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PubMed1983

Demonstration of beta 1H globulin in pemphigus.

beta 1H globulin is a plasma protein which regulates the biologic activities of the major fragment of the 3rd complement component, C3b. The role of beta 1H globulin in pemphigus was investigated using immunofluorescence in the present study. Lesional skin biopsies from patients with confirmed pemphigus demonstrated in-vivo deposition of beta 1H in addition to C3 in all of four biopsies. Eight serum samples containing C3 fixing intercellular antibodies were then tested for the capacity to fix beta 1H and other complement components. All eight pemphigus sera showed fixation of beta 1H to the intercellular areas of normal human skin. C1q and C4 fixation by pemphigus sera was also demonstrated in 7 of 8 sera, respectively. The experiment using C2-deficient serum indicated that the fixation of beta 1H by intercellular antibodies requires the activation of the classical complement pathway. These data suggested that beta 1H, a co-factor of C3b inactivator, plays a role in the in-vivo regulation of complement activity and supplies additional evidence for the participation of complement system in the pathogenesis of pemphigus.

Antibody Specificity↗
PubMed1987

Impaired surface membrane expression of C3bi but not C3b receptors on neonatal neutrophils.

Because increased complement receptor expression is necessary for optimal function of adult neutrophils, we tested the hypothesis that the increased susceptibility of neonates to infection might be due to an impaired ability of neonatal neutrophils to increase expression of complement receptors in response to chemotactic stimuli. We used monoclonal antibodies and flow cytometry to compare surface expression of the receptors for the complement components C3b (CR1) and C3bi (CR3) on adult and neonatal cord blood neutrophils (PMNs). We also compared receptor expression on PMNs from infants delivered by cesarean section without labor versus infants delivered vaginally. Expression of both CR1 and CR3 was minimal on resting adult and neonatal PMNs maintained at 0 degrees C. There was a modest increase in expression of both receptors when PMNs were warmed to 37 degrees C. This increase was similar on adult and neonatal cells, both unfractionated in whole blood and after isolation with Percoll density centrifugation, with one exception. Expression of CR1 was greater on isolated PMNs from vaginally delivered infants versus adults when the cells were warmed to 37 degrees C. This difference was not observed with cells from infants delivered by cesarean section without labor, suggesting this modest increase in receptor expression may be due to factors associated with labor. When isolated cells were stimulated with either N-formyl-methionyl-leucyl-phenylalanine or zymosan-activated serum, expression of CR1 increased to the same extent in both neonatal and adult PMNs. In contrast, maximal CR3 expression on cord PMNs stimulated with N-formyl-methionyl-leucyl-phenylalanine or zymosan-activated serum was only 75% of the adult values.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1981

Formation of the initial C3 convertase of the alternative complement pathway. Acquisition of C3b-like activities by spontaneous hydrolysis of the putative thioester in native C3.

Activation of the alternative pathway of complement commences with the formation of an initial fluid-phase C3 convertase. Treatment of C3 with the nucleophilic reagent methylamine has previously been shown to result in the cleavage of an intramolecular thioester bond and to induce C3b-like properties, including the ability to form a fluid-phase C3 convertase. This report examines the hypothesis that spontaneous hydrolysis of the thioester generates a derivative of C3 that is responsible for the formation of the initial C3 convertase of the alternative pathway. The rate of spontaneous decay of C3 hemolytic activity in buffer was found to be between 0.2 and 0.4%/h. In the presence of other alternative pathway proteins, the rate of inactivation was 1%/h. The rate of spontaneous inactivation was greatly accelerated by low concentrations of chaotrophic agents such as KSCN or guanidine. Liberation of a sulfhydryl group, not present in native C3, correlated with loss of hemolytic activity, indicating that exposure to chaotropic agents resulted in thioester hydrolysis. Unlike native C3, C3 bearing a single reactive sulfhydryl group was capable of generating fluid-phase C3 convertase with Factors B, D, and P and was cleaved by Factor I (C3b inactivator) in the presence of Factor H (beta 1H). The fragmentation patterns indicated that the C3a domain was covalently associated with the functionally C3b-like C3. Organomercurial agarose was employed for the rapid removal of sulfhydryl-bearing, hemolytically inactive forms of C3 and C3b from native hemolytically active C3.

Complement Activating Enzymes↗
PubMed1983

Anaphylatoxin formation during hemodialysis: comparison of new and re-used dialyzers.

Hemodialysis of 11 endstage renal failure patients with new cuprophan hollow fiber dialyzers produced significant leukopenia as well as increased plasma levels of both C3a and C5a antigens during the initial phases of the procedure. Formalin-fixed new dialyzers produced quantitatively similar phenomena in eight of these same patients. By contrast, hemodialysis with re-used dialyzers, that is dialyzers exposed to blood prior to formalin sterilization, produced only a 20 to 30% decline in peripheral blood leukocyte counts. Correspondingly, C3a antigen formation within re-used dialyzers was only 20% of that observed in new dialyzers. Re-used dialyzers also differed significantly from either new or formalin-fixed new dialyzers in that C3b antigen could be readily detected within them even after extensive washing. These observations suggest that C3b deposition on the cellulosic membrane surface during first use markedly diminishes the complement activating potential of cuprophan dialyzers when they are subsequently re-used.

Adult↗
PubMed1990

How naturally occurring anti-band 3 antibodies stimulate C3b deposition to senescent and oxidatively stressed red blood cells.

Several naturally occurring antibodies stimulate alternative complement pathway C3b deposition. One type of these antibodies, human anti-band 3 antibody, is involved in opsonizing senescent and oxidatively stressed red cells with C3b (Proc. Natl. Acad. Sci. USA 84[1987]7368). The mechanism was investigated, by which it stimulates alternative complement pathway. Anti-band 3 antibodies are preferred targets for nascent C3b which forms ester bonds with IgG and generates C3b-IgG complexes. Since C3b-IgG and free C3b can equally well nucleate alternative convertases and since C3b in C3b-IgG is protected from degradation (J. Exp. Med. 160[1984]1640), generation of C3b-IgG complexes means stimulation of alternative complement pathway. Anti-band 3 antibodies are preferred targets for the shortlived, nascent C3b since they bind to native C3 by a site distant from the antigen binding site. In vitro generation of C3b-IgG complexes confirmed a preferential formation of C3b-anti-band 3 complexes. Thus, an affinity for C3 may be all what is required to make an antibody a stimulator of the alternative complement pathway and thereby a potent opsonin.

Anion Exchange Protein 1, Erythrocyte↗
PubMed1979

Interference of propamidine with binding of the fifth component of complement to surface-fixed C3b, and with C5 activation.

The effects of propamidine and their dose dependency, on utilization of the third and fifth complement components in immune haemolysis have been compared. While C3 utilization is not disturbed that of C5 is markedly inhibited by propamidine in concentrations as low as 0.5 mM. Both, binding of C5 to surface-fixed C3b and cleavage of C5 by convertases C42 and C3bBb, are also inhibited in the presence of propamidine. Since neither C3 cleavage by these enzymes nor even C5 cleavage by the cobra venom factor-supported convertase CFVBb is significantly reduced a general convertase-inhibiting effect of propamidine is ruled out. Rather the effect on utilization of C5 is the result of interference with binding of C5 is the result of interference with binding of C5 to C3b and hence impairment of its accessibility to the convertases. These findings thus further support the role of surface fixed C3b in C5 activation proposed earlier.

Amidines↗
PubMed1979

Human megakaryocytes. I. Characterization of the membrane and cytoplasmic components of isolated marrow megakaryocytes.

Human marrow megakaryocytes have been isolated with high purity and yield by processing marrow cells sequentially through density centrifugation and velocity sedimentation. Analysis of the isolated cells for various platelet-associated components by immunofluorescence demonstrated that fibrinogen, plasma factor VIII antigen (factor VIII:AGN) platelet myosin, platelet glycoproteins I and III are present on the membrane and in the cytoplasm of over 90% of marrow megakaryocytes. Parallel studies of human and mouse megakaryocytes and platelets for IgG receptor (FcR), complement receptor type one (CR1) (C3b receptor), complement receptor type two (CR2) (C3d receptor), and Ia antigen by fluorescence and (or) rosette formation methods were performed. FcR were present on most human megakaryocytes and platelets. The Ia antigen was detected on a proportion (10-15%) of human megakaryocytes but it was undetectable on human platelets. CR1 was found on 20-40% of mouse megakaryocytes and also on a proportion of mouse platelets. These differentiation markers may be of use in monitoring megakaryocyte maturation.

Animals↗
PubMed2003

Vaccinia virus complement control protein increases early bacterial clearance during experimental peritonitis.

BACKGROUND: Complement is one of the first immunological pathways activated in peritonitis. It functions to initiate and augment the innate immune response. Complement activation has also been shown to contribute to multiple organ failure after sepsis. Vaccinia virus complement control protein (VCP) is an immunomodulatory protein encoded by vaccinia virus and binds complement components C3b and C4b of the complement cascade to inhibit both the classical and alternative pathways of complement activation. This study investigates the effect of complement inhibition by recombinant (r) VCP on bacterial clearance after cecal ligation and puncture (CLP). METHODS: Swiss Webster mice were intravenously given either 20 mg/kg rVCP in 0.2 mL of normal saline, or 0.2 mL of normal saline alone, at the time of CLP. After 4 and 18 h, samples of peritoneal washout, blood, liver, and lung were collected for bacteriology, myeloperoxidase (MPO) assay for neutrophil accumulation, differential cell counts, and interleukin (IL)12 ELISA. Statistical analysis was by Mann-Whitney U test for bacteriology, and analysis of variance (ANOVA) for MPO and IL-12 concentrations. RESULTS: Aerobic and anaerobic bacterial levels were significantly lower at 4 h after treatment with rVCP (p < 0.05) in peritoneal lavage, blood, and liver compared with controls. There were no differences in bacterial levels at 18 h. There were no differences in myeloperoxidase concentrations or in the differential cell counts between the groups at either 4 or 18 h after CLP. IL-12 concentrations in serum or peritoneal washout were also not different. CONCLUSIONS: rVCP enhances early bacterial clearance in mice after CLP, although not through neutrophil recruitment, as MPO concentrations and cell counts were not different. rVCP may, however, increase neutrophil function potentially by prevention of accumulation of complement factors that inhibit leukocytes. Further studies will be needed to elucidate this pathway.

Animals↗
PubMed1998

Complement factor H or a related protein is a marker for transitional cell cancer of the bladder.

The BTAstat and BTA TRAK tests are new immunoassays that detect and measure an antigen in the urine of individuals diagnosed with bladder cancer. As described in this report, the monoclonal antibodies used in these kits were developed by immunizing mice with partially purified protein preparations derived from the urine of patients with bladder cancer. The antigen that is recognized by the monoclonal antibodies was purified from the urine of bladder cancer patients by immunoaffinity chromatography and identified as being either complement factor H (FH) or a closely related protein (CFHrp) by partial amino acid sequence analysis. Like serum FH, the urine antigen was demonstrated to have a complement factor C3b binding site and to accelerate the degradation of C3b in the presence of complement factor I. The culture supernatants from several human bladder, cervical, and renal cancer cell lines contained antigen as determined by immunoassay, and antigen affinity-purified from HeLaS3 culture media was shown to have FH activity. Moreover, the cell lines were shown to make products of the expected sizes by reverse transcription-PCR using FH-specific primers. In contrast, normal human epithelial keratinocytes, a myeloid leukemia cell line, and the colon cancer line LS174T were negative for production of a FH-like protein (CFHrp). We propose that the expression of proteins with FH-like activities may confer a selective growth advantage to cancer cells in vivo by decreasing complement activity, thus aiding their escape from lysis by immune surveillance. Identification of these proteins as cancer products also suggests avenues of chemotherapy or immunotherapy of some cancers.

Amino Acid Sequence↗
PubMed2006

CRIg: a macrophage complement receptor required for phagocytosis of circulating pathogens.

The complement system serves an important role in clearance of pathogens, immune complexes, and apoptotic cells present in the circulation. Complement fragments deposited on the particle surface serve as targets for complement receptors present on phagocytic cells. Although Kupffer cells, the liver resident macrophages, play a dominant role in clearing particles in circulation, complement receptors involved in this process have yet to be identified. Here we report the identification and characterization of a Complement Receptor of the Immunoglobulin superfamily, CRIg, that binds complement fragments C3b and iC3b. CRIg expression on Kupffer cells is required for efficient binding and phagocytosis of complement C3-opsonized particles. In turn, Kupffer cells from CRIg-deficient mice are unable to efficiently clear C3-opsonized pathogens in the circulation, resulting in increased infection and mortality of the host. CRIg therefore represents a dominant component of the phagocytic system responsible for rapid clearance of C3-opsonized particles from the circulation.

Animals↗
PubMed1981

Interaction of particle-bound [125I]C3b, the third component of complement, with specific receptors on human B-lymphoblastoid cells (Raji).

Taking advantage of the high density of the complex formed between the C3b receptor on cultured B-lymphoblastoid cells and particle-bound C3b, some properties of their interaction were studied. The process had an apparent dissociation constant equal to 10(-7) M. Thus particle-bound C3b has a higher affinity for C3b receptor than soluble C3b. Moreover, analysis of dissociation rate of particle-bound C3b-C3b receptor complexes suggested that a cooperative effect was induced at the cell surface by particle-bound C3b but not by soluble C3b. The most suitable explanations of these data are discussed.

B-Lymphocytes↗
PubMed1982

Unique role of the complement receptor CR1 in the degradation of C3b associated with immune complexes.

The main finding of this paper is that CR1, the membrane receptor for C3b and C4b, together with C3b/C4b-inactivator (I), degrades C3b bound to immune complexes (C3b*). Two fragments are generated: C3c, which is released from the immune complexes, and C3d*. The C3c fragment released from the cell intermediate EAC1423b prepared with 125I-C3 was analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and radioautography. It has a 135,000 mol wt and contains disulfide bonded labeled polypeptide chains of 75,000 and 31,000 mol wt, which presumably represent the beta and a fragment of the alpha-chain of C3b*. Silver staining of the SDS-PAGE gels revealed other C3-derived bands with 39-42,000 mol wt. Human erythrocytes + I also cleave C3b* into C3c and C3d*. The activity of the erythrocytes is CR1 mediated because it can be totally inhibited by monoclonal antibodies to CR1. In contrast with these results, I together with the serum protein beta 1H (H) transform EAC1423b into hemolytically inactive EAC1423bi and cleave the alpha' chain of C3b* into fragments of 70,000 and 40,000 mol wt. Small amounts of C3c are also released at relatively high concentrations of H. On a molar basis, the efficiency of CR1 in the generation of C3c and C3d is 10(4)-10(5) greater than H. An additional observation was that C3c could be released by treating EAC1423bi with CR1 + I and that this reaction was also inhibited by monoclonal antibodies to CR1. Therefore, it is likely that CR1 has binding affinity for iC3b and that the degradation of C3b* proceeds as follows: C3b (formula, see text) C3c + C3d*. Taken together, our findings argue that the processing of C3b* in vivo occurs in solid phase, that is, on the surface of cells bearing CR1.

Antigen-Antibody Complex↗
PubMed1983

Importance of factors H and I for the adherence of C3b-coated erythrocytes to cells.

The role of cell membrane-associated human factor H for the binding of cell-bound C3b to complement receptor-carrying (CR+) cells was investigated. Pretreatment of CR+ cells with antibodies to factor H inhibited the adherence of C3b-coated red cells to human tonsil lymphocytes (TL) and peripheral blood monocytes (M phi). The C3b receptor reactivity of human polymorphonuclear leucocytes (PMN) was not influenced and the one of Raji lymphoblastoid cells only slightly influenced; iC3b and C3d receptor reactivity was in no case affected. When diisopropylfluorophosphate (DFP) in a concentration of 0.1 mM was present during pretreatment of the CR+ cells with anti H, the antibodies gained the capacity to inhibit the adherence of C3b-coated erythrocytes to Raji cells; this effect was dose-dependent with respect to DFP. In contrast, there was no influence of DFP on the inhibition pattern of anti H in the case of TL and M phi. The adherence of C3b-coated erythrocytes to PMN remained unaffected by anti-H antibodies in the presence of DFP. Polyclonal as well as monoclonal antibodies directed against human factor I inhibited the binding of C3b cells to Raji cells but not to TL. Additionally, when anti I and anti H antibodies were both present, C3b receptor reactivity of Raji cells was inhibited to a larger extent than with either antibody alone; again, TL remained unaffected. Results obtained by washing the Raji cells before and after treatment with anti H and anti I suggest that the respective antibodies act on factor H primarily on the level of the cell membrane and on factor I in the fluid phase.

Animals↗
PubMed1977

The activation of the C3b feedback cycle with human complement components. I. Through the classical pathway.

Reaction between the fourth, the oxidized second and the activated first components of human complement generated the stable enzyme C4oxy2 capable of cleaving the third component and depleting total complement in human serum. This enzyme was shown further to activate the C3b feedback cycle as shown by its ability to consume factor B in serum and the reduction in the extent of complement consumption in the presence of EDTA. OxyC2 on its own gave rise to C3 cleavage in normal human serum by a pathway needing classical pathway components. This unexpected finding suggests that there may be a 'C-1 tickover' in serum analogous to the 'C3b tickover'; the presence of oxyC2 allowing the 'capture' of the trivial amounts of C42 normally formed. In preliminary experiments in the rat, C4oxy2 was successfully formed in vivo, where it gave rise to cleavage of C3, consumption of C5, depletion of cobra venom factor cofactors and a biphasic change in the neutrophil count.

Animals↗
PubMed2002

Mapping of the sites responsible for factor I-cofactor activity for cleavage of C3b and C4b on human C4b-binding protein (C4bp) by deletion mutagenesis.

Human C4b-binding protein (C4bp) facilitates the factor I-mediated proteolytic cleavage of the active forms of complement effectors C3b and C4b into their inactive forms. C4bp comprises a disulfide-linked heptamer of alpha-chains with complement (C) regulatory activity and a beta-chain. Each alpha-chain contains 8 short consensus repeat (SCR) domains. Using SCR-deletion mutants of recombinant multimeric C4bp, we identified the domains responsible for the C3b/C4b-binding and C3b/C4b-inactivating cofactor activity. The C4bp mutant with deletion of SCR2 lost the C4b-binding ability, as judged on C3b/C4b-Sepharose binding assaying and ELISA. In contrast, the essential domains for C3b-binding extended more to the C-terminus, exceeding SCR4. Using fluid phase cofactor assaying and deletion mutants of C4bp, SCR2 and 3 were found to be indispensable for C4b cleavage by factor I, and SCR1 contributed to full expression of the factor I-mediated C4b cleaving activity. On the other hand, SCR1, 2, 3, 4, and 5 participated in the factor I-cofactor activity for C3b cleavage, and SCR2, 3, and 4 were absolutely required for C3b inactivation. Thus, different sets of SCRs participate in C3b and C4b inactivation, and the domain repertoire supporting C3b cofactor activity is broader than that supporting C4b inactivation by C4bp and factor I. Furthermore, the domains participating in C3b/C4b binding are not always identical to those responsible for cofactor activity. The necessity of the wide range of SCRs in C3b inactivation compared to C4b inactivation by C4bp and factor I may reflect the physiological properties of C4bp, which is mainly directed to C4b rather than C3b.

Antibodies, Monoclonal↗
PubMed1983

C3 cleavage products stimulate release of prostaglandins by human mononuclear phagocytes in vitro.

Human monocytes cultured for up to 48 hr in serum-free, chemically defined culture media released low levels of prostaglandin. C3b, C3bi, and C3c stimulated an indomethacin-sensitive, dose-responsive increase in the amount of monocyte prostaglandin released by 18 hr after treatment. Native C3 and C3d, which do not bind to monocyte receptors, failed to stimulate increased prostaglandin release. Lymphocytes, treated and untreated, produced 10(-2) to 10(-3) as much prostaglandin as the monocytes. These data support the concept that monocytes are a significant source of leukocyte prostaglandin. They also introduce an important new biologic function for the C3 fragments C3b, C3bi, and C3c.

Cells, Cultured↗
PubMed1990

Glycoprotein C of herpes simplex virus type 1 prevents complement-mediated cell lysis and virus neutralization.

Glycoprotein gC1 of herpes simplex virus type 1 (HSV-1) binds complement component C3b. To determine if gC1 modifies the interaction of complement with virus-infected cells or cell-free virus, ns-1, a mutant HSV-1 strain that does not express gC1 at the cell surface and does not bind C3b, was compared with its parental strain, NS. Cells infected with the gC1 mutant were more susceptible to cytolysis mediated by antibody and complement or complement alone. The gC1 or gD1 genes were expressed in mammalian cells under the control of an inducible promoter. Cells induced to express gC1 resisted complement cytolysis, while cells expressing gD1 did not. gC1 modified cytolysis of virus-infected or -transfected cells by blocking alternative complement pathway activation. gC1 also modified complement-dependent virus neutralization, which was mediated by inhibiting the classical complement pathway. These results indicate a protective role for gC1 on the virion and at the cell surface.

Antibody-Dependent Cell Cytotoxicity↗
PubMed1996

Ligand-binding renders the 160 kDa Trypanosoma cruzi complement regulatory protein susceptible to proteolytic cleavage.

It has been previously shown that a surface complement regulatory protein (CRP) of Trypanosoma cruzi trypomastigotes binds human complement components C3b and C4b, and inhibits C3 convertase formation, thus contributing to the resistance of the bloodstage parasites to complement-mediated lysis. The blood stage parasites rapidly and spontaneously release a limited set of membrane glycoproteins, including CRP, and the modulation of release of CRP following ligand binding was investigated. Incubation of the parasites with C3b results in the release of CRP at a reduced apparent molecular mass. To determine if proteolytic processing was responsible for the reduction in apparent molecular mass of the released CRP, the proteolytic activity present in trypomastigote membrane preparations was examined. In addition to a well described cysteine protease, a novel 75 kDa protease was identified in tissue culture-derived trypomastigotes and axenically-derived metacyclic trypomastigotes membrane preparations. This protease was inhibited by aprotinin and leupeptin, but not L-trans-epoxysuccinyl-leucylamide-(4-guanidinobutane), N-[N-L-3-transcarboxyirane-2-carbonyl-L-leucinyl]-agmatine (E64). Treatment of the parasites with protease inhibitors did not affect spontaneous shedding of proteins, however, protease inhibitors abrogated the effect of C3b-binding on CRP degradation. These results indicate that binding of C3b to the CRP renders the CRP susceptible to cleavage by a parasite protease, possibly as a means of removing the CRP-C3b from the parasite surface. This process may represent an additional immune evasion mechanism which allows the parasite to avoid both complement-mediated lysis and clearance.

Animals↗