The specific role of most of these newly discovered DC subsets in antigen sampling and presentation is unknown and it remains therefore unclear whether they work synergistically, as alternatives or have distinct functions in the recognition of the intestinal microbiota. The focus of effort in this field will be to understand how these diverse DC subsets cooperate in regulating the complex homeostasis and host defense in the different intestinal immune compartments. The intestinal lamina propria contains a DC network that serves as a newly discovered gateway for the uptake and transport of the intestinal microbiota. Specialized vesicular structures at the end of transepithelial dendrites serve as `luminal sensors' for the mucosal immune system. Characterization of the surface components of these luminal sensors may aid in developing strategies to prevent bacterial and viral pathogen entry. We propose that the lamina propria and M cell-dependent antigen sampling systems are associated with specific DC subsets. The discovery of functionally defined subsets of dendritic cells associated with distinct mechanisms responsible for the uptake of antigens across the intestinal barrier opens the path for strategies for targeting them specifically in the development of vaccines or treatment approaches for inflammatory bowel diseases. For these exciting possibilities to progress into practical applications, it needs to be established whether distinct DCs subsets are associated with multiple pathways or if their function is linked to specific innate or adaptive immune responses. Defining DC function in the intestine will be pivotal in finally progressing our understanding on how the mucosal immune system makes the distinction between commensal microbiota, pathogens and self antigens.
Forty healthy individuals with a low defecation frequency were selected, and the effects of intake of a fermented milk beverage that contains Lactobacillus casei strain Shirota (LcS) at 4 × 109 bacteria/bottle for 2 weeks (1 bottle/day) on the defecation frequency and intestinal microflora were evaluated by the placebo-controlled double-blind cross-over scheme. Defecation frequency both in times per week and days per week significantly increased in the LcS beverage period compared with the frequency before the beginning of intake. The differences were more notable in those with a stronger tendency to constipation (frequency of defecation before intake ≤ 4.0 times/week, n=21), and the frequency of defecation in the LcS beverage period was significantly higher than in the placebo period. The stool smell and feeling of completion of voiding improved significantly in the LcS beverage period compared with the placebo period, and in those with a stronger tendency to constipation, the stools were significantly softened compared with the state before intake. The number of bifidobacteria and their percentage in the total number of fecal bacteria in the LcS beverage period increased significantly compared with the levels before intake and were significantly higher than the values in the placebo period. No marked change due to the intake of the LcS beverage was observed in the other components of the microflora, the organic acid contents, stool pH, water content, or contents of putrefactive metabolites. These results suggest that intake of the probiotic fermented milk beverage conditions the intestines by improving the state of bowel movements and stool quality and increasing the fecal population level of bifidobacteria.