Abstract
The sensitivity of mice to mouse hepatitis virus 3 (MHV3) varies according to strain, age, and immune status of the animals. In semisusceptible strains, mice surviving the acute phase of infection develop a chronic disease characterized by the occurrence of paralysis, virus persistence, and immunodeficiency. Persistent MHV3 infections established in vitro in YAC and RDM -4 mouse lymphoid cell lines were characterized by virus production, presence of cytoplasmic viral antigens, and cell lysis. The occurrence of cell "crisis" in YAC cells was manifested by a sharp increase in cell lysis and in the number of fluorescent cells and, concomitantly, by a marked decrease in virus titers. A relationship was observed among the percentage of fluorescent cells, cell lysis, and virus yield and was modulated by renewal of culture media, change in temperature, or inhibition of cellular RNA synthesis. Cell cloning and antibody treatment experiments indicated that viral transmission was performed by viral infection of newly permissive cells produced by the division of uninfected cells in the culture and not by transmission of viral information by infected dividing cells. The biological and biochemical properties of MHV3 variants derived from persistently infected YAC lymphoid cells were characterized. Thermosensitivity and thermolability of cloned viruses originating from persistently infected YAC cells, as well as parent virus suspensions, were studied. A similar heterogeneity was observed when YAC-derived cloned substrains (YAC-MHV3) were compared with parent-derived cloned viruses, indicating that no selection of temperature-sensitive mutants was induced in persistently infected YAC cells. However, the capacity of MHV3 to induce a lethal acute disease when injected into susceptible mice was lost very rapidly. The absence of pathogenicity was related to the induction of a subclinical infection which elicited defense mechanisms. These data suggest, therefore, that MHV3 replication in lymphoid cell lines leads to induction or selection of variants which maintain pathogenicity in vitro but display reduced pathogenic effects in vivo.