Aspects of the developmental morphology of california encephalitis virus in cultured vertebrate and arthropod cells and in mouse brain

An electron microscopic study was made of the replication of California encephalitis (La Crosse strain) in cultured vertebrate cells (Vero, African green monkey kidney), in a line of cultured mosquito cells (Aedes albopictus), and in brain tissue of suckling mice. Morphologically similar virus particles, approximately 95 nm in diameter, were encountered in all three systems, and a common mode of virus assembly and maturation appeared to obtain. Virus assembly was shown to occur exclusively at internal cytomembrane interfaces, the Golgi complex appearing as the initial assembly site, which site became less focal as infection progressed due to the proliferation of Golgi smooth membranes and the dilation of cisternae and vesicles. The assembly process involved viral budding into cisternal and vesicular lumina, the virion thereby acquiring its limiting membrane. The envelope of such intracellular virions exhibited a poorly defined fringe, approximately 8 nm in width, which appeared to undergo a maturational change as the virions were discharged from the cell, such that, extracellularly, virions displayed a well-developed fringe, approximately 12 nm wide—a change especially noteworthy in the case of virions in infected mouse brain. The presence was noted in infected Vero cells—and in one instance in an infected mosquito cell—of crescent-shaped segments of thickened cisternal membrane, which possibly represented an early phase of viral assembly in which nucleocapsid aligned itself in close apposition to a membrane segment preparatory to the initiation of budding. In areas of the cytoplasm adjacent to sites of viral assembly in neurons, a fine granulofibrillar matrix was frequently found, enmeshed in which were numbers of 50–60 nm spherical structures. In a low proportion of cells from infected mosquito cultures, dense granulofibrillar masses were found in the cytoplasm. In Vero cells, infection with CE virus was cytolytic, while in A. albopictus cells, no gross cytopathic effects were manifest, and persistently infected cultures developed upon subcultivation. However, less than 10% of challenged mosquito cells became productively infected and for a proportion of these, at least as determined by electron microscopy, the infection was lethal.