Characterising the Mechanism of Airway Smooth Muscle β(2) Adrenoceptor Desensitization by Rhinovirus Infected Bronchial Epithelial Cells

Rhinovirus (RV) infections account for approximately two thirds of all virus-induced asthma exacerbations and often result in an impaired response to β(2) agonist therapy. Using an in vitro model of RV infection, we investigated the mechanisms underlying RV-induced β(2) adrenoceptor desensitization in primary human airway smooth muscle cells (ASMC). RV infection of primary human bronchial epithelial cells (HBEC) for 24 hours produced conditioned medium that caused β(2) adrenoceptor desensitization on ASMCs without an effect on ASMCs viability. Less than 3 kDa size fractionation together with trypsin digestion of RV-induced conditioned medium did not prevent β(2) adrenoceptor desensitization, suggesting it could potentially be mediated by a small peptide or lipid. RV infection of BECs, ASMCs and fibroblasts produced prostaglandins, of which PGE(2), PGF(2α) and PGI(2) had the ability to cause β(2) adrenoceptor desensitization on ASMCs. RV-induced conditioned medium from HBECs depleted of PGE(2) did not prevent ASMC β(2) adrenoceptor desensitization; however this medium induced PGE(2) from ASMCs, suggesting that autocrine prostaglandin production may be responsible. Using inhibitors of cyclooxygenase and prostaglandin receptor antagonists, we found that β(2) adrenoceptor desensitization was mediated through ASMC derived COX-2 induced prostaglandins. Since ASMC prostaglandin production is unlikely to be caused by RV-induced epithelial derived proteins or lipids we next investigated activation of toll-like receptors (TLR) by viral RNA. The combination of TLR agonists poly I:C and imiquimod induced PGE(2) and β(2) adrenoceptor desensitization on ASMC as did the RNA extracted from RV-induced conditioned medium. Viral RNA but not epithelial RNA caused β(2) adrenoceptor desensitization confirming that viral RNA and not endogenous human RNA was responsible. It was deduced that the mechanism by which β(2) adrenoceptor desensitization occurs was by pattern recognition receptor activation of COX-2 induced prostaglandins.