Klebsiella pneumoniae infection induces an S100A8/A9-mediated autocrine loop in human airway epithelium to amplify inflammation

<i>Klebsiella pneumoniae</i> pneumonia is marked by an excessive inflammatory response that drives lung injury. The initial stages of infection and the mechanisms driving hyper-inflammation at the airway epithelial barrier are not fully defined. The alarmins S100A8/A9 are potent inflammatory mediators, and studies have indicated their importance in the host response to <i>K. pneumoniae</i>. Here, we show that infection with live <i>K. pneumoniae</i> directly induces the transcription and secretion of the S100A8/A9 heterodimer in a primary human bronchial epithelial (HBE) cell model. Furthermore, the infection sensitizes epithelial cells by upregulating the expression of Toll-like receptor 4, a key receptor that mediates S100A8/A9 signaling. We also established that HBE cells are highly responsive to extracellular S100A8/A9, which stimulates pro-inflammatory cytokine production. Critically, silencing of endogenous S100A9 expression in HBE cells significantly attenuated NF-κB activation and the overall cytokine response to bacterial infection, providing direct evidence for an epithelium-intrinsic autocrine amplification loop. Functionally, disruption of this loop markedly reduced the ability of epithelial cell supernatants to induce neutrophil chemotaxis. These findings reveal a novel autocrine mechanism that positions the airway epithelium not just as a responder, but as an active initiator and amplifier of local inflammation during early <i>K. pneumoniae</i> infection.