Interaction between terminal complement proteins C5b-7 and anionic phospholipids.
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We have recently shown that C5b-6 binds to the erythrocyte membrane via an ionic interaction with sialic acid before the addition of C7 and subsequent membrane insertion. In this study we assessed the role of anionic lipids in the binding of the terminal complement proteins to the membrane and the efficiency of subsequent hemolysis. Human erythrocytes were modified by insertion of dipalmitoyl phosphatidylcholine (DPPC), dipalmitoyl phosphatidylserine (DPPS), dipalmitoyl phosphatidylethanolamine (DPPE), or dipalmitoyl phosphatidic acid (DPPA). Lipid incorporation and the hemolytic assays were done in the presence of 100 micromol/L sodium orthovanadate to prevent enzymatic redistribution of lipid. We found that the neutral lipids, DPPC and DPPE, did not affect C5b-7 uptake or hemolysis by C5b-9. In contrast, the two acidic phospholipids, DPPS and DPPA, caused a dose-dependent increase in both lysis and C5b-7 uptake. We conclude that the presence of anionic lipids on the exterior face of the membrane increases C5b-7 uptake and subsequent hemolysis. It is known that sickle cell erythrocytes have increased exposure of phosphatidylserine on their external face and are abnormally sensitive to lysis by C5b-9. The data presented here provide a plausible mechanism for this increased sensitivity. (+info)
Reaction of an activated complex of guinea-pig complement components, C56, with unsensitized erythrocytes and with erythrocytes carrying C3b molecule.
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During the interaction of guinea-pig complement intermediate cells, EAC423, with guinea-pig C5 and C6, an activated complex of C5 and C6, C56, was demonstrated in the fluid phase of the reaction mixture. C56 also was eluted from EAC42356 which had been generated by the interaction of EAC423 with C5 and C6. Both preparations of C56 showed quite similar characteristics and were not distinguished from one another. Both were capable of reacting with unsensitized erythrocytes (E) in the presence of C7 to form EC567. Further, they were able to react with EAC43 in the absence of C7 to form EAC43568 but did react with EAC43 pretreated with C3b inactivator, dithiothreitol or N-bromosuccinimide. These results indicate that guinea-pig C56 generated on EAC423 has a tendency to dissociate into the fluid phase. Nevertheless, the dissociated C56 can bind again to intact C3b molecule on the cells. The ability of cell-bound C3b to combine with C56 may lead to localization of C56 to the cell membrane carrying C3b, resulting in acceleration of attachment of C567 to the membrane. This assumption could be supported by the finding that the replacement of E by EAC43 increased the susceptibility of the cells to lytic action of complement induced by cobra venom factor. Thus, a new function of cell-bound C3b as localizing C56 to the membrane of sensitized cells was indicated. (+info)
rC5a directs the in vitro migration of human memory and naive tonsillar B lymphocytes: implications for B cell trafficking in secondary lymphoid tissues.
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Human C5a is a potent chemoattractant for granulocytes, monocytes, and dendritic cells. In mice C5a has been shown to be chemotactic for germinal center (GC) B cells. To date, no information is available on the effects of C5a on human B cell locomotion. Here we demonstrate that rC5a increases polarization and migration of human tonsillar B cells. The locomotory response was due to both chemokinetic and chemotactic activities of rC5a. Moreover, memory and, at a lesser extent, naive B cell fractions from purified tonsillar populations displayed rC5a-enhanced migratory properties, whereas GC cells did not. Flow cytometry revealed C5aR (CD88) on approximately 40% memory and 10% naive cells, respectively, whereas GC cells were negative. Immunohistochemistry showed that a few CD88+ cells were of the B cell lineage and localized in tonsillar subepithelial areas, where the majority of memory B cells settle. Pretreatment of memory B cells with the CD88 mAb abolished their migratory responsiveness to rC5a. Finally, the C5 gene was found to be expressed in naive, GC, and memory B lymphocytes at both the mRNA and the protein level. This study delineates a novel role for C5a as a regulator of the trafficking of human memory and naive B lymphocytes and supports the hypothesis that the B cells themselves may serve as source of C5 in secondary lymphoid tissues. (+info)
Active sites in complement components C5 and C3 identified by proximity to indels in the C3/4/5 protein family.
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We recently suggested that sites of length polymorphisms in protein families (indels) might serve as useful guides for locating protein:protein interaction sites. This report describes additional site-specific mutagenesis and synthetic peptide inhibition studies aimed at testing this idea for the paralogous complement C3, C4, and C5 proteins. A series of C5 mutants was constructed by altering the C5 sequence at each of the 27 indels in this protein family. Mutants were expressed in COS cells and were assayed for hemolytic activity and protease sensitivity. Mutants at five indels showed relatively normal expression but substantially reduced sp. act., indicating that the mutations damaged sites important for C5 function. Twenty-three synthetic peptides with C5 sequences and 10 with C3 sequences were also tested for the ability to inhibit C hemolytic activity. Three of the C5 peptides and one of the C3 peptides showed 50% inhibition of both C hemolytic and bactericidal activities at a concentration of 100 microM. In several cases both the mutational and peptide methods implicated the same indel site. Overall, the results suggest that regions important for function of both C3 and C5 lie proximal to residues 150-200 and 1600-1620 in the precursor sequences. Additional sites potentially important for C5 function are near residue 500 in the beta-chain and at two or three sites between the N-terminus of the alpha'-chain and the C5d fragment. One of the latter sites, near residue 865, appears to be important for proteolytic activation of C5. (+info)
In vitro and in vivo responses of murine granulocytes to human complement-derived, haemolytically inactive C5b67 (iC5b67).
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Haemolytically inactive C5b67 (iC5b67), which was made from purified human components and decayed to a haemolytically inactive form, was evaluated as an agonist for murine leucocytes both in vitro and in vivo. In an in vitro assay, iC5b67 stimulated chemotaxis for both neutrophils purified from mouse bone marrow and splenic eosinophils of IL-5 transgenic mice. The stimulation was dose-dependent, with high dose inhibition. As with human neutrophils, iC5b67 also failed to up-regulate CR3 (CD11b/CD18) expression and to stimulate superoxide generation in murine bone marrow neutrophils, in vitro. In vivo, iC5b67 elicited an inflammatory response in a mouse model of pleuritis. A marked infiltration of neutrophils, which peaked at 4 h, was followed by an infiltration of eosinophils and mononuclear leucocytes. This inflammatory response was dose- and time-dependent. However, the protein concentration in the pleural wash fluid did not increase, indicating that iC5b67 did not induce a capillary leak. Although the infiltration of neutrophils could not be reproduced by pure C7 or human serum albumin (HSA), C5b6 did induce an influx of neutrophils. We were able to document the existence of C7, both antigenically and functionally, in pleural washes of normal mice, making it likely that the activity of C5b6 resulted from the in situ formation of C5b67 and iC5b67. The mouse model of pleuritis promises to be a useful in vivo system in which to evaluate the pro- and anti-inflammatory effects of iC5b67 that have been noted in vitro. (+info)
Triggering a second T cell receptor on diabetogenic T cells can prevent induction of diabetes.
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In this paper, we test the hypothesis that triggering of a second T cell receptor (TCR) expressed on diabetogenic T cells might initiate the onset of diabetes. A cross between two TCR-transgenic strains, the BDC2.5 strain that carries diabetogenic TCRs and the A18 strain that carries receptors specific for C5, was set up to monitor development of diabetes after activation through the C5 TCR. F1 BDC2. 5 x A18 mice developed diabetes spontaneously beyond 3-4 mo of age. Although their T cells express both TCRs constitutively, the A18 receptor is expressed at extremely low levels. In vitro activation of dual TCR T cells followed by adoptive transfer into neonatal or adult F1 mice resulted in diabetes onset and death within 10 d after transfer. In contrast, in vivo immunization of F1 mice with different forms of C5 antigen not only failed to induce diabetes but protected mice from the spontaneous onset of diabetes. We propose that antigenic stimulation of cells with low levels of TCR produces signals inadequate for full activation, resulting instead in anergy. (+info)
Pneumococcal surface protein A inhibits complement activation by Streptococcus pneumoniae.
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Pneumococcal surface protein A (PspA) is a surface-exposed protein virulence factor for Streptococcus pneumoniae. In this study, no significant depletion of serum complement was observed for the serum of mice infected with pneumococci that express PspA. In contrast, in mice infected with an isogenic strain of pneumococci lacking PspA, significant activation of serum complement was detected within 30 min after infection. Also, the PspA-deficient strain but not the PspA-expressing strain was cleared from the blood within 6 h. The contribution of PspA to pneumococcal virulence was further investigated by using mice deficient for C5, C3, or factor B. In mice deficient for C3 or factor B, PspA-negative pneumococci became fully virulent. In contrast, in C5-deficient mice as in wild-type mice, PspA-deficient pneumococci were avirulent. These in vivo data suggest that, in nonimmune mice infected with pneumococci, PspA interferes with complement-dependent host defense mechanisms mediated by factor B. Immunoblots of pneumococci opsonized in vitro suggested that more C3b was deposited on PspA-negative than on PspA-positive pneumococci. This was observed with and without anticapsular antibody. Furthermore, processing of the alpha chain of C3b was reduced in the presence of PspA. We propose that PspA exerts its virulence function by interfering with deposition of C3b onto pneumococci and/or by inhibiting formation of a fully functional alternative pathway C3 convertase. By blocking recruitment of the alternative pathway, PspA reduces the amount of C3b deposited onto pneumococci, thereby reducing the effectiveness of complement receptor-mediated pathways of clearance. (+info)
Enhancement of lectin pathway haemolysis by immunoglobulins.
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We recently reported that indicator sheep erythrocytes (E) coated with mannan and sensitized with mannan-binding lectin (MBL) (E-M-MBL) are lysed by human serum in the absence of calcium via the lectin pathway of complement activation by a process which requires alternative pathway amplification and is associated with increased binding of and control by complement regulatory proteins C4 bp and factor H. In the present study, we investigated the effect of immunoglobulin (Ig) on this haemolysis. Co-sensitization of indicator E with anti-E haemolysin led to threefold enhancement of lectin pathway haemolysis in the absence of calcium, associated with increased binding of C3 and C5. Lysis was enhanced approximately twofold when E-M-MBL were chemically or immunologically coated with IgM or IgA, and fourfold when coated with IgG, prior to lysis in human serum-Mg-ethyleneglycol tetraacetic acid. The presence of haemolysin did not reduce the binding or inhibitory activity of C4 bp, and the enhancing activity of haemolysin was retained in serum depleted of C4 bp. By contrast, binding of factor H was greatly reduced in the presence of haemolysin, which had no enhancing effect in serum depleted of factor H. These experiments demonstrate the ability of IgG, IgM and IgA to enhance lectin pathway cytolysis, and that this enhancement occurs by neutralization of the inhibitory activity of factor H. Immunoglobulin enhancement of lectin pathway cytolysis represents another interaction between the innate and adaptive systems of immunity. (+info)