Isolation of a replication-defective murine leukaemia virus from cultured AKR leukaemia cells.

Mice of the AKR strain are characterised by a high incidence of spontaneous thymic lymphomas. AKR chromosomes contain the genomes of ecotropic murine leukaemia virus (MuLV) at two loci, termed Akv-1 and Akv-2 (refs 2-6). Shortly after birth, the normal tissues of AKR mice begin to produce high levels of this XC-positive MuLV (ref. 7) (that is, one that forms XC plaques). A second class of MuLV, termed mink cell focus-inducing virus (MCF), is produced specifically by preleukaemic and leukaemic AKR thymocytes. Nowinski et al. have established a series of tissue culture lines from AKR leukaemias and reported that the resulting cell lines produce virus particles, but that these particles, surprisingly, do not give rise to XC plaques. We have analysed the virus particles produced by one of these cell lines, termed AKRSL2. We show here that, unlike most or all of the nonmalignant tissues in the AKR mouse, these cultured lymphoma cells produce very little non-defective ecotropic MuLV; however, they do produce replication-defective ecotropic MuLV.