Symbiotic commensal bacteria direct maturation of the host immune system

Purpose of review Although commensal bacteria are known to play an important role in the proper maturation of the immune system of their mammalian hosts, the molecular mechanisms underlying this immunomodulation are poorly characterized. The present review summarizes recent findings in the field and describes new knowledge on the interplay of the innate and adaptive arms of the immune response induced by symbiotic bacterial carbohydrate antigens. Recent findings Commensal bacteria in the intestine not only interact directly with dendritic cells but also engage in cross-talk with epithelial cells. These interactions lead to the induction of tolerogenic antigen-presenting cells in the lamina propria and ultimately to the regulation of functional maturation of effector T cells. Upon recognition of capsular polysaccharide antigens of commensal bacteria by dendritic cells (through toll-like receptor 2), innate immune responses facilitate and act in conjunction with adaptive responses to promote optimal Th1 polarization. In contrast, adaptive immunoglobulin A responses to symbiotic bacteria regulate the magnitude of oxidative innate immune responses in the mucosa as well as bacterial epitope expression in the lumen. Summary Accumulating evidence is elucidating surface carbohydrate structures of symbiotic bacteria that drive the modulation of the intestinal immune system, resulting in mature, balanced immune responses and oral tolerance.