IN VITRO INTERACTION OF MOUSE HEPATITIS VIRUS AND MACROPHAGES FROM GENETICALLY RESISTANT MICE: II. BIOLOGICAL CHARACTERIZATION OF A VARIANT VIRUS MHV(C

A variant mouse hepatitis virus MHV(C 3 H) to which cultured peritoneal macrophages from both PRI and C 3 H mice were susceptible was isolated from stocks of the MHV(PRI) strain of mouse hepatitis virus. It was cloned on C 3 H macrophage monolayers and killed both adult PRI and C 3 H mice when injected intraperitoneally. This new variant was antigenically indistinguishable from the wild type virus. While the emergence of the variant virus was delayed in the course of infecting C 3 H macrophages with large inocula of MHV(PRI), the second passage grew to a high titer in both cell types without delay. Thus, adaptation to the new host was immediate. Interference, apparently not interferon-mediated, between the two variant viruses may have been the cause for the delay in the emergence of the variant virus. The delayed destruction of C 3 H-cultured macrophages by large inocula of MHV(PRI) uniformly resulted in the emergence of MHV(C 3 H). Whether the new variant emerged as a result of a selection of a pre-existing stable mutant or was conditioned by "growth" in the resistant host was not determined.