The respiratory tract is heavily populated with innate immune cells, but the mechanisms that control such cells are poorly defined. Here we found that the E3 ubiquitin ligase TRIM29 was a selective regulator of the activation of alveolar macrophages, the expression of type I interferons and the production of proinflammatory cytokines in the lungs. We found that deletion of TRIM29 enhanced macrophage production of type I interferons and protected mice from infection with influenza virus, while challenge of Trim29−/− mice with Haemophilus influenzae resulted in lethal lung inflammation due to massive production of proinflammatory cytokines by macrophages. Mechanistically, we demonstrated that TRIM29 inhibited interferon-regulatory factors and signaling via the transcription factor NF-κB by degrading the adaptor NEMO and that TRIM29 directly bound NEMO and subsequently induced its ubiquitination and proteolytic degradation. These data identify TRIM29 as a key negative regulator of alveolar macrophages and might have important clinical implications for local immunity and immunopathology.

Many double-stranded DNA viruses, such as Epstein-Barr virus, can establish persistent infection, but the underlying virus–host interactions remain poorly understood. Here we report that in human airway epithelial cells Epstein-Barr virus induces TRIM29, a member of the TRIM family of proteins, to inhibit innate immune activation. Knockdown of TRIM29 in airway epithelial cells enhances type I interferon production, and in human nasopharyngeal carcinoma cells results in almost complete Epstein-Barr virus clearance. TRIM29 is also highly induced by cytosolic double-stranded DNA in myeloid dendritic cells. TRIM29 −/− mice have lower adenovirus titers in the lung, and are resistant to lethal herpes simplex virus-1 infection due to enhanced production of type I interferon. Mechanistically, TRIM29 induces K48-linked ubiquitination of Stimulator of interferon genes, a key adaptor in double-stranded DNA-sensing pathway, followed by its rapid degradation. These data demonstrate that Epstein-Barr virus and possible other double-stranded DNA viruses use TRIM29 to suppress local innate immunity, leading to the persistence of DNA virus infections.

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Arenavirus pathogens cause a wide spectrum of diseases in humans ranging from central nervous system disease to lethal hemorrhagic fevers with few treatment options. The reason why some arenaviruses can cause severe human diseases while others cannot is unknown. We find that the Z proteins of all known pathogenic arenaviruses, lymphocytic choriomeningitis virus ( IMPORTANCEWe show that all known human-pathogenic arenaviruses share an innate immune suppression mechanism that is based on viral Z protein-mediated RLR inhibition. Our report offers important insights into the potential mechanism of arenavirus pathogenesis, provides a convenient way to evaluate the pathogenic potential of known and/or emerging arenaviruses, and reveals a novel target for the development of broad-spectrum therapies to treat this group of diverse pathogens. More broadly, our report provides a better understanding of the mechanisms of viral immune suppression and host-pathogen interactions. Intracellular RNA viruses are recognized by a family of cytosolic RNA helicase proteins called retinoic acid-inducible gene 1 (RIG-i)-like receptors (RLRs) to activate the antiviral and inflammatory signals (1, 2). The RLR members include RIG-i, Melanoma Differentiation-Associated protein 5 (MDA5), and Laboratory of Genetics and Physiology 2 (LGP2) (3-5). RIG-i recognizes short double-stranded RNA (dsRNA) with 5= triphosphate, while MDA5 recognizes long RNA duplexes (6). Upon ligand binding by the C-terminal domains (CTD) of RIG-i and MDA5, these proteins undergo conformational changes to activate the N-terminal CARD domains that mediate their interactions with the adaptor molecule mitochondrial antiviral signaling (MAVS)/IPS-1/virus-induced signaling adaptor (VISA)/Cardif to trigger the signaling cascades that consist of tumor necrosis factor (TNF) receptor-associated factors (TRAFs), TANK-binding kinase 1 (TBK1), and inhibitor-B kinase ε (IKKε) to activate transcription factors NF-B, interferon (IFN) regulatory factor 3 (IRF3), and IRF7, which induce the production of the type I IFNs and other cytokines (3). The RLR pathway is essential for host innate immunity to RNA viruses and is thus a major target of viral immune evasion mechanisms (4, 7). Influenza virus NS1 inhibits RIG-i activation by interacting with TRIM25 to prevent RIG-i ubiquitinylation (8). Paramyxovirus V protein binds and inhibits MDA5 (9). Ebola virus (EBOV) VP35 blocks RLR signaling through multiple mechanisms such as sequestering the RIG-i cofactor PKR activator (PACT), preventing the interactions of TBK1 and IKKε with IRFs, and inhibiting IRF7 activity (10-14). Arenaviral nucleoprotein (NP) strongly inhibits the production of type I IFNs through its DEDDH exoribonuclease (RNase) activity, possibly by degrading the immunostimulatory dsRNA substrates (15)(16)(17)(18)(19)(20).Arenaviruses are a diverse family of negative-strand enveloped RNA viruses with a bisegmented RNA genome, which encodes two proteins on each segment in an ambisense orientation-glycoprotein GPC and nuc...

Varicella-zoster virus (VZV) infection of differentiated cells within the host and establishment of latency likely requires evasion of innate immunity and limits secretion of antiviral cytokines. Here we report that its immediate-early protein ORF61 antagonizes the beta interferon (IFN-␤) pathway. VZV infection down-mod-Varicella-zoster virus (VZV) is ubiquitous in most of the population worldwide and is an alphaherpesvirus restricted to humans. Primary VZV infection begins with inoculation of the respiratory mucosa, followed by cell-associated viremia and the rash of chickenpox. During primary infection, the virus establishes latency in sensory ganglia and can subsequently reactivate to cause shingles (herpes zoster) (3). The VZV genome contains one copy of linear double-strand DNA approximately 125 kb in length and encodes about 70 unique proteins. Similar to other alphaherpesviruses, the expression of VZV genes is assorted temporally into three categories-immediate-early (IE), early (E), and late (L)-and the IE proteins usually play critical roles during VZV life cycles (29).ORF61 encodes a 62-to 66-kDa phosphoprotein (45), which is highly homologous to herpes simplex virus 1 (HSV-1) ICP0 in the RING finger domain and can partially complement the function of ICP0 in ICP0-null HSV-1 (28). Both proteins exhibit regulatory functions, but the difference between them is that ORF61 has either a transactivated or repressive function (16, 31), while ICP0 mostly shows transcriptional activation (12,14). Although ICP0 can inhibit the innate immunity pathway at many levels, such as IRF3 and PML (13,26), little is known about the function of ORF61 in interrupting the innate immunity.The innate immune system is an ancient and nonspecific system that provides the first line of defense against infection. One of the most effective innate antiviral responses is the production of alpha/beta interferon (IFN-␣/␤) and the subsequent induction of interferon-stimulated genes (ISGs) (38). Previous studies have demonstrated that VZV infection stimulates innate immune responses mainly by the production of IFN-␣ and IFN-␥ (1, 2, 27), while IFN-␤ is not detected in serum of VZV-infected patients. This might be explained by the evidence provided by one group that the production of IFN-␤ may be blocked by ORF62 during VZV infection (40).Increasing evidence has shown that ICP0 possesses various antagonistic functions against the host innate immune system

Arenaviruses cause severe hemorrhagic fever diseases in humans, and there are limited preventative and therapeutic measures against these diseases. Previous structural and functional analyses of arenavirus nucleoproteins (NPs) revealed a conserved DEDDH exoribonuclease (RNase) domain that is important for type I interferon (IFN) suppression, but the biological roles of the NP RNase in viral replication and host immune suppression have not been well characterized. Infection of guinea pigs with Pichinde virus (PICV), a prototype arenavirus, can serve as a surrogate small animal model for arenavirus hemorrhagic fevers. In this report, we show that mutation of each of the five RNase catalytic residues of PICV NP diminishes the IFN suppression activity and slightly reduces the viral RNA replication activity. Recombinant PICVs with RNase catalytic mutations can induce high levels of IFNs and barely grow in IFN-competent A549 cells, in sharp contrast to the wild-type (WT) virus, while in IFNdeficient Vero cells, both WT and mutant viruses can replicate at relatively high levels. Upon infection of guinea pigs, the RNase mutant viruses stimulate strong IFN responses, fail to replicate productively, and can become WT revertants. Serial passages of the RNase mutants in vitro can also generate WT revertants. Thus, the NP RNase function is essential for the innate immune suppression that allows the establishment of a productive early viral infection, and it may be partly involved in the process of viral RNA replication. IMPORTANCEArenaviruses, such as Lassa, Lujo, and Machupo viruses, can cause severe and deadly hemorrhagic fever diseases in humans, and there are limited preventative and treatment options against these diseases. Development of broad-spectrum antiviral drugs depends on a better mechanistic understanding of the conserved arenavirus proteins in viral infection. The nucleoprotein (NPs) of all arenaviruses carry a unique exoribonuclease (RNase) domain that has been shown to be critical for the suppression of type I interferons. However, the functional roles of the NP RNase in arenavirus replication and host immune suppression have not been characterized systematically. Using a prototype arenavirus, Pichinde virus (PICV), we characterized the viral growth and innate immune suppression of recombinant RNase-defective mutants in both cell culture and guinea pig models. Our study suggests that the NP RNase plays an essential role in the suppression of host innate immunity, and possibly in viral RNA replication, and that it can serve as a novel target for developing antiviral drugs against arenavirus pathogens.

Semaphorin-4A (Sema4A) has been implicated in the co-stimulation of T cells and drives Th1 immune responses by binding to the receptor T-cell immunoglobulin and mucin domain protein 2 (Tim-2) in mice. Here we show that human, but not murine, Sema4A is preferentially expressed on antigen-presenting cells, and co-stimulates CD4+ T-cell proliferation and drives Th2 responses. By employing two independent cloning strategies, we demonstrate that Immunoglobulin-like transcript 4 (ILT-4) is a receptor for human SEMA4A (hSEMA4A) on activated CD4+ T cells. We also find hSEMA4A to be highly expressed in human asthmatic lung tissue, implying its potential function in disease pathogenesis. Our study defines a different biological function of hSEMA4A from its murine homolog through its binding to the receptor of ILT-4 to co-stimulate CD4+T cells and regulate Th2 cells differentiation.