Generation of thymotropic envelope gene recombinant virus and induction of lymphoma by ecotropic Moloney murine leukemia virus.

Biologically cloned pure ecotropic Moloney MuLV was used to infect Balb/c and AKR mice to determine the replication of ecotropic virus, the possible generation of recombinant viruses, and the induction of disease. Infectious cell center (ICC) experiments carried out with lymphoid cells of individual Balb/c mice showed that e-M-MuLV rapidly infected up to 30% of lymphoid cells in liver, spleen, and especially in the thymus. No recombinant virus was seen until about Day 35 when a burst of RM-MuLV was observed only in the thymus. New RM-MuLV was found in all 32 preleukemic and leukemic mice tested and persisted at low levels until death. The RM-MuLV recovered early in the preleukemic phase had an env-related M-MuLV but grew very poorly. Cells from a late tumor which grew and cloned readily were examined to see whether the new RM-MuLV was present in every clone. Overtly, most tumor cells did not seem to contain RM-MuLV, but when "unmasking" was performed, every tumor cell contained identical RM-MuLV. In AKR mice, both e-M-MuLV and recombinant M-MuLV caused an acceleration of lymphoma. De novo appearance of a thymotropic RM-MuLV, which was of the Moloney RM-MuLV type and the absence of early detectable endogenous AKR-MCF-type recombinants, suggested that the early lymphoma was due to the induction of a new disease. Several theoretical approaches dealing with viral env-gene permutations are discussed.