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. 2017 Jul 28;8:838. doi: 10.3389/fimmu.2017.00838

Figure 2.

Figure 2

Interaction between diet, microbiota, and immune response at mucosal sites. (A) To keep a healthy state, the local microbiota and mucosal immune system are in homeostasis at mucosal sites. The microbiota educates and promotes the maturation of the immune system by induction of pro-inflammatory and anti-inflammatory immune cells, e.g., Th17 (SFB), T regulatory cells (Clostridia spp.), and Th1 (Bacteroides fragilis). Moreover, the immune system surveys microbial activities (e.g., antigen sampling at the mucosal barrier) and responds in a controlled fashion by producing, e.g., antimicrobial peptides, sIgA to prevent tissue damage. The integrity of the mucosal barrier is sustained by bacteria-produced metabolites (e.g., SCFA) such as butyrate resulting in high expression of tight-junction proteins and mucus production, thereby restricting interaction of microbes to the lumen and luminal epitheliums. The diet is involved in all processes, serving the microbiome with fermentable fibers and the immune system and epithelium with essential nutrients, e.g., vitamins and minerals. (B) During pathological conditions, such as inflammatory bowel disease and asthma, the homeostasis at the mucosal barrier is disrupted. A westernized diet, i.e., high in SFA, high ω-6/ω-3 ratio, high sucrose and iron (oral iron supplements), and low in fiber promotes inflammation and growth of pathogenic/pathobiont (disease causing) bacteria in the gut. The microbiota, which is rich in non-beneficial bacteria, favorably induces the maturation of pro-inflammatory immune cells, leading to uncontrolled inflammation resulting in tissue damage of the mucosal compartment. The damaged mucosa and shifted immune response fail to control the microbiota, which exaggerates the pathophysiological state. Under certain conditions, bacteria-derived LPS enters the systemic circulation and further stimulates the immune system toward a pro-inflammatory state. Abbreviations: LPS, lipopolysaccharide; SCFAs, short-chain fatty acids; SFAs, saturated fatty acids; SFB, segmented filamentous bacteria; sIgA, secretory immunoglobulin A; ω-6/ω-3, omega-6/omega-3 fatty acid ratio; Th, T helper.