IDENTIFICATION AND CHARACTERIZATION OF A 100-KD LIGAND FOR CD6 ON HUMAN THYMIC EPITHELIAL-CELLS

CD6 is a 130-kD glycoprotein expressed on the surface of thymocytes an d peripheral blood T cells that is involved in TCR-mediated T cell act ivation. In thymus, CD6 mediates interactions between thymocytes and t hymic epithelial (TE) cells. In indirect immunofluorescence assays, a recombinant CD6-immunoglobulin fusion protein (CD6-Rg) bound to cultur ed human TE cells and to thymic fibroblasts. CD6-Rg binding to TF and TE cells was trypsin sensitive, and 54 +/- 4% of binding was divalent cation dependent. By screening the blind panel of 479 monoclonal antib odies (mAbs) from the 5th International Workshop on Human Leukocyte Di fferentiation Antigens for expression on human TE cells and for the ab ility to block CD6-Rg binding to TE cells, we found one mAb (J4-81) th at significantly inhibited the binding of CD6-Rg to TE cells (60 +/- 7 % inhibition). A second mAb to the surface antigen identified by mAb J 4-81, J3-119, enhanced the binding of CD6-Rg to TE cells by 48 +/- 5%. Using covalent cross-linking and trypsin digestion, we found that mAb J4-81 and CD6-Rg both bound to the same 100-kD glycoprotein (CD6L-100 ) on the surface of TE cells. These data demonstrate that a 100-kD gly coprotein on TE cells detected by mAb 54-81 is a ligand for CD6.