Monitoring and cell-specific deletion of C5aR1 using a novel floxed GFP-C5aR1 reporter knock-in mouse

Local complement activation and modulation in mucosal immunity

The complement system is an evolutionarily conserved arm of innate immunity, which forms one of the first lines of host response to pathogens and assists in the clearance of debris. A deficiency in key activators/amplifiers of the cascade results in recurrent infection, whereas a deficiency in regulating the cascade predisposes to accelerated organ failure, as observed in colitis and transplant rejection. Given that there are over 60 proteins in this system, it has become an attractive target for immunotherapeutics, many of which are United States Food and Drug Administration-approved or in multiple phase 2/3 clinical trials. Moreover, there have been key advances in the last few years in the understanding of how the complement system operates locally in tissues, independent of its activities in circulation. In this review, we will put into perspective the abovementioned discoveries to optimally modulate the spatiotemporal nature of complement activation and regulation at mucosal surfaces.

The C5aR1 complement receptor: A novel immunomodulator of insulin action in skeletal muscle

The complement system constitutes an integral component of the innate immune system and plays a critical role in adaptive immunity. Activation of this system engenders the production of complement peptide fragments, including C5a, which engage G-protein coupled receptors predominantly expressed in immune-associated cells, such as neutrophils, initiating pro-inflammatory responses. Intriguingly, our investigation has unveiled the presence of C5a receptor 1 (C5aR1) expression within skeletal muscle, a key metabolic tissue and primary target of insulin. Herein, we demonstrate that C5aR1 activation by C5a in differentiated human skeletal muscle cells elicits acute suppression of insulin signalling. This suppression manifests as impaired insulin-dependent association between IRS1 and the p85 subunit of PI3-kinase, a 50% reduction in Akt phosphorylation, and a 60% decline in insulin-stimulated glucose uptake. This impairment in insulin signalling is associated with a three-fold elevation in intramyocellular diacylglycerol (DAG) levels and a two-fold increase in cytosolic calcium content, which promote PKC-mediated IRS1 inhibition via enhanced phosphorylation at IRS1 Ser1101. Significantly, our findings demonstrate that structurally diverse C5aR1 antagonists, along with genetic deletion or stable silencing of C5aR1 by 80% using short-hairpin RNA, effectively attenuate repression of insulin signalling by C5a in LHCN-M2 human skeletal myotubes. These results underscore the potential of heightened C5aR1 activation, characteristic of obesity and chronic inflammatory conditions, to detrimentally impact insulin function within skeletal muscle cells. Additionally, the study suggests that agents targeting the C5a-C5aR axis, originally devised for mitigating complement-dependent inflammatory conditions, may offer therapeutic avenues to ameliorate immune-driven insulin resistance in key peripheral metabolic tissues, including skeletal muscle.

Platelets regulate ischemia-induced revascularization and angiogenesis by secretion of growth factor-modulating factors

In ischemic tissue, platelets can modulate angiogenesis. The specific factors influencing this function, however, are poorly understood. Here, we characterized the complement anaphylatoxin C5a-mediated activation of C5a receptor 1 (C5aR1) expressed on platelets as a potent regulator of ischemia-driven revascularization. We assessed the relevance of the anaphylatoxin receptor C5aR1 on platelets in patients with coronary artery disease as well as those with peripheral artery disease and used genetic mouse models to characterize its significance for ischemia and growth factor-driven revascularization. The presence of C5aR1-expressing platelets was increased in the hindlimb ischemia model. Ischemia-driven angiogenesis was significantly improved in C5aR1-/- mice but not in C5-/- mice, suggesting a specific role of C5aR1. Experiments using the supernatant of C5a-stimulated platelets suggested a paracrine mechanism of angiogenesis inhibition by platelets by means of antiangiogenic CXC chemokine ligand 4 (CXCL4, PF4). Lineage-specific C5aR1 deletion verified that the secretion of CXCL4 depends on C5aR1 ligation on platelets. Using C5aR1-/-CXCL4-/- mice, we observed no additional effect in the revascularization response, underscoring a strong dependence of CXCL4 secretion on the C5a-C5aR1-axis. We identified a novel mechanism for inhibition of neovascularization via platelet C5aR1, which was mediated by the release of antiangiogenic CXCL4.

Revisiting the relationship between complement and ulcerative colitis

Ulcerative colitis (UC) is an inflammatory bowel disorder (IBD) characterized by relapsing chronic inflammation of the colon that causes continuous mucosal inflammation. The global incidence of UC is steadily increasing. Immune mechanisms are involved in the pathogenesis of UC, of which complement is shown to play a critical role by inducing local chronic inflammatory responses that promote tissue damage. However, the function of various complement components in the development of UC is complex and even paradoxical. Some components (e.g. C1q, CD46, CD55, CD59, and C6) are shown to safeguard the intestinal barrier and reduce intestinal inflammation, while others (e.g. C3, C5, C5a) can exacerbate intestinal damage and accelerate the development of UC. The complement system was originally thought to function primarily in an extracellular mode; however, recent evidence indicates that it can also act intracellularly as the complosome. The current study provides an overview of current studies on complement and its role in the development of UC. While there are few studies that describe how intracellular complement contributes to UC, we discuss potential future directions based on related publications. We also highlight novel methods that target complement for IBD treatment.

Complement-mediated immune mechanisms in allergy

Allergic conditions are associated with canonical and noncanonical activation of the complement system leading to the release of several bioactive mediators with inflammatory and immunoregulatory properties that regulate the immune response in response to allergens during the sensitization and/or the effector phase of allergic diseases. Further, immune sensors of complement and regulator proteins of the cascade impact on the development of allergies. These bioactive mediators comprise the small and large cleavage fragments of C3 and C5. Here, we provide an update on the multiple roles of immune sensors, regulators, and bioactive mediators of complement in allergic airway diseases, food allergies, and anaphylaxis. A particular emphasis is on the anaphylatoxins C3a and C5a and their receptors, which are expressed on many of the effector cells in allergy such as mast cells, eosinophils, basophils, macrophages, and neutrophils. Also, we will discuss the multiple pathways, by which the anaphylatoxins initiate and control the development of maladaptive type 2 immunity including their impact on innate lymphoid cell recruitment and activation. Finally, we briefly comment on the potential to therapeutically target the complement system in different allergic conditions.

The role of C5a receptors in autoimmunity

The complement system is an essential component of the innate immune response and plays a vital role in host defense and inflammation. Dysregulation of the complement system, particularly involving the anaphylatoxin C5a and its receptors (C5aR1 and C5aR2), has been linked to several autoimmune diseases, indicating the potential for targeted therapies. C5aR1 and C5aR2 are seven-transmembrane receptors with distinct signaling mechanisms that play both partially overlapping and opposing roles in immunity. Both receptors are expressed on a broad spectrum of immune and non-immune cells and are involved in cellular functions and physiological processes during homeostasis and inflammation. Dysregulated C5a-mediated inflammation contributes to autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, epidermolysis bullosa acquisita, antiphospholipid syndrome, and others. Therefore, targeting C5a or its receptors may yield therapeutic innovations in these autoimmune diseases by reducing the recruitment and activation of immune cells that lead to tissue inflammation and injury, thereby exacerbating the autoimmune response. Clinical trials focused on the inhibition of C5 cleavage or the C5a/C5aR1-axis using small molecules or monoclonal antibodies hold promise for bringing novel treatments for autoimmune diseases into practice. However, given the heterogeneous nature of (systemic) autoimmune diseases, there are still several challenges, such as patient selection, optimal dosing, and treatment duration, that require further investigation and development to realize the full therapeutic potential of C5a receptor inhibition, ideally in the context of a personalized medicine approach. Here, we aim to provide a brief overview of the current knowledge on the function of C5a receptors, the involvement of C5a receptors in autoimmune disorders, the molecular mechanisms underlying C5a receptor-mediated autoimmunity, and the potential for targeted therapies to modulate their activity.

C5a-licensed phagocytes drive sterilizing immunity during systemic fungal infection

Systemic candidiasis is a common, high-mortality, nosocomial fungal infection. Unexpectedly, it has emerged as a complication of anti-complement C5-targeted monoclonal antibody treatment, indicating a critical niche for C5 in antifungal immunity. We identified transcription of complement system genes as the top biological pathway induced in candidemic patients and as predictive of candidemia. Mechanistically, C5a-C5aR1 promoted fungal clearance and host survival in a mouse model of systemic candidiasis by stimulating phagocyte effector function and ERK- and AKT-dependent survival in infected tissues. C5ar1 ablation rewired macrophage metabolism downstream of mTOR, promoting their apoptosis and enhancing mortality through kidney injury. Besides hepatocyte-derived C5, local C5 produced intrinsically by phagocytes provided a key substrate for antifungal protection. Lower serum C5a concentrations or a C5 polymorphism that decreases leukocyte C5 expression correlated independently with poor patient outcomes. Thus, local, phagocyte-derived C5 production licenses phagocyte antimicrobial function and confers innate protection during systemic fungal infection.

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

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

FBXO11 governs macrophage cell death and inflammation in response to bacterial toxins

Staphylococcus aureus causes severe infections such as pneumonia and sepsis depending on the pore-forming toxin Panton-Valentine leukocidin (PVL). PVL kills and induces inflammation in macrophages and other myeloid cells by interacting with the human cell surface receptor, complement 5a receptor 1 (C5aR1). C5aR1 expression is tighly regulated and may thus modulate PVL activity, although the mechanisms involved remain incompletely understood. Here, we used a genome-wide CRISPR/Cas9 screen and identified F-box protein 11 (FBXO11), an E3 ubiquitin ligase complex member, to promote PVL toxicity. Genetic deletion of FBXO11 reduced the expression of C5aR1 at the mRNA level, whereas ectopic expression of C5aR1 in FBXO11-/- macrophages, or priming with LPS, restored C5aR1 expression and thereby PVL toxicity. In addition to promoting PVL-mediated killing, FBXO11 dampens secretion of IL-1β after NLRP3 activation in response to bacterial toxins by reducing mRNA levels in a BCL-6-dependent and BCL-6-independent manner. Overall, these findings highlight that FBXO11 regulates C5aR1 and IL-1β expression and controls macrophage cell death and inflammation following PVL exposure.

The complement receptor C5aR2 regulates neutrophil activation and function contributing to neutrophil-driven epidermolysis bullosa acquisita

Introduction

The function of the second receptor for the complement cleavage product C5a, C5aR2, is poorly understood and often neglected in the immunological context. Using mice with a global deficiency of C5aR2, we have previously reported an important role of this receptor in the pathogenesis of the neutrophil-driven autoimmune disease epidermolysis bullosa acquisita (EBA). Based on in vitro analyses, we hypothesized that the absence of C5aR2 specifically on neutrophils is the cause of the observed differences. Here, we report the generation of a new mouse line with a LysM-specific deficiency of C5aR2.

Methods

LysM-specific deletion of C5aR2 was achieved by crossing LysM^cre mice with tdTomato-C5ar2^fl/fl mice in which the tdTomato-C5ar2 gene is flanked by loxP sites. Passive EBA was induced by subcutaneous injection of rabbit anti-mouse collagen type VII IgG. The effects of targeted deletion of C5ar2 on C5a-induced effector functions of neutrophils were examined in in vitro assays.

Results

We confirm the successful deletion of C5aR2 at both the genetic and protein levels in neutrophils. The mice appeared healthy and the expression of C5aR1 in bone marrow and blood neutrophils was not negatively affected by LysM-specific deletion of C5aR2. Using the antibody transfer mouse model of EBA, we found that the absence of C5aR2 in LysM-positive cells resulted in an overall amelioration of disease progression, similar to what we had previously found in mice with global deficiency of C5aR2. Neutrophils lacking C5aR2 showed decreased activation after C5a stimulation and increased expression of the inhibitory Fcγ receptor FcγRIIb.

Discussion

Overall, with the data presented here, we confirm and extend our previous findings and show that C5aR2 in neutrophils regulates their activation and function in response to C5a by potentially affecting the expression of Fcγ receptors and CD11b. Thus, C5aR2 regulates the finely tuned interaction network between immune complexes, Fcγ receptors, CD11b, and C5aR1 that is important for neutrophil recruitment and sustained activation. This underscores the importance of C5aR2 in the pathogenesis of neutrophil-mediated autoimmune diseases.

C5aR1 activation in mice controls inflammatory eosinophil recruitment and functions in allergic asthma

BACKGROUND

Pulmonary eosinophils comprise at least two distinct populations of resident eosinophils (rEOS) and inflammatory eosinophils (iEOS), the latter recruited in response to pulmonary inflammation. Here, we determined the impact of complement activation on rEOS and iEOS trafficking and function in two models of pulmonary inflammation.

METHODS

BALB/c wild-type and C5ar1^-/- mice were exposed to different allergens or IL-33. Eosinophil populations in the airways, lung, or mediastinal lymph nodes (mLN) were characterized by FACS or immunohistochemistry. rEOS and iEOS functions were determined in vivo and in vitro.

RESULTS

HDM and IL-33 exposure induced a strong accumulation of iEOS but not rEOS in the airways, lungs, and mLNs. rEOS and iEOS expressed C3/C5 and C5aR1, which were significantly higher in iEOS. Initial pulmonary trafficking of iEOS was markedly reduced in C5ar1^-/- mice and associated with less IL-5 production from ILC2 cells. Functionally, adoptively transferred pulmonary iEOS from WT but not from C5ar1^-/- mice-induced airway hyperresponsiveness (AHR), which was associated with significantly reduced C5ar1^-/- iEOS degranulation. Pulmonary iEOS but not rEOS were frequently associated with T cells in lung tissue. After HDM or IL-33 exposure, iEOS but not rEOS were found in mLNs, which were significantly reduced in C5ar1^-/- mice. C5ar1^-/- iEOS expressed less costimulatory molecules, associated with a decreased potency to drive antigen-specific T cell proliferation and differentiation into memory T cells.

CONCLUSIONS

We uncovered novel roles for C5aR1 in iEOS trafficking and activation, which affects key aspects of allergic inflammation such as AHR, ILC2, and T cell activation.

Case report: Mafb promoter activity may define the alveolar macrophage dichotomy

Cre-LoxP system has been widely used to induce recombination of floxed genes of interest. Currently available macrophage promoter-specific Cre recombinase mice strains have various limitations that warrants the testing of additional Cre strains. V-maf musculoaponeurotic fibrosarcoma oncogene family, protein b -Cre (Mafb-Cre) mice label macrophages in most organs such as spleen, small intestine, lung, bone marrow, and peritoneal cavity. However, whether Mafb-Cre recombinase targets the gene recombination in alveolar macrophage remains untested. Here, we utilized Mafb^Cre/WTR26^mTmG/WT strain that expresses mTOM protein in all the cells of mouse body except for those that express Mafb-Cre-regulated mEGFP. We performed fluorescent microscopy and flow cytometry to analyze mTOM and mEGFP expression in alveolar macrophages from Mafb^Cre/WTR26^mTmG/WT mice. Our analyses revealed that the Mafb-Cre is active in only ~40% of the alveolar macrophages in an age-independent manner. While Mafb- (mTOM+/mEGFP-) and Mafb+ (mEGFP+) alveolar macrophages exhibit comparable expression of CD11b and CD11c surface markers, the surface expression of MHCII is elevated in the Mafb+ (mEGFP+) macrophages. The bone marrow-derived macrophages from Mafb^Cre/WTR26^mTmG/WT mice are highly amenable to Cre-LoxP recombination in vitro. The bone marrow depletion and reconstitution experiment revealed that ~98% of alveolar macrophages from Mafb^Cre/WTR26^mTmG/WT → WT chimera are amenable to the Mafb-Cre-mediated recombination. Finally, the Th2 stimulation and ozone exposure to the Mafb^Cre/WTR26^mTmG/WT mice promote the Mafb-Cre-mediated recombination in alveolar macrophages. In conclusion, while the Mafb-/Mafb+ dichotomy thwarts the use of Mafb-Cre for the induction of floxed alleles in the entire alveolar macrophage population, this strain provides a unique tool to induce gene deletion in alveolar macrophages that encounter Th2 microenvironment in the lung airspaces.

Folic acid-mediated fibrosis is driven by C5a receptor 1-mediated activation of kidney myeloid cells

We previously reported that increased expression and activation of kidney cell complement components play an important role in the pathogenesis of renal scarring. Here we used floxed green fluorescent protein (GFP)-C5a receptor 1 (C5aR1) knock-in mice (GFP-C5ar1^fl/fl) and the model of Folic acid-induced kidney injury to define the cell types and potential mechanisms by which increased C5aR1 activation leads to fibrosis. Using flow cytometry and confocal microscopy we identified macrophages as the major interstitial cell type showing increased expression of C5aR1 in FA-treated mice. C5ar1^fl/fl.Lyz2Cre^+/- mice, in which C5aR1 has been specifically deleted in lysozyme M (LysM)-expressing myeloid cells, experienced reduced fibrosis when compared to control C5ar1^fl/fl mice. Examination of C5aR1-expressing macrophage transcriptomes by gene set enrichment analysis (GSEA) demonstrated that these cells were enriched in pathways corresponding to the complement cascade, collagen formation and the NABA matrisome, strongly pointing to their critical roles in tissue repair/scarring. Since C5aR1 was also detected in a small population of PDGFBR+ GFP+ cells we develop C5ar1^fl/fl.Foxd1Cre^+/- mice, in which C5aR1 is deleted specifically in pericytes, and found reduced FA-induced fibrosis. Primary cell cultures of platelet-derived growth factor receptor beta (PDGFRβ)+ pericytes isolated from FA-treated C5ar1^fl/fl.Foxd1Cre^+/- mice showed reduced secretion of several cytokines, including IL-6, and macrophage inflammatory proteins (MIP) 2, when compared to pericytes isolated from FA-treated control GFP-C5ar1^fl/fl mice. Collectively, these data imply that the C5a/C5aR1 axis activation primarily in interstitial cells contributes to the development of renal fibrosis.

Complement receptor C5aR1 on osteoblasts regulates osteoclastogenesis in experimental postmenopausal osteoporosis

In recent years, evidence has accumulated that the complement system, an integral part of innate immunity, may be involved in the regulation of bone homeostasis as well as inflammatory bone loss, for example, in rheumatoid arthritis and periodontitis. Complement may also contribute to osteoporosis development, but investigation of the mechanism is limited. Using mice with a conditional deletion of the complement anaphylatoxin receptor C5aR1, we here demonstrated that C5aR1 in osteoblasts (C5aR1 ^Runx2-Cre mice) or osteoclasts (C5aR1 ^LysM-Cre mice) did not affect physiological bone turnover or age-related bone loss in either sex, as confirmed by micro-computed tomography, histomorphometry, and biomechanical analyses of the bone and by the measurement of bone turnover markers in the blood serum. When female mice were subjected to ovariectomy (OVX), a common model for postmenopausal osteoporosis, significant bone loss was induced in C5aR1 ^fl/fl and C5aR1 ^LysM-Cre mice, as demonstrated by a significantly reduced bone volume fraction, trabecular number and thickness as well as an increased trabecular separation in the trabecular bone compartment. Confirming this, the osteoclast number and the receptor activator of nuclear factor k-B (RANK) ligand (RANKL) serum level were significantly elevated in these mouse lines. By contrast, C5aR1 ^Runx2-Cre mice were protected from bone loss after OVX and the serum RANKL concentration was not increased after OVX. These data suggested that bone cell-specific C5aR1 may be redundant in bone homeostasis regulation under physiological conditions. However, C5aR1 on osteoblasts was crucial for the induction of bone resorption under osteoporotic conditions by stimulating RANKL release, whereas C5aR1 on osteoclasts did not regulate OVX-induced bone loss. Therefore, our results implicate C5aR1 on osteoblasts as a potential target for treating postmenopausal osteoporosis.

The complement cascade in the regulation of neuroinflammation, nociceptive sensitization, and pain

The complement cascade is a key component of the innate immune system that is rapidly recruited through a cascade of enzymatic reactions to enable the recognition and clearance of pathogens and promote tissue repair. Despite its well-understood role in immunology, recent studies have highlighted new and unexpected roles of the complement cascade in neuroimmune interaction and in the regulation of neuronal processes during development, aging, and in disease states. Complement signaling is particularly important in directing neuronal responses to tissue injury, neurotrauma, and nerve lesions. Under physiological conditions, complement-dependent changes in neuronal excitability, synaptic strength, and neurite remodeling promote nerve regeneration, tissue repair, and healing. However, in a variety of pathologies, dysregulation of the complement cascade leads to chronic inflammation, persistent pain, and neural dysfunction. This review describes recent advances in our understanding of the multifaceted cross-communication that takes place between the complement system and neurons. In particular, we focus on the molecular and cellular mechanisms through which complement signaling regulates neuronal excitability and synaptic plasticity in the nociceptive pathways involved in pain processing in both health and disease. Finally, we discuss the future of this rapidly growing field and what we believe to be the significant knowledge gaps that need to be addressed.

The C5a/C5a receptor 1 axis controls tissue neovascularization through CXCL4 release from platelets

Platelets contribute to the regulation of tissue neovascularization, although the specific factors underlying this function are unknown. Here, we identified the complement anaphylatoxin C5a-mediated activation of C5a receptor 1 (C5aR1) on platelets as a negative regulatory mechanism of vessel formation. We showed that platelets expressing C5aR1 exert an inhibitory effect on endothelial cell functions such as migration and 2D and 3D tube formation. Growth factor- and hypoxia-driven vascularization was markedly increased in C5ar1^-/- mice. Platelet-specific deletion of C5aR1 resulted in a proangiogenic phenotype with increased collateralization, capillarization and improved pericyte coverage. Mechanistically, we found that C5a induced preferential release of CXC chemokine ligand 4 (CXCL4, PF4) from platelets as an important antiangiogenic paracrine effector molecule. Interfering with the C5aR1-CXCL4 axis reversed the antiangiogenic effect of platelets both in vitro and in vivo.In conclusion, we identified a mechanism for the control of tissue neovascularization through C5a/C5aR1 axis activation in platelets and subsequent induction of the antiangiogenic factor CXCL4.

Complement C5a promotes antigen cross-presentation by Peyer's patch monocyte-derived dendritic cells and drives a protective CD8+ T cell response

The complement fragment C5a is closely associated with adaptive immune induction in the mucosa. However, the mechanisms that control CD8⁺ T cell responses by C5a have not been extensively explored. This study reveals that C5/C5a in the Peyer's patch (PP) subepithelial dome increases upon oral Listeria infection. We hypothesize that C5aR⁺ PP cells play an important role in the induction of antigen-specific T cell immunity. Using single-cell RNA sequencing, we identify C5aR- and lysozyme-expressing dendritic cells (C5aR⁺ LysoDCs) in PP and examine their role in CD8⁺ T cell immune induction. Stimulation of C5aR⁺ LysoDCs by C5a increases reactive oxygen species levels, leading to efficient antigen cross-presentation, which elicits an antigen-specific CD8⁺ T cell response. In C5-deficient mice, oral co-administration of C5a and Listeria enhances Listeria-specific cytotoxic T cell levels. Collectively, these findings suggest a role of the complement system in intestinal T cell immunity.

Immunohistochemical identification of complement peptide C5a receptor 1 (C5aR1) in non-neoplastic and neoplastic human tissues

The complement component C5a and its receptor C5aR1 are involved in the development of numerous inflammatory diseases. In addition to immune cells, C5aR1 is expressed in neoplastic cells of multiple tumour entities, where C5aR1 is associated with a higher proliferation rate, advanced tumour stage, and poor patient outcomes. The aim of the present study was to obtain a broad expression profile of C5aR1 in human non-neoplastic and neoplastic tissues, especially in tumour entities not investigated in this respect so far. For this purpose, we generated a novel polyclonal rabbit antibody, {5227}, against the carboxy-terminal tail of C5aR1. The antibody was initially characterised in Western blot analyses and immunocytochemistry using transfected human embryonic kidney (HEK) 293 cells. It was then applied to a large series of formalin-fixed, paraffin-embedded non-neoplastic and neoplastic human tissue samples. C5aR1 was strongly expressed by different types of immune cells in the majority of tissue samples investigated. C5aR1 was also present in alveolar macrophages, bronchial, gut, and bile duct epithelia, Kupffer cells, occasionally in hepatocytes, proximal renal tubule cells, placental syncytiotrophoblasts, and distinct stem cell populations of bone marrow. C5aR1 was also highly expressed in the vast majority of the 32 tumour entities investigated, where a hitherto unappreciated high prevalence of the receptor was detected in thyroid carcinomas, small-cell lung cancer, gastrointestinal stromal tumours, and endometrial carcinomas. In addition to confirming published findings, we found noticeable C5aR1 expression in many tumour entities for the first time. Here, it may serve as an interesting target for future therapies.

Nanoparticle-Induced Complement Activation: Implications for Cancer Nanomedicine

Nanoparticle-based anticancer medications were first approved for cancer treatment almost 2 decades ago. Patients benefit from these approaches because of the targeted-drug delivery and reduced toxicity, however, like other therapies, adverse reactions often limit their use. These reactions are linked to the interactions of nanoparticles with the immune system, including the activation of complement. This activation can cause well-characterized acute inflammatory reactions mediated by complement effectors. However, the long-term implications of chronic complement activation on the efficacy of drugs carried by nanoparticles remain obscured. The recent discovery of protumor roles of complement raises the possibility that nanoparticle-induced complement activation may actually reduce antitumor efficacy of drugs carried by nanoparticles. We discuss here the initial evidence supporting this notion. Better understanding of the complex interactions between nanoparticles, complement, and the tumor microenvironment appears to be critical for development of nanoparticle-based anticancer therapies that are safer and more efficacious.

Characterization of Anaphylatoxin Receptor Expression and C3a/C5a Functions in Anaphylatoxin Receptor Reporter Mice

The anaphylatoxins (AT) C3a and C5a are effector molecules of C3 and C5 exerting multiple biologic functions through binding and activation of their cognate G protein-coupled receptors. C3a interacts with the C3a receptor (C3aR), whereas C5a and its primary degradation product C5a-desArg engage C5aR1 and C5aR2. In the past, analysis of AT expression has been hampered by cross reaction of antibodies designed to recognize the different AT receptors. Furthermore, assessment of effects mediated by cell-specific activation has been difficult. Here, floxed AT receptor reporter mice are described as tools to monitor AT receptor expression in cells and tissues and to study the functions of C3a and C5a by cell-specific deletion of their cognate AT receptors. © 2020 The Authors. Basic Protocol 1: Genotyping of floxed GFP-C5aR1 knockin mice Support Protocol 1: Genotyping of LysMcre-C5ar1^-/- mice Basic Protocol 2: Genotyping of floxed tdTomato-C3aR and -tdTomato-C5aR2 knockin mice Support Protocol 2: Preparation of genomic DNA Basic Protocol 3: Determination of C5aR1, C5aR2, and C3aR expression using floxed AT receptor reporter mice Support Protocol 3: Determination of C3aR expression using a C3aR-specific antibody Support Protocol 4: Determination of C5aR1, C5aR2, and C3aR mRNA expression in floxed GFP-C5aR1, floxed tdTomato-C5aR2 or -tdTomato C3aR positive cells Basic Protocol 4: Analysis of C5aR1-driven ERK1/2 phosphorylation in GFP-C5aR1⁺ cells Basic Protocol 5: Assessment of C3aR functions in cells obtained from floxed tdTomato-C3aR knockin mice- Determination of C3aR internalization Alternate Protocol: C3a-induced increase in intracellular Ca²⁺ Basic Protocol 6: C5aR2-driven IFN-γ production from NK cells Support Protocol 5: Isolation of splenic NK cells by FACS.

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

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

Targeting NLRP3 and Staphylococcal pore-forming toxin receptors in human-induced pluripotent stem cell-derived macrophages

Staphylococcus aureus causes necrotizing pneumonia by secreting toxins such as leukocidins that target front-line immune cells. The mechanism by which leukocidins kill innate immune cells and trigger inflammation during S. aureus lung infection, however, remains unresolved. Here, we explored human-induced pluripotent stem cell-derived macrophages (hiPSC-dMs) to study the interaction of the leukocidins Panton-Valentine leukocidin (PVL) and LukAB with lung macrophages, which are the initial leukocidin targets during S. aureus lung invasion. hiPSC-dMs were susceptible to the leukocidins PVL and LukAB and both leukocidins triggered NLPR3 inflammasome activation resulting in IL-1β secretion. hiPSC-dM cell death after LukAB exposure, however, was only temporarily dependent of NLRP3, although NLRP3 triggered marked cell death after PVL treatment. CRISPR/Cas9-mediated deletion of the PVL receptor, C5aR1, protected hiPSC-dMs from PVL cytotoxicity, despite the expression of other leukocidin receptors, such as CD45. PVL-deficient S. aureus had reduced ability to induce lung IL-1β levels in human C5aR1 knock-in mice. Unexpectedly, inhibiting NLRP3 activity resulted in increased wild-type S. aureus lung burdens. Our findings suggest that NLRP3 induces macrophage death and IL-1β secretion after PVL exposure and controls S. aureus lung burdens.

New insights into the immune functions of complement

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

Allergen-Induced C5a/C5aR1 Axis Activation in Pulmonary CD11b+ cDCs Promotes Pulmonary Tolerance through Downregulation of CD40

Activation of the C5/C5a/C5a receptor 1 (C5aR1) axis during allergen sensitization protects from maladaptive T cell activation. To explore the underlying regulatory mechanisms, we analyzed the impact of C5aR1 activation on pulmonary CD11b⁺ conventional dendritic cells (cDCs) in the context of house-dust-mite (HDM) exposure. BALB/c mice were intratracheally immunized with an HDM/ovalbumin (OVA) mixture. After 24 h, we detected two CD11b⁺ cDC populations that could be distinguished on the basis of C5aR1 expression. C5aR1⁻ but not C5aR1⁺ cDCs strongly induced T cell proliferation of OVA-reactive transgenic CD4⁺ T cells after re-exposure to antigen in vitro. C5aR1⁻ cDCs expressed higher levels of MHC-II and CD40 than their C5aR1⁺ counterparts, which correlated directly with a higher frequency of interactions with cognate CD4⁺ T cells. Priming of OVA-specific T cells by C5aR1⁺ cDCs could be markedly increased by in vitro blockade of C5aR1 and this was associated with increased CD40 expression. Simultaneous blockade of C5aR1 and CD40L on C5aR1⁺ cDCs decreased T cell proliferation. Finally, pulsing with OVA-induced C5 production and its cleavage into C5a by both populations of CD11b⁺ cDCs. Thus, we propose a model in which allergen-induced autocrine C5a generation and subsequent C5aR1 activation in pulmonary CD11b⁺ cDCs promotes tolerance towards aeroallergens through downregulation of CD40.

Complement-Mediated Events in Alzheimer's Disease: Mechanisms and Potential Therapeutic Targets

An estimated 5.7 million Americans suffer from Alzheimer's disease in the United States, with no disease-modifying treatments to prevent or treat cognitive deficits associated with the disease. Genome-wide association studies suggest that an enhancement of clearance mechanisms and/or promotion of an anti-inflammatory response may slow or prevent disease progression. Increasing awareness of distinct roles of complement components in normal brain development and function and in neurodegenerative disorders align with complement-mediated responses, and thus, thorough understanding of these molecular pathways is needed to facilitate successful therapeutic design. Both beneficial and detrimental effects of C1q as well as contributions to local inflammation by C5a-C5aR1 signaling in brain highlight the need for precision of therapeutic design. The potential benefit of β-amyloid clearance from the circulation via CR1-mediated mechanisms is also reviewed. Therapies that suppress inflammation while preserving protective effects of complement could be tested now to slow the progression of this debilitating disease.

The Role of Complement in Organ Transplantation

The current immunosuppressive protocols used in transplant recipients have improved short-term outcomes, but long-term allograft failure remains an important clinical problem. Greater understanding of the immunologic mechanisms that cause allograft failure are needed, as well as new treatment strategies for protecting transplanted organs. The complement cascade is an important part of the innate immune system. Studies have shown that complement activation contributes to allograft injury in several clinical settings, including ischemia/reperfusion injury and antibody mediated rejection. Furthermore, the complement system plays critical roles in modulating the responses of T cells and B cells to antigens. Therapeutic complement inhibitors, therefore, may be effective for protecting transplanted organs from several causes of inflammatory injury. Although several anti-complement drugs have shown promise in selected patients, the role of these drugs in transplantation medicine requires further study.

C5a receptors C5aR1 and C5aR2 mediate opposing pathologies in a mouse model of melanoma

The canonical complement component 5a (C5a) receptor (C5aR) 1 has well-described roles in tumorigenesis but the contribution of the second receptor, C5aR2, is unclear. The present study demonstrates that B16.F0 melanoma cells express mRNA for both C5aR1 and C5aR2 and signal through ERK and p38 MAPKs in response to C5a. Despite this, C5a had no impact on melanoma cell proliferation or migration in vitro. In vivo studies demonstrated that the growth of B16.F0 melanoma tumors was increased in C5aR2^-/- mice but reduced in C5aR1^-/- mice and wild-type mice treated with a C5aR1 antagonist. Analysis of tumor-infiltrating leukocyte populations showed no significant differences between wild-type and C5aR2^-/- mice. Conversely, percentages of myeloid-derived suppressor cells, macrophages, and regulatory T lymphocytes were lower in tumors from C5aR1^-/- mice, whereas total (CD3⁺) T lymphocytes and CD4⁺ subsets were higher. Analysis of cytokine and chemokine levels also showed plasma IFN-γ was higher and tumor C-C motif chemokine ligand 2 was lower in the absence of C5aR1. The results suggest that C5aR1 signaling supports melanoma growth by promoting infiltration of immunosuppressive leukocyte populations into the tumor microenvironment, whereas C5aR2 has a more restricted but beneficial role in limiting tumor growth. Overall, these data support the potential of C5aR1-inhibitory therapies for melanoma.-Nabizadeh, J. A., Manthey, H. D., Panagides, N., Steyn, F. J., Lee, J. D., Li, X. X., Akhir, F. N. M., Chen, W., Boyle, G. M., Taylor, S. M., Woodruff, T. M., Rolfe, B. E. C5a receptors C5aR1 and C5aR2 mediate opposing pathologies in a mouse model of melanoma.

Differential effects of anaphylatoxin C5a on antigen presenting cells, roles for C5aR1 and C5aR2

The anaphylatoxin C5a is well-known for its role as chemoattractant and contributes to immune cell recruitment into inflamed tissue and local inflammation. C5a has recently been implicated in modulation of antigen presenting cell function, such as macrophages and dendritic cells, which are pivotal for T cell activation and final T cell effector function. The published data on the effect of C5a on APC function and subsequent adaptive immune responses are in part conflicting, as both pro and anti-inflammatory effects have been described. In this review the opposing effects of C5a on APC function in mice and human are summarized and discussed in relation to origin of the involved APC subset, being either of the monocyte-derived lineage or dendritic cell lineage. In addition, the current knowledge on the expression of C5aR1 and C5aR2 on the different APC subsets is summarized. Based on the combined data, we propose that the differential effects of C5a on APC function may be attributed to absence or presence of co-expression of C5aR2 and C5aR1 on the specific APC.

C5a receptor 1-/- mice are protected from the development of IgE-mediated experimental food allergy

BACKGROUND

Food-induced anaphylaxis is a serious allergic reaction caused by Fcε-receptor activation on mast cells (MCs). The exact mechanisms breaking oral tolerance and the effector pathways driving food allergy remain elusive. As complement is activated in food-induced anaphylaxis, we aimed to assess the role of C5a in disease pathogenesis.

METHODS

Oral antigen-induced food-induced anaphylaxis was induced in BALB/c wild-type (wt) and C5ar1-/- mice. Readouts included diarrhea development, changes in rectal temperature, hematocrit, antigen-specific serum IgE, MCPT-1, and intestinal MC numbers, as well as FcεR1-mediated MC functions including C5a receptor 1 (C5aR1) regulation. Further, histamine-mediated hypothermia and regulation of endothelial tight junctions were determined.

RESULTS

Repeated oral OVA challenge resulted in diarrhea, hypothermia, increased hematocrit, high OVA-specific serum IgE, and MCPT-1 levels in wt mice. Male C5ar1-/- mice were completely whereas female C5ar1-/- mice were partially protected from anaphylaxis development. Serum MCPT-1 levels were reduced gender-independent, whereas IgE levels were reduced in male but not in female C5ar1-/- mice. Mechanistically, IgE-mediated degranulation and IL-6 production from C5ar1-/- BMMCs of both sexes were significantly reduced. Importantly, FcεR1 cross-linking strongly upregulated C5aR1 MC expression in vitro and in vivo. Finally, C5ar1-/- male mice were largely protected from histamine-induced hypovolemic shock, which was associated with protection from histamine-induced barrier dysfunction in vitro following C5aR targeting.

CONCLUSIONS

Our findings identify C5aR1 activation as an important driver of IgE-mediated food allergy through regulation of allergen-specific IgE production, FcεR1-mediated MC degranulation, and histamine-driven effector functions preferentially in male mice.

Intracellular Activation of Complement C3 Leads to PD-L1 Antibody Treatment Resistance by Modulating Tumor-Associated Macrophages

Complement aids in the construction of an immunosuppressive tumor microenvironment. Tumor cell-derived C3 has been previously reported, but whether and how it acts on antitumor immunity remains to be elucidated. Here, we describe a mechanism for tumor cell-derived C3 in suppressing antitumor immunity. Tumor cell-derived C3 was activated intracellularly, which results in generation of C3a. C3a modulated tumor-associated macrophages via C3a-C3aR-PI3Kγ signaling, thereby repressing antitumor immunity. Deletion of C3 in tumor cells that had high C3 expression enhanced efficacy of anti-PD-L1 treatment. Collectively, our results suggest tumor cell-derived C3 may be a useful target for cancer immunotherapy and that targeting C3 in tumor cells may enhance antitumor immunity.

Distribution and Interaction of Murine Pulmonary Phagocytes in the Naive and Allergic Lung

The division of labor between pulmonary phagocytic subsets [macrophage/monocyte and dendritic cell (DC) subpopulations] has been described at the functional level. However, whether these lung phagocytes also display unique spatial distribution remains unclear. Here, to analyze cellular distribution in lung compartments and contacts between phagocyte subpopulations, we established an immunohistochemistry (IHC)-based method to clearly identify murine lung phagocyte subsets in situ based on differential expression of CD11c, CD11b, MHC-II, Langerin and mPDCA-1. Furthermore, we investigated subset-specific functional differences in antigen uptake and spatial changes upon allergic sensitization. Our staining allowed the distinction between alveolar macrophages (AMs), interstitial macrophage (IM) subpopulations, CD11b⁺ DC subpopulations, CD103⁺ DCs, and plasmacytoid DCs (pDCs). We identified interstitial regions between airways and around airways as regions of IM/CD11b⁺ DC/CD103⁺ DC clusters, where a subset of IMs (IM2) and CD103⁺ DCs formed intense contacts that decreased upon allergic sensitization. These data indicate functional interactions between both cell types either in steady state or after antigen encounter affecting the development of allergies or tolerance. Furthermore, we observed major antigen uptake in AMs and IMs rather than DC subpopulations that was not restricted to airways and adjacent areas. This will enable to focus future studies to immunologically relevant cellular interactions and to unravel which cells are tipping the balance between pro-inflammatory immune responses or tolerance.

Sparking Fire Under the Skin? Answers From the Association of Complement Genes With Pemphigus Foliaceus

Skin blisters of pemphigus foliaceus (PF) present concomitant deposition of autoantibodies and components of the complement system (CS), whose gene polymorphisms are associated with susceptibility to different autoimmune diseases. To investigate these in PF, we evaluated 992 single-nucleotide polymorphisms (SNPs) of 44 CS genes, genotyped through microarray hybridization in 229 PF patients and 194 controls. After excluding SNPs with minor allele frequency <1%, out of Hardy-Weinberg equilibrium in controls or in strong linkage disequilibrium (r2 ≥ 0.8), 201 SNPs remained for logistic regression. Polymorphisms of 11 genes were associated with PF. MASP1 encodes a crucial serine protease of the lectin pathway (rs13094773: OR = 0.5, p = 0.0316; rs850309: OR = 0.23, p = 0.03; rs3864098: OR = 1.53, p = 0.0383; rs698104: OR = 1.52, p = 0.0424; rs72549154: OR = 0.55, p = 0.0453). C9 (rs187875: OR = 1.46, p = 0.0189; rs700218: OR = 0.12, p = 0.0471) and C8A (rs11206934: OR = 4.02, p = 0.0323) encode proteins of the membrane attack complex (MAC) and C5AR1 (rs10404456: OR = 1.43, p = 0.0155), a potent anaphylatoxin-receptor. Two encode complement regulators: MAC-blocking CD59 (rs1047581: OR = 0.62, p = 0.0152) and alternative pathway-blocking CFH (rs34388368: OR = 2.57, p = 0.0195). One encodes opsonin: C3 (rs4807895: OR = 2.52, p = 0.0239), whereas four encode receptors for C3 fragments: CR1 (haplotype with rs6656401: OR = 1.37, p = 0.0382), CR2 (rs2182911: OR = 0.23, p = 0.0263), ITGAM (CR3, rs12928810: OR = 0.66, p = 0.0435), and ITGAX (CR4, rs11574637: OR = 0.63, p = 0.0056). Associations reinforced former findings, regarding differential gene expression, serum levels, C3, and MAC deposition on lesions. Deregulation of previously barely noticed processes, e.g., the lectin and alternative pathways and opsonization-mediated phagocytosis, also modulate PF susceptibility. The results open new crucial avenues for understanding disease etiology and may improve PF treatment through additional therapeutic targets.

Lymphocyte integration of complement cues

We address current data, views and puzzles on the emerging topic of regulation of lymphocytes by complement proteins or fragments. Such regulation is believed to take place through complement receptors (CR) and membrane complement regulators (CReg) involved in cell function or protection, respectively, including intracellular signalling. Original observations in B cells clearly support that complement cues through CR improve their performance. Other lymphocytes likely integrate complement-derived signals, as most lymphoid cells constitutively express or regulate CR and CReg upon activation. CR-induced signals, particularly by anaphylatoxins, clearly regulate lymphoid cell function. In contrast, data obtained by CReg crosslinking using antibodies are not always confirmed in human congenital deficiencies or knock-out mice, casting doubts on their physiological relevance. Unsurprisingly, human and mouse complement systems are not completely homologous, adding further complexity to our still fragmentary understanding of complement-lymphocyte interactions.

The intestinal complement system in inflammatory bowel disease: Shaping intestinal barrier function

The complement system is part of innate sensor and effector systems such as the Toll-like receptors (TLRs). It recognizes and quickly systemically and/or locally respond to microbial-associated molecular patterns (MAMPs) with a tailored defense reaction. MAMP recognition by intestinal epithelial cells (IECs) and appropriate immune responses are of major importance for the maintenance of intestinal barrier function. Enterocytes highly express various complement components that are suggested to be pivotal for proper IEC function. Appropriate activation of the intestinal complement system seems to play an important role in the resolution of chronic intestinal inflammation, while over-activation and/or dysregulation may worsen intestinal inflammation. Mice deficient for single complement components suffer from enhanced intestinal inflammation mimicking the phenotype of patients with chronic inflammatory bowel disease (IBD) such as Crohn's disease (CD) or ulcerative colitis (UC). However, the mechanisms leading to complement expression in IECs seem to differ markedly between UC and CD patients. Hence, how IECs, intestinal bacteria and epithelial cell expressed complement components interact in the course of IBD still remains to be mostly elucidated to define potential unique patterns contributing to the distinct subtypes of intestinal inflammation observed in CD and UC.

An unexpected player in Gaucher disease: The multiple roles of complement in disease development

The complement system is well appreciated for its role as an important effector of innate immunity that is activated by the classical, lectin or alternative pathway. C5a is one important mediator of the system that is generated in response to canonical and non-canonical C5 cleavage by circulating or cell-derived proteases. In addition to its function as a chemoattractant for neutrophils and other myeloid effectors, C5a and its sister molecule C3a have concerted roles in cell homeostasis and surveillance. Through activation of their cognate G protein coupled receptors, C3a and C5a regulate multiple intracellular pathways within the mitochondria and the lysosomal compartments that harbor multiple enzymes critical for protein, carbohydrate and lipid metabolism. Genetic mutations of such lysosomal enzymes or their receptors can result in the compartmental accumulation of specific classes of substrates in this organelle summarized as lysosomal storage diseases (LSD). A frequent LSD is Gaucher disease (GD), caused by autosomal recessively inherited mutations in GBA1, resulting in functional defects of the encoded enzyme, acid β-glucosidase (glucocerebrosidase, GCase). Such mutations promote excessive accumulation of β-glucosylceramide (GC or GL1) in innate and adaptive immune cells frequently associated with chronic inflammation. Recently, we uncovered an unexpected link between the C5a and C5a receptor 1 (C5aR1) axis and the accumulation of GL1 in experimental and clinical GD. Here, we will review the pathways of complement activation in GD, its role as a mediator of the inflammatory response, and its impact on glucosphingolipid metabolism. Further, we will discuss the potential role of the C5a/C5aR1 axis in GL1-specific autoantibody formation and as a novel therapeutic target in GD.

A Novel Role for C5a in B-1 Cell Homeostasis

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

Complement Receptor C5aR1 Plays an Evolutionarily Conserved Role in Successful Cardiac Regeneration

BACKGROUND

Defining conserved molecular pathways in animal models of successful cardiac regeneration could yield insight into why adult mammals have inadequate cardiac regeneration after injury. Insight into the transcriptomic landscape of early cardiac regeneration from model organisms will shed light on evolutionarily conserved pathways in successful cardiac regeneration.

METHODS

Here we describe a cross-species transcriptomic screen in 3 model organisms for cardiac regeneration: axolotl, neonatal mice, and zebrafish. Apical resection to remove ≈10% to 20% of ventricular mass was carried out in these model organisms. RNA-sequencing analysis was performed on the hearts harvested at 3 time points: 12, 24, and 48 hours after resection. Sham surgery was used as internal control.

RESULTS

Genes associated with inflammatory processes were found to be upregulated in a conserved manner. Complement receptors (activated by complement components, part of the innate immune system) were found to be highly upregulated in all 3 species. This approach revealed induction of gene expression for complement 5a receptor 1 in the regenerating hearts of zebrafish, axolotls, and mice. Inhibition of complement 5a receptor 1 significantly attenuated the cardiomyocyte proliferative response to heart injury in all 3 species. Furthermore, after left ventricular apical resection, the cardiomyocyte proliferative response was diminished in mice with genetic deletion of complement 5a receptor 1.

CONCLUSIONS

These data reveal that activation of complement 5a receptor 1 mediates an evolutionarily conserved response that promotes cardiomyocyte proliferation after cardiac injury and identify complement pathway activation as a common pathway of successful heart regeneration.

C5a receptor 1 promotes autoimmunity, neutrophil dysfunction and injury in experimental anti-myeloperoxidase glomerulonephritis

The prospects for complement-targeted therapy in ANCA-associated vasculitis have been enhanced by a recent clinical trial in which C5a receptor 1 (C5aR1) inhibition safely replaced glucocorticoids in induction treatment. C5aR1 primes neutrophils for activation by anti-neutrophil cytoplasmic antibody (ANCA) and is therefore required in models of glomerulonephritis induced by anti-myeloperoxidase antibody. Although humoral and cellular autoimmunity play essential roles in ANCA-associated vasculitis, a role for C5aR1 in these responses has not been described. Here, we use murine models to dissect the role of C5aR1 in the generation of anti-myeloperoxidase autoimmunity and the effector responses resulting in renal injury. The genetic absence or pharmacological inhibition of C5aR1 results in reduced autoimmunity to myeloperoxidase with an attenuated Th1 response, increased Foxp3⁺ regulatory T cells and reduction in generation of myeloperoxidase-ANCA. These changes are mediated by C5aR1 on dendritic cells, which promotes activation, and thus myeloperoxidase autoimmunity and glomerulonephritis. We also use renal intravital microscopy to determine the effect of C5aR1 inhibition on ANCA induced neutrophil dysfunction. We found that myeloperoxidase-ANCA induce neutrophil retention and reactive oxygen species burst within glomerular capillaries. These pathological behaviors are abrogated by C5aR1 inhibition. Thus, C5aR1 inhibition ameliorates both autoimmunity and intra-renal neutrophil activation in ANCA-associated vasculitis.

The C5a/C5aR1 axis controls the development of experimental allergic asthma independent of LysM-expressing pulmonary immune cells

C5a regulates the development of maladaptive immune responses in allergic asthma mainly through the activation of C5a receptor 1 (C5aR1). Yet, the cell types and the mechanisms underlying this regulation are ill-defined. Recently, we described increased C5aR1 expression in lung tissue eosinophils but decreased expression in airway and pulmonary macrophages as well as in pulmonary CD11b+ conventional dendritic cells (cDCs) and monocyte-derived DCs (moDCs) during the allergic effector phase using a floxed green fluorescent protein (GFP)-C5aR1 knock-in mouse. Here, we determined the role of C5aR1 signaling in neutrophils, moDCs and macrophages for the pulmonary recruitment of such cells and the importance of C5aR1-mediated activation of LysM-expressing cells for the development of allergic asthma. We used LysM-C5aR1 KO mice with a specific deletion of C5aR1 in LysMCre-expressing cells and confirmed the specific deletion of C5aR1 in neutrophils, macrophages and moDCs in the airways and/or the lung tissue. We found that alveolar macrophage numbers were significantly increased in LysM-C5aR1 KO mice. Induction of ovalbumin (OVA)-driven experimental allergic asthma in GFP-C5aR1fl/fl and LysM-C5aR1 KO mice resulted in strong but similar airway resistance, mucus production and Th2/Th17 cytokine production. In contrast, the number of airway but not of pulmonary neutrophils was lower in LysM-C5aR1 KO as compared with GFP-C5aR1fl/fl mice. The recruitment of macrophages, cDCs, moDCs, T cells and type 2 innate lymphoid cells was not altered in LysM-C5aR1 KO mice. Our findings demonstrate that C5aR1 is critical for steady state control of alveolar macrophage numbers and the transition of neutrophils from the lung into the airways in OVA-driven allergic asthma. However, C5aR1 activation of LysM-expressing cells plays a surprisingly minor role in the recruitment and activation of such cells and the development of the allergic phenotype in OVA-driven experimental allergic asthma.

Regulation and function of anaphylatoxins and their receptors in allergic asthma

Allergic asthma is a disease of the airways driven by maladaptive T helper 2 (Th2) and Th17 immune response against harmless, airborne substances. The hallmarks of this disease are airway hyperresponsiveness (AHR), eosinophilic and neutrophilic airway inflammation and mucus overproduction. Distinct dendric cell (DC) subsets together with airway epithelial and pulmonary vascular endothelial cells play critical roles in allergen sensing and in driving T cell differentiation towards Th2 and Th17 effector or regulatory T cells (Treg). Previous studies suggested already a pivotal role for the anaphylatoxins (C5a, C3a) in the pathogenesis of allergic asthma. During sensitization for example it is described, that C3a promotes, whereas C5a protects from the development of maladaptive immunity during allergen sensitization. Here we will discuss the role of the anaphylatoxins (C3a, C5a) and their receptors during the pathogenesis of allergic asthma, and specifically in lung DC biology. We will also have a look on canonical and non-canonical complement activation and we will discuss novel concepts on how the adaptive immune system can regulate the function of ATRs also in the context of allergic asthma.

Monitoring C5aR2 Expression Using a Floxed tdTomato-C5aR2 Knock-In Mouse

The biological significance of C5a receptor [(C5aR)2/C5L2], a seven-transmembrane receptor binding C5a and C5adesArg, remains ill-defined. Specific ligation of C5aR2 inhibits C5a-induced ERK1/2 activation, strengthening the view that C5aR2 regulates C5aR1-mediated effector functions. Although C5aR2 and C5aR1 are often coexpressed, a detailed picture of C5aR2 expression in murine cells and tissues is still lacking. To close this gap, we generated a floxed tandem dye (td)Tomato-C5aR2 knock-in mouse that we used to track C5aR2 expression in tissue-residing and circulating immune cells. We found the strongest C5aR2 expression in the brain, bone marrow, and airways. All myeloid-derived cells expressed C5aR2, although with different intensities. C5aR2 expression in blood and tissue neutrophils was strong and homogeneous. Specific ligation of C5aR2 in neutrophils from tdTomato-C5aR2 mice blocked C5a-driven ERK1/2 phosphorylation, demonstrating functionality of C5aR2 in the reporter mice. In contrast to neutrophils, we found tissue-specific differences in C5aR2 expression in eosinophils, macrophages, and dendritic cell subsets. Naive and activated T cells stained negative for C5aR2, whereas B cells from different tissues homogeneously expressed C5aR2. Also, NK cell subsets in blood and spleen strongly expressed C5aR2. Activation of C5aR2 in NK cells suppressed IL-12/IL-18-induced IFN-γ production. Intratracheal IL-33 challenge resulted in decreased C5aR2 expression in pulmonary eosinophils and monocyte-derived dendritic cells. In summary, we provide a detailed map of murine C5aR2 immune cell expression in different tissues under steady-state conditions and upon pulmonary inflammation. The C5aR2 knock-in mouse will help to reliably track and conditionally delete C5aR2 expression in experimental models of inflammation.

Regulation of Eosinophil and Group 2 Innate Lymphoid Cell Trafficking in Asthma

Asthma is an inflammatory disease usually characterized by increased Type 2 cytokines and by an infiltration of eosinophils to the airways. While the production of Type 2 cytokines has been associated with T_H2 lymphocytes, increasing evidence indicates that group 2 innate lymphoid cells (ILC2) play an important role in the production of the Type 2 cytokines interleukin (IL)-5 and IL-13, which likely amplifies the recruitment of eosinophils from the blood to the airways. In that regard, recent asthma treatments have been focusing on blocking Type 2 cytokines, notably IL-4, IL-5, and IL-13. These treatments mainly result in decreased blood or sputum eosinophil counts as well as decreased asthma symptoms. This supports that therapies blocking eosinophil recruitment and activation are valuable tools in the management of asthma and its severity. Herein, we review the mechanisms involved in eosinophil and ILC2 recruitment to the airways, with an emphasis on eotaxins, other chemokines as well as their receptors. We also discuss the involvement of other chemoattractants, notably the bioactive lipids 5-oxo-eicosatetraenoic acid, prostaglandin D₂, and 2-arachidonoyl-glycerol. Given that eosinophil biology differs between human and mice, we also highlight and discuss their responsiveness toward the different eosinophil chemoattractants.

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

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

Monitoring C3aR Expression Using a Floxed tdTomato-C3aR Reporter Knock-in Mouse

C3a exerts multiple biologic functions through activation of its cognate C3a receptor. C3^-/- and C3aR^-/- mice have been instrumental in defining important roles of the C3a/C3aR axis in the regulation of acute and chronic inflammatory diseases, including ischemia/reperfusion injury, allergic asthma, autoimmune nephritis, and rheumatoid arthritis. Surprisingly little is known about C3aR expression and function in immune and stromal cells. To close this gap, we generated a floxed tandem-dye Tomato (tdTomato)-C3aR reporter knock-in mouse, which we used to monitor C3aR expression in cells residing in the lung, airways, lamina propria (LP) of the small intestine, brain, visceral adipose tissue, bone marrow (BM), spleen, and the circulation. We found a strong expression of tdTomato-C3aR in the brain, lung, LP, and visceral adipose tissue, whereas it was minor in the spleen, blood, BM, and the airways. Most macrophage and eosinophil populations were tdTomato-C3aR⁺ Interestingly, most tissue eosinophils and some macrophage populations expressed C3aR intracellularly. BM-derived dendritic cells (DCs), lung-resident cluster of differentiation (CD) 11b⁺ conventional DCs (cDCs) and monocyte-derived DCs, LP CD103⁺, and CD11b⁺ cDCs but not pulmonary CD103⁺ cDCs and splenic DCs were tdTomato-C3aR⁺ Surprisingly, neither BM, blood, lung neutrophils, nor mast cells expressed C3aR. Similarly, all lymphoid-derived cells were tdTomato-C3aR^-, except some LP-derived type 3 innate lymphoid cells. Pulmonary and LP-derived epithelial cells expressed at best minor levels of C3aR. In summary, we provide novel insights into the expression pattern of C3aR in mice. The floxed C3aR knock-in mouse will help to reliably track and conditionally delete C3aR expression in experimental models of inflammation.

Local endothelial complement activation reverses endothelial quiescence, enabling t-cell homing, and tumor control during t-cell immunotherapy

Cancer immunotherapy relies upon the ability of T cells to infiltrate tumors. The endothelium constitutes a barrier between the tumor and effector T cells, and the ability to manipulate local vascular permeability could be translated into effective immunotherapy. Here, we show that in the context of adoptive T cell therapy, antitumor T cells, delivered at high enough doses, can overcome the endothelial barrier and infiltrate tumors, a process that requires local production of C3, complement activation on tumor endothelium and release of C5a. C5a, in turn, acts on endothelial cells promoting the upregulation of adhesion molecules and T-cell homing. Genetic deletion of C3 or the C5a receptor 1 (C5aR1), and pharmacological blockade of C5aR1, impaired the ability of T cells to overcome the endothelial barrier, infiltrate tumors, and control tumor progression in vivo, while genetic chimera mice demonstrated that C3 and C5aR1 expression by tumor stroma, and not leukocytes, governs T cell homing, acting on the local endothelium. In vitro, endothelial C3 and C5a expressions were required for endothelial activation by type 1 cytokines. Our data indicate that effective immunotherapy is a consequence of successful homing of T cells in response to local complement activation, which disrupts the tumor endothelial barrier.

Novel insights into the expression pattern of anaphylatoxin receptors in mice and men

The anaphylatoxins (AT) C3a and C5a play important roles as mediators of inflammation. Further, they regulate and control multiple innate and adaptive immune responses through binding and activation of their cognate G protein-coupled receptors, i.e. C3a receptor (C3aR), C5a receptor 1 (C5aR1) and C5a receptor 2 (C5aR2), although the latter lacks important sequence motifs for G protein-coupling. Based on their pleiotropic functions, they contribute not only to tissue homeostasis but drive, perpetuate and resolve immune responses in many inflammatory diseases including infections, malignancies, autoimmune as well as allergic diseases. During the past few years, transcriptome expression data provided detailed insights into AT receptor tissue mRNA expression. In contrast, our understanding of cellular AT receptor expression in human and mouse tissues under steady and inflammatory conditions is still sketchy. Ligand binding studies, flow cytometric and immunohistochemical analyses convincingly demonstrated tissue-specific C5aR1 expression in various cells of myeloid origin. However, a detailed map for C3aR or C5aR2 expression in human or mouse tissue cells is still lacking. Also, reports about AT expression in lymphoid cells is still controversial. To understand the multiple roles of the ATs in the innate and adaptive immune networks, a detailed understanding of their receptor expression in health and disease is required. Recent findings obtained with novel GFP or tdTomato AT-receptor knock-in mice provide detailed insights into their expression pattern in tissue immune and stroma cells. Here, we will provide an update about our current knowledge of AT receptor expression pattern in humans and mice.

Complement C5a-C5aR1 signalling drives skeletal muscle macrophage recruitment in the hSOD1G93A mouse model of amyotrophic lateral sclerosis

Background

The terminal pathway of the innate immune complement system is implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). Terminal complement activation leads to generation of C5a, which through its receptor, C5aR1, drives immune cell recruitment and activation. Importantly, genetic or pharmacological blockage of C5aR1 improves motor performance and reduces disease pathology in hSOD1^G93A rodent models of ALS. In this study, we aimed to explore the potential mechanisms of C5aR1-mediated pathology in hSOD1^G93A mice by examining their skeletal muscles.

Results

We found elevated levels of C1qB, C4, fB, C3, C5a, and C5aR1 in tibialis anterior muscles of hSOD1^G93A mice, which increased with disease progression. Macrophage cell numbers also progressively increased in hSOD1^G93A muscles in line with disease progression. Immuno-localisation demonstrated that C5aR1 was expressed predominantly on macrophages within hSOD1^G93A skeletal muscles. Notably, hSOD1^G93A × C5aR1^-/- mice showed markedly decreased numbers of infiltrating macrophages, along with reduced neuromuscular denervation and improved grip strength in hind limb skeletal muscles, when compared to hSOD1^G93A mice.

Conclusion

These results indicate that terminal complement activation and C5a production occur in skeletal muscle tissue of hSOD1^G93A mice, and that C5a-C5aR1 signalling contributes to the recruitment of macrophages that may accelerate muscle denervation in these ALS mice.