Virus particles associated with “nonproducer” Rous sarcoma cells

Abstract

Examination of four cultures of “nonproducer” (NP) Rous sarcoma cells in the electron microscope revealed the presence of extracellular particles with the morphology of avian leukosis viruses. Focus assays on fluids from these NP cell cultures failed to detect infectious virus. To confirm this observation, two lines of NP cells were carried in culture through seven serial passages and examined at intervals for evidence of infectious or noninfectious virus. A total of 160 ml of culture fluid was removed from the two NP cell lines at various times during passage and this fluid was tested for infectious virus in fresh cultures of chick fibroblasts. No evidence of infection was found when the indicator cells were tested by focus assay, RIF test, or COFAL test, and the indicator cells remained free of virus particles detectable in the electron microscope. NP cells were examined directly for the presence of group-specific complement-fixing antigen with hamster antiserum against Schmidt-Ruppin sarcoma virus. In early passages no antigen was detected, but the 6th and 7th passage NP cells contained small amounts of C'F antigen. Electron microscopic examination revealed the presence of virus particles in both cell lines through 7 passages in culture. Direct particle counts were not attempted; however, NP cell cultures were estimated to contain 1 10 to 1 100 the number of particles seen in cultures of Rous sarcoma cells that were releasing infectious virus.