The Major Histocompatibility Complex (MHC) locus encodes classical MHC class I and MHC class II molecules and nonclassical MHC-I molecules. The architecture of these molecules is ideally suited to capture and present an array of peptide antigens (Ags). In addition, the CD1 family members and MR1 are MHC class I-like molecules that bind lipid-based Ags and vitamin B precursors, respectively. These Ag-bound molecules are subsequently recognized by T cell antigen receptors (TCRs) expressed on the surface of T lymphocytes. Structural and associated functional studies have been highly informative in providing insight into these interactions, which are crucial to immunity, and how they can lead to aberrant T cell reactivity. Investigators have determined over thirty unique TCR-peptide-MHC-I complex structures and twenty unique TCR-peptide-MHC-II complex structures. These investigations have shown a broad consensus in docking geometry and provided insight into MHC restriction. Structural studies on TCR-mediated recognition of lipid and metabolite Ags have been mostly confined to TCRs from innate-like natural killer T cells and mucosal-associated invariant T cells, respectively. These studies revealed clear differences between TCR-lipid-CD1, TCR-metabolite-MR1, and TCR-peptide-MHC recognition. Accordingly, TCRs show remarkable structural and biological versatility in engaging different classes of Ag that are presented by polymorphic and monomorphic Ag-presenting molecules of the immune system.

Antigen-specific T-cell responses induced by infection, transplantation, autoimmunity or hypersensitivity are characterized by cells expressing biased profiles of T-cell receptors (TCRs) that are selected from a diverse, naive repertoire. Here, we review the evidence for these TCR biases, focusing on crystallographic analysis of the structural constraints that determine the binding of a TCR to its ligand and the persistence of certain TCRs in an immune repertoire. We discuss the ways in which diversity in a selected TCR repertoire can contribute to protective immunity and the implications of this for vaccine design and immunotherapy.

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The continued westward dissemination of H5N1 influenza A viruses in avian populations and the nearly 50% mortality rate of humans infected with H5N1 are a source of great international concern. A mutant H5N1 virus with the capability to spread rapidly between humans could cause a global catastrophe. Governments have reacted by developing national response plans, stockpiling antiviral drugs and speeding up the development and approval of vaccines. Here we summarize what is known about the interaction between influenza A viruses and the mammalian host response, specifically emphasizing issues that might be of interest to the broader immunology community.