The Response of Tissue Mast Cells to TLR3 Ligand Poly(I:C) Treatment

Inhibition of FGFR3 upregulates MHC-I and PD-L1 via TLR3/NF-kB pathway in muscle-invasive bladder cancer

BACKGROUND

Improving the potency of immune response is paramount among issues concerning immunotherapy of muscle-invasive bladder cancer (MIBC).

METHODS

On the basis of immune subtypes, we investigated possible molecular mechanisms involved in tumor immune escape in MIBC. According to the 312 immune-related genes, three MIBC immune subtypes were clustered.

RESULTS

Cluster 2 subtype is characterized by FGFR3 mutations and has a better clinical prognosis. However, the expression levels of MHC-I and immune checkpoints genes were the lowest, indicating that this subtype is subject to immune escape and has a low response rate to immunotherapy. Bioinformatics analysis and immunofluorescence staining of clinical samples revealed that the FGFR3 is involved in the immune escape in MIBC. Besides, after FGFR3 knockout with siRNA in RT112 and UMUC14 cells, the TLR3/NF-kB pathway was significantly activated and was accompanied by upregulation of MHC-I and PD-L1 gene expression. Furthermore, the use of TLR3 agonists poly(I:C) can further improve the effect.

CONCLUSION

Together, our results suggest that FGFR3 might involve in immunosuppression by inhibition of NF-kB pathway in BC. Considering that TLR3 agonists are currently approved for clinical treatment as immunoadjuvants, our study might provide more insights for improving the efficacy of immunotherapy in MIBC.

Intracellular TLRs of Mast Cells in Innate and Acquired Immunity

Mast cells (MCs) distribute to interface tissues with environment, such as skin, airway, and gut mucosa, thereby functioning as the sentinel against invading allergens and pathogens. To respond to and exclude these external substances promptly, MCs possess granules containing inflammatory mediators, including heparin, proteases, tumor necrosis factor, and histamine, and produce these mediators as a consequence of degranulation within minutes of activation. As a delayed response to external substances, MCs de novo synthesize inflammatory mediators, such as cytokines and chemokines, by sensing pathogen- and damage-associated molecular patterns through their pattern recognition receptors, including Toll-like receptors (TLRs). A substantial number of studies have reported immune responses by MCs through surface TLR signaling, particularly TLR2 and TLR4. However, less attention has been paid to immune responses through nucleic acid-recognizing intracellular TLRs. Among intracellular TLRs, human and rodent MCs express TLR3, TLR7, and TLR9, but not TLR8. Some virus infections modulate intracellular TLR expression in MCs. MC-derived mediators, such as histamine, cysteinyl leukotrienes, LL-37, and the granulocyte-macrophage colony-stimulating factor, have also been reported to modulate intracellular TLR expression in an autocrine and/or paracrine fashion. Synthetic ligands for intracellular TLRs and some viruses are sensed by intracellular TLRs of MCs, leading to the production of inflammatory cytokines and chemokines including type I interferons. These MC responses initiate and facilitate innate responses and the subsequent recruitment of additional innate effector cells. MCs also associate with the regulation of adaptive immunity. In this overview, the expression of intracellular TLRs in MCs and the recognition of pathogens, including viruses, by intracellular TLRs in MCs were critically evaluated.

Mast cells: Therapeutic targets for COVID-19 and beyond

Mast cells (MCs) are innate immune cells that widely distribute throughout all tissues and express a variety of cell surface receptors. Upon activation, MCs can rapidly release a diverse array of preformed mediators residing within their secretory granules and newly synthesize a broad spectrum of inflammatory and immunomodulatory mediators. These unique features of MCs enable them to act as sentinels in response to rapid changes within their microenvironment. There is increasing evidence now that MCs play prominent roles in other pathophysiological processes besides allergic inflammation. In this review, we highlight the recent findings on the emerging roles of MCs in the pathogenesis of coronavirus disease-2019 (COVID-19) and discuss the potential of MCs as novel therapeutic targets for COVID-19 and other non-allergic inflammatory diseases.

Toll-like receptor 3 (TLR3) regulation mechanisms and roles in antiviral innate immune responses

Toll-like receptor 3 (TLR3) is a member of the TLR family, mediating the transcriptional induction of type I interferons (IFNs), proinflammatory cytokines, and chemokines, thereby collectively establishing an antiviral host response. Studies have shown that unlike other TLR family members, TLR3 is the only RNA sensor that is utterly dependent on the Toll-interleukin-1 receptor (TIR)‍-domain-containing adaptor-inducing IFN-‍β (TRIF). However, the details of how the TLR3-TRIF signaling pathway works in an antiviral response and how it is regulated are unclear. In this review, we focus on recent advances in understanding the antiviral mechanism of the TRIF pathway and describe the essential characteristics of TLR3 and its antiviral effects. Advancing our understanding of TLR3 may contribute to disease diagnosis and could foster the development of novel treatments for viral diseases.

The pLysRS-Ap4A Pathway in Mast Cells Regulates the Switch from Host Defense to a Pathological State

The innate and adaptive immune systems play an essential role in host defense against pathogens. Various signal transduction pathways monitor and balance the immune system since an imbalance may promote pathological states such as allergy, inflammation, and cancer. Mast cells have a central role in the regulation of the innate/adaptive immune system and are involved in the pathogenesis of many inflammatory and allergic diseases by releasing inflammatory mediators such as histamines, proteases, chemotactic factors, and cytokines. Although various signaling pathways are associated with mast cell activation, our discovery and characterization of the pLysRS-Ap₄A signaling pathway in these cells provided an additional important step towards a full understanding of the intracellular mechanisms involved in mast cell activation. In the present review, we will discuss in depth this signaling pathway’s contribution to host defense and the pathological state.

The impact of TLR7 agonist R848 treatment on mast cell phenotype and activity

Bearing in mind that mast cell contribution to viral clearance is still not fully understood, in this study, we evaluated the effect of Toll-like receptor (TLR)7 viral single-stranded ribonucleic acid (ssRNA) mimic ligand, namely resiquimod (R)848, on mast cell phenotype and activity. We demonstrated that rat peritoneal mast cells exhibit surface and intracellular expression of ssRNA-specific TLR7 molecule, and that mimic ligand switches the self-expression of this receptor. We also detected other proteins associated with the cellular antiviral response: interferon-alpha receptor 1 (IFNAR1), interferon-gamma receptor 1 (IFNGR1), and major histocompatibility complex I (MHC I). Moreover, we showed that R848 caused the decrease of all molecule's expression after prolonged incubation. Interestingly, we found that R848 induced the increase of high-affinity IgE receptor (FcεRI) expression. Finally, we documented that TLR7 ligand-stimulated mast cells synthesize/release interferon (IFN)-α and -β, tumor necrosis factor (TNF), and chemokines CCL3, CXCL8, as well as pro-inflammatory lipid mediators. Our findings confirm that mast cells may respond to TLR7 ligand by altering their phenotype and synthesizing mediators and could serve as active participants in the antiviral immune response.

Harnessing the Power of Mast Cells in unconventional Immunotherapy Strategies and Vaccine Adjuvants

Mast cells are long-lived, granular, myeloid-derived leukocytes that have significant protective and repair functions in tissues. Mast cells sense disruptions in the local microenvironment and are first responders to physical, chemical and biological insults. When activated, mast cells release growth factors, proteases, chemotactic proteins and cytokines thereby mobilizing and amplifying the reactions of the innate and adaptive immune system. Mast cells are therefore significant regulators of homeostatic functions and may be essential in microenvironmental changes during pathogen invasion and disease. During infection by helminths, bacteria and viruses, mast cells release antimicrobial factors to facilitate pathogen expulsion and eradication. Mast cell-derived proteases and growth factors protect tissues from insect/snake bites and exposure to ultraviolet radiation. Finally, mast cells release mediators that promote wound healing in the inflammatory, proliferative and remodelling stages. Since mast cells have such a powerful repertoire of functions, targeting mast cells may be an effective new strategy for immunotherapy of disease and design of novel vaccine adjuvants. In this review, we will examine how certain strategies that specifically target and activate mast cells can be used to treat and resolve infections, augment vaccines and heal wounds. Although these strategies may be protective in certain circumstances, mast cells activation may be deleterious if not carefully controlled and any therapeutic strategy using mast cell activators must be carefully explored.

Signal Transduction Pathways Activated by Innate Immunity in Mast Cells: Translating Sensing of Changes into Specific Responses

Mast cells (MCs) constitute an essential cell lineage that participates in innate and adaptive immune responses and whose phenotype and function are influenced by tissue-specific conditions. Their mechanisms of activation in type I hypersensitivity reactions have been the subject of multiple studies, but the signaling pathways behind their activation by innate immunity stimuli are not so well described. Here, we review the recent evidence regarding the main molecular elements and signaling pathways connecting the innate immune receptors and hypoxic microenvironment to cytokine synthesis and the secretion of soluble or exosome-contained mediators in this cell type. When known, the positive and negative control mechanisms of those pathways are presented, together with their possible implications for the understanding of mast cell-driven chronic inflammation. Finally, we discuss the relevance of the knowledge about signaling in this cell type in the recognition of MCs as central elements on innate immunity, whose remarkable plasticity converts them in sensors of micro-environmental discontinuities and controllers of tissue homeostasis.