Identification of a novel mycobacterial histone H1 homologue (HupB) as an antigenic target of pANCA monoclonal antibody and serum immunoglobulin A from patients with Crohn's disease

pANCA is a marker antibody associated with inflammatory bowel disease (IBD) , including most patients with ulcerative colitis and a subset with Crohn's disease. This study addressed the hypothesis that pANCA reacts with an ant igen(s) of microbial agents potentially relevant to IBD pathogenesis. Using a pANCA monoclonal antibody, we have previously identified the C-terminal basic random coil domain of histone H1 as a pANCA autoantigen, BLAST analys is of the peptide databases revealed H1 epitope homologues in open reading frames of the Mycobacterium tuberculosis genome. Western analysis of extrac ts from sis mycobacterial species directly demonstrated reactivity to a sin gle, conserved similar to 32-kDa protein. Direct protein sequencing, follow ed by gene cloning, revealed a novel 214-amino-acid protein, an iron-regula ted protein recently termed HupB, Sequence analysis demonstrated its homolo gy with the mammalian histone H1 gene family, and recombinant protein expre ssion confirmed its reactivity with the 5-3 pANCA monoclonal antibody, Bind ing activity of patient serum immunoglobulin G (IgG) to HupB did not correl ate with reactivity to histone H1 or pANCA, indicating the complex characte r of the pANCA antigen. However, anti-HupB IgA nas strongly associated with Crohn's disease (P < 0.001). These findings indicate that the 5-3 pANCA mo noclonal antibody detects a structural domain recurrent among mycobacteria and cross-reactive with a DNA-binding domain of histone HI. The association of HupB-binding serum IgA with IBD provides new evidence for the associati on of a mycobacterial species with Crohn's disease.