Generation and pathogenicity of an NB-tropic SL3-3 murine leukemia virus

A 428-base pair region of the gag gene of a molecular clone of the SL3-3 murine leukemia virus (MuLV) was replaced with allelic sequences that contain the NB-tropic determinants of the Moloney MuLV. SL3-3NB, the ecotropic virus derived from this modified clone, rapidly induced T-cell lymphoblastic lymphomas in the Fv-1n mouse strains CWD and NIH Swiss and the Fv-1b strain, B10.Br. By Southern blot assay, each of the CWD tumors and all but one of the B10.Br tumors that were tested contained recombinant proviruses. The envelope genes of the B10.Br recombinant viruses retained the SL3-3NB 5' p15E (TM) gene sequences (type I env), whereas the CWD recombinants did not (type II env). The production of type I env recombinants by B10.Br mice is a characteristic that is shared with other mouse strains that express the H-2k haplotype. The results indicate that the inserted Moloney virus gag sequences conferred NB-tropism to SL3-3NB and did not interfere with the expression of SL3-3 pathogenic determinants or the formation of recombinant viruses.