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High mobility group box 1 (HMGB1) protein plays multiple roles in transcription, replication, and cellular differentiation. HMGB1 is also secreted by activated monocytes and macrophages and passively released by necrotic or damaged cells, stimulating inflammation. HMGB1 is a novel antigen of anti-neutrophil cytoplasmic antibodies (ANCA) observed in the sera of patients with ulcerative colitis and autoimmune hepatitis, suggesting that HMGB1 is secreted from neutrophils to the extracellular milieu. However, the actual distribution of HMGB1 in the cytoplasm of neutrophils and the mechanisms responsible for it are obscure. Here we show that HMGB1 in neutrophils is post-translationally mono-methylated at Lys42. The methylation alters the conformation of HMGB1 and weakens its DNA binding activity, causing it to become largely distributed in the cytoplasm by passive diffusion out of the nucleus. Thus, post-translational methylation of HMGB1 causes its cytoplasmic localization in neutrophils. This novel pathway explains the distribution of nuclear HMGB1 to the cytoplasm and is important for understanding how neutrophils release HMGB1 to the extracellular milieu.

HMGB1 and HMGB2 are abundant nonhistone chromosomal proteins in eukaryotic organisms. Their respective primary sequences are highly conserved. Our previous studies showed that these proteins are novel autoantigens of anti-neutrophil cytoplasmic antibodies in sera from patients with ulcerative colitis (UC), rheumatic disease and autoimmune hepatitis (AIH). In the present paper, we showed that anti-HMGB1 and HMGB2 antibodies in sera of patients with UC do not recognize HMGB1 in neutrophils while they recognize the protein in lymphocytes. Anti-HMGB2 monoclonal antibody FBH7, recognizing HMGB1 in lymphocytes, showed a similar profile to the antibodies in the patients' sera. In order to elucidate the difference in immunoreactivity to HMGB1 between neutrophils and lymphocytes, we mapped the epitope for FBH7 by means of several methods. The results showed that FBH7 recognizes the intact conformation composed of 52-56 residues of HMGB1 in lymphocytes. This suggested that HMGB1 in neutrophils is conformationally changed in the epitope or the peripheral structure of the epitope from the protein in lymphocytes. The apparent conformational change of HMGB1 between neutrophils and lymphocytes will be important for understanding the functional difference of HMGB1 in these cells.