Mycobacterium vaccae and immune responses: Implications for leprosy control

Abstract

Mycobacterium vaccae, common in some tropical environments, may have a beneficial effect on the incidence of leprosy by acting as a natural vaccine, or have a real or apparent harmful effect by interfering with the protection afforded by BCG vaccination. We are using an animal model to assess these possibilities. The results reported here show that a strain of M. vaccae isolated from Ugandan mud can evoke a significant immune response in mice sensitized subcutaneously or orally. Spleen cells from such mice responded equally well in vitro to M. vaccae and BCG as measured by two independent assays. Good responses were observed for at least 3 months after oral exposure to M. vaccae even though no visible organisms could be detected in the organs at this time showing that persistence of M. vaccae is not necessary for expression of sensitization. These experiments support the idea that people who become sensitized to M. vaccae or certain other environmental mycobacteria might be expected to show some resistance to leprosy. However, BCG vaccination might appear ineffective in that many individuals would already be sensitized to antigens common to all mycobacteria.