Immune regulation of epithelial cell function: Implications for GI pathologies

The mammalian immune system is a complex and dynamic network that recognizes, responds, and adapts to numerous foreign and self-molecules. CD4+ T cells orchestrate adaptive immune responses and upon stimulation by antigen, naïve CD4+ T cells proliferate and differentiate into various T cell subsets including T helper (Th) 1, Th2, and Th17 effector cells, and T regulatory cells (Treg). Each of the T cell subsets is characterized by distinct profiles of cytokines and carries out distinct and sometimes opposing activities. Initiated by IL-12 released from dendritic cells, the development of Th1 cells is the typical host response against the invasion of intracellular pathogens such as bacteria or viruses. Th1 cells deliver cell-mediated immunity through their secreted cytokines such as IFN-γ, TNF-α, IL-1β, IL-2, and IL-12, and are responsible for the clearance of intracellular pathogens. Th2 cells are initiated by IL-4 and develop in response to allergens or the invasion of extracellular pathogens. Th2 cytokines include IL-4, IL-5, and IL-13, and are particularly important for allergic responses and the clearance of parasites.