Helper-independent virus from a molecularly cloned stock of NB-ecotropic Friend MuLV (F-MuLV) induces in newborn NIH Swiss and BALB/c mice an erythropro-

A new class of murine leukemia viruses (MuLV) 1 has been designated as mink cell focus-inducing (MCF) for their ability to induce cytopathie foci when grown on mink lung fibroblasts (1). The classic MCF virus is an XC-negative, dual-tropic virus that appears to be an envelope gene recombinant between ecotropic and endogenous xenotropic viral sequences (2-7). An association between a variety of leukemias and MCF viruses is becoming increasingly apparent. Hartley et al. (1) first demonstrated that MCF viruses could be isolated from thymic lymphomas of AKR mice, and it was subsequently shown (8) that such viruses can accelerate leukemia in the AKR mouse. MCF viruses have also been implicated in diseases induced by Moloney (9) and Rauscher (10) MuLV and in x ray-induced lymphomas (11), graft vs. host-induced reticulum cell neoplasms (12), and plasmacytomas (13). Helper-independent virus from a molecularly cloned stock of NB-ecotropic Friend MuLV (F-MuLV) induces in newborn NIH Swiss and BALB/c mice an erythroproliferative disease characterized by splenomegaly and severe anemia (14-16). As demonstrated by molecular hybridization techniques, spleens from F-MuLV-infected mice, as compared with spleens from uninfected mice, express 100 to 1,000-fold higher levels of xenotropic virus-related RNA sequences, and subsequently MCF viruses were isolated from several of these spleens (14). In an effort to better understand F-MuLV-induced disease in the mouse and the role that MCF viruses play in this disease, a series of studies was carried out to determine the susceptibility of various strains of mice to disease induced by F-MuLV, to characterize the viruses and viral proteins expressed in the spleens of infected and uninfected mice, and to determine whether Friend MCF virus (Fr-MCF) induces the same disease. We report that the erythroproliferative disease developing in susceptible F-MuLV-infected mice is correlated with the expression of MCF-related envelope glycoproteins and that MCF viruses isolated from these mice are moderately pathogenic when infected as pseudotypes prepared by mixed infection with either ecotropic