Roles of lung epithelium in neutrophil recruitment during pneumococcal pneumonia.

Epithelial cells line the respiratory tract and interface with the external world. Epithelial cells contribute to pulmonary inflammation, but specific epithelial roles have proven difficult to define. To discover unique epithelial activities that influence immunity during infection, we generated mice with nuclear factor-κB RelA mutated throughout all epithelial cells of the lung and coupled this approach with epithelial cell isolation from infected and uninfected lungs for cell-specific analyses of gene induction. The RelA mutant mice appeared normal basally, but in response to pneumococcus in the lungs they were unable to rapidly recruit neutrophils to the air spaces. Epithelial cells expressed multiple neutrophil-stimulating cytokines during pneumonia, all of which depended on RelA. Cytokine expression by nonepithelial cells was unaltered by the epithelial mutation of RelA. Epithelial cells were the predominant sources of CXCL5 and granulocyte-macrophage colony-stimulating factor (GM-CSF), whereas nonepithelial cells were major sources for other neutrophil-activating cytokines. Epithelial RelA mutation decreased whole lung levels of CXCL5 and GM-CSF during pneumococcal pneumonia, whereas lung levels of other neutrophil-recruiting factors were unaffected. Defective neutrophil recruitment in epithelial mutant mice could be rescued by administration of CXCL5 or GM-CSF. These results reveal a specialized immune function for the pulmonary epithelium, the induction of CXCL5 and GM-CSF, to accelerate neutrophil recruitment in the infected lung.

Roles of STAT3 in protein secretion pathways during the acute-phase response.

The acute-phase response is characteristic of perhaps all infections, including bacterial pneumonia. In conjunction with the acute-phase response, additional biological pathways are induced in the liver and are dependent on the transcription factors STAT3 and NF-κB, but these responses are poorly understood. Here, we demonstrate that pneumococcal pneumonia and other severe infections increase expression of multiple components of the cellular secretory machinery in the mouse liver, including the endoplasmic reticulum (ER) translocon complex, which mediates protein translation into the ER, and the coat protein complexes (COPI and COPII), which mediate vesicular transport of proteins to and from the ER. Hepatocyte-specific mutation of STAT3 prevented the induction of these secretory pathways during pneumonia, with similar results observed following pharmacological activation of ER stress by using tunicamycin. These findings implicate STAT3 in the unfolded protein response and suggest that STAT3-dependent optimization of secretion may apply broadly. Pneumonia also stimulated the binding of phosphorylated STAT3 to promoter regions of secretion-related genes in the liver, supporting a direct role for STAT3 in their transcription. Altogether, these results identify a novel function of STAT3 during the acute-phase response, namely, the induction of secretory machinery in hepatocytes. This may facilitate the processing and delivery of newly synthesized loads of acute-phase proteins, enhancing innate immunity and preventing liver injury during infection.