Infection of AtT20 murine pituitary tumour cells by mouse hepatitis virus strain A59: virus budding is restricted to the Golgi region.

AtT20 cells, a line of murine pituitary tumour cells that secrete adrenocorticotropic hormone (ACTH), have been infected with the coronavirus mouse hepatitis virus strain A59 (MHV-A59). Between 5% and 10% of AtT20 cells are susceptible to the infection. Unlike infections of fibroblastic sac- and 17Cl 1 cells, the infection of AtT20 cells does not lead to cell fusion, despite the production of the fusogenic E2 viral spike glycoprotein. Within infected AtT20 cells the second viral envelope glycoprotein, E1, is located in a perinuclear region; at least until very late in the infection it fails to accumulate to detectable levels in the rough endoplasmic reticulum (RER). By contrast to infection of sac- and 17Cl 1 cells, where the RER is a major site of assembly of progeny virions, in AtT20 cells budding of progeny virions is restricted to the Golgi cisternae, which eventually vesiculate, and peri-Golgi smooth membraned vesicles. Apparently, therefore, the intracellular compartments into which wild-type MHV-A59 buds are determined not by the virus but by the host cells. MHV-A59 infected cultures of AtT20 cells can be serially passaged without loss of the infection or increase in the proportion of infected cells; they become persistently infected carrier cultures. The progeny virus from serially passaged, infected AtT20 cells is apparently wild-type. It infects sac- cells and induces them to form syncitia. Within the sac- syncitia the viral E1 glycoprotein accumulates in the RER and many virions assemble there.