Cell‐free Hemoglobin‐Oxidized LDL Axis Contributes to Microvascular Endothelial Barrier Dysfunction and Poor Outcomes During Sepsis

Abstract

IntroductionSepsis, a dysregulated host response to infection with high morbidity and mortality, is characterized by a systemic inflammatory response and widespread vascular hyperpermeability leading to edema, organ dysfunction, and death with no specific treatments. Disruption of the microvascular endothelial barrier is a critical pathological feature of sepsis‐induced organ dysfunction driven by circulating inflammatory mediators, oxidants, and proteolytic enzymes. Cell‐free hemoglobin (CFH) is one such factor, released from damaged red blood cells during sepsis due to their increased deformability and fragility. Though CFH is not detectable in healthy subjects, plasma CFH is elevated in patients with sepsis and independently associated with mortality. CFH contributes to endothelial dysfunction, but the signaling mechanisms are incompletely understood. CFH can oxidize circulating low‐density lipoproteins (oxLDL), which could potentiate endothelial barrier disruption. We hypothesized that elevated levels of CFH during sepsis are associated with increased oxLDL and contribute to microvascular endothelial barrier dysfunction through the oxLDL receptor, lectin‐like oxidized LDL receptor 1 (LOX‐1).MethodsCirculating levels of CFH and oxLDL were measured in sepsis patients. In human lung microvascular endothelial cells (HLMVECs), transendothelial electrical resistance (TER), a measure of barrier dysfunction, was assessed by Electric Cell‐substrate Impedance Sensing (ECIS).Results (Figure)In sepsis patients, plasma oxLDL levels correlated with CFH (r=0.686; p=0.016, n=12) and were higher in patients who died in hospital (33.24 U/L [IQR 28.08, 48.28] vs. 27.19 U/L [IQR 16.24, 38.24]; p=0.045, n=25). HLMVEC barrier dysfunction induced by CFH (p<0.0001, n=6‐7) or oxLDL (p=0.002, n=4‐6) was attenuated by blocking the LOX‐1 receptor with small molecule inhibitor BI‐0115 (Boehringer Ingelheim, 20 µM; p=0.018 vs CFH, n=5; p=0.0002 vs oxLDL, n=6).ConclusionIncreased plasma CFH and oxLDL are associated with HLMVEC barrier dysfunction and poor clinical outcomes during sepsis; one mechanism by which CFH may cause vascular hyperpermeability during sepsis is through oxidation of LDL which can drive signaling through the LOX‐1 receptor.