The duck EB66® cell substrate reveals a novel retrotransposon.

During the establishment of the duck EB66® cell line as a new cell substrate for vaccine production in industry, a very low level of reverse transcriptase (RT) activity was detected in the culture supernatant by product-enhanced RT assay but a whole battery of tests failed to evidence infectious particles. Results from extensive biochemical and physical testing demonstrated that RT activity was associated to an intracellular, non-enveloped and dense structure different from an infectious retroviruses. In silico analysis of Anas platyrhynchos genome revealed that the most likely candidates for encoding a ribonucleoprotein (RNP)-associated RT were nine copies of chicken repeat 1 (CR1)-like elements, belonging to the non-long terminal repeat retrotransposons. The presence of the full length Anas platyrhynchos chicken repeat 1-like sequence (APCR1) was confirmed in EB66® cells and the related ribonucleic acid was present in the RT-containing fraction of EB66® cells.

A role for CD81 on the late steps of HIV-1 replication in a chronically infected T cell line.

BACKGROUND: HIV-1 uses cellular co-factors for virion formation and release. The virus is able to incorporate into the viral particles host cellular proteins, such as tetraspanins which could serve to facilitate HIV-1 egress. Here, we investigated the implication of several tetraspanins on HIV-1 formation and release in chronically infected T-lymphoblastic cells, a model that permits the study of the late steps of HIV-1 replication. RESULTS: Our data revealed that HIV-1 Gag and Env structural proteins co-localized with tetraspanins in the form of clusters. Co-immunoprecipitation experiments showed that Gag proteins interact, directly or indirectly, with CD81, and less with CD82, in tetraspanin-enriched microdomains composed of CD81/CD82/CD63. In addition, when HIV-1 producing cells were treated with anti-CD81 antibodies, or upon CD81 silencing by RNA interference, HIV-1 release was significantly impaired, and its infectivity was modulated. Finally, CD81 downregulation resulted in Gag redistribution at the cell surface. CONCLUSION: Our findings not only extend the notion that HIV-1 assembly can occur on tetraspanin-enriched microdomains in T cells, but also highlight a critical role for the tetraspanin CD81 on the late steps of HIV replication.

Cell substrates for the production of viral vaccines.

Vaccines have been used for centuries to protect people and animals against infectious diseases. For vaccine production, it has become evident that cell culture technology can be considered as a key milestone and has been the result of decades of progress. The development and implementation of cell substrates have permitted massive and safe production of viral vaccines. The demand in new vaccines against emerging viral diseases, the increasing vaccine production volumes, and the stringent safety rules for manufacturing have made cell substrates mandatory viral vaccine producer factories. In this review, we focus on cell substrates for the production of vaccines against human viral diseases. Depending on the nature of the vaccine, choice of the cell substrate is critical. Each manufacturer intending to develop a new vaccine candidate should assess several cell substrates during the early development phase in order to select the most convenient for the application. First, as vaccine safety is quite naturally a central concern of Regulatory Agencies, the cell substrate has to answer the regulatory rules stringency. In addition, the cell substrate has to be competitive in terms of viral-specific production yields and manufacturing costs. No cell substrate, even the so-called "designer" cell lines, is able to fulfil all the requested criteria for all viral vaccines. Therefore, the availability of a variety of cell substrates for vaccine production is essential because it improves the chance to successfully respond to the current and future needs of vaccines linked to new emerging or re-emerging infectious diseases (e.g. pandemic flu, Ebola, and Chikungunya outbreaks).

Human Discs Large is a new negative regulator of human immunodeficiency virus-1 infectivity.

Human immunodeficiency virus (HIV)-1 replication is positively or negatively regulated through multiple interactions with host cell proteins. We report here that human Discs Large (Dlg1), a scaffold protein recruited beneath the plasma membrane and involved in the assembly of multiprotein complexes, restricts HIV-1 infectivity. The endogenous Dlg1 and HIV-1 Gag polyprotein spontaneously interact in HIV-1-chronically infected T cells. Depleting endogenous Dlg1 in either adherent cells or T cells does not affect Gag maturation, production, or release, but it enhances the infectivity of progeny viruses five- to sixfold. Conversely, overexpression of Dlg1 reduces virus infectivity by approximately 80%. Higher virus infectivity upon Dlg1 depletion correlates with increased Env content in cells and virions, whereas the amount of virus-associated Gag or genomic RNA remains identical. Dlg1 knockdown is also associated with the redistribution and colocalization of Gag and Env toward CD63 and CD82 positive vesicle-like structures, including structures that seem to still be connected to the plasma membrane. This study identifies both a new negative regulator that targets the very late steps of the HIV-1 life cycle, and an assembly pathway that optimizes HIV-1 infectivity.

Nedd4.1-mediated ubiquitination and subsequent recruitment of Tsg101 ensure HTLV-1 Gag trafficking towards the multivesicular body pathway prior to virus budding.

One of the most exciting recent developments in the field of retroviruses is the finding that their Gag proteins hijack cellular proteins from the mutivesicular body (MVB) pathway during the budding process. The Gag proteins of oncoretroviruses possess a PPxY motif that recruits a ubiquitin ligase from the Nedd4 family, whereas those of the human immunodeficiency virus interact through a PTAP motif with Tsg101, a protein of the ESCRT-1 complex. It is currently assumed that Nedd4 and Tsg101 represent equivalent entry gates towards the same cellular process leading to budding, and that both partners are recruited to the plasma membrane where viral budding occurs. However, we report here that the budding of the human oncoretrovirus HTLV-1, the Gag proteins of which possess tandem PPPY/PTAP motifs, requires both Nedd4 and Tsg101. We show that Nedd4.1, but not Nedd4.2, is recruited by the PPPY motif of Gag and subsequently catalyzes Gag ubiquitination. We also demonstrate that Gag interacts first with Nedd4.1 at the plasma membrane and then with Tsg101 in late endosomes/MVBs. Consistently, we found that HTLV-1 particles mutated in the PPPY motif remain underneath the plasma membrane, blocked at an early step of the budding process, whereas PTAP-mutated viruses accumulate in intracellular vesicles, blocked at a later step. Our findings indicate that Nedd4.1 and Tsg101 act successively in the assembly process of HTLV-1 to ensure proper Gag trafficking through the endocytic pathway up to late endosomes where the late steps of retroviral release occur.

Human Dlg protein binds to the envelope glycoproteins of human T-cell leukemia virus type 1 and regulates envelope mediated cell-cell fusion in T lymphocytes.

Human homologue of the Drosophila Dlg tumor suppressor (hDlg) is a widely expressed scaffold protein implicated in the organization of multi-protein complexes at cell adhesion sites such as the neuronal synapse. hDlg contains three PDZ domains that mediate its binding to the consensus motifs present at the C-termini of various cell surface proteins, thus inducing their clustering and/or stabilization at the plasma membrane. Using a yeast two-hybrid screen, we identified hDlg as a cellular binding partner of a viral membrane integral protein, the envelope glycoprotein (Env) of human T-cell leukemia virus type 1 (HTLV-1). HTLV-1 is a human retrovirus that infects CD4+ T lymphocytes and is preferentially transmitted via direct contacts between infected and target cells, through a structure referred to as the virological synapse. Here, we demonstrate that hDlg interacts with a classical PDZ domain-binding motif present at the C-terminus of the cytoplasmic domain of HTLV-1 Env and conserved in the related HTLV-2 virus. We further document that, in HTLV-1 infected primary T cells, hDlg and Env are concentrated in restricted areas of the plasma membrane, enriched in molecules involved in T-cell contacts. The presence of Gag proteins responsible for viral assembly and budding in these areas indicated that they constitute platforms for viral assembly and transmission. Finally, a mutant virus unable to bind hDlg exhibited a decreased ability to trigger Env mediated cell fusion between T lymphocytes. We thus propose that hDlg stabilizes HTLV-1 envelope glycoproteins at the virological synapse formed between infected and target cells, hence assisting the cell-to-cell transmission of the virus.