Modulation of T cell immunity by TCR/pMHC dwell time and activating/inhibitory receptor pairs on the antigen-presenting cell

The molecular interactions occurring at the interface between the antigen presenting cell (APC) and the T lymphocyte play an important role in the immune surveillance against infectious agents and tumors, as well as in autoimmunity and transplant rejection. The significance of the APC-T cell interaction in immunity is underscored by the observation that deficiencies in the function of either one of these two cell types cause extreme susceptibility to infections and tumor growth. Furthermore, a disregulated APC-T cell interaction can initiate autoimmunity. Thus, antigen recognition by T cells must be tightly regulated in order to ensure protection against pathogens and tumors, avoiding activation of self-reactive T cells. Efficient T cell activation requires two simultaneous signals provided by the APC: Antigen (or signal 1) and co-stimulation (or signal 2). The specificity of antigen recognition by T cells (signal 1) is controlled exclusively by the T cell receptor (TCR), an extremely diverse heterodimeric protein composed of disulfide-bonded alpha and beta chains. While it is clear that the TCR recognizes antigens as small peptides bound to molecules of the Major Histocompatibility Complex (MHC), the molecular explanation for the specificity of antigen recognition by the alphabetaTCR is just beginning to be elucidated. In this review are described some of the advances made in the understanding of the molecular interactions that define the antigen-specificity of the TCR, and the current models for T cell activation by antigen on APCs are discussed.