Innate Immune Responses to Transplants: A Significant Variable with Cadaver Donors

Abstract

Over the past several decades, advances in the study of T cell biology have led to numerous experimental protocols and strategies to induce transplantation tolerance or to dramatically inhibit rejection responses. Unfortunately, in general these approaches have met with considerably less success in the clinical setting. While differences in the immune systems of inbred laboratory rodents and heterogeneous human populations undoubtedly account for many of these discrepancies, relatively little attention has been given to potentially important differences between experimental transplant models and the practice of clinical transplantation. In most transplant experiments, organs and tissues are removed from unmanipulated healthy donors under ideal surgical conditions and immediately transplanted into healthy recipients. This simulates most closely the clinical conditions of a renal transplant performed from a living donor. However, most organs are harvested from cadaver donors. Unlike the carefully screened and controlled living donor, cadaveric organs are harvested under conditions of extreme physiologic stress (summarized in Table 1) , many of which can stimulate components of the innate immune system.Table 1Stresses Incurred by Transplanted Organs in Donor and Recipient that Can Alter Innate Immune ResponsesDonorTransportRecipientMortal injury (MVA, GSW, CVA)aMVA, motor vehicle accident; GSW, gunshot wound; CVA, cerebral vascular accident. Emergency interventions Brain death Postmortum support Organ harvestCold ischemiaWarm ischemia (Cardiac by-pass) Drug therapy Infection Alloimmunitya MVA, motor vehicle accident; GSW, gunshot wound; CVA, cerebral vascular accident. Open table in a new tab In this review, we highlight selected aspects of the innate immune system, in particular the complement cascade and natural killer (NK) cells, and the mechanisms by which they may have a significant impact on the alloimmune responses to organ transplants. Heightened interest in the potential impact of innate immune responses on subsequent alloimmune responses has emerged as the result of the recent use of renal transplants from living genetically unrelated donors. These transplants from histoincompatible living unrelated donors survive as well or better than transplants from “well-matched” cadaver donors (D'Alessandro et al. 1998D'Alessandro A.M. Pirsch J.D. Knechtle S.J. Odorico J.S. Van der Werf W.J. Collins B.H. Becker Y.T. Kalayoglu M. Armbrust M.J. Sollinger H.W. Living unrelated renal donation the University of Wisconsin experience.Surgery. 1998; 124: 604-610Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar). This disparity in survival is magnified when the renal transplant does not function immediately. Some of the major differences between organs harvested from cadaver and living donors are related to ischemia-reperfusion. Longer periods of “warm” and “cold” ischemia for organs harvested from cadaver donors eventuate from complexities of obtaining permission to use the organs, coordinating harvest of multiple organs, preserving organs for transportation, identifying appropriate recipients, and performing unscheduled surgery. Reperfusion of blood through ischemic tissue results in a set of reactions that can cause injury to vascular and parenchymal cells. Pathologically, these are characterized by deposition of complement, upregulation of adhesion molecules, inflammatory cell infiltration, and cytokine release (Weisman et al. 1990Weisman H.F. Bartow T. Leppo M.K. Marsh Jr., H.C. Carson G.R. Concino M.F. Boyle M.P. Roux K.H. Weisfeldt M.L. Fearon D.T. Soluble human complement receptor type 1 In vivo inhibitor of complement suppressing post-ischemic myocardial inflammation and necrosis.Science. 1990; 249: 146-151Crossref PubMed Scopus (844) Google Scholar, Halloran et al. 1992Halloran P.F. Autenried P. Ramassar V. Urmson J. Cockfield S. Local T cell responses induce widespread MHC expression Evidence that INFg induces its own expression in remote sites.J. Immunol. 1992; 148: 3837-3846PubMed Google Scholar, Weiser et al. 1996Weiser M.R. Williams J.P. Moore Jr., F.D. Kobzik L. Ma M. Hechtman H.B. Carroll M.C. Reperfusion injury of ischemic skeletal muscle is mediated by natural antibody and complement.J. Exp. Med. 1996; 183: 2343-2348Crossref PubMed Scopus (310) Google Scholar). Activation of complement, platelets, neutrophils, and macrophages following ischemia extends the initial damage. Ischemic injury can be compounded by the traumatic injury and emergency interventions that precede organ harvest from cadaver donors (Wilhelm et al. 2000Wilhelm M.J. Pratschke J. Beato F. Taal M. Kusaka M. Hancock W.W. Tilney N.L. Activation of the heart by donor brain death accelerates acute rejection after transplantation.Circulation. 2000; 102: 2426-2433Crossref PubMed Scopus (162) Google Scholar). In addition, recipients also are subjected to therapeutic treatments before and during transplantation that may amplify innate immune responses. Examples include the use of dialysis or ventricular assist devices prior to transplantation and cardiopulmonary bypass intraoperatively (Van Loo et al. 1998Van Loo A.A. Vanholder R.C. Bernaert P.R. Vermassen F.E. Van der Vennet M. Lameire N.H. Pretransplantation hemodialysis strategy influences early renal graft function.J. Am. Soc. Nephrol. 1998; 9: 473-481PubMed Google Scholar, Fitch et al. 1999Fitch J.C. Rollins S. Matis L. Alford B. Aranki S. Collard C.D. Dewar M. Elefteriades J. Hines R. Kopf G. et al.Pharmacology and biological efficacy of a recombinant, humanized, single-chain antibody C5 complement inhibitor in patients undergoing coronary artery bypass graft surgery with cardiopulmonary bypass.Circulation. 1999; 100: 2499-2506Crossref PubMed Scopus (232) Google Scholar). This is not to suggest that a concomitant innate response to the acute injury associated with the transplant procedure is required to elicit allograft rejection. Studies using T cell–deficient mice have shown that prompt rejection can occur after T cell reconstitution even when skin or cardiac allografts have been allowed to recover and heal for more than 100 days (Bingaman et al. 2000Bingaman A.W. Ha J. Waitze S.Y. Durham M. Cho H.R. Tucker-Burden C. Hendrix R. Cowan S.R. Pearson T.C. Larsen C.P. Vigorous allograft rejection in the absence of danger.J. Immunol. 2000; 164: 3065-3071PubMed Google Scholar). Rather, concomitant stimulation of the innate immune system leads to heightened expression of MHC molecules, costimulatory molecules, cytokines, and other factors that may alter the threshold for T cell activation, influence the cytokine profile of the adaptive immune response, promote the more rapid recruitment of allo-reactive T cells, or directly promote injury of the transplanted organ. For example, recent evidence suggests that ischemia reperfusion injury alone in the absence of an alloantigenic stimulus results in a histological and functional picture that is very similar to chronic transplant nephropathy. This process is associated with upregulation of B7 costimluatory and MHC class II molecules and is inhibited by administration of CTLA4-Ig (Chandraker et al. 1997Chandraker A. Takada M. Nadeau K.C. Peach R. Tilney N.L. Sayegh M.H. CD28-b7 blockade in organ dysfunction secondary to cold ischemia/reperfusion injury.Kidney Int. 1997; 52: 1678-1684Crossref PubMed Scopus (89) Google Scholar, Takada et al. 1997Takada M. Nadeau K.C. Shaw G.D. Marquette K.A. Tilney N.L. The cytokine-adhesion molecule cascade in ischemia/reperfusion injury of the rat kidney. Inhibition by a soluble P-selectin ligand.J. Clin. Invest. 1997; 99: 2682-2690Crossref PubMed Scopus (455) Google Scholar). This review will focus on the mechanisms by which two of the best-defined mediators of innate immunity, complement and natural killer cells, influence the adaptive immune response and promote transplant injury. Activation of complement is an essential feature of the inflammatory response generated by ischemia-reperfusion, dialysis, and cardiopulmonary bypass (Weisman et al. 1990Weisman H.F. Bartow T. Leppo M.K. Marsh Jr., H.C. Carson G.R. Concino M.F. Boyle M.P. Roux K.H. Weisfeldt M.L. Fearon D.T. Soluble human complement receptor type 1 In vivo inhibitor of complement suppressing post-ischemic myocardial inflammation and necrosis.Science. 1990; 249: 146-151Crossref PubMed Scopus (844) Google Scholar, Halloran et al. 1992Halloran P.F. Autenried P. Ramassar V. Urmson J. Cockfield S. Local T cell responses induce widespread MHC expression Evidence that INFg induces its own expression in remote sites.J. Immunol. 1992; 148: 3837-3846PubMed Google Scholar, Weiser et al. 1996Weiser M.R. Williams J.P. Moore Jr., F.D. Kobzik L. Ma M. Hechtman H.B. Carroll M.C. Reperfusion injury of ischemic skeletal muscle is mediated by natural antibody and complement.J. Exp. Med. 1996; 183: 2343-2348Crossref PubMed Scopus (310) Google Scholar, Van Loo et al. 1998Van Loo A.A. Vanholder R.C. Bernaert P.R. Vermassen F.E. Van der Vennet M. Lameire N.H. Pretransplantation hemodialysis strategy influences early renal graft function.J. Am. Soc. Nephrol. 1998; 9: 473-481PubMed Google Scholar, Fitch et al. 1999Fitch J.C. Rollins S. Matis L. Alford B. Aranki S. Collard C.D. Dewar M. Elefteriades J. Hines R. Kopf G. et al.Pharmacology and biological efficacy of a recombinant, humanized, single-chain antibody C5 complement inhibitor in patients undergoing coronary artery bypass graft surgery with cardiopulmonary bypass.Circulation. 1999; 100: 2499-2506Crossref PubMed Scopus (232) Google Scholar). Complement activation is integral to the tissue injury resulting from these interventions because interrupting the complement cascade by exogenous inhibitors or genetic deficiencies decreases the inflammatory cell infiltrates and tissue injury (Weisman et al. 1990Weisman H.F. Bartow T. Leppo M.K. Marsh Jr., H.C. Carson G.R. Concino M.F. Boyle M.P. Roux K.H. Weisfeldt M.L. Fearon D.T. Soluble human complement receptor type 1 In vivo inhibitor of complement suppressing post-ischemic myocardial inflammation and necrosis.Science. 1990; 249: 146-151Crossref PubMed Scopus (844) Google Scholar, Weiser et al. 1996Weiser M.R. Williams J.P. Moore Jr., F.D. Kobzik L. Ma M. Hechtman H.B. Carroll M.C. Reperfusion injury of ischemic skeletal muscle is mediated by natural antibody and complement.J. Exp. Med. 1996; 183: 2343-2348Crossref PubMed Scopus (310) Google Scholar, Fitch et al. 1999Fitch J.C. Rollins S. Matis L. Alford B. Aranki S. Collard C.D. Dewar M. Elefteriades J. Hines R. Kopf G. et al.Pharmacology and biological efficacy of a recombinant, humanized, single-chain antibody C5 complement inhibitor in patients undergoing coronary artery bypass graft surgery with cardiopulmonary bypass.Circulation. 1999; 100: 2499-2506Crossref PubMed Scopus (232) Google Scholar). Although there is general agreement that complement can contribute to ischemia-reperfusion injury, the mechanism of complement activation in this setting has not been clarified fully. The complement cascade can be activated through three different initial pathways: the classical (C1, C4, and C2), alternative (factor B), and mannose binding lectin (MBL) pathways (Figure 1). The majority of evidence indicates that ischemia-reperfusion leads to complement activation through the classical or MBL pathway. Antibodies are the most thoroughly investigated mechanism of activating the classical pathway of complement. Clinically, “natural” antibodies to the major blood group antigens and alloantibodies induced by previous transfusions, pregnancies, or transplants are known to activate complement. The degree of tissue injury caused by these antibodies depends on the isotype, affinity, and titer of the antibodies as well as the expression of the target antigen. Weiser and coworkers (Weiser et al. 1996Weiser M.R. Williams J.P. Moore Jr., F.D. Kobzik L. Ma M. Hechtman H.B. Carroll M.C. Reperfusion injury of ischemic skeletal muscle is mediated by natural antibody and complement.J. Exp. Med. 1996; 183: 2343-2348Crossref PubMed Scopus (310) Google Scholar) have raised the possibility that natural antibodies to normally cryptic antigens may contribute to complement-mediated ischemia-reperfusion injury. They demonstrated that vascular leakage following ischemia-reperfusion of skeletal muscle was associated with the deposition of IgM and complement. Tissue injury was decreased in both immunoglobulin and complement (C4 or C3) knockout mice. Furthermore, reconstitution of immunoglobulin knockout mice with serum from normal mice restored susceptibility to ischemia-reperfusion injury. To date, the potential target of these natural IgM antibodies has not been defined, but natural polyreactive autoantibodies are an important component of the normal B cell repertoire. Some natural polyreactive IgM autoantibodies have been defined that recognize epitopes on apoptotic cells (Shaw et al. 2000Shaw P.X. Horkko S. Chang M.K. Curtiss L.K. Palinski W. Silverman G.J. Witztum J.L. Natural antibodies with the T15 idiotype may act in atherosclerosis, apoptotic clearance, and protective immunity.J. Clin. Invest. 2000; 105: 1731-1740Crossref PubMed Scopus (524) Google Scholar). These would be good candidates for complement activation in ischemically injured tissues. Besides antibodies, several acute phase proteins can bind C1q of the classical pathway of complement. C-reactive protein (CRP) is the major acute-phase protein in humans (for recent review see Du Clos 2000Du Clos T.W. Function of C-reactive protein.Ann. Med. 2000; 32: 274-278Crossref PubMed Scopus (466) Google Scholar). Trauma, surgery, or infection can stimulate an acute synthesis of these proteins by the liver that results in as much as a 1,000-fold increase in plasma concentration of CRP within hours to days. This response is regulated by proinflammatory mediators, predominantly IL-6, and is largely uninhibited by anti-inflammatory drugs. Consequently, circulating CRP would be expected to be elevated in cadaver donors and transplant recipients. Studies have confirmed that CRP is elevated for several days after renal or cardiac transplantation (Harris et al. 1996Harris K.R. Digard N.J. Lee H.A. Serum C-reactive protein. A useful and economical marker of immune activation in renal transplantation.Transplantation. 1996; 61: 1593-1600Crossref PubMed Scopus (32) Google Scholar, Wolbink et al. 1996Wolbink G.J. Brouwer M.C. Buysmann S. ten Berge I.J. Hack C.E. CRP-mediated activation of complement in vivo assessment by measuring circulating complement-C-reactive protein complexes.J. Immunol. 1996; 157: 473-479PubMed Google Scholar). CRP is composed of five identical 206 amino acid subunits arranged in a planar ring structure. The pentameric structure and the capacity to activate complement are features CRP shares with other members of the pentraxin family of proteins including serum amyloid P component, another prototypical acute-phase protein, and the long pentraxin TSG-14/PTX3 that is produced by monocytes and endothelial cells (Rovere et al. 2000Rovere P. Peri G. Fazzini F. Bottazzi B. Doni A. Bondanza A. Zimmermann V.S. Garlanda C. Fascio U. Sabbadini M.G. et al.The long pentraxin PTX3 binds to apoptotic cells and regulates their clearance by antigen-presenting dendritic cells.Blood. 2000; 96: 4300-4306Crossref PubMed Google Scholar). CRP was discovered and named because of its reactivity to pneumococcal C-polysaccharide, but CRP also binds to phosphatidylcholine and sphingomyelin that are exposed on plasma membranes of injured cells and to chromatin that is exposed on apoptotic or necrotic cells. CRP can bind to other ligands exposed during tissue injury, including laminin and fibronectin, the principal adhesive molecules in basal lamina and in connective tissue, respectively. After CRP complexes with a tissue ligand through its five calcium-dependent binding sites on one surface, it can activate complement or Fc receptor bearing cells (Figure 2). CRP activates the classical pathway of complement by binding to the collagen-like region of C1q. CRP also interacts with the high- and low-affinity receptors for IgG, FcγRI and FcγRII, respectively. These interactions can lead to enhanced antigen presentation and production of IL-1 and TNF. CRP has been implicated in the pathogenesis of ischemic injury in vivo by its colocalization with complement in atherosclerotic lesions and acute myocardial infarcts. Griselli and colleagues (Griselli et al. 1999Griselli M. Herbert J. Hutchinson W.L. Taylor K.M. Sohail M. Krausz T. Pepys M.B. C-reactive protein and complement are important mediators of tissue damage in acute myocardial infarction.J. Exp. Med. 1999; 190: 1733-1740Crossref PubMed Scopus (424) Google Scholar) have exploited their finding that rat CRP does not activate complement to develop an in vivo model, in which human CRP binds to damaged cells and activates rat complement. In this model, an injection of human CRP into rats was found to increase the area of ischemia and infarct caused by a temporary ligation of a coronary artery. Both CRP and complement were deposited in the tissue at risk. The potential for CRP to activate complement shortly after transplantation is supported by measurements of circulating complexes of CRP and complement components. These complexes increase significantly within the first 3 days after clinical renal transplantation (Wolbink et al. 1996Wolbink G.J. Brouwer M.C. Buysmann S. ten Berge I.J. Hack C.E. CRP-mediated activation of complement in vivo assessment by measuring circulating complement-C-reactive protein complexes.J. Immunol. 1996; 157: 473-479PubMed Google Scholar). Similar to CRP, MBL is a component of the innate immune response to pathogens. MBL, like C1q, is a collectin composed of subunits with collagen-like tails attached to globular lectin heads. The complete MBL is formed by multimers of 2–6 subunits. Each head contains three C-type lectin domains that bind to terminal mannose, fucose, or glucosamine in oligosaccharides on pathogens. After binding, MBL can cleave C2 and C4 through two associated serine proteases (MASP-1 and -2) that are similar to C1r and C1s. In vitro and in vivo studies suggest that MBL might also activate complement following ischemia-reperfusion. MBL has been shown to colocalize with complement deposition on human vascular endothelium after oxidative stress in vitro and in rat hearts after temporary ligation of a coronary artery (Collard et al. 2000Collard C.D. Vakeva A. Morrissey M.A. Agah A. Rollins S.A. Reenstra W.R. Buras J.A. Meri S. Stahl G.L. Complement activation after oxidative stress role of the lectin complement pathway.Am. J. Pathol. 2000; 156: 1549-1556Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar). In this study, activation of complement was inhibited by free D-mannose or monoclonal antibodies to MBL. Like CRP, MBL is capable not only of activating the classical complement components C4 and C2 but also of serving as a ligand for receptors on macrophages (Bajtay et al. 2000Bajtay Z. Jozsi M. Banki Z. Thiel S. Thielens N. Erdei A. Mannan-binding lectin and C1q bind to distinct structures and exert differential effects on macrophages.Eur. J. Immunol. 2000; 30: 1706-1713Crossref PubMed Scopus (27) Google Scholar). C3 as hub of complement cascade: C3 is the hub of the complement cascade quantitatively and functionally. All three pathways of complement activation converge in the activation of C3. C3 is present in plasma at high concentrations (1.3 mg/ml). Additional C3 can be produced in tissues by infiltrating macrophages (Bajtay et al. 2000Bajtay Z. Jozsi M. Banki Z. Thiel S. Thielens N. Erdei A. Mannan-binding lectin and C1q bind to distinct structures and exert differential effects on macrophages.Eur. J. Immunol. 2000; 30: 1706-1713Crossref PubMed Scopus (27) Google Scholar) and certain parenchymal cells within a transplanted organ, including vascular endothelial cells, glomerular mesangial cells, renal tubular epithelial cells, and alveolar epithelial cells (Brooimans et al. 1991Brooimans R.A. Stegmann A.P. van Dorp W.T. van der Ark A.A. van der Woude F.J. van Es L. Daha M.R. Interleukin 2 mediates stimulation of complement C3 biosynthesis in human proximal tubular epithelial cells.J. Clin. Invest. 1991; 88: 379-384Crossref PubMed Scopus (166) Google Scholar, Takada et al. 1997Takada M. Nadeau K.C. Shaw G.D. Marquette K.A. Tilney N.L. The cytokine-adhesion molecule cascade in ischemia/reperfusion injury of the rat kidney. Inhibition by a soluble P-selectin ligand.J. Clin. Invest. 1997; 99: 2682-2690Crossref PubMed Scopus (455) Google Scholar, Pratt et al. 2000Pratt J.R. Abe K. Miyazaki M. Zhou W. Sacks S.H. In situ localization of C3 synthesis in experimental acute renal allograft rejection.Am. J. Pathol. 2000; 157: 825-831Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar). Within inflammatory foci, the production of C3 by different parenchymal cells may be augmented by proinflammatory cytokines, such as IL-1, IL-2, IL-6, and TNFα (Brooimans et al. 1991Brooimans R.A. Stegmann A.P. van Dorp W.T. van der Ark A.A. van der Woude F.J. van Es L. Daha M.R. Interleukin 2 mediates stimulation of complement C3 biosynthesis in human proximal tubular epithelial cells.J. Clin. Invest. 1991; 88: 379-384Crossref PubMed Scopus (166) Google Scholar). Macrophage production of C3 is also increased in the presence of C1q through the C1q receptor (Bajtay et al. 2000Bajtay Z. Jozsi M. Banki Z. Thiel S. Thielens N. Erdei A. Mannan-binding lectin and C1q bind to distinct structures and exert differential effects on macrophages.Eur. J. Immunol. 2000; 30: 1706-1713Crossref PubMed Scopus (27) Google Scholar). Activation of C3 is accomplished by enzymatic cleavage into two biologically active fragments: a small soluble mediator of chemotaxis and activation (C3a) and a large fragment that can bind covalently to proteins or carbohydrates (C3b). Once bound, C3b is subject to two competing processes: (1) an amplification loop through factor B of the alternative pathway that can augment the deposition of C3b and (2) regulation. Regulation of C3 is controlled by a group of regulators of complement activation (RCA). The RCA gene cluster codes for six inhibitors of C3 activation: membrane cofactor protein (MCP, CD46), complement receptor 1 (CR1, CD35) and complement receptor 2 (CR2, CD21), decay accelerating factor (DAF), C4 binding protein (C4bp), and factor H. The first four regulators are expressed as membrane bound proteins, while the last two circulate in the plasma. MCP and DAF are present on a wide variety of cells, including leukocytes and vascular endothelial cells, while CR1 and CR2 are expressed on selected leukocytes and antigen-presenting cells. When C3b is bound by CR1 or factor H, it can be cleaved successively by factor I, leaving first iC3b and then C3dg attached to the membrane (Figure 3). C3b, iC3b, and C3dg serve as ligands for cells with complement receptors CR1, CR3 and CR2, respectively. Macrophages, which possess CR1 and CR3, are characteristic features of ischemia-reperfusion damage in transplants (Baldwin et al. 1999Baldwin III, W.M. Samaniego-Picota M. Kasper E.K. Clark A. Czader M. Rohde C. Zachary A.A. Sanfilippo F. Hruban H. Complement deposition in early cardiac transplant biopsies is associated with ischemic injury and subsequent rejection episodes.Transplantation. 1999; 68: 894-900Crossref PubMed Scopus (103) Google Scholar). Factor H is particularly relevant to activation of complement by CRP. CRP binds factor H in addition to C1q, and thereby effectively truncates complement activation at C3 (Du Clos 2000Du Clos T.W. Function of C-reactive protein.Ann. Med. 2000; 32: 274-278Crossref PubMed Scopus (466) Google Scholar). This leads to the deposition of the ligands for CR1, CR3, and CR2 without generating the inflammatory C5a or terminal complement components (Figure 2). C3a and C5a as chemoattractants and activators: activation of C5 as well as C3 is accomplished by enzymatic cleavage into two biologically active fragments. The small fragments (C3a and C5a) mediate chemotaxis and activation through separate receptors (C3aR and C5aR) on leukocytes. C5a is quantitatively much more potent as a chemotatic agent than C3a and affects a wider range of cells, including neutrophils, monocytes, basophils, and eosinophils. C3a acts primarily on mast cells and eosinophils (DiScipio et al. 1999DiScipio R.G. Daffern P.J. Jagels M.A. Broide D.H. Sriramarao P. A comparison of C3a and C5a-mediated stable adhesion of rolling eosinophils in postcapillary venules and transendothelial migration in vitro and in vivo.J. Immunol. 1999; 162: 1127-1136PubMed Google Scholar). Activation of monocytes through their C5a receptor causes production of proinflammatory cytokines, such as IL-1, IL-6, IL-8, and TNFα, and upregulation of CR1 and CR3 (Arnaout et al. 1984Arnaout M.A. Spits H. Terhorst C. Pitt J. Todd III., R.F. Deficiency of a leukocyte surface glycoprotein (LFA-1) in two patients with Mo1 deficiency. Effects of cell activation on Mo1/LFA-1 surface expression in normal and deficient leukocytes.J. Clin. Invest. 1984; 74: 1291-1300Crossref PubMed Scopus (130) Google Scholar, Ember et al. 1994Ember J.A. Sanderson S.D. Hugli T.E. Morgan E.L. Induction of interleukin-8 synthesis from monocytes by human C5a anaphylatoxin.Am. J. Pathol. 1994; 144: 393-403PubMed Google Scholar). The increased synthesis of proinflammatory cytokines can act together with C5a to promote hepatic synthesis of CRP (Szalai et al. 2000Szalai A.J. van Ginkel F.W. Wang Y. McGhee J.R. Volanakis J.E. Complement-dependent acute-phase expression of C-reactive protein and serum amyloid P-component.J. Immunol. 2000; 165: 1030-1035PubMed Google Scholar). C5a has also been reported to activate endothelial cells directly through a C5a receptor (Foreman et al. 1994Foreman K.E. Vaporciyan A.A. Bonish B.K. Jones M.L. Johnson K.J. Glovsky M.M. Eddy S.M. Ward P.A. C5a-induced expression of P-selectin in endothelial cells.J. Clin. Invest. 1994; 94: 1147-1155Crossref PubMed Scopus (385) Google Scholar). Activation of endothelial cells by C5a can be augmented in the presence of TNFα. The activities of C5a are important early steps in initiating macrophage activation and possibly T cell recruitment (Nataf et al. 1999Nataf S. Davoust N. Ames R.S. Barnum S.R. Human T cells express the C5a receptor and are chemoattracted to C5a.J. Immunol. 1999; 162: 4018-4023PubMed Google Scholar, Tsuji et al. 2000Tsuji R.F. Kawikova I. Ramabhadran R. Akahira-Azuma M. Taub D. Hugli T.E. Gerard C. Askenase P.W. Early local generation of C5a initiates the elicitation of contact sensitivity by leading to early T cell recruitment.J. Immunol. 2000; 165: 1588-1598PubMed Google Scholar). In the clinical setting, a recombinant antibody to C5 has been found to inhibit leukocyte activation as measured by CR3 expression in patients undergoing surgery requiring cardiopulmonary bypass (Fitch et al. 1999Fitch J.C. Rollins S. Matis L. Alford B. Aranki S. Collard C.D. Dewar M. Elefteriades J. Hines R. Kopf G. et al.Pharmacology and biological efficacy of a recombinant, humanized, single-chain antibody C5 complement inhibitor in patients undergoing coronary artery bypass graft surgery with cardiopulmonary bypass.Circulation. 1999; 100: 2499-2506Crossref PubMed Scopus (232) Google Scholar). The bioincompatible materials in cardiopulmonary bypass pumps and dialysis machines are known to upregulate multiple aspects of the innate immune system, which can affect subsequent transplant function (Van Loo et al. 1998Van Loo A.A. Vanholder R.C. Bernaert P.R. Vermassen F.E. Van der Vennet M. Lameire N.H. Pretransplantation hemodialysis strategy influences early renal graft function.J. Am. Soc. Nephrol. 1998; 9: 473-481PubMed Google Scholar). Membrane attack complex (MAC) as an activator of leukocytes and vascular cells: the terminal components of complement (C5b-C9) form a tubular structure, the membrane attack complex. If only low densities of MAC are assembled on cells, complement regulatory proteins, such as CD59, inhibit lysis (Lachmann 1991Lachmann P.J. The control of homologous lysis.Immunol. Today. 1991; 12: 312-315Abstract Full Text PDF PubMed Scopus (160) Google Scholar). However, insertion of sublytic quantities of MAC into endothelial cells in vitro causes the translocatation of P-selectin and von Willebrand factor from Weibel-Palade storage granules to the plasma membrane (Hattori et al. 1989Hattori R. Hamilton K.K. McEver R.P. Sims P.J. Complement proteins C5b-C9 induce secretion of high molecular weight multimers of endothelial von Willebrand factor and translocation of granule membrane protein GMP-140 to the cell surface.J. Biol. Chem. 1989; 264: 9053-9060Abstract Full Text PDF PubMed Google Scholar). Similarly, MAC stimulates the translocation of P-selectin from platelet α-granules to the plasma membrane and the secretion of mediators from storage granules (Sims and Wiedmer 1991Sims P.J. Wiedmer T. The response of human platelets to activated components of the complement system.Immunol. Today. 1991; 12: 338-342Abstract Full Text PDF PubMed Scopus (136) Google Scholar). P-selectin initiates adhesion of monocytes and platelets to the endothelium and costimulates their production of inflammatory mediators (Celi et al. 1994Celi A. Pellegrini G. Lorenzet R. De Blasi A. Ready N. Furie B.C. Furie B. P-selectin induces the expression of tissue factor on monocytes.Proc. Natl. Acad. Sci. USA. 1994; 91: 8767-8771Crossref PubMed Scopus (532) Google Scholar, Weyrich et al. 1996Weyrich A.S. Elstad M.R. McEver R.P. McIntyre T.M. Moore K.L. Morrissey J.H. Prescott S.M. Zimmerman G.A. Activated platelets signal chemokine synthesis by human monocytes.J. Clin.