Role of C5a in multiorgan failure during sepsis

Crepuscular rays - The bright side of complement after tissue injury

Acute injuries trigger an intense activation of the body's defense mechanisms aiming to limit damage and initiate healing. Among the crucial components of the intravascular immune system, the complement system plays a significant role in traumatic injuries, albeit often negatively. It has been suggested that excessive activation of the complement system, transitioning from a localized and timed response to a systemic one, can lead to a loss of its host-protective characteristics. Complement activation products have been associated with the severity of injuries, which sometimes serve as predictors for the onset of organ dysfunctions. Animal studies utilizing complement-targeting agents have provided the basis for considering complement in the management of traumatic injuries in humans. However, numerous studies suggest that the spatial and temporal aspects of complement inhibition are crucial for its efficacy. Understanding the underlying mechanism of the injury is essential to determine where, when, and whether complement inhibition is warranted. Despite the detrimental effects of uncontrolled complement activation, its regulated activation may contribute to essential aspects of healing, such as waste removal and regeneration. This review focuses on the beneficial roles of complement activation in trauma, which are often overlooked or given less consideration but are of immense importance.

Glomerular injury after trauma, burn, and sepsis

Acute kidney injury development after trauma, burn, or sepsis occurs frequently but remains a scientific and clinical challenge. Whereas the pathophysiological focus has mainly been on hemodynamics and the downstream renal tubular system, little is known about alterations upstream within the glomerulus post trauma or during sepsis. Particularly for the glomerular endothelial cells, mesangial cells, basal membrane, and podocytes, all of which form the glomerular filter, there are numerous in vitro studies on the molecular and functional consequences upon exposure of single cell types to specific damage- or microbial-associated molecular patterns. By contrast, a lack of knowledge exists in the real world regarding the orchestrated inflammatory response of the glomerulus post trauma or burn or during sepsis. Therefore, we aim to provide an overview on the glomerulus as an immune target but also as a perpetrator of the danger response to traumatic and septic conditions, and present major players involved in the context of critical illness. Finally, we highlight research gaps of this rather neglected but worthwhile area to define future molecular targets and therapeutic strategies to prevent or improve the course of AKI after trauma, burn, or sepsis.

Research Progress on the Mechanism and Management of Septic Cardiomyopathy: A Comprehensive Review

Sepsis is defined as a kind of life-threatening organ dysfunction due to a dysregulated host immune response to infection and is a leading cause of mortality in the intensive care unit. Sepsis-induced myocardial dysfunction, also called septic cardiomyopathy, is a common and serious complication in patients with sepsis, which may indicate a bad prognosis. Although efforts have been made to uncover the pathophysiology of septic cardiomyopathy, a number of uncertainties remain. This article sought to review available literature to summarize the existing knowledge on current diagnostic tools and biomarkers, pathogenesis, and treatments for septic cardiomyopathy.

Complement and complement regulatory proteins are upregulated in lungs of COVID-19 patients

We explored the pathological changes and the activation of local complement system in COVID-19 pneumonia. Lung paraffin sections of COVID-19 infected patients were analyzed by HE (hematoxylin-eosin) staining. The deposition of complement C3, the deposition of C3b/iC3b/C3d and C5b-9, and the expression of complement regulatory proteins, CD59, CD46 and CD55 were detected by immunohistochemistry. In COVID-19 patients' lung tissues, fibrin exudation, mixed with erythrocyte, alveolar macrophage and shed pneumocyte are usually observed in the alveoli. The formation of an "alveolar emboli" structure may contribute to thrombosis and consolidation in lung tissue. In addition, we also found that compared to normal tissue, the lung tissues of COVID-19 patients displayed the hyper-activation of complement that is represented by extensive deposition of C3, C3b/iC3b/C3d and C5b-9, and the increased expression level of complement regulatory proteins CD55, and especially CD59 but not CD46. The thrombosis and consolidation in lung tissues may contribute to the pathogenesis of COVID-19. The increased expression of CD55 and CD59 may reflect a feedback of self-protection on the complement hyper-activation. Further, the increased C3 deposition and the strongly activated complement system in lung tissues may suggest the rationale of complement-targeted therapeutics in conquering COVID-19.

Increased Expression of Anaphylatoxin C5a-Receptor-1 in Neutrophils and Natural Killer Cells of Preterm Infants

Preterm infants are susceptible to infection and their defense against pathogens relies largely on innate immunity. The role of the complement system for the immunological vulnerability of preterm infants is less understood. Anaphylatoxin C5a and its receptors C5aR1 and -2 are known to be involved in sepsis pathogenesis, with C5aR1 mainly exerting pro-inflammatory effects. Our explorative study aimed to determine age-dependent changes in the expression of C5aR1 and C5aR2 in neonatal immune cell subsets. Via flow cytometry, we analyzed the expression pattern of C5a receptors on immune cells isolated from peripheral blood of preterm infants (n = 32) compared to those of their mothers (n = 25). Term infants and healthy adults served as controls. Preterm infants had a higher intracellular expression of C5aR1 on neutrophils than control individuals. We also found a higher expression of C5aR1 on NK cells, particularly on the cytotoxic CD56^dim subset and the CD56^- subset. Immune phenotyping of other leukocyte subpopulations revealed no gestational-age-related differences for the expression of and C5aR2. Elevated expression of C5aR1 on neutrophils and NK cells in preterm infants may contribute to the phenomenon of "immunoparalysis" caused by complement activation or to sustained hyper-inflammatory states. Further functional analyses are needed to elucidate the underlying mechanisms.

THE IMMUNOPATHOGENESIS OF POLYMICROBIAL SEPSIS

ABSTRACT

This report deals with the advances made in the areas of complement and its role in sepsis, both in mice and in humans. The study relates to work over the past 25 years (late 1990s to October 2022). During this period, there has been very rapid progress in understanding the activation pathways of complement and the activation products of complement, especially the anaphylatoxin C5a and its receptors, C5aR1 and C5aR2. Much has also been learned about these pathways of activation that trigger activation of the innate immune system and the array of strong proinflammatory cytokines that can cause cell and organ dysfunction, as well as complement products that cause immunosuppression. The work in septic humans and mice, along with patients who develop lung dysfunction caused by COVID-19, has taught us that there are many strategies for treatment of humans who are septic or develop COVID-19-related lung dysfunction. To date, treatments in humans with these disorders suggest that we are in the midst of a new and exciting area related to the complement system.

Complement Activation in Patients With Heat-Related Illnesses: Soluble CD59 Is a Novel Biomarker Indicating Severity of Heat-Related Illnesses

Although multiple organ dysfunction syndrome (MODS) is the main cause of death in patients with heat-related illnesses, its underlying pathophysiological mechanism remains elusive. Complement activation is considered one of the main causes of MODS in patients with sepsis and trauma. Considering the pathophysiological similarity of heat related-illnesses with sepsis and trauma, the complement system might be activated in patients with heat-related illnesses as well. Our aim was to investigate whether excessive complement activation occurs in patients with heat-related illnesses.

C5a and C5aR1 are key drivers of microvascular platelet aggregation in clinical entities spanning from aHUS to COVID-19

C5a/C5aR1 signaling in endothelial cells is a common prothrombogenic effector spanning from rare genetic diseases and viral infections.

C5a causes RalA-mediated exocytosis of vWF and P-selectin, which favor further vWF binding on the endothelium and platelet aggregates.

Unrestrained activation of the complement system till the terminal products, C5a and C5b-9, plays a pathogenetic role in acute and chronic inflammatory diseases. In endothelial cells, complement hyperactivation may translate into cell dysfunction, favoring thrombus formation. The aim of this study was to investigate the role of the C5a/C5aR1 axis as opposed to C5b-9 in inducing endothelial dysfunction and loss of antithrombogenic properties. In vitro and ex vivo assays with serum from patients with atypical hemolytic uremic syndrome (aHUS), a prototype rare disease of complement-mediated microvascular thrombosis due to genetically determined alternative pathway dysregulation, and cultured microvascular endothelial cells, demonstrated that the C5a/C5aR1 axis is a key player in endothelial thromboresistance loss. C5a added to normal human serum fully recapitulated the prothrombotic effects of aHUS serum. Mechanistic studies showed that C5a caused RalA-mediated exocytosis of von Willebrand factor (vWF) and P-selectin from Weibel-Palade bodies, which favored further vWF binding on the endothelium and platelet adhesion and aggregation. In patients with severe COVID-19 who suffered from acute activation of complement triggered by severe acute respiratory syndrome coronavirus 2 infection, we found the same C5a-dependent pathogenic mechanisms. These results highlight C5a/C5aR1 as a common prothrombogenic effector spanning from genetic rare diseases to viral infections, and it may have clinical implications. Selective C5a/C5aR1 blockade could have advantages over C5 inhibition because the former preserves the formation of C5b-9, which is critical for controlling bacterial infections that often develop as comorbidities in severely ill patients. The ACCESS trial registered at www.clinicaltrials.gov as #NCT02464891 accounts for the results related to aHUS patients treated with CCX168.

The case of complement inhibitors

Severe COVID-19 is characterized by lung and multiorgan inflammation and coagulation in the presence of overactivation of the complement system. Complement is a double edged-sward in SARS-Cov-2 infection. On one hand, it can control the viral infection in milder cases, on the other hand in cases with severe and prolonged infection massive complement activation occurs, which can intensify lung and systemic inflammation and promote a procoagulant and prothrombotic state. Several uncontrolled studies and controlled clinical trials with different complement inhibitors have been performed and others are ongoing. Results are promising in some but negative in others. Further studies are required to elucidate the benefit to risk profile of complement inhibitors in COVID-19 patients at different stages of the disease and to clarify the best targets in the complement cascade.

Complement Anaphylatoxins and Inflammatory Cytokines as Prognostic Markers for COVID-19 Severity and In-Hospital Mortality

COVID-19 severity due to innate immunity dysregulation accounts for prolonged hospitalization, critical complications, and mortality. Severe SARS-CoV-2 infections involve the complement pathway activation for cytokine storm development. Nevertheless, the role of complement in COVID-19 immunopathology, complement-modulating treatment strategies against COVID-19, and the complement and SARS-CoV-2 interaction with clinical disease outcomes remain elusive. This study investigated the potential changes in complement signaling, and the associated inflammatory mediators, in mild-to-critical COVID-19 patients and their clinical outcomes. A total of 53 patients infected with SARS-CoV-2 were enrolled in the study (26 critical and 27 mild cases), and additional 18 healthy control patients were also included. Complement proteins and inflammatory cytokines and chemokines were measured in the sera of patients with COVID-19 as well as healthy controls by specific enzyme-linked immunosorbent assay. C3a, C5a, and factor P (properdin), as well as interleukin (IL)-1β, IL-6, IL-8, tumor necrosis factor (TNF)-α, and IgM antibody levels, were higher in critical COVID-19 patients compared to mild COVID-19 patients. Additionally, compared to the mild COVID-19 patients, factor I and C4-BP levels were significantly decreased in the critical COVID-19 patients. Meanwhile, RANTES levels were significantly higher in the mild patients compared to critical patients. Furthermore, the critical COVID-19 intra-group analysis showed significantly higher C5a, C3a, and factor P levels in the critical COVID-19 non-survival group than in the survival group. Additionally, IL-1β, IL-6, and IL-8 were significantly upregulated in the critical COVID-19 non-survival group compared to the survival group. Finally, C5a, C3a, factor P, and serum IL-1β, IL-6, and IL-8 levels positively correlated with critical COVID-19 in-hospital deaths. These findings highlight the potential prognostic utility of the complement system for predicting COVID-19 severity and mortality while suggesting that complement anaphylatoxins and inflammatory cytokines are potential treatment targets against COVID-19.

An update: the emerging evidence of complement involvement in COVID-19

The current outbreak of coronavirus disease 2019 (COVID-19) has affected people around the world. Typically, COVID-19 originates in the lung, but lately it can extend to other organs and lead to tissue injury and multiorgan failure in severe patients, such as acute respiratory distress syndrome (ARDS), kidney failure and sepsis or systemic inflammation. Given that COVID-19 has been detected in a range of other organs, the COVID-19-associated disease is an alert of aberrant activation of host immune response which drives un-controlled inflammation that affects multiple organs. Complement is a vital component of innate immunity where it forms the first line of defense against potentially harmful microbes, but its role in COVID-19 is still not clear. Notably, the abnormal activation and continuous deposits of complement components were identified in the pre-clinical samples from COVID-19 patients, which have been confirmed in animal models. Recent evidence has revealed that the administration of complement inhibitors leads to relieve inflammatory response in ARDS. Hence, we speculate that the targeting complement system could be a potential treatment option for organ damage in COVID-19 patients.

Antioxidant and C5a-blocking strategy for hepatic ischemia-reperfusion injury repair

BACKGROUND

Nonspecific liver uptake of nanomaterials after intravenous injection has hindered nanomedicine for clinical translation. However, nanomaterials' propensity for liver distribution might enable their use in hepatic ischemia-reperfusion injury (IRI) repair. During hepatic IRI, reactive oxygen species (ROS) are generated and the fifth component of complement (C5a) is activated. In addition, C5a is confirmed to exacerbate the vicious cycle of oxidative stress and inflammatory damage. For these reasons, we have investigated the development of nanomaterials with liver uptake to scavenge ROS and block C5a for hepatic IRI repair.

RESULTS

To achieve this goal, a traditional nanoantioxidant of nanoceria was surface conjugated with the anti-C5a aptamers (Ceria@Apt) to scavenge the ROS and reduce C5a-mediated inflammation. High uptake of Ceria@Apt in the liver was confirmed by preclinical positron emission tomography (PET) imaging. The clinical symptoms of hepatic IRI were effectively alleviated by Ceria@Apt with ROS scavenging and C5a blocking in mice model. The released pro-inflammatory cytokines were significantly reduced, and subsequent inflammatory reaction involved in the liver was inhibited.

CONCLUSIONS

The synthesized Ceria@Apt has great potential of medical application in hepatic IRI repair, which could also be applied for other ischemic-related diseases.

Interleukin 8 Elicits Rapid Physiological Changes in Neutrophils That Are Altered by Inflammatory Conditions

A sufficient response of neutrophil granulocytes stimulated by interleukin (IL)-8 is vital during systemic inflammation, for example, in sepsis or severe trauma. Moreover, IL-8 is clinically used as biomarker of inflammatory processes. However, the effects of IL-8 on cellular key regulators of neutrophil properties such as the intracellular pH (pHi) in dependence of ion transport proteins and during inflammation remain to be elucidated. Therefore, we investigated in detail the fundamental changes in pHi, cellular shape, and chemotactic activity elicited by IL-8. Using flow cytometric methods, we determined that the IL-8-induced cellular activity was largely dependent on specific ion channels and transporters, such as the sodium-proton exchanger 1 (NHE1) and non-NHE1-dependent sodium flux. Exposing neutrophils in vitro to a proinflammatory micromilieu with N-formyl-Met-Leu-Phe, LPS, or IL-8 resulted in a diminished response regarding the increase in cellular size and pH. The detailed kinetics of the reduced reactivity of the neutrophil granulocytes could be illustrated in a near-real-time flow cytometric measurement. Last, the LPS-mediated impairment of the IL-8-induced response in neutrophils was confirmed in a translational, animal-free human whole blood model. Overall, we provide novel mechanistic insights for the interaction of IL-8 with neutrophil granulocytes and report in detail about its alteration during systemic inflammation.

Hemorrhagic Shock Induces a Rapid Transcriptomic Shift of the Immune Balance in Leukocytes after Experimental Multiple Injury

The immune response following trauma represents a major driving force of organ dysfunction and poor outcome. Therefore, we investigated the influence of an additional hemorrhagic shock (HS) on the early posttraumatic immune dysbalance in the whole population of blood leukocytes. A well-established murine polytrauma (PT) model with or without an additional pressure-controlled HS (mean arterial pressure of 30 mmHg (±5 mmHg) for 60 mins, afterwards fluid resuscitation with balanced electrolyte solution four times the volume of blood drawn) was used. C57BL/6 mice were randomized into a control, PT, and PT + HS group with three animals in each group. Four hours after trauma, corresponding to three hours after induction of hemorrhage, RNA was isolated from all peripheral blood leukocytes, and a microarray analysis was performed. Enrichment analysis was conducted on selected genes strongly modulated by the HS. After additional HS in PT mice, the gene expression of pathways related to the innate immunity, such as IL-6 production, neutrophil chemotaxis, cell adhesion, and toll-like receptor signaling was upregulated, whereas pathways of the adaptive immune system, such as B- and T-cell activation as well as the MHC class II protein complex, were downregulated. These results demonstrate that an additional HS plays an important role in the immune dysregulation early after PT by shifting the balance to increased innate and reduced adaptive immune responses.

Role of the C5a-C5a receptor axis in the inflammatory responses of the lungs after experimental polytrauma and hemorrhagic shock

Singular blockade of C5a in experimental models of sepsis is known to confer protection by rescuing lethality and decreasing pro-inflammatory responses. However, the role of inhibiting C5a has not been evaluated in the context of sterile systemic inflammatory responses, like polytrauma and hemorrhagic shock (PT + HS). In our presented study, a novel and highly specific C5a L-aptamer, NoxD21, was used to block C5a activity in an experimental murine model of PT + HS. The aim of the study was to assess early modulation of inflammatory responses and lung damage 4 h after PT + HS induction. NoxD21-treated PT + HS mice displayed greater polymorphonuclear cell recruitment in the lung, increased pro-inflammatory cytokine levels in the bronchoalveolar lavage fluids (BALF) and reduced myeloperoxidase levels within the lung tissue. An in vitro model of the alveolar-capillary barrier was established to confirm these in vivo observations. Treatment with a polytrauma cocktail induced barrier damage only after 16 h, and NoxD21 treatment in vitro did not rescue this effect. Furthermore, to test the exact role of both the cognate receptors of C5a (C5aR1 and C5aR2), experimental PT + HS was induced in C5aR1 knockout (C5aR1 KO) and C5aR2 KO mice. Following 4 h of PT + HS, C5aR2 KO mice had significantly reduced IL-6 and IL-17 levels in the BALF without significant lung damage, and both, C5aR1 KO and C5aR2 KO PT + HS animals displayed reduced MPO levels within the lungs. In conclusion, the C5aR2 could be a putative driver of early local inflammatory responses in the lung after PT + HS.

Targeting Complement C5a Receptor 1 for the Treatment of Immunosuppression in Sepsis

Complement factor C5a was originally identified as a powerful promoter of inflammation through activation of the C5a receptor 1 (C5ar1). Recent evidence suggests involvement of C5a not only in pro- but also in anti-inflammatory signaling. The present study aims to unveil the role of C5ar1 as potential therapeutic target in a murine sepsis model. Our study discloses a significantly increased survival in models of mild to moderate but not severe sepsis of C5ar1-deficient mice. The decreased mortality of C5ar1-deficient mice is accompanied by improved pathogen clearance and largely preserved liver function. C5ar1-deficient mice exhibited a significantly increased production of the pro-inflammatory mediator interferon-γ (IFN-γ) and a decreased production of the anti-inflammatory cytokine interleukin-10 (IL-10). Together, these data uncover C5a signaling as a mediator of immunosuppressive processes during sepsis and describe the C5ar1 and related changes of the IFN-γ to IL-10 ratio as markers for the immunological (dys)function accompanying sepsis.

SARS-CoV-2 and coagulation disorders in different organs

Coronavirus disease 2019 (COVID-19) is a prominent pandemic disease that emerged in China and hurriedly stretched worldwide. There are many reports on COVID-19 associated with the amplified incidence of thrombotic events. In this review, we focused on COVID-19 coupled with the coagulopathy contributes to severe outcome inclusive of comorbidities such as venous thromboembolism, stroke, diabetes, lung, heart attack, AKI, and liver injury. Initially, the COVID-19 patient associated coagulation disorders show an elevated level of the D-dimer, fibrinogen, and less lymphocyte count such as lymphopenia. COVID-19 associated with the Kawasaki disease has acute vasculitis in childhood which further affects the vessels found all over the body. COVID-19 linked with the thrombotic microangiopathy triggers the multiple vasculitis along with the arterioles thrombosis, medium, large venous and arterial vessels mediates the disseminated intravascular coagulation (DIC). SARS-Co-V-2 patients have reduced primary platelet production, increased destruction of the platelet, decreased circulating platelet leads to the condition of increased thrombocytopenia which contributes to the coagulation disorder. Endothelial dysfunction plays an important role in the coagulation disorders via increased generation of the thrombin and stops fibrinolysis further leads to hypercoagulopathy. Along with that endothelial dysfunction activates the complement system pathways and contributes to the acute and chronic inflammation via cytokine storm with the production of the cytokines and chemokines, coagulation in different organs such as lung, brain, liver, heart, kidney and further leads to multi-organ failure.

Complement in trauma-Traumatised complement?

Physical trauma represents a major global burden. The trauma-induced response, including activation of the innate immune system, strives for regeneration but can also lead to post-traumatic complications. The complement cascade is rapidly activated by damaged tissue, hypoxia, exogenous proteases and others. Activated complement can sense, mark and clear both damaged tissue and pathogens. However, excessive and insufficient activation of complement can result in a dysfunctional immune and organ response. Similar to acute coagulopathy, complementopathy can develop with enhanced anaphylatoxin generation and an impairment of complement effector functions. Various remote organ effects are induced or modulated by complement activation. Frequently, established trauma treatments are double-edged. On one hand, they help stabilising haemodynamics and oxygen supply as well as injured organs and on the other hand, they also drive complement activation. Immunomodulatory approaches aim to reset trauma-induced disbalance of complement activation and thus may change surgical trauma management procedures to improve outcome. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

Pathophysiology and potential future therapeutic targets using preclinical models of COVID-19

Severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) gains entry into the lung epithelial cells by binding to the surface protein angiotensin-converting enzyme 2. Severe SARS-CoV-2 infection, also known as coronavirus disease 2019 (COVID-19), can lead to death due to acute respiratory distress syndrome mediated by inflammatory immune cells and cytokines. In this review, we discuss the molecular and biochemical bases of the interaction between SARS-CoV-2 and human cells, and in doing so we highlight knowledge gaps currently precluding development of new effective therapies. In particular, discovery of novel treatment targets in COVID-19 will start from understanding pathologic changes based on a large number of autopsy lung tissue samples. Pathogenetic roles of potential molecular targets identified in human lung tissues must be validated in established animal models. Overall, this stepwise approach will enable appropriate selection of candidate therapeutic modalities targeting SARS-CoV2 and the host inflammatory response.

Pre-existing antibody-mediated adverse effects prevent the clinical development of a bacterial anti-inflammatory protein

Bacterial pathogens have evolved to secrete strong anti-inflammatory proteins that target the immune system. It was long speculated whether these virulence factors could serve as therapeutics in diseases in which abnormal immune activation plays a role. We adopted the secreted chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) as a model virulence factor-based therapeutic agent for diseases in which C5AR1 stimulation plays an important role. We show that the administration of CHIPS in human C5AR1 knock-in mice successfully dampens C5a-mediated neutrophil migration during immune complex-initiated inflammation. Subsequent CHIPS toxicology studies in animal models were promising. However, during a small phase I trial, healthy human volunteers showed adverse effects directly after CHIPS administration. Subjects showed clinical signs of anaphylaxis with mild leukocytopenia and increased C-reactive protein concentrations, which are possibly related to the presence of relatively high circulating anti-CHIPS antibodies and suggest an inflammatory response. Even though our data in mice show CHIPS as a potential anti-inflammatory agent, safety issues in human subjects temper the use of CHIPS in its current form as a therapeutic candidate. The use of staphylococcal proteins, or other bacterial proteins, as therapeutics or immune-modulators in humans is severely hampered by pre-existing circulating antibodies.

Immunopathophysiology of trauma-related acute kidney injury

Physical trauma can affect any individual and is globally accountable for more than one in every ten deaths. Although direct severe kidney trauma is relatively infrequent, extrarenal tissue trauma frequently results in the development of acute kidney injury (AKI). Various causes, including haemorrhagic shock, rhabdomyolysis, use of nephrotoxic drugs and infectious complications, can trigger and exacerbate trauma-related AKI (TRAKI), particularly in the presence of pre-existing or trauma-specific risk factors. Injured, hypoxic and ischaemic tissues expose the organism to damage-associated and pathogen-associated molecular patterns, and oxidative stress, all of which initiate a complex immunopathophysiological response that results in macrocirculatory and microcirculatory disturbances in the kidney, and functional impairment. The simultaneous activation of components of innate immunity, including leukocytes, coagulation factors and complement proteins, drives kidney inflammation, glomerular and tubular damage, and breakdown of the blood-urine barrier. This immune response is also an integral part of the intense post-trauma crosstalk between the kidneys, the nervous system and other organs, which aggravates multi-organ dysfunction. Necessary lifesaving procedures used in trauma management might have ambivalent effects as they stabilize injured tissue and organs while simultaneously exacerbating kidney injury. Consequently, only a small number of pathophysiological and immunomodulatory therapeutic targets for TRAKI prevention have been proposed and evaluated.

Immunopathogenesis of COVID-19: Summary and Possible Interventions

Since the onset of the COVID-19 pandemic in the fall of 2019 over 4 million people have been infected and over 280,000 have died (1). Information about the SARS-CoV2 virus is evolving rapidly. At this time there are no interventions proven to be effective for cases infected with SARS-CoV2. Current knowledge about the clinical and laboratory manifestations of COVID-19 infection is reviewed and combined with knowledge about the immunopathogenic mechanisms of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV1) and Middle East Respiratory Syndrome (MERS) to formulate theories and suggest possible therapeutic interventions. SARS-CoV2 immunopathogenic mechanisms vary from immunosuppression that initially enables viral escape to a hyperinflammatory immune response. Ultimately therapeutic intervention will be phase dependent.

C5a impairs phagosomal maturation in the neutrophil through phosphoproteomic remodeling

Critical illness is accompanied by the release of large amounts of the anaphylotoxin, C5a. C5a suppresses antimicrobial functions of neutrophils which is associated with adverse outcomes. The signaling pathways that mediate C5a-induced neutrophil dysfunction are incompletely understood. Healthy donor neutrophils exposed to purified C5a demonstrated a prolonged defect (7 hours) in phagocytosis of Staphylococcus aureus. Phosphoproteomic profiling of 2712 phosphoproteins identified persistent C5a signaling and selective impairment of phagosomal protein phosphorylation on exposure to S. aureus. Notable proteins included early endosomal marker ZFYVE16 and V-ATPase proton channel component ATPV1G1. An assay of phagosomal acidification demonstrated C5a-induced impairment of phagosomal acidification, which was recapitulated in neutrophils from critically ill patients. Examination of the C5a-impaired protein phosphorylation indicated a role for the PI3K VPS34 in phagosomal maturation. Inhibition of VPS34 impaired neutrophil phagosomal acidification and killing of S. aureus. This study provides a phosphoproteomic assessment of human neutrophil signaling in response to S. aureus and its disruption by C5a, identifying a defect in phagosomal maturation and mechanisms of immune failure in critical illness.

C5a disrupts the neutrophil phosphoproteomic response to bacteria, impairing phagosomal maturation and bacterial killing.

The case of complement activation in COVID-19 multiorgan impact

The novel coronavirus disease COVID-19 originates in the lungs, but it may extend to other organs, causing, in severe cases, multiorgan damage, including cardiac injury and acute kidney injury. In severe cases, the presence of kidney injury is associated with increased risk of death, highlighting the relevance of this organ as a target of SARS-CoV-2 infection. COVID-19-associated tissue injury is not primarily mediated by viral infection, but rather is a result of the inflammatory host immune response, which drives hypercytokinemia and aggressive inflammation that affect lung parenchymal cells, diminishing oxygen uptake, but also endothelial cells, resulting in endotheliitis and thrombotic events and intravascular coagulation. The complement system represents the first response of the host immune system to SARS-CoV-2 infection, but there is growing evidence that unrestrained activation of complement induced by the virus in the lungs and other organs plays a major role in acute and chronic inflammation, endothelial cell dysfunction, thrombus formation, and intravascular coagulation, and ultimately contributes to multiple organ failure and death. In this review, we discuss the relative role of the different complement activation products in the pathogenesis of COVID-19-associated tissue inflammation and thrombosis and propose the hypothesis that blockade of the terminal complement pathway may represent a potential therapeutic option for the prevention and treatment of lung and multiorgan damage.

Complement in sepsis-when science meets clinics

Sepsis as life-threatening organ dysfunction caused by microorganisms represents a dreadful challenge for the immune system. The role of the complement system as major column of innate immunity has been extensively studied in various sepsis models, but its translational value remains in the dark. Complement activation products, such as C3a and C5a, and their corresponding receptors provide useful diagnostic tools and promising targets to improve organ function and outcome. However, a monotherapeutic complement intervention irrespective of the current immune function seems insufficient to reverse the complex sepsis mechanisms. Indeed, sepsis-induced disturbances of cross talking complement, coagulation, and fibrinolytic cascades lead to systemic 'thromboinflammation', ultimately followed by multiple-organ failure. We propose to reliably monitor the complement function in the patient and to re-establish the immune balance by patient-tailored combined therapies, such as complement and Toll-like receptor inhibition. Our working hypothesis aims at blocking the 'explosive' innate immune recognition systems early on before downstream mediators are released and the inflammatory response becomes irreversible, a strategy that we name 'upstream approach'.

Complement levels in patients with bloodstream infection due to Staphylococcus aureus or Gram-negative bacteria

The complement system is a vital component of the innate immune system, though its role in bacteremia is poorly understood. We present complement levels in Staphylococcus aureus bacteremia (SAB) and Gram-negative bacteremia (GNB) and describe observed associations of complement levels with clinical outcomes. Complement and cytokine levels were measured in serum samples from 20 hospitalized patients with SAB, 20 hospitalized patients with GNB, 10 non-infected hospitalized patients, and 10 community controls. C5a levels were significantly higher in patients with SAB as compared to patients with GNB. Low C4 and C3 levels were associated with septic shock and 30-day mortality in patients with GNB, and elevated C3 was associated with a desirable outcome defined as absence of (1) septic shock, (2) acute renal failure, and (3) death within 30 days of bacteremia. Low levels of C9 were associated with septic shock in patients with GNB but not SAB. Elevated IL-10 was associated with increased 30-day mortality in patients with SAB. Complement profiles differ in patients with SAB and those with GNB. Measurement of IL-10 in patients with SAB and of C4, C3, and C9 in patients with GNB may help to identify those at higher risk for poor outcomes.

Requirement of Complement C6 for Intact Innate Immune Responses in Mice

Over the first days of polymicrobial sepsis, there is robust activation of the innate immune system, causing the appearance of proinflammatory cytokines and chemokines, along with the appearance of extracellular histones, which are highly proinflammatory and prothrombotic. In the current study, we studied different innate immune responses in mice with knockout (KO) of complement protein 6 (C6). Polymorphonuclear neutrophils (PMNs) from these KO mice had defective innate immune responses, including defective expression of surface adhesion molecules, generation of superoxide anion, and appearance of reactive oxygen species and histone release after activation of PMNs, along with defective phagocytosis. In addition, in C6-/- mice, the NLRP3 inflammasome was defective both in PMNs and in macrophages. When these KO mice were subjected to polymicrobial sepsis, their survival was improved, associated with reduced levels in the plasma of proinflammatory cytokines and chemokines and lower levels of histones in plasma. In addition, sepsis-induced cardiac dysfunction was attenuated in these KO mice. In a model of acute lung injury induced by LPS, C6-/- mice showed reduced PMN buildup and less lung epithelial/endothelial cell dysfunction (edema and hemorrhage). These data indicate that C6-/- mice have reduced innate immune responses that result in less organ injury and improved survival after polymicrobial sepsis.

Cronobacter sakazakii Infection in Early Postnatal Rats Impaired Contextual-Associated Learning: a Putative Role of C5a-Mediated NF-κβ and ASK1 Pathways

This study was designed to test whether the Cronobacter sakazakii infection-impaired contextual learning and memory are mediated by the activation of the complement system; subsequent activation of inflammatory signals leads to alternations in serotonin transporter (SERT). To test this, rat pups (postnatal day, PND 15) were treated with either C. sakazakii (10⁷ CFU) or Escherichia coli OP50 (10⁷ CFU) or Luria bertani broth (100 μL) through oral gavage and allowed to stay with their mothers until PND 24. Experimental groups' rats were allowed to explore (PNDs 31-35) and then trained in contextual learning task (PNDs 36-43). Five days after training, individuals were tested for memory retention (PNDs 49-56). Observed behavioural data showed that C. sakazakii infection impaired contextual-associative learning and memory. Furthermore, our analysis showed that C. sakazakii infection activates complement system complement anaphylatoxin (C5a) (a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS1)) and mitogen-activated protein kinase kinase1 (MEKK1). Subsequently, MEKK1 induces pro-inflammatory signals possibly through apoptosis signal-regulating kinase-1 (ASK-1), c-Jun N-terminal kinase (JNK1/3) and protein kinase B gamma (AKT-3). In parallel, activated nuclear factor kappa-light-chain-enhancer B cells (NF-κB) induces interleukin-6 (IL-6) and IFNα-1, which may alter the level of serotonin transporter (SERT). Observed results suggest that impaired contextual learning and memory could be correlated with C5a-mediated NF-κβ and ASK1 pathways.

Deficiency of Mouse FHR-1 Homolog, FHR-E, Accelerates Sepsis, and Acute Kidney Injury Through Enhancing the LPS-Induced Alternative Complement Pathway

Alternative complement pathway (AP) plays an important role in the development of sepsis, which is life threatening. Deficiency of factor H-related protein 1 (FHR-1), which is a regulator of AP, has been considered as a susceptible factor for atypical hemolytic uremic syndrome (aHUS) and other types of nephropathy when an inducer such as infection exists. However, the underlying mechanism of the disease development is largely unknown. There is no report on CFHR1 gene knockout in any animal infection model and its function in vivo is still unclear. Here, a Cfhr1 knockout mouse was generated for investigating AP in sepsis and sepsis-induced acute kidney injury (AKI). We found that murine FHR-1 homolog (FHR-E) deficiency enhanced lipopolysaccharide (LPS)-induced AP activation both in vitro and in vivo and that Cfhr1 knockout mice exhibited more severe sepsis and AKI in response to LPS challenge. These results indicated that FHR-E deficiency promoted LPS-induced sepsis and AKI through AP over-activation, providing a mouse model for studying AP regulation and sepsis. This study revealed the function of FHR-E in vivo, which may further provide hints to the pathogenesis of FHR-1 deficiency-related diseases by enhancing LPS-induced AP activation.

Complement as a Major Inducer of Harmful Events in Infectious Sepsis

There is abundant evidence that infectious sepsis both in humans and mice with polymicrobial sepsis results in robust activation of complement. Major complement activation products involved in sepsis include C5a anaphylatoxin and its receptors (C5aR1 and C5aR2) and, perhaps, the terminal complement activation product, C5b-9. These products (and others) also cause dysfunction of the innate immune system, with exaggerated early proinflammatory responses, followed by decline of the innate immune system, leading to immunosuppression and multiorgan dysfunction. Generation of C5a during sepsis also leads to activation of neutrophils and macrophages and ultimate appearance of extracellular histones, which have powerful proinflammatory and prothrombotic activities. The distal complement activation product, C5b-9, triggers intracellular Ca fluxes in epithelial and endothelial cells. Histones activate the NLRP3 inflammasome, products of which can damage cells. C5a also activates MAPKs and Akt signaling pathways in cardiomyocytes, causing buildup of [Ca]i, defective action potentials and substantial cell dysfunction, resulting in cardiac and other organ dysfunction. Cardiac dysfunction can be quantitated by ECHO-Doppler parameters. In vivo interventions that block these complement-dependent products responsible for organ dysfunction in sepsis reduce the intensity of sepsis. The obvious targets in sepsis are C5a and its receptors, histones, and perhaps the MAPK pathways. Blockade of C5 has been considered in sepsis, but the FDA-approved antibody (eculizumab) is known to compromise defenses against neisseria and pneumonococcal bacteria, and requires immunization before the mAb to C5 can be used clinically. Small molecular blocking agents for C5aRs are currently in development and may be therapeutically effective for treatment of sepsis.

Renal protective effects of zingerone in a mouse model of sepsis

Zingerone (ZGR), a phenolic alkanone isolated from ginger, has been reported to possess pharmacological activities such as anti-inflammatory and anti-apoptotic effects. This study was initiated to determine whether ZGR could modulate renal functional damage in a mouse model of sepsis and to elucidate the underlying mechanisms. The potential of ZGR treatment to reduce renal damage induced by cecal ligation and puncture (CLP) surgery in mice was measured by assessment of serum creatinine, blood urea nitrogen (BUN), lipid peroxidation, total glutathione, glutathione peroxidase activity, catalase activity, and superoxide dismutase activity. Treatment with ZGR resulted in elevated plasma levels of BUN and creatinine, and of protein in urine in mice with CLP-induced renal damage. Moreover, ZGR inhibited nuclear factor-κB activation and reduced the induction of nitric oxide synthase and excessive production of nitric acid. ZGR treatment also reduced the plasma levels of interleukin-6 and tumor necrosis factor-α, reduced lethality due to CLP-induced sepsis, increased lipid peroxidation, and markedly enhanced the antioxidant defense system by restoring the levels of superoxide dismutase, glutathione peroxidase, and catalase in kidney tissues. Our study showed renal suppressive effects of zingerone in a mouse model of sepsis, suggesting that ZGR protects mice against sepsis-triggered renal injury.

Renal protective effects of aloin in a mouse model of sepsis

Aloin is the major anthraquinone glycoside obtained from the Aloe species and exhibits anti-inflammatory and anti-oxidative activities. However, the renal protective effects of aloin and underlying molecular mechanism remain unclear. This study was initiated to determine whether aloin could modulate renal functional damage in a mouse model of sepsis and to elucidate the underlying mechanisms. The potential of aloin treatment to reduce renal damage induced by cecal ligation and puncture (CLP) surgery in mice was measured by assessment of serum creatinine, blood urea nitrogen (BUN), lipid peroxidation, total glutathione, glutathione peroxidase activity, catalase activity, and superoxide dismutase activity. Post-treatment with aloin resulted in a significant reduction in the deleterious renal functions by CLP, such as elevated BUN, creatinine, and urine protein. Moreover, aloin inhibited nuclear factor-κB activation and reduced the induction of nitric oxide synthase and excessive production of nitric acid. Aloin treatment also reduced the plasma levels of interleukin-6 and tumor necrosis factor-α, reduced lethality due to CLP-induced sepsis, increased lipid peroxidation, and markedly enhanced the antioxidant defense system by restoring the levels of superoxide dismutase, glutathione peroxidase, and catalase in kidney tissues. Our study suggested that aloin protects mice against sepsis-triggered renal injury.

Inter-α-inhibitor Ameliorates Endothelial Inflammation in Sepsis

PURPOSE

Vascular endothelial cells demonstrate severe injury in sepsis, and a reduction in endothelial inflammation would be beneficial. Inter-α-Inhibitor (IαI) is a family of abundant plasma proteins with anti-inflammatory properties and has been investigated in human and animal sepsis with encouraging results. We hypothesized that IαI may protect endothelia from sepsis-related inflammation.

METHODS

IαI-deficient or sufficient mice were treated with endotoxin or underwent complement-induced lung injury. VCAM-1 and ICAM-1 expression was measured in blood and lung as marker of endothelial activation. Human endothelia were exposed to activated complement C5a with or without IαI. Blood from human sepsis patients was examined for VCAM-1 and ICAM-1 and levels were correlated with blood levels of IαI.

RESULTS

IαI-deficient mice showed increased endothelial activation in endotoxin/sepsis- and complement-induced lung injury models. In vitro, levels of endothelial pro-inflammatory cytokines and cell growth factors induced by activated complement C5a were significantly decreased in the presence of IαI. This effect was associated with decreased ERK and NFκB activation. IαI levels were inversely associated with VCAM-1 and ICAM-1 levels in a human sepsis cohort.

CONCLUSIONS

IαI ameliorates endothelial inflammation and may be beneficial as a treatment of sepsis.

Renal protective effects of zingerone in a mouse model of sepsis

Zingerone (ZGR), a phenolic alkanone isolated from ginger, has been reported to possess pharmacological activities such as anti-inflammatory and anti-apoptotic effects. This study was initiated to determine whether ZGR could modulate renal functional damage in a mouse model of sepsis and to elucidate the underlying mechanisms. The potential of ZGR treatment to reduce renal damage induced by cecal ligation and puncture (CLP) surgery in mice was measured by assessment of serum creatinine, blood urea nitrogen (BUN), lipid peroxidation, total glutathione, glutathione peroxidase activity, catalase activity, and superoxide dismutase activity. Treatment with ZGR resulted in elevated plasma levels of BUN and creatinine, and of protein in urine in mice with CLP-induced renal damage. Moreover, ZGR inhibited nuclear factor-κB activation and reduced the induction of nitric oxide synthase and excessive production of nitric acid. ZGR treatment also reduced the plasma levels of interleukin-6 and tumor necrosis factor-α, reduced lethality due to CLP-induced sepsis, increased lipid peroxidation, and markedly enhanced the antioxidant defense system by restoring the levels of superoxide dismutase, glutathione peroxidase, and catalase in kidney tissues. Our study showed renal suppressive effects of zingerone in a mouse model of sepsis, suggesting that ZGR protects mice against sepsis-triggered renal injury. [BMB Reports 2019; 52(4): 271-276].

Targeting Complement Pathways in Polytrauma- and Sepsis-Induced Multiple-Organ Dysfunction

Exposure to traumatic or infectious insults results in a rapid activation of the complement cascade as major fluid defense system of innate immunity. The complement system acts as a master alarm system during the molecular danger response after trauma and significantly contributes to the clearance of DAMPs and PAMPs. However, depending on the origin and extent of the damaged macro- and micro -milieu, the complement system can also be either excessively activated or inhibited. In both cases, this can lead to a maladaptive immune response and subsequent multiple cellular and organ dysfunction. The arsenal of complement-specific drugs offers promising strategies for various critical conditions after trauma, hemorrhagic shock, sepsis, and multiple organ failure. The imbalanced immune response needs to be detected in a rational and real-time manner before the translational therapeutic potential of these drugs can be fully utilized. Overall, the temporal-spatial complement response after tissue trauma and during sepsis remains somewhat enigmatic and demands a clinical triad: reliable tissue damage assessment, complement activation monitoring, and potent complement targeting to highly specific rebalance the fluid phase innate immune response.

C5a anaphylatoxin and its role in critical illness-induced organ dysfunction

Critical illness is an aetiologically and clinically heterogeneous syndrome that is characterised by organ failure and immune dysfunction. Mortality in critically ill patients is driven by inflammation-associated organ damage and a profound vulnerability to nosocomial infection. Both factors are influenced by the activated complement protein C5a, released by unbridled activation of the complement system during critical illness. C5a exerts deleterious effects on organ systems directly and suppresses antimicrobial functions of key immune cells. Whilst several recent reports have added key knowledge of the cellular signalling pathways triggered by C5a, there remain a number of areas that are incompletely understood and therapeutic opportunities are still being evaluated. In this review, we summarise the cellular basis for C5a-induced vulnerability to nosocomial infection and organ dysfunction. We focus on cells of the innate immune system, highlighting the major areas in need of further research and potential avenues for targeted therapies.

Blood clotting and traumatic injury with shock mediates complement-dependent neutrophil priming for extracellular ROS, ROS-dependent organ injury and coagulopathy

Polymorphonuclear (PMN) leucocytes participate in acute inflammatory pathologies such as acute respiratory distress syndrome (ARDS) following traumatic injury and shock, which also activates the coagulation system systemically. Trauma can prime the PMN nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex for an enhanced respiratory burst, but the relative role of various priming agents in this process remains incompletely understood. We therefore set out to identify mediators of PMN priming during coagulation and trauma-shock and determine whether PMN reactive oxygen species (ROS) generated in this manner could influence organ injury and coagulation. Initial experiments demonstrated that PMN are primed for predominantly extracellular ROS production by products of coagulation, which was abrogated by CD88/C5a receptor(C5aR) inhibition. The importance of this was highlighted further by demonstrating that known PMN priming agents result in fractionally different amounts of extracellular versus intracellular ROS release depending on the agent used. Plasma from trauma patients in haemodynamic shock (n = 10) also primed PMN for extracellular ROS in a C5a-dependent manner, which correlated with both complement alternative pathway activation and thrombin generation. Furthermore, PMN primed by preincubation with products of blood coagulation directly caused loss of endothelial barrier function in vitro that was abrogated by C5aR blockade or NADPH oxidase inhibition. Finally, we show in a murine model of trauma-shock that p47phox knock-out (KO) mice with PMN incapable of generating ROS were protected from inflammatory end-organ injury and activated protein C-mediated coagulopathy. In summary, we demonstrate that trauma-shock and coagulation primes PMN for predominantly extracellular ROS production in a C5a-dependent manner that contributes to endothelial barrier loss and organ injury, and potentially enhances traumatic coagulopathy.

Role of complement C5a and histones in septic cardiomyopathy

Polymicrobial sepsis (after cecal ligation and puncture, CLP) causes robust complement activation with release of C5a. Many adverse events develop thereafter and will be discussed in this review article. Activation of complement system results in generation of C5a which interacts with its receptors (C5aR1, C5aR2). This leads to a series of harmful events, some of which are connected to the cardiomyopathy of sepsis, resulting in defective action potentials in cardiomyocytes (CMs), activation of the NLRP3 inflammasome in CMs and the appearance of extracellular histones, likely arising from activated neutrophils which form neutrophil extracellular traps (NETs). These events are associated with activation of mitogen-activated protein kinases (MAPKs) in CMs. The ensuing release of histones results in defective action potentials in CMs and reduced levels of [Ca²⁺]i-regulatory enzymes including sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA2) and Na⁺/Ca²⁺ exchanger (NCX) as well as Na⁺/K⁺-ATPase in CMs. There is also evidence that CLP causes release of IL-1β via activation of the NLRP3 inflammasome in CMs of septic hearts or in CMs incubated in vitro with C5a. Many of these events occur after in vivo or in vitro contact of CMs with histones. Together, these data emphasize the role of complement (C5a) and C5a receptors (C5aR1, C5aR2), as well as extracellular histones in events that lead to cardiac dysfunction of sepsis (septic cardiomyopathy).

Sulforaphane Suppresses Sepsis-Mediated Renal Functions in Cecal Ligation and Puncture Mouse Model

Sulforaphane (SFN), a natural isothiocyanate present in cruciferous vegetables such as broccoli and cabbage, is effective in preventing carcinogenesis, diabetes, and inflammatory responses. This study was initiated to determine whether SFN could modulate renal functional damage in a mouse model of sepsis and to elucidate the underlying mechanisms. The potential of SFN treatment to reduce renal damage induced by cecal ligation and puncture (CLP) surgery in mice was measured by assessment of serum creatinine, blood urea nitrogen (BUN), lipid peroxidation, total glutathione, glutathione peroxidase activity, catalase activity, and superoxide dismutase activity. Treatment with SFN resulted in elevated plasma levels of BUN and creatinine, and of protein in urine in mice with CLP-induced renal damage. SFN treatment also reduced the plasma levels of interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α),▢increased lipid peroxidation, and markedly enhanced the antioxidant defense system by restoring the levels of superoxide dismutase, glutathione peroxidase, and catalase in kidney tissues. The present results suggested that SFN protects mice against sepsis-triggered renal injury.

Complement C5a Alters the Membrane Potential of Neutrophils during Hemorrhagic Shock

Background

Polymorphonuclear granulocytes (PMN) play a crucial role in host defense. Physiologically, exposure of PMN to the complement activation product C5a results in a protective response against pathogens, whereas in the case of systemic inflammation, excessive C5a substantially impairs neutrophil functions. To further elucidate the inability of PMN to properly respond to C5a, this study investigates the role of the cellular membrane potential of PMN in response to C5a.

Methods

Electrophysiological changes in cellular and mitochondrial membrane potential and intracellular pH of PMN from human healthy volunteers were determined by flow cytometry after exposure to C5a. Furthermore, PMN from male Bretoncelles-Meishan-Willebrand cross-bred pigs before and three hours after severe hemorrhagic shock were analyzed for their electrophysiological response.

Results

PMN showed a significant dose- and time-dependent depolarization in response to C5a with a strong response after one minute. The chemotactic peptide fMLP also evoked a significant shift in the membrane potential of PMN. Acidification of the cellular microenvironment significantly enhanced depolarization of PMN. In a clinically relevant model of porcine hemorrhagic shock, the C5a-induced changes in membrane potential of PMN were markedly diminished compared to healthy littermates. Overall, these membrane potential changes may contribute to PMN dysfunction in an inflammatory environment.

Janus face of complement-driven neutrophil activation during sepsis

During local and systemic inflammation, the complement system and neutrophil granulocytes are activated not only by pathogens, but also by released endogenous danger signals. It is recognized increasingly that complement-mediated neutrophil activation plays an ambivalent role in sepsis pathophysiology. According to the current definition, the onset of organ dysfunction is a hallmark of sepsis. The preceding organ damage can be caused by excessive complement activation and neutrophil actions against the host, resulting in bystander injury of healthy tissue. However, in contrast, persistent and overwhelming inflammation also leads to a reduction in neutrophil responsiveness as well as complement components and thus may render patients at enhanced risk of spreading infection. This review provides an overview on the molecular and cellular processes that link complement with the two-faced functional alterations of neutrophils in sepsis. Finally, we describe novel tools to modulate this interplay beneficially in order to improve outcome.

Auxiliary activation of the complement system and its importance for the pathophysiology of clinical conditions

Activation and regulation of the cascade systems of the blood (the complement system, the coagulation/contact activation/kallikrein system, and the fibrinolytic system) occurs via activation of zymogen molecules to specific active proteolytic enzymes. Despite the fact that the generated proteases are all present together in the blood, under physiological conditions, the activity of the generated proteases is controlled by endogenous protease inhibitors. Consequently, there is remarkable little crosstalk between the different systems in the fluid phase. This concept review article aims at identifying and describing conditions where the strict system-related control is circumvented. These include clinical settings where massive amounts of proteolytic enzymes are released from tissues, e.g., during pancreatitis or post-traumatic tissue damage, resulting in consumption of the natural substrates of the specific proteases and the available protease inhibitor. Another example of cascade system dysregulation is disseminated intravascular coagulation, with canonical activation of all cascade systems of the blood, also leading to specific substrate and protease inhibitor elimination. The present review explains basic concepts in protease biochemistry of importance to understand clinical conditions with extensive protease activation.

The immune system's role in sepsis progression, resolution, and long-term outcome

Sepsis occurs when an infection exceeds local tissue containment and induces a series of dysregulated physiologic responses that result in organ dysfunction. A subset of patients with sepsis progress to septic shock, defined by profound circulatory, cellular, and metabolic abnormalities, and associated with a greater mortality. Historically, sepsis-induced organ dysfunction and lethality were attributed to the complex interplay between the initial inflammatory and later anti-inflammatory responses. With advances in intensive care medicine and goal-directed interventions, early 30-day sepsis mortality has diminished, only to steadily escalate long after "recovery" from acute events. As so many sepsis survivors succumb later to persistent, recurrent, nosocomial, and secondary infections, many investigators have turned their attention to the long-term sepsis-induced alterations in cellular immune function. Sepsis clearly alters the innate and adaptive immune responses for sustained periods of time after clinical recovery, with immune suppression, chronic inflammation, and persistence of bacterial representing such alterations. Understanding that sepsis-associated immune cell defects correlate with long-term mortality, more investigations have centered on the potential for immune modulatory therapy to improve long-term patient outcomes. These efforts are focused on more clearly defining and effectively reversing the persistent immune cell dysfunction associated with long-term sepsis mortality.

Cudratricusxanthone A attenuates renal injury in septic mice

As a natural compound extracted from the roots of Cudrania tricuspidata Bureau, Cudratricusxanthone A (CTXA) is known to possess hepatoprotective, anti-inflammatory, and anti-proliferative activities. This study was aimed to clarify the role of CTXA in modulating renal functional damage in a mouse model of sepsis and to elucidate its underlying mechanisms. We examined the renal protective effects of CTXA on cecal ligation and puncture (CLP)-induced renal damage by assessment of serum creatinine, blood urea nitrogen (BUN), lipid peroxidation, total glutathione, glutathione peroxidase activity, catalase activity, and superoxide dismutase activity. Post-treatment with CTXA resulted in a significant reduction in the deleterious renal functions by CLP, such as elevated BUN, creatinine, and urine protein. Induction of nitric oxide synthase and excessive production of nitric acid by CLP surgery were significantly reduced by post-treatment with CTXA via inhibiting nuclear factor-κB activation. Furthermore, the plasma levels of interleukin-6 and tumor necrosis factor-α were suppressed by CTXA post-treatment. Concurrently, CTXA treatment potently suppressed the CLP-induced septic lethality, rise of lipid peroxidation and markedly enhanced the antioxidant defense system by restoring the levels of superoxide dismutase, glutathione peroxidase, and catalase in kidney. The present results suggested that CTXA could protect against sepsis-triggered renal injury in mice.

Complement-induced activation of MAPKs and Akt during sepsis: role in cardiac dysfunction

Polymicrobial sepsis in mice causes myocardial dysfunction after generation of the complement anaphylatoxin, complement component 5a (C5a). C5a interacts with its receptors on cardiomyocytes (CMs), resulting in redox imbalance and cardiac dysfunction that can be functionally measured and quantitated using Doppler echocardiography. In this report we have evaluated activation of MAPKs and Akt in CMs exposed to C5a in vitro and after cecal ligation and puncture (CLP) in vivo In both cases, C5a in vitro caused activation (phosphorylation) of MAPKs and Akt in CMs, which required availability of both C5a receptors. Using immunofluorescence technology, activation of MAPKs and Akt occurred in left ventricular (LV) CMs, requiring both C5a receptors, C5aR1 and -2. Use of a water-soluble p38 inhibitor curtailed activation in vivo of MAPKs and Akt in LV CMs as well as the appearance of cytokines and histones in plasma from CLP mice. When mouse macrophages were exposed in vitro to LPS, activation of MAPKs and Akt also occurred. The copresence of the p38 inhibitor blocked these activation responses. Finally, the presence of the p38 inhibitor in CLP mice reduced the development of cardiac dysfunction. These data suggest that polymicrobial sepsis causes cardiac dysfunction that appears to be linked to activation of MAPKs and Akt in heart.-Fattahi, F., Kalbitz, M., Malan, E. A., Abe, E., Jajou, L., Huber-Lang, M. S., Bosmann, M., Russell, M. W., Zetoune, F. S., Ward, P. A. Complement-induced activation of MAPKs and Akt during sepsis: role in cardiac dysfunction.

Complement C5a Functions as a Master Switch for the pH Balance in Neutrophils Exerting Fundamental Immunometabolic Effects

During sepsis, excessive activation of the complement system with generation of the anaphylatoxin C5a results in profound disturbances in crucial neutrophil functions. Moreover, because neutrophil activity is highly dependent on intracellular pH (pH_i), we propose a direct mechanistic link between complement activation and neutrophil pH_i In this article, we demonstrate that in vitro exposure of human neutrophils to C5a significantly increased pH_i by selective activation of the sodium/hydrogen exchanger. Upstream signaling of C5a-mediated intracellular alkalinization was dependent on C5aR1, intracellular calcium, protein kinase C, and calmodulin, and downstream signaling regulated the release of antibacterial myeloperoxidase and lactoferrin. Notably, the pH shift caused by C5a increased the glucose uptake and activated glycolytic flux in neutrophils, resulting in a significant release of lactate. Furthermore, C5a induced acidification of the extracellular micromilieu. In experimental murine sepsis, pH_i of blood neutrophils was analogously alkalinized, which could be normalized by C5aR1 inhibition. In the clinical setting of sepsis, neutrophils from patients with septic shock likewise exhibited a significantly increased pH_i These data suggest a novel role for the anaphylatoxin C5a as a master switch of the delicate pH_i balance in neutrophils resulting in profound inflammatory and metabolic changes that contribute to hyperlactatemia during sepsis.

Selective Biological Responses of Phagocytes and Lungs to Purified Histones

Histones invoke strong proinflammatory responses in many different organs and cells. We assessed biological responses to purified or recombinant histones, using human and murine phagocytes and mouse lungs. H1 had the strongest ability in vitro to induce cell swelling independent of requirements for toll-like receptors (TLRs) 2 or 4. These responses were also associated with lactate dehydrogenase release. H3 and H2B were the strongest inducers of [Ca2+]i elevations in phagocytes. Cytokine and chemokine release from mouse and human phagocytes was predominately a function of H2A and H2B. Double TLR2 and TLR4 knockout (KO) mice had dramatically reduced cytokine release induced in macrophages exposed to individual histones. In contrast, macrophages from single TLR-KO mice showed few inhibitory effects on cytokine production. Using the NLRP3 inflammasome protocol, release of mature IL-1β was predominantly a feature of H1. Acute lung injury following the airway delivery of histones suggested that H1, H2A, and H2B were linked to alveolar leak of albumin and the buildup of polymorphonuclear neutrophils as well as the release of chemokines and cytokines into bronchoalveolar fluids. These results demonstrate distinct biological roles for individual histones in the context of inflammation biology and the requirement of both TLR2 and TLR4.

Assessment of interleukin-17A, C5a and RANTES for early diagnosis of neonatal sepsis - a preliminary study

The aim of the present study was to investigate serum levels of novel markers: interleukin 17A (IL-17A), anaphylatoxin C5a and chemokine regulated upon activation normal T-cell expressed and secreted (RANTES) in neonates with clinically suspected early-onset neonatal sepsis (EONS), and to compare their values with those of non-infected neonates. Eighteen neonates with clinical signs and symptoms of EONS were enrolled in this study. Fifty healthy, non-infected neonates served as the control group. In all neonates serum levels of IL-17A, C5a and RANTES were measured by solid-phase sandwich enzyme-linked immunosorbent assay (ELISA). At the time of investigation serum levels of anaphylatoxin C5a were significantly higher in neonates with clinical symptoms of EONS than in non-infected neonates (median 65.35 vs. 50.4 ng/ml, p = 0.034), whereas levels of RANTES were similar and levels of IL-17A were under detection limit of the method. Based on these preliminary results, serum levels of C5a may be a useful marker of inflammation in early onset neonatal sepsis. Because traditional methods of microbiological diagnostics in EONS are frequently unsuccessful, the search for an alternative laboratory biomarkers is of great clinical importance. Thus, there is a strong need for further studies evaluating usefulness of this anaphylatoxin in EONS diagnosis on a larger group of patients.