Mendez-Fernandez, Yanice

Abstract MHC class Ia molecules are regulators of antiviral immunity and are thought to function similarly in the presentation of self and antigenic peptides to T cells. However, differences in natural polymorphisms found among class Ia loci in humans and mice indicate these genes may not function equivalently. We used transgenic mice to evaluate the equivalence of MHC Ia gene function in directing cellular immunity against the picornavirus, Theiler’s encephalomyelitis virus, and found that effective antiviral immunity was promoted by H-2D transgenes but not by H-2K in this system. Because transgenes evaluated in our study encoded the peptide binding domain of the Db antigen presenting molecule expressed in the context of the Db or Kb genetic framework, our findings indicate that structural features distinguishing these genes mapping outside of these coding sequences determine ability to direct effective viral immunity. Irrespective of which transgene was expressed, the mice developed robust anti viral T cell responses against the immunodominant TMEV epitope and the T cells were recruited to infected tissue. However, only when the Db antigen binding domain was expressed in the context of the H-2D genetic framework was virus cleared. Our findings suggest that differences in tissue specific class I gene expression at the site of infection is a key factor in this differential function of class Ia genes. Such difference could underlie natural variations in class Ia polymorphism.

Abstract Peptide immunotherapy both activates and suppresses the T cell response against known peptide Ags. Although pretreatment with VP2121–130 peptide inhibits the development of antiviral CTL specific for the immunodominant Db:VP2121–130 epitope expressed during acute Theiler’s murine encephalomyelitis virus infection, i.v. injection of this same peptide or MHC tetramers containing the peptide during an ongoing antiviral CTL response results in a peptide-induced fatal syndrome (PIFS) within 48 h. Susceptibility to PIFS is dependent on peptide-specific CD8+ T cells, varies among inbred strains of mice, and is not mediated by traditionally defined mechanisms of shock. Analyses using bone marrow chimeras and mutant mice demonstrate that susceptibility to PIFS is determined by the genotype of bone marrow-derived cells and requires the expression of perforin. Animals responding to peptide treatment with PIFS develop classical stress responses in the brain. These findings raise important considerations for the development of peptide therapies for active diseases to modify immune responses involving expanded populations of T cells. In summary, treatment with peptides or MHC-tetramers during a peptide-specific immune response can result in a fatal shock-like syndrome. Susceptibility to the syndrome is genetically determined, is mediated by CD8+ T cells, and requires expression of perforin. These findings raise concerns about the use of peptides and MHC tetramers in therapeutic schemes.

Abstract Both CD8 and the TCR bind to MHC class I molecules during physiologic T cell activation. It has been shown that for optimal T cell activation to occur, CD8 must be able to bind the same class I molecule that is bound by the TCR. However, no direct evidence for the class I-dependent association of CD8 and the TCR has been demonstrated. Using fluorescence resonance energy transfer, we show directly that a single class I molecule causes TCR/CD8 interaction by serving as a docking molecule for both CD8 and the TCR. Furthermore, we show that CD3ε is brought into close proximity with CD8 upon TCR/CD8 association. These interactions are not dependent on the phosphorylation events characteristic of T cell activation. Thus, MHC class I molecules, by binding to both CD8 and the TCR, mediate the reorganization of T cell membrane components to promote cellular activation.

Abstract Natural selection drives diversification of MHC class I proteins, but the mechanism by which selection for polymorphism occurs is not known. New variant class I alleles differ from parental alleles both in the nature of the CD8 T cell repertoire formed and the ability to present pathogen-derived peptides. In the current study, we examined whether T cell repertoire differences, Ag presentation differences, or both account for differential viral resistance between mice bearing variant and parental alleles. We demonstrate that nonresponsive mice have inadequate presentation of viral Ag, but have T cell repertoires capable of mounting Ag-specific responses. Although previous work suggests a correlation between the ability to present an Ag and the ability to generate a repertoire responsive to that Ag, we show that the two functions of MHC class I are independent.

ABSTRACTTheiler's murine encephalomyelitis virus (TMEV) infection of the brain induces a virus-specific CD8+T-cell response in genetically resistant mice. The peak of the immune response to the virus occurs 7 days after infection, with an immunodominant CD8+T-cell response against a VP2-derived capsid peptide in the context of the Dbmolecule. The process of activation of antigen-specific T cells that migrate to the brain in the TMEV model has not been defined. The site of antigenic challenge in the TMEV model is directly into the brain parenchyma, a site that is considered immune privileged. We investigated the hypothesis that antiviral CD8+T-cell responses are initiated in situ upon intracranial inoculation with TMEV. To determine whether a brain parenchymal antigen-presenting cell is responsible for the activation of virus-specific CD8+T cells, we evaluated the CD8+T-cell response to the VP2 peptide in bone marrow chimeras and mutant mice lacking peripheral lymphoid organs. The generation of the anti-TMEV CD8+T-cell response in the brain requires priming by a bone marrow-derived antigen-presenting cell and the presence of peripheral lymphoid organs. Although our results show that activation of TMEV-specific CD8+T cells occurs in the peripheral lymphoid compartment, they do not exclude the possibility that the immune response to TMEV is initiated by a brain-resident, bone marrow-derived, antigen-presenting cell.

The interaction of the T cell receptor (TCR) with peptide in the binding site of the major histocompatibility complex molecule provides the basis for T cell recognition during immune surveillance, repertoire development, and tolerance. Little is known about the extent to which repertoire selection is influenced directly by variation of the structure of the class I heavy chain. We find that the 2C TCR, normally positively selected in the context of the K b molecule, is minimally selected into the CD8 lineage in the absence of antigen-processing genes. This finding underscores the importance of peptides in determining the positive-selecting class I ligands in the thymus. In contrast, K bm3 , a variant class I molecule that normally exerts a negative selection pressure on 2C-bearing T cells, positively selects 2C transgenic T cells into the CD8 lineage in an antigen-processing gene-deficient environment. These findings indicate that structural changes in the heavy chain can have direct influence in T cell recognition, from which we conclude that the nature of TCR interaction with class I heavy chain influences the array of TCRs selected during development of the functional adult repertoire.