vitro with HCV core-derived 25-mer lipopeptides. In addition, we studied expressions of Fas ligand and perforin and interferon-gamma (IFN-gamma) secretion in HLA-A2-HCV(core_35-44) tetramer-positive T cells generated with lipopeptide amino acid 20-44 (LP20-44).

RESULTS

CD8+ T cells induced in vitro with HCV core-derived lipopeptides only infrequently exerted HCV-specific cytotoxicity, irrespective of whether antigen-coated T2 cells or autologous B lymphoblasts were used as targets. Detailed analysis of HLA-A2-HCV(core_35-44) tetramer-positive T cells generated with LP20-44 revealed that in vitro immunization resulted in T cells that secreted IFN-gamma after antigen-specific restimulation and that upregulated expression of Fas ligand but not of perforin.

CONCLUSIONS

Our data confirm at the functional level that HCV lipopeptides induce antigen-specific T lymphocytes that produce IFN-gamma but exert significant cytotoxicity in only a minority of experiments, probably because expression of cytolytic effector molecules is not enhanced in their granules.

A Large Number of T Lymphocytes Recognize Moloney-Murine Leukemia Virus-Induced Antigens, but a Few Mediate Long-Lasting Tumor Immunosurveillance

The CD8(+) T cell response to Moloney-murine leukemia virus (M-MuLV)-induced Ags is almost entirely dominated by the exclusive expansion of lymphocytes that use preferential TCRVbeta chain rearrangements. In mice lacking T cells expressing these TCRVbeta, we demonstrate that alternative TCRVbeta can substitute for the lack of the dominant TCRVbeta in the H-2-restricted M-MuLV Ag recognition. We show that, at least for the H-2(b)-restricted response, the shift of TCR usage is not related to a variation of the immunodominant M-MuLV epitope recognition. After virus immunization, all the potentially M-MuLV-reactive lymphocytes are primed, but only the deletion of dominant Vbeta rescues the alternative Vbeta response. The mechanism of clonal T cell "immunodomination" that guides the preferential Vbeta expansion is likely the result of a proliferative advantage of T cells expressing dominant Vbeta, due to differences in TCR affinity and/or cosignal requirements. In this regard, a CD8 involvement is strictly required for the virus-specific cytotoxic activity of CTL expressing alternative, but not dominant, Vbeta gene rearrangements. The ability of T cells expressing alternative TCRVbeta rearrangements to mediate tumor protection was evaluated by a challenge with M-MuLV tumor cells. Although T cells expressing alternative Vbeta chains were activated and expanded, they were not able to control tumor growth in a long-lasting manner due to their incapacity of conversion and accumulation in the T central memory pool.

Pathogen-host standoff: immunity to polyomavirus infection and neoplasia

Polyomaviruses establish persistent infection in a variety of hosts, including humans, where they pose an oncogenic threat under conditions of depressed immune function. Control of persistent infection by these DNA tumor viruses requires continuous immunosurveillance by functionally competent antiviral CD8+ T cells. Repetitive antigen encounter by these T cells, however, often leads to their deletion or inactivation. Elucidation of the in vivo mechanisms that sustain antigen-specific CD8+ T-cell effector activity in the face of persistent antigen is essential for devising immunotherapeutic strategies against viral oncogenesis.

Effector and regulatory T?cells derived from the same T?cell clone differ in MHC class II-peptide multimer binding

MHC class II-peptide multimers are a valuable tool for antigen-specific detection of CD4(+) T cells. However, it has been proposed that T cells in a hypo-responsive state can have diminished binding of such multimers. In the present study, we investigated this phenomenon at the clonal level. We found that anergic CD4(+) T cells had a reduced capacity to bind MHC class II-peptide multimers compared to their non-anergic counterparts. Increasing the incubation temperature, time, or MHC-peptide valency could not equalize multimer binding by anergic and non-anergic T cells. Neither anergic T cells nor non-anergic T cells internalized the MHC class II-peptide dimers efficiently, and in both cases the dimers bound to the plasma membrane at locations containing a low amount of raft-associated lipids. Disruption of lipid rafts, however, led to decreased dimer binding by non-anergic T cells and to a lesser extent by anergic T cells. Finally, we show that the depth of the anergic state of the T cell, which determines its ability to regulate other T cell responses, correlates with the reduced dimer binding. We here demonstrate for the first time differential MHC class II-peptide multimer binding by regulatory (anergic) and effector T cells with identical TCR.

Functionally impaired HIV-specific CD8 T cells show high affinity TCR-ligand interactions

We eventually isolated two different clonotypic CD8 T cell subsets recognizing an HIV Pol-derived epitope peptide (IPLTEEAEL) in association with HLA-B35 from a chronic HIV-infected patient. By kinetic analysis experiments, the subsets showed a >3-fold difference in half-lives for the HLA tetramer in complex with the Pol peptide. In functional assays in vitro and ex vivo, both subsets showed substantial functional avidity toward peptide-loaded cells. However, the high affinity subset did not show cytolytic activity, cytokine production, or proliferation activity toward HIV-infected cells, whereas the moderate affinity one showed potent activities. Furthermore, using ectopic expression of each of the TCR genes into primary human CD8 T cells, the CD8 T cells transduced with the high affinity TCR showed greater binding activity toward the tetramer and impaired cytotoxic activity toward HIV-infected cells, corroborating the results obtained with parental CD8 T cells. Taken together, these data indicate that impaired responsiveness of T cells toward HIV-infected cells can occur at the level of TCR-ligand interactions, providing us further insight into the immune evasion mechanisms by HIV.

Preapoptotic phenotype of viral epitope-specific CD8 T cells precludes memory development and is an intrinsic property of the epitope

Virus-specific CD8 T cells after clearance of infection reduce their number in lymphoid organs by apoptotic death and by migration into peripheral tissues. During and after infection, many lymphocytic choriomeningitis virus (LCMV)-specific CD8 T cells in lymphoid but not peripheral tissues are in a preapoptotic state, as detected by the early apoptosis marker annexin V. In this report, we investigated the significance of this preapoptotic state and how it may be influenced by viral epitope specificity. Stimulation with anti-CD3 or IL-2 in vitro postponed DNA fragmentation in annexin V+ cells, but adoptive transfer studies in vivo showed that this preapoptotic phenotype precluded the development of functional memory. CD8 T cells specific to LCMV epitopes NP396 and gp33 differed in their preapoptotic state, with NP396-specific T cells binding more annexin V than gp33-specific T cells. These epitope- and tissue-dependent differences were seen in primary, memory, and secondary responses and in mice receiving different displays of Ag by infection with LCMV strains of different tropisms or by infection with vaccinia virus recombinants expressing LCMV proteins. Thus, the epitope-dependent differences in apoptosis were independent of virus tropisms, duration of Ag exposure, and competition within APCs, and were an intrinsic property of the epitope. The tissue-dependent and epitope-dependent preapoptotic state correlated with reduced expression of IL-7Ralpha.

Tapasin enhances MHC class I peptide presentation according to peptide half-life

Understanding how peptides are selected for presentation by MHC class I is crucial to vaccination strategies based on cytotoxic T lymphocyte priming. We have studied this selection of the MHC class I peptide repertoire in terms of the presentation of a series of individual peptides with a wide range of binding to MHC class I. This series was expressed as minigenes, and the presentation of each peptide variant was determined with the same MHC class I peptide-specific antibody. In wild-type cells, the hierarchy of presentation followed peptide half-life. This hierarchy broke down in cells lacking tapasin but not in cells lacking calreticulin or in cells lacking transporter associated with antigen processing-associated ERp57. We demonstrate a key role for tapasin in shaping the MHC class I peptide repertoire, as enhancement of presentation in the presence of tapasin correlated with peptide half-life.

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

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

A transgenic mouse model genetically tags all activated CD8 T cells

Identifying and characterizing Ag-specific CD8+ T cells are central to the study of immunological memory. Although powerful strategies such as MHC tetramers and peptide-induced cytokine production assays exist for identifying Ag-specific CD8+ T cells, alternate strategies that are not dependent upon a priori knowledge of the immunodominant and subdominant antigenic epitopes, as well as the MHC background of the animal are of obvious utility. In this study, we present a transgenic mouse model that uses Cre-loxP recombination to permanently mark all activated CD8+ T cells with beta-galactosidase. We used the lymphocytic choriomeningitis virus infection model to track the dynamics of the antiviral CD8+ T cell responses. We show that in this transgenic mouse model system, all of the antiviral effector and memory CD8+ T cells are contained within the beta-gal-marked CD8+ T cell population.

CD8 T cell responses to infectious pathogens

CD8 T cells respond to viral infections but also participate in defense against bacterial and protozoal infections. In the last few years, as new methods to accurately quantify and characterize pathogen-specific CD8 T cells have become available, our understanding of in vivo T cell responses has increased dramatically. Pathogen-specific T cells, once thought to be quite rare following infection, are now known to be present at very high frequencies, particularly in peripheral, nonlymphoid tissues. With the ability to visualize in vivo CD8 T cell responses has come the recognition that T cell expansion is programmed and, to a great extent, independent of antigen concentrations. Comparison of CD8 T cell responses to different pathogens also highlights the intricate relationship between microbially induced innate inflammatory responses and the kinetics, magnitude, and character of long-term T cell responses. This review describes recent progress in some of the major murine models of CD8 T cell-mediated immunity to viral, bacterial, and protozoal infection.

Activation of tumor antigen-specific cytotoxic T lymphocytes (CTLs) by human dendritic cells infected with an attenuated influenza A virus expressing a CTL epitope derived from the HER-2/neu proto-oncogene

The development of cancer vaccines requires approaches to induce expansion and functional differentiation of tumor antigen-specific cytotoxic T lymphocyte (CTL) effectors which posses cytolytic capability and produce cytokines. Efficient induction of such cells is hindered by the poor immunogenicity of tumor antigens and by the poor transduction efficiency of dendritic cells (DCs) with current nonreplicating vectors. We have investigated the use of influenza A virus, a potent viral inducer of CTLs, as a vector expressing the immunodominant HER-2 CTL epitope KIF (E75). For this purpose, an attenuated influenza A/PR8/34 virus with a truncated nonstructural (NS1) gene was generated containing the E75 epitope in its neuraminidase protein (KIF-NS virus). Stimulation of peripheral blood mononuclear cells from healthy donors and of tumor-associated lymphocytes from ovarian and breast cancer patients with DCs infected with KIF-NS virus (KIF-NS DC) induced CTLs that specifically recognized the peptide KIF and HER-2-expressing tumors in cytotoxicity assays and secreted gamma interferon (IFN-gamma) and interleukin-2 at recall with peptide. Priming with KIF-NS DCs increased the number of E75(+) CD45RO(+) cells by more than 10-fold compared to nonstimulated cells. In addition, KIF-NS virus induced high levels of IFN-alpha in DCs. This is the first report demonstrating induction of human epitope-specific CTLs against a tumor-associated antigen with a live attenuated recombinant influenza virus vector. Such vectors may provide a novel approach for tumor antigen delivery, lymphocyte activation, and differentiation in human cancer vaccine development.

Not all effector CD8+ T cells are alike

As naive CD8+ T cells circulate throughout the bloodstream and secondary lymphoid tissues (i.e. spleen and lymph nodes), they sample complexes of peptides and MHC class I molecules expressed on the surface of professional antigen presenting cells (APCs). A proper fit between lymphocyte and APCs sets into motion a complex series of events that result in the generation of activated cytotoxic T lymphocytes (CTLs) that are the principal immune effectors against infected and transformed cells. Owing to the severe immunopathology that can result from the aberrant stimulation of CTLs, the activation of naïve CD8(+) T cells is a tightly regulated process. A growing body of evidence suggests that the quality of stimulation naïve CD8+ T cells receive during the induction and maintenance of an immune response dictates the functional competency of the responding antigen-specific CTLs, and that CD8+ T cells and their progeny "effector cells" can exist long-term in vastly different activation states.

Transfer of TCR genes into mature T cells is accompanied by the maintenance of parental T cell avidity

The adoptive transfer of tumor-specific T cells expanded in vitro can be of significant therapeutic value in select cancer patients. This strategy is limited though, as it is often difficult, if not impossible, to obtain T cells of clinical value. The transfer of TCR genes to mature T cells to generate tumor-reactive T cells provides a potential mechanism to overcome these limitations. To evaluate the feasibility of such an approach and the quality of the resulting T cells, we generated replication-deficient retroviral vectors using the well-characterized OT-1 TCR genes. After transducing murine T cells, we were able to expand large numbers of Ag-specific T cells that were functionally active against tumor cells expressing the relevant Ag. Furthermore, we found that T cells expressing retrovirally encoded TCR had avidity that was similar to that of the parental clone. This maintenance of avidity was despite variable expression of the retrovirally encoded TCR and the presence of potentially competing endogenous TCRs. These results suggest that the inherent qualities of the TCR, as dictated by the coding sequence, are the most critical parameters in the generation of high-avidity T cells.

Alterations in T-cell receptor Vbeta repertoire of CD4 and CD8 T lymphocytes in human immunodeficiency virus-infected children

Perturbations in the T-cell receptor (TCR) Vbeta repertoire were assessed in the CD4 and CD8 T lymphocytes of human immunodeficiency virus (HIV)-infected children who were receiving therapy during the chronic phase of infection by flow cytometry (FC) and PCR analysis. By FC, representation of 21 TCR Vbeta subfamilies was assessed for an increased or decreased percentage in CD4 and CD8 T cells, and by PCR, 22 TCR Vbeta subfamilies of CD4 and CD8 T cells were analyzed by CDR3 spectratyping for perturbations and reduction in the number of peaks, loss of Gaussian distribution, or clonal dominance. The majority of the TCR Vbeta subfamilies were examined by both methods and assessed for deviation from the norm by comparison with cord blood samples. The CD8-T-lymphocyte population exhibited more perturbations than the CD4 subset, and clonal dominance was present exclusively in CD8 T cells. Of the 55 total CD8-TCR Vbeta families classified with clonal dominance by CDR3 spectratyping, only 18 of these exhibited increased expression by FC. Patients with high numbers of CD8-TCR Vbeta families with decreased percentages had reduced percentages of total CD4 T cells. Increases in the number of CD4-TCR Vbeta families with increased percentages showed a positive correlation with skewing. Overall, changes from normal were often discordant between the two methods. This study suggests that the assessment of HIV-induced alterations in TCR Vbeta families at cellular and molecular levels yields different information and that our understanding of the immune response to HIV is still evolving.

Lack of effector cell function and altered tetramer binding of tumor-infiltrating lymphocytes

Tumor-specific CD8 T cell responses to MCA102 fibrosarcoma cells expressing the cytotoxic T cell epitope gp33 from lymphocytic choriomeningitis virus were studied. MCA102(gp33) tumors grew progressively in C57BL/6 mice, despite induction of peripheral gp33-tetramer(+) T cells that were capable of mediating antiviral protection, specific cell rejection, and concomitant tumor immunity. MCA102(gp33) tumors were infiltrated with a high number ( approximately 20%) of CD11b(+)CD11c(-) macrophage-phenotype cells that were able to cross-present the gp33 epitope to T cells. Tumor-infiltrating CD8 T cells exhibited a highly activated phenotype but lacked effector cell function. Strikingly, a significant portion of tumor-infiltrating lymphocytes expressed TCRs specific for gp33 but bound MHC tetramers only after cell purification and a 24-h resting period in vitro. The phenomenon of "tetramer-negative T cells" was not restricted to tumor-infiltrating lymphocytes from MCA102(gp33) tumors, but was also observed when Ag-specific T cells derived from an environment with high Ag load were analyzed ex vivo. Thus, using a novel tumor model, allowing us to trace tumor-specific T cells at the single cell level in vivo, we demonstrate that the tumor microenvironment is able to alter the functional activity of T cells infiltrating the tumor mass.

Cell-surface expression and immune receptor recognition of HLA-B27 homodimers

OBJECTIVE

HLA-B27 is capable of forming in vitro a heavy-chain homodimer structure lacking beta(2)-microglobulin. We undertook this study to ascertain if patients with spondylarthritis express beta(2)-microglobulin-free HLA-B27 heavy chains in the form of homodimers and receptors for HLA-B27 homodimers.

METHODS

Expression of HLA-B27 heavy chains by mononuclear cells was analyzed by fluorescence-activated cell sorter staining, Western blotting with the monoclonal antibody HC-10, and 2-dimensional isoelectric focusing. Fluorescence-labeled tetrameric complexes of HLA-B27 heavy-chain homodimers were constructed in which each dimer comprised one His-tagged heavy chain and one biotinylated heavy chain, and were used to stain patient and control mononuclear cells and transfected cell lines.

RESULTS

Patients with spondylarthritis expressed cell-surface HLA-B27 homodimers. Populations of synovial and peripheral blood monocytes, and B and T lymphocytes from patients with spondylarthritis, and controls carried receptors for HLA-B27 homodimers. Experiments with transfected cell lines demonstrated that KIR3DL1 and KIR3DL2, and immunoglobulin-like transcript 4 (ILT4), but not ILT2, are receptors for HLA-B27 homodimers.

CONCLUSION

Patients with spondylarthritis express both HLA-B27 heavy-chain homodimers and receptors for HLA-B27 homodimers. This may be of significance with regard to disease pathogenesis.

Labeling antigen-specific CD4(+) T cells with class II MHC oligomers

Class I MHC-peptide oligomers (MHC tetramers) have become popular reagents for the detection and characterization of antigen-specific CD8(+) T cells. Class II MHC proteins can be produced by expression in Escherichia coli followed by in vitro folding, or by native expression in insect cells; biotin can be introduced by site-specific chemical modification of cysteine, or by enzymatic modification of a peptide tag; and a variety of fluorescent streptavidin preparations can be used for oligomerization. Here we review methodologies for production of fluorescent oligomers of soluble class II MHC proteins and discuss their use in analysis of antigen-specific CD4(+) T cells. We explore the experimental conditions necessary for efficient staining of CD4(+) T cells using oligomers of class II MHC proteins, and we establish a standard protocol. Finally, we consider complications and challenges associated with these reagents, discuss the interpretation of staining results, and suggest future directions for investigation, in particular the use of MHC oligomers for the study of T cell avidity modulation.

Induction of Tumor-Reactive CTL by C-Side Chain Variants of the CTL Epitope HER-2/neu Protooncogene (369-377) Selected by Molecular Modeling of the Peptide: HLA-A2 Complex

To design side chain variants for modulation of immunogenicity, we modeled the complex of the HLA-A2 molecule with an immunodominant peptide, E75, from the HER-2/neu protooncogene protein recognized by CTL. We identified the side chain orientation of E75. We modified E75 at the central Ser(5) (E75 wild-type), which points upward, by removing successively the HO (variant S5A) and the CH2-OH (variant S5G). Replacement of the OH with an aminopropyl (CH2)3-NH3 (variant S5K) maintained a similar upward orientation of the side chain. S5A and S5G were stronger stimulators while S5K was a weaker stimulator than E75 for induction of lytic function, indicating that the OH group and its extension hindered TCR activation. S5K-CTL survived longer than did CTL induced by E75 and the variants S5A and S5G, which became apoptotic after restimulation with the inducer. S5K-CTL also recognized E75 endogenously presented by the tumor by IFN-gamma production and specific cytolysis. S5K-CTL expanded at stimulation with E75 or with E75 plus agonistic anti-Fas mAb. Compared with S5K-CTL that had been restimulated with the inducer S5K, S5K-CTL stimulated with wild-type E75 expressed higher levels of E75(+) TCR and BCL-2. Activation of human tumor-reactive CTL by weaker agonists than the nominal Ag, followed by expansion with the nominal Ag, is a novel approach to antitumor CTL development. Fine tuning of activation of tumor-reactive CTL by weak agonists, designed by molecular modeling, may circumvent cell death or tolerization induced by tumor Ag, and thus, may provide a novel approach to the rational design of human cancer vaccines.

Avoiding the kiss of death: how HIV and other chronic viruses survive

Virus-specific CD8 T cells during chronic infection often exceed in numbers virus-replicating infected cells. Why then do antiviral CD8 T cells not do a better job of controlling infection? Although viral strategies for immune evasion are well known, this review will focus on changes in the CD8 T cell that interfere with cytolytic function. Most antiviral CD8 T cells in chronic infection do not express perforin, a molecule that is required for cytolysis. IL-2 and other costimulatory signals can restore cytotoxicity that has been impaired, suggesting a role for CD4 T cell anergy. The chance to eradicate an infection by T cell mediated lysis is undermined after an infection becomes established, in part because the effector immune response is impaired in the setting of chronic antigen.

Minor H antigens: genes and peptides

In this review, we describe the evidence from which the existence of non-MHC histocompatibility (H) antigens was deduced, the clinical setting of bone marrow transplantation in which they are important targets for T cell responses, and the current understanding of their molecular identity. We list the peptide epitopes, their MHC restriction molecules and the genes encoding them, of the human and murine minor H antigens now identified at the molecular level. Identification of the peptide epitopes allows T cell responses to these antigens following transplantation of MHC-matched, minor H-mismatched tissues to be enumerated using tetramers and elispot assays. This will facilitate analysis of correlations with HVG, GVH and GVL reactions in vivo. The potential to use minor H peptides to modulate in vivo responses to minor H antigens is discussed. Factors controlling immunodominance of T cell responses to one or a few of many potential minor H antigens remain to be elucidated but are important for making predictions of in vivo HVG, GVH and GVL responses and tailoring therapy after HLA-matched BMT and DLI.

T cell receptor V beta repertoire of the antigen specific CD8 T lymphocyte subset of HIV infected children

OBJECTIVE

Analysis of the T cell receptor V beta repertoire during HIV infection reveals expansions in multiple V beta families of CD8 T cells, but their antigenic specificity is ill-defined. We sought to determine the TCR V beta repertoire of HIV specific CD8 T lymphocytes in infected children.

DESIGN/METHODS

We performed flow cytometry to examine TCR V beta families as identified by specific monoclonal antibodies and binding of HIV peptide loaded tetrameric MHC complexes in peripheral blood samples from a group of HIV infected children.

RESULTS

Simultaneous assessment of 12 selected expanded V beta families amongst nine HIV infected patients for tetramer binding revealed only one child in whom the expanded V beta population bound HIV Gag or Pol tetramers. In four HIV infected children, percentage tetramer binding cells was determined in 21 TCR V beta families. The tetramer binding cells of three children exhibited a widely distributed TCR V beta repertoire while in the fourth patient they were preferentially localized within two TCR V beta families. Repeat analysis revealed that the TCR V beta repertoire of tetramer binding cells was stable.

CONCLUSIONS

These data provide evidence that the HIV-specific CD8 T cell response in children is usually distributed widely among many different TCR V beta families. The heterogeneity of the TCR V beta repertoire usage by the antigen specific CD8 T cells may reflect the dynamic interaction between host and pathogen during the course of HIV infection and may be influenced by the rate of viral mutation, CD4 T cell helper activity, or other factors.

The immunodominant, Ld-restricted T cell response to hepatitis B surface antigen (HBsAg) efficiently suppresses T cell priming to multiple Dd-, Kd-, and Kb-restricted HBsAg epitopes

MHC-I-restricted CTL responses of H-2(d) (L(d+) or L(d-)) and F(1) H-2(dxb) mice to hepatitis B surface Ag (HBsAg) are primed by either DNA vaccines or HBsAg particles. The D(d)/S(201-209) and K(d)/S(199-208) epitopes are generated by processing endogenous HBsAg; the K(b)/S(208-215) epitope is generated by processing exogenous HBsAg; and the L(d)/S(28-39) epitope is generated by exogenous as well as endogenous processing of HBsAg. DNA vaccination primed high numbers of CTL specific for the L(d)/S(28-39) HBsAg epitope, low numbers of CTL specific for the D(d)/S(201-209) or K(d)/S(199-208) HBsAg epitopes in BALB/c mice, and high numbers of D(d)/S(201-209)- and K(d)/S(199-208)-specific CTL in congenic H-2(d)/L(d-) dm2 mice. In F(1)(dxb) mice, the K(d)-, D(d)-, and K(b)-restricted CTL responses to HBsAg were strikingly suppressed in the presence but efficiently elicited in the absence of L(d)/S(28-39)-specific CTL. Once primed, the K(d)- and D(d)-restricted CTL responses to HBsAg were resistant to suppression by immunodominant L(d)/S(28-39)-specific CTL. The L(d)-restricted immunodominant CTL reactivity to HBsAg can thus suppress priming to multiple alternative epitopes of HBsAg, independent of the processing pathway that generates the epitope, of the background of the mouse strain used, and of the presence/absence of different allelic variants of the K and D MHC class I molecules.

Escaping high viral load exhaustion: CD8 cells with altered tetramer binding in chronic hepatitis B virus infection

Deletion, anergy, and a spectrum of functional impairments can affect virus-specific CD8 cells in chronic viral infections. Here we characterize a low frequency population of CD8 cells present in chronic hepatitis B virus (HBV) infection which survive in the face of a high quantity of viral antigen. Although they do not appear to exert immunological pressure in vivo, these CD8 cells are not classically "tolerant" since they proliferate, lyse, and produce antiviral cytokines in vitro. They are characterized by altered HLA/peptide tetramer reactivity, which is not explained by TCR down-regulation or reduced functional avidity and which can be reversed with repetitive stimulation. CD8 cells with altered tetramer binding appear to have a specificity restricted to envelope antigen and not to other HBV antigens, suggesting that mechanisms of CD8 cell dysfunction are differentially regulated according to the antigenic form and presentation of individual viral antigens.

Escaping high viral load exhaustion: CD8 cells with altered tetramer binding in chronic hepatitis B virus infection

Deletion, anergy, and a spectrum of functional impairments can affect virus-specific CD8 cells in chronic viral infections. Here we characterize a low frequency population of CD8 cells present in chronic hepatitis B virus (HBV) infection which survive in the face of a high quantity of viral antigen. Although they do not appear to exert immunological pressure in vivo, these CD8 cells are not classically "tolerant" since they proliferate, lyse, and produce antiviral cytokines in vitro. They are characterized by altered HLA/peptide tetramer reactivity, which is not explained by TCR down-regulation or reduced functional avidity and which can be reversed with repetitive stimulation. CD8 cells with altered tetramer binding appear to have a specificity restricted to envelope antigen and not to other HBV antigens, suggesting that mechanisms of CD8 cell dysfunction are differentially regulated according to the antigenic form and presentation of individual viral antigens.

Escaping high viral load exhaustion: CD8 cells with altered tetramer binding in chronic hepatitis B virus infection

Deletion, anergy, and a spectrum of functional impairments can affect virus-specific CD8 cells in chronic viral infections. Here we characterize a low frequency population of CD8 cells present in chronic hepatitis B virus (HBV) infection which survive in the face of a high quantity of viral antigen. Although they do not appear to exert immunological pressure in vivo, these CD8 cells are not classically "tolerant" since they proliferate, lyse, and produce antiviral cytokines in vitro. They are characterized by altered HLA/peptide tetramer reactivity, which is not explained by TCR down-regulation or reduced functional avidity and which can be reversed with repetitive stimulation. CD8 cells with altered tetramer binding appear to have a specificity restricted to envelope antigen and not to other HBV antigens, suggesting that mechanisms of CD8 cell dysfunction are differentially regulated according to the antigenic form and presentation of individual viral antigens.

Generation of CD8(+) T cell memory in response to low, high, and excessive levels of epitope

The magnitude of a virus-specific memory CTL population can dramatically influence the outcome of secondary infections, yet little is known about the determinants of memory size. We investigated the impact of epitope levels on CTL memory generation by using a recombinant vaccinia virus system that allows for a broad range of epitope expression with the same infectious dose of virus. The size of the memory pool was examined using MHC class I/peptide tetramer staining and IFN-gamma ELISPOT analysis following priming with viruses expressing low, high, or excessive epitope levels. The size of the epitope-specific CD8(+) T cell memory population correlates with Ag dose at the low and high levels of epitope expression. However, at excessive epitope levels, the number of functional, IFN-gamma-producing, epitope-specific memory cells is significantly reduced compared with the number of tetramer(+) cells. These results demonstrate that the level of epitope expressed during an acute viral infection in vivo can dramatically influence CTL memory size. Furthermore, when epitope is overexpressed, the quality of the response can be adversely affected. Therefore, epitope expression level is an important consideration when developing approaches to optimize CTL memory induction.

Age-associated decrease in virus-specific CD8+ T lymphocytes during primary influenza infection

The mechanism of the age-associated decrease in CD8+ T cell response of mice to virus infection was examined in young adult (6 months) and aged (22 months) C57BL/6 mice during primary pulmonary influenza A virus infection. A significant age-associated decrease in both the percentage (P<0.0001) and number (P<0.05) of CD8+ T cells binding MHC Class I tetramers containing influenza A nucleoprotein (NP) epitope and in virus-specific CTL activity (P<0.05) was observed with pulmonary lymphocytes. The percentage of NP+CD8+ cells of individual mice strongly correlated with NP-specific cytotoxic activity (r(2)=0.77, P<0.02) and with the percentage of CD8+ cells that produced interferon-gamma (r(2)=0.86, P<0.002) in both young and aged mice. Comparable expression of the CD28, CD25, and the memory CD44(hi)/CD62L(lo) phenotype was detected on NP+CD8+ lymphocytes from mice of both age groups. There was a delay in the maximal expansion of NP+CD8+ cells in aged compared to young mice that paralleled a delay in maximal cytotoxic activity and in virus clearance. These data suggest that the age-related impairment of CD8+ lymphocyte activity during a primary influenza A infection is due to a defect in the expansion, rather than in effector activity, of influenza-specific CD8+ T cells.

Epitope-specific evolution of human CD8(+) T cell responses from primary to persistent phases of Epstein-Barr virus infection

Primary virus infection often elicits a large CD8(+) T cell response which subsequently contracts to a smaller memory T cell pool; the relationship between these two virus-specific populations is not well understood. Here we follow the human CD8(+) T cell response to Epstein-Barr virus (EBV) from its primary phase in infectious mononucleosis (IM) through to the persistent carrier state. Using HLA-A2.1 or B8 tetramers specific for four lytic cycle and three latent cycle epitopes, we find marked differences in the epitope-specific composition of the T cell populations between the two phases of infection. The primary response is dominated by lytic epitope specificities which are severely culled (and in one case extinguished) with resolution of the acute infection; in contrast latent epitope specificities are less abundant, if present at all, in acute IM but often then increase their percentage representation in the CD8 pool. Even comparing epitopes of the same type, the relative size of responses seen in primary infection does not necessarily correlate with that seen in the longer term. We also follow the evolution of phenotypic change in these populations and show that, from a uniform CD45RA(-)RO(+)CCR7(-) phenotype in IM, lytic epitope responses show greater reversion to a CD45RA(+)RO(-) phenotype whereas latent epitope responses remain CD45RA(-)RO(+) with a greater tendency to acquire CCR7. Interestingly these phenotypic distinctions reflect the source of the epitope as lytic or latent, and not the extent to which the response has been amplified in vivo.

CD8(+) T cell-mediated injury in vivo progresses in the absence of effector T cells

Tissue injury is a common sequela of acute virus infection localized to a specific organ such as the lung. Tissue injury is an immediate consequence of infection with lytic viruses. It can also result from the direct destruction of infected cells by effector CD8(+) T lymphocytes and indirectly through the action of the T cell-derived proinflammatory cytokines and recruited inflammatory cells on infected and uninfected tissue. We have examined CD8(+) T cell-mediated pulmonary injury in a transgenic model in which adoptively transferred, virus-specific cytotoxic T lymphocytes (CTLs) produce lethal, progressive pulmonary injury in recipient mice expressing the viral target transgene exclusively in the lungs. We have found that over the 4-5 day course of the development of lethal pulmonary injury, the effector CTLs, while necessary for the induction of injury, are present only transiently (24-48 h) in the lung. We provide evidence that the target of the antiviral CD8(+) T cells, the transgene expressing type II alveolar cells, are not immediately destroyed by the effector T cells. Rather, after T cell-target interaction, the type II alveolar cells are stimulated to produce the chemokine monocyte chemoattractant protein 1. These results reinforce the concept that, in vivo, the cellular targets of specific CTLs may participate directly in the development of progressive tissue injury by activating in response to interaction with the T cells and producing proinflammatory mediators without sustained in vivo activation of CD8(+) T cell effectors.

Quantity and quality of virus-specific CD8 cell response: relevance to the design of a therapeutic vaccine for chronic HBV infection

Chronic hepatitis B virus (HBV) infection is a major cause of morbidity and mortality worldwide, yet currently available therapies fail to provide long-term control of viral replication in most patients. Strategies to boost the weak virus-specific T-cell response typically found in patients with chronic hepatitis B have been proposed as a means of terminating persistent HBV infection. The potential problems arising from the stimulation of virus-specific immunity in a disease caused by a non-cytopathic virus, where viral control and liver injury are mediated by the immune system, are discussed. Furthermore, the concept of augmenting the HBV-specific T-cell response, which has previously been focused solely on quantitative issues, is expanded in the light of new findings of qualitative differences in the HBV-specific CD8 cell response.

Regulation of IFN-gamma signaling is essential for the cytotoxic activity of CD8(+) T cells

Previous studies have demonstrated that, as naive murine CD4(+) cells differentiate into Th1 cells, they lose expression of the second chain of IFN-gammaR (IFN-gammaR2). Hence, the IFN-gamma-producing subset of Th cells is unresponsive to IFN-gamma. Analysis of IFN-gamma-producing CD8(+) T cells demonstrates that, like Th1 cells, these cells do not express IFN-gammaR2. To define the importance of IFN-gamma signaling for the development of functional CD8(+) T cells, mice either lacking IFN-gammaR2 or overexpressing this protein were examined. While CD8(+) T cell development and function appear normal in IFN-gammaR2(-/-) mice, CD8(+) T cell function in IFN-gammaR2 transgenic is altered. IFN-gammaR2 transgenic CD8(+) T cells are unable to lyse target cells in vitro. However, these cells produce Fas ligand, perforin, and granzyme B, the effector molecules required for killing. Interestingly, TG CD8(+) T cells proliferate normally and produce cytokines, such as IFN-gamma in response to antigenic stimulation. Therefore, although IFN-gamma signaling is not required for the generation of normal cytotoxic T cells, constitutive IFN-gamma signaling can selectively impair the cytotoxic function of CD8(+) T cells.

Minor H antigens: genes and peptides

In this review, we describe the evidence from which the existence of non-MHC histocompatibility (H) antigens was deduced, the clinical setting of bone marrow transplantation in which they are important targets for T-cell responses, and the current understanding of their molecular identity. We list the peptide epitopes of the human and murine minor H antigens now identified at the molecular level, their MHC restriction molecules and the genes encoding them. Identification of the peptide epitopes allows T-cell responses to these antigens following transplantation of MHC-matched, minor H-mismatched tissues to be enumerated using tetramers and elispot assays. This will facilitate analysis of correlations with host-versus-graft (HVG), graft-versus-host (GVH) and graft-versus-leukaemia (GVL) reactions in vivo. The potential to use minor H peptides to modulate in vivo responses to minor H antigens is discussed. Factors controlling immunodominance of T-cell responses to one or a few of many potential minor H antigens remain to be elucidated but are important for making predictions of in vivo HVG, GVH and GVL responses and tailoring therapy after HLA-matched bone marrow transplantation and donor lymphocyte infusion.

Effector cytolotic function but not IFN-gamma production in cytotoxic T cells triggered by virus-infected target cells in vitro

Cytolysis and interferon(IFN)-gamma production are two independent effector functions of activated cytotoxic T (Tc) cells. We have used the Tc-cell response against the flavivirus, Murray Valley encephalitis virus (MVE), to investigate the requirements for inducing these two functions with regard to antigen-concentration and CD8 coreceptor involvement. Cognate peptide-pulsed target cells triggered cytolysis by primary ex vivo MVE-immune as well as in vitro peptide-restimulated splenocytes at lower peptide concentrations than IFNgamma-production (100-fold lower in the case of primary ex vivo effectors). Little difference was observed in CD8 dependency. Importantly, neither of the effector populations were triggered to produce IFN-gamma by virus-infected target cells, although cytolysis occurred. This result raises the possibility that the levels of presentation of cognate antigen on virus-infected cells in vivo may be below the threshold required for the IFN-gamma production.

Immunodomination results from functional differences between competing CTL

The presence of dominant epitopes suppresses generation of CTL activity toward other non-dominant epitopes found on the same antigen-presenting cell (APC). This phenomenon, termed immunodomination, drastically restricts the diversity of the repertoire of CTL responses. Under various experimental conditions we assessed the in vivo expansion by tetramer staining and function by expression of O-glycans and intracellular perforin of CTL specific for a dominant (B6(dom1)) and a non-dominant (HY) H2D(b)-restricted epitope. Immunodomination abrogated expansion rather than differentiation of HY-specific CTL. When immunodomination was precluded because HY was presented alone or because high numbers of antigen-bearing APC were present, the numbers of HY-specific T cells detected after antigen priming were similar to those of B6(dom1)-specific T cells. The main difference between T cells that recognized B6(dom1) versus HY was functional rather than quantitative. The key feature of T cells specific for B6(dom1) is that they show striking up-regulation of molecules involved in CTL effector activity rather than accumulating to particularly high levels, as assessed by tetramer staining. These results support the emerging concept that following antigen priming, CTL populations of similar size can display important differences in effector function, and suggest that these functional differences are instrumental in shaping the repertoire of CTL responses.

Nonspecific down-regulation of CD8+ T-cell responses in mice expressing human papillomavirus type 16 E7 oncoprotein from the keratin-14 promoter

The E7 oncoprotein of human papillomavirus 16 (HPV16) transforms basal and suprabasal cervical epithelial cells and is a tumor-specific antigen in cervical carcinoma, to which immunotherapeutic strategies aimed at cytotoxic T-lymphocyte (CTL) induction are currently directed. By quantifying major histocompatibility complex class I tetramer-binding T cells and CTL in mice expressing an HPV16 E7 transgene from the keratin-14 (K14) promoter in basal and suprabasal keratinocytes and in thymic cortical epithelium, we show that antigen responsiveness of both E7- and non-E7-specific CD8+ cells is down-regulated compared to non-E7 transgenic control mice. We show that the effect is specific for E7, and not another transgene, expressed from the K14 promoter. Down-regulation did not involve deletion of CD8+ T cells of high affinity or high avidity, and T-cell receptor (TCR) Vbeta-chain usage and TCR receptor density were similar in antigen-responsive cells from E7 transgenic and non-E7 transgenic mice. These data indicate that E7 expressed chronically from the K14 promoter nonspecifically down-regulates CD8+ T-cell responses. The in vitro data correlated with the failure of immunized E7 transgenic mice to control the growth of an E7-expressing tumor challenge. We have previously shown that E7-directed CTL down-regulation correlates with E7 expression in peripheral but not thymic epithelium (T. Doan et al., J. Virol. 73:6166-6170, 1999). The findings have implications for the immunological consequences of E7-expressing tumor development and E7-directed immunization strategies. Generically, the findings illustrate a T-cell immunomodulatory function for a virally encoded human oncoprotein.

Cutting edge: lipid raft integrity affects the efficiency of MHC class I tetramer binding and cell surface TCR arrangement on CD8+ T cells

Physically distinct cholesterol/sphingolipid-rich plasma membrane microdomains, so-called lipid rafts, have been recognized to play an important regulatory role in various cellular processes, from membrane trafficking to signal transduction, in a number of cell types. We report here that the ability of TCR on activated, functional CD8+ T lymphocytes to efficiently bind MHC class I tetramer complexes is dependent on the integrity of lipid rafts on the T lymphocyte membrane. We further provide evidence that TCR interact (associate) with lipid raft elements on the T cell surface before receptor engagement and that the topological arrangement of TCR on the cell surface is likewise influenced by lipid raft integrity.

Modulation of CD8+ T cell response to antigen by the levels of self MHC class I

The response of H-Y-specific TCR-transgenic CD8(+) T cells to Ag is characterized by poor proliferation, cytolytic activity, and IFN-gamma secretion. IFN-gamma secretion, but not cytotoxic function, can be rescued by the B7.1 molecule, suggesting that costimulation can selectively enhance some, but not all, effector CD8(+) T cell responses. Although the H-Y epitope binds H-2D(b) relatively less well than some other epitopes, it can induce potent CTL responses in nontransgenic mice, suggesting that the observed poor responsiveness of transgenic CD8(+) T cells cannot be ascribed to the epitope itself. Previously reported reactivity of this TCR to H-2A(b) is also not the cause of the poor responsiveness of the H-Y-specific CD8(+) T cells, as H-Y-specific CD8(+) T cells obtained from genetic backgrounds lacking H-2A(b) also responded poorly. Rather, reducing the levels of H-2(b) class I molecules by breeding the mice to (C57BL/6 x B10.D2)F(1) or TAP1(+/-) backgrounds partially restored cytotoxic activity and enhanced proliferative responses. These findings demonstrate that the self MHC class I gene dosage may regulate the extent of CD8(+) T cell responsiveness to Ag.