Functions of C5a receptors

Complement Component C5a and Fungal Pathogen Induce Diverse Responses through Crosstalk between Transient Receptor Potential Channel (TRPs) Subtypes in Human Conjunctival Epithelial Cells

The conjunctiva has immune-responsive properties to protect the eye from infections. Its innate immune system reacts against external pathogens, such as fungi. The complement factor C5a is an important contributor to the initial immune response. It is known that activation of transient-receptor-potential-vanilloid 1 (TRPV1) and TRP-melastatin 8 (TRPM8) channels is involved in different immune reactions and inflammation in the human body. The aim of this study was to determine if C5a and mucor racemosus e voluminae cellulae (MR) modulate Ca2+-signaling through changes in TRPs activity in human conjunctival epithelial cells (HCjECs). Furthermore, crosstalk was examined between C5a and MR in mediating calcium regulation. Intracellular Ca2+-concentration ([Ca2+]i) was measured by fluorescence calcium imaging, and whole-cell currents were recorded using the planar-patch-clamp technique. MR was used as a purified extract. Application of C5a (0.05-50 ng/mL) increased both [Ca2+]i and whole-cell currents, which were suppressed by either the TRPV1-blocker AMG 9810 or the TRPM8-blocker AMTB (both 20 µM). The N-terminal peptide C5L2p (20-50 ng/mL) blocked rises in [Ca2+]i induced by C5a. Moreover, the MR-induced rise in Ca2+-influx was suppressed by AMG 9810 and AMTB, as well as 0.05 ng/mL C5a. In conclusion, crosstalk between C5a and MR controls human conjunctival cell function through modulating interactions between TRPV1 and TRPM8 channel activity.

Renal improvement and remission in a patient with refractory ANCA-associated vasculitis treated with avacopan

Antineutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis is associated with end-organ damage resulting in significant morbidity and mortality. Most recently, avacopan, an orally administered selective antagonist of the C5a receptor, was approved by the US Food and Drug Administration as an adjunctive treatment of adult patients with severe, active ANCA-associated vasculitis (granulomatosis with polyangiitis and microscopic polyangiitis) in combination with standard therapy including glucocorticoids. This case study describes a 58-year-old Asian female with severe ANCA-associated vasculitis and acute renal failure who responded to adjunctive therapy with avacopan despite being refractory to rituximab and glucocorticoid therapy.

C5a drives the inflammatory response with bacterial dose effect by binding to C5aR1 in zebrafish infected with Aeromonas hydrophila

The complement system is essential to host defense, but its excessive activation caused by severe pathogen invasion is a driving force in adverse inflammatory. The binding of complement component 5a (C5a) and complement component 5a receptor 1 (C5aR1) is the key to trigger complement-mediated inflammatory response in mammals. However, the role of C5a-C5aR1 axis in fish immune response remains obscure. In this study, the role of C5a-C5aR1 axis of zebrafish (Danio rerio) after serious infection with Aeromonas hydrophila was investigated. C5a and C5aR1 of zebrafish were cloned, with CDS sequences of 228 and 1041 bp, respectively, and they were widely expressed in various tissues with the highest expression in the liver and spleen, respectively. The survival of zebrafish was closely correlated to the dose of A. hydrophila. The cytokine storm occurred at high concentrations of A. hydrophila infection. At 24 h post infection (hpi), the expression of C5a and C5aR1 in the spleen increased 26.8-fold and 9.9-fold in treatment group 1 (TG1, 3.0 × 107 CFU/mL) (P < 0.01), and 4.7-fold and 3.4-fold in treatment group 2 (TG2, 1.0 × 107 CFU/mL) (P < 0.05), respectively. Correspondingly, proinflammatory cytokines interleukin-1β (IL-1β), interleukin-8 (IL-8), and interleukin-17 (IL-17) were positively correlated to C5a and C5aR1 at mRNA and protein expression levels. The expression of IL-1β was significantly increased in the spleen at 6 hpi, with a 599.2-fold and 203.2-fold upregulation in TG1 and TG2 (P < 0.001), respectively. Moreover, after inhibition of C5a-C5aR1 binding treated with C5aR1 antagonist (W-54011), zebrafish showed lower expression of C5a, C5aR1, and cytokines, less intestinal damage, and significantly enhancement of survival (P < 0.05) after A. hydrophila challenge. This study revealed that the inflammatory effect of C5a was achieved by binding to C5aR1 in zebrafish, providing novel insights into using C5a-C5aR1 axis as an effective target to reduce bacterial inflammation and disease in fish.

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.

Complement activation and increased anaphylatoxin receptor expression are associated with cortical grey matter lesions and the compartmentalised inflammatory response of multiple sclerosis

Background

The extent of cortical pathology is an important determinant of multiple sclerosis (MS) severity. Cortical demyelination and neurodegeneration are related to inflammation of the overlying leptomeninges, a more inflammatory CSF milieu and with parenchymal microglia and astroglia activation. These are all components of the compartmentalised inflammatory response. Compartmentalised inflammation is a feature of progressive MS, which is not targeted by disease modifying therapies. Complement is differentially expressed in the MS CSF and complement, and complement receptors, are associated with demyelination and neurodegeneration.

Methods

To better understand if complement activation in the leptomeninges is associated with underlying cortical demyelination, inflammation, and microglial activation, we performed a neuropathological study of progressive MS (n = 22, 14 females), neuroinflammatory (n = 8), and non-neurological disease controls (n = 10). We then quantified the relative extent of demyelination, connective tissue inflammation, complement, and complement receptor positive microglia/macrophages.

Results

Complement was elevated at the leptomeninges, subpial, and within and around vessels of the cortical grey matter. The extent of complement C1q immunoreactivity correlated with connective tissue infiltrates, whilst activation products C4d, Bb, and C3b associated with grey matter demyelination, and C3a receptor 1+ and C5a receptor 1+ microglia/macrophages closely apposed C3b labelled cells. The density of C3a receptor 1+ and C5a receptor 1+ cells was increased at the expanding edge of subpial and leukocortical lesions. C5a receptor 1+ cells expressed TNFα, iNOS and contained puncta immunoreactive for proteolipid protein, neurofilament and synaptophysin, suggesting their involvement in grey matter lesion expansion.

Interpretation

The presence of products of complement activation at the brain surfaces, their association with the extent of underlying pathology and increased complement anaphylatoxin receptor positive microglia/macrophages at expanding cortical grey matter lesions, could represent a target to modify compartmentalised inflammation and cortical demyelination.

Atomic force microscopy applied to interrogate nanoscale cellular chemistry and supramolecular bond dynamics for biomedical applications

Understanding biological interactions at a molecular level grants valuable information relevant to improving medical treatments and outcomes. Among the suite of technologies available, Atomic Force Microscopy (AFM) is unique in its ability to quantitatively probe forces and receptor-ligand interactions in real-time. The ability to assess the formation of supramolecular bonds and intermediates in real-time on surfaces and living cells generates important information relevant to understanding biological phenomena. Combining AFM with fluorescence-based techniques allows for an unprecedented level of insight not only concerning the formation and rupture of bonds, but understanding medically relevant interactions at a molecular level. As the ability of AFM to probe cells and more complex models improves, being able to assess binding kinetics, chemical topographies, and garner spectroscopic information will likely become key to developing further improvements in fields such as cancer, nanomaterials, and virology. The rapid response to the COVID-19 crisis, producing information regarding not just receptor affinities, but also strain-dependent efficacy of neutralizing nanobodies, demonstrates just how viable and integral to the pre-clinical development of information AFM techniques are in this era of medicine.

Complement Components in the Diagnosis and Treatment after Kidney Transplantation-Is There a Missing Link?

Currently, kidney transplantation is widely accepted as the renal replacement therapy allowing for the best quality of life and longest survival of patients developing end-stage renal disease. However, chronic transplant rejection, recurrence of previous kidney disease or newly acquired conditions, or immunosuppressive drug toxicity often lead to a deterioration of kidney allograft function over time. Complement components play an important role in the pathogenesis of kidney allograft impairment. Most studies on the role of complement in kidney graft function focus on humoral rejection; however, complement has also been associated with cell mediated rejection, post-transplant thrombotic microangiopathy, the recurrence of several glomerulopathies in the transplanted kidney, and transplant tolerance. Better understanding of the complement involvement in the transplanted kidney damage has led to the development of novel therapies that inhibit complement components and improve graft survival. The analysis of functional complotypes, based on the genotype of both graft recipient and donor, may become a valuable tool for assessing the risk of acute transplant rejection. The review summarizes current knowledge on the pathomechanisms of complement activation following kidney transplantation and the resulting diagnostic and therapeutic possibilities.

Targeting the Complement Pathway in Malignant Glioma Microenvironments

Malignant glioma is a highly fatal type of brain tumor, and its reoccurrence is largely due to the ordered interactions among the components present in the complex microenvironment. Besides its role in immune surveillance and clearance under physiological conditions, the complement system is expressed in a variety of tumor types and mediates the interactions within the tumor microenvironments. Recent studies have uncovered the broad expression spectrum of complement signaling molecules in the tumor microenvironment and various tumor cells, in particular, malignant glioma cells. Involvement of the complement system in tumor growth, immunosuppression and phenotype transition have also been elucidated. In this review, we enumerate the expression and function of complement molecules in multiple tumor types reported. Moreover, we elaborate the complement pathways in glioma cells and various components of malignant glioma microenvironments. Finally, we summarize the possibility of the complement molecules as prognostic factors and therapeutic targets in the treatment of malignant glioma. Specific targeting of the complement system maybe of great significance and value in the future treatment of multi-type tumors including malignant glioma.

"Complimenting the Complement": Mechanistic Insights and Opportunities for Therapeutics in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the most common primary malignancy of the liver and a leading cause of death in the US and worldwide. HCC remains a global health problem and is highly aggressive with unfavorable prognosis. Even with surgical interventions and newer medical treatment regimens, patients with HCC have poor survival rates. These limited therapeutic strategies and mechanistic understandings of HCC immunopathogenesis urgently warrant non-palliative treatment measures. Irrespective of the multitude etiologies, the liver microenvironment in HCC is intricately associated with chronic necroinflammation, progressive fibrosis, and cirrhosis as precedent events along with dysregulated innate and adaptive immune responses. Central to these immunological networks is the complement cascade (CC), a fundamental defense system inherent to the liver which tightly regulates humoral and cellular responses to noxious stimuli. Importantly, the liver is the primary source for biosynthesis of >80% of complement components and expresses a variety of complement receptors. Recent studies implicate the complement system in liver inflammation, abnormal regenerative responses, fibrosis, carcinogenesis, and development of HCC. Although complement activation differentially promotes immunosuppressive, stimulant, and angiogenic microenvironments conducive to HCC development, it remains under-investigated. Here, we review derangement of specific complement proteins in HCC in the context of altered complement regulatory factors, immune-activating components, and their implications in disease pathogenesis. We also summarize how complement molecules regulate cancer stem cells (CSCs), interact with complement-coagulation cascades, and provide therapeutic opportunities for targeted intervention in HCC.

Role of G Protein-Coupled Receptors in Hepatic Stellate Cells and Approaches to Anti-Fibrotic Treatment of Non-Alcoholic Fatty Liver Disease

The prevalence of non-alcoholic fatty liver disease (NAFLD) is globally increasing. Gaining control over disease-related events in non-alcoholic steatohepatitis (NASH), an advanced form of NAFLD, is currently an unmet medical need. Hepatic fibrosis is a critical prognostic factor in NAFLD/NASH. Therefore, a better understanding of the pathophysiology of hepatic fibrosis and the development of related therapies are of great importance. G protein-coupled receptors (GPCRs) are cell surface receptors that mediate the function of a great variety of extracellular ligands. GPCRs represent major drug targets, as indicated by the fact that about 40% of all drugs currently used in clinical practice mediate their therapeutic effects by acting on GPCRs. Like many other organs, various GPCRs play a role in regulating liver function. It is predicted that more than 50 GPCRs are expressed in the liver. However, our knowledge of how GPCRs regulate liver metabolism and fibrosis in the different cell types of the liver is very limited. In particular, a better understanding of the role of GPCRs in hepatic stellate cells (HSCs), the primary cells that regulate liver fibrosis, may lead to the development of drugs that can improve hepatic fibrosis in NAFLD/NASH. In this review, we describe the functions of multiple GPCRs expressed in HSCs, their roles in liver fibrogenesis, and finally speculate on the development of novel treatments for NAFLD/NASH.

From Classical to Unconventional: The Immune Receptors Facilitating Platelet Responses to Infection and Inflammation

Platelets have long been recognized for their role in maintaining the balance between hemostasis and thrombosis. While their contributions to blood clotting have been well established, it has been increasingly evident that their roles extend to both innate and adaptive immune functions during infection and inflammation. In this comprehensive review, we describe the various ways in which platelets interact with different microbes and elicit immune responses either directly, or through modulation of leukocyte behaviors.

Fc Gamma Receptors and Complement Component 3 Facilitate Anti-fVIII Antibody Formation

Anti-factor VIII (fVIII) alloantibodies, which can develop in patients with hemophilia A, limit the therapeutic options and increase morbidity and mortality of these patients. However, the factors that influence anti-fVIII antibody development remain incompletely understood. Recent studies suggest that Fc gamma receptors (FcγRs) may facilitate recognition and uptake of fVIII by recently developed or pre-existing naturally occurring anti-fVIII antibodies, providing a mechanism whereby the immune system may recognize fVIII following infusion. However, the role of FcγRs in anti-fVIII antibody formation remains unknown. In order to define the influence of FcγRs on the development of anti-fVIII antibodies, fVIII was injected into WT or FcγR knockout recipients, followed by evaluation of anti-fVIII antibodies. Anti-fVIII antibodies were readily observed following fVIII injection into FcγR knockouts, with similar anti-fVIII antibody levels occurring in FcγR knockouts as detected in WT mice injected in parallel. As antibodies can also fix complement, providing a potential mechanism whereby anti-fVIII antibodies may influence anti-fVIII antibody formation independent of FcγRs, fVIII was also injected into complement component 3 (C3) knockout recipients in parallel. Similar to FcγR knockouts, C3 knockout recipients developed a robust response to fVIII, which was likewise similar to that observed in WT recipients. As FcγRs or C3 may compensate for each other in recipients only deficient in FcγRs or C3 alone, we generated mice deficient in both FcγRs and C3 to test for potential antibody effector redundancy in anti-fVIII antibody formation. Infusion of fVIII into FcγRs and C3 (FcγR × C3) double knockouts likewise induced anti-fVIII antibodies. However, unlike individual knockouts, anti-fVIII antibodies in FcγRs × C3 knockouts were initially lower than WT recipients, although anti-fVIII antibodies increased to WT levels following additional fVIII exposure. In contrast, infusion of RBCs expressing distinct alloantigens into FcγRs, C3 or FcγR × C3 knockout recipients either failed to change anti-RBC levels when compared to WT recipients or actually increased antibody responses, depending on the target antigen. Taken together, these results suggest FcγRs and C3 can differentially impact antibody formation following exposure to distinct alloantigens and that FcγRs and C3 work in concert to facilitate early anti-fVIII antibody formation.

Analysis of the Complement System in the Clinical Immunology Laboratory

The complement system is a critical component of both the innate and adaptive immune systems that augments the function of antibodies and phagocytes. Antigen-antibody immune complexes, lectin binding, and accelerated C3 tick-over can activate this well-coordinated and carefully regulated process. The importance of this system is highlighted by the disorders that arise when complement components or regulators are deficient or dysregulated. This article describes the pathways involved in complement activation and function, the regulation of these various pathways, and the interpretation of laboratory testing performed for the diagnosis of diseases of complement deficiency, exuberant complement activation, and complement dysregulation.

Examining the Role of Complement in Predicting, Preventing, and Treating Hemolytic Transfusion Reactions

Red blood cell (RBC) transfusion is a critical component of optimal management for a broad range of conditions. Regardless of the indication, pretransfusion testing is required to appropriately match RBC donors and recipients to provide immunologically compatible blood. Although this approach is effective in the vast majority of situations, occasionally, patients will inadvertently receive an incompatible RBC transfusion, which can result in a hemolytic transfusion reaction (HTR). In addition, patients with life-threatening anemia and a complex alloantibody profile, which precludes rapid procurement of compatible RBCs, may also receive incompatible RBCs, placing them at risk for an HTR. Despite the rarity of these clinical situations, when incompatible blood transfusion results in an HTR, the consequences can be devastating. In this review, we will explore the challenges associated with actively preventing and treating acute HTRs following incompatible RBC transfusion. In doing so, we will focus primarily on the role of complement, not only as a key player in HTRs, but also as a potential target for the prevention and treatment of HTRs.

Deletions in Genes Participating in Innate Immune Response Modify the Clinical Course of Andes Orthohantavirus Infection

Andes orthohantavirus (ANDV) is an important human pathogen causing hantavirus cardiopulmonary syndrome (HCPS) with a fatality rate of 30% in Chile. Around 60% of all cases have a severe clinical course, while the others have a mild clinical course. The main goal of this study was to understand if the genetic variation of patients is associated with the clinical course they develop after ANDV infection. For this, the frequency of copy number variants (CNVs, i.e., deletions and duplications) was studied in 195 patients, 88 with mild and 107 with severe HCPS. CNVs were called from intensity data of the Affymetrix Genome-Wide SNP Array 6.0. The analysis of the data was performed with PennCNV, ParseCNV and R softwares; Results: a deletion of 19, 416 bp in the q31.3 region of chromosome 1 is found more frequently in severe patients (p < 0.05). This region contains Complement Factor H Related (CFHR1) and CFHR3 genes, regulators of the complement cascade. A second deletion of 1.81 kb located in the p13 region of chr20 was significantly more frequent in mild patients (p < 0.05). This region contains the SIRPB1 gene, which participates in the innate immune response, more specifically in neutrophil trans-epithelial migration. Both deletions are associated with the clinical course of HCPS, the first being a risk factor and the second being protective. The participation of genes contained in both deletions in ANDV infection pathophysiology deserves further investigation.

Deletion of Biliverdin Reductase A in Myeloid Cells Promotes Chemokine Expression and Chemotaxis in Part via a Complement C5a--C5aR1 Pathway

Biliverdin reductase (BVR)-A is a pleotropic enzyme converting biliverdin to bilirubin and a signaling molecule that has cytoprotective and immunomodulatory effects. We recently showed that biliverdin inhibits the expression of complement activation fragment 5a receptor one (C5aR1) in RAW 264.7 macrophages. In this study, we investigated the role of BVR-A in determining macrophage inflammatory phenotype and function via regulation of C5aR1. We assessed expression of C5aR1, M1-like macrophage markers, including chemokines (RANTES, IP-10), as well as chemotaxis in response to LPS and C5a in bone marrow-derived macrophages from BVR ^fl/fl and LysM-Cre:BVR ^{fl / fl} mice (conditional deletion of BVR-A in myeloid cells). In response to LPS, macrophages isolated from LysM-Cre:BVR ^fl/fl showed significantly elevated levels of C5aR1 as well as chemokines (RANTES, IP10) but not proinflammatory markers, such as iNOS and TNF. An increase in C5aR1 expression was also observed in peritoneal macrophages and several tissues from LysM-Cre:BVR ^fl/fl mice in a model of endotoxemia. In addition, knockdown of BVR-A resulted in enhanced macrophage chemotaxis toward C5a. Part of the effects of BVR-A deletion on chemotaxis and RANTES expression were blocked in the presence of a C5aR1 neutralizing Ab, confirming the role of C5a-C5aR1 signaling in mediating the effects of BVR. In summary, BVR-A plays an important role in regulating macrophage chemotaxis in response to C5a via modulation of C5aR1 expression. In addition, macrophages lacking BVR-A are characterized by the expression of M1 polarization-associated chemokines, the levels of which depend in part on C5aR1 signaling.

Myocardial expression of the anaphylatoxin receptor C3aR is associated with cardiac inflammation and prognosis in patients with non-ischaemic heart failure

Aim

The aim of this study is to analyse the prognostic value of complement anaphylatoxin receptors in patients with non‐ischaemic cardiomyopathy undergoing endomyocardial biopsy.

Methods and results

In 102 patients (72.5% male patients, median age 54 years) with non‐ischaemic cardiomyopathy, myocardial expression of C3aR was assessed among other parameters. The primary study endpoint was a composite of death, heart transplantation, heart failure‐related re‐hospitalization, and deterioration of left ventricular ejection fraction within a mean follow‐up of 11.9 months. The number of cells, which stained positive for C3aR, was significantly increased in patients with inflammatory compared with non‐inflammatory cardiomyopathy (1.75 ± 0.31 cells in inflammatory cardiomyopathy vs. 0.94 ± 0.26 in non‐inflammatory cardiomyopathy, P = 0.049). Subsequently, positive expression for C3aR was judged based on a semi‐quantitative scoring system. Significantly, more patients with positive MHCII and CD68 expression showed an increased number of C3aR‐positive cells. C3aR expression based on this score was more pronounced in patients with human herpesvirus 6 viral genome detection. Kaplan–Meier curves illustrate that the C3aR‐negative group reached the primary endpoint significantly more often (mean follow‐up 11.9 months, log rank 5.963, P = 0.015). Lack of C3aR expression was a strong independent predictor for the primary endpoint in Cox regression analysis [hazard ratio 0.46 (0.26–0.82, P = 0.009)].

Conclusions

C3aR‐positive cells are found more often in patients with inflammatory cardiomyopathy. The relevance of C3aR‐positive cells in patients with non‐ischaemic cardiomyopathy should be further evaluated as potential predictors or modulators of adverse cardiac remodelling, the substrate of progressive heart failure.

CD177-mediated nanoparticle targeting of human and mouse neutrophils

Neutrophils are the most abundant white blood cells, with a vital role in innate immune defense against bacterial and fungal pathogens. Although mostly associated with pathological processes directly related to immune defense, they can also play a detrimental role in inflammatory conditions and have been found to have a pro-metastatic role in the spread of cancer cells. Here, we explore ways to temporarily suppress these detrimental activities. We first examined the possibility of using siRNA and antisense oligonucleotides (ASOs) for transient knockdown of the human and mouse C5a receptor, an important chemoattractant receptor involved in neutrophil-mediated injury that is associated with myocardial infarction, sepsis, and neurodegenerative diseases. We found that siRNAs and ASOs transfected into cultured cell lines can eliminate 70–90% of C5a receptor mRNA and protein within 72 h of administration, a clinically relevant time frame after a cardiovascular event. Targeted drug delivery to specific cells or tissues of interest in a mammalian host, however, remains a major challenge. Here, using phage display technology, we have identified peptides that bind specifically to CD177, a neutrophil-specific surface molecule. We have attached these peptides to fluorescent, lipid-based nanoparticles and confirmed targeting and delivery to cultured cells ectopically presenting either human or mouse CD177. In addition, we have shown peptide-nanoparticle binding specifically to neutrophils in human and mouse blood. We anticipate that these or related tagged nanoparticles may be therapeutically useful for delivery of siRNAs or ASOs to neutrophils for transient knockdown of pro-inflammatory proteins such as the C5a receptor.

New tricks for an ancient system: Physiological and pathological roles of complement in the CNS

While the mechanisms underlying the functions of the complement system in the central nervous system (CNS) and systemically, namely opsonization, chemotaxis, membrane lysis, and regulation of inflammation are the same, the plethora of functions that complement orchestrates in the central nervous system (CNS) is complex. Strictly controlled expression of complement effector molecules, regulators and receptors across the gamut of life stages (embryogenesis, development and maturation, aging and disease) dictate fascinating contributions for this ancient system. Furthermore, it is becoming apparent that complement functions differ widely across distinct brain regions. This review provides a comprehensive overview of the newly identified roles for complement in the brain, including its roles in CNS development and function, during aging and in the processes of neurodegeneration. The diversity and selectively of beneficial and detrimental activities of complement, while challenging, should lead to precision targeting of specific components to provide disease modifying treatments for devastating psychiatric and neurodegenerative disorders that are still without effective treatment.

Polymorphonuclear Neutrophil Functions are Differentially Altered in Amnestic Mild Cognitive Impairment and Mild Alzheimer's Disease Patients

The mechanisms of neurodegeneration in Alzheimer's disease (AD) remain under investigation. Alterations in the blood-brain barrier facilitate exchange of inflammatory mediators and immune cells between the brain and the periphery in AD. Here, we report analysis of phenotype and functions of polymorphonuclear neutrophils (PMN) in peripheral blood from patients with amnestic mild cognitive impairment (aMCI, n = 13), patients with mild AD (mAD, n = 15), and healthy elderly controls (n = 13). Results showed an increased expression of CD177 in mAD but not in healthy or aMCI patients. IL-8 stimulated increased expression of the CD11b integrin in PMN of healthy subjects in vitro but PMN of aMCI and mAD patients failed to respond. CD14 and CD16 expression was lower in PMN of mAD but not in aMCI individuals relative to controls. Only PMN of aMCI subjects expressed lower levels of CD88. Phagocytosis toward opsonized E. coli was differentially impaired in PMN of aMCI and mAD subjects whereas the capacity to ingest Dextran particles was absent only in mAD subjects. Killing activity was severely impaired in aMCI and mAD subjects whereas free radical production was only impaired in mAD patients. Inflammatory cytokine (TNFα, IL-6, IL-1β, IL-12p70) and chemokine (MIP-1α, MIP-1β, IL-8) production in response to LPS stimulation was very low in aMCI and nearly absent in mAD subjects. TLR2 expression was low only in aMCI. Our data showed a differentially altered capacity of PMN of aMCI and mAD subjects to respond to pathological aggression that may impact impaired responses associated with AD development.

Systemic inhibition of the membrane attack complex impedes neuroinflammation in chronic relapsing experimental autoimmune encephalomyelitis

The complement system is a key driver of neuroinflammation. Activation of complement by all pathways, results in the formation of the anaphylatoxin C5a and the membrane attack complex (MAC). Both initiate pro-inflammatory responses which can contribute to neurological disease. In this study, we delineate the specific roles of C5a receptor signaling and MAC formation during the progression of experimental autoimmune encephalomyelitis (EAE)-mediated neuroinflammation. MAC inhibition was achieved by subcutaneous administration of an antisense oligonucleotide specifically targeting murine C6 mRNA (5 mg/kg). The C5a receptor 1 (C5aR1) was inhibited with the C5a receptor antagonist PMX205 (1.5 mg/kg). Both treatments were administered systemically and started after disease onset, at the symptomatic phase when lymphocytes are activated. We found that antisense-mediated knockdown of C6 expression outside the central nervous system prevented relapse of disease by impeding the activation of parenchymal neuroinflammatory responses, including the Nod-like receptor protein 3 (NLRP3) inflammasome. Furthermore, C6 antisense-mediated MAC inhibition protected from relapse-induced axonal and synaptic damage. In contrast, inhibition of C5aR1-mediated inflammation diminished expression of major pro-inflammatory mediators, but unlike C6 inhibition, it did not stop progression of neurological disability completely. Our study suggests that MAC is a key driver of neuroinflammation in this model, thereby MAC inhibition might be a relevant treatment for chronic neuroinflammatory diseases.

Antibodies to Signaling Molecules and Receptors in Alzheimer's Disease are Associated with Psychomotor Slowing, Depression, and Poor Visuospatial Function

BACKGROUND

Alzheimer's disease (AD) is associated with several antibodies as well as signaling molecules and receptors. These may be detrimental in the presence of a disrupted blood-brain barrier (BBB).

OBJECTIVE

To investigate whether the levels of antibodies toward 33 signaling molecules involved in neurotransmitter, vascular, and immune functions were associated with AD and, within the AD group; cognitive function and mood.

METHODS

Antibodies in sera from patients with mild AD [(n = 91) defined as a Mini-Mental State Examination ≥ 20 or a Clinical Dementia Rating Scale≤1] and healthy controls (n = 102) were measured with enzyme-linked immunosorbent assays. Levels in AD and controls were compared by Mann-Whitney test. In the AD group, associations between antibodies and psychometric test scores were analyzed by robust regression. The false discovery threshold was set to 0.05.

RESULTS

Antibodies to serotonin receptors [5-HT2AR (effect size (r) = 0.21, p = 0.004), 5-HT2CR (r = 0.25, p = 0.0005) and 5-HT7R (r = 0.21, p = 0.003)], vascular endothelial growth factor receptor 1 [VEGFR1 (r = 0.29, p < 0.001)] and immune-receptors (Stabilin-1 (r = 0.23, p = 0.001) and C5aR1 (r = 0.21, p = 0.004) were higher in AD. Psychomotor speed was associated with D1R-abs (β 0.49, p < 0.001), depression with ETAR-abs (β 0.31, p < 0.001), and visuospatial function with 5-HT1AR-abs (β 0.27, p = 0.004) despite similar antibody levels compared to controls.

CONCLUSIONS

Antibody levels to VEGFR1, serotonergic receptors, and receptors in the immune system were increased in AD. Antibodies at similar levels as in controls were associated cognitive dysfunction and depression in AD.

The Complement System Is Critical in Maintaining Retinal Integrity during Aging

The complement system is a key component of innate immunity comprised of soluble components that form a proteolytic cascade leading to the generation of effector molecules involved in cellular clearance. This system is highly activated not only under general inflammatory conditions such as infections, collagen diseases, nephritis, and liver diseases, but also in focal ocular diseases. However, little is known about the role of the complement system in retinal homeostasis during aging. Using young (6-week-old) and adult (6-month-old) mice in wild type (C57BL/6) and complement knockout strains (C1q^−/−, Mbl a/c^−/−, Fb^−/−, C3^−/−, and C5^−/−), we compared amplitudes of electroretinograms (ERG) and thicknesses of retinal layers in spectral domain optical coherence tomography between young and adult mice. The ERG amplitudes in adult mice were significantly decreased (p < 0.001, p < 0.0001) compared to that of young mice in all complement knockout strains, and there were significant decreases in the inner nuclear layer (INL) thickness in adult mice compared to young mice in all complement knockout strains (p < 0.0001). There were no significant differences in ERG amplitude or thickness of the INL between young and adult control mice. These data suggest that the complement system plays an important role in maintaining normal retinal integrity over time.

C5a/C5aR pathway accelerates renal ischemia-reperfusion injury by downregulating PGRN expression

Recent reports indicate that the complement C5a/C5aR pathway and progranulin (PGRN) deficiency both contribute to ischemia-reperfusion (IR)-induced acute kidney injury. However, the underlying relationship between the C5a/C5aR signaling pathway and PGRN expression during acute kidney injury is poorly understood. In this study, we showed that C5aR expression was significantly upregulated after renal IR, and that C5aR deficiency led to a marked increase in PGRN expression and a significant reduction in tubular damage and production of inflammatory cytokines. In accordance with these results, recombinant C5a caused downregulation of PGRN protein and mRNA levels in renal tubular epithelial cells (HK-2 cells), which could be negated by disruption of C5a/C5aR signaling by the C5aR antagonist, as confirmed by immunofluorescence, western blotting, and quantitative real-time PCR. Moreover, C5aR deficiency resulted in attenuated NF-κB expression 24h after IR, and recombinant C5a potentiated TNFα-induced NF-κB activation in HK-2 cells. Inhibition of NF-κB activation reversed C5a-induced downregulation of PGRN expression. Our results show for the first time that the complement C5a/C5aR pathway aggravates IR-induced acute kidney injury by suppressing PGRN expression and confirm that suppression of PGRN expression is associated with increased NF-κB activation induced by C5a.

The "ins and outs" of complement-driven immune responses

The complement system represents an evolutionary old and critical component of innate immunity where it forms the first line of defense against invading pathogens. Originally described as a heat-labile fraction of the serum responsible for the opsonization and subsequent lytic killing of bacteria, work over the last century firmly established complement as a key mediator of the general inflammatory response but also as an acknowledged vital bridge between innate and adaptive immunity. However, recent studies particularly spanning the last decade have provided new insights into the novel modes and locations of complement activation and highlighted unexpected additional biological functions for this ancient system, for example, in regulating basic processes of the cell. In this review, we will cover the current knowledge about complement's established and novel roles in innate and adaptive immunity with a focus on the functional differences between serum circulating and intracellularly active complement and will describe and discuss the newly discovered cross-talks of complement with other cell effector systems particularly during T-cell induction and contraction.

The role of the complement system in cancer

In addition to being a component of innate immunity and an ancient defense mechanism against invading pathogens, complement activation also participates in the adaptive immune response, inflammation, hemostasis, embryogenesis, and organ repair and development. Activation of the complement system via classical, lectin, or alternative pathways generates anaphylatoxins (C3a and C5a) and membrane attack complex (C5b-9) and opsonizes targeted cells. Complement activation end products and their receptors mediate cell-cell interactions that regulate several biological functions in the extravascular tissue. Signaling of anaphylatoxin receptors or assembly of membrane attack complex promotes cell dedifferentiation, proliferation, and migration in addition to reducing apoptosis. As a result, complement activation in the tumor microenvironment enhances tumor growth and increases metastasis. In this Review, I discuss immune and nonimmune functions of complement proteins and the tumor-promoting effect of complement activation.

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.

C5a of Cynoglossus semilaevis has anaphylatoxin-like properties and promotes antibacterial and antiviral defense

Activation of the complement system leads to the cleavage of component factor C5 into C5a and C5b. C5a can induce chemotaxis and inflammatory responses in mammals. The function of C5a in fish is poorly understood. In this study, we report the identification and analysis of a C5 homologue, CsC5, from tongue sole (Cynoglossus semilaevis). CsC5 is composed of 1683 amino acid residues that include an anaphylatoxin homologous domain. Expression of CsC5 could be detected in a variety of tissues and was up-regulated by bacterial or viral pathogen infection. Purified recombinant CsC5a (rCsC5a) could bind to peripheral blood leukocytes (PBL) and stimulate PBL chemotaxis, proliferation, respiratory burst, acid phosphatase activity, and phagocytosis. Tongue sole administered rCsC5a exhibited enhanced resistance against bacterial and viral infections. These results indicate that CsC5a is an anaphylatoxin with a role in innate immune defense against bacterial and viral infections.

Differential capacity for complement receptor-mediated immune evasion by Porphyromonas gingivalis depending on the type of innate leukocyte

The complement system plays a central role in immunity and inflammation, although certain pathogens can exploit complement to undermine protective immunity. In this context, the periodontal keystone pathogen Porphyromonas gingivalis was previously shown by our group to evade killing by neutrophils or macrophages through exploitation of complement C5a receptor 1 (C5aR1) and complement receptor 3 (CR3). Here, we examined whether P. gingivalis uses complement receptors to also subvert killing by dendritic cells. In line with earlier independent studies, intracellular viable P. gingivalis bacteria could be recovered from mouse bone-marrow-derived dendritic cells (BMDC) or human monocyte-derived dendritic cells (MDDC) exposed to the pathogen. However, in the presence of C5a, the intracellular survival of P. gingivalis was significantly decreased in a C5aR1-dependent way. Further work using wild-type and receptor-knockout BMDC showed that, in the presence of C3a, the C3a receptor (C3aR) similarly enhanced the intracellular killing of P. gingivalis. In contrast, C5aR2, an alternative receptor for C5a (G protein-coupled receptor 77), was associated with increased intracellular P. gingivalis viable counts, consistent with the notion that C5aR2 functions as a negative regulator of C5aR1 activity. Moreover, P. gingivalis failed to use CR3 as a phagocytic receptor in BMDC, in contrast to our earlier findings in macrophages where CR3-mediated uptake promotes P. gingivalis survival. Collectively, these data show that complement receptors mediate cell-type-specific effects on how innate leukocytes handle P. gingivalis, which appears to exploit complement to preferentially evade those cells (neutrophils and macrophages) that are most often encountered in its predominant niche, the periodontal pocket.

Pulp Fibroblasts Control Nerve Regeneration through Complement Activation

Dentin-pulp regeneration is closely linked to the presence of nerve fibers in the pulp and to the healing mechanism by sprouting of the nerve fiber’s terminal branches beneath the carious injury site. However, little is known about the initial mechanisms regulating this process in carious teeth. It has been recently demonstrated that the complement system activation, which is one of the first immune responses, contributes to tissue regeneration through the local production of anaphylatoxins such as C5a. While few pulp fibroblasts in intact teeth and in untreated fibroblast cultures express the C5a receptor (C5aR), here we show that all dental pulp fibroblasts, localized beneath the carious injury site, do express this receptor. This observation is consistent with our in vitro results, which showed expression of C5aR in lipoteichoic acid–stimulated pulp fibroblasts. The interaction of C5a, produced after complement synthesis and activation from pulp fibroblasts, with the C5aR of these cells mediated the local brain-derived neurotropic factor (BDNF) secretion. Overall, this activation guided the neuronal growth toward the lipoteichoic acid–stimulated fibroblasts. Thus, our findings highlight a new mechanism in one of the initial steps of the dentin-pulp regeneration process, linking pulp fibroblasts to the nerve sprouting through the complement system activation. This may provide a useful future therapeutic tool in targeting the fibroblasts in the dentin-pulp regeneration process.

Differential Contributions of the Complement Anaphylotoxin Receptors C5aR1 and C5aR2 to the Early Innate Immune Response against Staphylococcus aureus Infection

The complement anaphylatoxin C5a contributes to host defense against Staphylococcus aureus. In this study, we investigated the functional role of the two known C5a receptors, C5aR1 and C5aR2, in the host response to S. aureus. We found that C5aR1^−/− mice exhibited greater susceptibility to S. aureus bloodstream infection than wild type and C5aR2^−/− mice, as demonstrated by the significantly higher bacterial loads in the kidneys and heart at 24 h of infection, and by the higher levels of inflammatory IL-6 in serum. Histological and immunohistochemistry investigation of infected kidneys at 24 h after bacterial inoculation revealed a discrete infiltration of neutrophils in wild type mice but already well-developed abscesses consisting of bacterial clusters surrounded by a large number of neutrophils in both C5aR1^−/− and C5aR2^−/− mice. Furthermore, blood neutrophils from C5aR1^−/− mice were less efficient than those from wild type or C5aR2^−/− mice at killing S. aureus. The requirement of C5aR1 for efficient killing of S. aureus was also demonstrated in human blood after disrupting C5a-C5aR1 signaling using specific inhibitors. These results demonstrated a role for C5aR1 in S. aureus clearance as well as a role for both C5aR1 and C5aR2 in the orchestration of the inflammatory response during infection.

Complement Split Products C3a/C5a and Receptors: Are They Regulated by Circulating Angiotensin II Type 1 Receptor Autoantibody in Severe Preeclampsia?

BACKGROUND

This study measured the serum levels of complement component (C)3a and C5a and the placental expressions of C3a receptor (R) and C5aR to determine a potential correlation with circulating angiotensin II type 1 (AT1) receptor agonistic autoantibody (AT1-AA) in severe pre-eclampsia.

METHODS

A total of 118 women were recruited and divided into 2 groups: the control group (normotensive preterm pregnancies, n = 66) and severe pre-eclampsia group (n = 52). Levels of C3a, C5a and AT1-AA in serum were measured by enzyme-linked immunosorbent assay and C3aR and C5aR in placenta by Western blotting.

RESULTS

Levels of C3a, C5a and AT1-AA in serum from the severe pre-eclampsia group were significantly higher than in controls (p < 0.05). Placental expression of C3aR and C5aR in the pre-eclampsia group was lower than that in controls (p < 0.05). There were significant positive correlations between levels of C3a, C5a and AT1-AA in serum from the pre-eclampsia group (p < 0.05). In contrast, there was no correlation between C3aR and C5aR in the placenta and AT1-AA in serum in the pre-eclampsia group (p > 0.05).

CONCLUSION

Increased C3a, C5a and AT1-AA in the serum provide indirect evidence that AT1-AA-mediated activation contributes to activate complement, which is a key mechanism underlying the pathogenesis of severe pre-eclampsia.

Regulation of age-related macular degeneration-like pathology by complement factor H

Age-related macular degeneration (AMD) affects approximately one-third of Americans over 70 and is characterized by lipoprotein-rich sub-retinal pigmented epithelium (sub-RPE) deposits. Substantial evidence has emerged that implicates complement factor H (CFH) in the pathogenesis of AMD. Here, we conduct an in vivo analysis to elucidate the role of CFH in AMD pathology. We show that (i) CFH and lipoproteins compete for binding in the sub-RPE extracellular matrix such that decreasing CFH leads to lipoprotein accumulation and sub-RPE deposit formation; and (ii) detrimental complement activation within sub-RPE deposits leads to RPE damage and vision loss. This new understanding of the complicated interactions of CFH in development of AMD-like pathology paves the way for identifying more targeted therapeutic strategies for AMD.

Complement factor H (CFH) is a major susceptibility gene for age-related macular degeneration (AMD); however, its impact on AMD pathobiology is unresolved. Here, the role of CFH in the development of AMD pathology in vivo was interrogated by analyzing aged Cfh^+/− and Cfh^−/− mice fed a high-fat, cholesterol-enriched diet. Strikingly, decreased levels of CFH led to increased sub-retinal pigmented epithelium (sub-RPE) deposit formation, specifically basal laminar deposits, following high-fat diet. Mechanistically, our data show that deposits are due to CFH competition for lipoprotein binding sites in Bruch’s membrane. Interestingly and despite sub-RPE deposit formation occurring in both Cfh^+/− and Cfh^−/− mice, RPE damage accompanied by loss of vision occurred only in old Cfh^+/− mice. We demonstrate that such pathology is a function of excess complement activation in Cfh^+/− mice versus complement deficiency in Cfh^−/− animals. Due to the CFH-dependent increase in sub-RPE deposit height, we interrogated the potential of CFH as a previously unidentified regulator of Bruch’s membrane lipoprotein binding and show, using human Bruch’s membrane explants, that CFH removes endogenous human lipoproteins in aged donors. Thus, advanced age, high-fat diet, and decreased CFH induce sub-RPE deposit formation leading to complement activation, which contributes to RPE damage and visual function impairment. This new understanding of the complicated interactions of CFH in AMD-like pathology provides an improved foundation for the development of targeted therapies for AMD.

Expression of genes involved in the T cell signalling pathway in circulating immune cells of cattle 24 months following oral challenge with Bovine Amyloidotic Spongiform Encephalopathy (BASE)

Background

Bovine Amyloidotic Spongiform Encephalopathy (BASE) is a variant of classical BSE that affects cows and can be transmitted to primates and mice. BASE is biochemically different from BSE and shares some molecular and histo-pathological features with the MV2 sub-type of human sporadic Creutzfeld Jakob Disease (sCJD).

Results

The present work examined the effects of BASE on gene expression in circulating immune cells. Ontology analysis of genes differentially expressed between cattle orally challenged with brain homogenate from cattle following intracranial inoculation with BASE and control cattle identified three main pathways which were affected. Within the immune function pathway, the most affected genes were related to the T cell receptor-mediated T cell activation pathways. The differential expression of these genes in BASE challenged animals at 10,12 and 24 months following challenge, vs unchallenged controls, was investigated by real time PCR.

Conclusions

The results of this study show that the effects of prion diseases are not limited to the CNS, but involve the immune system and particularly T cell signalling during the early stage following challenge, before the appearance of clinical signs.

Complement and the control of HIV infection: an evolving story

PURPOSE OF REVIEW

Thirty years ago, investigators isolated and later determined the structure of HIV-1 and its envelope proteins. Using techniques that were effective with other viruses, they prepared vaccines designed to generate antibody or T-cell responses, but they were ineffective in clinical trials. In this article, we consider the role of complement in host defense against enveloped viruses, the role it might play in the antibody response and why complement has not controlled HIV-1 infection.

RECENT FINDINGS

Complement consists of a large group of cell-bound and plasma proteins that are an integral part of the innate immune system. They provide a first line of defense against microbes and also play a role in the immune response. Here we review the studies of complement-mediated HIV destruction and the role of complement in the HIV antibody response.

SUMMARY

HIV-1 has evolved a complex defense to prevent complement-mediated killing reviewed here. As part of these studies, we have discovered that HIV-1 envelope, on administration into animals, is rapidly broken down into small peptides that may prove to be very inefficient at provident the type of antigenic stimulation that leads to an effective immune response. Improving complement binding and stabilizing envelope may improve the vaccine response.

Complement system in pathogenesis of AMD: dual player in degeneration and protection of retinal tissue

Age-related macular degeneration (AMD) is the most common cause of blindness among the elderly, especially in Western countries. Although the prevalence, risk factors, and clinical course of the disease are well described, its pathogenesis is not entirely elucidated. AMD is associated with a variety of biochemical abnormalities, including complement components deposition in the retinal pigment epithelium-Bruch's membrane-choriocapillaris complex. Although the complement system (CS) is increasingly recognized as mediating important roles in retinal biology, its particular role in AMD pathogenesis has not been precisely defined. Unrestricted activation of the CS following injury may directly damage retinal tissue and recruit immune cells to the vicinity of active complement cascades, therefore detrimentally causing bystander damage to surrounding cells and tissues. On the other hand, recent evidence supports the notion that an active complement pathway is a necessity for the normal maintenance of the neurosensory retina. In this scenario, complement activation appears to have beneficial effect as it promotes cell survival and tissue remodeling by facilitating the rapid removal of dying cells and resulting cellular debris, thus demonstrating anti-inflammatory and neuroprotective activities. In this review, we discuss both the beneficial and detrimental roles of CS in degenerative retina, focusing on the diverse aspects of CS functions that may promote or inhibit macular disease.

Complement and spinal cord injury: traditional and non-traditional aspects of complement cascade function in the injured spinal cord microenvironment

The pathology associated with spinal cord injury (SCI) is caused not only by primary mechanical trauma, but also by secondary responses of the injured CNS. The inflammatory response to SCI is robust and plays an important but complex role in the progression of many secondary injury-associated pathways. Although recent studies have begun to dissect the beneficial and detrimental roles for inflammatory cells and proteins after SCI, many of these neuroimmune interactions are debated, not well understood, or completely unexplored. In this regard, the complement cascade is a key component of the inflammatory response to SCI, but is largely underappreciated, and our understanding of its diverse interactions and effects in this pathological environment is limited. In this review, we discuss complement in the context of SCI, first in relation to traditional functions for complement cascade activation, and then in relation to novel roles for complement proteins in a variety of models.

The complement system and its role in the pathogenesis of periodontitis: current concepts

Periodontitis is a highly prevalent inflammatory disease in tooth supporting tissues, induced by bacteria growing in a biofilm on tooth surfaces. Components of the complement system are present in the periodontal tissue and the system is activated in periodontitis. Continuous complement activation and modulation by bacteria within the biofilm in periodontal pockets, however, may enhance local tissue destruction, providing the biofilm with both essential nutrients and space to grow. A more profound understanding of the mechanisms involved in complement-derived tissue degradation may facilitate the development of new treatment concepts for periodontitis. Further studies on the role of complement in periodontitis pathogenesis may also contribute to the understanding of why some individuals fail to resolve periodontitis. Here, we review evidence that links complement to the pathogenesis of periodontitis with an emphasis on interaction of complement with bacteria from periodontitis-associated biofilm.

Nonhematopoietic β-Arrestin-1 inhibits inflammation in a murine model of polymicrobial sepsis

β-Arrestin-1 (βArr1), a scaffolding protein critical in G-protein coupled receptor desensitization has more recently been found to be important in the pathogenesis of various inflammatory diseases. We sought to understand the role of βArr1 in sepsis pathogenesis using a mouse model of polymicrobial sepsis. Although in previous studies we established that βArr1 deficiency protects mice from endotoxemia, here we demonstrate that the absence of βArr1 remarkably renders mice more susceptible to mortality in polymicrobial sepsis. In accordance with the mortality pattern, early production of inflammatory mediators was markedly enhanced in βArr1 knockout mice systemically and locally in various organs. In addition, enhanced inflammation in the heart was associated with increased NFκB activation. Compared to these effects, immune cell infiltration, thymic apoptosis, and immune suppression during polymicrobial sepsis were unaffected by a deficiency of βArr1. Additionally, enhanced inflammation and consequent higher mortality were not observed in heterozygous mice, suggesting that one allele of βArr1 was sufficient for this protective negative regulatory role. We further demonstrate that, unexpectedly, βArr1 in nonhematopoietic cells is critical and sufficient for inhibiting sepsis-induced inflammation, whereas hematopoietic βArr1 is likely redundant. Taken together, our results reveal a novel and previously unrecognized negative regulatory role of the nonhematopoietic βArr1 in sepsis-induced inflammation.

Anaphylatoxin C5a modulates hepatic stellate cell migration

BACKGROUND

C5a and its cognate receptor, C5a receptor (C5aR), key elements of complement, are critical modulators of liver immunity and fibrosis. However, the molecular mechanism for the cross talk between complement and liver fibrosis is not well understood. C5a is a potent chemokine regulating migration of cells in the innate immune system. Since activation and migration of hepatic stellate cells (HSC) are hallmarks of liver fibrosis, we hypothesized that C5a contributes to fibrosis by regulating HSC activation and/or migration.

RESULTS

Primary cultures of mouse HSC increased expression of alpha smooth muscle actin (α-SMA) and collagen 1A (Col1A1) mRNA in response to activation on plastic. Expression of mRNA for C5aR, but not C5L2, a second C5a receptor that acts as a negative regulator, increased in parallel with markers of HSC activation in culture. Increased expression of C5aR on activated HSC was confirmed by immunocytochemistry. Cell surface expression of C5aR was also detected by flow cytometry on activated HSC isolated from mice expressing GFP under the control of the collagen promoter after exposure to chronic carbon tetrachloride. To understand the functional significance of C5aR expression in HSC, we next investigated whether C5a influenced HSC activation and/or migration. Challenge of HSC with C5a during culture had no effect on expression of α-SMA and Col1A1, suggesting that C5a did not influence HSC activation. Another important characteristic of HSC is their migratory capacity; migration of HSC in response to platelet derived growth factor (PDGF) and monocyte chemoattractant protein-1 (MCP-1) has been well characterized. Challenge of HSC with C5a enhanced HSC migration almost as efficiently as PDGF in a two-dimensional wound healing and Boyden chamber migration assays. C5a also stimulated expression of MCP-1. C5a-induced cell migration was slowed, but not completely inhibited, in presence of 227016, a MCP-1 receptor antagonist, suggesting C5a-induced migration occurs via both MCP-1-dependent and -independent mechanisms.

CONCLUSIONS

These data reveal that C5a regulates migration of HSC and suggest a novel mechanism by which complement contributes to hepatic fibrosis. C5a and its receptors are therefore potential therapeutic targets for the prevention and/or treatment of liver fibrosis.

Fosb gene products contribute to excitotoxic microglial activation by regulating the expression of complement C5a receptors in microglia

The Fosb gene encodes subunits of the activator protein-1 transcription factor complex. Two mature mRNAs, Fosb and ΔFosb, encoding full-length FOSB and ΔFOSB proteins respectively, are formed by alternative splicing of Fosb mRNA. Fosb products are expressed in several brain regions. Moreover, Fosb-null mice exhibit depressive-like behaviors and adult-onset spontaneous epilepsy, demonstrating important roles in neurological and psychiatric disorders. Study of Fosb products has focused almost exclusively on neurons; their function in glial cells remains to be explored. In this study, we found that microglia express equivalent levels of Fosb and ΔFosb mRNAs to hippocampal neurons and, using microarray analysis, we identified six microglial genes whose expression is dependent on Fosb products. Of these genes, we focused on C5ar1 and C5ar2, which encode receptors for complement C5a. In isolated Fosb-null microglia, chemotactic responsiveness toward the truncated form of C5a was significantly lower than that in wild-type cells. Fosb-null mice were significantly resistant to kainate-induced seizures compared with wild-type mice. C5ar1 mRNA levels and C5aR1 immunoreactivity were increased in wild-type hippocampus 24 hours after kainate administration; however, such induction was significantly reduced in Fosb-null hippocampus. Furthermore, microglial activation after kainate administration was significantly diminished in Fosb-null hippocampus, as shown by significant reductions in CD68 immunoreactivity, morphological change and reduced levels of Il6 and Tnf mRNAs, although no change in the number of Iba-1-positive cells was observed. These findings demonstrate that, under excitotoxicity, Fosb products contribute to a neuroinflammatory response in the hippocampus through regulation of microglial C5ar1 and C5ar2 expression.

Complement and its receptors: new insights into human disease

Although new activation and regulatory mechanisms are still being identified, the basic architecture of the complement system has been known for decades. Two major roles of complement are to control certain bacterial infections and to promote clearance of apoptotic cells. In addition, although inappropriate complement activation has long been proposed to cause tissue damage in human inflammatory and autoimmune diseases, whether this is indeed true has been uncertain. However, recent studies in humans, especially those using newly available biological therapeutics, have now clearly demonstrated the pathophysiologic importance of the complement system in several rare diseases. Beyond these conditions, recent genetic studies have strongly supported an injurious role for complement in a wide array of human inflammatory, degenerative, and autoimmune diseases. This review includes an overview of complement activation, regulatory, and effector mechanisms. It then focuses on new understandings gained from genetic studies, ex vivo analyses, therapeutic trials, and animal models as well as on new research opportunities.

Pattern recognition receptors in innate immunity, host defense, and immunopathology

Infection by pathogenic microbes initiates a set of complex interactions between the pathogen and the host mediated by pattern recognition receptors. Innate immune responses play direct roles in host defense during the early stages of infection, and they also exert a profound influence on the generation of the adaptive immune responses that ensue. An improved understanding of the pattern recognition receptors that mediate innate responses and their downstream effects after receptor ligation has the potential to lead to new ways to improve vaccines and prevent autoimmunity. This review focuses on the control of innate immune activation and the role that innate immune receptors play in helping to maintain tissue homeostasis.

Complement activation and the resulting placental vascular insufficiency drives fetal growth restriction associated with placental malaria

Placental malaria (PM) is a major cause of fetal growth restriction, yet the underlying mechanism is unclear. Complement C5a and C5a receptor levels are increased with PM. C5a is implicated in fetal growth restriction in non-infection-based animal models. In a case-control study of 492 pregnant Malawian women, we find that elevated C5a levels are associated with an increased risk of delivering a small-for-gestational-age infant. C5a was significantly increased in PM and was negatively correlated with the angiogenic factor angiopoietin-1 and positively correlated with angiopoietin-2, soluble endoglin, and vascular endothelial growth factor. Genetic or pharmacological blockade of C5a or its receptor in a mouse model of PM resulted in greater fetoplacental vessel development, reduced placental vascular resistance, and improved fetal growth and survival. These data suggest that C5a drives fetal growth restriction in PM through dysregulation of angiogenic factors essential for placental vascular remodeling resulting in placental vascular insufficiency.

Complement factor H related proteins (CFHRs)

Factor H related proteins comprise a group of five plasma proteins: CFHR1, CFHR2, CFHR3, CFHR4 and CFHR5, and each member of this group binds to the central complement component C3b. Mutations, genetic deletions, duplications or rearrangements in the individual CFHR genes are associated with a number of diseases including atypical hemolytic uremic syndrome (aHUS), C3 glomerulopathies (C3 glomerulonephritis (C3GN), dense deposit disease (DDD) and CFHR5 nephropathy), IgA nephropathy, age related macular degeneration (AMD) and systemic lupus erythematosus (SLE). Although complement regulatory functions were attributed to most of the members of the CFHR protein family, the precise role of each CFHR protein in complement activation and the exact contribution to disease pathology is still unclear. Recent publications show that CFHR proteins form homo- as well as heterodimers. Genetic abnormalities within the CFHR gene locus can result in hybrid proteins with affected dimerization or recognition domains which cause defective functions. Here we summarize the recent data about CFHR genes and proteins in order to better understand the role of CFHR proteins in complement activation and in complement associated diseases.

A pro-inflammatory role of C5L2 in C5a-primed neutrophils for ANCA-induced activation

Background

The complement system is crucial for the development of antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). In particular, C5a and its receptor on neutrophils, CD88, play a central role. The functional role of the second receptor of C5a, C5L2, remains unclear. In the current study, we investigated the role of C5L2 in C5a-primed neutrophils for ANCA-induced activation.

Methods

The effect of blocking C5L2 by anti-human C5L2 blocking antibody were tested on respiratory burst and degranulation of C5a-primed neutrophils activated with ANCA, as well as on membrane-bound proteinase 3 (mPR3) and concentration of myeloperoxidase (MPO) in supernatant of C5a-primed neutrophils. An antagonist for CD88 was also employed.

Results

Blocking C5L2 resulted in a significantly decreased MPO concentration in the supernatant of C5a-primed neutrophils. mPR3 expression increased from 209.0±43.0 in untreated cells to 444.3±60.8 after C5a treatment (P<0.001), and decreased to 375.8±65.44, 342.2±54.3 and 313.7±43.6 by pre-incubating blocking C5L2 antibody at 2.5 µg/ml, 5 µg/ml or 10 µg/ml (compared with C5a-priming group, P<0.001, P<0.001, and P<0.001), respectively. In C5a-primed neutrophils, subsequently activating with MPO-ANCA-positive IgG, the MFI value was 425.8±160.6, which decreased to 292.8±141.2, 289.7±130.0 and 280.3±136.4 upon pre-incubation with mouse anti-human C5L2 blocking antibody at 2.5 µg/ml, 5 µg/ml or 10 µg/ml (compared with C5a-primed neutrophils, for MPO-ANCA-positive IgG-induced activation, P<0.05, P<0.05, and P<0.05), respectively. Blocking C5L2 also resulted in significantly decreased C5a-primed neutrophils for PR3-ANCA-positive IgG-induced activation. Moreover, the lactoferrin concentration in the supernant significantly decreased in pre-incubation with anti-human C5L2 blocking antibody, compared with C5a-primed neutrophils induced by PR3- or MPO-ANCA-positive IgG.

Conclusions

C5L2 may be implicated in the pro-inflammatory role in C5a-primed neutrophils for ANCA-induced activation.