High-avidity CTL exploit two complementary mechanisms to provide better protection against viral infection than low-avidity CTL

Superior control of HIV-1 replication by CD8+ T cells is reflected by their avidity, polyfunctionality, and clonal turnover

The key attributes of CD8⁺ T cell protective immunity in human immunodeficiency virus (HIV) infection remain unclear. We report that CD8⁺ T cell responses specific for Gag and, in particular, the immunodominant p24 epitope KK10 correlate with control of HIV-1 replication in human histocompatibility leukocyte antigen (HLA)–B27 patients. To understand further the nature of CD8⁺ T cell–mediated antiviral efficacy, we performed a comprehensive study of CD8⁺ T cells specific for the HLA-B27–restricted epitope KK10 in chronic HIV-1 infection based on the use of multiparametric flow cytometry together with molecular clonotypic analysis and viral sequencing. We show that B27-KK10–specific CD8⁺ T cells are characterized by polyfunctional capabilities, increased clonal turnover, and superior functional avidity. Such attributes are interlinked and constitute the basis for effective control of HIV-1 replication. These data on the features of effective CD8⁺ T cells in HIV infection may aid in the development of successful T cell vaccines.

Evolution of human immunodeficiency virus type 1 cytotoxic T-lymphocyte epitopes: fitness-balanced escape

CD8(+) cytotoxic T lymphocytes (CTL) are strong mediators of human immunodeficiency virus type 1 (HIV-1) control, yet HIV-1 frequently mutates to escape CTL recognition. In an analysis of sequences in the Los Alamos HIV-1 database, we show that emerging CTL escape mutations were more often present at lower frequencies than the amino acid(s) that they replaced. Furthermore, epitopes that underwent escape contained amino acid sites of high variability, whereas epitopes persisting at high frequencies lacked highly variable sites. We therefore infer that escape mutations are likely to be associated with weak functional constraints on the viral protein. This was supported by an extensive analysis of one subject for whom all escape mutations within defined CTL epitopes were studied and by an analysis of all reported escape mutations of defined CTL epitopes in the HIV Immunology Database. In one of these defined epitopes, escape mutations involving the substitution of amino acids with lower database frequencies occurred, and the epitope soon reverted back to the sensitive form. We further show that this escape mutation substantially diminished viral fitness in in vitro competition assays. Coincident with the reversion in vivo, we observed the fixation of a mutation 3 amino acids C terminal to the epitope, coincident with the ablation of the corresponding CTL response. The C-terminal mutation did not restore replication fitness reduced by the escape mutation in the epitope and by itself had little effect on replication fitness. Therefore, this C-terminal mutation presumably impaired the processing and presentation of the epitope. Finally, for one persistent epitope, CTL cross-reactivity to a mutant form may have suppressed the mutant to undetected levels, whereas for two other persistent epitopes, each of two mutants showed poor cross-reactivity and appeared in the subject at later time points. Thus, a viral dynamic exists between the advantage of immune escape, peptide cross-reactivity, and the disadvantage of lost replication fitness, with the balance playing an important role in determining whether a CTL epitope will persist or decline during infection.

Secondary immunisation with high-dose heterologous peptide leads to CD8 T cell populations with reduced functional avidity

Expansion of high- or low-avidity CD8 T cells in vitro inversely correlates with the concentration of peptide ligand present during culture. In contrast, the selective enrichment of high- or low-avidity T cell populations in vivo using peptide immunisation is not well documented. In our study, a single immunisation with different doses of wild-type peptide or a variant peptide able to stimulate CTL responses cross-reactive with wild-type peptide failed to shift the average avidity of the responding CD8 T cell population specific to either peptide. However, in contrast to homologous prime-boost immunisation, heterologous prime-boost immunisation incorporating high doses of the second immunogen resulted in peptide-specific CD8 T cell populations polarized toward a low average functional avidity. These data suggest that sequential exposure to structurally related viral peptides could impair rather than promote anti-viral immunity by lowering the avidity of the responding CD8 T cell population. This study has implications for improving vaccine strategies against viruses and tumours and enhances our understanding of heterologous immunity during sequential viral infection.

Epitope-dependent avidity thresholds for cytotoxic T-lymphocyte clearance of virus-infected cells

Cytotoxic T lymphocytes (CTLs) are crucial for immune control of viral infections. "Functional avidity," defined by the sensitizing dose of exogenously added epitope yielding half-maximal CTL triggering against uninfected target cells (SD(50)), has been utilized extensively as a measure of antiviral efficiency. However, CTLs recognize infected cells via endogenously produced epitopes, and the relationship of SD(50) to antiviral activity has never been directly revealed. We elucidate this relationship by comparing CTL killing of cells infected with panels of epitope-variant viruses to the corresponding SD(50) for the variant epitopes. This reveals a steeply sigmoid relationship between avidity and infected cell killing, with avidity thresholds (defined as the SD(50) required for CTL to achieve 50% efficiency of infected cell killing [KE(50)]), below which infected cell killing rapidly drops to none and above which killing efficiency rapidly plateaus. Three CTL clones recognizing the same viral epitope show the same KE(50) despite differential recognition of individual epitope variants, while CTLs recognizing another epitope show a 10-fold-higher KE(50), demonstrating epitope dependence of KE(50). Finally, the ability of CTLs to suppress viral replication depends on the same threshold KE(50). Thus, defining KE(50) values is required to interpret the significance of functional avidity measurements and predict CTL efficacy against virus-infected cells in pathogenesis and vaccine studies.

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

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

Mucosal HIV-1 Pox Virus Prime-Boost Immunization Induces High-Avidity CD8+ T Cells with Regime-Dependent Cytokine/Granzyme B Profiles

The quality of virus-specific CD8(+) CTL immune responses generated by mucosal and systemic poxvirus prime-boost vaccines were evaluated in terms of T cell avidity and single-cell analysis of effector gene expression. Intranasal (I.N.) immunization regimes generated higher avidity CTL responses specific for HIV K(d)Gag(197-205) (amino acid sequence AMQMLKETI; H-2K(d) binding) compared with i.m. immunization regime. Single-cell RT-PCR of K(d)Gag(197-205)-specific mucosal and systemic CTL revealed that the cytokine and granzyme B expression profiles were dependent on both the route and time after immunization. The I.N./i.m.-immunized group elicited elevated number of CTL-expressing granzyme B mRNA from the genitomucosal sites compared with the i.m./i.m. regime. Interestingly, CTL generated after both I.N. or i.m. immunization demonstrated expression of Th2 cytokine IL-4 mRNA that was constitutively expressed over time, although lower numbers were observed after I.N./I.N. immunization. Results suggest that after immunization, Ag-specific CTL expression of IL-4 may be an inherent property of the highly evolved poxvirus vectors. Current observations indicate that the quality of CTL immunity generated after immunization can be influenced by the inherent property of vaccine vectors and route of vaccine delivery. A greater understanding of these factors will be crucial for the development of effective vaccines in the future.

Redirecting T lymphocyte specificity by T cell receptor gene transfer – A new era for immunotherapy

The therapeutic efficacy of adoptively transferred cytotoxic T lymphocytes (CTL) has been demonstrated in clinical trials for the treatment of chronic myelogenous leukemia, cytomegalovirus-mediated disease, and Epstein-Barr virus-positive B cell lymphomas. It is however limited by the difficulty of generating sufficient amounts of CTLs in vitro, especially for the treatment of solid tumors. Recent gene therapy approaches, including two clinical trials, successfully apply genetic engineering of T cell specificity by T cell receptor (TCR) gene transfer. In this review we want to elucidate several principles of the redirection of T cell specificity. We cover basic aspects of retroviral gene transfer, regarding transduction efficacy and transgene expression levels. It was demonstrated that the number of TCR molecules on a T cell is important for its function. Therefore, an efficient transfer system that yields high transduction efficiency and strong and stable transgene expression is a prerequisite to achieve effector function by redirected T cells. Furthermore, we consider more recent aspects of T cell specificity engineering. These include the possibility of co-transferring coreceptors to create for example functional T helper cells by engrafting CD4(+) T cells with a MHC class I restricted TCR and the CD8 coreceptor and vice versa. Also, risks related to the adoptive transfer of TCR gene-modified T cells and possible safety mechanisms are discussed. Finally, we summarize recent findings describing transferred TCRs capable of displacing endogenous TCRs from the cell surface.

A single amino acid difference within the alpha-2 domain of two naturally occurring equine MHC class I molecules alters the recognition of Gag and Rev epitopes by equine infectious anemia virus-specific CTL

Although CTL are critical for control of lentiviruses, including equine infectious anemia virus, relatively little is known regarding the MHC class I molecules that present important epitopes to equine infectious anemia virus-specific CTL. The equine class I molecule 7-6 is associated with the equine leukocyte Ag (ELA)-A1 haplotype and presents the Env-RW12 and Gag-GW12 CTL epitopes. Some ELA-A1 target cells present both epitopes, whereas others are not recognized by Gag-GW12-specific CTL, suggesting that the ELA-A1 haplotype comprises functionally distinct alleles. The Rev-QW11 CTL epitope is also ELA-A1-restricted, but the molecule that presents Rev-QW11 is unknown. To determine whether functionally distinct class I molecules present ELA-A1-restricted CTL epitopes, we sequenced and expressed MHC class I genes from three ELA-A1 horses. Two horses had the 7-6 allele, which when expressed, presented Env-RW12, Gag-GW12, and Rev-QW11 to CTL. The other horse had a distinct allele, designated 141, encoding a molecule that differed from 7-6 by a single amino acid within the alpha-2 domain. This substitution did not affect recognition of Env-RW12, but resulted in more efficient recognition of Rev-QW11. Significantly, CTL recognition of Gag-GW12 was abrogated, despite Gag-GW12 binding to 141. Molecular modeling suggested that conformational changes in the 141/Gag-GW12 complex led to a loss of TCR recognition. These results confirmed that the ELA-A1 haplotype is comprised of functionally distinct alleles, and demonstrated for the first time that naturally occurring MHC class I molecules that vary by only a single amino acid can result in significantly different patterns of epitope recognition by lentivirus-specific CTL.

Induction of human immunodeficiency virus type 1 (HIV-1)-specific T-cell responses in HIV vaccine trial participants who subsequently acquire HIV-1 infection

Candidate human immunodeficiency virus type 1 (HIV-1) vaccines designed to elicit T-cell immunity in HIV-1-uninfected persons are under investigation in phase I to III clinical trials. Little is known about how these vaccines impact the immunologic response postinfection in persons who break through despite vaccination. Here, we describe the first comprehensive characterization of HIV-specific T-cell immunity in vaccine study participants following breakthrough HIV-1 infection in comparison to 16 nonvaccinated subjects with primary HIV-1 infection. Whereas none of the 16 breakthrough infections possessed vaccine-induced HIV-1-specific T-cell responses preinfection, 85% of vaccinees and 86% of nonvaccinees with primary HIV-1 infection developed HIV-specific T-cell responses postinfection. Breakthrough subjects' T cells recognized 43 unique HIV-1 T-cell epitopes, of which 8 are newly described, and 25% were present in the vaccine. The frequencies of gamma interferon (IFN-gamma)-secreting cells recognizing epitopes within gene products that were and were not encoded by the vaccine were not different (P = 0.64), which suggests that responses were not anamnestic. Epitopes within Nef and Gag proteins were the most commonly recognized in both vaccinated and nonvaccinated infected subjects. One individual controlled viral replication without antiretroviral therapy and, notably, mounted a novel HIV-specific HLA-C14-restricted Gag LYNTVATL-specific T-cell response. Longitudinally, HIV-specific T cells in this individual were able to secrete IFN-gamma and tumor necrosis factor alpha, as well as proliferate and degranulate in response to their cognate antigenic peptides up to 5 years postinfection. In conclusion, a vaccinee's ability to mount an HIV-specific T-cell response postinfection is not compromised by previous immunization, since the CD8+ T-cell responses postinfection are similar to those seen in vaccine-naïve individuals. Finding an individual who is controlling infection highlights the importance of comprehensive studies of breakthrough infections in vaccine trials to determine whether host genetics/immune responses and/or viral characteristics are responsible for controlling viral replication.

The hypervariable immunodominant NP418-426 epitope from the influenza A virus nucleoprotein is recognized by cytotoxic T lymphocytes with high functional avidity

Recently it was shown that influenza A viruses can accumulate mutations in epitopes associated with escape from recognition by human virus-specific cytotoxic T lymphocytes (CTL). It is unclear what drives diversification of CTL epitopes and why certain epitopes are variable and others remain conserved. It has been shown that simian immunodeficiency virus-specific CTL that recognize their epitope with high functional avidity eliminate virus-infected cells efficiently and drive diversification of CTL epitopes. T-cell functional avidity is defined by the density of major histocompatibility complex class I peptide complexes required to activate specific CTL. We hypothesized that functional avidity of CTL contributes to epitope diversification and escape from CTL also for influenza viruses. To test this hypothesis, the functional avidity of polyclonal CTL populations specific for nine individual epitopes was determined. To this end, peripheral blood mononuclear cells from HLA-A- and -B-genotyped individuals were stimulated in vitro with influenza virus-infected cells to allow expansion of virus-specific CTL, which were used to determine the functional avidity of CTL specific for nine individual epitopes in enzyme-linked immunospot assays. We found that the functional avidity for the respective epitopes varied widely. Furthermore, the functional avidity of CTL specific for the hypervariable NP(418-426) epitope was significantly higher than that of CTL recognizing other epitopes (P < 0.01). It was speculated that the high functional avidity of NP(418-426)-specific CTL was responsible for the diversification of this influenza A virus CTL epitope.

Protective immunity against hepatitis C virus infection

There is increasing evidence that a small percentage of individuals exposed to the hepatitis C virus have the capacity to generate a strong cellular immune response against the virus and avoid persistent infection, and perhaps do so repeatedly after re-exposure. This article reviews the evidence that the responses identified in this unique group of individuals represent the protective immunity that will need to be elicited by hepatitis C virus vaccines.

Impact of vaccine-induced mucosal high-avidity CD8+ CTLs in delay of AIDS viral dissemination from mucosa

Natural HIV transmission occurs through mucosa, but it is debated whether mucosal cytotoxic T lymphocytes (CTLs) can prevent or reduce dissemination from the initial mucosal site to the systemic circulation. Also, the role of CTL avidity in mucosal AIDS viral transmission is unknown. To address these questions, we used delay in acute-phase peak viremia after intrarectal challenge as an indicator of systemic dissemination. We found that a peptide-prime/poxviral boost vaccine inducing high levels of high-avidity mucosal CTLs can have an impact on dissemination of intrarectally administered pathogenic SHIV-ku2 in macaques and that such protection correlates better with mucosal than with systemic CTLs and particularly with levels of high-avidity mucosal CTLs.

Identification and epitope enhancement of a PAX-FKHR fusion protein breakpoint epitope in alveolar rhabdomyosarcoma cells created by a tumorigenic chromosomal translocation inducing CTL capable of lysing human tumors

Fusion proteins created by chromosomal translocations in tumors can create neoantigenic determinants at the breakpoint, which are unique to the tumor cells but shared by the vast majority of tumors of that histologic type. If the fusion protein is responsible for the malignant transformation, its expression cannot be lost by the tumor to escape immune responses against this tumor antigen. Here, we identify such a fusion protein breakpoint epitope in the PAX-FKHR fusion protein created by the t(2;13) translocation present in 80% of cases of alveolar rhabdomyosarcoma, a highly aggressive pediatric soft-tissue sarcoma. We use autologous dendritic cells pulsed with the RS10 breakpoint fusion peptide to raise a human CTL line from a normal healthy HLA-B7+ blood donor specific for this peptide. These CTLs are CD8+ (CD4-CD56-) and restricted by HLA-B7. These human peptide-specific CTL lyse human HLA-B7+ rhabdomyosarcoma tumor cells. Therefore, the fusion protein is endogenously processed to produce this natural epitope presented by HLA-B7 and thus this peptide is a bone fide human tumor antigen. We also define a substitution that increases the affinity for HLA-B7 without loss of antigenicity. This epitope-enhanced peptide may serve as a candidate cancer vaccine for HLA-B7+ patients with alveolar rhabdomyosarcoma.

Immunity to melanoma antigens: from self-tolerance to immunotherapy

The development of effective immune therapy for cancer is a central goal of immunologists in the 21st century. Our laboratories have been deeply involved in characterization of the immune response to melanoma and translation of laboratory discoveries into clinical trials. We have identified a cohort of peptide antigens presented by Major Histocompatibility Complex (MHC) molecules on melanoma cells and widely recognized by T cells from melanoma patients. These have been incorporated into peptide-based vaccines that induce CD8(+) and CD4(+) T-cell responses in 80-100% of patients. Major objective clinical tumor regressions have been observed in some patients, and overall survival in vaccinated patients exceeds expected stage-specific survival. New clinical trials will determine the value of combination of melanoma helper peptides (MHP) into multipeptide vaccines targeting CD8 cells. New trials will also evaluate new approaches to modulating the host-tumor relationship and will develop new combination therapies. Parallel investigations in murine models are elucidating the immunobiology of the melanoma-host relationship and addressing issues that are not feasible to approach in human trials. Based on the fact that the largest cohort of melanoma antigens are derived from normal proteins concerned with pigment production, we have evaluated the mechanisms of self-tolerance to tyrosinase (Tyr) and have determined how T cells in an environment of self-tolerance are impacted by immunization. Using peptide-pulsed dendritic cells as immunogens, we have also used the mouse model to establish strategies for quantitative and qualitative enhancement of antitumor immunity. This information creates opportunities for a new generation of therapeutic interventions using cancer vaccines.

Vaccines with enhanced costimulation maintain high avidity memory CTL

The avidity of Ag-specific CTL is a critical determinant for clearing viral infection and eliminating tumor. Although previous studies have demonstrated that vaccines using enhanced costimulation will enhance the level and avidity of Ag-specific T cells from naive mice, there are conflicting data about the effects of vaccines using enhanced costimulation (vector or dendritic cell based) on the survival of memory T cells. In this study we have first extended previous observations that primary vaccination with a recombinant vaccinia virus (rV-) expressing a model Ag (LacZ) and a triad of T cell costimulatory molecules (B7-1, ICAM-1, and LFA-3 (designated TRICOM)) enhances the level and avidity of T cells from naive vaccinated C57BL/6 (Thy1.2) mice. Adoptive transfer of Thy1.1 memory CD8(+) T cells into naive Thy1.2 C57BL/6 mice was followed by booster vaccinations with a recombinant fowlpox (rF-)-expressing LacZ (rF-LacZ) or booster vaccinations with rF-LacZ/TRICOM. Analysis of levels of beta-galactosidase tetramer-positive T cells and functional assays (IFN-gamma expression and lytic activity) determined that booster vaccinations with rF-LacZ/TRICOM were superior to booster vaccinations with rF-LacZ in terms of both maintenance and enhanced avidity of memory CD8(+) T cells. Antitumor experiments using a self-Ag (carcinoembryonic Ag (CEA) vaccines in CEA transgenic mice bearing CEA-expressing tumors) also demonstrated that the use of booster vaccinations with vaccines bearing enhanced costimulatory capacity had superior antitumor effects. These studies thus have implications in the design of more effective vaccine strategies.

Avidity for antigen shapes clonal dominance in CD8+ T cell populations specific for persistent DNA viruses

The forces that govern clonal selection during the genesis and maintenance of specific T cell responses are complex, but amenable to decryption by interrogation of constituent clonotypes within the antigen-experienced T cell pools. Here, we used point-mutated peptide-major histocompatibility complex class I (pMHCI) antigens, unbiased TCRB gene usage analysis, and polychromatic flow cytometry to probe directly ex vivo the clonal architecture of antigen-specific CD8(+) T cell populations under conditions of persistent exposure to structurally stable virus-derived epitopes. During chronic infection with cytomegalovirus and Epstein-Barr virus, CD8(+) T cell responses to immunodominant viral antigens were oligoclonal, highly skewed, and exhibited diverse clonotypic configurations; TCRB CDR3 sequence analysis indicated positive selection at the protein level. Dominant clonotypes demonstrated high intrinsic antigen avidity, defined strictly as a physical parameter, and were preferentially driven toward terminal differentiation in phenotypically heterogeneous populations. In contrast, subdominant clonotypes were characterized by lower intrinsic avidities and proportionately greater dependency on the pMHCI-CD8 interaction for antigen uptake and functional sensitivity. These findings provide evidence that interclonal competition for antigen operates in human T cell populations, while preferential CD8 coreceptor compensation mitigates this process to maintain clonotypic diversity. Vaccine strategies that reconstruct these biological processes could generate T cell populations that mediate optimal delivery of antiviral effector function.

The response of autologous T cells to a human melanoma is dominated by mutated neoantigens

Our understanding of pathways leading to antitumor immunity may depend on an undistorted knowledge of the primary antigenic targets of patients' autologous T cell responses. In the melanoma model derived from patient DT, we applied cryopreserved short-term autologous mixed lymphocyte-tumor cell cultures (MLTCs) in combination with an IFN-gamma enzyme-linked immunospot (ELISPOT) assay to cDNA expression screening. We identified three previously unknown peptides processed from melanosomal proteins tyrosinase (presented by HLA-A(*)2601 and -B(*)3801) and gp100 (presented by HLA-B(*)07021) and five neoantigens generated by somatic point mutations in the patient's melanoma. The mutations were found in the genes SIRT2, GPNMB, SNRP116, SNRPD1, and RBAF600. Peptides containing the mutated residues were presented by HLA-A(*)03011, -B(*)07021, and -B(*)3801. Mutation-induced functional impairment was so far demonstrated for SIRT2. Within MLTC responder populations that were independently expanded from the patient's peripheral blood lymphocytes of different years, T cells against mutated epitopes clearly predominated. These results document a high degree of individuality for the cellular antitumor response and support the need for individualizing the monitoring and therapeutic approaches to the primary targets of the autologous T cell response, which may finally lead to a more effective cancer immunotherapy.

Evaluation of high functional avidity CTL to Gag epitope clusters in EIAV carrier horses

Cytotoxic T lymphocytes (CTL) are critical for lentivirus control including EIAV. Since CTL from most EIAV carrier horses recognize Gag epitope clusters (EC), the hypothesis that carrier horses would have high functional avidity CTL to optimal epitopes in Gag EC was tested. Twenty-two optimal EC epitopes were identified; two in EC1, six in EC2, and seven each in EC3 and 4. However, only five of nine horses had high functional avidity CTL (<or=11 nM) recognizing six epitopes in EC; four in relatively conserved EC3; and one each in EC1 and 2. Horses with high functional avidity CTL had significantly more days since the last clinical episode than horses with low avidity CTL, and this was not explained by analyzing duration of infection. Furthermore, there was a significant inverse correlation between the CTL functional avidity of the nine horses and the days since the last clinical episode. Gag CTL epitope escape variants were found in three horses, but only one of these was recognized by high functional avidity CTL. Thus, not all carrier horses had high functional avidity CTL to Gag EC, but those that did had longer periods without disease episodes.

Multiple costimulatory modalities enhance CTL avidity

Recent studies in both animal models and clinical trials have demonstrated that the avidity of T cells is a major determinant of antitumor and antiviral immunity. In this study, we evaluated several different vaccine strategies for their ability to enhance both the quantity and avidity of CTL responses. CD8(+) T cell quantity was measured by tetramer binding precursor frequency, and avidity was measured by both tetramer dissociation and quantitative cytolytic function. We have evaluated a peptide, a viral vector expressing the Ag transgene alone, with one costimulatory molecule (B7-1), and with three costimulatory molecules (B7-1, ICAM-1, and LFA-3), with anti-CTLA-4 mAb, with GM-CSF, and combinations of the above. We have evaluated these strategies in both a foreign Ag model using beta-galactosidase as immunogen, and in a "self" Ag model, using carcinoembryonic Ag as immunogen in carcinoembryonic Ag transgenic mice. The combined use of several of these strategies was shown to enhance not only the quantity, but, to a greater magnitude, the avidity of T cells generated; a combination strategy is also shown to enhance antitumor effects. The results reported in this study thus demonstrate multiple strategies that can be used in both antitumor and antiviral vaccine settings to generate higher avidity host T cell responses.

Delayed expansion of a restricted T cell repertoire by low-density TCR ligands

The role of TCR ligand density (i.e. the number of antigen-MHC complexes) in modulating the diversity of a T cell response selected from a pool of naive precursors remains largely undefined. By measuring early-activation markers up-regulation and proliferation following stimulation with staphylococcal enterotoxin A (SEA), we demonstrate that decreasing the ligand dose below an optimal concentration leads to the delayed activation of a restricted set of TCRVbeta-bearing T cells, with the specific, non-stochastic exclusion of some TCRVbeta+ T cells from the activated pool. Our results suggest that the failure of these TCRVbeta-bearing T cells to reach the activation threshold at sub-optimal ligand concentration is due to the inefficiency of TCR engagement, as measured by TCR internalization, and does not correlate with the relative precursor frequency in the non-immune repertoire. Moreover, even at SEA concentrations that lead to the simultaneous proliferation of all SEA-reactive T cells, we observe marked differences in the ability to secrete cytokines among the different responsive TCRVbeta-bearing T cells. Altogether, our results indicate that the development of a T cell response to a scarce display of ligand significantly narrows TCR repertoire diversity by mechanisms that involve focusing of the repertoire on the expansion of those T cells with the highest avidity of TCR engagement.

A phase I vaccination study with tyrosinase in patients with stage II melanoma using recombinant modified vaccinia virus Ankara (MVA-hTyr)

A significant percentage of patients with stage II melanomas suffer a relapse after surgery and therefore need the development of adjuvant therapies. In the study reported here, safety and immunological response were analyzed after vaccination in an adjuvant setting with recombinant modified vaccinia virus Ankara carrying the cDNA for human tyrosinase (MVA-hTyr). A total of 20 patients were included and vaccinated three times at 4-week intervals with 5x10(8) IU of MVA-hTyr each time. The responses to the viral vector, to known HLA class I-restricted tyrosinase peptides, and to dendritic cells transfected with tyrosinase mRNA, were investigated by ELISpot assay on both ex vivo T cells and on T cells stimulated in vitro prior to testing. The delivery of MVA-hTyr was safe and did not cause any side effects above grade 2. A strong response to the viral vector was achieved, indicated by an increase in the frequency of MVA-specific CD4+ and CD8+ T cells and an increase in virus-specific antibody titers. However, no tyrosinase-specific T-cell or antibody response was observed with MVA-hTyr in any of the vaccinated patients. Although MVA-hTyr provides a safe and effective antigen-delivery system, it does not elicit a measurable immune response to its transgene product in patients with stage II melanoma after repeated combined intradermal and subcutaneous vaccination. We presume that modification of the antigen and/or prime-boost vaccination applying different approaches to antigen delivery may be required to induce an effective tyrosinase-specific immune response.

Exploiting T cell receptor genes for cancer immunotherapy

Adoptive antigen-specific immunotherapy is an attractive concept for the treatment of cancer because it does not require immunocompetence of patients, and the specificity of transferred lymphocytes can be targeted against tumour-associated antigens that are poorly immunogenic and thus fail to effectively trigger autologous T cell responses. As the isolation and in vitro expansion of antigen-specific lymphocytes is difficult, 'conventional' adoptive T cell therapy can only be carried out in specialized centres in small numbers of patients. However, T cell receptor (TCR) genes isolated from antigen-specific T cells can be exploited as generic therapeutic molecules for 'unconventional' antigen-specific immunotherapy. Retroviral TCR gene transfer into patient T cells can readily produce populations of antigen-specific lymphocytes after a single round of polyclonal T cell stimulation. TCR gene modified lymphocytes are functionally competent in vitro, and can have therapeutic efficacy in murine models in vivo. TCR gene expression is stable and modified lymphocytes can develop into memory T cells. Introduction of TCR genes into CD8(+) and CD4(+) lymphocytes provides an opportunity to use the same TCR specificity to produce antigen-specific killer and helper T lymphocytes. Thus, TCR gene therapy provides an attractive strategy to develop antigen-specific immunotherapy with autologous lymphocytes as a generic treatment option.

High responsiveness of HLA-B57-restricted Gag-specific CD8+ T cells in vitro may contribute to the protective effect of HLA-B57 in HIV-infection

HLA-B57 has been shown to be associated with long-term asymptomatic HIV-1 infection. To investigate the biological mechanism by which the HLA-B57 allele could protect from HIV-1 disease, we studied both the number of CD8(+) T cells as well as CD8(+) T cell responsiveness directed to different HIV-1 Gag peptides presented by HLA-A2, -B8 or -B57. T cells specific for the HLA-B57 peptide KAFSPEVIPMF responded more readily and to a higher extend to antigenic stimulation in vitro than T cells specific for the HLA-A2 peptide SLYNTVATL or the HLA-B8 peptide EIYKRWII. This phenomenon was reproducible with T cells from individuals expressing HLA-B57 in combination with one or both of the other alleles and was persistent during long-term follow-up. Lower reactivity of A2- and B8-restricted T cells was not explained by mutations in the B8- or A2-restricted Gag-peptides. Moreover, no correlation between peptide mutation frequency and IFN-gamma production by the corresponding Gag-specific T cells was observed. In conclusion, functional differences were observed between T cells specific for HIV epitopes derived from the same protein presented by different HLA molecules. B57-restricted KAFSPEVIPMF-specific CD8(+) T cells have relatively high responsiveness, which could contribute to the protective effect of HLA-B57 in HIV infection.

DNA vaccines employing intracellular targeting strategies and a strategy to prolong dendritic cell life generate a higher number of CD8+ memory T cells and better long-term antitumor effects compared with a DNA prime-vaccinia boost regimen

We have previously shown that intradermal coadministration of DNA encoding Bcl-x(L), an antiapoptotic protein, with DNA encoding E7 antigen linked to the sorting signal of the lysosome-associated membrane protein type 1 (Sig/E7/LAMP-1) prolongs dendritic cell life and enhances antigen presentation through the MHC class I and II pathways. In the current study, we compared this approach with a conventional DNA prime-vaccinia boost protocol on the basis of their ability to generate antigen-specific CD8(+) memory T cells and longterm antitumor effects against an E7-expressing tumor. Mice primed and boosted with Sig/E7/LAMP-1 DNA mixed with Bcl-x(L) DNA generated significantly higher numbers of E7-specific CD8(+) memory T cells and a better long-term protective antitumor effect compared with mice primed with Sig/E7/LAMP-1 DNA and boosted with Sig/E7/LAMP-1 vaccinia (Vac-Sig/E7/LAMP-1). Furthermore, coadministration of Sig/E7 /LAMP-1 DNA mixed with Bcl-x(L) DNA also generated higher avidity E7-specific CD8(+) T cells than did vaccination with Sig/E7/LAMP-1 DNA followed by a Vac-Sig/E7/LAMP-1 booster. Our results indicate that coadministration of a DNA vaccine employing intracellular targeting strategies and a DNA encoding antiapoptotic proteins may potentially generate a higher number of memory CD8(+) T cells and better long-term protective antitumor effects compared with the conventional DNA prime-vaccinia boost regimen.

IL-15/IL-15Ralpha-mediated avidity maturation of memory CD8+ T cells

T cell avidity is critical to viral clearance, but mechanisms of CD8(+) T cell avidity maturation are poorly understood. Here, we find that IL-15 mediates two mechanisms of avidity maturation. (i) By selection at the population level, IL-15 promotes greater survival of high- compared with low-avidity cytotoxic T lymphocytes (CTLs). High-avidity CTLs express higher levels of IL-15Ralpha and persist longer by homeostatic proliferation. (ii) At the individual cell level, IL-15 induces higher levels of surface coreceptor CD8alphabeta, increasing functional avidity. IL-15 during priming selects or induces higher-avidity CTLs. Conversely, high-avidity CTLs are diminished in IL-15Ralpha knockout mice. These results provide an explanation of CD8+ T cell avidity maturation and may contribute to the design of novel vaccines.

Partial activation of neonatal CD11c+ dendritic cells and induction of adult-like CD8+ cytotoxic T cell responses by synthetic microspheres

Neonatal cytotoxic T cell responses have only been elicited to date with immunogens or delivery systems inducing potent direct APC activation. To define the minimal activation requirements for the induction of neonatal CD8(+) cytotoxic responses, we used synthetic microspheres (MS) coated with a single CD8(+) T cell peptide from lymphocytic choriomeningitis virus (LCMV) or HIV-1. Unexpectedly, a single injection of peptide-conjugated MS without added adjuvant induced CD4-dependent Ag-specific neonatal murine cytotoxic responses with adult-like CTL precursor frequency, avidity for Ag, and frequency of IFN-gamma-secreting CD8(+) splenocytes. Neonatal CD8(+) T cell responses to MS-LCMV were elicited within 2 wk of a single immunization and, upon challenge, provided similar protection from viral replication as adult CTLs, demonstrating their in vivo competence. As previously reported, peptide-coated MS elicited no detectable activation of adult CD11c(+) dendritic cells (DC). In contrast, CTL responses were associated with a partial activation of neonatal CD11c(+) DC, reflected by the up-regulation of CD80 and CD86 expression but no concurrent changes in MHC class II or CD40 expression. However, this partial activation of neonatal DC was not sufficient to circumvent the requirement for CD4(+) T cell help. The effective induction of neonatal CD8(+) T cell responses by this minimal Ag delivery system demonstrates that neonatal CD11c(+) DC may mature sufficiently to stimulate naive CD8(+) neonatal T cells, even in the absence of strong maturation signals.

Progress on new vaccine strategies against chronic viral infections

Among the most cost-effective strategies for preventing viral infections, vaccines have proven effective primarily against viruses causing acute, self-limited infections. For these it has been sufficient for the vaccine to mimic the natural virus. However, viruses causing chronic infection do not elicit an immune response sufficient to clear the infection and, as a result, vaccines for these viruses must elicit more effective responses--quantitative and qualitative--than does the natural virus. Here we examine the immunologic and virologic basis for vaccines against three such viruses, HIV, hepatitis C virus, and human papillomavirus, and review progress in clinical trials to date. We also explore novel strategies for increasing the immunogenicity and efficacy of vaccines.

Modulation of CD8+ T cell avidity by increasing the turnover of viral antigen during infection

The increased potency of high avidity CD8+ T cells for the clearance of viral infections has been well documented. We have previously reported the novel finding that intranasal infection with the paramyxovirus SV5 induces a CD8+ T cell response to the SV5 P protein that is almost exclusively of high avidity. Based on our results that the level of peptide presentation is a critical factor in the selective expansion of high versus low avidity cells in vitro, we hypothesized that the avidity of the anti-viral response generated in vivo could be altered by increasing the turnover of the P protein during viral infection through linkage to ubiquitin (UbP). Infection with a virus expressing UbP (VV-UbP) elicited a significant increase in low avidity cells in both BALB/c and C3H mice compared to the almost exclusively high avidity response elicited by VV-P. Our results are the first demonstration of the control of avidity during the antiviral response through an engineered change to a viral antigen. The implications of our findings for vaccine development are discussed.

GAD65-specific CD4+ T-cells with high antigen avidity are prevalent in peripheral blood of patients with type 1 diabetes

Negative selection of self-reactive T-cells during thymic development, along with activation-induced cell death in peripheral lymphocytes, is designed to limit the expansion and persistence of autoreactive T-cells. Autoreactive T-cells are nevertheless present, both in patients with type 1 diabetes and in at-risk subjects. By using MHC class II tetramers to probe the T-cell receptor (TcR) specificity and avidity of GAD65 reactive T-cell clones isolated from patients with type 1 diabetes, we identified high-avidity CD4+ T-cells in peripheral blood, coexisting with low-avidity cells directed to the same GAD65 epitope specificity. A variety of cytokine patterns was observed, even among T-cells with high MHC-peptide avidity, and the clones utilize a biased set of TcR genes that favor two combinations, Valpha12-beta5.1 and Valpha17-Vbeta4. Presence of these high-avidity TcRs indicates a failure to delete autoreactive T-cells that likely arise from oligoclonal expansion in response to autoantigen exposure during the progression of type 1 diabetes.

Progress on new vaccine strategies against chronic viral infections

Among the most cost-effective strategies for preventing viral infections, vaccines have proven effective primarily against viruses causing acute, self-limited infections. For these it has been sufficient for the vaccine to mimic the natural virus. However, viruses causing chronic infection do not elicit an immune response sufficient to clear the infection and, as a result, vaccines for these viruses must elicit more effective responses--quantitative and qualitative--than does the natural virus. Here we examine the immunologic and virologic basis for vaccines against three such viruses, HIV, hepatitis C virus, and human papillomavirus, and review progress in clinical trials to date. We also explore novel strategies for increasing the immunogenicity and efficacy of vaccines.

Changes to peptide structure, not concentration, contribute to expansion of the lowest avidity cytotoxic T lymphocytes

The efficient in vitro expansion of antigen-specific CD8+ cytotoxic T lymphocytes (CTL) for use in adoptive immunotherapy represents an important clinical goal. Furthermore, the avidity of expanded CTL populations often correlates closely with clinical outcome. In our study, high-avidity CTL lines could be expanded ex vivo from an antigen-primed animal using low peptide concentration, and intermediate peptide concentrations favored the generation of lower avidity CTL. Further increases in peptide concentration during culture inhibited the expansion of all peptide-specific CD8+ cells. In contrast, a single amino acid variant peptide efficiently generated functional CTL populations at high or low peptide concentration, which responded to wild-type epitope with the lowest average avidity seen in this study. We propose that for some peptides, the efficient generation of low-avidity CTL responses will be favored by stimulation with altered peptide rather than high concentrations of wild-type epitope. In addition, some variant peptides designed to have improved binding to major histocompatibility complex class I may reduce rather than enhance the functional avidity for the wild-type peptide of ex vivo-expanded CTL. These observations are relevant to in vitro expansion of CTL for immunotherapy and strategies to elicit regulatory or therapeutic immunity to neo-self-antigen when central tolerance has eliminated high-avidity, cognate T cells.

A stochastic model of cytotoxic T cell responses

We have constructed a stochastic stage-structured model of the cytotoxic T lymphocyte (CTL) response to antigen and the maintenance of immunological memory. The model follows the dynamics of a viral infection and the stimulation, proliferation, and differentiation of naïve CD8(+) T cells into effector CTL, which can eliminate virally infected cells. The model is capable of following the dynamics of multiple T cell clones, each with a T cell receptor represented by a digit string. MHC-viral peptide complexes are also represented by strings and a string match rule is used to compute the affinity of a T cell receptor for a viral epitope. The avidities of interactions are also computed by taking into consideration the density of MHC-viral peptides on the surface of an infected cell. Lastly, the model allows the probability of T cell stimulation to depend on avidity but also incorporates the notion of an antigen-independent programmed proliferative response. We compare the model to experimental data on the cytotoxic T cell response to lymphocytic choriomeningitis virus infections.

Leishmania-induced inhibition of macrophage antigen presentation analyzed at the single-cell level

A number of studies have previously examined the capacity of intracellular Leishmania parasites to modulate the capacity of macrophages to process and present Ags to MHC class II-restricted CD4(+) T cells. However, the bulk culture approaches used for assessing T cell activation make interpretation of some of these studies difficult. To gain a more precise understanding of the interaction between Leishmania-infected macrophages and effector T cells, we have analyzed various parameters of T cell activation in individual macrophage-T cell conjugates. Leishmania-infected macrophages efficiently stimulate Ag-independent as well as Ag-dependent, TCR-mediated capping of cortical F-actin in DO.11 T cells. However, infected macrophages are less efficient at promoting the sustained TCR signaling necessary for reorientation of the T cell microtubule organizing center and for IFN-gamma production. A reduced ability to activate these T cell responses was not due to altered levels of surface-expressed MHC class II-peptide complexes. This study represents the first direct single-cell analysis of the impact of intracellular infection on the interaction of macrophages with T cells and serves to emphasize the subtle influence Leishmania has on APC function.

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

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

Modelling the impact of antigen kinetics on T‐cell activation and response

Cytotoxic T lymphocyte (CTL) responses are thought to be important for the control of many viral and other infections. Qualitative aspects of the CTL response, including the epitope specificity, affinity, and clonal composition, may affect the ability of T cells to mediate infection control. Although it is clear that the mode of introduction and the dose of antigen can affect these qualitative aspects of the response, little is understood of the mechanisms. We have developed an in silico model of the CTL response, which we use to study the impact of antigen dose, antigen kinetics and repeated antigen delivery on the response. The results suggest that recent observations on differences in response to killed antigen can be explained simply by differences in timing of T-cell activation. These findings may provide insight into how different vaccination strategies can quantitatively and qualitatively affect the outcome of the immune response.

Evolution of CD8+ T cell immunity and viral escape following acute HIV-1 infection

Induction of HIV-1-specific CD8(+) T cells during acute infection is associated with a decline in viremia. The role CD8(+) effectors play in subsequently establishing viral set point remains unclear. To address this, we focused on two acutely infected patients with the same initial Tat-specific CD8(+) response, analyzing their CD8(+) T cell responses longitudinally in conjunction with viral load and sequence evolution. In one patient initiating treatment during acute infection, the frequencies of Tat-specific CD8(+) T cells gradually diminished but persisted, and the Tat epitope sequence was unaltered. By contrast, in the second patient who declined treatment, the Tat-specific CD8(+) T cells disappeared below detection, in conjunction with Gag-specific CD4(+) T cell loss, as plasma viremia reached a set point. This coincided with the emergence of an escape variant within the Tat epitope and an additional Vpr epitope. New CD8(+) T cell responses emerged but with no further associated decline in viremia. These findings indicate that, in the absence of treatment, the initial CD8(+) T cell responses have the greatest impact on reducing viremia, and that later, continuously evolving responses are less efficient in further reducing viral load. The results also suggest that T cell help may contribute to the antiviral efficiency of the acute CD8(+) T cell response.

Immune modulation by flaviviruses

Flaviviruses cause pleomorphic disease with significant morbidity and mortality worldwide. Interestingly, in contrast to most viruses, which subvert or avoid host immune systems, members of the neurotropic Japanese encephalitis serocomplex cause functional changes associated with increased efficacy of the immune response. These viruses induce increased cell surface expression of immune recognition molecules, including class I and II major histocompatibility complex (MHC) and various adhesion molecules. Increases are functional: infected cells are significantly more susceptible to both virus- and MHC-specific cytotoxic T cell lysis. Induced changes are modulated positively or negatively by Th1 and Th2 cytokines, as well as by cell cycle position and adherence status at infection. Infection also increases costimulatory molecule expression on Langerhans cells in the skin. Local interleukin-1 beta production causes accelerated migration of phenotypically altered Langerhans cells to local draining lymph nodes, where initiation of antiviral immune responses occur. The exact mechanism(s) of upregulation is unclear, but changes are associated with NF-kappa B activation and increased MHC and ICAM-1 gene transcription, independently of interferon (IFN) or other proinflammatory cytokines. Increased MHC and adhesion molecule expression may contribute to the pathogenesis of flavivirus encephalitis. Results from a murine model of flavivirus encephalitis developed in this laboratory suggest that fatal disease is immunopathological in nature, with IFN-gamma playing a crucial role. We hypothesize that these viruses may decoy the adaptive immune system into generating low-affinity T cells, which clear virus poorly, as part of their survival strategy. This may enable viral growth and immune escape in cycling cells, which do not significantly upregulate cell surface molecules.

Enhancing DNA vaccine potency by combining a strategy to prolong dendritic cell life with intracellular targeting strategies

We have recently shown that intradermal coadministration of DNA encoding Ag with DNA encoding inhibitors of apoptosis, including Bcl-x(L), prolongs dendritic cell (DC) life and thereby enhances the potency of DNA vaccines in vivo. We have also demonstrated that DNA vaccines targeting Ag to subcellular compartments, using proteins such as Mycobacterium tuberculosis heat shock protein 70, calreticulin, or the sorting signal of the lysosome-associated membrane protein type 1 (LAMP-1), enhanced DNA vaccine potency. In this study, we reasoned that the combination of a strategy to prolong DC life with intracellular targeting strategies might produce a more effective DNA vaccine against human papillomavirus E7. We showed that coadministration of DNA encoding Bcl-x(L) with DNA encoding E7/heat shock protein 70, calreticulin/E7, or Sig/E7/LAMP-1 resulted in further enhancement of the E7-specific CD8(+) T cell response for all three constructs. Of these strategies, mice vaccinated with Sig/E7/LAMP-1 DNA mixed with Bcl-x(L) DNA showed the greatest increase in E7-specific CD8(+) T cells ( approximately 13-fold increase). This combination of strategies resulted in increased CD8(+) T cell functional avidity, an increased E7-specific CD4(+) Th1 cell response, enhanced tumor treatment ability, and stronger long-term tumor protection when compared with mice vaccinated without Bcl-x(L) DNA. Therefore, DNA vaccines that combine strategies to enhance intracellular Ag processing and prolong DC life have potential clinical implications for control of viral infection and neoplasia.

Duration of antiviral immunity after smallpox vaccination

Although naturally occurring smallpox was eliminated through the efforts of the World Health Organization Global Eradication Program, it remains possible that smallpox could be intentionally released. Here we examine the magnitude and duration of antiviral immunity induced by one or more smallpox vaccinations. We found that more than 90% of volunteers vaccinated 25-75 years ago still maintain substantial humoral or cellular immunity (or both) against vaccinia, the virus used to vaccinate against smallpox. Antiviral antibody responses remained stable between 1-75 years after vaccination, whereas antiviral T-cell responses declined slowly, with a half-life of 8-15 years. If these levels of immunity are considered to be at least partially protective, then the morbidity and mortality associated with an intentional smallpox outbreak would be substantially reduced because of pre-existing immunity in a large number of previously vaccinated individuals.

Epitope specificity is critical for high and moderate avidity cytotoxic T lymphocytes associated with control of viral load and clinical disease in horses with equine infectious anemia virus

Equine infectious anemia virus (EIAV) is a lentivirus that causes persistent infections in horses. We hypothesized that high-avidity CTL specific for nonvariable epitopes might be associated with low viral load and minimal disease in EIAV-infected horses. To test this hypothesis, memory CTL (CTLm) responses were analyzed in two infected horses with high plasma viral loads and recurrent disease (progressors), and in two infected horses with low-to-undetectable viral loads and mild disease (nonprogressors). High-avidity CTLm in one progressor recognized an envelope gp90 epitope, and the data documented for the first time in EIAV that viral variation led to CTL escape. Each of the nonprogressors had high-to-moderate avidity CTLm directed against epitopes within Rev, including the nuclear export and nuclear localization domains. These results suggested that the epitope specificity of high- and moderate-avidity CTLm was an important determinant for disease outcome in the EIAV-infected horses examined.

Presentation and binding affinity of equine infectious anemia virus CTL envelope and matrix protein epitopes by an expressed equine classical MHC class I molecule

Control of a naturally occurring lentivirus, equine infectious anemia virus (EIAV), occurs in most infected horses and involves MHC class I-restricted, virus-specific CTL. Two minimal 12-aa epitopes, Env-RW12 and Gag-GW12, were evaluated for presentation by target cells from horses with an equine lymphocyte Ag-A1 (ELA-A1) haplotype. Fifteen of 15 presented Env-RW12 to CTL, whereas 11 of 15 presented Gag-GW12. To determine whether these epitopes were presented by different molecules, MHC class I genes were identified in cDNA clones from Arabian horse A2152, which presented both epitopes. This horse was selected because it is heterozygous for the SCID trait and is used to breed heterozygous females. Offspring with SCID are used as recipients for CTL adoptive transfer, and normal offspring are used for CTL induction. Four classical and three putative nonclassical full-length MHC class I genes were found. Human 721.221 cells transduced with retroviral vectors expressing each gene had equine MHC class I on their surface. Following peptide pulsing, only cells expressing classical MHC class I molecule 7-6 presented Env-RW12 and Gag-GW12 to CTL. Unlabeled peptide inhibition of (125)I-labeled Env-RW12 binding to 7-6-transduced cells demonstrated that Env-RW12 affinity was 15-fold higher than Gag-GW12 affinity. Inhibition with truncated Env-RW12 demonstrated that amino acid positions 1 and 12 were necessary for binding, and single substitutions identified positions 2 and 3 as possible primary anchor residues. Since MHC class I 7-6 presented both epitopes, outbred horses with this allele can be immunized with these epitopes to optimize CTL responses and evaluate their effectiveness against lentiviral challenge.

CD8+ T cells, NK cells and IFN-gamma are important for control of tumor with downregulated MHC class I expression by DNA vaccination

One of the major hurdles facing cancer immunotherapy is that cancers may downregulate expression of MHC class I molecules. The development of a suitable tumor model with downregulated MHC class I expression is critical for designing vaccines and immunotherapeutic strategies to control such tumors. We developed an E7-expressing murine tumor model with downregulated MHC class I expression, TC-1 P3 (A15). Using this model, we tested DNA and vaccinia vaccines for their ability to control tumors with downregulated MHC class I expression. We found that vaccination with DNA encoding E7 linked to Mycobacterial heat shock protein 70 (HSP70) generated a significant antitumor effect against TC-1 P3 (A15), while vaccination with E7/HSP70 vaccinia did not generate an appreciable antitumor effect. Lymphocyte depletion experiments revealed that both CD8+ T cells and NK cells were essential for the antitumor effect generated by E7/HSP70 DNA against TC-1 P3 (A15). Furthermore, tumor protection experiments using IFN-gamma knockout mice revealed that IFN-gamma was essential for the antitumor effect generated by E7/HSP70 DNA against TC-1 P3 (A15). Our results demonstrate that vaccination with E7/HSP70 DNA results in a significant antitumor effect against a neoplasm with downregulated MHC class I expression and the importance of CD8+ T cells, NK cells, and IFN-gamma in generating this antitumor effect.

The race between initial T-helper expansion and virus growth upon HIV infection influences polyclonality of the response and viral set-point

Infection with HIV is characterized by very diverse disease-progression patterns across patients, associated with a wide variation in viral set-points. Progression is a multifactorial process, but an important role has been attributed to the HIV-specific T-cell response. To explore the conditions under which different set-points may be explained by differences in initial CD4 and CD8 T-cell responses and virus inoculum, we have formulated a model assuming that HIV-specific CD4 cells are both targets for infection and mediators of a monoclonal or polyclonal immune response. Clones differ in functional avidity for HIV epitopes. Importantly, in contrast to previous models, in this model we obtained coexistence of multiple clones at steady-state viral set-point, as seen in HIV infection. We found that, for certain parameter conditions, multiple steady states are possible: with few initial CD4 helper cells and high virus inoculum, no immune response is established and target-cell-limited infection follows, with associated high viral load; when CD4 clones are initially large and virus inoculum is low, infection can be controlled by several clones. The conditions for the dependence of viral set-point on initial inoculum and CD4 T-helper clone availability are investigated in terms of the effector mechanism of the clones involved.

Interaction of flaviviruses with cells of the vertebrate host and decoy of the immune response

Flaviviruses cause endemic and epidemic disease with significant morbidity and mortality throughout the world. In contrast to viruses that avoid the host immune response by down-regulating cell surface major histocompatibility complex expression, infection by members of the neurotropic Japanese encephalitis serogroup induce virus-directed functional increases in expression of class I and II major histocompatibility complex and various adhesion molecules, resulting in increased susceptibility to both virus- and major histocompatibility complex-specific cytotoxic T lymphocyte lysis. These changes are comodulated by T1 and T2 cytokines, as well as by cell cycle position and adherence status at infection. Infected skin dendritic (Langerhans) cells also show increased costimulatory molecule expression and local interleukin-1beta production causes accelerated migration of Langerhans cells to local draining lymph nodes, where initiation of antiviral immune responses occur. The exact mechanism(s) of up-regulation is unclear, but changes are associated with NF-kappaB activation and increased MHC and ICAM-1 gene transcription, independently of interferon or other pro-inflammatory cytokines. We hypothesize that these viruses may decoy the adaptive immune system into generating low-affinity, self-reactive T cells which clear virus poorly, as part of their survival strategy. This may enable viral growth and immune escape in cycling cells, which do not significantly up-regulate cell surface molecules. A possible side-effect of this might be immunopathology, caused by 'autoimmune' cross-reactive damage of uninfected high major histocompatibility complex and adhesion molecule-expressing cells, with consequent exacerbation of encephalitic disease. Results from a murine model of flavivirus encephalitis developed in this laboratory further suggest that interferon-gamma plays a crucial role in fatal immunopathology.

Enhanced induction of hepatitis C virus-specific cytotoxic T lymphocytes and protective efficacy in mice by DNA vaccination followed by adenovirus boosting in combination with the interleukin-12 expression plasmid

We evaluated the prime-boost immunization consisting of hepatitis C virus (HCV)-core expression plasmid (pCEP4-core) and replication-defective adenovirus expressing HCV-core (Adex1SR3ST) for core-specific CTL induction in mice. Compared to a single booster, double boosters after priming enhance CTL induction. The prime-double boosts immunization involving pCEP4-core priming followed by pCEP4-core and Adex1SR3ST boostings (pC/pC/aC) can induce core-specific CTLs as well as other combinations: pC/aC/aC; aC/pC/pC; aC/aC/aC, whereas pC/pC/pC does not induce CTLs. Furthermore, co-administration of interleukin-12 (IL-12) expression plasmid leads to the highly efficient CTL induction and clearance of HCV-core expressing vaccinia virus challenged. Thus, the prime-double boosts immunization together with IL-12 may be promising for HCV vaccine.

Selective induction of high avidity CTL by altering the balance of signals from APC

High avidity CTL are most effective at clearing viruses and cancer cells. Therefore, understanding the mechanisms involved in induction of high avidity CTL is critical for effective vaccines. However, no vaccine approach to selectively induce high avidity CTL in vivo has been discovered. In a new approach, signals from MHC class I (signal 1) and costimulatory molecules (signal 2) were adjusted by varying Ag dose and by use of recombinant poxvirus expressing a triad of costimulatory molecules (B7-1, ICAM-1, and LFA-3), respectively. Independent of CTL avidity, a strong signal 1 resulted in an increased frequency of CD8(+) CTL. However, a strong signal 2 was necessary for the induction of high avidity CD8(+) CTL that killed target cells more efficiently, and signal 2 played a more crucial role in the absence of a strong signal 1. Only CTL induced with strong signal 2 killed tumor cells endogenously expressing low levels of Ag. Signal 2 contributed to the induction of high avidity CD8(+) CTL in both primary and secondary responses. Thus, although signal 2 has been known to increase the quantity of CTL response, in this study we show that it also improves the quality of CTL response. Our data also suggested that dendritic cells play an important role in induction of high avidity CD8(+) CTL in vivo. This strategy to selectively induce higher avidity CTL may lead to more effective vaccines for viruses and cancer.

CD4-independent protective cytotoxic T cells induced in early life by a non-replicative delivery system based on virus-like particles

The relative immaturity of the neonatal immune system limits CD4(+) Th1 and cytotoxic T lymphocyte (CTL) responses, and represents a significant challenge for the development of vaccines against intracellular pathogens. In this report, we demonstrate the ability of a non-replicative delivery system based on parvovirus-like particles (VLP) to induce CTL responses in the neonatal period. A single immunization of 1-week-old BALB/c mice with recombinant VLP carrying a CD8(+) T cell determinant from lymphocytic choriomeningitis virus (VLP-LCMV) induced antigen-specific CD8(+) cytotoxic T cells that were similar to those elicited by adult immunization, as assessed by cytotoxic activity, interferon (IFN)-gamma secretion, cytotoxic precursor cell frequencies, in vitro avidity for antigen and protective activity against viral challenge. These CTL responses are elicited within 2 weeks of a single immunization, in the absence of adjuvant and independently of the presence and help of CD4(+) T cells, highlighting the potential of VLP as candidate vaccine vectors in early life.

High avidity CD8+ T cells are the initial population elicited following viral infection of the respiratory tract

Following intranasal administration, the model paramyxovirus simian virus 5 (SV5) establishes an infection in the respiratory tract of mice, which is subsequently cleared by CD8+ T cells. In this study, we sought to understand the maturation of the antiviral immune response over time by assessing the functional avidity of the responding T cells and the expansion of immunodominant populations. Surprisingly, we determined that the initial response to Ag at day 3 (d3) in the mediastinal lymph node was exclusively high avidity. However, by d5 postinfection, low avidity cells were approximately 50% of the responding T cell population. Following secondary exposure to SV5, high avidity CD8+ T cells again are the exclusive cell type present at early times postinfection (d2). Similarly, high avidity cells were preferentially elicited at d3 following infection with the unrelated vaccinia virus. We also made the observation that the immunodominance profile has not been established at d3 postinfection with SV5. However, by d5 a clear immunodominance pattern arises and is permanently maintained. These data indicate that high avidity cells are the predominant population responding at early times postinfection following respiratory infection with SV5 or vaccinia virus. However, as the response progresses, low avidity cells are activated/expanded to a greater extent compared with high avidity cells.

Prospects for immunotherapy of malignant disease

The majority of T cell-recognized tumour antigens in humans are encoded by genes that are also present in normal tissues. Low levels of gene expression in normal cells can lead to the inactivation of high-avidity T cells by immunological tolerance mechanisms. As a consequence, low-avidity T cell responses in patients are often inadequate in providing tumour protection. Recently, several technologies have been developed to overcome tolerance, allowing the isolation of high-affinity, HLA-restricted receptors specific for tumour-associated peptide epitopes. Furthermore, transfer of HLA-restricted antigen receptors provides an opportunity to empower patient T cells with new tumour-reactive specificities that cannot be retrieved from the autologous T cell repertoire.