The interplay between the gut immune system and microbiota in health and disease: nutraceutical intervention for restoring intestinal homeostasis.

Gut immune system is daily exposed to a plethora of antigens contained in the environment as well as in food. Both secondary lymphoid tissue, such as Peyer's patches, and lymphoid follicles (tertiary lymphoid tissue) are able to respond to antigenic stimuli releasing cytokines or producing antibodies (secretory IgA). Intestinal epithelial cells are in close cooperation with intraepithelial lymphocytes and possess Toll-like receptors on their surface and Nod-like receptors (NLRs) which sense pathogens or pathogen-associated molecular patterns. Intestinal microbiota, mainly composed of Bacteroidetes and Firmicutes, generates tolerogenic response acting on gut dendritic cells and inhibiting the T helper (h)-17 cell anti-inflammatory pathway. This is the case of Bacteroides fragilis which leads to the production of interleukin-10, an anti-inflammatory cytokine, from both T regulatory cells and lamina propria macrophages. Conversely, segmented filamentous bacteria rather induce Th17 cells, thus promoting intestinal inflammation. Intestinal microbiota and its toxic components have been shown to act on both Nod1 and Nod2 receptors and their defective signaling accounts for the development of inflammatory bowel disease (IBD). In IBD a loss of normal tolerance to intestinal microbiota seems to be the main trigger of mucosal damage. In addition, intestinal microbiota thanks to its regulatory function of gut immune response can prevent or retard neoplastic growth. In fact, chronic exposure to environmental microorganisms seems to be associated with low frequency of cancer risk. Major nutraceuticals or functional foods employed in the modulation of intestinal microbiota are represented by prebiotics, probiotics, polyunsaturated fatty acids, amino acids and polyphenols. The cellular and molecular effects performed by these natural products in terms of modulation of the intestinal microbiota and mostly attenuation of the inflammatory pathway are described.