Physical and Functional Association between Thymic Shared Antigen-1/Stem Cell Antigen-2 and the T Cell Receptor Complex

Abstract Thymic shared antigen-1 (TSA-1)/stem cell Ag-2 (Sca-2) is a glycosylphosphatidylinositol (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 elucidate a molecular mechanism for TSA-1-mediated modulation of the TCR-signaling pathway, we examined whether TSA-1 is physically coupled to the TCR in the present study. TSA-1 was clearly associated with CD3ζ chains in T cell hybridomas, activated T cells, and COS-7 cells transfected with TSA-1 and CD3ζ cDNA. The physical association was confirmed on the surface of T cells in immunoprecipitation and confocal microscopy. The analysis using stable and transient transfectants expressing a transmembrane form of TSA-1 revealed that the association of CD3ζ did not require the GPI anchor of TSA-1. Finally, tyrosine phosphorylation of CD3ζ chains was induced after stimulation with anti-TSA-1, suggesting that a functional association between these two molecules also exists. These results imply that the physical association to CD3ζ underlies a regulatory role of TSA-1/Sca-2 in the TCR-signaling pathway.