- Title
- Soluble fibre as an anti-inflammatory treatment in asthma
- Creator
- Williams, Lily May
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2023
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Asthma is a chronic inflammatory disease of the airways affecting >300 million people worldwide, including 2.7 million Australians (1, 2). Despite the benefits of inhaled corticosteroid (ICS) therapy in asthma, issues with medication adherence, side effects, and variability in responsiveness highlights the limitations of this mainstay therapy (3-5). As such, non- pharmacological adjunctive interventions in asthma management are needed. There are a number of lifestyle interventions, including diet and exercise, which have been summarised in detail previously (6). Genetics are a key determinant of asthma development (7, 8). However, environmental exposures from the in utero environment to adulthood can modulate disease development and progression (9, 10). Diet is a modifiable risk factor in asthma, with epidemiological evidence demonstrating fibre intake in subjects with asthma is associated with reduced eosinophilic airway inflammation and improved lung function (11). Further, in the past decade the therapeutic benefit of soluble fibre intervention in adults with asthma has been demonstrated (12-15). Soluble fibre has anti-inflammatory and immunomodulatory effects, which are hypothesised to be attributed to the generation of bacterial metabolites (16). Short-chain fatty acids (SCFAs), including acetate, propionate and butyrate, are bacterial metabolites which are synthesised during the fermentation of soluble fibre within the large intestine. These SCFAs act on immune cells in circulation and some end organs via the activation of free fatty acid receptors (FFARs), namely FFAR2 and FFAR3, and through histone deacetylase (HDAC) inhibition (HDACi), specifically Class I and IIa HDAC subtypes. This thesis aims to: summarise the evidence available for prebiotics, synbiotics, and SCFA on markers of immune function, and respiratory tract infections; examine the effects of different administration regimes of a soluble fibre (oligosaccharide blend containing inulin) supplement on clinical asthma outcomes and ex vivo inflammatory cytokine production in lipopolysaccharide (LPS) and rhinovirus A1 (RVA1) stimulated peripheral blood mononuclear cells (PBMCs) isolated from subjects with poorly-controlled asthma; examine how in vitro SCFA treatment of PBMCs modify inflammatory cytokine production following LPS and RVA1 stimulation and the gene expression of FFARs, nuclear factor kappa B (NFκB) pathway, and HDAC enzymes; and examine in vitro SCFA treatment of PBMCs isolated from subjects with asthma on type 2 (T2) inflammatory cytokine production and gene expression of GATA3, IL17RB and ST2 following exposure to recombinant epithelial-derived cytokines (IL-25, IL-33) with and without RVA1. Chapter 2 presents the available evidence for prebiotics, synbiotics and SCFAs on respiratory tract infections (RTIs) and immune function in human subjects across all age categories. This systematic review and meta-analysis demonstrated protective effects of pre- and synbiotic supplementation on RTIs, with meta-analysis demonstrating a 27% and 25% reduction, respectively, in the odds of ≥1 RTI episode during the study period. However, there was considerable variability in study dose and duration. There was conflicting evidence as to whether prebiotic supplementation led to altered immune functions. Though, this is possibly a consequence of the scarcity of studies examining this intervention and outcome together. However, meta-analysis indicated synbiotic supplementation increased NK cell activity compared to control. Overall, this systematic review also demonstrated a protective role for prebiotics and synbiotics for RTI, and modulated NK cell activity. In Chapter 3, the effects of soluble fibre (oligosaccharide blend containing inulin) supplementation in different timing and dosage regimes in subjects (n=35) with poorly-controlled asthma (ACQ7 score >0.75) were investigated. We found supplementation with single-daily low dose (LD, 6g/day) soluble fibre improved asthma control (ACQ7, p=0.033), and reduced blood (p=0.048) and sputum (p=0.033) eosinophil count, compared to pre-intervention. In subjects with elevated sputum eosinophils (≥3% sputum eosinophils, n=5), we found a trend towards reduced IL4 (p=0.068), IL5 (p=0.068), and IL13 (p=0.068) gene expression following the LD compared to pre-intervention. Single-daily high dose (HD12, 12g/day) soluble fibre supplementation led to an increase in total plasma SCFAs (p=0.027) and plasma acetate (p=0.022) compared to placebo. Following HD12, the mRNA expression of HDAC1 in PBMCs was increased compared to pre- intervention (p=0.043). In subjects with low airway eosinophils (sputum eosinophils < 3%, n=12), LPS-induced TNF-α decreased following HD12 compared to LD (p=0.017), while LPS-induced IL-10 increased following HD12 compared to pre-intervention (p=0.049). Following HD12, there was a decrease in RVA1-induced TNF-α (p=0.010), IL-10 (p=0.017), and IFN-γ (p=0.028) in this subgroup compared to pre-intervention. Overall, single daily intervention with low dose soluble fibre had the greatest clinical benefit in subjects with poorly controlled asthma, associated with decreased type 2 inflammation. Single daily high dose soluble fibre supplementation modulates inflammatory responses by PBMCs from subjects with asthma in response to LPS and RVA1, particularly those with low airway eosinophils. SCFAs appear to contribute to changes in ex vivo PBMC responses, and may be linked with increased HDAC1 expression. Chapter 4 reports 20mM acetate treatment significantly (p<0.05) lowered pro-inflammatory cytokine (IL-1β, TNF-α, IFN-γ) protein release by LPS stimulated PBMCs from subjects with asthma. Propionate or butyrate treatment reduced IL-1β, IFN-γ, and IL-10 protein expression by LPS stimulated cells. Treatment with 20mM propionate lowered RVA1-induced TNF-α and IFN- γ protein expression. Similarly, butyrate reduced RVA1-induced IFN-γ (2mM, 20mM) and IL-10 (20mM only) protein expression. Further, there were also trends for lower RVA1-induced IL-1β (p=0.076) and TNF-α (p=0.076) protein expression with butyrate treatment. In the absence of LPS or RVA1 stimulation, there were no significant effects of acetate treatment on PBMC mRNA expression of HDAC subtypes, FFARs, or components of the NFκB pathway. Propionate and butyrate treatment led to altered mRNA expression of genes encoding molecules in the FFAR (propionate: ARRB2; butyrate: FFAR2, ARRB2) and NFκB (propionate: NFKB1, RELA; butyrate, NFKB1, NFKBIA) pathways. Propionate and butyrate treatment upregulated the gene expression of Class I HDACs (HDAC1, HDAC3), and downregulated genes encoding Class IIa HDACs (HDAC4, HDAC7, HDAC9). Overall, these results suggest interventions increasing propionate and butyrate in circulation, including soluble fibre intake, may modulate inflammatory responses to viral and bacterial infections in asthma through HDAC-mediated mechanisms. In Chapter 5, in vitro SCFA treatment in PBMCs from subjects with asthma with propionate and butyrate (1mM) lowered IL-5 (propionate, p=0.004; butyrate, p<0.001) and IL-13 (propionate, p=0.001; butyrate, p=0.008) induced by recombinant IL-2, IL-25 and IL-33. The mRNA expression of GATA3 was not significantly changed with IL-2, IL-25, and IL-33 or SCFA treatment. IL-2, IL-25, and IL-33 upregulated IL17RB and ST2 mRNA expression 30- and 6-fold, respectively. Propionate treatment downregulated IL17RB (p=0.020), while there was lower expression IL17RB (p<0.001) and ST2 (p=0.012) with butyrate treatment. The induction of T2 cytokines and epithelial-derived cytokine receptor gene expression (IL17RB, ST2) with IL-2, IL- 25 and IL-33 was seen consistently in both PBMCs from subjects with high (FeNO ≥30ppb, n=7) and low (FeNO <30ppb, n=11) FeNO. This data indicated T2 inflammatory responses from immune cells in asthma can be targeted with SCFAs, propionate and butyrate, via downregulation of IL-25 and IL-33 receptors, IL17RB and ST2. Overall, this thesis has demonstrated that soluble fibre intake is advantageous for health, with a specific benefit in adults with poorly-controlled asthma. Intervention with low dose soluble fibre supplementation demonstrated improvements in asthma control and reduced T2 airway inflammation. Further, T2 inflammation appears to be targeted by SCFAs, generated by bacterial fermentation of soluble fibre, which act directly on immune cells through HDAC-mediated mechanisms. As such, we conclude interventions increasing SCFAs in circulation, including soluble fibre intake, is a promising adjunct treatment strategy in asthma management.
- Subject
- asthma; inflammation; soluble fibre; short-chain fatty acids
- Identifier
- http://hdl.handle.net/1959.13/1509662
- Identifier
- uon:56278
- Rights
- Copyright 2023 Lily May Williams
- Language
- eng
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