Inducible translational regulation of the NF-IL6 transcription factor by respiratory syncytial virus infection in pulmonary epithelial cells.

Respiratory syncytial virus (RSV), the most common etiologic agent of epidemic pediatric respiratory disease, infects and replicates in the human airway epithelium, resulting in the induction of cellular gene products essential for immune and inflammatory responses. We describe the effect of RSV infection on nuclear factor-IL6 (NF-IL6) expression, a human basic domain-leucine zipper-containing transcription factor that alone in combination with other inducible transcription factors regulates the expression of cytokine and adhesion molecule genes. RSV-infected human type II pulmonary alveolar epithelial cells (A549) synthesize a single 45.7-kDa isoform of NF-IL6 rapidly and in a time-dependent manner. NF-IL6 is first detectable after 3 h of infection and continues to accumulate until 48 h (until the cells lose viability). NF-IL6 production could not be induced by UV-inactivated virus, demonstrating the requirement of viral replication for NF-IL6 synthesis. Immunoprecipitation after [35S]methionine metabolic labeling was done to investigate the mechanism for NF-IL6 production. There was robust NF-IL6 protein synthesis within RSV-infected (24 h) cells. Protein synthesis occurred without detectable changes in the abundance or size of the single 1.8-kb NF-IL6 mRNA. RNase protection assay of transfected chloramphenicol acetyltransferase reporter genes driven by either wild-type or mutated NF-IL6 binding sites show a virus-induced increase in NF-IL6-dependent transcription. These studies have demonstrated a novel inducible mechanism for translational control of NF-IL6 synthesis and identify this transcription factor as a potential effector of the host response to RSV infection.