Defective virus is associated with induction of murine retrovirus-induced immunodeficiency syndrome

C57BL/6 mice infected with a mixture of murine leukemia viruses (MuLV) develop a syndrome characterized by lymphoproliferation and profound immunodeficiency. Analyses of this viral mixture (LP-BM5 MuLV) showed that it includes replication-competent ecotropic and mink cell focus-inducing MuLV and defective viruses with genome sizes of 3.8-6.5 kilobases. The ecotropic and mink cell focus-inducing MuLV biologically cloned from the mixture did not induce disease, whereas viral preparations containing the ecotropic MuLV and 4.8-kilobase defective virus were active. Cells producing the 4.8-kilobase defective virus expressed an unusual gag-encoded polyprotein of Mr 60,000.