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Neuroinflammation and oxidative stress contribute to the progression of sepsis-associated encephalopathy (SAE). Angiotensin-converting enzyme 2 (ACE2) is considered to be a neuroprotective factor due to its anti-inflammatory and antioxidant properties. However, the role of ACE2 on myeloid cells in regulating SAE and the underlying mechanism warrants further exploration. SAE was induced in ACE2 transgenic (TG), knockout (KO), and bone marrow (BM) chimeric mice by cecal ligation and puncture (CLP). The expression levels of apoptosis-, oxidation- and neuroinflammation-associated mediators and morphological changes were monitored by quantitative real-time PCR analyses and histological examinations in the cortex of septic mice. The contents of angiotensin (Ang) II and Ang-(1–7) along with the activity of ACE2 were examined with commercial kits. The expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and Sestrin2 was detected by immunoblotting analysis. Our results indicated that the expression of cortical ACE2 was significantly reduced in the early phase of CLP-induced sepsis. Moreover, ACE2 overexpression in TG mice conferred neuroprotection against sepsis, as evidenced by alleviated neuronal apoptosis, oxidative stress, and proinflammatory M1-like microglial polarization, accompanied by upregulation of the Ang-(1–7), Nrf2, and Sestrin2 protein levels. Conversely, ACE2 deficiency in KO mice exacerbated SAE. The neuroprotective effects of ACE2 were further confirmed in wild-type mice transplanted with ACE2-TG and KO BM cells. Therefore, our data suggest that myeloid ACE2 exerts a protective role in the pathogenesis of SAE, potentially by activating Ang-(1–7)-Nrf2/sestrin2 signaling pathway, and highlight that upregulating ACE2 expression and activity may represent a promising approach for the treatment of SAE in patients with sepsis.

Sepsis-associated encephalopathy (SAE) is a neurological dysfunction induced by sepsis, with symptoms ranging from mild delirium to deep coma. About 70% of patients with severe systemic infection develop SAE and with more than half of surviving patients suffering from long-term cognitive deficits, which seriously damaged the quality of their daily life and brought a heavy burden to society. The pathogenesis of SAE is multifactorial, including activated inflammation, blood- brain barrier (BBB) disruption, cerebral blood flow impairment, and neurotransmitter disturbances. Microglia mediate multiple SAE pathologies. In this review, we summarized the most recent findings in the roles of microglia in every stage of SAE pathogenesis, focusing on the molecular pathways in microglia activation and downstream effects. We also demonstrated the novel therapeutic studies targeting microglia in SAE. Deep insight into the role of microglia in SAE is of great importance in exploring pathogenesis and developing effective remedies of SAE.

Sepsis-associated encephalopathy (SAE) is a severe neurological syndrome marked by widespread brain dysfunctions due to sepsis. Despite increasing data supporting the hypothesis of neuronal damage, the exact mechanism of sepsis-related cognitive disorders and therapeutic strategies remain unclear and need further investigation. In this study, a sepsis model was established in C57 mice using lipopolysaccharide (LPS). The findings demonstrated that LPS exposure induced neuronal loss, synaptic and cognitive deficits accompanied by mitochondrial damage. Bioinformatics and western blot analyses demonstrated a significant increase in Lipocalin-2 (LCN2) during sepsis as a key hub gene involved in immune and neurological inflammation. Interestingly, the recombinant LCN2 protein exhibited similar effects on synaptic dysfunction and cognitive deficits in C57 mice. Conversely, downregulating LCN2 effectively nullified the impact of LPS, leading to the amelioration of synaptic and cognitive deficits, neuronal loss, and reactive oxygen species (ROS)-associated mitochondrial damage. These findings suggest a novel etiopathogenic mechanism of SAE, which is initiated by the increased LCN2, leading to neuronal loss and cognitive deficit. Inhibition of LCN2 could be therapeutically beneficial in treating sepsis-induced synaptic and cognitive impairments.

Key Points Sepsis is a life-threatening organ dysfunction that is caused by a dysregulated host response to infection. The sepsis-associated host response is characterized by concurrent excessive inflammatory, catabolic, metabolic and immune-suppressive features, and a failure to return to homeostasis, which often results in a condition referred to as chronic critical illness and is not fundamentally different from the sustained host response aberrations that are induced by severe non-infectious injuries. Sepsis is a very heterogeneous syndrome, and current knowledge does not enable the stratification of patients into more homogeneous subgroups in which specific and potentially targetable host response derailments drive pathology. Key pro-inflammatory responses during sepsis include the activation of the complement system, the coagulation system, the vascular endothelium, neutrophils and platelets, whereas immune suppression is primarily caused by the reprogramming of antigen-presenting cells, and the apoptosis and exhaustion of lymphocytes. Individuals who survive sepsis frequently suffer from long-term cognitive and physical impairments, the aetiology of which is uncertain. Strategies to modulate the aberrant host response have been unsuccessful in a large number of clinical trials, which may at least in part be related to the inadequate selection of therapeutic targets and an inability to select the patients who might benefit from a certain intervention. Future research should focus the discovery and validation of biomarkers that reflect the predominant pathophysiological mechanisms at different body sites, and that can guide the selection of patients for targeted therapies and the monitoring thereof. Sepsis — which is caused by a dysregulated host response to infection — is a life-threatening organ dysfunction. This Review describes the recent advances in our understanding of sepsis pathogenesis and discusses strategies for the development of successful therapies. Sepsis is defined as a life-threatening organ dysfunction that is caused by a dysregulated host response to infection. In sepsis, the immune response that is initiated by an invading pathogen fails to return to homeostasis, thus culminating in a pathological syndrome that is characterized by sustained excessive inflammation and immune suppression. Our understanding of the key mechanisms involved in the pathogenesis of sepsis has increased tremendously, yet this still needs to be translated into novel targeted therapeutic strategies. Pivotal for the clinical development of new sepsis therapies is the selection of patients on the basis of biomarkers and/or functional defects that provide specific insights into the expression or activity of the therapeutic target.

Sepsis is a leading cause of death in critical illness, and its pathophysiology varies depending on preexisting medical conditions. Here we identified nonalcoholic fatty liver disease (NAFLD) as an independent risk factor for sepsis in a large clinical cohort and showed a link between mortality in NAFLD-associated sepsis and hepatic mitochondrial and energetic metabolism dysfunction. Using in vivo and in vitro models of liver lipid overload, we discovered a metabolic coordination between hepatocyte mitochondria and liver macrophages that express triggering receptor expressed on myeloid cells-2 (TREM2). Trem2-deficient macrophages released exosomes that impaired hepatocytic mitochondrial structure and energy supply because of their high content of miR-106b-5p, which blocks Mitofusin 2 (Mfn2). In a mouse model of NAFLD-associated sepsis, TREM2 deficiency accelerated the initial progression of NAFLD and subsequent susceptibility to sepsis. Conversely, overexpression of TREM2 in liver macrophages improved hepatic energy supply and sepsis outcome. This study demonstrates that NAFLD is a risk factor for sepsis, providing a basis for precision treatment, and identifies hepatocyte-macrophage metabolic coordination and TREM2 as potential targets for future clinical trials.

The blood–brain barrier (BBB) protects the central nervous system from infections or harmful substances 1 ; its impairment can lead to or exacerbate various diseases of the central nervous system 2 – 4 . However, the mechanisms of BBB disruption during infection and inflammatory conditions 5 , 6 remain poorly defined. Here we find that activation of the pore-forming protein GSDMD by the cytosolic lipopolysaccharide (LPS) sensor caspase-11 (refs. 7 – 9 ), but not by TLR4-induced cytokines, mediates BBB breakdown in response to circulating LPS or during LPS-induced sepsis. Mice deficient in the LBP–CD14 LPS transfer and internalization pathway 10 – 12 resist BBB disruption. Single-cell RNA-sequencing analysis reveals that brain endothelial cells (bECs), which express high levels of GSDMD, have a prominent response to circulating LPS. LPS acting on bECs primes Casp11 and Cd14 expression and induces GSDMD-mediated plasma membrane permeabilization and pyroptosis in vitro and in mice. Electron microscopy shows that this features ultrastructural changes in the disrupted BBB, including pyroptotic endothelia, abnormal appearance of tight junctions and vasculature detachment from the basement membrane. Comprehensive mouse genetic analyses, combined with a bEC-targeting adeno-associated virus system, establish that GSDMD activation in bECs underlies BBB disruption by LPS. Delivery of active GSDMD into bECs bypasses LPS stimulation and opens the BBB. In CASP4 -humanized mice, Gram-negative Klebsiella pneumoniae infection disrupts the BBB; this is blocked by expression of a GSDMD-neutralizing nanobody in bECs. Our findings outline a mechanism for inflammatory BBB breakdown, and suggest potential therapies for diseases of the central nervous system associated with BBB impairment. Lipopolysaccharide-induced breakdown of the blood–brain barrier requires activation of GSDMD-mediated plasma membrane permeabilization and pyroptosis in brain endothelial cells.

Mitochondria have emerged as key actors of innate and adaptive immunity. Mitophagy has a pivotal role in cell homeostasis, but its contribution to macrophage functions and host defense remains to be delineated. Here, we showed that lipopolysaccharide (LPS) in combination with IFN-γ inhibited PINK1-dependent mitophagy in macrophages through a STAT1-dependent activation of the inflammatory caspases 1 and 11. In addition, we demonstrated that the inhibition of mitophagy triggered classical macrophage activation in a mitochondrial ROS-dependent manner. In a murine model of polymicrobial infection (cecal ligature and puncture), adoptive transfer of Pink1-deficient bone marrow or pharmacological inhibition of mitophagy promoted macrophage activation, which favored bactericidal clearance and led to a better survival rate. Reciprocally, mitochondrial uncouplers that promote mitophagy reversed LPS/IFN-γ-mediated activation of macrophages and led to immunoparalysis with impaired bacterial clearance and lowered survival. In critically ill patients, we showed that mitophagy was inhibited in blood monocytes of patients with sepsis as compared with nonseptic patients. Overall, this work demonstrates that the inhibition of mitophagy is a physiological mechanism that contributes to the activation of myeloid cells and improves the outcome of sepsis.

Sepsis is a life-threatening organ dysfunction syndrome caused by dysregulated host response to infection that leads to uncontrolled inflammatory response followed by immunosuppression. However, despite the high mortality rate, no specific treatment modality or drugs with high efficacy is available for sepsis to date. Although improved treatment strategies have increased the survival rate during the initial state of excessive inflammatory response, recent trends in sepsis show that mortality occurs at a period of continuous immunosuppressive state in which patients succumb to secondary infections within a few weeks or months due to post-sepsis “immune paralysis.” Immune cell alteration induced by uncontrolled apoptosis has been considered a major cause of significant immunosuppression. Particularly, apoptosis of lymphocytes, including innate immune cells and adaptive immune cells, is associated with a higher risk of secondary infections and poor outcomes. Multiple postmortem studies have confirmed that sepsis-induced immune cell apoptosis occurs in all age groups, including neonates, pediatric, and adult patients, and it is considered to be a primary contributing factor to the immunosuppressive pathophysiology of sepsis. Therapeutic perspectives targeting apoptosis through various strategies could improve survival in sepsis. In this review article, we will focus on describing the major apoptosis process of immune cells with respect to physiologic and molecular mechanisms. Further, advances in apoptosis-targeted treatment modalities for sepsis will also be discussed.

Background. Sepsis-associated encephalopathy (SAE) is a transient and reversible brain dysfunction, that occurs when the source of sepsis is located outside of the central nervous system; SAE affects nearly 30% of septic patients at admission and is a risk factor for mortality. In our study, we sought to determine whether metabolite changes in plasma could be a potential biomarker for the early diagnosis and/or the prediction of the prognosis of sepsis. Method. A total of 31 SAE patients and 28 healthy controls matched by age, gender, and body mass index (BMI) participated in our study. SAE patients were divided into four groups according to the Glasgow Coma Score (GCS). Plasma samples were collected and used to detect metabolism changes by gas chromatography-mass spectrometry (GC-MS). Analysis of variance was used to determine which metabolites significantly differed between the control and SAE groups. Results. We identified a total of 63 metabolites that showed significant differences among the SAE and control groups. In particular, the 4 common metabolites in the four groups were 4-hydroxyphenylacetic acid; carbostyril, 3-ethyl-4,7-dimethoxy (35.8%); malic acid peak 1; and oxalic acid. The concentration of 4-hydroxyphenylacetic acid in sepsis patients decreased with a decrease of the GCS. Conclusions. According to recent research on SAE, metabolic disturbances in tissue and cells may be the main pathophysiology of this condition. In our study, we found a correlation between the concentration of 4-hydroxyphenylacetic acid and the severity of consciousness disorders. We suggest that 4-hydroxyphenylacetic acid may be a potential biomarker for SAE and useful in predicting patient prognosis.

Sepsis is the third leading cause of death worldwide and the main cause of mortality in hospitals 1 – 3 , but the best treatment strategy remains uncertain. In particular, evidence suggests that current practices in the administration of intravenous fluids and vasopressors are suboptimal and likely induce harm in a proportion of patients 1 , 4 – 6 . To tackle this sequential decision-making problem, we developed a reinforcement learning agent, the Artificial Intelligence (AI) Clinician, which extracted implicit knowledge from an amount of patient data that exceeds by many-fold the life-time experience of human clinicians and learned optimal treatment by analyzing a myriad of (mostly suboptimal) treatment decisions. We demonstrate that the value of the AI Clinician’s selected treatment is on average reliably higher than human clinicians. In a large validation cohort independent of the training data, mortality was lowest in patients for whom clinicians’ actual doses matched the AI decisions. Our model provides individualized and clinically interpretable treatment decisions for sepsis that could improve patient outcomes. A reinforcement learning agent, the AI Clinician, can assist physicians by providing individualized and clinically interpretable treatment decisions to improve patient outcomes.