Characterization of a Receptor for C5a Anaphylatoxin on Human Eosinophils

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.

Emerging Approaches for the Management of Chemotherapy-Induced Peripheral Neuropathy (CIPN): Therapeutic Potential of the C5a/C5aR Axis

Chemotherapy-induced peripheral neuropathy (CIPN) is the most common neurologic complication of chemotherapy, resulting in symptoms like pain, sensory loss, and numbness in the hands and feet that cause lots of uneasiness in patients with cancer. They often suffer from pain so severe that it interrupts the treatment, thus invalidating the entire chemotherapy-based healing process, and significantly reducing their quality of life. In this paper, we underline the role of the complement system in CIPN, highlighting the relevance of the C5a fragment and its receptor C5aR1, whose activation is thought to be involved in triggering a cascade of events that can lead to CIPN onset. Recent experimental data showed the ability of docetaxel and paclitaxel to specifically bind and activate C5aR1, thus shining light on one of the molecular mechanisms by which taxanes may activate a cascade of events leading to neuropathy. According to these new evidence, it was possible to suggest new mechanisms underlying the pathophysiology of CIPN. Hence, the C5a/C5aR1 axis may represent a new target for CIPN treatment, and the use of C5aR1 inhibitors can be proposed as a potential new therapeutic option to manage this high unmet medical need.

Mass Cytometry Phenotyping of Human Granulocytes Reveals Novel Basophil Functional Heterogeneity

Basophils, the rarest granulocyte, play critical roles in parasite- and allergen-induced inflammation. We applied mass cytometry (CyTOF) to simultaneously asses 44 proteins to phenotype and functionally characterize neutrophils, eosinophils, and basophils from 19 healthy donors. There was minimal heterogeneity seen in eosinophils and neutrophils, but data-driven analyses revealed four unique subpopulations within phenotypically basophilic granulocytes (PBG; CD45+HLA-DR-CD123+). Through CyTOF and fluorescence-activated cell sorting (FACS), we classified these four PBG subpopulations as (I) CD16lowFcεRIhighCD244high (88.5 ± 1.2%), (II) CD16highFcεRIhighCD244high (9.1 ± 0.4%), (III) CD16lowFcεRIlowCD244low (2.3 ± 1.3), and (IV) CD16highFcεRIlowCD244low (0.4 ± 0.1%). Prospective isolation confirmed basophilic-morphology of PBG I-III, but neutrophilic-morphology of PBG IV. Functional interrogation via IgE-crosslinking or IL-3 stimulation demonstrated that PBG I-II had significant increases in CD203c expression, whereas PBG III-IV remained unchanged compared with media-alone conditions. Thus, PBG III-IV could serve roles in non-IgE-mediated immunity. Our findings offer new perspectives in human basophil heterogeneity and the varying functional potential of these new subsets in health and disease.

The Complement Receptors C3aR and C5aR Are a New Class of Immune Checkpoint Receptor in Cancer Immunotherapy

Cancer immunotherapy has made remarkable clinical advances in recent years. Antibodies targeting the immune checkpoint receptors PD-1 and CTLA-4 and adoptive cell therapy (ACT) based on ex vivo expanded peripheral CTLs, tumor infiltrating lymphocytes (TILs), gene-engineered TCR- and chimeric antigen receptor (CAR)-T cells have all shown durable clinical efficacies in multiple types of cancers. However, these immunotherapeutic approaches only benefit a small fraction of cancer patients as various immune resistance mechanisms and limitations make their effective use a challenge in the majority of cancer patients. For example, adaptive resistance to therapeutic PD-1 blockade is associated with an upregulation of some additional immune checkpoint receptors. The efficacy of transferred tumor-specific T cells under the current clinical ACT protocol is often limited by their inefficient engraftment, poor persistence, and weak capability to attack tumor cells. Recent studies demonstrate that the complement receptor C3aR and C5aR function as a new class of immune checkpoint receptors. Complement signaling through C3aR and C5aR expressed on effector T lymphocytes prevent the production of the cytokine interleukin-10 (IL-10). Removing C3aR/C5aR-mediated transcriptional suppression of IL-10 expression results in endogenous IL-10 production by antitumor effector T cells, which drives T cell expansion and enhances T cell-mediated antitumor immunity. Importantly, preclinical, and clinical data suggest that a signaling axis consisting of complement/C3aR/C5aR/IL-10 critically regulates T cell mediated antitumor immunity and manipulation of the pathway ex vivo and in vivo is an effective strategy for cancer immunotherapy. Furthermore, a combination of treatment strategies targeting the complement/C3aR/C5aR/IL-10 pathway with other treatment modalities may improve cancer therapeutic efficacy.

Aptamers that bind to the human complement component receptor hC5aR1 interfere with hC5aR1 interaction to its hC5a ligand

The complement system plays an important role in inflammation and immunity. In this system, a potent inflammatory ligand is C5a, which initiates its effects by activating its core receptor C5aR1. Thus, compounds that interfere with the C5a-C5aR1 interaction could alleviate some inflammatory conditions. Consequently, several ligands that bind to either C5a or C5aR1 have previously been isolated and evaluated. In the present study, two RNA aptamers, aptamer 1 and aptamer 9, that specifically bind to hC5aR1 with much higher affinity than antibodies were isolated. These two aptamers were tested for their ability to interfere with the cognate ligand of hC5aR1, C5a, using a chemotaxis assay. Both aptamer 1 and 9 interfered with the C5a interaction, suggesting that the aptamers recognized the extracellular domain of hC5aR1 responsible for hC5a ligand binding. Considering the higher affinity of aptamers to the hC5aR1 and their interference with hC5a ligand binding, further study is warranted to explore not only their applications in the diagnosis of inflammatory diseases but also their usefulness in modulating hC5a and hC5aR1 interactions.

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.

Osteoblast-specific overexpression of complement receptor C5aR1 impairs fracture healing

The anaphylatoxin receptor C5aR1 plays an important role not only in innate immune responses, but also in bone metabolism and fracture healing, being highly expressed on immune and bone cells, including osteoblasts and osteoclasts. C5aR1 induces osteoblast migration, cytokine generation and osteoclastogenesis, however, the exact role of C5aR1-mediated signaling in osteoblasts is not entirely known. Therefore, we hypothesized that osteoblasts are essential target cells for C5a and that fracture healing should be disturbed in mice with an osteoblast-specific C5aR1 overexpression (Col1a1-C5aR1). Osteoblast activity in vitro, bone phenotype and fracture healing after isolated osteotomy and after combined osteotomy with additional thoracic trauma were analyzed. The systemic and local inflammatory reactions were analyzed by determining C5a and IL-6 concentrations in blood, bronchoalveolar lavage fluid and fracture callus and the recruitment of immune cells. In vitro, osteoblast proliferation and differentiation were similar to wildtype cells, and phosphorylation of p38 and expression of IL-6 and RANKL were increased in osteoblasts derived from Col1a1-C5aR1 mice. Bone phenotype and the inflammatory reaction were unaffected in Col1a1-C5aR1 mice. Fracture healing was significantly impaired as demonstrated by significantly reduced bone content, bone mineral density and flexural rigidity, possibly due to significantly increased osteoclast numbers. C5aR1 signaling in osteoblasts might possibly affect RANKL/OPG balance, leading to increased bone resorption. Additional trauma significantly impaired fracture healing, particularly in Col1a1-C5aR1 mice. In conclusion, the data indicate that C5aR1 signaling in osteoblasts plays a detrimental role in bone regeneration after fracture.

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.

Differential regulation of C5a receptor 1 in innate immune cells during the allergic asthma effector phase

C5a drives airway constriction and inflammation during the effector phase of allergic asthma, mainly through the activation of C5a receptor 1 (C5aR1). Yet, C5aR1 expression on myeloid and lymphoid cells during the allergic effector phase is ill-defined. Recently, we generated and characterized a floxed green fluorescent protein (GFP)-C5aR1 knock-in mouse. Here, we used this reporter strain to monitor C5aR1 expression in airway, pulmonary and lymph node cells during the effector phase of OVA-driven allergic asthma. C5aR1 reporter and wildtype mice developed a similar allergic phenotype with comparable airway resistance, mucus production, eosinophilic/neutrophilic airway inflammation and Th2/Th17 cytokine production. During the allergic effector phase, C5aR1 expression increased in lung tissue eosinophils but decreased in airway and pulmonary macrophages as well as in pulmonary CD11b⁺ conventional dendritic cells (cDCs) and monocyte-derived DCs (moDCs). Surprisingly, expression in neutrophils was not affected. Of note, moDCs but not CD11b⁺ cDCs from mediastinal lymph nodes (mLN) expressed less C5aR1 than DCs residing in the lung after OVA challenge. Finally, neither CD103⁺ cDCs nor cells of the lymphoid lineage such as Th2 or Th17-differentiated CD4⁺ T cells, B cells or type 2 innate lymphoid cells (ILC2) expressed C5aR1 under allergic conditions. Our findings demonstrate a complex regulation pattern of C5aR1 in the airways, lung tissue and mLN of mice, suggesting that the C5a/C5aR1 axis controls airway constriction and inflammation through activation of myeloid cells in all three compartments in an experimental model of allergic asthma.

Current and future biomarkers in allergic asthma

Diagnosis early in life, sensitization, asthma endotypes, monitoring of disease and treatment progression are key motivations for the exploration of biomarkers for allergic rhinitis and allergic asthma. The number of genes related to allergic rhinitis and allergic asthma increases steadily; however, prognostic genes have not yet entered clinical application. We hypothesize that the combination of multiple genes may generate biomarkers with prognostic potential. The current review attempts to group more than 161 different potential biomarkers involved in respiratory inflammation to pave the way for future classifiers. The potential biomarkers are categorized into either epithelial or infiltrate-derived or mixed origin, epithelial biomarkers. Furthermore, surface markers were grouped into cell-type-specific categories. The current literature provides multiple biomarkers for potential asthma endotypes that are related to T-cell phenotypes such as Th1, Th2, Th9, Th17, Th22 and Tregs and their lead cytokines. Eosinophilic and neutrophilic asthma endotypes are also classified by epithelium-derived CCL-26 and osteopontin, respectively. There are currently about 20 epithelium-derived biomarkers exclusively derived from epithelium, which are likely to innovate biomarker panels as they are easy to sample. This article systematically reviews and categorizes genes and collects current evidence that may promote these biomarkers to become part of allergic rhinitis or allergic asthma classifiers with high prognostic value.

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

Many of the biological properties of C5a are mediated through activation of its receptor (C5aR1), the expression of which has been demonstrated convincingly on myeloid cells, such as neutrophils, monocytes, and macrophages. In contrast, conflicting results exist regarding C5aR1 expression in dendritic cells (DCs) and lymphoid lineage cells. In this article, we report the generation of a floxed GFP-C5aR1 reporter knock-in mouse. Using this mouse strain, we confirmed strong C5aR1 expression in neutrophils from bone marrow, blood, lung, and spleen, as well as in peritoneal macrophages. Further, we show C5aR1 expression in lung eosinophils, lung- and lamina propria-resident and alveolar macrophages, bone marrow-derived DCs, and lung-resident CD11b(+) and monocyte-derived DCs, whereas intestinal and pulmonary CD103(+) DCs stained negative. Also, some splenic NKT cells expressed GFP, whereas naive NK cells and B2 cells lacked GFP expression. Finally, we did not observe any C5aR1 expression in naive or activated CD4(+) Th cells in vitro or in vivo. Mating the floxed GFP-C5aR1 mouse strain with LysMCre mice, we were able to specifically delete C5aR1 in neutrophils and macrophages, whereas C5aR1 expression was retained in DCs. In summary, our findings suggest that C5aR1 expression in mice is largely restricted to cells of the myeloid lineage. The novel floxed C5aR1 reporter knock-in mouse will prove useful to track C5aR1 expression in experimental models of acute and chronic inflammation and to conditionally delete C5aR1 in immune cells.

LPS Induces Pulp Progenitor Cell Recruitment via Complement Activation

Complement system, a major component of the natural immunity, has been recently identified as an important mediator of the dentin-pulp regeneration process through STRO-1 pulp cell recruitment by the C5a active fragment. Moreover, it has been shown recently that under stimulation with lipoteichoic acid, a complex component of the Gram-positive bacteria cell wall, human pulp fibroblasts are able to synthesize all proteins required for complement activation. However, Gram-negative bacteria, which are also involved in tooth decay, are known as powerful activators of complement system and inflammation. Here, we investigated the role of Gram-negative bacteria-induced complement activation on the pulp progenitor cell recruitment using lipopolysaccharide (LPS), a major component of all Gram-negative bacteria. Our results show that incubating pulp fibroblasts with LPS induced membrane attack complex formation and C5a release in serum-free fibroblast cultures. The produced C5a binds to the pulp progenitor cells’ membrane and induces their migration toward the LPS stimulation chamber, as revealed by the dynamic transwell migration assays. The inhibition of this migration by the C5aR-specific antagonist W54011 indicates that the pulp progenitor migration is mediated by the interaction between C5a and C5aR. Our findings demonstrate, for the first time, a direct interaction between the recruitment of progenitor pulp cells and the activation of complement system generated by pulp fibroblast stimulation with LPS.

Complement anaphylatoxins as immune regulators in cancer

The role of the complement system in innate immunity is well characterized. However, a recent body of research implicates the complement anaphylatoxins C3a and C5a as insidious propagators of tumor growth and progression. It is now recognized that certain tumors elaborate C3a and C5a and that complement, as a mediator of chronic inflammation and regulator of immune function, may in fact foster rather than defend against tumor growth. A putative mechanism for this function is complement-mediated suppression of immune effector cells responsible for immunosurveillance within the tumor microenvironment. This paradigm accords with models of immune dysregulation, such as autoimmunity and infectious disease, which have defined a pathophysiological role for abnormal complement signaling. Several types of immune cells express the cognate receptors for the complement anaphylatoxins, C3aR and C5aR, and demonstrate functional modulation in response to complement stimulation. In turn, impairment of antitumor immunity has been intimately tied to tumor progression in animal models of cancer. In this article, the literature was systematically reviewed to identify studies that have characterized the effects of the complement anaphylatoxins on the composition and function of immune cells within the tumor microenvironment. The search identified six studies based upon models of lymphoma and ovarian, cervical, lung, breast, and mammary cancer, which collectively support the paradigm of complement as an immune regulator in the tumor microenvironment.

Functional analysis of C5a effector responses in vitro and in vivo

C5a is a powerful proinflammatory and immunomodulatory mediator as evidenced by its involvement in infectious, allergic, and autoimmune diseases as well as in cancer. C5a exerts most of its biologic functions through binding and activation of the G protein-coupled C5a receptor (C5aR). C5aR activation promotes complex signaling pathways eventually resulting in increased intracellular calcium (Ca(2+))i concentration. One of the prototypic effector functions associated with C5aR activation is the reconfiguration of the cell cytoskeleton resulting in increased cell motility and ligand-specific cell migration. Here, we describe three assays to assess C5a-mediated effector functions: (1) increase in (Ca(2+))i in bone marrow-derived neutrophils using the intracellular fluorescent Ca(2+)-sensitive indicator Fluo-4 AM and flow cytometry-based methods; (2) in vitro migration of murine, bone marrow-derived neutrophils using a modified Boyden chamber; and (3) in vivo migration of neutrophils from the circulation into the peritoneal cavity. To control for the specificity of the C5a-mediated effects, C5aR-deficient mice, neutralizing C5aR-specific antibodies and potent pharmacologic C5aR inhibitors are available.

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.

Global H3K4me3 genome mapping reveals alterations of innate immunity signaling and overexpression of JMJD3 in human myelodysplastic syndrome CD34+ cells

The molecular bases of myelodysplastic syndromes (MDS) are not fully understood. Trimethylated histone 3 lysine 4 (H3K4me3) is present in promoters of actively transcribed genes and has been shown to be involved in hematopoietic differentiation. We performed a genome-wide H3K4me3 CHIP-Seq (chromatin immunoprecipitation coupled with whole genome sequencing) analysis of primary MDS bone marrow (BM) CD34+ cells. This resulted in the identification of 36 genes marked by distinct higher levels of promoter H3K4me3 in MDS. A majority of these genes are involved in nuclear factor (NF)-κB activation and innate immunity signaling. We then analyzed expression of histone demethylases and observed significant overexpression of the JmjC-domain histone demethylase JMJD3 (KDM6b) in MDS CD34+ cells. Furthermore, we demonstrate that JMJD3 has a positive effect on transcription of multiple CHIP-Seq identified genes involved in NF-κB activation. Inhibition of JMJD3 using shRNA in primary BM MDS CD34+ cells resulted in an increased number of erythroid colonies in samples isolated from patients with lower-risk MDS. Taken together, these data indicate the deregulation of H3K4me3 and associated abnormal activation of innate immunity signals have a role in the pathogenesis of MDS and that targeting these signals may have potential therapeutic value in MDS.

Adenovirus-mediated in vivo silencing of anaphylatoxin receptor C5aR

C5a, one of the most potent inflammatory peptides, induces its inflammatory functions by interacting with C5a receptor (C5aR) that belongs to the rhodopsin family of seven-transmembrane G protein-coupled receptors. C5a/C5aR signaling has been implicated in the pathogenesis of many inflammatory and immunological diseases such as sepsis and acute lung injury. Widespread upregulation of C5aR has been seen at both the protein level and transcriptional level under pathological conditions. Here, we show that C5aR gene expression can be specifically suppressed by siRNA, both in vitro and in vivo. A panel of chemically siRNA oligonucleotides was first synthesized to identify the functional siRNA sequences. The short hairpin RNAs (shRNAs) were also designed, cloned, and tested for the silencing effects in C5aR transfected cells. The effective shRNA expression cassettes were then transferred to an adenovirus DNA vector. ShRNA-expressing adenoviruses were intratracheally administered into mouse lung, and a significant in vivo silencing of C5aR was obtained four days after administration. Thus, C5aR shRNA-expressing adenoviruses appear to be an alternative strategy for the treatment of complement-induced disorders.

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

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

Attenuation of IgG immune complex-induced acute lung injury by silencing C5aR in lung epithelial cells

Acute lung injury (ALI) in mouse lung occurs after distal airway deposition of IgG immune complexes (IgGICs), resulting in a breakdown of the vascular-airway barrier, causing intrapulmonary edema, hemorrhage, and accumulation of neutrophils [polymorphonuclear leukocytes (PMNs)] in the alveolar compartment, these changes being complement (C5a) and C5a receptor (C5aR) dependent. In this ALI model, C5aR expression (protein) was found to occur on upper (bronchial) and lower (alveolar) airway epithelial cells. An adenovirus construct (siRNA) was used to silence mRNA for C5aR in the lung. Under such conditions, C5aR protein was markedly reduced on lung epithelial cells, resulting in much reduced leakage of albumin into the lung, diminished buildup of PMNs, and lower levels of proinflammatory mediators in bronchoalveolar lavage fluids. These studies indicate that bronchial and alveolar epithelial cell C5aR is up-regulated and greatly contributes to inflammation and injury in the lung. The use of siRNA administered into the airways avoids systemic suppression of C5aR, which might compromise innate immunity. It is possible that such an intervention might be employed in humans with ALI or acute respiratory distress syndrome as well as in upper-airway inflammatory diseases, such as chronic obstructive pulmonary disease and asthma, where there is evidence for complement activation and buildup of PMNs.

Mechanisms of complement activation, C4d deposition, and their contribution to the pathogenesis of antibody-mediated rejection

Complement split products have emerged as useful markers of antibody-mediated rejection in solid organ transplants. One split product, C4d, is now widely accepted as a marker for antibody-mediated rejection in renal and cardiac allografts. This review summarizes the rationale for the use of C4d as a marker of antibody-mediated rejection, along with the clinical evidence supporting its use in the clinical diagnosis of antibody-mediated rejection. Antibody-independent mechanisms by which C4d can be activated by the classical and lectin pathways of complement activation are also identified. Finally, mechanisms by which complement activation stimulates effector cells (neutrophils, monocytes, macrophages, platelets, and B and T lymphocytes) as well as target cells (endothelial cells) are discussed in relation to antibody-mediated allograft rejection.

The role of the anaphylatoxins in health and disease

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

Overexpression of the anaphylatoxin receptors, complement anaphylatoxin 3a receptor and complement anaphylatoxin 5a receptor, in the nasal mucosa of patients with mild and severe persistent allergic rhinitis

BACKGROUND

Although the complement anaphylatoxin peptides, complement anaphylatoxin 3a (C3a) and complement anaphylatoxin 5a (C5a), are implicated in the inflammatory process in allergic rhinitis, a direct interaction between allergic mucosa and complement receptors has not been demonstrated.

OBJECTIVE

We investigated the expressional levels and distributional patterns of the C3a receptor (C3aR) and C5a receptor (C5aR) in normal, mild, and severe persistent allergic nasal mucosa.

METHODS

Immunohistochemistry and Western blotting using C3aR and C5aR antibodies were applied to normal nasal, mild, and severe persistent allergic nasal mucosa.

RESULTS

Complement anaphylatoxin 5a receptor was detected in the inflammatory cells of normal and allergic nasal mucosa, and its expression level was significantly higher in allergic nasal mucosa than normal nasal mucosa. C3aR in normal and allergic nasal mucosa was commonly expressed in nonmyeloid cells such as epithelial cells, submucosal glands, and nerve fibers. In addition, C3aR was expressed in the endothelium of cavernous sinuses and the surrounding perivascular muscle layer in severe persistent allergic nasal mucosa, but not in normal and mild allergic nasal mucosa. Western blotting demonstrated that the expression level of C3aR was significantly increased in severe persistent allergic nasal mucosa compared with normal and mild nasal mucosa.

CONCLUSION

On the basis of the location of C3aR and C5aR, C5aR may play a role in activation of inflammatory cells, whereas C3aR may mediate mucus secretion and mucosal swelling in allergic nasal mucosa, especially severe persistent allergic nasal mucosa.

Role of complement in immune regulation and its exploitation by virus

Complement is activated during the early phase of viral infection and promotes destruction of virus particles as well as the initiation of inflammatory responses. Recently, complement and complement receptors have been reported to play an important role in the regulation of innate as well as adaptive immune responses during infection. The regulation of host immune responses by complement involves modulation of dendritic cell activity in addition to direct effects on T-cell function. Intriguingly, many viruses encode homologs of complement regulatory molecules or proteins that interact with complement receptors on antigen-presenting cells and lymphocytes. The evolution of viral mechanisms to alter complement function may augment pathogen persistence and limit immune-mediated tissue destruction. These observations suggest that complement may play an important role in both innate and adaptive immune responses to infection as well as virus-mediated modulation of host immunity.

Conformational movement of F251 contributes to the molecular mechanism of constitutive activation in the C5a receptor

The activation mechanism of G-protein-coupled receptors triggered upon binding of a ligand represents a very important 'conformational switch' in the biological array of signal transduction. However, the molecular and functional details for this activation switch remain unknown. Random saturation mutagenesis data on the complement factor 5a receptor has provided a large data set of mutants including several constitutively active mutants. In the present study, we employed computational modeling to rationalize the constitutive activity for two constitutively active mutants, NQ (I124N/L127Q) and F251A, and we then made predictions for a series of mutants that either promote or constrain constitutive activity. Biological testing of the site-directed mutants confirmed most of the predictions of the computational modeling. These results support a molecular mechanism of constitutive activity in complement factor 5a receptor mutants that is associated with conformational changes in a network of residues neighboring F251 as the focal point of origin.

Role of complement C5a in mechanical inflammatory hypernociception: potential use of C5a receptor antagonists to control inflammatory pain

BACKGROUND AND PURPOSE

C5a, a complement activation product, exhibits a broad spectrum of inflammatory activities particularly neutrophil chemoattraction. Herein, the role of C5a in the genesis of inflammatory hypernociception was investigated in rats and mice using the specific C5a receptor antagonist PMX53 (AcF-[OP(D-Cha)WR]).

EXPERIMENTAL APPROACH

Mechanical hypernociception was evaluated with a modification of the Randall-Selitto test in rats and electronic pressure meter paw test in mice. Cytokines were measured by ELISA and neutrophil migration was determined by myeloperoxidase activity.

KEY RESULTS

Local pretreatment of rats with PMX53 (60-180 microg per paw) inhibited zymosan-, carrageenan-, lipopolysaccharide (LPS)- and antigen-induced hypernociception. These effects were associated with C5a receptor blockade since PMX53 also inhibited the hypernociception induced by zymosan-activated serum and C5a but not by the direct-acting hypernociceptive mediators, prostaglandin E(2) and dopamine. Underlying the C5a hypernociceptive mechanisms, PMX53 did not alter the cytokine release induced by inflammatory stimuli. However, PMX53 inhibited cytokine-induced hypernociception. PMX53 also inhibited the recruitment of neutrophils induced by zymosan but not by carrageenan or LPS, indicating an involvement of neutrophils in the hypernociceptive effect of C5a. Furthermore, the C5a-induced hypernociception was reduced in neutrophil-depleted rats. Extending these findings in rats, blocking C5a receptors also reduced zymosan-induced joint hypernociception in mice.

CONCLUSIONS AND IMPLICATIONS

These results suggest that C5a is an important inflammatory hypernociceptive mediator, acting by a mechanism independent of hypernociceptive cytokine release, but dependent on the presence of neutrophils. Therefore, we suggest that inhibiting the action of C5a has therapeutic potential in the control of inflammatory pain.

Anaphylatoxins: their role in bacterial infection and inflammation

Activation of the complement system plays a crucial role in the pathogenesis of infection and inflammation. Especially the complement activation products C3a and C5a, known as the anaphylatoxins, are potent proinflammatory mediators. In addition to their evident role in innate immunity, it is clear that the anaphylatoxins also play a role in regulation of adaptive immune responses. The anaphylatoxins play a role in a variety of infectious and inflammatory diseases like sepsis, ischemia-reperfusion injury, immune complex diseases, and hypersensitivity diseases like asthma. In this review we discuss the role of anaphylatoxins in infection and inflammation. Furthermore, we focus on bacterial complement evasion strategies that can provide tools for further research on pathogenesis of infectious diseases and a better understanding of the role of complement and anaphylatoxins in infection and inflammation.

Eosinophils develop in distinct stages and are recruited to peripheral sites by alternatively activated macrophages

Eosinophils are associated with allergic diseases and helminth infections. Development of these cells and recruitment to peripheral tissues are only partially understood. Distinct stages of eosinophil development in fetal liver, bone marrow, and blood could be identified using IL-4 reporter mice and mAb against FIRE, Siglec-F, and CCR3. Immature eosinophils were present in the fetal liver and could reconstitute the eosinophil compartment in irradiated recipient mice. In adult mice, eosinophil maturation proceeded from CCR3(-) to CCR3(+) cells in the bone marrow and was accompanied with changes in the transcriptional profile. Eosinophils appeared as activated cells in lung, thymus, lymph nodes, and Peyer's patches but remained in a resting state in bone marrow, blood, and spleen. Mixed bone marrow chimeras revealed that recruitment to lung and peritoneum was dependent on Stat6 expression in noneosinophils. Alternatively activated macrophages contributed substantially to tissue recruitment of eosinophils, providing a novel basis for development of therapeutic approaches to lower tissue eosinophilia.

Complement-related molecular events in sepsis leading to heart failure

Despite intensive ongoing research efforts, the mortality of patients with sepsis remains unacceptably high. A significant number of clinical trials have failed to produce sufficient therapeutic strategies despite showing promising results in animal models. So far, many studies have focused on deterioration of the humoral and cellular components of the immune system, the main cause of death in septic patients being multi-organ failure. However, not much is known about the effects of the complement system on parenchymal cells of organs such as the heart. Recently, septic cardiomyopathy has been recognized as one of the major complications during sepsis, often determining the clinical outcome. In this review, we describe molecular events which are thought to be related to cardiac dysfunction during sepsis. A special emphasis will be placed on the complement system, which generates powerful anaphylatoxins (such as C5a) and which has recently been associated with septic cardiomyopathy. Together with the impact on cardiac function of various cytokines we will provide a synopsis of the current knowledge regarding the pathophysiology underlying cardiac failure during sepsis with a special emphasis on C5a and C5aR.

Genetic analysis of the first and third extracellular loops of the C5a receptor reveals an essential WXFG motif in the first loop

The extracellular loops of G protein-coupled receptors (GPCRs) frequently contain binding sites for peptide ligands. However, the mechanism of receptor activation following ligand binding and the influence of the extracellular loops in other aspects of receptor function are poorly understood. Here we report a structure-function analysis of the first and third extracellular loops of the human C5a receptor, a GPCR that binds a 74-amino acid peptide ligand. Amino acid substitutions were randomly incorporated into each loop, and functional receptors were identified in yeast. The first extracellular loop contains a large number of positions that cannot tolerate amino acid substitutions, especially residues within the WXFG motif found in many rhodopsin-like GPCRs, yet disruption of these residues does not alter C5a binding affinity. These results demonstrate an unanticipated role for the first extracellular loop, and the WXFG motif in particular, in ligand-mediated activation of the C5a receptor. This motif likely serves a similar role in other GPCRs. The third extracellular loop, in contrast, contains far fewer preserved residues and appears to play a less essential role in receptor activation.

High expression of eosinophil chemoattractant ecalectin/galectin-9 in drug-induced liver injury

BACKGROUND

Ecalectin/galectin-9 (ECL/GL9) is an eosinophil chemoattractant isolated from T lymphocytes. Drug-induced liver injury (DILI), often caused by an allergic mechanism, is occasionally accompanied by eosinophilic infiltration. In this study, we intended to determine whether DILI can induce augmentation of ECL/GL9 expression. Further, we investigated whether this augmentation is associated with tissue eosinophilia.

METHODS

We examined the expression of ECL/GL9 in biopsy specimens of DILI using the immunohistochemical technique. A rabbit anti-ECL/GL9 antibody was produced by immunizing rabbits with synthetic peptide corresponding to a molecular epitope of ECL/GL9. Thereafter, immunohistochemical staining with the use of this antibody was performed on 16 DILI needle biopsy specimens, and on biopsy specimens of chronic viral hepatitis, liver cirrhosis, and normal liver tissues as controls.

RESULTS

In all cases of DILI specimens, but not in control liver specimens, a clear positive staining for ECL/GL9 was observed. Such positive staining was noted on Kupffer cells, fibroblasts, and histiocytes, but not on lymphocytes or hepatocytes. However, the intensity of immunolabeling did not correlate with the extent of eosinophile leukocyte infiltration.

CONCLUSION

High expression of ECL/GL9 is suggested to be a specific finding of DILI. However, tissue eosinophilia in DILI cannot be explained by the augmentation of ECL/GL9 expression.

Complement-induced impairment of the innate immune system during sepsis

The complement system is an integral part of innate immunity and is chiefly responsible for controlling bacterial infections, especially those involving gram- negative organisms. To accomplish this task, serum proteins engage in a series of enzymatic cascades. The cleaved proteins assemble pores on membranous structures, which lead to cell lysis. During this process, powerful inflammatory mediators are produced, including the anaphylatoxins, C5a, C3a, and the membrane attack complex (MAC). Under systemic inflammatory conditions, an overactive complement system may compromise the effectiveness of innate immunity. We review the detrimental effects that are caused by uncontrolled complement activation during sepsis.

Novel C5a regulators in inflammatory disease

Complement is part of the innate immune system, acting to protect the host from microorganisms such as bacteria, and other foreign and abnormal cells. Although primarily protective, complement activation can also cause damage to the host. In a number of inflammatory diseases, including rheumatoid arthritis and dermatitis, there is excessive and inappropriate complement activation. Many of the toxic effects seen in these conditions are attributable to the excessive production of the anaphylatoxin C5a, which may contribute to both the initiation and progression of the disease. Therefore, the regulation of C5a production and modulation of its function are good pharmacological targets in these disorders. As yet, there are no effective agents for the therapeutic regulation of C5a in routine clinical practice. This review describes the role of C5a in inflammatory disease, animal models used to study C5a-related effects, and current strategies aimed at regulating C5a. There is also a discussion of the strengths and weaknesses of these approaches, and an outline of the likely progress of this class of drugs in the future.

Cigarette smoke extract increases C5a receptor expression in human bronchial epithelial cells

We have shown that exposing human bronchial epithelial cells (HBECs) to 5% cigarette smoke extract (CSE) up-regulates C5a anaphylatoxin receptor (C5aR) expression as determined by flow cytometric analysis and immunohistochemistry. In this study, we conducted whole-cell saturation studies to quantitate the receptor number. After exposing an HBEC line (BEAS-2B) to CSE, radiolabeled C5a bound saturably with Kd = 2.71 +/- 1.03 nM (n = 4) and Bmax = 15,044 +/- 5702 receptors/cells. Without 5% CSE, no C5a binding was detected. Competitive binding studies revealed two classes of sites with distinct affinities for C5a (Ki1 = 3.28 x 10(-16) M; Ki2 = 1.60 x 10(-9) M). BEAS-2Bs were transfected with wild-type (WT) or mutant dominant-negative (DN) protein kinase C-alpha (PKC-alpha) to investigate the relationship between PKC-alpha and C5aR availability and affinity. Western blot analysis revealed a 75-kDa lysate band from cells expressing WT and DN PKC-alpha, but DN cells exposed to 5% CSE had no functional PKC activity. Pretreatment with Gö6976 [12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole] (PKC-alpha inhibitor) had no effect on DN but significantly decreased WT PKC activity. Competitive binding studies conducted on either WT or DN PKC-alpha-transfected cells also revealed two classes of binding sites for C5a having different affinities. There was a significant rightward shift of the binding curve when WT cells were pretreated with Gö6976. These data suggest that C5aR is detectable on bronchial epithelial cells exposed to CSE and that exposure to CSE increases the availability of C5a binding sites. The data also indicate that PKC-alpha may play an important role in modulating C5aR binding.

ROLE OF C5A IN INFLAMMATORY RESPONSES

The complement system not only represents an effective innate immune mechanism of host defense to eradicate microbial pathogens, but it is also widely involved in many forms of acute and chronic inflammatory diseases including sepsis, acute lung injury, ischemia-reperfusion injury, and asthma, to give just a few examples. The complement-activated product, C5a, displays powerful biological activities that lead to inflammatory sequelae. C5a is a strong chemoattractant and is involved in the recruitment of inflammatory cells such as neutrophils, eosinophils, monocytes, and T lymphocytes, in activation of phagocytic cells and release of granule-based enzymes and generation of oxidants, all of which may contribute to innate immune functions or tissue damage. Accumulating data suggest that C5a provides a vital bridge between innate and adaptive immune functions, extending the roles of C5a in inflammation. Herein, we review human and animal data describing the cellular and molecular mechanisms of C5a in the development of inflammatory disorders, sepsis, acute lung injury, ischemia-reperfusion injury, and asthma.

Evidence for a functional role of the second C5a receptor C5L2

During experimental sepsis in rodents after cecal ligation and puncture (CLP), excessive C5a is generated, leading to interactions with C5aR, loss of innate immune functions of neutrophils, and lethality. In the current study, we have analyzed the expression of the second C5a receptor C5L2, the putative "default" or nonsignaling receptor for C5a. Rat C5L2 was cloned, and antibody was developed to C5L2 protein. After CLP, blood neutrophils showed a reduction in C5aR followed by its restoration, while C5L2 levels gradually increased, accompanied by the appearance of mRNA for C5L2. mRNA for C5L2 increased in lung and liver during CLP. Substantially increased C5L2 protein (defined by binding of 125I-anti-C5L2 IgG) occurred in lung, liver, heart, and kidney after CLP. With the use of serum IL-6 as a marker for sepsis, infusion of anti-C5aR dramatically reduced serum IL-6 levels, while anti-C5L2 caused a nearly fourfold increase in IL-6 when compared with CLP controls treated with normal IgG. When normal blood neutrophils were stimulated in vitro with LPS and C5a, the antibodies had similar effects on release of IL-6. These data provide the first evidence for a role for C5L2 in balancing the biological responses to C5a.

Transcriptional regulation of the murine C5a receptor gene: NF-Y is required for basal and LPS induced expression in macrophages and endothelial cells

The anaphylatoxin receptors of the complement system are important in immune defense but also play a role in autoimmune disease. Reports have demonstrated induced C5a receptor (C5aR) expression in a number of disease states, yet little is known about the regulation of this gene. We have examined sequences in the presumptive promoter-enhancer region in order to study the regulation of this gene. Rapid amplification of cDNA ends (RACE) analyses were used to identify the transcriptional start site, and we then cloned 2278 bp of sequence from this region for use in luciferase assays. Deletion analyses of 5' sequences demonstrated that the majority of this region is dispensable for expression in macrophages and endothelial cells (ECs). A 232 bp region proximal to the transcription start site was fully capable of directing expression in macrophages and ECs, while being minimally active in cells that do not express the receptor. The transcriptional regulatory site most critical for this expression matches the consensus sequence for nuclear factor-Y (NF-Y) at position -96. Site-directed mutagenesis of this site resulted in a 70-90% decrease in luciferase activity depending on the cell type. Electrophoretic mobility shift/supershift assay (EMSA) analyses demonstrated the specific binding of NF-Y to labeled oligonucleotides containing the putative CCAAT site with macrophages and EC nuclear extracts, and antibodies to NF-Y were able to supershift this -96 NF-Y complex. We also demonstrate LPS leads to enhanced C5aR transcription and this is mediated predominantly through the NF-Y site. The data reported in this study might be critical for determination of transcription factors that can be targeted pharmacologically to modulate the expression of the C5aR in infectious disease or autoimmunity.

Role of C5a-C5aR interaction in sepsis

C5a-C5aR signaling plays an essential role in innate immunity of neutrophils. However, excessive interaction of C5a-C5aR results in harmful effects in these cells. In sepsis, robust generation of C5a occurs; blockade of either C5a or C5aR greatly improves survival in experimental sepsis following cecal ligation and puncture (CLP). The beneficial effects derived from C5a-C5aR interaction are associated with preservation of neutrophil innate immune functions (chemotaxis, phagocytosis, respiratory burst), attenuation of the inflammatory reaction, amelioration of coagulopathy, alteration in adhesion molecule expression, and modulation of apoptosis. Following CLP, C5aR expression is significantly elevated in organs, perhaps setting the stage for C5a-induced organ dysfunction. In contrast, C5aR content on neutrophils drops significantly at early stages of sepsis and progressively increases at later time points. Re-expression of C5aR on neutrophils during sepsis appears to be associated with the functional recovery of neutrophil innate immune functions. Following CLP, there is a positive correlation between C5aR content on blood neutrophils and survival of individual animals; high levels of C5aR on neutrophils are associated with survival, whereas low levels of C5aR on neutrophils predict mortality. These data suggest that in sepsis C5a-C5aR signaling is excessive, resulting in paralysis of neutrophil function. Interception of either C5a or C5aR dramatically improves survival during experimental sepsis.

Complement-induced impairment of the innate immune system during sepsis

The complement system is an integral part of innate immunity and is chiefly responsible for controlling bacterial infections, especially those involving gram- negative organisms. To accomplish this task, serum proteins engage in a series of enzymatic cascades. The cleaved proteins assemble pores on membranous structures, which lead to cell lysis. During this process, powerful inflammatory mediators are produced, including the anaphylatoxins, C5a, C3a, and the membrane attack complex (MAC). Under systemic inflammatory conditions, an overactive complement system may compromise the effectiveness of innate immunity. We review the detrimental effects that are caused by uncontrolled complement activation during sepsis.

Characterization of C3a and C5a receptors in rat cerebellar granule neurons during maturation. Neuroprotective effect of C5a against apoptotic cell death

There is now clear evidence that the Complement anaphylatoxin C3a and C5a receptors (C3aR and C5aR) are expressed in glial cells, notably in astrocytes and microglia. In contrast, very few data are available concerning the possible expression of these receptors in neurons. Here, we show that transient expression of C3aR and C5aR occurs in cerebellar granule neurons in vivo with a maximal density in 12-day-old rat, suggesting a role of these receptors during development of the cerebellum. Expression of C3aR and C5aR mRNAs and proteins was also observed in vitro in cultured cerebellar granule cells. Quantification of the mRNAs by real-time reverse transcription-PCR showed a peak of expression at day 2 in vitro (DIV 2); the C3aR and C5aR proteins were detected by Western blot analysis at DIV 4 and by flow cytometry and immunocytochemistry in differentiating neurons with a maximum density at DIV 4-9. Apoptosis of granule cells plays a crucial role for the harmonious development of the cerebellar cortex. We found that, in cultured granule neurons in which apoptosis was induced by serum deprivation and low potassium concentration, a C5aR agonist promoted cell survival and inhibited caspase-3 activation and DNA fragmentation. The neuroprotective effect of the C5aR agonist was associated with a marked inhibition of caspase-9 activity and partial restoration of mitochondrial integrity. Our results provide the first evidence that C3aR and C5aR are both expressed in cerebellar granule cells during development and that C5a, but not C3a, is a potent inhibitor of apoptotic cell death in cultured granule neurons.

A novel promoter polymorphism in the gene encoding complement component 5 receptor 1 on chromosome 19q13.3 is not associated with asthma and atopy in three independent populations

BACKGROUND

The inflammatory functions of complement component 5 (C5) are mediated by its receptor, C5R1, which is expressed on bronchial, epithelial, vascular endothelial and smooth muscle cells. A susceptibility locus for murine allergen-induced airway hyper-responsiveness was identified in a region syntenic to human chromosome 19q13, where linkage to asthma has been demonstrated and where the gene encoding C5R1 is localized.

OBJECTIVE

The aim of this study was to screen for novel polymorphisms in the C5R1 gene and to determine whether any identified polymorphisms are associated with asthma and/or atopy and whether they are functional.

METHODS

Single-nucleotide polymorphism (SNP) detection in the gene encoding C5R1 was performed by direct sequencing. Genotyping was performed in three populations characterized for asthma and/or atopy: (1) 823 German children from The Multicenter Allergy Study; (2) 146 individuals from Tangier Island, Virginia, a Caucasian isolate; and (3) asthma case-parent trios selected from 134 families (N=783) in Barbados. Functional studies were performed to evaluate differences between the wild-type and the variant alleles.

RESULTS

We identified a novel SNP in the promoter region of C5R1 at position -245 (T/C). Frequency of the -245C allele was similar in the German (31.5%) and Tangier Island (36.3%) populations, but higher in the Afro-Caribbean population (53.0%; P=0.0039 to <0.0001). We observed no significant associations between the -245 polymorphism and asthma or atopy phenotypes. Upon examination of the functional consequences of the -245T/C polymorphism, we did not observe any change in promoter activity.

CONCLUSION

This new marker may provide a valuable tool to assess the risk for C5a-associated disorders, but it does not appear to be associated with asthma and/or atopy.

Neutrophil C5a receptor and the outcome in a rat model of sepsis

Complement fragment 5a (C5a)-C5a receptor (C5aR) signaling plays an essential role in neutrophil innate immunity. Blockade of either the ligand or the receptor improves survival rates in experimental sepsis. In the current study, sepsis was induced in rats by cecal ligation/puncture. Early in sepsis C5aR content on neutrophils significantly dropped, reached the nadir at 24 h after onset of sepsis, and progressively elevated thereafter. Western-blot, RT-PCR, and confocal microscopy analyses revealed that the loss and re-expression of C5aR during sepsis might be due, at least in part, to the receptor internalization and reconstitution. The reduction and reconstitution of C5aR correlate with the loss and restoration of innate immune functions of blood neutrophils (chemotaxis and reactive oxygen species production), respectively. Quantitative measurements of C5aR on blood neutrophils are highly predictive of survival or death during sepsis. These data suggest that neutrophil C5aR content represents an essential component of an efficient defense system in sepsis and may serve as a prognostic marker for the outcome.

Innate immunity in viral encephalitis: role of C5

The complement system is a critical component of both the innate and acquired immune systems. It is important in host defense against viruses, bacteria, and fungi for opsonization and for lysis of pathogens. However, activated complement can also cause tissue damage. There is compelling evidence that complement factors are presented in the central nervous system (CNS). Complement activation (by any of the three pathways: classical, alternate, and lectin) can lead to inflammation and tissue damage, while at the same time may also restrict certain pathogens in the CNS. C5a is formed by proteolytic cleavage C5. C5a is considered the most potent proinflammatory mediator, often called an anaphylotoxin. In this communication, we examine the roles of C5 (C5a) in vesicular stomatitis virus (VSV)-induced encephalitis. We found that C5a is produced during VSV infection, but C5-deficient mice had similar pathology as their controls. We concluded that C5 is not a critical factor in mediating the host response during VSV encephalitis.

Protective effects of IL-6 blockade in sepsis are linked to reduced C5a receptor expression

IL-6 is known to be an important pro- and anti-inflammatory cytokine, which is up-regulated during sepsis. Our previous work has suggested a role for IL-6 in the up-regulation of C5aR in sepsis. We reported earlier that interception of C5a or C5aR results in improved outcomes in experimental sepsis. Using the cecal ligation/puncture (CLP) model in mice, we now demonstrate that treatment with anti-IL-6 Ab (anti-IL-6) results in significantly improved survival, dependent on the amount of Ab infused. CLP animals showed significantly increased binding of 125I-labeled anti-C5aR to organs when compared to either control mice at 0 h or CLP animals infused with normal rabbit 125I-labeled IgG. Binding of 125I-labeled anti-C5aR to lung, liver, kidney, and heart was significantly decreased in anti-IL-6-treated animals 6 h after CLP. RT-PCR experiments with mRNA isolated from various organs obtained 3, 6, and 12 h after CLP demonstrated increased C5aR mRNA expression during the onset of sepsis, which was greatly suppressed in CLP mice treated with anti-IL-6. These data suggest that IL-6 plays an important role in the increased expression of C5aR in lung, liver, kidney, and heart during the development of sepsis in mice and that interception of IL-6 leads to reduced expression of C5aR and improved survival.

Expression and function of C5a receptor in mouse microvascular endothelial cells

The complement-derived anaphylatoxin, C5a, is a potent phlogistic molecule that mediates its effects by binding to C5a receptor (C5aR; CD88). We now demonstrate specific binding of radiolabeled recombinant mouse C5a to mouse dermal microvascular endothelial cells (MDMEC) with a K(d50) of 3.6 nM and to approximately 15,000-20,000 receptors/cell. Recombinant mC5a competed effectively with binding of [(125)I]rmC5a to MDMEC. Enhanced binding of C5a occurred, as well as increased mRNA for C5aR, after in vitro exposure of MDMEC to LPS, IFN-gamma, or IL-6 in a time- and dose-dependent manner. By confocal microscopy, C5aR could be detected on surfaces of MDMEC using anti-C5aR Ab. In vitro expression of macrophage inflammatory protein-2 (MIP-2) and monocyte chemoattractant protein-1 (MCP-1) by MDMEC was also measured. Exposure of MDMEC to C5a or IL-6 did not result in changes in MIP-2 or MCP-1 production, but initial exposure of MDMEC to IL-6, followed by exposure to C5a, resulted in significantly enhanced production of MIP-2 and MCP-1 (but not TNF-alpha and MIP-1alpha). Although LPS or IFN-gamma alone induced some release of MCP-1 and MIP-2, pre-exposure of these monolayers to LPS or IFN-gamma, followed by addition of C5a, resulted in synergistic production of MIP-2 and MCP-1. Following i.v. infusion of LPS into mice, up-regulation of C5aR occurred in the capillary endothelium of mouse lung, as determined by immunostaining. These results support the hypothesis that C5aR expression on MDMEC and on the microvascular endothelium of lung can be up-regulated, suggesting that C5a in the co-presence of additional agonists may mediate pro-inflammatory effects of endothelial cells.

C5a receptor and thymocyte apoptosis in sepsis

In sepsis, apoptosis occurs in many different organs. The mediators responsible for induction of apoptosis are not clearly known, although there are some suggestions that C5a and the C5a receptor (C5aR) might be directly linked to apoptosis. In the cecal ligation/puncture (CLP) model of sepsis in rats, apoptosis occurs early in a variety of organs, especially in the thymus. We demonstrate that thymocytes from normal rats show specific, saturable, and high affinity binding of 125I-labeled recombinant rat C5a. C5a binding to thymocytes was significantly increased 3 h after CLP and also when thymocytes from normal rats were first incubated in vitro with lipopolysaccharide (LPS) or IL-6. The expression of C5aR mRNA in thymocytes was markedly increased 3, 6, and 12 h after CLP and increased similarly when normal thymocytes were first exposed to LPS or IL-6 in vitro. Thymocytes obtained 2 or 3 h after CLP and exposed in vitro to C5a, but not normal thymocytes, underwent increased apoptosis, as demonstrated by annexin-V binding, coinciding with increased activation of caspases 3, 6, and 8. These data provide the first direct evidence that in the early onset of sepsis, increased expression of C5aR occurs in thymocytes, which increases their susceptibility to C5a-induced apoptosis.