Identification and characterization of serologically active mycobacterial
antigens are prerequisites for the development of diagnostic
reagents. We examined the humoral immune responses of active
tuberculosis (TB) patients against Triton-soluble
proteins extracted from Mycobacterium tuberculosis by immunoblotting. A 29-kDa
protein reacted with
immunoglobulin M (
IgM) in the pooled sera of the patients, and its N-terminal amino acid sequence matched that of the
heparin-binding hemagglutinin (HBHA). Recombinant full-length HBHA was expressed in Escherichia coli (rEC-HBHA) and M. smegmatis (rMS-HBHA). In immunoblot analysis, the
IgM antibodies of the TB patients reacted strongly with rMS-HBHA but not with rEC-HBHA, whereas the
IgG antibodies of these patients reacted weakly with both recombinant HBHA
proteins. In
enzyme-linked
immunosorbent assay analysis using rMS-HBHA and 85B as
antigens, the mean levels and sensitivities of the anti-HBHA
IgM antibodies of the TB patients were significantly higher than those of the anti-
antigen 85B
IgM antibodies, while the
IgG antibodies showed the opposite results. Of interest in this respect, the pooled sera from the TB patients that contained anti-HBHA
IgM antibodies neutralized the entry of M.
tuberculosis into epithelial cells. These findings suggest that
IgM antibody to HBHA may play a role in protection against extrapulmonary dissemination.