Antimicrobial-resistant (AMR) bacteria, such asKlebsiellaspecies, are an increasingly common cause of hospital-acquired pneumonia, resulting in high mortality and morbidity. Harnessing the host immune response to AMR bacterial infection using mesenchymal stem cells (MSCs) is a promising approach to bypass bacterial AMR mechanisms. The administration of single doses of naïve MSCs to ARDS clinical trial patient cohorts has been shown to be safe, although efficacy is unclear. The study tested whether repeated MSC dosing and/or preactivation, would attenuate AMRKlebsiella pneumonia-induced established pneumonia. Rat models of establishedK. pneumoniae-induced pneumonia were randomised to receive intravenous naïve or cytomix-preactivated umbilical cord MSCs as a single dose at 24 h post pneumonia induction with or without a subsequent dose at 48 h. Physiological indices, bronchoalveolar lavage (BAL), and tissues were obtained at 72 h post pneumonia induction. A single dose of naïve MSCs was largely ineffective, whereas two doses of MSCs were effective in attenuatingKlebsiellapneumosepsis, improving lung compliance and oxygenation, while reducing bacteria and injury in the lung. Cytomix-preactivated MSCs were superior to naïve MSCs. BAL neutrophil counts and activation were reduced, and apoptosis increased. MSC therapy reduced cytotoxic BAL T cells, and increased CD4+/CD8+ratios. Systemically, granulocytes, classical monocytes, and the CD4+/CD8+ratio were reduced, and nonclassical monocytes were increased. Repeated doses of MSCs—particularly preactivated MSCs—enhance their therapeutic potential in a clinically relevant model of established AMRK. pneumoniae-induced pneumosepsis.