Intestinal colonization with bifidobacteria affects the expression of galectins in extraintestinal organs

This study aimed at determining the contribution of intestinal bifidobacteria to the immune system activation using widely distributed galectins as markers of immune cell homoeostasis. In human flora-associated mice, bacteria were enumerated in the gut, blood, spleen, liver and lungs, while the expression of galectin-1 (Gal-1) and galectin-3 (Gal-3) was estimated by PCR in the intestine and real-time quantitative PCR in the other organs. Gal-1 and -3 were rarely expressed in the intestine. In blood, only Gal-1 was expressed while both galectins were expressed in all other organs. A high prevalence of colonic bifidobacteria was associated with a lower expression of both pulmonary galectins, whose levels negatively correlated with bifidobacterial counts. Caecal bifidobacterial counts also negatively correlated with pulmonary Gal-3 mRNA levels. The spleen was the only organ showing an upregulation of Gal-1 expression related to its bacterial contamination. However, this upregulation was only observed when bifidobacteria were not detected in the colon. A putative mechanism explaining the reduced expression of galectins when bifidobacteria highly colonize the mouse intestine could be that, by reducing the bacterial translocation, bifidobacteria also lead to a decreased blood concentration of substances produced by intestinal bacteria.