Myeloid-Related Protein-14 Contributes to Protective Immunity in Gram-Negative Pneumonia Derived Sepsis

Klebsiella (K.) pneumoniae is a common cause of pneumonia-derived sepsis. Myeloid related protein 8 (MRP8, S100A8) and MRP14 (S100A9) are the most abundant cytoplasmic proteins in neutrophils. They can form MRP8/14 heterodimers that are released upon cell stress stimuli. MRP8/14 reportedly exerts antimicrobial activity, but in acute fulminant sepsis models MRP8/14 has been found to contribute to organ damage and death. We here determined the role of MRP8/14 in K. pneumoniae sepsis originating from the lungs, using an established model characterized by gradual growth of bacteria with subsequent dissemination. Infection resulted in gradually increasing MRP8/14 levels in lungs and plasma. Mrp14 deficient (mrp14−/−) mice, unable to form MRP8/14 heterodimers, showed enhanced bacterial dissemination accompanied by increased organ damage and a reduced survival. Mrp14−/− macrophages were reduced in their capacity to phagocytose Klebsiella. In addition, recombinant MRP8/14 heterodimers, but not MRP8 or MRP14 alone, prevented growth of Klebsiella in vitro through chelation of divalent cations. Neutrophil extracellular traps (NETs) prepared from wildtype but not from mrp14−/− neutrophils inhibited Klebsiella growth; in accordance, the capacity of human NETs to kill Klebsiella was strongly impaired by an anti-MRP14 antibody or the addition of zinc. These results identify MRP8/14 as key player in protective innate immunity during Klebsiella pneumonia.肺炎克雷伯菌（Klebsiella pneumoniae）是引发肺炎相关性脓毒症的常见病原菌。髓系相关蛋白8（MRP8，S100A8）与MRP14（S100A9）是中性粒细胞中丰度最高的胞质蛋白，二者可形成MRP8/14异二聚体，并在细胞应激刺激下被释放至胞外。既往研究表明MRP8/14具有抗菌活性，但在急性暴发性脓毒症模型中，MRP8/14却可加剧器官损伤并升高机体死亡率。本研究针对肺部起源的肺炎克雷伯菌脓毒症，明确了MRP8/14的作用，采用的模型以细菌逐步增殖并随后发生播散为特征。实验结果显示，感染后小鼠肺部与血浆中的MRP8/14水平呈渐进式升高。无法形成MRP8/14异二聚体的Mrp14基因敲除（mrp14−/−）小鼠，其细菌播散能力增强，同时伴随器官损伤加重与存活率下降。Mrp14−/−巨噬细胞吞噬肺炎克雷伯菌的能力显著受损。此外，重组MRP8/14异二聚体（而非单独的MRP8或MRP14）可通过螯合二价阳离子在体外抑制肺炎克雷伯菌的增殖。从野生型小鼠（而非mrp14−/−小鼠）中性粒细胞中分离得到的中性粒细胞胞外陷阱（NETs）可抑制肺炎克雷伯菌生长；与之相符的是，抗MRP14抗体或添加锌离子可显著削弱人源NETs杀灭肺炎克雷伯菌的能力。上述结果证实，MRP8/14是肺炎克雷伯菌肺炎病程中保护性固有免疫的关键介导因子。