Immunomodulatory Roles of Probiotics: From Intestinal Barrier Regulation to Clinical Applications

The mammalian gut is a highly dynamic physiological ecosystem that harbors an enormous number of commensal bacteria that play an essential role in the development of various cells of the immune system. The gut bacteria, both resident and diet-derived, share a habitat with immune cells located within the lamina propria layer of the intestinal villi. The intestinal epithelial cells do not come into direct contact with the gut microbiota, but microbial metabolites play a significant role in their maturation and appropriate function. According to the World Health Organization, probiotics are "live microorganisms which, when administered in adequate amounts, confer a health benefit on the host." The microbiota found within the human gut has been evolutionarily selected based on their suitability to survive within the GT. Lactic acid bacteria, belonging to Pediococcus, Lactobacilli, Streptococci, Bifidobacteria, Enterococcus, and yeast cells like Saccharomyces boulardii, are found to be predominantly used as a source of probiotics. However, these properties are strain-specific and not a general feature of all bacteria. To maintain immunological homeostasis and control both the innate and adaptive immune responses, immune cells, nutrients, and commensal microbes continuously interact in dynamic processes in a stable environment. This interaction allows the host to enhance immunity through improved immunological responses to diseases, including inflammatory and infectious conditions. Among specific benefits are infection prevention, alleviation of symptoms associated with Helicobacter pylori suppression, irritable bowel syndrome management, cancer prevention, reduction of gut inflammation, and allergy, diabetes, and drug-resistant pathogen prevention. This review focuses on compiling key findings up to date and discussing their implications in the interaction between probiotics and immune cells towards immune function.