Spontaneous lymphomas in SJL/J-(v+) mice: Ecotropic and dualtropic virus expression in normal and lymphomatous tissues

Approximately 60% of inbred SJL/J-(v⁺) adult mice having high levels of ecotropic endogenous XC⁺ virus showed virus activation within the first month of life, while the others produced virus at comparable levels later on. In an attempt to correlate the time of virus activation with the incidence and latency of lymphomas, the tails of 57 1- and 2-month-old mice were tested for virus presence, and the mice were then observed for lymphoma appearance. While all 2-month-old mice expressed ecotropic virus, only 63% of the 1-month-old mice were virus-positive. However no relationship existed between early virus production (within 1 month) or late virus production and lymphoma latency, total lymphoma incidence, and histopathology. In contrast with high titers of XC⁺ virus in tail tissues of diseased mice, a markedly low virus content was found in lymphomatous organs. This difference was not due to selective growth of poor virus-producer cells or to inhibitory factors possibly released by the inflammatory cell component. Viral protein content and XC⁺ virus titer were not closely correlated in the neoplastic organs. Search for XC^- viruses revealed that only 1 of 6 aged normal and 16 of 19 lymphomatous mice produced viruses that grew on mink lung cells. By use of a standard limiting dilution cloning procedure, four isolates were obtained that showed tropism for both murine and heterologous cells. Three of these isolates induced cytopathic changes similar to those induced by MCF viruses on mink lung cells but not on mouse cells. Interference and neutralization assays performed to better characterize the virus envelope properties further indicated that SJL/J isolates had features typical of MCF dualtropic viruses.