Murine K-papovavirus was serially passaged 10 times in primary cultures of mouse embryo cells, using cell suspensions and media from infected cultures to inoculate fresh flasks at each passage level. Titers of K virus infectivity in cell suspensions and media rose during serial passage, but viral hemagglutinating activity was not detected in cells or media before the 10th passage. Although the infectivity of K virus for mouse embryo cultures was enhanced by serial passage, lethality of the virus for suckling mice was lost. The present study represents the first successful serial transmission of K virus infection in vitro.

Papovavirus K (K virus) is a murine papovavirus that produces a fatal interstitial pneumonia in newborn mice and a clinically inapparent infection in older animals. The present study was conducted to determine whether the virus produces latent infection in animals surviving acute infection and whether the infection can be reactivated by immunosuppression. Mice were inoculated by the oral route with 100 newborn mouse 50% lethal doses at 12 days of age and followed for 8 months by using immunofluorescence staining. Cells positive for K virus capsid antigen were found in lungs, livers, kidneys, intestines, and brains for 6 months, but not thereafter. Organ examined at 8 months were negative for virus by tissue culture assay, mouse inoculation, explantation, and cocultivation. Immunosuppression of the remaining animals with 8 weekly injections of cyclophosphamide (150 mg/kg) resulted in the reappearance of viral antigen and infectious virus in multiple organs including brains. The highest titers of virus were present in kidneys. One animal sacrificed after 42 days of immunosuppression was found to have a small pulmonary adenoma or alveologenic carcinoma, but efforts to explant this tumor into tissue culture were unsuccessful. The present study demonstrates that K virus produces a latent infection that is reactivated by immunosuppression, and our results raise questions as to whether reactivated infection may occasionally be associated with the development of neoplasia.

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.