Abstract
Discussion and Summary
These findings on the reappearance of the SPV-antigen, when shifted to the lower temperature, suggest that the genome of SPV has been persisting in the SPV-infected CRK cells even after the disappearance of the viral antigen detectable by immunofluorescence in their sixth passage. The idea of transferring the cells to 30°C was primarily considered because of the fact that the temperature at the surface of the skin of domestic rabbits rarely exceeded 30°C when measured under the environmental temperature of 15°C (9).
Although the procedure of lowering the temperature appears to provide a favorable condition for the functional expression of the “masked” SPV genome and subsequently leads to viral antigen synthesis, the mechanism of this phenomenon remains to be clarified. An alternative interpretation which cannot be excluded completely is that the reappearance of the specific fluorescent reaction could be due to the “accumulation” of the SPV-antigen by the slowdown of cellular activities as a result of exposure to the lower temperature.
The kidney tissue of cottontail rabbit was utilized in the attempt for the in vitro synthesis of SPV merely because of a well-known fact that this wild animal is the only host in which SPV can be produced in vivo. The observation of the specific intranuclear fluorescence among the cells inoculated with SPV in vitro clearly suggests that the synthesis of the SPV-virion antigen is taking place at the reactive site. Such localization of the viral antigen in vitro is consistent with the site of the viral antigen synthesis in the cells of the Shope papillomas (10,11) and also with the antigen localization in other papova virus infections (12).
The difference in the sites of the immunofluorescent reaction observed in this study, i.e., intranuclear fluorescence in the CRK cells primarily infected with SPV and cytoplasmic fluorescence in the cells in later passages exposed to 30°C, requires further comments. In the previous observations with in vitro cultured cells of the Shope system, both nuclear and cytoplasmic fluorescent reactions have been noticed. The primary cultured cells derived from Shope papilloma exhibited conspicuous intranuclear fluorescence when tested against anti-SPV antiserum (11) whereas the cultured cells derived from the transplantable carcinomas V × 2 and V × 7 both showed immunofluorescent reaction mostly in the cytoplasm (6). At the present time, suitable explanation is not available on this point. Studies are in progress to investigate further the sequence of events occurring in this system.
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