Inhibited apoptosis of lymphocytes present in synovial fluid (SFL) and persistently infiltrating synovial tissue may be crucial in the pathogenesis of rheumatoid arthritis (RA). Similarly, this may be the case in juvenile idiopathic arthritis (JIA). Little is known about lymphocyte apoptosis in this disease. Recently, we reported significantly enhanced apoptosis of peripheral blood lymphocytes (JIA-PBL) compared to synovial fluid (JIA-SFL) or healthy lymphocytes, with downregulation of p53 in JIA-SFL. In this study we assessed other possible molecular mechanisms of this phenomenon.
PBL from 31 children with JIA and 26 healthy children were examined. SFL obtained from 18 patients was also studied. Apoptosis was assessed by TdT-mediated dUTP-biotin nick-end labeling (TUNEL) method. Expression of several apoptosis-regulating proteins was analyzed, including myeloid cell leukemia 1 (Mcl-1), cross-linked inhibitor of apoptosis (XIAP), FLICE-inhibitory protein (FLIP), or Bcl-xL inhibitors and proapoptotic p53, Bcl-w, Bak, and Bid.
We found significant overexpression of Mcl-1 and XIAP in JIA-SFL (p < 0.001 and p < 0.02, respectively). Expression of Mcl-1 and XIAP in SFL correlated inversely with the apoptotic index (p < 0.002 and p < 0.01, respectively). FLIP expression was also distinctly higher in SFL than in JIA-PBL; however, the difference was not statistically significant (p = 0.061). No statistically significant differences were found in the expression of other proteins between SFL and PBL.
This is the first study showing that upregulation of anti-apoptotic Mcl-1 and XIAP proteins, along with downregulation of p53 protein, is correlated with inhibition of JIA-SFL apoptosis.