Is IκBζ constitutively expressed in mammalian airway epithelium

IκBζ is a transcriptional factor induced primarily in immune cells upon Toll-like receptor (TLR) activation that drives important cytokine responses. Recent studies have demonstrated constitutive IκBζ expression in the epithelial cells of mouse skin and eyes, possibly reflecting the activation of TLRs by pathogen-associated molecular patterns (PAMPs). In this context, another mucosal surface, the lung epithelium, may not be as actively exposed to the external environment as the skin and the conjunctiva, especially since the lower lung airways are typically conceived to be sterile. Whether IκBζ expression in the lungs is constitutive or induced remains largely unexplored. This is especially important since IκBζ has been shown to promote the expression of protective cytokine and antimicrobial peptide responses, supporting a role for IκBζ in lung host defense. We hence evaluated IκBζ expression in airway epithelia of both humans and mice using immunostaining with antiserum raised against recombinant IκBζ in our laboratory. We observed positive signal in the nuclei of ciliated epithelial cells lining the central airways. Airway cells of gnotobiotic mice also stained positive, suggesting that IκBζ expression does not require induction by bacterial PAMPs. Unexpectedly, we also observed staining in the lung epithelia of IκBζ knockout mice, indicating possible false positive signals from our immunohistochemistry experiments. In this context, 2D gel analysis followed by mass spectrometry revealed that our IκBζ antiserum also detected a nuclear protein lamin B1. Nevertheless, immunoblotting tissue homogenates from gnotobiotic mouse lungs and primary human airway epithelial cells showed the appropriate 86 kDa band for IκBζ. Together, these results demonstrate constitutive IκBζ expression in airway epithelium, suggesting that lung epithelial cells may depend upon IκBζ expression for airway protection.