Electron microscopic observations on granulosis virus entry, uncoating and replication processes during infection of the midgut cells of Trichoplusia ni

Abstract

A detailed ultrastructural study is presented showing the processes whereby intact granulosis virus particles invade and infect the midgut epithelial cells of Trichoplusia ni larvae by per os routes. Intact, enveloped nucleocapsids released from the proteinic crystals in the gut lumen apparently entered the columnar cells by interactions with the microvilli not yet defined and understood. The results of this study suggest that the viral envelope is lost at, or during, the entry process since only nonenveloped nucleocapsids were seen in cells shortly after infection. Phagocytosis of enveloped or naked nucleocapsids was not observed, nor were such virions observed in cytoplasmic vesicles shortly after infection. After entry, observations indicate that naked nucleocapsids interact end-on with the nuclear pore of susceptible cell nuclei to directly release viral contents into the nuclear region. Such observations as described above were frequently made shortly after introducing highly purified, occluded virions into the gut lumen. Virus progeny were observed in columnar cell nuclei 24 hours after virus entry. Certain aberrant forms of progeny nucleocapsids were also observed at that time. Assembled virions were not occluded in a proteinaceous crystal in midgut cells, but, for the most part, were released from the nuclei and caught up in vesicles which migrated toward the base of the cell, and into elements of the basement membrane; thus suggesting that virus progeny are released into the insect hemocoel by such processes. Plaque elements or structures were frequently seen on the surface of the virus-containing vesicles before and after release from the columnar cell into the hemocoel. Discussions are presented considering the source of the membranous structures associated with the virions.