Antibodies to the collagen-like region of C1q and type II collagen are independent non-cross-reactive populations in systemic lupus erythematosus and rheumatoid arthritis.

The collagen-like region (CLR) of the first component of complement, C1q, and type II collagen are structurally similar, raising the possibility of epitopes in common, and of the existence of autoantibodies that are cross-reactive. Accordingly, antibodies to the CLR of C1q and to type II collagen were measured in patients' sera with systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) by an ELISA. IgG antibodies to the CLR of C1q were present in 17% of patients with SLE but none with RA, and IgA antibodies were present in 10% and 8% of patients with SLE and RA, respectively. IgG antibodies to type II collagen were present in 15% and 25% of patients with SLE and RA, respectively, and IgA antibodies in 15% and 28% of patients with SLE and RA, respectively. There was no correlation in either disease between the serum levels of antibodies to the CLR of C1q and antibodies to type II collagen. For sera with antibodies to both antigens, neither competitive inhibition by ELISA nor preabsorption with the alternative antigen affected the level of reactivity to the other antigen. Thus antibodies to the CLR of C1q and antibodies to type II collagen are independent and non-cross-reactive populations, and presumably occur by different types of immunogenic stimulation.