Evidence for the expression of a second CD6 ligand by synovial fibroblasts

Objective. CD6, a cell surface glycoprotein expressed primarily on T cells, may function as a costimulatory molecule and may play a role in autoreacti ve immune responses. Recently, a CD6 ligand termed CD166 (previously known as activated leukocyte cell adhesion molecule [ALCAM]) has been identified and shown to be expressed on activated T cells, B cells, thymic epithelium, keratinocytes, and in rheumatoid arthritis synovial tissue. However, the r esults of functional studies have suggested the existence of a second CD6 l igand, The present study was undertaken to seek evidence for a second CD6 l igand on cultured synovial fibroblasts. Methods. Flow cytometric and biochemical techniques were applied, using ant i-CD166 monoclonal antibody (mAb) and a recombinant CD6 fusion protein, to determine whether cultured synovial fibroblasts and other cell types expres sed a non-ALCAM CD6 ligand. Results. CD14- fibroblastic synoviocytes showed greater binding of a recomb inant CD6 fusion protein than of anti-ALCAM mAb, With interferon-gamma trea tment of synovial fibroblasts, binding of both reagents increased, but this was more marked for binding of CD6 fusion protein. Exposure of synovial fi broblasts to other cytokines or to the superantigen staphylococcal enteroto xin A also regulated binding of CD6 fusion protein and anti-ALCAM mAb in a discordant manner. Immunoprecipitation of proteins from membrane extracts o f synovial fibroblasts with a CD6-Ig fusion protein revealed a novel 130-kd band distinct from CD166; an identical molecule was also precipitated from membranes of HBL-100 tumor cells. Conclusion. Taken together with previous data regarding CD6 and CD166 funct ion, the present findings strongly suggest the existence of a second CD6 li gand distinct from CD166, which can be expressed by synovial fibroblasts as well as other cells.