The cellular protein IFI16 colocalizes with the herpes simplex virus 1 (HSV-1) ubiquitin ligase ICP0 at early times of infection and is degraded as infection progresses. Here, we report that the factors governing the degradation of IFI16 and its colocalization with ICP0 are distinct from those of promyelocytic leukemia protein (PML), a well-characterized ICP0 substrate. Unlike PML, IFI16 colocalization with ICP0 was dependent on the ICP0 RING finger and did not occur when proteasome activity was inhibited. Expression of ICP0 in the absence of infection did not destabilize IFI16, the degradation occurred efficiently in the absence of ICP0 if infection was progressing efficiently, and IFI16 was relatively stable in wild-type (wt) HSV-1-infected U2OS cells. Therefore, IFI16 stability appears to be regulated by cellular factors in response to active HSV-1 infection rather than directly by ICP0. Because IFI16 is a DNA sensor that becomes associated with viral genomes during the early stages of infection, we investigated its role in the recruitment of PML nuclear body (PML NB) components to viral genomes. Recruitment of PML and hDaxx was less efficient in a proportion of IFI16-depleted cells, and this correlated with improved replication efficiency of ICP0-null mutant HSV-1. Because the absence of interferon regulatory factor 3 (IRF3) does not increase the plaque formation efficiency of ICP0-null mutant HSV-1, we speculate that IFI16 contributes to cell-mediated restriction of HSV-1 in a manner that is separable from its roles in IRF3-mediated interferon induction, but that may be linked to the PML NB response to viral infection. Intrinsic and innate immunity mechanisms are highly important arms of cellular defense against viral infections. A prominent aspect of innate immunity involves interferon (IFN)-related signaling mechanisms that activate expression of IFN-stimulated genes (ISGs), several of which have antiviral activity. While many sensors that initiate these pathways reside on the cell surface or in the cytoplasm (reviewed in reference 1), a prominent strand of recent research involves sensors of "foreign" DNA within the nucleus, notably IFI16 (2, 3). IFI16 is a member of the IFN-inducible p200 family of proteins which has both human and murine homologues (4). All members of the p200 protein family contain at least one copy of a 200-amino-acid motif located toward its C terminus which is involved in DNA binding and protein-protein interactions (4, 5). Some family members, including IFI16, also have an amino-terminal Pyrin domain. Pyrin domains are thought to be involved in regulation of cytokine responses, supporting a role for IFI16 in innate immunity (5-7). A model emerging from recent work proposes that IFI16 signals to STING, a cytoplasmic protein that is required for the IFN response to pathogen DNA, which in turn activates TBK1-mediated phosphorylation of interferon regulatory factor 3 (IRF3) and subsequent transcriptional activation of the beta IFN (IFN-␤) gene (8).Several recent studies indicate that ...

Covalent linkage to members of the small ubiquitin-like (SUMO) family of proteins is an important mechanism by which the functions of many cellular proteins are regulated. Sumoylation has roles in the control of protein stability, activity and localization, and is involved in the regulation of transcription, gene expression, chromatin structure, nuclear transport and RNA metabolism. Sumoylation is also linked, both positively and negatively, with the replication of many different viruses both in terms of modification of viral proteins and modulation of sumoylated cellular proteins that influence the efficiency of infection. One prominent example of the latter is the widespread reduction in the levels of cellular sumoylated species induced by herpes simplex virus type 1 (HSV-1) ubiquitin ligase ICP0. This activity correlates with relief from intrinsic immunity antiviral defence mechanisms. Previous work has shown that ICP0 is selective in substrate choice, with some sumoylated proteins such the promyelocytic leukemia protein PML being extremely sensitive, while RanGAP is completely resistant. Here we present a comprehensive proteomic analysis of changes in the cellular SUMO2 proteome during HSV-1 infection. Amongst the 877 potentially sumoylated species detected, we identified 124 whose abundance was decreased by a factor of 3 or more by the virus, several of which were validated by western blot and expression analysis. We found many previously undescribed substrates of ICP0 whose degradation occurs by a range of mechanisms, influenced or not by sumoylation and/or the SUMO2 interaction motif within ICP0. Many of these proteins are known or are predicted to be involved in the regulation of transcription, chromatin assembly or modification. These results present novel insights into mechanisms and host cell proteins that might influence the efficiency of HSV-1 infection.

We previously reported that MORC3, a protein associated with promyelocytic leukemia nuclear bodies (PML NBs), is a target of herpes simplex virus 1 (HSV-1) ICP0-mediated degradation (E. Sloan, et al., PLoS Pathog 11:e1005059, 2015, http: //dx.doi.org/10.1371/journal.ppat.1005059). Since it is well known that certain other components of the PML NB complex play an important role during an intrinsic immune response to HSV-1 and are also degraded or inactivated by ICP0, here we further investigate the role of MORC3 during HSV-1 infection. We demonstrate that MORC3 has antiviral activity during HSV-1 infection and that this antiviral role is counteracted by ICP0. In addition, MORC3's antiviral role extends to wild-type (wt) human cytomegalovirus (HCMV) infection, as its plaque-forming efficiency increased in MORC3-depleted cells. We found that MORC3 is recruited to sites associated with HSV-1 genomes after their entry into the nucleus of an infected cell, and in wt infections this is followed by its association with ICP0 foci prior to its degradation. The RING finger domain of ICP0 was required for degradation of MORC3, and we confirmed that no other HSV-1 protein is required for the loss of MORC3. We also found that MORC3 is required for fully efficient recruitment of PML, Sp100, hDaxx, and ␥H2AX to sites associated with HSV-1 genomes entering the host cell nucleus. This study further unravels the intricate ways in which HSV-1 has evolved to counteract the host immune response and reveals a novel function for MORC3 during the host intrinsic immune response. IMPORTANCEHerpesviruses have devised ways to manipulate the host intrinsic immune response to promote their own survival and persistence within the human population. One way in which this is achieved is through degradation or functional inactivation of PML NB proteins, which are recruited to viral genomes in order to repress viral transcription. Because MORC3 associates with PML NBs in uninfected cells and is a target for HSV-1-mediated degradation, we investigated the role of MORC3 during HSV-1 infection. We found that MORC3 is also recruited to viral HSV-1 genomes, and importantly it contributes to the fully efficient recruitment of PML, hDaxx, Sp100, and ␥H2AX to these sites. Depletion of MORC3 resulted in an increase in ICP0-null HSV-1 and wt HCMV replication and plaque formation; therefore, this study reveals that MORC3 is an antiviral factor which plays an important role during HSV-1 and HCMV infection. Herpes simplex virus 1 (HSV-1) is prevalent in populations throughout the world and is responsible for a number of clinically important diseases that range from facial and genital lesions to encephalitis (1, 2). This alphaherpesvirus establishes lifelong persistence within the host, remaining latent within sensory ganglia after the primary infection is resolved. Periodically the virus is reactivated from its latent state, resulting in recurrent lesions. HSV-1 has the capacity to remain persistent within the host and allow transmission within the popu...

Highlights d A mechanism of hybrid gene birth is employed by many families of RNA viruses d Human RNA and viral RNA encode new genes together d Hybrid genes either make extensions of viral proteins or novel proteins (UFOs) d Human-virus genes and proteins play roles in pathogenesis and are conserved

V aricella-zoster virus (VZV) is an alphaherpesvirus causing chickenpox (varicella) during primary infection and shingles (herpes zoster) following reactivation from a latent infection. Following initial exposure to the virus, there is a 10-to 21-day incubation period before the appearance of the varicella rash. During this time it has been proposed that VZV actively evades immune recognition in this period, since the development of adaptive immunity is delayed (reviewed in reference 1). We have postulated that VZV infection of dendritic cells (DCs) and/or modulation of the immune function of these potent antigen-presenting cells would provide a strategy that would enhance the capacity of the virus to be transported from the site of inoculation to the draining lymph nodes to infect T cells while also evading immune detection.We have previously shown that VZV can productively infect human DCs in vitro and in vivo (2,16,22). These studies included demonstration that productively infected immature monocytederived DCs (MDDCs) are unable to upregulate the functionally important immune molecules CD80, CD83, CD86, major histocompatibility complex I, and CCR7, which are required for DC maturation and induction of an effective antiviral immune response (2). The expression of the immune molecules inhibited by VZV are largely regulated by the nuclear factor B (NF-B) signal transduction pathway (4,6,(12)(13)(14). The NF-B signal transduction pathway is an important regulator of innate immunity and inflammation that is triggered by a wide variety of stimuli, including virus infection, tumor necrosis factor alpha (TNF-␣), and other cytokines and pathogens (26,29). Activation of the NF-B pathway via pattern recognition receptors results in the phosphorylation of inhibitor of B kinase complex (IKK), which in turn phosphorylates IB, targeting it for ubiquitination and degradation, allowing NF-B proteins (p50 and p65) to translocate into the nucleus and bind to promoters containing NF-B response elements, initiating transcription of target genes (reviewed in references 26 and 29).Herpesviruses encode multiple proteins that function in immune evasion, and several herpesvirus proteins target and disrupt the NF-B pathway. Viral genes encoded by 27,28), cytomegalovirus (23,34), and herpes simplex virus 1 (HSV-1) (3, 9, 24) have been identified to regulate the NF-B pathway in a cell type-dependent manner. Jones and Arvin (17) reported that VZV inhibits the NF-B pathway in human fibroblasts in vitro and in vivo following the phosphorylation and ubiquitination of IB␣ but prior to the translocation of NF-B proteins into the nucleus.In the present study, we sought to extend these studies and examine the effect of VZV on the NF-B pathway within VZVinfected human MDDCs. Using flow cytometry, immunofluorescent staining, and Western blotting, we establish the point where VZV impacts the NF-B pathway in VZV antigen-positive DCs. In addition, using a transient-transfection approach and flow cytometry, we identified the E3 ubiquitin ligase domai...

Influenza viruses can interact during coinfections, allowing viral fitness to be altered by genome complementation and competition, and increasing population diversity through reassortment. However, opportunities for these interactions are limited, as coinfection is blocked shortly after primary infection by a process known as superinfection exclusion (SIE). We asked whether SIE, which occurs at the level of individual cells, could limit within-host interactions between populations of influenza viruses as they spread across regions of cells. We first created a simplified model of within-host spread by infecting monolayers of cells with two isogenic influenza A viruses, each encoding a different fluorophore, and measuring the proportion of coinfected cells. In this system SIE begins within 2-4 hours of primary infection, with the kinetics of onset defined by the dose of primary virus. We then asked how SIE controls opportunities for coinfection as viruses spread across a monolayer of cells. We observed that viruses spreading from a single coinfected focus continued to coinfect cells as they spread, as all new infections were of cells that had not yet established SIE. In contrast, viruses spreading towards each other from separately infected foci could only establish minimal regions of coinfection before SIE blocked further coinfection. This patterning was recapitulated in the lungs of infected mice and is likely to apply to other viruses that exhibit SIE. It suggests that the kinetics of SIE onset separate a spreading infection into discrete regions, within which interactions between virus populations can occur freely, and between which they are blocked.

The induction of antiviral effector proteins as part of a homeostatically controlled innate immune response to infection plays a critical role in limiting the propagation and transmission of respiratory pathogens. However, the prolonged induction of this immune response can lead to lung hyperinflammation, tissue damage, and respiratory failure. We hypothesized that tissues exposed to the constant threat of infection may constitutively express higher levels of antiviral effector proteins to reduce the need to activate potentially harmful innate immune defences. By analysing transcriptomic data derived from a range of human tissues, we identify lung tissue to express constitutively higher levels of antiviral effector genes relative to that of other mucosal and non-mucosal tissues. By using primary cell lines and the airways of rhesus macaques, we show the interferon-stimulated antiviral effector protein TRIM22 (TRIpartite Motif 22) to be constitutively expressed in the lung independently of viral infection or innate immune stimulation. These findings contrast with previous reports that have shown TRIM22 expression in laboratory-adapted cell lines to require interferon stimulation. We demonstrate that constitutive levels of TRIM22 are sufficient to inhibit the onset of human and avian influenza A virus (IAV) infection by restricting the onset of viral transcription independently of interferon-mediated innate immune defences. Thus, we identify TRIM22 to confer a pre-existing (intrinsic) intracellular defence against IAV infection in cells derived from the respiratory tract. Our data highlight the importance of tissue-specific and cell-type dependent patterns of pre-existing immune gene expression in the intracellular restriction of IAV from the outset of infection.

Interactions between viruses during coinfections can influence viral fitness and population diversity, as seen in the generation of reassortant pandemic influenza A virus (IAV) strains. However, opportunities for interactions between closely related viruses are limited by a process known as superinfection exclusion (SIE), which blocks coinfection shortly after primary infection. Using IAVs, we asked whether SIE, an effect which occurs at the level of individual cells, could limit interactions between populations of viruses as they spread across multiple cells within a host. To address this, we first measured the kinetics of SIE in individual cells by infecting them sequentially with 2 isogenic IAVs, each encoding a different fluorophore. By varying the interval between addition of the 2 IAVs, we showed that early in infection SIE does not prevent coinfection, but that after this initial lag phase the potential for coinfection decreases exponentially. We then asked how the kinetics of SIE onset controlled coinfections as IAVs spread asynchronously across monolayers of cells. We observed that viruses at individual coinfected foci continued to coinfect cells as they spread, because all new infections were of cells that had not yet established SIE. In contrast, viruses spreading towards each other from separately infected foci could only establish minimal regions of coinfection before reaching cells where coinfection was blocked. This created a pattern of separate foci of infection, which was recapitulated in the lungs of infected mice, and which is likely to be applicable to many other viruses that induce SIE. We conclude that the kinetics of SIE onset segregate spreading viral infections into discrete regions, within which interactions between virus populations can occur freely, and between which they are blocked.