Prevention trumps treatment of antibody-mediated transplant rejection

Belying the spectacular success of solid organ transplantation and improvements in immunosuppressive therapy is the reality that long-term graft survival rates remain relatively unchanged, in large part due to chronic and insidious alloantibody-mediated graft injury. Half of heart transplant recipients develop chronic rejection within 10 years — a daunting statistic, particularly for young patients expecting to achieve longevity by enduring the rigors of a transplant. The current immunosuppressive pharmacopeia is relatively ineffective in preventing late alloantibody-associated chronic rejection. In this issue of the JCI, Kelishadi et al. report that preemptive deletion of B cells prior to heart transplantation in cynomolgus monkeys, in addition to conventional posttransplant immunosuppressive therapy with cyclosporine, markedly attenuated not only acute graft rejection but also alloantibody elaboration and chronic graft rejection. The success of this preemptive strike implies a central role for B cells in graft rejection, and this approach may help to delay or prevent chronic rejection after solid organ transplantation.

Newly transplanted organs are susceptible within a week to acute rejection, mediated dominantly by T cells, but are usually effectively protected from this form of inflammation and injury by currently used immunosuppressive agents such as calcineurin inhibitors, antiproliferative agents, mTOR inhibitors, and prophylactic therapy with T cell–specific antibodies. When acute rejection occurs, as it does in 5%–25% of solid organ recipients within the first year, it can typically be successfully treated with steroid therapy or, if needed, T cell–specific antibodies. However, the current immunosuppressive pharmacopeia is relatively ineffective in preventing or treating rejection mediated by B cells and the antibodies they produce. Antibody-mediated allograft injury, which occurs in 50% of heart transplant patients within 10 years, typically manifests more than a year after transplantation, more insidiously than T cell–mediated injury, and in a process characterized by complement deposition and microvascular obliteration that leads to tissue ischemia and eventually fibrosis with loss of graft function. Chronic graft rejection refers to this antibody-mediated process.

While factors contributing to chronic injury of organ transplants are multiple and include ischemia/reperfusion injury, preexisting donor disease, drug toxicities, and recurrence of original disease, the subtle development in the graft recipient of antibodies specific for the foreign donor tissue (alloantibodies) in the months and years following organ transplantation has been shown to be an accurate predictor of graft failure (1, 2). The magnitude of this problem is compounded by the practical difficulties in designing feasible clinical trials to evaluate methods for preventing alloantibody development and by the paucity of proven strategies to prevent alloantibody development in large animal models or humans. Nevertheless, data suggest that if preexisting alloantibody levels can be reduced, the risk of graft loss is lower (3).

In the medical literature, organ transplant patients experiencing antibody-mediated rejection have been treated with rituximab (a CD20-specific monoclonal antibody that depletes the B cell population) or by targeting of their plasma cells (antibody-secreting differentiated B cells), and in most cases these patients possessed preexisting alloantibody or suffered from early antibody-mediated rejection (4, 5). As expected, it is difficult to reverse the damage done by alloantibody in the setting of an established B cell immune response, and the efficacy of targeting B cells with rituximab under these posttransplant circumstances has been difficult to clearly establish. The combination of B cell depletion with profound T cell immunosuppression may also be complicated by loss of protective immunity (6). In other words, infection or malignancy may ensue, especially when both T cell– and B cell–depleting antibodies are administered simultaneously or sequentially. Therefore, an alternative strategy, that being prevention as opposed to treatment of the B cell alloimmune response, even if resorting to B cell depletion, may be attractive.

In their study in this issue of the JCI, Kelishadi et al. (7) show that preemptive treatment of cynomolgus monkeys transplanted with an allogeneic heart with rituximab on the day of the transplant substantially eliminated the injury attributable to B cells. In particular, infiltration of the graft by B cells was markedly reduced, as were intragraft levels of B cell–activating factor (BAFF; also known as B lymphocyte survival factor [BlyS]) and the B cell costimulatory molecules CD80 and CD86. In addition, the downstream effects of B cell activation were attenuated; for example, levels of alloantibody in the blood were reduced and less complement deposition in the graft was observed. Perhaps most importantly, these mechanistic changes were reflected by a substantial improvement in the microvascular integrity of the transplanted hearts (i.e., there was less chronic allograft vasculopathy) and by improved cardiac function, with four of four hearts beating well by 90 days compared with only three of seven in cynomolgus monkeys treated with cyclosporine alone. As seen from the control animals treated with cyclosporine alone, calcineurin inhibitors alone were unable to effectively prevent the injury inflicted by B cell–mediated rejection. Cotreatment with rituximab and cyclosporine also effectively prevented acute rejection compared with cyclosporine treatment alone, suggesting a role for B cells in acute rejection as well as chronic rejection.

Therapeutic targeting of CD20 in transplantation may be appealing because of CD20’s stable expression primarily on B cells in the peripheral blood and its absence from plasma cells, pro-B cells, and hematopoietic stem cells, thus permitting maintenance of serum IgG levels and posttreatment recovery by spared pro-B and stem cells (8). For the same reasons, therapeutic targeting of CD20 may not be as effective in treating recipients known to have donor-specific alloantibody prior to transplantation, since memory B cells and plasma cells capable of producing antibody specific for the donor organ would already be primed. Since many T cell–mediated immune responses include a B cell component, the impact of B cell depletion may extend beyond suppression of measurable antibody (9), as is suggested by the observation in the current study that acute rejection was reduced from a 57% incidence in cynomolgus monkeys treated with cyclosporine alone to zero by addition of rituximab to the treatment regimen (7).

Nonhuman primate (NHP) models, such as the one used by Kelishadi et al. (7), are far closer, genetically, to the human condition than any rodent model might be, and thus the current report is expected to predict better than any rodent model of transplantation how humans might respond to B cell depletion. Nevertheless, it is worth noting that even observations in NHPs in the field of organ transplantation have sometimes been difficult to translate directly into the clinic (10, 11). By analogy, human heart transplant patients usually receive three or four simultaneous immunosuppressive agents to prevent T cell–mediated rejection, whereas the cynomolgus monkeys in the study by Kelishadi et al. received high-dose cyclosporine as their sole immunosuppressive agent (7). The applicability of the findings of the current study to human organ transplantation will therefore require rigorous testing in order to determine whether preemptive CD20 monoclonal antibody treatment in the setting of more intense T cell immunosuppression is accompanied by opportunistic infection.

Targeting B cell immunity without depleting these cells in order to prevent alloantibody development may also lead to opportunities to prevent allograft injury (Figure 1). Such strategies include targeting complement pathway components (12) and B cell cytokines and/or chemokines such as BAFF and/or a proliferation-inducing ligand (APRIL), which may influence both B and T cell responses (13, 14). Other biologics being considered for development for the targeting of B cell responses in the setting of transplantation are those that affect the costimulatory pathways. Interactions between CD28 on CD4⁺ T cells and CD80/CD86 on B cells, as well as between CD40 ligand (CD40L; also known as CD154) on activated CD4⁺ T cells and CD40 on B cells have been shown to participate in providing T cell help to B cells (15). The CD40/CD40L interaction stimulates B cell proliferation and isotype switching in the appropriate cytokine milieu (16, 17). CD28 and CTLA4 expression have also been shown to be involved in germinal center formation (18).

Figure 1

B cell– and antibody-related biologics in transplantation. (i) CD20-specific mAb (i.e., rituximab) (anti-CD20), as reported in the current issue of the JCI by Kelishadi et al (7), binds and selectively depletes CD20⁺ B cells, thereby reducing alloantibody levels. Third generation CD20-specific mAbs are under development (e.g., ocrelizumab, ofatumumab). (ii) Inhibitors such as belimumab neutralize BAFF, while inhibitors such as atacicept (TACI-Ig) inhibit both BAFF and APRIL. (iii) Proteasome inhibitors (e.g., bortezomib) reversibly bind to the proteasome and disrupt various cell signaling pathways including the NF-κB pathway. (iv) Complement inhibitors, such as eculizumab (an antibody specific for complement component 5 [C5]), bind the complement protein C5, leading to cessation of complement-mediated cell lysis via the membrane attack complex (MAC). Since activation of the complement system is initiated by binding of 2 alloantibody molecules to a multivalent antigen followed by formation of the C1 complex, C1 inhibitor (C1-INH) prevents initiation of the serial complement cascade by inhibiting proteolytic cleavage of later complement components (specifically C2 and C4) and formation of C3 convertase. (v) Abatacept and belatacept (LEA29Y) are CTLA4-Ig molecules that bind the B7 costimulation molecule and block T cell costimulation of B cell activation and thereby production of alloantibodies. (vi) CD40-specific mAb (anti-CD40) binds the CD40 costimulation molecule. Blocking CD40L/CD40 interactions with CD40-specific antibody prevents T cell help to B cell activation, and consequently alloantibody production is inhibited.

Each of these potential therapies is under active investigation. It will be important to compare the relative safety and efficacy of such strategies with that of profound B cell depletion with rituximab. Additionally, it will be necessary to determine the durability and need for repeated application of B cell therapy in the setting of constant exposure to alloantigen, as is the case with an organ transplant. Nevertheless, the current report by Kelishadi et al. (7) offers clear experimental evidence in a large animal model that B cell targeting in parallel with T cell inhibition can prevent alloantibody development and lead to improved long-term graft histology and better small blood vessel patency. Prevention of chronic rejection would represent a major advance for the field of transplantation, and prevention of alloantibody development is more likely to succeed than are strategies to reverse ongoing antibody-mediated graft injury.

The authors are supported by NIH grant AI074635 (to S.J. Knechtle) and by Roche Organ Transplantation Research Foundation grant 962141545 (to N. Iwakoshi).

Address correspondence to: Stuart J. Knechtle, Emory University School of Medicine, 5105 WMB, 101 Woodruff Circle, Atlanta, GA 30322. Phone: 404.712.9910; Fax: 404.727.3660; E-mail: Stuart.Knechtle@emoryhealthcare.org.

Conflict of interest: Stuart J. Knechtle acknowledges stock ownership in Bristol-Myers Squibb.

Reference information: J Clin Invest. 2010;120(4):1036–1309. doi:10.1172/JCI42532

See the related article at Preemptive CD20+ B cell depletion attenuates cardiac allograft vasculopathy in cyclosporine-treated monkeys.

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