Cytokine induction by circulating immune complexes and signs of in‐vivo complement activation in systemic lupus erythematosus are associated with the occurrence of anti‐Sjögren's syndrome A antibodies

Summary Circulating immune complexes (IC) and levels of IC‐induced cytokines have been correlated with complement activation and autoantibody profiles in systemic lupus erythematosus (SLE). SLE sera were analysed concerning levels of immune complexes (IC), classical complement function and different antinuclear and anti‐C‐reactive protein (CRP) autoantibodies. Blood mononuclear cells from healthy donors were stimulated with isolated IC and production of interleukin (IL)‐10, IL‐6 and IL‐12p40 was measured. Functional experiments revealed that increased levels of IC‐induced cytokines were associated with both increased classical complement activation and the occurrence of anti‐Sjögren's syndrome A (SSA) and anti‐SSB but not other autoantibodies. Biochemical measurement of circulating IC showed that the degree of complement activation and the occurrence of anti‐SSA were synergistically associated with levels of circulating IC in SLE sera, as complement activation was a prerequisite for the enhancing effect of anti‐SSA. Anti‐CRP was associated with complement activation, but not with other autoantibodies. Our results indicate that anti‐SSA and possibly anti‐SSB antibodies influence IC formation and subsequent IC‐induced cytokine induction, and that they thereby participate in the inflammatory process in active SLE.