DPP4 and rheumatoid arthritis: Diminished dipeptidyl peptidase IV (DPP-IV, soluble CD26) activity in both serum and synovial fluid of RA patients has been previously reported [4,5] and we have also found a relationship between serum sCD26 levels and RA activity [6] (not all DPP-IV activity is ascribed to sCD26 [7]).