Mucins in the mucosal barrier to infection (original) (raw)

nature publishing group REVIEW MUCINS-AN INTEGRAL PART OF THE MUCOSAL BARRIER Mucosal epithelial tissues have evolved multiple mechanisms of defense in response to their vulnerability to microbial attack due to their exposure to the external environment. The mucosal epithelial cells form a contiguous lining that acts as a barrier between the moist exterior environment and the remainder of the host. In addition, these cells, both constitutively and in response to microbes, together with underlying leukocytes, secrete many defensive compounds into the mucosal fluid, including mucins, antibodies, defensins, protegrins, collectins, cathlecidins, lysozyme, histatins, and nitric oxide. 1-3 Together, these different defensive compounds form a physical barrier and have direct antimicrobial activity, and the ability to opsonize microbes to aid clearance. Mucin glycoproteins, however, can fulfill all of these roles individually. Mucosal pathogens, almost by definition, have evolved mechanisms to subvert these mucosal defensive measures. The first barrier the pathogen encounters is the highly hydrated mucus gel that covers the mucosal surface and protects the epithelial cells against chemical, enzymatic, microbial, and mechanical insult. Mucosal surfaces are coated with a layer of viscous mucus ranging in thickness from 10 m in the eye 4 and trachea 5 to 300 m in the stomach and 700 m in the intestine. 6-8 This mucus layer is not static but moves to clear trapped material. In the gastrointestinal tract, the outer mucus layer is continually removed by movement of the luminal contents, whereas in the