PATHOGENESIS OF TUBERCULOSIS - INTERACTION OF MYCOBACTERIUM-TUBERCULOSIS WITH MACROPHAGES

Central to understanding the pathogenesis of tuberculosis is the inter action between the pathogen and mononuclear phagocytes. A key question about that interaction is whether Mycobacterium tuberculosis exerts a n effect on phagolysosome fusion. We have reexamined the dynamics of p hagolysosome fusion and its effect on intracellular bacterial replicat ion in M. tuberculosis-infected macrophages by performing an extensive study at the electron microscopic level. Thoria-labelled murine and h uman macrophages were infected with a virulent (H37Rv) or avirulent (H 37Ra) strain of M. tuberculosis or with Mycobacterium bovis BCG vaccin e for times ranging from 2 h to 7 days. In all cases, by 2 h postinfec tion, approximately 85% of the bacteria clearly resided in fused vacuo les. However, at 4 days postinfection, fusion levels for viable H37Rv and H37Ra were reduced by half, whereas the fusion profiles of BCG and of heat-killed H37Rv and H37Ra were unchanged. A comparison of the nu mbers of bacteria per fused and nonfused vacuoles suggests both a net transfer of bacteria out of fused vacuoles and preferential bacterial multiplication in nonfused vacuoles. H37Rv and H37Ra appeared to bud f rom the phagolysosomes into tightly apposed membrane vesicles that did not fuse with secondary lysosomes. In some cases, no such membrane wa s seen and the bacteria appeared to be free in the cytoplasm. Only via ble H37Rv showed a significant increase in bacterial counts during the course of infection. Thus, both of the attenuated strains we examined differed from the virulent strain H37Rv in their abilities to replica te successfully within macrophages, but each diverged from H37Rv at a different point in the process. Viable tubercle bacilli H37Rv and H37R a had the capacity to escape from fused vesicles as the infection prog ressed; BCG did not. After extrusion from the phagolysosome, H37Rv, bu t not H37Ra, was able to multiply. These results suggest a novel mecha nism by which virulent M. tuberculosis eludes the microbicidal mechani sms of macrophages by escaping from fused phagolysosomes into nonfused vesicles or the cytoplasm.