Antimicrobial peptides as a major part of the innate immune defense at the ocular surface

The ocular surface is in constant contact with the environment (e.g. when using one's fingers to insert a contact lens) and thus also with diverse bacteria, bacterial components and their pathogen associated molecules. Dysfunctions of the tear film structure or decreased moistening of the ocular surface, as in dry eye (keratoconjunctivitis sicca) for example, often lead to inflammatory and infectious complications resulting in severe functional disorders, particularly concerning the cornea. Besides different protective antimicrobial substances in the tear fluid (mucins, lysozyme, lactoferrin), the epithelia of cornea and conjunctiva can also protect themselves from microbial invasion by producing an arsenal of antimicrobial peptides (AMPs). A number of different studies have revealed that small cationic AMPs, which display antimicrobial activity against a broad spectrum of microorganisms, are a major component of the innate immune system at the human ocular surface. Furthermore, several AMPs modulate cellular activation processes like migration, proliferation, chemotaxis and cytokine production, and in this way also affect the adaptive immune system. In this article, we have summarized current knowledge of the mechanisms of activity and functional roles of AMPs, with a focus on potential multifunctional roles of human beta-defensins and S100 peptide psoriasin (S100A7) at the ocular surface.