A blood atlas of COVID-19 defines hallmarks of disease severity and specificity.

Treatment of severe COVID-19 is currently limited by clinical heterogeneity and incomplete description of specific immune biomarkers. We present here a comprehensive multi-omic blood atlas for patients with varying COVID-19 severity in an integrated comparison with influenza and sepsis patients versus healthy volunteers. We identify immune signatures and correlates of host response. Hallmarks of disease severity involved cells, their inflammatory mediators and networks, including progenitor cells and specific myeloid and lymphocyte subsets, features of the immune repertoire, acute phase response, metabolism, and coagulation. Persisting immune activation involving AP-1/p38MAPK was a specific feature of COVID-19. The plasma proteome enabled sub-phenotyping into patient clusters, predictive of severity and outcome. Systems-based integrative analyses including tensor and matrix decomposition of all modalities revealed feature groupings linked with severity and specificity compared to influenza and sepsis. Our approach and blood atlas will support future drug development, clinical trial design, and personalized medicine approaches for COVID-19.

The pathophysiology of sepsis and precision-medicine-based immunotherapy.

Sepsis remains a major cause of morbidity and mortality in both low- and high-income countries. Antibiotic therapy and supportive care have significantly improved survival following sepsis in the twentieth century, but further progress has been challenging. Immunotherapy trials for sepsis, mainly aimed at suppressing the immune response, from the 1990s and 2000s, have largely failed, in part owing to unresolved patient heterogeneity in the underlying immune disbalance. The past decade has brought the promise to break this blockade through technological developments based on omics-based technologies and systems medicine that can provide a much larger data space to describe in greater detail the immune endotypes in sepsis. Patient stratification opens new avenues towards precision medicine approaches that aim to apply immunotherapies to sepsis, on the basis of precise biomarkers and molecular mechanisms defining specific immune endotypes. This approach has the potential to lead to the establishment of immunotherapy as a successful pillar in the treatment of sepsis for future generations.

A pairwise cytokine code explains the organism-wide response to sepsis.

Sepsis is a systemic response to infection with life-threatening consequences. Our understanding of the molecular and cellular impact of sepsis across organs remains rudimentary. Here, we characterize the pathogenesis of sepsis by measuring dynamic changes in gene expression across organs. To pinpoint molecules controlling organ states in sepsis, we compare the effects of sepsis on organ gene expression to those of 6 singles and 15 pairs of recombinant cytokines. Strikingly, we find that the pairwise effects of tumor necrosis factor plus interleukin (IL)-18, interferon-gamma or IL-1ß suffice to mirror the impact of sepsis across tissues. Mechanistically, we map the cellular effects of sepsis and cytokines by computing changes in the abundance of 195 cell types across 9 organs, which we validate by whole-mouse spatial profiling. Our work decodes the cytokine cacophony in sepsis into a pairwise cytokine message capturing the gene, cell and tissue responses of the host to the disease.

Sepsis-trained macrophages promote antitumoral tissue-resident T cells.

Sepsis induces immune alterations, which last for months after the resolution of illness. The effect of this immunological reprogramming on the risk of developing cancer is unclear. Here we use a national claims database to show that sepsis survivors had a lower cumulative incidence of cancers than matched nonsevere infection survivors. We identify a chemokine network released from sepsis-trained resident macrophages that triggers tissue residency of T cells via CCR2 and CXCR6 stimulations as the immune mechanism responsible for this decreased risk of de novo tumor development after sepsis cure. While nonseptic inflammation does not provoke this network, laminarin injection could therapeutically reproduce the protective sepsis effect. This chemokine network and CXCR6 tissue-resident T cell accumulation were detected in humans with sepsis and were associated with prolonged survival in humans with cancer. These findings identify a therapeutically relevant antitumor consequence of sepsis-induced trained immunity.

Neutrophils and emergency granulopoiesis drive immune suppression and an extreme response endotype during sepsis.

Sepsis arises from diverse and incompletely understood dysregulated host response processes following infection that leads to life-threatening organ dysfunction. Here we showed that neutrophils and emergency granulopoiesis drove a maladaptive response during sepsis. We generated a whole-blood single-cell multiomic atlas (272,993 cells, n = 39 individuals) of the sepsis immune response that identified populations of immunosuppressive mature and immature neutrophils. In co-culture, CD66b+ sepsis neutrophils inhibited proliferation and activation of CD4+ T cells. Single-cell multiomic mapping of circulating hematopoietic stem and progenitor cells (HSPCs) (29,366 cells, n = 27) indicated altered granulopoiesis in patients with sepsis. These features were enriched in a patient subset with poor outcome and a specific sepsis response signature that displayed higher frequencies of IL1R2+ immature neutrophils, epigenetic and transcriptomic signatures of emergency granulopoiesis in HSPCs and STAT3-mediated gene regulation across different infectious etiologies and syndromes. Our findings offer potential therapeutic targets and opportunities for stratified medicine in severe infection.

GDF15 Is an Inflammation-Induced Central Mediator of Tissue Tolerance.

Growth and differentiation factor 15 (GDF15) is an inflammation-associated hormone with poorly defined biology. Here, we investigated the role of GDF15 in bacterial and viral infections. We found that inflammation induced GDF15, and that GDF15 was necessary for surviving both bacterial and viral infections, as well as sepsis. The protective effects of GDF15 were largely independent of pathogen control or the magnitude of inflammatory response, suggesting a role in disease tolerance. Indeed, we found that GDF15 was required for hepatic sympathetic outflow and triglyceride metabolism. Failure to defend the lower limit of plasma triglyceride levels was associated with impaired cardiac function and maintenance of body temperature, effects that could be rescued by exogenous administration of lipids. Together, we show that GDF15 coordinates tolerance to inflammatory damage through regulation of triglyceride metabolism.

Intracellular immune sensing promotes inflammation via gasdermin D-driven release of a lectin alarmin.

Inflammatory caspase sensing of cytosolic lipopolysaccharide (LPS) triggers pyroptosis and the concurrent release of damage-associated molecular patterns (DAMPs). Collectively, DAMPs are key determinants that shape the aftermath of inflammatory cell death. However, the identity and function of the individual DAMPs released are poorly defined. Our proteomics study revealed that cytosolic LPS sensing triggered the release of galectin-1, a ß-galactoside-binding lectin. Galectin-1 release is a common feature of inflammatory cell death, including necroptosis. In vivo studies using galectin-1-deficient mice, recombinant galectin-1 and galectin-1-neutralizing antibody showed that galectin-1 promotes inflammation and plays a detrimental role in LPS-induced lethality. Mechanistically, galectin-1 inhibition of CD45 (Ptprc) underlies its unfavorable role in endotoxin shock. Finally, we found increased galectin-1 in sera from human patients with sepsis. Overall, we uncovered galectin-1 as a bona fide DAMP released as a consequence of cytosolic LPS sensing, identifying a new outcome of inflammatory cell death.

FDA-approved disulfiram inhibits pyroptosis by blocking gasdermin D pore formation.

Cytosolic sensing of pathogens and damage by myeloid and barrier epithelial cells assembles large complexes called inflammasomes, which activate inflammatory caspases to process cytokines (IL-1ß) and gasdermin D (GSDMD). Cleaved GSDMD forms membrane pores, leading to cytokine release and inflammatory cell death (pyroptosis). Inhibiting GSDMD is an attractive strategy to curb inflammation. Here we identify disulfiram, a drug for treating alcohol addiction, as an inhibitor of pore formation by GSDMD but not other members of the GSDM family. Disulfiram blocks pyroptosis and cytokine release in cells and lipopolysaccharide-induced septic death in mice. At nanomolar concentration, disulfiram covalently modifies human/mouse Cys191/Cys192 in GSDMD to block pore formation. Disulfiram still allows IL-1ß and GSDMD processing, but abrogates pore formation, thereby preventing IL-1ß release and pyroptosis. The role of disulfiram in inhibiting GSDMD provides new therapeutic indications for repurposing this safe drug to counteract inflammation, which contributes to many human diseases.

TREML4 receptor regulates inflammation and innate immune cell death during polymicrobial sepsis.

Sepsis is a biphasic disease characterized by an acute inflammatory response, followed by a prolonged immunosuppressive phase. Therapies aimed at controlling inflammation help to reduce the time patients with sepsis spend in intensive care units, but they do not lead to a reduction in overall mortality. Recently, the focus has been on addressing the immunosuppressive phase, often caused by apoptosis of immune cells. However, molecular triggers of these events are not yet known. Using whole-genome CRISPR screening in mice, we identified a triggering receptor expressed on myeloid cells (TREM) family receptor, TREML4, as a key regulator of inflammation and immune cell death in sepsis. Genetic ablation of Treml4 in mice demonstrated that TREML4 regulates calcium homeostasis, the inflammatory cytokine response, myeloperoxidase activation, the endoplasmic reticulum stress response and apoptotic cell death in innate immune cells, leading to an overall increase in survival rate, both during the acute and chronic phases of polymicrobial sepsis.

Targeted removal of macrophage-secreted interleukin-1 receptor antagonist protects against lethal Candida albicans sepsis.

Invasive fungal infections are associated with high mortality rates, and the lack of efficient treatment options emphasizes an urgency to identify underlying disease mechanisms. We report that disseminated Candida albicans infection is facilitated by interleukin-1 receptor antagonist (IL-1Ra) secreted from macrophages in two temporally and spatially distinct waves. Splenic CD169+ macrophages release IL-1Ra into the bloodstream, impeding early neutrophil recruitment. IL-1Ra secreted by monocyte-derived tissue macrophages further impairs pathogen containment. Therapeutic IL-1Ra neutralization restored the functional competence of neutrophils, corrected maladapted hyper-inflammation, and eradicated the otherwise lethal infection. Conversely, augmentation of macrophage-secreted IL-1Ra by type I interferon severely aggravated disease mortality. Our study uncovers how a fundamental immunoregulatory mechanism mediates the high disease susceptibility to invasive candidiasis. Furthermore, interferon-stimulated IL-1Ra secretion may exacerbate fungal dissemination in human patients with secondary candidemia. Macrophage-secreted IL-1Ra should be considered as an additional biomarker and potential therapeutic target in severe systemic candidiasis.

STING activation in platelets aggravates septic thrombosis by enhancing platelet activation and granule secretion.

Sepsis is a dysregulated inflammatory consequence of systemic infection. As a result, excessive platelet activation leads to thrombosis and coagulopathy, but we currently lack sufficient understanding of these processes. Here, using the cecal ligation and puncture (CLP) model of sepsis, we observed septic thrombosis and neutrophil extracellular trap formation (NETosis) within the mouse vasculature by intravital microscopy. STING activation in platelets was a critical driver of sepsis-induced pathology. Platelet-specific STING deficiency suppressed platelet activation and granule secretion, which alleviated sepsis-induced intravascular thrombosis and NETosis in mice. Mechanistically, sepsis-derived cGAMP promoted the binding of STING to STXBP2, the assembly of SNARE complex, granule secretion, and subsequent septic thrombosis, which probably depended on the palmitoylation of STING. We generated a peptide, C-ST5, to block STING binding to STXBP2. Septic mice treated with C-ST5 showed reduced thrombosis. Overall, platelet activation via STING reveals a potential strategy for limiting life-threatening sepsis-mediated coagulopathy.

Pregnancy-induced changes to the gut microbiota drive macrophage pyroptosis and exacerbate septic inflammation.

The physiological and immune changes that occur during pregnancy are associated with worsened disease outcomes during infection and sepsis. How these perturbations exacerbate inflammation has not been explored. Here, using antibiotic treatment and fecal microbial transfers, we showed that sepsis susceptibility is driven by pregnancy-induced changes to gut microbiome in mice and humans. Integrative multiomics and genetically engineered bacteria revealed that reduced Parabacteroides merdae (P. merdae) abundance during pregnancy led to decreased formononetin (FMN) and increased macrophage death. Mechanistically, FMN inhibited macrophage pyroptosis by suppressing nuclear accumulation of hnRNPUL2 and subsequent binding to the Nlrp3 promoter. Treatment with FMN or deletion of murine hnRNPUL2 protected against septic inflammation. Intestinal abundances of P. merdae and FMN inversely correlated with the progression of septic patients. Our data reveal a microbe-immune axis that is disrupted in pregnant septic hosts, highlighting the potential of the FMN-hnRNPUL2-NLRP3 axis in providing promising therapeutic strategies for sepsis.

Gut microbiota in pregnancy, please don't change me now.

Pregnancy predisposes women to develop severe sepsis. However, the mechanisms regulating this remain unclear. In this issue of Immunity, Chen et al. describe the critical role of gut dysbiosis during pregnancy in driving excessive macrophage pyroptosis, increasing susceptibility to sepsis.

Just a Spoonful of Sugar Helps the Tolerance Go Up.

Survival of deleterious infections depends significantly on how much stress the affected organism can tolerate. In this issue, Weis et al. find that mice can survive sepsis by maintaining normoglycemia through ferritin's capacity to inactivate Fe2+ ions that otherwise induce free radicals impacting gluconeogenesis in the liver.

Metabolic Adaptation Establishes Disease Tolerance to Sepsis.

Sepsis is an often lethal syndrome resulting from maladaptive immune and metabolic responses to infection, compromising host homeostasis. Disease tolerance is a defense strategy against infection that preserves host homeostasis without exerting a direct negative impact on pathogens. Here, we demonstrate that induction of the iron-sequestering ferritin H chain (FTH) in response to polymicrobial infections is critical to establish disease tolerance to sepsis. The protective effect of FTH is exerted via a mechanism that counters iron-driven oxidative inhibition of the liver glucose-6-phosphatase (G6Pase), and in doing so, sustains endogenous glucose production via liver gluconeogenesis. This is required to prevent the development of hypoglycemia that otherwise compromises disease tolerance to sepsis. FTH overexpression or ferritin administration establish disease tolerance therapeutically. In conclusion, disease tolerance to sepsis relies on a crosstalk between adaptive responses controlling iron and glucose metabolism, required to maintain blood glucose within a physiologic range compatible with host survival.

Functional proteomic profiling links deficient DNA clearance with increased mortality in individuals with severe COVID-19 pneumonia.

The factors that influence survival during severe infection are unclear. Extracellular chromatin drives pathology, but the mechanisms enabling its accumulation remain elusive. Here, we show that in murine sepsis models, splenocyte death interferes with chromatin clearance through the release of the DNase I inhibitor actin. Actin-mediated inhibition was compensated by upregulation of DNase I or the actin scavenger gelsolin. Splenocyte death and neutrophil extracellular trap (NET) clearance deficiencies were prevalent in individuals with severe COVID-19 pneumonia or microbial sepsis. Activity tracing by plasma proteomic profiling uncovered an association between low NET clearance and increased COVID-19 pathology and mortality. Low NET clearance activity with comparable proteome associations was prevalent in healthy donors with low-grade inflammation, implicating defective chromatin clearance in the development of cardiovascular disease and linking COVID-19 susceptibility to pre-existing conditions. Hence, the combination of aberrant chromatin release with defects in protective clearance mechanisms lead to poor survival outcomes.

Inhibition of transcription factor NFAT activity in activated platelets enhances their aggregation and exacerbates gram-negative bacterial septicemia.

During gram-negative septicemia, interactions between platelets and neutrophils initiate a detrimental feedback loop that sustains neutrophil extracellular trap (NET) induction, disseminated intravascular coagulation, and inflammation. Understanding intracellular pathways that control platelet-neutrophil interactions is essential for identifying new therapeutic targets. Here, we found that thrombin signaling induced activation of the transcription factor NFAT in platelets. Using genetic and pharmacologic approaches, as well as iNFATuation, a newly developed mouse model in which NFAT activation can be abrogated in a cell-specific manner, we demonstrated that NFAT inhibition in activated murine and human platelets enhanced their activation and aggregation, as well as their interactions with neutrophils and NET induction. During gram-negative septicemia, NFAT inhibition in platelets promoted disease severity by increasing disseminated coagulation and NETosis. NFAT inhibition also partially restored coagulation ex vivo in patients with hypoactive platelets. Our results define non-transcriptional roles for NFAT that could be harnessed to address pressing clinical needs.

ß-Glucan Reverses the Epigenetic State of LPS-Induced Immunological Tolerance.

Innate immune memory is the phenomenon whereby innate immune cells such as monocytes or macrophages undergo functional reprogramming after exposure to microbial components such as lipopolysaccharide (LPS). We apply an integrated epigenomic approach to characterize the molecular events involved in LPS-induced tolerance in a time-dependent manner. Mechanistically, LPS-treated monocytes fail to accumulate active histone marks at promoter and enhancers of genes in the lipid metabolism and phagocytic pathways. Transcriptional inactivity in response to a second LPS exposure in tolerized macrophages is accompanied by failure to deposit active histone marks at promoters of tolerized genes. In contrast, ß-glucan partially reverses the LPS-induced tolerance in vitro. Importantly, ex vivo ß-glucan treatment of monocytes from volunteers with experimental endotoxemia re-instates their capacity for cytokine production. Tolerance is reversed at the level of distal element histone modification and transcriptional reactivation of otherwise unresponsive genes. VIDEO ABSTRACT.

Opposing Effects of Fasting Metabolism on Tissue Tolerance in Bacterial and Viral Inflammation.

Acute infections are associated with a set of stereotypic behavioral responses, including anorexia, lethargy, and social withdrawal. Although these so-called sickness behaviors are the most common and familiar symptoms of infections, their roles in host defense are largely unknown. Here, we investigated the role of anorexia in models of bacterial and viral infections. We found that anorexia was protective while nutritional supplementation was detrimental in bacterial sepsis. Furthermore, glucose was necessary and sufficient for these effects. In contrast, nutritional supplementation protected against mortality from influenza infection and viral sepsis, whereas blocking glucose utilization was lethal. In both bacterial and viral models, these effects were largely independent of pathogen load and magnitude of inflammation. Instead, we identify opposing metabolic requirements tied to cellular stress adaptations critical for tolerance of differential inflammatory states. VIDEO ABSTRACT.