Mechanisms of Mycobacterium avium subsp. paratuberculosis induced apoptosis and necrosis in bovine macrophages

The interaction between Mycobacterium avium subsp. paratuberculosis (Map) and macrophages is a complex process to maximize the chances of their respective survival. Previous studies have shown that Map induces cell death in macrophages, but the mechanism is not known. In the present study, we investigated the mechanism by which Map induces cell death in bovine macrophages using the fluorescent and electron microscopic techniques. The macrophages infected with an equal number of Map (i.e., multiplicity of infection, MOI = 1) showed no changes of cell death, but those macrophages infected at MOI = 10 showed the morphological changes consistent with apoptosis. Strikingly, the macrophages infected by Map at MOI = 50 showed the changes of apoptosis and necrosis. The Map-induced apoptosis was a caspase-dependent mechanism at MOI = 10 while it was caspase- and nitric oxide-independent at MOI = 50. The results of the present study suggest that the mitochondrial damage following Map infection initiates the cell death processes in macrophages.