TLISA1 (PTA1) ACTIVATION ANTIGEN IMPLICATED IN T-CELL DIFFERENTIATIONAND PLATELET ACTIVATION IS A MEMBER OF THE IMMUNOGLOBULIN SUPERFAMILYEXHIBITING DISTINCTIVE REGULATION OF EXPRESSION

T lineage-specific activation antigen 1 (TLiSA1) antigen was initially described as a T lineage-specific activation antigen involved in the differentiation of human cytotoxic T cells, Subsequently, the antigen was identified on platelets and was shown to be involved in platelet a ctivation, hence it was renamed platelet and T cell antigen 1 (PTA1), although identity between the two antigens was not established, In the present study we have cloned the cDNA encoding TLiSA1 from Jurkat cel ls and show it to be a novel member of the immunoglobulin superfamily with the unusual structure of two V domains only, Identity between TLi SA1 and platelet PTA1 is established by immunological criteria, by int ernal peptide sequences obtained from the purified platelet glycoprote in and by sequencing the platelet transcript after reverse transcripta se-polymerase chain reaction, In Jurkat cells, TLiSA1/PTA1 mRNA and su rface protein expression is greatly stimulated by treatment of the cel ls with phorbol ester, but the T cell proliferative signal of phorbol ester and ionophore combined greatly reduces or abrogates this respons e, and this suppressive effect of the ionophore is not reversed by inc orporating FK506 to inhibit calcineurin. Together with the known signa ling role of PTA1, these data substantiate the notion that this molecu le is implicated in T cell differentiation, perhaps by engagement of a n adhesive ligand.