Roles and Functions of Toll-Like Receptors in Coronavirus Infections: Forthcoming Vaccine and Therapeutic Strategies for Confronting COVID-19

Expression of Functional Toll-Like Receptors on Cultured Human Epidermal Keratinocytes

Expression and function of Toll-like receptors in human endometrial epithelial cell lines

Toll-like receptors (TLR) are pattern recognition receptors that recognize conserved molecular patterns on microbes and link innate and adaptive immune systems. We investigated whether the enhanced susceptibility to bacterial, yeast, and viral infections and poor adaptive immune responses in aging are a result of diminished expression and function of TLRs. We examined the expression and function of all murine TLRs on macrophages from young and aged mice. Both splenic and activated peritoneal macrophages from aged mice expressed significantly lower levels of all TLRs. Furthermore, macrophages from aged mice secreted significantly lower levels of IL-6 and TNF-alpha when stimulated with known ligands for TLR1 and 2, 2 and 6,TLR3, TLR4, TLR5, and TLR9 when compared with those from young mice. These results support the concept that increased susceptibility to infections and poor adaptive immune responses in aging may be due to the decline in TLR expression and function.

ObjectiveTo investigate the expression and function of the Toll-like receptor (TLR) family in peripheral blood mononuclear cells (PBMCs) of patients with polymyalgia rheumatica (PMR) and giant cell arteritis (GCA).MethodsThe authors...

Subtle increases in immaturity of immune function in early infancy have been implicated in the rising susceptibility to allergic disease, particularly relative impairment of type 1 interferon (IFN)-γ responses in the neonatal period. Although genetic predisposition is a clear risk factor, the escalating rates of allergic disease in infancy suggest that environmental factors are also implicated. We previously showed that maternal smoking in pregnancy may impair neonatal IFN-γ responses. Our more recent studies now indicate that this common avoidable toxic exposure is also associated with attenuation of innate immune function, with attenuated Toll-like receptor (TLR)-mediated microbial responses (including TLR-2, -3, -4, and -9 responses). Most notably, the effects were more marked if the mothers were also allergic. In this review, we discuss the significance of these observations in the context of the emerging hypothesis that variations in TLR function in early life may be implicated in allergic propensity. There is now growing evidence that many of the key pathways involved in subsequent T-cell programming and regulation (namely, antigen-presenting cells and regulatory T cells) rely heavily on microbe-driven TLR activation for maturation and function. Factors that influence the function and activity of these innate pathways in early life may contribute to the increasing predisposition for allergic disease. Although "cleaner" environments have been implicated, here we explore the possibility that other common environmental exposures (such as maternal smoking) could also play a role.

As sensors of invading microorganisms, Toll-like receptors (TLRs) are expressed not only on macrophages and dendritic cells (DCs) but also on epithelial cells. In the TLR family, Tlr11 appears to have the unique feature in that it is expressed primarily on epithelial cells, although it is also expressed on DCs and macrophages. Here, we demonstrate that transcription of the Tlr11 gene is regulated through two cis-acting elements, one Ets-binding site and one interferon regulatory factor (IRF)-binding site. The Ets element interacts with the epithelium-specific transcription factors, ESE-1 and ESE-3, and the IRF motif interacts with IRF-8. Thus, Tlr11 expression on epithelial cells is regulated by the transcription factors that are presumably distinct from transcription factors that regulate the expression of TLRs in innate immune cells such as macrophages and DCs. Our results imply that the distinctive transcription regulatory machinery for TLRs on epithelium may represent a promising new avenue for the development of epithelia-specific therapeutic interventions.

This unit summarizes a combination of methods that can be optimized for measuring toll-like receptor (TLR) function. TLRs serve as primary innate immune sensors and exhibit high specificity towards evolutionarily conserved microbial and viral structures. The unit focuses specifically on TLR4, the principal Gram-negative lipopolysaccharide (LPS) sensor. Methods described include transient transfections, analyses of activation of various promoters in reporter-gene assays, and induction of IL-8 secretion. Other topics that will be briefly discussed include the necessity for the assessment of surface expression of transmembrane receptors (e.g., TLR4) using FACS analysis, and a permutation of the TLR functional analysis approach using site-directed mutagenesis.

IFN-α Enhances Poly-IC Responses in Human Keratinocytes by Inducing Expression of Cytosolic Innate RNA Receptors: Relevance for Psoriasis

TLR-2 independent recognition of Mycobacterium tuberculosis by CD11c + pulmonary cells from old mice

Toll-like receptors in B cells and obesity.

Varying susceptibility exists among mammalian species to the development of potentially fatal endotoxemia due to gram-negative bacteria molecular component, lipopolysaccharide (LPS). Toll-like receptor 4 (TLR4) is responsible for LPS-associated immune response and is expressed on numerous immune cells including B lymphocytes. TLR4 is expressed in a functional form on mouse B lymphocytes, a species much less susceptible to endotoxemia compared with humans who are highly sensitive to endotoxin. Humans possess B lymphocytes that are not responsive to LPS. Likewise, horses are highly susceptible to endotoxemia but the expression and function of TLR4 on horse B lymphocytes is not known. Colic, the major cause of mortality in horses, is often complicated by resultant endotoxemia. The objective of this study was to determine the expression and function of TLR4 on equine B lymphocytes. Lymphocytes were isolated from peripheral blood mononuclear cells that were collected from six horses, and the expression and function of TLR4 was analyzed for each horse. Flow cytometry results indicate TLR4 is expressed on horse B lymphocytes but stimulation with LPS did not alter this expression (P = 0.99) compared with unstimulated B lymphocytes after 24 h. After 72 h of in vitro LPS stimulation, analysis of cell proliferation dye by flow cytometry demonstrated that equine B lymphocytes did not proliferate, while mouse B lymphocytes predictably did. Furthermore, the total number of LPS stimulated equine B lymphocytes did not significantly differ from unstimulated cells after 72 h of culture (P = 0.92). Horse lymphocytes exhibited no significant differences in the measured TLR4 signaling pathway genes (TLR4, IL-10, IL-6, IFNβ, and TNFα) when expression was compared with LPS stimulated vs. unstimulated cells. In conclusion, while TLR4 is expressed on horse B lymphocytes, it appears minimally responsive to LPS in vitro, similar to results seen in human B lymphocytes. While further studies are still needed, our work reveals a potential link between B lymphocyte TLR4 expression and endotoxin sensitivity.

Roles of aging or immune responses mediated by Toll-like receptors and natural killer cell in the onset or progression of human candidiasis remain unclear. This study was designed to elucidate the roles using peripheral blood mononuclear cells from healthy donors and patients with oral candidiasis. Subjects tested were healthy volunteers and patients who visited Dental Clinical Division of Hokkaido University Hospital. The patients with oral candidiasis included 39 individuals (25-89years of age) with major complaints on pain in oral mucosa and/or dysgeusia. Healthy volunteers include students (25-35years of age) and teaching staffs (50-65years of age) of Hokkaido University Graduate School of Dental Medicine. Functions of Toll-like receptors 2 and 4 were downregulated significantly and the natural killer activity was slightly, but not significantly downregulated in aged healthy volunteers compared with healthy young volunteers. Functions of Toll-like receptors 2 and 4 and the natural killer activity were significantly downregulated in patients with oral candidiasis compared with healthy volunteers. Downregulation of functions of Toll-like receptors 2 and 4 as well as natural killer activity is suggested to be associated with the onset or progression of oral candidiasis in human.

Recent data have demonstrated that Toll-like receptors (TLRs) play a key role in both innate and adaptive immune responses. The human skin is consistently exposed to a wide range of exogenous stimuli and can exhibit anti-infectious properties. The aim of this review is to describe TLRs and assess the expression of TLRs in human epidermis and the function of these receptors within the epidermis under physiological and pathological conditions.

The human endometrium is an important site for contact between the host and pathogens ascending the reproductive tract, and thus plays an important role in female reproductive tract immunity. Toll-like receptors (TLRs) are involved in recognition of pathogens and ligation of TLRs results in the production of cytokines and chemokines important for both immune and reproductive functions of the endometrium. The reproductive tract is unique since it hormonally controls expression levels of soluble immune mediators and influx of immune cells to mediate these functions. Since TLRs have the potential to induce alterations in cytokine production through ligand recognition, TLR ligation could significantly affect endometrial health. This review focuses on the importance of TLR expression and function within the endometrium and examines the impact of steroid hormones on TLR function. The importance of TLR3 is also reviewed, as it is expressed at high levels during the window of implantation (WOI), when other TLRs and antiviral molecules are either not expressed or expressed at very low levels. Although TLR3 is not the only TLR demonstrating cycle-dependent expression, its upregulation during the WOI may indicate a unique role for TLR3 in the functioning of the endometrium and in preparation for embryo implantation. Keywords: Toll-like receptor 3 (TLR3), human endometrium, mucosal immunity, innate immunity, steroid hormones, menstrual cycle

The family of the toll-like receptors comprises a minimum of 10 members identified in humans so far. These transmembrane receptors act as important signaling intermediates between the host and the invading pathogens. The following review describes the complexities encountered by researchers studying toll-like receptor (TLR) expression changes during bacterial infections. Mutations in some of the TLRs, most prominently TLR4 and TLR2, have been associated with increased susceptibility to infectious diseases. While it is tempting to correct the phenotypic effect of such mutations, in vitro and in vivo research has shown that TLR activity and function comprises a complex regulatory network. Heterodimer formation, synergy, and cross-tolerance have previously been described. More recently, interdependence of TLR2 and TLR4 expression has been identified. In addition, TLR expression follows a specific timeline that may be dependent on the invading pathogen. Lastly, mutations in invading pathogens have been shown to alter the expression profile of TLR2 and TLR4, indicating that therapies against bacterial pathogens will have to target multiple TLRs. Despite the complexities involved in TLR function, the significant progress made in our understanding of the role these proteins play in human diseases also indicates their potential value as therapeutic agents.