Protective effects of anti-C5a in sepsis-induced thymocyte apoptosis

Oxidative stress, defective proteostasis and immunometabolic complications in critically ill patients

Oxidative stress (OS) develops in critically ill patients as a metabolic consequence of the immunoinflammatory and degenerative processes of the tissues. These induce increased and/or dysregulated fluxes of reactive species enhancing their pro-oxidant activity and toxicity. At the same time, OS sustains its own inflammatory and immunometabolic pathogenesis, leading to a pervasive and vitious cycle of events that contribute to defective immunity, organ dysfunction and poor prognosis. Protein damage is a key player of these OS effects; it generates increased levels of protein oxidation products and misfolded proteins in both the cellular and extracellular environment, and contributes to forms DAMPs and other proteinaceous material to be removed by endocytosis and proteostasis processes of different cell types, as endothelial cells, tissue resident monocytes-macrophages and peripheral immune cells. An excess of OS and protein damage in critical illness can overwhelm such cellular processes ultimately interfering with systemic proteostasis, and consequently with innate immunity and cell death pathways of the tissues thus sustaining organ dysfunction mechanisms. Extracorporeal therapies based on biocompatible/bioactive membranes and new adsorption techniques may hold some potential in reducing the impact of OS on the defective proteostasis of patients with critical illness. These can help neutralizing reactive and toxic species, also removing solutes in a wide spectrum of molecular weights thus improving proteostasis and its immunometabolic corelates. Pharmacological therapy is also moving steps forward which could help to enhance the efficacy of extracorporeal treatments. This narrative review article explores the aspects behind the origin and pathogenic role of OS in intensive care and critically ill patients, with a focus on protein damage as a cause of impaired systemic proteostasis and immune dysfunction in critical illness.

The anaphylatoxin C5a: Structure, function, signaling, physiology, disease, and therapeutics

The complement system is one of the oldest known tightly regulated host defense systems evolved for efficiently functioning cell-based immune systems and antibodies. Essentially, the complement system acts as a pivot between the innate and adaptive arms of the immune system. The complement system collectively represents a cocktail of ∼50 cell-bound/soluble glycoproteins directly involved in controlling infection and inflammation. Activation of the complement cascade generates complement fragments like C3a, C4a, and C5a as anaphylatoxins. C5a is the most potent proinflammatory anaphylatoxin, which is involved in inflammatory signaling in a myriad of tissues. This review provides a comprehensive overview of human C5a in the context of its structure and signaling under several pathophysiological conditions, including the current and future therapeutic applications targeting C5a.

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 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.

Inhibition of complement C5a receptor protects lung cells and tissues against lipopolysaccharide-induced injury via blocking pyroptosis

Acute lung injury (ALI) is the injury of alveolar epithelial cells and capillary endothelial cells caused by various factors. Complement system and pyroptosis have been proved to be involved in ALI, and inhibition of C5a/C5a receptor (C5aR) could alleviate ALI. This study aimed to investigate whether C5a/C5aR inhibition could protect against LPS-induced ALI via mediating pyroptosis. Rats were assigned into four groups: Control, LPS, LPS+W-54011 1mg/kg, and LPS+W-54011 5mg/kg. Beas-2B cells pretreated with or without C5a and W-54011, alone and in combination, were challenged with LPS+ATP. Results unveiled that LPS caused lung tissue injury and inflammatory response, increased pyroptotic and apoptotic factors, along with elevated C5a concentration and C5aR expressions. However, W-54011 pretreatment alleviated lung damage and pulmonary edema, reduced inflammation and prevented cell pyroptosis. In vitro studies confirmed that LPS+ATP reduced cell viability, promoted cell death, generated inflammatory factors and promoted expressions of pyroptosis-related proteins, which could be prevented by W-54011 pretreatment while intensified by C5a pretreatment. The co-treatment of C5a and W-54011 could blunt the effects of C5a on LPS+ATP-induced cytotoxicity. In conclusion, inhibition of C5a/C5aR developed protective effects against LPS-induced ALI and the cytotoxicity of Beas-2B cells, and these effects may depend on blocking pyroptosis.

The evolutionary analysis of complement component C5 and the gene co-expression network and putative interaction between C5a and C5a anaphylatoxin receptor (C5AR/CD88) in human and two Cyprinid fish

The complement system is a complex network of soluble and membrane-associated serum proteins that regulate immune response. Activation of the complement C5 generates C5a and C5b which generate chemoattractive effect on myeloid cells and initiate the membrane attack complex (MAC) assembly. However, the study of evolutionary process and systematic function of C5 are still limited. In this study, we performed an evolutionary analysis of C5. Phylogeny analysis indicated that C5 sequences underwent complete divergence in fish and non-fish vertebrate. It was found that codon usage bias improved and provided evolution evidence of C5 in species. Notably, the codon usage bias of grass carp was evolutionarily closer to the zebrafish genome compared with humans and stickleback. This suggested that the zebrafish cell line may provide an alternative environment for heterologous protein expression of grass carp. Sequence comparison showed a higher similarity between human and mouse, grass carp, and zebrafish. Moreover, selective pressure analysis revealed that the C5 genes in fish and non-fish vertebrates exhibited different evolutionary patterns. To study the function of C5, gene co-expression networks of human and zebrafish were built which revealed the complexity of C5 function networks in different species. The protein structure simulation of C5 indicated that grass carp and zebrafish are more similar than to human, however, differences between species in C5a proteins are extremely smaller. Spatial conformations of C5a-C5AR (CD88) protein complex were constructed, which showed that possible interaction may exist between C5a and CD88 proteins. Furthermore, the protein docking sites/residues were measured and calculated according to the minimum distance for all atoms from C5a and CD88 proteins. In summary, this study provides insights into the evolutionary history, function and potential regulatory mechanism of C5 in fish immune responses.

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).

Apoptosis-induced lymphopenia in sepsis and other severe injuries

Sepsis and other acute injuries such as severe trauma, extensive burns, or major surgeries, are usually followed by a period of marked immunosuppression. In particular, while lymphocytes play a pivotal role in immune response, their functions and numbers are profoundly altered after severe injuries. Apoptosis plays a central role in this process by affecting immune response at various levels. Indeed, apoptosis-induced lymphopenia duration and depth have been associated with higher risk of infection and mortality in various clinical settings. Therapies modulating apoptosis represent an interesting approach to restore immune competence after acute injury, although their use in clinical practice still presents several limitations. After briefly describing the apoptosis process in physiology and during severe injuries, we will explore the immunological consequences of injury-induced lymphocyte apoptosis, and describe associations with clinically relevant outcomes in patients. Therapeutic perspectives targeting apoptosis will also be discussed.

Complement C5a induces mesenchymal stem cell apoptosis during the progression of chronic diabetic complications

AIMS/HYPOTHESIS

Regeneration and repair mediated by mesenchymal stem cells (MSCs) are key self-protection mechanisms against diabetic complications, a reflection of diabetes-related cell/tissue damage and dysfunction. MSC abnormalities have been reported during the progression of diabetic complications, but little is known about whether a deficiency in these cells plays a role in the pathogenesis of this disease. In addition to MSC resident sites, peripheral circulation is a major source of MSCs that participate in the regeneration and repair of damaged tissue. Therefore, we investigated whether there is a deficiency of circulating MSC-like cells in people with diabetes and explored the underlying mechanisms.

METHODS

The abundance of MSC-like cells in peripheral blood was evaluated by FACS. Selected diabetic and non-diabetic serum (DS and NDS, respectively) samples were used to mimic diabetic and non-diabetic microenvironments, respectively. The proliferation and survival of MSCs under different serum conditions were analysed using several detection methods. The survival of MSCs in diabetic microenvironments was also investigated in vivo using leptin receptor mutant (Lepr db/db ) mice.

RESULTS

Our data showed a significant decrease in the abundance of circulating MSC-like cells, which was correlated with complications in individuals with type 2 diabetes. DS strongly impaired the proliferation and survival of culture-expanded MSCs through the complement system but not through exposure to high glucose levels. DS-induced MSC apoptosis was mediated, at least in part, by the complement C5a-dependent upregulation of Fas-associated protein with death domain (FADD) and the Bcl-2-associated X protein (BAX)/B cell lymphoma 2 (Bcl-2) ratio, which was significantly inhibited by neutralising C5a or by the pharmacological or genetic inhibition of the C5a receptor (C5aR) on MSCs. Moreover, blockade of the C5a/C5aR pathway significantly inhibited the apoptosis of transplanted MSCs in Lepr db/db recipient mice.

CONCLUSIONS/INTERPRETATION

C5a-dependent apoptotic death is probably involved in MSC deficiency and in the progression of complications in individuals with type 2 diabetes. Therefore, anticomplement therapy may be a novel intervention for diabetic complications.

Complement and sepsis-induced heart dysfunction

It is well known that cardiac dysfunction develops during sepsis in both humans and in rodents (rats, mice). These defects appear to be reversible, since after "recovery" from sepsis, cardiac dysfunction disappears and the heart returns to its function that was present before the onset of sepsis. Our studies, using in vivo and in vitro models, have demonstrated that C5a and its receptors (C5aR1 and C5aR2) play key roles in cardiac dysfunction developing during sepsis. Use of a neutralizing antibody to C5a largely attenuates cardiac dysfunction and other adverse events developing during sepsis. The molecular basis for cardiac dysfunctions is linked to generation of C5a and its interaction with C5a receptors present on surfaces of cardiomyocytes (CMs). It is established that C5a interactions with C5a receptors leads to significant reductions involving faulty contractility and relaxation in CMs. In addition, C5a interactions with C5a receptors on CMs results in reductions in Na⁺/K⁺-ATPase in CMs. This ATPase is essential for intact action potentials in CMs. The enzymatic activity and protein for this ATPase were strikingly reduced in CMs during sepsis by unknown mechanisms. In addition, C5a interactions with C5aRs also caused reductions in CM homeostatic proteins that regulate cytosolic [Ca²⁺]i in CMs: sarco/endoplasmic reticulum Ca²⁺-ATPase2 (SERCA2) and Na⁺/Ca²⁺ exchanger (NCX). In the absence of C5a receptors, defects in SERCA2 and NCX in CMs after sepsis are strikingly attenuated. These observations suggest new strategies to protect the heart from dysfunction developing during sepsis.

Temporal Progression of Pneumonic Plague in Blood of Nonhuman Primate: A Transcriptomic Analysis

Early identification of impending illness during widespread exposure to a pathogenic agent offers a potential means to initiate treatment during a timeframe when it would be most likely to be effective and has the potential to identify novel therapeutic strategies. The latter could be critical, especially as antibiotic resistance is becoming widespread. In order to examine pre-symptomatic illness, African green monkeys were challenged intranasally with aerosolized Yersinia pestis strain CO92 and blood samples were collected in short intervals from 45 m till 42 h post-exposure. Presenting one of the first genomic investigations of a NHP model challenged by pneumonic plague, whole genome analysis was annotated in silico and validated by qPCR assay. Transcriptomic profiles of blood showed early perturbation with the number of differentially expressed genes increasing until 24 h. By then, Y. pestis had paralyzed the host defense, as suggested by the functional analyses. Early activation of the apoptotic networks possibly facilitated the pathogen to overwhelm the defense mechanisms, despite the activation of the pro-inflammatory mechanism, toll-like receptors and microtubules at the port-of-entry. The overexpressed transcripts encoding an early pro-inflammatory response particularly manifested in active lymphocytes and ubiquitin networks were a potential deviation from the rodent models, which needs further verification. In summary, the present study recognized a pattern of Y. pestis pathogenesis potentially more applicable to the human system. Independent validation using the complementary omics approach with comprehensive evaluation of the organs, such as lungs which showed early bacterial infection, is essential.

Complement 5a receptor-mediated neutrophil dysfunction is associated with a poor outcome in sepsis

Complement 5a (C5a) has been implicated in the pathogenesis of sepsis by inducing the functional impairment of neutrophils; however, the utility of C5a receptors (C5aRs; C5aR and C5L2) as biomarkers for the management of sepsis is uncertain. This study investigated the dynamic expression of C5aR and C5L2 on neutrophils and their effects on neutrophil function. We found that sepsis patients displayed low expression levels of C5aR and C5L2 on neutrophils compared to healthy and systemic inflammatory response syndrome (SIRS) subjects, and this expression pattern was correlated with disease severity. Additionally, the expression levels of C5aR and C5L2 were associated with the survival of sepsis patients. In vitro, the addition of C5a significantly reduced C5aR and C5L2 expression levels and IL-8 production in neutrophils from sepsis patients. Those findings suggest that the reduced expression of C5aRs was associated with the functional impairment of neutrophils and a poor prognosis for sepsis patients. Overall, these findings may help establish C5aRs expression levels as early markers to predict the severity of sepsis.

Complement-mediated cellular injury

Complement activation and recruitment of inflammatory leukocytes is an important defense mechanism against bacterial infection. However, complement also can mediate cellular injury and contribute to the pathogenesis of various diseases. With the appreciation that the C5b-9 membrane attack complex can injure cells in the absence of leukocytes, a role for the terminal complement pathway in inducing cell injury and kidney disease was shown in several experimental models, including the rat passive Heymann nephritis model of human membranous nephropathy. In podocytes, sublytic C5b-9 activates a variety of downstream pathways including protein kinases, lipid metabolism, reactive oxygen species, growth factors/gene transcription, endoplasmic reticulum stress, and the ubiquitin-proteasome system, and it impacts the integrity of the cytoskeleton and slit diaphragm proteins. C5b-9 also injures other kidney cells, including mesangial, glomerular endothelial, and tubular epithelial cells, and it contributes to the pathogenesis of mesangial-proliferative glomerulonephritis, thrombotic microangiopathy, and acute kidney injury. Conversely, certain C5b-9 signals limit complement-induced injury, or promote recovery of cells. In addition to C5b-9, complement cleavage products, such as C5a and C1q, can injure kidney cells. Thus, the complement system contributes to various kidney pathologies by causing cellular damage in both an inflammation-dependent and inflammation-independent manner.

Protein kinase C-δ mediates sepsis-induced activation of complement 5a and urokinase-type plasminogen activator signaling in macrophages

OBJECTIVE AND DESIGN

Activations of the complement C5a (C5a) and the urokinase-type plasminogen activator (uPA) are commonly seen together during sepsis. However, the mechanism linking these two important pathways remains elusive.

MATERIAL, METHODS AND TREATMENT

We used the C57BL/6 J mice model of sepsis induced by cecal ligation puncture (CLP) procedure, injected anti-C5aR or rottlerin through the tail vein to neutralize C5aR or PKC-δ, and then isolated peritoneal macrophages. Total RNA was isolated from the cells and analyzed by quantitative PCR.

RESULTS

Our study revealed that neutralizing C5aR markedly inhibited sepsis-induced uPA receptor (uPAR) expression and its downstream signaling in macrophage. Similarly, neutralizing uPAR suppressed sepsis activation of C5a signaling. Importantly, inhibition of PKC-δ largely blocked sepsis-induced expression of C5aR and uPAR.

CONCLUSIONS

Our study demonstrates a crosstalk between the complement C5a signaling and the fibrinolytic uPA pathways, which may depend on each other to maintain their expression and signaling, and reveals a central role of PKC-δ in mediating sepsis-induced activation of these pathways.

Erythropoietin prevents lymphoid apoptosis but has no effect on survival in experimental sepsis

BACKGROUND

Lymphoid apoptosis in sepsis is associated with poor outcome, and prevention of apoptosis frequently improves survival in experimental models of sepsis. Recently, erythropoietin (EPO) was shown to protect against lipopolysaccharide (LPS)-induced mortality. As cecal ligation and puncture (CLP) is a clinically more relevant model of sepsis, we evaluated the effect of EPO on CLP-induced lymphoid tissue apoptosis and mortality.

METHODS

Young Wistar rats were subjected to polymicrobial sepsis by CLP. EPO (5,000 U/kg intraperitoneal) was administered 30 min before CLP and then 1 and 4 h after CLP. Spleen, thymus, and small intestine were harvested at 24 h and assessed for apoptosis by terminal deoxynucleotidyl transferase nick-end labeling (TUNEL) and caspase-3 staining. A separate group of animals was followed up for mortality.

RESULTS

Splenic, thymic, and intestinal apoptosis was increased after CLP; administration of EPO significantly decreased apoptosis as determined by TUNEL and caspase-3 staining. Final survival in the CLP mortality study was 30% in both saline and EPO groups.

CONCLUSION

Our results provide the first evidence that EPO attenuates lymphoid apoptosis in the CLP model of sepsis. However, EPO is not associated with a survival benefit in the CLP model of sepsis.

C5a regulates IL-12+ DC migration to induce pathogenic Th1 and Th17 cells in sepsis

OBJECTIVE

It is well known that complement system C5a is excessively activated during the onset of sepsis. However, it is unclear whether C5a can regulate dentritic cells (DCs) to stimulate adaptive immune cells such as Th1 and Th17 in sepsis.

METHODS

Sepsis was induced by cecal ligation and puncture (CLP). CLP-induced sepsis was treated with anti-C5a or IL-12. IL-12(+)DC, IFNγ(+)Th1, and IL-17(+)Th17 cells were analyzed by flow cytometry. IL-12 was measured by ELISA.

RESULTS

Our studies here showed that C5a induced IL-12(+)DC cell migration from the peritoneal cavity to peripheral blood and lymph nodes. Furthermore, IL-12(+)DC cells induced the expansion of pathogenic IFNγ(+)Th1 and IL-17(+)Th17 cells in peripheral blood and lymph nodes. Moreover, IL-12, secreted by DC cells in the peritoneal cavity, is an important factor that prevents the development of sepsis.

CONCLUSION

Our data suggests that C5a regulates IL-12(+)DC cell migration to induce pathogenic Th1 and Th17 cells in sepsis.

Complement in animal development: unexpected roles of a highly conserved pathway

The complement pathway is most famous for its role in immunity, orchestrating an exquisitely refined system for immune surveillance. At its core lies a cascade of proteolytic events that ultimately serve to recognise microbes, infected cells or debris and target them for elimination. Mounting evidence has shown that a number of the proteolytic intermediaries in this cascade have, in themselves, other functions in the body, signalling through receptors to drive events that appear to be unrelated to immune surveillance. It seems, then, that the complement system not only functions as an immunological effector, but also has cell-cell signalling properties that are utilised by a number of non-immunological processes. In this review we examine a number of these processes in the context of animal development, all of which share a requirement for precise control of cell behaviour in time and space. As we will see, the scope of the complement system's function is indeed much greater than we might have imagined only a few years ago.

An upstream initiator caspase 10 of snakehead murrel Channa striatus, containing DED, p20 and p10 subunits: molecular cloning, gene expression and proteolytic activity

Caspase 10 (CsCasp10) was identified from a constructed cDNA library of freshwater murrel (otherwise called snakehead) Channa striatus. The CsCasp10 is 1838 base pairs (bp) in length and it is encoding 549 amino acid (aa) residues. CsCasp10 amino acid contains two death effector domains (DED) in the N-terminal at 2-77 and 87-154 and it contains caspase family p20 domain (large subunit) and caspase family p10 domain (small subunit) in the C-terminal at 299-425 and 449-536 respectively. Pairwise analysis of CsCasp10 showed the highest sequence similarity (79%) with caspase 10 of Paralichthys olivaceus. Moreover, the phylogenetic analysis showed that CsCasp10 is clustered together with other fish caspase 10, formed a sister group with caspase 10 from other lower vertebrates including amphibian, reptile and birds and finally clustered together with higher vertebrates such as mammals. Significantly (P < 0.05) highest CsCasp10 gene expression was noticed in gills and lowest in intestine. Furthermore, the CsCasp10 gene expression in C. striatus was up-regulated in gills by fungus Aphanomyces invadans and bacteria Aeromonas hydrophila induction. The proteolytic activity was analyzed using the purified recombinant CsCasp10 protein. The results showed the proteolytic activity of CsCasp10 for caspase 10 substrate was 2.5 units per μg protein. Moreover, the proteolytic activities of CsCasp10 in kidney and spleen induced by A. invadans and A. hydrophila stimulation were analyzed by caspase 10 activity assay kit. All these results showed that CsCasp10 are participated in immunity of C. striatus against A. invadans and A. hydrophila infection.

New insights for C5a and C5a receptors in sepsis

The complement system plays a central role in inflammation and immunity. Among the complement activation products, C5a is one of the most potent inflammatory peptides with a broad spectrum of functions. There is strong evidence for complement activation including elevated plasma level of C5a in humans and animals with sepsis. C5a exerts its effects through the C5a receptors. Of the two receptors that bind C5a, the C5aR (CD88) is known to mediate signaling activity, whereas the function of another C5a binding receptor, C5L2, remains largely unknown. Here, we review the critical role of C5a in sepsis and summarize evidence indicating that both C5aR and C5L2 act as regulating receptors for C5a during sepsis.

Manipulation of the complement system for benefit in sepsis

There is evidence in sepsis, both in rodents and in humans, that activation of the complement system results in excessive production of C5a, which triggers a series of events leading to septic shock, multiorgan failure, and lethality. In rodents following cecal ligation and puncture (CLP), which induces polymicrobial sepsis, in vivo blockade of C5a using neutralizing antibodies dramatically improved survival, reduced apoptosis of lymphoid cells, and attenuated the ensuing coagulopathy. Based on these data, it seems reasonable to consider therapeutic blockade of C5a in humans entering into sepsis and septic shock. Strategies for the development of such an antibody for use in humans are presented.

Molecular mechanisms of inflammation and tissue injury after major trauma--is complement the "bad guy"?

Trauma represents the leading cause of death among young people in industrialized countries. Recent clinical and experimental studies have brought increasing evidence for activation of the innate immune system in contributing to the pathogenesis of trauma-induced sequelae and adverse outcome. As the "first line of defense", the complement system represents a potent effector arm of innate immunity, and has been implicated in mediating the early posttraumatic inflammatory response. Despite its generic beneficial functions, including pathogen elimination and immediate response to danger signals, complement activation may exert detrimental effects after trauma, in terms of mounting an "innocent bystander" attack on host tissue. Posttraumatic ischemia/reperfusion injuries represent the classic entity of complement-mediated tissue damage, adding to the "antigenic load" by exacerbation of local and systemic inflammation and release of toxic mediators. These pathophysiological sequelae have been shown to sustain the systemic inflammatory response syndrome after major trauma, and can ultimately contribute to remote organ injury and death. Numerous experimental models have been designed in recent years with the aim of mimicking the inflammatory reaction after trauma and to allow the testing of new pharmacological approaches, including the emergent concept of site-targeted complement inhibition. The present review provides an overview on the current understanding of the cellular and molecular mechanisms of complement activation after major trauma, with an emphasis of emerging therapeutic concepts which may provide the rationale for a "bench-to-bedside" approach in the design of future pharmacological strategies.

Ecto-5'-nucleotidase (CD73) decreases mortality and organ injury in sepsis

The extracellular concentrations of adenosine are increased during sepsis, and adenosine receptors regulate the host's response to sepsis. In this study, we investigated the role of the adenosine-generating ectoenzyme, ecto-5'-nucleotidase (CD73), in regulating immune and organ function during sepsis. Polymicrobial sepsis was induced by subjecting CD73 knockout (KO) and wild type (WT) mice to cecal ligation and puncture. CD73 KO mice showed increased mortality in comparison with WT mice, which was associated with increased bacterial counts and elevated inflammatory cytokine and chemokine concentrations in the blood and peritoneum. CD73 deficiency promoted lung injury, as indicated by increased myeloperoxidase activity and neutrophil infiltration, and elevated pulmonary cytokine levels. CD73 KO mice had increased apoptosis in the thymus, as evidenced by increased cleavage of caspase-3 and poly(ADP-ribose) polymerase and increased activation of NF-κB. Septic CD73 KO mice had higher blood urea nitrogen levels and increased cytokine levels in the kidney, indicating increased renal dysfunction. The increased kidney injury of CD73 KO mice was associated with augmented activation of p38 MAPK and decreased phosphorylation of Akt. Pharmacological inactivation of CD73 in WT mice using α, β-methylene ADP augmented cytokine levels in the blood and peritoneal lavage fluid. These findings suggest that CD73-derived adenosine may be beneficial in sepsis.

The effects of CD59 gene as a target gene on breast cancer cells

The retroviral-vector-targeted CD59 gene (pSUPER-siCD59) was constructed and transfected into breast cells (MCF-7). The results demonstrated that the retroviral vector-mediated RNAi successfully suppressed human CD59 gene. The expression of CD59 decreased at both mRNA and protein levels. Knockdown of CD59 abrogated its protective effect on complement-mediated cytolysis. Fas and caspase-3 were remarkably upregulated, which induced apoptosis and tumor growth suppression in MCF-7 cells. In addition, overexpression of CD59 promoted the proliferation of MCF-7 cells and inhibited anti-apoptotic Bcl-2 expression. In conclusion, CD59 may be a promising target in the gene therapy of breast cancer.

γδT-cell function in sepsis is modulated by C5a receptor signalling

We previously showed that γδT cells are involved in the pathogenesis of sepsis, but, the underlying mechanisms remained unclear. The present study demonstrates, for the first time, that γδT cells express the complement C5a receptor (C5aR, CD88) and that CD88 expression in γδT cells was up-regulated in mice following sepsis both at protein and mRNA levels. Complement C5a itself contributed to the regulation of C5aR expression on γδT cells, as (i) neutralization of C5a in vivo prevented the expression of C5aR on γδT cells in septic mice and (ii) incubation of mouse spleen cells or purified γδT cells with recombinant C5a in vitro increased CD88 expression by γδT cells at both protein and mRNA levels. C5a receptor on γδT cells also mediates increased interleukin-17 (IL-17) expression as incubation of mouse spleen cells or purified γδT cells with recombinant C5a promotes the IL-17 expression by γδT cells. Ligation of the C5aR on γδT cells activated the phosphoinositide 3-kinase (PI3K)/Akt signalling pathway, which enhances CD88 expression and promotes IL-17 secretion. These results demonstrate that C5a acts directly on the C5aR expressed on γδT cells, resulting in cell activation, and subsequently enhances their capacity for IL-17 production. The up-regulation of the PI3K/Akt pathway following C5a stimulation contributes to up-regulation of γδT-cell function.

New insights of an old defense system: structure, function, and clinical relevance of the complement system

The complement system was discovered a century ago as a potent defense cascade of innate immunity. After its first description, continuous experimental and clinical research was performed, and three canonical pathways of activation were established. Upon activation by traumatic or surgical tissue damage, complement reveals beneficial functions of pathogen and danger defense by sensing and clearing injured cells. However, the latest research efforts have provided a more distinct insight into the complement system and its clinical subsequences. Complement has been shown to play a significant role in the pathogenesis of various inflammatory processes such as sepsis, multiorgan dysfunction, ischemia/reperfusion, cardiovascular diseases and many others. The three well-known activation pathways of the complement system have been challenged by newer findings that demonstrate direct production of central complement effectors (for example, C5a) by serine proteases of the coagulation cascade. In particular, thrombin is capable of producing C5a, which not only plays a decisive role on pathogens and infected/damaged tissues, but also acts systemically. In the case of uncontrolled complement activation, “friendly fire” is generated, resulting in the destruction of healthy host tissue. Therefore, the traditional research that focuses on a mainly positive-acting cascade has now shifted to the negative effects and how tissue damage originated by the activation of the complement can be contained. In a translational approach including structure-function relations of this ancient defense system, this review provides new insights of complement-mediated clinical relevant diseases and the development of complement modulation strategies and current research aspects.

C5a Receptor Targeting in Neointima Formation After Arterial Injury in Atherosclerosis-Prone Mice

Background— Receptor binding of complement C5a leads to proinflammatory activation of many cell types, but the role of receptor-mediated action during arterial remodeling after injury has not been studied. In the present study, we examined the contribution of the C5a receptor (C5aR) to neointima formation in apolipoprotein E–deficient mice employing a C5aR antagonist (C5aRA) and a C5aR-blocking monoclonal antibody.

Methods and Results— Mice fed an atherogenic diet were subjected to wire-induced endothelial denudation of the carotid artery and treated with C5aRA and anti-C5aR-blocking monoclonal antibody or vehicle control. Compared with controls, neointima formation was significantly reduced in mice receiving C5aRA or anti-C5aR-blocking monoclonal antibody for 1 week but not for 3 weeks, attributable to an increased content of vascular smooth muscle cells, whereas a marked decrease in monocyte and neutrophil content was associated with reduced vascular cell adhesion molecule-1. As assessed by immunohistochemistry, reverse transcription polymerase chain reaction, and flow cytometry, C5aR was expressed in lesional and cultured vascular smooth muscle cells, upregulated by injury or tumor necrosis factor-α, and reduced by C5aRA. Plasma levels and neointimal plasminogen activator inhibitor-1 peaked 1 week after injury and were downregulated in C5aRA-treated mice. In vitro, C5a induced plasminogen activator inhibitor-1 expression in endothelial cells and vascular smooth muscle cells in a C5aRA-dependent manner, possibly accounting for higher vascular smooth muscle cell immigration.

Conclusions— One-week treatment with C5aRA or anti-C5aR-blocking monoclonal antibody limited neointimal hyperplasia and inflammatory cell content and was associated with reduced vascular cell adhesion molecule-1 expression. However, treatment for 3 weeks failed to reduce but rather stabilized plaques, likely by reducing vascular plasminogen activator inhibitor-1 and increasing vascular smooth muscle cell migration.

Complement C5a regulates IL-17 by affecting the crosstalk between DC and gammadelta T cells in CLP-induced sepsis

Complement 5a (C5a) and Interleukin-17 (IL-17) are two important inflammatory mediators in sepsis. Here we studied the mechanisms underlying regulation of IL-17 by anaphylatoxin C5a. We found that C5a blockade increased the survival rate of mice following cecal ligation and puncture (CLP)-induced sepsis and decreased IL-17 expression in vivo. IL-17 was secreted mainly by gammadelta T cells in this model. Importantly, our data suggest that C5a participates in the regulation of IL-17 secretion by gammadelta T cells. Dendritic cells (DC) were found to act as a "bridge" between C5a and gammadelta T cells in a mechanism involving IL-6 and transforming growth factor beta (TGF-beta). These results imply that C5a affects the crosstalk between DC and gammadelta T cells during sepsis development, and this may result in a large production of inflammatory mediators such as IL-17.

Complement activation in emergency department patients with severe sepsis

OBJECTIVES

This study assessed the extent and mechanism of complement activation in community-acquired sepsis at presentation to the emergency department (ED) and following 24 hours of quantitative resuscitation.

METHODS

A prospective pilot study of patients with severe sepsis and healthy controls was conducted among individuals presenting to a tertiary care ED. Resuscitation, including antibiotics and therapies to normalize central venous and mean arterial pressure (MAP) and central venous oxygenation, was performed on all patients. Serum levels of Factor Bb (alternative pathway), C4d (classical and mannose-binding lectin [MBL] pathway), C3, C3a, and C5a were determined at presentation and 24 hours later among patients.

RESULTS

Twenty patients and 10 healthy volunteer controls were enrolled. Compared to volunteers, all proteins measured were abnormally higher among septic patients (C4d 3.5-fold; Factor Bb 6.1-fold; C3 0.8-fold; C3a 11.6-fold; C5a 1.8-fold). Elevations in C5a were most strongly correlated with alternative pathway activation. Surprisingly, a slight but significant inverse relationship between illness severity (by sequential organ failure assessment [SOFA] score) and C5a levels at presentation was noted. Twenty-four hours of structured resuscitation did not, on average, affect any of the mediators studied.

CONCLUSIONS

Patients with community-acquired sepsis have extensive complement activation, particularly of the alternative pathway, at the time of presentation that was not significantly reversed by 24 hours of aggressive resuscitation.

Different apoptosis modalities in periprosthetic membranes

This study reports on an investigation into apoptotic and proliferation signals in leukocyte and membrane fibroblasts in periprosthetic membranes collected during revision surgery for loosened total hip joint arthroplasty. Cementless and cemented prosthesis were studied under both aseptic and septic conditions. Fluorescence colocalization immunohistochemistry and colorimetric immunohistochemistry were used to investigate cell death signals. In aseptic cementless prosthesis macrophages and membrane fibroblasts show high bax signal, implying the occurrence of toxic/oxidative cell death caused by the debris of titanium alloy metal implant. Instead in aseptic cemented prosthesis only a moderate number of apoptotic leukocytes were observed, whilst the fibroblasts were affected by a diffuse apoptotic-like cell death, the Co-Cr ions debris released from cemented stem, may be at basis of apoptotic cell death induction. Furthermore cement debris is recognized to induce macrophages to produce cytokine, that may be responsible for the cell death observed and implant failure. The septic environment seems to protect leukocytes cell death. Septic cementless prosthesis showed only a few apoptotic leukocytes, instead fibroblasts remain affected by cell death signals. Similarly in septic cemented prosthesis, scanty apoptotic leukocytes were detected, whereas membrane fibroblasts showed an increase in proliferation index (Ki-67) along with caspase-3 activation. These findings indicate some kind of caspase-3 involvement in tissue proliferation, rather than in cell death pathway. Apoptotic periprosthetic sites have been interpreted as signs of inflammation resolution and normal tissue turnover. Nevertheless apoptosis may also be a sign of cell renewal associated to tissue proliferation.

VX-166: a novel potent small molecule caspase inhibitor as a potential therapy for sepsis

Introduction

Prevention of lymphocyte apoptosis by caspase inhibition has been proposed as a novel treatment approach in sepsis. However, it has not been clearly demonstrated that caspase inhibitors improve survival in sepsis models when dosed post-insult. Also, there are concerns that caspase inhibitors might suppress the immune response. Here we characterize VX-166, a broad caspase inhibitor, as a novel potential treatment for sepsis.

Methods

VX-166 was studied in a number of enzymatic and cellular assays. The compound was then tested in a murine model of endotoxic shock (lipopolysaccharide (LPS), 20 mg/kg IV) and a 10 d rat model of polymicrobial sepsis by caecal ligation and puncture (CLP).

Results

VX-166 showed potent anti-apoptotic activity in vitro and inhibited the release of interleukin (IL)-1beta and IL-18. In the LPS model, VX-166 administered 0, 4, 8 and 12 h post-LPS significantly improved survival in a dose-dependent fashion (P < 0.0028). In the CLP model, VX-166 continuously administered by mini-osmotic pump significantly improved survival when dosed 3 h after insult, (40% to 92%, P = 0.009). When dosed 8 h post-CLP, VX-166 improved survival from 40% to 66% (P = 0.19). Mode of action studies in the CLP model confirmed that VX-166 significantly inhibited thymic atrophy and lymphocyte apoptosis as determined by flow cytometry (P < 0.01). VX-166 reduced plasma endotoxin levels (P < 0.05), suggesting an improved clearance of bacteria from the bloodstream. Release of IL-1beta in vivo or T-cell activation in vitro were moderately affected.

Conclusions

Our studies enhance the case for the use of caspase inhibitors in sepsis. VX-166 itself has promise as a therapy for the treatment of sepsis in man.

Sepsis, complement and the dysregulated inflammatory response

Sepsis in human beings is a major problem involving many individuals and with a high death rate. Except for a single drug (recombinant activated protein C) that has been approved for treatment of septic patients, supportive measures represent the main clinical approach. There are many models of experimental sepsis, mostly in rodents. A commonly used model is cecal ligation and puncture (CLP). In this model, robust activation of complement occurs together with up-regulation of C5a receptors (C5aR, C5L2) in a variety of different organs (lungs, kidneys, liver, heart). In septic human beings there is abundant evidence for complement activation. Interception of C5a or its receptors in the CLP model greatly improves survival in septic rodents. There is compelling evidence that CLP causes an intense pro-inflammatory state and that C5a interaction with its receptors can be linked to apoptosis of the lymphoid system and cells of the adrenal medulla, loss of innate immune functions of blood neutrophils, consumptive coagulopathy and cardiac dysfunction. These findings may have implications for therapeutic interventions in human beings with sepsis.

Anti-complement Strategies in Experimental Sepsis

Using the cecal ligation/puncture (CLP) model of sepsis in rodents, evidence was obtained for excessive activation of the complement system, which leads to nearly total loss of innate immune protective functions of blood neutrophils. These defects are associated with profound defects in chemotaxis, respiratory burst (H2O2 production) and phagocytosis. The molecular mechanisms of these defects are linked to the complement activation product C5a. In CLP rats and mice, the C5a receptor (C5aR) is widely up-regulated in organs, in part owing to the production of interleukin-6 (IL-6). The up-regulation of C5aR in the thymus is linked to C5a-dependent induction of apoptosis in thymocytes, as revealed by caspase activation, increased binding of C5a and DNA laddering. Such events in thymocytes are prevented if rats first are treated with anti-C5a or with anti-C5aR at the time of CLP. Treatment of CLP rats and mice with anti-C5a, anti-IL-6 or anti-C5aR dramatically improves survival rates after CLP, indicating a linkage between C5a and C5aR in the harmful outcomes of sepsis in rodents. Studies are underway in humans with sepsis to determine whether similar mechanisms are in play.

Innate immune responses to danger signals in systemic inflammatory response syndrome and sepsis

The systemic immune response induced by non-infectious agents is called systemic inflammatory response syndrome (SIRS) and infection-induced systemic immune response is called sepsis. The host inflammatory response in SIRS and sepsis is similar and may lead to multiple organ dysfunction syndrome (MODS) and ultimately death. The mortality and morbidity in SIRS and sepsis (i.e. critical illness) remain high despite advances in diagnostic and organ supporting possibilities in intensive care units. In critical illness, the acute immune response is organized and executed by innate immunity influenced by the neuroendocrine system. This response starts with sensing of danger by pattern-recognition receptors on the immune competent cells and endothelium. The sensed danger signals, through specific signalling pathways, activate nuclear transcription factor kappaB and other transcription factors and gene regulatory systems which up-regulate the expression of pro-inflammatory mediators. The plasma cascades are also activated which together with the produced pro-inflammatory mediators stimulate further the production of inflammatory biomarkers. The acute inflammatory response underlies the pathophysiological mechanisms involved in the development of MODS. The inflammatory mediators directly affect organ function and cause a decline in remote organ function by mediating the production of nitric oxide leading to mitochondrial anergy and cytopathic hypoxia, a condition of cellular inability to use oxygen. Understanding the mechanisms of acute immune responses in critical illness is necessary for the development of urgently needed therapeutics. The aim of this review is to provide a description of the key components and mechanisms involved in the immune response in SIRS and sepsis.

The role of the anaphylatoxins in health and disease

The anaphylatoxin (AT) C3a, C5a and C5a-desArg are generally considered pro-inflammatory polypeptides generated after proteolytic cleavage of C3 and C5 in response to complement activation. Their well-appreciated effector functions include chemotaxis and activation of granulocytes, mast cells and macrophages. Recent evidence suggests that ATs are also generated locally within tissues by pathogen-, cell-, or contact system-derived proteases. This local generation of ATs is important for their pleiotropic biologic effects beyond inflammation. The ATs exert most of the biologic activities through ligation of three cognate receptors, i.e. the C3a receptor, the C5a receptor and the C5a receptor-like, C5L2. Here, we will discuss recent findings suggesting that ATs regulate cell apoptosis, lipid metabolism as well as innate and adaptive immune responses through their impact on antigen-presenting cells and T cells. As we will outline, such regulatory functions of ATs and their receptors play important roles in the pathogenesis of allergy, autoimmunity, neurodegenerative diseases, cancer and infections with intracellular pathogens.

Milk fat globule epidermal growth factor-factor VIII is down-regulated in sepsis via the lipopolysaccharide-CD14 pathway

Phagocytosis prevents the release of potentially harmful or immunogenic materials from dying cells. Milk fat globule epidermal growth factor (EGF)-factor VIII (MFG-E8) mediates the clearance of apoptotic cells. We have previously shown that the administration of MFG-E8-rich exosomes from immature dendritic cells promotes the phagocytosis of apoptotic cells and improves survival in sepsis. Because endotoxin is elevated in polymicrobial sepsis, we hypothesized that down-regulation of MFG-E8 is mediated via the LPS-CD14 pathway, eventually leading to the accruement of apoptotic cells. Polymicrobial sepsis was induced by cecal ligation and puncture (CLP) in CD14-deficient (CD14(-/-)), TLR4-mutated and wild-type (WT) mice. In addition, endotoxemia was elicited by i.p. injection of LPS. LPS was also neutralized by pretreating CLP-induced WT mice with polymyxin B. Splenic MFG-E8 expression, phagocytic activity, and apoptosis were assessed 5 and 20 h after CLP or 5 h after LPS administration. In septic WT mice, MFG-E8 mRNA and protein levels were suppressed by 49 and 33%, respectively. Endotoxemia reduced MFG-E8 mRNA expression in a dose dependent manner and the down-regulation of MFG-E8 mRNA expression in CLP-induced sepsis was attenuated by polymyxin B. This CLP-induced suppression was not observed in both CD14(-/-) and TLR4-mutated mice. CLP significantly decreased phagocytic activity of peritoneal macrophages in WT (by 30%), but not in CD14(-/-) mice. CLP also induced significant apoptosis in the spleen of WT (by 61%), but less in CD14(-/-) mice. Thus, MFG-E8 production is down-regulated in sepsis by LPS-CD14 dependent fashion, leading to a reduction of phagocytosis of apoptotic cells.

Inhibition of complement C5a prevents breakdown of the blood-brain barrier and pituitary dysfunction in experimental sepsis

Introduction

Septic encephalopathy secondary to a breakdown of the blood-brain barrier (BBB) is a known complication of sepsis. However, its pathophysiology remains unclear. The present study investigated the effect of complement C5a blockade in preventing BBB damage and pituitary dysfunction during experimental sepsis.

Methods

Using the standardised caecal ligation and puncture (CLP) model, Sprague-Dawley rats were treated with either neutralising anti-C5a antibody or pre-immune immunoglobulin (Ig) G as a placebo. Sham-operated animals served as internal controls.

Results

Placebo-treated septic rats showed severe BBB dysfunction within 24 hours, accompanied by a significant upregulation of pituitary C5a receptor and pro-inflammatory cytokine expression, although gene levels of growth hormone were significantly attenuated. The pathophysiological changes in placebo-treated septic rats were restored by administration of neutralising anti-C5a antibody to the normal levels of BBB and pituitary function seen in the sham-operated group.

Conclusions

Collectively, the neutralisation of C5a greatly ameliorated pathophysiological changes associated with septic encephalopathy, implying a further rationale for the concept of pharmacological C5a inhibition in sepsis.

Activation by C5a of endothelial cell caspase 8 and cFLIP

OBJECTIVES AND DESIGN

In this study, we examine the relationship between C5a and activation of cysteine aspartic acid protease 8 (caspase 8) in human umbilical vein endothelial cells (HUVEC).

MATERIALS OR SUBJECTS

Primary cultures of HUVEC were used.

TREATMENTS

Recombinant human C5a (50 ng/ml) was used in the presence or absence of 10 microg/ml cycloheximide (CHX).

METHODS

HUVEC were treated with C5a alone and in the presence of CHX, then monitored for cell viability, poly- ADP-ribose 1 (PARP-1) and caspase 8 activities. Gene and protein expressions were assessed for caspase 8 and the caspase 8 homologue, FLICE -inhibitory protein (cFLIP).

RESULTS

We found a 43.1 +/- 6.9 percent reduction in viability of HUVEC stimulated for 18 h with 50 ng/ml C5a in the presence of 10 microg/ml CHX (p < 0.05). In contrast, the cell viability of cells stimulated for 18 h with 50 ng/ml C5a or 10 microg/ml CHX alone was not significantly different compared to the non-stimulated control. Treatment of HUVEC with C5a induced an increase in caspase 8 activity but did not significantly affect cFLIP levels.

CONCLUSIONS

These data suggest caspase 8 activation induced by C5a leads to cell death if protein synthesis of antiapoptotic protein(s) is blocked.

Identification and characterization of NDT 9513727 [N,N-bis(1,3-benzodioxol-5-ylmethyl)-1-butyl-2,4-diphenyl-1H-imidazole-5-methanamine], a novel, orally bioavailable C5a receptor inverse agonist

The complement system represents an innate immune mechanism of host defense that has three effector arms, the C3a receptor, the C5a receptor (C5aR), and the membrane attack complex. Because of its inflammatory and immune-enhancing properties, the biological activity of C5a and its classical receptor have been widely studied. Because specific antagonism of the C5aR could have therapeutic benefit without affecting the protective immune response, the C5aR continues to be a promising target for pharmaceutical research. The lack of specific, potent and orally bioavailable small-molecule antagonists has limited the clinical investigation of the C5aR. We report the discovery of NDT 9513727 [N,N-bis(1,3-benzodioxol-5-ylmethyl)-1-butyl-2,4-diphenyl-1H-imidazole-5-methanamine], a small-molecule, orally bioavailable, selective, and potent inverse agonist of the human C5aR. NDT 9513727 was discovered based on the integrated use of in vitro affinity and functional assays in conjunction with medicinal chemistry. NDT 9513727 inhibited C5a-stimulated responses, including guanosine 5'-3-O-(thio)triphosphate binding, Ca(2+) mobilization, oxidative burst, degranulation, cell surface CD11b expression and chemotaxis in various cell types with IC(50)s from 1.1 to 9.2 nM, respectively. In C5a competition radioligand binding experiments, NDT 9513727 exhibited an IC(50) of 11.6 nM. NDT 9513727 effectively inhibited C5a-induced neutropenia in gerbil and cynomolgus macaque in vivo. The findings suggest that NDT 9513727 may be a promising new entity for the treatment of human inflammatory diseases.

Blockade of apoptosis as a rational therapeutic strategy for the treatment of sepsis

Over time it has become clear that, much like other organ systems, the function and responsiveness of the immune system is impaired during the course of sepsis and that this is a precipitous event in the decline of the critically ill patient/animal. One hypothesis put forward to explain the development of septic immune dysfunction is that it is a pathological result of increased immune cell apoptosis. Alternatively, it has been proposed that the clearance of increased numbers of apoptotic cells may actively drive immune suppression through the cells that handle them. Here we review the data from studies involving septic animals and patients, which indicate that loss of immune cells, as well as non-immune cells, in some cases, is a result of dysregulated apoptosis. Subsequently, we will consider the cell death pathways, i.e. 'extrinsic' and/or 'intrinsic', which are activated and what cell populations may orchestrate this dysfunctional apoptotic process, immune and/or non-immune. Finally, we will discuss potentially novel therapeutic targets, such as caspases, death receptor family members (e.g. tumour necrosis factor, Fas) and pro-/anti apoptotic Bcl-family members, and approaches such as caspase inhibitors, the use of fusion proteins, peptidomimetics and siRNA, which might be considered for the treatment of the septic patient.

Sepsis, apoptosis and complement

Programmed cell death (apoptosis) is a prominent feature in human and experimental sepsis, especially as it involves the lymphoid system with resulting immunoparalysis. In addition, sepsis is associated with strong activation of the complement system, resulting in generation of the powerful anaphylatoxin, C5a, as well as the upregulation of the C5a receptor (C5aR) in a variety of different organs. The consequences of C5a interactions with C5aR can be directly linked to apoptosis of thymocytes and adrenal medullary cells after cecal ligation and puncture (CLP)-induced sepsis in rodents, as well as with other accompanying complications of CLP: cardiac dysfunction, consumptive coagulopathy, organ dysfunction, and lethality. This communication reviews the evidence for the adverse roles of C5a and C5aR in the setting of experimental sepsis and linkages to the various complications of sepsis, especially apoptosis as well as the roles of the two C5a receptors (C5aR and C5L2) in experimental sepsis.

Complement 5a receptor inhibition improves renal allograft survival

Complement activation plays a key role in mediating apoptosis, inflammation, and transplant rejection. In this study, the role of the complement 5a receptor (C5aR) was examined in human renal allografts and in an allogenic mouse model of renal transplant rejection. In human kidney transplants with acute rejection, C5aR expression was increased in renal tissue and in cells infiltrating the tubulointerstitium. Similar findings were observed in mice. When recipient mice were treated once daily with a C5aR antagonist before transplantation, long-term renal allograft survival was markedly improved compared with vehicle-treatment (75 versus 0%), and apoptosis was reduced. Furthermore, treatment with a C5aR antagonist significantly attenuated monocyte/macrophage infiltration, perhaps a result of reduced levels of monocyte chemoattractant protein 1 and the intercellular adhesion molecule 1. In vitro, C5aR antagonism inhibited intercellular adhesion molecule 1 upregulation in primary mouse aortic endothelial cells and reduced adhesion of peripheral blood mononuclear cells. Furthermore, C5aR blockade markedly reduced alloreactive T cell priming. These results demonstrate that C5aR plays an important role in mediating acute kidney allograft rejection, suggesting that pharmaceutical targeting of C5aR may have potential in transplantation medicine.

The first fifty years in research

It has been an honor for me to write the prefatory article for Volume 4 of the Annual Review of Pathology: Mechanisms of Disease. I decided to describe the first 50 years of my career in research, which started with my entry into medical school. I have tried to outline the numerous scientific mentors who played such an important role in my development as an independent scientific investigator. In general, I have tried to avoid mention in the text of the many, many colleagues who carried out the scientific work, as I would inevitably fail to cite many of them. Rather, I have cited what I think are my most important publications, which identify many of these scientific colleagues. I am now engaged nearly full-time in research and look forward to the next period of research progress.

The complement anaphylatoxin C5a induces apoptosis in adrenomedullary cells during experimental sepsis

Sepsis remains a poorly understood, enigmatic disease. One of the cascades crucially involved in its pathogenesis is the complement system. Especially the anaphylatoxin C5a has been shown to have numerous harmful effects during sepsis. We have investigated the impact of high levels of C5a on the adrenal medulla following cecal ligation and puncture (CLP)-induced sepsis in rats as well as the role of C5a on catecholamine production from pheochromocytoma-derived PC12 cells. There was significant apoptosis of adrenal medulla cells in rats 24 hrs after CLP, as assessed by the TUNEL technique. These effects could be reversed by dual-blockade of the C5a receptors, C5aR and C5L2. When rats were subjected to CLP, levels of C5a and norepinephrine were found to be antipodal as a function of time. PC12 cell production of norepinephrine and dopamine was significantly blunted following exposure to recombinant rat C5a in a time-dependent and dose-dependent manner. This impaired production could be related to C5a-induced initiation of apoptosis as defined by binding of Annexin V and Propidium Iodine to PC12 cells. Collectively, we describe a C5a-dependent induction of apoptotic events in cells of adrenal medulla in vivo and pheochromocytoma PC12 cells in vitro. These data suggest that experimental sepsis induces apoptosis of adrenomedullary cells, which are responsible for the bulk of endogenous catecholamines. Septic shock may be linked to these events. Since blockade of both C5a receptors virtually abolished adrenomedullary apoptosis in vivo, C5aR and C5L2 become promising targets with implications on future complement-blocking strategies in the clinical setting of sepsis.

Functions of the complement components C3 and C5 during sepsis

Activation of the complement system is a key event in the pathogenesis of sepsis. Nevertheless, the exact mechanisms remain inadequately understood. In the current study, we examined the role of complement C3 and C5 in sepsis in wild-type and C3- or C5-deficient mice induced by cecal ligation and puncture. When compared to wild-type mice, C5(-/-) showed identical survival, and C3(-/-) presented significantly reduced survival. Interestingly, this was associated with significant decreases in plasma levels of proinflammatory mediators. Moreover, although septic C3(-/-) animals displayed a 10-fold increase of blood-borne bacteria, C5(-/-) animals exhibited a 400-fold increase in bacteremia when compared to wild-type mice. These effects were linked to the inability of C5(-/-) mice to assemble the terminal membrane attack complex (MAC), as determined by complement hemolytic activity (CH-50). Surprisingly, although negative control C3(-/-) mice failed to generate the MAC, significant increases of MAC formation was found in septic C3(-/-) mice. In conclusion, our data corroborate that hemolytic complement activity is essential for control of bacteremia in septic mice. Thus, during sepsis, blockade of C5a or its receptors (rather than C5) seems a more promising strategy, because C5a-blockade still allows for MAC formation while the adverse effects of C5a are prevented.