Perforin-2 Protects Host Cells and Mice by Restricting the Vacuole to Cytosol Transitioning of a Bacterial Pathogen.

The host-encoded Perforin-2 (encoded by the macrophage-expressed gene 1,Mpeg1), which possesses a pore-forming MACPF domain, reduces the viability of bacterial pathogens that reside within membrane-bound compartments. Here, it is shown that Perforin-2 also restricts the proliferation of the intracytosolic pathogenListeria monocytogenes Within a few hours of systemic infection, the massive proliferation ofL. monocytogenesinPerforin-2(-/-)mice leads to a rapid appearance of acute disease symptoms. We go on to show in culturedPerforin-2(-/-)cells that the vacuole-to-cytosol transitioning ofL. monocytogenesis greatly accelerated. Unexpectedly, we found that inPerforin-2(-/-)macrophages,Listeria-containing vacuoles quickly (≤15 min) acidify, and that this was coincident with greater virulence gene expression, likely accounting for the more rapid translocation ofL. monocytogenesto its replicative niche in the cytosol. This hypothesis was supported by our finding that aL. monocytogenesstrain expressing virulence factors at a constitutively high level replicated equally well inPerforin-2(+/+)andPerforin-2(-/-)macrophages. Our findings suggest that the protective role of Perforin-2 against listeriosis is based on it limiting the intracellular replication of the pathogen. This cellular activity of Perforin-2 may derive from it regulating the acidification ofListeria-containing vacuoles, thereby depriving the pathogen of favorable intracellular conditions that promote its virulence gene activity.