Immunogenicity and cytoadherence of recombinant Heparin Binding Haemagglutinin (HBHA) of Mycobacterium avium subsp. paratuberculosis: functional promiscuity or a role in virulence?

Mycobacterium avium subsp. paratuberculosis (MAP) is the causative agent of Johne's Disease, a chronic granulomatous enteritis of ruminants. Recently, an association of MAP bacilli with Crohn's disease in humans has been proposed. Due to genetic similarities and serological cross-reactivity of the M. avium complex with other mycobacteria, functional analysis of species-specific proteins may allow new insights into the pathogenesis of mycobacterial diseases. We report production and molecular characterization of the recombinant HBHA from the MAP complex bacilli. The HBHA was expressed in Escherichia coli and Mycobacterium smegmatis using efficient expression vector systems. The recombinant HBHA was found to be immunogenic and therefore induced antibody responses in cattle against the MAP bacilli with a possible cross reactivity with M. bovis infection. The MAP complex HBHA was thus found to be a target of the host humoral responses in Johne's disease. The recombinant HBHA protein was also found to be adherent to the Caco2 cell lines in-vitro, a significant observation to understand possible virulence mechanisms. Since M. tuberculosis HBHA was earlier shown to be involved in dissemination of the tubercle bacilli, the immunogenicity and cytoadherent nature of this MAP protein possibly suggests functional promiscuity.