Characterization of two distinct major histocompatibility complex class I Kk-restricted T-cell epitopes within the influenza A/PR/8/34 virus hemagglutinin

Harnessing an RNA-mediated chaperone for the assembly of influenza hemagglutinin in an immunologically relevant conformation

A novel protein-folding function of RNA has been recognized, which can outperform previously known molecular chaperone proteins. The RNA as a molecular chaperone (chaperna) activity is intrinsic to some ribozymes and is operational during viral infections. Our purpose was to test whether influenza hemagglutinin (HA) can be assembled in a soluble, trimeric, and immunologically activating conformation by means of an RNA molecular chaperone (chaperna) activity. An RNA-interacting domain (RID) from the host being immunized was selected as a docking tag for RNA binding, which served as a transducer for the chaperna function for de novo folding and trimeric assembly of RID-HA1. Mutations that affect tRNA binding greatly increased the soluble aggregation defective in trimer assembly, suggesting that RNA interaction critically controls the kinetic network in the folding/assembly pathway. Immunization of mice resulted in strong hemagglutination inhibition and high titers of a neutralizing antibody, providing sterile protection against a lethal challenge and confirming the immunologically relevant HA conformation. The results may be translated into a rapid response to a new influenza pandemic. The harnessing of the novel chaperna described herein with immunologically tailored antigen-folding functions should serve as a robust prophylactic and diagnostic tool for viral infections.-Yang, S. W., Jang, Y. H., Kwon, S. B., Lee, Y. J., Chae, W., Byun, Y. H., Kim, P., Park, C., Lee, Y. J., Kim, C. K., Kim, Y. S., Choi, S. I., Seong, B. L. Harnessing an RNA-mediated chaperone for the assembly of influenza hemagglutinin in an immunologically relevant conformation.

Effect of MHC class I diversification on influenza epitope-specific CD8+ T cell precursor frequency and subsequent effector function

Earlier studies of influenza-specific CD8(+) T cell immunodominance hierarchies indicated that expression of the H2K(k) MHC class I allele greatly diminishes responses to the H2D(b)-restriced D(b)PA(224) epitope (acid polymerase, residues 224-233 complexed with H2D(b)). The results suggested that the presence of H2K(k) during thymic differentiation led to the deletion of a prominent Vβ7(+) subset of D(b)PA(224)-specific TCRs. The more recent definition of D(b)PA(224)-specific TCR CDR3β repertoires in H2(b) mice provides a new baseline for looking again at this possible H2K(k) effect on D(b)PA(224)-specific TCR selection. We found that immune responses to several H2D(b)- and H2K(b)-restricted influenza epitopes were indeed diminished in H2(bxk) F(1) versus homozygous mice. In the case of D(b)PA(224), lower numbers of naive precursors were part of the explanation, though a similar decrease in those specific for the D(b)NP(366) epitope did not affect response magnitude. Changes in precursor frequency were not associated with any major loss of TCR diversity and could not fully account for the diminished D(b)PA(224)-specific response. Further functional and phenotypic characterization of influenza-specific CD8(+) T cells suggested that the expansion and differentiation of the D(b)PA(224)-specific set is impaired in the H2(bxk) F(1) environment. Thus, the D(b)PA(224) response in H2(bxk) F(1) mice is modulated by factors that affect the generation of naive epitope-specific precursors and the expansion and differentiation of these T cells during infection, rather than clonal deletion of a prominent Vβ7(+) subset. Such findings illustrate the difficulties of predicting and defining the effects of MHC class I diversification on epitope-specific responses.

In silico characterization of the functional and structural modules of the hemagglutinin protein from the swine-origin influenza virus A (H1N1)-2009

The 2009 swine-origin influenza virus (S-OIV, H1N1 subtype) has developed into a new pandemic influenza as announced by the World Health Organization. In order to uncover clues about the determinants for virulence and pathogenicity of the virus, we characterized the functional modules of the surface glycoprotein hemagglutinin (HA), the most important protein in molecular epidemiology and pathogenesis of influenza viruses. We analyzed receptor binding sites, basic patch, neutralization antibody epitopes and T cell epitopes in the HA protein of the current S-OIV according to the corresponding functional and structural modules previously characterized in other H1 HA molecules or HA molecules of other subtypes. We compared their differences and similarities systematically. Based on the amino acids defined as the functional and structural modules, the HA protein of 2009 S-OIV should specifically bind to the human 2,6-receptor. The D225G/E mutation in HA, which is found in some isolates, may confer dual binding specificity to the 2,3- and 2,6-receptor based on previously reported work. This HA variant contains two basic patches, one of which results in increased basicity, suggesting enhanced membrane fusion function. The 2009 S-OIV HA also has an extra glycosylation site at position 276. Four of the five antibody neutralization epitopes identified in A/RP/8/34(H1N1) were exposed, but the other was hidden by a glycosylation site. The previously identified cytotoxic T cell epitopes in various HA molecules were summarized and their corresponding sequences in 2009 S-OIV HA were defined. These results are critical for understanding the pathogenicity of the virus and host immune response against the virus.

A single-chain H-2Db molecule presenting an influenza virus nucleoprotein epitope shows enhanced ability at stimulating CD8+ T cell responses in vivo

We have generated a construct encoding a single-chain H-2D(b) mouse MHC class I molecule in which an influenza virus nucleoprotein (NP) epitope, amino acid sequence ASNENMDAM, is fused to mouse beta(2)-microglobulin and the D(b) H chain via flexible linker sequences. This single-chain trimer (SCT) was efficiently expressed at the cell surface independently of TAP and endogenous beta(2)-microglobulin, and it was recognized directly and efficiently by specific T cells in vitro. A recombinant vaccinia virus encoding the D(b) NP SCT primed a CD8(+) T cell response in C57BL/6 mice 4-fold greater than an equivalent virus expressing the NP epitope as a minigene, as shown by tetramer staining, whether or not the minigene was directed into the endoplasmic reticulum by a signal sequence. This response was functional as shown by in vivo lysis assays with peptide-pulsed target cells, and it was greatly expanded following secondary challenge in vivo with influenza virus. The SCT was also significantly more immunostimulatory for CD8(+) cells than the NP minigene in adoptive transfer experiments using F5 TCR transgenic spleen cells, in which the magnitude of the T cell response was much greater. Our results extend previous DNA vaccination studies using SCTs, which demonstrated that such molecules are capable of generating functional CD8(+) T cell responses. We have shown that class I SCTs are more immunogenic than even preprocessed Ag in the form of an epitope minigene, and they therefore should be considered for use when the generation of optimal CD8(+) T cell responses is required.

CD8 T cells require gamma interferon to clear borna disease virus from the brain and prevent immune system-mediated neuronal damage

Borna disease virus (BDV) frequently causes meningoencephalitis and fatal neurological disease in young but not old mice of strain MRL. Disease does not result from the virus-induced destruction of infected neurons. Rather, it is mediated by H-2(k)-restricted antiviral CD8 T cells that recognize a peptide derived from the BDV nucleoprotein N. Persistent BDV infection in mice is not spontaneously cleared. We report here that N-specific vaccination can protect wild-type MRL mice but not mutant MRL mice lacking gamma interferon (IFN-gamma) from persistent infection with BDV. Furthermore, we observed a significant degree of resistance of old MRL mice to persistent BDV infection that depended on the presence of CD8 T cells. We found that virus initially infected hippocampal neurons around 2 weeks after intracerebral infection but was eventually cleared in most wild-type MRL mice. Unexpectedly, young as well as old IFN-gamma-deficient MRL mice were completely susceptible to infection with BDV. Moreover, neurons in the CA1 region of the hippocampus were severely damaged in most diseased IFN-gamma-deficient mice but not in wild-type mice. Furthermore, large numbers of eosinophils were present in the inflamed brains of IFN-gamma-deficient mice but not in those of wild-type mice, presumably because of increased intracerebral synthesis of interleukin-13 and the chemokines CCL1 and CCL11, which can attract eosinophils. These results demonstrate that IFN-gamma plays a central role in host resistance against infection of the central nervous system with BDV and in clearance of BDV from neurons. They further indicate that IFN-gamma may function as a neuroprotective factor that can limit the loss of neurons in the course of antiviral immune responses in the brain.

The H-2KkMHC Peptide-Binding Groove Anchors the Backbone of an Octameric Antigenic Peptide in an Unprecedented Mode

A wealth of data has accumulated on the structure of mouse MHC class I (MHCI) molecules encoded by the H-2(b) and H-2(d) haplotypes. In contrast, there is a dearth of structural data regarding H-2(k)-encoded molecules. Therefore, the structures of H-2K(k) complexed to an octameric peptide from influenza A virus (HA(259-266)) and to a nonameric peptide from SV40 (SV40(560-568)) have been determined by x-ray crystallography at 2.5 and 3.0 A resolutions, respectively. The structure of the H-2K(k)-HA(259-266) complex reveals that residues located on the floor of the peptide-binding groove contact directly the backbone of the octameric peptide and force it to lie deep within the H-2K(k) groove. This unprecedented mode of peptide binding occurs despite the presence of bulky residues in the middle of the floor of the H-2K(k) peptide-binding groove. As a result, the Calpha atoms of peptide residues P5 and P6 are more buried than the corresponding residues of H-2K(b)-bound octapeptides, making them even less accessible to TCR contact. When bound to H-2K(k), the backbone of the SV40(560-568) nonapeptide bulges out of the peptide-binding groove and adopts a conformation reminiscent of that observed for peptides bound to H-2L(d). This structural convergence occurs despite the totally different architectures of the H-2L(d) and H-2K(k) peptide-binding grooves. Therefore, these two H-2K(k)-peptide complexes provide insights into the mechanisms through which MHC polymorphism outside primary peptide pockets influences the conformation of the bound peptides and have implications for TCR recognition and vaccine design.

Peptide selection by class I molecules of the major histocompatibility complex

Class I molecules of the major histocompatibility complex (MHC) bind peptides derived from cytoplasmic proteins. Comparison of over 100 such peptides reveals the importance of the carboxy-terminal residue in selective binding. Recent evidence implicates the proteases and transporters of the processing pathway in providing peptides with the correct residues at the carboxyl terminus.

The Generation of CD25+CD4+Regulatory T Cells That Prevent Allograft Rejection Does Not Compromise Immunity to a Viral Pathogen

In all but a small minority of cases, continued survival of solid organ grafts after transplantation depends on lifelong, nonselective immunosuppression that, although effective, results in increased rates of infection, cancer, and vascular disease. Therapeutic strategies that engage or mimic self-tolerance may allow prolonged allograft survival without the disadvantages of nonspecific immunotherapy. Pretreatment of recipient mice with donor alloantigen combined with transient modulation of the peripheral T cell pool with anti-CD4 Ab leads to the indefinite survival of MHC-incompatible cardiac allografts without further therapy. Tolerance is dependent on CD25+ CD4+ regulatory T cells that arise from naive CD25- precursors and regulate rejection via both IL-10 and CTLA-4. Although these cells are clearly effective at controlling rejection, the proven ability of recently activated CD25+ cells to mediate bystander regulation raises the possibility that tolerized individuals might also have a reduced capacity to respond to environmental pathogens. We have examined anti-influenza responses in tolerized primary heart recipients, secondary recipients following adoptive transfer of regulatory populations, and tolerized mice in which bystander regulation has been deliberately induced. Neither virus-specific CTL activity in vitro nor the clearance of virus in vivo was significantly diminished in any of these treatment groups compared with infected unmanipulated controls. The data suggest that the induction of dominant allograft tolerance dependent on regulatory T cells does not necessarily result in attenuated responses to pathogens providing further support for the development of tolerance induction protocols in clinical transplantation.

Immunization with dendritic cells can break immunological ignorance toward a persisting virus in the central nervous system and induce partial protection against intracerebral viral challenge

Dendritic cells (DCs) have been used successfully to induce CD8 T cells that control virus infections and growth of tumours. The efficacy of DC-mediated immunization for the control of neurotropic Borna disease virus (BDV) in mice was evaluated. Certain strains of mice only rarely develop spontaneous neurological disease, despite massive BDV replication in the brain. Resistance to disease is due to immunological ignorance toward BDV antigen in the central nervous system. Ignorance in mice can be broken by immunization with DCs coated with TELEISSI, a peptide derived from the N protein of BDV, which represents the immunodominant cytotoxic T lymphocyte epitope in H-2(k) mice. Immunization with TELEISSI-coated DCs further induced solid protective immunity against intravenous challenge with a recombinant vaccinia virus expressing BDV-N. Interestingly, however, this immunization scheme induced only moderate protection against intracerebral challenge with BDV, suggesting that immune memory raised against a shared antigen may be sufficient to control a peripherally replicating virus, but not a highly neurotropic virus that is able to avoid activation of T cells. This difference might be due to the lack of BDV-specific CD4 T cells and/or inefficient reactivation of DC-primed, BDV-specific CD8 T cells by the locally restricted BDV infection. Thus, a successful vaccine against persistent viruses with strong neurotropism should probably induce antiviral CD8 (as well as CD4) T-cell responses and should favour the accumulation of virus-specific memory T cells in cervical lymph nodes.

Transgenic mice expressing the nucleoprotein of Borna disease virus in either neurons or astrocytes: decreased susceptibility to homotypic infection and disease

The nucleoprotein (N) of Borna disease virus (BDV) is the major target of the disease-inducing antiviral CD8 T-cell response in the central nervous system of mice. We established two transgenic mouse lines which express BDV-N in either neurons (Neuro-N) or astrocytes (Astro-N). Despite strong transgene expression, neurological disease or gross behavioral abnormalities were not observed in these animals. When Neuro-N mice were infected as adults, replication of BDV was severely impaired and was restricted to brain areas with a low density of transgene-expressing cells. Notably, the virus failed to replicate in the transgene-expressing granular and pyramidal neurons of the hippocampus (which are usually the preferred host cells of BDV). When Neuro-N mice were infected within the first 5 days of life, replication of BDV was not suppressed in most neurons, presumably because the onset of transgene expression in the brain occurred after these cells became infected with BDV. Astro-N mice remained susceptible to BDV infection, but they were resistant to BDV-induced neurological disorder. Unlike their nontransgenic littermates, Neuro-N mice with persistent BDV infection did not develop neurological disease after immunization with a vaccinia virus vector expressing BDV-N. In contrast to the situation in wild-type mice, this treatment also failed to induce N-specific CD8 T cells in the spleens of both transgenic mouse lines. Thus, while resistance to BDV infection in N-expressing neurons appeared to result from untimely expression of a viral nucleocapsid component, the resistance to BDV-induced neuropathology probably resulted from immunological tolerance.

Competition between MHC class I alleles for cell surface expression alters CTL responses to influenza A virus

Mammalian cells express up to six different MHC class I alleles, many of which differ in terms of their interaction with components of the Ag presentation pathway and level of cell surface expression. However, it is often assumed in Ag presentation studies that class I alleles function independently of each other. We have compared cell surface expression levels and function of MHC class I molecules in F(1) hybrid mice with those in the homozygous parental strains. The level of cell surface expression of certain alleles in F(1) mice differed significantly from 50% of that found on the same cell type in the corresponding parental strain, suggesting allele-specific competition for cell surface expression, and not expression solely according to gene dosage. The strongest effect was observed in H-2(b) x H-2(k) F(1) mice, in which the H-2(b) class I molecules dominated over the H-2(k) class I molecules. The magnitude of H-2(k)-restricted CTL responses to influenza A virus infection was similar in the F(1) hybrid and parental H-2(k) mice. However, in H-2(k) mice expressing a K(b) transgene, cell surface levels of the endogenous class I molecules were down-regulated to a greater degree than in F(1) hybrid mice, and H-2(k)-restricted CTL responses against influenza A virus were greatly reduced, although the CTL repertoire was apparently present. Therefore, certain MHC class I molecules compete with each other for cell surface expression, and the resulting low cell surface expression of specific alleles can lead to a severe reduction in the ability to generate a CTL response.

Presentation of a new H-2D(k)-restricted epitope in the Tax protein of human T-lymphotropic virus type I is enhanced by the proteasome inhibitor lactacystin

Tax, the trans-activator of human T-lymphotropic virus type I (HTLV-I), is the dominant target antigen for cytotoxic T lymphocytes (CTLs) in the majority of infected individuals, although the reason for this immunodominance is not clear. Tax has been shown to associate physically with the proteasome, a protease that is responsible for the generation of the majority of major histocompatibility complex (MHC) class I ligands recognized by CTLs. This association could lead to the preferential targeting of Tax to the MHC class I pathway and account for its high immunogenicity. Here, the CTL response to Tax was investigated in mice by priming with a Tax expression vector and boosting with a Tax recombinant vaccinia virus (modified vaccinia virus Ankara strain). This approach led to the identification of a new H-2D(k)-restricted epitope in Tax, amino acid residues 38-46, sequence ARLHRHALL. Surprisingly, presentation of this epitope was found to be enhanced by the proteasome inhibitor lactacystin, although Tax was shown to associate with proteasomes in murine cells. The difficulties encountered in generating Tax-specific CTL responses and the results of enzyme-linked immunospot (ELISpot) analysis suggested that Tax is only poorly immunogenic for CTLs in mice. Therefore, the immunodominance of Tax in human CTL responses to HTLV-I is probably not due to an intrinsic property of the protein itself, such as an association with the proteasome, but instead may result from the fact that Tax is the predominant protein synthesized early after infection.

Identification of the immunodominant H-2K(k)-restricted cytotoxic T-cell epitope in the Borna disease virus nucleoprotein

Borna disease virus (BDV)-induced immunopathology in mice is most prominent in strains carrying the major histocompatibility complex H-2k allele and is mediated by CD8(+) T cells that are directed against the viral nucleoprotein p40. We now identified the highly conserved octamer peptide TELEISSI, located between amino acid residues 129 and 136 of BDV p40, as a potent H-2K(k)-restricted cytotoxic T-cell (CTL) epitope. When added to the culture medium of L929 target cells, TELEISSI conferred sensitivity to lysis by CTLs isolated from brains of BDV-infected MRL mice with acute neurological disease. Vaccinia virus-mediated expression of a p40 variant with mutations in the two K(k)-specific anchor residues of the TELEISSI peptide (p40(E130K,I136T)) did not sensitize L929 target cells for lysis by BDV-specific CTLs, whereas expression of wild-type p40 did. Furthermore, unlike vaccination with wild-type p40, vaccination of persistently infected symptomless B10.BR mice with p40(E130K,I136T) did not result in central nervous system inflammation and neurological disease. These results demonstrate that TELEISSI is the immunodominant CTL epitope of BDV p40 in H-2k mice.

Characterization of a new H-2D(k)-restricted epitope prominent in primary influenza A virus infection

Influenza A virus infection of mice has been used extensively as a model to investigate the mechanisms of antigen presentation to cytotoxic T lymphocytes (CTL) and the phenomenon of immunodominance in antiviral CTL responses. The different virus-encoded epitopes that are recognized in H-2(b) and H-2(d) mice have been characterized and their relative immunodominance has been well-studied. In H-2(k) mice, four different K(k)-restricted influenza virus epitopes have been described, but the dominance hierarchy of these epitopes is unknown and there is also an uncharacterized D(k)-restricted response against the virus. In this study, a D(k)-restricted epitope derived from the influenza virus A/PR/8/34 polymerase protein PB1, corresponding to amino acid residues 349-357 (ARLGKGYMF), was identified. This peptide is the major epitope within the PB1 polymerase and is at least as dominant as any of the four K(k)-restricted epitopes that are recognized in CBA mice following primary influenza virus infection. The PB1 epitope is only the fourth D(k)-presented peptide to be reported and the sequence of this epitope confirms a D(k)-restricted peptide motif, consisting of arginine at position two, arginine or lysine at position five and a hydrophobic residue at the carboxy terminus.

Differential processing and presentation of the H-2D(b)-restricted epitope from two different strains of influenza virus nucleoprotein

The influenza virus strains A/NT/60/68 and A/PR/8/34 both have an immunodominant D(b)-restricted epitope in their nucleoprotein (NP) at amino acid residues 366-374, with two amino acid differences between the epitopes. Cross-reactive cytotoxic T lymphocytes (CTLs) were generated by priming mice with the influenza virus A/NT/60/68 NP and restimulating in vitro with influenza virus A/PR/8/34. CTLs that gave high levels of specific lysis recognized target cells infected with either strain of influenza virus with similar efficiency. Surprisingly, when target cells were infected with recombinant vaccinia viruses (VV) expressing the two different NPs, presentation of the D(b)-restricted epitope from the A/NT/60/68 NP was extremely poor, whereas presentation of the equivalent epitope from the A/PR/8/34 NP was as efficient as in influenza virus-infected cells. This difference was observed in spite of the fact that the two NP sequences show 94% identity at the amino acid sequence level. Experiments with additional cross-reactive CTL cell lines which recognized target cells less efficiently revealed a similar difference in presentation between the two NP epitopes in influenza virus-infected cells and showed a difference in the efficiency of presentation of the D(b)-restricted epitope from the two NP molecules independent of VV infection. The results show that two equivalent epitopes in highly similar proteins are processed with very different efficiency, even though they are both immunodominant epitopes. They also suggest that the previously described inhibition of antigen presentation by VV is a general, non-specific effect, which is more apparent for epitopes that are processed and presented less efficiently.

Contemporary analysis of MHC-related immunodominance hierarchies in the CD8+ T cell response to influenza A viruses

Early studies of influenza virus-specific CD8+ T cell-mediated immunity indicated that the level of CTL activity associated with H2Db is greatly diminished in mice that also express H2Kk. Such MHC-related immunodominance hierarchies are of some interest, as they could lead to variable outcomes for peptide-based vaccination protocols in human populations. The influence of H2Kk on the H2Db-restricted response profile has thus been looked at again using a contemporary, quantitative, IFN-gamma-based flow cytometric assay. The depressive effect of H2Kk was very apparent for the influenza DbPA224 epitope and was also reproduced when CTL activity was measured for H2Db-expressing targets pulsed with the immunodominant NP366 peptide. The secondary CD8+IFN-gamma+ DbNP366-specific response was much greater in parental H2b than in H2kxbF1 mice, but the sizes of the CD8+ sets specific for KkNP50 and DbNP366 were essentially equivalent in the F1 animals. Thus, although the immunodominance profile associated with DbNP366 is lost when H2Kk is also present, the response is still substantial. A further, MHC-related effect was also identified for the KkNS1152 epitope, which was consistently associated with a greater CD8+IFN-gamma+ response in H2KkDb recombinant than in (H2KkDk x H2KbDb)F1 mice. The diminished DbPA224 response in H2kxbF1 mice was characterized by loss of a prominent Vbeta7 TCR responder phenotype, supporting the idea that TCR deletion during ontogeny shapes the available repertoire. The overall conclusion is that these MHC-related immunodominance hierarchies are more subtle than the early CTL assays suggested and, although inherently unpredictable, are unlikely to cause a problem for peptide-based vaccine strategies.

Poor correspondence between predicted and experimental binding of peptides to class I MHC molecules

Naturally processed peptides presented by class I major histocompatibility complex (MHC) molecules display a characteristic allele specific motif of two or more essential amino acid side chains, the so-called peptide anchor residues, in the context of an 8-10 amino acid long peptide. Knowledge of the peptide binding motif of individual class I MHC molecules permits the selection of potential peptide antigens from proteins of infectious organisms that could induce protective T-cell-mediated immunity. Several methods have been developed for the prediction of potential class I MHC binding peptides. One is based on a simple scanning for the presence of primary peptide anchor residues in the sequence of interest. A more sophisticated technology is the utilization of predictive computer algorithms. Here, we have analyzed the experimental binding of 84 peptides selected on the basis of the presence of peptide binding motifs for individual class I MHC molecules. The actual binding was compared with the results obtained when analyzing the same peptides by two well-known, publicly available computer algorithms. We conclude that there is no strong correlation between actual and predicted binding when using predictive computer algorithms. Furthermore, we found a high number of false-negatives when using a predictive algorithm compared to simple scanning for the presence of primary anchor residues. We conclude that the peptide binding assay remains an important step in the identification of cytotoxic T lymphocyte (CTL) epitopes which can not be substituted by predictive algorithms.

A CTL assay requiring only 150 microliter of mouse blood

In this paper, we present a method for measuring antigen specific cytotoxic T lymphocyte (CTL) activity from individual mouse peripheral blood samples without animal sacrifice. Peripheral blood cells are stimulated in vitro with a cocktail of antigen, cytokines, costimulatory molecules and irradiated feeder cells resulting, 7 days later, in a readily detectable antigen specific signal from a well plated under limiting dilution conditions. This highly sensitive and antigen specific assay is more efficient than conventional CTL assays and thus increases the number of mice that can be tested in a single assay. Since blood samples can be assayed from an individual mouse at multiple times during the course of an in vivo study, the assay can facilitate and strengthen correlative studies on CTL responses and in vivo results.

Conserved MHC class I peptide binding motif between humans and rhesus macaques

Since the onset of the HIV pandemic, the use of nonhuman primate models of infection has increasingly become important. An excellent model to study HIV infection and immunological responses, in particular cell-mediated immune responses, is SIV infection of rhesus macaques. CTL epitopes have been mapped using SIV-infected rhesus macaques, but, to date, a peptide binding motif has been described for only one rhesus class I MHC molecule, Mamu-A*01. Herein, we have established peptide-live cell binding assays for four rhesus MHC class I molecules: Mamu-A*11, -B*03, -B*04, and -B*17. Using such assays, peptide binding motifs have been established for all four of these rhesus MHC class I molecules. With respect to the nature and spacing of crucial anchor positions, the motifs defined for Mamu-B*04 and -B*17 present unique features not previously observed for other primate species. The motifs identified for Mamu-A*11 and -B*03 are very similar to the peptide binding motifs previously described for human HLA-B*44 and -B*27, respectively. Accordingly, naturally processed peptides derived from HLA-B*44 and HLA-B*27 specifically bind Mamu-A*11 and Mamu-B*03, respectively, indicating that conserved MHC class I binding capabilities exist between rhesus macaques and humans. The definition of four rhesus MHC class I-specific motifs expands our ability to accurately detect and quantitate immune responses to MHC class I-restricted epitopes in rhesus macaques and to rationally design peptide epitope-based model vaccine constructs destined for use in nonhuman primates.

Induction of cytotoxic T-cell responses by gene gun DNA vaccination with minigenes encoding influenza A virus HA and NP CTL-epitopes

Cytotoxic T-lymphocyte (CTL) response is an important component of anti-viral immunity. CTLs are specific to short peptides presented by MHC-I molecules and immunisation with the exact peptide sequence introduced in the cytosol is therefore a minimal approach, which potentially affords a high degree of controllability. We have examined the induction of murine CTL's by this approach using DNA plasmid minigene vaccines encoding known mouse K(k) minimal CTL epitopes (8 amino acids) from the influenza A virus hemagglutinin and nucleoprotein. We here report that such an approach is feasible and that wild type influenza virus flanking amino acid sequences can influence the CTL response but are not essential for optimal CTL induction. We also examined the effect of different new amino acid sequences flanking the CTL epitopes. In one version, two CTL epitopes were linked together as 'string of beads'. This did not improve CTL induction. In another version, one CTL epitope was inserted into a known T-helper protein (HBsAg). This did significantly augment the response probably due to immunological help from HBsAg Th epitopes. Finally, the CTL inducing minigene DNA vaccines were compared with Flu-induced CTL responses and tested for their protective effect against a lethal influenza A virus infection in mice and no effect was found. We conclude that a specific and highly directed CTL induction is possible by unlinked minigene DNA immunisation, but that CTL induction solely is not always sufficient to provide protection.

Phosphorylated Peptides Can Be Transported by TAP Molecules, Presented by Class I MHC Molecules, and Recognized by Phosphopeptide-Specific CTL

CTL recognize short peptide fragments presented by class I MHC molecules. In this study, we examined the effect of phosphorylation on TAP transport, binding to class I MHC molecules, and recognition by CTL of peptide fragments from known phosphorylated oncogene proteins or virus phosphoproteins. We show that phosphopeptides can be efficiently transported from the cytosol to the endoplasmic reticulum by the TAP. Furthermore, we show that phosphorylation can have a neutral, negative, or even a positive effect on peptide binding to class I MHC. Finally, we have generated phosphopeptide-specific CTL that discriminate between the phosphorylated and the nonphosphorylated versions of the peptide. We conclude that phosphopeptide-specific CTL responses are likely to constitute a subset of the class I MHC-restricted CTL repertoire in vivo.

Cross-Recognition of Two Middle T Protein Epitopes by Immunodominant Polyoma Virus-Specific CTL

We recently identified the immunodominant epitope for polyoma virus-specific CTL as the Dk-associated peptide MT389–397 derived from the middle T (MT) viral oncoprotein. Another Dk-restricted peptide corresponding to residues 236–244 of MT was recognized by nearly all MT389–397-reactive CTL clones, but required concentrations at least 2 logs higher to sensitize syngeneic target cells for lysis. Except for identity at the three putative Dk-peptide anchor residues, MT236–244 shares no homology with MT389–397. Using a novel europium-based class I MHC-peptide binding immunoassay, we determined that MT236–244 bound Dk 2–3 logs less well than MT389–397. Infection with a mutant polyoma virus whose MT is truncated just before the MT389–397 epitope or immunization with MT389–397 or MT236–244 peptides elicited CTL that recognized both MT389–397 and MT236–244. Importantly, infection with a polyoma virus lacking MT389–397 and mutated in an MT236–244 Dk anchor position induced polyoma virus-specific CTL recognizing neither MT389–397 nor MT236–244 epitopes. Despite predominant usage of the Vβ6 gene segment, MT389–397/MT236–244 cross-reactive CTL clones possess diverse complementarity-determining region 3β domains; this is functionally reflected in their heterogeneous recognition patterns of alanine-monosubstituted MT389–397 peptides. Using Dk/MT389–397 tetramers, we directly visualized MT236–244 peptide-induced TCR down-modulation of virtually all MT389–397-specific CD8+ T cells freshly explanted from polyoma-infected mice, suggesting that a single TCR recognizes both Dk-restricted epitopes. The availability of immunodominant epitope-specific CTL capable of recognizing a second epitope in MT, a viral protein essential for tumorigenesis, may serve to amplify the CTL response to the immunodominant epitope and prevent the emergence of immunodominant epitope-loss viruses and virus-induced tumors.

The Importance of Pairwise Interactions Between Peptide Residues in the Delineation of TCR Specificity

A minimal, nonamer epitope (TEMEKEGKI) from the reverse transcriptase protein of HIV-1, restricted by H-2Kk, was identified and the function of individual residues determined. Besides classical anchor residues at positions 2 and 9, methionine at position 3 was identified as an important MHC anchor and improved binding of a different (malarial) nonamer epitope to H-2Kk, albeit while also abolishing CTL recognition. Lysine at position 5 was replaceable by alanine for CTL raised against wild-type peptide but abolished recognition for CTL raised against the variant 5ALA peptide, indicating a unidirectional cross-reactivity. Interestingly, one CTL line raised against the 5ALA substituted peptide was permissive for a double substitution at positions 5 and 6, in which lysine was permissive at position 5 only if the adjacent glutamic acid was replaced by alanine. Extensive analysis revealed three distinct patterns of responses with peptides doubly substituted in this region: recognition of both single substitutions but not the double substitution, recognition of only one single substitution but also the double substitution, or recognition of both single substitutions and the double substitution. A second complementary substitution can therefore restore function lost through a first substitution. Thus, no residue acts independently of its neighbors, and pairs of substitutions may give results not predictable from the effects of each taken singly. This finding may have bearing on viral infections (such as HIV), in which the accumulation of two mutations in the epitope may lead to the reengagement of memory CTL previously silenced by the initial mutation.

Dissociation of Proteasomal Degradation of Biosynthesized Viral Proteins from Generation of MHC Class I-Associated Antigenic Peptides

To study the role of proteasomes in Ag presentation, we analyzed the effects of proteasome inhibitors Cbz-Leu-Leu-Leucinal and lactacystin on the ability of mouse fibroblast cells to present recombinant vaccinia virus gene products to MHC class I-restricted T cells. The effects of the inhibitors depended on the determinant analyzed. For influenza virus nucleoprotein (NP), presentation of the immunodominant Kk-restricted determinant (NP50–57) was marginally inhibited, whereas presentation of the immunodominant Kd-restricted determinant (NP147–155) was enhanced, particularly by lactacystin. Biochemical purification of peptides confirmed that lactacystin enhanced the generation of Kd-NP147–155 complexes fourfold. Lactacystin also enhanced the recovery of one Kd-restricted vaccinia virus determinant from HPLC fractions, while inhibiting recovery of another. The inhibitors were used at sufficient concentrations to block presentation of biosynthesized full-length OVA and to completely stabilize a rapidly degraded chimeric ubiquitin-NP fusion protein. Strikingly, presentation of antigenic peptides from this protein was unaffected by proteasome inhibitors. We also observed that proteasome inhibitors induced expression of cytosolic and endoplasmic reticulum stress-responsive proteins. These data demonstrate first that the processes of protein degradation and generation of antigenic peptides from cytosolic proteins can be dissociated, and second that effects of proteasome inhibitors on Ag presentation may reflect secondary effects on cellular metabolism.

Recombinant viruses expressing a human malaria antigen can elicit potentially protective immune CD8+ responses in mice

Extensive studies on protective immunity to rodent malaria provided the basis for the current experiments in which mice were immunized with recombinant (re) influenza and vaccinia viruses expressing selected sequences of the circumsporozoite (CS) protein of the human malaria parasite Plasmodium falciparum. Mice of different H-2 haplotypes immunized with re influenza viruses expressing the immunodominant B cell epitope of this CS protein produced high titers of antibodies to the parasite. A cytotoxic T lymphocyte epitope of the CS protein of P. falciparum, PF3, recognized by CD8+ T cells of H-2(k) mice, was expressed in a re vaccinia virus (VacPf) and a re influenza virus (FluPf). Immunization of mice with either FluPf or VacPf elicited a modest CS-specific CD8+ T cell response detected by interferon gamma secretion of individual immune cells. Priming of mice with FluPf, followed by a booster with VacPf, resulted in a striking enhancement of this T cell response. The reverse protocol, i.e., priming with VacPf followed by a booster with FluPf, failed to enhance the primary response. VacPf also greatly enhanced the primary response of mice injected with P. falciparum sporozoites or with a lipopeptide containing PF3. A booster with FluPf also amplified the response of lipopeptide- or sporozoite-primed mice but less than a VacPf booster did. Although mice are not susceptible to infection by P. falciparum sporozoites, we demonstrated that administration of two distinct immunogens expressing PF3 elicited activated, extravasating CS-specific T cells that protected against an intracerebral VacPf challenge.

Resistance to Polyoma Virus-Induced Tumors Correlates with CTL Recognition of an Immunodominant H-2Dk-Restricted Epitope in the Middle T Protein

The natural mouse pathogen polyoma virus is highly oncogenic in H-2k mice carrying the endogenous superantigen encoded by the mouse mammary tumor provirus Mtv-7. This superantigen results in deletion of Vβ6 TCR-expressing polyoma-specific CD8+ CTL, which appear to be critical effectors against polyoma tumorigenesis. Here we have isolated cloned lines of CD8+ T cells from resistant (i.e., Mtv-7−) H-2k mice that specifically lyse syngeneic polyoma virus-infected cells and polyoma tumor cells. Nearly all these CTL clones express Vβ6 and are restricted in their recognition of virus-infected cells by H-2Dk. Screening a panel of synthetic peptides predicted to bind to Dk, for which no consensus peptide binding motif is known, we identified a peptide corresponding to a nine-amino acid sequence in the carboxyl-terminus of the middle T (MT) protein (amino acids 389–397) that was recognized by all the Vβ6+CD8+ CTL clones. The inability of MT389–397-reactive CTL to recognize cells infected with a mutant polyoma virus encoding a MT truncated just proximal to this sequence indicates that MT389-397 is a naturally processed peptide. The frequencies of precursor CTL specific for polyoma virus and MT389–397 peptide were similar, indicating that MT389–397 is the immunodominant epitope in H-2k mice. In addition, polyoma-infected resistant mice possess a 10- to 20-fold higher MT389-397-specific precursor CTL frequency than susceptible mice. This highly focused CTL response to polyoma virus provides a valuable animal model to investigate the in vivo activity of CTL against virus-induced neoplasia.

An improved assembly assay for peptide binding to HLA-B*2705 and H-2K(k) class I MHC molecules

The assembly assay for peptide binding to class I major histocompatibility complex (MHC) is based on the ability to stabilise MHC class I molecules from mutant cell lines by the addition of suitable peptides. Such cell lines lack a functional transporter associated with antigen presentation (TAP) and as a result accumulate empty, unstable class I molecules in the ER. These dissociate rapidly in cell lysates unless they are stabilised by the addition of an appropriate binding peptide during lysis. The extent of stabilisation of class I molecules is directly related to the binding affinity of the added peptide. However, some MHC class I molecules, including HLA-B * 2705 and H-2Kk are unusually stable in their peptide-receptive state making them inappropriate for analysis using this assay or assays which depend on the ability of peptides to stabilise MHC class I molecules at the cell surface. Here we present an improved method that permits reliable measurements of peptide binding to such class I MHC molecules that are unusually stable in the absence of peptide. Cells are lysed in the presence of peptide and incubated at 4 degrees C. After 2 h, during which peptide binding to empty MHC molecules occurs, the lysate is heated to a temperature which preferentially destabilises those MHC molecules that remain empty. We have used this technique to assay peptide binding to HLA-B * 2705, as well as to the murine allele H-2Kk which also displays a stable phenotype when transfected into TAP-deficient T2 cells and show that this method represents a marked improvement over previous methods in terms of lower background signal and higher recovery of peptide bound molecules.

The proteasome-specific inhibitor lactacystin blocks presentation of cytotoxic T lymphocyte epitopes in human and murine cells

We describe the effect of the proteasome specific inhibitor lactacystin on the metabolic stability of influenza nucleoprotein (NP) and on the generation of antigens presented by human and murine class I molecules of the major histocompatibility complex to cytotoxic T lymphocytes (CTL). We show that cells treated with lactacystin fail to present influenza antigens to influenza-specific CTL, but retain the capacity to present defined epitopes expressed as peptides intracellularly by recombinant vaccinia viruses. This block in antigen presentation can be overcome by expressing the viral protein within the lumen of the endoplasmic reticulum, confirming the specificity of lactacystin for cytosolic proteases. We also show that the effect of lactacystin on antigen presentation correlates with the block of breakdown of a rapidly degraded form of the influenza NP linked to ubiquitin. These results demonstrate that proteasome-dependent degradation plays an important role in the cytosolic generation of CTL epitopes.

Peptide binding specificity of major histocompatibility complex class I resolved into an array of apparently independent subspecificities: quantitation by peptide libraries and improved prediction of binding

Considerable interest has focused on understanding how major histocompatibility complex (MHC) specificity is generated and characterizing the specificity of MHC molecules with the ultimate goal being to predict peptide binding. We have used a strategy where all possible peptides of a particular size are distributed into positional scanning combinatorial peptide libraries (PSCPL) to develop a highly efficient, universal and unbiased approach to address MHC specificity. The PSCPL approach appeared qualitatively and quantitatively superior to other currently used strategies. The average effect of any amino acid in each position was quantitated, allowing a detailed description of extended peptide binding motifs including primary and secondary anchor residues. It also identified disfavored residues which were found to be surprisingly important in shaping MHC class I specificity. Assuming that MHC class I specificity is the result of largely independently acting subsites, the binding of unknown peptides could be predicted. Conversely, this argues that MHC class I specificities consist of an array of subspecificities acting in a combinatorial mode.

Immunodominance of major histocompatibility complex class I-restricted influenza virus epitopes can be influenced by the T-cell receptor repertoire

We have used T-cell receptor beta-chain transgenic mice to determine the effects of a limited T-cell receptor repertoire on major histocompatibility complex class I-restricted epitope selection during the course of an influenza virus infection. Analysis of T-cell hybridomas generated from wild-type and transgenic mice demonstrated that the viral epitope recognized depended on the available T-cell receptor repertoire. Wild-type T-cell hybridomas recognized epitopes derived from the nucleoprotein and basic polymerase molecules, whereas hybridomas generated from transgenic mice recognized epitopes derived from the nonstructural protein and the matrix protein. There was no overlap in specificity between the two panels of hybridomas. This reciprocal pattern of specificity was also apparent in cytoxicity assays with brochoalveolar lavage cells isolated from the lungs of influenza virus-infected mice. T-cell receptor usage in the transgenic hybridomas was very restricted, with only one V alpha element used for ech of the two viral epitopes recognized. In the case of the hybridomas reactive to the nonstructural protein, sequence analysis showed that they all expressed V alpha 4J alpha 32 chains associated with the same junctional amino acids (Leu-Leu) that were encoded by five different nucleotide sequences, indicating a strong selection for T-cell receptor usage. Taken together, these data demonstrate that the available T-cell receptor repertoire can have a profound effect on the immunodominance of class I-restricted epitopes during a viral infection.

Identification of Kk-restricted T-cell epitope within influenza virus nucleoprotein

In order to determine the epitope structure in peptide NP50-63, which has been reported to be the only Kk-restricted T-cell antigen within the influenza virus (A/PR/8/34) nucleoprotein, a series of 13 peptides truncated from C- and N-termini of NP50-63 were synthesized and their sensitizing activities against Kk-restricted nucleoprotein-specific cytotoxic T lymphocytes (CTLs) were examined. One of the 13 peptides, NP50-57, sensitized L929 cells at the nM level, which was 100-1000 times lower in concentration than that at which the other peptides sensitized these cells. The presence of NP50-57 in A/PR/8/34-infected L929 cells was also investigated. Acid extracts of virus-infected cells were separated on a reverse-phase HPLC column and then anion-exchange column. By both separations, only one peak of sensitizing activity against nucleoprotein-specific CTLs was observed. The position of the peak coincided with that of the elution of NP50-57. These results strongly suggest that NP50-57 is the natural epitope in the antigenic structure, NP50-63.

LMP2+ proteasomes are required for the presentation of specific antigens to cytotoxic T lymphocytes

BACKGROUND

Major histocompatibility complex (MHC) class I molecules present short peptides generated by intracellular protein degradation to cytotoxic T lymphocytes (CTL). The multisubunit, non-lysosomal proteinases known as proteasomes have been implicated in the generation of these peptides. Two interferon-gamma (IFN-gamma)-inducible proteasome subunits, LMP2 and LMP7, are encoded within the MHC gene cluster in a region associated with antigen presentation. The incorporation of these LMP subunits into proteasomes may alter their activity so as to favour the generation of peptides able to bind to MHC class I molecules. It has been difficult, however, to demonstrate a specific requirement for LMP2 or LMP7 in the presentation of peptide epitopes to CTL.

RESULTS

We describe a T-cell lymphoma, termed SP3, that displays a novel selective defect in MHC class I-restricted presentation of influenza virus antigens. Of the MHC-encoded genes implicated in the class I pathway, only LMP2 is underexpressed in SP3 cells. Expression of IFN-gamma in transfected SP3 cells simultaneously restores LMP2 expression and antigen presentation to CTL. Expression of antisense-LMP2 mRNA in these IFN-gamma-transfected cells selectively represses antigen recognition and the induction of surface class I MHC expression. Moreover, the expression of this antisense-LMP2 mRNA in L929 fibroblast cells, which constitutively express LMP2 and have no presentation defect, blocks the presentation of the same influenza virus antigens that SP3 cells are defective in presenting.

CONCLUSIONS

Our results show that the LMP2 proteasome subunit can directly influence both MHC class I-restricted antigen presentation and class I surface expression.

Antigen processing in vivo and the elicitation of primary CTL responses

CD8+ T lymphocytes (TCD8+) play an important role in cellular immune responses. TCD8+ recognize MHC class I molecules complexed to peptides of 8 to 10 residues derived largely from cytosolic proteins. Proteins are generally thought to be fragmented in the cytoplasm and delivered to nascent class I molecules in the endoplasmic reticulum (ER) by a peptide transporter encoded by the MHC. To explore the extent to which TCD8+ induction in vivo is limited by proteolysis or peptide transport into the ER, mice were immunized with recombinant vaccinia viruses containing mini-genes encoding antigenic peptides (bypassing the need for proteolysis), or these peptides with a NH2-terminal ER insertion sequence (bypassing the requirements for both proteolysis and transport). Additionally, mice were immunized with recombinant vaccinia viruses encoding rapidly degraded fragments of proteins. We report that limitations in induction of TCD8+ responses vary among Ags: for some, full length proteins are as immunogenic as other forms tested; for others, maximal responses are induced by peptides or by peptides targeted to the ER. Most importantly, in every circumstance examined, targeting peptides to the ER never diminished, and in some cases greatly enhanced, the TCD8+ immune response and provide an important alternative strategy in the design of live viral or naked DNA vaccines for the treatment of cancer and infectious diseases.

Processing of major histocompatibility class I-restricted antigens in the endoplasmic reticulum

We have introduced long precursor peptides directly into the endoplasmic reticulum (ER) of a mutant cell line (T2-Db) that lacks the ability to transport peptides from the cytosol to the ER in a transporter associated with antigen processing (TAP) dependent way. This was done by expressing various influenza A-derived peptides containing the naturally processed epitope ASNENMDAM (366-374) preceded by the influenza hemagglutinin ER translocation sequence. Peptides derived from these minigenes that became associated with Db were isolated and identified by combined reversed phase liquid chromatography and detection by cytotoxic T lymphocytes. Our results establish that NH2-terminal extensions of at least 40 residues can be trimmed from peptides entering the ER, but that proteolysis of larger proteins may be limited.

Characterization of defectiveness in endogenous antigen presentation of novel murine cells established from methylcholanthrene-induced fibrosarcomas

Three cell lines (4A1, 4C2 and 6D1 cells) derived from fibrosarcoma induced by the inoculation of 3-methylcholanthrene into C3H/HeN (H-2k) mice were examined for their ability to present antigens to CD8+ cytotoxic T lymphocytes (CTL). 6D1 and 4C2 cells were deficient in presenting endogenously synthesized influenza virus antigens to CTL, but they were able to present antigens when they were sensitized with a synthetic epitope peptide. The expression of the H-2 Kk gene in 4C2 and 6D1 cells was much reduced and was detectable only with Northern blot hybridization. The expression of two transporter genes (TAP1 and TAP2), examined by Northern hybridization, was also reduced in both cells, and negligible particularly in 4C2 cells. Interferon-gamma (IFN-gamma) treatment of these cells induced expression of Kk, TAP1 and TAP2 genes and rescued the defect of class I-restricted antigen presentation in 4C2 and 6D1 cells. Even after this treatment, however, antigen-presentation capability of 4C2 cells was still much lower than that of normal 4A1 cells. This finding suggests that 4C2 cells might have an additional defective gene(s), whose products are involved in the processing of class I-restricted antigen, besides the Kk and TAP genes, and this may explain the difficulty of 4C2 cells to induce tumour-specific immunity, as described previously. To our knowledge, the 4C2 cell is the first tumour cell postulated to have more than three defective genes involved in class I-restricted antigen presentation.

Preformed purified peptide/major histocompatibility class I complexes are potent stimulators of class I-restricted T cell hybridomas

A panel of antigen-specific, major histocompatibility complex class I-restricted T cell hybridomas has been generated to examine the capacity of peptide/class I complexes to stimulate T cells at the molecular level. Peptide/class I complexes were generated in detergent solution, purified and quantitated. Latex particles were subsequently coated with known amounts of preformed complexes and used to stimulate the T cell hybridomas. Stimulation was specific, i.e. only the appropriate peptide/class I combination were stimulatory, and quite sensitive, i.e. as little as 300 complexes per bead could be detected by the T cells. Preformed complexes were about 500,000 times more potent than free peptide in terms of T cell stimulation, demonstrating the physiological relevancy of the biochemically generated complexes. Surprisingly, the majority (including the most sensitive of the hybridomas) had lost CD8 expression, suggesting that antigen-specific stimulation of class I-restricted T cell hybridomas, as assessed by IL-2 release, does not depend on CD8.

Virus-specific CD8+ T-cell responses in mice transgenic for a T-cell receptor beta chain selected at random

The consequences of severely limiting the T-cell receptor (TCR) repertoire available for the response to intranasal infection with an influenza A virus or with Sendai virus have been analyzed by using H-2k mice (TG8.1) transgenic for a TCR beta-chain gene (V beta 8.1D beta 2J beta 2.3C beta 2). Analyzing the prevalence of V beta 8.1+ CD8+ T cells in lymph node cultures from nontransgenic (non-TG) H-2k controls primed with either virus and then stimulated in vitro with the homologous virus or with anti-CD3 epsilon showed that this TCR is not normally selected from the CD8+ T-cell repertoire during these infections. However, the TG8.1 mice cleared both viruses and generated virus-specific effector cytotoxic T lymphocytes (CTL) and memory CTL precursors, though the responses were delayed compared with the non-TG controls. Depletion of the CD4+ T-cell subset had little effect on the course of influenza virus infection but substantially slowed the development of the Sendai virus-specific CTL response and virus elimination in both the TG8.1 and non-TG mice, indicating that CD4+ helpers are promoting the CD8+ T-cell response in the Sendai virus model. Even so, restricting the available T-cell repertoire to lymphocytes expressing a single TCR beta chain still allows sufficient TCR diversity for CD8+ T cells (acting in the presence or absence of the CD4+ subset) to limit infection with an influenza A virus and a parainfluenza type 1 virus.

Human transporters associated with antigen processing possess a promiscuous peptide-binding site

The peptide selectivity of the human transporters associated with antigen processing (TAP) was investigated using a panel of peptides of varying length and sequence. Peptides were assayed for their ability to compete for the translocation of a labeled reporter peptide containing an N-linked glycosylation acceptor site in Streptolysin O (SLO)-permeabilized cells. We find that human TAP is very promiscuous for peptides in the 8-12 amino acid range, while showing increased selectivity and lower translocation efficiency for peptides in the 13-30 amino acid range. The minimum peptide length appears to be 8 amino acids, while the maximum length appears to be approximately 25 amino acids. Furthermore, a photoactive peptide analogue was synthesized that can photolabel TAP molecules. Using this analogue, we showed that an ATP-independent peptide-binding site exists on TAP, and that competition for translocation reflects competition for peptide binding.

Structure of peptides associated with MHC class I molecules

Recent progress in understanding the structures of MHC class I molecules and the peptides that they bind has led to a generalized model for peptide binding, and an understanding of allelic specificity. Prediction on the basis of motifs and new techniques for peptide analysis have recently resulted in the identification of several peptides that comprise epitopes for antigen-specific T cells.

A quantitative assay to measure the interaction between immunogenic peptides and purified class I major histocompatibility complex molecules

A direct and sensitive biochemical assay to measure the interaction in solution between peptides and affinity-purified major histocompatibility complex (MHC) class I molecules has been generated. Specific binding reflecting the known class I restriction of cytotoxic T cell responses was obtained. Adding an excess of beta 2-microglobulin (beta 2m) significantly increased the rate of peptide association, but it did not affect the rate of dissociation. Binding was complicated by a rapid and apparently irreversible loss of functional MHC class I at 37 degrees C which might limit the life span of empty MHC class I thereby preventing the inadvertent exchange of peptides at the target cell surface. All class I molecules tested bound peptides of the canonical octa- to nona-meric length. However, one class I molecule, Kk, also bound peptides, which were much longer suggesting that the preference of class I molecules for short epitopes is not absolute and may be caused by factors other than the peptide-MHC class I binding event itself.

H-2Dd exploits a four residue peptide binding motif

We have characterized the amino acid sequences of over 20 endogenous peptides bound by a soluble analog of H-2Dd, H-2Dds. Synthetic analogs corresponding to self, viral, tumor, or motif peptides were then tested for their ability to bind to H-2Dd by serologic epitope induction assays using both purified soluble protein and cell surface H-2Dd. The dominant primary sequence motif included glycine at position 2, proline at position 3, and a hydrophobic COOH terminus: leucine, isoleucine, or phenylalanine at position 9 or 10. Ancillary support for high affinity binding was contributed by a positively charged residue at position 5. Three-dimensional computer models of H-2Dds/peptide complexes, based on the crystallographic structure of the human HLA-B27/peptide complex, showed that the basic residue at position 5 was in position to form a salt bridge with aspartic acid at position 156, a polymorphic residue of the H-2Dd heavy (H) chain. Analysis of 28 such models, including 17 based on nonamer self-peptides, revealed considerable variation in the structure of the major histocompatibility complex (MHC) surrounding peptide residue 1, depending on the size and charge of the side chain. Interactions between the side chains of peptide residues 5 and 7, and 6 and 8 commonly occurred. Those peptide positions with limited sequence variability and least solvent accessibility may satisfy structural requirements for high affinity binding of the peptide to the MHC class I H chain, whereas the highly variable positions of the peptide (such as positions 4, 6, and 8) may contribute more to the T cell epitopes.

Presentation of numerous viral peptides to mouse major histocompatibility complex (MHC) class I-restricted T lymphocytes is mediated by the human MHC-encoded transporter or by a hybrid mouse-human transporter

The major histocompatibility complex-encoded transporter associated with antigen processing (TAP) is required for the efficient presentation of cytosolic antigens to class I-restricted T cells. TAP is thought to be formed by the interaction of two gene products, termed TAP1 and TAP2. We find that TAPs consisting either of human subunits, or mouse TAP1 and human TAP2, facilitate the presentation of numerous defined viral peptides to mouse class I-restricted T cells. As human and mouse TAP2 and TAP1 differ in 23 and 28% of their residues, respectively, this indicates that TAP1 and TAP2 can form a functional complex with partners considerably different from those they coevolved with. Moreover, these findings indicate that widely disparate TAPs facilitate delivery of the same peptides to class I molecules. These findings suggest that TAP polymorphism does not greatly influence the types of peptides presented to the immune system.

Protective cross-reactive epitope on the nonstructural protein NS1 of influenza A virus

We reported previously that adoptive immunization with an influenza A virus NS1-specific H-2Ld-restricted, cross-reactive, CTL clone A-11 established by stimulation with A/PR/8/34 virus (H1N1) reduced lung virus titers in mice challenged with virus in vivo (Virology 178:174-179, 1990). Using a set of recombinant vaccinia virus constructs containing truncated portions of the NS gene we have localized this cross-protective CTL epitope to the N-terminal region of the NS1 protein. This region of NS1 is active in inducing CD8+ CTL in vivo because virus-stimulated BALB/c immune spleen cells in bulk cultures also recognized the N-terminal region of the NS1 protein.