DNA binding to TLR9 expressed by red blood cells promotes innate immune activation and anemia.

Red blood cells (RBCs) are essential for aerobic respiration through delivery of oxygen to distant tissues. However, RBCs are currently considered immunologically inert, and few, if any, secondary functions of RBCs have been identified. Here, we showed that RBCs serve as critical immune sensors through surface expression of the nucleic acid­sensing Toll-like receptor 9 (TLR9). Mammalian RBCs expressed TLR9 on their surface and bound CpG-containing DNA derived from bacteria, plasmodia, and mitochondria. RBC-bound mitochondrial DNA was increased during human and murine sepsis and pneumonia. In vivo, CpG-carrying RBCs drove accelerated erythrophagocytosis and innate immune activation characterized by increased interferon signaling. Erythroid-specific deletion of TLR9 abrogated erythrophagocytosis and decreased local and systemic cytokine production during CpG-induced inflammation and polymicrobial sepsis. Thus, detection and capture of nucleic acid by TLR9-expressing RBCs regulated red cell clearance and inflammatory cytokine production, demonstrating that RBCs function as immune sentinels during pathologic states. Consistent with these findings, RBC-bound mitochondrial DNA was elevated in individuals with viral pneumonia and sepsis secondary to coronavirus disease 2019 (COVID-19) and associated with anemia and severity of disease. These findings uncover a previously unappreciated role of RBCs as critical players in inflammation distinct from their function in gas transport.

Chemically synthesized Secoisolariciresinol diglucoside (LGM2605) improves mitochondrial function in cardiac myocytes and alleviates septic cardiomyopathy.

Sepsis is the overwhelming systemic immune response to infection, which can result in multiple organ dysfunction and septic shock. Myocardial dysfunction during sepsis is associated with advanced disease and significantly increased in-hospital mortality. Our group has shown that energetic failure and excess reactive oxygen species (ROS) generation constitute major components of myocardial dysfunction in sepsis. Because ROS production is central to cellular metabolic health, we tested if the synthetic anti-oxidant lignan secoisolariciresinol diglucoside (SDG; LGM2605) would alleviate septic cardiac dysfunction and investigated the underlying mechanism. Using the cecal ligation and puncture (CLP) mouse model of peritonitis-induced sepsis, we observed impairment of cardiac function beginning at 4 h post-CLP surgery. Treatment of mice with LGM2605 (100 mg/kg body weight, i.p.) 6 h post-CLP surgery reduced cardiac ROS accumulation and restored cardiac function. Assessment of mitochondrial respiration (Seahorse XF) in primary cardiomyocytes obtained from adult C57BL/6 mice that had undergone CLP and treatment with LGM2605 showed restored basal and maximal respiration, as well as preserved oxygen consumption rate (OCR) associated with spare capacity. Further analyses aiming to identify the cellular mechanisms that may account for improved cardiac function showed that LGM2605 restored mitochondria abundance, increased mitochondrial calcium uptake and preserved mitochondrial membrane potential. In addition to protecting against cardiac dysfunction, daily treatment with LGM2605 and antibiotic ertapenem (70 mg/kg) protected against CLP-associated mortality and reversed hypothermia when compared against mice receiving ertapenem and saline. Therefore, treatment of septic mice with LGM2605 emerges as a novel pharmacological approach that reduces cardiac ROS accumulation, protects cardiac mitochondrial function, alleviates cardiac dysfunction, and improves survival.

Inhibition of NADPH oxidase 2 (NOX2) prevents sepsis-induced cardiomyopathy by improving calcium handling and mitochondrial function.

Cardiomyopathy frequently complicates sepsis and is associated with increased mortality. Increased cardiac oxidative stress and mitochondrial dysfunction have been observed during sepsis, but the mechanisms responsible for these abnormalities have not been determined. We hypothesized that NADPH oxidase 2 (NOX2) activation could be responsible for sepsis-induced oxidative stress and cardiomyopathy. Treatment of isolated adult mouse cardiomyocytes with low concentrations of the endotoxin lipopolysaccharide (LPS) increased total cellular reactive oxygen species (ROS) and mitochondrial superoxide. Elevated mitochondrial superoxide was accompanied by depolarization of the mitochondrial inner membrane potential, an indication of mitochondrial dysfunction, and mitochondrial calcium overload. NOX2 inhibition decreased LPS-induced superoxide and prevented mitochondrial dysfunction. Further, cardiomyocytes from mice with genetic ablation of NOX2 did not have LPS-induced superoxide or mitochondrial dysfunction. LPS decreased contractility and calcium transient amplitude in isolated cardiomyocytes, and these abnormalities were prevented by inhibition of NOX2. LPS decreased systolic function in mice, measured by echocardiography. NOX2 inhibition was cardioprotective in 2 mouse models of sepsis, preserving systolic function after LPS injection or cecal ligation and puncture (CLP). These data show that inhibition of NOX2 decreases oxidative stress, preserves intracellular calcium handling and mitochondrial function, and alleviates sepsis-induced systolic dysfunction in vivo. Thus, NOX2 is a potential target for pharmacotherapy of sepsis-induced cardiomyopathy.