Implication of phagosome-lysosome fusion in restriction of Mycobacterium avium growth in bone marrow macrophages from genetically resistant mice

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 (Bcg(r)) restrict bacterial growth, whereas susceptible strains (Bcg(s)) allow bacterial growth. Expression of the Bcg allele is known to influence the priming of host macrophages (Mφs) for bactericidal function. In the present work, bone marrow-derived Mφs from congenic BALB/c (Bcg(s)) and C.D2 (BALB/c.Bcg(r)) 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 Mφs from resistant mice. Using acid phosphatase as a lysosomal marker, we provide evidence that the hydrolytic activity of Mφs, 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 Bcg(r) Mφs as in Bcg(s) Mφs, and (ii) the percentage of intact viable bacteria residing in acid phosphatase-negative phagosomes was twice as low in Bcg(r) Mφs as in their Bcg(s) counterparts. These differences are not due to a lower activity of the enzyme in Bcg(r) Mφs. The mechanism by which the Bcg gene exerts control over phagolysosomal fusion is discussed.