A quantitative spleen colony assay method for tumor cells induced by Friend leukemia virus infection in mice.

are characteristic of the early disease induced by this virus (3, 17, 18, 30). Late in the course of Friend virus infection, cells have been demonstrated in the spleen which have the capacity for continuous proliferation when transplanted into normal hosts or cultivated in vitro (8, 16, 19, 20, 34). But whether the infected cells acquire these neoplastic character istics late in the disease or whether tumor cells already exist during the early phases of infection and form an integral part of the new cell population that arises in response to the virus is still an open question. Because of their inherent interest for studies on virus-induced abnormality of growth regulation and because so little is known about their properties, we felt that it would be valuable to try to devise an assay method for investigating these early cells quantitatively. The spleen colony method seemed an obvious choice in view of its successful application in quantitative studies on lym phoma (5†" 7,9) and plasma cell tumor populations (D. E. Bergsagel and F. A. Valeriote, in preparation). This method was based on the observation (7) that, following the intra venous injection of neoplastic (but not normal) cell suspensions into normal, genetically compatible mice, discrete colonies de veloped in the spleen. These colonies, which were large enough to be visible to the naked eye 8†" 12days after injection, could be shown to be derived from the extensive proliferation of donor cells that had settled in the normal host spleen and could thus be regarded as tumor colonies. Since the number of tumor colonies formed per spleen was found to be directly proportional to the number of nucleated cells injected, counts of these colonies could be used to obtain a direct measure of the number of tumor colony-forming units (TCFU) in the in jected cell suspension, no matter how heterogeneous its cell population, and without the necessity for any assumptions me garding morphologic properties of the tumor cells responsible for colony formation. It was not unreasonable to expect that if the spleens of mice, recently infected with virus, contained tumor cells, these cells might also form colonies in the spleens of normal hosts. How ever, it was evident from the beginning of the present study that a difficulty existed here which was not encountered with the lymphomas or with the plasma cell tumor. Cell suspensions from Friend virus-infected mice may contain …