Acute Inflammation Induces Lactate Release By Bone Marrow Neutrophils That Promotes Their Mobilization Via Endothelial GPR81 Signaling

Innate immune neutrophils provide the first line of host defense against bacterial infections. Neutrophils under steady state rely almost entirely on glycolysis and exhibit very low levels of oxidative phosphorylation. The metabolite lactate has long been considered a “waste byproduct” of cell metabolism which accumulates during inflammation and sepsis. Increased plasma lactate levels in human patients is used as a marker for sepsis diagnosis. However, the direct effector actions of lactate, particularly in regulating neutrophil mobilization and function during inflammation has remained obscure. To better understand the metabolic consequences of BM neutrophil activation during the onset of inflammation, we tested how bacterial lipopolysaccharides (mimicking gram negative bacterial inflammation) introduced intraperitoneally (i.p.) affect neutrophil metabolism and mobilization. RNAseq of sorted BM neutrophils revealed that LPS-activated neutrophils upregulate enzymes catalyzing the first part of glycolysis (hexokinase and PFKL) and downregulate the expression of TCA cycle enzymatic genes. In addition, LPS enhanced neutrophil lactate production and release as indicated by higher levels of BM lactate and higher expression of LDHA and MCT4. In addition, LPS increased NADPH oxidase (NOX)-mediated reactive oxygen species and HIF-1α levels in BM neutrophils, which are up-stream of glycolytic enzymes and lactate production and release. Recently, we reported that i.p. lactate administration rapidly activated and mobilized neutrophils from BM to the circulation (ASH, 2017). To test if lactate acts preferentially on neutrophils, we also examined other types of hematopoietic cells. Interestingly, we found that lactate specifically and rapidly (i.e., within 4 hrs) mobilized neutrophils to the circulation whereas the levels of peripheral blood (PB) monocytes, lymphocytes, granulocyte monocyte progenitors (GMPs) and hematopoietic progenitor stem cells (LSK) were reduced following lactate administration. LPS treatment failed to mobilize activated ROShigh neutrophils to the PB in NOX-/- mice, while lactate administration partially rescued this defect following LPS treatment. Our data also reveal that the NOX/ROS axis operates upstream of lactate production in BM neutrophils since abnormal metabolic rates were found in NOX-/- neutrophils during the onset of the acute inflammatory responses. Moreover, we found that BM endothelial cells (BMEC) abundantly express the highly sel