HIV-1 induced generation of C5a attracts immature dendriticcells and promotes infection of autologous T cells

For the recruitment of dendritic cells (DC) to the site of infection, DC express several sensors for danger signals, such as receptors for C5a. This anaphylatoxin is generated upon complement activation. As HIV-1 triggers the complement cascade, we determined whether C5a is generated by the virus and tested the functional activity of C5a in migration and infection assays. The immature (i)DC responded in migration assays to recombinant C5a and native C5a, which was generated in situ upon activation of the complement system by HIV-1. In combined migration and infection assays, a C5a-dependent enhancement of HIV-1 infection in DC-T cell cocultures was observed. These results indicate that HIV induces generation of C5a and thereby attracts iDC, which in turn promote the productive infection of autologous primary T cells.     

Expression of complement C3, C5, C3aR and C5aR1 genes in resting and activated CD4+ T cells

Complement activation is traditionally thought to occur in the extracellular space. However, it has been suggested that complement proteins are activated and function at additional locations. T cells contain intracellular stores of C3 and C5 that can be cleaved into C3a and C5a and bind to intracellular receptors, which have been shown to be of vital importance for the differentiation and function of these cells. However, whether the origin of the complement proteins located within T cells is derived from endogenous produced complement or from an uptake dependent mechanism is unknown.The presence of intracellular C3 in T cells from normal donors was investigated by fluorescence microscopy and flow cytometry. Moreover, mRNA expression levels of several genes encoding for complement proteins with primary focus on C3, C3aR, C5 and C5aR1 during resting state and upon activation of CD4⁺ T cells were investigated by a quantitative PCR technique. Furthermore, the gene expression level was evaluated at different time points.We confirmed the presence of intracellular C3 protein in normal T-cells. However, we could not see any increase in mRNA levels using any activation strategy tested. On the contrary, we observed a slight increase in C3 and C5aR1 mRNA only in the non-activated T-cells compared to the activated T cells, and a decrease in the activated T-cells at different incubation time points.Our results show that there is a baseline intracellular expression of the complement C3, C5, C3aR and C5aR1 genes in normal CD4⁺ T cells, but that expression is not increased during T-cell activation, but rather down regulated. Thus, the pool of intracellular complement in CD4⁺ T cells may either be due to accumulated complement due low-grade expression or arise from the circulation from an uptake dependent mechanism, but these possibilities are not mutually exclusive.     

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.     

Mast cells and complement system: Ancient interactions between components of innate immunity

The emergence and evolution of the complement system and mast cells (MCs) can be traced back to sea urchins and the ascidian Styela plicata, respectively. Acting as a cascade of enzymatic reactions, complement is activated through the classical (CP), the alternative (AP), and the lectin pathway (LP) based on the recognized molecules. The system's main biological functions include lysis, opsonization, and recruitment of phagocytes. MCs, beyond their classic role as master cells of allergic reactions, play a role in other settings, as well. Thus, MCs are considered as extrahepatic producers of complement proteins. They express various complement receptors, including those for C3a and C5a. C3a and C5a not only activate the C3aR and C5aR expressing MCs but also act as chemoattractants for MCs derived from different anatomic sites, such as from the bone marrow, human umbilical cord blood, or skin in vitro. Cross talk between MCs and complement is facilitated by the production of complement proteins by MCs and their activation by the MC tryptase. The coordinated activity between MCs and the complement system plays a key role, for example, in a number of allergic, cutaneous, and vascular diseases. At a molecular level, MCs and complement system interactions are based on the production of several complement zymogens by MCs and their activation by MC-released proteases. Additionally, at a cellular level, MCs act as potent effector cells of complement activation by expressing receptors for C3a and C5a through which their chemoattraction and activation are mediated by anaphylatoxins in a paracrine and autocrine fashion.     

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。     

肾癌荷瘤小鼠组织中补体 C5a / C5aR 通路活化与肿瘤高凝状态 的关系及其机制研究

目的 研究肾癌荷瘤小鼠组织中补体 C5a / C5aR 通路活化与肿瘤高凝状态的关系及其机制。 方法 收集 2016 年 7 月至 2019 年 10 月于唐山市人民医院收治的 32 例经病理学活检确诊的肾癌患者的癌组织和 癌旁组织。 采用 Western 印迹法检测组织中 C5a / C5aR 通路活化相关蛋白表达情况。 于 BALB/ c-nu / nu 裸鼠 前肢腋窝皮下接种人 ACHN 肾癌细胞株, 接种成功后随机分为干预组、 模型组, 其中干预组尾静脉注射 C5a 联合 C5aR 阻断剂 (C5aR antagonist, C5aRA), 模型组等体积注射生理盐水, 另取 6 只空白小鼠作为对 照组。 观察小鼠肿瘤生长情况, 检测高凝状态相关指标水平。 脱颈处死小鼠, 收集癌组织, 采用 Western 印迹法检测组织中 C5a / C5aR 通路活化相关蛋白表达情况。 结果 两组小鼠肿瘤体积于荷瘤 6 ~ 21 d 差异具 有统计学意义, 且干预组的肿瘤体积明显低于模型组 ( P< 0. 05)。 3 组小鼠于荷瘤第 1 天的血清 D-D、 vWF、 TF 水平差异无统计学意义, 荷瘤第 14 天、 第 21 天干预组、 模型组血清凝血指标水平明显增加, 其 中干预组均明显低于模型组 (P< 0. 05)。 干预组癌组织中 C5aR、 C5b-9、 FGL2、 P38、 p-P38 的表达水平均 明显低于模型组 (P< 0. 05)。 肾癌患者癌组织中 C5aR、 C5b-9、 FGL2、 P38、 p-P38 的表达水平均明显高于 癌旁组织 (P< 0. 05)。 32 例肾癌患者中高凝状态患者 14 例, 非高凝状态患者 18 例, 高凝状态组癌组织中 C5aR、 C5b-9、 FGL2、 P38、 p-P38 的表达水平均明显高于非高凝状态组 (P< 0. 05)。 结论 补体 C5a / C5aR 通路可通过调控 FGL2 的表达而诱导肾癌的高凝状态, 进而参与肾癌的发病过程。     

HIV-1-induced neuronal injury in the developing brain

HIV-1 infection of the nervous system causes neuronal injury and death, resulting in cognitive, motor, and behavioral dysfunction in both adults and children. In infants a characteristic feature of HIV-1 infection is impaired brain growth resulting in secondary microcephaly with onset between 2 and 4 months of age. This post-natal period of brain development is particularly vulnerable to excitotoxic neuronal injury due to the active synaptogenesis and pruning that takes place at this age associated with over-expression of excitatory amino acid (EAA) receptors. HIV-1 infection of brain microglia and perivascular macrophages results in chronic inflammation manifest pathologically as diffuse microglial activation and reactive astrogliosis. Several inflammatory products of activated microglia, including tumor necrosis factor alpha (TNF-alpha) and platelet-activating factor (PAF) have been shown to act as neuronal toxins. This toxic effect can be antagonized by blocking NMDA (or AMPA) glutamate receptors, suggesting that (weak) excitotoxicity leads to oxidative stress, neuronal injury, and apoptosis. HIV-1 infection and chronic inflammation may also contribute disruption of the blood-brain barrier and could result in further entry into the CNS of toxic viral or cellular products or additional HIV-1-infected cells. We hypothesize that prolonged microglial activation during HIV-1 infection underlies the neuronal injury and impaired brain growth in affected infants. Further investigation of the interaction between HIV-1-infected/activated microglia and developing neurons seems warranted. The current understanding of HIV neuropathogenesis implies that therapeutic strategies should target the sustained immune activation in microglia, attempt to repair the integrity of the blood-brain barrier, and provide "neuroprotection" from excitotoxic neuronal injury.     

Targeting C3a/C5a receptors inhibits human mesangial cell proliferation and alleviates immunoglobulin A nephropathy in mice

Complement activation has a deep pathogenic influence in immunoglobulin (Ig)A nephropathy (IgAN). C3a and C5a, small cleavage fragments generated by complement activation, are key mediators of inflammation. The fragments exert broad proinflammatory effects by binding to specific receptors (C3aR and C5aR, respectively). However, no studies thus far have investigated the effects of C3a, C5a and their receptors on IgAN. We observed that C3aR and C5aR antagonists repressed IgA‐induced cell proliferation and interleukin (IL)‐6 and monocyte chemotactic protein 1 (MCP‐1) production in cultured human mesangial cells (HMCs). Furthermore, an IgAN mouse model induced by Sendai virus infection was employed to investigate the effects of C3aR and C5aR on IgAN in vivo for the first time. Wild‐type (WT) and several knock‐out mouse strains (C3aR^–/– or C5aR^–/–) were immunized intranasally with increasing doses of inactivated virus for 14 weeks and were subjected to two intravenous viral challenges during the time‐period indicated. In the Sendai virus‐induced IgAN model, C3aR/C5aR‐deficient mice had significantly reduced proteinuria, lower renal IgA and C3 deposition, less histological damage and reduced mesangial proliferation compared with WT mice. Both C3aR deficiency and C5aR deficiency, especially C3aR deficiency, inhibited renal tumour necrosis factor (TNF)‐α, transforming growth factor (TGF)‐β, IL‐1β, IL‐6 and MCP‐1 expression significantly. However, C3aR/C5aR‐deficient and WT mice with IgAN did not differ with respect to their blood urea nitrogen (BUN) and serum creatinine levels. Our findings provide further support for the idea that C3aR and C5aR are crucially important in IgAN, and suggest that pharmaceutically targeting C3aR/C5aR may hold promise for the treatment of IgAN.     

Complement activation in diabetic ketoacidosis brains

The metabolic crisis of diabetic ketoacidosis (DKA) and its treatment can result in the life-threatening complication of clinical brain edema. However, there is limited information available regarding either the pathophysiology or histology of this acute complication. It has been reported that DKA and its treatment are associated with a systemic inflammatory response involving the activation of the complement cascade with increases of SC5b-9 serum level. We studied the brains of two patients, both of whom died as the result of DKA-related brain edema, for the presence of C5b-9, C1q and the expression of the CD59. Apoptosis was also evaluated by the TUNEL method. All regions of the brain demonstrated varying degrees of C5b-9 deposits on neurons, oligodendrocytes and blood vessels. C5b-9 was co-localized with C1q, suggesting the activation of classical pathway. No expression of CD59 was found on neurons, oligodendrocytes or blood vessels in DKA brain, but this complement inhibitor was present on these cells in the normal brain. Rarely, C5b-9 was co-localized with apoptotic neurons and OLG. Our data demonstrate that the metabolic crisis of DKA results in a loss of CD59 expression and assembly of C5b-9 on neurons and oligodendrocytes, suggesting that complement activation and C5b-9 may play a role in the pathophysiology of the brain edema of DKA.     

Expression of the receptor for complement C5a (CD88) is up-regulated on reactive astrocytes, microglia, and endothelial cells in the inflamed human central nervous system.

C5a receptor (C5aR, CD88) is a receptor originally described on neutrophils and monocyte-macrophages but recently found on hepatocytes, epithelial cells, endothelial cells, and tissue mast cells. We recently reported that human fetal astrocytes expressed a functional C5aR in vitro. Here we examine C5aR expression in adult brain cultures by immunostaining with six different anti-C5aRs and show that C5aR is expressed constitutively by astrocytes, microglia, and fibroblast-like cells but not by oligodendrocytes. In fetal brain cultures we confirmed that astrocytes constitutively expressed C5aR and demonstrated that fetal microglia and fibroblast-like cells but not oligodendrocytes and neurones expressed C5aR. Incubation with inflammatory cytokines (interferon gamma, interleukin-1, and tumor necrosis factor alpha) or phorbol ester failed to induce or up-regulate C5aR expression on fetal or adult brain cells. Immunohistochemistry was performed to determine the expression and distribution of C5aR in the normal and inflamed brain. In the normal brain C5aR was minimally expressed, whereas in inflamed brains from a variety of pathologies, C5aR expression was greatly up-regulated on reactive astrocytes and microglia and to a lesser extent on endothelial cells. We propose that expression of C5aR is a marker of central nervous system inflammation, and that C5aR expression on brain cells in inflammation plays an important role in cell activation and recruitment (gliosis).     

Treatment with anti-C5aR mAb leads to early-onset clinical and mechanistic effects in the murine delayed-type hypersensitivity arthritis model

Abstract

Blockade of the complement cascade at the C5a/C5a receptor (C5aR)-axis is believed to be an attractive treatment avenue in rheumatoid arthritis (RA). However, the effects of such interventions during the early phases of arthritis remain to be clarified. In this study we use the murine delayed-type hypersensitivity arthritis (DTHA) model to study the very early effects of a blocking, non-depleting anti-C5aR mAb on joint inflammation with treatment synchronised with disease onset, an approach not previously described. The DTHA model is a single-paw inflammatory arthritis model characterised by synchronised and rapid disease onset driven by T-cells, immune complexes and neutrophils. We show that a reduction in paw swelling, bone erosion, cartilage destruction, synovitis and new bone formation is apparent as little as 60?h after administration of a single dose of a blocking, non-depleting anti-mouse C5aR mAb. Importantly, infiltration of neutrophils into the joint and synovium is also reduced following a single dose, demonstrating that C5aR signalling during the early stage of arthritis regulates neutrophil infiltration and activation. Furthermore, the number of T-cells in circulation and in the draining popliteal lymph node is also reduced following a single dose of anti-C5aR, suggesting that modulation of the C5a/C5aR axis results in effects on the T cell compartment in inflammatory arthritis. In summary, these data demonstrate that blockade of C5aR leads to rapid and significant effects on arthritic disease development in a DTHA model strengthening the rationale of C5aR-blockade as a treatment strategy for RA, especially during the early stages of arthritis flare.     

Microglial and Astrocyte Chemokines Regulate Monocyte Migration through the Blood-Brain Barrier in Human Immunodeficiency Virus-1 Encephalitis

The numbers of immune-activated brain mononuclear phagocytes (MPs) affect the progression of human immunodeficiency virus (HIV)-1-associated dementia (HAD). Such MPs originate, in measure, from a pool of circulating monocytes. To address the mechanism(s) for monocyte penetration across the blood-brain barrier (BBB), we performed cross-validating laboratory, animal model, and human brain tissue investigations into HAD pathogenesis. First, an artificial BBB was constructed in which human brain microvascular endothelial and glial cells-astrocytes, microglia, and/or monocyte-derived macrophages (MDM)-were placed on opposite sides of a matrix-coated porous membrane. Second, a SCID mouse model of HIV-1 encephalitis (HIVE) was used to determine in vivo monocyte blood-to-brain migration. Third, immunohistochemical analyses of human HIVE tissue defined the relationships between astrogliosis, activation of microglia, virus infection, monocyte brain infiltration, and beta-chemokine expression. The results, taken together, showed that HIV-1-infected microglia increased monocyte migration through an artificial BBB 2 to 3.5 times more than replicate numbers of MDM. In the HIVE SCID mice, a marked accumulation of murine MDM was found in areas surrounding virus-infected human microglia but not MDM. For human HIVE, microglial activation and virus infection correlated with astrogliosis, monocyte transendothelial migration, and beta-chemokine expression. Pure cultures of virus-infected and activated microglia or astrocytes exposed to microglial conditioned media produced significant quantities of beta-chemokines. We conclude that microglial activation alone and/or through its interactions with astrocytes induces beta-chemokine-mediated monocyte migration in HAD.     

Crosstalk Between Components of the Blood Brain Barrier and Cells of the CNS in Microglial Activation in AIDS

During the progression of AIDS, a majority of patients develop cognitive disorders such as HIV encephalitis (HIVE) and AIDS dementia complex (ADC), which correlate closely with macrophage infiltration into the brain and microglial activation. Microglial activation occurs in response to infection, inflammation and neurological disorders including HIVE, Alzheimer's disease, Parkinson's disease and multiple sclerosis. Microglia can be activated by immunoreactive cells independent of, but enhanced by HIV infection, from at least two routes. Activation may occur from signals originating from activated monocytes and lymphocytes in the blood stream, which initiate a cascade of stimuli that ultimately reach microglia in the brain or from activated macrophages/microglia/astrocytes within the brain. Effects of microglial activation stemming from both systemic and CNS HIV infection act together to commence signaling feedback, leading to HIVE and increased neurodegeneration. Most recent data indicate that in AIDS patients, microglial activation in the brain with subsequent release of excitotoxins, cytokines and chemokines leads to neurodegeneration and cognitive impairment. Since the presence of HIV in the brain results from migration of infected monocytes and lymphocytes across the vascular boundary, the development of novel therapies aimed at protecting the integrity of the blood brain barrier (BBB) upon systemic HIV infection is critical for controlling CNS infection.     

New concepts on the therapeutic control of complement anaphylatoxin receptors

The complement system is a pivotal driver of innate immunity, coordinating the host response to protect against pathogens. At the heart of the complement response lie the active fragments, C3a and C5a, acting through their specific receptors, C3aR, C5aR1, and C5aR2, to direct the cellular response to inflammation. Their potent function however, places them at risk of damaging the host, with aberrant C3a and C5a signaling activity linked to a wide range of disorders of inflammatory, autoimmune, and neurodegenerative etiologies. As such, the therapeutic control of these receptors represents an attractive drug target, though, the realization of this clinical potential remains limited. With the success of eculizumab, and the progression of a number of novel C5a-C5aR1 targeted drugs to phase II and III clinical trials, there is great promise for complement therapeutics in future clinical practice. In contrast, the toolbox of drugs available to modulate C3aR and C5aR2 signaling remains limited, however, the emergence of new selective ligands and molecular tools, and an increased understanding of the function of these receptors in disease, has highlighted their unique potential for clinical applications. This review provides an update on the growing arsenal of drugs now available to target C5, and C5a and C3a receptor signaling, and discusses their utility in both clinical and pre-clinical development.     

Activation of human microglial cells by HIV-1 gp41 and Tat proteins

The viral proteins, Tat (HIV-1 nuclear protein) and gp41 (HIV-1 coat protein), detected in the brains of HIV-1-infected patients have been shown to be neurotoxic. We investigated the effects of HIV-1 Tat and gp41 proteins on cytokine, chemokine, and superoxide anion (O(-)(2)) production by microglia, the resident macrophages of the brain. Tat and gp41 dose-dependently stimulated cytokine and chemokine production by microglia. Peak production of these cytokines and chemokines differed in microglial cells treated with gp41 and Tat. Expression of cytokine and chemokine mRNA was also stimulated in gp41- and Tat-treated microglia. Neither gp41 nor Tat alone stimulated O(-)(2) production by microglia. Treatment of microglial cells with Tat but not with gp41 evoked an increase in intracellular Ca(2+). The results of this study suggest that HIV-1 Tat and gp41 proteins impact several key functions of microglial cells which could contribute to the neuropathogenesis of HIV-1.     

Human immunodeficiency virus encephalitis in SCID mice.

The human immunodeficiency virus (HIV) is neuroinvasive and commonly causes cognitive and motor deficits during the later stages of viral infection. (referred to as HIV dementia). The mechanism(s) for disease revolves around secretory products produced from immune-activated brain macrophages/microglia. Recently, we developed an animal model system for HIV dementia that contains xenografts of HIV-1-infected cells inoculated into brains of mice with severe combined immunodeficiency (SCID). This animal system was used to quantitatively evaluate HIV-induced neuropathology. Xenografts of HIV-1-infected human monocytes (placed into the putamen and cortex of SCID mice) remained viable for 5 weeks. HIV-1 p24 antigen expression in mouse brain was persistent. Progressive inflammatory responses (including astrogliosis and cytokine production), which began at 3 days, peaked at day 12. The range of astrocyte proliferative reactions exceeded the inoculation site by > 1000 microns. Brains with virus-infected monocytes showed a > or = 1.6-fold increase in glial fibrillary acidic protein (staining distribution and intensity) as compared with similarly inoculated brains with uninfected control monocytes. These findings paralleled the accumulation and activation of murine microglia (increased branching of cell processes, formation of microglial nodules, interleukin (IL)-1 beta and IL-6 expression). An inflammatory reaction of human monocytes (as defined by HLA-DR, IL-1 beta, IL-6, and tumor necrosis factor-alpha expression) and neuronal injury (apoptosis) also developed after virus-infected monocyte xenograft placement into mouse brain tissue. These data, taken together, demonstrate that this SCID mouse model of HIV-1 neuropathogenesis can reproduce key aspects of disease (virus-infected macrophages, astrocytosis, microglial activation, and neuronal damage). This model may serve as an important means for therapeutic development directed toward improving mental function in HIV-infected subjects with cognitive and motor dysfunction.     

Increased activity of the complement system in the liver of patients with alcoholic hepatitis

Inflammation has been suggested as a mechanism underlying the development of alcoholic hepatitis (AH). The activation of the complement system plays an important role in inflammation. Although it has been shown that ethanol-induced activation of the complement system contributes to the pathophysiology of ethanol-induced liver injury in mice, whether ethanol consumption activates the complement system in the human liver has not been investigated. Using antibodies against C1q, C3, and C5, the immunoreactivity of the complement system in patients with AH was examined by immunohistochemistry and quantified by morphometric image analysis. The immunoreactivity intensity of C1q, C3, and C5 in patients with AH was significantly higher than that seen in normal controls. Further, the gene expression of C1q, C3, and C5 was examined using real-time PCR. There were increases in the levels of C1q and C5, but not C3 mRNA in AH. Moreover, the immunoreactivity of C5a receptor (C5aR) also increased in AH. To explore the functional implication of the activation of the complement system in AH, we examined the colocalization of C5aR in Mallory–Denk bodies (MDBs) forming balloon hepatocytes. C5aR was focally overexpressed in the MDB forming cells. Collectively, our study suggests that alcohol consumption increases the activity of the complement system in the liver cells, which contributes to the inflammation-associated pathogenesis of AH.     

短发夹状RNA基因沉默补体受体C5aR及抑制LPS诱导的肾脏上皮细胞凋亡

目的:探讨短发夹状RNA基因沉默补体受体C5aR及抑制LPS诱导的肾脏上皮细胞凋亡的作用效果。方法:构建针对大鼠补体受体C5aR基因编码区的短发夹状RNA(shRNA)真核表达载体质粒pRNAT-U6.1-C5aR shRNA,采用电穿孔的方法转染RK3E细胞,经G418筛选后,形成稳定的表达C5aR shRNA的细胞系。实验分为3组,①正常对照组:未转染的RK3E细胞;②阴性对照组:转染空载体pRNAT-U6.1的RK3E细胞系;③实验组:转染C5aR shRNA的RK3E细胞系。经脂多糖(LPS)孵育12小时后,流式细胞仪检测各组细胞凋亡率,RT-PCR检测mRNA的表达,γ计数仪测定125I标记的C5a与RK3E的结合情况。结果:与正常对照组和阴性对照组相比,实验组的细胞凋亡率显著降低(P0.01),C5aR mRNA水平显著降低(P0.01),125I标记的C5a与RK3E结合活性显著下降。结论:针对C5aR的特异性短发夹RNA可以明显引起靶基因的沉默,进而抑制LPS诱导的肾脏上皮细胞凋亡的发生。     

Comparison of the suppressive effects of soluble CR1 and C5a receptor antagonist in acute arthritis induced in rats by blocking of CD59

We investigated the effects of suppression of complement activation at C3 level and inhibition of C5a on acute synovitis in rats. Acute synovitis was induced in Wistar rats by intra-articular (i.a.) injection into one knee of 0.3 mg of MoAb 6D1 (anti-rat CD59 antibody). In the treatment groups, soluble CR1 (sCR1) or C5a receptor (C5aR) antagonist was administered intra-articularly or intravenously and effects on the course of the acute synovitis were monitored. Synovitis induced by 6D1 was characterized by joint swelling, thickening of synovial tissue, cellular infiltration and deposition of membrane attack complex (MAC) on the synovial surface. Neither inflammatory change nor MAC deposition was found in rats which received an i.a. injection of sCR1 to suppress complement activity in the joint. Intra-articular injection of sCR1 did not reduce plasma complement activity. Intravenous administration of sCR1 suppressed plasma complement activity but had no effect on the course of the arthritis and synovitis with MAC deposition was observed. Neither i.a. nor i.v. injection of C5aR antagonist had any suppressive effects on inflammatory change or MAC deposition in synovium. The data show that inflammatory change induced by 6D1 was mediated by local complement activation and was not accompanied by systemic complement activation. C5a generation was not responsible for the observed inflammation, suggesting that other complement activation products, possibly MAC, mediate the inflammatory change observed in this model of acute synovitis in rats.