The beneficial effects of probiotics have been demonstrated in many diseases. One of the major mechanisms of probiotic action is through the regulation of host immune response. This review highlights the recent scientific research findings that advance our understanding of probiotic regulation of the host immune response with potential application for disease prevention and treatment.
Probiotic genomic and proteomic studies have identified several genes and specific compounds derived from probiotics, which mediate immunoregulatory effects. Studies regarding the biological consequences of probiotics in host immunity suggested that they regulate the functions of systemic and mucosal immune cells and intestinal epithelial cells. Thus, probiotics showed therapeutic potential for diseases, including several immune response-related diseases, such as allergy, eczema, viral infection, and potentiating vaccination responses.
Probiotics may provide novel approaches for both disease prevention and treatment. However, the results of clinical studies regarding probiotic application are preliminary and require further confirmation.
The immune response is initiated by innate immunity following exposure to foreign substances or tissue injury. Innate immunity exerts protective roles in host homeostasis in part by priming adaptive immune responses against persisting insults and inducing inflammation. However, the unbalanced immune response leads to severe inflammation and uncontrolled tissue damage and disease. Sensing of the intestinal microbiota by the host mucosal immune system plays significant roles in maintaining intestinal homeostasis and inducing systemic protective responses. Thus, manipulation of the intestinal microbiota is a potential alternative approach for maintaining health and preventing and/or treating diseases. Probiotics were defined as ‘live microorganisms which, when consumed in adequate amounts as part of food, confer a health benefit on the host’. Lactobacillus, Bifidobacterium, and Saccharomyces are three extensively studied and commonly used probiotics in humans and animals.
Several beneficial effects of probiotics on the host intestinal mucosal defenses system have been identified. These include blocking pathogenic bacterial effects by producing bacteriocidal substances and competing with pathogens and toxins for adherence to the intestinal epithelium. For intestinal epithelial homeostasis, probiotics promote intestinal epithelial cell survival, enhance barrier function, and stimulate protective responses from intestinal epithelial cells. Most importantly, modulation of the immune system is one of the most plausible mechanisms underlying the beneficial effects of probiotics on human health. Probiotics have been found to enhance the innate immunity and modulate pathogen-induced inflammation via toll-like receptor-regulated signaling pathways .
The purpose of this review is to address the most recent findings regarding probiotic regulation of immune health (published after January 2010). Clinical applications and mechanisms of action are highlighted, which include probiotic genes and probiotic-derived factors involved in the regulation of host immunity, molecular targets of probiotic action responsible for the host immune responses, and roles and mechanisms of probiotics in vaccination and prevention and treatment of diseases, such as allergy, eczema, and viral infections.
Host immune responses regulated by probiotics
Probiotics play a role in defining and maintaining the delicate balance between necessary and excessive defense mechanisms including innate and adaptive immune responses. Points of interaction with the immune regulation for probiotics include bacteria direct interaction with intestinal epithelial cells, or following internalization by M cells through interaction with dendritic cells and follicle-associated epithelial cells, initiating responses mediated by macrophages and T and B lymphocytes. Regulation of gene expression and signaling pathways in the host cells are two major mechanisms underlying probiotic action leading to immunomodulation.
Probiotics for immune disease prevention and treatment
Results of evidence-based analysis from human studies and animal models suggest that probiotics have potential for clinical effectiveness on intestinal diseases, including infectious diarrhea, antibiotic-associated diarrhea, atopic diseases, necrotizing enterocolitis, ulcerative colitis, and irritable bowel syndrome, and extraintestinal diseases, such as allergy.
Current evidence shows promise for further developing health benefits and the efficacy of probiotics and probiotic-derived factors on the regulation of host homeostasis, including immune health. However, as probiotic research goes into the next stage, several questions have emerged that are to be answered to elucidate the mechanisms of probiotic action and to better apply probiotics for clinical uses. For example, what host factors need to be considered when designing studies and evaluating results? In addition, identifying biomarkers for evaluation of therapies, including probiotics in hosts is an emergent topic for translational and clinical research. As shown in human studies, there is person-to-person variation in gene expression patterns upon probiotic administration [10••]. Given the potential demand for personalized medicine, future clinical trial study populations may be selected or characterized based on their baseline individual microflora and their individual genetic pattern responses to probiotic introduction. Thus, adding an intriguing and unexpected dimension to probiotic application in human disease prevention and treatment.