Acetate reduces neutrophil migration via a mechanism mediated by GPR43.

<p>The total number of leukocytes in the peritoneal cavity was quantified in wildtype and <i>Gpr43</i>^{<i>−/−</i>}mice at 24 h following sham or cecal ligation and puncture (CLP; (<b>A</b>)), or at 2 h after saline, fMLP or fMLP + acetate (<b>B</b>). The peritoneal neutrophil numbers of acetate-treated wildtype and <i>Gpr43</i>^{<i>−/−</i>}mice at 2 h after saline or fMLP treatment were also measured (<b>C</b>). N ≥ 3 mice per group, **<i>p</i> < 0.01, *<i>p</i> < 0.05 vs corresponding Sham or Saline group, <i>t</i> test. ##<i>p</i> < 0.01, #<i>p</i> < 0.05 vs treated Wildtype, <i>t</i> test. <i>&p <</i> 0.05 vs fMLP-treated Wildtype, <i>t</i> test. N.S denotes not statistically significant vs fMLP-treated <i>Gpr43</i>^{<i>−/−</i>}.</p

Exacerbated intestinal inflammation in <i>Gpr43</i>^{<i>−/−</i>}mice in response to LPS.

<p>(<b>A</b>) Representative histological images of jejunum of wildtype and <i>Gpr43</i>^{<i>−/−</i>}mice following 4 h of saline or LPS challenge. Neutrophils are denoted by the black arrowheads. Scale bar = 100 μm. (<b>B</b>) Histological quantification of neutrophils in the (<b>i</b>) duodenum, (<b>ii</b>) jejunum and (<b>iii</b>) ileum of wildtype and <i>Gpr43</i>^{<i>−/−</i>}mice following 4 h of saline or LPS challenge. N ≥ 5 mice per group, **<i>p</i> < 0.01, *<i>p</i> < 0.05 vs corresponding Saline group, <i>t</i> test. #<i>p</i> < 0.05 vs Wildtype, <i>t</i> test.</p

Baseline parameters are comparable between wildtype and <i>Gpr43</i>^{<i>−/−</i>}mice.

<p>The number of circulating leukocytes (<b>A</b>) and neutrophils (<b>B</b>) in the peripheral blood of wildtype and <i>Gpr43</i>^{<i>−/−</i>}mice were quantitated. Bone marrow leukocytes (<b>C</b>) and neutrophils (<b>D</b>) isolated from wildtype and <i>Gpr43</i>^{<i>−/−</i>}mice were quantitated. N ≥ 8 per group. Baseline cell surface expression of L-selectin (<b>E</b>), and PMA-induced L-selectin shedding (<b>F</b>) and respiratory burst (<b>G</b>) was measured and compared between wildtype and <i>Gpr43</i>^{<i>−/−</i>}bone marrow neutrophils. N ≥ 4 mice per group or independent <i>in vitro</i> experiments performed in duplicates, N.S. denote not statistical significant, <i>t</i> test.</p

GPR43-deficient neutrophils display slow rolling following 4 h of LPS challenge.

<p>Intravital microscopy was utilised to visualise and quantitate the number of intravascular neutrophils that were rolling (<b>A</b>), and their rolling velocity (<b>B</b>) or adherent (<b>C</b>) following 4 h of saline or LPS challenge in wildtype and <i>Gpr43</i>^{<i>−/−</i>}mice. N ≥ 5 mice per group, ***<i>p</i> < 0.001 vs corresponding Saline group, <i>t</i> test. # <i>p</i> < 0.05 vs Wildtype, <i>t</i> test.</p

GPR43 deficiency induces migration, accelerated neutrophil rolling and adhesion following 1 h of LPS challenge.

<p>Neutrophils were isolated from the bone marrow of wildtype and <i>Gpr43</i>^{<i>−/−</i>}mice, and analysed for chemotaxis towards increasing concentration of CXCL1 (<b>A</b>) and LPS (<b>B</b>). N ≥ 4 independent <i>in vitro</i> experiments performed in duplicates, ***<i>p</i> < 0.001, *<i>p</i> < 0.05 vs Wildtype, 2-way ANOVA. Intravital microscopy was utilised to visualise and quantitate the number of intravascular neutrophils that were rolling (<b>C</b>) or adherent (<b>D</b>) following 1 h of saline or LPS challenge in wildtype and <i>Gpr43</i>^{<i>−/−</i>}mice. N ≥ 5 mice per group, ***<i>p</i> < 0.001, **<i>p</i> < 0.01 vs corresponding Saline group, <i>t</i> test. #<i>p</i> < 0.05 vs Wildtype, <i>t</i> test.</p

Enhanced migratory behaviour of neutrophils is maintained after microbiota transfer.

<p>Neutrophils of GF mice reconstituted with microbiota from SPF mice fed on Ctrl or NF diet were analyzed for chemotaxis towards increasing concentration of CXCL1 (<b>A</b>) and fMLP (<b>B</b>). N ≥ 4 independent <i>in vitro</i> experiments performed in duplicates, **<i>p</i> < 0.01, *<i>p</i> < 0.05 vs Ctrl diet, 2-way ANOVA.</p

Deficiency of dietary fiber modulates gut microbiota composition, neutrophil recruitment and worsens experimental colitis

Ulcerative colitis is an inflammatory disease of the colon that is associated with colonic neutrophil accumulation. Recent evidence indicates that diet alters the composition of the gut microbiota and influences host–pathogen interactions. Specifically, bacterial fermentation of dietary fiber produces metabolites called short-chain fatty acids (SCFAs), which have been shown to protect against various inflammatory diseases. However, the effect of fiber deficiency on the key initial steps of inflammation, such as leukocyte–endothelial cell interactions, is unknown. Moreover, the impact of fiber deficiency on neutrophil recruitment under basal conditions and during inflammation in vivo is unknown. Herein, we hypothesized that a fiber-deficient diet promotes an inflammatory state in the colon at baseline and predisposes the host to more severe colitis pathology. Mice fed a no-fiber diet for 14 days showed significant changes in the gut microbiota and exhibited increased neutrophil-endothelial interactions in the colonic microvasculature. Although mice fed a no-fiber diet alone did not have observable colitis-associated symptoms, these animals were highly susceptible to low dose (0.5%) dextran sodium sulphate (DSS)-induced model of colitis. Supplementation of the most abundant SCFA, acetate, prevented no-fiber diet-mediated enrichment of colonic neutrophils and colitis pathology. Therefore, dietary fiber, possibly through the actions of acetate, plays an important role in regulating neutrophil recruitment and host protection against inflammatory colonic damage in an experimental model of colitis