IMPLICATION OF PHAGOSOME-LYSOSOME FUSION IN RESTRICTION OF MYCOBACTERIUM-AVIUM GROWTH IN BONE-MARROW MACROPHAGES FROM GENETICALLY RESISTANTMICE

The ability of the host to resist infection to a variety of intracellu lar 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 str ains (Bcg(s)) allow bacterial growth. Expression of the Bcg allele is known to influence the priming of host macrophages (Mphis) for bacteri cidal function. In the present work, bone marrow-derived Mphis from co ngenic BALB/c (Bcg(s)) and C.D2 (BALB/c.Bcg(r)) mice were infected wit h the virulent strain Mycobacterium avium TMC 724 to define the mechan ism involved in growth restriction of M. avium. By combining CFU measu rements and ultrastructural analyses, we show that growth of this bact erium is restricted in marrow Mphis from resistant mice. Using acid ph osphatase as a lysosomal marker, we provide evidence that the hydrolyt ic activity of Mphis, 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 phenome non is a key antibacterial activity responsible for growth restriction of M. avium: (i) the percentage of phagosome-lysosome fusions was twi ce as high in Bcg(r) Mphis as in Bcg(s) Mphis, and (ii) the percentage of intact viable bacteria residing in acid phosphatase-negative phago somes was twice as low in Bcg(r) Mphis as in their Bcg(s) counterparts . These differences are not due to a lower activity of the enzyme in B cg(r) Mphis. The mechanism by which the Bcg gene exerts control over p hagolysosomal fusion is discussed.