Inhibition of cleavage of the third component of human complement (C3) by its small cleavage fragment, C3a: inhibition occurs with the classical-pathway, but not the alternative-pathway, C3 convertase

Activation of the third component of complement (C), C3, is central to the functioning of the C system in inflammation. Cleavage of C3 by the C3 convertases of both the classical and alternative pathways results in the formation of two split products, C3b and C3a. C3a inhibited cleavage of C3 by the classical-pathway C3 convertase. The inhibition varied in a concn-dependent relationship, with a concn of approximately 40 micrograms/ml yielding 50% inhibition. Removal of the carboxy terminal arginine from the C3a did not alter the inhibition. C3a did not inhibit cleavage of C3 by the alternative C pathway C3 convertase, or cleavage of C5 by C5 convertase. The C3-cleaving capacity of EAC142oxy that had been previously incubated with C3a could be recovered completely by washing the cells, indicating that the C3a binding to the EAC42oxy cell must have been reversed without having had an effect on the amount of C2 bound. Ribonuclease, a molecule of similar size and charge to C3a, did not affect C3 cleavage and C3a inhibition was not reduced by providing a surface for non-specific adsorption of the C3a, suggesting that the effect of C3a on C3 cleavage was not mediated by non-specific interaction with cell surfaces. C3a inhibited the C3-cleaving capacity of the fluid-phase enzyme, C42oxy, to the same degree as it inhibited the cell-bound enzyme, EAC42oxy, indicating that the C3a must interact with the C42 complex directly. Inhibition of C3 cleavage by C3a is the first demonstration of product inhibition of a complement enzyme. It may provide another control of C3 activation.     

The third complement factor (C3) and its in vivo cleavage products: interaction with lectins and precipitation with polyethylene glycol

Five molecular forms of C3 expressing D but not C epitopes were identified following in vivo activation of the complement system. Examination of concanavalin A (Con-A) reactivity in crossed immunoelectrophoresis revealed that native C3, C3c and the beta mobile form 4 of C3d were completely precipitated by 100 micrograms Con A/cm2. The alpha-1 mobile form 1 of C3d did not interact with Con A, whereas the alpha-2 mobile forms 2 and 3 were retarded in electrophoretic migration by Con A. Native C3, C3c, and forms 4 and 5 of C3d were precipitated by 12% (w/v) polyethylene glycol (PEG). Form 1 of C3d was soluble in these PEG concentrations, whereas forms 2 and 3 were partially precipitated.     

A possible role for activated complement component 3 in phagocytic activity exhibited by the mouse trophoblast.

PROBLEM: To determine whether any blood plasma factor may play a regulatory role in trophoblast phagocytosis in rodent early pregnancy. METHOD OF STUDY: The effects of alloplasma on the phagocytosis of cultured mouse trophoblast cells (TCs) were evaluated using erythrocytes as target cells, in the presence of 10% fresh, normal plasma; 10% heat-inactivated plasma; 10% component 3 (C3)-depleted plasma; or medium alone. The possible activation of C3 complement, the phagocytosis of zymosan bound or unbound to C3b, and immunoreactivity to C3b receptor were also estimated. Phagocytic activity was expressed as the percentage of phagocytic TCs, and as the number of phagosomes/TCs. RESULTS: The use of complement sufficient plasma significantly enhanced the phagocytosis of the TCs while the use of heat-inactivated plasma eliminated the erythrophagocytosis. Very low levels of phagocytic activity were seen when the plasma was C3-complement deficient. Phagocytosis of C3b-bound zymosan was remarkable in comparison to zymosan alone, and immunoreactivity to C3b-receptors was seen on the TCs. CONCLUSION: These results indicate the participation of thermosensitive molecules mediating the phagocytosis of TCs and suggest, as in macrophages, a role for C3-C3b in this process.     

Internal cleavages of hepatitis C virus NS3 induced by P1 mutations at the NS3/4A cleavage site

Despite being the focus of intensive investigation for its enzymatic activities and its roles in HCV virus replication, little is known about the internal processing of NS3. Here we show that single mutations at P1 position of the NS3/4A junction lead to alternative cleavages. Among the multiple novel cleavage products observed, there were two predominant species of about 12 kDa (p12) and 67 kDa (p67). This p12 species consists of the NS4A and about a 6 kDa long C-terminal region of NS3 and forms a complex with NS3. The remaining NS3 corresponds to the p67 species. This alternative cleavage is an NS3 protease-mediated intra-molecular event and more interestingly can also be induced with low concentrations of one NS3 protease inhibitor examined. Our results led us to propose a model explaining the alternative cleavage observed and its functional role.     

C3研究进展

C3是3条补体激活途径的交汇点，系机体防御体系中的关键分子，是连接天然免疫和获得性免疫的桥梁之一，但其活化失调则会导致组织细胞损害，且其与病原体相互作用为微生物逃避补体攻击提供了可能。因此，深入了解C3的分子结构及其与其它蛋白、病原体、细胞的相互作用，对补体系统研究以及补体相关疾病和感染性疾病的治疗均有重要意义。     

ICAM-1 isoforms: specific activity and sensitivity to cleavage by leukocyte elastase and cathepsin G

The extracellular moiety of ICAM-1 consists of five Ig-like domains, the first and third domains mediating adhesion to integrin ligands. The ICAM-1 gene, however, gives rise to the expression of five alternative splice variants containing two, three, or four Ig-like domains. In this work, we have investigated whether the rearrangement of the architecture of ICAM-1 affects its structural properties and function. We showed that, in contrast to the common form, all alternative isoforms of ICAM-1 were susceptible to cleavage by leukocyte elastase and cathepsin G. We found that the length of an isoform did not influence the susceptibility to proteolysis. The molecular diversity provided by the skipping of entire Ig domains and the level of expression on the APC, however, significantly influenced their ability to potentiate the proliferation of T cells. Finally, we found that the expression of minor ICAM-1 isoforms encoding the third Ig-like domains was sufficient to sustain neutrophil infiltration in the liver and confer exon-5-targeted ICAM-1-deficient mice susceptibility to LPS-induced septic shock. These findings not only demonstrate that ICAM-1 isoforms are fully functional, but support the concept that alternative RNA splicing in the Ig superfamily may fulfill distinct roles during the development of the immune response.     

Secretion and function of the third component of complement (C3) by murine leukocytes.

Our new finding of de novo synthesis and secretion of C3 by both murine peritoneal macrophages and polymorphonuclear leukocytes (PMN) was confirmed by the incorporation of [35S]methionine into C3 molecules and their complete inhibition by cycloheximide. The methods of secretion of C3 from these two types of cells were compared by examining the C3 contents in their culture supernatants. Completely different modes of secretion were observed, i.e. although macrophages synthesize and secrete C3 constitutively, PMN has a mechanism to store the already synthesized C3 in the cell and secrete it in response to stimuli. Protein kinase C (PKC) activators, e.g. 12-O-tetradecanoyl-phorbol 13-acetate, dioctanoyl glycerol, and mezerein, as well as calcium ionophore A23187 stimulate the secretion of C3 from PMN. These results suggest the involvement of PKC and the calmodulin pathway. A very sensitive method for measuring C3 activity was developed which enabled us to show for the first time that C3 secreted by PMN had opsonizing activity and that particles cultured with PMN were phagocytosed effectively.     

A new single-nucleotide polymorphism in the seventh component of complement (C7) gene

A new single-nucleotide polymorphism has been found in the 3′ untranslated region of the complement component C7 gene. It is present with similar frequencies in the Japanese and Germans. This polymorphism would be a useful marker in the genetic study of C6 and C7 deficiencies. Received: March 10, 1999 / Accepted: April 9, 1999     

Inherited complement C3 deficiency: A defect in C3 secretion

The molecular basis of inherited complement C3 deficiency in a 20-year-old newly diagnosed male patient was studied. Using an enzyme-linked immunosorbent assay, the patient's C3 serum level was found to be approximately 7 μg/ml, which is less than 1 % of normal. In contrast, Northern analysis indicated that the patient's C3 mRNA was of normal size and quantity. Peripheral blood monocytes (PBM) and skin fibroblast cultures (F) from the patient and from healthy donors were labeled for 2 h with [³⁵S] methionine. Analysis of cell lysates and supernatants by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) demonstrated normal levels of C3 in lysates of patient's PBM and F. However, C3 secretion in the patient's cells was extremely reduced, with pulse-chase experiments demonstrating a long delay in the disappearance of intracellular C3. Secretion of C1r and factor B by the patient's cells was normal. Lipopolysaccharide and interleukin-1 increased C3 synthesis in the patient's PBM and F, but had no effect on the secretion. SDS-PAGE analysis of trypsin-cleaved intracellular C3 revealed an aberrant cleavage profile for the patient's C3. Collectively, these data indicate that C3 deficiency in this patient is due to a defect in the C3 secretion, probably as the result of abnormality in the proC3 structure.     

Approximate linkage equilibrium between two polymorphic sites within the gene for human complement component 3

Haplotypes have been determined for alleles at two separate polymorphic sites within the human structural gene for complement component 3, identified as a protein polymorphism and as a restriction fragment length polymorphism by hybridization using a cloned gene probe. No evidence was found for appreciable disequilibrium between alleles at these sites, despite their very close linkage.     

Differential localization of complement component 3 within the capsular matrix of Cryptococcus neoformans

The polysaccharide capsule of Cryptococcus neoformans is a powerful activator of the complement system. The goal of the present study was to assess serum and cellular variables that influence the sites for C3 binding within the capsular matrix. Confocal microscopy using fluorophore-labeled polyclonal anti-C3 and anticapsular monoclonal antibodies and rosetting of fluorescent microspheres coated with anti-C3 were used to identify sites of C3 binding relative to the capsular edge. The results showed that the source of serum was a major variable influencing localization of C3. C3 bound at or very near the capsular edge in the case of human serum. C3 deposition was further from the capsule edge with guinea pig and rat sera; in the case of mouse serum, there was no binding of C3 in the outer region of the capsule. Addition of human C3 to mouse serum led to deposition of the C3 at the capsular edge, indicating that distinct properties of mouse and human C3 account for the differential localization of C3. Finally, the density of the capsular matrix was an important variable in determining sites for C3 deposition. Yeast cells with a high concentration of polysaccharide near the capsule edge supported deposition of mouse C3 at or near the capsular edge, whereas cells with a low matrix density showed deposition well beneath the edge. Taken together, these results indicate that the spatial deposition of C3 within the capsular matrix is a complex process that is influenced by the serum source and the density of the capsular matrix.     

Chemical cleavage of mismatch: A new look at an established method

Chemical cleavage of mismatch (CCM), also known as chemical mismatch cleavage (CMC) or the HOT (hydroxylamine/osmium tetroxide) chemical method, has been used for detection of sequence variability with many systems since it was first described. Recently, adaptation to fluorescence-based detection systems has fundamentally changed both the execution and analysis of CCM. This review will outline major advances in the methodology of CCM, from the advent of PCR through fluorescent analysis, and includes applications and modifications of CCM. Hum Mutat 11:345–353, 1998. © 1998 Wiley-Liss, Inc.     

Human complement C3 deficiency: Th1 induction requires T cell-derived complement C3a and CD46 activation

Human T helper type 1 (Th1) responses are essential in defense. Although T cell receptor (TCR) and co-stimulator engagement are indispensable for T cell activation, stimulation of additional receptor pathways are also necessary for effector induction. For example, engagement of the complement regulator CD46 by its ligand C3b generated upon TCR activation is required for IFN-γ production as CD46-deficient patients lack Th1 responses. Utilizing T cells from two C3-deficient patients we demonstrate here that normal Th1 responses also depend on signals mediated by the anaphylatoxin C3a receptor (C3aR). Importantly, and like in CD46-deficient patients, whilst Th1 induction are impaired in C3-deficient patients in vitro, their Th2 responses are unaffected. Furthermore, C3-deficient CD4⁺ T cells present with reduced expression of CD25 and CD122, further substantiating the growing notion that complement fragments regulate interleukin-2 receptor (IL-2R) assembly and that disturbance of complement-guided IL-2R assembly contributes to aberrant Th1 effector responses. Lastly, sustained intrinsic production of complement fragments may participate in the Th1 contraction phase as both C3a and CD46 engagement regulate IL-10 co-expression in Th1 cells. These data suggest that C3aR and CD46 activation via intrinsic generation of their respective ligands is an integral part of human Th1 (but not Th2) immunity.     

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 physiological breakdown of the third component of human complement

The C3b INA-dependent breakdown of fluid phase C3b has been shown to have an absolute requirement for a second factor. This factor is contained in catalytic amounts (with respect to C3b) of highly purified β1H. β1H alone does not cause proteolysis of C3b. In the presence of C3b INA and β1H, proteolysis of the larger3 polypeptide chain (116K) of C3b occurs. Initially, a single scission gives two chains of 68K and 46K. A subsequent split of the 46K chain yields a smaller product of 43K. All of these chains remain covalently bonded to the β chain of C3b. These initial reactions are the same whether purified components or whether low concentrations of serum as a source of C3b INA and β1H are used. Pre-treatment of C3b, C3b INA and β1H with DFP has no effect on these events.Further protelysis of the 68K. chain requires a DFP-sensitive protease and leads to the formation of an additional high molecular weight breakdown product of C3. This has the same polypeptide chain composition as the high molecular weight breakdown product of C3 purified from aged serum, and is therefore identified as C3c.     

Increased production of the third component of complement (C3) by monocytes from patients with systemic lupus erythematosus.

We measured in vitro C3 production by peripheral blood monocytes from patients with systemic lupus erythematosus (SLE), and found it to be significantly greater than that from normal controls. We also found that monocytes from SLE patients with active disease produced a markedly larger amount of C3 than those from SLE patients with inactive disease. Production of C3 by monocytes correlated with serum levels of anti-dsDNA antibodies and inversely correlated with serum C3 levels in SLE patients. Serial measurement of C3 in the culture supernatant from each SLE patient showed that C3 production by monocytes fell in parallel with a decrease of disease activity. The effect of corticosteroids was ruled out as there was no relation between the level of C3 production by monocytes and the dose of prednisolone. This seems to be the first study in which the C3 production was assayed at a cellular level in SLE patients, and this study suggests that the local C3 production is increased in SLE patients.     

In vitro expression of the beta subunit of human complement component C8

Human C8 is one of five components of the cytolytic C5b-9 complex of complement. It is an oligomeric protein composed of three subunits (α, β, γ) encoded in separate genes. These are arranged as a disulfide-linked α-γ dimer and a noncovalently associated β chain. Biosynthesis studies and analyses of humans with hereditary C8 deficiencies suggest that C8α-γ synthesis and secretion can occur independently of C8β, but that serum levels of C8β are dependent on C8α-γ. One aim of the present study was to determine if functional human C8β could be synthesized in the absence of C8α-γ. Human C8β expression constructs were prepared and used to produce recombinant C8β (rC8β) in insect and COS-7 cells. Both cell types secreted rC8β that was similar in size to human C8β and exhibited similar ability to associate with human C8α-γ and form functional C8. A mutant form of C8β in which N-glycosylation sites were eliminated was also expressed and found to be functionally similar to rC8β and human C8β. These results indicate that C8α-γ is not required for intracellular processing and secretion of C8β. Furthermore, N-linked carbohydrate on C8β is not necessary for association with C8α-γ or for C8 activity.     

Murine polymorphonuclear leukocytes synthesize and secrete the third component and factor B of complement

Complement proteins in serum are synthesized mostly by hepatocytes and many other cell-types have also been shown to synthesize complement in various tissues. However, polymorphonuclear leukocytes (PMNs) have never been reported to secrete complement. This paper demonstrates the synthesis and secretion of C3 and factor B by murine peritoneal exudate PMNs elicited with OK432 (Streptococcus preparation). Using [35S]methionine incorporation and immunoprecipitation, C3 and factor B produced by PMN are found to be antigenically and physically identical to macrophage C3 and factor B. ELISA analysis reveals that culture supernatant of PMN--free of macrophage contamination--contains C3 antigen, and both flow cytometric analysis and immunoperoxidase staining also demonstrate the presence of intracellular C3 using special precautions to eliminate non-specific staining. The role of complement produced by PMN is currently unknown, but it is very important to take this new finding into consideration for further clarification of the roles of complement in extravascular inflammatory sites.     

Reference typing report for complement component C3

A comparison of five C3 variant samples has been performed by conventional high-voltage gel electrophoresis in three laboratories (Palermo, Berlin and Mainz). Local designation was shown within SD = +/-0.75 mm migration distance in the Mainz laboratory. Methodological modifications by laboratories were not accounted for (cooling temperature, relative mobilities between runs). In parallel, all reference samples were also sequenced after exon-specific amplification. As a result, two variants with identical final designations and two variants with different mobilities were shown to conform at the molecular basis exhibiting an amino acid exchange that causes the corresponding change in relative mobility as compared to normal C3F and C3S.