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Infection of Swiss mouse 3T3FL cells with a clonal isolate of Moloney leukemia virus (MLV-IC) resulted in virus progeny composed of at least three different murine helper oncornaviruses. Each entity was purified in appropriate cells by several sequential terminal dilution isolations and was grouwn to high titers. Besides ecotropic MLV-IC there was a pure xenotropic virus and a third novel virus with properties of both eco- and xenotropic viruses. The purified xenotropic virus had a wide host range, was restricted in mouse cells, and was inactivated by normal mouse sera like other xenotropic isolates. The purified virus with hybrid properties (HIX) could infect a wide range of mammalian cells, which included both N and B mouse cells. HIX gave single-hit titrations with equal titers on both mouse and cat indicator cells. Envelope properties of HIX were examined by virus preinfection interference, by interference involving viral glycoprotein, and by neutralization with specific antisera. Both xenotropic and MLV-IC type ecotropic determinants were found on the virus coat. The origins of HIX and the xenotropic virus were investigated in detail. The original MLV-IC stock had HIX type virus in low titer but no detectable pure xenotropic virus. Infection of mouse cells with a single infectious unit of the ecotropic virus from the MLV-IC virus stocks could at times give rise to HIX type virus. HIX type virus, passed once through heterologous rat cells, was subjected to long-term passage either in infected mouse or cat cells. After several months HIX type virus disappeared from some mouse and cat cell systems. The possible hybrid nature of HIX and the origins of newly appearing xenotropic viruses are discussed.
Fischinger et al. (Mon,) studied this question.