An avian leukosis virus associated with stocks of rous sarcoma virus

Abstract

Isolation of a second virus from stocks of the Bryan high-titer strain of Rous sarcoma virus (RSV) is reported. The newly isolated agent, designated as Rous-associated virus (RAV), can be detected in tissue culture, despite its failure to produce discrete cytological alterations, because it interferes with infection and focus formation by RSV. There is a strong selection for RAV when cells infected with RSV are cloned, so that most of the clones produce much RAV and little RSV. In stocks of the Bryan strain the titer of RAV is about 4 times higher than that of RSV. RAV induces resistance to RSV much more rapidly than does RIF, a naturally occurring leukosis virus. RAV can be accurately assayed in vitro, since the level of resistance induced to RSV is proportional, on a logarithmic scale, to the logarithm of the concentration of RAV.

RAV is more closely related immunologically to RSV than to RIF. Fluorescent antibody studies show that it matures at the cell membrane, as does RSV. When viewed in the electron microscope RAV is morphologically indistinguishable from RSV and other chicken tumor viruses. It produces no tumor at the site of inoculation in chickens, but produces erythroblastosis following intravenous inoculation of embryos.