Inactivation of lysis of oncornaviruses by human serum.

The phenomenon of oncornavirus inactivation by human serum was examined. As judged by 50% inhibition of plaque or focus formation, every fresh human serum tested inactivated eight different oncornaviruses at serum dilution endpoints of 1:16. Fresh human serum released RNA-dependent DNA polymerase (RDDP) activity from each of 10 tested oncornaviruses. Thus, at least one mechanism of the human serum-induced virus inactivation was by viral lysis. The RDDP assay provided us with a simple method to screen the virolytic properties of many sera against a number of viruses. Oncornaviruses were lysed by sera from monkeys and cats, but not by sera from guinea pigs, rabbits, chickens, rats, goats, and mice ($\text{BALB}\text{c}$, CBA, NIH Swiss). Heated normal human sera did not inactivate or lyse oncornaviruses, nor did they sensitize virions to guinea pig serum-mediated lysis. However, serum from a human vaccinated against Rauscher leukemia virus did sensitize the virus to guinea pig serum-mediated lysis. This ability of human sera to lyse oncornaviruses may be a cause of the difficulties in finding oncornaviruses in humans.