Structure-function relationship between homogalacturonan pectins and intestinal immunity: Microbiota-(in)dependent effects on the gastrointestinal immune barrier

Pectins are dietary fibers that have been recognized to reduce the incidence of lifestyle related disease. Pectins can contain several chemical structures, including homogalacturonan regions which consist of a galacturonan backbone. These galacturonans can be methyl-esterified causing a difference in degree and distribution of methyl-esters in pectins. It is known that pectins can influence the intestinal immune system, but which specific structures are responsible for these effects is unknown. Therefore, the immunomodulatory role of the degree of methyl-esterification (DM) and the distribution of methyl-esters in homogalacturonan pectins on the intestinal immune system was investigated. Current studies demonstrate that low DM pectins and pectins with an intermediate DM and blockwise distribution of methyl-esters inhibit the activation of Toll-like receptor (TLR) 2-1 and prevent the development of TLR2-mediated inflammation of the small intestine in mice. Besides, administration of a low DM pectin that contains a more blockwise distribution of methyl-esters and intermediate DM pectins influence T cell immunity and the microbiota composition in the intestine of healthy mice. In addition, pectins inhibit the growth of Citrobacter rodentium and prevent inflammation caused by this pathogen in the large intestine. These effects of pectins on C. rodentium were independent of structural differences. In conclusion, homogalacturonan pectins can influence the intestinal immune system through direct effects on TLR2, modulation of intestinal microbiota composition or by inhibiting the growth of enteric pathogens in the intestine. The degree and distribution of methyl-esters of pectins appear to play an important role in these effects.