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Current and future challenges in therapy for antibody-mediated rejection

2011; Elsevier BV; Volume: 30; Issue: 6 Linguagem: Inglês

10.1016/j.healun.2011.02.002

1557-3117

Nandini Nair, Timothy Ball, Patricia A. Uber, Mandeep R. Mehra,

Viral Infections and Immunology Research

Antibody-mediated rejection (AMR) continues to present a challenge for the survival of the cardiac allograft. AMR appears to be on the rise, likely secondary to changing trends in clinical practice, including selection of patients for transplantation on mechanical circulatory support and development of more effective combinations of immunosuppressive drugs against acute cellular rejection. Most current strategies are aimed at treating acute AMR, but the treatment of chronic AMR is still not well defined. Clinically, AMR can often be more severe than cellular rejection and more difficult to treat, often not responding to typical protocols of increased immunosuppression. Complex steps involved in the antibody response allows for several potential targets for therapeutic intervention, including suppression of T and B cells, elimination of circulating antibodies, and inhibition of residual antibodies. Existing evidence suggests a multiregimen approach is the best option. Sustenance of accommodation and induction of tolerance could be viewed as viable options if adequate immune surveillance can be achieved in this setting. This review discusses the challenges in treating AMR and provides a critical analysis of current and possible future therapies. Antibody-mediated rejection (AMR) continues to present a challenge for the survival of the cardiac allograft. AMR appears to be on the rise, likely secondary to changing trends in clinical practice, including selection of patients for transplantation on mechanical circulatory support and development of more effective combinations of immunosuppressive drugs against acute cellular rejection. Most current strategies are aimed at treating acute AMR, but the treatment of chronic AMR is still not well defined. Clinically, AMR can often be more severe than cellular rejection and more difficult to treat, often not responding to typical protocols of increased immunosuppression. Complex steps involved in the antibody response allows for several potential targets for therapeutic intervention, including suppression of T and B cells, elimination of circulating antibodies, and inhibition of residual antibodies. Existing evidence suggests a multiregimen approach is the best option. Sustenance of accommodation and induction of tolerance could be viewed as viable options if adequate immune surveillance can be achieved in this setting. This review discusses the challenges in treating AMR and provides a critical analysis of current and possible future therapies. Antibody mediated rejection (AMR) continues to be one of the major obstacles in transplant survival.1Kfoury A.G. Hammond M.E. Controversies in defining cardiac antibody-mediated rejection: need for updated criteria.J Heart Lung Transplant. 2010; 29: 389-394Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar Characterized by host antibodies targeting human leukocyte antigen (HLA) molecules, blood group antigens, and endothelial cells in the donor tissue,2Montgomery R.A. Hardy M.A. Jordan S.C. et al.Consensus opinion from the antibody working group on the diagnosis, reporting, and risk assessment for antibody-mediated rejection and desensitization protocols.Transplantation. 2004; 7: 181-185Crossref Scopus (95) Google Scholar the long-term effects are not fully understood. Although pioneering work on AMR by Hammond et al3Hammond E.H. Yowell R.L. Nunoda S. et al.Vascular (humoral) rejection in heart transplantation: pathologic observations and clinical implications.J Heart Lung Transplant. 1989; 8: 430-433Google Scholar was described in 1989, AMR failed to be recognized as a definite entity until 2005, when it was included in the rejection nomenclature.4Stewart S. Winters G. Fishbein M. et al.Revision of the 1990 working formulation for the standardization of nomenclature in the diagnosis of heart rejection.J Heart Lung Transplant. 2005; 24: 1710-1720Abstract Full Text Full Text PDF PubMed Scopus (1337) Google Scholar, 5Reed E. Demetris A. Hammond M.E. et al.Acute antibody-mediated rejection of cardiac transplants.J Heart Lung Transplant. 2006; 25: 153-159Abstract Full Text Full Text PDF PubMed Scopus (245) Google Scholar AMR is a significant risk factor for poor outcomes after organ transplantation and is associated with, increased mortality, increased graft loss, and accelerated allograft coronary artery disease.1Kfoury A.G. Hammond M.E. Controversies in defining cardiac antibody-mediated rejection: need for updated criteria.J Heart Lung Transplant. 2010; 29: 389-394Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar, 2Montgomery R.A. Hardy M.A. Jordan S.C. et al.Consensus opinion from the antibody working group on the diagnosis, reporting, and risk assessment for antibody-mediated rejection and desensitization protocols.Transplantation. 2004; 7: 181-185Crossref Scopus (95) Google Scholar, 3Hammond E.H. Yowell R.L. Nunoda S. et al.Vascular (humoral) rejection in heart transplantation: pathologic observations and clinical implications.J Heart Lung Transplant. 1989; 8: 430-433Google Scholar, 4Stewart S. Winters G. Fishbein M. et al.Revision of the 1990 working formulation for the standardization of nomenclature in the diagnosis of heart rejection.J Heart Lung Transplant. 2005; 24: 1710-1720Abstract Full Text Full Text PDF PubMed Scopus (1337) Google Scholar, 5Reed E. Demetris A. Hammond M.E. et al.Acute antibody-mediated rejection of cardiac transplants.J Heart Lung Transplant. 2006; 25: 153-159Abstract Full Text Full Text PDF PubMed Scopus (245) Google Scholar, 6Hammond E.H. Yowell R.L. Price G.D. et al.Vascular rejection and its relationship to allograft coronary artery disease.J Heart Lung Transplant. 1992; 11: S111-S119PubMed Google Scholar Risk factors for AMR include prior antibody exposure, multiparity, repeat transplantation, blood transfusions, use of ventricular assist devices, positive B-cell flow cytometry crossmatch, and elevated panel-reactive antibodies. AMR appears to be on the rise, likely secondary to changing trends in clinical practice, including selection of patients for transplantation on mechanical circulatory support and the development of more effective combinations of immunosuppressive drugs against acute cellular rejection (ACR).7Hunt S.A. Haddad F. The changing face of transplantation.J Am Coll Cardiol. 2008; 52: 587-598Abstract Full Text Full Text PDF PubMed Scopus (177) Google Scholar The decision to treat AMR is essentially based on clinical manifestations of graft failure and/or hemodynamic compromise in the absence of an endomyocardial biopsy specimen negative for cellular rejection. Finding complement deposition alone would not justify treatment in the absence of clinical manifestations.8Rodriguez E.R. Skojec D.V. Tan C.D. et al.Antibody-mediated rejection in human cardiac allografts: evaluation of immunoglobulins and complement activation products C4d and C3d as markers.Am J Transplant. 2005; 5: 2778-2785Crossref PubMed Scopus (137) Google Scholar AMR has been theoretically divided into 4 stages when described in renal allografts.9Colvin R.B. Antibody-mediated renal allograft rejection: diagnosis and pathogenesis.J Am Soc Nephrol. 2007; 18: 1046-1056Crossref PubMed Scopus (437) Google Scholar The first 2 stages have the sub-clinical presence of complement activation and C4d deposition in the graft but do not quite result in pathology to the graft, which is described as accommodation. Randomized trials are needed to further elucidate the need to treat accommodation. The rate of progress from stages 2 to 3 and 4 are variable and may lose the complement activation and revert to a normal course.9Colvin R.B. Antibody-mediated renal allograft rejection: diagnosis and pathogenesis.J Am Soc Nephrol. 2007; 18: 1046-1056Crossref PubMed Scopus (437) Google Scholar The significance of treating asymptomatic AMR is still unclear and warrants further investigation.9Colvin R.B. Antibody-mediated renal allograft rejection: diagnosis and pathogenesis.J Am Soc Nephrol. 2007; 18: 1046-1056Crossref PubMed Scopus (437) Google Scholar Early complement activation can be aborted by immunoregulatory molecules such as decay acceleration factor.10Gonzalez-Stawinski G.V. Tan C.D. et al.Decay acceleration factor may provide immunoprotection against antibody-mediated cardiac allograft rejection.J Heart Lung Transplant. 2008; 27: 357-361Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar Most current strategies are aimed at treating clinically overt AMR, but treatment of chronic AMR still remains a challenge. The spectrum of AMR can extend from an asymptomatic sub-clinical phase to refractory disease. We propose that AMR can be divided into 6 clinical and sub-clinical phenotypes (Table 1).Table 1Clinical Stages of Antibody-Mediated RejectionStage 1Sub-clinicalAppearance of antibodies without symptomsStage 2Pre-clinicalActivation of the complement system without symptomsStage 3Acute clinicalPathology suggestive of graft injuryStage 4Persistent sub-clinicalMinimum symptoms with low level of circulating antibodiesStage 5Recurrent clinicalIncreasing levels of circulating antibodies with overt graft dysfunctionStage 6Refractory diseaseChronic rejection with sequelae (cardiac allograft vasculopathy) Open table in a new tab Clinically, AMR can often be more severe than cellular rejection and more difficult to treat, often not responding to typical protocols of increased immunosuppression. Multiple complex steps involved in the antibody response allows for several potential targets for therapeutic intervention. Treatment traditionally relied on plasmapheresis11Olsen S.L. Wagoner L.E. Hammond E.H. et al.Vascular rejection in heart transplantation: clinical correlation, treatment options, and future considerations.J Heart Lung Transplant. 1993; 12: S135-S142PubMed Google Scholar, 12Miller L.W. Wesp A. Jennison S.H. et al.Vascular rejection in heart transplant recipients.J Heart Lung Transplant. 1993; 12: S147-S152PubMed Google Scholar; however, this method does not slow the production of antibodies and often leads to disease recurrence after treatment. Multitreatment regimens have focused on 4 main areas: suppression of T cells, elimination of circulating antibodies, inhibition of residual antibodies, and suppression of B cells (Table 2). Pulse steroid therapy has been shown to increase left ventricular function in patients with noncellular rejection.1Kfoury A.G. Hammond M.E. Controversies in defining cardiac antibody-mediated rejection: need for updated criteria.J Heart Lung Transplant. 2010; 29: 389-394Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar, 2Montgomery R.A. Hardy M.A. Jordan S.C. et al.Consensus opinion from the antibody working group on the diagnosis, reporting, and risk assessment for antibody-mediated rejection and desensitization protocols.Transplantation. 2004; 7: 181-185Crossref Scopus (95) Google Scholar, 3Hammond E.H. Yowell R.L. Nunoda S. et al.Vascular (humoral) rejection in heart transplantation: pathologic observations and clinical implications.J Heart Lung Transplant. 1989; 8: 430-433Google Scholar, 4Stewart S. Winters G. Fishbein M. et al.Revision of the 1990 working formulation for the standardization of nomenclature in the diagnosis of heart rejection.J Heart Lung Transplant. 2005; 24: 1710-1720Abstract Full Text Full Text PDF PubMed Scopus (1337) Google Scholar, 5Reed E. Demetris A. Hammond M.E. et al.Acute antibody-mediated rejection of cardiac transplants.J Heart Lung Transplant. 2006; 25: 153-159Abstract Full Text Full Text PDF PubMed Scopus (245) Google Scholar, 6Hammond E.H. Yowell R.L. Price G.D. et al.Vascular rejection and its relationship to allograft coronary artery disease.J Heart Lung Transplant. 1992; 11: S111-S119PubMed Google Scholar, 11Olsen S.L. Wagoner L.E. Hammond E.H. et al.Vascular rejection in heart transplantation: clinical correlation, treatment options, and future considerations.J Heart Lung Transplant. 1993; 12: S135-S142PubMed Google ScholarTable 2Targets of TherapyElimination of circulating antibodiesInhibition of circulating antibodiesSuppression of B cellsPlasma cell depletionComplement inhibitionPlasma exchangeIntravenous immunoglobulinRituximabBortezomibEculizumabDouble filtration plasmapheresisCytomegalovirus hyperimmunoglobulinSplenectomyImmunoadsorption plasmapheresisCalcineurin inhibitors (anti-proliferative/apoptotic effects)Belimumab (anti-Blys antibody)?Epratuzumab (anti-CD22 antibody)? Open table in a new tab T lymphocytes are a favored target of many of the current treatment protocols. B-lymphocyte activation requires T-lymphocyte–dependent stimulation to induce memory B lymphocytes, isotype switching, and production of high-affinity antibodies. T lymphocytes have also been a target in cellular rejection, resulting in numerous available drugs that effectively reduce the number of cells in circulation. Most protocols currently use or have used OK-T3, antithymocyte globulin (ATG), tacrolimus, or mycophenolic acid (MPA)-derived drugs.13Bonnefoy-Berard N.Revillard Jean-Pierre. Mechanisms of immunosuppression induced by antithymocyte globulins and OKT3.J Heart Lung Transplant. 1996; 15: 435-442PubMed Google Scholar Some studies suggest that ATG may also directly inhibit B lymphocytes, likely targeting B-lymphocyte–specific CD-20 or plasma cell markers like CD38 and CD138.14Zand M.S. B-cell activity of polyclonal antithymocyte globulins.Transplantation. 2006; 82: 1387-1395Crossref PubMed Scopus (44) Google Scholar, 15Zand M.S. Vo T. Huggins J. et al.Polyclonal rabbit antithymocyte globulin triggers B-cell and plasma cell apoptosis by multiple pathways.Transplantation. 2005; 79: 1507-1515Crossref PubMed Scopus (201) Google Scholar The inhibitory effects of ATG remain to be completely understood. Plasma exchange (PE), double filtration plasmapheresis (DFPP), and immunoadsorption plasmapheresis (IAPP) have all been used in various studies to treat AMR by reducing the amount of antibodies in the serum. DFPP and IAPP have been used more often in Japan and Europe, likely secondary to cost issues. PE or DFPP remove all plasma components non- or semi-selectively. Replacement fluids, such as fresh frozen plasma and albumin, are necessary during PE or DFPP, thus increasing the risk of infection and allergic reactions. IAPP does not require any replacement fluid16Yang K.S. Kenpe K. Yamaji K. et al.Plasma adsorption in critical care.Ther Apher. 2002; 6: 184-188Crossref PubMed Scopus (18) Google Scholar and effectively removes anti-HLA cytotoxic antibodies and shows definite benefits in C4d-positive AMR.17Bohmig G.A. Regele H. Exner M. et al.C4d-positive acute humoral renal allograft rejection: effective treatment by immunoadsorption.J Am Soc Nephrol. 2001; 12: 2482-2489PubMed Google Scholar Clinical use remains limited due to increased cost and lack of available filter membranes. PE and DFPP are not selective enough to remove all of the circulating antibodies and are associated with a high rate of recurrence. This necessitates immunosuppression with drugs. Intravenous immunoglobulin (IVIG) is currently the primary method used to inhibit residual antibodies. IVIG is produced from pooled plasma and is composed of 95% IgG. Originally used to treat viral diseases and congenital immunodeficiency, IVIG preparations contain antibodies against various cytokines, with both neutralizing and stabilizing effects, and may decrease the synthesis of cytokines. IVIG and PP have both shown similar effect, with IVIG having few side effects.18John R. Lietz K. Burke E. et al.Intravenous immunoglobulin reduces anti-HLA alloreactivity and shortens waiting time to cardiac transplantation in highly sensitized left ventricular assist device recipients.Circulation. 1999; 100: II229-II235Crossref PubMed Google Scholar One study found the combination of IVIG and PP was superior to either treatment alone.19Leech S.H. Lopez-Cepero M. LeFor W.M. et al.Management of the sensitized cardiac recipient: the use of plasmapheresis and intravenous immunoglobulin.Clin Transplant. 2006; 20: 476-484Crossref PubMed Scopus (59) Google Scholar IVIG has been shown to have several effects, including inhibition of B and T lymphocytes, in part by balancing Th1 and Th2 activities and their cytokine production. The effect of IVIG on the complement system remains controversial. Undesirable complement activation and complement system inhibition (membrane-attack complex, C3b, and C4b) have both been reported.20Singh N. Pirsch J. Samaniego M. Antibody-mediated rejection: treatment alternatives and outcomes.Transplant Rev (Orlando). 2009; 23: 34-46Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar In a study of 16 sensitized patients receiving IVIG treatment over a period of 1 to 3 months, a 33% reduction in antibodies reactivity within 1 week of treatment was observed, whereas an additional 20% reduction could be achieved in resistant patients by using a high-dose regimen.18John R. Lietz K. Burke E. et al.Intravenous immunoglobulin reduces anti-HLA alloreactivity and shortens waiting time to cardiac transplantation in highly sensitized left ventricular assist device recipients.Circulation. 1999; 100: II229-II235Crossref PubMed Google Scholar Low-dose IVIG prophylaxis therapy after cardiac transplantation had no significant effect on the risk of developing reactive antibodies.20Singh N. Pirsch J. Samaniego M. Antibody-mediated rejection: treatment alternatives and outcomes.Transplant Rev (Orlando). 2009; 23: 34-46Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar Such non-specific therapies remain a temporary means for symptomatic relief and cannot be extended to treat chronic AMR. Specific inhibition of CD20 molecules has emerged as a useful strategy in the treatment of acute AMR. Rituximab is a chimeric monoclonal antibody made up of human IgG1 heavy chain and κ light chain constant regions fused to mouse Ig variable regions specific for CD20. CD20 is expressed on pre-B and mature B lymphocytes throughout the antigen-independent stage of development until the early stages of B-cell activation. CD20 is a transmembrane protein that plays a key role in B-lymphocyte maturation, regulating the early steps of cell cycle initiation and differentiation, and may function as a calcium channel. The primary mechanism of rituximab-induced B-lymphocyte depletion is antibody-dependent cell cytotoxicity (ADCC). This may present a challenge in some patients. Significant interpatient variability in the depletion activity of rituximab has been noted, likely secondary to polymorphisms in the Fc-γ receptor affecting the ADCC mechanism. Rituximab also appears to be involved in complement-dependent cell killing and induction of apoptosis. Rituximab has no direct effect on antibody-producing plasma cells. The likely mechanism of antibody reduction is related to the reduction in the memory B-lymphocyte population. Most rituximab research has centered on renal transplant patients when used as induction therapy for sensitized patients and in a small number of trials as a treatment for AMR. Limited case studies suggest that it has been effective in treating refractory and recurrent AMR in cardiac transplant patients.21Garrett Jr, H.E. Groshart K. Duvall-Seaman D. et al.Treatment of humoral rejection with rituximab.Ann Thorac Surg. 2002; 74: 1240-1242Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar, 22Baran D.A. Lubitz S. Alvi S. et al.Refractory humoral cardiac allograft rejection successfully treated with a single dose of rituximab.Transplant Proc. 2004; 36: 3164-3166Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar Rituximab could be more effective in a multidrug regimen for AMR than as a monotherapeutic agent. Current data appear to dictate an important role for rituximab in a multidrug regimen for rejection. Two of the most recent additions to the list of agents that target B cells are belimumab and epratuzumab, both of which are being tested in ongoing trials for B-cell suppression in systemic lupus erythematosus (SLE). Belimumab is a human monoclonal antibody that inhibits binding of B-Lyss to its receptors, and epratuzumab is a monoclonal antibody that is directed to the B-cell surface antigen CD22.23Looney R.J. B cell-targeted therapies for systemic lupus erythematosus: an update on clinical trial data.Drugs. 2010; 70: 529-540Crossref PubMed Scopus (59) Google Scholar These monoclonal antibodies have been shown to decrease the number of circulating B cells in small studies of SLE patients. This effect on B-cell populations may well be used to combat AMR. An ongoing clinical trial is currently addressing the use of belimumab in desensitizing sensitized patients waiting for renal transplantation.23Looney R.J. B cell-targeted therapies for systemic lupus erythematosus: an update on clinical trial data.Drugs. 2010; 70: 529-540Crossref PubMed Scopus (59) Google Scholar The need for the development of effective future therapies is imperative because current strategies continue to provide only symptomatic relief and no long-term solutions. Effective treatment of AMR requires immune suppression to maintain long-term remission. An objective comparison of the current treatment regimens is difficult due to the low number of patients, short follow-up period, and lack of a strict randomized controlled design. The next section of this review targets promising future therapies in development. Stegall and Gloor24Stegall M.D. Gloor J.M. Deciphering antibody-mediated rejection: new insights into mechanisms and treatment.Curr Opin Organ Transplant. 2010; 15: 8-10Crossref PubMed Scopus (93) Google Scholar summarize 2 novel approaches to combat AMR, such as use of bortezomib, a proteasome inhibitor, for plasma cell depletion and the inhibition of terminal complement activation with eculizumab, a humanized anti-C5 antibody. Most of the cellular damage in AMR results from the activation of the complement cascade. The complement system can be activated by 3 separate pathways that converge into C5, leading to the formation of the membrane attack complex (C5b-C9), which has proinflammatory, chemotactic, and cell lytic properties. C5 appears to be a promising target to inhibit the complement system. Removal of C5 also prevents the formation of C5a, a potent immunomodulator involved in chemotaxis, macrophage cytokine production, and ischemia and reperfusion–mediated organ damage. In addition, targeting the terminal components of the complement system will allow the early components to remain active in immune defense. C3b is important in microbial opsonization and immune complex clearance as well as in antigenic tolerance. C3a is another important player and has been shown to downregulate the Th2 response in murine models: Wang et al25Wang H. Arp J. Liu W. et al.Inhibition of terminal complement components in presensitized transplant recipients prevents antibody mediated rejection leading to long-term graft survival and accommodation.J Immunol. 2007; 179: 4451-4463Crossref PubMed Scopus (83) Google Scholar used studies of anti-C5 monoclonal antibody in a pre-sensitized (via previous skin graft) mouse cardiac allograft model to show that triple therapy, consisting of anti-C5 monoclonal antibody, cyclosporine A (CsA), and cyclophosphamide effectively prevented AMR and achieved indefinite heart graft survival for about 100 days in pre-sensitized recipients, thus supporting a multiregimen approach. Rother et al26Rother R.P. Arp J. Jiang J. et al.C5 blockade with conventional immunosuppression induces long-term graft survival in presensitized recipients.Am J Transplant. 2008; 8: 1129-1142Crossref PubMed Scopus (55) Google Scholar evaluated triple therapy with CsA, LF15-0195 (LF), and anti-C5 monoclonal antibody on a pre-sensitized mouse kidney allograft model, which prevented graft rejection and resulted in indefinite (>100 days) kidney survival and function, as indicated by serum creatinine levels within normal reference ranges (range, 35.3–88.4 mmol/liter). In a study by Gueler et al27Gueler F. Rong S. Gwinner W. et al.Complement 5a receptor inhibition improves renal allograft survival.J Am Soc Nephrol. 2008; 19: 2302-2312Crossref PubMed Scopus (87) Google Scholar of human renal transplant, C5aR expression was increased in renal tissue. When recipient mice were treated once daily with a C5aR antagonist before transplantation, long-term renal allograft survival was markedly improved compared with vehicle treatment (75% vs 0%), with a concomitant reduction in apoptosis. Treatment with a C5aR antagonist significantly attenuated monocyte/macrophage infiltration, possibly due to reduced levels of monocyte chemoattractant protein 1(MCP-1) and intercellular adhesion molecule 1 (ICAM-1). In vitro, C5aR antagonism inhibited ICAM-1 upregulated in primary mouse aortic endothelial cells and reduced adhesion of peripheral blood mononuclear cells. C5aR blockade markedly reduced alloreactive T-cell priming and improved renal allograft survival. B lymphocytes have been a target of therapy; however, once these cells are activated by T lymphocytes, they become antigen-producing plasma cells. Drugs that target proteasomes show potential in reducing plasma cells. Bortezomib is the first therapeutic proteasome inhibitor to be tested in humans. It is currently approved in the United States for treating relapses of multiple myeloma and mantle cell lymphoma. Bortezomib binds the catalytic site of the 26S proteasome. In normal cells, proteasomes are responsible for degrading ubiquitinated and misfolded proteins. Correlation has been found between the high rate of protein synthesis in plasma cells, which inherently results in defective ribosomal products and unfolded proteins, and their susceptibility to proteasome inhibition. Bortezomib eliminates both short-lived and long-lived plasma cells by activation of the terminal unfolded protein response. In a recent study by Trivedi et al,28Trivedi H.L. Terasaki P.I. Feroz A. et al.Abrogation of anti-HLA antibodies via proteasome inhibition.Transplantation. 2009; 87: 1555-1561Crossref PubMed Scopus (135) Google Scholar 11 patients with anti-HLA alloantibodies were treated with bortezomib (proteasome inhibition). Bortezomib elicited a substantial reduction in both donor-specific antibody (DSA) and non-DSA levels. At a mean follow-up of approximately 4 months after treatment, all patients had stable graft function, with minimal transient side effects such as gastrointestinal toxicity, thrombocytopenia, and paresthesias. In another small study of 5 patients (2 kidney/pancreas and 3 kidney transplant recipients) with AMR and coexisting ACR, treatment with bortezomib (median follow-up, 6.9 months) led to prompt reversal of ACR and AMR. Patients were treated after other known anti-humoral therapies had failed to reverse the acute rejection episode. DSA levels decreased significantly in all patients (except 1 patient with short follow-up). Side effects of bortezomib included a transient grade III thrombocytopenia in 1 patient and mild-to-moderate gastrointestinal toxicities in 3 of 5 patients. No opportunistic infections were observed.29Everly M.J. Everly J.J. Susskind B. et al.Proteasome inhibition reduces donor-specific antibody levels.Transplant Proc. 2009; 41: 105-107Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar Identification of molecular targets by genomic, proteomic, and metabolomic profiling seems an exciting alternative and an elegant solution, but still remains in its infancy. Two markers have been identified in the rat and the mouse (TOAG-1, α-1,2-mannosidase), which are present in leucocytes in the graft and in the peripheral blood with high specificity and reproducibility. These molecules are highly expressed during induction and maintenance of acceptance, and are downregulated during rejection. Transforming growth factor-β2, preproenkephalin, GM2a, glucocorticoid-induced tumor necrosis factor receptor–related protein, and interleukin (IL)-1R2 have been shown to be specifically upregulated in regulatory T cells associated with tolerance of skin allografts. Study of murine cardiac allografts identified 2 genes, H2-Ea and Frzb, which are highly expressed in long-term surviving heart allografts. In cases of cellular rejection, increases in cytotoxic T-lymphocyte transcripts, such as perforin, granzyme B, and CD154, have been detected blood mononuclear cells at the time or even before acute rejection. A major effort in identification of biomarkers in transplantation is underway in Canada.30Lin D. Hollander Z. Meredith A. McManus B.M. Searching for ‘omic' biomarkers.Can J Cardiol. 2009; 25: 9A-14AAbstract Full Text PDF PubMed Scopus (13) Google Scholar The field has great promise, but it seems a little early to declare any clear successes. Molecular profiling would help not just in diagnosis but also in our understanding of the pathophysiology and in the development appropriate therapeutics. Improvisation of methods to sustain accommodation at the molecular level is a relatively unexplored area that could be of immense utility. C4d-positivity can be found in solid-organ transplants in cases of “accommodation,” a form of humoral rejection without graft dysfunction. Accommodation might reflect a change in antibodies or in the antigen, or the graft acquiring a resistance to injury by antibodies and complement. Therefore, methods to sustain accommodation would be relevant to graft survival. The difference between accommodation and AMR essentially lies in the partial vs complete activation of the endothelial cells. Hence, methods to inhibit complement activation may be promising. However, isolated complement inhibition may not be enough to sustain accommodation. Concomitant induction of cytoprotective cytokines like IL-4 and IL-13 appears to be necessary for graft survival by accommodation. This protection appears to be mediated by the PI3/Akt pathway, possibly through the induction of the anti-apoptotic proteins Bcl-2 and Bcl-XL, as shown in vitro models. Long-term use of MTOR inhibitors (rapamycin) decreases major histocompatibility complex (MHC) class 1–induced Bcl-2 expression. This is consistent with the finding that rapamycin affects the Akt signaling pathway. Hence, this drug regimen may sustain accommodation to prevent chronic graft rejection. However, sustaining accommodation indefinitely could have its own pitfalls because this leads to poor tumor surveillance and reduced resistance to viral infections.31Jindra J.T. Jin Y.P. Rozengurt E. et al.HLA class1 antibody–mediated endothelial cell proliferation via the mTOR pathway.J Immunol. 2008; 180: 2357-2366Crossref PubMed Scopus (127) Google Scholar Benefits of mixed allogenicity vs full allogenicity include a decreased risk of graft-vs-host disease and a relative increased immunologic competence to respond to infectious threats. Mixed chimerism is associated with donor-specific transplant tolerance and may result in long-term cardiac allograft survival without rejection. The induction of stable chimeras through pre-conditioning of mice with donor immature dendritic cells, followed by BMT, led to tolerance, allowing the long-term survival of mismatched cardiac allografts. Regulatory T cells (Tregs) can induce and maintain tolerance in animal models. It has also been postulated from animal models of transplantation that a combination of rapamycin and Tregs may be successful in inducing and maintaining tolerance.32Long E. Wood K.J. Regulatory T cells in transplantation: transferring mouse studies to the clinic.Transplantation. 2009; 88: 100-156Crossref Scopus (62) Google Scholar, 33Zhang C. Shan J. Lu J. et al.Rapamycin in combination with donor-specific CD4+CD25+Treg cells amplified in vitro might be realize the immune tolerance in clinical organ transplantation.Cell Immunol. 2010; 264: 111-113Crossref PubMed Scopus (12) Google Scholar Fully effective structural and conditional tolerance might preclude the mounting of a protective immune response against common pathogens. Therefore, it may be postulated that future strategies should consider an orchestrated combination of accommodation and tolerance because a certain amount of residual immunity against self or graft would be needed for protection when battling invasive organisms. Management of acute AMR routinely involves removal of circulating antibodies with plasmapheresis every other day, followed by IVIGs. This should be followed by methods of suppression of B cells, such as the use of rituximab. Other agents that could have a possible use in suppressing B cells are belimumab and epratuzumab. Concomitant or subsequent treatment with bortezomib for plasma cell inhibition and eculizumab for terminal complement inhibition would constitute a multiregimen treatment. Such multiregimen treatment protocols should be followed by surveillance every 4 weeks for anti-HLA antibodies. If AMR continues to recur, repeat cycles of suppression of B cells, depletion of plasma cells, and inhibition of terminal complement should be administered until results of surveillance tests of anti-HLA antibody are negative. Aggressive treatment of acute AMR would in essence eliminate the existence of chronic AMR and its sequelae, such as graft vascular disease. In summary AMR remains a difficult challenge and a threat to long-term graft survival. From the existing evidence, a multiregimen approach appears to be the best option, although this is based on small studies. Better understanding of AMR at the molecular level will continue to open new therapeutic modalities in the pre-sensitized recipient. Long-term sustenance of accommodation and complete tolerance seem to be attractive options but should be viewed with caution because of the possibility of increased incidence of cancers and reduced resistance to infections in an immune-suppressed host (Figure 1). Dr Mehra reports research funding from the National Institutes of Health. He has also led the National Heart, Lung and Blood Institute Working Group on Cardiac Transplantation Task Force. None of the other authors has a financial relationship with a commercial entity that has an interest in the subject of the presented manuscript or other conflicts of interest to disclose.