Clinical significance of U1-RNP immune complexes in mixed connective tissue disease and systemic lupus erythematosus.

We measured U1-RNP: anti-U1-RNP immune complexes (U1-RNP ICs) in patients with mixed connective tissue disease (MCTD) and systemic lupus erythematosus (SLE) to examine the clinical significance of circulating U1-RNP ICs. The level of U1-RNP ICs in 11 patients with MCTD was significantly higher than that in 22 normal subjects and there was a close correlation between the level of U1-RNP ICs and the clinical disease activity index of MCTD. In contrast, the level of U1-RNP ICs in 31 patients with SLE was not significantly higher than that in normal subjects and that was not correlated with the clinical disease activity index of SLE or the renal histologic activity index of lupus nephritis. We conclude that U1-RNP ICs are present in sera of patients with MCTD and SLE, and that the level of U1-RNP ICs may be closely associated with clinical disease activity in patients with MCTD.