Viral-induced cytotoxicity was determined by measuring adenylate kinase activity in apical washes using a commercially available assay (Lonza, Inc.). Apical samples were centrifuged prior to freezing to remove any cellular contaminants present in the wash. Luminescence detected in samples from infected HAE were normalized to uninfected HAE and expressed as fold change over AK measured in uninfected (mock) HAE. Morphological assessment of cytotoxicity in HAE was performed with paraformaldehyde (PFA, 4%)-fixed histological sections (5 mm) stained with hematoxylin and eosin.