CD6 dependent interactions of T cells and keratinocytes: functional evidence for a second CD6 ligand on gamma-interferon activated keratinocytes.

The CD6 glycoprotein is expressed by T lymphocytes and is hypothesized to interact with one or more ligands expressed on antigen presenting cells (APCs). We show that CD6 mediates binding of the transformed CD4+ T cell line Hut 78 to gamma-interferon activated keratinocytes (KCs). A recombinant CD6-Ig fusion protein has been reported to bind to a CD6 ligand ALCAM, but this is the first demonstration that cell-cell adhesion of human T lymphocytes can be CD6 dependent. The known CD6 ligand ALCAM (CD166) is expressed on cultured KCs but does not appear to mediate KC-Hut 78 binding, suggesting the existence of additional CD6 ligands expressed on KCs. In functional studies using autologous KCs as APCs for tetanus toxoid specific T cell clones, KCs +/- gamma-interferon are unable to stimulate autologous T cells with recall antigen. Therefore interaction of T cell CD6 with CD6 ligands on KCs does not provide sufficient co-stimulation of primed T cells to support responses to nominal antigen.