IgG autoantibody to the collagen-like region of Clq in hypocomplementemic urticarial vasculitis syndrome, systemic lupus erythematosus, and 6 other musculoskeletal or rheumatic diseases.

We previously found that the Clq precipitin in sera from patients with hypocomplementemic urticarial vasculitis syndrome is an IgG autoantibody to Clq. We report here a prevalence study of this autoantibody in 162 patients with musculoskeletal or rheumatic diseases including hypocomplementemic urticarial vasculitis syndrome, systemic lupus erythematosus (SLE), and rheumatoid arthritis uncomplicated by vasculitis. The autoantibody, which binds only to the collagen-like region of Clq, was found almost exclusively in hypocomplementemic urticarial vasculitis syndrome (100%) and SLE (35%) sera. Our results support the idea that among rheumatic diseases, anti-Clq autoantibody develops in disorders characterized by immune complex mediated injury, particularly of cutaneous and glomerular microvasculature.