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Aims/hypothesisThe islet vasculature, including its constituent islet endothelial cells, is a key contributor to the microenvironment necessary for normal beta cell health and function. In type 2 diabetes, islet amyloid polypeptide (IAPP) aggregates, forming amyloid deposits that accumulate between beta cells and islet capillaries. This process is known to be toxic to beta cells but its impact on the islet vasculature has not previously been studied. Here, we report the first characterisation of the effects of IAPP aggregation on islet endothelial cells/capillaries using cell-based and animal models.MethodsPrimary and immortalised islet endothelial cells were treated with amyloidogenic human IAPP (hIAPP) alone or in the presence of the amyloid blocker Congo Red or the Toll-like receptor (TLR) 2/4 antagonist OxPAPc. Cell viability was determined0 along with mRNA and protein levels of inflammatory markers. Islet capillary abundance, morphology and pericyte coverage were determined in pancreases from transgenic mice with beta cell expression of hIAPP using conventional and confocal microscopy.ResultsAggregated hIAPP decreased endothelial cell viability in immortalised and primary islet endothelial cells (by 78% and 60%, respectively) and significantly increased expression of inflammatory markers Il6, Vcam1 and Edn1 mRNA relative to vehicle treatment in both cell types (p<0.05; n=4). Both cytotoxicity and the proinflammatory response were ameliorated by Congo Red (p<0.05; n=4); whereas TLR2/4-inhibition blocked inflammatory gene expression (p<0.05; n=6) without improving viability. Islets from high-fat-diet-fed amyloid-laden hIAPP transgenic mice also exhibited significantly increased expression of most markers of endothelial inflammation (p<0.05; n=5) along with decreased capillary density compared with non-transgenic littermates fed the same diet (p<0.01). Moreover, a 16% increase in capillary diameter was observed in amyloid-adjacent capillaries (p<0.01), accompanied by a doubling in pericyte structures positive for neuron-glial antigen 2 (p<0.001).Conclusions/interpretationIslet endothelial cells are susceptible to hIAPP-induced cytotoxicity and exhibit a TLR2/4-dependent proinflammatory response to aggregated hIAPP. Additionally, we observed amyloid-selective effects that decreased islet capillary density, accompanied by increased capillary diameter and increased pericyte number. Together, these data demonstrate that the islet vasculature is a target of the cytotoxic and proinflammatory effects of aggregated hIAPP that likely contribute to the detrimental effects of hIAPP aggregation on beta cell function and survival in type 2 diabetes.

The transcription factor XBP1 is activated after endoplasmic reticulum stress. Glimcher and colleagues show that XBP1 can also be activated by TLR2 and TLR4 signaling pathways, in which it sustains proinflammatory cytokine production. Sensors of pathogens, such as Toll-like receptors (TLRs), detect microbes to activate transcriptional programs that orchestrate adaptive responses to specific insults. Here we report that TLR4 and TLR2 specifically activated the endoplasmic reticulum (ER) stress sensor kinase IRE1α and its downstream target, the transcription factor XBP1. Previously described ER-stress target genes of XBP1 were not induced by TLR signaling. Instead, TLR-activated XBP1 was required for optimal and sustained production of proinflammatory cytokines in macrophages. Consistent with that finding, activation of IRE1α by ER stress acted in synergy with TLR activation for cytokine production. Moreover, XBP1 deficiency resulted in a much greater bacterial burden in mice infected with the TLR2-activating human intracellular pathogen Francisella tularensis . Our findings identify an unsuspected critical function for XBP1 in mammalian host defenses.

Significance Excess Toll-like receptor 2 (TLR2) signaling has been implicated in numerous inflammatory diseases, yet there is no TLR2 inhibitor licensed for human use. Using computer-aided drug design (CADD), we identified a compound, C ₁₆H ₁₅NO ₄ (C29), and a derivative, ortho -vanillin, that inhibit TLR2 signaling in vitro and in vivo. Our findings also revealed unexpected differences between TLR2/1 and TLR2/6 signaling in mice vs. humans. Importantly, our data provide proof of principle that the CADD-targeted BB loop pocket residues are critical for TLR2 signaling and may be targeted therapeutically. Toll-like receptor (TLR) signaling is initiated by dimerization of intracellular Toll/IL-1 receptor resistance (TIR) domains. For all TLRs except TLR3, recruitment of the adapter, myeloid differentiation primary response gene 88 (MyD88), to TLR TIR domains results in downstream signaling culminating in proinflammatory cytokine production. Therefore, blocking TLR TIR dimerization may ameliorate TLR2-mediated hyperinflammatory states. The BB loop within the TLR TIR domain is critical for mediating certain protein–protein interactions. Examination of the human TLR2 TIR domain crystal structure revealed a pocket adjacent to the highly conserved P681 and G682 BB loop residues. Using computer-aided drug design (CADD), we sought to identify a small molecule inhibitor(s) that would fit within this pocket and potentially disrupt TLR2 signaling. In silico screening identified 149 compounds and 20 US Food and Drug Administration-approved drugs based on their predicted ability to bind in the BB loop pocket. These compounds were screened in HEK293T-TLR2 transfectants for the ability to inhibit TLR2-mediated IL-8 mRNA. C ₁₆H ₁₅NO ₄ (C29) was identified as a potential TLR2 inhibitor. C29, and its derivative, ortho -vanillin ( o -vanillin), inhibited TLR2/1 and TLR2/6 signaling induced by synthetic and bacterial TLR2 agonists in human HEK-TLR2 and THP-1 cells, but only TLR2/1 signaling in murine macrophages. C29 failed to inhibit signaling induced by other TLR agonists and TNF-α. Mutagenesis of BB loop pocket residues revealed an indispensable role for TLR2/1, but not TLR2/6, signaling, suggesting divergent roles. Mice treated with o -vanillin exhibited reduced TLR2-induced inflammation. Our data provide proof of principle that targeting the BB loop pocket is an effective approach for identification of TLR2 signaling inhibitors.

Recent studies have implicated the innate immunity system in the pathogenesis of myelodysplastic syndromes (MDS). Toll-like receptor ( TLR ) genes encode key innate immunity signal initiators. We recently identified multiple genes, known to be regulated by TLRs, to be overexpressed in MDS bone marrow (BM) CD34+ cells, and hypothesized that TLR signaling is abnormally activated in MDS. We analyzed a large cohort of MDS cases and identified TLR1, TLR2 and TLR6 to be significantly overexpressed in MDS BM CD34+ cells. Deep sequencing followed by Sanger resequencing of TLR1 , TLR2 , TLR4 and TLR6 genes uncovered a recurrent genetic variant, TLR2-F217S, in 11% of 149 patients. Functionally, TLR2-F217S results in enhanced activation of downstream signaling including NF-κB activity after TLR2 agonist treatment. In cultured primary BM CD34+ cells of normal donors, TLR2 agonists induced histone demethylase JMJD3 and interleukin-8 gene expression. Inhibition of TLR2 in BM CD34+ cells from patients with lower-risk MDS using short hairpin RNA resulted in increased erythroid colony formation. Finally, RNA expression levels of TLR2 and TLR6, as well as presence of TLR2-F217S, are associated with distinct prognosis and clinical characteristics. These findings indicate that TLR2-centered signaling is deregulated in MDS, and that its targeting may have potential therapeutic benefit in MDS.

OBJECTIVE: Individuals with type 2 diabetes have a myriad of metabolic aberrations including increased inflammation, increasing their cardiovascular risk. Toll-like receptors (TLRs) and their ligands play a key role in insulin resistance and atherosclerosis. However, there is a paucity of data examining the expression and activity of TLRs in type 2 diabetes. Thus, in the present study, we examined TLR2 and TLR4 mRNA and protein expression, their ligands, and signaling in monocytes of recently diagnosed type 2 diabetic patients. RESEARCH DESIGN AND METHODS: TLR mRNA, protein expression, TLR ligands, and TLR signaling were measured in freshly isolated monocytes from healthy human control subjects (n = 23) and type 2 diabetic subjects (n = 23) using real-time RT-PCR, Western blot, and flow cytometric assays. RESULTS: Type 2 diabetic subjects had significantly increased TLR2, TLR4 mRNA, and protein in monocytes compared with control subjects (P < 0.05). Increased TLR2 and TLR4 expression correlated with BMI, homeostasis model assessment-insulin resistance (HOMA-IR), glucose, A1C, Nε-(carboxymethyl) lysine (CML), and free fatty acid (FFA). Ligands of TLR2 and TLR4, namely, HSP60, HSP70, HMGB1, endotoxin, and hyaluronan levels, were elevated in type 2 diabetic subjects and positively correlated with TLR2 and TLR4. Type 2 diabetic subjects showed increased MyD88, phosphorylated IRAK-1, Trif, TICAM-1, IRF-3, and NF-κB p65 expression in monocytes compared with control subjects. Furthermore, TLR-MyD88-NF-κB signaling resulted in elevated levels of cytokines (P < 0.05), but increased interleukin (IL)-1β, interferon (IFN)-γ, and endotoxin were not significant when adjusted for BMI. CONCLUSIONS: In this comprehensive study, we make the novel observation that TLR2 and TLR4 expression and their ligands, signaling, and functional activation are increased in recently diagnosed type 2 diabetes and contribute to the proinflammatory state.

Gut barrier function is key in maintaining a balanced response between the host and its microbiome. The microbiota can modulate changes in gut barrier as well as metabolic and inflammatory responses. This highly complex system involves numerous microbiota-derived factors. The gut symbiont Akkermansia muciniphila is positively correlated with a lean phenotype, reduced body weight gain, amelioration of metabolic responses and restoration of gut barrier function by modulation of mucus layer thickness. However, the molecular mechanisms behind its metabolic and immunological regulatory properties are unexplored. Herein, we identify a highly abundant outer membrane pili-like protein of A. muciniphila MucT that is directly involved in immune regulation and enhancement of trans-epithelial resistance. The purified Amuc_1100 protein and enrichments containing all its associated proteins induced production of specific cytokines through activation of Toll-like receptor (TLR) 2 and TLR4. This mainly leads to high levels of IL-10 similar to those induced by the other beneficial immune suppressive microorganisms such as Faecalibacterium prausnitzii A2-165 and Lactobacillus plantarum WCFS1. Together these results indicate that outer membrane protein composition and particularly the newly identified highly abundant pili-like protein Amuc_1100 of A. muciniphila are involved in host immunological homeostasis at the gut mucosa, and improvement of gut barrier function.

Successful cancer immunotherapy entails activation of innate immune receptors to promote dendritic cell (DC) maturation, antigen presentation, up-regulation of costimulatory molecules, and cytokine secretion, leading to activation of tumor antigen-specific cytotoxic T lymphocytes (CTLs). Here we screened a synthetic library of 100,000 compounds for innate immune activators using TNF production by THP-1 cells as a readout. We identified and optimized a potent human and mouse Toll-like receptor (TLR)1/TLR2 agonist, Diprovocim, which exhibited an EC50 of 110 pM in human THP-1 cells and 1.3 nM in primary mouse peritoneal macrophages. In mice, Diprovocim-adjuvanted ovalbumin immunization promoted antigen-specific humoral and CTL responses and synergized with anti–PD-L1 treatment to inhibit tumor growth, generating long-term antitumor memory, curing or prolonging survival of mice engrafted with the murine melanoma B16-OVA. Diprovocim induced greater frequencies of tumor-infiltrating leukocytes than alum, of which CD8 T cells were necessary for the antitumor effect of immunization plus anti–PD-L1 treatment.

Candida albicans can switch from commensal to pathogenic mode, causing mucosal or disseminated candidiasis. The host relies on pattern-recognition receptors including Toll-like receptors (TLRs) and C-type lectin receptors (CLRs) to sense invading fungal pathogens and launch immune defense mechanisms. However, the complex interplay between fungus and host innate immunity remains incompletely understood. Here we report that C . albicans upregulates expression of a small secreted cysteine-rich protein Sel1 upon encountering limited nitrogen and abundant serum. Sel1 activates NF-κB and MAPK signaling pathways, leading to expression of proinflammatory cytokines and chemokines. Comprehensive genetic and biochemical analyses reveal both TLR2 and TLR4 are required for the recognition of Sel1. Further, SEL1 -deficient C . albicans display an impaired immune response in vivo, causing increased morbidity and mortality in a bloodstream infection model. We identify a critical component in the Candida-host interaction that opens a new avenue to tackle Candida infection and inflammation. The interplay between fungal pathogens and the innate immune system remains incompletely understood. Here, Wang et al. show that a small protein secreted by Candida albicans induces a TLR2- and TLR4-mediated inflammatory response in a mouse infection model.

Bacteroides fragilis (BF) are Gram-negative anaerobe symbionts present in the colon. Recent studies have reported the beneficial role of BF in maintaining intestinal homeostasis, stimulating host immunologic development, and preventing infectious colitis caused by pathogenic bacteria. Our previous studies showed that monocolonization of germ-free mice with BF significantly reduced colon inflammations and damage. In order to investigate the Toll-like receptor-2 (TLR2), TLR4, and interleukin 10 (IL-10) molecular signaling pathways involved in BF-mediated prevention of dextran sulfate sodium (DSS)-induced colitis. The wild-type (WT), TLR4, TLR2, and IL-10 knockout (-/-) germ-free mice grown were with or without BF colonization for 28 days, and then administered 1% DSS in drinking water for 7 day to induce acute ulcerative colitis. We compared phenotypes such as weight loss, disease activity, intestinal histological scores, and immunohistochemistry for inflammatory cells. Unlike WT and TLR4-/- mice, the severity of DSS-colitis did not improve in TLR2-/- animals after BF colonization. The BF enhanced anti-inflammatory cytokines IL-10 expression and inhibited pro-inflammatory-related tumor necrosis factor (TNF-α) and IL-6 mRNA expression in both WT and TLR4-/- mice. In contrast, the failed to up-regulated IL-10 and down-regulated the TNF-α and IL-6 in BF colonization TLR2-/- mice. In addition, we further perform IL-10-/- mice to clarify whether the BF through TLR2 /IL-10 pathway to alleviate DSS-colitis. There were no significant differences in colitis severity and pro-inflammatory related genes expression in the IL-10-/- mice with or without BF colonization. These results indicate the disease-preventing effects of BF in acute DSS-induced colitis may occur through the TLR2/IL-10 signal pathway.

The pathogenesis of COVID-19 is associated with a hyperinflammatory response; however, the precise mechanism of SARS-CoV-2-induced inflammation is poorly understood. Here, we investigated direct inflammatory functions of major structural proteins of SARS-CoV-2. We observed that spike (S) protein potently induced inflammatory cytokines and chemokines, including IL-6, IL-1β, TNFα, CXCL1, CXCL2, and CCL2, but not IFNs in human and mouse macrophages. No such inflammatory response was observed in response to membrane (M), envelope (E), and nucleocapsid (N) proteins. When stimulated with extracellular S protein, human and mouse lung epithelial cells also produced inflammatory cytokines and chemokines. Interestingly, epithelial cells expressing S protein intracellularly were non-inflammatory, but elicited an inflammatory response in macrophages when co-cultured. Biochemical studies revealed that S protein triggers inflammation via activation of the NF-κB pathway in a MyD88-dependent manner. Further, such an activation of the NF-κB pathway was abrogated in Tlr2-deficient macrophages. Consistently, administration of S protein-induced IL-6, TNF-α, and IL-1β in wild-type, but not Tlr2-deficient mice. Notably, upon recognition of S protein, TLR2 dimerizes with TLR1 or TLR6 to activate the NF-κB pathway. Taken together, these data reveal a mechanism for the cytokine storm during SARS-CoV-2 infection and suggest that TLR2 could be a potential therapeutic target for COVID-19.