Objective: To elucidate the nature of the antigen reactive with the "lupus erythematosus (LE) cell factor," the autoantibody involved in the LE cell phenomenon.
Methods: Serum samples from systemic lupus erythematosus (SLE) patients who were positive for the LE cell phenomenon (LEc+) and SLE patients who were negative for the LE cell phenomenon (LEc-) were used to characterize the nuclear antigen bound by the LE cell factor, by immunoblotting and immunoprecipitation techniques.
Results: All LEc+ sera, but none of the LEc- sera, uniformly reacted with a double band of MW approximately 30 kd in nuclear extracts. Depletion of nuclear protein extracts of antigens bound by pooled LEc- serum allowed precipitation of a low molecular weight protein by pooled LEc+ serum. This protein was able to block LE cell formation by LEc+ serum. Based on its reactivity with antihistone antibody and an electrophoretic mobility identical with that of precipitated and purified histone H1, this protein was identified as histone H1. Moreover, all LEc+ sera, but none of the LEc- sera, reacted with purified histone H1 by immunoblotting, whereas other histones were reactive with both types of sera. In addition, purified histone H1, but none of the other histones, strongly inhibited the induction of LE cells by LEc+ serum.
Conclusion: Histone H1 represents the major antigenic component recognized by the LE cell factor. Thus, the LE cell phenomenon appears to be due primarily to anti-histone H1 reactivity.