Soluble recombinant HTLV-1 surface glycoprotein competitively inhibits syncytia formation and viral infection of cells.

Efficient entry into, and infection of, human cells by human T-cell leukaemia virus type-1 (HTLV-1) is mediated by the viral envelope glycoproteins, gp46 and gp21. The gp46 surface glycoprotein binds to an as yet unidentified cell surface receptor, thereby, allowing the gp21 transmembrane glycoprotein to initiate fusion of the viral and cellular membranes. In the absence of membrane fusion viral penetration and entry into the host cell cannot occur. The envelope glycoproteins are also a major target for neutralising antibodies and cytotoxic T lymphocytes following a protective immune response, and represent ideal constituents for a recombinant HTLV-1 vaccine. Given the importance of the envelope proteins in HTLV-1 pathogenesis there is increasing interest in obtaining sufficient quantities of these proteins for biochemical, biophysical and biological analyses. We have now developed a system for production of large amounts of a glycosylated and functional form of soluble recombinant gp46 (sRgp46), and have used this recombinant material for analysis of envelope function and receptor binding activity. We find that, the sRgp46 molecules expressed in our system are immunologically indistinguishable from the native virally expressed surface glycoproteins; that sRgp46 binds to T-cells in a dose dependent and saturable manner; and that cell surface binding by sRgp46 can be inhibited by neutralising antibodies. Importantly, we demonstrate that these sRgp46 molecules potently inhibit syncytia formation and viral infection of target cells, and that regions outwith the SU domain of envelope are not required for binding to target cells or for inhibiting membrane fusion. The sRgp46 produced in our study will provide new opportunities to investigate envelope-receptor interactions, and will be of utility in defining the conformationally sensitive antigenic determinants of the HTLV-1 surface glycoprotein.

Kappa B-specific DNA binding proteins: role in the regulation of human interleukin-2 gene expression.

Transcriptional activation of the human interleukin-2 (IL-2) gene, like induction of the IL-2 receptor alpha (IL-2R alpha) gene and the type 1 human immunodeficiency virus (HIV-1), is shown to be modulated by a kappa B-like enhancer element. Mutation of a kappa B core sequence identified in the IL-2 promoter (-206 to -195) partially inhibits both mitogen- and HTLV-I Tax-mediated activation of this transcription unit and blocks the specific binding of two inducible cellular factors. These kappa B-specific proteins (80 to 90 and 50 to 55 kilodaltons) similarly interact with the functional kappa B enhancer present in the IL-2R alpha promoter. These data suggest that these kappa B-specific proteins have a role in the coordinate regulation of this growth factor-growth factor receptor gene system that controls T cell proliferation.

Functional replacement of the HIV-1 rev protein by the HTLV-1 rex protein.

Two evolutionarily distinct families of human retroviruses, the human immunodeficiency viruses (HIV) and the human T-cell leukaemia viruses (HTLV), have been defined (reviewed in ref. 1). Although these virus groups share tropism for human CD4+ T cells, they differ markedly in primary sequence, genetic organization and disease association (AIDS versus adult T-cell leukaemia), but show similar general strategies for the regulation of viral gene expression. Each encodes a protein able to trans-activate transcription from the homologous viral long terminal repeat (tat in HIV, tax in HTLV), although these proteins act by different mechanisms and do not appear to be interchangeable. Each virus also produces a second trans-acting protein that induces the expression of the unspliced messenger RNAs encoding the viral structural proteins (rev in HIV and rex in HTLV). Here we show that the rex protein of HTLV-I can functionally replace the rev protein of HIV-1 in transient expression assays. This genetic complementation by rex is adequate for the rescue of a replication-defective rev mutant of HIV-1. This unexpected shared function between the structurally distinct rex and rev proteins emphasizes the importance of this highly conserved pathway for the regulation of human retrovirus gene expression.

Cell-line specific activation of SV40 transcriptional enhancer by p40tax of HTLV-1.

A transcriptional trans-activator p40tax of HTLV-1 was reported to activate HTLV-1 enhancer, but not SV40 or Rous sarcoma virus enhancer. However, in certain cell lines, we found that SV40 enhancer was activated by p40tax. These cell lines were mostly T cells, where the SV40 enhancer showed only low activity without p40tax. Since p40tax-mediated activation of the LTR is not cell line-specific, the activation of enhancers by p40tax depends on the combination of enhancer and cell type used for the test. Thus, apparent activation by p40tax depends on variable cellular components involved in transcriptional regulation.