Endosomal Toll-Like Receptors 7 and 9 Cooperate in Detection of Murine Gammaherpesvirus 68 Infection

Intrinsic p53 Activation Restricts Gammaherpesvirus-Driven Germinal Center B Cell Expansion during Latency Establishment

Gammaherpesviruses (GHV) are DNA tumor viruses that establish lifelong latent infections in lymphocytes. For viruses such as Epstein-Barr virus (EBV) and murine gammaherpesvirus 68 (MHV68), this is accomplished through a viral gene-expression program that promotes cellular proliferation and differentiation, especially of germinal center (GC) B cells. Intrinsic host mechanisms that control virus-driven cellular expansion are incompletely defined. Using a small-animal model of GHV pathogenesis, we demonstrate in vivo that tumor suppressor p53 is activated specifically in B cells that are latently infected by MHV68. In the absence of p53, the early expansion of MHV68 latency was greatly increased, especially in GC B cells, a cell-type whose proliferation was conversely restricted by p53. We identify the B cell-specific latency gene M2, a viral promoter of GC B cell differentiation, as a viral protein sufficient to elicit a p53-dependent anti-proliferative response caused by Src-family kinase activation. We further demonstrate that EBV-encoded latent membrane protein 1 (LMP1) similarly triggers a p53 response in primary B cells. Our data highlight a model in which GHV latency gene-expression programs that promote B cell proliferation and differentiation to facilitate viral colonization of the host trigger aberrant cellular proliferation that is controlled by p53.

IMPORTANCE

Gammaherpesviruses cause lifelong infections of their hosts, commonly referred to as latency, that can lead to cancer. Latency establishment benefits from the functions of viral proteins that augment and amplify B cell activation, proliferation, and differentiation signals. In uninfected cells, off-schedule cellular differentiation would typically trigger anti-proliferative responses by effector proteins known as tumor suppressors. However, tumor suppressor responses to gammaherpesvirus manipulation of cellular processes remain understudied, especially those that occur during latency establishment in a living organism. Here we identify p53, a tumor suppressor commonly mutated in cancer, as a host factor that limits virus-driven B cell proliferation and differentiation, and thus, viral colonization of a host. We demonstrate that p53 activation occurs in response to viral latency proteins that induce B cell activation. This work informs a gap in our understanding of intrinsic cellular defense mechanisms that restrict lifelong GHV infection.

Immunity and Immune Evasion Mechanisms of Epstein-Barr Virus

Epstein-Barr virus (EBV) is the first human oncogenic virus to be identified, which evades the body's immune surveillance through multiple mechanisms that allow long-term latent infection. Under certain pathological conditions, EBVs undergo a transition from the latent phase to the lytic phase and cause targeted dysregulation of the host immune system, leading to the development of EBV-related diseases. Therefore, an in-depth understanding of the mechanism of developing an immune response to EBV and the evasion of immune recognition by EBV is important for the understanding of the pathogenesis of EBV, which is of great significance for finding strategies to prevent EBV infection, and developing a therapy to treat EBV-associated diseases. In this review, we will discuss the molecular mechanisms of host immunological responses to EBV infection and the mechanisms of EBV-mediated immune evasion during chronic active infection.

The innate and T-cell mediated immune response during acute and chronic gammaherpesvirus infection

Immediately after entry into host cells, viruses are sensed by the innate immune system, leading to the activation of innate antiviral effector mechanisms including the type I interferon (IFN) response and natural killer (NK) cells. This innate immune response helps to shape an effective adaptive T cell immune response mediated by cytotoxic T cells and CD4+ T helper cells and is also critical for the maintenance of protective T cells during chronic infection. The human gammaherpesvirus Epstein-Barr virus (EBV) is a highly prevalent lymphotropic oncovirus that establishes chronic lifelong infections in the vast majority of the adult population. Although acute EBV infection is controlled in an immunocompetent host, chronic EBV infection can lead to severe complications in immunosuppressed patients. Given that EBV is strictly host-specific, its murine homolog murid herpesvirus 4 or MHV68 is a widely used model to obtain in vivo insights into the interaction between gammaherpesviruses and their host. Despite the fact that EBV and MHV68 have developed strategies to evade the innate and adaptive immune response, innate antiviral effector mechanisms still play a vital role in not only controlling the acute infection but also shaping an efficient long-lasting adaptive immune response. Here, we summarize the current knowledge about the innate immune response mediated by the type I IFN system and NK cells, and the adaptive T cell-mediated response during EBV and MHV68 infection. Investigating the fine-tuned interplay between the innate immune and T cell response will provide valuable insights which may be exploited to design better therapeutic strategies to vanquish chronic herpesviral infection.

Age-associated B cells are long-lasting effectors that impede latent γHV68 reactivation

Age-associated B cells (ABCs; CD19⁺CD11c⁺T-bet⁺) are a unique population that are increased in an array of viral infections, though their role during latent infection is largely unexplored. Here, we use murine gammaherpesvirus 68 (γHV68) to demonstrate that ABCs remain elevated long-term during latent infection and express IFNγ and TNF. Using a recombinant γHV68 that is cleared following acute infection, we show that ABCs persist in the absence of latent virus, though their expression of IFNγ and TNF is decreased. With a fluorescent reporter gene-expressing γHV68 we demonstrate that ABCs are infected with γHV68 at similar rates to other previously activated B cells. We find that mice without ABCs display defects in anti-viral IgG2a/c antibodies and are more susceptible to reactivation of γHV68 following virus challenges that typically do not break latency. Together, these results indicate that ABCs are a persistent effector subset during latent viral infection that impedes γHV68 reactivation.

Age-associated B cells are long-lasting effectors that impede latent γHV68 reactivation

Age-associated B cells (ABCs; CD19⁺CD11c⁺T-bet⁺) are a unique population that are increased in an array of viral infections, though their role during latent infection is largely unexplored. Here, we use murine gammaherpesvirus 68 (γHV68) to demonstrate that ABCs remain elevated long-term during latent infection and express IFNγ and TNF. Using a recombinant γHV68 that is cleared following acute infection, we show that ABCs persist in the absence of latent virus, though their expression of IFNγ and TNF is decreased. With a fluorescent reporter gene-expressing γHV68 we demonstrate that ABCs are infected with γHV68 at similar rates to other previously activated B cells. We find that mice without ABCs display defects in anti-viral IgG2a/c antibodies and are more susceptible to reactivation of γHV68 following virus challenges that typically do not break latency. Together, these results indicate that ABCs are a persistent effector subset during latent viral infection that impedes γHV68 reactivation.

Anti-viral and pro-inflammatory functions of Toll-like receptors during gamma-herpesvirus infections

Toll-like receptors (TLRs) control anti-viral responses both directly in infected cells and in responding cells of the immune systems. Therefore, they are crucial for responses against the oncogenic γ-herpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus and the related murine virus MHV68, which directly infect immune system cells. However, since these viruses also cause lifelong persistent infections, TLRs may also be involved in modulation of inflammation during latent infection and contribute to virus-driven tumorigenesis. This review summarizes work on both of these aspects of TLR/γ-herpesvirus interactions, as well as results showing that TLR activity can drive these viruses' re-entry into the replicative lytic cycle.

UNC93B1 Is Widely Expressed in the Murine CNS and Is Required for Neuroinflammation and Neuronal Injury Induced by MicroRNA let-7b

The chaperone protein Unc-93 homolog B1 (UNC93B1) regulates internalization, trafficking, and stabilization of nucleic acid-sensing Toll-like receptors (TLR) in peripheral immune cells. We sought to determine UNC93B1 expression and its functional relevance in inflammatory and injurious processes in the central nervous system (CNS). We found that UNC93B1 is expressed in various CNS cells including microglia, astrocytes, oligodendrocytes, and neurons, as assessed by PCR, immunocyto-/histochemistry, and flow cytometry. UNC93B1 expression in the murine brain increased during development. Exposure to the microRNA let-7b, a recently discovered endogenous TLR7 activator, but also to TLR3 and TLR4 agonists, led to increased UNC93B1 expression in microglia and neurons. Microglial activation by extracellular let-7b required functional UNC93B1, as assessed by TNF ELISA. Neuronal injury induced by extracellular let-7b was dependent on UNC93B1, as UNC93B1-deficient neurons were unaffected by the microRNA's neurotoxicity in vitro. Intrathecal application of let-7b triggered neurodegeneration in wild-type mice, whereas mice deficient for UNC93B1 were protected against injurious effects on neurons and axons. In summary, our data demonstrate broad UNC93B1 expression in the murine brain and establish this chaperone as a modulator of neuroinflammation and neuronal injury triggered by extracellular microRNA and subsequent induction of TLR signaling.

Conquering the Host: Determinants of Pathogenesis Learned from Murine Gammaherpesvirus 68

Gammaherpesviruses are an important class of oncogenic pathogens that are exquisitely evolved to their respective hosts. As such, the human gammaherpesviruses Epstein-Barr virus (EBV) and Kaposi sarcoma herpesvirus (KSHV) do not naturally infect nonhuman primates or rodents. There is a clear need to fully explore mechanisms of gammaherpesvirus pathogenesis, host control, and immune evasion in the host. A gammaherpesvirus pathogen isolated from murid rodents was first reported in 1980; 40 years later, murine gammaherpesvirus 68 (MHV68, MuHV-4, γHV68) infection of laboratory mice is a well-established pathogenesis system recognized for its utility in applying state-of-the-art approaches to investigate virus-host interactions ranging from the whole host to the individual cell. Here, we highlight recent advancements in our understanding of the processes by which MHV68 colonizes the host and drives disease. Lessons that inform KSHV and EBV pathogenesis and provide future avenues for novel interventions against infection and virus-associated cancers are emphasized.

Detection of Viral Infections by Innate Immunity

Pattern recognition receptors (PRRs) and inflammasomes are a key part of the anti-viral innate immune system as they detect conserved viral pathogen-associated molecular patterns (PAMPs). A successful host response to viral infections critically depend on the initial activation of PRRs by viruses, mainly by viral DNA and RNA. The signalling pathways activated by PRRs leads to the expression of pro-inflammatory cytokines, to recruit immune cells, and type I and type III interferons which leads to the induction of interferon stimulated genes (ISG), powerful virus restriction factors that establish the "antiviral state". Inflammasomes contribute to anti-viral responses through the maturation of interleukin (IL)-1 and IL-18 and through triggering pyroptotic cell death. The activity of the innate immune system along with the adaptive immune response normally leads to successful virus elimination, although disproportionate innate responses contribute to viral pathology. In this review we will discuss recent insights into the influence of PRR activation and inflammasomes on viral infections and what this means for the mammalian host. We will also comment on how specific PRRs and inflammasomes may be relevant to how SARS-CoV-2, the virus responsible for the current COVID-19 pandemic, interacts with host innate immunity.

Reactive oxygen species oxidize STING and suppress interferon production

Reactive oxygen species (ROS) are by-products of cellular respiration that can promote oxidative stress and damage cellular proteins and lipids. One canonical role of ROS is to defend the cell against invading bacterial and viral pathogens. Curiously, some viruses, including herpesviruses, thrive despite the induction of ROS, suggesting that ROS are beneficial for the virus. However, the underlying mechanisms remain unclear. Here, we found that ROS impaired interferon response during murine herpesvirus infection and that the inhibition occurred downstream of cytoplasmic DNA sensing. We further demonstrated that ROS suppressed the type I interferon response by oxidizing Cysteine 147 on murine stimulator of interferon genes (STING), an ER-associated protein that mediates interferon response after cytoplasmic DNA sensing. This inhibited STING polymerization and activation of downstream signaling events. These data indicate that redox regulation of Cysteine 147 of mouse STING, which is equivalent to Cysteine 148 of human STING, controls interferon production. Together, our findings reveal that ROS orchestrates anti-viral immune responses, which can be exploited by viruses to evade cellular defenses.

Conserved Herpesvirus Kinase ORF36 Activates B2 Retrotransposons during Murine Gammaherpesvirus Infection

Short interspersed nuclear elements (SINEs) are RNA polymerase III (RNAPIII)-transcribed, retrotransposable noncoding RNA (ncRNA) elements ubiquitously spread throughout mammalian genomes. While normally silenced in healthy somatic tissue, SINEs can be induced during infection with DNA viruses, including the model murine gammaherpesvirus 68 (MHV68). Here, we explored the mechanisms underlying MHV68 activation of SINE ncRNAs. We demonstrate that lytic MHV68 infection of B cells, macrophages, and fibroblasts leads to robust activation of the B2 family of SINEs in a cell-autonomous manner. B2 ncRNA induction requires neither host innate immune signaling factors nor involvement of the RNAPIII master regulator Maf1. However, we identified MHV68 ORF36, the conserved herpesviral kinase, as playing a key role in B2 induction during lytic infection. SINE activation is linked to ORF36 kinase activity and can also be induced by inhibition of histone deacetylases 1 and 2 (HCAC 1/2), which is one of the known ORF36 functions. Collectively, our data suggest that ORF36-mediated changes in chromatin modification contribute to B2 activation during MHV68 infection and that this activity is conserved in other herpesviral protein kinase homologs.IMPORTANCE Viral infection dramatically changes the levels of many types of RNA in a cell. In particular, certain oncogenic viruses activate expression of repetitive genes called retrotransposons, which are normally silenced due to their ability to copy and spread throughout the genome. Here, we established that infection with the gammaherpesvirus MHV68 leads to a dramatic induction of a class of noncoding retrotransposons called B2 SINEs in multiple cell types. We then explored how MHV68 activates B2 SINEs, revealing a role for the conserved herpesviral protein kinase ORF36. Both ORF36 kinase-dependent and kinase-independent functions contribute to B2 induction, perhaps through ORF36 targeting of proteins involved in controlling the accessibility of chromatin surrounding SINE loci. Understanding the features underlying induction of these elements following MHV68 infection should provide insight into core elements of SINE regulation, as well as disregulation of SINE elements associated with disease.

Differential immunomodulatory effect of vitamin D (1,25 (OH)2 D3) on the innate immune response in different types of cells infected in vitro with infectious bursal disease virus

It has been demonstrated that vitamin D (Vit D) included in diets offers a beneficial effect by improving innate immune responses in chickens. However, its mechanisms of action and the effect on immunosuppressive pathogens, such as infectious bursal disease virus, are not yet known. In the present study, we have studied the immunomodulatory effect of Vit D on the innate immune response in 3 cell lines: fibroblast cells (DF-1), macrophages (HD11), and B cells (DT-40) infected with IBDV (intermediate vaccine) at 2 multiplicity of infections (MOI) (1 and 0.1). Genes associated with innate immune responses (TLR-3, TLR-21, MDA-5, MyD88, TRIF, IRF-7, INF-α, INF-β, PKR, OAS, viperin, IL-1β, IL-6, and IL-12) were evaluated at different time points (3, 6, 12, 24, and 36 h after infection, h.p.i). Virus production reached a maximum at 24 h.p.i., which was significantly (P < 0.05) higher in DF-1 cells, followed by HD-11 and DT-40 cells. Mainly in HD-11 cells, there was a significant (P < 0.05) effect of Vit D supplementation on receptors TLR-3, TLR-21, and MDA-5 after 12 h.p.i, independent of MOI. DT-40 cells showed the highest antiviral activity, with a significant (P < 0.05) effect on IRF-7, IFN-β, OAS, and PKR gene expression, where expression of IRF-7 and IFN-β correlated positively with Vit D supplementation, while OAS and PKR were independent of Vit D. Proinflammatory cytokines were significantly (P < 0.05) upregulated and found to be Vit D and MOI dependent. In conclusion, this study demonstrated the capacity of IBDV to trigger a strong innate immune response in chicken cells and contributes to the understanding of the activation pathways of innate immunity induced by IBDV and further shows the benefitial effect of Vit D supplementation as an immunomodulator.

Protein tyrosine phosphatase 1B is involved in efficient type I interferon secretion upon viral infection

Protein tyrosine phosphatase 1B (PTP1B, also known as PTPN1) is a negative regulator of the leptin and insulin signalling pathways. This phosphatase is of great interest as PTP1B-knockout mice are protected against the development of obesity and diabetes. Here, we provide evidence for a novel function of PTP1B that is independent of its phosphatase activity, but requires its localisation to the membrane of the endoplasmic reticulum. Upon activation of pattern recognition receptors, macrophages and plasmacytoid dendritic cells from PTP1B-knockout mice secrete lower amounts of type I interferon (IFN) than cells from wild-type mice. In contrast, secretion of the proinflammatory cytokines TNFα and IL6 was unaltered. While PTP1B deficiency did not affect Ifnb1 transcription, type I IFN accumulated in macrophages, suggesting a role for PTP1B in mediating secretion of type I IFN. In summary, we have uncovered that PTP1B positively regulates the type I IFN response by promoting secretion of key antiviral cytokines.

The induced decrease in TLR2 and TLR4 by cerebrolysin in the alcoholic liver of rats

INTRODUCTION

Toll-like receptors (TLRs) are innate immunity receptors, which have an important role in modulating inflammation in disease. Cerebrolysin is a biotechnologically prepared peptide that stimulates neurotrophic regulation in the central nervous system. The aim of the present study was to investigate the effect of experimenting cerebrolysin on TLR2 and TLR4 in alcoholic liver disease (ALD).

MATERIALS AND METHODS

TLR2 and TLR4 expressions were determined using real-time polymerase chain reaction in rats, which have used alcohol and they were separated into five groups.

RESULTS

The results of the present study showed that in mild dose of cerebrolysin, the expression of TLR2 and TLR4 was decreased significantly than other groups. Also, the results of the western blot analysis proved the same.

CONCLUSION

The present study demonstrated that the anti-inflammatory effect of cerebrolysin can decrease the TLR2 and TLR4 expressions through downregulating nuclear factor-κB pathway in the ALD disease.

Epstein-Barr Virus and Innate Immunity: Friends or Foes?

Epstein–Barr virus (EBV) successfully persists in the vast majority of adults but causes lymphoid and epithelial malignancies in a small fraction of latently infected individuals. Innate immunity is the first-line antiviral defense, which EBV has to evade in favor of its own replication and infection. EBV uses multiple strategies to perturb innate immune signaling pathways activated by Toll-like, RIG-I-like, NOD-like, and AIM2-like receptors as well as cyclic GMP-AMP synthase. EBV also counteracts interferon production and signaling, including TBK1-IRF3 and JAK-STAT pathways. However, activation of innate immunity also triggers pro-inflammatory response and proteolytic cleavage of caspases, both of which exhibit proviral activity under some circumstances. Pathogenic inflammation also contributes to EBV oncogenesis. EBV activates NFκB signaling and induces pro-inflammatory cytokines. Through differential modulation of the proviral and antiviral roles of caspases and other host factors at different stages of infection, EBV usurps cellular programs for death and inflammation to its own benefits. The outcome of EBV infection is governed by a delicate interplay between innate immunity and EBV. A better understanding of this interplay will instruct prevention and intervention of EBV-associated cancers.