CD6 and rheumatoid arthritis: While the nature of the endogenous CD5 ligand is yet uncertain, one of the most-well studied CD6 ligands is CD166/ALCAM, a cell adhesion molecule overexpressed in pSS salivary gland epithelial cells (8, 9, 25), but also RA synovium (61), MS blood–brain barrier endothelium (62), and lupus nephritis kidneys (63), thus contributing to T and B cell migration and infiltration at inflamed tissues in autoimmune processes.