Innate Immunity in the Skin: How Keratinocytes Fight Against Pathogens

The human skin is constantly exposed to a large number of external pathogens, due to the daily contact with the environment. The observation that extensive skin infections are relatively rare suggests the presence of an efficient host defense system at the skin surface. While the physical barrier function of the epidermis was formerly believed to play the major role in the protection against infections, the recent description of Toll-like receptors (TLRs) and antimicrobial peptides in the epidermis indicates that keratinocytes play an active role in innate immunity. To date TLR1, TLR2, TLR3, TLR4, TLR5, TLR6 and TLR9 have been described to be expressed in a constitutive or inducible manner in keratinocytes and to mediate the recognition of pathogen-associated molecular patterns (PAMPs). Recognition of PAMPs results in the production of proinflammatory mediators such as cytokines, chemokines and antimicrobial peptides in keratinocytes. Recent findings indicate the importance of these peptides as effector molecules of innate immunity, but also as regulators of acquired immune responses, inflammation and wound repair. This review considers the current findings regarding TLR expression in the epidermis and the role these receptors might serve in host defense. Finally, the clinical relevance of the functions of keratinocytes in the innate immunity will be discussed in this review.