Class I major histocompatibility complex-restricted cytolytic T lymphocytes recognize a limited number of sites on the influenza hemagglutinin

Evolutionary pressures rendered by animal husbandry practices for avian influenza viruses to adapt to humans

Commercial poultry operations produce and crowd billions of birds every year, which is a source of inexpensive animal protein. Commercial poultry is intensely bred for desirable production traits, and currently presents very low variability at the major histocompatibility complex. This situation dampens the advantages conferred by the MHC’s high genetic variability, and crowding generates immunosuppressive stress. We address the proteins of influenza A viruses directly and indirectly involved in host specificities. We discuss how mutants with increased virulence and/or altered host specificity may arise if few class I alleles are the sole selective pressure on avian viruses circulating in immunocompromised poultry. This hypothesis is testable with peptidomics of MHC ligands. Breeding strategies for commercial poultry can easily and inexpensively include high variability of MHC as a trait of interest, to help save billions of dollars as a disease burden caused by influenza and decrease the risk of selecting highly virulent strains.

Peptidylarginine Deiminase 2 in Murine Antiviral and Autoimmune Antibody Responses

The peptidylarginine deiminases (PADs) and the citrullinated proteins that they generate have key roles in innate immunity and rheumatoid arthritis, an inflammatory arthritis with antibodies that target citrullinated proteins. However, the importance of PADs, particularly PAD2, in the adaptive immune response, both normal and pathogenic, is newly emerging. In this study, we evaluated a requirement for PAD2 in the antibody response in collagen-induced arthritis (CIA), a T and B cell-driven murine model of rheumatoid arthritis, and in the protective antibody response to murine influenza infection. Using PAD2-/- and PAD2+/+ mice on the DBA/1J background, we found that PAD2 is required for maximal anti-collagen antibody levels, but not collagen-specific plasma cell numbers, T cell activation or polarization, or arthritis severity in CIA. Also, we found that PAD2 is required not just for normal levels of persistent hemagglutination inhibiting antibodies but also for full protection from lethal influenza rechallenge. Together, these data provide evidence for a novel modest requirement for PAD2 in a normal antiviral antibody response and in an abnormal autoantibody response in inflammatory arthritis.

Targets for the induction of protective immunity against influenza a viruses

The current pandemic caused by the new influenza A(H1N1) virus of swine origin and the current pandemic threat caused by the highly pathogenic avian influenza A viruses of the H5N1 subtype have renewed the interest in the development of vaccines that can induce broad protective immunity. Preferably, vaccines not only provide protection against the homologous strains, but also against heterologous strains, even of another subtype. Here we describe viral targets and the arms of the immune response involved in protection against influenza virus infections such as antibodies directed against the hemagglutinin, neuraminidase and the M2 protein and cellular immune responses directed against the internal viral proteins.

Identification and structural definition of H5-specific CTL epitopes restricted by HLA-A*0201 derived from the H5N1 subtype of influenza A viruses

The haemagglutinin (HA) glycoprotein of influenza A virus is a major antigen that initiates humoral immunity against infection; however, the cellular immune response against HA is poorly understood. Furthermore, HA-derived cytotoxic T-lymphocyte (CTL) epitopes are relatively rare in comparison to other internal gene products. Here, CTL epitopes of the HA serotype H5 protein were screened. By using in silico prediction, in vitro refolding and a T2 cell-binding assay, followed by immunization of HLA-A2.1/K(b) transgenic mice, an HLA-A*0201-restricted decameric epitope, RI-10 (H5 HA205-214, RLYQNPTTYI), was shown to elicit a robust CTL epitope-specific response. In addition, RI-10 and its variant, KI-10 (KLYQNPTTYI), were also demonstrated to be able to induce a higher CTL epitope-specific response than the influenza A virus dominant CTL epitope GL-9 (GILGFVFTL) in peripheral blood mononuclear cells of HLA-A*0201-positive patients who had recovered from H5N1 virus infection. Furthermore, the crystal structures of RI-10-HLA-A*0201 and KI-10-HLA-A*0201 complexes were determined at 2.3 and 2.2 A resolution, respectively, showing typical HLA-A*0201-restricted epitopes. The conformations of RI-10 and KI-10 in the antigen-presenting grooves in crystal structures of the two complexes show significant differences, despite their nearly identical sequences. These results provide implications for the discovery of diagnostic markers and the design of novel influenza vaccines.

Distinct contributions of vaccine-induced immunoglobulin G1 (IgG1) and IgG2a antibodies to protective immunity against influenza

Vaccination represents the most effective form of protection against influenza infection. While neutralizing antibodies are typically measured as a correlate of vaccine-induced protective immunity against influenza, nonneutralizing antibodies may contribute to protection or amelioration of disease. The goal of this study was to dissect the individual contributions of the immunoglobulin G1 (IgG1) and IgG2a antibody isotypes to vaccine-induced immunity against influenza virus. To accomplish this, we utilized an influenza vaccine regimen that selectively enhanced IgG1 or IgG2a antibodies by using either DNA or viral replicon particle (VRP) vectors expressing influenza virus hemagglutinin (HA) (HA-DNA or HA-VRP, respectively). After HA-DNA vaccination, neutralizing antibodies were detected by both in vitro (microneutralization) and in vivo (lung viral titer) methods and were associated with increased IgG1 expression by enzyme-linked immunosorbent assay (ELISA). Vaccination with HA-VRP did not strongly stimulate either neutralizing or IgG1 antibodies but did induce IgG2a antibodies. Expression of IgG2a antibodies in this context correlated with clearance of virus and increased protection against lethal influenza challenge. Increased induction of both antibody isotypes as measured by ELISA was a better correlate for vaccine efficacy than neutralization alone. This study details separate but important roles for both IgG1 and IgG2a expression in vaccination against influenza and argues for the development of vaccine regimens that stimulate and measure expression of both antibody isotypes.

Multiple synergizing factors contribute to the strength of the CD8+ T cell response against listeriolysin O

Immunodominance in CD8+ T cell responses against Listeria monocytogenes is a well-recognized but still not fully understood phenomenon. From listeriolysin, the major virulence factor of L. monocytogenes, only a single epitope, pLLO91-99, is presented by MHC class I molecules in BALB/c mice which dominates the cytotoxic T cell response against this bacterial pathogen. To obtain more insights into the molecular and cellular mechanisms underlying immunodominance of this particular epitope, we compared the various steps involved in the presentation and recognition of pLLO91-99 derived from a wild-type toxin with an equivalent epitope from a mutated toxin. This fully functional variant contains within the pLLO91-99 epitope a conservative isoleucine to alanine replacement at the C-terminal anchor residue which results in loss of antigenicity. The binding properties of the variant peptide to soluble Kd remained unaffected and cytotoxic T cells capable of recognizing the pLLO99A/Kd complex were detectable in BALB/c mice. However, such T cells required higher concentrations of antigen in order to be optimally activated in vitro. A comparison between the TAP translocation efficiency of wild-type and mutant peptide demonstrated that the mutation at the C-terminus leads to a reduced transportation rate. Furthermore, the amino acid substitution changes the in vitro proteasomal cleavage pattern, resulting in a reduced liberation of the correct peptide from a polypeptide precursor. Thus, in all assays employed the immunodominant epitope performs optimally while the variant was found to be inferior. The synergy of all these steps most likely is the decisive factor in the immunodominance of pLLO91-99.

Frequency, Specificity, and Sites of Expansion of CD8+T Cells during Primary Pulmonary Influenza Virus Infection

We have used intracellular cytokine staining and MHC class I tetramer binding in conjunction with granzyme B protease expression and in vivo BrdU uptake to characterize the primary murine CD8(+) T cell response to pulmonary influenza virus infection. We have observed that the majority (>90%) of the CD8(+) T cell response to the A/Japan/305/57 virus in the lung at the peak of the response (days 9-11) is directed to four epitopes (three dominant and one subdominant). Using induction of granzyme B as a surrogate to identify specific activated CD8(+) T cells, we found that an unexpectedly large fraction ( approximately 70%) of lung-infiltrating CD8(+) T cells expressed granzyme B on day 6 of infection when estimates by MHC tetramer/intracellular cytokine staining yielded substantially lower frequencies ( approximately 30%). In addition, by using intranasal administration of BrdU during infection, we obtained evidence for proliferative expansion of activated CD8(+) T cells in the infected lung early (days 5-7) in the primary response. These results suggest that the frequency and number of specific CTL present in the lung early in infection may be underestimated by standard detection methods, and primary CD8(+) T cell expansion may occur in both secondary lymphoid organs and the infected lung.

Accelerated migration of respiratory dendritic cells to the regional lymph nodes is limited to the early phase of pulmonary infection

Respiratory dendritic cells (RDC) are believed to play a central role in the induction of adaptive immune responses to pulmonary infection. Herein we examine the basal migration of RDC from the lungs to secondary lymphoid tissues and their enhanced maturation/migration after pulmonary infection/inflammation. We demonstrate that the accelerated migration of RDC to the draining peribronchial lymph nodes occurs only during the first 24 hr after pulmonary virus infection. RDC are refractory to further migration thereafter in spite of ongoing virus replication and pulmonary inflammation. We further demonstrate that induction of this RDC refractory state suppresses additional RDC mobilization to subsequent pulmonary virus infection and results in concomitant suppression of an antiviral pulmonary CD8(+) T cell response.

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

CD8(+) cytotoxic T lymphocytes (CTLs) recognize antigen in the context of major histocompatibility complex (MHC) class I molecules. Class I epitopes have been classified as dominant or subdominant depending on the magnitude of the CTL response to the epitope. In this report, we have examined the in vitro memory CTL response of H-2(d) haplotype murine CD8(+) T lymphocytes specific for a dominant and subdominant epitope of influenza hemagglutinin using activation marker expression and staining with soluble tetrameric MHC-peptide complexes. Immune CD8(+) T lymphocytes specific for the dominant HA204-210 epitope give rise to CTL effectors that display activation markers, stain with the HA204 tetramer, and exhibit effector functions (i.e., cytolytic activity and cytokine synthesis). In contrast, stimulation of memory CD8(+) T lymphocytes directed to the subdominant HA210-219 epitope results in the generation of a large population of activated CD8(+) T cells that exhibit weak cytolytic activity and fail to stain with the HA210 tetramer. After additional rounds of restimulation with antigen, the HA210-219-specific subdominant CD8(+) T lymphocytes give rise to daughter cells that acquire antigen-specific CTL effector activity and transition from a HA210 tetramer-negative to a tetramer-positive phenotype. These results suggest a novel mechanism to account for weak CD8(+) CTL responses to subdominant epitopes at the level of CD8(+) T lymphocyte differentiation into effector CTL. The implications of these findings for CD8(+) T lymphocyte activation are discussed.

Recognition of a sequestered self peptide by influenza virus-specific CD8+ cytolytic T lymphocytes

The Ag receptors on CD8+ CTL recognize foreign antigenic peptides associated with cell surface MHC class I molecules. Peptides derived from self proteins are also normally presented by MHC class I molecules. Here we report that an H-2Kd-restricted murine CD8+ CTL clone directed to an influenza hemagglutinin epitope can recognize a peptide derived from the murine mitochondrial aconitase enzyme in association with H-2Kd molecules. Surprisingly, this self peptide is not normally displayed on the cell surface associated with the restricting MHC class I molecule. Several lines of evidence suggest that this self peptide, although requiring association with the Kd molecule for CTL recognition, is not associated with this or other MHC class I allele under physiologic conditions in intact cells. Rather, it is sequestered in the cytoplasm associated with a carrier protein and is released only upon cell disruption. These results suggest a means of restricting the entry of self peptide into the class I pathway. In addition, this finding raises the possibility that self peptides sequestered within the cell can, after release from damaged cells, interact with MHC class I molecules on bystander cells and trigger autoimmune injury by virus-specific CTLs during viral infection.

Generation of IL-2-Dependent Cytolytic T Lymphocytes (CTLs) with Altered TCR Responses Derived from Antigen-Dependent CTL Clones

Ag-specific CD8+ CTL clones require TCR stimulation to respond to IL-2 for growth. Because IL-2 may be produced in the vicinity of CD8+ CTLs when Ag is limiting at the end of an immune response, we have examined the effect of culturing viral-specific CTL clones in IL-2 in the absence of antigenic stimulation. Limiting dilution analysis revealed a high precursor frequency for CTL clones derived from IL-2 propagation (termed CTL-factor dependent (FD)) that are dependent upon exogenous IL-2 for growth and survival and no longer require TCR stimulation to proliferate. Culturing CTL-FDs with infected splenocytes presenting Ag and IL-2 did not revert the clones but did lead to a TCR-induced inhibition of proliferation. The derived CTL-FDs have lost the ability to kill via the perforin/granule exocytosis mechanism of killing, although they express similar levels of TCR, CD3ε, CD8αβ, CD45, and LFA-1 compared with the parental clones. The CTL-FDs retain Fas ligand/Fas-mediated cytotoxicity, and IFN-γ production and regulate the expression of CD69 and IL-2Rα when triggered through the TCR. A parental CTL protected BALB/c mice from a lethal challenge of influenza virus, whereas a CTL-FD did not. These findings represent a novel regulatory function of IL-2 in vitro that, if functional in vivo, may serve to down-regulate cellular immune responses.

Comparative organization and function of the major histocompatibility complex of domesticated cattle

This review focuses on recent advances in research on the bovine major histocompatibility complex (BoLA), with specific reference to the genetic organization, polymorphism and function of the class II genes. The BoLA region is unlike the MHC of humans and mice in that a large inversion has moved several class II genes, including the TAP/LMP cluster, close to the centromere of bovine chromosome 23. Therefore, close linkage of MHC genes and other genes associated with the MHC in humans and mice does not appear to be required for normal immunological function. In cattle, polymorphism in the class IIa genes influences both the magnitude and the epitope specificity of antigen-specific T-cell responses to foot-and-mouth disease virus peptides. Disease association studies have demonstrated that BoLA alleles affect the subclinical progression of bovine leukemia virus (BLV) infection. This association is strongly correlated with the presence of specific amino acid motifs within the DRB3 antigen-binding domain. In addition to the practical significance of these findings, the association between BoLA and BLV provides a unique model to study host resistance to retrovirus infection in a non-inbred species. These studies contribute to our understanding of the evolution of the MHC in mammals, to the development of broadly effective vaccines, and to breeding strategies aimed at improving resistance to infectious diseases.

Gag protein epitopes recognized by ELA-A-restricted cytotoxic T lymphocytes from horses with long-term equine infectious anemia virus infection

Most equine infectious anemia virus (EIAV)-infected horses have acute clinical disease, but they eventually control the disease and become lifelong carriers. Cytotoxic T lymphocytes (CTL) are considered an important immune component in the control of infections with lentiviruses including EIAV, but definitive evidence for CTL in the control of disease in carrier horses is lacking. By using retroviral vector-transduced target cells expressing different Gag proteins and overlapping synthetic peptides of 16 to 25 amino acids, peptides containing at least 12 Gag CTL epitopes recognized by virus-stimulated PBMC from six long-term EIAV-infected horses were identified. All identified peptides were located within Gag matrix (p15) and capsid (p26) proteins, as no killing of target cells expressing p11 and p9 occurred. Each of the six horses had CTL recognizing at least one Gag epitope, while CTL from one horse recognized at least eight different Gag epitopes. None of the identified peptides were recognized by CTL from all six horses. Two nonamer peptide epitopes were defined from Gag p26; one (18a) was likely restricted by class I equine leukocyte alloantigen A5.1 (ELA-A5.1) molecules, and the other (28b-1) was likely restricted by ELA-A9 molecules. Sensitization of equine kidney target cells for CTLm killing required 10 nM peptide 18a and 1 nM 28b-1. The results demonstrated that diverse CTL responses against Gag epitopes were generated in long-term EIAV-infected horses and indicated that ELA-A class I molecules were responsible for the diversity of CTL epitopes recognized. This information indicates that multiple epitopes or whole proteins will be needed to induce CTL in horses with different ELA-A alleles in order to evaluate their role in controlling EIAV.

HLA-B27 Subtype Polymorphism and CTL Epitope Choice: Studies with EBV Peptides Link Immunogenicity with Stability of the B27:Peptide Complex

HLA-B27-restricted CTL responses to EBV are principally directed against two of the EBV nuclear Ags, EBNAs 3B and 3C. We have previously described a target epitope derived from EBNA 3C (residues 258–266, sequence RRIYDLIEL) that is immunodominant in the context of at least three different B27 subtypes, including B*2705 and B*2702. In this study, we show that this peptide binds well to B*2705 and B*2702 in a cell surface binding assay, and that the two B27:peptide complexes are relatively stable, with t1/2 of 20 and 37 h, respectively. We now identify another B27-restricted epitope derived from EBNA 3B (residues 243–253, sequence RRARSLSAERY), which again accords well with the B*2705/B*2702 consensus motifs, having an arginine residue at position 2 and a tyrosine residue at the carboxyl terminus. In this case, five of five B*2702-positive donors respond to the epitope, whereas there was no response in any B*2705-positive donor studied. This peptide binds at least as well to B*2705 as to its restriction element B*2702; however, the two class I:peptide complexes show marked differences in stability, with t1/2 of 9 and 42 h, respectively. Thus, the stability of B27:peptide complexes can vary markedly between different B27 subtypes in ways that may not be apparent from cell surface binding assays and cannot be predicted from currently known peptide consensus motifs, yet which may critically influence CTL epitope choice.

Selective Induction of CD8+ Cytotoxic T Lymphocyte Effector Function by Staphylococcus Enterotoxin B

Upon encounter with its antigenic stimulus, CTL characteristically proliferate, produce cytokines, and lyse the Ag-presenting cell in an attempt to impede further infection. Superantigens are extremely efficient immunostimulatory proteins that promote high levels of proliferation and massive cytokine production in reactive T cells. We compared the activation of murine influenza-specific CD8+ CTL clones stimulated with either influenza peptide or the superantigen staphylococcus enterotoxin B (SEB). We found that influenza peptide/MHC and SEB appeared equally capable of eliciting proliferation and IFN-γ production. However, while influenza peptide/MHC elicited both perforin- and Fas ligand (FasL)/Fas (CD95L/CD95)-mediated cytolytic mechanisms, SEB was unable to trigger perforin-mediated cytolysis or serine esterase release. Examination of intracellular Ca2+ mobilization events revealed that the ability to trigger intracellular Ca2+ flux was not comparable between influenza peptide and SEB. SEB stimulated only a small rise in levels of intracellular Ca2+, at times indistinguishable from background. These findings indicate that the short-term cytolytic potential of superantigen-activated CD8+ CTL clones appears to be restricted to FasL/Fas (CD95L/CD95) mediated cytolysis.

Ca2+ signaling modulates cytolytic T lymphocyte effector functions

Cytolytic T cells use two mechanisms to kill virally infected cells, tumor cells, or other potentially autoreactive T cells in short-term in vitro assays. The perforin/granule exocytosis mechanism uses preformed cytolytic granules that are delivered to the target cell to induce apoptosis and eventual lysis. FasL/Fas (CD95 ligand/CD95)-mediated cytolysis requires de novo protein synthesis of FasL by the CTL and the presence of the death receptor Fas on the target cell to induce apoptosis. Using a CD8(+) CTL clone that kills via both the perforin/granule exocytosis and FasL/Fas mechanisms, and a clone that kills via the FasL/Fas mechanism only, we have examined the requirement of intra- and extracellular Ca2+ in TCR-triggered cytolytic effector function. These two clones, a panel of Ca2+ antagonists, and agonists were used to determine that a large biphasic increase in intracellular calcium concentration, characterized by release of Ca2+ from intracellular stores followed by a sustained influx of extracellular Ca2+, is required for perforin/granule exocytosis. Only the sustained influx of extracellular Ca2+ is required for FasL induction and killing. Thapsigargin, at low concentrations, induces this small but sustained increase in [Ca2+]i and selectively induces FasL/Fas-mediated cytolysis but not granule exocytosis. These results further define the role of Ca2+ in perforin and FasL/Fas killing and demonstrate that differential Ca2+ signaling can modulate T cell effector functions.

A lung-specific neo-antigen elicits specific CD8+ T cell tolerance with preserved CD4+ T cell reactivity. Implications for immune-mediated lung disease

The A/Japan/57 influenza hemagglutinin (HA) was expressed in BALB/c mice under the transcriptional control of the surfactant protein C (SP-C) promoter, resulting in expression of HA in type II alveolar epithelial cells, as well as low level variable expression in other tissues, including the thymus in some of the founder lines. Transgenic animals were able to recover from infection with A/Japan/57 influenza, and they were able to mount antibody responses to A/Japan/57 HA in titers similar to wild type. We therefore tested their CD4+ T lymphocyte responses to HA and found them to be similar to wild type responses. However, CD8+ T cells from A/Japan/57-infected transgenic animals were unable to express cytolytic activity against target cells expressing the A/Japan/57 HA. The CD8+ T cell tolerance was also extremely specific, since transgenics immunized with an influenza strain containing a single amino acid substitution in a dominant HA epitope were able to mount full cytolytic responses to that epitope, but not the wild-type epitope. Adoptive transfer of CD8+ T cell clones into transgenic animals resulted extensive interstitial pneumonitis that was antigen-specific and associated with significant morbidity and mortality. We conclude that a lung-specific transgene may lead to specific CD8+ T cell tolerance, with CD4+ T cell and B cell reactivity to the antigen, and that CD4+ T cell reactivity may remain intact to an antigen expressed in the thymus, even when CD8+ T cell tolerance exists. This observation may have profound implications concerning immune-mediated lung diseases, particularly those mediated by CD4+ T cells.

The requirement for proteasome activity class I major histocompatibility complex antigen presentation is dictated by the length of preprocessed antigen

Accumulating evidence has implicated the proteasome in the processing of protein along the major histocompatibility complex (MHC) class I presentation pathway. The availability of potent proteasome inhibitors provides an opportunity to examine the role of proteasome function in antigen presentation by MHC class I molecules to CD8+ cytotoxic T lymphocytes (CTLs). We have investigated the processing and presenting of antigenic epitopes from influenza hemagglutinin in target cells treated with the inhibitor of proteasome activity MG132. In the absence of proteasome activity, the processing and presentation of the full-length hemagglutinin was abolished, suggesting the requirement for proteasome function in the processing and presentation of the hemagglutinin glycoprotein. Epitope-containing translation products as short as 21 amino acids when expressed in target cells required proteasome activity for processing and presentation of the hemagglutin epitope to CTLs. However, when endogenous peptides of 17 amino acids or shorter were expressed in target cells, the processing and presentation of epitopes contained in these peptides were insensitive to the proteasome inhibitor. Our results support the hypothesis that proteasome activity is required for the generation of peptides presented by MHC class I molecules and that the requirement for proteasome activity is dependent on the size of the translation product expressed in the target cell. The implications of these findings are discussed.

Distinct T cell receptor signaling requirements for perforin- or FasL-mediated cytotoxicity

A diverse array of signals are generated in a cytotoxic T lymphocyte (CTL) after the T cell receptor (TCR) engages the class I major histocompatibility complex (MHC) peptide complex. These signals result in a multitude of CTL effector functions, including cellular cytotoxicity, cell surface receptor expression, and cytokine secretion. We have examined signaling through the TCR in a wild type CD8+, MHC-restricted, antigen-specific CTL clone, 14-7, and its interleukin 2-dependent variant clone 14-7FD. We report here that 14-7FD is unable to kill via the perforin mechanism of killing, yet is able to kill via the Fas ligand/Fas mechanism and secrete interferon-gamma in an antigen-specific manner. 14-7FD has cytolytic granules that contain perforin and serine esterases, which are secreted after phorbol ester and Ca2+ ionophore treatment. Lastly, to investigate which TCR signaling requirements were operational in 14-7FD, we examined TCR-triggered intracellular Ca2+ mobilization in the two clones. After TCR engagement, 14-7FD failed to mobilize intracellular Ca2+, which may be the cause for its inability to trigger the perforin/granule exocytosis mechanism of killing. These results indicate that the signal transduction events that trigger perforin killing and the signaling requirements to induce FasL expression are distinct. We hypothesize that these two distinct TCR signal transduction requirements allow for separate activation of these two mechanisms of killing relating to their role in eradication of infected cells or regulation of immune responses.

Predominance of MHC class II-restricted CD4+ cytotoxic T cells against mouse hepatitis virus A59

Coronavirus-induced acute hepatitis is a complex event and the role of different components of the immune system with regard to defined viral proteins and the course of the infection is not yet clear. We have analysed the cytotoxic T-lymphocyte (CTL) response in mouse hepatitis virus (MHV-A59) infection. Surprisingly, we detected only a very clear virus-specific major histocompatibility complex (MHC) class II-restricted cytotoxicity in mice infected with MHV-A59. We found no evidence of activation of the classical CD8+ MHC class I-restricted CTL. The virus-specific CD4+ CTL derived from two different mouse strains having different MHC haplotypes recognized the same immunodominant epitope. This epitope, comprising the amino acid residues 329-343 of the viral S-glycoprotein, was recognized both at the polyclonal level and by virus-specific CTL clones. Transfer studies using a MHV-A59-specific CD4+ CTL clone showed significant protection against a lethal challenge with MHV-A59, implicating that these CD4+ CTL play a pivotal role in the protection against MHV-A59 infections.

Multispecific and heterogeneous recognition of the gag protein by cytotoxic T lymphocytes (CTL) from HIV-infected patients: factors other than the MHC control the epitopic specificities

The HIV gag polyprotein is a major target for recognition by CTL in infected humans. Using recombinant vaccinia viruses (rVV) expressing truncations of the p24gag, and the p18gag, p15gag and HIV-2 p56gag proteins, the characterization of epitope regions recognized by in vitro-stimulated peripheral blood mononuclear cells (PBMC) from 18 infected patients has been studied. The gag-specific response of most individuals is polyclonal and multispecific, and interindividual variations between target epitope regions were frequently observed, despite shared MHC alleles. As CTL may play an important role in the control of HIV replication in infected hosts, these results have important implications for designing vaccine strategies.

Downregulation of peptide transporter genes in cell lines transformed with the highly oncogenic adenovirus 12

The expression of class I major histocompatibility complex antigens on the surface of cells transformed by adenovirus 12 (Ad12) is generally very low, and correlates with the high oncogenicity of this virus. In primary embryonal fibroblasts from transgenic mice that express both endogenous H-2 genes and a miniature swine class I gene (PD1), Ad12-mediated transformation results in suppression of cell surface expression of all class I antigens. Although class I mRNA levels of PD1 and H-2Db are similar to those in nonvirally transformed cells, recognition of newly synthesized class I molecules by a panel of monoclonal antibodies is impaired, presumably as a result of inefficient assembly and transport of the class I molecules. Class I expression can be partially induced by culturing cells at 26 degrees C, or by coculture of cells with class I binding peptides at 37 degrees C. Analysis of steady state mRNA levels of the TAP1 and TAP2 transporter genes for Ad12-transformed cell lines revealed that they both are significantly reduced, TAP2 by about 100-fold and TAP1 by 5-10-fold. Reconstitution of PD1 and H-2Db, but not H-2Kb, expression is achieved in an Ad12-transformed cell line by stable transfection with a TAP2, but not a TAP1, expression construct. From these data it may be concluded that suppressed expression of peptide transporter genes, especially TAP2, in Ad12-transformed cells inhibits cell surface expression of class I molecules. The failure to fully reconstitute H-2Db and H-2Kb expression indicates that additional factors are involved in controlling class I gene expression in Ad12-transformed cells. Nevertheless, these results suggest that suppression of peptide transporter genes might be an important mechanism whereby virus-transformed cells escape immune recognition in vivo.

Induction of a major histocompatibility complex class I-restricted cytotoxic T-lymphocyte response to a highly conserved region of human immunodeficiency virus type 1 (HIV-1) gp120 in seronegative humans immunized with a candidate HIV-1 vaccine

Efforts to induce broadly reactive immunity against human immunodeficiency virus type 1 (HIV-1) have been impaired by the extent of sequence variation exhibited by this lentivirus. Cytotoxic T lymphocytes (CTL) specific for other viruses such as influenza virus have been shown to mediate immunity against divergent viral strains, a property that is related to the ability of CTL to recognize processed antigen derived from conserved viral proteins. A recent candidate HIV-1 vaccine regimen has been described in which subjects receive a primary immunization with a recombinant vaccinia virus expressing gp160 and then a booster immunization with recombinant gp160. Volunteers immunized with this regimen have exhibited augmented humoral responses and have also developed CD4+ and CD8+ CTL specific for gp160. In this report, we have identified the epitopes recognized by CD4+ and CD8+ CTL obtained from two vaccines. An immunodominant CD8+ CTL response was HLA-A3.1 restricted and recognized a 10-amino-acid epitope (gp120/38-47) in a highly conserved region of gp120. CTL specific for the epitope gp120/38-47 were able to lyse targets sensitized with peptides corresponding to all known natural sequence variants in this region. In addition, other HLA class I-restricted CTL epitopes were identified in relatively conserved regions of gp120 and gp41, and CD4+ CTL were shown to recognize two different regions of gp120. Thus, in these two volunteers, immunization with a single strain of HIV-1 induced CD4+ and CD8+ CTL that are specific for multiple conserved regions of HIV-1 and would be expected to recognize a broad range of viral isolates.

Recognition of an immunoglobulin VH epitope by influenza virus-specific class I major histocompatibility complex-restricted cytolytic T lymphocytes

There are two immunogenic sites on the type A influenza A/Japan/57 (H2N2) hemagglutinin (HA) that can be recognized by class I major histocompatibility complex (MHC), H-2Kd-restricted cytolytic T lymphocytes (CTLs). One of these sites encompasses two distinct partially overlapping epitopes, which span HA residues 204-212 and 210-219. During the analysis of the fine specificity of CTL clones directed to the HA 210-219 epitope, we found that one clone 40-2 also recognized the myeloma cell line P3x63-Ag8. P3x63-Ag8 is derived from the MOPC 21 myeloma and expresses an immunoglobulin (Ig) heavy chain variable region (VH) gene which is a member of the murine 7183 VH gene family. Recognition was specific for the endogenously processed MOPC 21 heavy chain in association with the Kd molecules, since the SP2/0 derivative of P3x63-Ag8, which does not make a functional Ig H chain, is not recognized. The VH epitope recognized by clone 40-2 could be mapped to a 10 amino acid peptide spanning MOPC 21 VH residues 49-58. Cross-reactivity for the VH gene product was also demonstrable in some heterogeneous populations of CTL generated in response to influenza virus infection. These results represent the first demonstration of cross-reactivity for an endogenously processed product of a self-Ig by the CTL directed to a foreign antigen and raise the possibility that the Ig VH expression may regulate the CD8+ T cell response to foreign antigens.

T-helper epitopes on human nicotinic acetylcholine receptor in myasthenia gravis

The synthesis of AChR antibodies requires intervention of AChR-specific Th cells. Because of the paucity of anti-AChR Th cells in the blood of myasthenia gravis (MG) patients, direct studies of these autoimmune cells in the blood are seldom possible. Propagation in vitro of anti-AChR T cells from MG patients by cycles of stimulation with AChR antigens selectively enriches and expands the autoimmune T-cell clones, allowing investigation of their function and epitope specificity. Torpedo electroplax AChR was initially used for propagation of anti-AChR T-cell lines. Those studies demonstrated the feasibility of in vitro propagation of AChR-specific T cells. These are bona fide CD4+ Th cells, which stimulate production in vitro of anti-AChR antibodies by B cells of myasthenic patients and recognize equally well denatured and native AChR, suggesting the usefulness of synthetic human AChR sequences as antigens for propagation of the autoimmune Th cells. We used pools of overlapping synthetic peptides, corresponding to the complete sequences of the human AChR alpha-, beta-, gamma-, and delta-subunits, to propagate AChR-specific Th cells from the blood of MG patients. The AChR sequence regions forming epitopes recognized by the autoimmune T cells were determined by challenging the lines with individual synthetic peptides, 20 residues long, screening the AChR subunit sequences. Although each line had an individual pattern of epitope recognition--as expected from their different HLA-DR haplotype--some peptides were recognized by most of all the CD4+ T-cell lines, irrespective of their DR haplotype. The existence of immunodominant regions of the AChR sequence was verified by investigating the response of unselected CD4+ cells from the blood of a relatively large number of MG patients to the individual peptides screening the human alpha-, gamma-, and delta-subunit sequences. Those studies confirmed that each patient has an individual pattern of peptide recognition. The studies also identified a large number of T epitopes of the human AChR and verified the existence of sequence regions immunodominant for T-helper sensitization, because a limited number of sequence regions, including all those immunodominant for the T-helper lines, were recognized by most patients. Anti-AChR CD4+ T lines could be propagated from some healthy controls only for a brief period of time. They recognized AChR sequences poorly, suggesting a low affinity of their T-cell receptors for the corresponding AChR epitopes.(ABSTRACT TRUNCATED AT 400 WORDS)

A pattern search method for putative anchor residues in T cell epitopes

The binding affinity between an antigenic peptide and its particular major histocompatibility complex (MHC) molecule seems to be largely determined by only a few residues. These residues have been called "anchors" because of their property of fitting into "pockets" inside the groove of the MHC molecule. To predict natural antigenic epitopes within a longer sequence, it therefore appears to be important to know the motif or pattern describing the anchors, i.e. the anchors amino acid residue preference and the distance between anchor residues. A large set of MHC class I-restricted peptides has been described. Peptide sequences vary in length and lack an obvious common sequence motif. For a list of peptides belonging to one type of MHC class I molecule, we describe a method to find the most prominent sequence motif with at least two anchor residues. Briefly, antigenic sequences are aligned, and two anchor positions are searched for, where all anchor residues share a high similarity. The alignments are scored according to the similarity of their anchor residues. We show that the motifs predicted for the MHC alleles A2.1, B27, Kb, Kd, Db are in substantial agreement with experimental data. We derive binding motifs for the MHC class I alleles HLA-A1, A11, B8, B14, H-2Ld and for the MHC class II alleles I-Ab and I-As. In some cases, higher scores were obtained by allowing a slight variation in the number of residues between anchors. Therefore, we support the view that the length of epitopes belonging to a particular class I MHC is not uniform. This method can be used to predict the natural short epitope inside longer antigenic peptides and to predict the epitopes anchor residues. Anchor motifs can be used to search for antigenic regions in sequences of infectious viruses, bacteria and parasites.

Recognition of a highly conserved region of human immunodeficiency virus type 1 gp120 by an HLA-Cw4-restricted cytotoxic T-lymphocyte clone

Human immunodeficiency virus type 1 (HIV-1) isolates exhibit extensive sequence variation, particularly in the gp120 subunit of the envelope glycoprotein, and the degree of this variation has raised questions as to whether conserved regions of the HIV-1 envelope can be recognized by the host immune response. A CD8+ cytotoxic T-lymphocyte (CTL) clone specific for the HIV-1 envelope was derived by culturing peripheral blood mononuclear cells from an HIV-1 seropositive subject in the presence of a CD3-specific monoclonal antibody, interleukin-2, and irradiated allogeneic peripheral blood mononuclear cells. Lysis of target cells was restricted by an HLA-C molecule, Cw4, which has not been previously shown to present viral antigen to CTL. Mapping of the specificity of this CTL clone by using synthetic HIV-1 peptides localized the epitope to an 8-amino-acid region of gp120 (amino acids 376 to 383) which is conserved among approximately 90% of sequenced viral isolates. Examination of the recognition of variant peptides by this CTL clone demonstrated that a single, nonconservative amino acid substitution within the 8-amino-acid minimal epitope could abrogate lysis of targets incubated with the variant peptide. The identification of a CTL epitope in a highly conserved region of gp120 documents the ability of cellular immune responses of infected persons to respond to relatively invariant portions of this highly variable envelope glycoprotein. However, the ability of even a single-amino-acid change in gp120 to abolish lysis by CTL supports the hypothesis that sequence variation in HIV-1 may serve as a mechanism of immune escape. In addition, the identification of an HLA-C molecule presenting viral antigen to CTL supports a functional role for these molecules.

Infectious Sindbis virus transient expression vectors for studying antigen processing and presentation

Sindbis virus (SIN) is a small positive-strand enveloped RNA virus that infects a broad range of vertebrate and insect cells. A SIN vector (called dsSIN), designed for transient expression of heterologous RNAs and proteins, was engineered by inserting a second subgenomic mRNA promoter sequence into a nonessential region of the SIN genome. By using this vector, dsSIN recombinants have been constructed that express either bacterial chloramphenicol acetyltransferase, a truncated form of the influenza hemagglutinin (HA), or mini-genes encoding two distinct immunodominant cytotoxic T lymphocyte (CTL) HA epitopes. Infection of murine cell lines with these recombinants resulted in the expression of approximately 10(6)-10(7) chloramphenicol acetyltransferase polypeptides per cell and efficient sensitization of target cells for lysis by appropriate major histocompatibility complex-restricted HA-specific CTL clones in vitro. In addition, priming of an influenza-specific T-cell response was observed after immunizing mice with dsSIN recombinants expressing either a truncated form of HA or the immunodominant influenza CTL epitopes. This SIN expression system allows the generation of high-titered recombinant virus stocks in a matter of days and should facilitate mapping and mutational analysis of class I major histocompatibility complex-restricted T-cell epitopes expressed via the endogenous pathway of antigen processing and presentation.

Failure of Epstein-Barr virus-specific cytotoxic T lymphocytes to lyse B cells transformed with the B95-8 strain is mapped to an epitope that associates with the HLA-B8 antigen

There are two types, A and B, of Epstein-Barr virus (EBV) and B95-8 represents the common type A laboratory strain. Herein, we show in a family study that paternal EBV-specific cytotoxic T lymphocytes (CTL) generated in short-term cultures following stimulation with the autologous B95-8-transformed lymphoblastoid cell line (LCL) or B cells freshly infected with the B95-8 isolate did not lyse haploidentical B95-8 LCL expressing the HLA-A1, -B8, -DR3 paternal haplotype. In contrast, the haploidentical B95-8 LCL expressing the HLA-A11, -B51, -DR7 paternal haplotype was strongly lysed. Moreover, paternal CTL generated in response to stimulation with the B95-8 LCL expressing the haploidentical HLA-A1, -B8, -DR3 paternal haplotype included an allogeneic response against the maternal haplotype but no EBV-specific response as shown by the poor lysis of the autologous LCL target cells. However, stimulation with the haploidentical HLA-A11, -B51, -DR7 paternal haplotype resulted in the generation of both an allogeneic and an EBV-specific response. CTL clones were generated from two HLA-B8+ donors in response to stimulation with the autologous type A LCL transformed with wildtype EBV. The clones were cross-reactive for an immunodominant B95-8-associated peptide epitope that interacted with the HLA-B8 allele but failed to lyse B95-8-transformed LCL targets unless the targets were pre-coated with the exogenous peptide. A CTL clone that was initially stimulated with the autologous BL74 LCL lysed the spontaneous autologous LCL and spontaneous LCL from an HLA-B8+ donor, but failed to lyse the B95-8 LCL from that donor. The observed haplotype preference can be explained in terms of sequence variation between the B95-8 and the corresponding wildtype epitope. Our findings may help to clarify the role of EBV in the pathogenesis of primary Sjögren's syndrome which is closely associated with HLA-B8.

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

Cytotoxic T-lymphocyte (CTL) clones specific for the influenza A/PR/8/34 virus hemagglutinin (HA) were isolated by priming CBA mice with a recombinant vaccinia virus expressing the HA molecule. The epitopes recognized by two of these clones, which were CD8+, Kk restricted, and HA subtype specific, were defined by using a combination of recombinant vaccinia viruses expressing HA fragments and synthetic peptides. One epitope is in the HA1 subunit at residues 259 to 266 (numbering from the initiator methionine), amino acid sequence FEANGNLI, and the other epitope is in the HA2 subunit at residues 10 to 18 (numbering from the amino terminus of the HA2 subunit), sequence IEGGWTGMI. These two peptides are good candidates for naturally processed HA epitopes presented during influenza infection, as they are the same length (eight and nine residues) as other naturally processed viral peptides presented to CTL. A comparison of the sequences of these two new epitopes with those of the three previously published Kk-restricted T-cell epitopes showed some homology among all of the epitopes, suggesting a binding motif. In particular, an isoleucine residue at the carboxy-terminal end is present in all of the epitopes. On the basis of this homology, we predicted that the Kk-restricted epitope in influenza virus nucleoprotein, previously defined as residues 50 to 63, was contained within residues 50 to 57, sequence SDYEGRLI. This shorter peptide was found to sensitize target cells at a 200-fold lower concentration than did nucleoprotein residues 50 to 63 when tested with a CTL clone, confirming the alignment of Kk-restricted epitopes.

Identification of naturally processed viral nonapeptides allows their quantification in infected cells and suggests an allele-specific T cell epitope forecast

Virus-specific cytotoxic T lymphocytes (CTL) recognize virus-derived peptides presented by major histocompatibility complex (MHC) class I molecules on virus-infected cells. Such peptides have been isolated from infected cells and were compared to synthetic peptides. We found previously the Kd- or Db-restricted natural influenza nucleoprotein peptides to coelute on reversed phase high performance liquid chromatography columns with certain peptidic by-products present in synthetic peptide preparations. Here we show by extensive biochemical and immunological comparison that the natural peptides in all respects behave as the surmised synthetic nonapeptides, and thus, must be identical to them. The absolute amounts of these natural peptides contained in infected cells could be determined to be between 220 and 540 copies by comparing with defined amounts of pure synthetic nonapeptides. The comparison of the natural Kd-restricted peptide with published synthetic peptides known to contain other Kd-restricted CTL epitopes suggested a new MHC allele-specific T cell epitope forecast method, based on the defined length of nine amino acid residues and on critical amino acid residues at the second and the last position.

Nonconventional (TL-encoded) major histocompatibility complex molecules present processed viral antigen to cytotoxic T lymphocytes

A large number of class I-like genes are located distal to the K and D regions of the murine major histocompatibility complex (MHC) within the Q and TL region. The function of the molecules encoded within this region is obscure since unlike conventional MHC gene products, these molecules have not been reported to present processed environmental antigens to T cells. In the present report, we demonstrate that a peptide corresponding to processed influenza virus hemagglutinin can be recognized by CD8+ T cell receptor alpha/beta-positive cytotoxic T lymphocytes (CTL) in association with a MHC class I-like product encoded within the TL region. Thus, nonconventional class I MHC molecules can bind and present processed environmental antigens, and TCR-alpha/beta CTL directed to such peptide MHC complexes are represented in the mature T cell pool. Our data imply that Q/TL region products may be charged by peptides generated through an antigen processing and presentation pathway distinct from the pathway used by conventional MHC molecules and not normally available to environmental antigens.

Allele-specific motifs revealed by sequencing of self-peptides eluted from MHC molecules

The crystal structures of major histocompatibility complex (MHC) molecules contain a groove occupied by heterogeneous material thought to represent peptides central to immune recognition, although until now relatively little characterization of the peptides has been possible. Exact information about the contents of MHC grooves is now provided. Moreover, each MHC class I allele has its individual rules to which peptides presented in the groove adhere.

Overlapping cytotoxic T-lymphocyte and B-cell antigenic sites on the influenza virus H5 hemagglutinin

To define the recognition site of cytotoxic T lymphocytes (CTLs) on influenza virus H5 hemagglutinin (HA), an H5 HA-specific CTL clone was examined for the ability to recognize monoclonal antibody-selected HA variants of influenza virus A/Turkey/Ontario/7732/66 (H5N9). On the basis of 51Cr release assays with the variants, a CTL epitope was located near residue 168 of H5 HA. To define the epitope more precisely, a series of overlapping peptides corresponding to this region was synthesized and tested for CTL recognition. The minimum peptide recognized by the CTL clone encompassed residues 158 to 169 of H5 HA. Relative to the H3 HA three-dimensional structure, this CTL epitope is located near the distal tip of the HA molecule, also known as a major B-cell epitope on H3 HA. A single mutation at residue 168 (Lys to Glu) in the H5 HA variants abolished CTL recognition; this same amino acid was shown previously to be critical for B-cell recognition (M. Philpott, C. Hioe, M. Sheerar, and V. S. Hinshaw, J. Virol. 64:2941-2947, 1990). Additionally, mutations within this region of the HA molecule were associated with attenuation of the highly virulent A/Turkey/Ontario/7732/66 (H5N9) (M. Philpott, B. C. Easterday, and V.S. Hinshaw, J. Virol. 63:3453-3458, 1989). When tested for recognition of other H5 viruses, the CTL clone recognized the HA of A/Turkey/Ireland/1378/83 (H5N8) but not that of A/Chicken/Pennsylvania/1370/83 (H5N2), even though these viruses contain identical HA amino acid 158-to-169 sequences. These results suggest that differences outside the CTL epitope affected CTL recognition of the intact HA molecule. The H5 HA site defined in these studies is, therefore, important in both CTL and B-cell recognition, as well as the pathogenesis of the virus.

Allelic genealogy under overdominant and frequency-dependent selection and polymorphism of major histocompatibility complex loci

To explain the long-term persistence of polymorphic alleles (trans-specific polymorphism) at the major histocompatibility complex (MHC) loci in rodents and primates, a computer simulation study was conducted about the coalescence time of different alleles sampled under various forms of selection. At the same time, average heterozygosity, the number of alleles in a sample, and the rate of codon substitution were examined to explain the mechanism of maintenance of polymorphism at the MHC loci. The results obtained are as follows. (1) The coalescence time for neutral alleles is too short to explain the trans-specific polymorphism at the MHC loci. (2) Under overdominant selection, the coalescence time can be tens of millions of years, depending on the parameter values used. The average heterozygosity and the number of alleles observed are also high enough to explain MHC polymorphism. (3) The pathogen adaptation model proposed by Snell is incapable of explaining MHC polymorphism, since the coalescence time for this model is too short and the expected heterozygosity and the expected number of alleles are too small. (4) From the mathematical point of view, the minority advantage model of frequency-dependent selection is capable of explaining a high degree of polymorphism and trans-specific polymorphism. (5) The molecular mimicry hypothesis also gives a sufficiently long coalescence time when the mutation rate is low in the host but very high in the parasite. However, the expected heterozygosity and the expected number of alleles tend to be too small. (6) Consideration of the molecular mechanism of the function of MHC molecules and other biological observations suggest that the most important factor for the maintenance of MHC polymorphism is overdominant selection. However, some experiments are necessary to distinguish between the overdominance and frequency-dependent selection hypotheses.

Class I H-2d-restricted cytotoxic T lymphocytes recognize the neuraminidase glycoprotein of influenza virus subtype N1

Class I major histocompatibility complex-restricted cytotoxic T lymphocytes (CTL) that recognize the neuraminidase (NA) glycoprotein of subtype N1 influenza A viruses have been demonstrated in BALB/c mice. Responses to NA were obtained only in protocols that use two in vivo inoculations of virus, including a recombinant vaccinia virus containing the NA of subtype N1 influenza virus (NA-VAC) to prime or boost. Restimulation in vitro was also required for CTL recognition of NA and strongly depended on the specific N1 virus used. Influenza viruses A/Puerto Rico/8/34 (H1N1), A/CAM/46 (H1N1), J1 (H3N1), and JAP/BEL (H2N1), but not A/Bellamy (H1N1) or MEM/BEL (H3N1) virus, were able to stimulate NA-specific memory T cells in vitro. Single or double in vivo inoculation of any of the N1 viruses or a single injection of NA-VAC failed to elicit restimulatable NA-specific CTL. Lysis of NA-VAC-infected cells at low effector/target ratios was comparable to that observed toward other influenza virus proteins known to be major targets of CTL in BALB/c mice, indicating that antigenic determinants of the subtype N1 NA molecule can be efficiently presented in the context of major histocompatibility complex class I.

Cytotoxic T cells recognize a peptide from the circumsporozoite protein on malaria-infected hepatocytes

Irradiated malaria sporozoites can induce CD8+ T cells that are required for protection against infection. However, the parasite antigens targeted by this immune response are unknown. We have discovered a 16-amino acid epitope from the Plasmodium yoelii circumsporozoite (CS) protein that is recognized by cytotoxic T cells from immune mice. Lymphocytes stimulated with this peptide can kill P. yoelii liver stage parasites in vitro in an MHC-restricted, antigen-specific manner. Thus, epitopes from the CS protein are presented on the surface of infected hepatocytes and can be targets for T cells, even though intact CS protein has not been detected on the surface of the infected hepatocyte. A vaccine that induced CTL to parasite antigens might protect humans against malaria by eliminating liver stage parasites.

Structural requirements for class I MHC molecule-mediated antigen presentation and cytotoxic T cell recognition of an immunodominant determinant of the human immunodeficiency virus envelope protein

In H-2d mice, the immunodominant determinant of the HIV-1-IIIB gp160 envelope glycoprotein recognized by CD8+ CTL is represented by a 15-residue synthetic peptide (315-329: RIQRGPGRAFVTIGK). This peptide is seen in association with the Dd class I MHC molecule expressed on H-2k L cell fibroblast targets. We explored the structural requirements for CTL recognition of this peptide at the levels of both the peptide molecule and the class I MHC molecule. Using several transfectants expressing recombinant Dd/Ld molecules, we found that presentation of this epitope required both the alpha 1 and alpha 2 domains of the Dd molecule, in contrast to certain instances of allorecognition for which alpha 1 of Dd was sufficient in association with alpha 2 of Ld. Because this peptide derives from a hypervariable segment of the HIV envelope, substituted peptides could be used to define not only the structures affecting interaction of peptide with class I MHC molecule and with the TCR, but also the structural basis for the effect of naturally occurring viral variation on CTL recognition. The CTL-LINE specific for this HIV-1-IIIB-derived sequence could not recognize the HIV-1-RF variant-derived sequence from exactly the same site (315-329:--HIGPGRVIYATGQ). Peptides with single amino acid substitutions from the HIV-1-IIIB sequence toward the HIV-1-RF sequence were made to test the effect of each residue significantly affected recognition, and only one, 324(F), was obligatory. Moreover, both 322(R) and 324(F) substituted peptides failed to inhibit the binding of the wild type peptide to the MHC molecule. Therefore, the amino-acids 322(R) and 324(F) seem to be involved in regulating peptide interaction with the Dd class I MHC molecule. In contrast, 325(V) appeared to affect interaction with the TCR. We suggest that sequence variations among known HIV-1 isolates that affect peptide binding to MHC such as those described here, if occurring during the course of infection of an individual, could result in failure of the MHC molecules of that individual to present the peptide. If the number of dominant HIV CTL epitopes is indeed very limited, such a blind spot could allow the virus to escape immune control, proliferate rapidly, and cause AIDS.

Recognition of pre-processed endogenous antigen by class I but not class II MHC-restricted T cells

Class I and class II MHC-restricted T lymphocytes recognize non-native forms of antigen. The presentation of antigen to these two classes of T lymphocytes can occur through distinct pathways. Several mechanisms, including differences in antigen processing in different intracellular compartments, have been proposed to account for these pathway differences. Here we describe a T-cell epitope located on the influenza virus haemaglutinin, which is recognized by both class I and class II MHC-restricted cytolytic T lymphocytes (CTL). When expressed de novo in target cells, from a synthetic minigene encoding only the epitope, this pre-processed antigenic site is recognized by class I but not class II MHC-restricted T lymphocytes, even though target cells treated with the exogenously introduced peptide can be recognized by both classes of T cells. Because endogenous expression of the pre-processed antigenic fragment results in differential presentation to class I and class II MHC-restricted CTL, differences between the two different pathways of presentation could lie not at the level of processing but at the level of targeting and/or interaction of processed antigen with MHC.

A single amino acid interchange yields reciprocal CTL specificities for HIV-1 gp160

For the IIIB isolate of human immunodeficiency virus type-1 (HIV-1), the immunodominant determinant of the envelope protein gp160 for cytotoxic T lymphocytes (CTLs) of H-2d mice is in a region of high sequence variability among HIV-1 isolates. The general requirements for CTL recognition of peptide antigens and the relation of recognition requirements to the natural variation in sequence of the HIV were investigated. For this purpose, a CTL line specific for the homologous segment of the envelope from the MN isolate of HIV-1 and restricted by the same class I major histocompatibility (MHC) molecule (Dd) as the IIIB-specific CTLs was raised from mice immunized with MN-env-recombinant vaccinia virus. The IIIB-specific and MN-specific CTLs were completely non-cross-reactive. Reciprocal exchange of a single amino acid between the two peptide sequences, which differed in 6 of 15 residues, led to a complete reversal of the specificity of the peptides in sensitizing targets, such that the IIIB-specific CTLs lysed targets exposed to the singly substituted MN peptide and vice versa. These data indicate the importance of single residues in defining peptide epitopic specificity and have implications for both the effect of immune pressure on selection of viral mutants and the design of effective vaccines.

Class I major histocompatibility complex-restricted T lymphocyte recognition of the influenza hemagglutinin. Overlap between class I cytotoxic T lymphocytes and antibody sites

The influenza hemagglutinin is a critical regulator of disease expression during influenza virus infection and serves as a major target for the host immune response to this pathogen. In this report, we have analyzed an immunodominant site on the hemagglutinin (residues 202-221) recognized by murine class I MHC-restricted T lymphocytes. This analysis has revealed evidence for the duplication of a T cell recognition site within the region 202-221. We have also identified critical amino acids necessary for class I-restricted T cell recognition within these two epitopes. In addition, we provide evidence that a site on the influenza hemagglutinin recognized by neutralizing antibody directly overlaps with an epitope recognized by class I MHC-restricted CTL.

Molecularly engineered vaccine which expresses an immunodominant T-cell epitope induces cytotoxic T lymphocytes that confer protection from lethal virus infection

Identification of a single viral T-cell epitope, associated with greater than 95% of the virus-specific cytotoxic T-lymphocyte (CTL) activity in BALB/c (H-2d) mice (J. L. Whitton, A. Tishon, H. Lewicki, J. Gebhard, T. Cook, M. Salvato, E. Joly, and M. B. A. Oldstone, J. Virol. 63:4303-4310, 1989), permitted us to design a CTL vaccine and test its ability to protect against a lethal virus challenge. Here we show that a single immunization with a recombinant vaccinia virus-lymphocytic choriomeningitis virus (LCMV) vaccine (VVNPaa1-201) expressing the immunodominant epitope completely protected H-2d mice from lethal infection with LCMV but did not protect H-2b mice. Furthermore, we show that the success or failure of immunization was determined entirely by the host class I major histocompatibility glycoproteins. The difference in outcome between mice of these two haplotypes was consistent with the presence or absence in the immunizing sequences of an epitope for CTL recognition and is correlated with the induction of LCMV-specific H-2-restricted CTL in H-2d mice. Protection is not conferred by a humoral immune response, since LCMV-specific antibodies were not detectable in sera from VVNPaa1-201-immunized mice. In addition, passive transfer of sera from vaccinated mice did not confer protection upon naive recipients challenged with LCMV. Hence, the molecular dissection of viral proteins can uncover immunodominant CTL epitope(s) that can be engineered into vaccines that elicit CTL. A single CTL epitope can protect against a lethal virus infection, but the efficacy of the vaccine varies in a major histocompatibility complex-dependent manner.

Cloned cytotoxic T cells recognize an epitope in the circumsporozoite protein and protect against malaria

Protective immunity against malaria is induced by vaccination of hosts with irradiation-attenuated sporozoites. This immunity is mediated in part by neutralizing antibodies that are directed mainly against the repeat domain of the circumsporozoite protein. Early experiments showed, however, that B-cell-depleted mice that are immunized with sporozoites can resist challenge, indicating that T-cell effector mechanisms may also have a role in protection. This idea was supported by the recent observation that protective immunity also requires T-cells expressing the CD8 antigen (CD8+ T cells) whose target is probably the developing liver-stage parasites. Moreover, an oral Salmonella vaccine that expresses the circumsporozoite protein is able to protect against murine malaria in the absence of antibodies. Here we report the identification of an epitope contained within amino acids 249-260 of the Plasmodium berghei circumsporozoite protein that is recognized by H-2Kd-restricted cytotoxic T cells. Passive transfer into mice of cytotoxic-T-cell clones that recognize this epitope conferred a high degree of protection against challenge. These results provide the first direct evidence that CD8+ T cells that are specific for a defined epitope can confer protection against a parasitic infection.

Lack of immunodominance in the T cell response to herpes simplex virus glycoprotein D after administration of infectious virus

Glycoprotein D (gD) of HSV has been shown to be a potent immunogen. To analyze the T cell antigenic determinants on gD, a series of 28 overlapping 20-mer peptides that span the extracellular portion of gD-1 were examined for their ability to stimulate T cells from rgD-1 or infectious HSV-1-primed H-2d mice in vitro. rgD-1-primed cells responded exclusively to peptide 241-260, the immunodominant determinant of gD in H-2d mice. In contrast, infectious HSV-primed T cells were shown to respond to 17 (and up to 22) of 28 synthetic gD peptides. These results indicate an extensive diversity in the T cell repertoire to gD in H-2d mice with T cells directed to a broad array of peptide determinants being recruited during the acute phase of an HSV infection.