Immunity to Friend virus complex: analysis of protection with a sarcoma virus pseudotype

To specify the mechanism whereby inoculation of sarcoma viruses protects against leukemia development by leukoviruses, several experiments were performed using Friend virus complex (FVC) and MSV-F, a sarcoma virus pseudotype prepared in vitro with FVC as source of the helper component. The sarcomatous lesions induced by MSV-F were self-limited, irrespective of dose and route of inoculation and protected against the progressive, lethal erythroleukemia induced fy FVC. The sera of MSV-F primed mice had high FVC-neutralizing titers, Out of 3 properties titered in FVC, immunogenicity, XC-syncytium-forming activity and lethality, the frmer two were retained at high titers in MSV-F, while lethality was almost entirely lost: the dose-range between lethal immunizing dilutions was 2-3 logs broader for MSV-F than for FVC. It is postulated that the lethal, early erythroleukemia inducing component (Spleen Focus Forrming Virus) is rapidly lost from FVC upon in vitro propagation. The rescued MSV-F would carry a hightitered, immunogenic and Xc-syncytium forming component (Lymphatic Leukemia Virus), thus behaving as an immunogenic virus of limited pathogenicity.