Conformational difference in HMGB1 proteins of human neutrophils and lymphocytes revealed by epitope mapping of a monoclonal antibody

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.