Serological analysis of C-terminal region of α antigen from Mycobacterium avium-intracellulare complex and Mycobacterium tuberculosis

The α antigen, which is a 30 kDa protein secreted by mycobacterial species, is an immunodominant antigen. The C-terminal regions of α antigens are highly divergent, though there are regions where the amino acid sequence of α antigen is conserved. We investigated whether the C-terminal regions of the Mycobacterium avium α antigen, M. intracellulare α antigen and M. tuberculosis α antigen contain sequence-specific B-cell epitopes. The C- terminal regions of M. avium α antigen and M. intracellulare α antigen reacted to anti-M. avium α antigen but not to anti-M. tuberculosis α antigen derived from rabbits. Thus, M. avium and M. intracellulare have an antigenic determinant in common with rabbit. The C-terminal region of M. tuberculosis α antigen did not react to anti-M. avium α antigen or anti-M. tuberculosis α antigen. An enzyme-linked immunosorbent assay revealed that only the C-terminal region of M. avium α antigen reacted to the sera of two of six patients with M. avium-intracellulare (MAC) but not to the sera of patients with M. tuberculosis. In contrast, the C-terminal regions of M. intracellulare α antigen and M. tuberculosis α antigen were not recognized by the sera from patients with MAC or M. tuberculosis. This region of M. avium α antigen can produce a sequence-specific B-cell epitope in humans.