Generation of new oncogenic murine retroviruses by cotransfection of cloned AKR and MH2 proviruses

We have obtained a set of oncogenic recombinant retroviruses, the 3RV complex, by cotransfecting murine fibroblasts (SC-1 cells) with plasmids containing the cloned genomes of the avian MH2 and murine AKR viruses. The transfected culture (TAM-2) was shown to release murine transforming viruses by means of reverse transcription and focus formation assays. Analysis of TAM-2 intracellular RNA revealed new transcripts hybridizing with the oncogenes myc and mil and cross-hybridizing with an AKR probe. The biological activity of the 3RV complex was tested for the induction of murine macrophage proliferation in the absence of exogenous growth factors, a property described as the result of mil and myc cooperativity. Cell-free supernatants from 3RV transformed fibroblasts were indeed able to induce the proliferation of macrophage-like cells from murine bone marrow and spleen primary cultures. Such cultures were capable of continuous growth and showed independence from exogenous myeloid growth factors. The cells expressed antigenic markers and functional properties specific of the monocytic-macrophage lineage. These results suggest that transfection-induced recombination could be a novel way to generate biologically active recombinant retroviruses.