Antigenic and chromosomal changes in thymus and spleen of Balb/Mo mice during leukemogenesis

The Balb/Mo mouse strain carries a single copy of the germ line integrated Moloney leukemia virus (M-MuL V) and shows a high leukemia frequency. According to the clinical manifestations lymphomas can be divided into two major categories. In one type the thymus appears to be the primary site for lymphoma development. The second type is dominated by the generalized enlargement of spleen and lymph nodes. Individual lymphomas differ in the cell surface expression of Thy 1.2, MCSA (designated operationally as the M-MuL V-determined cell surface antigen) and viral p30 antigens. In addition, spleen and thymus of the same leukemic animal often differ antigenically. The karyotype of cells from enlarged organs is diploid or shows trisomy of chromosome 15. Lymphomas developing in Balb/Mo mice are thus heterogeneous with regard to clinical manifestations, cell surface antigens and karyotype and, in this respect, do not differ from lymphomas arising after the inoculation of exogenous M-MuL V. Amplification of the M-MuL V genome in young Balb/Mo mice is not accompanied by the appearance of MCSA on thymus cells. Still, 32% of lymphomas are MCSA positive. The results suggest that MCSA is related to a virus activated in a minority of Balb/Mo mice during the late phase of leukemogenesis.