Infection of mouse macrophages with viable Mycobacterium leprae does not induce apoptosis.

The role played by apoptosis in host response to Mycobacterium leprae is unclear. Here, we studied in vitro induction of apoptosis in mouse bone marrow-derived macrophages infected with live and irradiated M. leprae, as a function of multiplicity of infection under permissive (33 degrees C) and nonpermissive (37 degrees C) temperatures. The infected macrophages were scored for apoptosis by using DAPI (4',6-diamindino-2-phenylindole dihydrochloride) and Annexin V staining, along with activated Caspases 3 and 9 and TUNEL (terminal dUTP nick end labeling) assay. Our results show that, in contrast to uninfected cells, murine macrophages infected with live M. leprae demonstrated little, if any, apoptosis, even when macrophages had a heavy burden of live leprosy bacilli. In contrast, elevated levels of apoptosis were observed when macrophages were infected with irradiated M. leprae. The results strongly suggest that the viability and purity of the leprosy bacilli used for in vitro studies determines the extent of apoptosis observed in infected host cells.