Anti-histone autoantibodies recognize centromeric heterochromatin in metaphase chromosomes and hidden epitopes in interphase cells.

IgM autoantibodies from a subset of patients with undifferentiated rheumatic disease syndromes stained mouse kidney nuclei with a distinctive variable large-speckled (VLS) indirect immunofluorescence (IIF) pattern. However, these antibodies did not stain nuclei of tissue culture cells prepared with conventional fixation. These sera were shown to react with histone H3 by an enzyme-linked immunosorbent assay (ELISA), and adsorption with H3 reduced or eliminated the IIF reaction. Sera yielding a VLS-IIF pattern reacted in ELISA with all three H3 variants as well as the native (H3-H4)2 tetramer, but the reactive determinants were unavailable for binding when chromatin was the substrate. By IIF assay, the epitopes were exposed after treatment of tissue culture cells with 0.5 M NaCl, and were removed by 1.5 M NaCl. These sera also stained the centromeric region of metaphase chromosomes. These observations suggest that the VLS-IIF pattern is due to antibodies that recognize epitopes on constitutive heterochromatin near the centromere. The epitopes are exposed in differentiated cells but hidden in dividing cells. Histone in heterochromatin, or CENP-A, a histone-like protein in the centromere with a sequence similarity to histone H3, are candidates for the target antigen.