Central Role of the NF-κB Pathway in the
Scgb1a1
-Expressing Epithelium in Mediating Respiratory Syncytial Virus-Induced Airway Inflammation
Authors: Tian Bing , Yang Jun , Zhao Yingxin , Ivanciuc Teodora , Sun Hong , Wakamiya Maki , Garofalo Roberto P. , Brasier Allan R. 
Published: March 2018

Lower respiratory tract infection with respiratory syncytial virus (RSV) produces profound inflammation. Despite an understanding of the role of adaptive immunity in RSV infection, the identity of the major sentinel cells initially triggering inflammation is controversial. Here we evaluate the role of nonciliated secretoglobin (
Scgb1a1
)-expressing bronchiolar epithelial cells in RSV infection. Mice expressing a tamoxifen (TMX)-inducible Cre recombinase-estrogen receptor fusion protein (CreERTM) knocked into the
Scgb1a1
locus were crossed with mice that harbor a
RelA
conditional allele (

RelA
fl

), with loxP sites flanking exons 5 to 8 of the Rel homology domain. The
Scgb1a1
CreERTM/+
× RelA
fl/fl
mouse is a
RelA
conditional knockout (RelA
CKO
) of a nonciliated epithelial cell population enriched in the small bronchioles. TMX-treated RelA
CKO
mice have reduced pulmonary neutrophilic infiltration and impaired expression and secretion of NF-κB-dependent cytokines in response to RSV. In addition, RelA
CKO
mice had reduced expression levels of interferon (IFN) regulatory factor 1/7 (IRF1/7) and retinoic acid-inducible gene I (RIG-I), components of the mucosal IFN positive-feedback loop. We demonstrate that RSV replication induces RelA to complex with bromodomain-containing protein 4 (BRD4), a cofactor required for RNA polymerase II (Pol II) phosphorylation, activating the atypical histone acetyltransferase (HAT) activity of BRD4 required for phospho-Ser2 Pol II formation, histone H3K122 acetylation, and cytokine secretion
in vitro
and
in vivo
. TMX-treated RelA
CKO
mice have less weight loss and reduced airway obstruction/hyperreactivity yet similar levels of IFN-γ production despite higher levels of virus production. These data indicate that the nonciliated
Scgb1a1
-expressing epithelium is a major innate sensor for restricting RSV infection by mediating neutrophilic inflammation and chemokine and mucosal IFN production via the RelA-BRD4 pathway.

IMPORTANCE
RSV infection is the most common cause of infant hospitalizations in the United States, resulting in 2.1 million children annually requiring medical attention. RSV primarily infects nasal epithelial cells, spreading distally to produce severe lower respiratory tract infections. Our study examines the role of a nonciliated respiratory epithelial cell population in RSV infection. We genetically engineered a mouse that can be selectively depleted of the NF-κB/RelA transcription factor in this subset of epithelial cells. These mice show an impaired activation of the bromodomain-containing protein 4 (BRD4) coactivator, resulting in reduced cytokine expression and neutrophilic inflammation. During the course of RSV infection, epithelial RelA-depleted mice have reduced disease scores and airway hyperreactivity yet increased levels of virus replication. We conclude that RelA-BRD4 signaling in nonciliated bronchiolar epithelial cells mediates neutrophilic airway inflammation and disease severity. This complex is an attractive target to reduce the severity of infection.