C1q binding to surface-bound IgG is stabilized by C1r2s2 proteases

Complement is an important effector mechanism for antibody-mediated clearance of infections and tumor cells. Upon binding to target cells, the antibody’s constant (Fc) domain recruits complement component C1 to initiate a proteolytic cascade that generates lytic pores and stimulates phagocytosis. The C1 complex (C1qr₂s₂) consists of the large recognition protein C1q and a heterotetramer of proteases C1r and C1s (C1r₂s₂). While interactions between C1 and IgG-Fc are believed to be mediated by the globular heads of C1q, we here find that C1r₂s₂ proteases affect the capacity of C1q to form an avid complex with surface-bound IgG molecules (on various 2,4-dinitrophenol [DNP]-coated surfaces and pathogenic Staphylococcus aureus). The extent to which C1r₂s₂ contributes to C1q–IgG stability strongly differs between human IgG subclasses. Using antibody engineering of monoclonal IgG, we reveal that hexamer-enhancing mutations improve C1q–IgG stability, both in the absence and presence of C1r₂s₂. In addition, hexamer-enhanced IgGs targeting S. aureus mediate improved complement-dependent phagocytosis by human neutrophils. Altogether, these molecular insights into complement binding to surface-bound IgGs could be important for optimal design of antibody therapies.

Antibodies are important mediators of the human complement response, which offers critical protection against microbial infections and damaged host cells (1). In order to initiate a complement response, an antibody molecule first needs to bind antigens on the target cell via its antigen-binding (Fab) domains (2–5). Subsequently, the antibody’s constant (Fc) domain recruits the first complement protein complex, C1, to the cell surface (SI Appendix, Fig. S1A). The large C1 complex (also denoted as C1qr₂s₂, 766 kDa) consists of the recognition protein C1q (410 kDa) and a heterotetramer of serine proteases C1r and C1s (denoted C1r₂s₂, 356 kDa) (SI Appendix, Fig. S1B). While C1q is responsible for antibody recognition, its attached proteases C1r₂s₂ induce the activation of downstream enzymatic complexes (i.e., C3 convertases [C4b2b (6)]) that catalyze the covalent deposition of C3-derived molecules (e.g., C3b and its degradation product iC3b) onto the target cell surface (SI Appendix, Fig. S1A) (7, 8). C3b opsonizes the target cell surface and can induce formation of lytic pores (membrane attack complex [MAC]) in the target cell membrane (9–11). In contrast to human cells and gram-negative bacteria, gram-positive bacteria are not susceptible to the MAC due to their thick cell wall (12). On these bacteria, efficient decoration with C3b and iC3b is essential for triggering effective phagocytic uptake of target cells via complement receptors (CR) expressed on phagocytes of which the integrin CR3 (also denoted CD11b/CD18) is considered most important (13, 14).In recent years, our insights into IgG-dependent complement activation have increased significantly. A combination of structural, biophysical, and functional studies revealed that surface-bound IgG molecules (after Fab-mediated antigen binding) require organization into higher-ordered structures, namely hexamers, to induce complement activation most effectively (15–19). Hexamerized IgGs are being held together by noncovalent Fc–Fc interactions and form an optimal platform for C1q docking (SI Appendix, Fig. S1A). C1q has a “bunch of tulips–” like structure, consisting of six collagen arms that each end in a globular (gC1q) domain (SI Appendix, Fig. S1B) that binds the Fc region of an IgG. As the affinity of C1q for a single IgG is very weak (20, 21), avidity achieved through simultaneous binding of its globular domains to six oligomerized IgG molecules is paramount for a strong response (15, 17–19). Furthermore, it was found that IgG hexamerization could be manipulated by specific point mutations in the Fc–Fc contact region that enhance such oligomerization (15, 18, 22). While these hexamer-enhancing mutations in IgG potentiate the efficacy of MAC-dependent cytotoxicity on tumor cells and gram-negative bacteria (15, 23), their effect on complement-dependent phagocytosis is not known.Because complement is an important effector mechanism to kill bacteria and tumor cells, development of complement-enhancing antibodies represents an attractive strategy for immune therapies (1, 24). Immunotherapy based on human monoclonal antibodies is not yet available for bacterial infections (25–28). Such developments are mainly hampered by the fact that little is known about the basic mechanisms of complement activation on bacterial cells. For instance, we do not understand why certain antibodies induce complement activation on bacteria and others do not. In this study, we set out to investigate how antibacterial IgGs induce an effective complement response. By surprise, we noticed that C1q–IgG stability differs between human IgG subclasses. More detailed molecular investigations revealed that C1r₂s₂ proteases are important for generating stable C1q–IgG complexes on various target surfaces. Furthermore, we demonstrate that C1q–IgG stability is influenced by antibody oligomerization. These molecular insights into C1q binding to surface-bound IgGs may pave the way for optimal design of antibody therapies.     

Detection of igg sensitization of red cells with 125I staphylococcal protein A

Most cases of immune hemolytic anemia are associated with a positive direct antiglobulin test. However, in some cases, the antiglobulin test is not sensitive enough to detect low levels of red-cell bound antibodies. This report describes a method using radiolabelled purified staphylococcal protein A which is capable of detecting IgG sensitization of red cells beyond the threshold of serologic techniques. It is less cumbersome than previously described methods and does not require antibody purification procedures. Its effectiveness was demonstrated for the detection of red-cell alloantibodies and in evaluation of patients with acquired hemolytic anemias associated with a negative direct antiglobulin test.     

Possible participation of IgG4 in the activation of complement in IgG4-related disease with hypocomplementemia

Objective: To investigate which IgG subclasses contribute to the activation of the complement pathway in IgG4-related disease (IgG4RD) patients with hypocomplementemia.

Methods: Sera of IgG4RD patients were analyzed for the binding ability of IgG subclasses to complement component 1q (C1q). Polyethylene glycol (PEG) precipitates containing immune complexes (ICs) in sera of IgG4RD patients were analyzed for IgG subclass composition by Western blotting. PEG precipitates containing ICs (PEG-ICs) in sera of patients were also analyzed for their ability to consume complement in normal human serum (NHS) using a total complement hemolytic (CH50) assay and a commercial kit to measure the complement capacity of all three individual complement pathways.

Results: The C1q binding assay revealed high serum levels of C1q-binding IgG4 in IgG4RD patients with hypocomplementemia. ICs in PEG precipitates were formed with IgG4 in IgG4RD patients, regardless of the presence or absence of hypocomplementemia. We observed a marked reduction of CH50 and reduced complement activity in the classical complement pathway as well as the mannan-binding lectin complement pathway in NHS incubated with PEG-IC isolated from IgG4RD patients with hypocomplementemia.

Conclusion: Our results suggest that IgG4 may participate in the activation of complement in IgG4RD patients with hypocomplementemia.     

Temperature dependent activation of the alternate complement pathway by an IgG cryoglobulin.

A patient with chronic membranoproliferative glomerulonephritis is presented whose serum contains a monoclonal IgG3 cryoglobulin. The presence of persistent hypocomplementemia suggested the possibility that the cryoglobulin, upon cold-induced precipitation, was capable of activating the complement system. Because visible cryoprecipitation commenced in vitro at 30 degrees C, the patient's serum and normal serum had been added the isolated cryoglobulin were repeatedly cooled to 30 degrees C and rewarmed to 37 degrees C. This reproduction of the in vivo counterpart of blood circulating through an extremity exposed to the cold resulted in activation of C3-proactivator (properdin factor B), C3 cleavage, and a 78% reduction in total hemolytic complement. This study demonstrates that IgG is capable of activating complement via the alternate pathway and reveals a mechanism through which this can occur in vivo; namely, by means of temperature dependent polymerization. In addition, we postulate that episodic complement activation initiated by the cryoglobulin contributed to the development of glomerulonephritis in this patient.     

Anti-prothrombin IgG from patients with anti-phospholipid antibodies inhibits the inactivation of factor Va by activated protein C

Interference of anti-phospholipid antibodies with the protein C pathway has been suggested to play a role in the development of thrombosis in the anti-phospholipid syndrome. We studied the effect of IgG preparations containing anti-prothrombin antibodies of 17 lupus anticoagulant-positive patients and 12 controls on the inactivation of factor Va (FVa) by activated protein C (APC) in a system with purified coagulation factors. Test IgG was incubated with human prothrombin, phospholipid vesicles and CaCl(2). Protein S, FVa and APC were added and the APC-dependent loss of FVa activity was monitored over time. The residual amount of FVa remaining after 10 min was 14 +/- 4% (mean +/- SD) when 1.5 mg/ml normal IgG was present and ranged between 17% and 82% with 1.5 mg/ml patient IgG. Twelve patients IgG gave values of residual FVa >22% (i.e. 2 SD above the mean of controls), indicating that APC-mediated inactivation of FVa was significantly inhibited. The inhibition was strictly dependent on the presence of prothrombin, proportional to the concentration of IgG and strongly diminished at a 20-fold higher phospholipid concentration. Most, although not all, IgG containing anti-prothrombin antibodies inhibit the APC-catalysed FVa inactivation, which may contribute to the increased risk of thrombosis in patients with the anti-phospholipid syndrome.     

免疫吸附治疗ANCA相关血管炎的初步观察

目的:探讨葡萄球菌蛋白A免疫吸附(IA)联合霉酚酸酯(MMF)疗法对抗中性粒细胞胞浆抗体(ANCA)相关血管炎血清ANCA水平的影响及临床疗效。方法:6例ANCA相关血管炎伴活动性肾血管炎患者(男、女各3例,年龄19～60岁)临床均为微型多血管炎,ANCA均为MPO-ANCA,治疗前BVAS评分15～22分,均有肾功能不全[SCr(354.6±296.1)μmol/L,其中需即时透析1例],肾脏病理新月体比例(51.0±27.1)%。采用甲基泼尼松龙静脉冲击联合葡萄球菌蛋白A行IA治疗(IA隔日1次,每次吸附血浆4.5L,共3～10次)。IA结束后口服泼尼松及霉酚酸酯(1.5g/d)。观察IA过程中ANCA水平及病情变化,IA后MMF治疗过程中血管炎活动性(BVAS评分)、肾功能与尿检变化。结果:首次IA后1例血清ANCA转阴,其余5例ANCA下降37.2%～70.4%[平均(52.6±14.0)%]。3次IA后ANCA水平下降67.8%～91.5%[平均(81.8±10.0)%],吸附结束后ANCA下降至基础值(9.6±6.3)%,5例治疗前无需透析患者SCr均降至(206.9±106.1)μmol/L,其中1例降至正常,1例治疗前需透析患者治疗后摆脱透析(SCr由929.1μmol/L降至363.3μmol/L)。IA后MMF治疗随访3～12月,3例缓解,2例稳定,1例死亡。不良反应:IA治疗过程中1例发生低血压1次,随访中各有1例发生带状疱疹和口角疱疹,1例随访3月并发严重肺部感染死亡。结论:葡萄球菌蛋白A免疫吸附治疗能快速、有效降低血清ANCA水平,抑制血管炎活动性,改善肾功能,但IA后免疫抑制剂治疗过程中应注意预防感染。     

致皮肤软组织感染社区获得性金黄色葡萄球菌的分子生物特征分析

目的了解解放军总医院第一附属医院致皮肤软组织感染社区获得性金黄色葡萄球菌(CA-SA)的流行情况、耐药特点、毒力和致病因素信息。方法收集2009年1月至2010年8月皮肤科、门诊、急诊55例患者化脓性皮肤软组织感染检测标本,进行细菌鉴定和药敏试验,采用多位点序列分型(MIST)、金黄色葡萄球菌A蛋白(SPA)分型和毒素基因检测筛查。结果 55例中分离到社区获得性甲氧西林敏感的金黄色葡萄球菌(CA-MSSA)12株;药敏结果显示CA-MSSA除对红霉素、克林霉素、四环素、庆大霉素和左氧氟沙星耐药性较高(8.3%~50.0%),对其他抗菌药物均敏感。毒素基因检测显示杀白细胞素(pvl)、肠毒素C(sec)和毒性休克综合征毒素-1(tsst-1)在CA-MSSA中的阳性率分别为33.3%,25.0%和8.3%,未检测到葡萄球菌肠毒素H(seh)和葡萄球菌表皮剥脱毒素(et);结合MLST和SPA分型,发现CA-MSSA中的克隆有ST5-t002(2株)、ST22-t309(2株)及ST398-t034、ST15-t5864、ST7-t091、ST25-t078、ST30-t318、ST121-t1425、ST800-t1425、ST630-t377各1株。结论我院致皮肤软组织感染CA-MSSA菌株对抗菌药物的敏感性较高,分子分型具有多样性,携带多种毒素。     

牛源金黄色葡萄球菌spa基因多态性分型研究

目的为了研究引起奶牛乳房炎的主要病原菌金黄色葡萄球菌的35个分离株进行spa基因多态性分型研究。方法在河北、内蒙古、山东、黑龙江、湖北、河南等省份采集奶牛乳房炎奶样,经实验室分离鉴定,从中择35株金黄色葡萄球菌,采用PCR方法扩增spa基因的X区域,测序后根据数据库(http://www.ridom.de/spaserver/)进行分型。结果35株菌可分成11个型,分别为t521型有8株,t518型和t2788型各5株,t246型和t519型各4株,t359型和t2756型各3株,t189型t、224型t、267型t、2759型各有1株;同时发现3个新型,分别为t2788、t2756t、2759。结论研究显示spa基因分型方法具有快速、分辨力强、易于标准化等特点,可用于牛源金黄色葡萄球菌的基因分型及生态学等研究。     

Activation of complement in C-reactive protein positive sera by phosphorylcholine-bearing component isolated from parasite extract

Summary Phosphorylcholine-bearing component levels in extracts of various parasites were determined by a capillary precipitin test using anti-phosphorylcho-line Ig A myeloma protein, TEPC-15. Phosphorylcholine was demonstrated as a structural component not only in nematodes but also in trematodes and cestodes. The phosphorylcholine-bearing component was isolated from an extract of Toxocara canis larvae using a TEPC-15-Sepharose 4B column. The component reacted with C-reactive protein in sera to form one precipitin line in Immunoelectrophoresis. The component provided two Brilliant Coomassie Blue positive bands in SDS-polyacrylamide gel electrophoresis. It reacted with C-reactive protein to activate complement in serum.     

Anti-SARS CoV-2 IgG in COVID-19 Patients with Hematological Diseases: A Single-center,Retrospective Study in Japan

Objective Coronavirus disease (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has spread globally. Although the relationship between anti-SARS-CoV-2 immunoglobulin G (IgG) antibodies and COVID-19 severity has been reported, information is lacking regarding the seropositivity of patients with particular types of diseases, including hematological diseases. Methods In this single-center, retrospective study, we compared SARS-CoV-2 IgG positivity between patients with hematological diseases and those with non-hematological diseases. Results In total, 77 adult COVID-19 patients were enrolled. Of these, 30 had hematological disorders, and 47 had non-hematological disorders. The IgG antibody against the receptor-binding domain of the spike protein was detected less frequently in patients with hematological diseases (60.0%) than in those with non-hematological diseases (91.5%; p=0.029). Rituximab use was significantly associated with seronegativity (p=0.010). Conclusion Patients with hematological diseases are less likely to develop anti-SARS-CoV-2 antibodies than those with non-hematological diseases, which may explain the poor outcomes of COVID-19 patients in this high-risk group.     

IgG subclasses of anti-FVIII antibodies during immune tolerance induction in patients with hemophilia A

The eradication of inhibitory antibodies in patients with haemophilia A can be accomplished by frequent administration of high or intermediate doses of factor VIII (FVIII), so-called immune tolerance induction (ITI). This study monitored the distribution of IgG subclasses of anti-FVIII antibodies during ITI. FVIII-specific antibodies of subclass IgG1 were detected in all inhibitor patients tested, anti-FVIII IgG4 in 16, IgG2 in 10 and IgG3 in one of 20 patients analysed. Levels of anti-FVIII IgG1 and IgG4 correlated well with inhibitor titres as measured by Bethesda assay. In low-titre inhibitor patients, anti-FVIII antibodies consisted primarily of subclass IgG1 whereas, anti-FVIII antibodies of subclass IgG4 were more prominent in patients with high titre inhibitors who needed prolonged treatment or who failed ITI. Longitudinal analysis of 14 patients undergoing ITI revealed that the relative contribution of IgG subclasses was constant for most of the patients analysed. In two patients, the relative contribution of IgG4 increased during ITI. Overall, our findings document the distribution and dynamics of anti-FVIII IgG subclasses during ITI. Future studies will need to address whether monitoring the relative contribution of anti-FVIII subclasses IgG1 and IgG4 may be useful for the identification of patients who are at risk of failing ITI.     

Serum IgG3 to the Plasmodium falciparum merozoite surface protein 2 is strongly associated with a reduced prospective risk of malaria

The merozoite surface protein 2 (MSP2) of Plasmodium falciparum is recognized by human antibodies elicited during natural infections, and may be a target of protective immunity. In this prospective study, serum IgG antibodies to MSP2 were determined in a cohort of 329 Gambian children immediately before the annual malaria transmission season, and the incidence of clinical malaria in the following 5 months was monitored. Three recombinant MSP2 antigens were used, representing each of the two major allelic serogroups and a conserved region. The prevalence of serum IgG to each antigen correlated positively with age and with the presence of parasitaemia at the time of sampling. These antibodies were associated with a reduced subsequent incidence of clinical malaria during the follow-up. This trend was seen for both IgG1 and IgG3, although the statistical significance was greater for IgG3, the most common subclass against MSP2. After adjusting for potentially confounding effects of age and pre-season parasitaemia, IgG3 reactivities against each of the major serogroups of MSP2 remained significantly associated with a lower prospective risk of clinical malaria. Individuals who had IgG3 reactivity to both of the MSP2 serogroup antigens had an even more significantly reduced risk. Importantly, this effect remained significant after adjusting for a simultaneous strong protective association of antibodies to another antigen (MSP1 block 2) which itself remained highly significant.     

急性脑梗死患者血清C反应蛋白与C_3水平变化及意义

目的进一步探讨急性脑梗死(ACI)发生、发展机制。方法选择ACI患者118例(ACI组)和健康体检者56例(对照组),应用免疫比浊法测定血清C反应蛋白(CRP)和C3水平。结果ACI组血清CRP和C3水平明显高于对照组,且随病情进展及梗死面积增大而升高;随血清CRP和C3水平升高,进展型ACI发病率升高。结论血清CRP和C3水平升高在ACI发病及进展中发挥重要作用,可作为预测ACI严重程度及进展的相关性生化指标。     

老年高血压患者颈动脉硬化与C-反应蛋白、血尿酸和补体的相关性

目的比较老年高血压患者血清C-反应蛋白（CRP）、血尿酸（UA）水平、补体C3与血压及颈动脉硬化程度的相关性。方法将132例老年高血压患者按血压值的不同分为3组，比较各组间CRP、UA与血压的相关性，对各组患者分别检测血生化指标：血清CRP、UA、补体C3，B超观察颈动脉内膜变化，按B超结果将患者分为内膜正常组、内膜增厚组、斑块形成组和管腔狭窄组，各组间CRP、UA、C3水平。结果高血压3级患者CRP、UA显著高于高血压1级、2级患者，差异有统计学意义（P〈0．05）。3组高血压患者CRP、UA值各组间比较差异有统计学意义（P〈0.05）。颈动脉内膜异常组CRP、UA、补体C，值明显高于内膜正常组，差异有统计学意义（P〈0．01）。随着颈动脉内膜-中层厚度（IMT）的增加，UA、CRP的浓度亦逐渐增高；颈动脉内膜异常组各组间比较差异均有显著性（P〈0．05）。结论老年高血压患者颈动脉硬化程度与血压、UA、CRP和补体C3浓度密切相关。老年高血压患者血压、CRP、UA浓度与IMT呈正相关关系（P〈0．01）。     

Metformin reduces C-reactive protein but not complement factor C3 in overweight patients with Type 2 diabetes mellitus.

AIMS: To determine the influence of metformin treatment on plasma C-reactive protein (CRP) and complement factor C3. METHODS: A double-blind, placebo-controlled trial of metformin in patients with poorly controlled Type 2 diabetes mellitus and body mass index > 25 kg/m2. CRP and C3 were analysed in stored plasma samples by in-house ELISAs. Patients attended two baseline visits before randomization and subsequently attended at 3, 6, 12 and 24 weeks post randomization. All patients gave informed consent according to a protocol approved by the Leeds Teaching Hospitals Research Ethics Committee. RESULTS: Baseline CRP in the patients randomized to placebo [1.33 (0.79, 2.25) mg/l] and metformin [1.24 (0.89, 1.71) mg/l] were similar (P = 0.8). Baseline CRP correlated with baseline C3 (r = 0.366) and HbA1c (r = 0.327). The difference in ratios of CRP levels at each visit to baseline between placebo- (n = 16) and metformin-treated (n = 26) subjects was significantly different at the 12-week (P = 0.002) and 24-week (P = 0.03) visits. The difference in CRP ratios between the two treatment groups remained significant after accounting for glycaemic control at both visits (P = 0.001 and P = 0.003, respectively). Baseline C3 was correlated with CRP. Baseline C3 was lower in the placebo-treated group [0.97 (0.88, 1.05) mg/ml] compared with the metformin-treated group [1.09 (1.02, 1.17) mg/ml, P = 0.03]. There was no difference in the mean change in C3 at any visit from baseline between placebo- and metformin-treated groups. CONCLUSION: Metformin may have a specific interaction with mechanisms involved in CRP synthesis or secretion, not directly related to improved insulin sensitivity and dampening of chronic inflammation.     

Activated protein C inhibits thrombus formation in a system with flowing blood

We studied the effect of increasing concentrations of protein C (PC) and activated protein C (APC) on haemostasis in an in vitro thrombosis model. Blood from healthy donors was anticoagulated with citrate–phosphate–dextrose (final citrate concentration 19 m m ) or a low molecular weight heparin (LMWH, 20 IU/ml). Enzymatically denuded rabbit aorta segments were exposed to flowing blood for 10 min in an annular perfusion chamber. PC and APC were added to the perfusate immediately prior to exposure. In citrated blood at a shear rate of 800/s, PC and APC induced a statistically significant decrease in platelet deposit at 16 μg/ml and 32 μg/ml. In perfusions performed with blood anticoagulated with LMWH, there was no effect on platelet deposition at 16 and 32 μg/ml either at shear rates of 300/s or 800/s. Addition of PC showed no effect on fibrin deposition at a shear rate of 300/s; in contrast, a non-statistically significant 40% reduction was seen at a shear rate of 800/s, compared to controls. Addition of APC caused a 100% reduction in fibrin formation at 16 and 32 μg/ml at both shear rates studied. PC and APC inhibited platelet deposition on the exposed subendothelial surface, in a dose-dependent manner. Effects of PC and APC on platelet function might be mediated through inhibition of thrombin generation at the platelet microenvironment.     

S protein/vitronectin in chronic liver diseases: correlations with serum cholinesterase, coagulation factor X and complement component C3

S protein/vitronectin plays an important role as a regulatory component in the terminal steps of the complement- and coagulation cascades. In patients suffering from chronic liver diseases, plasma S protein concentration was measured and compared with changes in serum cholinesterase activity, coagulation factor X activity and complement component C3 concentration. Significant decreases of all these proteins were seen in liver cirrhosis. Changes in S protein concentration correlated closely with those of cholinesterase, factor X and complement C3. The data give support for the liver as the main organ of plasma S protein/vitronectin synthesis.     

RNA干扰技术阻断小鼠巨噬细胞受体相互作用蛋白2表达的实验研究

目的观察阻断受体相互作用蛋白2（Rip2）对巨噬细胞产生炎症细胞因子的影响,以及对内毒素血症小鼠的保护作用。方法构建Rip2小干扰RNA（siRNh）重组表达质粒,转染细胞后RT．PCR和Western blot检测Rip2的mRNA和蛋白表达,四甲基偶氮唑盐（M1Tr）法检测细胞增殖水平,脂多糖（LPS）刺激后,测定TNFa和高迁移率组蛋白1（HMGB1）的水平。Rip2 siRNA质粒转染小鼠后,观察小鼠病死率,测定血清TNFct水平和肝组织Rip2和HMGB1表达。结果Rip2 siRNA表达质粒可阻断Rip2 mRNA和蛋白表达。Rip2阻断的细胞增殖明显,LPS刺激后产生TNFα、HMGB1减少;Rip2阻断的小鼠生存率较其他组高（P〈0．05）,肝组织中HMGB1[（40．21±11．03）Pg／g]表达和血清TNFα[（300．43±59．26）ng／L]水平均较其他组低（P〈0．05）。结论Rip2 siRNA表达质粒可阻断Rip2的表达,从而减少TNFα、HMGB1等炎症细胞因子的产生,降低小鼠内毒素血症的病死率。