TLR5 Activation Exacerbates Airway Inflammation in Asthma

When the allergy alarm bells toll: The role of Toll-like receptors in allergic diseases and treatment

Toll-like receptors of the human immune system are specialized pathogen detectors able to link innate and adaptive immune responses. TLR ligands include among others bacteria-, mycoplasma- or virus-derived compounds such as lipids, lipo- and glycoproteins and nucleic acids. Not only are genetic variations in TLR-related genes associated with the pathogenesis of allergic diseases, including asthma and allergic rhinitis, their expression also differs between allergic and non-allergic individuals. Due to a complex interplay of genes, environmental factors, and allergen sources the interpretation of TLRs involved in immunoglobulin E-mediated diseases remains challenging. Therefore, it is imperative to dissect the role of TLRs in allergies. In this review, we discuss i) the expression of TLRs in organs and cell types involved in the allergic immune response, ii) their involvement in modulating allergy-associated or -protective immune responses, and iii) how differential activation of TLRs by environmental factors, such as microbial, viral or air pollutant exposure, results in allergy development. However, we focus on iv) allergen sources interacting with TLRs, and v) how targeting TLRs could be employed in novel therapeutic strategies. Understanding the contributions of TLRs to allergy development allow the identification of knowledge gaps, provide guidance for ongoing research efforts, and built the foundation for future exploitation of TLRs in vaccine design.

Bronchial epithelial cells release inflammatory markers linked to airway inflammation and remodeling in response to TLR5 ligand flagellin

Background/Aims

Flagellin, which is abundant in gram-negative bacteria, including Pseudomonas, is reported to influence on inflammatory responses in various lung diseases. However, its effect on airway epithelial cells in contribution to asthma pathogenesis is not elucidated yet. We aimed to investigate the effect of TLR5 ligand flagellin on the transcriptomic profile of primary human epithelial cells and to determine the markers of airway inflammation.

Methods

Normal human bronchial epithelial (NHBE) cells were grown and differentiated in air-liquid interface (ALI) culture for 14-16 days. The cells were treated with flagellin in vitro at 10 and 100 ng/ml for 3 and 24 h. The conditioned media and cells were harvested to validate inflammatory markers involved in airway inflammation using ELISA, Western blot, and quantitative PCR methods. RNA-sequencing was performed to investigate the transcriptional response to flagellin in ALI-NHBE cells.

Results

Altered transcriptional responses to flagellin in differentiated bronchial epithelial cells were determined, including genes encoding chemokines, matrix metalloproteinases, and antimicrobial biomolecules. Pathway analysis of the transcriptionally responsive genes revealed enrichment of signaling pathways. Flagellin induced the mRNA expressions of proinflammatory cytokines and chemokines, and secretion of GM-CSF, CXCL5, CCL5 and CXCL10. Flagellin enhanced the protein expression of MMP-13 in TGF-β1 and TGF-β2 pretreated cell lysates and Wnt/β-catenin signaling.

Conclusions

These findings suggest that flagellin could be a potent inducer of inflammatory markers that may contribute to airway inflammation and remodeling.

Ozone impairs endogenous compensatory responses in allergic asthma

Asthma is a chronic inflammatory airway disease characterized by acute exacerbations triggered by inhaled allergens, respiratory infections, or air pollution. Ozone (O3), a major component of air pollution, can damage the lung epithelium in healthy individuals. Despite this association, little is known about the effects of O3 and its impact on chronic lung disease. Epidemiological data have demonstrated that elevations in ambient O3 are associated with increased asthma exacerbations. To identify mechanisms by which O3 exposure leads to asthma exacerbations, we developed a two-hit mouse model where mice were sensitized and challenged with three common allergens (dust mite, ragweed and Aspergillus fumigates, DRA) to induce allergic inflammation prior to exposure to O3 (DRAO3). Changes in lung physiology, inflammatory cells, and inflammation were measured. Exposure to O3 following DRA significantly increased airway hyperreactivity (AHR), which was independent of TLR4. DRA exposure resulted in increased BAL eosinophilia while O3 exposure resulted in neutrophilia. Additionally, O3 exposure following DRA blunted anti-inflammatory and antioxidant responses. Finally, there were significantly less monocytes and innate lymphoid type 2 cells (ILC2s) in the dual challenged DRA-O3 group suggesting that the lack of these immune cells may influence O3-induced AHR in the setting of allergic inflammation. In summary, we developed a mouse model that mirrors some aspects of the clinical course of asthma exacerbations due to air pollution and identified that O3 exposure in the asthmatic lung leads to impaired endogenous anti-inflammatory and antioxidant responses and alterations inflammatory cell populations.

Emerging Insights into the Impact of Air Pollution on Immune-Mediated Asthma Pathogenesis

PURPOSE OF REVIEW

Increases in ambient levels of air pollutants have been linked to lung inflammation and remodeling, processes that lead to the development and exacerbation of allergic asthma. Conventional research has focused on the role of CD4+ T helper 2 (TH2) cells in the pathogenesis of air pollution-induced asthma. However, much work in the past decade has uncovered an array of air pollution-induced non-TH2 immune mechanisms that contribute to allergic airway inflammation and disease.

RECENT FINDINGS

In this article, we review current research demonstrating the connection between common air pollutants and their downstream effects on non-TH2 immune responses emerging as key players in asthma, including PRRs, ILCs, and non-TH2 T cell subsets. We also discuss the proposed mechanisms by which air pollution increases immune-mediated asthma risk, including pre-existing genetic risk, epigenetic alterations in immune cells, and perturbation of the composition and function of the lung and gut microbiomes. Together, these studies reveal the multifaceted impacts of various air pollutants on innate and adaptive immune functions via genetic, epigenetic, and microbiome-based mechanisms that facilitate the induction and worsening of asthma.

Oxidant-induced epithelial alarmin pathway mediates lung inflammation and functional decline following ultrafine carbon and ozone inhalation co-exposure

Environmental inhalation exposures are inherently mixed (gases and particles), yet regulations are still based on single toxicant exposures. While the impacts of individual components of environmental pollution have received substantial attention, the impact of inhalation co-exposures is poorly understood. Here, we mechanistically investigated pulmonary inflammation and lung function decline after inhalation co-exposure and individual exposures to ozone (O₃) and ultrafine carbon black (CB). Environmentally/occupationally relevant lung deposition levels in mice were achieved after inhalation of stable aerosols with similar aerodynamic and mass median distributions. X-ray photoemission spectroscopy detected increased surface oxygen contents on particles in co-exposure aerosols. Compared with individual exposures, co-exposure aerosols produced greater acellular and cellular oxidants detected by electron paramagnetic resonance (EPR) spectroscopy, and in vivo immune-spin trapping (IST), as well as synergistically increased lavage neutrophils, lavage proteins and inflammation related gene/protein expression. Co-exposure induced a significantly greater respiratory function decline compared to individual exposure. A synthetic catalase-superoxide dismutase mimetic (EUK-134) significantly blunted lung inflammation and respiratory function decline confirming the role of oxidant imbalance. We identified a significant induction of epithelial alarmin (thymic stromal lymphopoietin-TSLP)-dependent interleukin-13 pathway after co-exposure, associated with increased mucin and interferon gene expression. We provided evidence of interactive outcomes after air pollution constituent co-exposure and identified a key mechanistic pathway that can potentially explain epidemiological observation of lung function decline after an acute peak of air pollution. Developing and studying the co-exposure scenario in a standardized and controlled fashion will enable a better mechanistic understanding of how environmental exposures result in adverse outcomes.

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Interaction with O₃ mediates free radical production on the surface of carbon black (CB) particles.

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Oxidants mediate co-exposure (CB + O₃)-induced lung function decline.

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EUK-134 (a synthetic superoxide-catalase mimetic) abrogates CB + O₃-induced lung inflammation.

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CB + O₃ co-exposure induces greater lung inflammation than individual exposures.

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Epithelial alarmin (TSLP) contributes significantly to the CB + O₃ toxicity.

Downregulation of miR-3934 in Peripheral Blood Mononuclear Cells of Asthmatic Patients and Its Potential Diagnostic Value

Background

The present study focused on the potential clinical significance of miR-3934 in the occurrence and development of asthma.

Methods

80 asthma and 80 healthy controls were recruited in this study. The peripheral blood mononuclear cells (PBMCs) and serum samples of the asthma patients as well as the healthy controls were isolated, and the expression levels of miR-3934 in PBMCs were examined by RT-qPCR methods. Furthermore, the relationship between the level of miR-3934 in PBMCs and the disease severity has been analyzed, and the potential diagnostic value of miR-3934 was evaluated by the receiver operating characteristics (ROC) curve. Finally, the expression level of IL-6, IL-8, and IL-33 have been detected using the ELISA kits, and Pearson's correlation analysis was performed to investigate the relationship between the level of miR-3934 in PBMCs and the serum expression of those inflammatory cytokines in asthma patients.

Results

miR-3934 was dramatically decreased in PBMCs of the asthma patients, and miR-3934 was markedly reduced in PBMCs of patients with severe asthma vs. mild asthma. Furthermore, ROC analysis showed that levels of miR-3934 in PBMCs can distinguish asthma patient, especially the severe asthma patients from the controls. Finally, the levels of miR-3934 in PBMCs were negatively correlated with the serum levels of IL-6, IL-8, and IL-33 in asthma patients, respectively.

Conclusions

miR-3934 was downregulated in PBMCs of asthmatic patients and may function as a potential diagnosis biomarker.

B cells and the microbiota: a missing connection in food allergy

Food allergies are a major public health concern due to their widespread and rising prevalence. The increase in food allergy is partially due to Western lifestyle habits which deplete protective commensal microbiota. These microbial perturbations can result in adverse host-microbe interactions, altering the phenotype of various immune cells and instigating allergic sensitization. Although B cells are critical to allergic pathology, microbial influences on B cells have been somewhat overlooked. Here, we focus on direct and indirect interactions between bacteria and B cells and how such interactions regulate B-cell phenotype, namely antibody production (IgA, IgE, IgG1, and IgG4) and regulatory B-cell (Breg) function. Understanding how microbes modulate B-cell activity in the context of food allergies is critical to both tracing the development of disease and assessing future treatment options.

Roseburia intestinalis‑derived flagellin ameliorates colitis by targeting miR‑223‑3p‑mediated activation of NLRP3 inflammasome and pyroptosis

Ulcerative colitis (UC) is a type of inflammatory bowel disease (IBD), which is a chronic, relapsing condition associated with the disorder of gut microbial communities. A previous study reported that levels of Roseburia intestinalis (R.I), a butyrate‑producing bacterium, are significantly decreased in patients with IBD and exert an anti‑inflammatory function in dextran sulfate sodium (DSS)‑induced colitis. However, the role of R.I flagellin in UC and its underlying molecular mechanism are not yet fully understood. Therefore, a DSS‑induced colitis model in C57Bl/6 mice and the LPS/ATP‑induced THP‑1 macrophages were treated with R.I flagellin, which were used to investigate the anti‑inflammatory effects of R.I flagellin. The results demonstrated that R.I flagellin decreased colitis‑associated disease activity index, colonic shortening and the pathological damage of the colon tissues in murine colitis models. Furthermore, R.I flagellin decreased the serum levels of proinflammatory cytokines and inhibited activation of the nucleotide‑binding oligomerization segment‑like receptor family 3 (NLRP3) inflammasome in murine colitis. R.I flagellin was also demonstrated to decrease the Gasdermin D to yield the N‑terminal fragment membrane pore and inhibit inflammasome‑triggered pyroptosis. In vitro analysis indicated that microRNA (miR)‑223‑3p was involved in the regulation of R.I flagellin on NLRP3 inflammasome activation. Taken together, the results of the present study demonstrated that R.I flagellin inhibited activation of the NLRP3 inflammasome and pyroptosis via miR‑223‑3p/NLRP3 signaling in macrophages, suggesting that R.I flagellin may be used as a novel probiotic product for the treatment of UC.

Role of Environmental Adjuvants in Asthma Development

PURPOSE OF REVIEW

This review summarizes recent progress in our understanding how environmental adjuvants promote the development of asthma.

RECENT FINDINGS

Asthma is a heterogeneous set of lung pathologies with overlapping features. Human studies and animal models suggest that exposure to different environmental adjuvants activate distinct immune pathways, which in turn give rise to distinct forms, or endotypes, of allergic asthma. Depending on their concentrations, inhaled TLR ligands can activate either type 2 inflammation, or Th17 differentiation, along with regulatory responses that function to attenuate inflammation. By contrast, a different category of environmental adjuvants, proteases, activate distinct immune pathways and prime predominantly type 2 immune responses. Asthma is not a single disease, but rather a group of pathologies with overlapping features. Different endotypes of asthma likely arise from perturbations of distinct immunologic pathways during allergic sensitization.