Isolation and Identification of a Helper Virus Found in the Moloney Sarcoma-Leukemia Virus Complex

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Abstract
A leukemia helper virus indigenous to the Moloney isolate of sarcoma virus was isolated in pure form from an infected clone of 3T3 cells. The helper virus has been propagated in chronically infected 3T3 cell subline (IC line) for over 70 weekly transfers. Morphologically the IC cell line does not show overt signs of viral transformation. The IC cells contained no detectable sarcoma virus but they produced high titers of a helper virus (IC) for defective sarcoma. This helper virus could be propagated indefinitely in other susceptible murine cells. Attempts at superinfection of IC cells by either Rauscher or Moloney pseudotypes of sarcoma virus showed that IC cells were completely resistant to either sarcoma variant. Both the helper virus and the defective sarcoma virus could be specifically neutralized by anti-Moloney leukemia virus antisera. Inoculation of newborn Swiss mice with the IC virus resulted in a lymphoma essentially identical to the Moloney leukemia syndrome. Cell-free extracts of lymphomatous organs contained high titers of leukemogenic virus. It is concluded that the helper virus found in the Moloney isolate of murine sarcoma is true Moloney leukemia virus.

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