Serine proteases in immune protection of the small intestine

The gastrointestinal tract is subject to a huge antigenic load, which is especially significant in the intestinal lumen. Being the connecting link between the organism and the external environment, the small intestine fulfils not only digestive and transport functions, but also protective ones and acts as a selective barrier for the flow of nutrients. This review considers proteases of the protective system of small intestine cells, their biochemical properties and activation mechanisms, and involvement in biochemical processes responsible for normal functioning and defense reactions of the intestine. Serine proteases of intestinal immunity are multifunctional enzymes making proteolytic attack aimed to immediately exterminate aggressive elements of the intestinal contents (allergens, toxins), to activate (inactivate) zymogens, receptors, and peptide hormones, and to hydrolyze protein precursors and other biologically active factors. Proteases of intestinal immunity control the inflammatory response, proliferation of B-lymphocytes, apoptosis, and secretory and contractive activity of the intestine; they release neurogenic factors, inactivate biologically active substances, and are involved in degradation of the intercellular matrix and in tissue remodeling.