Fiber effects on the immune system
Dietary fibers can have direct and indirect effects on the host immune system.21 Before being fermented by microbes in the colon, dietary fibers are known to have a substantial impact on the intestine via modulation of intestinal barrier function and immune responses. Specific fiber subunits are recognised by pattern recognition receptors (PRRs) such as C-type lectin receptors (CLRs), galectins or Toll-like receptors (mainly TLR-2, 3 and 4) on epithelial cells and cells of the innate immune system. Via PRRs, different fiber ligands support a functional intestinal epithelial barrier by modulation of tight junction protein assembly, goblet cell function, regulation of epithelial cell growth and glycocalyx maturation.22,23Pectins, for example, have been shown to strengthen the mucus layer by activating goblet cells and by forming hydrogen bonds with mucins.24 Dietary fibers directly modulate chemokine and cytokine production by intestinal epithelial cells, dendritic cells and macrophages, in part mediated by activation of PPARγ.21
Following microbial fermentation, a wide range of potent immunological metabolites are produced (Figure 2). The best described metabolites are short-chain fatty acids (SCFAs), which are primarily acetate, propionate, and butyrate (Table 4).25 SCFAs are potent immunomodulators that promote IL-10 secretion by dendritic cells and lymphocytes, influence Treg numbers and effectiveness, influence bone marrow haematopoiesis, reduce effector T cell activity, improve epithelial barrier, support IgA secretion by B lymphocytes, inhibit mast cell degranulation and modulate ILC2 activation.26-30Fiber consumption or SCFA administration in experimental models protects against colitis, inflammatory arthritis, respiratory syncytial virus infection, allergic airway inflammation and food allergy.31-34 SCFAs exert effects on the host immune system via binding to G protein-coupled receptors (GPCRs) such as GPR41, GPR43 and GPR109A, via epigenetic modifications that inhibit histone deacetylase (HDAC) activity, and most recently butyrate has been described as an aryl hydrocarbon receptor (AhR) ligand. Epigenetic mechanisms seem particularly important for the induction of T regulatory cells in the gut as butyrate enhances histone acetylation of the Foxp3 promoter thereby driving Treg development.35,36Importantly, consumption of fruits and vegetables during the first year of life is associated with increased levels of fecal butyrate and those children with the highest fecal levels of butyrate and propionate were less likely to develop allergies and asthma later in life.32