RECOMBINANT MINK CELL FOCUS-INDUCING VIRUS AND LONG TERMINAL REPEAT ALTERATIONS ACCOMPANY THE INCREASED LEUKEMOGENICITY OF THE MO-VIRUS AFTER INTRAPERITONEAL INOCULATION(PYF101 VARIANT OF MOLONEY MURINE LEUKEMIA)

We recently showed that different routes of inoculation affect the leu kemogenicity of the Mo+PyF101 variant of Moloney murine leukemia virus (M-MuLV). Intraperitoneal (i,p.) inoculation of neonatal mice,vith Mo +PyF101 M-MuLV greatly enhanced its leukemogenicity compared,vith subc utaneous (s.c.) inoculation. We previously also suggested that the leu kemogenic defect of Mo+PyF101 M-MuLV when inoculated s.c. may result f rom the inability of this virus to form env gene recombinant (mink cel l focus-inducing [MCF]) virus. In this study, virus present in end-sta ge tumors and in preleukemic animals inoculated i.p. by Mo+PyF101 M-Mu LV was characterized. In contrast to s.c. inoculation, all tumors from i.p.-inoculated mice contained high levels of recombinant MCF virus. Furthermore, Southern blot analyses demonstrated that the majority of the tumors contained altered Mo+PyF101 M-MuLV long terminal repeats. T he U3 regions from several tumors with altered long terminal repeats w ere cloned by PCR amplification. Sequence analyses indicated that the M-MuLV 75-bp tandem repeat in the enhancer region aas triplicated. Thi s amplification was also previously observed in mice infected s.c. wit h a pseudotypic mixture of Mo+PyF101 M-MuLV and Mo+PyF101 MCF virus. T he enhancer triplication was an early event, and it occurred within 2 weeks postinfection. Recombinant MCF viruses were not detected by Sout hern blot analyses until 4 weeks postinfection. Thus, the M-MuLV enhan cer triplication event was initially important for efficient propagati on of ecotropic Mo+PyF101 M-MuLV. The increased leukemogenicity follow ing i.p. inoculation could be explained if the triplication enhances M o+PyF101 M-MuLV replication in the bone marrow and bone marrow infecti on is required for recombinant MCF virus formation.