Elevated Spleen Tyrosine Kinase in Low-Density Neutrophils During Bacterial Sepsis in a Nonhuman Primate Model

BACKGROUND: Sepsis is a leading cause of death worldwide. Identifying novel host-directed therapeutic targets may improve sepsis outcomes. METHODS: Six nonhuman primates were infected with Klebsiella pneumoniae to induce septic shock and provided supportive care for up to 72 hours. Flow cytometry was used to characterize whole-blood neutrophils (WBNs) and low-density neutrophils (LDNs) at time 0 (T0), T6, T24, and T48 hours postinfection, and postmortem examination (ie, necropsy). Dimensional reduction with clustering via FlowSOM and traditional gating strategies were used to compare WBNs to LDNs and delineate spleen tyrosine kinase (SYK) expression across neutrophils subsets. We measured soluble biomarkers of end-organ dysfunction and neutrophil activation, and quantified SYK and myeloperoxidase in tissue. RESULTS: At T6, we identified populations of active immature WBNs and a population of LDNs not detected at baseline. At T24, neutrophil heterogeneity increased across WBNs and LDNs with differential expression of myeloperoxidase (MPO). Compared to WBNs, LDNs were more activated with increased MPO expression. At T6, SYK expression surged in WBNs and LDNs and SYK+ WBNs and LDNs expressed higher levels of MPO and lactoferrin compared to SYK- neutrophils. Circulating levels of SYK+ LDNs significantly correlated with serum creatinine levels, indicative of acute kidney injury; prolonged prothrombin time and decreased fibrinogen, indicative of consumptive coagulopathy; and SYK expression in tissues. CONCLUSIONS: Bacterial sepsis leads to heterogenous populations of circulating neutrophils, including LDNs. Elevated SYK expression in WBNs and LDNs correlates with end-organ dysfunction, highlighting SYK as a potential therapeutic target in bacterial sepsis.