Characterization of a novel baboon virus closely resembling human T-cell leukemia virus

We report the isolation of a virus from a baboon imported from Kenya and the analysis of the nucleotide sequence of the env gene. Comparison of the complete nucleotide sequence of the env gene of different HTLV-1 strains and the baboon T-cell leukemia virus (designated BTLV) indicated similarities ranging from 92.5 to 97.4%. In contrast, only 89.1% similarity was observed between the BTLV env sequence and that of simian T-cell leukemia virus (PtM3). The sequences corresponding to the glycosylation sites, endoproteolytic processing site, and major immunological determinants were strictly conserved between BTLV and HTLV-1. To characterize the expressed protein we used a vaccinia expression system, which indicated that a protein of 62 kDa is encoded by the envelope gene. The protein acquired mostly high mannose modifications and was localized predominantly in the endoplasmic reticulum. A fraction of the protein was expressed at the cell surface, where it could induce membrane fusion of target cells. The existence of HTLV-1-like viruses in baboons indicates the potential risk of transmission of such virus from these nonhuman primates to humans, thus highlighting the need for specific screening for such viruses during xenotransplantation.