In vivo priming of cytotoxic T lymphocyte responses in relation to in vitro up-regulation of major histocompatibility complex class I molecules by short synthetic peptides

Influenza Virus-Derived CD8 T Cell Epitopes: Implications for the Development of Universal Influenza Vaccines

The influenza virus poses a global health burden. Currently, an annual vaccine is used to reduce influenza virus-associated morbidity and mortality. Most influenza vaccines have been developed to elicit neutralizing Abs against influenza virus. These Abs primarily target immunodominant epitopes derived from hemagglutinin (HA) or neuraminidase (NA) of the influenza virus incorporated in vaccines. However, HA and NA are highly variable proteins that are prone to antigenic changes, which can reduce vaccine efficacy. Therefore, it is essential to develop universal vaccines that target immunodominant epitopes derived from conserved regions of the influenza virus, enabling cross-protection among different virus variants. The internal proteins of the influenza virus serve as ideal targets for universal vaccines. These internal proteins are presented by MHC class I molecules on Ag-presenting cells, such as dendritic cells, and recognized by CD8 T cells, which elicit CD8 T cell responses, reducing the likelihood of disease and influenza viral spread by inducing virus-infected cell apoptosis. In this review, we highlight the importance of CD8 T cell-mediated immunity against influenza viruses and that of viral epitopes for developing CD8 T cell-based influenza vaccines.

The Immunogenicity of a Proline-Substituted Altered Peptide Ligand toward the Cancer-Associated TEIPP Neoepitope Trh4 Is Unrelated to Complex Stability

Human cancers frequently display defects in Ag processing and presentation allowing for immune evasion, and they therefore constitute a significant challenge for T cell-based immunotherapy. We have previously demonstrated that the antigenicity of tumor-associated Ags can be significantly enhanced through unconventional residue modifications as a novel tool for MHC class I (MHC-I)-based immunotherapy approaches. We have also previously identified a novel category of cancer neo-epitopes, that is, T cell epitopes associated with impaired peptide processing (TEIPP), that are selectively presented by MHC-I on cells lacking the peptide transporter TAP. In this study, we demonstrate that substitution of the nonanchoring position 3 into a proline residue of the first identified TEIPP peptide, the murine Trh4, results in significantly enhanced recognition by antitumor CTLs toward the wild-type epitope. Although higher immunogenicity has in most cases been associated with increased MHC/peptide complex stability, our results demonstrate that the overall stability of H-2D^b in complex with the highly immunogenic altered peptide ligand Trh4-p3P is significantly reduced compared with wild-type H-2D^b/Trh4. Comparison of the crystal structures of the H-2D^b/Trh4-p3P and H-2D^b/Trh4 complexes revealed that the conformation of the nonconventional methionine anchor residue p5M is altered, deleting its capacity to form adequate sulfur-π interactions with H-2D^b residues, thus reducing the overall longevity of the complex. Collectively, our results indicate that vaccination with Thr4-p3P significantly enhances T cell recognition of targets presenting the wild-type TEIPP epitope and that higher immunogenicity is not necessarily directly related to MHC/peptide complex stability, opening for the possibility to design novel peptide vaccines with reduced MHC/peptide complex stability.

Characterization of T-cell immunogenicity of two PE/PPE proteins of Mycobacterium tuberculosis

The PE and PPE proteins of Mycobacterium tuberculosis form a source of antigenic variation among different strains of this bacterium. Two of the PE_PGRS protein-encoding genes, rv3812 and rv3018c, are expressed in pathogenic mycobacteria and are implicated, respectively, in the persistence of the organism in macrophages and in virulence. Peptides derived from these proteins have been predicted to bind major histocompatibility complex (MHC) class I with high affinity on the basis of immunoinformatics analysis, suggesting a possible role for these proteins in antimycobacterial immunity. In the present work, using DNA constructs containing the rv3812 and rv3018c genes of M. tuberculosis, the immunogenicity of these proteins was demonstrated in BALB/c mice. Immunization with either DNA construct induced a significant number of CD8+-type T cells and a strong Th1-type response, with high gamma interferon (IFN-gamma) and low interleukin-4 responses. Three nonameric peptides of Rv3812 and two of Rv3018c elicited a strong T-cell response in an MHC-restricted manner. An epitope-specific response was demonstrated by the lysis of peptide-pulsed antigen-presenting cells, release of perforin and IFN-gamma production. Experimentally, these peptides bound with high affinity to MHC H-2Kd and showed low dissociation rates of peptide-MHC complexes. This study suggests that the identified T-cell epitopes may contribute to immunity against tuberculosis if included in a vaccine.

Conversion of tyrosine to the inflammation-associated analog 3'-nitrotyrosine at either TCR- or MHC-contact positions can profoundly affect recognition of the MHC class I-restricted epitope of lymphocytic choriomeningitis virus glycoprotein 33 by CD8 T cells

Immunohistochemical detection of increased levels of protein-associated nitrotyrosine has become widely used as a surrogate marker of in situ inflammation. However, the potential consequences of protein-associated nitrotyrosine formation in terms of cellular immune recognition has received surprisingly little attention. Using a well-defined I-E(K)-restricted epitope of pigeon cytochrome c, we previously demonstrated that conversion of a single tyrosine residue to nitrotyrosine can have a profound effect on recognition by CD4 T cells. In this study, we used the MHC class I-restricted epitope of lymphocytic choriomeningitis virus glycoprotein (gp33) to demonstrate that conversion of tyrosine to nitrotyrosine can also profoundly affect recognition of MHC class I-restricted epitopes. Conversion of the Y4 residue of the gp33 epitope to nitrotyrosine completely abrogated recognition by gp33-specific T cells from P14 TCR-transgenic mice. In contrast, CD8(+) T cells specific for "nitrated gp33" (NY-gp33) can be readily elicited in C57BL/6 mice after immunization with NY-gp33 peptide. Interestingly, T-T hybridomas specific for NY-gp33 peptide were found to fall into two distinct subsets, being specific for NY-gp33 presented in the context of either H-2D(b) or H-2K(b). This latter result is surprising in light of previous structural studies showing that Y4 comprises a critical TCR-contact residue when presented by H-2D(b) but that the same residue points downward into the peptide-binding groove of the MHC when presented by H-2K(b). Together, these results indicate that nitrotyrosine formation can impact T cell recognition both directly, through alteration of TCR-contact residues, or indirectly, through alterations in MHC-contact positions.

HLA-A*0201-restricted cytotoxic T-cell epitopes in three PE/PPE family proteins of Mycobacterium tuberculosis

CD8+ T cells are thought to play an important role in protective immunity against tuberculosis. We report the identification of three peptides derived from Rv1818c, Rv3812 and Rv3018c proteins of Mycobacterium tuberculosis that bound to HLA-A*0201 molecules and their ability to induce in vitro T-cell response in peripheral blood lymphocytes from HLA-A*0201-positive healthy individuals (PPD+) and patients with TB. The peptide-specific cytotoxic T lymphocytes (CTL) generated were capable of recognizing peptide pulsed targets. Three 9-mer peptides bound with high affinity to HLA-A*0201 and displayed low dissociation rates of the bound peptide from HLA. Epitope-specific recognition was demonstrated by the release of perforin and gamma-interferon. Overall, our results demonstrate the presence of HLA class I-restricted CD8+ CTL against proteins from PE and PPE proteins of M. tuberculosis and identify epitopes that are strongly recognized by HLA-A*0201-restricted CD8+ T cells in humans. These epitopes thus represent potential subunit components for the design of vaccines against tuberculosis.

Evaluation of T-cell responses to peptides with MHC class I-binding motifs derived from PE_PGRS 33 protein of Mycobacterium tuberculosis

The PE and PPE proteins of Mycobacterium tuberculosis form a source of antigenic variation among different strains of M. tuberculosis. One of the PE_PGRS proteins, Rv1818c, plays a role in the pathogenesis of mycobacterial infection and specifically influences host-cell responses to tuberculosis infection. Although little is known about these two classes of protein, an immunoinformatics approach has indicated the possibility of their participation in eliciting a major histocompatibility complex (MHC) class I-mediated immune response against tuberculosis, as peptides derived from Rv1818c are predicted to bind to MHC class I molecules with high affinity. In the present work, a DNA vaccine was constructed encoding the full-length Rv1818c protein of M. tuberculosis and its immunogenicity was analysed in BALB/c mice. Immunization with Rv1818c DNA induced a strong CD8+ cytotoxic lymphocyte and Th1-type response, with high levels of gamma interferon (IFN-gamma) and low levels of interleukin-4. Two nonameric peptides (Peptide(6-14) and Peptide(385-393)) from Rv1818c were identified by their ability to induce the production of IFN-gamma by CD8+ T cells in mice immunized with Rv1818c DNA. An epitope-specific response was demonstrated by the lysis of peptide-pulsed antigen-presenting cells, release of cytotoxic granules and IFN-gamma production. These peptides bound with high affinity to MHC H-2K(d) and showed low dissociation rates of peptide-MHC complexes. These results could form the basis for testing the identified T-cell epitopes of PE_PGRS proteins in the induction of protective immunity against M. tuberculosis challenge in the mouse model.

Computational prediction and identification of HLA-A2.1-specific Ebola virus CTL epitopes

Ebola virus (EBOV) is known to cause a severe hemorrhagic fever resulting in high mortality. Although the precise host defense mechanism(s) that afford protection against EBOV is not completely understood, T cell-mediated immune responses is believed to play a pivotal role in controlling virus replication and EBOV infection. There have been no reports on mapping of MHC Class I-binding CTL epitopes for EBOV till to date. In this study, we identified five HLA-A2-binding 9-mer peptides of EBOV nucleoprotein (NP) using computer-assisted algorithm. The peptides were synthesized and examined for their ability to bind to MHC class I molecules using a flow cytometry based MHC stabilization assay. Three of the EBOV-NP peptides tested (FLSFASLFL, RLMRTNFLI and KLTEAITAA) stabilized HLA-A2. The ability of the HLA-A2-binding EBOV-NP peptides to generate peptide-specific CTLs was evaluated in HLA-A2.1 transgenic mice. Epitope-specific CTL responses were confirmed by cytotoxic assays against peptide-pulsed target cells and interferon-gamma ELISPOT assay. Each of the EBOV-NP peptides induced CTL responses in HLA-A2-transgenic mice. Interestingly, all the three peptides were conserved in three different strains of Ebola (Zaire and Reston and Sudan). Taken together, these findings provide direct evidence for the existence of EBOV-derived NP epitopes that may be useful in the development of protective immunogens for this hemorrhagic virus.

Lassa fever virus peptides predicted by computational analysis induce epitope-specific cytotoxic-T-lymphocyte responses in HLA-A2.1 transgenic mice

Lassa fever is a hemorrhagic disease caused by Lassa fever virus (LV). Although the precise host defense mechanism(s) that affords protection against LV is not completely understood, cellular immunity mediated by cytotoxic T lymphocytes (CTLs) plays a pivotal role in controlling viral replication and LV infection. To date, there have been no reports mapping major histocompatibility complex (MHC) class I-binding CTL epitopes for LV. Using computer-assisted algorithms, we identified five HLA-A2.1-binding peptides of LV glycoprotein (GP) and two peptides from LV nucleoprotein (NP). Synthesized peptides were examined for their ability to bind to MHC class I molecules using a flow cytometric assay that measures peptide stabilization of class I. Three of the LV-GP peptides tested (LLGTFTWTL, SLYKGVYEL, and YLISIFLHL) stabilized HLA-A2. The LV-NP peptides tested failed to stabilize this HLA-A2. We then investigated the ability of the HLA-A2-binding LV-GP peptides to generate peptide-specific CTLs in HLA-A2.1 transgenic mice. Functional assays used to confirm CTL activation included gamma interferon enzyme-linked immunospot (ELISPOT) assays and intracellular cytokine staining of CD8+ T cells from peptide-primed mice. CTL assays were also performed to verify the cytolytic activity of peptide-pulsed target cells. Each of the LV-GP peptides induced CTL responses in HLA-A2-transgenic mice. MHC class I tetramers prepared using one LV-GP peptide that showed the highest cytolytic index (LLGTFTWTL) confirmed that peptide-binding CD8+ T cells were present in pooled lymphocytes harvested from peptide-primed mice. These findings provide direct evidence for the existence of LV-derived GP epitopes that may be useful in the development of protective immunogens for this hemorrhagic virus.

Flow cytometric analysis of peptide binding to major histocampatibility complex class I for hepatitis C virus core T-cell epitopes

BACKGROUND/METHODS

To characterize the repertoire of T-cell epitopes on the hepatitis C virus (HCV) core protein, we studied major histocompatibility complex (MHC) class I binding of 75 decapeptides on 20 human B-cell lines and murine spleen cells using a flow cytometric assay. The results were compared with MHC class I stabilization on T2 cells, the SYFPEITHI algorithm, and known T-cell epitopes from the literature.

RESULTS

Binding of peptides proved to be specific for MHC class I molecules. We observed peak fluorescence signals at positions amino acids (aa) 35-44, aa 87-96, aa 131-140, and aa 167-176 in virtually all HLA-A2-positive cell lines. These sites corresponded to T-cell epitopes predicted by SYFPEITHI and the positions of known T-cell epitopes, whereas T2 stabilization was at variance for two peptides. The assay was applied to HLA-A2-negative cells and murine spleen cells without further modification, and identified additional peptides, corresponding to known T-cell epitopes.

CONCLUSIONS

Peptide binding to different MHC class I alleles can be mapped rapidly by a flow cytometric assay and enables a first orientation on the sites of possible T-cell epitopes. Application of this assay to HCV core suggests a rather limited repertoire of epitopes in the Caucasoid population.

In vivo induction of a high-avidity, high-frequency cytotoxic T-lymphocyte response is associated with antiviral protective immunity

Many approaches are currently being developed to deliver exogenous antigen into the major histocompatibility complex class I-restricted antigen pathway, leading to in vivo priming of CD8(+) cytotoxic T cells. One attractive possibility consists of targeting the antigen to phagocytic or macropinocytic antigen-presenting cells. In this study, we demonstrate that strong CD8(+) class I-restricted cytotoxic responses are induced upon intraperitoneal immunization of mice with different peptides, characterized as CD8(+) T-cell epitopes, bound to 1-microm synthetic latex microspheres and injected in the absence of adjuvant. The cytotoxic response induced against a lymphocytic choriomeningitis virus (LCMV) peptide linked to these microspheres was compared to the cytotoxic T-lymphocyte (CTL) response obtained upon immunization with the nonreplicative porcine parvovirus-like particles (PPV:VLP) carrying the same peptide (PPV:VLP-LCMV) previously described (C. Sedlik, M. F. Saron, J. Sarraseca, I. Casal, and C. Leclerc, Proc. Natl. Acad. Sci. USA 94:7503-7508, 1997). We show that the induction of specific CTL activity by peptides bound to microspheres requires CD4(+) T-cell help in contrast to the CTL response obtained with the peptide delivered by viral pseudoparticles. Furthermore, PPV:VLP are 100-fold more efficient than microspheres in generating a strong CTL response characterized by a high frequency of specific T cells of high avidity. Moreover, PPV:VLP-LCMV are able to protect mice against a lethal LCMV challenge whereas microspheres carrying the LCMV epitope fail to confer such protection. This study demonstrates the crucial involvement of the frequency and avidity of CTLs in conferring antiviral protective immunity and highlights the importance of considering these parameters when developing new vaccine strategies.

Induction of a polarized Th1 response by insertion of multiple copies of a viral T-cell epitope into adenylate cyclase of Bordetella pertussis

The adenylate cyclase (CyaA) of Bordetella pertussis delivers the N-terminal catalytic domain into the cytosol of a large number of eukaryotic cells, in particular, professional antigen-presenting cells. This allows the delivery of CD8(+) T-cell epitopes to the major histocompatibility complex class I presentation pathway. We have previously shown that immunization of mice with CyaA carrying a single CD8(+) T-cell epitope leads to antiviral protection as well as to protective and therapeutic antitumor immunity associated with the induction of specific cytotoxic T-lymphocyte (CTL) responses. Here, we evaluated the capacity of CyaA carrying one to four copies of the CD8(+) CD4(+) T-cell epitope from the nucleoprotein of the lymphocytic choriomeningitis virus to induce T-cell responses. Both CTL and Th1-like specific responses were detected in mice immunized with recombinant CyaA with or without adjuvant. Although the insertion of the larger peptides resulted in partial loss of the invasive capacity of recombinant CyaA, insertion of several copies of the same epitope led to a strong enhancement of Th1 responses and, to a lesser degree, CTL responses. These results underscore the potency of CyaA for vaccine design with a new impact on diseases in which the Th1 response has been described to have a beneficial effect.

Evaluation of T-cell responses to peptides and lipopeptides with MHC class I binding motifs derived from the amino acid sequence of the 19-kDa lipoprotein of Mycobacterium tuberculosis

Cytotoxic T-lymphocyte (CTL) epitopes on the 19-kDa lipoprotein from Mycobacterium tuberculosis were identified by the use of lipopeptides and their cytokine profile studied. Selection of candidate CTL epitopes was based on synthetic peptides derived from the amino acid sequence of the 19-kDa lipoprotein showing major histocompatibility complex class I (MHC-I) binding motifs (H-2D(b) and H-2L(d)). Their ability to up-regulate and stabilize MHC-I molecules on the mouse lymphoma cell line RMA-S was studied. Similar studies were performed with peptides, in which the anchor amino acid of the H-2D(b) MHC-I motif was replaced by alanine. Three out of five peptides with H-2D(b) or H-2L(d) binding motifs and their corresponding lipopeptides as well, up-regulated and stabilized the H-2D(b) molecules on RMA-S cells. Replacement of the anchor amino acid residues of the H-2D(b) MHC-I motif by alanine revealed that the anchor amino acid asparagine at position 5, contributed more to binding of peptide to H-2D(b) molecules than leucine at position 11. The closely related lipopeptides LP19c and LP19d, in combination with incomplete Freund's adjuvant (IFA), induced CTL responses in C57BL/6 (H-2(b)) mice. These CTLs could recognize the naturally processed antigen, i.e. the 19-kDa antigen protein produced and processed by the EX-19 cell line. The capacity of the various lipopeptides to induce CTL correlated well with the ability of the (lipo)peptide to up-regulate and to stabilize H-2D(b) molecules. Lipopeptide LP19c primed spleen cells showed a T helper type one profile after in vitro stimulation with P19c and P19d 19 kDa peptides. The approach to characterize presumptive 19-kDa CTL epitopes might lead to selection of promising CTL epitopes, which can be applied in the development of subunit tuberculosis vaccines.

Identification of new cytotoxic T-cell epitopes on the 38-kilodalton lipoglycoprotein of Mycobacterium tuberculosis by using lipopeptides

Induction of cytotoxic T lymphocytes (CTLs) by vaccination has been shown to protect against bacterial, viral, and tumoral challenge. The aim of this study was to identify CTL epitopes on the 38-kDa lipoglycoprotein from Mycobacterium tuberculosis. The identification of these CTL epitopes was based on synthesizing peptides designed from the 38-kDa lipoglycoprotein, with known major histocompatibility complex class I (MHC-I) binding motifs (H-2Db), and studying their ability to up-regulate and stabilize MHC-I molecules on the mouse lymphoma cell line RMA-S. To improve the capacity of the identified peptides to induce CTL responses in mice, palmitic acid with a cysteine-serine-serine spacer amino acid sequence was attached to the amino terminus of the peptide. Two of five peptides with H-2Db binding motifs and their corresponding lipopeptides up-regulated and stabilized the H-2Db molecules on RMA-S cells. Both lipopeptides, in combination with incomplete Freund's adjuvant, induced CTL responses in C57BL/6 (H-2(b)) mice. Moreover, the lipopeptide induced stronger CTL responses than the peptide. The capacity of the various lipopeptides to induce CTL displayed a good relationship with the ability of the (lipo)peptide to up-regulate and to stabilize H-2Db molecules. The capacity of the peptides and lipopeptides to up-regulate and stabilize MHC-I expression can therefore be used to predict their potential to function as a CTL epitope. The newly identified CTL epitopes and their lipid derivatives provide us with important information for future M. tuberculosis vaccine design.

Recombinant parvovirus-like particles as an antigen carrier: a novel nonreplicative exogenous antigen to elicit protective antiviral cytotoxic T cells

To develop a strategy that promotes efficient antiviral immunity, hybrid virus-like particles (VLP) were prepared by self-assembly of the modified porcine parvovirus VP2 capsid protein carrying a CD8(+) T cell epitope from the lymphocytic choriomeningitis virus nucleoprotein. Immunization of mice with these hybrid pseudoparticles, without adjuvant, induced strong cytotoxic T lymphocyte (CTL) responses against both peptide-coated- or virus-infected-target cells. This CD8(+) class I-restricted cytotoxic activity persisted in vivo for at least 9 months. Furthermore, the hybrid parvovirus-like particles were able to induce a complete protection of mice against a lethal lymphocytic choriomeningitis virus infection. To our knowledge, this study represents the first demonstration that hybrid nonreplicative VLP carrying a single viral CTL epitope can induce protection against a viral lethal challenge, in the absence of any adjuvant. These recombinant particles containing a single type of protein are easily produced by the baculovirus expression system and, therefore, represent a promising and safe strategy to induce strong CTL responses for the elimination of virus-infected cells.

Induction of cytotoxic T-cell response by optimal-length peptides does not require CD4+ T-cell help

In several experimental models, synthetic peptides were shown to activate efficiently cytotoxic T-lymphocyte (CTL) responses and therefore represent an attractive strategy to develop new vaccines. However, the mechanisms by which they induce CTL responses are not yet fully understood. Several studies using 15 16-mer peptides previously demonstrated that CD4 helper T cells are required to induce optimal CTL responses with synthetic peptides. However, recently it was suggested that shorter 8 12-mer peptides could have an increased in vivo immunogenicity. In the present study, we therefore investigated if such optimal-length peptides still require CD4+ T-cell help to activate CTL responses. To address this question three synthetic peptides containing different viral CTL epitopes were injected into mice depleted of CD4+ or CD8+ T cells using specific monoclonal antibodies or into mice genetically deficient in those T-cell populations. Our results clearly established that activation of CTL responses by those short optimal peptides does not require CD4+ T-cell help and therefore suggested that high-density binding of peptides to major histocompatibility complex class I molecules on the surface of antigen-presenting cells is required for direct activation of CD8+ T cells, independently of CD4+ T-cell help.

Comparison of adjuvant formulations for cytotoxic T cell induction using synthetic peptides

We have investigated the capacity of synthetic peptides delivered in different adjuvant formulations to induce cytotoxic T lymphocyte (CTL) responses to a class I H-2Kd-restricted Plasmodium berghei circumsporozoite epitope, CS 252-260. Using three immunogen formulations: soybean emulsion; Montanide ISA720; and lipopeptide (P3-CS), we first evaluated the effects of immunization routes on CTL induction. No CTL response was induced in mice immunized s.c. or i.p. with CS peptide formulated in soybean emulsion. In contrast, immunization with lipopeptide P3-CS either s.c. or i.p. effectively primed for CTL. Interestingly, CS peptide emulsified in Montanide ISA720 induced a CTL response only when delivered s.c. and not i.p., indicating the critical influence of immunization routes on CTL induction. We then compared the effectiveness of eight adjuvant formulations to induce CTL response following a single s.c. immunization. Notably, lipopeptide P3-CS and CS peptide admixed with P3 or POE lipid molecules stimulated a vigorous CTL response. However, only mice immunized with P3-CS and CS peptide admixed with P3 molecule generated long-lived CTL which persisted in vivo for 5 months. Thus, based on a simultaneous comparison of the different adjuvant formulations, we demonstrated that the conjugated and unconjugated P3 lipopeptides were the most effective immunogens for eliciting primary and memory CTL in mice.

Major histocompatibility complex class I binding glycopeptides for the estimation of 'empty' class I molecules

Different forms of major histocompatibility complex (MHC) class I heavy chains are known to be expressed on the cell surface, including molecules which are functionally 'empty'. Direct peptide binding to cells is obvious during sensitization of target cells in vitro for cytotoxic T lymphocyte killing and 'empty' MHC-I molecules are comparatively abundant on TAP-1/2 peptide transporter mutant cells. In the present work we have estimated the fraction of 'empty' MHC class I molecules using glycosylated peptides and cellular staining with carbohydrate specific monoclonal antibodies. Synthetic Db and Kb binding peptides were coupled at different positions with different di- or trisaccharides, using different spacing between the carbohydrate and the peptide backbone. Binding of sugar specific mAbs was compared in ELISA and cellular assays. An optimal Db binding glycopeptide was used for comparative staining with anti-Db and anti-carbohydrate monoclonal antibodies to estimate fractions of 'empty' molecules on different T lymphoid cells. On activated normal T cells, a large fraction of Db molecules were found to be 'empty'. The functional role of such 'empty' MHC class I molecules on T cells is presently unclear. However, on antigen presenting cells they might participate in the antigen presentation process.

Cell-invasive activity of epitope-tagged adenylate cyclase of Bordetella pertussis allows in vitro presentation of a foreign epitope to CD8+ cytotoxic T cells

The adenylate cyclase (AC) toxin (CyaA) of Bordetella pertussis has an invasive catalytic domain (AC domain) which penetrates the cytoplasmic membrane of a variety of eukaryotic cells and intoxicates them by unregulated synthesis of cyclic AMP. Previous work led to identification of five permissive sites in the AC domain at which heterologous peptides are accommodated without affecting its enzymatic properties. We have constructed a set of CyaA toxins tagged at these permissive sites by insertion of a CD8+ T-cell epitope, RPQASGVYMGNLTAQ, from the nucleoprotein of lymphocytic choriomeningitis virus. Introduction of the epitope at any of the five sites did not affect the capacity of the toxin to deliver its AC domain into target cells. Moreover, the toxin with the inserted epitope was shown to sensitize target cells for lysis by epitope-specific CD8+ cytotoxic T lymphocytes in vitro, showing that the tagged AC was processed for presentation of the lymphocytic choriomeningitis virus epitope in association with the major histocompatibility complex class I molecules. This finding indicates that by virtue of delivery of foreign epitopes into the antigen-presenting cells, purpose-designed recombinant CyaAs may be useful for induction of specific major histocompatibility complex class I-restricted cell-mediated immunity also in vivo.

Characterization of TAP-independent and brefeldin A-resistant presentation of Sendai virus antigen to CD8+ cytotoxic T lymphocytes

H-2Kb-transfected T2 cells, which lack both TAP1/2 and LMP2/7 genes, are able to efficiently process and present Sendai virus Antigen to Kb-restricted Sendai virus-specific CTL. This presentation is not inhibited by Brefeldin A (BFA). Here we extend our analysis of this novel antigen presentation pathway. We show that presentation of Sendai virus antigen was not due to sensitization of T2Kb cells by peptides in the virus preparation or peptides released from virus infected cells. Also, the ability to present Sendai virus in a BFA resistant fashion was specific for cells of the T2 lineage. Re-expression of TAP1/2 genes in T2Kb cells did not alter the capability to present antigen in a BFA resistant fashion, i.e. the presence of a functional TAP transporter complex did not relocate (all) peptides to the classical pathway for antigen processing and presentation. We found that co-infection of T2Kb cells with either Sendai virus plus influenza virus or Sendai virus plus VSV did not relocate presentation of influenza or VSV antigen to the TAP independent BFA resistant antigen presentation pathway. Peptide elution experiments and studies with peptide-specific CTL firmly demonstrated that the antigen presented by T2Kb cells after infection with Sendai virus was the natural Sendai virus epitope NP324-332. The same epitope, when expressed as a minigene in vaccinia virus, could be presented also by T2Kb cells but this presentation could be blocked by BFA. Thus, the TAP independent BFA resistant presentation of antigen seem cell (T2 lineage) and virus (Sendai virus) specific, but not epitope specific. The ability of T2Kb cells to present Sendai virus antigen in a TAP independent BFA resistant fashion was only partially blocked by lysosomal inhibitors such as methylamine, ammonium chloride and chloroquine. These findings demonstrate that TAP1/2-independent and BFA-resistant class I processing is only expressed in certain cell types, in parallel with classical MHC class I processing, and that Sendai virus selectively can enter this pathway. Hypothetical models for the TAP-independent class I processing are discussed.

Efficient MHC class I-peptide binding is required but does not ensure MHC class I-restricted immunogenicity

Cytotoxic T lymphocyte (CTL) epitopes are naturally processed peptides bound and presented by major histocompatibility (MHC) class I molecules. Since they are expressed at the cell surface in sufficient amounts to be recognized by CTL, it is generally believed, and in some cases demonstrated, that they bind efficiently to MHC class I molecules in vivo. Based on this knowledge, candidate CTL epitopes are now searched for by identifying peptides that efficiently bind to MHC class I molecules in vitro. We analysed whether this approach is valid by systematically investigating the relationship between MHC class I-peptide binding and peptide immunogenicity. Fifteen peptides that represent known CTL epitopes were tested for their MHC class I binding ability. In a comparative study with 83 peptides that bear the appropriate MHC class I allele-specific motifs but do not contain known CTL epitopes, the CTL epitope-bearing peptides showed the highest binding affinity for MHC class I. This was true for two MHC class I alleles in two different assay systems that monitor peptide-MHC class I binding. Furthermore, selected motif-bearing Kb binding peptides were used to induce peptide-specific CTL responses in mice. Only a subset of the high affinity Kb binding peptides induced reproducible peptide-specific CTL responses, whereas none of the low affinity Kb binding peptides induced a response. Taken together, these results indicate that efficient peptide-MHC class I binding is required for immunogenicity. Vice versa, immunogenicity is not guaranteed by efficient peptide-MHC class I binding, implying that additional factors are involved. Nevertheless, selection of candidate CTL epitopes on the basis of MHC class I binding seems valid. Our data indicate that, although an excess of peptides might be selected, the chance of missing immunogenic peptides is minimal.

Major histocompatibility complex class I-binding peptides are recycled to the cell surface after internalization

Cytotoxic T lymphocytes (CTL) recognize target antigens as short, processed peptides bound to major histocompatibility complex class I (MHC-I) heavy and light chains (beta 2-microglobulin; beta 2-m). The heavy chain, which comprise the actual peptide binding alpha-1 and alpha-2 domains, can exist at the cell surface in different forms, either free, bound to beta 2-m or as a ternary complex with beta 2-m and peptides. MHC-I chains are also known to internalize, and recycle to the cell surface, and this has been suggested to be important in peptide presentation. Whether MHC-I-bound peptides also can recycle is not known. We have investigated this by using both peptide transporter mutant RMA-S cells and EL4 cells loaded with Db-binding peptides, by two different approaches. First, peptides were covalently linked with galabiose (Gal alpha 4Gal) at a position which did not interfere with Db binding or immunogenicity, and peptide recycling tested with Gal2-specific monoclonal antibodies. By flow cytometry, a return of Gal2 epitopes to the cell surface was found, after cellular internalization and cell surface clearance by pronase treatment. This peptide recycling could be discriminated from free fluid-phase uptake and was inhibited by methylamine, chloroquine and low temperature (18 degrees C) but not by leupeptin. Second, specific CTL were reacted with peptide-loaded target cells after complete removal of surface Db molecules by pronase, and after different times of incubation at 37 degrees C to allow reexpression. By this procedure, reappearance of target cell susceptibility was confirmed. The results are in agreement with a model for optimizing peptide presentation by recycling through an intracellular compartment similar to early endosomes in certain antigen-presenting cells.

In vitro induction of human cytotoxic T lymphocyte responses against peptides of mutant and wild-type p53

The central role of the p53 tumor suppressor gene product in oncogenesis is gradually being clarified. Point mutations in the p53 tumor suppressor gene are common in most human cancers and are often associated with p53 protein overexpression. Overexpressed wild-type or mutant determinants of the p53 protein thus represent an attractive target for immunotherapy of cancer directed against a structure involved in malignant transformation. An important step towards this goal is identification of epitopes of p53 that can be recognized by human cytotoxic T lymphocytes. We identified peptides of (mutant) p53 capable of binding to HLA-A2.1 in an in vitro assay. These HLA-A2.1-binding peptides were utilized for in vitro induction of primary cytotoxic T lymphocyte responses using a human processing-defective cell line (174CEM.T2) as antigen-presenting cell. These cells display "empty" HLA class I surface molecules, that can efficiently be loaded with a single peptide. We obtained CD8+ cytotoxic T lymphocyte clones capable of specifically lysing target cells loaded with wild-type or tumor-specific mutant p53 peptides. This strategy allows the in vitro initiation of human cytotoxic T lymphocyte responses against target molecules of choice.

Antigen processing mutant T2 cells present viral antigen restricted through H-2Kb

Cytotoxic T lymphocytes (CTL) recognize foreign antigens as short peptides presented by class I molecules of the major histocompatibility complex (MHC). T2 cells are profoundly defective in the presentation of endogenously synthesized antigens to CTL due to a deletion of MHC class II-encoded genes for transporters associated with antigen presentation (TAP1/TAP2). Surprisingly, we here demonstrate that T2 cells, after infection with Sendai virus, are readily killed by H-2Kb restricted CD8+ T cells. In contrast to classical class I-mediated antigen presentation, the presentation of Sendai virus antigen in T2Kb cells is brefeldin A (BFA) insensitive. The present findings may suggest the presence of an alternative pathway for MHC class I-mediated antigen presentation in T2 cells.

TAP2-defective RMA-S cells present Sendai virus antigen to cytotoxic T lymphocytes

The murine antigen-processing-defective mutant cell line RMA-S is leaky in the presentation of certain endogenously synthesized minor histocompatibility and viral antigens to major histocompatibility complex (MHC) class I-restricted cytotoxic T lymphocytes (CTL). The viral antigens include influenza virus nucleoprotein, vesicular stomatitis virus (VSV) nucleocapsid and Rauscher murine leukemia virus (MuLV) antigen. Here we demonstrate Sendai virus antigen presentation by the HAM2 (murine TAP2, transporter associated with antigen presentation type 2)-defective RMA-S cell line and compare antigen presentation after restoration of the defect by murine TAP1/2 gene transfection. Kinetic studies revealed that RMA-S cells required 2-3 h longer incubation and approximately 10 times higher doses of Sendai virus to reach the same level of killing as the RMA parental line. After transfection of RMA-S cells with the murine TAP1/2 gene, Sendai virus antigen presentation was restored to levels of the RMA wild-type line with regard to time of virus infection and dose of virus needed for sensitizing target cells. The presentation of Sendai virus antigen in RMA-S cells was sensitive to brefeldin A (BFA), suggesting that the presentation was mediated via the endogenous pathway. Our findings confirmed leakiness of antigen presentation in RMA-S cells and extended it to Sendai virus. The results underscored the role for intact expression of the TAP 1/2 molecules for efficient MHC class I-mediated antigen presentation.