Abstract
Murine mammary tumour virus (MTV) induced an increased net synthesis of DNA, RNA and protein in short-term cultures of baby mouse kidney cells (BMKC) made from two different BALB c mouse substrains (He.A and Crgl.A). The dose-response ratio went through a maximum but growth inhibition was not observed. Similar effects were induced in baby hamster kidney cells ( BHK 21 C 13 ). In longer-term cultures of MTV-infected BMKC foci of piled-up small rounded epithelioid cells appeared on the monolayers. Cell lines derived from these foci grew well in media without serum and produced biologically active MTV (B-particles) and murine leukemia virus (MLV; C-particles). Higher in vitro passages of these cell lines induced less early (< 300 days) mammary tumours and more cases of leukemia than the lower passages after intraperitoneal injection. Cell-free extracts induced similar tumours. Not-infected control BMKC did not acquire the described properties. The nature of the induced MTV and MLV is discussed.
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