White blood cells called T lymphocytes play critical roles in immune defense against viruses, bacteria, fungi, protozoa, and cancer cells. They are also involved in allergies/asthma due to the development of an unwanted or excessive type of immune response to substances in our environment and in autoimmune diseases that result from the inappropriate attack of these cells on the body?s own tissues. The effector functions of T cells are mediated largely by proteins termed cytokines that either be expressed at the cell surface or secreted. Because T cells see foreign substances (antigens) in the form of peptide- major histocompatibility complex (MHC) molecule complexes on cell surfaces, we wish to know how such complexes interact with specific receptors to evoke the effector activities of mature T cells in the body, as well as regulate their growth, inactivation, or death. In particular, we want to understand in molecular detail the protein-protein interactions that turn recognition of antigen by T cells into signals guiding the normal survival and effector functions of these cells, how variations in these recognition and signaling events leads to desirable versus undesirable forms of immunity, and how we can manipulate these events to augment useful immune responses and inhibit damaging ones. Our studies currently focus on the intracellular events evoked by binding of the T-cell receptor with peptide:MHC molecule ligands and on new biochemical regulatory pathways that help T-cells discriminate between self- and foreign peptide:MHC molecule complexes, as well as show very high sensitivity to antigen on presenting cells of the types studied in LI545. During the past year we have worked out the molecular details of two novel regulatory pathways controlling early signaling by the T-cell receptor that we identified last year(SHP-1 phosphatase dependent negative control and MAPK mediated positive control). Using retroviral gene transduction we have confirmed by functional studies in nontransformed T lymphocytes the pathways we discovered using biochemical tools. Our studies are also beginning to reveal how the specific location of membrane and cytoplasmic molecules supports the sensitivity and antigen discrimination capacity of the antigen receptor of T cells. We have developed real time imaging methods for tracking protein movements during antigen -dependent T cell activation. We have also demonstrated a role for self-recognition in generating intracellular signals in naive peripheral T-cells and shown that this signaling can be divorced from those maintaining lymphocyte viability. Lastly, our experiments have uncovered evidence that recently activated T cells acquire overt reactivity to self antigens, which we believe helps expand clonal precursors during the period of antigen limitation early in an infectious process.