Emergence of tumorigenic cells during the course of Friend virus leukemias.

Appearance of tumorigenic cells was studied in DBA/2 and ICFW mice infected either with the polycythemia-inducing or the anemia-inducing strain of Friend leukemia virus. Tumorigenicity was defined by transplantability of virus-infected cells into the omentum of an isogeneic preirradiated host. Tumorigenic cells were detected in 50% of the leukemic donors 3 wk after infection by the polycythemia-inducing strain and 7-8 wk after infection by the anemia-inducing strain. These cells appeared first in the spleen and later in peripheral blood, bone marrow, and liver. They consisted of a heterogeneous population at different degrees of malignancy as determined by successive transfers in vivo and in vitro. The observations clearly show that leukemias induced by Friend viruses evolve by multistep processes, in which different stages of malignancy can be detected.