High-molecular-weight kininogen is a critical component of host defense against Escherichia coli sepsis.

High-molecular-weight kininogen (HK) is known to bind lipopolysaccharides (LPS) with high affinity and serves as a crucial LPS carrier in circulation, supporting endotoxemia. However, its role in host defense against Gram-negative bacterial infection remains unclear. Here we demonstrate that HK directly binds to Escherichia coli (E. coli) via LPS and rapidly localizes to sites of infection. HK-deficient mice (Kng1-/-) showed increased susceptibility to infection, with increased bacterial dissemination, lung injury, and proinflammatory cytokine production. In contrast, endogenous expression of human HK in Kng1-/- mice restored survival, limited bacterial spread, and reduced tissue damage. Mechanistically, HK promoted neutrophil antimicrobial responses by enhancing reactive oxygen species production and microbicidal activity. Consistently, liver-specific HK deficiency recapitulated the impaired bacterial clearance and reduced survival upon E. coli challenge, highlighting the importance of plasma HK. Together, these findings identify HK as a new soluble pattern recognition molecule that senses E. coli invasion and initiates neutrophil-mediated antimicrobial responses, revealing a previously unrecognized protective function of the contact system in innate immunity.