The role of complement in danger sensing and transmission

Complement activity and autophagy are dysregulated in the lungs of patients with nonresolvable COVID-19 requiring lung transplantation

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-induced coronavirus disease 2019 (COVID-19) pandemic has challenged the current understanding of the complement cascade mechanisms of host immune responses during infection-induced nonresolvable lung disease. While the complement system is involved in opsonization and phagocytosis of the invading pathogens, uncontrolled complement activation also leads to aberrant autophagic response and tissue damage. Our recent study revealed unique pathologic and fibrotic signature genes associated with epithelial bronchiolization in the lung tissues of patients with nonresolvable COVID-19 (NR-COVID-19) requiring lung transplantation. However, there is a knowledge gap if complement components are modulated to contribute to tissue damage and the fibrotic phenotype during NR-COVID-19. We, therefore, aimed to study the role of the complement factors and their corresponding regulatory proteins in the pathogenesis of NR-COVID-19. We further examined the association of complement components with mediators of the host autophagic response. We observed significant upregulation of the expression of the classical pathway factor C1qrs and alternative complement factors C3 and C5a, as well as the anaphylatoxin receptor C5aR1, in NR-COVID-19 lung tissues. Of note, complement regulatory protein, decay accelerating factor (DAF; CD55) was significantly downregulated at both transcript and protein levels in the NR-COVID-19 lungs, indicating a dampened host protective response. Furthermore, we observed significantly decreased levels of the autophagy mediators PPARγ and LC3a/b, which was corroborated by decreased expression of factor P and the C3b receptor CR1, indicating impaired clearance of damaged cells that may contribute to the fibrotic phenotype in NR-COVID-19 patients. Thus, our study revealed previously unrecognized complement dysregulation associated with impaired cell death and clearance of damaged cells, which may promote NR-COVID-19 in patients, ultimately necessitating lung transplantation. The identified network of dysregulated complement cascade activity indicates the interplay of regulatory factors and the receptor-mediated modulation of host immune and autophagic responses as potential therapeutic targets for treating NR-COVID-19.

Stress hyperglycaemia following trauma - a survival benefit or an outcome detriment?

PURPOSE OF REVIEW

Stress hyperglycaemia occur often in critically injured patients. To gain new consideration about it, this review compile current as well as known immunological and biochemical findings about causes and emergence.

RECENT FINDINGS

Glucose is the preferred energy substrate for fending immune cells, reparative tissue and the cardiovascular system following trauma. To fulfil these energy needs, the liver is metabolically reprogrammed to rebuild glucose from lactate and glucogenic amino acids (hepatic insulin resistance) at the expenses of muscles mass and - to a less extent - fat tissue (proteolysis, lipolysis, peripheral insulin resistance). This inevitably leads to stress hyperglycaemia, which is evolutionary preserved and seems to be an essential and beneficial survival response. It is initiated by damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs), intensified by immune cells itself and mainly ruled by tumour necrosis factor (TNF)α and catecholamines with lactate and hypoxia inducible factor (HIF)-1α as intracellular signals and lactate as an energy shuttle. Important biochemical mechanisms involved in this response are the Warburg effect as an efficient metabolic shortcut and the extended Cori cycle.

SUMMARY

Stress hyperglycaemia is beneficial in an acute life-threatening situation, but further research is necessary, to prevent trauma patients from the detrimental effects of persisting hyperglycaemia.

The Role of Inflammation in Cholestatic Liver Injury

Cholestasis is a common clinical event in which bile formation and excretion are blocked, leading to retention of bile acids or bile salts; whether it occurs intra- or extrahepatically, primary or secondary, its pathogenesis is still unclear and is influenced by a combination of factors. In a variety of inflammatory and immune cells such as neutrophils, macrophages (intrahepatic macrophages are also known as Kupffer cells), mast cells, NK cells, and even T cells in humoral immunity and B cells in cellular immunity, inflammation can be a "second strike" against cholestatic liver injury. These cells, stimulated by a variety of factors such as bile acids, inflammatory chemokines, and complement, can be activated and accumulate in the cholestatic liver, and with the involvement of inflammatory mediators and modulation by cytokines, can lead to destruction of hepatocytes and bile duct epithelial cells and exacerbate (and occasionally retard) the progression of cholestatic liver disease. In this paper, we summarized the new research advances proposed so far regarding the relationship between inflammation and cholestasis, aiming to provide reference for researchers and clinicians in the field of cholestatic liver injury research.

The role of fibromodulin in inflammatory responses and diseases associated with inflammation

Inflammation is an immune response that the host organism eliminates threats from foreign objects or endogenous signals. It plays a key role in the progression, prognosis as well as therapy of diseases. Chronic inflammatory diseases have been regarded as the main cause of death worldwide at present, which greatly affect a vast number of individuals, producing economic and social burdens. Thus, developing drugs targeting inflammation has become necessary and attractive in the world. Currently, accumulating evidence suggests that small leucine-rich proteoglycans (SLRPs) exhibit essential roles in various inflammatory responses by acting as an anti-inflammatory or pro-inflammatory role in different scenarios of diseases. Of particular interest was a well-studied member, termed fibromodulin (FMOD), which has been largely explored in the role of inflammatory responses in inflammatory-related diseases. In this review, particular focus is given to the role of FMOD in inflammatory response including the relationship of FMOD with the complement system and immune cells, as well as the role of FMOD in the diseases associated with inflammation, such as skin wounding healing, osteoarthritis (OA), tendinopathy, atherosclerosis, and heart failure (HF). By conducting this review, we intend to gain insight into the role of FMOD in inflammation, which may open the way for the development of new anti-inflammation drugs in the scenarios of different inflammatory-related diseases.

Role of the inflammasome in insulin resistance and type 2 diabetes mellitus

The inflammasome is a protein complex composed of a variety of proteins in cells and which participates in the innate immune response of the body. It can be activated by upstream signal regulation and plays an important role in pyroptosis, apoptosis, inflammation, tumor regulation, etc. In recent years, the number of metabolic syndrome patients with insulin resistance (IR) has increased year by year, and the inflammasome is closely related to the occurrence and development of metabolic diseases. The inflammasome can directly or indirectly affect conduction of the insulin signaling pathway, involvement the occurrence of IR and type 2 diabetes mellitus (T2DM). Moreover, various therapeutic agents also work through the inflammasome to treat with diabetes. This review focuses on the role of inflammasome on IR and T2DM, pointing out the association and utility value. Briefly, we have discussed the main inflammasomes, including NLRP1, NLRP3, NLRC4, NLRP6 and AIM2, as well as their structure, activation and regulation in IR were described in detail. Finally, we discussed the current therapeutic options-associated with inflammasome for the treatment of T2DM. Specially, the NLRP3-related therapeutic agents and options are widely developed. In summary, this article reviews the role of and research progress on the inflammasome in IR and T2DM.

Molecular Actors of Inflammation and Their Signaling Pathways: Mechanistic Insights from Zebrafish

Inflammation is a hallmark of the physiological response to aggressions. It is orchestrated by a plethora of molecules that detect the danger, signal intracellularly, and activate immune mechanisms to fight the threat. Understanding these processes at a level that allows to modulate their fate in a pathological context strongly relies on in vivo studies, as these can capture the complexity of the whole process and integrate the intricate interplay between the cellular and molecular actors of inflammation. Over the years, zebrafish has proven to be a well-recognized model to study immune responses linked to human physiopathology. We here provide a systematic review of the molecular effectors of inflammation known in this vertebrate and recapitulate their modes of action, as inferred from sterile or infection-based inflammatory models. We present a comprehensive analysis of their sequence, expression, and tissue distribution and summarize the tools that have been developed to study their function. We further highlight how these tools helped gain insights into the mechanisms of immune cell activation, induction, or resolution of inflammation, by uncovering downstream receptors and signaling pathways. These progresses pave the way for more refined models of inflammation, mimicking human diseases and enabling drug development using zebrafish models.

Intracellular complement: Evidence, definitions, controversies, and solutions

The term "intracellular complement" has been introduced recently as an umbrella term to distinguish functions of complement proteins that take place intracellularly, rather than in the extracellular environment. However, this rather undefined term leaves some confusion as to the classification of what intracellular complement really is, and as to which intracellular compartment(s) it should refer to. In this review, we will describe the evidence for both canonical and non-canonical functions of intracellular complement proteins, as well as the current controversies and unanswered questions as to the nature of the intracellular complement. We also suggest new terms to facilitate the accurate description and discussion of specific forms of intracellular complement and call for future experiments that will be required to provide more definitive evidence and a better understanding of the mechanisms of intracellular complement activity.

Dysregulation of complement system in neuropsychiatric disorders: A mini review

Complement system is one of the most important defense mechanisms of the innate immune system. In addition to their roles in immune regulation, complement proteins are also involved in neurodevelopment and adult brain plasticity. Complement dysregulation has been shown in neurodevelopmental disorders including schizophrenia and autism spectrum disorder as well as in mood disorders. A number of clinical as well as genetic studies suggest the role of complement proteins in the cortical thinning and excessive synaptic pruning frequently associated with schizophrenia. The changes in complement proteins are also associated with the pathophysiology of autism spectrum disorder, major depressive disorder and bipolar disorder, but warrant further research. In addition, rodent models suggest a strong case for complement system in anxiety-like behavior. In this article, we review the recent findings on the role of complement system in neuropsychiatric disorders. The possible uses for future complement targeted therapies are also discussed.

The Role of the Inflammatory Response in Mediating Functional Recovery Following Composite Tissue Injuries

Composite tissue injuries (CTI) are common among US Military Service members during combat operations, and carry a high potential of morbidity. Furthermore, CTI are often complicated due to an altered wound healing response, resulting in part from a dysregulation of the innate and adaptive immune responses. Unlike normal wound healing, in CTI, disruptions occur in innate immune responses, altering neutrophil functions, macrophage activation and polarization, further impacting the functions of T regulatory cells. Additionally, the biological underpinnings of these unfavorable wound healing conditions are multifactorial, including various processes, such as: ischemia, hypoxia, low nutrient levels, and altered cell metabolic pathways, among others, all of which are thought to trigger anergy in immune cells and destabilize adaptive immune responses. As a result, impaired wound healing is common in CTI. Herein, we review the altered innate and adaptive immune cells and their metabolic status and responses following CTI, and discuss the role a multi-pronged immunomodulatory approach may play in facilitating improved outcomes for afflicted patients.

Renal diseases and the role of complement: Linking complement to immune effector pathways and therapeutics

The complement system is an ancient and phylogenetically conserved key danger sensing system that is critical for host defense against pathogens. Activation of the complement system is a vital component of innate immunity required for the detection and removal of pathogens. It is also a central orchestrator of adaptive immune responses and a constituent of normal tissue homeostasis. Once complement activation occurs, this system deposits indiscriminately on any cell surface in the vicinity and has the potential to cause unwanted and excessive tissue injury. Deposition of complement components is recognized as a hallmark of a variety of kidney diseases, where it is indeed associated with damage to the self. The provenance and the pathophysiological role(s) played by complement in each kidney disease is not fully understood. However, in recent years there has been a renaissance in the study of complement, with greater appreciation of its intracellular roles as a cell-intrinsic system and its interplay with immune effector pathways. This has been paired with a profusion of novel therapeutic agents antagonizing complement components, including approved inhibitors against complement components (C)1, C3, C5 and C5aR1. A number of clinical trials have investigated the use of these more targeted approaches for the management of kidney diseases. In this review we present and summarize the evidence for the roles of complement in kidney diseases and discuss the available clinical evidence for complement inhibition.

Alterations of Plasma Galectin-3 and C3 Levels in Patients with Parkinson's Disease

Parkinson’s disease (PD) is characterized by progressive neurodegeneration of dopaminergic neurons in the ventral midbrain. The complement-phagosome pathway is involved in the pathogenesis of PD. Here we measured levels of complement-phagocytosis molecules, including galectin-3, C3, C4, and cathepsin D, in the plasma of 56 patients with PD, and 46 normal controls (NCs). Plasma levels of galectin-3 (9.93 ± 3.94 ng/mL) were significantly higher in PD patients compared with NCs (8.39 ± 1.95 ng/mL, p = 0.012), and demonstrated a positive correlation with Hoehn and Yahr stages in PD patients (R² = 0.218, p < 0.001). On the other hand, plasma C3 levels were significantly lower in PD patients (305.27 ± 205.16 μg/mL) compared with NCs (444.34 ± 245.54 μg/mL, p = 0.002). However, the levels did not correlate with Hoehn and Yahr stages (R² = 0.010, p = 0.469). Plasma levels of C4 and cathepsin D in PD patients were similar to those in NCs. Our results show possible altered complement-phagocytosis signals in the peripheral blood of PD patients, highlighting the potential of galectin-3 as a biomarker of PD.

Neutrophil and remnant clearance in immunity and inflammation

Neutrophil-centred inflammation and flawed clearance of neutrophils cause and exuberate multiple pathological conditions. These most abundant leukocytes exhibit very high daily turnover in steady-state and stress conditions. Various armours including oxidative burst, NETs and proteases function against pathogens, but also dispose neutrophils to spawn pro-inflammatory responses. Neutrophils undergo death through different pathways upon ageing, infection, executing the intruder's elimination. These include non-lytic apoptosis and other lytic deaths including NETosis, necroptosis and pyroptosis with distinct disintegration of the cellular membrane. This causes release and presence of different intracellular cytotoxic, and tissue-damaging content as cell remnants in the extracellular environment. The apoptotic cells and apoptotic bodies get cleared with non-inflammatory outcomes, while lytic deaths associated remnants including histones and cell-free DNA cause pro-inflammatory responses. Indeed, the enhanced frequencies of neutrophil-associated proteases, cell-free DNA and autoantibodies in diverse pathologies including sepsis, asthma, lupus and rheumatoid arthritis, imply disturbed neutrophil resolution programmes in inflammatory and autoimmune diseases. Thus, the clearance mechanisms of neutrophils and associated remnants are vital for therapeutics. Though studies focused on clearance mechanisms of senescent or apoptotic neutrophils so far generated a good understanding of the same, clearance of neutrophils undergoing distinct lytic deaths, including NETs, are being the subjects of intense investigations. Here, in this review, we are providing the current updates in the clearance mechanisms of apoptotic neutrophils and focusing on not so well-defined recognition, uptake and degradation of neutrophils undergoing lytic death and associated remnants that may provide new therapeutic approaches in inflammation and autoimmunity.

The Persistent Pain Transcriptome: Identification of Cells and Molecules Activated by Hyperalgesia

During persistent pain, the dorsal spinal cord responds to painful inputs from the site of injury, but the molecular modulatory processes have not been comprehensively examined. Using transcriptomics and multiplex in situ hybridization, we identified the most highly regulated receptors and signaling molecules in rat dorsal spinal cord in peripheral inflammatory and post-surgical incisional pain models. We examined a time course of the response including acute (2 hours) and longer term (2 day) time points after peripheral injury representing the early onset and instantiation of hyperalgesic processes. From this analysis, we identify a key population of superficial dorsal spinal cord neurons marked by somatotopic upregulation of the opioid neuropeptide precursor prodynorphin, and 2 receptors: the neurokinin 1 receptor, and anaplastic lymphoma kinase. These alterations occur specifically in the glutamatergic subpopulation of superficial dynorphinergic neurons. In addition to specific neuronal gene regulation, both models showed induction of broad transcriptional signatures for tissue remodeling, synaptic rearrangement, and immune signaling defined by complement and interferon induction. These signatures were predominantly induced ipsilateral to tissue injury, implying linkage to primary afferent drive. We present a comprehensive set of gene regulatory events across 2 models that can be targeted for the development of non-opioid analgesics. PERSPECTIVE: The deadly impact of the opioid crisis and the need to replace morphine and other opioids in clinical practice is well recognized. Embedded within this research is an overarching goal of obtaining foundational knowledge from transcriptomics to search for non-opioid analgesic targets. Developing such analgesics would address unmet clinical needs.

Complement Factor C5a Inhibits Apoptosis of Neutrophils-A Mechanism in Polytrauma?

Life-threatening polytrauma results in early activation of the complement and apoptotic system, as well as leukocytes, ultimately leading to the clearance of damaged cells. However, little is known about interactions between the complement and apoptotic systems in PMN (polymorphonuclear neutrophils) after multiple injuries. PMN from polytrauma patients and healthy volunteers were obtained and assessed for apoptotic events along the post-traumatic time course. In vitro studies simulated complement activation by the exposure of PMN to C3a or C5a and addressed both the intrinsic and extrinsic apoptotic pathway. Specific blockade of the C5a-receptor 1 (C5aR1) on PMN was evaluated for efficacy to reverse complement-driven alterations. PMN from polytrauma patients exhibited significantly reduced apoptotic rates up to 10 days post trauma compared to healthy controls. Polytrauma-induced resistance was associated with significantly reduced Fas-ligand (FasL) and Fas-receptor (FasR) on PMN and in contrast, significantly enhanced FasL and FasR in serum. Simulation of systemic complement activation revealed for C5a, but not for C3a, a dose-dependent abrogation of PMN apoptosis in both intrinsic and extrinsic pathways. Furthermore, specific blockade of the C5aR1 reversed C5a-induced PMN resistance to apoptosis. The data suggest an important regulatory and putative mechanistic and therapeutic role of the C5a/C5aR1 interaction on PMN apoptosis after polytrauma.

Role of intracellular complement activation in kidney fibrosis

Increased expression of complement C1r, C1s and C3 in kidney cells plays an important role in the pathogenesis of kidney fibrosis. Our studies suggest that activation of complement in kidney cells with increased generation of C3 and its fragments occurs by activation of classical and alternative pathways. Single nuclei RNA sequencing studies in kidney tissue from unilateral ureteral obstruction mice show that increased synthesis of complement C3 and C5 occurs primarily in renal tubular epithelial cells (proximal and distal), while increased expression of complement receptors C3ar1 and C5ar1 occurs in interstitial cells including immune cells like monocytes/macrophages suggesting compartmentalization of complement components during kidney injury. Although global deletion of C3 and macrophage ablation prevent inflammation and reduced kidney tissue scarring, the development of mice with cell-specific deletion of complement components and their regulators could bring further insights into the mechanisms by which intracellular complement activation leads to fibrosis and progressive kidney disease. 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.

Identification of significant potential signaling pathways and differentially expressed proteins in patients with wheat intolerance based on quantitative proteomics

Wheat intolerance has various systemic manifestations that can affect people's quality of life, and few studies have focused on the mechanism of wheat intolerance and the signaling pathways involved in wheat intolerance have not been fully identified. We compared the protein profiles of patients with wheat intolerance with those of healthy controls using LASSO (least absolute shrinkage and selection operator) and PLS (partial least squares regression) to obtain DEPs (differentially expressed proteins) for GO (Gene Ontology) analysis, KEGG (Kyoto Encyclopedia of Genes and Genomes) pathway enrichment analysis, and PPI (protein-protein interaction) network analysis. Internal validation and external validation were conducted for target proteomics testing. The correlation between differently expressed protein and the wheat-specific IgG antibody concentration was analyzed. Then ROC curve (receiver operating characteristic curve) was generated to validate the differentially expressed proteins. We identified 33 DEPs as significant candidate proteins of wheat intolerance. These proteins were mainly enriched in complement and coagulation cascade pathways, immune activation, and immune response-related pathways. After internal and external target proteomics validation, CFHR3 (complement factor H-related protein 3) was identified as a key protein that may have an important role in wheat intolerance. We found CFHR3 protein expression abundance and the wheat-specific IgG antibody concentration were significantly negatively correlated (P = 0.035; Spearman correlation coefficient r = -0.565). The AUC (median area under the ROC curve) of CFHR3 is 0.857 in external verification data. This study provides insights into wheat intolerance that can be used to further explore the pathogenesis of this condition. SIGNIFICANCE: Proteomics has performed important potential in food allergy research and is conducive to improving our comprehension on molecular mechanisms of food allergy. The present study identified significant signaling pathways and differentially expressed proteins in patients with wheat intolerance by means of bioinformatics from the viewpoint of mass spectrometry-based proteomics, which provided insights into further research on the pathogenesis and timely diagnosis of wheat intolerance.

Complement in metabolic disease: metaflammation and a two-edged sword

We are currently experiencing an enduring global epidemic of obesity and diabetes. It is now understood that chronic low-grade tissue inflammation plays an important role in metabolic disease, brought upon by increased uptake of a so-called Western diet, and a more sedentary lifestyle. Many evolutionarily conserved links exist between metabolism and the immune system, and an imbalance in this system induced by chronic over-nutrition has been termed 'metaflammation'. The complement system is an important and evolutionarily ancient part of innate immunity, but recent work has revealed that complement not only is involved in the recognition of pathogens and induction of inflammation, but also plays important roles in cellular and tissue homeostasis. Complement can therefore contribute both positively and negatively to metabolic control, depending on the nature and anatomical site of its activity. This review will therefore focus on the interactions of complement with mechanisms and tissues relevant for metabolic control, obesity and diabetes.

Neuroinflammatory Mechanisms of Mitochondrial Dysfunction and Neurodegeneration in Glaucoma

The exact mechanism of retinal ganglion cell loss in the pathogenesis of glaucoma is yet to be understood. Mitochondrial damage-associated molecular patterns (DAMPs) resulting from mitochondrial dysfunction have been linked to Leber's hereditary optic neuropathy and autosomal dominant optic atrophy, as well as to brain neurodegenerative diseases. Recent evidence shows that, in conditions where mitochondria are damaged, a sustained inflammatory response and downstream pathological inflammation may ensue. Mitochondrial damage has been linked to the accumulation of age-related mitochondrial DNA mutations and mitochondrial dysfunction, possibly through aberrant reactive oxygen species production and defective mitophagy. The present review focuses on how mitochondrial dysfunction may overwhelm the ability of neurons and glial cells to adequately maintain homeostasis and how mitochondria-derived DAMPs trigger the immune system and induce neurodegeneration.

Heat-Inactivation of Human Serum Destroys C1 Inhibitor, Pro-motes Immune Complex Formation, and Improves Human T Cell Function

Heat-inactivation of sera is used to reduce possible disturbing effects of complement factors in cell-culture experiments, but it is controversially discussed whether this procedure is appropriate or could be neglected. Here, we report a strong impact of heat-inactivation of human sera on the activation and effector functions of human CD4+ T cells. While T cells cultured with native sera were characterized by a higher proliferation rate and higher expression of CD28, heat-inactivated sera shaped T cells towards on-blast formation, higher cytokine secretion (interferon γ, tumor necrosis factor, and interleukin-17), stronger CD69 and PD-1 expression, and increased metabolic activity. Heat-inactivated sera contained reduced amounts of complement factors and regulators like C1 inhibitor, but increased concentrations of circulating immune complexes. Substitution of C1 inhibitor reduced the beneficial effect of heat-inactivation in terms of cytokine release, whereas surface-molecule expression was affected by the addition of complex forming anti-C1q antibody. Our data clearly demonstrate a beneficial effect of heat-inactivation of human sera for T cell experiments but indicate that beside complement regulators and immune complexes other components might be relevant. Beyond that, this study further underpins the strong impact of the complement system on T cell function.

Soluble pattern recognition molecules: Guardians and regulators of homeostasis at airway mucosal surfaces

Maintenance of homeostasis at body barriers that are constantly challenged by microbes, toxins and potentially bioactive (macro)molecules requires complex, highly orchestrated mechanisms of protection. Recent discoveries in respiratory research have shed light on the unprecedented role of airway epithelial cells (AEC), which, besides immune cells homing to the lung, also significantly contribute to host defence by expressing membrane‐bound and soluble pattern recognition receptors (sPRR). Recent evidence suggests that distinct, evolutionary ancient, sPRR secreted by AEC might become activated by usually innocuous proteins, commonly referred to as allergens. We here provide a systematic overview on sPRR detectable in the mucus lining of AEC. Some of them become actively produced and secreted by AECs (like the pentraxins C‐reactive protein and pentraxin 3; the collectins mannose binding protein and surfactant proteins A and D; H‐ficolin; serum amyloid A; and the complement components C3 and C5). Others are elaborated by innate and adaptive immune cells such as monocytes/macrophages and T cells (like the pentraxins C‐reactive protein and pentraxin 3; L‐ficolin; serum amyloid A; and the complement components C3 and C5). Herein we discuss how sPRRs may contribute to homeostasis but sometimes also to overt disease (e.g. airway hyperreactivity and asthma) at the alveolar–air interface.

Outside in: Roles of complement in autophagy

The complement system is a well-characterized cascade of extracellular serum proteins that is activated by pathogens and unwanted waste material. Products of activated complement signal to the host cells via cell surface receptors, eliciting responses such as removal of the stimulus by phagocytosis. The complement system therefore functions as a warning system, resulting in removal of unwanted material. This review describes how extracellular activation of the complement system can also trigger autophagic responses within cells, up-regulating protective homeostatic autophagy in response to perceived stress, but also initiating targeted anti-microbial autophagy in order to kill intracellular cytoinvasive pathogens. In particular, we will focus on recent discoveries that indicate that complement may also have roles in detection and autophagy-mediated disposal of unwanted materials within the intracellular environment. We therefore summarize the current evidence for complement involvement in autophagy, both by transducing signals across the cell membrane, as well as roles within the cellular environment. 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.

C5aR1 Activation Drives Early IFN-γ Production to Control Experimental Toxoplasma gondii Infection

Toxoplasma gondii (T. gondii) is a parasite infecting animals and humans. In intermediate hosts, such as humans or rodents, rapidly replicating tachyzoites drive vigorous innate and adaptive immune responses resulting in bradyzoites that survive within tissue cysts. Activation of the innate immune system is critical during the early phase of infection to limit pathogen growth and to instruct parasite-specific adaptive immunity. In rodents, dendritic cells (DCs) sense T. gondii through TLR11/12, leading to IL-12 production, which activates NK cells to produce IFN-γ as an essential mechanism for early parasite control. Further, C3 can bind to T. gondii resulting in limited complement activation. Here, we determined the role of C5a/C5aR1 axis activation for the early innate immune response in a mouse model of peritoneal T. gondii infection. We found that C5ar1^−/− animals suffered from significantly higher weight loss, disease severity, mortality, and parasite burden in the brain than wild type control animals. Severe infection in C5ar1^−/− mice was associated with diminished serum concentrations of IL-12, IL-27, and IFN-γ. Importantly, the serum levels of pro-inflammatory cytokines, including IL-1α, IL-6, and TNF-α, as well as several CXC and CC chemokines, were decreased in comparison to wt animals, whereas anti-inflammatory IL-10 was elevated. The defect in IFN-γ production was associated with diminished Ifng mRNA expression in the spleen and the brain, reduced frequency of IFN-γ⁺ NK cells in the spleen, and decreased Nos2 expression in the brain of C5ar1^−/− mice. Mechanistically, DCs from the spleen of C5ar1^−/− mice produced significantly less IL-12 in response to soluble tachyzoite antigen (STAg) stimulation in vivo and in vitro. Our findings suggest a model in which the C5a/C5aR1 axis promotes IL-12 induction in splenic DCs that is critical for IFN-γ production from NK cells and subsequent iNOS expression in the brain as a critical mechanism to control acute T. gondii infection.

Complement and human T cell metabolism: Location, location, location

The complement system represents one of the evolutionary oldest arms of our immune system and is commonly recognized as a liver-derived and serum-active system critical for providing protection against invading pathogens. Recent unexpected findings, however, have defined novel and rather "uncommon" locations and activities of complement. Specifically, the discovery of an intracellularly active complement system-the complosome-and its key role in the regulation of cell metabolic pathways that underly normal human T cell responses have taught us that there is still much to be discovered about this system. Here, we summarize the current knowledge about the emerging functions of the complosome in T cell metabolism. We further place complosome activities among the non-canonical roles of other intracellular innate danger sensing systems and argue that a "location-centric" view of complement evolution could logically justify its close connection with the regulation of basic cell physiology.

TLR4 Cross-Talk With NLRP3 Inflammasome and Complement Signaling Pathways in Alzheimer's Disease

Amyloid plaques, mainly composed of abnormally aggregated amyloid β-protein (Aβ) in the brain parenchyma, and neurofibrillary tangles (NFTs), consisting of hyperphosphorylated tau protein aggregates in neurons, are two pathological hallmarks of Alzheimer's disease (AD). Aβ fibrils and tau aggregates in the brain are closely associated with neuroinflammation and synapse loss, characterized by activated microglia and dystrophic neurites. Genome-wide genetic association studies revealed important roles of innate immune cells in the pathogenesis of late-onset AD by recognizing a dozen genetic risk loci that modulate innate immune activities. Furthermore, microglia, brain resident innate immune cells, have been increasingly recognized to play key, opposing roles in AD pathogenesis by either eliminating toxic Aβ aggregates and enhancing neuronal plasticity or producing proinflammatory cytokines, reactive oxygen species, and synaptotoxicity. Aggregated Aβ binds to toll-like receptor 4 (TLR4) and activates microglia, resulting in increased phagocytosis and cytokine production. Complement components are associated with amyloid plaques and NFTs. Aggregated Aβ can activate complement, leading to synapse pruning and loss by microglial phagocytosis. Systemic inflammation can activate microglial TLR4, NLRP3 inflammasome, and complement in the brain, leading to neuroinflammation, Aβ accumulation, synapse loss and neurodegeneration. The host immune response has been shown to function through complex crosstalk between the TLR, complement and inflammasome signaling pathways. Accordingly, targeting the molecular mechanisms underlying the TLR-complement-NLRP3 inflammasome signaling pathways can be a preventive and therapeutic approach for AD.

New insights into the immune functions of complement

The recognition of microbial or danger-associated molecular patterns by complement proteins initiates a cascade of events that culminates in the activation of surface complement receptors on immune cells. Such signalling pathways converge with those activated downstream of pattern recognition receptors to determine the type and magnitude of the immune response. Intensive investigation in the field has uncovered novel pathways that link complement-mediated signalling with homeostatic and pathological T cell responses. More recently, the observation that complement proteins also act in the intracellular space to shape T cell fates has added a new layer of complexity. Here, we consider fundamental mechanisms and novel concepts at the interface of complement biology and immunity and discuss how these affect the maintenance of homeostasis and the development of human pathology.

Complement Component C5 and TLR Molecule CD14 Mediate Heme-Induced Thromboinflammation in Human Blood

Heme is a critical danger molecule liberated from hemeproteins in various conditions, including from hemoglobin in hemolytic diseases. Heme may cause thromboinflammatory damage by activating inflammatory and hemostatic pathways, such as complement, the TLRs, coagulation, and platelets. In this study, we explored the effect of single and dual inhibition of complement component C5 and TLR coreceptor CD14 on heme-induced thromboinflammation in an ex vivo human whole blood model. Heme induced a dose-dependent activation of complement via the alternative pathway. Single inhibition of C5 by eculizumab attenuated the release of IL-6, IL-8, TNF, MCP-1, MIP-1α, IFN-γ, LTB-4, MMP-8 and -9, and IL-1Ra with more than 60% (p < 0.05 for all) reduced the upregulation of CD11b on granulocytes and monocytes by 59 and 40%, respectively (p < 0.05), and attenuated monocytic tissue factor expression by 33% (p < 0.001). Blocking CD14 attenuated IL-6 and TNF by more than 50% (p < 0.05). In contrast to single inhibition, combined C5 and CD14 was required for a significantly attenuated prothrombin cleavage (72%, p < 0.05). Markers of thromboinflammation were also quantified in two patients admitted to the hospital with sickle cell disease (SCD) crisis. Both SCD patients had pronounced hemolysis and depleted plasma hemopexin and haptoglobin. Plasma heme and complement activation was markedly increased in one patient, a coinciding observation as demonstrated ex vivo. In conclusion, heme-induced thromboinflammation was largely attenuated by C5 inhibition alone, with a beneficial effect of adding a CD14 inhibitor to attenuate prothrombin activation. Targeting C5 has the potential to reduce thromboinflammation in SCD crisis patients.

Early complementopathy predicts the outcomes of patients with trauma

Background

Complementopathy (rapid complement activation and consumption after trauma) has been reported in trauma patients, but the underlying mechanism of these phenomena and their clinical significance remain unclear. This study aimed to determine the complement/complement pathway activation and identify the association of complement activation with clinical outcomes in trauma patients.

Methods

We studied 33 trauma patients with mean Injury Severity Score of 25, and 25 healthy volunteers. Sera were collected on patients’ arrival at the emergency department, as well as 1, 2, 3, 5, and 7 days after trauma, to measure the levels of terminal complement activation product soluble C5b-9 (sC5b-9) by ELISA. In addition, the functional complement activation pathway was evaluated using a commercial complement system screening kit.

Results

Serum concentrations of sC5b-9 (complement terminal pathway activity) were significantly increased in trauma patients throughout the entire observation period except on day 1. Complement terminal activities were significantly higher in 27 of 33 patients with systemic inflammatory response syndrome (SIRS) than non-SIRS patients on day 2, day 5, and day 7. Increased serum levels of sC5b-9 positively correlated with SIRS. Functional complement analysis revealed that the classical pathway was the predominant pathway responsible for complement activation. Burn patients tended to have a greater and prolonged classical pathway activation than non-burn patients, and burn injury and blunt injury were associated with higher blood levels of sC5b-9 than penetrating injury.

Discussion

Early complement activation through the classical pathway after trauma is observed and positively correlated with the development of SIRS. Thus, monitoring of the complement system might be beneficial in the care of critically injured patients.

Level of evidence

III.

Study type

Prognostic.

Toll-like receptor 2 confers partial neuroprotection during prion disease

Neuroinflammation and neurodegeneration are common during prion infection, but the mechanisms that underlie these pathological features are not well understood. Several components of innate immunity, such as Toll-like receptor (TLR) 4 and Complement C1q, have been shown to influence prion disease. To identify additional components of innate immunity that might impact prion disease within the central nervous system (CNS), we screened RNA from brains of pre-clinical and clinical 22L-infected mice for alterations in genes associated with innate immunity. Transcription of several genes encoding damage-associated molecular pattern (DAMP) proteins and receptors were increased in the brains of prion-infected mice. To investigate the role of some of these proteins in prion disease of the CNS, we infected mice deficient in DAMP receptor genes Tlr2, C3ar1, and C5ar1 with 22L scrapie. Elimination of TLR2 accelerated disease by a median of 10 days, while lack of C3aR1 or C5aR1 had no effect on disease tempo. Histopathologically, all knockout mouse strains tested were similar to infected control mice in gliosis, vacuolation, and PrPSc deposition. Analysis of proinflammatory markers in the brains of infected knockout mice indicated only a few alterations in gene expression suggesting that C5aR1 and TLR2 signaling did not act synergistically in the brains of prion-infected mice. These results indicate that signaling through TLR2 confers partial neuroprotection during prion infection.

Expression of complement C3, C5, C3aR and C5aR1 genes in resting and activated CD4+ T cells

Complement activation is traditionally thought to occur in the extracellular space. However, it has been suggested that complement proteins are activated and function at additional locations. T cells contain intracellular stores of C3 and C5 that can be cleaved into C3a and C5a and bind to intracellular receptors, which have been shown to be of vital importance for the differentiation and function of these cells. However, whether the origin of the complement proteins located within T cells is derived from endogenous produced complement or from an uptake dependent mechanism is unknown. The presence of intracellular C3 in T cells from normal donors was investigated by fluorescence microscopy and flow cytometry. Moreover, mRNA expression levels of several genes encoding for complement proteins with primary focus on C3, C3aR, C5 and C5aR1 during resting state and upon activation of CD4⁺ T cells were investigated by a quantitative PCR technique. Furthermore, the gene expression level was evaluated at different time points. We confirmed the presence of intracellular C3 protein in normal T-cells. However, we could not see any increase in mRNA levels using any activation strategy tested. On the contrary, we observed a slight increase in C3 and C5aR1 mRNA only in the non-activated T-cells compared to the activated T cells, and a decrease in the activated T-cells at different incubation time points. Our results show that there is a baseline intracellular expression of the complement C3, C5, C3aR and C5aR1 genes in normal CD4⁺ T cells, but that expression is not increased during T-cell activation, but rather down regulated. Thus, the pool of intracellular complement in CD4⁺ T cells may either be due to accumulated complement due low-grade expression or arise from the circulation from an uptake dependent mechanism, but these possibilities are not mutually exclusive.

Complement and the Regulation of T Cell Responses

The complement system is an evolutionarily ancient key component of innate immunity required for the detection and removal of invading pathogens. It was discovered more than 100 years ago and was originally defined as a liver-derived, blood-circulating sentinel system that classically mediates the opsonization and lytic killing of dangerous microbes and the initiation of the general inflammatory reaction. More recently, complement has also emerged as a critical player in adaptive immunity via its ability to instruct both B and T cell responses. In particular, work on the impact of complement on T cell responses led to the surprising discoveries that the complement system also functions within cells and is involved in regulating basic cellular processes, predominantly those of metabolic nature. Here, we review current knowledge about complement's role in T cell biology, with a focus on the novel intracellular and noncanonical activities of this ancient system.

Unexpected Roles for Intracellular Complement in the Regulation of Th1 Responses

The complement system is generally recognized as an evolutionarily ancient and critical part of innate immunity required for the removal of pathogens that have breached the protective host barriers. It was originally defined as a liver-derived serum surveillance system that induces the opsonization and killing of invading microbes and amplifies the general inflammatory reactions. However, studies spanning the last four decades have established complement also as a vital bridge between innate and adaptive immunity. Furthermore, recent work on complement, and in particular its impact on human T helper 1 (Th1) responses, has led to the unexpected findings that the complement system also functions within cells and that it participates in the regulation of basic processes of the cell, including metabolism. These recent new insights into the unanticipated noncanonical activities of this ancient system suggest that the functions of complement extend well beyond mere host protection and into cellular physiology.

A Novel Role for C5a in B-1 Cell Homeostasis

B-1 cells constitute a unique subpopulation of lymphocytes residing mainly in body cavities like the peritoneal cavity (PerC) but are also found in spleen and bone marrow (BM). As innate-like B cells, they mediate first line immune defense through low-affinity natural IgM (nIgM) antibodies. PerC B-1 cells can egress to the spleen and differentiate into nIgM antibody-secreting plasma cells that recognize conserved exogenous and endogenous cellular structures. Homing to and homeostasis within the PerC are regulated by the chemokine CXCL13 released by PerC macrophages and stroma cells. However, the exact mechanisms underlying the regulation of CXCL13 and B-1 homeostasis are not fully explored. B-1 cells play important roles in the inflammatory response to infection, autoimmunity, ischemia/reperfusion injury, obesity, and atherosclerosis. Remarkably, this list of inflammatory entities has a strong overlap with diseases that are regulated by complement suggesting a link between B-1 cells and the complement system. Interestingly, up to now, no data exist regarding the role of complement in B-1 cell biology. Here, we demonstrate for the first time that C5a regulates B-1 cell steady-state dynamics within the peritoneum, the spleen, and the BM. We found decreased B-1a cell numbers in the peritoneum and the spleen of C5aR1^−/− mice associated with increased B1-a and B1-b numbers in the spleen and high serum titers of nIgM antibodies directed against phosphorylcholine and several pneumococcal polysaccharides. Similarly, peritoneal B-1a cells were decreased in the peritoneum and splenic B-1a and B-1b cells were increased in C5aR2^−/− mice. The decrease in peritoneal B-1 cell numbers was associated with decreased peritoneal CXCL13 levels in C5aR1^−/− and C5aR2^−/− mice. In search for mechanisms, we found that combined TLR2 and IL-10 receptor activation in PerC macrophages induced strong CXCL13 production, which was significantly reduced in cells from C5aR1- and C5aR2-deficient mice and after combined C5aR-targeting. Such stimulation also induced marked local C5 production by PerC macrophages and C5a generation. Importantly, peritoneal in vivo administration of C5a increased CXCL13 production. Taken together, our findings suggest that local non-canonical C5 activation in PerC macrophages fuels CXCL13 production as a novel mechanism to control B-1 cell homeostasis.

Old dogs-new tricks: immunoregulatory properties of C3 and C5 cleavage fragments

The activation of the complement system by canonical and non-canonical mechanisms results in the generation of multiple C3 and C5 cleavage fragments including anaphylatoxins C3a and C5a as well as opsonizing C3b/iC3b. It is now well appreciated that anaphylatoxins not only act as pro-inflammatory mediators but as immunoregulatory molecules that control the activation status of cells and tissue at several levels. Likewise, C3b/iC3b is more than the opsonizing fragment that facilitates engulfment and destruction of targets by phagocytes. In the circulation, it also facilitates the transport and delivery of bacteria and immune complexes to phagocytes, through a process known as immune adherence, with consequences for adaptive immunity. Here, we will discuss non-classical immunoregulatory properties of C3 and C5 cleavage fragments. We highlight the influence of anaphylatoxins on Th2 and Th17 cell development during allergic asthma with a particular emphasis on their role in the modulation of CD11b⁺ conventional dendritic cells and monocyte-derived dendritic cells. Furthermore, we discuss the control of anaphylatoxin-mediated activation of dendritic cells and allergic effector cells by adaptive immune mechanisms that involve allergen-specific IgG1 antibodies and plasma or regulatory T cell-derived IL-10 production. Finally, we take a fresh look at immune adherence with a particular focus on the development of antibacterial cytotoxic T-cell responses.

Fc microparticles can modulate the physical extent and magnitude of complement activity

The complement system is an integral component of the humoral immune system, and describes a cascade of interacting proteins responsible for the opsonization and lysis of foreign pathogens, in addition to the recruitment of immune cells. However, complement activation is also implicated in the progression and complication of immune dysfunctions such as sepsis. Microparticle (MP) biomaterials capable of tuning the local magnitude of serum complement activation could improve complement-mediated cytotoxicity to serum-resistant bacteria or calm an overactive immune response during sepsis. We demonstrate that model Fc-functionalized microparticles can be designed to either enhance or diminish the local cytotoxic effect of complement activation in human serum. The particles were formed with either the antibody Fc domains oriented outward from the particle surface or randomly adsorbed in a non-oriented fashion. In the oriented Fc form, complement products were directly sequestered to the particle surface, including C5a, a potent anaphylatoxin that, when elevated, is associated with poor sepsis prognosis. The oriented particle also lowered the cytotoxicity of serum and thus decreased the antibiotic effect when compared to serum alone. Conversely, the non-oriented microparticles were found to sequester similar levels of C5a, but much lower levels of iC3b and TCC on the microparticle surface, thereby increasing the amount of the soluble terminal complement complex. In addition, the non-oriented microparticles extend the distance over which TCC forms and enhance serum cytotoxicity to bacteria. Together, these two types of complement-modulating particles provide the first biomaterial that can functionally modify the range of complement activation at sites distant from the particle surface. Thus, biomaterials that exploit Fc presentation provide new possibilities to functionally modulate complement activation to achieve a desired clinical result.

Impaired Surface Expression of HLA-DR, TLR2, TLR4, and TLR9 in Ex Vivo-In Vitro Stimulated Monocytes from Severely Injured Trauma Patients

Objective. Trauma patients (TP) frequently develop an imbalanced immune response that often causes infectious postinjury complications. Monocytes show a diminished capability of both producing proinflammatory cytokines and antigen presentation after trauma. TLR2, TLR4, and TLR9 recognize pathogens and subsequently activate monocytes. While there are conflictive data about TLR2 and TLR4 expression after trauma, no studies about the expression of TLR2, TLR4, TLR9, and HLA-DR on monocytes from TP after their secondary ex vivo-in vitro "hit" have been reported. Methods/Results. Ex vivo-in vitro lipopolysaccharide- (LPS-) stimulated blood from TP showed diminished interleukin- (IL-) 1β-release in TP for five postinjury days compared to healthy volunteers (HV). The recovery was observed at day 5. In parallel, monocytes from TP showed an impaired capability of TLR2, TLR4, and TLR9 expression after secondary stimulation compared to HV, while the measurement of unstimulated samples showed significant reduction of TLR4 and TLR9 at ED. Furthermore, HLA-DR decreased after trauma and was even more profound by stimulation of monocytes. Ratio of monocytes to leukocytes was significantly increased at days 6 and 7 after trauma compared to HV. Conclusion. Impaired expression of TLRs and HLA-DR in acute inflammatory conditions may be responsible for the well-described monocyte paralysis after severe trauma.

Complement activation by cholesterol crystals triggers a subsequent cytokine response

In the host a diverse collection of endogenous danger signals is constantly generated consisting of waste material as protein aggregates or crystalline materials that are recognized and handled by soluble pattern recognition receptors and phagocytic cells of the innate immune system. These signals may under certain circumstances drive processes leading to adverse inflammation. One example is cholesterol crystals (CC) that accumulate in the vessel wall during early phases of atherogenesis and represent an important endogenous danger signal promoting inflammation. CC is recognized by the lectin- and classical pathways of the complement system resulting in activation of C3 and C5 with release of inflammatory mediators like the potent C5a fragment. Complement activation by CC leads to crosstalk with the NLRP3 inflammasome-caspase-1 pathway and production of IL-1β. Neutralization of IL-1β may have beneficial effects on atherosclerosis and a large clinical trial with an IL-1β inhibitor is currently in progress (the CANTOS study). However, upstream inhibition of CC-induced inflammation by using a complement inhibitor may be more efficient in treating atherosclerosis since this will block initiation of inflammation processes before downstream release of cytokines including IL-1β. Another therapeutic candidate can be broad-acting 2-hydroxypropyl-β-cyclodextrin, a compound that targets several mechanisms such as cholesterol efflux, complement gene expression, and the NLRP3 pathway. In summary, emerging evidence show that complement is a key upstream player in the pathophysiology of atherosclerosis and that therapy aiming at inhibiting complement could be effective in controlling atherosclerosis.

Dual inhibition of complement and Toll-like receptors as a novel approach to treat inflammatory diseases-C3 or C5 emerge together with CD14 as promising targets

Review of how targeting key upstream molecules at the recognition phase of innate immunity exert anti-inflammatory effects; a potential therapeutic regimen for inflammatory diseases.

The host is protected by pattern recognition systems, including complement and TLRs, which are closely cross-talking. If improperly activated, these systems might induce tissue damage and disease. Inhibition of single downstream proinflammatory cytokines, such as TNF, IL-1β, and IL-6, have failed in clinical sepsis trials, which might not be unexpected, given the substantial amounts of mediators involved in the pathogenesis of this condition. Instead, we have put forward a hypothesis of inhibition at the recognition phase by “dual blockade” of bottleneck molecules of complement and TLRs. By acting upstream and broadly, the dual blockade could be beneficial in conditions with improper or uncontrolled innate immune activation threatening the host. Key bottleneck molecules in these systems that could be targets for inhibition are the central complement molecules C3 and C5 and the important CD14 molecule, which is a coreceptor for several TLRs, including TLR4 and TLR2. This review summarizes current knowledge of inhibition of complement and TLRs alone and in combination, in both sterile and nonsterile inflammatory processes, where activation of these systems is of crucial importance for tissue damage and disease. Thus, dual blockade might provide a general, broad-acting therapeutic regimen against a number of diseases where innate immunity is improperly activated.

Complement-Mediated Regulation of Metabolism and Basic Cellular Processes

Complement is well appreciated as a critical arm of innate immunity. It is required for the removal of invading pathogens and works by directly destroying them through the activation of innate and adaptive immune cells. However, complement activation and function is not confined to the extracellular space but also occurs within cells. Recent work indicates that complement activation regulates key metabolic pathways and thus can impact fundamental cellular processes, such as survival, proliferation, and autophagy. Newly identified functions of complement include a key role in shaping metabolic reprogramming, which underlies T cell effector differentiation, and a role as a nexus for interactions with other effector systems, in particular the inflammasome and Notch transcription-factor networks. This review focuses on the contributions of complement to basic processes of the cell, in particular the integration of complement with cellular metabolism and the potential implications in infection and other disease settings.

Complement inhibition in C3 glomerulopathy

C3 glomerulopathy (C3G) describes a spectrum of glomerular diseases defined by shared renal biopsy pathology: a predominance of C3 deposition on immunofluorescence with electron microscopy permitting disease sub-classification. Complement dysregulation underlies the observed pathology, a causal relationship that is supported by well described studies of genetic and acquired drivers of disease. In this article, we provide an overview of the features of C3G, including a discussion of disease definition and a review of the causal role of complement. We discuss molecular markers of disease and how biomarkers are informing our evolving understanding of underlying pathology. Research advances are laying the foundation for complement inhibition as a targeted approach to treatment of C3G.

A novel "complement-metabolism-inflammasome axis" as a key regulator of immune cell effector function

The inflammasomes are intracellular multiprotein complexes that induce and regulate the generation of the key pro‐inflammatory cytokines IL‐1β and IL‐18 in response to infectious microbes and cellular stress. The activation of inflammasomes involves several upstream signals including classic pattern or danger recognition systems such as the TLRs. Recently, however, the activation of complement receptors, such as the anaphylatoxin C3a and C5a receptors and the complement regulator CD46, in conjunction with the sensing of cell metabolic changes, for instance increased amino acid influx and glycolysis (via mTORC1), have emerged as additional critical activators of the inflammasome. This review summarizes recent advances in our knowledge about complement‐mediated inflammasome activation, with a specific focus on a novel “complement – metabolism – NLRP3 inflammasome axis.”

Complement in disease: a defence system turning offensive

Although the complement system is primarily perceived as a host defence system, a more versatile, yet potentially more harmful side of this innate immune pathway as an inflammatory mediator also exists. The activities that define the ability of the complement system to control microbial threats and eliminate cellular debris - such as sensing molecular danger patterns, generating immediate effectors, and extensively coordinating with other defence pathways - can quickly turn complement from a defence system to an aggressor that drives immune and inflammatory diseases. These host-offensive actions become more pronounced with age and are exacerbated by a variety of genetic factors and autoimmune responses. Complement can also be activated inappropriately, for example in response to biomaterials or transplants. A wealth of research over the past two decades has led to an increasingly finely tuned understanding of complement activation, identified tipping points between physiological and pathological behaviour, and revealed avenues for therapeutic intervention. This Review summarizes our current view of the key activating, regulatory, and effector mechanisms of the complement system, highlighting important crosstalk connections, and, with an emphasis on kidney disease and transplantation, discusses the involvement of complement in clinical conditions and promising therapeutic approaches.

The relationship of circulating proteins in early pregnancy with preterm birth

BACKGROUND

Preterm birth (PTB) (< 37 completed weeks' gestation) is a pathological outcome of pregnancy and a major global health problem. Babies born preterm have an elevated risk for long-term adverse medical and neurodevelopmental sequelae. Substantial evidence implicates intrauterine infection and/or inflammation in PTB. However, these are often relatively late findings in the process, when PTB is inevitable. Identification of earlier markers of PTB may make successful intervention possible. Although select proteins, notably those related to the inflammatory pathways, have been associated with PTB, there has been a lack of research into the role of other protein pathways in the development of PTB. The purpose of this study was to investigate, using a previously described biomarker discovery approach, a subset of circulating proteins and their association with PTB focusing on samples from early pregnancy.

OBJECTIVES

The objectives of the study were as follows: (1) to perform a large-scale biomarker discovery, utilizing an innovative platform to identify proteins associated with preterm birth in plasma taken between 10 and 15 weeks' gestation and, (2) to determine which protein pathways are most strongly associated with preterm birth. To address these aims, we measured 1129 proteins in a plasma sample from early pregnancy using a multiplexed aptamer-based proteomic technology developed in Colorado by SomaLogic.

STUDY DESIGN

Using a nested case-control approach, we measured proteins at a single time point in early pregnancy in 41 women who subsequently delivered preterm and 88 women who had term uncomplicated deliveries. We measured 1129 proteins using a multiplexed aptamer-based proteomic technology developed by SomaLogic. Logistic regressions and random forests were used to compare protein levels.

RESULTS

The complement factors B and H and the coagulation factors IX and IX ab were the highest-ranking proteins distinguishing cases of preterm birth from term controls. The top 3 pathways associated with preterm birth were the complement cascade, the immune system, and the clotting cascade.

CONCLUSION

Using a discovery approach, these data provide further confirmation that there is an association of immune- and coagulation-related events in early pregnancy with preterm birth. Thus, plasma protein profiles at 10-15 weeks of gestation are related to the development of preterm birth later in pregnancy.

The anti-inflammatory effect of combined complement and CD14 inhibition is preserved during escalating bacterial load

Combined inhibition of complement and CD14 is known to attenuate bacterial-induced inflammation, but the dependency of the bacterial load on this effect is unknown. Thus, we investigated whether the effect of such combined inhibition on Escherichia coli- and Staphylococcus aureus-induced inflammation was preserved during increasing bacterial concentrations. Human whole blood was preincubated with anti-CD14, eculizumab (C5-inhibitor) or compstatin (C3-inhibitor), or combinations thereof. Then heat-inactivated bacteria were added at final concentrations of 5 × 10(4) -1 × 10(8) /ml (E. coli) or 5 × 10(7) -4 × 10(8) /ml (S. aureus). Inflammatory markers were measured using enzyme-linked immunosorbent assay (ELISA), multiplex technology and flow cytometry. Combined inhibition of complement and CD14 significantly (P < 0.05) reduced E. coli-induced interleukin (IL)-6 by 40-92% at all bacterial concentrations. IL-1β, IL-8 and macrophage inflammatory protein (MIP)-1α were significantly (P < 0.05) inhibited by 53-100%, and the effect was lost only at the highest bacterial concentration. Tumour necrosis factor (TNF) and MIP-1β were significantly (P < 0.05) reduced by 80-97% at the lowest bacterial concentration. Monocyte and granulocyte CD11b were significantly (P < 0.05) reduced by 63-91% at all bacterial doses. Lactoferrin was significantly (P < 0.05) attenuated to the level of background activity at the lowest bacterial concentration. Similar effects were observed for S. aureus, but the attenuation was, in general, less pronounced. Compared to E. coli, much higher concentrations of S. aureus were required to induce the same cytokine responses. This study demonstrates generally preserved effects of combined complement and CD14 inhibition on Gram-negative and Gram-positive bacterial-induced inflammation during escalating bacterial load. The implications of these findings for future therapy of sepsis are discussed.

Organ inflammation in porcine Escherichia coli sepsis is markedly attenuated by combined inhibition of C5 and CD14

Sepsis is an infection-induced systemic inflammatory syndrome, potentially causing organ failure. We previously showed attenuating effects on inflammation, thrombogenicity and haemodynamics by inhibiting the Toll-like receptor co-factor CD14 and complement factor C5 in a porcine Escherichia coli-induced sepsis model. The present study explored the effect on organ inflammation in these pigs. Tissue samples were examined from the combined treatment group (n = 8), the positive (n = 8) and negative (n = 6) control groups after 4h of sepsis. Inflammatory biomarkers were measured using ELISA, multiplex and qPCR analysis. Combined inhibition of C5 and CD14 markedly attenuated IL-1β by 31-66% (P < 0.05) and IL-6 by 54-96% (P < 0.01) in liver, kidney, lung and spleen; IL-8 by 65-100% in kidney, lung, spleen, and heart (P < 0.05) and MCP-1 by 46-69% in liver, kidney, spleen and heart (P < 0.05). Combined inhibition significantly attenuated tissue factor mRNA upregulation in spleen (P < 0.05) and IP-10 mRNA upregulation in four out of five organs. Finally, C5aR mRNA downregulation was prevented in heart and kidney (P < 0.05). Combined inhibition of C5 and CD14 thus markedly attenuated inflammatory responses in all organs examined. The anti-inflammatory effects observed in lung and heart may explain the delayed haemodynamic disturbances observed in septic pigs receiving combined inhibition of C5 and CD14.

Crucial role of IL1beta and C3a in the in vitro-response of multipotent mesenchymal stromal cells to inflammatory mediators of polytrauma

Multipotent mesenchymal stromal cells (MSC) exert immune-modulatory effects and support tissue regeneration in various local trauma models. In case of a polytrauma, high amounts of danger-associated molecular patterns are released, leading to a systemic increase of inflammatory mediators. The influence of such a complex inflammatory microenvironment on human MSC is mainly unknown so far. Therefore, we investigated the effects of a defined serum-free polytrauma “cocktail” containing ILͳbeta, IL6, IL8 and the anaphylatoxins C3a and C5a, in concentrations corresponding to those measured in the blood of polytrauma patients, on human MSC in vitro. The polytrauma cocktail induced directed migration of MSC with C3a representing its major soluble chemoattractive agent. Furthermore, the polytrauma cocktail and IL1beta upregulated the expression of MMP1 indicating a potential role of IL1beta to enhance MSC migration in the tissue context. COX2, PTGES and TSG6 were also found to be upregulated upon stimulation with the polytrauma cocktail or IL1beta, but not through other single factors of the polytrauma cocktail in pathophysiologically relevant concentrations. An RNA expression array of 84 inflammation-related genes revealed that both the polytrauma cocktail and IL1beta induced C3, CSF1, TLR3 and various chemokines without major qualitative or quantitative differences. These results indicate that IL1beta is a crucial mediator of the polytrauma cocktail in terms of immune-modulation and MMP1 expression. Thus, upon encountering the primary sterile, inflammatory milieu of a polytrauma, endogenous or systemically transfused MSC might be able to migrate to sites of injury, secrete TSG6 and PGE2 and to influence macrophage biology as observed in local trauma models.