Immortalization of murine primary spleen cells by v-myc, v-ras, and v-raf

Growth factor-independent cell lines, including four lines characterized as macrophages, were isolated by infection of BALB/c mouse primary spleen cells with combinations of three retroviruses encoding v-myc, v-ras, and v-myc/v-raf. Proliferating cell lines were isolated only rarely, and after long crisis periods, following the introduction of myc and raf by infection with J2 virus, or of myc and ras by coinfection with myc309 and raszip6 viruses. However, sequential infections with all three viruses--myc plus ras cells reinfected with J2, or J2 followed by myc plus ras coinfection--resulted in rapid outgrowth of cell lines which grew at high growth rates to high densities. When cells were treated with anti-IgG F(ab')2/IL-4/IL-5 to specifically stimulate B cells, cell lines were isolated readily by infection with myc plus ras alone, J2 alone, or all three viruses. These cell lines arose after shorter crisis times and all grew at high growth rates and to high densities. Analysis of cell surface markers and immunoglobulin gene arrangement revealed no lymphoid characteristics in any of the lines. Four cell lines express all three macrophage markers analyzed (F4/80, Mac1, FcR), and many others are Mac1+ and/or FcR+. Out of 20 immortalized cell lines tested, 13 show clonal growth in soft agar, and 3/6 of these produced tumors in BALB/c mice, indicating that fully transformed cells may be isolated by these procedures. In at least one of the cell lines, integration of all three infecting viruses has occurred.