The open reading frame I (ORF I)/ORF II part of the human T-cell leukemia virus type I X region is dispensable for p40tax, p27rex, or envelope expression.

The X region of the human T-cell leukemia virus type I contains the second coding exon of the tax and rex regulatory proteins (open reading frame IV [ORF IV] and ORF III, respectively), as well as coding regions for more recently described proteins, p30II (or the tof protein) and p13II in ORF II and the putative rof protein and p12I in ORF I. Deletions and transcomplementation experiments showed that expression of the envelope, as well as that of the tax and rex proteins, was independent of the proteins encoded in the ORF I/ORF II region. Furthermore, p30II and p12I proteins could not replace the rex protein in a rex-dependent envelope or Gag protein expression system.     

Lymphosarcoma with lymphoblastic leukemia in a New Zealand white rabbit.

An 8 to 10-week-old female New Zealand white rabbit, Oryctolagus cuniculus, which exhibited clinical signs of anorexia, depression, and torticollis was found to have lymphosarcoma with lymphoblastic leukemia. The multiple visceral involvement with neoplastic lymphoid cells observed in this animal was similar to previously reprted cases of lymphosarcoma in the rabbit. An unusual finding was the occurrence of lymphoblastic leukemia since lymphosarcoma in the rabbit has previously been reported as aleukemic.     

The Rex proteins of human T-cell leukemia virus type II differ by serine phosphorylation.

The Rex proteins of human T-cell leukemia virus types I and II (HTLV-I and HTLV-II) induce cytoplasmic expression of unspliced gag-pol mRNA and singly spliced env mRNA and are critical for virus replication. Two rex gene products, p27rex and p21rex of HTLV-I and p26rex and p24rex of HTLV-II, have been detected in HTLV-infected cells; however, the structural and biological relationship of the proteins has not been clearly elucidated. Endoproteinase digestion and phosphoamino acid analysis of HTLV-II Rex indicated that p24rex has the same amino acid backbone as p26rex and that the larger apparent molecular size of p26rex is attributable to serine phosphorylation.     

Biochemical and immunological characterization of the major envelope glycoprotein of bovine leukemia virus.

The major envelope glycoprotein of bovine leukemia virus was isolated by lectin-bound Sepharose and DEAE-cellulose column chromatography. This protein was shown to have a molecular weight of about 41,000 and to lack detectable immunological cross-reactivity with glycoproteins of other oncornaviruses. Sera obtained from 100% of cattle examined with clinically diagnosed lymphosarcoma contained high-titered antibody to 125I-labeled bovine leukemia virus glycoprotein, whereas sera from animals in a disease-free herd were antibody negative.     

Polymerase chain reaction analysis of defective human T-cell leukemia virus type I proviral genomes in leukemic cells of patients with adult T-cell leukemia.

Human T-cell leukemia virus type I (HTLV-I) is the etiologic agent of adult T-cell leukemia, and the clonally derived leukemic cells all contain proviral genomes. Polymerase chain reaction with a variety of primers which span the HTLV-I genome was used to determine that a significant fraction of patients (at least 32%) carry deleted viral genomes in their leukemic cells. The pX region of the HTLV-I genome encoding the regulatory genes tax and rex was preferentially retained. The fact that the tax coding region was retained provides supporting evidence that the tax protein contributes to leukemogenesis in vivo. The reasonably high fraction of patients with adult T-cell leukemia carrying deleted genomes in their tumor cells suggests that the deletions have a role in leukemogenesis.     

Role of human T-cell leukemia virus type 1 X region proteins in immortalization of primary human lymphocytes in culture.

Human T-cell leukemia virus type 1 (HTLV-1) immortalizes human CD4+ T lymphocytes in culture. Previous studies show that in the context of a herpesvirus saimiri vector, the sequence of the X region at the 3' end of the HTLV-1 genome is also capable of immortalizing CD4+ lymphocytes in the absence of HTLV-1 structural proteins. The X region of HTLV-1 encodes two trans-acting viral proteins, the 42-kDa Tax protein and the 27-kDa Rex protein. Infection of human cord blood cells with herpesvirus saimiri recombinants which contain HTLV-1 X region sequences defective for expression of tax, rex, or both tax and rex demonstrates that tax function is necessary and sufficient for immortalization of primary human CD4+ cord blood lymphocytes in culture in the context of the herpesvirus saimiri vector.     

Functional analysis of human T-cell leukemia virus type I rex-response element: direct RNA binding of Rex protein correlates with in vivo activity.

The human T-cell leukemia virus type I rex gene product plays a critical role in the expression of the retroviral structural proteins Gag and Env from incompletely spliced mRNAs. Rex protein acts through a cis element (rex-response element [RxRE]) which is located in the U3/R region of the 3' long terminal repeat and is present on all human T-cell leukemia virus type I-specific mRNAs. Two domains of the predicted secondary structure of the RxRE are crucially important for Rex action in vivo as measured by two assay systems. In vitro studies using highly purified recombinant Rex protein revealed a specific and direct interaction with radiolabeled RxRE sequences. The correlation between our in vivo results and the direct binding of Rex protein to mutant and wild-type RxRE sequences supports both the existence of the predicted secondary structure and the importance of this direct interaction with the cis-acting RNA sequence for Rex function in vivo.     

In vivo infection of sheep by bovine leukemia virus mutants.

Direct inoculation of a cloned bovine leukemia virus (BLV) provirus into sheep has allowed study of the viral infectivity of genetic mutants in vivo. Three BLV variants cloned from BLV-induced tumors and 12 in vitro-modified proviruses were isolated and analyzed for viral expression in cell culture. The proviruses were then inoculated into sheep in order to assess viral infectivity in vivo. Of three variants cloned from BLV-induced tumors (344, 395, and 1345), one (344) was found infectious in vivo. This particular provirus was used to engineer 12 BLV mutants. A hybrid between the 5' region of the complete but noninfectious provirus 395 and the 3' end of mutant 344 was infectious in vivo, suggesting that the tax/rex sequences were altered in virus 395. As expected, several regions of the BLV genome appeared to be essential for viral infection: the protease, pol, and env genes. Even discrete modifications in the fusion peptide located at the NH2 end of the transmembrane gp30 glycoprotein destroyed the infectious potential. In contrast, mutations and deletions in the X3 region present between the env gene and the 3' tax/rex region did not interfere with viral infection in vivo. This region of unknown function could thus be used to introduce foreign sequences. A BLV recombinant carrying a ribozyme directed against the tax/rex sequences was still infectious in vivo. Cotransfection of two noninfectious mutants carrying deletions led to infection in two of four independent injections, the infectious virus being then a recombinant between the two deletants. The experimental approach described here should help to gain insight into essential mechanisms such as in vivo viral replication, cooperation between deletants for viral infectivity, and viral superinfections. The gene products in the X3 and X4 region which are dispensable for in vivo infection could be involved in leukemogenesis, and thus proviruses deleted in these sequences could constitute the basis for a live attenuated vaccine.