Immunodominance of cytotoxic T lymphocyte epitopes co-injected in vivo and modulation by interleukin-12

Vaccination with Gag, Vif, and Nef gene fragments affords partial control of viral replication after mucosal challenge with SIVmac239

Broadly targeted cellular immune responses are thought to be important for controlling replication of human and simian immunodeficiency viruses (HIV and SIV). However, eliciting such responses by vaccination is complicated by immunodominance, the preferential targeting of only a few of the many possible epitopes of a given antigen. This phenomenon may be due to the coexpression of dominant and subdominant epitopes by the same antigen-presenting cell and may be overcome by distributing these sequences among several different vaccine constructs. Accordingly, we tested whether vaccinating rhesus macaques with "minigenes" encoding fragments of Gag, Vif, and Nef resulted in broadened cellular responses capable of controlling SIV replication. We delivered these minigenes through combinations of recombinant Mycobacterium bovis BCG (rBCG), electroporated recombinant DNA (rDNA) along with an interleukin-12 (IL-12)-expressing plasmid (EP rDNA plus pIL-12), yellow fever vaccine virus 17D (rYF17D), and recombinant adenovirus serotype 5 (rAd5). Although priming with EP rDNA plus pIL-12 increased the breadth of vaccine-induced T-cell responses, this effect was likely due to the improved antigen delivery afforded by electroporation rather than modulation of immunodominance. Indeed, Mamu-A*01(+) vaccinees mounted CD8(+) T cells directed against only one subdominant epitope, regardless of the vaccination regimen. After challenge with SIVmac239, vaccine efficacy was limited to a modest reduction in set point in some of the groups and did not correlate with standard T-cell measurements. These findings suggest that broad T-cell responses elicited by conventional vectors may not be sufficient to substantially contain AIDS virus replication.

IMPORTANCE

Immunodominance poses a major obstacle to the generation of broadly targeted, HIV-specific cellular responses by vaccination. Here we attempted to circumvent this phenomenon and thereby broaden the repertoire of SIV-specific cellular responses by vaccinating rhesus macaques with minigenes encoding fragments of Gag, Vif, and Nef. In contrast to previous mouse studies, this strategy appeared to minimally affect monkey CD8(+) T-cell immundominance hierarchies, as seen by the detection of only one subdominant epitope in Mamu-A*01(+) vaccinees. This finding underscores the difficulty of inducing subdominant CD8(+) T cells by vaccination and demonstrates that strategies other than gene fragmentation may be required to significantly alter immunodominance in primates. Although some of the regimens tested here were extremely immunogenic, vaccine efficacy was limited to a modest reduction in set point viremia after challenge with SIVmac239. No correlates of protection were identified. These results reinforce the notion that vaccine immunogenicity does not predict control of AIDS virus replication.

CD8 T cell epitope distribution in viruses reveals patterns of protein biosynthesis

Distinguishing T cell epitope distribution patterns is relevant for epitope-vaccine design. To that end, we invest0069gated the distribution of known CD8 T cell epitopes from Hepatitis C Virus, Human Immunodeficiency Virus-1 and Influenza A Virus using χ² statistics. We found that epitopes are not distributed in the viral proteomes proportionally to the size of the source proteins. Specifically, capsid and matrix proteins pack significantly more epitopes than those expected by their size. Such non-homogeneous distribution cannot be accounted by underlying MHC I-peptide binding preferences nor it is related to sequence variability. Instead, we propose that it might be related to preferential protein translation/biosynthesis. Overall, these results support the prioritization of structural antigens for epitope identification and vaccine design.

CD8+ T cell immunodominance in lymphocytic choriomeningitis virus infection is modified in the presence of toll-like receptor agonists

Currently, we have limited understanding of how Toll-like receptor (TLR) engagement by microbial products influences the immune response during a concurrent virus infection. In this study, we established that dual TLR2 plus TLR3 (designated TLR2+3) stimulation alters the immunodominance hierarchies of lymphocytic choriomeningitis virus (LCMV) epitopes by reducing NP396-specific CD8+ T cell responses and shifting it to a subdominant position. The shift in immunodominance occurred due to a reduction in antigen uptake and the reduced cross-presentation of NP396, a major LCMV immunodominant epitope that is efficiently cross-presented. Moreover, the altered immunodominance was dependent on TLR stimulation occurring at the site of infection. Finally, as lipopolysaccharide failed to induce the same phenomenon, the data suggest that these findings are dependent not only on the dual engagement of the TRIF/MyD88 pathways but also on how TLR agonists activate antigen-presenting cells. Taken together, our data demonstrate a novel role for TLR ligands in regulating antiviral CD8+ T cell responses due to the regulation of the cross-presentation of cell-associated antigens.

Immunotherapy of malignant brain tumors

Despite aggressive multi-modality therapy including surgery, radiation, and chemotherapy, the prognosis for patients with malignant primary brain tumors remains very poor. Moreover, the non-specific nature of conventional therapy for brain tumors often results in incapacitating damage to surrounding normal brain and systemic tissues. Thus, there is an urgent need for the development of therapeutic strategies that precisely target tumor cells while minimizing collateral damage to neighboring eloquent cerebral cortex. The rationale for using the immune system to target brain tumors is based on the premise that the inherent specificity of immunologic reactivity could meet the clear need for more specific and precise therapy. The success of this modality is dependent on our ability to understand the mechanisms of immune regulation within the central nervous system (CNS), as well as counter the broad defects in host cell-mediated immunity that malignant gliomas are known to elicit. Recent advances in our understanding of tumor-induced and host-mediated immunosuppressive mechanisms, the development of effective strategies to combat these suppressive effects, and a better understanding of how to deliver immunologic effector molecules more efficiently to CNS tumors have all facilitated significant progress toward the realization of true clinical benefit from immunotherapeutic treatment of malignant gliomas.

Epitope mapping of full-length glycoprotein D from HSV-2 reveals a novel CD4+ CTL epitope located at the transmembrane-cytoplasmic junction

The glycoprotein D of HSV-2 (gD2) is currently a leading candidate vaccine target for genital herpes vaccines as both cellular and humoral responses can be generated against it. However, little is known about how vaccine composition will affect T cell epitope selection. A panel of 15-mer peptides (with 11 amino acid overlap) spanning full-length gD2 was used to investigate the fine specificity of T cell responses to gD2 as well as the role of vaccine composition on epitope selection. Spleen cells from BALB/c mice (H-2(d)) immunized with gD2, formulated with or without AlPO(4) and/or IL-12, were stimulated in vitro with overlapping gD2 peptides. Cellular responses (lymphoproliferation and IFN-gamma expression) were mapped to four epitopes within the gD2 molecule: gD2(49-63), gD2(105-119), gD2(245-259), and gD2(333-347). CTL analysis of these four epitopes indicated that not all of them could serve as a CTL epitope. Mice immunized with gD2 expressed from a viral vector mounted CTL responses primarily to one epitope located in the extracellular domain of gD2 (gD2(245-259)). More importantly, mice immunized with gD2 co-administered with IL-12 mounted CTL responses to an additional epitope located at the transmembrane-cytoplasmic junction of gD2 (gD2(333-347)). The location of this novel epitope emphasizes the benefit of using full-length versions of glycoproteins when designing vaccine components.

Immunodomination in the evolution of dominant epitope-specific CD8+ T lymphocyte responses in simian immunodeficiency virus-infected rhesus monkeys

Because the control of HIV-1 replication is largely dependent on CD8+ T lymphocyte responses specific for immunodominant viral epitopes, vaccine strategies that increase the breadth of dominant epitope-specific responses should contribute to containing HIV-1 spread. Developing strategies to elicit such broad immune responses will require an understanding of the mechanisms responsible for focusing CD8+ T lymphocyte recognition on a limited number of epitopes. To explore this biology, we identified cohorts of rhesus monkeys that expressed the MHC class I molecules Mamu-A*01, Mamu-A*02, or both, and assessed the evolution of their dominant epitope-specific CD8+ T lymphocyte responses (Gag p11C- and Tat TL8-specific in the Mamu-A*01+ and Nef p199RY-specific in the Mamu-A*02+ monkeys) following acute SIV infection. The Mamu-A*02+ monkeys that also expressed Mamu-A*01 exhibited a significant delay in the evolution of the CD8+ T lymphocyte responses specific for the dominant Mamu-A*02-restricted SIV epitope, Nef p199RY. This delay in kinetics was not due to differences in viral load kinetics or magnitude or in viral escape mutations, but was associated with the evolution of the Mamu-A*01-restricted CD8+ T lymphocyte responses to the highly dominant SIV epitopes Gag p11C and Tat TL8. Thus, the evolution of dominant epitope-specific CD8+ T lymphocyte responses can be suppressed by other dominant epitope-specific responses, and this immunodomination is important in determining the kinetics of dominant epitope-specific responses.

A dose sparing effect by plasmid encoded IL-12 adjuvant on a SIVgag-plasmid DNA vaccine in rhesus macaques

An experimental pDNA vaccine adjuvant expressing IL-12 was evaluated for its ability to augment the humoral and cellular immune responses elicited by a SIVmac239 gag p39 expressing pDNA vaccine. To determine the effect of vaccine dose on the immune response, rhesus macaques were immunized with 1.5 mg or 5.0 mg of SIVmac239 gag pDNA, with or without co-immunization of IL-12 pDNA at 1.5 mg and 5.0 mg, respectively. Serum antibody responses to simian immunodeficiency virus (SIV) gag were increased 10-fold (p=0.044, 0.002) in macaques receiving IL-12 pDNA. Cellular immune responses, monitored by SIV gag-specific IFN-gamma ELISpot assay, were also significantly higher (p=0.007, 0.019) when the pDNA vaccine was co-immunized with IL-12 pDNA at high and low doses. There was no statistical difference between the immune responses elicited by the high and low dose of IL-12 pDNA (p=0.221, 0.917), a finding which could allow a dose reduction of vaccine without the concomitant loss of imunogenicity. Furthermore, analysis of the breadth of the T-cell response during the vaccination schedule, using overlapping peptides to SIV gag, demonstrated a significant correlation (p=0.0002) between the magnitude and breadth of the immune responses in the vaccines. These results have important implications for the continuing development of an effective, safe low dose pDNA vaccine adjuvant suitable for human use.

Relative dominance of HLA-B*07 restricted CD8+ T-lymphocyte immune responses to human cytomegalovirus pp65 in persons sharing HLA-A*02 and HLA-B*07 alleles

CD8(+) T-cell responses to three human cytomegalovirus (CMV) pp65 epitopes were studied in panels of healthy seropositive HLA-A*02/HLA-B*07 individuals, and HLA-A*02 donors mismatched for HLA-B*07. The majority of the latter had significant responses to a HLA-A*02-restricted epitope within the CMV pp65 antigen. By contrast, the strongest responses to CMV in the first group were to HLA-B*07-restricted epitopes. Similar immunodominance of HLA-B*07 over HLA-A*02 was found in two immunocompromised HIV-infected HLA-A*02/HLA-B*07 patients, and in the reconstituting immune system of three stem cell transplant recipients. In vitro stimulation of peripheral blood mononuclear cells (PBMC) from two immunocompetent HLA-A*02/HLA-B*07 individuals indicated that cytotoxic T lymphocyte (CTL) precursors specific for both HLA-A*02 and HLA-B*07 restricted epitopes were present and could be expanded by stimulation with the cognate peptides. However, if stimulation was performed by antigen presenting cells infected with recombinant vaccinia expressing full-length native pp65, only HLA-B*07 epitope-specific cells were seen. In one patient the HLA-B*07 dominance was partially broken by using recombinant vaccinia expressing ubiquitinated pp65, suggesting that enhanced protein processing can reveal weaker immune responses. Our results indicate that CMV-specific cellular immune responses restricted by HLA-B*07 dominate those restricted by HLA-A*02 in both immunocompetent and immunocompromised individuals. This may have significant consequences for the design of epitope-specific vaccines.

Effects of granulocyte-macrophage colony-stimulating factor and foreign helper protein as immunologic adjuvants on the T-cell response to vaccination with tyrosinase peptides

Immunologic adjuvants are used to augment the immunogenicity of MHC class I-restricted peptide vaccines, but this effect has rarely been systematically evaluated in a clinical trial. We have investigated, in a phase I study, whether addition of the 2 adjuvants GM-CSF and KLH can enhance the T-cell response to MHC class I peptide vaccines. Forty-three high-risk melanoma patients who were clinically free of disease received 6 vaccinations with MHC class I-restricted tyrosinase peptides alone, with either GM-CSF or KLH or with a combination of both adjuvants. The primary end point was induction of tyrosinase-specific T cells, and serial T-cell monitoring was performed in unstimulated peripheral blood samples before and after the second, fourth and sixth vaccinations by ELISPOT assay. Tyrosinase-specific IFN-gamma-producing T cells were detected as early as 2 weeks after the second vaccination in 5 of 9 patients vaccinated with tyrosinase peptides in combination with GM-CSF and KLH but not in any patient vaccinated with tyrosinase peptides without adjuvants or in combination with either adjuvant alone. After 6 vaccinations, tyrosinase-specific T cells were found in patients immunized with peptides either without adjuvants (3 of 9 patients) or in combination with the single adjuvant GM-CSF (4 of 9 patients) but not with KLH (0 of 10 patients). Our results suggest that addition of either GM-CSF or KLH as a single adjuvant has little impact on the immunogenicity of tyrosinase peptides. The combined application of GM-CSF and KLH was associated with early induction of T-cell responses.

IFN-gamma induction by SCG, 1,3-beta-D-glucan from Sparassis crispa, in DBA/2 mice in vitro

Sparassis crispa Fr. in an edible mushroom recently cultivable in Japan. A branched beta-glucan from S. crispa (SCG) is a major 6-branched 1,3-beta-D-glucan showing antitumor activity. In this study, we examined interferon-gamma (IFN-gamma) induction by SCG from splenocytes in DBA/2 mice in vitro. In the splenocytes derived from almost all inbred strains of mice except for DBA/1 and DBA/2 mice, IFN-gamma production was not induced by SCG. The breeder and genders of DBA/2 mice showed no influence on IFN-gamma induction by SCG. On the other hand, the magnitude of IFN-gamma induction was lower in young mice than in their older counterparts. IFN-gamma was induced by SCG in adherent splenocytes, but IFN-gamma production was most significantly increased by SCG in instances involving coexistence of adherent and nonadherent splenocytes. In fact, inhibition of cell-cell contact reduced IFN-gamma induction by SCG. In addition, interleukin-12 p70 (IL-12p70) was induced by SCG in DBA/2 mice. It was suggested that soluble factors and cell-cell contact mediate synergistic effects on SCG-induced IFN-gamma production.

IL-12 administration leads to a transient depletion of T cells, B cells, and APCs and concomitant abrogation of the HLA-A2.1-restricted CTL response in transgenic mice

The injection of a mixture of bona fide T cell epitopes can lead to the occurrence of immunodominance, meaning that the immune response is focused on the recognition of a single epitope or a small portion of the epitopes injected. We have previously demonstrated that the administration of rIL-12 can counteract immunodominance in BALB/c mice. In this study, we show that the administration of rIL-12 to HLA-A2.1 transgenic mice (A2k(b) mice) abrogates specifically the immune response against HLA-A2.1-restricted HIV epitopes in the spleen. This lack of immune response is most probably due to a transient depletion of B cells, T cells, macrophages, and dendritic cells in this organ. Therefore, our study explains the mechanism of immunosuppression by rIL-12 in vivo.

Stimulation of both humoral and cellular immune responses to HIV-1 gp120 by interleukin-12 in Rhesus macaques

The adjuvant effect of recombinant Rhesus macaque interleukin-12 (RhIL-12) on the induction of cellular and humoral immune responses elicited by the HIV-1 subunit vaccine protein gp120 in Rhesus macaques was examined. RhIL-12 in conjunction with gp120 was given at day 0, 28 and 84 intramuscularly. Coadministration resulted in an approximate 10-fold increase in plasma anti-gp120 antibody levels as compared to levels generated in control monkeys receiving gp120 alone. Potentiation of the humoral arm of the immune response was evident by both ELISA and an antiviral bioassay. In addition, RhIL-12 was found to produce a significant increase in gp120-specific proliferative responses and in the frequency of antigen-specific IFN-gamma and IL-2 producing T cells after restimulation of PBMC with gp120 in vitro indicating that RhIL-12 potentiates cell-mediated immune responses as well. A critical finding was that during the course of the study, RhIL-12 did not induce a neutralizing antibody response to the administered cytokine. The doses of RhIL-12 were well tolerated and no detectable adverse side-effects on hematopoietic and hepatic parameters were noted. The data revealed that IL-12, when coadministered intramuscularly, acts as a potent adjuvant which is able to enhance not only cellular but also humoral immune responses to gp120 in non-human primates and may have to be considered in future HIV vaccine strategies.

Contribution of HLA class I allele expression to CD8+ T-cell responses against Epstein-Barr virus

Most immune responses to viral infections involve CD8+ T cells recognizing viral peptides of typically 9-10 amino acids in the groove of major histocompatibility complex (MHC) class I. Importantly, CD8+ T-cell responses appear to focus on few viral epitopes, a phenomenon termed immunodominance. While the understanding of this phenomenon has been based largely on experimental mice models, it is imperative to evaluate its contribution in humans, as the design of peptide-based vaccines may be influenced by immunodomination. Here, we present evidence that immunodominance can be detected among Epstein-Barr virus (EBV) epitopes associated with two of the most frequent class I alleles in Western Europe, human leucocyte antigen (HLA)-A2 and HLA-B7. CD8+ T-cell responses to HLA-A2-associated EBV epitopes were significantly reduced in individuals coexpressing HLA-B7. The impairment of HLA-A2-associated responses correlated with a dominant response to an HLA-B7 epitope. The data demonstrate a hierarchy in the human cellular immune response to immunodominant EBV epitopes presented by separate HLA class I alleles. This may have implications for EBV vaccine development as well as for the interpretation of isolated analysis of immunodominant responses to EBV.

Induction of a cytotoxic T-cell response to HIV-1 proteins with short synthetic peptides and human compatible adjuvants

The goal of this study was the induction of a strong CTL response against multiple CTL epitopes present in HIV proteins using short synthetic peptides. Four HLA-A2.1 restricted peptides (RT 476-484, p17 77-85, gp41 814-823, RT 956-964) that showed stable binding to the HLA-A2.1 molecule in an in vitro binding assay were able to elicit a strong specific immune response in HLA-A2.1 transgenic mice when injected with IFA or Montanide. The use of biodegradable microspheres (MS) as adjuvant was also successfully tested for all peptides. When the peptides were injected as a mixture the response was weaker as compared to individual injections of the peptides indicating the occurrence of immunodominance (ID). We are currently investigating whether ID can be overcome by a combined injection of peptide loaded MS with different release patterns. Taken together, it seems feasible to induce a specific CTL response in HLA-A2.1 transgenic mice against several HIV proteins using short synthetic peptides and human compatible adjuvants.

IL-12/IL-18-dependent IFN-gamma release by murine dendritic cells

Dendritic cells (DC) develop in GM-CSF-stimulated cultures from murine bone marrow progenitors in serum-free (or low serum) medium. CD11c(+) myeloid DC from 7-day cultures stimulated with TNF-alpha, IFN-alpha, IFN-gamma, or LPS up-regulated surface expression of CD40 and CD86 costimulator and MHC class II molecules, did not up-regulate the low "spontaneous" release of IL-18, and did not release IFN-gamma. Stimulation of in vitro-generated DC with exogenous IL-12 and IL-18 (but not with IL-4 or LPS plus IL-18) induced IFN-gamma expression and release in 15-20% of the DC (detectable by FACS analyses or ELISA). Endogenous IL-12 p70 produced by DC in response to ligation of CD40 stimulated IFN-gamma release when exogenous IL-18 was supplied. In vivo-generated, splenic CD8alpha(+) and CD8alpha(-) DC (from immunocompetent and immunodeficient H-2(d) and H-2(b) mice) cultured with IL-12 and IL-18 released IFN-gamma. The presence of LPS during the stimulation of DC with IL-18 plus endogenous (CD40 ligation) or exogenous IL-12 did not affect their IFN-gamma release. In contrast, splenic DC pretreated in vitro or in vivo by LPS strikingly down-regulated IFN-gamma release in response to stimulation by IL-18 and (endogenous or exogenous) IL-12. Hence, DC are a source of early IFN-gamma generated in response to a cascade of cytokine- and/or cell-derived signals that can be positively and negatively regulated.

A subdominant CD8(+) cytotoxic T lymphocyte (CTL) epitope from the Plasmodium yoelii circumsporozoite protein induces CTLs that eliminate infected hepatocytes from culture

Previous studies indicated that the Plasmodium yoelii circumsporozoite protein (PyCSP) 57-70 region elicits T cells capable of eliminating infected hepatocytes in vitro. Herein, we report that the PyCSP58-67 sequence contains an H-2(d) binding motif, which binds purified K(d) molecules in vitro with low affinity (3, 267 nM) and encodes an H-2(d)-restricted cytotoxic T lymphocyte (CTL) epitope. Immunization of BALB/c mice with three doses of a multiple antigen peptide (MAP) construct containing four branches of amino acids 57 to 70 linked to a lysine-glycine core [MAP4(PyCSP57-70)] and Lipofectin as the adjuvant induced both T-cell proliferation and a peptide-specific CTL response that was PyCSP59-67 specific, H-2(d) restricted, and CD8(+) T cell dependent. Immunization with either DNA encoding the PyCSP or irradiated sporozoites demonstrated that this CTL epitope is subdominant since it is not recognized in the context of whole CSP immunization. The biological relevance of this CTL response was underlined by the demonstration that it could mediate genetically restricted, CD8(+)- and nitric-oxide-dependent elimination of infected hepatocytes in vitro, as well as partial protection of BALB/c mice against sporozoite challenge. These findings indicate that subdominant epitopes with low major histocompatibility complex affinity can be used to engineer epitope-based vaccines and have implications for the selection of epitopes for subunit-based vaccines.

IL-12-Dependent enhancement of CTL response to weak class I-restricted peptide immunogens requires coimmunization with T helper cell immunogens

The effect of in vivo administration of rmIL-12 on the CTL response to immunization with a weakly immunogenic class I-restricted peptide emulsified in incomplete Freund's adjuvant was investigated. In the absence of IL-12, peptide-specific CTL responses were significantly greater following coimmunization with class I-restricted peptide and T helper cell antigens than following immunization with the class I-restricted peptide alone. IL-12-dependent enhancement of the CTL response to peptide immunization was demonstrated in the presence of, but not in the absence of, coimmunization with T helper cell antigen. These findings indicate that IL-12 enhancement of the CTL response to weak class I-restricted immunogens is T helper cell dependent. Treatment with rmIL-12 also enhanced the CTL response to immunization with cDNA encoding both CTL and T helper cell epitopes. These findings are relevant to the design of vaccines containing tumor-associated class I-restricted peptides currently being tested as an immunotherapy for cancer patients.

Interleukin-12 as an adjuvant for cancer immunotherapy

Interleukin-12 (IL-12) is a cytokine whose main effect is to drive Th-cell differentiation throughout a T helper type 1 cell type of response, thus inducing interferon gamma (IFNgamma) and favoring a switch from Ig to IgG2a. These properties make IL-12 a candidate adjuvant for vaccination against cancer and infection disease. Enthusiasm was generated in many animal studies where IL-12 was given either systemically or locally. The experience of some toxicity in humans has hampered its further development into clinical applications, which, however, are still possible if restricted to local administration. Gene transfer seems to be the preferred approach to obtain this local release of cytokine. Here we review the applications of IL-12 as adjuvant.

T cell competition for the antigen-presenting cell as a model for immunodominance in the cytotoxic T lymphocyte response against minor histocompatibility antigens

We recently demonstrated that spleen cells primed against dominant BALB.B antigens can inhibit the cytotoxic T lymphocyte (CTL) response against subdominant antigens in vitro. In this study, we show that this interference is dependent on CD8+, but not CD4+, T cells directed against dominant antigens. Similar to immunodominance in vivo, T cell interference in vitro required presentation of dominant and subdominant antigens by the same antigen-presenting cell. In vivo priming with cells expressing dominant and subdominant antigens did not induce long-lasting unresponsiveness against the latter. These results support a model in which immunodominance is mediated by T cell competition. In line with this, we found that the immunodominance effects in the CTL response against these minor histocompatibility antigens could be broken by immunization with live bone marrow-derived dendritic cells.

Immunodominance in major histocompatibility complex class I-restricted T lymphocyte responses

Of the many thousands of peptides encoded by a complex foreign antigen that can potentially be presented to CD8+ T cells (TCD8+), only a small fraction induce measurable responses in association with any given major histocompatibility complex class I allele. To design vaccines that elicit optimal TCD8+ responses, a thorough understanding of this phenomenon, known as immunodominance, is imperative. Here we review recent progress in unraveling the molecular and cellular basis for immunodominance. Of foremost importance is peptide binding to class I molecules; only approximately 1/200 of potential determinants bind at greater than the threshold affinity (Kd > 500 nM) associated with immunogenicity. Limitations in the TCD8+ repertoire render approximately half of these peptides nonimmunogenic, and inefficient antigen processing further thins the ranks by approximately four fifths. As a result, only approximately 1/2000 of the peptides in a foreign antigen expressed by an appropriate antigen presenting cell achieve immunodominant status with a given class I allele. A roughly equal fraction of peptides have subdominant status, i.e. they induce weak-to-nondetectable primary TCD8+ responses in the context of their natural antigen. Subdominant determinants may be expressed at or above levels of immunodominant determinants, at least on antigen presenting cells in vitro. The immunogenicity of subdominant determinants is often limited by immunodomination: suppression mediated by TCD8+ specific for immunodominant determinants. Immunodomination is a central feature of TCD8+ responses, as it even occurs among clones responding to the same immunodominant determinant. Little is known about how immunodominant and subdominant determinants are distinguished by the TCD8+ repertoire, or how (and why) immunodomination occurs, but new tools are available to address these questions.

Cytotoxic T lymphocyte response against non-immunoselected tumor antigens predicts the outcome of gene therapy with IL-12-transduced tumor cell vaccine

The colon adenocarcinoma C26, carrying two endogenous tumor-associated antigens (TAA) recognized by CTL, has been transduced with the gene coding for the human folate receptor alpha (FR alpha) as an additional antigen in order to study the efficacy of vaccination against a tumor expressing multiple antigens. A dicistronic vector was used to transduce the IL-12 genes to create C26/IL-12/FR alpha that has been used as a cellular vaccine to treat mice bearing lung metastases of C26/FR alpha. After vaccination mice were partially splenectomized and splenic lymphocytes frozen and used retrospectively to study in vitro CD8 T cell response related to the treatment outcome. Vaccination cured 50% of mice and the effect was CD8 T cell dependent. Mice either cured (responders) or not cured (nonresponders) by vaccination developed tumor-specific CTL. However, analysis of CTL specificity and pCTL frequencies revealed that responders had a predominant CTL activity against endogenous C26-related tumor antigens, whereas nonresponders had CTL that recognized preferentially the FR alpha antigen. CD8 from responder mice were characterized to release high levels of granulocyte-macrophage (GM)-CSF upon antigen stimulation. Tumors obtained from mice that died despite vaccination lost expression of the FR alpha transgene but maintained expression of endogenous C26 antigens. Immunoselection against FR alpha antigen was not observed in tumors from non-vaccinated controls and from CD8-depleted vaccinated mice. Down-regulation of FR alpha antigen expression was due, at least in part, to methylation of retroviral vector long terminal repeat promoter since FR alpha expression was partially restored, ex vivo, by treatment with 5-aza-2'-deoxy-cytidine (aza). These results indicate that CD8 T cell-mediated immunoselection and production of GM-CSF are determining factors for the efficacy of tumor vaccines.

Immunodominance in the CTL Response Against Minor Histocompatibility Antigens: Interference Between Responding T Cells, Rather than with Presentation of Epitopes

We have investigated mechanisms involved in immunodominance of the CTL response of C57BL/6 (B6) mice against cells of BALB.B origin. This transplantation barrier consists of at least 40 minor histocompatibility (H) Ags. Insufficient presentation of nondominant epitopes in the presence of dominant epitopes was investigated as a possible mechanism for immunodominance. Ag presentation was assessed by recognition of dendritic cells of BALB.B origin, MLC restimulatory capacity, and quantification of cell surface presentation by peptide elution from intact cells. Cells from BALB.B mice, which fail to elicit CTL against nondominant epitopes, presented nondominant epitopes to a similar extent as cells from minor H congenic mice; the latter do elicit CTL against nondominant minor H Ags. Nevertheless, presentation of nondominant and dominant epitopes by the same APC appeared to be an important factor for immunodominance to occur, since simultaneous immunization with the epitopes on separate cells elicited CTL against both types of epitopes. This suggested that immunodominance is determined in the interaction between different responding T cells and the APC. Support for this was obtained in an in vitro model in which the CTL response against a nondominant epitope was inhibited by the concomitant response against a dominant epitope. This study suggests that immunodominance in the CTL response against certain minor H Ags results from interference between T cell responses and not from insufficient presentation of peptide epitopes. The study also provides an in vitro model for further investigations of the immunodominance phenomenon.

The effect of human beta2-microglobulin on major histocompatibility complex I peptide loading and the engineering of a high affinity variant. Implications for peptide-based vaccines

The ability to directly load cell surface major histocompatibility complex (MHC) class I molecules with peptides provides a potentially powerful approach toward the development of vaccines to generate cell-mediated immunity. We demonstrate that exogenous beta2-microglobulin (beta2m) stabilizes human cell surface MHC I molecules and facilitates their loading with exogenous peptides. Additionally, using three-dimensional crystal structures and known interaction sites between MHC I heavy chains and beta2m, we engineered variants of human beta2m (hbeta2m) with a single serine substitution at residue 55. This alteration was predicted to promote hydrophobic interactions at the MHC I heavy chain/beta2m interface and displace an ordered water molecule. Compared with hbeta2m, the serine to valine substitution at residue 55 had improved ability to bind to cell surface HLA-A1, HLA-A2, and HLA-A3 molecules, facilitate exogenous peptide loading, and promote recognition by peptide-specific T cells. The inclusion of hbeta2m or higher affinity variants when pulsing cells with MHC-restricted peptides increases the efficiency of peptide loading 50-80-fold. Therefore, the inclusion of hbeta2m in peptide-based vaccines may increase cell surface antigen densities above thresholds that allow recognition of peptide antigens by the immune system, particularly for cryptic, subdominant, or marginally antigenic peptides.

Superdominance Among Immunodominant H-2Kb-Restricted Epitopes and Reversal by Dendritic Cell-Mediated Antigen Delivery

To examine possible interference patterns between immunodominant CTL Ags, we analyzed the response to mixtures of five well-characterized H-2Kb-restricted epitopes, each of which had earlier been described as immunodominant within its antigenic system. Clear patterns of dominance were observed between peptides in the mixture, with the CTL response focusing on the Sendai virus nucleoprotein 324–332 and vesicular stomatitis virus nucleoprotein 52–59 epitopes. The dominance of these epitopes correlated with high CTL availability. Subdominance of the OVA257–264 and the MCF1233 murine leukemia virus envelope 574–581 peptides could not be explained by inferior ability to bind and stabilize MHC class I molecules. Interestingly, immunodominance was broken if the peptide mixture was pulsed on bone marrow-derived dendritic cells, a mode of immunization allowing efficient recognition of a broader set of specificities. Our results show that immunodominance is neither an absolute feature of a given epitope nor does it apply only in relation to other epitopes within the same protein, micro-organism, or cell. Novel “superdominant” hierarchies emerge in the response against multiple “dominant” epitopes. A T cell competition model to explain the data in terms of a balance influenced by CTL frequencies and available APC capacity is discussed.

Relationship Among Immunodominance of Single CD8+ T Cell Epitopes, Virus Load, and Kinetics of Primary Antiviral CTL Response

The primary CTL response of BALB/c mice infected with the lymphocytic choriomeningitis (LCM) virus strain WE is directed exclusively against one major epitope, n118, whereas a viral variant, ESC, that does not express n118 induces CTL against minor epitopes. We identified one minor epitope, g283, that induces primary lytic activity in ESC-infected mice. Infections of mice with WE and ESC were used to study the hierarchical control of a T cell response. Presentation of minor epitopes is not reduced in WE-infected cells. Generation of CTL against n118 does not suppress the generation of minor epitope-specific CTL systemically, as mice coinfected with WE and ESC developed CTL against n118 and g283. However, elimination of ESC and development of minor epitope-specific CTL in ESC infection were slower than elimination of WE and development of CTL against n118. CD8+ T cells against the minor epitope were activated in ESC and WE infection, but did not expand in the latter to show lytic activity in a primary response. We explain the absence of minor epitope-specific lytic activity in WE infection by the fast reduction of virus load due to the early developing n118-specific CTL. Immunodominance of CTL epitopes in primary virus infections thus can be explained as a kinetic phenomenon composed of 1) expansion of CD8+ T cells specific for individual epitopes, 2) stimulatory effect of virus load, and 3) negative feedback control on virus load by the fastest CTL population.

Characterizing immunodominant and protective influenza hemagglutinin epitopes by functional activity and relative binding to major histocompatibility complex class II sites

In the present study the analysis of functional activity and major histocompatibility complex (MHC) binding of two adjacent MHC class II-restricted epitopes, located in the C-terminal 306-329 region of human influenza A virus hemagglutinin 1 subunit (HA1) conserved with subtype sequences and not affected by antigenic drift, was undertaken to explore the hierarchy of local immunodominance. The functional activity of two T cell hybridomas of the memory/effector Th1 phenotype in combination with in vivo immunization studies provided a good tool for investigating the functional characteristics of the T cell response. The in vitro binding assays performed with a series of overlapping, N-terminal biotinylated peptides covering the 306-341 sequence enabled us to compare the relative binding efficiency of peptides, comprising two distinct epitopes of this region, to I-Ed expressed on living antigen-presenting cells. Our studies revealed that (i) immunization of BALB/c mice with the 306-329 H1 or H2 peptides resulted in the activation and proliferation of T cells recognizing both the 306-318 and the 317-329 epitopes, while the 306-329 H3 peptide elicits predominantly 306-318-specific T cells, (ii) the 317-329 HA1 epitope of the H1 and H2 but not the H3 sequence is recognized by T cells and is available for recognition not only in the 317-329 peptide but also in the extended 306-329 or 306-341 peptides, (iii) the 306-318 and the 317-329 hemagglutinin peptides encompassing the H1, H2 but not the H3 sequence bind with an apparently similar affinity to and therefore compete for I-Ed binding sites, and (iv) the 317-341, the 317-329 peptides and their truncated analogs show subtype-dependent differences in MHC binding and those with lower binding capacity represent the H3 subtype sequences. These results demonstrate that differences in the binding capacity of peptides comprising two non-overlapping epitopes located in the C-terminal 306-329 region of HA1 of all three subtype-specific sequences to MHC class II provide a rationale for the local and also for the previously observed in vivo immunodominance of the 306-318 region over the 317-329 epitope in the H3 but not in the H1 or H2 sequences. In good correlation with the results of the binding and functional inhibition assays, these data demonstrate that in the H1 and H2 subtypes both regions are available for T cell recognition, they compete for the same restriction element with an apparently similar binding efficiency and, therefore, function as co-dominant epitopes. Due to the stabilizing effect of the fusion peptide, peptides comprising the 306-341 or 317-341 H1 sequences are highly immunogenic and elicit a protective immune response which involves the production of antibodies and interleukin-2 and tumor necrosis factor producing effector Th1 cells both directed against the 317-329 region. Based on the similarity of the I-Ed and HLA-DR1 peptide binding grooves and motifs, these results suggest that amino acid substitutions inserted to the H3 subtype sequence during viral evolution can modify the relative MHC binding capacity and invert the local hierarchy of immunodominance of two closely situated epitopes that are able to bind to the same MHC class II molecule.