The CD4 and CD8 antigens play important roles in immunity by binding to major histocompatibility complex (MHC) antigens and by synergizing with the TCR/CD3 complex in the generation of signals leading to T-cell proliferation. An important issue has been to establish the molecular mechanism by which the CD4/CD8 antigens regulate cell growth. Recently, I reported the novel finding that the CD4 and CD8 antigens are coupled to a protein-tyrosine kinase (p56-lck) from T lymphocytes. Importantly p56-lck is a member of the src family of tyrosine kinases (c-src, c-yes, c-abl, etc.) which have been causally linked to the activation and transformation of mammalian cells. p56-lck is also related to kinase domains which comprise the cytoplasmic tails of the insulin, and epidermal growth factor receptors. Thus, p56-lck may constitute the underlying molecular mechanism of CD4 and CD8 function in the regulation of T-cell growth. Regulation of p56-lck activity may also be important in the development of T cell malignancies and autoimmune disorders. Importantly, the CD4/CD8:p56-lck complex is catalytically active as shown by its ability to undergo autophosphorylation and to phosphorylate various subunits of the CD3 complex (including the CD3 zeta chain). The CD4/CD8:p56-lck complex may thus mediate the cooperative interaction between the Ti/CD3 complex and the CD4/CD8 antigens. In this proposal, we propose to extend this work with a detailed biochemical analysis of the CD4/CD8:p56-lck complex (its size, the identity of other components (4OKd, 8OKd) in the complex, whether p56-lck activity is regulated by ligand binding to CD4/CD8 and the TcR complex etc.). In addition, we will attempt to map the site of p56-lck binding to the cytoplasmic tail of the CD4/CD8 antigens by use of peptides, site-directed mutagenesis and transfection analysis. Lastly, we propose to study the function of P56-lck in the activation and transformation of T cells by transfection with sense and anti-sense p56-lck constructs in various antigen-specific T-cell hybridomas and transformed T cells. This comprehensive approach should provide valuable information on the role played by CD4/CD8:p56-lck complex in T-cell activation and potentially on the development of T-cell leukemias and various lymphoproliferative disorders.
