Murine cytomegalovirus infection of epithelial cells in mouse tracheal ring organ culture

Animal cytomegaloviruses

Cytomegaloviruses are agents that infect a variety of animals. Human cytomegalovirus is associated with infections that may be inapparent or may result in severe body malformation. More recently, human cytomegalovirus infections have been recognized as causing severe complications in immunosuppressed individuals. In other animals, cytomegaloviruses are often associated with infections having relatively mild sequelae. Many of these sequelae parallel symptoms associated with human cytomegalovirus infections. Recent advances in biotechnology have permitted the study of many of the animal cytomegaloviruses in vitro. Consequently, animal cytomegaloviruses can be used as model systems for studying the pathogenesis, immunobiology, and molecular biology of cytomegalovirus-host and cytomegalovirus-cell interactions.

Effects of cell source, mouse strain, and immunosuppressive treatment on production of virulent and attenuated murine cytomegalovirus

Murine cytomegalovirus pools from various in vitro and in vivo sources were compared for virulence in suckling mice in an effort to identify the conditions which were necessary for the production of virulent and attenuated viruses. Virus passaged in tracheal ring and salivary gland organ cultures, where virus is produced primarily by epithelial cells, was even more attenuated than virus passaged in mouse embryo fibroblasts. The attenuation observed after passage in all three of these in vitro systems did not appear to be due to defective interfering particles. We also found that virus produced in vivo in salivary glands became attenuated with time after infection. Virus harvested from salivary glands 5 to 6 weeks after infection was highly attenuated compared with both salivary gland-passaged virus harvested 2 to 3 weeks after infection and tissue culture-passaged virus. The attenuation of salivary gland-passaged virus with time was reversed when animals were treated with cyclophosphamide before the virus was harvested. A comparison of virus pools harvested from susceptible and resistant mouse strains indicated that the mouse strain had little effect on the virulence of the virus produced. When the various sources of virus tested in this study were ranked in terms of the virulence of the virus produced, salivary glands in intact mice either 2 to 3 weeks after infection or after cyclophosphamide treatment produced the most virulent virus, followed by mouse embryo fibroblast cultures, tracheal ring and salivary gland organ cultures, and, finally, salivary glands in intact mice 5 to 6 weeks after infection.

Increased susceptibility to cytomegalovirus infection in beige mutant mice

Mice homozygous for the beige gene (bg/bg) are a homologue of the Chédiak-Higashi syndrome of man and are known to be selectively defective in natural killer (NK) cells. We have compared the susceptibility of bg/bg and bg/+ C57BL/6J mice to infection with murine cytomegalovirus (MCMV). Beige mice are more susceptible to lethal infection and develop 33- to 43-fold higher virus titers in the liver, spleen, and kidney than do bg/+ mice after a sublethal infection, although virus replication is the same in vitro in cultured fibroblasts or epithelial cells from these mice. Inoculation with a sublethal dose of virus stimulates a NK cell response, although this is lower in bg/bg mice despite higher titers of interferon type 1 than in bg/+. A dose of MCMV that is lethal only to bg/bg augments cytotoxicity within 12 hr in bg/+ mice, whereas cytotoxicity in bg/bg remains very low. In bone marrow chimeras, recipients of bg/bg marrow were more susceptible to MCMV and had lower NK cell responses after virus inoculation than did recipients of marrow from bg/+ donors. The greater susceptibility of beige mice to the virus suggests that NK cells may contribute to resistance early in McMV infection.

Replication of murine cytomegalovirus in murine Y-1 cells

Murine Y-1 cells, a continuous epithelial cell line derived from an adrenal adenocarcinoma, are highly susceptible to murine cytomegalovirus (MCMV) infection. Inoculation of these cells with MCMV results in production of infectious virus and is associated with development of both cytopathic changes and the expression of viral-specific nuclear and cytoplasmic antigens. Examination of the ultrastructure of infected Y-1 cells reveals MCMV in various stages of replication. Cytomegalovirus replication in this hormone-responsive epithelial cell line should provide a model system in which virus--host cell interactions and their effects on cellular metabolism can be studied.