Persistence of One Species of Oncovirus in Mixed Infection after Antiserum Treatment or Transspecies Rescue

Laboratory of Viral Carcinogenesis National Cancer Institute, Frederick, Maryland 21701, U.S.A. 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. Keywords: genomic masking (pseudotypes), oncoviruses, virus envelope Received 13 January 1982; accepted 11 May 1982.