James E. Marsh

Predominant role for C5b-9 in renal ischemia/reperfusion injury

Previous work has indicated that complement is a mediator of ischemia/reperfusion (I/R) injury. To investigate the components of complement responsible for this effect, we examined a model of renal I/R injury in C3-, C4-, C5-, and C6-deficient mice. We occluded the renal arteries and veins (40-58 minutes) and, after reperfusion (0-72 hours), assessed renal structural and functional injury. C3-, C5-, and C6-deficient mice were protected from renal I/R injury, whereas C4-deficient mice were not protected. C6-deficient mice treated with antibody to block C5a generation showed no additional protection from I/R injury. Reconstitution with C6 alone restored the I/R injury in C6-deficient mice. Tubular epithelial cells were the main structures damaged by complement-mediated attack, and, in contrast, the renal vessels were spared. Neutrophil infiltration and myeloperoxidase activity were reduced in C-deficient mouse kidney, but by a similar extent in C3-deficient and C6-deficient mice. We conclude that the membrane attack complex of complement (in which C5 and C6 participate) may account for the effect of complement on mouse renal I/R injury. Neither C5a-mediated neutrophil infiltration nor the classic pathway, in which C4 participates, appears to contribute to I/R injury in this model. By contrast with other organs, such as the heart, the primary effect of complement in the ischemic area is on the parenchymal cell rather than the vascular endothelial cell. The membrane attack complex of complement is a potential target for prevention of I/R injury in this model.

Fc-dependent depletion of activated T cells occurs through CD40L-specific antibody rather than costimulation blockade

Although the underlying mechanisms are not well understood, it is generally believed that antigen recognition by T cells in the absence of costimulation may alter the immune response, leading to anergy or tolerance. Further support for this concept comes from animal models of autoimmunity and transplantation, where treatments based on costimulation blockade, in particular CD40 ligand (CD40L)-specific antibodies, have been highly effective. We investigated the mechanisms of action of an antibody to CD40L and provide evidence that its effects are dependent on the constant (Fc) region. Prolongation of graft survival is dependent on both complement- and Fc receptor–mediated mechanisms in a major histocompatibility complex (MHC)-mismatched skin transplant model. These data suggest that antibodies to CD40L act through selective depletion of activated T cells, rather than exerting immune modulation by costimulation blockade as currently postulated. This finding opens new avenues for treatment of immune disorders based on selective targeting of activated T cells.

The allogeneic T and B cell response is strongly dependent on complement components C3 and C4.

BACKGROUND The mechanisms controlling the production of antibodies against histocompatibility antigens are of prime importance in organ transplantation. METHODS We investigated the role of complement in the response to allogeneic stimulation, using mice deficient in C3, C4, or C5 to dissect the role of the alternative, classical, and terminal complement pathways. RESULTS After fully major histocompatibility complex disparate skin grafts, the allospecific immunoglobulin (Ig)G response was markedly impaired in C3- and C4-, but not in C5-deficient mice. This defect was most pronounced for second set responses. C3-deficient mice also demonstrated a decreased range of IgG isotypes. In contrast, there was no impairment of the allospecific IgM response. In functional T cell assays, the proliferative response and interferon-gamma secretion of recipient lymphocytes restimulated in vitro with donor antigen was decreased two- to threefold in C3-deficient mice. CONCLUSIONS These data show impairment of allogeneic T cell and B cell function in mice with defective complement activation and suggest a predominant role for the classical pathway in stimulating alloimmunity. The terminal pathway seems unimportant in this regard. This extends the results reported for soluble protein antigens and demonstrates a surprisingly marked effect on the alloresponse despite the presence of a stringent antigenic stimulus. These results have implications for the prevention of sensitization in naïve transplant recipients.

BK virus nephropathy in renal transplant patients in London.

Background. BK nephropathy (BKN) is an important cause of renal transplant dysfunction, believed to be associated with higher levels of immunosuppression. We assessed the experience of BKN in renal transplant patients in the London region. Methods. All six London transplant centers participated and case notes of patients with BKN in 2004 to 2005 were reviewed. Results. There were 17 cases of BKN, giving an incidence of 2.1%. Median time to diagnosis was 9 months. Median baseline creatinine rose from 150 to 196 &mgr;mol/L. At diagnosis, 16 patients were on tacrolimus, 15 on mycophenolate mofetil, and 10 on triple therapy with tacrolimus, mycophenolate mofetil, and prednisolone. Management of BKN involved reducing immunosuppression; cidofovir was used in two patients and methylprednisolone in five for acute rejection. Median follow-up time was 29.2 months. Creatinine returned to baseline in four patients, remained elevated in 12 and one patient lost his graft. The new median baseline creatinine was 216 &mgr;mol/L. Eight patients underwent repeat biopsies of which four became negative for BKV and three subsequently cleared the virus on blood and urine polymerase chain reaction and urine decoy cells. Overall, eight patients cleared the virus. None of age, sex, viral load, or biopsy characteristics (Banff ct score, Drachenberg grade, and number of BKV positive cells) were associated with poorer outcome when patients with increase in creatinine of less than 30% (n=7) or more than 30% (n=10) from baseline were compared. Conclusion. The incidence of BKN in this study is comparable with previous studies, with more favorable outcomes. It supports the association of BKN with potent immunosuppression.

Targeting the complement system.

Interest has blossomed in the development of complement inhibitors, in parallel with a growth in our understanding of the biology of the complement cascade. The first generation of designed inhibitors was based on naturally occurring complement receptors and regulatory molecules. These agents provided useful tools for exploring the role of complement in experimental models of disease, but may have limited therapeutic application in humans because of their short half-lives, limited bioavailability and possible antigenicity. More recently, humanized antibodies and synthetic molecules that block the activation of complement have been developed, which look as though they may overcome some of these difficulties. The possibility for precision inhibition of a limited part of the complement cascade, or for inhibition confined to a single organ, may offer effective therapeutic results, while avoiding the disadvantages of nonselective complement blockade. This review examines the recent evidence that complement inhibition will reduce tissue damage resulting from organ transplantation, ischaemia-reperfusion injury, cancer, glomerulonephritis and the use of extracorporeal circuits.

Long‐ and short‐term outcomes in renal allografts with deceased donors: A large recipient and donor genome‐wide association study

Improvements in immunosuppression have modified short‐term survival of deceased‐donor allografts, but not their rate of long‐term failure. Mismatches between donor and recipient HLA play an important role in the acute and chronic allogeneic immune response against the graft. Perfect matching at clinically relevant HLA loci does not obviate the need for immunosuppression, suggesting that additional genetic variation plays a critical role in both short‐ and long‐term graft outcomes. By combining patient data and samples from supranational cohorts across the United Kingdom and European Union, we performed the first large‐scale genome‐wide association study analyzing both donor and recipient DNA in 2094 complete renal transplant‐pairs with replication in 5866 complete pairs. We studied deceased‐donor grafts allocated on the basis of preferential HLA matching, which provided some control for HLA genetic effects. No strong donor or recipient genetic effects contributing to long‐ or short‐term allograft survival were found outside the HLA region. We discuss the implications for future research and clinical application.

HLA matching: still important despite modern immunosuppression.

Registry reports and most single-center studies have historically shown a significant positive association between the number of human leukocyte antigen (HLA) mismatches and increased graft loss (1). Thus, most organ allocation systems have incorporated HLA matching as an important, and often the predominant, determinant of kidney allocation. In recent years, an argument has emerged that HLA matching has become less important with modern immunosuppressive protocols, especially after the introduction of anti-interleukin (IL)-2 receptor antibodies for induction immunosuppression. In support, large registry studies have highlighted that there have been reductions in the incidence of acute rejection and improvements in graft survival in association with the progressive introduction of calcineurin inhibitors, mycophenolate mofetil, and antiIL2 receptor antibodies. For example, Su and coworkers analyzed the United Network for Organ Sharing data, and reported a reduction in the influence of HLA matching on graft survival for more recent kidney transplants (2). They suggested that, based on this, organ allocation algorithms might need to be revised. In contrast, a recent analysis of the Collaborative Transplant Study (CTS) database highlighted that HLA mismatching retained the same relative risk of graft loss in very recent cohorts of recipients (2000 – 2004) compared with earlier cohorts (1985–1994) (3). The authors of this report argued that HLA matching remained important even with modern immunosuppression, which was likely to include anti-IL2 receptor antibodies. Registry studies such as these contribute to hypothesis generation and to debate, but by their nature lack the precision, control, and uniformity of single-center experiences or randomized controlled trials. For instance, immunosuppressive policies may differ across the different participating centers, as will the stringency of HLA typing, whereas the CTS database incorporates data from 363 centers in three different continents, many utilizing different models for organ allocation. It is not surprising that seemingly contradictory conclusions may emerge. Given the above, the study by Wissing et al. in this issue of Transplantation is a welcome addition to the literature (4). The authors report a retrospective single-center experience of 208 consecutive patients who received a kidney transplant and induction immunosuppression with an anti-IL2 receptor antibody, calcineurin inhibitor, mycophenolate mofetil, and steroids. Multivariable logistic regression modeling was performed to assess risk factors for acute rejection within the first year and subsequent graft survival. Protocol biopsies were not undertaken. They identified that a progressively higher number of HLA mismatches remained an independent predictor of acute rejection, with each unit increase in HLA-A, -B, and -DR mismatches conferring an odds ratio for acute rejection of 1.65 (P 0.007). In multivariate analysis, the only other factor that remained an independent risk factor for acute rejection was a prolonged period ( 4 years) of dialysis prior to transplantation. In concordance with the published literature (5), the authors found that acute rejection within the first year remained associated with a significant reduction in overall and death-censored graft survival at 5 years posttransplantation. They suggest a causal link between episodes of rejection in the first year and subsequent graft loss. In support of this assertion, they observed that 11 of the 27 patients who experienced acute rejection suffered either permanent reduction in graft function or early graft loss. Clearly, as a single-center report, the number of patients in this paper is relatively small. Hence, the power of this study is limited and other confounding influences cannot be excluded. However, this is the first study that has specifically investigated the role of HLA matching in patients receiving quadruple induction immunosuppression including an antiIL2 receptor antibody. In an era when acute rejection rates of less than 15% within the first year are routinely reported, we should not be complacent about immunological factors and their significant contribution towards graft loss.