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

Advances in assay of complement function and activation

The main function of the complement system is pattern recognition of danger. Typical exogenous danger signals are pathogen associated molecular patterns inducing a protective inflammatory response. Other examples are exposure to foreign surfaces of biomedical materials including nanoparticles, which principally induce the same inflammatory response. If a surface is "foreign" to the host, it induces complement activation. Development of monoclonal antibodies to neoepitopes on complement activation products introduced an entirely new set of methods for assay of complement activation. Activation of complement by a surface occurs by impairment of the fine balance of the control system, e.g. by preferred binding of factor B at the expense of factor H. Sensitive methods to detect complement activation on surfaces and in the fluid phase are a prerequisite for investigation of the biocompatibility of artificial materials. This information can be used to develop new materials with enhanced biocompatibility. Here we review available methods to study human and animal complement function and activation in vitro and in vivo.

Complement analysis in the 21st century

Complement analysis in the clinic is usually associated with the quantification of C3 and C4, measurement of C1-inhibitor and screening for complement activity. These analyses have been available in routine diagnostic laboratories for decades. In recent years, however, the field of complement analysis has expanded considerably, with the introduction of novel assays to detect complement activation products, and spreading still further towards genetic analysis to reveal the basis of complement deficiencies and identify mutations and polymorphisms associated with defined diseases such as atypical haemolytic uraemic syndrome and age related macular degeneration. Here we review the current status of complement analysis, including assays for the quantification of complement activity and complement activation products, together with genetic methods for the detection of deficiencies, mutations and polymorphisms. This is an area where significant developments have been made recently, paralleling the research advances into the role of complement in human disease. It is clear, however, that there is a need for consensus and standardisation of analytical methods. This will be a major challenge for the complement society in the future.

The role of complement C3 opsonization, C5a receptor, and CD14 in E. coli-induced up-regulation of granulocyte and monocyte CD11b/CD18 (CR3), phagocytosis, and oxidative burst in human whole blood

The relative role of complement and CD14 in Escherichia coli-induced leukocyte CD11b up-regulation, phagocytosis, and oxidative burst in human whole blood was examined. The highly specific thrombin inhibitor lepirudin was used as anticoagulant, as it does not affect complement activation. Complement inhibition at the level of C3 (anti-C2 and anti-factor D) and C5 (C5a receptor antagonist and anti-C5/C5a) efficiently inhibited CD11b up-regulation, phagocytosis, and oxidative burst in granulocytes. Monocyte activation was generally less complement-dependent, but when C3 activation was blocked, a pronounced inhibition of phagocytosis and oxidative burst was obtained. Only the combination of anti-C2 and antifactor D blocked E. coli C3 opsonization completely. Whole E. coli, disrupted E. coli, and the C3-convertase activator cobra venom factor up-regulated CD11b rapidly on both cell types, proportional to their complement activation potential in the fluid phase. In comparison, purified LPS at concentrations comparable with that present in the E. coli preparations did not activate complement. Oxidative burst was induced only by whole bacteria. Finally, the combination of complement inhibition and anti-CD14 completely blocked E. coli-induced granulocyte and monocyte CD11b up-regulation and quantitatively, virtually abolished phagocytosis. The results indicate that complement and CD14, despite differential effects on granulocytes and monocytes, are the two crucial, quantitative factors responsible for E. coli-induced CD11b, phagocytosis, and oxidative burst in both cell types.

In vivo visualization of hemodialysis-induced alterations in leukocyte-endothelial interactions

BACKGROUND

The aim of this study was to develop a model for hemodialysis (HD) in small animals using conventional dialysis equipment that would allow the intravital microscopic observation of leukocyte-endothelial interactions in vivo.

METHODS

Cuprophan dialyzers were adapted to obtain a similar ratio of membrane area to blood volume as in clinical HD. A silicone ring was inserted into the dialyzer's inlet to limit the number of blood-perfused capillaries. Rabbits were dialyzed for one hour without a dialysate flow.

RESULTS

Extracorporeal circulation with the cuprophan dialyzer resulted in a transient leukopenia and complement activation. At the nadir of leukopenia, leukocytes that rolled along the venular wall were scarcely observed, whereas rolling was abundant (54 +/- 9 per min) prior to extracorporeal circulation. The adhesion of leukocytes to the vascular endothelium was not induced. After 60 minutes, rolling of leukocytes was still reduced by 73 +/- 5.5%, despite the full recovery of circulating leukocyte counts. Extracorporeal circulation without a dialyzer also tended to reduce leukocyte rolling, although systemic leukocyte counts were not affected.

CONCLUSIONS

The use of adapted conventional cuprophan hemodialyzers in rabbits yielded a transient leukopenia similar to that in clinical HD. Using intravital microscopy, we demonstrated impairment of leukocyte-endothelial interactions. In addition, our data indicate that tissues, in which leukocytes can roll and adhere, are not automatically sites of leukocyte sequestration during HD-induced leukopenia.

Surprisingly high levels of anaphylatoxin C5a des Arg are extractable from psoriatic scales

Water-soluble extracts from psoriatic scales and normal human skin were prepared using either phosphate-buffered saline, pH 7.2, or 0.1 M carbonate buffer, pH 10.8. Anaphylatoxin C5a des Arg was quantified using a novel sandwich enzyme-linked immunosorbent assay (ELISA) using neoepitope-specific monoclonal antibodies. Alkali was about five to eight times more efficient than PBS in extracting C5a des Arg from scales, probably via dissociation of bound C5a des Arg. C5a des Arg concentration in scales from three patients suffering from psoriasis vulgaris varied between 2.5 and 4.6 ng/mg scale. No C5a des Arg was detectable in normal skin extracts. The biological activity of alkali-extractable C5a des Arg, i.e. chemotaxis, was preserved. The concentration of C5a des Arg relative to the concentration of albumin was taken as a parameter of the degree of complement activation in the psoriatic lesions, and was found to be more than six times higher than values attained in serum after maximum complement activation by zymosan. We conclude that complement activation may play a quantitatively important role in the inflammatory process in psoriasis.

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.

Complement activation and pulmonary dysfunction in experimental Escherichia coli septicaemia

Septicaemia was induced in anaesthetized and artificially ventilated rabbits. Three groups receiving different doses of Escherichia coli 055:B5 and a control group were studied. The animals were followed for 240 min after the start of the bacterial infusion. One animal in the high-dose and one in the medium-dose group died. Septic shock developed in the high-dose group. The infusion of E. coli led to a severe, sustained neutropenia and a dose-dependent thrombocytopenia. A severe arterial hypoxaemia developed during the infusion of bacteria in three of four animals, and in one of three animals in the high- and medium-dose groups, respectively. Both the levels of C5a in plasma and the accumulation of granulocytes in the lung tissue, as assessed by a morphometric method, were dependent on the dose of bacteria. A strong positive correlation was found between levels of C5a and endotoxin. The levels of both C5a and endotoxin correlated positively to the degree of accumulation of granulocytes in the lung tissue. The results from the present study suggest that the magnitude of bacteraemia and the degree of complement activation are probably important factors determining the course and the severity of septicaemia.

The Expression of Retrovirus-Like Particles in Psoriasis

Retrovirus-like particles have been isolated from patients with psoriasis. Antigens crossreacting with the major internal proteins pso p27, of these particles have been demonstrated in the wall of dermal vessels and in a subfraction of cells in psoriatic lesions. The antigen has also been observed in blood lymphocytes from psoriatic patients. Pso p27 antigen and anti-pso p27 antibodies are present as complement-activating immune complexes in psoriatic scale and in the blood of patients with psoriasis and psoriatic arthritis. The potential contribution of the circulating immune complexes to the inflammatory process in psoriasis is discussed.

The psoriasis-associated antigen, pso p27, participates in the formation of complement activating immune-complexes in psoriatic scale

The psoriasis-associated antigen, pso p27, and antibodies recognizing this antigen were demonstrated in extracts from psoriatic scales. It was demonstrated that the antigen was present in complexes containing IgG as well as complement factor C1q. The complexes were studied with respect to complement-activating potential. This was measured as the ability of the complexes to induce the generation of the complement factor C5a using rabbit serum as the source of complement. The data obtained showed that the pso p27-containing complexes were able to activate the complement system, indicating that the pso p27 antigen may contribute to the inflammatory process in psoriasis.