Nontypeable Haemophilus influenzae challenge during gammaherpesvirus infection enhances viral reactivation and latency.

Gammaherpesviruses are ubiquitous, lifelong pathogens associated with multiple cancers that infect over 95% of the adult population. Increases in viral reactivation, due to stress and other unknown factors impacting the immune response, frequently precedes lymphomagenesis. One potential stressor that could promote viral reactivation and increase viral latency would be the myriad of infections from bacterial and viral pathogens that we experience throughout our lives. Using murine gammaherpesvirus 68 (MHV68), a mouse model of gammaherpesvirus infection, we examined the impact of bacterial challenge on gammaherpesvirus infection. We challenged MHV68 infected mice during the establishment of latency with nontypeable Haemophilus influenzae (NTHi) to determine the impact of bacterial infection on viral reactivation and latency. Mice infected with MHV68 and then challenged with NTHi, saw increases in viral reactivation and viral latency. These data support the hypothesis that bacterial challenge can promote gammaherpesvirus reactivation and latency establishment, with possible consequences for viral lymphomagenesis.

Clonal lineage tracing reveals mechanisms skewing CD8+ T cell fate decisions in chronic infection.

Although recent evidence demonstrates heterogeneity among CD8+ T cells during chronic infection, developmental relationships and mechanisms underlying their fate decisions remain incompletely understood. Using single-cell RNA and TCR sequencing, we traced the clonal expansion and differentiation of CD8+ T cells during chronic LCMV infection. We identified immense clonal and phenotypic diversity, including a subset termed intermediate cells. Trajectory analyses and infection models showed intermediate cells arise from progenitor cells before bifurcating into terminal effector and exhausted subsets. Genetic ablation experiments identified that type I IFN drives exhaustion through an IRF7-dependent mechanism, possibly through an IFN-stimulated subset bridging progenitor and exhausted cells. Conversely, Zeb2 was critical for generating effector cells. Intriguingly, some T cell clones exhibited lineage bias. Mechanistically, we identified that TCR avidity correlates with an exhausted fate, whereas SHP-1 selectively restricts low-avidity effector cell accumulation. Thus, our work elucidates novel mechanisms underlying CD8+ T cell fate determination during persistent infection and suggests two potential pathways leading to exhaustion.

Mouse Homologue of Human HLA-DO Does Not Preempt Autoimmunity but Controls Murine Gammaherpesvirus MHV68.

H2-O (human HLA-DO) is a relatively conserved nonclassical MHC class II (MHCII)-like molecule. H2-O interaction with human HLA-DM edits the repertoire of peptides presented to TCRs by MHCII. It was long hypothesized that human HLA-DM inhibition by H2-O provides protection from autoimmunity by preventing binding of the high-affinity self-peptides to MHCII. The available evidence supporting this hypothesis, however, was inconclusive. A possibility still remained that the effect of H2-O deficiency on autoimmunity could be better revealed by using H2-O-deficient mice that were already genetically predisposed to autoimmunity. In this study, we generated and used autoimmunity-prone mouse models for systemic lupus erythematosus and organ-specific autoimmunity (type 1 diabetes and multiple sclerosis) to definitively test whether H2-O prevents autoimmune pathology. Whereas our data failed to support any significance of H2-O in protection from autoimmunity, we found that it was critical for controlling a γ-herpesvirus, MHV68. Thus, we propose that H2-O editing of the MHCII peptide repertoire may have evolved as a safeguard against specific highly prevalent viral pathogens.

MyD88 is an essential regulator of NK cell-mediated clearance of MCMV infection.

The signaling adapter MyD88 is critical for immune cell activation in response to viral or bacterial pathogens via several TLRs, IL-1ßR and IL-18R. However, the essential role of MyD88 during activations mediated by germline-encoded NK cell receptors (NKRs), such as Ly49H or NKG2D, has yet to be investigated. To define the NK cell-intrinsic function of MyD88, we generated a novel NK cell conditional knockout mouse for MyD88 (Myd88fl/flNcr1Cre/+). Phenotypic characterization of these mice demonstrated that MyD88 is dispensable for NK cell development and maturation. However, the MyD88-deficient NK cells exhibited significantly reduced cytotoxic potentials in vivo. In addition, the lack of MyD88 significantly reduced the NKG2D-mediated inflammatory cytokine production in vitro. Consistent with this, mice lacking MyD88 were unable to respond and clear MCMV infection. Transcriptomic analyses of splenic NK cells following MCMV infection revealed that inflammatory gene signatures were upregulated in Ly49H+. In contrast, Ly49H- NK cells have significant enrichment in G2M checkpoint genes, revealing distinct transcriptomic profiles of these subsets. Our results identify a central role for MyD88 in Ly49H-dependent gene signatures, including alterations in genes regulating proliferation in Ly49H+ NK cells. In summary, our study reveals a previously unknown function of MyD88 in Ly49H-dependent signaling and in vivo functions of NK cells.

MHC Class II Presentation Is Affected by Polymorphism in the H2-Ob Gene and Additional Loci.

Pathogen-derived peptides are loaded on MHC class II (MHCII) and presented to CD4+ T cells for their activation. Peptide loading of MHCII occurs in specialized endosomal compartments and is controlled by the nonclassical MHCII molecules H2-M and H2-O, which are both constitutive αß heterodimers. H2-M catalyzes MHCII peptide loading, whereas H2-O modulates H2-M activity by acting as an MHCII mimic. Recently, we discovered that the H2-Ob allele inherited by retrovirus-resistant I/LnJ mice results in nonfunctional H2-O. I/LnJ H2-O binds to but does not inhibit H2-M. Compared with H2-Oß from virus-susceptible mice, H2-Oß from I/LnJ mice has four unique amino acid substitutions, three in the Ig domain and one in the cytoplasmic tail. In this study we show that the three amino acids in the Ig domain of I/LnJ Oß are critical for the H2-O inhibitory activity of H2-M. Unexpectedly, we found that MHCII presentation was significantly different in Ag-presenting cells from two closely related mouse strains, B6J and B6N, which carry identical alleles of MHCII, H2-O, and H2-M. Using a positional cloning approach, we have identified two loci, polymorphic between B6J and B6N, that mediate the difference in MHCII presentation. Collectively, these studies reveal extra complexity in MHCII/H2-M/H-2O interactions that likely involve yet to be identified modulators of the pathway.

STING Activated Tumor-Intrinsic Type I Interferon Signaling Promotes CXCR3 Dependent Antitumor Immunity in Pancreatic Cancer.

BACKGROUND & AIMS: Pancreatic ductal adenocarcinoma (PDA) is a lethal chemoresistant cancer that exhibits early metastatic spread. The highly immunosuppressive PDA tumor microenvironment renders patients resistant to emerging immune-targeted therapies. Building from our prior work, we evaluated stimulator of interferon genes (STING) agonist activation of PDA cell interferon-α/ß-receptor (IFNAR) signaling in systemic antitumor immune responses. METHODS: PDA cells were implanted subcutaneously to wild-type, IFNAR-, or CXCR3-knockout mice. Tumor growth was monitored, and immune responses were comprehensively profiled. RESULTS: Human and mouse STING agonist ADU-S100 reduced local and distal tumor burden and activated systemic antitumor immune responses in PDA-bearing mice. Effector T-cell infiltration and inflammatory cytokine and chemokine production, including IFN-dependent CXCR3-agonist chemokines, were elevated, whereas suppressive immune populations were decreased in treated tumors. Intratumoral STING agonist treatment also generated inflammation in distal noninjected tumors and peripheral immune tissues. STING agonist treatment of type I IFN-responsive PDA tumors engrafted to IFNAR-/- recipient mice was sufficient to contract tumors and stimulate local and systemic T-cell activation. Tumor regression and CD8+ T-cell infiltration were abolished in PDA engrafted to CXCR3-/- mice treated with STING agonist. CONCLUSIONS: These data indicate that STING agonists promote T-cell infiltration and counteract immune suppression in locally treated and distant tumors. Tumor-intrinsic type I IFN signaling initiated systemic STING-mediated antitumor inflammation and required CXCR3 expression. STING-mediated induction of systemic immune responses provides an approach to harness the immune system to treat primary and disseminated pancreatic cancers.

Innate immunity and alpha/gammaherpesviruses: first impressions last a lifetime.

Innate immune system is considered the first line of defense during viral invasion, with the wealth of the literature demonstrating innate immune control of diverse viruses during acute infection. What is far less clear is the role of innate immune system during chronic virus infections. This short review focuses on alphaherpesviruses and gammaherpesviruses, two highly prevalent herpesvirus subfamilies that, following a brief, once in a lifetime period of acute lytic infection, establish life-long latent infection that is characterized by sporadic reactivation in an immunocompetent host. In spite of many similarities, these two viral families are characterized by distinct cellular tropism and pathogenesis. Here we focus on the published in vivo studies to review known interactions of these two viral subfamilies with the innate immunity of the intact host, both during acute and, particularly, chronic virus infection.

Gammaherpesviruses and B Cells: A Relationship That Lasts a Lifetime.

Gammaherpesviruses are highly prevalent pathogens that establish life-long infection and are associated with diverse malignancies, including lymphoproliferative diseases and B cell lymphomas. Unlike other viruses that either do not infect B cells or infect B cells transiently, gammaherpesviruses manipulate physiological B cell differentiation to establish life-long infection in memory B cells. Disruption of such viral manipulation by genetic or environmental causes is likely to seed viral lymphomagenesis. In this review, we discuss physiological and unique host and viral mechanisms usurped by gammaherpesviruses to fine tune host B cell biology for optimal infection establishment and maintenance.

Antiviral activity of a purine synthesis enzyme reveals a key role of deamidation in regulating protein nuclear import.

Protein nuclear translocation is highly regulated and crucial for diverse biological processes. However, our understanding concerning protein nuclear import is incomplete. Here we report that a cellular purine synthesis enzyme inhibits protein nuclear import via deamidation. Employing human Kaposi's sarcoma-associated herpesvirus (KSHV) to probe the role of protein deamidation, we identified a purine synthesis enzyme, phosphoribosylformylglycinamidine synthetase (PFAS) that inhibits KSHV transcriptional activation. PFAS deamidates the replication transactivator (RTA), a transcription factor crucial for KSHV lytic replication. Mechanistically, deamidation of two asparagines flanking a positively charged nuclear localization signal impaired the binding of RTA to an importin ß subunit, thus diminishing RTA nuclear localization and transcriptional activation. Finally, RTA proteins of all gamma herpesviruses appear to be regulated by PFAS-mediated deamidation. These findings uncover an unexpected function of a metabolic enzyme in restricting viral replication and a key role of deamidation in regulating protein nuclear import.

The Role of Metabolic Flexibility in the Regulation of the DNA Damage Response by Nitric Oxide.

In this report, we show that nitric oxide suppresses DNA damage response (DDR) signaling in the pancreatic ß-cell line INS 832/13 and rat islets by inhibiting intermediary metabolism. Nitric oxide is known to inhibit complex IV of the electron transport chain and aconitase of the Krebs cycle. Non-ß cells compensate by increasing glycolytic metabolism to maintain ATP levels; however, ß cells lack this metabolic flexibility, resulting in a nitric oxide-dependent decrease in ATP and NAD+ Like nitric oxide, mitochondrial toxins inhibit DDR signaling in ß cells by a mechanism that is associated with a decrease in ATP. Non-ß cells compensate for the effects of mitochondrial toxins with an adaptive shift to glycolytic ATP generation that allows for DDR signaling. Forcing non-ß cells to derive ATP via mitochondrial respiration (replacing glucose with galactose in the medium) and glucose deprivation sensitizes these cells to nitric oxide-mediated inhibition of DDR signaling. These findings indicate that metabolic flexibility is necessary to maintain DDR signaling under conditions in which mitochondrial oxidative metabolism is inhibited and support the inhibition of oxidative metabolism (decreased ATP) as one protective mechanism by which nitric oxide attenuates DDR-dependent ß-cell apoptosis.

γ-herpesvirus latency attenuates Mycobacterium tuberculosis infection in mice.

Tuberculosis is caused by Mycobacterium tuberculosis (Mtb), a bacterial pathogen which is transmitted via aerosol and establishes a chronic lung infection. In naïve hosts, Mtb grows for several weeks without being restricted by IFNγ-producing T cells, which eventually accumulate and limit Mtb dissemination. In this study, we used a mouse model of Mtb/γ-herpesvirus (γHV) coinfection to test the hypothesis that latent γHV infection alters host resistance to Mtb. γHVs are DNA viruses which elicit a polyclonal T cell response and attenuate some acute bacterial pathogens in mice; whether γHVs modulate infection with Mtb is unknown. Here, mice harboring latent mouse gammaherpesvirus 68 (MHV68)-a γHV genetically and biologically related to human Epstein Barr virus (EBV)-were infected via aerosol with a low dose of virulent Mtb. Mtb burdens and IFNγ+ T cell frequencies in mice with latent MHV68 (MHV68POS mice) were subsequently measured and compared to control mice that did not harbor latent MHV68 (MHV68NEG mice). Relative to MHV68NEG controls, MHV68POS mice more effectively limited Mtb growth and dissemination, and had higher frequencies of CD4+IFNγ+ cells in lung-draining lymph nodes. Collectively, our results support a model wherein latent γHV confers moderate protection against subsequent Mtb infection.

Chewing the Fat: The Conserved Ability of DNA Viruses to Hijack Cellular Lipid Metabolism.

Viruses manipulate numerous host factors and cellular pathways to facilitate the replication of viral genomes and the production of infectious progeny. One way in which viruses interact with cells is through the utilization and exploitation of the host lipid metabolism. While it is likely that most-if not all-viruses require lipids or intermediates of lipid synthesis to replicate, many viruses also actively induce lipid metabolic pathways to sustain a favorable replication environment. From the formation of membranous replication compartments, to the generation of ATP or protein modifications, viruses exhibit differing requirements for host lipids. Thus, while the exploitation of lipid metabolism is a common replication strategy, diverse viruses employ a plethora of mechanisms to co-opt these critical cellular pathways. Here, we review recent literature regarding the exploitation of host lipids and lipid metabolism specifically by DNA viruses. Importantly, furthering the understanding of the viral requirements for host lipids may offer new targets for antiviral therapeutics and provide opportunities to repurpose the numerous FDA-approved compounds targeting lipid metabolic pathways as antiviral agents.

Conserved Gammaherpesvirus Protein Kinase Selectively Promotes Irrelevant B Cell Responses.

Gammaherpesviruses are ubiquitous pathogens that are associated with B cell lymphomas. In the early stages of chronic infection, these viruses infect naive B cells and subsequently usurp the B cell differentiation process through the germinal center response to ensure latent infection of long-lived memory B cells. A unique feature of early gammaherpesvirus chronic infection is a robust differentiation of irrelevant, virus-nonspecific B cells with reactivities against self-antigens and antigens of other species. In contrast, protective, virus-specific humoral responses do not reach peak levels until a much later time. While several host factors are known to either promote or selectively restrict gammaherpesvirus-driven germinal center response, viral mechanisms that contribute to the irrelevant B cell response have not been defined. In this report we show that the expression and the enzymatic activity of the gammaherpesvirus-encoded conserved protein kinase selectively facilitates the irrelevant, but not virus-specific, B cell responses. Further, we show that lack of interleukin-1 (IL-1) receptor attenuates gammaherpesvirus-driven B cell differentiation and viral reactivation. Because germinal center B cells are thought to be the target of malignant transformation during gammaherpesvirus-driven lymphomagenesis, identification of host and viral factors that promote germinal center responses during gammaherpesvirus infection may offer an insight into the mechanism of gammaherpesvirus pathogenesis.IMPORTANCE Gammaherpesviruses are ubiquitous cancer-associated pathogens that usurp the B cell differentiation process to establish life-long latent infection in memory B cells. A unique feature of early gammaherpesvirus infection is the robust increase in differentiation of B cells that are not specific for viral antigens and instead encode antibodies that react with self-antigens and antigens of other species. Viral mechanisms that are involved in driving such irrelevant B cell differentiation are not known. Here, we show that gammaherpesvirus-encoded conserved protein kinase and host IL-1 signaling promote irrelevant B cell responses and gammaherpesvirus-driven germinal center responses, with the latter thought to be the target of viral transformation.

Single-cell RNA sequencing unveils an IL-10-producing helper subset that sustains humoral immunity during persistent infection.

During chronic viral infection, the inflammatory function of CD4 T-cells becomes gradually attenuated. Concurrently, Th1 cells progressively acquire the capacity to secrete the cytokine IL-10, a potent suppressor of antiviral T cell responses. To determine the transcriptional changes that underlie this adaption process, we applied a single-cell RNA-sequencing approach and assessed the heterogeneity of IL-10-expressing CD4 T-cells during chronic infection. Here we show an IL-10-producing population with a robust Tfh-signature. Using IL-10 and IL-21 double-reporter mice, we further demonstrate that IL-10+IL-21+co-producing Tfh cells arise predominantly during chronic but not acute LCMV infection. Importantly, depletion of IL-10+IL-21+co-producing CD4 T-cells or deletion of Il10 specifically in Tfh cells results in impaired humoral immunity and viral control. Mechanistically, B cell-intrinsic IL-10 signaling is required for sustaining germinal center reactions. Thus, our findings elucidate a critical role for Tfh-derived IL-10 in promoting humoral immunity during persistent viral infection.

ATM supports gammaherpesvirus replication by attenuating type I interferon pathway.

Ataxia-Telangiectasia mutated (ATM) kinase participates in multiple networks, including DNA damage response, oxidative stress, and mitophagy. ATM also supports replication of diverse DNA and RNA viruses. Gammaherpesviruses are prevalent cancer-associated viruses that benefit from ATM expression during replication. This proviral role of ATM had been ascribed to its signaling within the DNA damage response network; other functions of ATM have not been considered. In this study increased type I interferon (IFN) responses were observed in ATM deficient gammaherpesvirus-infected macrophages. Using a mouse model that combines ATM and type I IFN receptor deficiencies we show that increased type I IFN response in the absence of ATM fully accounts for the proviral role of ATM during gammaherpesvirus replication. Further, increased type I IFN response rendered ATM deficient macrophages more susceptible to antiviral effects of type II IFN. This study identifies attenuation of type I IFN responses as the primary mechanism underlying proviral function of ATM during gammaherpesvirus infection.

B Cell-Specific Expression of Ataxia-Telangiectasia Mutated Protein Kinase Promotes Chronic Gammaherpesvirus Infection.

Manipulation of host cellular pathways is a strategy employed by gammaherpesviruses, including mouse gammaherpesvirus 68 (MHV68), in order to negotiate a chronic infection. Ataxia-telangiectasia mutated (ATM) plays a unique yet incompletely understood role in gammaherpesvirus infection, as it has both proviral and antiviral effects. Chronic gammaherpesvirus infection is poorly controlled in a host with global ATM insufficiency, whether the host is a mouse or a human. In contrast, ATM facilitates replication, reactivation, and latency establishment of several gammaherpesviruses in vitro, suggesting that ATM is proviral in the context of infected cell cultures. The proviral role of ATM is also evident in vivo, as myeloid-specific ATM expression facilitates MHV68 reactivation during the establishment of viral latency. In order to better understand the complex relationship between host ATM and gammaherpesvirus infection, we depleted ATM specifically in B cells, a cell type critical for chronic gammaherpesvirus infection. B cell-specific ATM deficiency attenuated the establishment of viral latency due to compromised differentiation of ATM-deficient B cells. Further, we found that during long-term infection, peritoneal B-1b, but not related B-1a, B cells display the highest frequency of gammaherpesvirus infection. While ATM expression did not affect gammaherpesvirus tropism for B-1 B cells, B cell-specific ATM expression was necessary to support viral reactivation from peritoneal cells during long-term infection. Thus, our study reveals a role of ATM as a host factor that promotes chronic gammaherpesvirus infection of B cells.IMPORTANCE Gammaherpesviruses infect a majority of the human population and are associated with cancer, including B cell lymphomas. ATM is a unique host kinase that has both proviral and antiviral roles in the context of gammaherpesvirus infection. Further, there is insufficient understanding of the interplay of these roles in vivo during chronic infection. In this study, we show that ATM expression by splenic B cells is required for efficient establishment of gammaherpesvirus latency. We also show that ATM expression by peritoneal B cells is required to facilitate viral reactivation during long-term infection. Thus, our study defines a proviral role of B cell-specific ATM expression during chronic gammaherpesvirus infection.

Interferon Regulatory Factor 1 and Type I Interferon Cooperate To Control Acute Gammaherpesvirus Infection.

Gammaherpesviruses are ubiquitous pathogens that establish lifelong infection in >95% of adults worldwide and are associated with a variety of malignancies. Coevolution of gammaherpesviruses with their hosts has resulted in an intricate relationship between the virus and the host immune system, and perturbation of the virus-host balance results in pathology. Interferon regulatory factor 1 (IRF-1) is a tumor suppressor that is also involved in the regulation of innate and adaptive immune responses. Here, we show that type I interferon (IFN) and IRF-1 cooperate to control acute gammaherpesvirus infection. Specifically, we demonstrate that a combination of IRF-1 and type I IFN signaling ensures host survival during acute gammaherpesvirus infection and supports IFN gamma-mediated suppression of viral replication. Thus, our studies reveal an intriguing cross talk between IRF-1 and type I and II IFNs in the induction of the antiviral state during acute gammaherpesvirus infection. IMPORTANCE: Gammaherpesviruses establish chronic infection in a majority of adults, and this long-term infection is associated with virus-driven development of a range of malignancies. In contrast, a brief period of active gammaherpesvirus replication during acute infection of a naive host is subclinical in most individuals. Here, we discovered that a combination of type I interferon (IFN) signaling and interferon regulatory factor 1 (IRF-1) expression is required to ensure survival of a gammaherpesvirus-infected host past the first 8 days of infection. Specifically, both type I IFN receptor and IRF-1 expression potentiated antiviral effects of type II IFN to restrict gammaherpesvirus replication in vivo, in the lungs, and in vitro, in primary macrophage cultures.

Nitric Oxide Suppresses ß-Cell Apoptosis by Inhibiting the DNA Damage Response.

Nitric oxide, produced in pancreatic ß cells in response to proinflammatory cytokines, plays a dual role in the regulation of ß-cell fate. While nitric oxide induces cellular damage and impairs ß-cell function, it also promotes ß-cell survival through activation of protective pathways that promote ß-cell recovery. In this study, we identify a novel mechanism in which nitric oxide prevents ß-cell apoptosis by attenuating the DNA damage response (DDR). Nitric oxide suppresses activation of the DDR (as measured by γH2AX formation and the phosphorylation of KAP1 and p53) in response to multiple genotoxic agents, including camptothecin, H2O2, and nitric oxide itself, despite the presence of DNA damage. While camptothecin and H2O2 both induce DDR activation, nitric oxide suppresses only camptothecin-induced apoptosis and not H2O2-induced necrosis. The ability of nitric oxide to suppress the DDR appears to be selective for pancreatic ß cells, as nitric oxide fails to inhibit DDR signaling in macrophages, hepatocytes, and fibroblasts, three additional cell types examined. While originally described as the damaging agent responsible for cytokine-induced ß-cell death, these studies identify a novel role for nitric oxide as a protective molecule that promotes ß-cell survival by suppressing DDR signaling and attenuating DNA damage-induced apoptosis.

Gammaherpesvirus targets peritoneal B-1 B cells for long-term latency.

Gammaherpesviruses establish life-long infection in most adults and are associated with the development of B cell lymphomas. While the interaction between gammaherpesviruses and splenic B cells has been explored, very little is known about gammaherpesvirus infection of B-1 B cells, innate-like B cells that primarily reside in body cavities. This study demonstrates that B-1 B cells harbor the highest frequency of latently infected cells in the peritoneum throughout chronic infection, highlighting a previously unappreciated feature of gammaherpesvirus biology.