Surface tissues of the body such as the skin and intestinal tract are in direct contact with the external environment and are thus continuously exposed to large numbers of microorganisms. To cope with the substantial microbial exposure, epithelial surfaces produce a diverse arsenal of antimicrobial proteins that directly kill or inhibit the growth of microorganisms. In this Review, we highlight new advances in our understanding of how epithelial antimicrobial proteins protect against pathogens and contribute to microbiota-host homeostasis at the skin and gut mucosae. Further, we discuss recent insights into the regulatory mechanisms that control antimicrobial protein expression. Finally, we consider how impaired antimicrobial protein expression and function can contribute to disease.
"Antimicrobial peptides (AMPs) are evolutionarily conserved peptides found in almost all plants and animals. In mammals, they are expressed by epithelial cells under steady-state conditions and by infiltrating immune cells during inflammation (Gallo and Hooper, 2012). AMPs are cationic polypeptides with the ability to kill microorganisms by disrupting the integrity of their membrane (Zasloff, 2002). "
"These functions, in addition to those mentioned above, also include the activities of the epidermal and follicular pigmentary systems protecting the skin against the damaging effects of solar radiation and other physicochemical stresses  , and by determining skin pigmentation playing a role in social communication and camouflage . The innate and adaptive skin immune systems provide defense against infectious agents (viruses, rickets, bacteria, fungi and parasites), and are also involved in the integration of the response to foreign and self-antigens through interactions with the central immune system, or protect against cancer development   . Adnexal (secretory in nature) structures participate in thermoregulatory and sensory functions, regulation of electrolyte balance, release of substances strengthening the epidermal barrier, as well as being involved in social communication   . "
"The mucosal immune system in fish is in direct contact with the external environment and represents one of the primary barriers against pathogens. In higher vertebrates, intestinal epithelial cells (IECs) function as both physical barrier and immunocompetent cells producing a number of immune related molecules such as cytokines and host defense peptides (Gallo and Hooper, 2012). It is well established that teleost fish IECs produce a number of innate humoral defense factors (Gomez et al., 2013; Lazado and Caipang, 2014) and that they are also capable of overexpressing proinflammatory cytokines like IL-1β and TNF-α following a bacterial infection, suggesting their active contribution to the fish mucosal immune system (Komatsu et al., 2009; Mulder et al., 2007). "
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