Attempts were made to change the envelope of murine sarcoma virus (MSV) from B-MuX (a xenotropic murine leukaemia virus isolate) to feline leukaemia virus (FeLV) by infecting cat cells with MSV(B-MuX), adding excess FeLV as helper and using mouse serum oncovirus-inactivating factor or anti-B-MuX serum. In several attempts, virus from single foci of MSV initially contained only MSV(FeLV) but on passage each showed a minimal persistence of B-MuX. To eliminate the B-MuX component, MSV(B-MuX) was subjected to two consecutive transspecies rescues. The first was performed by co-cultivation of MSV(B-MuX)-producing quail cells with mouse 3T3FL cells, which are completely non-permissive for B-MuX, and pure ecotropic Friend Eveline strain of murine leukaemia virus (F-MuLV); this resulted in apparently pure MSV(F-MuLV). Second, these MSV(F-MuLV)-infected 3T3FL cells were co-cultivated with FeLV and cat cells, which are completely nonpermissive for F-MuLV; this resulted in the generation of MSV(FeLV). Passage of this apparently pure FeLV pseudotype in cells permissive only for the replication of B-MuX surprisingly revealed residual murine xenotropic virus. It appears that pressure for survival resulted in genomic masking of B-MuX by heterologous virus envelopes. This phenomenon, which also occurs extensively in nature, implies that if absolute oncovirus genetic purity is required, even extensive attempts at purification may be inadequate.


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