Infections with rhinovirus (RV), a single-stranded Picornavirus , are the most frequent cause of upper respiratory tract infections (“common colds”). RV also causes exacerbations of chronic lower airways diseases such as asthma, a major source of morbidity and health care expenditure. Little is known about how RV causes exacerbations of lower respiratory tract disease. This thesis examines mechanisms underlying the innate immune response to RV infection, focusing on the pattern recognition receptors required for interferon (IFN) responses. Viral double-stranded RNA produced during replication may be recognized by either Toll-like receptor (TLR)-3, retinoic acid-inducible gene (RIG)-I or melanoma-differentiation-associated gene (MDA)-5.
We examined the requirements of RIG-I, MDA5 and IFN response factor (IRF)-3 for RV-induced gene expression in human bronchial epithelial cells. RV-1B induced IRF3 phosphorylation and dimerization, as well as mRNA expression of IFNs and IFN-stimulated genes. siRNA against IRF3, MDA5 and TRIF, but not RIG-I, decreased RV1B-induced gene expression and IRF3 dimerization. These results suggest that, in cultured epithelial cells, TLR3 and MDA5, but not RIG-I, are required for maximal sensing of RV dsRNA, and that TLR3 and MDA5 signal through IRF3.
The overall airway response to viral infection represents an integration of antiviral and pro-inflammatory pathways. Also, other cells besides airway epithelial cells may contribute to the RV immune response. Therefore, we examined the responses of TLR3- and MDA5-deficient mice to RV1B infection. TLR3-/- mice showed normal IFN responses and viral clearance. MDA5-/- mice showed delayed type I and attenuated type III IFN responses, leading to a transient defect in viral clearance. RV-infected TLR3-/- and MDA5-/- mice showed reduced airway inflammation and responsiveness. Finally, RV-infected MDA5-/- mice with allergic airways disease showed enhanced viral clearance despite deficient IFN responses, as well as decreased inflammatory and contractile responses.
Together, these results suggest that MDA5 is the major receptor required for sensing RV dsRNA and initiating innate immune responses. Also, TLR3 and MDA5 initiate pro-inflammatory signaling pathways leading to airways inflammation. Thus, in the context of RV infection, TLR3 and MDA5 signaling is maladaptive. Therapies against TLR3 and MDA5 may be useful in the treatment or prevention of RV-induced asthma exacerbations.