An interferon-sensitive early step in the establishment of infection of murine cells by murine sarcoma/leukaemia virus.

NIH 3T3 mouse cells were infected at very high multiplicity with murine sarcoma/leukaemia virus (MSV/MLV) and then cloned. All of the 48 clones obtained were morphologically transformed, all but one showed anchorage-independence of growth, typical of MSV-transformed NIH 3T3 cells and most (91%) produced MSV/MLV. When cells which had been pre-treated with 10(4) units/ml of purified interferon (IF) were infected under the same conditions and then cloned in the presence of the same amount of IF, only 6 of a total of 63 clones were morphologically transformed. All but these 6 showed a degree of anchorage-independence typical of the uninfected parental cells and very few (2.4%) produced virus. Furthermore, the MSV genome could not be rescued in any of the 23 clones tested and only 1 out of 10 clones produced tumours. The properties of these clones remained stable over a period of 10 to 20 passages in the absence of interferon. We conclude that interferon can irreversibly block an early step in the MSV/MLV infectious process.