Prion diseases are neurodegenerative disorders associated in most cases with the accumulation in the central nervous system of PrPSc (conformationally altered isoform of cellular prion protein (PrPC); Sc for scrapie), a partially protease-resistant isoform of the PrPC. PrPSc is thought to be the causative agent of transmissible spongiform encephalopathies. The mechanisms involved in the intercellular transfer of PrPSc are still enigmatic. Recently, small cellular vesicles of endosomal origin called exosomes have been proposed to contribute to the spread of prions in cell culture models. Retroviruses such as murine leukemia virus (MuLV) or human immunodeficiency virus type 1 (HIV-1) have been shown to assemble and bud into detergent-resistant microdomains and into intracellular compartments such as late endosomes/multivesicular bodies. Here we report that moloney murine leukemia virus (MoMuLV) infection strongly enhances the release of scrapie infectivity in the supernatant of coinfected cells. Under these conditions, we found that PrPC, PrPSc and scrapie infectivity are recruited by both MuLV virions and exosomes. We propose that retroviruses can be important cofactors involved in the spread of the pathological prion agent.

Human T-cell leukemia virus type 1 (HTLV-1)-infected CD4؉ T cells and dendritic cells (DCs) are present in peripheral blood from HTLV-1 carriers. While T-cell infection requires cell-cell contact, DCs might be infected with cell-free virus, at least in vitro. However, a thorough comparison of the susceptibilities of the two cell types to HTLV-1 infection using cell-associated and cell-free viral sources has not been performed. We first determined that human primary monocyte-derived dendritic cells (MDDCs) were more susceptible to HTLV-1 infection than their autologous lymphocyte counterparts after contact with chronically infected cells. Next, a comparison of infection efficiency using nonconcentrated or concentrated supernatants from infected cells as well as purified viral biofilm was performed. Integrated provirus was found after exposure of MDDCs or primary lymphocytes to viral biofilm but not to a viral supernatant. Using a large series of primary cell samples (n ‫؍‬ 21), we demonstrated a higher proviral load in MDDCs exposed to viral biofilm than in lymphocytes. This higher susceptibility is correlated to a higher expression of neuropilin-1 on MDDCs than on autologous activated T lymphocytes. Moreover, we show that MDDCs infected with viral biofilm can transmit the virus to lymphocytes. In conclusion, MDDCs are more susceptible to HTLV-1 infection than autologous lymphocytes in vitro, supporting a model in which DC infection might represent an important step during primoinfection in vivo. IMPORTANCEHTLV-1 is able to infect several cell types, but viral DNA is mainly found in T lymphocytes in vivo. This supports a model in which T lymphocytes are the main target of infection. However, during the primo-infection of new individuals, incoming viruses might first encounter dendritic cells (DCs), the specialized immune cells responsible for the antiviral response of the host. HTLV-1 cell-free purified viruses can infect dendritic cells in vitro, while T-cell infection is restricted to cell-to-cell transmission. In order to understand the sequence of HTLV-1 dissemination, we undertook a direct comparison of the susceptibilities of the two cell types using cell-associated and cell-free viral sources. We report here that MDDCs are more susceptible to HTLV-1 infection than autologous lymphocytes in vitro and are able to efficiently transmit the virus to lymphocytes. Our results suggest that DCs may represent a true viral reservoir, as the first cell type to be infected in vivo.H uman T-cell leukemia virus type 1 (HTLV-1) infects 5 to 20 million people worldwide (1) and is the etiological agent of both an aggressive CD4 ϩ T-cell leukemia named adult T-cell leukemia (ATL) (2) and a neurological disease named tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM) (3, 4). HTLV-1 is transmitted through contact with infected cells present in maternal milk, semen, or blood and is mainly found in CD4 ϩ T cells, but other cell types might also be infected or functionally altered in vivo and thus involv...

Background information. TSEs (transmissible spongiform encephalopathies) are neurodegenerative disorders affecting humans and animals. PrPSc, a conformationally altered isoform of the normal prion protein (PrPC), is thought to be the pathogenic agent. However, the biochemical composition of the prion agent is still matter of debate. The potential transmission risk of the prion agent through biological fluids has been shown, but the development of competitive diagnostic tests and treatment for TSEs requires a more comprehensive knowledge of the agent and the cellular mechanisms by which it is disseminated. With this aim, we initiated characterization of the prion agent and the pathways by which it can be propagated using the cellular model system neuroblastoma (N2a). Results. The present study shows that N2a cells infected with scrapie release the prion agent into the cell culture medium in association with exosome‐like structures and viral particles of endogenous origin. We found that both prion proteins and scrapie infectivity are mainly associated with exosome‐like structures that contain viral envelope glycoprotein and nucleic acids, such as RNAs. Conclusions. The dissemination of prions in N2a cell culture is mediated through the exosomal pathway.

ISG20 is a broad spectrum antiviral protein thought to directly degrade viral RNA. However, this mechanism of inhibition remains controversial. Using the Vesicular Stomatitis Virus (VSV) as a model RNA virus, we show here that ISG20 interferes with viral replication by decreasing protein synthesis in the absence of RNA degradation. Importantly, we demonstrate that ISG20 exerts a translational control over a large panel of non-self RNA substrates including those originating from transfected DNA, while sparing endogenous transcripts. This activity correlates with the protein’s ability to localize in cytoplasmic processing bodies. Finally, these functions are conserved in the ISG20 murine ortholog, whose genetic ablation results in mice with increased susceptibility to viral infection. Overall, our results posit ISG20 as an important defense factor able to discriminate the self/non-self origins of the RNA through translation modulation.

Human T lymphotropic Virus type 1 (HTLV-1) is the etiological agent of Adult T cell Leukemia/Lymphoma (ATLL) and HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP). Both CD4+ T-cells and dendritic cells (DCs) infected with HTLV-1 are found in peripheral blood from HTLV-1 carriers. We previously demonstrated that monocyte-derived IL-4 DCs are more susceptible to HTLV-1 infection than autologous primary T-cells, suggesting that DC infection precedes T-cell infection. However, during blood transmission, breast-feeding or sexual transmission, HTLV-1 may encounter different DC subsets present in the blood, the intestinal or genital mucosa respectively. These different contacts may impact HTLV-1 ability to infect DCs and its subsequent transfer to T-cells. Using in vitro monocyte-derived IL-4 DCs, TGF-β DCs and IFN-α DCs that mimic DCs contacting HTLV-1 in vivo, we show here that despite their increased ability to capture HTLV-1 virions, IFN-α DCs restrict HTLV-1 productive infection. Surprisingly, we then demonstrate that it is not due to the antiviral activity of type–I interferon produced by IFN-α DCs, but that it is likely to be linked to a distinct trafficking route of HTLV-1 in IL-4 DCs vs. IFN-α DCs. Finally, we demonstrate that, in contrast to IL-4 DCs, IFN-α DCs are impaired in their capacity to transfer HTLV-1 to CD4 T-cells, both after viral capture and trans-infection and after their productive infection. In conclusion, the nature of the DCs encountered by HTLV-1 upon primo-infection and the viral trafficking route through the vesicular pathway of these cells determine the efficiency of viral transmission to T-cells, which may condition the fate of infection.

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Despite intense research in the context of neurodegenerative diseases associated with its misfolding, the endogenous human prion protein PrP(C) (or PRNP) has poorly understood physiological functions. Whereas most PrP(C) is exposed to the extracellular environment, conserved domains result in transmembrane forms of PrP(C) that traffic in the endolysosomal system and are linked to inherited and infectious neuropathologies. One transmembrane PrP(C) variant orients the N-terminal 'octarepeat' domain into the cytoplasm. Here we demonstrate that the octarepeat domain of human PrP(C) contains GW/WG motifs that bind Argonaute (AGO) proteins, the essential components of microRNA (miRNA)-induced silencing complexes (miRISCs). Transmembrane PrP(C) preferentially binds AGO, and PrP(C) promotes formation or stability of miRISC effector complexes containing the trinucleotide repeat-containing gene 6 proteins (TNRC6) and miRNA-repressed mRNA. Accordingly, effective repression of several miRNA targets requires PrP(C). We propose that dynamic interactions between PrP(C)-enriched endosomes and subcellular foci of AGO underpin these effects.

Exosomes are secreted membrane vesicles of endosomal origin present in biological fluids. Exosomes may serve as shuttles for amyloidogenic proteins, notably infectious prions, and may participate in their spreading in vivo. To explore the significance of the exosome pathway on prion infectivity and release, we investigated the role of the endosomal sorting complex required for transport (ESCRT) machinery and the need for ceramide, both involved in exosome biogenesis. Silencing of HRS-ESCRT-0 subunit drastically impairs the formation of cellular infectious prion due to an altered trafficking of cholesterol. Depletion of Tsg101-ESCRT-I subunit or impairment of the production of ceramide significantly strongly decreases infectious prion release. Together, our data reveal that ESCRT-dependent and -independent pathways can concomitantly regulate the exosomal secretion of infectious prion, showing that both pathways operate for the exosomal trafficking of a particular cargo. These data open up a new avenue to regulate prion release and propagation.