Capsule-mediated O-antigen masking protects hypervirulent Klebsiella pneumoniae from neutrophil killing

Klebsiella pneumoniae is listed as a critical priority pathogen by the WHO due to prevalent extended-spectrum β-lactamase and carbapenem resistance and high mortality rates Hypervirulent K. pneumoniae (hvKp) causes life-threatening infections in immunocompetent individuals, while classical K. pneumoniae (cKp) causes nosocomial infections in immunosuppressed patients. The convergence of hypervirulence with antibiotic resistance is now a major global health threat. Using a murine pulmonary infection model, we show that hvKp resisted clearance despite significant neutrophil recruitment. Using clinical hvKp and cKp isolates, bone marrow derived neutrophils, cell biology, pharmacology and imaging (both light and electron microscopy) in ex vivo assays revealed that the hvKp hypermucoviscous capsule prevents bacterial recognition and neutrophil activation, blocking ROS production, degranulation, and neutrophil extracellular traps (NETs) formation. In the absence of capsule (Δ wcaJ ) or hypermucoid capsule determinants (Δ rmpADC ), hvKp activates neutrophils, leading to degranulation, NET formation and bacterial killing. While superimposing the absence of O-antigen (Δ rfb ) on the Δ wc aJ abrogates degranulation, neutrophils still elaborate NETs that eliminate the Δ wcaJ /Δ rfb mutant at high efficiency. This suggests that O-antigen is a double-edged sword, functioning as a PAMP triggering neutrophil degranulation yet assisting the pathogen to evade NET-mediated destruction. These findings shed light on the mechanism underpinning immune evasion by the hypervirulent capsule in hvKp.