Objective: CD6, a cell surface glycoprotein expressed primarily on T cells, may function as a costimulatory molecule and may play a role in autoreactive 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 results of functional studies have suggested the existence of a second CD6 ligand. The present study was undertaken to seek evidence for a second CD6 ligand on cultured synovial fibroblasts.
Methods: Flow cytometric and biochemical techniques were applied, using anti-CD166 monoclonal antibody (mAb) and a recombinant CD6 fusion protein, to determine whether cultured synovial fibroblasts and other cell types expressed a non-ALCAM CD6 ligand.
Results: CD14- fibroblastic synoviocytes showed greater binding of a recombinant CD6 fusion protein than of anti-ALCAM mAb. With interferon-gamma treatment of synovial fibroblasts, binding of both reagents increased, but this was more marked for binding of CD6 fusion protein. Exposure of synovial fibroblasts to other cytokines or to the superantigen staphylococcal enterotoxin A also regulated binding of CD6 fusion protein and anti-ALCAM mAb in a discordant manner. Immunoprecipitation of proteins from membrane extracts of 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 function, the present findings strongly suggest the existence of a second CD6 ligand distinct from CD166, which can be expressed by synovial fibroblasts as well as other cells.