CHARACTERIZATION OF A LIVE-ATTENUATED RETROVIRAL VACCINE DEMONSTRATESPROTECTION VIA IMMUNE-MECHANISMS

Live-attenuated retroviruses have been shown to be effective retrovira l vaccines, but currently little is known regarding the mechanisms of protection. In the present studies, we used Friend virus as a model to analyze characteristics of a live-attenuated vaccine in protection ag ainst virus-induced disease. Highly susceptible mice were immunized wi th nonpathogenic Friend murine leukemia helper virus (F-MuLV), which r eplicates poorly in adult mice. Further attenuation of the vaccine vir us was achieved by crossing the Fv-l genetic resistance barrier. The m inimum dose of vaccine virus required to protect 100% of the mice agai nst challenge with pathogenic Friend virus complex was determined to b e 10(3) focus-forming units of attenuated virus. Live vaccine virus wa s necessary for induction of immunity, since inactivated F-MuLV did no t induce protection. To determine whether immune cells mediated protec tion, spleen cells from vaccinated donor mice were adoptively transfer red into syngeneic recipients. The results indicated that immune mecha nisms rather than viral interference mediated protection.