Emerging role of RIG-I-like receptors in antiviral innate immunity in airway structural cells

Abstract: Respiratory viral infections, especially involving rhinovirus, are main triggers of exacerbations of asthma and COPD. These severe conditions are a great burden to both the individual and society and currently lack effective treatments. Hence, a clear need exists for research into the cellular and molecular mechanisms by which viral infections evoke exacerbations of airway disease in order to better design effective therapeutics in the future. The aim of this thesis work has been to explore the immune response of airway structural cells to rhinoviral infection. To this end, we have employed in vitro cultures of primary human bronchial epithelial cells (BECs) and bronchial smooth muscle cells (BSMCs) from healthy subjects and patients with COPD and asthma. In general, these cells were infected with rhinovirus (RV) or stimulated with agonists for TLR3 and RIG-I-like receptors, and expression and/or release of specific genes and proteins were analysed by RT-qPCR, Western Blot and ELISA. In summary, our results show that BECs from severe COPD patients overproduce an upstream Th2-driving cytokine, TSLP, in response to RV infection and the RV replication intermediate dsRNA in a TLR3-dependent manner. Exaggerated production of the proinflammatory mediators TNF-α and CXCL8 was also observed in viral-challenged COPD-BECs and involved activation of both endosomal TLR3 and cytosolic RIG-I-like receptors. We further provide novel data that BSMCs from healthy individuals express the RIG-I-like receptors, RIG-I and MDA5. RV infection and activation of these innate immune sensors as well as TLR3 also lead to production of the antiviral cytokines IFN-β and IFN-λ1, suggesting a role for BSMCs in antiviral host defence. Finally, we have explored expression and regulation of another Th2-promoting cytokine, IL-33, in BSMCs from both healthy and asthmatic subjects. Our results demonstrate that IL-33 is upregulated by soluble mediators released from cultured BECs, and that dsRNA stimulation enhances this response, especially in asthmatic BSMCs. We further show that BSMCs also produce IL-33 in response to RV infection and activation of TLR3 and RIG-I-like receptors, and that ATP, via purinergic receptor signalling, is potentially involved as a mediator of IL-33 induction in BSMCs upon challenge with both epithelial-derived mediators and viral stimuli. In conclusion, our findings have provided new insights into regulatory functions of viral-challenged airway epithelial and smooth muscle cells in relation to airway inflammation and antiviral innate immunity. Intrinsic differences in health and disease have been identified, encouraging future studies of involved mechanisms that may serve as targets for novel therapeutic interventions in viral-induced exacerbations of asthma and COPD.