Naturally processed chromatin peptides reveal a major autoepitope that primes pathogenic T and B cells of lupus

Major autoepitopes for pathogenic Th cells of lupus were previously found in core histones of nucleosomes by testing overlapping synthetic peptides. To detect other dominant epitopes, we eluted peptides from MHC class II molecules of a murine lupus APC line that was fed with crude chromatin. The eluted peptides were purified by reverse-phase HPLC and tested for their ability to stimulate autoimmune Th clones, and then analyzed by mass spectrometry. Amino acid sequences of stimulatory fractions revealed three new autoepitopes. Two of the epitopes were homologous to brain transcription factor BRN-3, whereas the third sequence was homologous to histone H1′_22-42. H1′_22-42 stimulated autoimmune Th cells to augment the production of pathogenic antinuclear Abs, and was much more potent than other nucleosomal epitopes in accelerating glomerulonephritis in lupus-prone (SWR x NZB)F₁ (SNF₁) mice. Remarkably, a marked expansion of Th1 cells recognizing the H1′_22-42 epitope occurred spontaneously in SNF₁mice very early in life. A significant proportion of H1′_22-42-specific T cell clones cross-reacted with one or more core histone epitopes, but not with epitopes in other lupus autoantigens. The H1′_22-42 epitope was also recognized by autoimmune B cells, and with the onset of lupus nephritis, serum autoantibodies to the H1′_22-42epitope become increasingly cross-reactive with nuclear autoantigens. Convergence of T and B cell epitopes in H1′_22-42 and its ability to elicit a cross-reactive response make it a highly dominant epitope that could be targeted for therapy and for tracking autoimmune T and B cells.