Negative regulation of human mononuclear phagocyte function

Pattern Recognition Receptor Signaling and Cytokine Networks in Microbial Defenses and Regulation of Intestinal Barriers: Implications for Inflammatory Bowel Disease

Inflammatory bowel disease is characterized by defects in epithelial function and dysregulated inflammatory signaling by lamina propria mononuclear cells including macrophages and dendritic cells in response to microbiota. In this review, we focus on the role of pattern recognition receptors in the inflammatory response as well as epithelial barrier regulation. We explore cytokine networks that increase inflammation, regulate paracellular permeability, cause epithelial damage, up-regulate epithelial proliferation, and trigger restitutive processes. We focus on studies using patient samples as well as speculate on pathways that can be targeted to more holistically treat patients with inflammatory bowel disease.

The Exercise Training Modulatory Effects on the Obesity-Induced Immunometabolic Dysfunctions

Reduced physical activity rate in people's lifestyle is a global concern associated with the prevalence of health disorders such as obesity and metabolic disturbance. Ample evidence has indicated a critical role of the immune system in the aggravation of obesity. The type, duration, and production of adipose tissue-released mediators may change subsequent inactive lifestyle-induced obesity, leading to the chronic systematic inflammation and monocyte/macrophage (MON/MФ) phenotype polarization. Preliminary adipose tissue expansion can be inhibited by changing the lifestyle. In this context, exercise training is widely recommended due to a definite improvement of energy balance and the potential impacts on the inflammatory signaling cascades. How exercise training affects the immune system has not yet been fully elucidated, because its anti-inflammatory, pro-inflammatory, or even immunosuppressive impacts have been indicated in the literature. A thorough understanding of the mechanisms triggered by exercise can suggest a new approach to combat meta-inflammation-induced metabolic diseases. In this review, we summarized the obesity-induced inflammatory pathways, the roles of MON/MФ polarization in adipose tissue and systemic inflammation, and the underlying inflammatory mechanisms triggered by exercise during obesity.

MicroRNA in gastrointestinal cell signalling

Our gut forms an important organ and its formation, functioning and homeostasis are maintained by several factors including cell signalling pathways and commensal microflora. These factors affect pathological, physiological and immunological parameters to maintain gut health and prevent its inflammation. Among these, different intracellular signalling pathways play an important role in regulating gut homeostasis. These pathways are in turn regulated by various microRNAs that play a key role in maintaining the balance between tolerance and inflammation. This review highlights the importance of various cell signalling pathways in modulating gut homeostasis and the role specific miRNAs play in their regulation.

Diversity and functions of intestinal mononuclear phagocytes

The intestinal lamina propria (LP) contains a diverse array of mononuclear phagocyte (MNP) subsets, including conventional dendritic cells (cDC), monocytes and tissue-resident macrophages (mφ) that collectively play an essential role in mucosal homeostasis, infection and inflammation. In the current review we discuss the function of intestinal cDC and monocyte-derived MNP, highlighting how these subsets play several non-redundant roles in the regulation of intestinal immune responses. While much remains to be learnt, recent findings also underline how the various populations of MNP adapt to deal with the challenges specific to their environment. Understanding these processes should help target individual subsets for 'fine tuning' immunological responses within the intestine, a process that may be of relevance both for the treatment of inflammatory bowel disease (IBD) and for optimized vaccine design.

Flagellin suppresses experimental asthma by generating regulatory dendritic cells and T cells

BACKGROUND

Although the hygiene hypothesis suggests that microbial infections could subvert asthma and thus a microbial product might serve as a therapeutic adjuvant for asthma, the relationship between bacterial components and asthma is complex. Recently, low levels of flagellin, the Toll-like receptor (TLR) 5 ligand, have been reported to promote asthma.

OBJECTIVE

We show that a therapeutic dose of flagellin suppresses asthma and that the effect occurs through generating regulatory dendritic cells (rDCs) and regulatory T (Treg) cells.

METHODS

Ovalbumin (OVA)-induced wild-type and TLR5 knockout asthmatic mice were treated intranasally with a mixture of OVA and 10 μg of a flagellin B (FlaB; of Vibrio vulnificus). OVA/FlaB-treated rDCs were adoptively transferred to mice with OVA-induced asthma. Anti-CD25 mAb was used to deplete Treg cells. A mixture of house dust mite (HDM) and FlaB was used to treat mice with HDM-induced asthma. Blood CD14(+) monocyte-derived dendritic cells from HDM-sensitive asthmatic patients were treated with FlaB and incubated with autologous CD4(+) T cells.

RESULTS

An OVA/FlaB mixture ameliorated OVA-induced asthma by inhibiting TH1/TH2/TH17 responses in a TLR5-dependent manner through generating rDCs and Treg cells. The adoptive transfer of OVA/FlaB-treated dendritic cells inhibited OVA-induced asthma, whereas the depletion of CD25(+) cells eliminated the inhibitory effect. A similar effect of FlaB was observed in mice with HDM-induced asthma. In patients with HDM-sensitive asthma, FlaB-treated rDCs inhibited HDM-stimulated TH1/TH2 responses while enhancing Treg cells in an IL-10-dependent manner.

CONCLUSION

These findings collectively suggest that flagellin could be used as a tolerogenic adjuvant to treat allergic asthma.

TAM receptor-dependent regulation of SOCS3 and MAPKs contributes to proinflammatory cytokine downregulation following chronic NOD2 stimulation of human macrophages

Microbial-induced cytokine regulation is critical to intestinal immune homeostasis. Acute stimulation of nucleotide-binding oligomerization domain 2 (NOD2), the Crohn's disease-associated sensor of bacterial peptidoglycan, induces cytokines. However, cytokines are attenuated after chronic NOD2 and pattern recognition receptor stimulation of macrophages; similar attenuation is observed in intestinal macrophages. The role of Tyro3, Axl, and Mer (TAM) receptors in regulating chronic pattern recognition receptor stimulation and NOD2-induced outcomes has not been examined. Moreover, TAM receptors have been relatively less investigated in human macrophages. Whereas TAM receptors did not downregulate acute NOD2-induced cytokines in primary human macrophages, they were essential for downregulating signaling and proinflammatory cytokine secretion after chronic NOD2 and TLR4 stimulation. Axl and Mer were similarly required in mice for cytokine downregulation after chronic NOD2 stimulation in vivo and in intestinal tissues. Consistently, TAM expression was increased in human intestinal myeloid-derived cells. Chronic NOD2 stimulation led to IL-10- and TGF-β-dependent TAM upregulation in human macrophages, which, in turn, upregulated suppressor of cytokine signaling 3 expression. Restoring suppressor of cytokine signaling 3 expression under TAM knockdown conditions restored chronic NOD2-mediated proinflammatory cytokine downregulation. In contrast to the upregulated proinflammatory cytokines, attenuated IL-10 secretion was maintained in TAM-deficient macrophages upon chronic NOD2 stimulation. The level of MAPK activation in TAM-deficient macrophages after chronic NOD2 stimulation was insufficient to upregulate IL-10 secretion; however, full restoration of MAPK activation under these conditions restored c-Fos, c-Jun, musculoaponeurotic fibrosarcoma oncogene homolog K, and PU.1 binding to the IL-10 promoter and IL-10 secretion. Therefore, TAM receptors are critical for downregulating proinflammatory cytokines under the chronic NOD2 stimulation conditions observed in the intestinal environment.

Salmonella enterica serovar Typhimurium ΔmsbB triggers exacerbated inflammation in Nod2 deficient mice

The intracellular pathogen Salmonella enterica serovar Typhimurium causes intestinal inflammation characterized by edema, neutrophil influx and increased pro-inflammatory cytokine expression. A major bacterial factor inducing pro-inflammatory host responses is lipopolysaccharide (LPS). S. Typhimurium ΔmsbB possesses a modified lipid A, has reduced virulence in mice, and is being considered as a potential anti-cancer vaccine strain. The lack of a late myristoyl transferase, encoded by MsbB leads to attenuated TLR4 stimulation. However, whether other host receptor pathways are also altered remains unclear. Nod1 and Nod2 are cytosolic pattern recognition receptors recognizing bacterial peptidoglycan. They play important roles in the host's immune response to enteric pathogens and in immune homeostasis. Here, we investigated how deletion of msbB affects Salmonella's interaction with Nod1 and Nod2. S. Typhimurium Δ msbB-induced inflammation was significantly exacerbated in Nod2^−/− mice compared to C57Bl/6 mice. In addition, S. Typhimurium ΔmsbB maintained robust intestinal colonization in Nod2^−/− mice from day 2 to day 7 p.i., whereas colonization levels significantly decreased in C57Bl/6 mice during this time. Similarly, infection of Nod1^−/− and Nod1/Nod2 double-knockout mice revealed that both Nod1 and Nod2 play a protective role in S. Typhimurium ΔmsbB-induced colitis. To elucidate why S. Typhimurium ΔmsbB, but not wild-type S. Typhimurium, induced an exacerbated inflammatory response in Nod2^−/− mice, we used HEK293 cells which were transiently transfected with pathogen recognition receptors. Stimulation of TLR2-transfected cells with S. Typhimurium ΔmsbB resulted in increased IL-8 production compared to wild-type S. Typhimurium. Our results indicate that S. Typhimurium ΔmsbB triggers exacerbated colitis in the absence of Nod1 and/or Nod2, which is likely due to increased TLR2 stimulation. How bacteria with “genetically detoxified” LPS stimulate various innate responses has important implications for the development of safe and effective bacterial vaccines and adjuvants.

A TNFSF15 disease-risk polymorphism increases pattern-recognition receptor-induced signaling through caspase-8-induced IL-1

Inflammatory diseases are characterized by dysregulated cytokine production. Altered functions for most risk loci, including the inflammatory bowel disease and leprosy-associated tumor necrosis factor ligand superfamily member 15 (TNFSF15) region, are unclear. Regulation of pattern-recognition-receptor (PRR)-induced signaling and cytokines is crucial for immune homeostasis; TNFSF15:death receptor 3 (DR3) contributions to PRR responses have not been described. We found that human macrophages expressed DR3 and that TNFSF15:DR3 interactions were critical for amplifying PRR-initiated MAPK/NF-κB/PI3K signaling and cytokine secretion in macrophages. Mechanisms mediating TNFSF15:DR3 contributions to PRR outcomes included TACE-induced TNFSF15 cleavage to soluble TNFSF15; soluble TNFSF15 then led to TRADD/FADD/MALT-1- and caspase-8-mediated autocrine IL-1 secretion. Notably, TNFSF15 treatment also induced cytokine secretion through a caspase-8-dependent pathway in intestinal myeloid cells. Importantly, rs6478108 A disease risk-carrier macrophages demonstrated increased TNFSF15 expression and PRR-induced signaling and cytokines. Taken together, TNFSF15:DR3 interactions amplify PRR-induced signaling and cytokines, and the rs6478108 TNFSF15 disease-risk polymorphism results in a gain of function.

The IL18RAP region disease polymorphism decreases IL-18RAP/IL-18R1/IL-1R1 expression and signaling through innate receptor-initiated pathways

Fine-tuning of cytokine-inducing pathways is essential for immune homeostasis. Consistently, a dysregulated increase or decrease in pattern-recognition receptor (PRR)-induced signaling and cytokine secretion can lead to inflammatory bowel disease. Multiple gene loci are associated with inflammatory bowel disease, but their functional effects are largely unknown. One such region in chromosome 2q12 (rs917997), also associated with other immune-mediated diseases, encompasses IL18RAP. We found that human monocyte-derived macrophages (MDMs) from rs917997 AA risk carriers secrete significantly less cytokines than G carriers upon stimulation of multiple PRRs, including nucleotide-binding oligomerization domain 2 (NOD2). We identified that IL-18 signaling through IL-18RAP was critical in amplifying PRR-induced cytokine secretion in MDMs. IL-18RAP responded to NOD2-initiated early, caspase-1-dependent autocrine IL-18, which dramatically enhanced MAPK, NF-κB, PI3K, and calcium signaling. Reconstituting MAPK activation was sufficient to rescue decreased cytokines in NOD2-stimulated IL-18RAP-deficient MDMs. Relative to GG carriers, MDM from rs917997 AA carriers had decreased expression of cell-surface IL-18RAP protein, as well as of IL-18R1 and IL-1R1, genes also located in the IL18RAP region. Accordingly, these risk-carrier MDMs show diminished PRR-, IL-18-, and IL-1-induced MAPK and NF-κB signaling. Taken together, our results demonstrate clear functional consequences of the rs917997 risk polymorphism; this polymorphism leads to a loss-of-function through decreased IL-18RAP, IL-18R1, and IL-1R1 protein expression, which impairs autocrine IL-18 and IL-1 signaling, thereby leading to decreased cytokine secretion in MDMs upon stimulation of a broad range of PRRs.

Propofol reduces lipopolysaccharide-induced, NADPH oxidase (NOX 2) mediated TNF- α and IL-6 production in macrophages

During an infection, lipopolysaccharide (LPS) stimulates the production of reactive oxygen species (ROS), which is mediated, in large part, by nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs); NOX2 is the major NOX isoform found in the macrophage cell membrane. While the immunomodulatory activity of propofol is highly documented, its effect on the LPS-induced NOX2/ROS/NF-κB signaling pathway in macrophages has not been addressed. In present study, we used murine macrophage cell line RAW264.7 pretreated with propofol and stimulated with LPS. IL-6 and TNF-α expression, ROS production, and NOX activity were determined. Results showed that propofol attenuated LPS-induced TNF-α and IL-6 expression. Moreover, LPS-stimulated phosphorylation of NF-κB and generation of ROS were weakened in response to propofol. Propofol also reduced LPS-induced NOX activity and expression of gp91phox and p47phox. We conclude that propofol modulates LPS signaling in macrophages by reducing NOX-mediated production of TNF-α and IL-6.

NF-κB1 inhibits NOD2-induced cytokine secretion through ATF3-dependent mechanisms

Regulation of microbially induced cytokine secretion is critical in intestinal immune homeostasis. NOD2, the Crohn's disease-associated bacterial peptidoglycan sensor, activates the NF-κB pathway. After chronic NOD2 stimulation in human macrophages, cytokine secretion is significantly attenuated, similar to the situation in the intestinal environment. We find that NF-κB1 (p105/p50) expression is upregulated with chronic NOD2 stimulation and is required for attenuation of cytokine secretion in vitro in human macrophages and in vivo in mice. Upon chronic NOD2 stimulation, regulation of both activating (H3K4Me2 and H4Ac) and inhibitory (H3K27Me3) histone modifications was observed within cytokine gene promoters; these outcomes were NF-κB1 dependent. In addition to enhanced binding to cytokine gene promoters with chronic NOD2 stimulation, NF-κB1 bound to the promoter of the transcriptional repressor, ATF3. ATF3 was then induced and bound to cytokine gene promoters; both features were impaired upon NF-κB1 knockdown. Restoring ATF3 expression under NF-κB1 knockdown conditions restored NOD2-mediated cytokine downregulation. Finally, NF-κB1 and ATF3 cooperate with other inhibitory pathways, including IRAKM and secreted mediators, to downregulate cytokine secretion after chronic NOD2 stimulation. Therefore, we identify NF-κB1 and ATF3 as critical mechanisms through which NOD2 downregulates cytokines and contributes to intestinal immune homeostasis.