Anaphylatoxins in organ transplantation

C3a and C5a (also called anaphylatoxins) are inflammatory peptides generated during complement activation. They do not only play important roles in innate immunity through the initiation and regulation of inflammatory responses, but also significantly influence adaptive immune responses. Organ transplantation triggers an initial inflammatory response and subsequent to the specific immune response (also called the alloimmune response), both of which contribute to graft rejection. Emerging evidence suggests that anaphylatoxins, particularly C5a, are significantly involved in both inflammatory and alloimmune responses following organ transplantation, thus influencing graft outcome. This review will provide the information on our current understanding of the roles for anaphylatoxins in ischemia–reperfusion injury, graft rejection, and transplant tolerance, and the therapeutic potential of targeting anaphylatoxin receptors in organ transplantation.     

Nontransgenic hyperexpression of a complement regulator in donor kidney modulates transplant ischemia/reperfusion damage, acute rejection, and chronic nephropathy

Complement activation during ischemia and reperfusion contributes to the development of tissue injury with severe negative impact on outcomes in transplantation. To counter the effect of complement, we present a strategy to deliver a novel complement regulator stabilized on cell surfaces within donor organs. The membrane-bound complement regulator is able to inhibit complement activation when the donor organ is revascularized and exposed to host-circulating complement. Application of this construct to donor kidneys protected transplanted tissues from ischemia/reperfusion injury and reduced the deposition of activated complement and histological signs of damage under conditions in which a nontargeted control construct was ineffective. Treatment of donor organs in this way improved graft performance in the short and long term. An analysis of the immune response in allograft recipients showed that reducing graft damage at the time of transplantation through complement regulation also modulated the alloresponse. Additionally, the results of perfusion studies with human kidneys demonstrated the feasibility of targeting endothelial and epithelial surfaces with this construct, to allow investigation in clinical transplantation.     

The effect of locally synthesised complement on acute renal allograft rejection

The complement system of components and receptors is one of the earliest forms of defence. Excessive or inappropriate activation can result in tissue damage, classically illustrated in immune-mediated nephritis. In addition, complement forms a bridge between innate and adaptive immunity, helping to prepare and focus T and B lymphocyte responses. More recent research in renal allograft models has shown that complement-inhibited and complement-deficient animals have reduced inflammatory injury and lowered antidonor immune responses. Furthermore, it is known that the transplanted kidney is a significant site of local synthesis of C3, although until recently the relative contribution of locally produced C3 to transplant injury was unknown. Current evidence indicates that defective local synthesis of C3 both reduces tissue injury and lowers the antidonor T cell response, substantially increasing graft survival. Among various possible explanations to account for these findings, the data favours a direct effect of complement on alloreactive T cell stimulation. Study of complement gene regulation by common immunosuppressive agents suggests that they do not influence local complement synthesis. Alternative approaches are therefore required to control the local effect of complement in the extravascular tissue compartment of the graft.     

Exploration of complement split products in plasma and urine as biomarkers of kidney graft rejection

IntroductionThe complement system, consisting of more than thirty different soluble and cell-bound proteins, exerts essential functions both in the innate and adaptive immune systems and is believed to be an important contributor to allograft injury in kidney transplantation. The anaphylatoxins C3a and C5a are powerful chemoattractants, recruiting immune effector cells toward the site of complement activation and enhance T-cell response, while C3dg binding to CR2 on B-cells, enhances B-cell immunity at several stages of the B-cell differentiation.Complement split products in plasma and urine could reflect ongoing inflammation and tissue injury. We, therefore, investigated if complement split products increase in plasma and urine in kidney transplant recipients with rejection.MethodIn this case-control feasibility study, complement factors C3a, C3dg, C4a, and C5a were measured in plasma and C3dg and sC5b-9 associated C9 neoantigen in urine in 15 kidney transplant recipients with rejection (cases) and 15 kidney transplant recipients without (controls). The groups were matched on the type of transplantation and the time from transplantation to sampling. The complement split products were compared (i) between cases and controls and (ii) within the rejection group over time, comparing the measurements at rejection with measurements where the kidney transplant recipients were clinically stable. Possible moderators were explored, and results adjusted accordingly. P values < 0.05 were considered significant.Plasma C3dg was analyzed by immune-electrophoresis, plasma C3a, plasma C4a, and plasma C5a by flow cytometry, and urine C3dg and urine C9neo by ELISA.ResultsIn plasma, there were no significant differences between the rejection and the control group. However, steroids and pretransplant C3dg levels significantly influenced C3dg.Within the rejection group, plasma C3a and C3dg were significantly higher at the time of rejection compared to the stable phase (p < 0.01).In urine, C3dg/creatinine and C9 neoantigen/creatinine ratios were not different between the rejection and the control group. Urine C3dg/creatinine and urine C9 neoantigen/creatinine ratios correlated to urine albumin and significantly increased after the transplantation (p < 0.001).ConclusionThis study shows increased plasma C3a and C3dg in kidney transplant recipients, primarily with T cell mediated rejection. This finding suggests that consecutive measurements of C3a and C3dg in plasma could be applicable to monitor alloreactivity in kidney transplant recipients. Urine complement split products are unsuitable as rejection biomarkers since the permeability of the glomerular filtration barrier strongly influences them.Prospective longitudinal studies on plasma C3a and C3dg dynamics will be needed to validate present findings.     

Lectin complement pathway gene profile of the donor and recipient does not influence graft outcome after kidney transplantation

In kidney transplantation, complement activation was found to be induced by donor brain death, renal ischemia-reperfusion injury and allograft rejection. There are three known pathways of complement activation: the classical, lectin and the alternative pathway. The lectin complement pathway can be activated upon pattern recognition by mannan binding lectin (MBL) or ficolins (FCN). Single nucleotide polymorphisms (SNPs) in the genes encoding the lectin pathway proteins determine their functional activity and serum levels. The aim of this study was to investigate the role of the lectin gene profile of the donor and recipient on post-transplant outcome. A total of 12 functional SNPs in the MBL2, FCN2 and MBL-associated serine proteases 2 (MASP2) genes of 1271 donor-recipient pairs were determined. Lectin genotypic variants were analyzed for association with primary non-function (PNF), delayed graft function (DGF), biopsy proven acute rejection, death-censored graft survival and patient survival. Multivariate analyses found no association of donor and recipient MBL2 and MASP2 genotype with allograft outcome. Analysis of separate functional SNPs and haplotypes in the FCN2 gene of the donor and recipient did not reveal an association with transplant outcome. Also, the joint effect of the MBL2 and FCN2 genotype was not associated with allograft outcome.This study shows that the genetic profile of the lectin pathway of complement activation of the donor and recipient is not associated with allograft outcome after kidney transplantation.     

Complement in renal transplantation: The road to translation

Renal transplantation is the treatment of choice for patients with end-stage renal disease. The vital role of the complement system in renal transplantation is widely recognized. This review discusses the role of complement in the different phases of renal transplantation: in the donor, during preservation, in reperfusion and at the time of rejection. Here we examine the current literature to determine the importance of both local and systemic complement production and how complement activation contributes to the pathogenesis of renal transplant injury. In addition, we dissect the complement pathways involved in the different phases of renal transplantation. We also review the therapeutic strategies that have been tested to inhibit complement during the kidney transplantation. Several clinical trials are currently underway to evaluate the therapeutic potential of complement inhibition for the treatment of brain death-induced renal injury, renal ischemia-reperfusion injury and acute rejection. We conclude that it is expected that in the near future, complement-targeted therapeutics will be used clinically in renal transplantation. This will hopefully result in improved renal graft function and increased graft survival.     

Low molecular weight dextran sulfate as complement inhibitor and cytoprotectant in solid organ and islet transplantation

Complement is an essential part of the innate immune system and plays a crucial role in organ and islet transplantation. Its activation, triggered for example by ischemia/reperfusion (I/R), significantly influences graft survival, and blocking of complement by inhibitors has been shown to attenuate I/R injury. Another player of innate immunity are the dendritic cells (DC), which form an important link between innate and adaptive immunity. DC are relevant in the induction of an immune response as well as in the maintenance of tolerance. Modulation or inhibition of both components, complement and DC, may be crucial to improve the clinical outcome of solid organ as well as islet transplantation. Low molecular weight dextran sulfate (DXS), a well-known complement inhibitor, has been shown to prevent complement-mediated damage of the donor graft endothelium and is thus acting as an endothelial protectant. In this review we will discuss the evidence for this cytoprotective effect of DXS and also highlight recent data which show that DXS inhibits the maturation of human DC. Taken together the available data suggest that DXS may be a useful reagent to prevent the activation of innate immunity, both in solid organ and islet transplantation.     

Deficiency of C4 from donor or recipient mouse fails to prevent renal allograft rejection

Complement effector products generated in the transplanted kidney are known to mediate transplant rejection, but which of the three main activation pathways of complement trigger this response is unclear. Here we assessed the role of the classical and lectin pathways by studying the common component C4 in mouse kidney transplant rejection. We transplanted wild-type or C4-null H-2(b) donor kidneys into H-2(k) or H-2(d) recipients, or vice-versa, to assess the roles of donor kidney and recipient expression of complement. Intragraft C4 gene expression rose substantially during rejection. However, we found no significant association between graft acceptance and the presence of C4 in either the donor kidney or recipient mouse. At the time of rejection, we found no significant differences in alloantibody response in the different groups. Tubular deposition of C3 to C9 occurred regardless of the absence or presence of C4 in either the donor or recipient mouse, indicating that C4 was dispensable for complement activation at this site. These data suggest that complement activation and renal allograft rejection are independent of the classical and lectin pathways in these models, implying that in the absence of these pathways the alternative pathway is the main trigger for complement-mediated rejection.     

Effects of complement inhibition with soluble complement receptor-1 on vascular injury and inflammation during renal allograft rejection in the rat.

Complement is both an effector of the humoral immune response and a stimulator of leukocyte activation. To examine the influence of complement on the allograft response, we inhibited complement using recombinant human soluble complement receptor-1 (sCR1; TP10), in an unsensitized model of rat renal allograft rejection. Lewis to DA renal transplant recipients were treated daily with 25 mg/kg sCR1 or saline and sacrificed on days 1 to 5 after transplant. Transplanted organs were examined histologically and immunohistochemically for leukocyte subset markers and for the third component of complement, C3, and membrane attack complex deposition. A second set of recipients was followed from day 5 to day 9 to assess graft survival. sCR1-treated recipients displayed > 90% inhibition of plasma complement activity and a marked reduction in tissue C3 and membrane attack complex deposition. Inactivation of complement reduced the vascular injury such that there was almost complete sparing of vascular damage in day 5 sCR1-treated rats. There was a significant reduction in infiltrating leukocytes by day 5 after transplant, and complement inhibition delayed the time to reach a histologically defined end point of graft survival from 5 days in controls to 9 days in the sCR1-treated group. These results imply that the vascular and cell-mediated injury arises, in part, from complement activation. The partial inhibition of these injuries by sCR1 may have functional implications for strategies to inhibit allograft rejection.     

Low molecular weight dextran sulfate as complement inhibitor and cytoprotectant in solid organ and islet transplantation

Complement is an essential part of the innate immune system and plays a crucial role in organ and islet transplantation. Its activation, triggered for example by ischemia/reperfusion (I/R), significantly influences graft survival, and blocking of complement by inhibitors has been shown to attenuate I/R injury. Another player of innate immunity are the dendritic cells (DC), which form an important link between innate and adaptive immunity. DC are relevant in the induction of an immune response as well as in the maintenance of tolerance. Modulation or inhibition of both components, complement and DC, may be crucial to improve the clinical outcome of solid organ as well as islet transplantation. Low molecular weight dextran sulfate (DXS), a well-known complement inhibitor, has been shown to prevent complement-mediated damage of the donor graft endothelium and is thus acting as an endothelial protectant. In this review we will discuss the evidence for this cytoprotective effect of DXS and also highlight recent data which show that DXS inhibits the maturation of human DC. Taken together the available data suggest that DXS may be a useful reagent to prevent the activation of innate immunity, both in solid organ and islet transplantation.     

Tertiary lymphoid organs in renal allografts can be associated with donor-specific tolerance rather than rejection

Tertiary lymphoid organs can form at sites of chronic inflammation. Their presence has been mainly associated with tissue destruction. In transplantation, there is a dynamic immune response as in chronic inflammation. Indeed, the presence of tertiary lymphoid organs has been associated with chronic rejection. In addition to a destructive alloimmune response, secondary lymphoid organs are also important in transplant tolerance. We hypothesised that tertiary lymphoid organs may also form during transplantation tolerance as this process also requires an active local immune response. If so, their presence may enhance tolerance, resulting in better graft function rather than chronic rejection. Using a mouse kidney allograft model of tolerance, we demonstrate the formation of tertiary lymphoid organs within tolerated allografts. Tertiary lymphoid organs are supplied by high endothelial venules, and contain T and B cells, macrophages, DC, Foxp3(+) T cells, donor MHC class II(+) cells and recipient cells presenting donor-derived allopeptides. Formation of tertiary lymphoid organs and the presence of immune cells within them are associated with superior graft function, suggesting that tertiary lymphoid organs act to amplify the prevailing immune response, be it a tolerant and beneficial immune response or the previously described destructive alloimmunity.     

Complement-mediated inflammation and injury in brain dead organ donors

The importance of the complement system in renal ischemia-reperfusion injury and acute rejection is widely recognized, however its contribution to the pathogenesis of tissue damage in the donor remains underexposed. Brain-dead (BD) organ donors are still the primary source of organs for transplantation. Brain death is characterized by hemodynamic changes, hormonal dysregulation, and immunological activation. Recently, the complement system has been shown to be involved. In BD organ donors, complement is activated systemically and locally and is an important mediator of inflammation and graft injury. Furthermore, complement activation can be used as a clinical marker for the prediction of graft function after transplantation. Experimental models of BD have shown that inhibition of the complement cascade is a successful method to reduce inflammation and injury of donor grafts, thereby improving graft function and survival after transplantation. Consequently, complement-targeted therapeutics in BD organ donors form a new opportunity to improve organ quality for transplantation. Future studies should further elucidate the mechanism responsible for complement activation in BD organ donors.     

Genetics of allogeneic hematopoietic cell transplantation. Role of HLA matching,functional variation in immune response genes

Successful outcome following hematopoietic stem cell transplantation (HSCT) is ultimately determined by the ability to achieve sustained engraftment and immune reconstitution, control of graft-versus-host disease (GVHD), and in patients with hematological malignancy the complete eradication of abnormal or malignant cells. GVHD, which can be a serious and fatal complication, is an immune reaction that is initiated by donor T cells in response to recipient alloantigens. Genetic variation in both patient and donor can significantly affect transplant outcome by causing disparity for transplant antigens, and by altering the function of immune response genes that control cellular activation and inflammation. Genetic variation can modulate the intensity of the alloimmune response, the risk of transplant-related organ toxicity and mortality, and may also affect the development of tolerance and the reconstitution of the immune system following HSCT.     

Targeted complement inhibition and microvasculature in transplants: a therapeutic perspective

Active complement mediators play a key role in graft‐versus‐host diseases, but little attention has been given to the angiogenic balance and complement modulation during allograft acceptance. The complement cascade releases the powerful proinflammatory mediators C3a and C5a anaphylatoxins, C3b, C5b opsonins and terminal membrane attack complex into tissues, which are deleterious if unchecked. Blocking complement mediators has been considered to be a promising approach in the modern drug discovery plan, and a significant number of therapeutic alternatives have been developed to dampen complement activation and protect host cells. Numerous immune cells, especially macrophages, develop both anaphylatoxin and opsonin receptors on their cell surface and their binding affects the macrophage phenotype and their angiogenic properties. This review discusses the mechanism that complement contributes to angiogenic injury, and the development of future therapeutic targets by antagonizing activated complement mediators to preserve microvasculature in rejecting the transplanted organ.     

Chronic rejection: a significant role for Th17-mediated autoimmune responses to self-antigens

Despite progress in the field of organ transplantation for improvement in graft survival and function, long-term graft function is still limited by the development of chronic allograft rejection. Various immune-mediated and nonimmune-mediated processes have been postulated in the pathogenesis of chronic rejection. In this review, the authors discuss the important role of alloimmune responses to donor-specific antigens and autoimmune responses to tissue restricted self-antigens in the immunopathogenesis of chronic rejection following solid organ transplantation. In particular, the authors discuss the role of induction of Th17-type autoimmune responses and the crosstalk between autoimmune and alloimmune responses. These self-perpetuate each other leading to activation of profibrotic and proinflammatory cascades that ultimately result in the development of chronic rejection.     

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.     

The role of autoimmunity in the pathogenesis of lung allograft rejection

For many patients, lung transplantation is the only definitive treatment modality for different forms of end-stage lung disease. However, the lung is rejected more often than any other type of solid organ allografts, and the 5-year survival rate is less than that of other transplanted organs. While alloimmunity directed against donor transplantation antigens is believed to be the key mechanism that mediates rejection responses, newer immunosuppressive regimens designed to abrogate alloimmune activation have not improved survival. Accordingly, these data suggest that other antigens are involved in rejection. Autoimmune responses, reported to occur during allograft rejection, could participate in graft destruction. This review article discusses the role of autoimmune responses to type V collagen, a minor collagen in the lung, in the pathogenesis of lung allograft rejection. By recognizing that lung transplant rejection involves both alloimmune and autoimmune responses, scientific investigation may uncover novel targets for therapeutic intervention that could prolong the life of the lung transplant recipient.     

Relevance of MICA and other non-HLA antibodies in clinical transplantation

The clinical importance of HLA-specific antibodies for organ allograft outcome is well established. In the past few years, there has been an increasing interest in non-HLA antigens as targets of injury in organ transplant recipients. This increased interest has been spurred by the fact that HLA-identical kidney transplants also undergo immunological rejections. Polymorphisms within non-HLA genes associated with evoking an immune response to alloantigens are currently being studied for their association with transplant outcome. Non-HLA antigens, such as the polymorphic MHC class I-related chain A (MICA), expressed on endothelial cells have been implicated in the pathogenesis of hyperacute, acute and chronic organ allograft rejections. Use of endothelial cells as targets may clarify the specificities of other clinically relevant non-HLA antibodies in graft rejections. This review summarizes past and current knowledge of the clinical importance and specificities of non-HLA antibodies, and mechanisms by which these antibodies may contribute to graft destruction in clinical transplantation. The aims of current research into the role of non-HLA antigens and their genetics in predicting outcome are to develop an improved insight into the basic science of transplantation and to develop a risk or prognostic index for use in the clinical setting. Non-HLA antibody responses are receiving increasing interest in acute and chronic rejection and specificity, affinity, and pathogenicity need to be investigated to estimate their contribution. Undoubtedly, this will continue to be an area of interest in terms of fully understanding the role of non-HLA antigens as targets of immune-mediated injury and the potential for clinical intervention.     

Vitamin D: a new player in kidney transplantation?

Vitamin D is a hormone with pleiotropic effects. It mainly regulates calcium and phosphate metabolism through interactions with FGF23 and its receptor klotho. In addition, it has been shown that Vitamin D also regulates the immune response and has protective effects from cardiovascular disease, cancer and infections. Most renal transplant recipients have overt Vitamin D deficiency, a condition that may be associated with a decline in graft function and other complications. After kidney transplantation, elevated levels of FGF23 may predict increased risks of death and allograft loss. Theoretically, an optimal Vitamin D supplementation might favor operational tolerance and protect transplant recipients from the triad cardiovascular disease-cancer-infection. However, more solid data are needed to confirm this and to set the optimal level of serum Vitamin D supplementation in order to attain the best clinical outcome.