中和C5a过敏毒素的分子设计研究

目的 从蛋白质结构与功能的关系出发,探讨Ｃ５ａＲ与其配体Ｃ５ａ的结合位。方法 按分子设计理论,采用亲水性方案寻打Ｃ５ａ胞外区高亲水性区域,Ｆｍｏｃ方案人工合成Ｃ５ａ第９～３０位氨基酸基序（Ｐ２２肽）,经高压液相色谱纯化,毛细管电泳鉴定。结果合成多肽（Ｐ２２）的纯度为９５．１９％,每次缩合的平均效率为９９．７８％;能与ａｎｔｉ－Ｃ５ａＲ ＭｃＡｂ（Ｓ５／Ⅰ,Ｓｅｒｏｔｉｃ公司）有效地结合,酶联ＯＤ４     

Expression of the anaphylatoxin C5a receptor in non-myeloid cells

C5, a 74 amino acid peptide cleaved from the complement protein C5, represents the most potent anaphylatoxin and possesses inflammatory as well as immunomodulatory activities. In the past, expression of the receptor for the anaphylatoxin C5a (C5aR) has been thought to be restricted to cells of myeloid origin. However, recent evidence suggests that the C5aR is constitutively expressed in non-myeloid cells including epithelial, endothelial and smooth muscle cells in the human liver and lung. These findings are contrasted by results from our laboratory which demonstrated that in the normal human liver and lung C5aR expression is detectable exclusively in macrophages and macrophage-derived cells (Kupffer cells). Interestingly, we found evidence that C5aR expression may be inducible in epithelial cells as C5aR mRNA was observed in vivo in human keratinocytes of the inflamed but not of the normal skin. Herein we review the work of our laboratory and others on the expression of the C5aR in various human non-myeloid cells types. A better understanding of the expression patterns of this important anaphylatoxin receptor may provide new insights in the pathophysiological role of C5a in vivo.     

C5a promotes development of experimental lupus nephritis which can be blocked with a specific receptor antagonist

The MRL/lpr murine SLE model has widespread complement activation and deposition of complement fragments in affected tissues. The potent anaphylatoxin C5a has the potential to play a key role in the pathogenesis of lupus nephritis. We found that renal expression of C5aR mRNA and protein was significantly increased in MRL/lpr mice compared to control MRL/+ mice. To examine the role of C5a signaling through C5aR, a specific small molecule antagonist (a) of C5aR was administered continuously to MRL/lpr mice from 13 to 19 wks of age. Littermate controls were given vehicle alone. The progressive impairment in renal function exhibited in the control group was prevented by C5aRa treatment. Infiltration of neutrophils and macrophages into kidneys was significantly reduced in animals treated with C5aRa compared to controls. Furthermore, renal expression of IL-1beta and MIP-2 mRNA as well as the extent of apoptosis were significantly decreased with blockade of C5aR, indicating their dependence upon signals delivered through C5aR. Thus, pharmacological blockade of C5aR reduces disease manifestations in experimental lupus nephritis. These data support an important role for the C5a anaphylatoxin in lupus nephritis, and that blockade of C5aR represents a potentially viable treatment for human lupus nephritis.     

The Human Mast Cell Line HMC-1 Expresses C5a Receptors and Responds to C5a but not to C5a(desArg)

The expression of the receptor for the anaphylatoxin C5a (C5aR, CD88) on the human mast cell line HMC-1 was studied with four anti-C5aR monoclonal antibodies directed to the N-terminal domain of the receptor. All antibodies bound to the human mast cell line HMC-1. The binding could be blocked by recombinant C5a and by peptide EX-1 representing amino residues 1–31 on the N-terminal domain of the C5aR. In addition, FITC-labelled C5a bound to HMC-1, and this binding could be blocked by unlabelled C5a or C5aR antibodies. C5aR-specific mRNA was detected in HMC-1 cells by RT-PCR which confirmed the expression of the C5aR gene made by these cells. Lymphocyte-conditioned medium, interferon-γ or phorbol esters which have been shown to induce a down-regulation of C5aR on myeloid cells did not influence the expression of C5aR on HMC-1. C5a let to a transient mobilization of intracellular calcium in HMC-1 which could be inhibited by pre incubation of C5a with a C5a-specific antibody. In contrast to findings with granulocytes, HMC-1 did not respond to C5a(desArg), confirming previous findings with human skin mast cells. The findings show that (i) although HMC-1 differ from granulocytes in their responsiveness to C5a(desArg), they express similar C5aR and (ii) HMC-1 resemble skin mast cells in the expression and function of C5aR and may therefore serve as a model in future studies addressing the biology of this anaphylatoxin receptor on skin mast cells.     

Cloning and characterization of the guinea pig C5a anaphylatoxin receptor: interspecies diversity among the C5a receptors

The anaphylatoxin C5a receptor (C5aR, CD88 in man) plays a prominent role in mediating inflammatory and host defense processes. Direct evidence of C5aR involvement in host defense mechanisms was demonstrated recently using C5aR knockout mice. Mice deficient in C5aR were unable to clear intrapulmonary-instilled bacteria. The guinea pig system is perhaps unique for exhibiting cross-reactivity with human complement components and its high sensitivity to anaphylatoxins. Therefore, we cloned the guinea pig C5aR from a megakaryocyte cDNA library. The deduced amino acid sequence of guinea pig C5aR is 67% identical to human, 61.6% to dog, 60.2% to mouse and 63.6% to rat C5aR. Transient expression of guinea pig C5aR in COS-7 cells and stable expression on L cell fibroblasts were confirmed by FACS analysis. Competitive binding studies using [125I]C5a and stimulation of calcium mobilization by C5a proved that functional C5aR was expressed on these stably transfected L cells. The N-terminal extracellular region of guinea pig C5aR was five to seven residues shorter than the same region in C5aR from other species and sequence homology was limited to 11%. Other outer membrane loops were also poorly conserved (8-33%) when compared across five species. Transmembrane segments were highly conserved between these various species (46-86%). Guinea pig C5aR binds human C5a, therefore residues critical for C5a binding have been conserved between these species. Sequence comparison of C5aR from multiple species permits conserved elements of the ligand binding sites to be elucidated.      

Disruption of the C5a receptor gene increases resistance to acute Gram-negative bacteremia and endotoxic shock: opposing roles of C3a and C5a

The host response to intravascular, Gram-negative bacteria includes profound immunologic, hematologic and physiologic changes. Numerous host defense mechanisms are activated by Gram-negative bacteria, including the complement system. Activation of the complement system leads to cleavage of C5 with subsequent generation of the C5a anaphylatoxin peptide. C5a mediates potent, proinflammatory activities by binding to the C5a receptor (C5aR, CD88). In this study, we report the targeted disruption of the murine C5aR gene (C5aR-/- mice) and define the role of the C5aR in a model of Gram-negative bacteremia. Following an intravenous infusion of heat-killed Escherichia coli, the C5aR-/- mice were completely protected from the mortality suffered by their wild-type littermates (P<0.001). The C5aR-/- mice were also significantly (P=0.008) more resistant to mortality following an intravenous infusion of purified E. coli endotoxin compared to the wild-type littermates. In addition, the C5aR-/- mice were resistant to the thrombocytopenia and hemoconcentration observed in wild-type animals. Lethality in the wild-type mice was reversed by pre-treatment with either the histamine antagonist diphenhydramine or triprolidine. The wild-type littermates were also rescued following pre-treatment with the basophil and mast cell-stabilizing agent - cromolyn sodium. Collectively, these data demonstrate that not only is the absence of the C5aR protective in E. coli bacteremia, but that C5aR-dependent histamine release plays a major role in shock induced by Gram-negative septicemia. Moreover, they provide additional in vivo evidence that C3a and C5a have divergent biological functions in Gram-negative bacteremia and shock.     

Functional identification of the stable transfection C5aR cell line Molt-4

The complement C5 anaphylatoxin receptor is a member of the seven transmembrane-spanning G protein-coupled receptor superfamily that signals through Gcxi and Gtz16. C5aR is mostly expressed on neutrophils, macrophages and endothelial cells. C5a and C5aR interaction plays an important role in numerous biological effects such as in vivo cytokine storm which results in inflammatory damage. Considering the limitation of collection of human peripheral blood neutrophils and their short half life, the stably transfected cell line for studying the biological effects of C5aR is needed. In this study, we transfected C5aR gene into Molt-4 cell line and examined the function of ectopic C5aR. Our results showed stable expression of the C5aR in Molt-4 cell line and their interaction with human C5a induced ERKI/2 phosphorylation, Ca＋＋ influx. This stable transfected cell line may provide a useful tool for studying signal pathways related to C5a and C5aR interplay and antibody development specific for C5aR. Cellular ＆ Molecular Immunology.     

人C5a过敏毒素拮抗剂的分子设计及其活性检测

目的：从蛋白质结构与功能的关系出发,探讨C5aR与其配体的C5a的结合位。方法：按分子设计理论,采用亲水性方案寻找C5aR胞外区高亲水性区域,Fmoc方案人工合成C5aR第9－30位氨基酸基序（P22肽）,经高效液相色谱纯化,毛细管电泳鉴定。结果：合成多肽（P22）的纯度为95.19％,每次缩合的平均效率为99.78％;能与anti-C5aR McAb（S5／1,Serotic公司）有效地结合,酶联OD490极显著高于同浓度对照多肽C20;还可被配体rh-C5a(10ng/ml)明显抑制而降低其酶联OD值（P＜0.05）,此外10ng/mlP22还可抑制rhC5a所致U937细胞胞浆Ca^2 升高（P＜0.01）。结论：从C5a分子中寻找C5a结合位基序,可拮抗C5a的过敏毒素作用,为治疗C5a及其相关疾病进行新型的药物设计。     

补体裂解产物C5a的研究进展

C5a由C5裂解而来,由74个氨基酸构成,是一种潜在的前炎症因子。C5a在羧肽酶的作用下迅速变成C5adesArg,通过G蛋白受体经典的通路与C5aR结合后在天然免疫中和适应性免疫中均发挥着重要的作用,非G蛋白受体通路目前的研究还不明朗。研究发现,C5a是许多自身免疫性疾病的重要病原起始物,所以抑制C5a的释放在未来的治疗中是一个很好的策略和方向。     

The C terminus of the human C5a receptor (CD88) is required for normal ligand-dependent receptor internalization

The biological effects of the potent inflammatory mediator C5a, a complement split product, on human neutrophils and monocytes are limited by the rapid internalization of its specific receptor (C5aR, CD88). The C terminus of the C5aR is phosphorylated after stimulation with C5a of phorbol ester, and this phosphorylation might lead to receptor internalization. In this context, we have studied the effects on C5aR internalization of C5a, phorbol 12-myristate 13-acetate (PMA), the protein kinase inhibitor staurosporine, and pertussis toxin on rat basophilic RBL.2H3 cells stably transfected with the human wild-type or mutant C5aR. C5aR mutants lacked either part of the cytosolic C terminus, including suggested major phosphorylation sites, or a putative phosphorylation motif for protein kinase C in the third cytosolic loop. Additionally, agonist-induced internalization was analyzed on HEK293 cells co-transfected with C5aR and the pertussis toxin-resistant G protein alpha subunit, Gα16. Staurosporine-sensitive agonist-dependent C5aR internalization could be detected, suggesting that C5aR phosphorylation, most likely of the C terminus, participates in this type of internalization. In contrast, PMA-induced C5aR internalization seems to be independent of putative phosphorylation sites in either the truncated section of the C terminus or the third cytosolic loop. The phorbol ester-induced C5aR internalization may, therefore, be caused by an indirect and less specific effect of protein kinase C on the internalization machinery. Manipulation of the pertussis toxin-sensitive or -resistant G protein-dependent signal transduction had no effect on ligand-induced internalization.     

Antagonistic peptides against human anaphylatoxin C5a.

Multivalent synthetic peptides derived from C5a were prepared in order to examine their effects on the C5a receptor (C5aR). Multiple antigen peptide (MAP) of the C5a C-terminal region (MAP61-74) bound to cells expressing C5aR with high affinity. On the other hand, N-terminal peptides (MAP3-16 and MAP12-26) and one with a sequence from the mid-portion of C5a (MAP37-53) did not bind to the cells. In addition, MAP61-74 inhibited Ca2+ mobilization and release of beta-hexosaminidase by C5a from dibutyryl cAMP-activated U937 cells. This Ca2+ mobilization was also inhibited by MAP12-26 and Mono61-74, the monomeric C-terminal peptide. Taken together, these data indicate that C5a binds to the C5aR via its C-terminal region. Furthermore, MAP61-74, a 14mer peptide that has additional amino acids at the N-terminal compared with the C-terminal octapaptide, can bind to C5aR and can be considered an antagonist of C5a which may prove useful as an agent for controlling the allergic response caused by complement activation.     

Analysis of the C5a anaphylatoxin core domain using a C5a phage library selected on differentiated U937 cells.

The human anaphylatoxin C5a is a 74-amino acid comprising polypeptide with a plethora of biological functions. Site directed mutagenesis studies suggest that several residues within the core and the C-terminus mediate the interaction with the C5a receptor. However, the contribution of particular core residues to receptor binding remained to be clarified. By means of the phage display technique, the loop between positions 35-40 was randomly mutated and the resulting C5a[35-40] fusion phage library affinity selected on C5a receptor expressing U937 cells. After five rounds of affinity enrichment, residues Arg37 and Arg40 were preferably selected. Enrichment was as high as 100% for Arg37 and 79% for Arg40. No significant enrichment of consensus residues could be obtained for positions 35, 36, 38 and 39. The core mutant C5a[A35E36R37A38S39R40], in which only Arg37/40 and Ala38 are of the native C5a sequence, was as potent as native C5a in both receptor binding and enzyme release examined on U937 cells. In contrast, replacement of Arg40 as in the mutant C5a[Q35E36R37I38L39N40] resulted in a 10-fold decrease in both binding and functional activities. Thus, selected out of a multiplicity of possibilities by the natural binding partner, Arg37 as well as Arg40 appear to be anchor residues in binding to the C5a receptor.     

Complement gene expression is regulated by pro-inflammatory cytokines and the anaphylatoxin C3a in human tenocytes

Interplay between complement factors, regulatory proteins, anaphylatoxins and cytokines could be involved in tendon healing and scar formation. The expression and regulation of complement factors by cytokines or anaphylatoxins are completely unclear in tendon.Hence, the gene expression of the anaphylatoxin receptors C3aR, C5aR and cytoprotective complement regulatory proteins (CRPs) was analysed in human tendon, cultured primary tenocytes and to directly compare the general expression level, additionally in human leukocytes. Time-dependent regulation of complement by cytokines and the anaphylatoxin C3a was assessed in cultured tenocytes.Gene expression of the anaphylatoxin receptors C3aR, C5aR and the CRPs CD46, CD55 and CD59 was detected in tendon, cultured tenocytes and leukocytes, whereas CD35 could only be found in tendon and leukocytes. Compared with cultured tenocytes, complement expression was higher in tendon and compared with leukocytes C3aR, C5aR, CD35 and CD55, but not CD46 and CD59 gene expression levels were lower in tendon. C3aR mRNA was up-regulated by both TNFα and C3a in cultured tenocytes in a time-dependent manner whereby C5aR gene expression was only induced by C3a. IL-6 or C3a impaired the CRP gene expression. C3a stimulation lead to an up-regulation of TNFα and IL-1β mRNA in tenocytes. Degenerated tendons revealed an increased C5aR and a reduced CD55 expression.The expression profile of the investigated complement components in tendon and cultured tenocytes clearly differed from that of leukocytes. Tenocytes respond to the complement split fragment C3a with CRP suppression and enhanced pro-inflammatory cytokine gene expression suggesting their sensitivity to complement activation.     

A key role of C5a/C5aR activation for the development of sepsis

In recent studies, evidence has been provided for complement activation early during the onset of experimental sepsis. Excessive production of the anaphylatoxin C5a thereby appears to elicit various harmful effects. Blockade of C5a or C5a receptor (C5aR) at the start of experimental sepsis has been demonstrated to greatly improve survival in rodents. There is evidence that C5a, during the onset of sepsis, enhances the production of various proinflammatory mediators in different cell types. Besides its known, other proinflammatory effects, recent work suggested an inhibitory role of C5a for innate-immune functions of phagocytic cells (phagocytosis, reactive oxygen species production, chemotaxis) during experimental sepsis. This review article provides an overview of the important role of C5a/C5aR activation for the onset and development of sepsis.     

TLR activation enhances C5a-induced pro-inflammatory responses by negatively modulating the second C5a receptor, C5L2

TLR and complement activation ensures efficient clearance of infection. Previous studies documented synergism between TLRs and the receptor for the pro-inflammatory complement peptide C5a (C5aR/CD88), and regulation of TLR-induced pro-inflammatory responses by C5aR, suggesting crosstalk between TLRs and C5aR. However, it is unclear whether and how TLRs modulate C5a-induced pro-inflammatory responses. We demonstrate a marked positive modulatory effect of TLR activation on cell sensitivity to C5a in vitro and ex vivo and identify an underlying mechanistic target. Pre-exposure of PBMCs and whole blood to diverse TLR ligands or bacteria enhanced C5a-induced pro-inflammatory responses. This effect was not observed in TLR4 signalling-deficient mice. TLR-induced hypersensitivity to C5a did not result from C5aR upregulation or modulation of C5a-induced Ca(2+) mobilization. Rather, TLRs targeted another C5a receptor, C5L2 (acting as a negative modulator of C5aR), by reducing C5L2 activity. TLR-induced hypersensitivity to C5a was mimicked by blocking C5L2 and was not observed in C5L2KO mice. Furthermore, TLR activation inhibited C5L2 expression upon C5a stimulation. These findings identify a novel pathway of crosstalk within the innate immune system that amplifies innate host defense at the TLR-complement interface. Unravelling the mutually regulated activities of TLRs and complement may reveal new therapeutic avenues to control inflammation.     

Role of complement anaphylatoxin receptors (C3aR, C5aR) in the development of the rat cerebellum

There is now strong evidence for non-immune or inflammatory functions of complement, notably in the central nervous system. In particular, it has been recently reported that the anaphylatoxin receptors C3aR and C5aR are transiently expressed in the cerebellar cortex of newborn rat, suggesting that anaphylatoxins are involved in the histogenesis of the cerebellum. In the present study, we have investigated the effects of C3aR and C5aR agonists and antagonists on the development of the cerebellum of 11-12-day-old rats in vivo and in vitro. Sub-dural injection of C3aR and C5aR agonists at the surface of the cerebellum transiently modified the thickness of the cortical layers. The C5aR agonist provoked an enlargement of the external granule cell layer (EGL) that was due to increased proliferation of immature granule neurons. Conversely, the C3aR agonist decreased the thickness of the EGL and increased the thickness of the internal granule cell layer (IGL), suggesting that C3a accelerates the migration process of granule cells from the EGL to the IGL. Video-microscopy examination of cultured granule neurons confirmed the role of C3aR in cell motility. These results provide clear evidence for the involvement of anaphylatoxin receptors in the histogenesis of the cerebellar cortex.     

Deletion of both the C3a and C5a receptors fails to protect against experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is an autoimmune disease in which inflammation, leukocyte infiltration, and ultimately, demyelination occur as a result of innate and adaptive immune-mediated mechanisms. The pathophysiological role of the complement system, a major component of innate immunity, in the development and progression of experimental autoimmune encephalomyelitis (EAE), the animal model for MS has been extensively examined. Previous studies from our lab have shown that the complement receptor for the anaphylatoxin C3a, but not for C5a plays an important role in EAE. Based on the important contributions of the complement anaphylatoxin receptors to other inflammatory conditions in the CNS, we reasoned that deletion of both receptors may reveal underlying interactions between them that are important to EAE pathology. We performed EAE in C3aR/C5aR double knockout mice (C3aR/C5aR^−/−) and observed delayed onset of disease but no attenuation of disease severity compared to wild type mice. Interestingly there was trend toward greater infiltration of CD4⁺, but not CD8⁺ T cells, in C3aR/C5aR^−/− mice with EAE, suggesting altered trafficking of these cells. Antigen-specific T cells isolated from C3aR/C5aR^−/− mice during acute EAE produced elevated levels of TNF-α, but markedly reduced levels of IFN-γ and IL-12 compared to wild type mice. It remains unclear how the changes in these disease parameters contribute to the loss of the protective effect seen in C3aR^−/− mice, however our data indicate a level of cross-modulation between the C3aR and C5aR during EAE.     

Differential cytokine expression of human retinal pigment epithelial cells in response to stimulation by C5a

Human retinal pigment epithelial (RPE) cells form part of the blood-retina barrier where they potentially can regulate leucocyte function. RPE cells are known to secrete several cytokines in response to stimulation by other cytokines. Anaphylatoxin C5a, a potent inflammatory mediator produced during complement activation, binds to G-protein coupled C5a receptors (C5aR) on monocytes/macrophages and releases various cytokines from the cells. We previously reported that the human RPE cell line ARPE-19 possesses C5aR and expresses IL-8 mRNA in response to C5a stimulation. In this study, we used a primary human RPE cell line (RPE43) and found that C5a induces increased expression of IL-1beta, IL-6, MCP-1 and GM-CSF mRNAs as well as IL-8 mRNA. ARPE-19 cells showed similar increases in the same cytokines. Interestingly, the kinetics of expression of the various cytokines differed. These results provide further evidence that C5a stimulation of RPE cells may play a role in regulating leucocyte function during ocular inflammation in which there is complement activation.