Sequences between the Enhancer and Promoter in the Long Terminal Repeat Affect Murine Leukemia Virus Pathogenicity and Replication in the Thymus

We previously showed that the 93-bp region between the enhancer and promoter (named DEN for downstream of enhancer) of the long terminal repeat (LTR) of the MCF13 murine leukemia virus is an important determinant of the ability of this virus to induce thymic lymphoma. In this study we observed that DEN plays a role in the regulation of virus replication in the thymus during the preleukemic period. A NF-κB site in the DEN region partially contributes to the effect of DEN on both lymphomagenicity and virus replication. To further study the effects of DEN and the NF-κB site on viral pathogenicity during the preleukemic period, we examined replication of wild-type and mutant viruses with a deletion of the NF-κB site or the entire DEN region in the thymus. Thymic lymphocytes which were infected with wild-type and mutant viruses were predominantly the CD3(−) CD4(+) CD8(+) and CD3(+) CD4(+) CD8(+) cells. The increase in infection by wild-type virus and both mutant viruses of these two subpopulations during the preleukemic period ranged from 9- to 84-fold, depending upon the time point and virus. The major difference between the wild-type and both mutant viruses was the lower rate and lower level of mutant virus replication in these thymic subpopulations. Significant differences in replication between wild-type and both mutant viruses were seen in the CD3(−) CD4(+) CD8(+) and CD3(−) CD4(−) CD8(−) subpopulations, suggesting that these thymic cell types are important targets for viral transformation.