Detection of native human complement components C3 and C5 and their primary activation peptides C3a and C5a (anaphylatoxic peptides) by ELISAs with monoclonal antibodies

Succinimide-Functionalized Reduced Graphene Oxide Nanosheets: A High-throughput Resistive Sensing Platform for Age-Related Macular Degeneration Biomarker Determination Using Human Tears

Age-related macular degeneration (AMD) is a well-recognized affliction among the elderly, causing vision impairment ranging from blurred vision to complete blindness. This underscores the critical need for accurate, precise, and early detection methods. Herein, we developed a noninvasive, label-free electrical biosensor, constructed on an economical printed circuit board (PCB) substrate, designed specifically for the precise quantification of AMD biomarker: complement component III (C3). The hydrothermally reduced graphene oxide (rGO) was deposited between gold-interdigitated microelectrodes, forming a conductive channel. The fabricated C3 biosensor exhibits a low detection limit of 0.4342 ng/mL and an impressive sensitivity of 9.238 ((ΔR/R)/ng.mL-1)/cm2 with a regression coefficient of 0.9815 calibrated within the clinical C3 range of 10-30 ng/mL. This excellent performance is ascribed to the synergistic effects of 1-pyrenebutanoic acid succinimidyl ester (PBASE) linker and conducting properties of rGO as they generate large active sites for higher anti-C3 antibody immobilization, thereby enhancing sensitivity and specificity. Furthermore, the performance of this proposed C3 sensor chip was validated with enzyme-linked immunosorbent assay (ELISA) using five human tear samples exhibiting an outstanding correlation of a regression value of 0.9774. The unparalleled merits of this newly crafted C3 biosensor transcend those of preceding platforms, boasting superior accuracy and precision in quantifying C3 levels in human tears, accelerated operational speed with results attainable within a mere 15 min, cost-effectiveness, and excellent sensitivity.

Development of the Complement C5 Assay by LC-MS/MS in Monkey Serum and Comparison with Enzyme-Linked Immunosorbent Assay

Complement C5 (C5) is the key component for the complement activation pathway, which is important for innate immunity, and inhibition of C5 is considered to be effective in antibody-mediated rejection in organ transplantation. Thus determination of C5 levels in systemic circulation is a simple way to understand efficacy of drugs that aim to inhibit C5 production. We have developed a simple liquid chromatography with tandem mass spectrometry (LC-MS/MS) assay for C5 in cynomolgus monkey serum. C5 in monkey serum was subjected to tryptic digestion, and two signature peptides, DSSVPNTGTAR and LQGTLPVEAR, were assayed by LC-MS/MS with electrospray ionization in the positive ion mode. Assay reproducibility in serum samples was evaluated, and the assay was applied to the C5 assay in monkey serum after administration of C5 siRNA encapsulated in lipid nanoparticles to monkeys. The time profiles of C5 after administration of C5 siRNA were comparable between the two signature peptides by LC-MS/MS and were also similar to those by an enzyme-linked immunosorbent assay using an assay kit. These findings suggest that the established LC-MS/MS assay of C5 is reliable to determine C5 levels in monkey serum.

Cell-inspired selective potassium removal towards hyperkalemia therapy by microphase-isolated core-shell microspheres

Hyperkalemia is a common metabolic problem in patients with chronic kidney disease. Although oral medications and hemodialysis are clinically applied for lowering serum potassium, the intrinsic limitations encourage alternative therapy in the trend of adsorbent-based miniaturized blood purification devices. Cells serve as the biological K⁺ storage units that accumulate K⁺ through multiple mechanisms. Inspired by cells, our strategy aims at favorable permeation and enrichment of K⁺ in the microsphere. We incorporate cation-affinitive groups into core-shell structures with submicron-sized phase separation. These nano-spaced side-groups cooperate to form interlinked clusters, where crown ethers with Angstrom-scale ring for size-matched complexation, while ionic sulfonic acid groups for hydrophilicity and charge-buffering. The unique structure with such non-covalent interactions facilitates K⁺ for permeation across the shell and binding to the core while also ensuring mechanical strength and anti-swelling durability in biofluids. The microspheres exhibit high selectivity ratios of K⁺ (S_K/Na, S_K/Ca, S_K/Mg up to 9.8, 21.6, and 17.7). As column adsorbents for hemoperfusion simulation, they effectively lower elevated K⁺ levels to the normal range (clearance rates up to 44.4%/45.3% for hyperkalemic human serum/blood). Blood compatibility tests show low protein adsorption, preferable hemocyte compatibility, and anticoagulation property in vitro. This promising strategy has clinical potential for hyperkalemia in high-risk patients. STATEMENT OF SIGNIFICANCE: Hyperkalemia (serum potassium >5 mmol/L) is a common complication in chronic renal failure patients. The limitations of existing treatments prompt a shift to wearable artificial kidney technology for clinical convenience and efficacy. Existing treatments have limitations, and we turn to adsorbent-based miniaturized blood purification devices in the prospect of wearable artificial kidney technology. There exists a lack of ion-specific adsorbents applied in extracorporeal circuits to redress electrolyte imbalances like hyperkalemia. Inspired by cells, we aim at the favorable permeation and enrichment of K⁺ by microspheres. The microspheres have a microphase-isolated core-shell structure, whose nano-spaced groups form cation-affinitive clusters. Selective K⁺ removal and blood compatibility are achieved. We expect this strategy to enlighten alternative hyperkalemia therapy for these high-risk patients.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors

The activation of the complement cascade, a cornerstone of the innate immune response, produces a number of small (74-77 amino acid) fragments, originally termed anaphylatoxins, that are potent chemoattractants and secretagogues that act on a wide variety of cell types. These fragments, C5a, C4a, and C3a, participate at all levels of the immune response and are also involved in other processes such as neural development and organ regeneration. Their primary function, however, is in inflammation, so they are important targets for the development of antiinflammatory therapies. Only three receptors for complement peptides have been found, but there are no satisfactory antagonists as yet, despite intensive investigation. In humans, there is a single receptor for C3a (C3a receptor), no known receptor for C4a, and two receptors for C5a (C5a₁ receptor and C5a₂ receptor). The most recently characterized receptor, the C5a₂ receptor (previously known as C5L2 or GPR77), has been regarded as a passive binding protein, but signaling activities are now ascribed to it, so we propose that it be formally identified as a receptor and be given a name to reflect this. Here, we describe the complex biology of the complement peptides, introduce a new suggested nomenclature, and review our current knowledge of receptor pharmacology.

Sleep and circadian rhythm regulate circulating complement factors and immunoregulatory properties of C5a

The sleep-wake cycle is characterized by complex interactions among the central nervous, the endocrine and the immune systems. Continuous 24-h wakefulness prevents sleep-associated hormone regulation resulting in impaired pro-inflammatory cytokine production. Importantly, cytokines and hormones also modulate the complement system, which in turn regulates several adaptive immune responses. However, it is unknown whether sleep affects the activation and the immunoregulatory properties of the complement system. Here, we determined whether the 24-h sleep-wake cycle has an impact on: (i) the levels of circulating complement factors; and (ii) TLR4-mediated IL-12 production from human IFN-γ primed monocytes in the presence or absence of C5a receptor signaling. For this purpose, we analyzed the blood and blood-derived monocytes of 13 healthy donors during a regular sleep-wake cycle in comparison to 24 h of continuous wakefulness. We found decreased plasma levels of C3 and C4 during nighttime hours that were not affected by sleep. In contrast, sleep was associated with increased complement activation as reflected by elevated C3a plasma levels during nighttime sleep. Sleep deprivation prevented such activation. At the cellular level, C5a negatively regulated TLR4-mediated IL-12p40 and p70 production from human monocytes. Importantly, this regulatory effect of C5a on IL-12p70 production was effective only during daytime hours. Thus, similar to hormones, some complement factors and immunoregulatory properties of C5a are influenced by sleep and the circadian rhythm. Our findings that continuous wakefulness has a negative impact on complement activation may provide a rationale for the immunosupportive functions of sleep.

Cobra venom factor: Structure, function, and humanization for therapeutic complement depletion

Cobra venom factor (CVF) is the complement-activating protein in cobra venom. This manuscript reviews the structure and function of CVF, how it interacts with the complement system, the structural and functional homology to complement component C3, and the use of CVF as an experimental tool to decomplement laboratory animals to study the functions of complement in host defense and immune response as well as in the pathogenesis of diseases. This manuscript also reviews the recent progress in using the homology between CVF and C3 to study C3 structure and function, and to develop human C3 derivatives with the complement-depleting function of CVF. These human C3 derivatives represent humanized CVF, and are a conceptually different concept for pharmacological intervention of the complement system, therapeutic complement depletion. The use of humanized CVF for therapeutic complement depletion in several pre-clinical models of human diseases is also reviewed.

Defining targets for complement components C4b and C3b on the pathogenic neisseriae

Complement is a key arm of the innate immune defenses against the pathogenic neisseriae. We previously identified lipooligosaccharide on Neisseria meningitidis as an acceptor for complement C4b. Little is known about other neisserial targets for complement proteins C3 and C4, which covalently attach to bacterial surfaces and initiate opsonization and killing. In this study we demonstrate that Neisseria gonorrhoeae porin (Por) 1B selectively binds C4b via amide linkages and C3b via ester linkages. Using strains expressing hybrid Por1A/1B molecules, a region spanned by loops 4 and 5 of Por1B was identified as the preferred binding site for C4b. We also identified the opacity protein (Opa), a major adhesin of pathogenic neisseriae, as a target for C4b and C3b on both N. meningitidis and N. gonorrhoeae. Using N. gonorrhoeae variants that predominantly expressed individual Opa proteins, we found that all Opa proteins tested (A, B, C, D, E, F, and I) bound C4b and C3b via amide and ester linkages, respectively. Amide linkages with Por1B and Opa were confirmed using serum containing only the C4A isoform, which exclusively forms amide linkages with targets. While monomers and heterodimers of C4Ab were detected on bacterial targets, C4Bb appeared to preferentially participate in heterodimer (C5 convertase) formation. Our data provide another explanation for the enhanced serum sensitivity of Por1B-bearing gonococci. The binding of C3b and C4b to Opa provides a rationale for the recovery of predominantly "transparent" (Opa-negative) neisserial isolates from persons with invasive disease, where the bacteria encounter high levels of complement.

C5a mutants are potent antagonists of the C5a receptor (CD88) and of C5L2: position 69 is the locus that determines agonism or antagonism

The anaphylatoxin C5a exerts a plethora of biologic activities critical in the pathogenesis of systemic inflammatory diseases. Recently, we reported on a C5a mutant, jun/fos-A8, as a potent antagonist for the human and mouse C5a receptor (CD88). Addressing the molecular mechanism accounting for CD88 receptor antagonism by site-directed mutagenesis, we found that a positively charged amino acid at position 69 is crucial. Replacements by either hydrophobic or negatively charged amino acids switched the CD88 antagonist jun/fos-A8 to a CD88 agonist. In addition to CD88, the seven-transmembrane receptor C5L2 has recently been found to provide high affinity binding sites for C5a and its desarginated form, C5adesArg74. A jun/fos-A8 mutant in which the jun/ fos moieties and amino acids at positions 71-73 were deleted, A8Delta71-73, blocked C5a and C5adesArg74 binding to CD88 and C5L2. In contrast, the cyclic C5a C-terminal analog peptide AcF-[OP-d-ChaWR] inhibited binding of the two anaphylatoxins to CD88 but not to C5L2, suggesting that the C5a core segment is important for high affinity binding to C5L2. Both receptors are coexpressed on human monocytes and the human mast cell line HMC-1; however, C5L2 expression on monocytes is weaker as compared with HMC-1 cells and highly variable. In contrast, no C5L2 expression was found on human neutrophils. A8Delta71-73 is the first antagonist that blocks C5a and C5adesArg74 binding to both C5a receptors, CD88 and C5L2, making it a valuable tool for studying C5L2 functions and for blocking the biological activities of C5a and C5adesArg74 in mice and humans.

Pre-neutralization of C5a-mediated effects by the monoclonal antibody 137-26 reacting with the C5a moiety of native C5 without preventing C5 cleavage

Complement C5a is aetiologically linked to inflammatory tissue damage in conditions like septicaemia, immune complex diseases and ischaemia-reperfusion injury. We here describe a monoclonal antibody (mAb), 137-26, that binds to the C5a moiety of human C5 and neutralizes the effects of C5a without interfering with C5 cleavage and the subsequent formation of lytic C5b-9 complex. Mouse anti-human C5 mAbs were generated and the reactivity with C5 and C5a was detected by ELISA and surface plasmon resonance. The inhibition of C5a binding to C5a receptor was studied using a radioligand binding assay. The effects of the antibody on C5a functions were examined using isolated neutrophils and a novel human whole blood model of inflammation. Haemolytic assays were used to study the effect on complement-mediated lysis. mAb 137-26 reacted with both solid- and solution-phase C5 and C5a in a dose-dependent manner with high affinity. The antibody competed C5a binding to C5a receptor and inhibited C5a-mediated chemotaxis of neutrophils. Furthermore, the antibody effectively abrogated complement-dependent E. coli-induced CD11b up-regulation and oxidative burst in neutrophils of human whole blood. mAb 137-26 was more potent than a C5a receptor antagonist and a previously described anti-C5a antibody. mAb 137-26 did not inhibit complement-mediated lysis, nor did it activate complement itself. Together, mAb 137-26 binds both the C5a moiety of native C5 and free C5a, thereby effectively neutralizing the biological effects of C5a. The antibody may have therapeutic potential in inflammatory diseases where C5a inhibition combined with an operative lytic pathway of C5b-9 is particularly desired.

Biocompatibility of hemodialysis membranes: interrelations between plasma complement and cytokine levels

Hemodialysis (HD) membrane biocompatibility is defined as absence of complement activation. We have recently shown that circulating levels of interleukin (IL) 1 and IL-2 predict death and survival, respectively, of HD patients. Studies have assessed IL-1 in treatments with biocompatible and less biocompatible dialysis membranes, but no study has correlated circulating levels of all these immunoreactants. We assessed these immunoreactants, and temperature as an outcome, during HD in patients treated with different membranes. Twelve stable patients, receiving thrice-weekly chronic bicarbonate HD, were randomly dialyzed with three different types of membranes, composed of: Cuprophan, cuprammonium rayon modified cellulose, and Hemophan. Blood was drawn from the arterial line port before (Pre) and 15, 30, and 60 min during and after (Post) HD. Patients' temperatures were measured before and after each treatment. The plasma concentrations of IL-1 and IL-2 and factors C3a and C5a were assessed by ELISA. There were no differences between baseline levels of any of the immunoreactants in patients treated with different dialyzers. C3a, C5a, and IL-1 levels increased significantly during HD treatments with all three different membranes. C3a, C5a, and IL-1 levels during Cuprophan and Hemophan treatments were significantly higher than the levels during modified cellulose treatment at 30 and 60 min and Post (p < 0.01). For all the immunoreactants, however, the Post levels were higher than the Pre levels. In contrast to IL-1, there were no differences in mean IL-2 levels during treatments when different membranes were compared. There were few correlations of plasma C3a and C5a levels with plasma IL-1 levels, but there was only one treatment time in one dialyzer group during which IL-2 and any of the other factors were correlated. Pre and Post temperature values and percent change in temperature were not correlated with any of the immunoreactants measured. These data show that C3a, C5a, and IL-1 responses are similar, but not identical, during treatments with different membranes. The response of circulating IL-2 levels to treatments is quite different from that of plasma C3a, C5a and IL-1 levels and suggests that these changes are not solely due to treatment factors. Treatment with modified cellulose membranes is associated with a different immunoreactive profile as compared with patients dialyzed using other cellulose membranes. We suggest that circulating IL-1 levels are good biocompatibility markers.

Guinea pig C3 specific rabbit single chain Fv antibodies from bone marrow, spleen and blood derived phage libraries

We constructed combinatorial immunoglobulin libraries from the whole rabbit antibody repertoire of bone marrow, spleen and peripheral blood of a rabbit immunized with guinea pig complement protein C3. By means of the phage display technology we selected guinea pig C3 specific single chain Fv (scFv) antibodies from each of the libraries. None of the scFv antibodies cross reacted with guinea pig C3a, human C3 or rat C3. The frequency of bone marrow derived C3 positive clones was much higher as compared to blood or spleen derived clones. Additionally bone marrow and spleen derived clones show higher diversity than clones, obtained from blood, as determined by fingerprint analysis with the restriction enzyme AluI. Dissociation rate constants for all scFvs were similar, indicating that the source of the scFvs had no influence on affinities. The antibody fragments were used to analyze complement activation during xenotransplantation. Several blood or bone marrow derived scFvs bound to C3 located on rat liver endothelium after hyperacute rejection of a heterotopically transplanted rat liver into guinea pig. These data demonstrate that monoclonal rabbit scFvs can be easily generated from recombinant phage display libraries, constructed from spleen, blood or bone marrow. The selected guinea pig C3 specific scFvs appear to be useful to detect complement activation during xenotransplantation in guinea pigs.

Complement and contact activation related to surfactant response in respiratory distress syndrome

The activation of inflammation and coagulation cascades is part of the pathogenesis of adult respiratory distress syndrome (RDS). Previous studies have demonstrated contact activation in preterm infants with RDS, whereas no concordant results have been found with complement activation. In this study, both systems were investigated in preterm infants with severe RDS and related to surfactant response. Thirty preterm newborns with severe respiratory distress (FiO2 > 0.5), but with no evidence of infection or fetal acidosis, were studied. Eighteen healthy preterm newborns of similar gestational age and birth weight served as controls. The study group was divided into two subgroups, according to their response to a porcine natural surfactant 6 h after administration: responders (FiO2 reduction > 50%) and poor responders (FiO2 reduction < or = 50%). C1q, C4, factor B, C3a, C5a, complement, and C1-inhibitor activity, as well as factor XIIa, were determined in blood samples, drawn 24 h after birth. Except for C1-inhibitor concentration and C1-inhibitor activity, all parameters for infants with severe RDS were different from controls. Complement precursor proteins were lower, and activated split products of the complement and contact system were higher. Infants with a poor response after application of surfactant showed higher amounts of C3a, C5a, and factor XIIa but lower C1q and C4 levels compared with infants with a good response to surfactant. Activation of the complement and the contact system was demonstrated in all respiratory distress patients. This activation was more pronounced in poor responders to exogenous surfactant.

Complement activation during plasma production depends on the apheresis technique

Elevated anaphylatoxin concentrations have been found in fresh frozen plasma packs produced by apheresis. The aim of this study was to investigate anaphylatoxin generation during apheresis production in relation to two frequently used techniques: employing either centrifugation alone or centrifugation with simultaneous filtration. The concentrations of C3a. C5a and sC5b-9 were measured in 50 plasmas after apheresis and before freezing generated by combined centrifugation and filtration and in 50 plasmas generated solely by centrifugation and in the corresponding 100 donors before apheresis. The median C3a concentration increased during apheresis by centrifugation alone from 62 (donor) to 380 micrograms L-1 (FFP pack) and during apheresis by combined centrifugation and filtration from 70 to 992 micrograms L-1. The median C5a concentration increased during apheresis by centrifugation alone from 0.38 to 0.83 microgram L-1 and during apheresis by combined centrifugation and filtration from 0.29 to 4.9 micrograms L-1. The soluble terminal complement complex increased only by combined centrifugation and filtration apheresis from 162 to 426 micrograms L-1. Complement activation during apheresis by combined centrifugation and filtration is more pronounced than that obtained by the centrifugation method alone. Changes in the construction or the materials used in the membrane-type separation units may reveal possibilities of prevention.

Complement system in healthy term newborns: reference values in umbilical cord blood

Activation of the complement system occurs in several diseases. For reliable identification of complement activation in neonates, we establish reference ranges of several components in cord blood of healthy term newborns. For this study, cord blood samples were taken from 125 healthy term newborns. Concentrations of C1r, C2, C5, C7, Properdin, and factors D, H, and I were determined by single radial immunodiffusion. C3a and C5a were measured by specific EIA and complement function was measured by hemolytic assays. The results were expressed as 5th percentile, median, and 95th percentile. The following respective concentrations were found: C1r: 27, 47, 65 mg/l; C2: 12.0, 18.0, 24.0 mg/l; C5: 64, 92, 127 mg/l; C7: 32, 60, 89 mg/l; Properdin: 5.6, 9.7, 14.2 mg/l; factor D: 3.6, 5.2, 7.3 mg/l; factor H: 178, 234, 296 mg/l; and factor I: 15, 24, 32 mg/l. The functional activity of the whole complement system was 24%, 43%, 97% and for the alternative pathway 39%, 58%, 76%. The concentration of the activated split products C3a was 4, 65, 255 microg/l and of C5a, 0.11, 0.26, 1.19 microg/l. These reference values may be important for the detection of deficiencies of native complement proteins or perinatal processes leading to an activation of the complement system.

Complement and contact activation in term neonates after fetal acidosis

AIMS

To evaluate complement and contact activation after fetal acidosis.

METHODS

Fifteen term neonates with hypoxic-ischaemic encephalopathy after umbilical arterial pH < 7.10 were compared with 15 healthy neonates with umbilical arterial pH > 7.20. Determinations of the complement function and C1-inhibitor activity were performed as kinetic tests 22-28 hours after birth. C1q, C1-inhibitor, and factor B concentrations were determined by radial immunodiffusion and those of C3a, C5a, and factor XIIa by enzyme immunoabsorbent assay.

RESULTS

Median complement function (46 vs 73%), C1q (4.3 vs 9.1 mg/dl), and factor B (5.2 vs 7.7 mg/dl) decreased after fetal acidosis. The activated split products C3a (260 vs 185 micrograms/l), C5a (5.0 vs 0.6 micrograms/l), and factor XIIa (3.2 vs 1.3 micrograms/l) increased in the neonates after fetal acidosis. No differences were found in the concentration and activity of C1-inhibitor.

CONCLUSIONS

Complement and contact activation occurred in the newborns with hypoxic-ischaemic encephalopathy. Activation of these systems generates mediators which can trigger inflammation and tissue injury.

Complement and contact activation during cardiovascular operations in infants

BACKGROUND

By comparing the results of cardiac operations with or without cardiopulmonary bypass (CPB) in infants in a prospective study, we sought to determine which part of the postoperative systemic inflammatory response was caused by CPB.

METHODS

Thirty-five patients were divided into two groups: 11 infants operated on without CPB and 24 infants operated on with CPB. Blood samples were drawn before, during, and after the operation. We assessed complement function and the concentrations or activities of C1q, C3, C4, C1 inhibitor, factor B, the activated split product C3a, and prekallikrein and factor XIIa of the contact system.

RESULTS

All of the patients exhibited a decrease of complement proteins. This was greater in infants who underwent CPB. A increase in C3a and factor XIIa and changes in prekallikrein activity occurred only in infants during CPB.

CONCLUSIONS

Complement activation occurs in all infants, but is significantly higher in the group with CPB. Contact activation only occurs in patients who undergo CPB. Thus, the inflammatory response is caused by the use of a CPB circuit and to a lesser degree by surgical procedures and anesthesia.

Acidosis activates complement system in vitro

We investigated the in vitro effect of different forms of acidosis (pH 7.0) on the formation of anaphylatoxins C3a and C5a. Metabolic acidosis due to addition of hydrochloric acid (10 micromol/ml blood) or lactic acid (5.5 micromol/ml) to heparin blood (N=12) caused significant activation of C3a and C5a compared to control (both p=0.002). Respiratory acidosis activated C3a (p=0.007) and C5a (p=0.003) compared to normocapnic controls. Making blood samples with lactic acidosis hypocapnic resulted in a median pH of 7.37. In this respiratory compensated metabolic acidosis, C3a and C5a were not increased. These experiments show that acidosis itself and not lactate trigger for activation of complement components C3 and C5.

In vitro activation of complement and contact system by lactic acidosis

The activation of complement and contact systems occurs in reperfusion injuries with initial tissue hypoxia, and lactic acidosis such as mycardial infarction and birth asphyxia. The aim of our experiment was the formal proof of activation by sole lactic acidosis. Lactic acid was added to blood and plasma samples from 10 healthy volunteers. C5a and factor XIIa were measured by EIA after incubation at 37 degrees C for 1 h. Both concentrations increased (P < 0.0001 by Friedman analysis) in blood and plasma samples with increasing amount of added lactic acid. Lactic acidosis can activate C5 from the complement system and factor XII from the contact system directly, even in the absence of cellular components.

Complement factor C3 deposition and serum resistance in isogenic capsule and lipooligosaccharide sialic acid mutants of serogroup B Neisseria meningitidis

Serogroup B meningococci express sialic acids on their surfaces as a modification of the lipooligosaccharide (LOS) and as capsular material consisting of alpha2,8-linked sialic acid homopolymers. The aim of this study was to elucidate the impact of each sialic acid component on the deposition of complement factor C3 and serum resistance. For this purpose, we used isogenic mutants deficient in capsule expression (a polysialyltransferase mutant) or sialylation of the LOS (a galE mutant) or both (a mutant with a deletion of the cps gene locus). Bactericidal assays using 40% normal human serum (NHS) demonstrated that both the capsule and LOS sialic acid are indispensable for serum resistance. By immunoblotting with monoclonal antibody MAb755 that is specific for the C3 alpha-chain, we were able to demonstrate that C3 from 40% NHS was covalently linked to the surface structures of meningococci as C3b and iC3b, irrespective of the surface sialic acid compounds. However, C3b linkage was more pronounced and occurred on a larger number of target molecules in galE mutants with nonsialylated LOS than in meningococci with wild-type LOS, irrespective of the capsule phenotype. C3b deposition was caused by both the classical pathway (CP) and the alternative pathway of complement activation. Use of 10% NHS revealed that at low serum concentrations, C3 deposition occurred via the CP and was detected primarily on nonsialylated-LOS galE mutants, irrespective of the capsular phenotype. Accordingly, immunoglobulin M (IgM) binding to meningococci from heat-inactivated NHS was demonstrated only in both encapsulated and unencapsulated galE mutants. In contrast, inhibition of IgA binding required both encapsulation and LOS sialylation. We conclude that serum resistance in wild-type serogroup B meningococci can only be partly explained by an alteration of the C3b linkage pattern, which seems to depend primarily on the presence of wild-type LOS, since a serum-resistant phenotype also requires capsule expression.

Anaphylatoxins in fresh-frozen plasma

BACKGROUND

Fresh-frozen plasma (FFP) is widely used in patients with coagulation disorders and simultaneous complement activation. Complement activation in FFP itself is poorly investigated.

STUDY DESIGN AND METHODS

The concentration of anaphylatoxins C3a and C5a, the complement precursors C1q and factor B, and complement function were measured in 40 consecutively administered FFP units in two pediatric neonatal intensive care units. In 12 samples, the measurements were also performed after incubation with inulin.

RESULTS

In 15 of 40 FFP units, both anaphylatoxin concentrations were below the upper cutoff levels reported for healthy humans (C3a, 500 microg/L; C5a, 5 microg/L). Anaphylatoxin levels were higher in FFP units produced by apheresis than in those from blood donation. Complement activation of FFP by inulin increased anaphylatoxin concentration, whereas C1q and factor B levels, and complement function remained unchanged.

CONCLUSION

Elevated concentrations of anaphylatoxin are frequently found in FFP units produced by apheresis. Studies are necessary to investigate the reasons for complement activation and the possibilities of prevention during apheresis. As the concentrations of complement precursors and complement function did not change with activation in FFP, these studies should include measurement of the anaphylatoxins C3a and C5a.

C3a and C5a Stimulate Chemotaxis of Human Mast Cells

The factors that control migration of mast cells to sites of inflammation and tissue repair remain largely undefined. Whereas several recent studies have described chemotactic factors that induce migration of murine mast cells, only stem cell factor (SCF ) is known to induce migration of human mast cells. We report here that the anaphylatoxins C3a and C5a are chemotactic factors for the human mast cell line HMC-1, human cord blood-derived mast cells (CBMC) and cutaneous mast cells in vitro. The presence of an extracellular matrix protein, laminin, was required for chemotaxis in response to complement peptides. Migration of mast cells towards C3a and C5a was dose-dependent, peaking at 1 μg/mL (100 nmol/L), and was inhibited by specific antibodies. Pretreatment with pertussis toxin inhibited the anaphylatoxin-mediated migration of HMC-1 cells, indicating that Gi proteins are involved in complement-activated signal transduction pathways in human mast cells. Both C3a and C5a also induced a rapid and transient mobilization of intracellular free calcium ([Ca2+]i ) in HMC-1 cells. Besides SCF, other chemotactic factors tested, such as interleukin-3, nerve growth factor, transforming growth factor β, RANTES (regulated upon activation, normal T cell expressed and secreted), monocyte chemotactic protein-1 (MCP-1), MCP-2, MCP-3, macrophage inflammatory protein-1α (MIP-1α), and MIP-1β, failed to stimulate migration of human mast cells. In summary, these findings indicate that C3a and C5a serve as chemotaxins for human mast cells. Anaphylatoxin-mediated recruitment of mast cells might play an important role in hypersensitivity and inflammatory processes.

Both plasma- and leukocyte-associated C5a are essential for assessment of C5a generation in vivo

BACKGROUND

Measurement of C5a in plasma is hampered by the rapid clearance of C5a as a result of cell binding. Therefore, an assessment of whether cell-bound C5a might better reflect C5a generation in vivo is essential.

METHODS

We quantified plasma and leukocyte-bound C5a in samples from patients undergoing cardiopulmonary bypass, which is known to be associated with complement activation. C3 activation products and the terminal complement complex were measured as well.

RESULTS

Plasma levels of C3 activation products and the terminal complement complex increased rapidly and significantly after the onset of cardiopulmonary bypass until they reached a plateau after 30 minutes. The concentration of plasma C5a increased steadily to twice baseline at the end of bypass. The concentration of leukocyte-associated C5a increased threefold after 10 minutes of cardiopulmonary bypass, when a plateau was reached. A positive correlation was found between levels of plasma C3 activation products or terminal complement complex and plasma C5a plus cell-associated C5a but not between C3 activation products or terminal complement complex and either one of the C5a variables.

CONCLUSIONS

We conclude that both plasma C5a and leukocyte-associated C5a are needed for monitoring in vivo C5a generation.

A selection system to study C5a-C5a-receptor interactions: phage display of a novel C5a anaphylatoxin, Fos-C5aAla27

Binding and effector domains of the human anaphylatoxin C5a have been determined by either site directed mutagenesis or synthetic peptide studies. However, the lack of specific selection methods, which allow direct investigation of C5a-C5a-receptor interaction made these studies laborious. To overcome these limitations we have constructed a novel Fos-C5a expressed on the tip of a filamentous phage. To guarantee for a free C-terminus which is required for C5a activity C5a cDNA was cloned into the phagemid vector pJuFo. Helper phage infection of pJuFc-C5a transformed cells resulted in a mutant phage displaying Fos-C5a on its surface. However studies with Bt2cAMP differentiated U937 cells revealed that phage displayed Fos-C5a is functional inactive. Subsequently we replaced a nonconserved cysteine residue at position 27 by alanine and obtained Fos-C5aAla27. Both the purified and the phage displayed Fos-C5aAla27 proteins were functional active and induced enzyme release from differentiated U937 cells. In addition, purified Fos-C5aAla27 exhibited the same binding profile as compared to rhC5a. Fos-C5aAla27 displaying phages were mixed with phage harboring only the pJuFo plasmid at a ratio of 10(6). After four successive rounds of panning on differentiated U937 cells Fos-C5aAla27 phages were enriched to 100% as shown by C5a-specific ELISA. We expect this approach to prove helpful for studying C5a-C5a-receptor interactions. i.e. to screen C5a libraries for high affinity binders with agonistic or antagonistic properties directly on cells.

Detection of renal allograft rejection by complement components C5A and TCC in plasma and urine

Allograft rejection is associated with complement activation. Yet inconsistent results were obtained in evaluating plasma levels of complement factors or activation products as rejection markers. Therefore the human anaphylatoxin C5a and the soluble terminal complement complex (TCC) were measured by daily enzyme immunoassays on plasma (P) and urine (U) samples from 28 patients undergoing renal transplantation over a mean postoperative period of 25.8 days. The complement levels were evaluated longitudinally (cutoff of 100% increase on the previous day's level) during periods of rejection, stable graft function, acute tubular necrosis, and cytomegalovirus disease. Regarding the detection of 13 acute rejection episodes, U-C5a showed a diagnostic accuracy of 81% (sensitivity of 85%, specificity of 77%), P-C5a one of 62%, and P-TCC one of only 30%. The U-C5a increment (mean rise of 379%) preceded the clinical diagnosis of rejection by an average of 1.6 days. Cytomegalovirus diseases (n = 4) were associated with high P-C5a levels (mean increase of 251% by the time of the first detection of viral DNA). In contrast, resumption of kidney function after acute tubular necrosis (n = 10 periods) was heralded by marked peaks of U-C5a (x = 43.7 microg/l). U-TCC was not detected in any clinical setting. In conclusion, as opposed to P-TCC, U-TCC, and P-C5a, the anaphylatoxin C5a, measured daily in urine, might have potential as an early and reliable marker for acute renal allograft rejection.

Epitope mapping of a C5a neutralizing mAb using a combined approach of phage display, synthetic peptides and site-directed mutagenesis

BACKGROUND

The anaphylatoxin C5a is a powerful proinflammatory protein generated on activation of the complement system. Recently, we described an anti-hC5a neoepitope specific mAb, mAb 2925, which was raised against the nonapeptide ISHKDMQLG (C5a-(65-73). This mAb is unique in that it recognizes both hC5a and hC5adesArg, even when it is denatured. It inhibits binding of [125I]C5a to its receptor on Bt2-cAMP differentiated U937 cells.

OBJECTIVES

To define the epitope of mAb 2925, we used a combined approach of a bacteriophage random octapeptide library, synthetic peptides and site-directed mutagenesis.

STUDY DESIGN

First a phage peptide library was screened with the anti C5a mAb 2925. Then synthetic peptides were synthesized with respect to the sequence information yielded from the phage approach, and used for binding studies. Site-directed mutagenesis was performed to confirm the results from the mapping experiments.

RESULTS AND CONCLUSION

Most phages selected by biotinylated Fab 2925 displayed sequences on the minor coat protein which correspond to residues within the C-terminus of human C5a. A first consensus motif comprised amino acids His-Lys or His-Arg, which allowed us to define position 67 and 68 as part of the epitope. A second consensus motif was selected, comprising Arg/Lys-Trp-Trp. This motif did not match any residues within the C5a C-terminus. However, when expressed together with the consensus motif His-Arg, as in HRWWXXXX or in HRXKWWXX, binding of these peptides to Fab 2925 increased as compared to peptides expressing the His-Arg motif only. Thus, the Arg/Lys-Trp-Trp motif serves to stabilize the binding of His-Arg to mAb 2925. Synthetic peptide studies revealed further N-terminal residues Ile65 and Ser66 as part of the epitope. A C5a mutant with an exchange Lys68Glu (C5aGlu68) confirmed the participation of Lys68 as a contact residue within the epitope of mAb 2925. Hence, the epitope recognized by mAb 2925 is linear and comprises residues Ile65, Ser66, His67, and Lys68. Thus, we could demonstrate for the first time that a mAb inhibits C5a receptor binding through specific interaction with receptor binding residues of the ligand.

The anaphylatoxin C5a, a new parameter in the diagnosis of renal allograft rejection

In the underlying study the diagnostic value of the anaphylatoxin C5a was evaluated in kidney transplantation. In 49 transplant patients the following parameters were measured daily for a mean period of 25.1 days: plasma C5a [P-C5a], urine C5a [U-C5a], serum amyloid A [SAA], serum neopterin [S-NEOP] and urine neopterin [U-NEOP]. Sensitivity, specificity and day of first significant parameter increase (exceeding a cut-off level of > 50%) were evaluated retrospectively during 30 periods of rejection and 30 periods of stable graft function. U-C5a was the parameter with the highest sensitivity (84%) and specificity (84%), increasing in the mean 1.3 days before clinical diagnosis of rejection. Sensitivity and specificity of the other markers was lower: SAA 77% and 77%, U-NEOP 68% and 65%, S-NEOP 45% and 77%, and P-C5a 45% and 48%, respectively. During four instances of cytomegalovirus disease extremely high U-NEOP (> or = 1520 +/- 518 mumol/mol creatinine) and slightly increased P-C5a levels (> or = 1.5 +/- 1.4 ng/ml) occurred. Elevated urinary excretion of C5a seems to be a reliable and early marker of renal allograft rejection. In combination with SAA and U-NEOP, the daily assessment of U-C5a differentiates between viral infection and allograft rejection.

Re-evaluation of the storage conditions for blood samples which are used for determination of complement activation

EDTA-blood samples derived either from healthy staff or septic patients were investigated for in vitro complement activation during the first 48 h after blood drawing at 4 degrees C and 20 degrees C. For this purpose C3a/C3a desArg plasma levels were determined by the ABICAP C3a assay. Within the septic group no complement activation was detectable during the whole observation period. However, if blood from healthy persons was stored for longer than 6 h at 20 degrees C complement activation occurred. The most profound activation was found in EDTA-blood stored for 48 h at 20 degrees C. C3a values in this sample increased four-fold from 56 +/- 7 ng/ml to 222 +/- 38 ng/ml. From these data we conclude that both immediate cooling of EDTA-blood to 4 degrees C, as well as the immediate separation of plasma as proposed by Mollnes et al. (Clin. Exp. Immunol. (1988) 73, 484), is not necessary for determination of anaphylatoxin plasma values. Storage of EDTA-blood samples for up to 6 h without the need to perform centrifugation should allow anaphylatoxin measurement to become a routine parameter for diagnosis of inflammatory diseases.

Site-specific mutagenesis of residues in the human C5a anaphylatoxin which are involved in possible interaction with the C5a receptor

To check and clarify existing data on receptor-interacting residues in the human C5a anaphylatoxin, we tested mutant C5a proteins obtained by site-directed mutagenesis of a recombinant human C5a (rhC5a) cDNA clone for structural and functional integrity. Amino acid positions in three different regions of the molecule were investigated: Arg74 at the C-terminus, Arg40 and Pro45 located in the core region, and Lys14 and Lys19, Lys20 in the N-terminus. Des-Arg74-rhC5a displayed only a residual 3-4% functional activity in the myeloperoxidase-release assay from human granulocytes while retaining the three-dimensional solution structure of wild-type (wt)-rhC5a as shown by circular dichroism (CD) spectroscopy. Des-Arg74-rhC5a was able to activate the human C5a receptor transiently expressed in Xenopus oocytes, but was inactive in the heterologous guinea pig (gp) ileum-contraction assay. These results reveal profound differences between the guinea pig and human C5a-receptor ligand-binding characteristics. Exchange of the core residue Arg40 by a glycine did not significantly affect functional C5a activity, in contrast to a previous observation [Mollison, K. W., Mandecki, W., Zuiderweg, E. P., Fayer, L., Fey, T. A., Krause, R. A., Conway, R. G., Miller, L., Edalji, R. P., Shallcross, M. A., Lane, B., Fox, J. L., Greer, J. & Carter, G. W. (1989) Identification of receptor-interacting residues in the inflammatory complement protein C5a by site-directed mutagenesis, Proc. Natl Acad. Sci. USA 86, 292-296], nor did exchange of the conserved Pro45 residue by the C3a analogue glutamic acid, a mutation expected to alter the whole geometry of the loop connecting helix III-helix IV (including Arg40) of the C5a molecule. Thus, participation of this loop in receptor interaction appears unlikely. While exchange of the N-terminal Lys14 residue by alanine did not significantly affect functional activity, a double replacement of Lys19 and Lys20 by alanine residues reduced activity more than 30-fold. These results confirm Lys19 and/or Lys20 as a putative receptor-interacting site, although we could not obtain a CD spectrum of this important mutant due to poor expression.

Rapid quantification of C3a and C5a using a combination of chromatographic and immunoassay procedures

Monoclonal antibodies were isolated which reacted specifically with the complement cleavage products C3a, C3adR, C5a, and C5adR but not with the parent molecules C3 or C5. In both cases the mAbs showed a higher affinity towards the desArg forms. These mAbs were used as capture antibodies in immunoassays for C3a/C3adR and C5a/C5adR. The immunoassays are based on the ABICAP technology which ensures for a rapid measurement. Due to the large binding capacity and the very short diffusion pathways in the gel-matrix the binding equilibrium between capture antibodies and the antigen is reached whilst the sample is flowing through the column. Therefore this test represents an endpoint assay offering the possibility of using a single calibration curve for a large number of measurements. With the C3adR assay concentrations down to 16 ng/ml C3adR can be detected. The lower detection limit of the C5adR assay is 1 ng/ml C5adR. The tests for C3a/C3adR, and C5a/C5adR can be performed in 20 to 25 min and this rapid processing of plasma samples should permit the application of these parameters for diagnostic purposes and patient management.

A recombinant hybrid anaphylatoxin with dual C3a/C5a activity

By site-directed mutagenesis of a human complement factor C5a cDNA clone, we have designed a hybrid anaphylatoxin in which three amino acid residues in the C-terminal sequence of human C5a were exchanged to create the native C-terminal human C3a (hC3a) sequence Leu-Gly-Leu-Ala-Arg. This hybrid anaphylatoxin rC5a-(1-69)-LGLAR exhibited true C3a and C5a activity when tested in the guinea pig ileum contraction assay. Quantitative measurements of ATP release from guinea pig platelets revealed about 1% intrinsic C3a activity for this hybrid, while the C5a activity was essentially unchanged. Competitive binding assays confirmed that the rC5a-(1-69)-LGLAR mutant was able to displace radioiodinated rhC5a with a KI of approx. 40 nM and hC3a with a KI of approx. 3.7 microM from guinea pig platelets. Since the C-termini of both human C3a and C5a anaphylatoxins are known to interact with their respective receptors, we conclude that the same peptidic sequence, LGLAR, is able to bind to and activate two different receptors, the C3a receptor as well as the C5a receptor. This clone provides a novel tool for the identification of further receptor-binding residues in both anaphylatoxins, since any mutants may be tested for altered C3a and C5a activity simultaneously.

Assessment of complement activation in vivo

Hemolytic assays that measure the functional integrity of the complement system and the quantitation of individual components by immunochemical techniques have been widely used in the past for the assessment of in vivo complement activation. However, the complement system comprises a large number of interacting serum proteins which are subject to independent synthetic and catabolic processes. The fact that complement proteins are rapidly metabolized under in vivo conditions adds to the complexity of complement analysis. Assays that are based on monoclonal antibodies with specificities for activation-dependent neoepitopes now allow the direct determination of complement fragments in plasma. These methods are superior to the quantitation of native proteins. Several parameters that differentially affect the generation or the catabolism of individual complement activation products still have to be taken into account when elevated plasma levels of complement fragments suggest in vivo complement activation. These factors include the binding to complement fragment receptors, the degradation by serum proteases and renal or hepatic clearance. An accurate estimation of complement activation in vivo requires the simultaneous determination of both the native components and the activation products.

Production of monoclonal antibodies against the human anaphylatoxin C5a des Arg and their application in the neoepitope-specific sandwich-ELISA for the quantification of C5a des Arg in plasma

A panel of ten murine monoclonal antibodies (MAbs) was raised against the human anaphylatoxin C5a des Arg. The MAbs were shown to abrogate or significantly inhibit the chemotactic activity in zymosan activated serum. MAb 4A2E10E2 and MAb 3G3C4 were used as capture and detecting antibody, respectively, in a sandwich enzyme-linked immunosorbent assay (ELISA) for the quantification of C5a des Arg. This ELISA was shown to be very sensitive (detection limit 20 pg/ml) and could be applied directly to plasma/serum samples. The lack of interference by plasma components, in particular C5, suggested specificity for an epitope on C5a (des Arg) which is concealed in native C5 and exposed on the activation fragment only, i.e., a 'neoepitope'. The mean C5a des Arg level in EDTA-plasma from 25 healthy individuals assessed at a 1/20 dilution was 11.2 ng/ml (SD 3.4; range 6.4-16.8 ng/ml). The applicability of the assay was investigated in patients treated with haemodialysis using different membranes. Markedly elevated plasma levels of C5a des Arg were found in blood returning from the dialyzer following contact with cuprophane membranes.

Human C5a anaphylatoxin: gene cloning and expression in Escherichia coli

A gene coding for the human anaphylatoxin C5a was cloned and expressed in Escherichia coli. A combination of reverse transcription of mRNA of the U937 cell line with subsequent preparative polymerase chain reaction was employed to obtain the gene. The sequence was cloned into the plasmid vector pKK 233-2 behind an ATG initiation codon under the control of a trc promotor. After purification by ion exchange chromatography and reversed phase FPLC a mixture of predominantly non-glycosylated recombinant human C5a with a beta-mercaptoethanol adduct at cysteine 27 and the N-methionyl derivative was obtained which was homogeneous on silver-stained gels, immunoreactive with C5a-specific monoclonal antibodies and functionally active in releasing myeloperoxidase from human granulocytes and ATP from guinea pig platelets. The final yield was about 0.4-0.8 mg purified recombinant C5a per liter bacterial culture.

The apolipoprotein(a) moiety of lipoprotein(a) interacts with the complement activation fragment iC3b but does not functionally affect C3 activation or degradation

A previous study has shown that complement component C3 binds to recombinant apolipoprotein(a) (r-apo(a)). In the present report we have investigated the interactions between lipoprotein(a) (Lp(a)), r-apo(a) and C3 in relation to complement activation and degradation. Neither Lp(a) nor r-apo(a) affected complement activation as indicated by sheep and rabbit red blood cell hemolytic assays, and by assessment of the amount of C3a generated in zymosan-activated human serum in the presence or absence of Lp(a). Crossed immunoelectrophoretic analyses indicated that Lp(a) retarded the migration of iC3b in complement-activated serum but had no effects on C3, C3b, C3c or C3dg. Recombinant apo(a) exhibited the same properties as intact Lp(a) indicating that it is the apo(a) portion of Lp(a) that mediates this effect and not the lipid moiety. Low density lipoprotein had no effect on the migration of C3 cleavage fragments. Treatment of Lp(a) or apo(a) with neuraminidase abolished their capacity to alter iC3b migration. SDS-PAGE immunoblotting analysis of C3 activation fragments generated in the presence of Lp(a) demonstrated the usual physiologic C3 cleavage fragments. Rocket intermediate gel immunoelectrophoresis of complement-activated serum demonstrated that Lp(a) did not hinder or accelerate the generation of C3c and C3dg breakdown fragments of iC3b. The results indicate that the apo(a) moiety of Lp(a) alters the migration of iC3b in an electric field but does not affect complement activation or degradation of activated C3. The sialic acid residues on apo(a) are necessary for the apo(a)-iC3b interaction.

Stimulation of neutrophil functions by C5a(desArg): an in vitro model of haemodialysis

Cuprophane membranes during haemodialysis significantly increase the plasma levels of C5a(desArg) (maximal 55 mug C5a(adesArg)/1 blood after 30 min) whereas Hemophane or Polysulphonemembranes induce only low plasma levels of C5a(desArg). C5a(desArg) generated in vitro by yeast incubation of autologous plasma stimulates PMN chemotaxis and oxidative metabolism but has no effect on enzyme release. Preincubation of whole blood with C5a(desArg) causes aggregation and changed oxidative burst activity of the isolated PMN. These changes are similar to those found in cells from patients after haemodialysis with cuprophane membranes. So the elevated plasma levels of C5a(desArg) after haemodialysis explain some of the changes in PMN functions, but additional mechanisms have to be assumed.

Characterization of the C5a receptor on guinea pig platelets

Guinea pig (gp) platelets react to nanomolar doses of the complement-derived anaphylatoxin C5a with a shape change, aggregation and release of biogenic amines and nucleotides from their granules. We have investigated the specific receptor for C5a on gp platelets which mediates these biological effects. Competitive binding studies with 125I-labeled guinea pig C5a (125I-gpC5a) revealed approx. 4000 binding sites/cell with Kd = 6 x 10(-9) M. The more than 60-fold higher biological activity (ATP-release from gp platelets) of gpC5a versus recombinant human C5a (rhuC5a) and the different binding behavior of gpC5a and rhuC5a point to a species restriction in the gp platelet system. Cross-linking of 125I-gpC5a to gp platelets (250 microM DSS) and analysis by SDS-PAGE under reducing conditions resulted in labeling of a single band with a molecular mass of 32 kDa (ligand-receptor complex). Because of these characteristics, the C5a receptor on gp platelets clearly differs from all previously described C5a receptors.

Formation of C5a during cardiopulmonary bypass: inhibition by precoating with heparin

A novel enzyme immunoassay based on direct detection of C5a by a monoclonal antibody (C17/5) specific for a neoepitope exposed in C5a/C5adesArg was used to measure in vivo and in vitro C5a formation during cardiopulmonary bypass. In vivo, we observed a significant threefold to fourfold increase in patient plasma C5a/C5adesArg levels from baseline values (5.6; 1.6 to 12.9 ng/mL) (median and range) up to 42 hours postoperatively (17.5; 6.5 to 46.0 ng/mL) when two different uncoated cardiopulmonary bypass circuits were used. Coating of the extracorporeal circuit with end-point-attached heparin completely abolished C5a formation in vitro during circulation of blood through the circuit for 120 minutes. The C5a concentration (median and range) was 3.2 (2.6 to 15.9) ng/mL at the start and 3.1 (2.7 to 15.0) ng/mL at the end of the experiment. In the uncoated setups the corresponding C5a concentrations were 10.1 (6.2 to 17.5) and 19.7 (13.1 to 24.3) ng/mL. Finally, heparin-coated cardiopulmonary bypass circuits were examined in vivo. C5a levels did not increase significantly during the cardiopulmonary bypass period in the heparin-coated group in contrast to the uncoated group, but the postoperative increase in C5a levels was similar in the two groups. We conclude that heparin coating improves biocompatibility by completely abolishing C5a formation in vitro. The discrepancy between the in vitro and the in vivo findings is probably related to the complicated biological turnover of C5a.

Quantitation of the anaphylatoxin C3a in the presence of C3 by a novel sandwich ELISA using monoclonal antibody to a C3a neoepitope

C3a levels in plasma are usually measured by a competitive inhibition radioimmunoassay (RIA) using 125I-labelled C3a-desArg and antibodies to C3a capable of detecting C3a determinants which are also present on the native C3. Therefore, prior to the assay native, non-cleaved C3 has to be removed completely from the C3a-containing sample by precipitation. We developed a new rapid two-site sandwich ELISA system for the quantitation of C3a-desArg in plasma. This immunoassay uses a monoclonal antibody (mAb H466) reacting with C3a-desArg but not with C3. The reactivity of mAb H466 with a neoantigenic determinant of C3a-desArg permitted the direct quantitation of C3a-desArg without removal of C3 from the sample. The mAb H466 was used as a capture antibody and bound C3a-desArg was detected with a second peroxidase-labelled anti-C3a mAb. The lower limit of detection of C3a-desArg in this ELISA was 1 ng/ml. The C3a-desArg levels measured in the plasma samples of various patients were found to differ over a wide range. A good correlation was observed between the results obtained in the RIA and those obtained in the ELISA (r = 0.95). High levels of C3a-desArg were detected in plasma from patients with multiple trauma and patients undergoing haemodialysis. The C3a-desArg assay described should facilitate the routine quantitation of C3a in samples of plasma.

Characterization of C3a receptor-proteins on guinea pig platelets and human polymorphonuclear leukocytes

The expression of specific membrane receptors for C3a was determined on guinea pig C3a-sensitive (gp R+) platelets and human polymorphonuclear leukocytes (hu PMNL). Binding studies with 125I-labeled C3a from gp or hu sources and Scatchard analysis applied to the binding data revealed the existence of two receptor classes on gp R+ platelets; a high-affinity class with about 200 binding sites/cell and Kd = 1.7 x 10(-9) M, and a relatively low-affinity class with Kd = 10(-8) M and about 500 sites/cell. Hu PMNL express a homogeneous receptor class with Kd = 3 x 10(-8) M and 40,000 sites/cell. Molecular characterization of the C3a receptor on gp R+ platelets was achieved by (a) cross-linking photoaffinity-labeled receptors to bound 125I-labeled C3a; (b) photoaffinity labeling receptors with a 13-amino acid residue C3a analogue 125I-Nap-Ahx-13; and (c) use of chemical cross-linkers like disuccinimidylsuberate to cross-link receptors with 125I-C3a. All three techniques gave rise to very similar labeling patterns. With the photoaffinity labeling methods, a diffuse band pattern was observed with an apparent molecular mass of 95-123 kDa with 125I-C3a as label, and 85-105 kDa with 125I-Nap-Ahx-13 as label. Chemical cross-linking of 125I-C3a revealed three distinct bands with molecular masses of approximately 123, 108 and 95 kDa. Subtracting the contribution of the cross-linked ligands, the C3a receptor on gp R+ platelets appears to be a protein complex, consisting of one to three components with estimated molecular masses between 83-114 kDa.

Measurement of complement activation in rabbit plasma or serum using monoclonal antibodies against C5a

Eight murine monoclonal antibodies (MoAb) raised against the major zymosan-induced chemotactic factor in rabbit serum were found to neutralize the chemotactic activity induced by lipopolysaccharides (LPS) and antigen-antibody complexes. A 15 kDa antigen was identified in plasma incubated with LPS by immunoblot analysis with MoAb. This is similar to the molecular weight of the major zymosan-induced chemotactic factor. Both the generation of this 15 kDa antigen and chemotactic activity were abrogated in a heat-inactivated plasma. A cross-reaction to human C5a was demonstrated for three MoAb (5H8B9, 4B1C11, and 2A5E3) in an indirect enzyme-linked immunosorbent assay (ELISA) of partially purified C5a and by the isolation of zymosan-induced chemotactic activity by affinity chromatography. MoAb 5H8B9 and 4B1C11 were able to neutralize the chemotactic activity in human zymosan-activated serum. MoAb 2A5E3 was able to bind 125I-labelled human C5a des Arg. We conclude that these MoAb are directed against rabbit C5a. MoAb 5B2C5 and 2B1A2, which are directed to different antigenic binding sites on C5a, may be applied in a sandwich ELISA for the detection and quantification of C5a des Arg in rabbit serum or plasma. The sandwich ELISA can be performed directly on serum or plasma samples without having to precipitate native C5. Complement activation is demonstrated by measuring the increased generation of C5a des Arg in rabbit plasma or serum activated with LPS, zymosan, antigen-antibody complexes, or cobra venom factor.