The production of rheumatoid factor (RF) and antinuclear antibody by B-cells could depend on different cytokines action. We evaluated IL-13 serum levels in 230 patients with autoimmune rheumatic diseases including rheumatoid arthritis (RA) [M/F=22/62; mean age=55.2 (25-76) yrs; mean disease duration = 116 (5-605) months], SLE [M/F=17/97; mean age=38.3 (15-70) yrs; mean disease duration = 77 (1-456) months], Sjögren's syndrome (SS) [M/F=2/50; mean age = 55.2 (26-81) yrs; mean disease duration = 82 (3-540) months], and systemic sclerosis (ScS) [M/F = 1/31; mean age=50.6 (20-73) yrs; mean disease duration = 113 (12-276) months], in order to investigate the relationship of this cytokine with the autoantibody profile.
Serum levels of IL-13 (pg/ml) were significantly increased in patients with RA (P < 0.00003), with SLE (P < 0.03), with SS (P < 0.0007), with ScS (P < 0.025) as compared to controls. IL-13 serum levels correlated with those of RF in RA (P < 0.00001), SLE (P < 0.003) and ScS (P < 0.03). IL-13 levels were higher in RA (P < 0.0002), SLE (P < 0.004) and ScS (P < 0.05) patients with RF than patients without RF. SS patients with anti-SSA/Ro antibodies had significantly higher IL-13 levels than SS patients without this autoantibody (P < 0.036). No statistically significative correlation was found between IL-13 levels and any other antinuclear autoantibody or total immunoglobulin levels or main clinical features of each disease.
This study suggests that IL-13 may be involved in the pathogenesis of autoimmune rheumatic diseases, with a relevant role on RF production. In SS, the lack of correlation between IL-13 and RF is probably due to the peculiar characteristics of this antibody in the disease. We can conclude that the mechanisms involved in RF synthesis recognise different pathways depending on the underlying autoimmune disease.