Oxidative epithelial host defense is regulated by infectious and inflammatory stimuli

Epithelia express oxidative antimicrobial protection that uses lactoperoxidase (LPO), hydrogen peroxide (H₂O₂), and thiocyanate to generate the reactive hypothiocyanite. Duox1 and Duox2, found in epithelia, are hypothesized to provide H₂O₂ for use by LPO. To investigate the regulation of oxidative LPO-mediated host defense by bacterial and inflammatory stimuli, LPO and Duox mRNA were followed in differentiated primary human airway epithelial cells challenged with Pseudomonas aeruginosa flagellin or IFN-γ. Flagellin upregulated Duox2 mRNA 20-fold, but upregulated LPO mRNA only 2.5-fold. IFN-γ increased Duox2 mRNA 127-fold and upregulated LPO mRNA 10-fold. DuoxA2, needed for Duox2 activity, was also upregulated by flagellin and IFN-γ. Both stimuli increased H₂O₂ synthesis and LPO-dependent killing of P. aeruginosa. Reduction of Duox1 by siRNA showed little effect on basal H₂O₂ production, whereas Duox2 siRNA markedly reduced basal H₂O₂ production and resulted in an 8-fold increase in Nox4 mRNA. In conclusion, large increases in Duox2-mediated H₂O₂ production seem to be coordinated with increases in LPO mRNA and, without increased LPO, H₂O₂ levels in airway secretion are expected to increase substantially. The data suggest that Duox2 is the major contributor to basal H₂O₂ synthesis despite the presence of greater amounts of Duox1.