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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.

There is increasing evidence that coronavirus disease 2019 (COVID-19) produces more severe symptoms and higher mortality among men than among women 1 – 5 . However, whether immune responses against severe acute respiratory syndrome coronavirus (SARS-CoV-2) differ between sexes, and whether such differences correlate with the sex difference in the disease course of COVID-19, is currently unknown. Here we examined sex differences in viral loads, SARS-CoV-2-specific antibody titres, plasma cytokines and blood-cell phenotyping in patients with moderate COVID-19 who had not received immunomodulatory medications. Male patients had higher plasma levels of innate immune cytokines such as IL-8 and IL-18 along with more robust induction of non-classical monocytes. By contrast, female patients had more robust T cell activation than male patients during SARS-CoV-2 infection. Notably, we found that a poor T cell response negatively correlated with patients’ age and was associated with worse disease outcome in male patients, but not in female patients. By contrast, higher levels of innate immune cytokines were associated with worse disease progression in female patients, but not in male patients. These findings provide a possible explanation for the observed sex biases in COVID-19, and provide an important basis for the development of a sex-based approach to the treatment and care of male and female patients with COVID-19. Male patients with COVID-19 have higher plasma levels of innate immune cytokines and chemokines such as IL-8, IL-18 and CCL5 and more non-classical monocytes than female patients, whereas female patients mount robust T cell activation maintained even in older age.

Intercellular communication mediated by cytokines is the main mechanism by which cells of the immune system talk to each other. Many aspects of cytokine signalling in the immune system have been explored in great detail at the structural, biophysical, biochemical and cellular levels. However, a systematic understanding of the quantitative rules that govern cytokine-mediated cell-to-cell communication is still lacking. Here, we discuss recent efforts in the field of systems immunology to bring about a quantitative understanding of cytokine-mediated communication between leukocytes and to provide novel insights into the orchestration of immune responses and inflammation.Recent advances in systems immunology are beginning to elucidate the quantitative rules that govern cytokine-mediated cell-to-cell communication. This Review describes how combining theoretical analysis with experimental validation can lead to a better understanding of cytokine-mediated communication between cells of the immune system.

Aging accounts for increased risk and dismal outcome of ischemic stroke. Here, we investigated the impact of age-related changes in the immune system on stroke. Upon experimental stroke, compared with young mice, aged mice had increased neutrophil clogging of the ischemic brain microcirculation, leading to worse no-reflow and outcomes. Aged mice showed an enhanced granulopoietic response to stroke that led to the accumulation of CD101 + CD62L lo mature and CD177 hi CD101 lo CD62L lo and CD177 lo CD101 lo CD62L hi immature atypical neutrophils in the blood, endowed with increased oxidative stress, phagocytosis and procoagulant features. Production of CXCL3 by CD62L lo neutrophils of the aged had a key role in the development and pathogenicity of aging-associated neutrophils. Hematopoietic stem cell rejuvenation reverted aging-associated neutropoiesis and improved stroke outcome. In elderly patients with ischemic stroke, single-cell proteome profile of blood leukocytes identified CD62L lo neutrophil subsets associated with worse reperfusion and outcome. Our results unveil how stroke in aging leads to a dysregulated emergency granulopoiesis impacting neurological outcome. Bacigaluppi and colleagues report that the accumulation of atypical mature and immature neutrophil subsets and a dysregulated emergency granulopoiesis response in aged mice and humans affect the outcome of stroke.

Macrophages promote both injury and repair after myocardial infarction, but discriminating functions within mixed populations remains challenging. Here we used fate mapping, parabiosis and single-cell transcriptomics to demonstrate that at steady state, TIMD4 + LYVE1 + MHC-II lo CCR2 − resident cardiac macrophages self-renew with negligible blood monocyte input. Monocytes partially replaced resident TIMD4 – LYVE1 – MHC-II hi CCR2 − macrophages and fully replaced TIMD4 − LYVE1 − MHC-II hi CCR2 + macrophages, revealing a hierarchy of monocyte contribution to functionally distinct macrophage subsets. Ischemic injury reduced TIMD4 + and TIMD4 – resident macrophage abundance, whereas CCR2 + monocyte-derived macrophages adopted multiple cell fates within infarcted tissue, including those nearly indistinguishable from resident macrophages. Recruited macrophages did not express TIMD4, highlighting the ability of TIMD4 to track a subset of resident macrophages in the absence of fate mapping. Despite this similarity, inducible depletion of resident macrophages using a Cx3cr1 -based system led to impaired cardiac function and promoted adverse remodeling primarily within the peri-infarct zone, revealing a nonredundant, cardioprotective role of resident cardiac macrophages. Epelman and colleagues use fate mapping and single-cell transcriptomics to describe the dynamics of resident and recruited cardiac macrophages during ischemic injury.

How glutamine metabolism orchestrates macrophage activation is unclear. Ho and colleagues show glutamine metabolism tailors the immune responses of macrophages through metabolic and epigenetic reprogramming. Glutamine metabolism provides synergistic support for macrophage activation and elicitation of desirable immune responses; however, the underlying mechanisms regulated by glutamine metabolism to orchestrate macrophage activation remain unclear. Here we show that the production of α-ketoglutarate (αKG) via glutaminolysis is important for alternative (M2) activation of macrophages, including engagement of fatty acid oxidation (FAO) and Jmjd3-dependent epigenetic reprogramming of M2 genes. This M2-promoting mechanism is further modulated by a high αKG/succinate ratio, whereas a low ratio strengthens the proinflammatory phenotype in classically activated (M1) macrophages. As such, αKG contributes to endotoxin tolerance after M1 activation. This study reveals new mechanistic regulations by which glutamine metabolism tailors the immune responses of macrophages through metabolic and epigenetic reprogramming.

Neutrophils are frontline cells of the innate immune system. These effector leukocytes are equipped with intriguing antimicrobial machinery and consequently display high cytotoxic potential. Accurate neutrophil recruitment is essential to combat microbes and to restore homeostasis, for inflammation modulation and resolution, wound healing and tissue repair. After fulfilling the appropriate effector functions, however, dampening neutrophil activation and infiltration is crucial to prevent damage to the host. In humans, chemoattractant molecules can be categorized into four biochemical families, i.e., chemotactic lipids, formyl peptides, complement anaphylatoxins and chemokines. They are critically involved in the tight regulation of neutrophil bone marrow storage and egress and in spatial and temporal neutrophil trafficking between organs. Chemoattractants function by activating dedicated heptahelical G protein-coupled receptors (GPCRs). In addition, emerging evidence suggests an important role for atypical chemoattractant receptors (ACKRs) that do not couple to G proteins in fine-tuning neutrophil migratory and functional responses. The expression levels of chemoattractant receptors are dependent on the level of neutrophil maturation and state of activation, with a pivotal modulatory role for the (inflammatory) environment. Here, we provide an overview of chemoattractant receptors expressed by neutrophils in health and disease. Depending on the (patho)physiological context, specific chemoattractant receptors may be up- or downregulated on distinct neutrophil subsets with beneficial or detrimental consequences, thus opening new windows for the identification of disease biomarkers and potential drug targets.

Tissue-resident macrophages (TRMs) are long-lived cells that maintain locally and can be phenotypically distinct from monocyte-derived macrophages. Whether TRMs and monocyte-derived macrophages have district roles under differing pathologies is not understood. Here, we showed that a substantial portion of the macrophages that accumulated during pancreatitis and pancreatic cancer in mice had expanded from TRMs. Pancreas TRMs had an extracellular matrix remodeling phenotype that was important for maintaining tissue homeostasis during inflammation. Loss of TRMs led to exacerbation of severe pancreatitis and death, due to impaired acinar cell survival and recovery. During pancreatitis, TRMs elicited protective effects by triggering the accumulation and activation of fibroblasts, which was necessary for initiating fibrosis as a wound healing response. The same TRM-driven fibrosis, however, drove pancreas cancer pathogenesis and progression. Together, these findings indicate that TRMs play divergent roles in the pathogenesis of pancreatitis and cancer through regulation of stromagenesis. DeNardo and colleagues show that tissue-resident macrophages (TRMs) have a protective role during pancreas inflammation by triggering the activation of fibroblasts, but that TRM-driven fibrosis drives pancreas cancer pathogenesis.

Acute respiratory distress syndrome (ARDS) is a common respiratory critical syndrome with no effective therapeutic intervention. Neutrophils function in the overwhelming inflammatory process of acute lung injury (ALI) caused by ARDS; however, the phenotypic heterogeneity of pulmonary neutrophils in ALI/ARDS remains largely unknown. Here, using single-cell RNA sequencing, we identify two transcriptionally and functionally heterogeneous neutrophil populations (Fth1 hi Neu and Prok2 hi Neu) with distinct locations in LPS-induced ALI mouse lungs. Exposure to LPS promotes the Fth1 hi Neu subtype, with more inflammatory factors, stronger antioxidant, and decreased apoptosis under the regulation of interleukin-10. Furthermore, prolonged retention of Fth1 hi Neu within lung tissue aggravates inflammatory injury throughout the development of ALI/ARDS. Notably, ARDS patients have high ratios of Fth1 to Prok2 expression in pulmonary neutrophils, suggesting that the Fth1 hi Neu population may promote the pathological development and provide a marker of poor outcome. Acute respiratory distress syndrome (ARDS) is an acute pulmonary disease involving neutrophils. Here the authors characterise lung neutrophil infiltration during ARDS and show that there are neutrophils with different function and transcriptional profile which are regulated by IL-10.

High-dimensional approaches have revealed heterogeneity amongst dendritic cells (DCs), including a population of transitional DCs (tDCs) in mice and humans. However, the origin and relationship of tDCs to other DC subsets has been unclear. Here we show that tDCs are distinct from other well-characterized DCs and conventional DC precursors (pre-cDCs). We demonstrate that tDCs originate from bone marrow progenitors shared with plasmacytoid DCs (pDCs). In the periphery, tDCs contribute to the pool of ESAM + type 2 DCs (DC2s), and these DC2s have pDC-related developmental features. Different from pre-cDCs, tDCs have less turnover, capture antigen, respond to stimuli and activate antigen-specific naïve T cells, all characteristics of differentiated DCs. Different from pDCs, viral sensing by tDCs results in IL-1β secretion and fatal immune pathology in a murine coronavirus model. Our findings suggest that tDCs are a distinct pDC-related subset with a DC2 differentiation potential and unique proinflammatory function during viral infections. Establishment of antiviral immune responses depends on dendritic cells (DCs), but how transitional DCs compare with other DC subsets in this regard is unclear. Here the authors show the origins of these cells and their proinflammatory function during viral infection.