Complement as a mediator of vascular inflammation and activation in allografts

Effects of Caloric Restriction with Protein Supplementation on Plasma Protein Profiles in Middle-Aged Women with Metabolic Syndrome-A Preliminary Open Study

Background: Clinical studies have demonstrated that higher protein intake based on caloric restriction (CR) alleviates metabolic abnormalities. However, no study has examined the effects of plasma protein profiles on caloric restriction with protein supplementation (CRPS) in metabolic syndrome (MetS). Therefore, using a proteomic perspective, this pilot study investigated whether CRPS ameliorated metabolic abnormalities associated with MetS in middle-aged women. Methods: Plasma samples of middle-aged women with MetS in CR (n = 7) and CRPS (n = 6) groups for a 12-week intervention were obtained and their protein profiles were analysed. Briefly, blood samples from qualified participants were drawn before and after the dietary treatment. Anthropometric, clinical, and biochemical variables were measured and correlated with plasma proteomics. Results: In results, we found that body mass index, total body fat, and fasting blood glucose decreased significantly after the interventions but were not different between the CR and CRPS groups. After liquid chromatography–tandem mass spectrometry analysis, the relative plasma levels of alpha-2-macroglobulin (A2M), C4b-binding protein alpha chain (C4BPA), complement C1r subcomponent-like protein (C1RL), complement component C6 (C6), complement component C8 gamma chain (C8G), and vitamin K-dependent protein S (PROS) were significantly different between the CRPS and CR groups. These proteins are involved in inflammation, the immune system, and coagulation responses. Moreover, blood low-density lipoprotein cholesterol levels were significantly and positively correlated with C6 plasma levels in both groups. Conclusions: These findings suggest that CRPS improves inflammatory responses in middle-aged women with MetS. Specific plasma protein expression (i.e., A2M, C4BPA, C1RL, C6, C8G, and PROS) associated with the complement system was highly correlated with fasting blood glucose (FBG), blood lipids (BLs), and body fat.

Role of anti-vimentin antibodies in allograft rejection

Production of anti-vimentin antibodies (AVA) after solid organ transplantation are common. Although classically thought to be expressed mainly within the cytosol, recent evidence demonstrates that extracellular or cell surface expression of vimentin is not unusual. This review examines the evidence to assess whether AVA contribute to allograft pathology. Clinical studies suggest that AVA are associated with cardiac allograft vasculopathy in heart transplant recipients. Studies in non-human primates confirm that production of AVA after renal and heart transplantation are not inhibited by Cyclosporine. Experimental studies have demonstrated that mice pre-immunised with vimentin undergo accelerated acute rejection and vascular intimal occlusion of cardiac allografts. Adoptive transfer of hyperimmune sera containing AVA into B-cell-knock-out mice caused accelerated rejection of allografted hearts, this is clear evidence that antibodies to vimentin accelerate rejection. AVA act in concert with the alloimmune response and AVA do not damage syngeneic or native heart allografts. Confocal microscopy of allografted organs in vimentin immunised mice shows extensive expression of vimentin on endothelial cells, apoptotic leukocytes and platelet/leukocyte conjugates, co-localising with C4d. One explanation for the ability of AVA to accelerate rejection would be fixation of complement within the graft and subsequent pro-inflammatory effects; there may also be interactions with platelets within the vasculature.

Complement-mediated microvascular injury leads to chronic rejection

Microvascular loss may be an unappreciated root cause of chronic rejection for all solid organ transplants. As the only solid organ transplant that does not undergo primary systemic arterial revascularization at the time of surgery, lung transplants rely on the establishment of a microcirculation and are especially vulnerable to the effects of microvascular loss. Microangiopathy, with its attendant ischemia, can lead to tissue infarction and airway fibrosis. Maintaining healthy vasculature in lung allografts may be critical for preventing terminal airway fibrosis, also known as the bronchiolitis obliterans syndrome (BOS). BOS is the major obstacle to lung transplant success and affects up to 60% of patients surviving 5 years. The role of complement in causing acute microvascular loss and ischemia during rejection has recently been examined using the mouse orthotopic tracheal transplantation; this is an ideal model for parsing the role of airway vasculature in rejection. Prior to the development of airway fibrosis in rejecting tracheal allografts, C3 deposits on the vascular endothelium just as tissue hypoxia is first detected. With the eventual destruction of vessels, microvascular blood flow to the graft stops altogether for several days. Complement deficiency and complement inhibition lead to markedly improved tissue oxygenation in transplants, diminished airway remodeling, and accelerated vascular repair. CD4+ T cells and antibody-dependent complement activity independently mediate vascular destruction and sustained tissue ischemia during acute rejection. Consequently, interceding against complement-mediated microvascular injury with adjunctive therapy during acute rejection episodes, in addition to standard immunosuppression which targets CD4+ T cells, may help prevent the subsequent development of chronic rejection.

CD4+ T cells and complement independently mediate graft ischemia in the rejection of mouse orthotopic tracheal transplants

RATIONALE

While microvascular injury is associated with chronic rejection, the cause of tissue ischemia during alloimmune injury is not yet elucidated.

OBJECTIVE

We investigated the contribution of T lymphocytes and complement to microvascular injury-associated ischemia during acute rejection of mouse tracheal transplants.

METHODS AND RESULTS

Using novel techniques to assess microvascular integrity and function, we evaluated how lymphocyte subsets and complement specifically affect microvascular perfusion and tissue oxygenation in MHC-mismatched transplants. To characterize T cell effects on microvessel loss and recovery, we transplanted functional airway grafts in the presence and absence of CD4(+) and CD8(+) T cells. To establish the contribution of complement-mediated injury to the allograft microcirculation, we transplanted C3-deficient and C3-inhibited recipients. We demonstrated that CD4(+) T cells and complement are independently sufficient to cause graft ischemia. CD8(+) T cells were required for airway neovascularization to occur following CD4-mediated rejection. Activation of antibody-dependent complement pathways mediated tissue ischemia even in the absence of cellular rejection. Complement inhibition by CR2-Crry attenuated graft hypoxia, complement/antibody deposition on vascular endothelium and promoted vascular perfusion by enhanced angiogenesis. Finally, there was a clear relationship between the burden of tissue hypoxia (ischemia×time duration) and the development of subsequent airway remodeling.

CONCLUSIONS

These studies demonstrated that CD4(+) T cells and complement operate independently to cause transplant ischemia during acute rejection and that sustained ischemia is a precursor to chronic rejection.

The link between major histocompatibility complex antibodies and cell proliferation

Experimental evidence indicates that donor-specific antibodies targeting major histocompatibility complex classes I and II molecules can elicit the key features of transplant vasculopathy by acting on the graft vasculature in 3 ways: directly activating proliferative, prosurvival, and migratory signaling in the target endothelial and smooth muscle cells; increasing expression of mitogenic factors in vascular endothelial cells, creating a potential proliferative autocrine loop; and promoting recruitment of inflammatory cells that produce mitogenic factors and elicit chronic inflammation, proliferation, and fibrosis. Here, we review the experimental literature showing the complement and Fc-independent effects of major histocompatibility complex classes I and II antibodies on graft vascular cells that may directly contribute to the proliferative aspect of transplant vasculopathy.

A reevaluation of the role of IgM Non-HLA antibodies in cardiac transplantation

BACKGROUND

Preexisting IgG antibodies to donor human leukocyte antigens (HLA) are a risk factor for rapid allograft rejection. However, non-HLA antibodies, of the IgM class, also called autoreactive antibodies, are not believed to affect graft survival. The aim of this study was to determine the incidence and clinical relevance of pretransplant lymphocytotoxic non-HLA IgM antibodies on long-term cardiac allograft survival.

METHODS

A retrospective study of 616 adult recipients of cardiac allografts, transplanted at this center between 1991 and 2003, has been performed. Antibodies in pretransplant sera were initially defined using complement-dependent cytotoxicity assays, and subsequently analyzed for HLA specificities using solid phase assays.

RESULTS

HLA antibodies were present in 69 of 616 heart recipients (58 IgG, 11 IgM); in 22 of these, the antibodies were donor-specific. Non-HLA IgM antibodies were detected in 59 of 616 recipients who did not have HLA-specific antibodies; these patients had a 1, 2, 5, and 10 year survival of 55.9%, 54.2%, 49.9%, and 43.3% compared with 75.8%, 73.7%, 66.6%, and 52.8% for those without antibodies (P=0.0085 log-rank test). Multivariate analysis demonstrated pretransplant non-HLA IgM antibodies to be an independent risk factor for mortality (P=0.0001). Myocardial histology of postmortem heart and cardiac biopsies suggested an association with ischemic damage and "primary" allograft failure.

CONCLUSIONS

We propose the hypothesis that the presence of cytotoxic IgM antibodies to non-HLAs before heart transplantation maybe a risk factor for early allograft failure.

Autoantibodies to vimentin cause accelerated rejection of cardiac allografts

Autoimmune responses to vimentin occur after solid organ transplantation, but their pathogenic effects are unclear. The aim of these studies was to investigate the effects of vimentin preimmunization on allogeneic and isografted hearts in a murine transplant model. Immunization of C57BL/6 mice with murine vimentin in complete Freund's adjuvant resulted in anti-vimentin antibodies and vimentin-reactive Th-1 cells. Transplantation of 129/sv hearts into vimentin-immunized C57BL/6 recipients resulted in accelerated rejection (8.4 +/- 1.5 days; n = 18), compared with hen egg lysozyme-immunized C57BL/6 (13.3 +/- 2.2 days; n = 10; P < 0.0001, log-rank test). In contrast, isografts continued to beat beyond 90 days. Immunohistochemical analysis of allografts from vimentin/complete Freund's adjuvant mice demonstrated increased numbers of T cells and enhanced microvascular deposition of C3d, CD41, and P-selectin compared with controls. Antibodies were necessary for accelerated rejection, shown by the fact that vimentin-immunized B-cell-deficient IgH6 mice did not show accelerated rejection of 129/sv allografts, but rejection was restored by adoptive transfer of serum containing anti-vimentin antibodies. Eluates from donor hearts placed in vimentin/complete Freund's adjuvant recipients contained anti-vimentin antibodies, shown by Western blotting. Confocal imaging of rejected hearts de-monstrated presence of vimentin and C3d on apoptosed leukocytes, endothelial cells, and platelet/leukocyte conjugates. These results demonstrate that autoantibodies to vimentin, in conjunction with the alloimmune response, have a pathogenic role in allograft rejection.

Impact of Repetitive Episodes of Antibody-mediated or Cellular Rejection on Cardiovascular Mortality in Cardiac Transplant Recipients: Defining Rejection Patterns

BACKGROUND

In our previously published work dealing with antibody-mediated (vascular) rejection (AMR), we defined patterns of rejection (AMR and cellular rejection [CR]) based on a review of biopsy diagnoses taken in the first 6 to 12 weeks post-transplant. We have shown the significance of these pattern designations in relation to patient and allograft outcome in five outcome analyses. The current retrospective analysis was done to determine whether our previous criteria for pattern designations provided the greatest degree of discrimination between AMR and CR.

METHODS

Six hundred sixty-five patients from the U.T.A.H. Cardiac Transplant Program were included in our study. Patients induced with OKT3 immunosuppression were excluded. We analyzed the relationship of a number of either AMR or CR episodes to cardiovascular mortality. We constructed Kaplan-Meier survival curves to assess the impact of incremental numbers of AMR or CR episodes on cardiovascular mortality.

RESULTS

Three or more episodes of AMR resulted in a statistically significant increase in cardiovascular mortality. By contrast, CR episodes did not increase the risk of cardiovascular mortality.

CONCLUSIONS

Based on our findings, we believe that clinical trials should be designed to test treatments based on predominant rejection patterns and that end-points for trials should be defined by number of biopsies positive for either CR or AMR. This approach may lead to improved patient and allograft survival.

Glomerular C3c deposition and intravascular macrophage accumulation in early humoral renal allograft rejection signifies a poor short-term outcome

C4d deposition in the walls of peritubular capillaries is considered the key phenomenon in the histopathological diagnosis of humoral, i.e. antibody-mediated, allograft rejection. We have earlier proposed that deposition of C3c in glomerular capillaries and simultaneous intravascular accumulation of macrophages in allografts with immediate or early humoral rejection indicates a potentially serious condition with very poor prognosis. The clinical outcome of 45 cadaveric grafts with this phenomenon among 1960 renal allografts transplanted at our centre during 1984-1999, and the recipients of the contralateral kidneys, was retrospectively evaluated. Graft failure occurred in 44/45 grafts within 3 weeks, with graft loss in 33/45 (77%) within 4 months and 37/45 (82%) within 1 year. From the contralateral kidneys, 5/33 (15%) were lost within 1 year. In a recent series of early biopsies, we recognised that of 13 cases showing C4d positivity in peritubular capillaries but lacking C3c in glomeruli, 10/13 (77%) were still functioning after 4 months. The mean number of CD68(+) macrophages per glomerular profile, i.e. the glomerular macrophage index, shows a significant difference between C4d(+)C3c(-) and C4d(+)C3c(+) cases (p<0.001). Our results indicate the existence of a clinically important subgroup of early humoral rejection with particular morphological features.

C4d Deposition in Cardiac Allografts Correlates With Alloantibody

BACKGROUND

The presence of C4d along the peritubular capillaries in kidney allografts correlates with the presence of anti-donor serum alloantibodies. We applied C4d staining to cardiac allograft and non-allograft biopsies to determine if C4d staining in heart allografts correlates with anti-donor serum alloantibodies.

METHODS

We stained for C4d all available frozen tissue biopsies from cardiac transplant recipients between 1997 and 2002, including autopsies. Two hundred twenty-one tissue samples from 124 patients were analyzed. Included in both groups were a variety of International Society for Heart and Lung Transplantation (ISHLT) grades of rejection plus post-implant cardiac ischemic injury (PIMI), and biopsies from patients who had received OKT3. Patients were matched by age, gender and interval after transplantation. Forty-four additional controls were included from patients biopsied for non-transplant-related cardiac disease.

RESULTS

C4d staining of the myocardial capillaries correlated well with the presence of anti-donor alloantibodies. Twenty-one of 25 biopsies from patients with anti-donor alloantibodies showed C4d staining (84%), whereas only 7 of 60 without anti-donor alloantibodies stained for C4d. C4d staining did not correlate with ischemia or OKT3 therapy. Only 4 of 44 non-transplant biopsies stained for C4d (9%). An example of the clinical utility of C4d staining in patient care is presented.

CONCLUSIONS

C4d staining of the capillaries in cardiac allografts correlates well with anti-donor serum alloantibodies, is a useful assay to verify alloantibody deposition, and can be used to establish one of the criteria for antibody-mediated cardiac rejections.

Low mannose-binding lectin and increased complement activation correlate to allograft vasculopathy, ischaemia, and rejection after human heart transplantation

AIMS

Transplant-associated coronary artery disease (TxCAD) is a major cause of post-transplant graft failure. The aim of this study was to investigate a possible role of mannose-binding lectin (MBL) deficiency and complement activation in TxCAD.

METHODS AND RESULTS

In a prospective study of heart transplant recipients (n=38) with a follow-up of 5.3+/-1.3 years (range: 0.9-6.6), angiographically verified TxCAD (n=6) was correlated to plasma MBL, complement activation, and endothelial activation (soluble E-selectin). MBL deficiency (<100 ng/mL) was detected in 3/6 patients with TxCAD and in 3/32 with non-TxCAD (Kaplan-Meier, P=0.020). Furthermore, one or more acute rejection episodes were observed in 6/6 of the MBL-deficient patients and in 15/32 of the MBL-sufficient patients (chi(2); P=0.016). Complement activation (C4bc) correlated with soluble E-selectin (r=0.36; P=0.027), both being significantly higher in patients with ischaemia detected in the first biopsy (C4bc: 13.4+/-6.1 AU/mL; E-selectin: 96+/-13 ng/mL) than in those without ischaemia (C4bc: 6.3+/-0.5; E-selectin: 51+/-6; P=0.037 and 0.002). Finally, terminal complement complex correlated closely with mortality (P=0.002).

CONCLUSION

Low MBL was related to the development of TxCAD and acute rejection and increased complement activation correlated to histopathologic ischaemia and mortality after heart transplantation.

Capillary deposition of the complement fragment C4d in cardiac allograft biopsies is associated with allograft vasculopathy

Cardiac allograft vasculopathy (CAV) is a long-term threat in heart transplant recipients and its exact pathogenesis remains to be established. As complement activation contributes to early and late allograft dysfunction, we hypothesized that deposition of the complement fragment, C4d, in capillaries of cardiac allograft biopsies may be associated with CAV. A polyclonal anti-C4d antibody was used for immunohistochemistry on endomyocardial biopsies obtained from heart transplant recipients during the first year post-transplantation. CAV was assessed by intracoronary ultrasound performed at 1-year post-transplantation. We were able to show that CAV is highly associated with C4d deposition in capillaries of cardiac allografts and that serial C4d studies may predict development of CAV at 1-year post-transplantation.

Antithymocyte globulin is associated with complement deposition in cardiac transplant biopsies

Polyclonal antithymocyte globulin preparations contain antibodies with reactivity to endothelial cells. Therefore, we investigated whether treatment with this reagent caused complement deposition in human cardiac transplants. Frozen tissue was available from endomyocardial biopsies of 75 patients, who were transplanted between April 1995 and April 2000. Nine of these patients were converted from cyclosporin A (CsA) to horse antithymocyte globulin (ATGAM) in the first month after transplantation. All of the biopsies were stained by immunofluorescence for C4d as evidence of activation of the classical pathway of complement. In addition, biopsies from patients treated with ATGAM and control patients were stained for deposition of horse immunoglobulin (Ig)G. All nine patients who received ATGAM had deposition of horse IgG and C4d. Two color stains demonstrated that the horse IgG colocalized with the C4d staining. No staining for horse IgG or C4d was evident in biopsies obtained before ATGAM treatment. Likewise, no staining for horse IgG was detected in seven control patients who had C4d staining. Most patients treated with ATGAM had no histologic evidence of rejection, but did have myocyte damage and macrophage infiltration. Thus prophylactic treatment with ATGAM is associated with the deposition of horse IgG and activation of complement in the transplant.

Immunosuppression and transplant vascular disease: benefits and adverse effects

Cardiac allograft vasculopathy (CAV) occurs within 5 years of transplantation surgery and represents the main cause of death in long-term heart transplant survivors. The detailed pathogenesis of CAV is unknown, but there are strong indications that immunologic mechanisms, which are regulated by nonimmunologic factors, are the major cause of this phenomenon. Cyclosporine A (CsA) is a frequently used immunosuppressive agent in transplant medicine to prevent rejection. The mechanism of action of CsA involves initial binding to cyclophilin to form a complex that then inhibits calcineurin (CN), leading to reduced interleukin (IL)-2 production as part of the signal transduction pathway for the activation of B-lymphocytes and T-lymphocytes. Based on this proposed mechanism, it was expected that CsA should be an effective strategy in attenuating the host immune response against transplanted allograft tissue; however, CsA has not changed the outcome of CAV. Several mechanisms have been suggested for the ineffectiveness of CsA in long-term prevention of CAV. For example, routine therapeutic doses of CsA may block CN incompletely (50%), whereas complete blockade requires doses that are not clinically tolerable. Another explanation is the possible activation of T-cell receptors directly (CN independent) by the immune response, which induces protein kinase C theta (PKCtheta) and leads to IL-2 production and immune rejection. Moreover, there may be a role for nonimmunologic mechanisms, such as complement, which cannot be controlled by CsA, or CsA may cause hypercholesterolemia or induce overexpression of transforming growth factor-beta (TGF-beta). This review also compares the effect of CsA with other immunosuppressants in allograft artery preservation and their clinical efficacy.

Beyond C4d: other complement-related diagnostic approaches to antibody-mediated rejection

Complement is a multifunctional system of receptors and regulators as well as effector molecules. Both the pathogenic and diagnostic power of complement is based on the capacity of the complement system to amplify innate and adaptive immunity. This amplification is accomplished through two strategies: (1) enzymatic reactions in the complement cascade, and (2) stimulation of leukocytes, platelets and parenchymal cells through specific receptors or receptor-independent pore formation. The mechanisms by which complement mediates and modifies nonspecific inflammation, antibody-mediated injury and T-cell responses are of particular significance to the pathogenesis of transplant rejection. Understanding the mechanisms by which complement integrates the interactions of leukocytes, platelets and parenchymal cells offers opportunities to further refine the diagnosis of rejection.

Membrane attack complex contributes to destruction of vascular integrity in acute lung allograft rejection

The lung is known to be particularly susceptible to complement-mediated injury. Both C5a and the membrane attack complex (MAC), which is formed by the terminal components of complement (C5b-C9), can cause acute pulmonary distress in nontransplanted lungs. We used C6-deficient rats to investigate whether MAC causes injury to lung allografts. PVG.R8 lungs were transplanted orthotopically to MHC class I-incompatible PVG.1U recipients. Allografts from C6-sufficient (C6(+)) donors to C6(+) recipients were rejected with an intense vascular infiltration and diffuse alveolar hemorrhage 7 days after transplantation (n = 5). Ab and complement (C3d) deposition was accompanied by extensive vascular endothelial injury and intravascular release of von Willebrand factor. In contrast, lung allografts from C6-deficient (C6(-)) donors to C6(-) recipients survived 13-17 days (n = 5). In the absence of C6, perivascular mononuclear infiltrates of ED1(+) macrophages and CD8(+) T lymphocytes were present 7 days after transplantation, but vascular endothelial cells were quiescent, with minimal von Willebrand factor release and no evidence of alveolar hemorrhage or edema. Lung allografts were performed from C6(-) donors to C6(+) recipients (n = 5) and from C6(+) donors to C6(-) recipients (n = 5) to separate the effects of systemic and local C6 production. Lungs transplanted from C6(+) donors to C6(-) recipients had increased alveolar macrophages and capillary injury. C6 production by lung allografts was demonstrated at the mRNA and protein levels. These results demonstrate that MAC causes vascular injury in lung allografts and that the location of injury is dependent on the source of C6.

C4d staining of perioperative renal transplant biopsies

BACKGROUND

Deposition of C4d in peritubular capillaries (PTCs) has been shown to be a sensitive marker for antibody-mediated (humoral) rejection in renal transplant biopsies. Some studies also suggest that C4d in PTCs is specific for humoral rejection or, at least, for the presence of donor-specific antibodies. However, in other studies, PTC C4d deposits were noted in more than 40% of renal transplant biopsies performed for graft dysfunction and capillary C4d deposition in heart transplants may result from ischemic injury.

METHODS

To test the specificity of C4d staining as a marker for acute humoral rejection ACR in renal allografts, indirect immunofluorescence using a monoclonal anti-C4d antibody and a fluorescein-isothiocyanate-conjugated secondary antibody was performed on cryostat sections of 90 renal transplant biopsies, including 35 pairs of preimplantation and 1-hr postreperfusion biopsies of the same graft, postreperfusion biopsies of 12 additional grafts, and 8 positive controls (biopsies with known C4d-positive AHR). Eighteen grafts were cadaveric, 17 grafts were liviing-related, and 12 grafts were living-unrelated (excluding controls). Included in these grafts were 13 grafts that developed AHR 3 to 34 days posttransplantation.

RESULTS

Only 2 of 82 perioperative biopsies showed C4d staining in PTCs. Both perioperative biopsies were postreperfusion biopsies of grafts diagnosed with AHR 5 and 34 days posttransplantation, respectively, and, in each case, the recipient had been treated with plasmapheresis before transplantation because of a positive crossmatch (cytotoxic and flow cytometric) and continued to have a weakly positive flow crossmatch at the time of transplantation. In one biopsy, C4d staining was focal, and in the other biopsy, it was diffuse; in both biopsies, C4d staining was relatively mild (1+ on a 0-4+ scale). No C4d staining was noted on preimplantation biopsies of each graft. All biopsies that contained glomeruli showed linear capillary loop or blotchy mesangial staining, or both, which was similar in prereperfusion and postreperfusion biopsies. All positive controls showed diffuse C4d staining in PTCs.

CONCLUSIONS

C4d staining in PTCs may be seen as early as 1 hr posttransplantation in some recipients with low levels of antidonor antibodies. However, this was not observed as a feature of ischemic or ischemia-reperfusion injury in perioperative renal transplant biopsies, including those of cadaveric grafts with cold ischemia times of as long as 41 hr.

Antibody-mediated activation of the classical complement pathway in xenograft rejection

Transplant rejection is a multifactorial process involving complex interactions between components of the innate and the acquired immune system. In view of the shortage of donor organs available for transplantation, xenotransplantation of pig organs into man has been considered as a potential solution. However, in comparison to allografts, xenografts are subject to extremely potent rejection processes that are currently incompletely defined. Consequently, an appropriate and safe treatment protocol ensuring long-term graft survival is not yet available. The first barrier that has to be taken for a xenograft is hyperacute rejection, a rapid process induced by the binding of pre-formed antibodies from the host to the graft endothelium, followed by activation of the classical complement pathway. The present review concentrates on the role of antibodies and complement in xenograft rejection as well as on the approaches for treatment that target these components. The first part focuses on porcine xenoantigens that are recognized by human xenoreactive antibodies and the different treatment strategies that aim on interference in antibody binding. The second part of the review deals with complement activation by xenoreactive antibodies, and summarizes the role of complement in the induction of endothelial cell damage and cell activation. Finally, various options that are currently under development for complement inhibition are discussed, with special reference to the specific inhibition of the classical complement pathway by soluble complement inhibitors.

The endothelium in clinical cardiac transplantation

Cardiac transplantation is the most successful therapy for refractory heart failure, but clinical transplantation is still confronted with the problems of acute rejection and acute pump failure. The limiting factor in achieving prolonged survival remains cardiac allograft vasculopathy. In recent years it has become apparent that from brain death onward, the cardiac endothelium plays a key role in these acute and chronic events. Brain death is associated with an inflammatory response that primes the endothelium for cumulative injury during the subsequent stages of ischemic cold storage, reperfusion and allorecognition. As a structural and functional interface, the endothelium is the site at which inflammatory cells move from the bloodstream through the vessel wall into the parenchyma. The endothelium interacts with the complement system, the coagulation and inflammatory cascades, circulating leukocytes, the immune system, the smooth muscle in the vessel wall, and the surrounding matrix and cardiomyocytes. A better understanding of its many roles may lead to expansion of our therapeutic possibilities and better outcomes overall. This article reviews the possible roles of the endothelium in relation to cardiac transplantation, and discusses the diagnostic and therapeutic modalities that are available to date.

Accelerated graft arteriosclerosis in cardiac transplants: complement activation promotes progression of lesions from medium to large arteries

BACKGROUND

A critical role for the terminal components of complement (C5b-C9) has been demonstrated previously in acute allograft rejection with the use of C6-deficient PVG congenic rat strains. The C6 deficiency prevents the formation of membrane attack complex (MAC) by C5b-C9. Hearts transplanted from PVG.1A (RT1a) rats are rejected acutely (7-9 days) by fully MHC-incompatible C6-sufficient PVG.1L (RT11) recipients, but they survive significantly longer in untreated C6-deficient PVG.1L recipients (19 to >60 days).

METHODS

To investigate the contribution of MAC to chronic rejection and accelerated graft arteriosclerosis (AGA) in long-term cardiac allografts, hearts were transplanted heterotopically from PVG.1A donors to C6-sufficient and C6-deficient PVG.1L hosts that were treated with cyclosporine 15 mg/kg/day for 14 days after cardiac grafting. Alloantibody responses in hosts were measured by flow cytometry at 4, 8, 12, and 16 weeks after transplantation. Vigorously contracting grafts were removed at 60 days (n=5) and at 90-128 days (n=12) after surgery for morphological evaluation. Computerized planimetry measurements were made in complete cross-sections of grafts on all assessable arteries larger than 16 microns in diameter.

RESULTS

The survival of most (six of seven) cardiac allografts in C6-deficient recipients was prolonged by cyclosporine treatment to greater than 90 days. In contrast, 14 of 25 hearts that were transplanted to C6-sufficient recipients were rejected between 21 and 84 days with severe vascular injury. AGA, defined as smooth muscle cells forming a neointima inside the internal elastic lamina and luminal compromise, affected a greater percentage of arteries in C6-sufficient than in C6-deficient recipients. AGA developed earlier and more frequently in arteries of medium (<100 micron) diameter than those of large diameter in both C6-sufficient and C6-deficient recipients. Serial sections demonstrated the lesions in medium arteries to be located adjacent to the smooth muscle sphincters at the junction of arteriolar branches.

CONCLUSIONS

These results demonstrate that MAC promotes the pathogenesis of AGA in long-term cardiac allografts.