Macrophage interaction with mycobacteria including M. leprae

Resistance properties of pathogenic mycobacteria to macrophage bactericidal activity seems to be due mostly to the composition and constitution of their cell walls. In the case of Mycobacterium tuberculosis, sulfatides and polyglutamic acid could be implicated in the phenomenon of fusion inhibition between phagosomes and lysosomes. M. leprae and M. lepraemurium, which do not seem to inhibit fusions are protected by a thick electron transparent zone (ETZ) that seems to be composed of mycosides. This layer would inhibit lysosomal enzyme diffusion inside phagosomes. As ETZ does not exist in mycobacteria before their phagocytosis, we have tried to see when and how it is formed inside macrophages. We have compared ETZ formation in M. leprae and M. avium which both contain mycosides. These two species were allowed to be phagocytized by mouse bone-marrow derived macrophage and samples were taken for electron microscopy during the first hours of phagocytosis and also during several weeks of incubation. In M. avium ETZ appeared within 1 to 2 hours after phagocytosis. It seems to be formed by a sort of swelling of the thin electron transparent layer of the bacterial cell wall. This swelling occurs only in regions where the external polysaccharide layer of M. avium starts to disappear. After 1 to 3 hours, this layer was completely absent and all bacteria were enveloped in a thick ETZ. In M. leprae, the ETZ is also formed within one hour after ingestion. However, the presence in some bacteria of a very thin dense layer located at the original place of the outer dense layer of the cell wall does not fit well with the idea of ETL swelling. In addition, the appearance of a thick dense layer located between the ETZ and the phagosome membrane is not yet understood. The ETZ formed also rapidly in macrophages infected with heat killed cells of M. avium or M. leprae. This shows that its formation does not require the active participation of the bacterium. As already proposed ETZ seems to lessen considerably the diffusion of lysosomal enzymes towards the bacterium in both species. In M. leprae it seems especially efficient because despite acid phosphatase activity found in many phagosomes, neither the number of bacteria per macrophage nor their state of degradation changed during 3 and a half months of macrophage culture.(ABSTRACT TRUNCATED AT 400 WORDS)