Scrutinizing Mechanisms of the ‘Obesity Paradox in Sepsis’: Obesity Is Accompanied by Diminished Formation of Neutrophil Extracellular Traps (NETs) Due to Restricted Neutrophil–Platelet Interactions

Abstract

Systemic inflammation is a detrimental condition associated with high mortality. However, obese individuals seem to have higher chances of surviving sepsis. To elucidate what immunological differences exist between obese and lean individuals we studied the course of endotoxemia in mice fed high-fat diet (HFD) and ob/ob animals. Intravital microscopy revealed that neutrophil extracellular trap (NET) formation in liver vasculature is negligible in obese mice in sharp contrast to their lean counterparts (ND). Unlike in lean individuals, neutrophil influx is not driven by leptin or interleukin 33 (IL-33), nor occurs via a chemokine receptor CXCR2. In obese mice less platelets interact with neutrophils forming less aggregates. Platelets transfer from ND to HFD mice partially restores NET formation, and even further so upon P-selectin blockage on them. The study reveals that in obesity the overexaggerated inflammation and NET formation are limited during sepsis due to dysfunctional platelets suggesting their targeting as a therapeutic tool in systemic inflammation.

Metabolic Pathways Involved in Formation of Spontaneous and Lipopolysaccharide-Induced Neutrophil Extracellular Traps (NETs) Differ in Obesity and Systemic Inflammation

“…This pattern was altered when neutrophils were isolated from mice subjected to sepsis induced by LPS (endotoxemia). One should keep in mind that during systemic inflammation neutrophils become highly activated, extensively phagocytize and cast NETs as we recently showed by following 24-h kinetics of NET release with IVM [ 14 ]. During endotoxemia, HFD neutrophils released less NETs in vivo in the vasculature than ND cells and this was not because of any intrinsic malfunctioning but impaired neutrophil-platelet interactions as revealed for the 6-h time mark [ 14 ].…”

“…One should keep in mind that during systemic inflammation neutrophils become highly activated, extensively phagocytize and cast NETs as we recently showed by following 24-h kinetics of NET release with IVM [ 14 ]. During endotoxemia, HFD neutrophils released less NETs in vivo in the vasculature than ND cells and this was not because of any intrinsic malfunctioning but impaired neutrophil-platelet interactions as revealed for the 6-h time mark [ 14 ]. However, we show here that when we isolated neutrophils from septic mice and kept them ex vivo for 1 h, HFD cells were releasing more NETs spontaneously.…”

“…Such exhausted neutrophils show diminished response to following activating stimuli either because they have already secreted their stored granules and NETs [ 65 ] or because they become refractory to the immunologic stimulation [ 64 ]. We hypothesize that this phenotype was observed only in the case of HFD neutrophils because they are isolated from the restraining in vivo milieu by which we understand, among others, the lack of neutrophil/platelet interactions during endotoxemia [ 14 ]; thus, ex vivo, they start to release NETs spontaneously. Correspondingly, stronger release of spontaneous NETs was also observed in the case of human neutrophils isolated from T2D patients [ 32 ].…”

“…NETs, apart from phagocytosis and degranulation, represent the third weapon utilized by neutrophils but they are two-faced; on the one hand they do catch, immobilize, and (frequently) kill trapped pathogens, but when continually released, overproduced, or not removed efficiently, they cause bystander damage to the endothelium and other tissues [ 13 ]. Previously, we studied NET formation in vivo in the vasculature of obese mice, both healthy and with systemic inflammation (endotoxemia), and in either model of obesity—high-fat diet (HFD) or genetic (ob/ob mice)—we observed weaker NET release at the 24-h mark [ 14 ]. This phenomenon turned out to be linked to neutrophil-platelet interactions that timewise proceed NET release.…”

“…This phenomenon turned out to be linked to neutrophil-platelet interactions that timewise proceed NET release. Isolated neutrophils of HFD mice casted similar quantities of NETs than those isolated from lean animals (normal diet, ND) 6 h after lipopolysaccharide (LPS) stimulation [ 14 ]. To understand metabolic causes behind these phenomena, herein we aimed at comparing metabolic pathways leading to NET release by neutrophils of HFD and ND mice.…”