CD6 and rheumatoid arthritis: The first clue that CD6 ligands might be involved in chemotaxis came from Enyindah-Asonye et al., who demonstrated that CD318 is shed from fibroblast-like synoviocytes (FLS) and accumulates in a soluble form in rheumatoid arthritis (RA) synovial fluid at levels higher than found in normal or RA sera.