Characterization of a distinct nuclear acidic protein antigen (MA) and clinical findings in systemic lupus erythematosus patients with MA antibodies.

Circulating antibodies against certain nuclear acidic protein antigens have been shown to have diagnostic and prognostic importance in connective tissue disease. We describe a new precipitin system found in the sera of patients with systemic lupus erythematosus. The antigen, called MA, was prepared from calf thymus nuclei, and was shown to be distinct from other nuclear acidic protein antigens by physicochemical and immunologic techniques. MA antibodies were detected in the serum of 12 of 66 lupus patients and in none of 554 sera from normal controls or patients with other rheumatic diseases. Lupus patients having MA antibodies had more severe disease than did lupus patients with Sm or native DNA antibodies, manifested by recalcitrant skin rashes and a significantly greater incidence of hypocomplementemia, serious renal disease, hypertension, hepatosplenomegaly, lymphadenopathy, and neurological disease (P values range from 0.025 to 0.005). The presence of circulating MA antigen was demonstrated in three lupus patients immediately before a flare of nephritis. These data suggest that MA is a nuclear acidic protein antigen that may identify a subset of lupus patients with very severe disease. The presence of the antigen in the circulation before clinical flares suggests a possible biologic role for the MA system in an immune complex nephritis.