An in vitro assay for K-papovavirus of mice.

Primary cultures of mouse embryo cells were inoculated with K virus, a murine papovavirus, and were examined for cytopathic effect (CPE) of for the development of fluorescent antibody staining specific for K virus V antigen. CPE was not observed. However, numerous cells in infected cultures exhibited positive nuclear fluorescence, and the presence of papovavirus virions was demonstrated by electron microscopy. Extracts from infected cultures produced typical K virus pneumonia in newborn mice. Inoculation of cultures with serial dilutions of virus demonstrated that these cells provide a fluorescent antibody assay for K virus equal in sensitivity to animal inoculation methods. Although specific K virus fluorescence was also detected in cultures of fetal mouse endocardial cells, livers, placentas, and brains, positive cells were much less abundant in these cultures than in cultures of mouse embryo cells. The mouse embryo culture assay described in the present paper represents the first method of measuring K virus infectivity in vitro.