Monoclonal antibodies protect against pandrug-resistant Klebsiella pneumoniae

: The 'silent pandemic' caused by antimicrobial resistance requires innovative therapeutic approaches. Human monoclonal antibodies (mAbs), which are among the most transformative and safe drugs in oncology1 and autoimmunity2, are rarely used for infectious diseases and not yet used for antimicrobial resistance3. Here we applied an antigen-agnostic strategy to isolate extremely potent human mAbs against Klebsiella pneumoniae sequence type 147 (ST147), a hypervirulent and pandrug-resistant lineage that is spreading globally. Isolated mAbs target the KL64 capsule and the O-antigen. However, although mAbs displayed bactericidal activity in the picomolar range in vitro, only the capsule-specific mAbs were protective against fulminant bloodstream infection by ST147 and two geographically and genetically distant carbapenem-resistant KL64-bearing K. pneumoniae. Protection observed in vivo correlated with in vitro bacterial uptake by macrophages and enchained bacterial growth. Our study thus describes a mAb that protects against pandrug-resistant K. pneumoniae and provides a strategy to isolate mAbs and identify mAbs that confer protection against bacteria with antimicrobial resistance.