Lung epithelial cells: therapeutically inducible effectors of antimicrobial defense

Abstract

Lung epithelial cells are increasingly recognized to be active effectors of microbial defense, contributing to both innate and adaptive immune function in the lower respiratory tract. As immune sentinels, lung epithelial cells detect diverse pathogens through an ample repertoire of membrane-bound, endosomal, and cytosolic pattern-recognition receptors (PRRs). The highly plastic epithelial barrier responds to detected threats via modulation of paracellular flux, intercellular communications, mucin production, and periciliary fluid composition. Epithelial PRR stimulation also induces production of cytokines that recruit and sculpt leukocyte-mediated responses, and promotes epithelial generation of antimicrobial effector molecules that are directly microbicidal. The epithelium can alternately enhance tolerance to pathogens, preventing tissue damage through PRR-induced inhibitory signals, opsonization of pathogen-associated molecular patterns, and attenuation of injurious leukocyte responses. The inducibility of these protective responses has prompted attempts to therapeutically harness epithelial defense mechanisms to protect against pneumonias. Recent reports describe successful strategies for manipulation of epithelial defenses to protect against a wide range of respiratory pathogens. The lung epithelium is capable of both significant antimicrobial responses that reduce pathogen burdens and tolerance mechanisms that attenuate immunopathology. This manuscript reviews inducible lung epithelial defense mechanisms that offer opportunities for therapeutic manipulation to protect vulnerable populations against pneumonia.

Figure 1. Inducible antimicrobial resistance mechanisms of lung epithelial cells

(A) Cells contributing to inducible epithelial airspace defense. (B) Inducible responses in the conducting airways. Pattern recognition and cytokine receptors detect local danger signals in the conducting airways, responding with enhanced barrier and mucociliary functions to improve pathogen exclusion, increased production of microbicidal antimicrobial peptides and volatile species, and secretion of mediators of leukocyte recruitment and activation. (C) Inducible responses in the alveolar compartment. Epithelial cells in the gas exchange units of the lungs detect pathogen associated molecular patterns, perceive stress signals and communicate with lung resident leukocytes, and respond through inducible modulation of barrier function, enhanced production of antimicrobial peptides, collectins and volatile species, and secretion of leukocyte-active cytokines. TLR, Toll-like receptor; NTHi, non-typeable Haemophillus influenza; LTA, lipotechtoic acid; DLP, diacylated lipopeptides; ODN, oligodeoxynucleotide; ROS, reactive oxygen species; TEER, transepithelial electrical resistance; HβD, human β-defensin; SP, surfactant protein.