PHYSICAL AND FUNCTIONAL ASSOCIATION BETWEEN THYMIC SHARED ANTIGEN-1 STEM-CELL ANTIGEN-2 AND THE T-CELL RECEPTOR COMPLEX

Thymic shared antigen-1 (TSA-1)/stem cell Ag-2 (Sca-2) is a glycosylph osphatidylinositol (GPI)-anchored antigen expressed on lymphocytes. We have previously demonstrated that a signal via TSA-1/Sca-2 inhibits T cell receptor (TCR)-mediated T cell activation and apoptosis. To eluc idate a molecular mechanism for TSA-1-mediated modulation of the TCR-s ignaling pathway, we examined whether TSA-1 is physically coupled to t he TCR in the present study. TSA-1 was clearly associated with CD3 zet a chains in T cell hybridomas, activated T cells, and COS-7 cells tran sfected with TSA-1 and CD3 zeta cDNA. The physical association was con firmed on the surface of T cells in immunoprecipitation and confocal m icroscopy. The analysis using stable and transient transfectants expre ssing a transmembrane form of TSA-1 revealed that the association of C D3 zeta did not require the GPI anchor of TSA-1. Finally, tyrosine pho sphorylation of CD3 zeta chains was induced after stimulation with ant i-TSA-1, suggesting that a functional association between these two mo lecules also exists. These results imply that the physical association to CD3 zeta underlies a regulatory role of TSA-1/Sca-2 in the TCR-sig naling pathway.