Cytoplasmic distinction of avirulent and virulent Rickettsia prowazekii: fusion of infected fibroblasts with macrophage-like cells.

L-929 cells with virulent or avirulent Rickettsia prowazekii growing in their cytoplasm were fused with cells of the macrophage-like cell line RAW264.7. Fusion occurred between the two cell types, producing heterokaryons containing R. prowazekii. Both the number of rickettsiae per infected heterokaryon and the percentage of heterokaryons infected with rickettsiae decreased in experiments with the avirulent strain compared to those with the virulent strain. These results indicate that the differential survival of the virulent strain in macrophages is some unknown function of the cytoplasm of the macrophage and that the distinction is not only at the phagolysosome level.