Innate immunity in the respiratory epithelium

Abstract

The airway epithelium represents the first point of contact for inhaled foreign organisms. The protective arsenal of the airway epithelium is provided in the form of physical barriers and a vast array of receptors and antimicrobial compounds that constitute the innate immune system. Many of the known innate immune receptors, including the Toll-like receptors and nucleotide oligomerization domain-like receptors, are expressed by the airway epithelium, which leads to the production of proinflammatory cytokines and chemokines that affect microorganisms directly and recruit immune cells, such as neutrophils and T cells, to the site of infection. The airway epithelium also produces a number of resident antimicrobial proteins, such as lysozyme, lactoferrin, and mucins, as well as a swathe of cationic proteins. Dysregulation of the airway epithelial innate immune system is associated with a number of medical conditions that can result in compromised immunity and chronic inflammation of the lung. This review focuses on the innate immune capabilities of the airway epithelium and its role in protecting the lung from infection as well as the outcomes when its function is compromised.

Figure 1.

Innate immunity in the respiratory epithelium. Shown is an airway epithelial cell with the innate molecules discussed in this review and their ligands and surface receptors (Toll-like receptor (TLR)1, -2, -4, -5, -6; TNF receptor [TNFR]; and epidermal growth factor receptor (EGFR), endosomal receptors (TLR3, -4, -7, -8, and -9), cytosolic receptors (retinoic acid inducible gene [RIG]-I, melanoma differentiation–associated protein [MDA]5, nucleotide oligomerization domain [NOD]1, NOD2, IL1-β–converting enzyme protease activating factor [IPAF], and NOD-like receptor pyrin domain [NLRP3]) and antimicrobial proteins.