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