Phenotypic expression of in vivo sensitivity to mouse hepatitis vims type 3 (MHV3) was studied in vitro in macrophages and lymphocytes. MHV3 infections were induced in peritoneal exudate (PE), nonadherent spleen (NAS) and thymus (THY) cells from resistant A/J, susceptible C57BL/6 or semisusceptible (C57BL/6xA/J)Fl mice. Differences in cytopathic effect, cell viability and virus titers were found only at 48 hrs postinfection (p.i.). "Carrier state'infections were performed at 48 hrs p.i by transfer of supernatants of infected cells to newly collected cells originating from the same strain of mice. A passage-dependent restriction of viral replication was detected in vitro and was expressed in PE, NAS and THY cells as a recessive phenotype. No defective-interfering viral particles were involved in the restriction of viral replication. Results obtained with crossed infections and determination of the number of productively infected cells demonstrated that restriction of viral replication in macrophages and lymphoid cells from resistant A/J mice is controlled by a genetically-determined intrinsic cellular mechanism acting principally on the level of production of infectious viral particles by the infected cell.
