MPB70 and MPB83--major antigens of Mycobacterium bovis

MPB70 and MPB83 are among the most studied mycobacterial antigens. They are highly homologous proteins within Mycobacterium tuberculosis complex members, and the orthologs in different members of the complex are virtually identical. They are major antigens highly expressed by Mycobacterium bovis and considerably less abundantly expressed by M. tuberculosis. There are two genes encoding these proteins within an operon of six genes. MPB70 and MPB83 are encoded as precursor proteins with typical signal peptides for export through the general secretory pathway. MPB70 is a soluble secreted protein cleaved by signal peptidase I, while MPB83 is a glycosylated lipoprotein processed by signal peptidase II and located at the surface, possibly with the lipid tail coupled to the N-terminal cysteine embedded in the mycobacterial outer membrane. The expression of these genes is controlled by the transcriptional regulator SigK, and a point mutation in SigK explains why some Bacille Calmette-Guérin (BCG) strains express only minute amounts of MPB70 and MPB83. BCG strains that have retained the high expression of MPB70 and MPB83 of wild type M. bovis are linked to a risk of BCG osteitis in <1-year-old children as a complication to BCG vaccination at birth. The proposed mechanism for this is based on homology of these proteins with osteoblast-specific factor II which is a homofilic adhesion protein expressed on osteoblasts in growing bone tissue. T-cell responses and antibody responses to these proteins have been extensively explored for sensitive and specific diagnosis of bovine tuberculosis.