Natália Tassi (Langen / DE), Bernhard Kerscher (Langen / DE), Yvonne Pfeifer (Wernigerode / DE), Guido Werner (Wernigerode / DE), Isabelle Bekeredjian-Ding (Langen / DE; Marburg / DE)
Introduction: Multidrug-resistant bacteria such as K. pneumoniae represent important health threats due inefficacy of antibiotics. First line innate immune defense through neutrophils plays a crucial role in the resolution of infections. However, the contribution of the immune system to effective antibiotic therapy remains unappreciated and hard-to-measure.
Goals: Here we hypothesized that neutrophils might enhance the effectiveness of antibiotic treatment against antibiotic-resistant K. pneumoniae.
Methods: For this study, the Robert-Koch-Institut provided clinical isolates of K. pneumoniae. AST was performed and interpreted according to EUCAST criteria. Isolates were selected based on MICs. Neutrophils were isolated from healthy volunteers (ethics approval #2021-253). Phagocytosis was assessed with gentamycin protection assay. IL-8 was measured by ELISA. Expression of CD62L, CD66b and CD63 was analyzed by flow cytometry. Serum resistance was assessed in 50% normal human serum.
Results: Neutrophils failed to kill clinical isolates of K. pneumoniae. However, their presence synergistically increased the effect of meropenem and cefotaxime on bacterial killing. This effect was specific for cell wall-active antibiotics, e.g. 3rd generation cephalosporins and carbapenems and was not obtained with tigecycline. It was further dependent on the strain and on the respective MIC. Of note, the synergy of neutrophils and antibiotics enables killing of bacteria tested as intermediate and resistant. It did not correlate with susceptibility to complement-mediated killing nor expression of activation markers on neutrophils. Moreover, most strains were not phagocytosed, suggesting an extracellular mechanism underlying the observed synergy.
Summary: Although K. pneumoniae was resistant to neutrophil killing, our data suggest that these cells can play a role in control of the infection and bacterial killing in the presence of cell wall-targeting antibiotics. Further experiments will elucidate the cellular events supporting K. pneumoniae killing.
Funding: European Commission Innovative Training Network (ITN) Training towards Innovative Personalized Antibiotic Therapy (TIPAT).