Observations on lymphomagenesis and lymphoma in AKR mice. A description of prelymphoma changes in the thymus and phenotypic diversity of lymphomas induced by SL3-3 virus.

These studies were designed to look for a correlation of intrathymic survival of virus-infected thymocytes with lymphomagenesis. Cells from the normal-appearing prelymphoma thymus of SL3-3 virus-treated AKR mice were studied. Also, phenotypic properties of the malignant cells from the virus-induced lymphomas are described. In this model system, 100% of mice inoculated with virus at three days of age develop thymic lymphoma between 60 and 90 days of age. The experiments show that cells with malignant potential do not appear in the thymus until 36 days after virus inoculation. These cells are initially thymus-dependent (TD) in that they produce lymphoma of donor-type in recipients after intrathymic inoculation with long latency. They do not produce lymphoma after subcutaneous inoculation in syngeneic hosts. At 39 days after virus inoculation, the first thymus independent (TI) lymphoma cells appear. These cells, like the cells isolated from thymi with overt tumors, produce lymphoma of donor-type after a short latency when inoculated by the intrathymic or subcutaneous route. Thymocytes from normal-appearing thymi of mice at 42 days after virus inoculation, which could be expected to include TD, TI or no lymphoma cells, were evaluated for their ability to survive in a recipient thymus for three weeks after intrathymic inoculation. They were compared to thymocytes from age-matched control mice. Thymi receiving the virus-infected thymocytes showed 15% to 80% donor cells at three weeks. The highest numbers of donor cells were from thymi which were shown to contain TI lymphoma cells. However, cells from thymi with TD and no lymphoma cells could also be detected in significant numbers at three weeks after intrathymic inoculation. Less than 2% of donor-type thymocytes could be found after inoculation of thymocytes from normal control AKR mice. These data provide evidence that virus infection of thymocytes, even before the appearance of cells with lymphomagenic potential, endows them with a capacity for prolonged intrathymic survival. This appears to be a necessary step for tumor progression in this model. A remarkable phenotypic diversity of the virus-induced lymphomas was shown. The effect of various growth environments, intrathymic, subcutaneous, and in vitro on lymphoma cell phenotypic expression revealed individual differences in each tumor and in each environment.