THE CENTRAL ROLE OF CHROMATIN IN AUTOIMMUNE RESPONSES TO HISTONES ANDDNA IN SYSTEMIC LUPUS-ERYTHEMATOSUS

To gain insight into the mechanisms of autoantibody induction, sera fr om 40 patients with systemic lupus erythematosus (SLE) were tested by ELISAs for antibody binding to denatured individual histones, native h istone-histone complexes, histone-DNA subnucleosome complexes, three f orms of chromatin, and DN4. Whole chromatin was the most reactive subs trate, with 88% of the patients positive. By chi-square analysis, only the presence of anti-(H2A-H2B), anti-[(H2A-H2B)-DNA], and antichromat in were correlated with kidney disease measured by proteinuria > 0.5 g /d. SLE patients could be divided into two groups based on their antib ody-binding pattern to the above substrates. Antibodies from about hal f of the patients reacted with chromatin and the (H2A-H2B)-DNA subnucl eosome complex but displayed very low or no reactivity with native DNA or the (H3-H4)(2)-DNA subnucleosome complex. An additional third of t he patients had antibody reactivity to chromatin, as well as to both s ubnucleosome structures and DNA. Strikingly, all sera that bound to an y of the components of chromatin also bound to whole chromatin, and ad sorption with chromatin removed 85-100% of reactivity to (H2A-H2B)-DNA , (H3-H4)(2)-DNA, and native DNA. Individual sera often bound to sever al different epitopes on chromatin, with some epitopes requiring quate rnary protein-DNA interactions. These results are consistent with chro matin being a potent immunogenic stimulus in SLE. Taken together with previous studies, we suggest that antibody activity to the (H2A-H2B)-D NA component signals the initial breakdown of immune tolerance whereas responses to (H3-H4)(2)-DNA and native DNA reflect subsequent global loss of tolerance to chromatin.