MRP8/14 is a Protective Mediator in Murine Klebsiella (K.) Pneumoniae Induced Pneumonia

Abstract

BACKGROUND: K. pneumoniae is a common cause of pneumonia-derived sepsis. MRP8 and MRP14 are the most abundant cytoplasmic proteins in neutrophils. They can form MRP8/14 heterodimers that are released during invasive infectious diseases. MRP8/14 reportedly exerts antimicrobial activity and enhances the pro-inflammatory response via Toll-like receptor (TLR) 4. Our objective was to determine the role of MRP8/14 in the host defense during murine Klebsiella pneumonia. METHODS: We intranasally challenged Wildtype (WT) and MRP14 knockout (KO) mice with K. pneumoniae. Mice were sacrificed 6, 24 and 48 hours post-infection for the determination of bacterial loads, MRP8/14 and cytokines in blood and organs or monitored in a survival study. Whole blood obtained from WT, MRP14 KO and TLR4 KO mice were stimulated with growth-arrested Klebsiella to determine the role of MRP8/14 on the cytokine response in vitro. Growth inhibitory effects of MRP8/14 were assessed by incubation of Klebsiella with recombinant MRP8/14 in the presence or absence of zinc. In addition we determined the contribution of MRP8/14 on antimicrobial activity in neutrophil extracellular traps (NETs) induced in WT and MRP14 KO neutrophils. RESULTS: Klebsiella infection resulted in local and systemic MRP8/14 release. MRP14 KO mice were more susceptible to K. pneumoniae, as reflected by a worsened survival and increased bacterial outgrowth in blood and organs. Cytokine measurements in WT and MRP14 KO displayed minor differences. Whole blood stimulation showed a minimal contribution of MRP8/14 on the TLR4 mediated Klebsiella cytokine response. Recombinant MRP8/14 strongly reduced Klebsiella growth, which was reversed by the addition of zinc. Accordingly, NETs induced in MRP14 KO neutrophils showed a reduced capacity to inhibit Klebsiella growth compared to WT NETs CONCLUSION: MRP8/14 contributes to a protective innate defense in Klebsiella pneumonia. Our data indicate that MRP8/14 exerts direct effects on Klebsiella trapped in NETs by chelation of zinc required for bacterial growth.