Characterization of a CD6 ligand(s) expressed on human- and murine-derived cell lines and murine lymphoid tissues.

CD6, a type I cell surface glycoprotein expressed predominantly by thymocytes and mature T lymphocytes, becomes phosphorylated on tyrosine residues following T cell activation and has been implicated as an accessory molecule in T cell activation. The purpose of this study was to identify cell lines and tissues which express CD6 ligand(s), determine the requirements for CD6 binding, and biochemically characterize the putative CD6 ligand(s). Binding studies with a CD6 immunoglobulin fusion protein, CD6-Rg, allowed the identification of a number of human cell lines which express a CD6 ligand(s). The binding to these cell lines was trypsin sensitive, in part required divalent cations, was blocked by an anti-CD6 mAb, and could be downregulated by tumor necrosis factor alpha (TNF alpha), interleukin-1 beta (IL-1 beta) and interferon-gamma (IFN-gamma). Among the cell lines tested, the human breast carcinoma-derived cell line HBL-100 expressed the highest levels of CD6 ligand(s) and was used for immunoprecipitation studies. Following metabolic labeling, CD6-Rg immunoprecipitated glycoproteins of approximately 100, approximately 90, and approximately 45 kDa from HBL-100 cells. Using CD6-Rg we were able to show that murine thymus, lymph nodes, and skin express high levels of CD6 ligand(s) and that CD6-Rg bound to a murine thymic epithelial cell line and to cultured human epidermal keratinocytes.