Diurnal changes in bacterial settlement on the Peyer's patch and surrounding mucosa in the rat ileum and its effect against the intestinal immune system.

Our previous study revealed the diurnal change in the indigenous bacteria settling on the terminal region of the rat ileum. In the present study, we investigated the diurnal change in indigenous bacteria on the most distal ileal Peyer's patch (PP) and surrounding ileal mucosa and explored how stimulation from indigenous bacteria for a day affects the intestinal immune system at the beginning of the light phase. Histological measurement revealed that bacteria adjacent to the follicle-associated epithelium of PP and to the villous epithelium of the surrounding ileal mucosa are more abundant at zeitgeber time (ZT)0 and ZT18 than at ZT12. On the other hand, tissue-section 16S rRNA amplicon sequencing revealed no significant difference between ZT0 and ZT12 in the bacterial composition on the ileal tissue including the PP. One-day treatment with an antibiotic (Abx) successfully impaired the settlement of bacteria around the ileal PP. In transcriptome analysis, 1-day Abx treatment led to the downregulation of several chemokines in both PP and ordinary ileal mucosa at ZT0. Histological analysis of the 1-day Abx group revealed decreases in both CD68 macrophages in PP and naphthol AS-D chloroacetate esterase stain-positive mast cells in the ileal villi. Together, these findings suggest that the colonies of indigenous bacteria on the distal ileal PP and surrounding mucosa expand during the dark phase, which might lead to the expression of genes to regulate the intestinal immune system and contribute to the homeostasis of at least macrophages in PP and mast cells in the ileal mucosa.