Modulation of complement activation on hemodialysis membranes by immobilized heparin

Complement in Kidney Transplantation

Transplantation is the treatment of choice in most patients with kidney failure. The complement system plays a vital role in transplantation. The complement system forms a major part of innate immunity and acts as a bridge between innate and acquired immunity. Many diseases, particularly concerning the kidneys, result from complement system dysregulation, like atypical hemolytic uremic syndrome (aHUS), C3 glomerulopathy (C3GN), systemic lupus erythematosus (SLE and some other immune complex diseases. The complement system activation is a very important part of post-transplant events like ischemia-reperfusion injury (IRI), delayed graft function (DGF), antibody-mediated rejection (ABMR) and thrombotic microangiopathy (TMA). A better understanding of the complement cascade can help to plan strategies to prevent and manage complement-related problems before and after kidney transplantation. Many newer molecules are either being developed or in the pipeline, which target the complement system at various stages. These novel therapeutics are now considered additional measures to improve graft survival. This review summarises the complement cascade, its role in kidney diseases and kidney transplantation, and possible areas of target and novel therapeutics.

Immunological Effects of a Single Hemodialysis Treatment

Immune disorders, involving both innate and adaptive response, are common in patients with end-stage renal disease under chronic hemodialysis. Endogenous and exogenous factors, such as uremic toxins and the extracorporeal treatment itself, alter the immune balance, leading to chronic inflammation and higher risk of cardiovascular events. Several studies have previously described the immune effects of chronic hemodialysis and the possibility to modulate inflammation through more biocompatible dialyzers and innovative techniques. On the other hand, very limited data are available on the possible immunological effects of a single hemodialysis treatment. In spite of the lacking information about the immunological reactivity related to a single session, there is evidence to indicate that mediators of innate and adaptive response, above all complement cascade and T cells, are implicated in immune system modulation during hemodialysis treatment. Expanding our understanding of these modulations represents a necessary basis to develop pro-tolerogenic strategies in specific conditions, like hemodialysis in septic patients or the last session prior to kidney transplant in candidates for receiving a graft.

A Clinical Study to Assess the Effect of Heparin in Dialyzer Rinsing Solutions

The solution usually recommended for rinsing the blood side, which is an indispensable step in preparing a dialyzer for hemodialysis (HD), contains saline and heparin. The heparin used for rinsing is said to reduce the thrombogenic properties of the dialysis membrane and, hence, also the need for systemic heparinization during the whole procedure. The aim of our study was to establish whether this postulate also applies to polysulphone steam-sterilized dialyzers. To do so, 16 patients on long-term dialysis were randomized into two groups of eight. One group was subsequently treated with polysulphone low-flux dialyzers (F6HPS), the other with polysulphone high-flux dialyzers (F60S). Both groups were examined, in a crossover manner during HD using a dialyzer previously rinsed with 1000 ml of saline plus 2,000 IU of heparin, and during HD using a dialyzer previously rinsed with 500 ml of saline without heparin. Except for the rinsing, HD conditions were completely identical. Blood obtained before HD, and at 15, 60 and 240 min of HD at the dialyzer inlet, was used to determine the activated partial thromboplastin time (to test heparinization control), the thrombin-antithrombin III complex (ELISA, to evaluate coagulation system activation), platelet factor 4 (ELISA, a substance with antiheparin activity), and platelet count. None of the above parameters showed, at any of the collecting intervals, a statistically significant difference between HD with and without heparin with a reduced volume of rinsing solution, or between HD using low- and high-flux dialyzers. It is concluded that heparin used to rinse polysulphone dialyzers before HD has no effect on blood coagulation or on the need for heparin during the procedure.

Surface modification of polysulfone based hemodialysis membranes with layer by layer self assembly of polyethyleneimine/alginate-heparin: a simple polyelectrolyte blend approach for heparin immobilization

This study intends to improve blood compatibility of polysulfone (PSF) membranes by generating a nonthrombogenic surface through heparin immobilization. To achieve this task, the support membrane prepared from a blend of PSF and sulfonated polysulfone (SPSF) was modified with layer by layer (LBL) deposition of polyethyleneimine (PEI) and alginate (ALG) and heparin blended with ALG was immobilized only on the outermost surface of the LBL assembly. The results have shown that the adsorption of human plasma proteins and platelet activation on the LBL modified membranes decreased significantly compared with the unmodified PSF and PSF-SPSF blend membranes. Furthermore, blending ALG with a small amount of heparin remarkably prolonged the APTT values of heparin free PEI/ALG coated membranes. It is envisaged that the use of a blend of HEP and ALG only in the terminating layer of the LBL assembly can be an economical and alternative modification technique to create nonthrombogenic surfaces.

In vitro hemocompatibility of albumin-heparin multilayer coatings on polyethersulfone prepared by the layer-by-layer technique

Polyethersulfone foils (PES)--a unique material for blood purification membranes--were coated with a multilayer assembly of heparin (unfractionated or high anticoagulant activity fraction heparin) and albumin (albumin-heparin coatings), or with a multilayer of albumin (albumin coating), using the layer-by-layer technique. The coatings combine advantages of albumin (reduction of nonspecific interactions) and heparin (specific interactions with blood coagulation proteins). The differences between the two heparins, while significant for their biological activity, had only a minor effect on the multilayer assembly with albumin monitored in situ by reflection infrared spectroscopy (FTIR MIRS). Uncoated as well as modified PES surfaces were evaluated using an in vitro assay with freshly drawn, slightly heparinized (1.5 IU heparin/mL) human whole blood. The blood was circulated with a roller pump over the sample surfaces in shear flow across rectangular slit channels ( app. 6 mL/min and 120 s(-1)) for 1.5 h at 37 degrees C. All coatings effectively reduced platelet adhesion and activation according to the PF4 release. The activation of coagulation evaluated as TAT generation was significantly lowered for the coating composed of albumin and high activity heparin. A further beneficial effect of the heparin containing coatings was reduced complement activation as determined by different complement fragments.

The effect of heparin rinse on the biocompatibility of continuous veno-venous hemodiafiltration

The aims of our cross-over randomized study were (1) to assess hemostasis in patients with acute renal failure (ARF) and (2) to determine whether or not the generally recommended heparin rinse of the extracorporeal circuit (ECC) prior to the procedure affects thrombogenicity, complement activation, and leukocyte count in blood during continuous venovenous hemodiafiltration (CVVHDF). Eleven critically ill ARF patients were treated, in random order, using CVVHDF in postdilution setup following ECC rinse with saline (A) with heparin at a concentration of 2,000 IU/L (10 procedures), (B) with heparin at a concentration of 10,000 IU/L (7 procedures), and (C) without heparin (9 procedures). Except for the rinse, anticoagulation therapy did not differ in individual patients during the procedures. Blood was withdrawn before, and at minutes 15, 60, and 360 invariably at diafilter inlet and outlet. Compared with healthy individuals, patients showed lower blood thrombocyte counts (153 vs 233*10(9)/L, p<0.01, arithmetic means, Student's t test), longer aPTT (44 vs 36 s, p<0.05), higher plasma levels of heparin (0.1 vs 0.0 U/mL, p<0.05), D-dimer (1129 vs 36 ng/mL, p<0.001) and beta-thromboglobulin (BTG) (159 vs 37 U/mL, p<0.001) prior to CVVHDF. The comparison of procedures with different rinsing technique did not reveal any significant difference in their effects on blood thrombocyte and leukocyte counts, aPTT, plasma levels of heparin, BTG, thrombin-antithrombin III complexes, D-dimer, or the C5a complement component.

CONCLUSIONS

(1) Patients indicated for CVVHDF show impaired hemostasis involving thrombocytes, coagulation, and fibrinolysis, (2) no beneficial effect of heparin rinse on CVVHDF ECC thrombogenicity, complement activation or blood leukocyte counts was demonstrated.

Cell apoptosis and hemodialysis-induced inflammation

Hemodialysis patients exhibit a defective immune response leading to an increased susceptibility of infections and neoplasms. Far from being helpful, dialytic therapy per se also may be responsible for this acquired immunodeficiency. Dialysis membranes and bacterial products present in dialysis water may trigger and even perpetuate an abnormal mononuclear cell activation. Upon contact with cellulosic dialysis membranes, monocytes display an increased expression of surface markers of cell activation, such as adhesion molecules CD18, CD49, CD54 and the lipopolysaccharide (LPS) ligand (CD14). Moreover, proinflammatory cytokines as IL-1beta and TNF-alpha are released both in vivo and in vitro when monocytes are exposed to cellulosic membranes. Of special interest is the fact that end-stage renal disease patients undergoing hemodialysis exhibit an increased mononuclear cell apoptosis. This apoptosis is directly related to the degree of biocompatibility of the dialysis membrane. Apoptosis is activated when monocytes enter in contact with the cellulosic dialysis membrane through cell surface receptors linked to G-proteins. In early steps of apoptosis signaling, pertussis toxin-sensitive G proteins are coupled to protein kinase C (PKC)-dependent phosphorylative mechanisms. Furthermore, recent evidence support that the execution phase of apoptosis is mediated by a caspase-3 dependent pathway. Finally, very recent available data support that monocytes subjected to repeated activation suffer a process of accelerated senescence, as demonstrated by the senescent phenotype (CD14 and CD32) expressed and their shortened telomeric length. This senescent profile may generage a defective cellular response in acute stress situations, explaining (at least in part) the altered immune response observed in hemodialysis patients.

Interaction of poly(2-acrylamido 2-methylpropane sulfonate)-grafted polystyrene beads with cationic complement proteins

Influence of various biomaterials on the complement system in serum has been intensively studied by many research groups, since activation of the complement pathway in vivo has been known to give rise to some pathological conditions, such as inflammation and anaphylaxis. Much effort has been devoted to develop new materials that do not activate or deteriorate the complement system. The present work is aimed at revealing the mode of reactions of anionic poly(2-acrylamido 2-methylpropane sulfonate) grafted on polystyrene bead (PAMPS-g-bead) with serum complement. Complement activity assay, determination of complement proteins levels, and immunoblot analysis were carried out for sera pretreated with PAMPS-g-beads. The results clearly showed that, when PAMPS-g-beads were incubated with serum, those beads adsorbed several complement proteins, i.e. C1q, factor D, factor P, C6, and C8, but the generation of activation fragments of complement components was not observed. Especially, factor D was most effectively removed from serum, resulting in potential inhibition of the alternative pathway. A larger amount of PAMPS-g-beads was needed to decrease the serum CH50 level. That may be caused by removal of C6. Although some polyanions, such as dextran sulfate, were reported to activate the complement system, the obtained results indicate that the PAMPS-g-bead is not an activator of the complement pathway, but acts as an adsorbent of complement components. One possible clinical application of the PAMPS-g-beads is adsorption of serum factor D by extracorporeal treatment of patients with renal failure with a high level of factor D, because the increased quantity of factor D in serum may cause consistent activation of the alternative pathway.

Activation and control of complement, inflammation, and infection associated with the use of biomedical polymers

It is generally acknowledged that artificial biomaterials are much less immunologically active than transplants or tissue derived biomaterials. However, activation of both the coagulation cascade and the complement system is a common occurrence when human blood is exposed to biomaterial surfaces during extracorporeal procedures, such as renal hemodialysis or cardiopulmonary bypass. Both individual and collective activation of these cascades often produce local and systemic effects. A number of complement activation products function as the mediators of inflammation. They serve as ligands for specific receptors on polymorphonuclear leukocytes, monocytes, macrophages, mast cells, and other cells. Such an interaction leads to induction of cellular responses in adhered cells, including release of oxidative products, lysosomal enzymes, or both, which often contribute to a number of pathologic conditions. Most pathogens invading the human body are attacked by the immune system directly following entry, especially when they are in contact with blood. However, bacteria and parasites have developed a large number of specific strategies to overcome immune defense among others by avoiding either recognition or eradication by complement. In this aspect, of concern are several microorganisms responsible for formation of antibiotic resistant biofilms on biomaterial surfaces, namely Staphylococcus epidermidis, Staphylococcus aureus, and Pseudomonas aeruginosa.

Albumin and heparin multilayer coatings for blood-contacting medical devices

Three types of covalently crosslinked assemblies consisting of multiple (1) molecular layers of human serum albumin (HSA); (2) alternating layers of HSA and unfractionated heparin; and (3) alternating layers of HSA and partly depolymerized heparin fixed with one end to HSA were prepared on various surfaces. Adsorption of fibrinogen, IgG, and antithrombin (ATIII) from human citrated plasma on coated surfaces was evaluated by ELISA. Fibrinogen adsorption on coated ELISA plates was lower than that on bare polystyrene. There was no IgG adsorption on the HSA coating alone, but considerably high IgG adsorption was detected on the heparin-containing surface. The adsorption of ATIII increased with increasing heparin on the surface. The effect of multilayer coatings on platelets was tested by incubation of modified vascular prostheses with citrated blood. The most favorable interaction with platelets was observed on the HSA assembly. The interaction of platelets with the surface bearing unfractionated heparin was higher than that of the surface covered with partly depolymerized heparin. The long-term durability of the HSA-heparin coating was proven by a 21-day implantation of coated polyurethane plates in goat heart.

Rapid activation of the complement system by cuprophane depends on complement component C4

Hemodialysis with cuprophane dialyzer membranes promotes rapid activation of the complement system, which is thought to be mediated by the alternative pathway. Complete hereditary deficiency of complement C4, a classical pathway component, in two hemodialysis patients provided the opportunity to investigate a possible role of the classical pathway. In two hemodialysis patients with both C4 isotypes, C4A and C4B, and in one patient with C4B deficiency complement activation occurred immediately after the onset of hemodialysis, with peak levels of C3a and terminal complement complex (TCC) after ten to fifteen minutes. In patients with complete C4 deficiency, C3a and TCC remained unchanged for fifteen minutes and increased thereafter, reaching the highest level after thirty minutes. The leukocyte nadir was also delayed from fifteen to thirty minutes. In vitro incubation of normal, C4A- or C4B-deficient serum with cuprophane caused complement activation after fifteen minutes. In contrast, no activation was observed in sera of four C4-deficient patients. The addition of normal serum or purified human C4 restored the capacity for rapid complement activation. In one patient with severe immunoglobulin deficiency, C3a and TCC levels increased only moderately after 25 minutes of cuprophane dialysis. This patient's serum also exhibited delayed complement activation in vitro, which was normalized after pretreatment of cuprophane with immunoglobulins. Preincubation of normal serum with MgEGTA, a blocker of the classical pathway, inhibited rapid complement activation through cuprophane. As basal levels of C4a are markedly increased in hemodialysis patients (3450 +/- 850 ng/ml) compared to healthy controls (224 +/- 81 ng/ml), no further elevation of C4a was detectable during cuprophane hemodialysis. Incubation of normal serum with cuprophane, however, caused a slight increase in C4a after five minutes. These results indicate that the initial deposition of complement C3b on the cuprophane membrane, necessary for activation of the amplification loop of the alternative pathway, is mediated by the classical pathway C3-convertase C4b2a. We propose an extended concept of complement activation through cuprophane, which is based on four steps: (a) binding of anti-polysaccharide antibodies, (b) classical pathway activation, (c) alternative pathway activation and (d) terminal pathway activation.

Controlling the complement system in inflammation

Inappropriate or excessive activation of the complement system can lead to harmful, potentially life-threatening consequences due to severe inflammatory tissue destruction. These consequences are clinically manifested in various disorders, including septic shock, multiple organ failure and hyperacute graft rejection. Genetic complement deficiencies or complement depletion have been proven to be beneficial in reducing tissue injury in a number of animal models of severe complement-dependent inflammation. It is therefore believed that therapeutic inhibition of complement is likely to arrest the process of certain diseases. Attempts to efficiently inhibit complement include the application of endogenous soluble complement inhibitors (C1-inhibitor, recombinant soluble complement receptor 1- rsCR1), the administration of antibodies, either blocking key proteins of the cascade reaction (e.g. C3, C5), neutralizing the action of the complement-derived anaphylatoxin C5a, or interfering with complement receptor 3 (CR3, CD18/11b)-mediated adhesion of inflammatory cells to the vascular endothelium. In addition, incorporation of membrane-bound complement regulators (DAF-CD55, MCP-CD46, CD59) has become possible by transfection of the correspondent cDNA into xenogeneic cells. Thereby, protection against complement-mediated inflammatory tissue damage could be achieved in various animal models of sepsis, myocardial as well as intestinal ischemia/reperfusion injury, adult respiratory distress syndrome, nephritis and graft rejection. Supported by results from first clinical trials, complement inhibition appears to be a suitable therapeutic approach to control inflammation. Current strategies to specifically inhibit complement in inflammation have been discussed at a recent meeting on the 'Immune Consequences of Trauma, Shock and Sepsis', held from March 4-8, 1997, in Munich, Germany. The Congress (chairman: E. Faist, Munich, Germany), which was held in close cooperation with various national and international shock and trauma societies, was attended by about 2000 delegates from 40 countries. The major objective of the meeting was to provide an overview on the most state-of-the-art methods to prevent multiple organ dysfunction syndrome (MODS)/multiple organ failure (MOF) following the systemic inflammatory response (SIRS) to severe trauma. One of the largest symposia held within the Congress was devoted to current aspects of controlling complement in inflammation (for abstracts see: Shock 1997, 7 Suppl., 71-75). After providing the audience with information on the scientific background by addressing the clinical relevance of complement activation (G.O. Till, Ann Arbor, MI, USA) and discussing recent developments in modern complement diagnosis (J. Köhl, Hannover, Germany), B.P. Morgan (Cardiff, UK) introduced the symposium's special issue by giving an overview on complement regulatory molecules. Selected topics included overviews on the application of C1 inhibitor (C.E. Hack, Amsterdam, NL), sCR1 (U.S. Ryan, Needham, MA, USA), antibodies to C5 (Y. Wang, New Haven CT, USA) and to the anaphylatoxin C5a (M. Oppermann, Göttingen, Germany), and a report on complement inhibition in cardiopulmonary bypass (T.E. Mollnes, Bodø, Norway). The growing interest of clinicians in complement-directed anti-inflammatory therapy, and the fact that only some of the various aspects of therapeutic complement inhibition could be addressed on the meeting, has motivated the author to expand a Congress report into a short comprehensive review on recent strategies to control complement in inflammation.

In vitro evaluation of heparinized Cuprophan hemodialysis membranes

Cuprophan hemodialysis membranes can be heparinized using N,N'-carbonyldiimidazole (CDI) as a coupling agent. In this study, the characteristics of heparinized Cuprophan membranes have been evaluated. After immobilization, heparin partially retained its biologic activity. An anticoagulant activity of 12.4 +/- 4.2 mU/cm2 was measured using a thrombin inactivation assay. Immobilized heparin also displayed an anti-complement activity. After contact with human serum; heparinized Cuprophan induced no generation of significant amounts of fluid phase terminal complement complex (TCC), whereas untreated Cuprophan induced the generation of substantial amounts of TCC. Heparinization did not affect the permeability of Cuprophan for model solutes with molecular weights up to 12,000 g/mol except for sulfobromophthalein sodium salt. The permeability of Cuprophan for sulfobromophthalein sodium salt was slightly decreased after heparinization. The ultrafiltration rate of Cuprophan increased by about 30% after heparinization, probably owing to an increased swelling of the membrane in water. Heparinized Cuprophan incubated in phosphate-buffered saline at 37 degrees C showed some release of heparin. These amounts of released heparin, however, were very low as compared to the amounts of heparin which are systemically administered during clinical hemodialysis treatment. It is concluded that Cuprophan membranes heparinized by means of the CDI-activation procedure are highly promising for application in hemodialyzers to be used for the treatment of patients with reduced or without systemic administration of heparin.

Tubing loops as a model for cardiopulmonary bypass circuits: both the biomaterial and the blood-gas phase interfaces induce complement activation in an in vitro model

We describe here a model for the study of blood/surface and blood/air interaction as encountered in cardiopulmonary bypass (CPB) circuits. Polyethylene tubing was filled with serum or blood and closed end to end into loops whereby the volume of the remaining air bubble was inversely varied with respect to that of the fluid. The loops were rotated vertically in a water bath at 37 degrees C. The profiles of C3a, iC3, and TCC generation were similar to those observed at surgery, involving CPB. Soluble heparin and heparan sulfate inhibited both C3a and TCC formation, but surface-conjugated heparin had only a minor effect. Binding of C3 and/or C3 fragments to the heparin surface was much reduced compared to the amine matrix to which heparin was linked, but compared with the polyethylene surface the effect was less pronounced. These data suggest that, in addition to the biomaterial surface, the blood-gas interface seems to play an important role in the activation of complement and that this activation is inhibitable by high concentrations of soluble glucose aminoglycans.

Relationships between chemical characteristics and anticomplementary activity of fucans

We have shown previously that a low-molecular-weight fucan extracted from the brown seaweed Ascophylum nodosum strongly inhibited human complement activation in vitro and its mechanism of action was largely elucidated. We further investigated the influence of molecular weight and chemical composition of fucan on its anticomplementary activity. The capacity of 12 fragments of fucan (ranging from a molecular weight of 4100 to 214,000) to prevent complement-mediated haemolysis of sheep erythrocytes (classical pathway) and of rabbit erythrocytes (alternative pathway) increased with increasing molecular weight, and reached a plateau for 40,000 and 13,500, respectively. The most potent fucan fractions were 40-fold more active than heparin in inhibiting the classical pathway. They were, however, as active as heparin in inhibiting the alternative pathway. In addition, we have developed a haemolytic test based on the CH50 protocol, which allows discrimination between activators and inhibitors of complement proteins. Although the mannose content within the different fucan fragments did not vary, the galactose and glucuronic acid contents increased with increasing activity, suggesting that these residues should be essential for full anticomplementary activity. Meanwhile, sulphate groups appeared to be necessary, but were clearly not a sufficient requirement for anticomplementary activity of fucans. Taken together, these data illustrate the prospects for the use of fucans as potential anti-inflammatory agents.

Surface modification of chitosan membranes by complexation-interpenetration of anionic polysaccharides for improved blood compatibility in hemodialysis

Chitosan membrane surface was modified by complexation and interpenetration of anionic polysaccharides--heparin and dextran sulfate--for improved blood compatibility in hemodialysis. Electron spectroscopy for chemical analysis results showed a characteristic sulfur (S) and sodium (Na) peaks after modification with dextran sulfate. The sulfur/carbon (S/C) atomic composition ratio increased from 0.03 to 0.08 when the bulk dextran sulfate concentration used for modification was increased from 2.5 to 10 mg ml-1. The permeability of urea and creatinine did not change significantly upon modification with heparin or dextran sulfate. Surface modification, however, did decrease the permeability coefficients of glucose, vitamin B-2, and vitamin B-12. Unlike Cuprophan, chitosan and surface-modified chitosan membranes did not significantly activate the complement system as measured by the serum iC3b concentration. Compared to forty and sixty fully-activated platelets present on control surfaces, surface modification with heparin and dextran sulfate significantly reduced the number of adherent platelets per 25,000 microns 2 area and the extent of platelet activation. Surface modification with anionic polysaccharides, however, did significantly shorten the plasma recalcification time leading to fibrin clot formation. The results of this study show that chitosan membrane surface can be modified by complexation-interpenetration of anionic modifying agents. The modified membranes do resist complement activation and platelet adhesion and activation.

A new model for evaluation of biocompatibility: combined determination of neoepitopes in blood and on artificial surfaces demonstrates reduced complement activation by immobilization of heparin

An in vitro technique was developed for assessment of the biocompatibility of materials for use in clinical applications. Artificial materials were exposed to blood, and the resulting complement activation was quantified both in the plasma and on the material surface by enzyme immunoassays based on monoclonal antibodies specific for neoepitopes exposed in complement activation products. Several materials were evaluated, and the effect of surface modifications, including end-point immobilized heparin, was studied. The results revealed widely varying complement activation properties of the different materials and confirmed that heparin markedly improves biocompatibility. The present method is superior to analysis limited to either the fluid phase or solid phase since certain materials adsorb activation products (exemplified by Tecoflex) whereas others do not although activation was evident from fluid-phase assay (silicone). Furthermore, direct determination of activation-specific neoepitopes on the surface represents an improvement compared with previously described methods for detection of adsorbed components.

Role of bioincompatibility in dialysis morbidity and mortality

This review examines the mechanisms by which bioincompatibility in dialysis systems may have an effect on morbidity and mortality in the dialysis population. Direct toxic effects of membrane materials and various chemical substances have been well demonstrated in the chronic dialysis population. Activation of the complement cascade and stimulation of cytokine production may have autocrine effects on leukocyte function with sequelae such as enhanced rates of infection and the development of B2-microglobulin amyloidosis. The variable effect of different membrane materials on each of these effector systems is examined. Bioincompatibility may effect the incidence of infection, malignancy, cardiopulmonary disease, and malnutrition as well as induce novel disease processes. All these confounding variables must be considered when evaluating the effect of dialysis on mortality and morbidity.

Soluble complement receptor type 1 inhibits complement activation induced by hemodialysis membranes in vitro

A variety of bioincompatible events that occur during hemodialysis have been attributed to complement activation. However, cause-effect relationships have been based primarily on indirect evidence and the results of in vitro studies, because an acceptable method for inhibiting complement activation during clinical hemodialysis has been unavailable. Methods for inactivating complement in vitro are available, but the most commonly used of these techniques (heat inactivation and chelation of divalent cations) lack specificity. A recombinant, soluble form of human complement receptor type 1 (sCR1) has been developed recently and shown to inhibit complement activation in vivo. Here, we report studies aimed at determining the effects of sCR1 on dialysis-induced complement activation and neutrophil degranulation. In a concentration dependent fashion, sCR1 inhibited plasma complement activation by cuprophan membrane in vitro. Using a maximally inhibitory concentration (30 micrograms/ml), sCR1 blocked generation of C3a(desArg) by cuprophan, cellulose acetate, and polymethylmethacrylate membranes by 90%, 84%, and 84%, respectively. In contrast, elastase release (a measure of neutrophil degranulation) was inhibited by 70%, 70%, and 44%, respectively, suggesting that dialysis-induced neutrophil activation is mediated in part by noncomplement dependent mechanisms. Both heat- and EDTA-treatment of plasma abolished dialysis membrane-induced complement activation, but these treatments also affected noncomplement dependent components of the degranulation process. These observations show that, compared with other commonly used methods for inhibiting dialysis induced complement activation, sCR1 is more specific. An additional advantage of sCR1 is its potential for use in the clinical setting.