CD27 and rheumatoid arthritis: Apart from their surface expression, the soluble form of CD27 (sCD27) is extracellularly released upon TCR activation via proteolytic cleavage of membrane-bound CD27 by metalloproteinase (37), which is evident in the clinical specimens of patients with autoimmune inflammatory diseases such as rheumatoid arthritis (RA) (38)and systemic lupus erythematosus (SLE) (39).