The intestinal epithelial cell surface represents the largest exposed surface of the body that must be protected by the immune system against toxic substance and pathogenic bacteria. All intestinal epithelial cells are usually capable of regulating the immune response through different mechanisms, one of which is the secretion of anti-inflammatory cytokines. Throughout the present study, we have focused on the role of IL-10 in regulating epithelial cell function. IL-10 is a potent inhibitor of pro-inflammatory cytokine production, and has been shown to inhibit production of IL-6 and IL-1β in macrophages [18
]. Supporting evidence for a role for IL-10 in inflammation is derived from studies in mice deficient in IL-10 or harboring mutated IL-10, which are a model of enterocolitis [20
]. These IL-10−/−
mice under normal conditions show increased inflammatory responses and develop inflammatory bowel disease. Moreover, these IL-10−/−
mice are extremely susceptible to infection-induced immunopathology [21
]. All these data suggest that endogenous IL-10 synthesis plays an important role in vivo in down-regulating immune responses and preventing host immunopathology. Moreover, beneficial effects in colitis patients have been obtained via probiotic bacteria-induced IL-10 production [22
In our current study, C. butyricum stimulates elevated levels of IL-10 in HT-29 cells. Because this probiotic strain is frequently used in the management of allergic diseases or gastroenteritis, it is hypothesized that it promotes mucosal tolerance mediated through IL-10. Therefore, we further assessed the role of IL-10 in probiotic-mediated immune modulation by neutralizing or knocking down IL-10 in HT-29 cells. It was found that disruption of IL-10 enhanced effects of C. butyricum-induced NF-κB activation and IL-8 secretion. The results demonstrate that C. butyricum modifies the mucosal immune response to modulate the levels of specific molecules such as cytokines by increasing IL-10 levels and consequently decreasing inflammatory cytokines.
The viability of cells is dependent on cytokines. However, high-dose cytokines can induce apoptosis and necrosis. Bacteria and their metabolites can induce an anti-proliferative effect through induction of apoptosis [23
]. In the current study, disruption of IL-10 enhanced C. butyricum
-induced IL-8 secretion. We further assessed whether this probiotic strain induced apoptosis and necrosis of HT-29 cells due to a lack of effect of IL-10. The results showed that the number of abnormal cells significantly increased compared to the control, indicating that disruption of IL-10 caused a loss of suppression of the mucosal immune response and even excessive apoptosis and necrosis. This study confirmed that C. butyricum
exerts anti-inflammatory effects and enhances tolerance to bacteria through increasing IL-10 production. Moreover, some subsets of Crohn’s disease patients are actually deficient in the production of IL-10, and therefore in this subset this probiotic therapy may be of limited use.
The adaptive co-evolution of humans and bacteria has resulted in the establishment of commensal relationships where neither partner is disadvantaged, or symbiotic relationships where both partners benefit [26
]. In our current study, intestinal epithelial cells can secrete IL-10 to down-regulate inflammatory cascades through suppressing the secretion of pro-inflammatory cytokines. On the other hand, C. butyricum
can drive the secretion of IL-10 to enhance tolerance to bacteria. Such mechanisms allow the host to recognize symbiotic bacteria without eliciting a deleterious immune response, and enable the symbiotic bacteria to reside in the gut, thus providing unique metabolic traits or other benefits. This pathway may be part of an evolutionarily primitive form of adaptive immunity.