Drug-induced anti-histone autoantibodies display two patterns of reactivity with substructures of chromatin

Increasing evidence suggests that autoantibodies in the rheumatic diseases are a consequence of immune selection by self-material, but the nature of the in vivo immunogen is unknown. Insight into this problem may be obtained by measuring autoantibody binding to various forms of a target antigen. Antihistone antibodies arising as a side effect of therapy with various drugs offer an opportunity to explore this premise because many forms of histone have been characterized and adapted to ELISA formats. Two patterns of antibody reactivity were observed. All 21 patients with symptomatic procainamide-induced lupus and 7 of 12 patients with quinidine-induced lupus had IgG antibodies reacting predominantly with the (H2A-H2B)-DNA complex and with chromatin. In contrast, antibodies in 19 of 24 patients taking procainamide without accompanying lupus-like symptoms did not show any pattern. The second pattern was observed in 18/19 chlorpromazine-treated patients and 14/17 patients with hydralazine-induced lupus in which IgM antibodies displayed more reactivity with DNA-free histones than with the corresponding histone-DNA complexes and almost no binding to H1-stripped chromatin. Absorption studies were entirely consistent with these results. Thus, the two patterns of reactivity with nucleosomal components reflect the molecular substructure of chromatin, suggesting that two processes underlie antihistone antibody induction by drugs. In one, IgG autoantibodies appear to be elicited by chromatin, whereas in the other, autoimmune tolerance to native chromatin appears largely intact, and IgM antibodies may be driven by DNA-free histone.