Detection of human c-type “helper” viruses in human leukemic bone marrow with murine sarcoma virus-transformed human and rat non-producer cells

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Abstract
Bone-marrow cells from two leukemic children were co-cultivated with the canine thymus cell line A 7573. In early passages, C-type oncornaviruses were released as detected by extracellular reverse transcriptase assay. Co-cultivation of the infected canine cells with the non-producing cell lines R-970-5 (human) or K-NRK (rat) both transformed by Kirsten mouse sarcoma virus (MSV) yielded a new pseudotype of MSV that could transform rat embryo, rabbit SIRC and human kidney cells but not mouse embryo cells. The focus formation could be inhibited by an antiserum to the simian sarcoma virus but not by a serum directed against murine leukemia virus. A cell line derived from a focus of transformed cells became a high virus producer. By immunofluorescence it could be established that the virus is related to the simian sarcoma virus. It is concluded that the leukemic bone-marrow cells produce a C-type oncornavirus that can serve as a helper virus to the defective MSV.