Prevention trumps treatment of antibody-mediated transplant rejection
Stuart J. Knechtle, Jean Kwun, and Neal Iwakoshi
Emory University School of Medicine, Atlanta, Georgia, USA.
Conflict of interest: SJK acknowledges stock ownership in Bristol Myers Squibb
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Abstract
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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 Journal, Kelishadi et al. report that pre-emptive deletion of B cells prior to
heart transplantation in cynomolgus monkeys, in addition to conventional post-transplant
immunosuppressive therapy with cyclosporine, markedly attenuated not only acute
rejection but also alloantibody elaboration and chronic graft rejection in treated animals
(1). 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.
Acute and Chronic Rejection
Newly transplanted organs are susceptible within a week to acute rejection,
mediated dominantly by T lymphocytes, but are usually effectively protected from this
form of inflammation and injury by currently used immunosuppressive agents such as
calcineurin-inhibitors, anti-proliferative agents, mTOR inhibitors, and prophylactic T cell
antibody therapy. When acute rejection occurs, as it does in 5-25% human of solid organ
recipients within the first year, such episodes typically respond to steroid therapy or, if
needed, T cell antibodies. However, the current immunosuppressive pharmacopeia is
relatively ineffective in preventing or treating rejection mediated by B lymphocytes and
their antibody effector arm. Antibody injury typically manifests later, more insidiously,
and is characterized by complement deposition and microvascular obliteration leading to
tissue ischemia and eventually fibrosis with loss of function. Chronic rejection refers to
such immunologically mediated allograft injury.
While factors contributing to chronic injury of organ transplants are multiple and
include ischemia/reperfusion injury, pre-existing donor disease, drug toxicities, and
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recurrence of original disease, the subtle development in the graft recipient of antibodies
against the foreign donor tissue (alloantibodies) in the months and years following organ
transplantation has been shown to be an accurate predictor of graft failure (2, 3). 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 antibody can be
reduced, the risk of graft loss is lower (4).
B Cell Depletion as Treatment of Established Antibody-Mediated Rejection
In the medical literature, organ transplant patients experiencing antibodymediated rejection have been treated with rituximab (an anti-CD20 monoclonal antibody
that depletes the B cell population) or by targeting of their plasma cells, and in most cases
these patients possessed pre-existing alloantibody or suffered from early antibodymediated rejection ( 5, 6). 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 post-transplant 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 (7). In
other words, infection or malignancy may ensue, especially when both T 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.
Pre-emptive B Cell Depletion
In their study in this issue of the Journal, Kelishadi et al. (1) show that preemptive treatment of cynomologous monkey allogeneic heart transplant recipients with
rituximab on the day of the transplant substantially eliminated the injury attributable to B
lymphocytes: infiltration of the graft by B cells, B cell activating factor (BAFF), B cell
costimulatory molecules CD80 and 86. In addition, the downstream effects of B cell
activation were attenuated including reduced alloantibody in blood and less complement
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deposition in the graft. Perhaps most importantly, these mechanistic changes were
reflected by a significant improvement in the microvascular integrity of the transplanted
hearts (less chronic allograft vasculopathy) and by improved function with four of four
hearts beating well by 90 days compared to only three of seven treated with cyclosporine
alone. As seen from the control animals treated with cyclosporine alone, calcineurininhibitors alone were unable to effectively prevent the injury inflicted by B cell mediated
rejection.
Implications for Human Transplant Patients
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, or hematopoietic stem cells, thus permitting maintenance
of serum IgG levels and post-treatment 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 antibody prior to transplantation since memory
B cells and plasma cells capable of producing antibody to the donor organ would already
be primed. Since many T lymphocyte–mediated immune responses include a B
lymphocyte 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 with cyclosporine alone to zero by addition
of rituximab in experimental animals.
A non-human primate (NHP) model is far closer, genetically, to the human
condition than any rodent model might be, and thus the current report would be 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 cardiac transplant patients usually receive three or
four simultaneous immunosuppressive agents to prevent T cell–mediated rejection,
compared to the monkeys in the current study by Kelishadi et al., which received high
dose cyclosporine as their sole immunosuppressive agent (1). The applicability of the
findings of the current study to human organ transplantation will require rigorous testing
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in order to determine if pre-emptive CD20 monoclonal antibody treatment in the setting
of more intense T cell immunosuppression will not be attended by opportunistic
infection.
Other B Cell Strategies for Transplantation
Targeting B cell immunity without depleting these cells in order to prevent
alloantibody development may also lead to opportunities to prevent graft injury (Figure
1). Such strategies include targeting complement pathway components (12) and B cell
cytokines/chemokines such as BAFF/APRIL (B cell–activating factor belonging to the
TNF family / a proliferation-inducing ligand), 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 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 CD28 on activated CD4+ T cells and CD40 ligand (CD40L, also known as
CD154) 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/CTLA-4 (cytotoxic T-lymphocyte antigen 4)
expression has also been shown to be involved in germinal center formation (18).
Each of these potential therapies is under active investigation. The relative safety
and efficacy of such strategies compared to profound B cell depletion with rituxumab
will be important to compare. 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 (1) offers clear experimental evidence in a large animal model that B-cell targeting
in parallel with T-cell inhibition may 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.
Acknowledgments
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The authors are supported by grants # AI_______ to SJK and by ROTRF grant to NI.
Address correspondence to: Stuart J. Knechtle, Emory University School of Medicine,
5105 WMB, 101 Woodruff Circle, Atlanta, Georgia 30322, USA. Phone: (404) 7129910; Fax: (404) 727-3660; Email: Stuart.Knechtle@emoryhealthcare.org.
References
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Everly, M.J., et al. 2009. Reducing de novo donor-specific antibody levels during acute
rejection diminishes renal allograft loss. Am J Transplant 9:1063-1071.
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populations in vivo. Am J Transplant 7:402-407.
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Zarkhin, V., et al. 2008. A randomized, prospective trial of rituximab for acute rejection
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Teeling, J.L., et al. 2006. The biological activity of human CD20 monoclonal antibodies
is linked to unique epitopes on CD20. J Immunol 177:362-371.
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beta-cell function. N Engl J Med 361:2143-2152.
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Hale, D.A., Dhanireddy, K., Bruno, D., and Kirk, A.D. 2005. Induction of transplantation
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Larsen, C.P. et al. 2005. Rational development of LEA29Y (belatacept), a high-affinity
variant of CTLA4-Ig with potent immunosuppressive properties. Am J Transplant 5: 445453.
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Li, Y., Ma, L., Yin, D., Shen, J., and Chong, A.S. 2008. Long-term control of alloreactive
B cell responses by the suppression of T cell help. J Immunol 180:6077-6084.
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Quezada, S.A., Jarvinen, L.Z., Lind, E.F., and Noelle, R.J. 2004. CD40/CD154
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germinal center formation. J Immunol 156:4576-4581.
Figure 1. B cell– and antibody-related biologics in transplantation. (I) Anti-CD20 Ab
(rituximab), binds and selectively depletes CD20+ B cells. Third generation anti-CD20
mAbs are under development (e.g., ocrelizumab, ofatumumab). (II) BAFF/BlyS
inhibitors such as belimumab neutralize B cell activation factor (BAFF) also known as B
lymphocyte survival factor (BlyS), or both BAFF/BlyS and APRIL (atacicept, TACI-Ig).
(III) Proteosome inhibitors (e.g., bortezomib) reversibly bind to the proteosome and
disrupt various cell signaling pathways including NF-B. (IV) Complement inhibitors
(AUTHOR: Are these the 3 blue lines with blue circles on the end? Please clarify the
figure labeling), such as eculizumab (anti-C5), bind the complement protein C5
(AUTHOR: Please label C5. Only C5b is labeled), leading cessation of complementmediated cell destruction. (AUTHOR: Please include some comment about C3
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convertase and it’s effect on MAC. Please define MAC. Please also comment on the
presence of the C1 complex) (V) CTLA4-Ig/LEA29Y (AUTHOR: Please define
LEA29Y in full) (abatacept/belatacept) bind the B7 costimulation molecule. (AUTHOR:
Please clarify what the result of this blockade is) (VI) Anti-CD40L (AUTOHR: in the
figure the label just reads “Anti-CD40”. Please revise for consistency), binds the
CD40L (CD154) costimulation molecule (AUTHOR: please clarify what the effect of
this binding is).
(AUTHOR: The figure legend is rather stilted. It would be more helpful if it was
more narrative about the effects of these molecules in transplantation and graft
rejection. Can you also include a brief mention of what part of the figure relates to
the results reported in the current report by Kelishadi et al.?)
(AUTHOR: once the figure is suitably revised, our in-house illustrator will redraw it
prior to publication and you will have the opportunity to review the image at the
proof stage and request any more modifications prior to printing)
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