Nasal epithelial cells : innate immunity and inflammation

Abstract

The surface epithelium that lines the nasal passages is often the first tissue in the airway to encounter inhaled pathogens. It collaborates closely with the innate immune system, a subsystem of the immune system that defends the host from infection by organisms, mainly by initiating a local inflammatory reaction. Pattern-recognition receptors (PRRs) are important in pathogen recognition, cell activation and regulation of immune responses and include Toll-like receptors (TLRs), nucleotide-binding oligomerization domain-like receptors (NLRs) and retinoic acid-inducible gene 1 (RIG-I)-like receptors (RLRs). The transforming growth-factor beta (TGF-β) superfamily and their type I receptors, the activin receptor-like kinases (ALKs), are important mediators that promote remodelling and have recently also been shown to regulate airway inflammation. Even though PRRs and ALKs are essential in preventing disease, disruption of these systems is generally believed to be involved in the pathogenesis of airway inflammatory diseases such as asthma and chronic rhinosinusitis with nasal polyp (CRSwNP). Hence, the overall aim with this thesis was to investigate the role of PRRs and ALKs in airway inflammation.

Human airway smooth muscle cells (HASMCs) are essential for the regulation of airflow; importantly, they are also involved in the shortness of breath that characterises microbialinduced exacerbations of asthma. The present thesis showed that stimulation of TLR2, TLR3, TLR4, TLR7 and NOD1 on HASMCs resulted in cytokine release, upregulation of inflammatory cell surface markers and downregulation of receptors involved in smooth muscle cell contraction. The nasal epithelium was found to express TLR3, TLR7, TLR9, RIG-I and MDA-5 and stimulation resulted in an increased inflammatory response characterised by the release of chemokines and cytokines. In addition, a specific role for TLR9 was found in patients with CRSwNP that might be linked to polyp growth via downregulation of VEGFR expression and lowered release of inflammatory cytokines. Virus-related ligand stimulation of TLR7 induced a rapid release of the neuropeptide, substance P (SP), from human nasal epithelial cells (HNECs) and sensory neurons. The released SP promptly upregulated the epithelial TLR expression. This suggests a role for SP in rapid priming of the innate immune system during viral infections. Polyp epithelial cells from patients with CRSwNP expressed high levels of ALK1-6. Polyp epithelial cells stimulated with ALK-ligands demonstrated a potential anti-inflammatory role for ALKs in polyps. Previous reports have demonstrated low levels of ALK-ligands in patients with CRSwNP, suggesting that ALKs could contribute to uncontrolled inflammation promoting the progression of CRSwNP. BMP4, an ALK-ligand, suppressed inflammation and hyperplasia in the turbinate tissue of patients with CRSwNP. This effect was absent in the corresponding polyp, suggesting that BMP4-ALK3 interaction might be involved in polyp growth in patients.

In summary, this thesis demonstrates a role for specific epithelial PRRs and ALKs in CRSwNP and for smooth muscle PRRs in asthma. In addition, it proposes a novel role for substance P in kick starting the innate immune system by upregulating PRRs in response to microbial stimulation. These findings could generate new potential targets for the treatment of inflammatory airway diseases.