Regulation of cytotoxic T lymphocyte-associated molecule-4 by Src kinases

Cytotoxic T lymphocyte-associated molecule-4 (CTLA-4) is a cell surface rec eptor expressed on activated T cells that can inhibit T cell responses indu ced by activation of the TCR and CD28. Studies with phosphorylated peptides based on the CTLA-4 intracellular domain have suggested that tyrosine phos phorylation of CTLA-4 may regulate its interactions with cytoplasmic protei ns that could determine its intracellular trafficking and/or signal transdu ction, However, the kinase(s) that phosphorylate CTLA-4 remain uncharacteri zed, In this report, we show that CTLA-4 can associate with the Src kinases Fyn and Lck and that transfection of Fyn or Lck, but not the unrelated kin ase ZAP70, can induce tyrosine phosphorylation of CTLA-4 on residues Y201 a nd Y218, A similar pattern of tyrosine phosphorylation was found in pervana date-treated Jurkat T cells stably expressing CTLA-4, Phosphorylation of CT LA-4 Y201 in Jurkat cells correlated with cell surface accumulation of CTLA -4, CTLA-4 phosphorylation induced the association of CTLA-4 with the tyros ine phosphatase SHP-2, but not with phosphatidylinositol 3-kinase. In contr ast, Lck-induced phosphorylation of CD28 resulted in the recruitment of pho sphatidylinositol 3-kinase, but not SHP-2. These findings suggest that phos phorylation of CD28 and CTLA-4 by Lck activates distinct intracellular sign aling pathways, The association of CTLA-4 with Src kinases and with SHP-2 r esults in the formation of a CTLA-4 complex with the potential to regulate T cell activation.