The antigenic targets of the rheumatoid arthritis (RA)-associated autoantibodies to "citrullinated" proteins are generated following citrullination by a peptidylarginine deiminase (PAD). Of the five PAD isotypes, two - PAD2 and PAD4 - are expressed in RA synovial tissue. Within this tissue, the activation of macrophages and fibroblasts mediated by T-cell contact or driven by cytokines plays a prominent part in the pathogenesis. We wanted to determine whether cytokine stimulation and contact with T cells could play a role in PAD expression by peripheral blood monocytes and fibroblastic synoviocytes.
Human monocytes and T lymphocytes were derived from the blood of healthy donors. HUT-78 cells and T lymphocytes were stimulated with PHA and PMA. Human synovial fibroblasts were isolated after surgical synoviectomy. The expression of PAD was determined by real-time PCR and immunoblot.
In monocytes, the PADI2 and PADI4 mRNAs were transiently up-regulated by contact with stimulated HUT-78 and/or T lymphocytes. Positive modulation of the PAD2 and PAD4 proteins were also observed upon contact with stimulated HUT-78 T cells. Stimulation by IL-1β or IFN-β did not modify the PADI2 and PADI4 mRNAs, but enhanced PAD4 protein expression. No isotype of PAD was detected at the mRNA or protein level in resting or stimulated synovial fibroblasts.
Contact between stimulated T cells and monocyte-macrophages or cytokine-activated monocyte-macrophages constitutes a highly likely source of PAD2 and PAD4, which are observed in inflamed synovial tissues. In contrast, it is most unlikely that fibroblastic synoviocytes contribute to PAD expression in rheumatoid synovial membranes.