In an attempt to express the small (transmembrane) envelope protein p2le of type 1 human T-cell leukemia (lymphotrophic) virus (HTLV-1) exclusive of other viral gene products, we have constructed a recombinant plasmid clone (pMBE-1) in a bovine papillomavirus-derived mammalian expression vector. Mouse C127 cells transfected with the pMBE-1 plasmid expressed the introduced HTLV-1 viral gene(s) as demonstrated by Northern blot and indirect immunofluorescence with natural human antisera. The transfected mouse cells were injected into BALB/c mice, and a monoclonal antibody was recovered which specifically recognizes a 21-kilodalton protein present in HTLV-1 virions, indicating that the pMBE-1 plasmid encodes the small envelope protein.Type 1 human T-cell leukemia (lymphotropic) virus (HTLV-1) is a retrovirus isolated from a variety of cell lines established from patients with adult T-cell leukemia or lymphoma (16,23). The HTLV-1 retroviral genome consists of four separate genes encoding distinct viral proteins (19), specifically, the gag, pol, and env genes and a region, pX, located between the 3' end of the envelope gene and the 3' long terminal repeat U3. Only recently have the viral envelope proteins been characterized (7,18,21). These include a glycosylated protein of 62 kilodaltons (kDa), derived from an amino acid backbone of 46 kDa, which serves as the precursor for the small envelope protein p21e and the large envelope protein gp46.The small envelope gene product (plSe) of murine and feline retroviruses has been associated with profound immunosuppressive effects (20). These effects include inhibition of lymphocyte blastogenesis (12) and depressed macrophage-mediated functions (2). The HTLV-1 small envelope protein p21e shares 73% amino acid sequence homology with the pl5e proteins of various murine and feline retroviral isolates (1). Infection of immunocompetent T cells in vitro with HTLV-1 results in the abrogation of various immune functions, including specific recognition of antigen, cytotoxic function, and the requirement for macrophages in antigen recognition (14). It is not clear whether the p2le of HTLV-1 shares the immune inhibitory properties of the plSe proteins. To begin to address this issue, we designed a mammalian expression vector (pMBE-1) to direct the synthesis of most of the HTLV-1 p21e exclusive of the rest of the envelope gene product. We show here results obtained by using a bovine papillomavirus (BPV)-derived vector in which an HTLV-1 DNA segment containing most of the coding region of the putative small envelope protein, the pX region, and the 3' long terminal repeat is located downstream from a mouse metallothionein promoter. The results demonstrate the utility of this vector system in expressing a subgenic region of the HTLV-1 genome in mammalian cells * Corresponding author. in the absence of other virally coded or induced cell proteins. In addition, it provides a rapid and simple means of producing specific monoclonal antibodies by using the transfected cells rather than cells i...
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