Measles (MV) is an aerosol-transmitted virus that affects more than 10 million children each year and accounts for approximately 120,000 deaths1,2. While it was long believed to replicate in the respiratory epithelium before disseminating, it was recently shown to initially infect macrophages and dendritic cells of the airways using the signaling lymphocytic activation molecule (SLAM, CD150) as receptor3-6. These cells then cross the respiratory epithelium and ferry the infection to lymphatic organs where MV replicates vigorously7. How and where the virus crosses back into the airways has remained unknown. Based on functional analyses of surface proteins preferentially expressed on virus-permissive epithelial cell lines, we identified nectin-48 (poliovirus-receptor-like-4) as a candidate host exit receptor. This adherens junction protein of the immunoglobulin superfamily interacts with the viral attachment protein with high affinity through its membrane-distal domain. Nectin-4 sustains MV entry and non-cytopathic lateral spread in well-differentiated primary human airway epithelial sheets infected basolaterally. It is down-regulated in infected epithelial cells, including those of macaque tracheas. While other viruses use receptors to enter hosts or transit through their epithelial barriers, we suggest that MV targets nectin-4 to emerge in the airways. Nectin-4 is a cellular marker of several types of cancer9-11, which has implications for ongoing MV-based clinical trials of oncolysis12.

The Ebolavirus VP24 protein counteracts alpha/beta interferon (IFN-␣/␤) and IFN-␥ signaling by blocking the nuclear accumulation of tyrosine-phosphorylated STAT1 (PY-STAT1). According to the proposed model, VP24 binding to members of the NPI-1 subfamily of karyopherin alpha (KPN␣) nuclear localization signal receptors prevents their binding to PY-STAT1, thereby preventing PY-STAT1 nuclear accumulation. This study now identifies two domains of VP24 required for inhibition of IFN-␤-induced gene expression and PY-STAT1 nuclear accumulation. We demonstrate that loss of function correlates with loss of binding to KPN␣ proteins. Thus, the VP24 IFN antagonist function requires the ability of VP24 to interact with KPN␣.In the order Mononegavirales, Ebolavirus and Marburgvirus belong to the family Filoviridae (12). Ebolaviruses are responsible for outbreaks in central Africa of severe hemorrhagic fever in humans and nonhuman primates, with human fatality rates of up to 90% (16). Currently, there are no licensed vaccines or approved treatments available for filovirus infections. Ebolavirus counteracts synthesis of alpha/beta interferon (IFN-␣/␤) and cellular responses to IFN-␣/␤/␥ (7,8,10,11,23). VP35 blocks production of IFN-␣/␤ by inhibiting the activation of interferon regulatory factors by blocking interferon regulatory factor 3 (IRF-3) kinases IKKepsilon and TBK-1 (1, 2, 21) and by increasing SUMOylation of IRF-7 (3). VP24 impairs cellular responses to IFN-␣/␤/␥ by blocking nuclear accumulation of tyrosine-phosphorylated STAT1 (PY-STAT1). Nuclear translocation of PY-STAT1 is essential for transcriptional activation of numerous IFN-responsive genes and occurs through interaction with karyopherin ␣-1 (KPN␣1) and perhaps other members of the NPI-1 subfamily of KPN␣ proteins (14,15,18,19,22,25). Correspondingly, VP24 interacts with the KPN␣ proteins that mediate PY-STAT1 nuclear accumulation (23, 24). Other pathogenic viruses also impair innate immune signaling via interaction with KPN␣ proteins, suggesting that this may be a common immune evasion strategy. For example, the severe acute respiratory syndrome coronavirus (SARS-CoV) ORF6 protein tethers a karyopherin ␣-2/karyopherin ␤ complex to the endoplasmic reticulum (ER)/Golgi membrane to prevent PY-STAT1 nuclear import complex formation (6), and Hantaan virus nucleocapsid protein inhibits NF-B activation through interaction with KPN␣ (26). Our previous work suggests that the interaction of VP24 with select KPN␣ proteins is responsible for the ability of VP24 to inhibit IFN signaling (23,24).In this study, mutations that impair VP24 inhibition of IFNinduced gene expression were identified. Plasmids expressing wild-type or mutated VP24 proteins were cotransfected into 293T cells together with plasmids expressing Renilla luciferase (expressed from a constitutive cytomegalovirus [CMV] promoter) and firefly luciferase (expressed from an IFN-inducible ISG54 promoter). At 1 day posttransfection, the cells were treated with 1,000 U/ml of human IFN-␤ (PBL Interferon Source) fo...

In patients who are receiving prolonged antiretroviral treatment (ART), HIV can persist within a small pool of long-lived resting memory CD4+ T cells infected with integrated latent virus. This latent reservoir involves distinct memory subsets. Here we provide results that suggest a progressive reduction of the size of the blood latent reservoir around a core of less-differentiated memory subsets (central memory and stem cell-like memory (TSCM) CD4+ T cells). This process appears to be driven by the differences in initial sizes and decay rates between latently infected memory subsets. Our results also suggest an extreme stability of the TSCM sub-reservoir, the size of which is directly related to cumulative plasma virus exposure before the onset of ART, stressing the importance of early initiation of effective ART. The presence of these intrinsic dynamics within the latent reservoir may have implications for the design of optimal HIV therapeutic purging strategies.

Lassa fever is a major threat in Western Africa. The large number of people living at risk for this disease calls for the development of a vaccine against Lassa virus (LASV). We generated live-attenuated LASV vaccines based on measles virus and Mopeia virus platforms and expressing different LASV antigens, with the aim to develop a vaccine able to protect after a single shot. We compared the efficacy of these vaccines against LASV in cynomolgus monkeys. The vaccines were well tolerated and protected the animals from LASV infection and disease after a single immunization but with varying efficacy. Analysis of the immune responses showed that complete protection was associated with robust secondary T cell and antibody responses against LASV. Transcriptomic and proteomic analyses showed an early activation of innate immunity and T cell priming after immunization with the most effective vaccines, with changes detectable as early as 2 days after immunization. The most efficacious vaccine candidate, a measles vector simultaneously expressing LASV glycoprotein and nucleoprotein, has been selected for further clinical evaluation.

Marburg virus (MARV) has a high fatality rate in humans, causing hemorrhagic fever characterized by massive viral replication and dysregulated inflammation. Here, we demonstrate that VP24 of MARV binds Kelch-like ECH-associated protein 1 (Keap1), a negative regulator of nuclear transcription factor erythroid-derived 2 (Nrf2). Binding of VP24 to Keap1 Kelch domain releases Nrf2 from Keap1-mediated inhibition promoting persistent activation of a panoply of cytoprotective genes implicated in cellular responses to oxidative stress and regulation of inflammatory responses. Increased expression of Nrf2-dependent genes was demonstrated both during MARV infection and upon ectopic expression of MARV VP24. We also show that Nrf2-deficient mice can control MARV infection when compared to lethal infection in wild-type animals, indicating that Nrf2 is critical for MARV infection. We conclude that VP24-driven activation of the Nrf2-dependent pathway is likely to contribute to dysregulation of host antiviral inflammatory responses and that it ensures survival of MARV-infected cells despite these responses.

The epithelium is a highly organized type of animal tissue. Except for blood and lymph vessels, epithelial cells cover the body, line its cavities in single or stratified layers and support exchange between compartments. In addition, epithelia offer to the body a barrier to pathogen invasion. To transit through or to replicate in epithelia, viruses have to face several obstacles, starting from cilia and glycocalyx where they can be neutralized by secreted immunoglobulins. Tight junctions and adherens junctions also prevent viruses to cross the epithelial barrier. However, viruses have developed multiple strategies to blaze their path through the epithelium by utilizing components of cell–cell adhesion structures as receptors. In this Commentary, we discuss how viruses take advantage of the apical junction complex to spread. Whereas some viruses quickly disrupt epithelium integrity, others carefully preserve it and use cell adhesion proteins and their cytoskeletal connections to rapidly spread laterally. This is exemplified by the hidden transmission of enveloped viruses that use nectins as receptors. Finally, several viruses that replicate preferentially in cancer cells are currently used as experimental cancer therapeutics. Remarkably, these viruses use cell adhesion molecules as receptors, probably because – to reach tumors and metastases – oncolytic viruses must efficiently traverse or break epithelia.

The discovery that measles virus (MV) uses the adherens junction protein nectin-4 as its epithelial receptor provides a new vantage point from which to characterize its rapid spread in the airway epithelium. We show here that in well-differentiated primary cultures of airway epithelial cells from human donors (HAE), MV infectious centers form rapidly and become larger than those of other respiratory pathogens: human respiratory syncytial virus, parainfluenza virus 5, and Sendai virus. While visible syncytia do not form after MV infection of HAE, the cytoplasm of an infected cell suddenly flows into an adjacent cell, as visualized through wild-type MV-expressed cytoplasmic green fluorescent protein (GFP). High-resolution video microscopy documents that GFP flows through openings that form on the lateral surfaces between columnar epithelial cells. To assess the relevance of the protein afadin, which connects nectin-4 to the actin cytoskeleton, we knocked down its mRNA. This resulted in more-limited infectious-center formation. We also generated a nectin-4 mutant without the afadin-binding site in its cytoplasmic tail. This mutant was less effective than wild-type human nectin-4 at promoting MV infection in primary cultures of porcine airway epithelia. Thus, in airway epithelial cells, MV spread requires the nectin-4/afadin complex and is based on cytoplasm transfer between columnar cells. Since the viral membrane fusion apparatus may open the passages that allow cytoplasm transfer, we refer to them as intercellular membrane pores. Virus-induced intercellular pores may contribute to extremely efficient measles contagion by promoting the rapid spread of the virus through the upper respiratory epithelium. IMPORTANCE Measles virus (MV

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