Abstract
The ability of the host to resist infection to a variety of intracellular pathogens, including mycobacteria, is strongly dependent upon the expression of the Bcg gene. Mouse strains which express the resistance phenotype (Bcgr) restrict bacterial growth, whereas susceptible strains (Bcgs) allow bacterial growth. Expression of the Bcg allele is known to influence the priming of host macrophages (M phi s) for bactericidal function. In the present work, bone marrow-derived M phi s from congenic BALB/c (Bcgs) and C.D2 (BALB/c.Bcgr) mice were infected with the virulent strain Mycobacterium avium TMC 724 to define the mechanism involved in growth restriction of M. avium. By combining CFU measurements and ultrastructural analyses, we show that growth of this bacterium is restricted in marrow M phi s from resistant mice. Using acid phosphatase as a lysosomal marker, we provide evidence that the hydrolytic activity of M phi s, as measured by the capacity of lysosomes to fuse with and transfer active hydrolytic enzymes to phagosomes in which M. avium resides, is an expression of the Bcg gene and that this phenomenon is a key antibacterial activity responsible for growth restriction of M. avium: (i) the percentage of phagosome-lysosome fusions was twice as high in Bcgr M phi s as in Bcgs M phi s, and (ii) the percentage of intact viable bacteria residing in acid phosphatase-negative phagosomes was twice as low in Bcgr M phi s as in the Bcgs counterparts. These differences are not due to a lower activity of the enzyme in Bcgr M phi s. The mechanism by which the Bcg gene exerts control over the phagolysosomal fusion is discussed.