IN-VIVO ASSOCIATION OF CD5 WITH TYROSINE-PHOSPHORYLATED ZAP-70 AND P21 PHOSPHO-ZETA MOLECULES IN HUMAN CD3(+) THYMOCYTES

CD5 is a 67-kDa T cell surface Ag that can be found physically associa ted with the CDS-TCR molecular complex. In different experimental mode ls it has been shown to act as a costimulatory receptor for T cell act ivation. Unexpectedly, studies in CD5-deficient mice suggested a negat ive role for the CD5 Ag in CD3-TCR signaling in the thymus, In this re port we assessed the constitutive interactions of CD5 in freshly isola ted human thymocytes with signaling elements of the CD3-TCR complex, W e determined that the ZAP-70 protein tyrosine kinase was present in CD 5 immunoprecipitates. The two molecules were constitutively tyrosine p hosphorylated in a complex also associating with unphosphorylated as w ell as phosphorylated zeta-chains. Although both p21 and p23 tyrosine- phosphorylated forms of zeta as well as phospho-CD3 epsilon molecules were constitutively present in human thymocytes and could be immunopre cipitated with ZAP-70- or CD3 epsilon-specific Abs, the p21 species of zeta was predominant in CD5 immune complexes. The interaction between CD5 and ZAP-70 was not observed in CD3-negative thymocytes, where the constitutive tyrosine phosphorylation of ZAP-70 was very low. We conc lude that CD5 may affect in vivo the signaling capacity of TCRs expres sed by human thymocytes by altering the phosphorylation state of CD3 a nd/or by retaining ZAP-70 with the p21 species of zeta.