Most cells are believed to be capable of producing type I interferons (IFN I) as part of an innate immune response against, for instance, viral infections. In macrophages, IFN I is potently induced upon cytoplasmic exposure to foreign nucleic acids. Infection of these cells with herpesviruses leads to triggering of the DNA sensors interferon-inducible protein 16 (IFI16) and cyclic GMP-AMP (cGAMP) synthase (cGAS). Thereby, the stimulator of interferon genes (STING) and the downstream molecules TANK-binding kinase 1 (TBK1) and interferon regulatory factor 3 (IRF3) are sequentially activated culminating in IFN I secretion. Human gamma-herpesviruses, such as Epstein-Barr virus (EBV), exploit B cells as a reservoir for persistent infection. In this study, we investigated whether human B cells, similar to macrophages, engage the cytoplasmic DNA sensing pathway to induce an innate immune response. We found that the B cells fail to secrete IFN I upon cytoplasmic DNA exposure, although they express the DNA sensors cGAS and IFI16 and the signaling components TBK1 and IRF3. In primary human B lymphocytes and EBV-negative B cell lines, this deficiency is explained by a lack of detectable levels of the central adaptor protein STING. In contrast, EBV-transformed B cell lines did express STING, yet both these lines as well as STING-reconstituted EBV-negative B cells did not produce IFN I upon dsDNA or cGAMP stimulation. Our combined data show that the cytoplasmic DNA sensing pathway is dysfunctional in human B cells. This exemplifies that certain cell types cannot induce IFN I in response to cytoplasmic DNA exposure providing a potential niche for viral persistence.

The detection of infectious pathogens is essential for the induction of antimicrobial immune responses. The innate immune system detects a wide array of microbes using a limited set of pattern-recognition receptors (PRRs). One family of PRRs with a central role in innate immunity are the Toll-like receptors (TLRs). Upon ligation, these receptors initiate signaling pathways culminating in the release of pro-inflammatory cytokines and/or type I interferons (IFN-I). In recent years, it has become evident that the specific subcellular location and timing of TLR activation affect signaling outcome. The subtlety of this signaling has led to a growing demand for chemical tools that provide the ability to conditionally control TLR activation. In this review, we survey current models for TLR signaling in time and space, discuss how chemical tools have contributed to our understanding of TLR ligands, and describe how they can aid further elucidation of the dynamic aspects of TLR signaling.

Cell-mediated immunity plays a key role in host control of viral infection. This is exemplified by life-threatening reactivations of e.g. herpesviruses in individuals with impaired T-cell and/or iNKT cell responses. To allow lifelong persistence and virus production in the face of primed immunity, herpesviruses exploit immune evasion strategies. These include a reduction in viral antigen expression during latency and a number of escape mechanisms that target antigen presentation pathways. Given the plethora of foreign antigens expressed in virus-producing cells, herpesviruses are conceivably most vulnerable to elimination by cell-mediated immunity during the replicative phase of infection. Here, we show that a prototypic herpesvirus, Epstein-Barr virus (EBV), encodes a novel, broadly acting immunoevasin, gp150, that is expressed during the late phase of viral replication. In particular, EBV gp150 inhibits antigen presentation by HLA class I, HLA class II, and the non-classical, lipid-presenting CD1d molecules. The mechanism of gp150-mediated T-cell escape does not depend on degradation of the antigen-presenting molecules nor does it require gp150’s cytoplasmic tail. Through its abundant glycosylation, gp150 creates a shield that impedes surface presentation of antigen. This is an unprecedented immune evasion mechanism for herpesviruses. In view of its likely broader target range, gp150 could additionally have an impact beyond escape of T cell activation. Importantly, B cells infected with a gp150-null mutant EBV displayed rescued levels of surface antigen presentation by HLA class I, HLA class II, and CD1d, supporting an important role for iNKT cells next to classical T cells in fighting EBV infection. At the same time, our results indicate that EBV gp150 prolongs the timespan for producing viral offspring at the most vulnerable stage of the viral life cycle.

RNA editing by the adenosine deaminase ADAR1 prevents innate immune responses to endogenous RNAs. In ADAR1‐deficient cells, unedited self RNAs form base‐paired structures that resemble viral RNAs and inadvertently activate the cytosolic RIG‐I‐like receptor (RLR) MDA5, leading to an antiviral type I interferon (IFN) response. Mutations in ADAR1 cause Aicardi‐Goutières Syndrome (AGS), an autoinflammatory syndrome characterized by chronic type I IFN production. Conversely, ADAR1 loss and the consequent type I IFN production restricts tumor growth and potentiates the activity of some chemotherapeutics. Here, we show that another RIG‐I‐like receptor, LGP2, also has an essential role in the induction of a type I IFN response in ADAR1‐deficient human cells. This requires the canonical function of LGP2 as an RNA sensor and facilitator of MDA5‐dependent signaling. Furthermore, we show that the sensitivity of tumor cells to ADAR1 loss requires LGP2 expression. Finally, type I IFN induction in tumor cells depleted of ADAR1 and treated with some chemotherapeutics fully depends on LGP2 expression. These findings highlight a central role for LGP2 in self RNA sensing with important clinical implications.

Surface-exposed Toll-like receptors (TLRs) such as TLR2 and TLR4 survey the extracellular environment for pathogens. TLR activation initiates the production of various cytokines and chemokines including type I interferons (IFN-I). Downstream of TLR4, IFNβ secretion is only vigorously triggered in macrophages when the receptor undergoes endocytosis and switches signaling adaptor; surface TLR4 engagement predominantly induces proinflammatory cytokines via the signaling adaptor MyD88. It is unclear if this dichotomy is generally applicable to other TLRs, cell types, or differentiation states. Here, we report that diverse TLR2 ligands induce an IFN-I response in human monocyte-like cells, but not in differentiated macrophages. This TLR2-dependent IFN-I signaling originates from the cell surface and is dependent on MyD88; it involves combined activation of the transcription factors IRF3 and NF-κB, driven by the kinases TBK1 and TAK1-IKKβ, respectively. TLR2-stimulated monocytes produced modest IFNβ levels that caused productive downstream signaling, reflected by STAT1-phosphorylation and expression of numerous interferon-stimulated genes (ISGs). Our findings reveal that the outcome of TLR2 signaling includes an IFN-I response in human monocytes, which is lost upon macrophage differentiation, and differs mechanistically from IFN-I-induction through TLR4. These findings point to molecular mechanisms tailored to the differentiation state of a cell and the nature of receptors activated to control and limit TLR-triggered IFN-I responses.