Necrotic death of Rhodococcus equi-infected macrophages is regulated by virulence-associated plasmids

Rhodococcus equi is a gram-positive intracellular pathogen that can cause severe bronchopneumonia in foals and AIDS patients. It has been reported that advanced infection of foals is characterized by tissue necrosis, coinciding with the presence of degenerate bacteria-laden macrophages. Here, we report that the possession of the VapA-expressing plasmid, which has been previously correlated with a high level of virulence for foals and mice, strongly increases cytotoxicity of R. equi for murine macrophage-like (J774E) cells. Isolates containing different, VapB-expressing plasmids are less virulent and also have a lower cytotoxic potential. Isogenic strains lacking either plasmid are avirulent and have a very low cytotoxic potential. We show, using fluorescence-activated cell sorter analysis (annexin V/7-amino-actinomycin D and sub-G(1)-analysis), Western blotting [poly(ADP-ribose) polymerase processing analysis], and electron microscopy (macrophage and nucleus morphologies) that the deaths of murine macrophages are the result of necrotic rather than apoptotic events. We demonstrate that the bacteria must be alive in order to act cytotoxic. Therefore, one effect of the virulence-associated plasmids during infection with R. equi is the promotion of necrotic damage to the host.