Activation of the third component of complement (C3) detected by a monoclonal anti-C3'g' neoantigen antibody in a one-step enzyme immunoassay

Disparity in blood activation by two different heparin-coated cardiopulmonary bypass systems

BACKGROUND

Several studies have indicated reduced "blood activation" in heparin-coated cardiopulmonary bypass systems. The present study compares the effect of two different heparin coatings on different blood activation indices.

METHODS

Low-risk patients (n = 40) were randomized to coronary artery bypass grafting using cardiopulmonary bypass with surfaces coated entirely by either the Duraflo II heparin coat or the Carmeda Biological Active Surface, or with identical uncoated equipment. In all cases, a standard systemic heparin dosage was used. Complement activation (C3 activation products C3bc and C3a and formation of fluid phase terminal SC5b-9 complement complex), neutrophil activation (lactoferrin and myeloperoxidase), and lytic inhibitors (vitronectin and clusterin) were quantified during cardiopulmonary bypass and 6 hours postoperatively.

RESULTS

Heparin coating by either method reduced the formation of terminal SC5b-9 complement complex and the release of lactoferrin and myeloperoxidase compared with uncoated systems. Lactoferrin and myeloperoxidase levels increased significantly during cardiopulmonary bypass in the Duraflo II group, whereas no significant increase was observed in the Carmeda Biological Active Surface group. The least formation of terminal SC5b-9 complement complex and neutrophil activation was observed with the Maxima Carmeda Biological Active Surface-coated equipment. The vitronectin and clusterin concentrations were significantly less reduced in the Duraflo II compared with the control group. This study underlines the importance of terminal SC5b-9 complement complex as a suitable marker in the evaluation of complement activation during cardiopulmonary bypass.

CONCLUSIONS

Both heparin coatings reduce blood activation, probably more so with Carmeda Biological Active Surface than with Duraflo II.

A novel ELISA for the assessment of classical pathway of complement activation in vivo by measurement of C4-C3 complexes

Measurements of complement split products by enzyme-linked immunosorbent assays (ELISA) are well established for the assessment of in vivo complement activation. We have combined two monoclonal antibodies (mAb) with specificities for C3b/iC3b/C3dg (mAb I3/15) and C4/C4b/C4d (mAb M4d2), respectively, in a sandwich ELISA to quantitate C4-C3 complexes as an indicator of complement activation. Serum incubated with heat aggregated IgG (HAG) was used as a standard and the C4-C3 levels expressed as microgram equivalent HAG/ml (microgram HAG-equ/ml). Normal values of C4-C3 complexes in plasma (EDTA) of healthy probands (n = 11) were 6.3 micrograms HAG-equ/ml +/- 1.5 (mean +/- 1 standard deviation (SD), with a range from 3.6 to 9.1). In patients with systemic lupus erythematosus (SLE, n = 23) C4-C3 values were clearly elevated (48.8 micrograms HAG-equ/ml +/- 52.9, range 7.5-184.7) as compared to samples from patients with idiopathic hypertension (IDH, n = 10) (6.5 micrograms HAG-equ/ml +/- 1.7, range 4.1-9.4). For SLE patients C4-C3 levels significantly correlated with values for C3b/iC3b/C3d (r = 0.69, p < 0.001) and C3 containing immune complexes (r = 0.68, p < 0.001), but not with the C4d fragment (r = 0.26). C4-C3 levels of 96% of the studied SLE patients were increased more than 2 SD above the normal mean as compared to 74% of C4d and activated C3 values, respectively. Serum treated with zymosan as an activator of the alternative pathway of complement did not exhibit higher C4-C3 values. These results demonstrate that the quantitation of in vivo generated C4-C3 complexes by ELISA provide a novel, sensitive parameter for classical pathway of complement activation.

Direct and indirect effects of E. coli lipopolysaccharide on isolated human polymorphonuclear granulocytes and mixed leukocytes

Polymorphonuclear neutrophil granulocytes (PMN) may contribute to the lung injury induced by nonpulmonary infections with gram-negative bacteria. The direct effect of E. coli lipopolysaccharide (LPS) on isolated human PMN or mixed leukocytes (ML), as well as the priming effect of preincubating cells with LPS, was examined in assays measuring the maximal rate of oxygen consumption (OC), cell chemiluminescence (CHML), and aggregation (AGG). LPS, 1-10 micrograms/ml, caused no acute response in PMN or ML suspended in Fisher's-HEPES medium with BSA (FHA), but increased both CHML and AGG of cells suspended in autologous plasma. Preincubation in FHA with LPS, 1 microgram/ml, for more than 15 min increased the OC of PMN activated with zymosan-activated plasma (ZAP) or n-formyl-methionyl-leu-cyl-phenylalanine (FMLP) by more than 100%. A similar increase in the CHML of such cells was seen after FMLP, but not after ZAP. ZAP, however, primed the CHML response of the cells to subsequent activation with FMLP more than did preincubation with LPS. Previous exposure to both agents had an additive effect. Preincubation of PMN with LPS decreased the time interval from addition of phorbol myristate acetate (PMA) to peak OC response, but less so than previous activation with FMLP. Neither agent affected the maximal rate of OC after addition of PMA. LPS also increased the PMN aggregation induced by ZAP and FMLP, but not by PMA. Cells preincubated with LPS, 0.01 microgram/ml, increased their CHML in response to FMLP if suspended in Krebs-Ringer balanced salt solution, but not if suspended in FHA. Such preincubation had no effect on OC of similarly activated cells in any of the media.

Methods for assessing complement activation in the clinical immunology laboratory

Complement activation is a key component of the pathogenesis of immune-mediated tissue damage in many diseases. Assessment of complement activation in current practice is largely based on the measurement of intact C3 and C4 or the determination of complement haemolytic function. These parameters reflect activation only indirectly, are insensitive and open to influence by factors other than complement conversion. New approaches to evaluate complement activation directly using sensitive techniques have been developed, and several could be adopted easily in most laboratories. These concentrate on the detection of activation fragments, neoantigens or complexes that only arise as a direct result of complement activation. The wide application of these techniques in research and clinical practice would enhance our understanding of the pathogenesis of a range of inflammatory and infectious diseases.

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.

Quantitation of components of the alternative pathway of complement (APC) by enzyme-linked immunosorbent assays

Sensitive enzyme-linked immunosorbent assays (ELISA) using monoclonal antibodies have been developed to specifically detect components of the alternative pathway of complement in human blood plasma. Normal values of the factor B split products Ba (1.01 +/- 0.30 micrograms/ml, mean +/- SD), Bb (0.65 +/- 0.23 micrograms/ml), of the C3-fragments C3b/iC3b/C3dg (17.9 +/- 5.7 micrograms/ml), native factor B (238 +/- 48 micrograms/ml), factor D (1.05 +/- 0.27 micrograms/ml), and factor H (702 +/- 292 micrograms/ml) were determined in the EDTA-plasma of healthy probands (n = 55). The simultaneous quantitation of the main cleavage products and of control proteins in the plasma samples permits precise analysis of the activation of the alternative pathway of complement in various disease states. In addition, we describe a method for the specific depletion of factor B prior to fragment-specific assays utilizing monoclonal antibodies conjugated to paramagnetic beads. The latter should permit the quantitation of other complement split products.

Covalent binding of detergent-solubilized membrane glycoproteins to 'Chemobond' plates for ELISA

An ELISA method is presented which is based on covalent binding of detergent-solubilized membrane proteins to surface-modified polystyrene plates (Chemobond plates). These plates carried 0.52-0.65 nmol of aldehyde groups per well (150 microliters) and allowed coupling of protein by Schiff base formation either at high pH and subsequent reduction with NaBH4 or by trapping reduced imines at pH 6-6.8 with cyanoborohydride. They bound 15 times the amount of normal plates. Sodium chloride (0.5 M) increased binding 2-3-fold. Binding was essentially resistant to elution by 1% sodium dodecyl sulfate. Reduction of uncoated plates with NaBH4 eliminated the high extent of binding. ELISA tests on Chemobond plates with a rabbit anti-band 3 antibody gave a ten-fold higher signal than plates to which band 3 protein was merely adsorbed. The use of an antigen-enzyme conjugate to detect bound antibody allowed to perform antibody binding and detection of bound antibody simultaneously in the presence of 0.05% Triton X-100. A competitive, one step ELISA system allowed determination of rabbit anti-band 3 antibodies in diluted serum with a sensitivity range of 0.02-0.4 microgram/ml.

Preparation of monoclonal antibodies to C3b by immunization with C3b(i)-sepharose

We have prepared and characterized four monoclonal antibodies (MAbs) to human C3b of high specificity and affinity. Our procedure did not require a purified source of C3b for immunization. Instead, C3b and C3bi were deposited on Sepharose 4B via the alternative pathway of complement activation in normal human serum, and this C3b(i)-Sepharose served as the immunogen. C3b(i)-Sepharose was also prepared from a number of primate and non-primate sources, and this allowed us to demonstrate that the anti-human C3b MAbs cross-reacted with primate-derived C3b, but not with C3b from non-primates. The procedures we have developed may be useful in the further investigation of species-specific C3 fragment-binding proteins from both primate and non-primate sources.

Complement activation in patients with renal failure as detected through the quantitation of fragments of the complement proteins C3, C5, and factor B

Using sensitive and highly specific enzyme-linked immunosorbent assays fragments of the complement proteins C3, C5, and factor B were quantitated in patients with renal failure. During hemodialysis on new cuprophan membranes raised levels not only of C3a, but in addition of activated C3, C5a, and Ba were demonstrated. In patients with chronic renal failure and end-stage renal disease plasma concentrations of Ba and activated C3 were markedly elevated independent of hemodialysis. This finding is taken as an indication of a continuous recruitment of the alternative pathway of complement in these patients. As the detected complement protein fragments are known to exert immune regulatory functions these findings may imply that these peptides are involved in the maintenance of the immune suppressed state in renal failure.

Characterization of a monoclonal antibody MoAb bH6 reacting with a neoepitope of human C3 expressed on C3b, iC3b, and C3c

Activation products of the complement cascade contain neoepitopes that are not present in the individual native components. Monoclonal antibodies detecting neoepitopes have been used for direct quantification of activation at different steps in the cascade. These methods are suggested to be more sensitive and reliable than conventional complement activation tests, which are hampered by precipitation or fractionation procedures. The present study describes production screening and characterization of a monoclonal antibody (MoAb) bH6. MoAb bH6 exhibited a significantly higher binding capacity to ELISA plates coated with zymosan-activated human serum than to plates coated with EDTA plasma. When fixed to the enzyme-linked immunosorbent assay (ELISA) plates, MoAb bH6 retained material from zymosan-activated serum that only reacted with anti-C3 antibodies. Crossed immunoelectrophoresis performed on zymosan-activated serum demonstrated that MoAb bH6 co-precipitated with anti-C3c antibodies. In experiments using highly purified cell-bound fragments MoAb bH6 showed reactivity with C3b and iC3b, but not with C3d. MoAb bH6 reacted in ELISA with purified C3c, but not with C3dg, both as capture antibody and in tests with the fragments absorbed to the solid phase. Thus, MoAb bH6 is highly specific for a neoepitope of human C3 expressed on the cleavage fragments of C3b, iC3b, and C3c.

Quantification in enzyme-linked immunosorbent assay of a C3 neoepitope expressed on activated human complement factor C3

A sensitive double antibody enzyme-linked immunosorbent assay (ELISA) for quantification of C3 activation products in human plasma, synovial fluid, and cerebrospinal fluid is described. The monoclonal antibody MoAb bH6, which is specific for a C3 neoepitope expressed on C3b, iC3b, and C3c, was used as capture antibody. Detection antibody was a polyclonal rabbit anti-human C3c followed by development with a peroxidase-conjugated anti-rabbit Ig antiserum. The activity in normal human EDTA plasma was 1.5% of that in zymosan-activated serum (ZAS). The interassay and intra-assay coefficients of variation were 15% and 3%, respectively. The lower detection limit was 0.0005% of the ZAS standard. Reference range (1.1-2.1% of ZAS) was obtained by measuring EDTA plasma from 40 healthy blood donors. A positive correlation rs = +0.92; P less than 0.0002) was found between the present assay and an already established C3'g' activation ELISA, when samples from 16 patients were examined in both assays simultaneously. The present assay and an assay detecting the terminal complement complex showed virtually identical activation patterns in consecutively drawn samples from a patient undergoing extracorporal circulation.