Preemptive CD20+ B cell depletion attenuates cardiac allograft vasculopathy in cyclosporine-treated monkeys

Anti-CD20 Antibody and Calcineurin Inhibitor Combination Therapy Effectively Suppresses Antibody-Mediated Rejection in Murine Orthotopic Lung Transplantation

Antibody-mediated rejection (AMR) is a risk factor for chronic lung allograft dysfunction, which impedes long-term survival after lung transplantation. There are no reports evaluating the efficacy of the single use of anti-CD20 antibodies (aCD20s) in addition to calcineurin inhibitors in preventing AMR. Thus, this study aimed to evaluate the efficacy of aCD20 treatment in a murine orthotopic lung transplantation model. Murine left lung transplantation was performed using a major alloantigen strain mismatch model (BALBc (H-2d) → C57BL/6 (BL/6) (H-2b)). There were four groups: isograft (BL/6→BL/6) (Iso control), no-medication (Allo control), cyclosporine A (CyA) treated, and CyA plus murine aCD20 (CyA+aCD20) treated groups. Severe neutrophil capillaritis, arteritis, and positive lung C4d staining were observed in the allograft model and CyA-only-treated groups. These findings were significantly improved in the CyA+aCD20 group compared with those in the Allo control and CyA groups. The B cell population in the spleen, lymph node, and graft lung as well as the levels of serum donor-specific IgM and interferon γ were significantly lower in the CyA+aCD20 group than in the CyA group. Calcineurin inhibitor-mediated immunosuppression combined with aCD20 therapy effectively suppressed AMR in lung transplantation by reducing donor-specific antibodies and complement activation.

TNX-1500, a crystallizable fragment-modified anti-CD154 antibody, prolongs nonhuman primate cardiac allograft survival

Blockade of the CD40/CD154 T cell costimulation pathway is a promising approach to supplement or replace current clinical immunosuppression in solid organ transplantation. We evaluated the tolerability and activity of a novel humanized anti-CD154 monoclonal antibody, TNX-1500 (TNX), in a nonhuman primate heterotopic cardiac allogeneic (allo) transplant model. TNX-1500 contains a rupluzimab fragment antigen-binding region and an immunoglobin G4 crystallizable fragment region engineered to reduce binding to the crystallizable fragment gamma receptor IIa and associated risks of thrombosis. Recipients were treated for 6 months with standard-dose TNX (sTNX) monotherapy, low-dose TNX monotherapy (loTNX), or loTNX with mycophenolate mofetil (MMF) (loTNX + MMF). Results were compared with historical data using chimeric humanized 5c8 monotherapy dosed as for loTNX but discontinued at 3 months. Median survival time was similar for humanized 5c8 and both loTNX groups, but significantly longer with sTNX (>265 days) than with loTNX (99 days) or loTNX + MMF (88 days) (P < 0.05 for both comparisons against sTNX). Standard-dose TNX prevented antidonor alloantibody elaboration, inhibited chronic rejection, and was associated with a significantly reduced effector T cells/regulatory T cells ratio relative to loTNX with MMF. No thrombotic complications were observed. This study demonstrated that TNX was well tolerated, prolongs allograft survival, and prevents alloantibody production and cardiac allograft vasculopathy in a stringent preclinical nonhuman primate heart allotransplant model.

Metformin attenuates chronic lung allograft dysfunction: evidence in rat models

BACKGROUND

Chronic lung allograft dysfunction (CLAD) directly causes an abysmal long-term prognosis after lung transplantation (LTx), but effective and safe drugs are not available. Metformin exhibits high therapeutic potential due to its antifibrotic and immunomodulatory effects; however, it is unclear whether metformin exerts a therapeutic effect in CLAD. We sought to investigate the effect of metformin on CLAD based on rat models.

METHODS

Allogeneic LTx rats were treated with Cyclosporin A (CsA) in the first week, followed by metformin, CsA, or vehicle treatment. Syngeneic LTx rats received only vehicles. All rats were sacrificed on post-transplant week 4. Pathology of lung graft, spleen, and thymus, extent of lung fibrosis, activity of profibrotic cytokines and signaling pathway, adaptive immunity, and AMPK activity were then studied.

RESULTS

Allogeneic recipients without maintenance CsA treatment manifested CLAD pathological characteristics, but these changes were not observed in rats treated with metformin. For the antifibrotic effect, metformin suppressed the fibrosis extent and profibrotic cytokine expression in lung grafts. Regarding immunomodulatory effect, metformin reduced T- and B-cell infiltration in lung grafts, spleen and thymus weights, the T- and B-cell zone areas in the spleen, and the thymic medullary area. In addition, metformin activated AMPK in lung allografts and in α-SMA⁺ cells and T cells in the lung grafts.

CONCLUSIONS

Metformin attenuates CLAD in rat models, which could be attributed to the antifibrotic and immunomodulatory effects. AMPK activation suggests the potential molecular mechanism. Our study provides an experimental rationale for further clinical trials.

Effects of the complement system on antibody formation and function: implications for transplantation

PURPOSE OF REVIEW

In antibody-mediated allograft rejection, donor-reactive antibodies cause transplant injury in part via complement activation. New mechanistic insights indicate complement also modulates development of humoral immune responses. Herein we review recent data that describes how complement affects antibody formation and we discuss therapeutic implications.

RECENT FINDINGS

Extravasating T cells interacting with integrins express and activate intracellular complement that drives immune-metabolic adaptations vital for CD4 + helper cells. Marginal zone B cells can acquire intact major histocompatibility complexes from dendritic cells via complement-dependent trogocytosis for presentation to T cells. Activated B cells in germinal centers receive co-stimulatory signals from T-helper cells. These germinal center B cells undergo coordinate shifts in surface complement regulator expression that permit complement receptor signaling on the germinal center B cells required for affinity maturation. The positively selected, high-affinity B cells can differentiate into plasma cells that produce donor-HLA-reactive antibodies capable of ligating endothelial, among other, graft cells. Subsequent sublytic complement attack can stimulate endothelial cells to activate CD4 + and CD8 + T cells, promoting cellular and humoral rejection. Newly developed complement inhibitors are being tested to prevent/treat transplant rejection.

SUMMARY

The complement system influences T-cell, B-cell and endothelial-cell activation, and thereby contributes allograft injury. Emerging therapeutic strategies targeting complement activation have the potential to prevent or abrogate transplant injury and improve transplant outcomes.

Allo-Specific Humoral Responses: New Methods for Screening Donor-Specific Antibody and Characterization of HLA-Specific Memory B Cells

Antibody-mediated allograft rejection (AMR) causes more kidney transplant failure than any other single cause. AMR is mediated by antibodies recognizing antigens expressed by the graft, and antibodies generated against major histocompatibility complex (MHC) mismatches are especially problematic. Most research directed towards the management of clinical AMR has focused on identifying and characterizing circulating donor-specific HLA antibody (DSA) and optimizing therapies that reduce B-cell activation and/or block antibody secretion by inhibiting plasmacyte survival. Here we describe a novel set of reagents and techniques to allow more specific measurements of MHC sensitization across different animal transplant models. Additionally, we have used these approaches to isolate and clone individual HLA-specific B cells from patients sensitized by pregnancy or transplantation. We have identified and characterized the phenotypes of individual HLA-specific B cells, determined the V(D)J rearrangements of their paired H and L chains, and generated recombinant antibodies to determine affinity and specificity. Knowledge of the BCR genes of individual HLA-specific B cells will allow identification of clonally related B cells by high-throughput sequence analysis of peripheral blood mononuclear cells and permit us to re-construct the origins of HLA-specific B cells and follow their somatic evolution by mutation and selection.

Experimental modeling of desensitization: What have we learned about preventing AMR?

During the past 5 decades, short-term outcomes in kidney transplant have significantly improved, in large part due to reduced rates and severity of acute rejection. Development of better immunosuppressive maintenance agents, as well as new induction therapies, helped make these advances. Nonhuman primate models provided a rigorous testing platform to evaluate candidate biologics during this process. However, antibody-mediated rejection remains a major cause of late failure of kidney allografts despite advances made in pharmacologic immunosuppression and strategies developed to facilitate improved donor-recipient matching. Our laboratory has been actively working to develop strategies to prevent and treat antibody-mediated rejection and immunologic sensitization in organ transplant, relying largely on a nonhuman primate model of kidney transplant. In this review, we will cover outcomes achieved by managing antibody-mediated rejection or sensitization in nonhuman primate models and discuss promises, limitations, and future directions for this model.

Accelerated Allograft Vasculopathy With Rituximab After Cardiac Transplantation

BACKGROUND

The CTOT-11 (Prevention of Cardiac Allograft Vasculopathy Using Rituximab Therapy in Cardiac Transplantation [Clinical Trials in Organ Transplantation-11]) study was a randomized, placebo-controlled, multicenter, double-blinded clinical trial in nonsensitized primary heart transplant (HTX) recipients.

OBJECTIVES

The study sought to determine whether B cell depletion therapy would attenuate the development of cardiac allograft vasculopathy.

METHODS

A total of 163 HTX recipients were randomized to rituximab 1,000 mg intravenous or placebo on days 0 and 12 post-transplant. Primary outcome was change in percent atheroma volume (PAV) from baseline to 1 year measured by intravascular ultrasound. Secondary outcomes included treated episodes of acute rejection, de novo anti-HLA antibodies (including donor-specific antibodies), and phenotypic differentiation of B cells.

RESULTS

There were no significant differences at study entry between the rituximab and placebo groups. Paired intravascular ultrasound measures were available at baseline and 1 year in 86 subjects (49 rituximab, 37 placebo). The mean ± SD change in PAV at 12 months was +6.8 ± 8.2% rituximab versus +1.9 ± 4.4% placebo (p = 0.0019). Mortality at 12 months was 3.4% rituximab versus 6.8% placebo (p = 0.47); there were no retransplants or post-transplant lymphoproliferative disorder. The rate of treated rejection was 24.7% rituximab versus 32.4% placebo (p = 0.28). Rituximab therapy effectively eliminated CD20⁺/CD19⁺ B cells followed by a gradual expansion of a CD19^- cell population in the rituximab-treated group.

CONCLUSIONS

A marked, unexpected increase in coronary artery PAV with rituximab was observed during the first year in HTX recipients. One-year mortality was not impacted; however, longer-term follow-up and mechanistic explanations are required. (Prevention of Cardiac Allograft Vasculopathy Using Rituximab [Rituxan] Therapy in Cardiac Transplantation; NCT01278745).

B cell clonal expansion within immune infiltrates in human cardiac allograft vasculopathy

Cardiac allograft vasculopathy (CAV) is associated with intragraft B cell infiltrates. Here, we studied the clonal composition of B cell infiltrates using 4 graft specimens with CAV. Using deep sequencing, we analyzed the immunoglobulin heavy chain variable region repertoire in both graft and blood. Results showed robust B cell clonal expansion in the graft but not in the blood for all cases. Several expanded B cell clones, characterized by their uniquely rearranged complementarity-determining region 3, were detected in different locations in the graft. Sequences from intragraft B cells also showed elevated levels of mutated rearrangements in the graft compared to blood B cells. The number of somatic mutations per rearrangement was also higher in the graft than in the blood, suggesting that B cells continued maturing in situ. Overall, our studies demonstrated B cell clonal expansion in human cardiac allografts with CAV. This local B cell response may contribute to the pathophysiology of CAV through a mechanism that needs to be identified.

Multi-gene technical assessment of qPCR and NanoString n-Counter analysis platforms in cynomolgus monkey cardiac allograft recipients

Quantitative gene expression profiling of cardiac allografts characterizes the phenotype of the alloimmune response, yields information regarding differential effects that may be associated with various anti-rejection drug regimens, and generates testable hypotheses regarding the pathogenesis of the chronic rejection lesions typically observed in non-human primate heart transplant models. The goal of this study was to assess interplatform performance and variability between the relatively novel NanoString nCounter Analysis System, ΔΔCT (relative) RT-qPCR, and standard curve (absolute) RT-qPCR utilizing cynomolgus monkey cardiac allografts. Methods for RNA isolation and preamplification were also systematically evaluated and effective methods are proposed. In this study, we demonstrate strong correlation between the two RT-qPCR methods, but variable and, at times, weak correlation between RT-qPCR and NanoString. NanoString fold change results demonstrate less sensitivity to small changes in gene expression than RT-qPCR. These findings appear to be driven by technical aspects of each platform that influence the conditions under which each technique is ideal. Collectively, our data contribute to the general effort to optimally utilize gene expression profiling techniques, not only for transplanted tissues, but for many other applications where accurate rank-order of gene expression versus precise quantification of absolute gene transcript number may be relatively valuable.

Translational impact of NIH-funded nonhuman primate research in transplantation

The National Institutes of Health (NIH) has long supported using nonhuman primate (NHP) models for research on kidney, pancreatic islet, heart, and lung transplantation. The primary purpose of this research has been to develop new treatments for down-modulating or preventing deleterious immune responses after transplantation in human patients. Here, we discuss NIH-funded NHP studies of immune cell depletion, costimulation blockade, regulatory cell therapy, desensitization, and mixed hematopoietic chimerism that either preceded clinical trials or prevented the human application of therapies that were toxic or ineffective.

Transplant research in nonhuman primates to evaluate clinically relevant immune strategies in organ transplantation

Research in transplant immunology using non-human primate (NHP) species to evaluate immunologic strategies to prevent rejection and prolong allograft survival has yielded results that have translated successfully into human organ transplant patient management. Other therapies have not proceeded to human translation due to failure in NHP testing, arguably sparing humans the futility and risk of such testing. The NHP transplant models are ethically necessary for drug development in this field and provide the closest analogue to human transplant patients available. The refinement of this resource with respect to colony MHC typing, reagent and assay development, and availability to the research community has greatly enhanced knowledge about transplant immunology and drug development.

Selective CD28 Inhibition Modulates Alloimmunity and Cardiac Allograft Vasculopathy in Anti-CD154-Treated Monkeys

BACKGROUND

Selective CD28 inhibition is actively pursued as an alternative to B7 blockade using cytotoxic T lymphocyte antigen 4 Ig based on the hypothesis that the checkpoint immune regulators cytotoxic T lymphocyte antigen 4 and programmed death ligand 1 will induce tolerogenic immune signals. We previously showed that blocking CD28 using a monovalent nonactivating reagent (single-chain anti-CD28 Fv fragment linked to alpha-1 antitrypsin [sc28AT]) synergizes with calcineurin inhibitors in nonhuman primate (NHP) kidney and heart transplantation. Here, we explored the efficacy of combining a 3-week "induction" sc28AT treatment with prolonged CD154 blockade.

METHODS

Cynomolgus monkey heterotopic cardiac allograft recipients received sc28AT (10 mg/kg, d0-20, n = 3), hu5C8 (10-30 mg/kg, d0-84, n = 4), or combination (n = 6). Graft survival was monitored by telemetry. Protocol biopsies and graft explants were analyzed for International Society of Heart and Lung Transplantation acute rejection grade and cardiac allograft vasculopathy score. Alloantibody, T-cell phenotype and regulatory T cells were analyzed by flow cytometry. Immunochemistry and gene expression (NanoString) characterized intra-graft cellular infiltration.

RESULTS

Relative to modest prolongation of median graft survival time with sc28AT alone (34 days), hu5C8 (133 days), and sc28AT + hu5C8 (141 days) prolonged survival to a similar extent. CD28 blockade at induction, added to hu5C8, significantly attenuated the severity of acute rejection and cardiac allograft vasculopathy during the first 3 months after transplantation relative to hu5C8 alone. These findings were associated with decreased proportions of circulating CD8 and CD3CD28 T cells, and modulation of inflammatory gene expression within allografts.

CONCLUSIONS

Induction with sc28AT promotes early cardiac allograft protection in hu5C8-treated NHPs. These results support further investigation of prolonged selective CD28 inhibition with CD40/CD154 blockade in NHP transplants.

Replacement of mycophenolate mofetil with a JAK inhibitor, AS2553627, in combination with low-dose tacrolimus, for renal allograft rejection in non-human primates

In renal transplant patients, using mycophenolate mofetil (MMF) with calcineurin inhibitors (CNIs; cyclosporine and tacrolimus [TAC]) has led to a significant improvement in graft survival. However, reducing or withholding MMF due to its gastrointestinal adverse events increases rejection risk. CNI-sparing strategies are important to avoid CNI-related nephrotoxicity in clinical settings. Here, we investigated AS2553627, a JAK inhibitor replacing MMF in combination with a sub-therapeutic dose of TAC to treat allograft rejection in a monkey model. AS2553627 inhibited proliferation of IL-2 stimulated T cells with little species difference between monkeys and humans. In MMF monotherapy, oral administration of 20 or 40 mg/kg/day prolonged graft survival with median survival times (MSTs) of 16.5 days and 33 days, respectively, whereas untreated animals showed MST of 6 days. In MMF/TAC (1 mg/kg/day, p.o.) combination therapy, pharmacokinetic analysis indicated that MMF 20 mg/kg/day achieved the clinical target AUC_0-24h and prolonged renal allograft survival, with MST of 24 days. Oral administration of AS2553627 0.24 mg/kg/day in combination with TAC significantly prolonged renal allograft survival to MST of >90 days with low plasma creatinine levels. Histopathological analysis revealed that acute T cell-mediated rejection events such as vasculitis and interstitial mononuclear cell infiltration were significantly inhibited in AS2553627/TAC-treated allografts compared with MMF/TAC-treated allografts. All AS2553627/TAC-treated monkeys surviving >90 days exhibited less interstitial fibrosis/tubular atrophy than monkeys in the MMF/TAC group. These results suggest that AS2553627 replacing MMF is an attractive CNI-sparing strategy to prevent renal allograft rejection.

Pilot Study of Delayed ICOS/ICOS-L Blockade With αCD40 to Modulate Pathogenic Alloimmunity in a Primate Cardiac Allograft Model

Supplemental digital content is available in the text.

Background

Inducible costimulator (ICOS) is rapidly upregulated with T-cell stimulation and may represent an escape pathway for T-cell costimulation in the setting of CD40/CD154 costimulation blockade. Induction treatment exhibited no efficacy in a primate renal allograft model, but rodent transplant models suggest that the addition of delayed ICOS/ICOS-L blockade may prolong allograft survival and prevent chronic rejection. Here, we ask whether ICOS-Ig treatment, timed to anticipate ICOS upregulation, prolongs NHP cardiac allograft survival or attenuates pathogenic alloimmunity.

Methods

Cynomolgus monkey heterotopic cardiac allograft recipients were treated with αCD40 (2C10R4, d0-90) either alone or with the addition of delayed ICOS-Ig (d63-110).

Results

Median allograft survival was similar between ICOS-Ig + αCD40 (120 days, 120-125 days) and αCD40 (124 days, 89-178 days) treated animals, and delayed ICOS-Ig treatment did not prevent allograft rejection in animals with complete CD40 receptor coverage. Although CD4⁺ T_EM cells were decreased in peripheral blood (115 ± 24) and mLNs (49 ± 1.9%) during ICOS-Ig treatment compared with monotherapy (214 ± 27%, P = 0.01; 72 ± 9.9%, P = 0.01, respectively), acute and chronic rejection scores and kinetics of alloAb elaboration were similar between groups.

Conclusions

Delayed ICOS-Ig treatment with the reagent tested is probably ineffective in modulating pathogenic primate alloimmunity in this model.

Prevalence of polyreactive innate clones among graft--infiltrating B cells in human cardiac allograft vasculopathy

BACKGROUND

Cardiac allograft vasculopathy (CAV) has been associated with graft-infiltrating B cells, although their characteristics are still unclear. In this study we examined the frequency, localization and reactivity profile of graft-infiltrating B cells to determine their contribution to the pathophysiology of CAV.

METHODS

B cells, plasma cells and macrophages were examined by immunohistochemistry in 56 allografts with CAV, 49 native failed hearts and 25 autopsy specimens. A total of 102 B-cell clones were immortalized directly from the infiltrates of 3 fresh cardiac samples with CAV. Their secreted antibodies were assessed using enzyme-linked immunoassay and flow cytometry.

RESULTS

B-cell infiltration was observed around coronary arteries in 93% of allograft explants with CAV. Comparatively, intragraft B cells were less frequent and less dense in the intraventricular myocardium from where routine biopsies are obtained. Plasma cells and macrophages were also detected in 85% and 95% of explants, respectively. Remarkably, B-cell infiltrates were not associated with circulating donor-specific antibodies (DSA) or prior episodes of antibody-mediated rejection (AMR). Among all B-cell clones generated from 3 explants with CAV, a majority secreted natural antibodies reactive to multiple autoantigens and apoptotic cells, a characteristic of innate B cells.

CONCLUSIONS

Our study reveals a high frequency of infiltrating B cells around the coronary arteries of allografts with CAV, independent of DSA or AMR. These cells are enriched for innate B cells with a polyreactive profile. The findings shift the focus from conventional DSA-producing B cells to the potentially pathogenic polyreactive B cells in the development of clinical CAV.

Comparative Evaluation of αCD40 (2C10R4) and αCD154 (5C8H1 and IDEC-131) in a Nonhuman Primate Cardiac Allotransplant Model

BACKGROUND

Specific blockade of T cell costimulation pathway is a promising immunomodulatory approach being developed to replace our current clinical immunosuppression therapies. The goal of this study is to compare results associated with 3 monoclonal antibodies directed against the CD40/CD154 T cell costimulation pathway.

METHODS

Cynomolgus monkey heterotopic cardiac allograft recipients were treated with either IDEC-131 (humanized αCD154, n = 9), 5C8H1 (mouse-human chimeric αCD154, n = 5), or 2C10R4 (mouse-rhesus chimeric αCD40, n = 6) monotherapy using a consistent, comparable dosing regimen for 3 months after transplant.

RESULTS

Relative to the previously reported IDEC-131-treated allografts, median survival time (35 ± 31 days) was significantly prolonged in both 5C8H1-treated (142 ± 26, P < 0.002) and 2C10R4-treated (124 ± 37, P < 0.020) allografts. IDEC-131-treated grafts had higher cardiac allograft vasculopathy severity scores during treatment relative to either 5C8H1 (P = 0.008) or 2C10R4 (P = 0.0002). Both 5C8H1 (5 of 5 animals, P = 0.02) and 2C10R4 (6/6, P = 0.007), but not IDEC-131 (2/9), completely attenuated IgM antidonor alloantibody (alloAb) production during treatment; 5C8H1 (5/5) more consistently attenuated IgG alloAb production compared to 2C10R4 (4/6) and IDEC-131 (0/9). All evaluable explanted grafts experienced antibody-mediated rejection. Only 2C10R4-treated animals exhibited a modest, transient drop in CD20 lymphocytes from baseline at day 14 after transplant (-457 ± 152 cells/μL) compared with 5C8H1-treated animals (16 ± 25, P = 0.037), and the resurgent B cells were primarily of a naive phenotype.

CONCLUSIONS

In this model, CD154/CD40 axis blockade using IDEC-131 is an inferior immunomodulatory treatment than 5C8H1 or 2C10R4, which have similar efficacy to prolong graft survival and to delay cardiac allograft vasculopathy development and antidonor alloAb production during treatment.

Renal Allograft Survival in Nonhuman Primates Infused With Donor Antigen-Pulsed Autologous Regulatory Dendritic Cells

Systemic administration of autologous regulatory dendritic cells (DCreg; unpulsed or pulsed with donor antigen [Ag]), prolongs allograft survival and promotes transplant tolerance in rodents. Here, we demonstrate that nonhuman primate (NHP) monocyte-derived DCreg preloaded with cell membrane vesicles from allogeneic peripheral blood mononuclear cells induce T cell hyporesponsiveness to donor alloantigen (alloAg) in vitro. These donor alloAg-pulsed autologous DCreg (1.4-3.6 × 10⁶ /kg) were administered intravenously, 1 day before MHC-mismatched renal transplantation to rhesus monkeys treated with costimulation blockade (cytotoxic T lymphocyte Ag 4 immunoglobulin [CTLA4] Ig) and tapered rapamycin. Prolongation of graft median survival time from 39.5 days (no DCreg infusion; n = 6 historical controls) and 29 days with control unpulsed DCreg (n = 2), to 56 days with donor Ag-pulsed DCreg (n = 5) was associated with evidence of modulated host CD4⁺ and CD8⁺ T cell responses to donor Ag and attenuation of systemic IL-17 production. Circulating anti-donor antibody (Ab) was not detected until CTLA4 Ig withdrawal. One monkey treated with donor Ag-pulsed DCreg rejected its graft in association with progressively elevated anti-donor Ab, 525 days posttransplant (160 days after withdrawal of immunosuppression). These findings indicate a modest but not statistically significant beneficial effect of donor Ag-pulsed autologous DCreg infusion on NHP graft survival when administered with a minimal immunosuppressive drug regimen.

What's Hot, What's New From the 2016 American Transplant Congress

From June 11-15, 2016 the American Transplant Congress, the joint meeting of the American Society of Transplantation and the American Society of Transplant Surgeons, held its annual meeting in Boston, MA. The meeting, attended by 5200 registrants, included pre-meeting conferences, focused topic sessions, and hundreds of high-quality presentations from the transplant field. This meeting report highlights key findings from specific basic science, translational, and clinical research presentations deemed to have notable impact in thematic areas. In particular, there were a number of transformative studies indicating important advances in the understanding of alloimmunity, chronic rejection, tolerance, and organ-specific outcomes. Many of these results are discussed in the context of the published literature to showcase rapid advances in the transplant field.

Inflammation Causes Resistance to Anti-CD20-Mediated B Cell Depletion

B cells play a central role in antibody-mediated rejection and certain autoimmune diseases. However, B cell-targeted therapy such as anti-CD20 B cell-depleting antibody (aCD20) has yielded mixed results in improving outcomes. In this study, we investigated whether an accelerated B cell reconstitution leading to aCD20 depletion resistance could account for these discrepancies. Using a transplantation model, we found that antigen-independent inflammation, likely through toll-like receptor (TLR) signaling, was sufficient to mitigate B cell depletion. Secondary lymphoid organs had a quicker recovery of B cells when compared to peripheral blood. Inflammation altered the pharmacokinetics (PK) and pharmacodynamics (PD) of aCD20 therapy by shortening drug half-life and accelerating the reconstitution of the peripheral B cell pool by bone marrow-derived B cell precursors. IVIG (intravenous immunoglobulin) coadministration also shortened aCD20 drug half-life and led to accelerated B cell recovery. Repeated aCD20 dosing restored B cell depletion and delayed allograft rejection, especially B cell-dependent, antibody-independent allograft rejection. These data demonstrate the importance of further clinical studies of the PK/PD of monoclonal antibody treatment in inflammatory conditions. The data also highlight the disconnect between B cell depletion on peripheral blood compared to secondary lymphoid organs, the deleterious effect of IVIG when given with aCD20 and the relevance of redosing of aCD20 for effective B cell depletion in alloimmunity.

Preemptive CD20+ B cell Depletion Attenuates Cardiac Allograft Vasculopathy in CD154-Treated Monkeys

BACKGROUND

Anti-CD154 monotherapy is associated with antidonor allo-antibody (Ab) elaboration, cardiac allograft vasculopathy (CAV), and allograft failure in preclinical primate cell and organ transplant models. In the context of calcineurin inhibitors (CNI), these pathogenic phenomena are delayed by preemptive "induction" B cell depletion.

METHODS

αCD154 (IDEC-131)-treated cynomolgus monkey heart allograft recipients were given peritransplant rituximab (αCD20) alone or with rabbit antihuman thymocyte globulin.

RESULTS

Relative to previously reported reference groups, αCD20 significantly prolonged survival, delayed Ab detection, and attenuated CAV within 3 months in αCD154-treated recipients (αCD154 + αCD20 graft median survival time > 90 days, n = 7, vs 28 days for αCD154 alone (IDEC-131), n = 21; P = 0.05). Addition of rabbit antihuman thymocyte globulin to αCD154 (n = 6) or αCD154 + αCD20 (n = 10) improved graft protection from graft rejection and failure during treatment but was associated with significant morbidity in 8 of 16 recipients (6 infections, 2 drug-related complications). In αCD20-treated animals, detection of antidonor Ab and relatively severe CAV were anticipated by appearance of CD20 cells (>1% of lymphocytes) in peripheral blood and were associated with low αCD154 trough levels (below 100 μg/mL).

CONCLUSIONS

These observations support the hypothesis that efficient preemptive "induction" CD20 B cell depletion consistently modulates pathogenic alloimmunity and attenuates CAV in this translational model, extending our prior findings with calcineurin inhibitors to the context of CD154 blockade.

Regulatory T Cell Infusion Can Enhance Memory T Cell and Alloantibody Responses in Lymphodepleted Nonhuman Primate Heart Allograft Recipients

The ability of regulatory T cells (Treg) to prolong allograft survival and promote transplant tolerance in lymphodepleted rodents is well established. Few studies, however, have addressed the therapeutic potential of adoptively transferred, CD4(+) CD25(+) CD127(-) Foxp3(+) (Treg) in clinically relevant large animal models. We infused ex vivo-expanded, functionally stable, nonselected Treg (up to a maximum cumulative dose of 1.87 billion cells) into antithymocyte globulin-lymphodepleted, MHC-mismatched cynomolgus monkey heart graft recipients before homeostatic recovery of effector T cells. The monkeys also received tacrolimus, anti-interleukin-6 receptor monoclonal antibodies and tapered rapamycin maintenance therapy. Treg administration in single or multiple doses during the early postsurgical period (up to 1 month posttransplantation), when host T cells were profoundly depleted, resulted in inferior graft function compared with controls. This was accompanied by increased incidences of effector memory T cells, enhanced interferon-γ production by host CD8(+) T cells, elevated levels of proinflammatory cytokines, and antidonor alloantibodies. The findings caution against infusion of Treg during the early posttransplantation period after lymphodepletion. Despite marked but transient increases in Treg relative to endogenous effector T cells and use of reputed "Treg-friendly" agents, the host environment/immune effector mechanisms instigated under these conditions can perturb rather than favor the potential therapeutic efficacy of adoptively transferred Treg.

Elevated immune monitoring as measured by increased adenosine triphosphate production in activated lymphocytes is associated with accelerated development of cardiac allograft vasculopathy after cardiac transplantation

BACKGROUND

Elevated immune monitoring (IM), as measured by adenosine triphosphate (ATP) release from activated lymphocytes, has been suggested to represent an under-immunosuppressed state. Its association with the development of angiographic cardiac allograft vasculopathy (CAV) is unknown.

METHODS

Patients transplanted between January 2007 and December 2011 with annual angiograms and at least 1 IM assay were included in the analysis. Peak IM scores were determined for each patient. Patients with peak IM in the highest quartile (Group 2) were compared with those with scores in the lower quartiles (Group 1). Mild disease was scored as Grade 1 (CAV1) and moderate or severe disease was scored as Grades 2 or 3 (CAV2/3).

RESULTS

Two hundred forty patients were included. The mean age at transplant was 54.2 ± 12.1 years. Time to peak IM assay was 105.9 ± 44.1 days and average number of assays obtained per patient was 3.1 ± 1.8. Patients in the highest quartile (Group 2) had peak IM ≥446 ng ATP/ml. Mean clinical follow-up was 4.6 ± 1.7 years. CAV1 was observed in 86 of 180 (47.8%) patients in Group 1 and 39 of 60 (65.0%) in Group 2. Freedom from CAV1 was significantly lower in patients in Group 2 (log rank, p = 0.012). CAV2/3 occurred in 7 of 180 (3.7%) patients in Group 1 and 9 of 60 (15.0%) patients in Group 2. Freedom from CAV2/3 was significantly lower in patients in Group 2 (p = 0.003). In multivariate analysis elevated peak IM assay was still found to be associated with angiographic CAV (hazard ratio 1.647, confidence interval 1.020 to 2.661, p = 0.041).

CONCLUSION

Elevated peak IM, as measured by increased ATP production, in activated lymphocytes is associated with decreased freedom from angiographic CAV.

Interleukin-10 From Marginal Zone Precursor B-Cell Subset Is Required for Costimulatory Blockade-Induced Transplantation Tolerance

BACKGROUND

Blocking CD40-CD40L costimulatory signals induces transplantation tolerance. Although B-cell depletion prevents alloantibody formation, nonhumoral functions of B cells in tolerance have not been well characterized. We investigated whether specific subsets of B cell or B cell-derived interleukin (IL)-10 contribute to tolerance.

METHODS

Wild type C57BL/6, or B cell-specific interleukin (IL)-10 (CD19-Cre::IL-10) mice, received vascularized BALB/c cardiac allografts. BALB/c donor-specific splenocyte transfusion and anti-CD40L monoclonal antibody were used as tolerogen. B cells were depleted with antimouse CD20 monoclonal antibody. Various B-cell subsets were purified and characterized by flow cytometry, reverse transcription polymerase chain reaction, and adoptive transfer.

RESULTS

B-cell depletion prevented costimulatory blockade-induced allogeneic tolerance. Costimulatory blockade increased IL-10 in marginal zone precursor (MZP) B cells, but not other subsets. In particular, costimulatory blockade did not change other previously defined regulatory B-cell subsets (Breg), including CD5CD1d Breg or expression of TIM1 or TIM4 on these Breg or other Breg cell subsets. Costimulatory blockade also induced IL-21R expression in MZP B cells, and IL-21R MZP B cells expressed even more IL-10. B-cell depletion or IL-10 deficiency in B cells prevented tolerance in a cardiac allograft model, resulting in rapid acute cardiac allograft rejection. Adoptive transfer of wild type MZP B cells but not other subsets to B cell-specific IL-10 deficient mice prevented graft rejection.

CONCLUSIONS

CD40 costimulatory blockade induces MZP B cell IL-10 which is necessary for tolerance. These observations have implications for understanding tolerance induction and how B cell depletion may prevent tolerance.

Combined induction therapy with rabbit antithymocyte globulin and rituximab in highly sensitized renal recipients

Compared to non-sensitized renal transplant recipients, patients with preformed alloantibodies are at greater risk of cellular and humoral rejection and premature graft failure. We explored the effects of adding B-cell depleting agent (rituximab) to standard rabbit anti-thymocyte globulin (rATG) induction regimen for patients with panel reactive antibody levels >50%. Following induction therapy, 14 recipients were given two doses of rituximab (375 mg/m(2)) within the first month post-transplantation. Their long-term outcomes were compared to a historical control group of 23 recipients who received rATG alone. Graft survival at 5 years was superior with combination therapy compared to induction therapy alone (92.9 versus 48.3%, respectively, p = 0.02). While 30% of the rATG alone group experienced cellular rejection and 26% humoral rejection, none of rituximab plus rATG renal transplant recipients group had rejection. Thus, addition of rituximab to rATG provided superior outcomes to rATG alone. This combination induction therapy should be considered for a high-risk population.

From immunosuppression to tolerance

The past three decades have seen liver transplantation becoming a major therapeutic approach in the management of end-stage liver diseases. This is due to the dramatic improvement in survival after liver transplantation as a consequence of the improvement of surgical and anaesthetic techniques, of post-transplant medico-surgical management and of prevention of disease recurrence and other post-transplant complications. Improved use of post-transplant immunosuppression to prevent acute and chronic rejection is a major factor in these improved results. The liver has been shown to be more tolerogenic than other organs, and matching of donor and recipients is mainly limited to ABO blood group compatibility. However, long-term immunosuppression is required to avoid severe acute and chronic rejection and graft loss. With the current immunosuppression protocols, the risk of acute rejection requiring additional therapy is 10-40% and the risk of chronic rejection is below 5%. However, the development of histological lesions in the graft in long-term survivors suggest atypical forms of graft rejection may develop as a consequence of under-immunosuppression. The backbone of immunosuppression remains calcineurin inhibitors (CNI) mostly in association with steroids in the short-term and mycophenolate mofetil or mTOR inhibitors (everolimus). The occurrence of post-transplant complications related to the immunosuppressive therapy has led to the development of new protocols aimed at protecting renal function and preventing the development of de novo cancer and of dysmetabolic syndrome. However, there is no new class of immunosuppressive drugs in the pipeline able to replace current protocols in the near future. The aim of a full immune tolerance of the graft is rarely achieved since only 20% of selected patients can be weaned successfully off immunosuppression. In the future, immunosuppression will probably be more case oriented aiming to protect the graft from rejection and at reducing the risk of disease recurrence and complications related to immunosuppressive therapy. Such approaches will include strategies aiming to promote stable long-term immunological tolerance of the liver graft.

Advances and challenges in immunotherapy for solid organ and hematopoietic stem cell transplantation

Although major advances have been made in solid organ and hematopoietic stem cell transplantation in the last 50 years, big challenges remain. This review outlines the current immunological limitations for hematopoietic stem cell and solid organ transplantation and discusses new immune-modulating therapies in preclinical development and in clinical trials that may allow these obstacles to be overcome.

Heart transplantation: challenges facing the field

There has been significant progress in the field of heart transplantation over the last 45 years. The 1-yr survival rates following heart transplantation have improved from 30% in the 1970s to almost 90% in the 2000s. However, there has been little change in long-term outcomes. This is mainly due to chronic rejection, malignancy, and the detrimental side effects of chronic immunosuppression. In addition, over the last decade, new challenges have arisen such as increasingly complicated recipients and antibody-mediated rejection. Most, if not all, of these obstacles to long-term survival could be prevented or ameliorated by the induction of transplant tolerance wherein the recipient's immune system is persuaded not to mount a damaging immune response against donor antigens, thus eliminating the need for chronic immunosuppression. However, the heart, as opposed to other allografts like kidneys, appears to be a tolerance-resistant organ. Understanding why organs like kidneys and livers are prone to tolerance induction, whereas others like hearts and lungs are tolerance-resistant, could aid in our attempts to achieve long-term, immunosuppression-free survival in human heart transplant recipients. It could also advance the field of pig-to-human xenotransplantation, which, if successful, would eliminate the organ shortage problem. Of course, there are alternative futures to the field of heart transplantation that may include the application of total mechanical support, stem cells, or bioengineered whole organs. Which modality will be the first to reach the ultimate goal of achieving unlimited, long-term, circulatory support with minimal risk to longevity or lifestyle is unknown, but significant progress in being made in each of these areas.

B cells mediate chronic allograft rejection independently of antibody production

Chronic rejection is the primary cause of long-term failure of transplanted organs and is often viewed as an antibody-dependent process. Chronic rejection, however, is also observed in mice and humans with no detectable circulating alloantibodies, suggesting that antibody-independent pathways may also contribute to pathogenesis of transplant rejection. Here, we have provided direct evidence that chronic rejection of vascularized heart allografts occurs in the complete absence of antibodies, but requires the presence of B cells. Mice that were deficient for antibodies but not B cells experienced the same chronic allograft vasculopathy (CAV), which is a pathognomonic feature of chronic rejection, as WT mice; however, mice that were deficient for both B cells and antibodies were protected from CAV. B cells contributed to CAV by supporting splenic lymphoid architecture, T cell cytokine production, and infiltration of T cells into graft vessels. In chimeric mice, in which B cells were present but could not present antigen, both T cell responses and CAV were markedly reduced. These findings establish that chronic rejection can occur in the complete absence of antibodies and that B cells contribute to this process by supporting T cell responses through antigen presentation and maintenance of lymphoid architecture.

B-cell depletion improves islet allograft survival with anti-CD45RB

A short course of anti-CD45RB leads to long-term islet allograft survival and donor-specific tolerance in approximately half of immunocompetent mice. We have previously demonstrated that anti-CD45RB antibody-mediated tolerance requires B-cells for cardiac allograft survival. We therefore asked whether B-cells were also required for anti-CD45RB antibody-mediated survival of islets. Unexpectedly, we found that nearly 100% of islet allografts survive long term in B-cell-deficient mice. Similarly, B-cell depletion by anti-CD22/cal augmented anti-CD45RB-mediated tolerance when administered pretransplant, although it had no effect on tolerance induction when administered posttransplant. Our results demonstrate that the role of B-cells in promoting tolerance with anti-CD45RB is graft specific, promoting tolerance in cardiac grafts but resisting tolerance in islet transplantation. These findings may help elucidate the varied action of B-cells in promoting tolerance versus rejection.

Costimulation blockade alters germinal center responses and prevents antibody-mediated rejection

De novo donor-specific antibody (DSA) after organ transplantation promotes antibody-mediated rejection (AMR) and causes late graft loss. Previously, we demonstrated that depletion using anti-CD3 immunotoxin combined with tacrolimus and alefacept (AMR regimen) reliably induced early DSA production with AMR in a nonhuman primate kidney transplant model. Five animals were assigned as positive AMR controls, four received additional belatacept and four received additional anti-CD40 mAb (2C10R4). Notably, production of early de novo DSA was completely attenuated with additional belatacept or 2C10R4 treatment. In accordance with this, while positive controls experienced a decrease in peripheral IgM(+) B cells, bela- and 2C10R4-added groups maintained a predominant population of IgM(+) B cells, potentially indicating decreased isotype switching. Central memory T cells (CD4(+) CD28(+) CD95(+)) as well as PD-1(hi) CD4(+) T cells were decreased in both bela-added and 2C10R4-added groups. In analyzing germinal center (GC) reactions in situ, lymph nodes further revealed a reduction of B cell clonal expansion, GC-follicular helper T (Tfh) cells, and IL-21 production inside GCs with additional belatacept or 2C10R4 treatment. Here we provide evidence that belatacept and 2C10R4 selectively suppresses the humoral response via regulating Tfh cells and prevents AMR in this nonhuman primate model.

Evolving concepts and treatment strategies for cardiac allograft vasculopathy

OPINION STATEMENT

The central event in the development of allograft vasculopathy is the inflammatory response to immune-mediated and nonimmune-mediated endothelial damage. This response is characterized by the release of inflammatory cytokines, upregulation of cell-surface adhesion molecules, and subsequent binding of leukocytes. Growth factors stimulate smooth muscle cell proliferation and circulating progenitor cells are recruited to sites of arterial injury leading to neointima formation. Because of its diffuse nature, intravascular ultrasound is more sensitive than angiography for early diagnosis. Proliferation signal inhibitors (PSIs) have the capacity to slow vasculopathy progression by inhibiting smooth muscle cell proliferation, but its side effects profile makes its use as a first line agent difficult. Retransplantation is still the only definitive therapy but is available only in selected cases. The current hope is that immunomodulation at the time of transplant could induce long-term tolerance and graft accommodation, leading to less vasculopathy.

Mechanisms and rescue strategies of calcineurin inhibitor mediated tolerance abrogation induced by anti-CD4 mAb treatment

To ensure safety tolerance induction protocols are accompanied by conventional immunosuppressive drugs (IS). But IS such as calcineurin inhibitors (CNI), for example, cyclosporin A (CsA), can interfere with tolerance induction. We investigated the effect of an additional transient CsA treatment on anti-CD4mAb-induced tolerance induction upon rat kidney transplantation. Additional CsA treatment induced deteriorated graft function, resulting in chronic rejection characterized by glomerulosclerosis, interstitial fibrosis, tubular atrophy and vascular changes. Microarray analysis revealed enhanced intragraft expression of the B cell attracting chemokine CXCL13 early during CsA treatment. Increase in CXCL13 expression is accompanied by enhanced B cell infiltration with local and systemic IgG production and C3d deposition as early as 5 days upon CsA withdrawal. Adding different CNIs to cultures of primary mesangial cells isolated from glomeruli resulted in a concentration-dependent increase in CXCL13 transcription. CsA in synergy with TNF-α can enhance the B cell attracting and activating potential of mesangial cells. Transient B cell depletion or transfer of splenocytes from tolerant recipients 3 weeks after transplantation could rescue tolerance induction and did inhibit intragraft B cell accumulation, alloantibody production and ameliorate chronic rejection.

Transient B-cell depletion combined with apoptotic donor splenocytes induces xeno-specific T- and B-cell tolerance to islet xenografts

Peritransplant infusion of apoptotic donor splenocytes cross-linked with ethylene carbodiimide (ECDI-SPs) has been demonstrated to effectively induce allogeneic donor-specific tolerance. The objective of the current study is to determine the effectiveness and additional requirements for tolerance induction for xenogeneic islet transplantation using donor ECDI-SPs. In a rat-to-mouse xenogeneic islet transplant model, we show that rat ECDI-SPs alone significantly prolonged islet xenograft survival but failed to induce tolerance. In contrast to allogeneic donor ECDI-SPs, xenogeneic donor ECDI-SPs induced production of xenodonor-specific antibodies partially responsible for the eventual islet xenograft rejection. Consequently, depletion of B cells prior to infusions of rat ECDI-SPs effectively prevented such antibody production and led to the indefinite survival of rat islet xenografts. In addition to controlling antibody responses, transient B-cell depletion combined with ECDI-SPs synergistically suppressed xenodonor-specific T-cell priming as well as memory T-cell generation. Reciprocally, after initial depletion, the recovered B cells in long-term tolerized mice exhibited xenodonor-specific hyporesponsiveness. We conclude that transient B-cell depletion combined with donor ECDI-SPs is a robust strategy for induction of xenodonor-specific T- and B-cell tolerance. This combinatorial therapy may be a promising strategy for tolerance induction for clinical xenogeneic islet transplantation.

Primate models in organ transplantation

Large animal models have long served as the proving grounds for advances in transplantation, bridging the gap between inbred mouse experimentation and human clinical trials. Although a variety of species have been and continue to be used, the emergence of highly targeted biologic- and antibody-based therapies has required models to have a high degree of homology with humans. Thus, the nonhuman primate has become the model of choice in many settings. This article will provide an overview of nonhuman primate models of transplantation. Issues of primate genetics and care will be introduced, and a brief overview of technical aspects for various transplant models will be discussed. Finally, several prominent immunosuppressive and tolerance strategies used in primates will be reviewed.

Lymphodepletional strategies in transplantation

Because lymphocytes were shown to mediate transplant rejection, their depletion has been studied as a mechanism of preventing rejection and perhaps inducing immunologic tolerance. Agents that profoundly deplete lymphocytes have included monoclonal antibodies, cytotoxic drugs, and radiation. We have studied several such agents but focused on antibodies that deplete not only peripheral blood lymphocytes, but also lymph node lymphocytes. Depletion of lymph node T lymphocytes appears to permit peripheral tolerance at least for T cells in animal models. Nevertheless, B-cell responses may be resistant to such approaches, and T memory cells are likewise relatively resistant to depleting antibodies. We review the experimental and clinical approaches to depletion strategies and outline some of the pitfalls of depletion, such as limitations of currently available agents, duration of tolerance, infection, and malignancy. It is notable that most tolerogenic strategies that have been attempted experimentally and clinically include depleting agents even when they are not named as the underlying strategy. Thus, there is an implicitly acknowledged role for reducing the precursor frequency of donor antigen-specific lymphocytes when approaching the daunting goal of transplant tolerance.

Novel functions of B cells in transplantation

PURPOSE OF REVIEW

This manuscript reviews current knowledge and recent findings regarding antibody-independent functions of B cells in transplantation.

RECENT FINDINGS

Until recently the functions of B cells in transplantation have been attributed almost entirely to the antibodies they produce. However, the results of recent trials of B-cell-depleting agents for treatment of antibody-mediated rejection as well as auto-immune disease raised awareness that B cells mediate functions independent of antibody synthesis.

SUMMARY

These 'nonclassical' functions place B cells at the center of immune regulation with the power to enhance or inhibit immunity.

Adaptive transfer of B10 cells: a novel therapy for chronic rejection after solid organ transplantation

Chronic rejection occurs between almost all MHC-mismatched donors and recipients after transplantation. Immunosuppressive agents have been administrated indiscriminately to manage potential rejection, but complications from lifelong immunosuppressive therapy threaten transplant recipients. Recent studies demonstrated that a number of regulatory B cells (B10 cells) negatively regulate T cell mediated immune responses without inducing systemic immune suppression. Therefore, we propose that adaptive transfer of B10 cells suppresses alloreactive CD8(+) cytotoxic T cell activation induced by allogeneic solid organ transplantation, reduces T cell mediated rejection and prolongs allograft survival.

The paradoxical functions of B cells and antibodies in transplantation

Scarcely anyone would dispute that donor-specific B cells and the Abs that they produce can cause rejection of transplants. Less clear and more controversial, however, is the possibility that donor-specific B cells and the Abs that they produce are one or more means by which transplants can be protected from injury. In this article, we review and discuss this possibility and consider how less well-known functions of B cells and Abs might impact on the design of therapeutics and the management of transplant recipients.

Donor-specific antibodies to class II antigens are associated with accelerated cardiac allograft vasculopathy: a three-dimensional volumetric intravascular ultrasound study

BACKGROUND

Although a link between donor-specific antibodies against human leukocyte antigens type II (DSA II+) and transplant glomerulopathy has been clearly established, its role in cardiac allograft vasculopathy (CAV) is unclear.

METHODS

Donor-specific antibodies were evaluated using solid-phase single-antigen bead assay before transplantation in 51 heart transplant recipients. Coronary angiography and three-dimensional intravascular ultrasound were performed at baseline and approximately 1 year after the baseline examination.

RESULTS

There were 4 (7.8 %), 11 (21.5%), and 2 (3.9%) patients who had DSA against donor class I (DSA I+), DSA II+, or both, respectively. All patients had negative complement-dependent cytotoxic crossmatch. There was accelerated progression of CAV in the DSA II+ group demonstrated by accelerated progression in plaque index (plaque volume/vessel volume) compared to patients with no DSA II+ antibodies (13.8% [12%] vs. -7.9% [37%], P=0.01). The development of any angiographic CAV was also more common in DSA II+ patients as compared to the DSA- patients at 4 years (100% [0%] vs. 64.2% [10%], P=0.05). All other traditional risk factors for CAV or immunosuppression were similar between the groups (P>0.2 for all).

CONCLUSIONS

This is the first preliminary study demonstrating that heart transplant recipients with preformed class II DSA may be at an increased risk for accelerated CAV as detected by consecutive volumetric three-dimensional intravascular ultrasound.

Humoral theory of transplantation: some hot topics

INTRODUCTION

Antibody is a major cause of allograft injury. However, it has not been routinely tested post-transplant.

SOURCES OF DATA

A literature search was performed using PubMed on the topics of 'antibody monitoring', 'autoantibody and allograft dysfunction' and 'prevention and treatment of antibody-mediated rejection (AMR)'.

AREAS OF AGREEMENT

Donor-specific antibody (DSA) monitoring not only helps to identify patients at risk of AMR, but also serves as a biomarker to personalize patient's maintenance immunosuppression. Development of autoantibody is a secondary response following primary tissue injury. Some autoantibodies are directly involved in allograft injury, while others only serve as biomarkers of tissue injury.

AREAS OF CONTROVERSY

It remains controversial whether DSA-positive patients without symptoms need to be treated. In addition, given the variation in study designs and patient's characteristics, there is discrepancy regarding which treatment regimens provide optimal clinical outcome in preventing/treating AMR.

GROWING POINTS

Efficacy of B-cell and/or antibody-targeted therapies in treating or preventing AMR would be better measured by the incorporation of antibody monitoring into current functional and pathological assays.

AREAS TIMELY FOR DEVELOPING RESEARCH

Research in B-cell targeted therapies to prevent and treat AMR is rapidly growing, which includes monoclonal antibodies against B-cell markers CD20, CD40, CD19, BlyS, etc. It requires extensive clinical research to determine the best approach to inhibit or delete antibody and how to balance the drug efficacy with safety.

Endothelial dysfunction and cardiac allograft vasculopathy

Cardiac allograft vasculopathy remains a major challenge to long-term survival after heart transplantation. Endothelial injury and dysfunction, as a result of multifactorial immunologic and nonimmunologic insults in the donor and the recipient, are prevalent early after transplant and may be precursors to overt cardiac allograft vasculopathy. Current strategies for managing cardiac allograft vasculopathy, however, rely on the identification and treatment of established disease. Improved understanding of mechanisms leading to endothelial dysfunction in heart transplant recipients may provide the foundation for the development of sensitive screening techniques and preventive therapies.

Enhanced de novo alloantibody and antibody-mediated injury in rhesus macaques

Chronic allograft rejection is a major impediment to long-term transplant success. Humoral immune responses to alloantigens are a growing clinical problem in transplantation, with mounting evidence associating alloantibodies with the development of chronic rejection. Nearly a third of transplant recipients develop de novo antibodies, for which no established therapies are effective at preventing or eliminating, highlighting the need for a nonhuman primate model of antibody-mediated rejection. In this report, we demonstrate that depletion using anti-CD3 immunotoxin (IT) combined with maintenance immunosuppression that included tacrolimus with or without alefacept reliably prolonged renal allograft survival in rhesus monkeys. In these animals, a preferential skewing toward CD4 repopulation and proliferation was observed, particularly with the addition of alefacept. Furthermore, alefacept-treated animals demonstrated increased alloantibody production (100%) and morphologic features of antibody-mediated injury. In vitro, alefacept was found to enhance CD4 effector memory T cell proliferation. In conclusion, alefacept administration after depletion and with tacrolimus promotes a CD4+memory T cell and alloantibody response, with morphologic changes reflecting antibody-mediated allograft injury. Early and consistent de novo alloantibody production with associated histological changes makes this nonhuman primate model an attractive candidate for evaluating targeted therapeutics.

Anti-CD45RB/anti-TIM-1-induced tolerance requires regulatory B cells

The role of B cells in transplant tolerance remains unclear. Although B-cell depletion often prolongs graft survival, sometimes it results in more rapid rejection, suggesting that B cells may have regulatory activity. We previously demonstrated that tolerance induction by anti-CD45RB antibody requires recipient B cells. Here, we show that anti-CD45RB in combination with anti-TIM-1 antibody has a synergistic effect, inducing tolerance in all recipients in a mouse islet allograft model. This effect depends on the presence of recipient B cells, requires B-cell IL-10 activity, and is antigen-specific. These data suggest the existence of a regulatory B-cell population that promotes tolerance via an IL-10-dependent pathway.

Tolerogenic therapies in transplantation

Since the concept of immunologic tolerance was discovered in the 1940s, the pursuit of tolerance induction in human transplantation has led to a rapid development of pharmacologic and biologic agents. Short-term graft survival remains an all-time high, but successful withdrawal of immunosuppression to achieve operational tolerance rarely occurs outside of liver transplantation. Collaborative efforts through the NIH sponsored Immune Tolerance Network and the European Commission sponsored Reprogramming the Immune System for Establishment of Tolerance consortia have afforded researchers opportunity to evaluate the safety and efficacy of tolerogenic strategies, investigate mechanisms of tolerance, and identify molecular and genetic markers that distinguish the tolerance phenotype. In this article, we review traditional and novel approaches to inducing tolerance for organ transplantation, with an emphasis on their translation into clinical trials.

Essential role for B cells in transplantation tolerance

T lymphocytes are the primary targets of immunotherapy in clinical transplantation. However, B lymphocytes are detrimental to graft survival by virtue of their capacity to present antigen to T cells via the indirect pathway of allorecognition and the generation of donor specific alloantibody. Furthermore, the long-term survival of organ allografts remains challenged by chronic rejection, a process in which activated B cells have been found to play a significant role. Therefore, the achievement of transplantation tolerance will likely require induction of both T and B cell tolerance to alloantigens. Moreover, human and animal investigations have shown that subsets of B cells, Transitional and Regulatory, are inherently tolerogenic. Developing therapeutic strategies that exploit these populations may be key to achieving transplantation tolerance. In this review we describe the current evidence for the essential role of B cells in transplant tolerance and discuss emerging B cell directed strategies to achieve allograft tolerance.

Inotuzumab ozogamicin murine analog-mediated B-cell depletion reduces anti-islet allo- and autoimmune responses

B cells participate in the priming of the allo- and autoimmune responses, and their depletion can thus be advantageous for islet transplantation. Herein, we provide an extensive study of the effect of B-cell depletion in murine models of islet transplantation. Islet transplantation was performed in hyperglycemic B-cell-deficient(μMT) mice, in a purely alloimmune setting (BALB/c into hyperglycemic C57BL/6), in a purely autoimmune setting (NOD.SCID into hyperglycemic NOD), and in a mixed allo-/autoimmune setting (BALB/c into hyperglycemic NOD). Inotuzumab ozogamicin murine analog (anti-CD22 monoclonal antibody conjugated with calicheamicin [anti-CD22/cal]) efficiently depleted B cells in all three models of islet transplantation examined. Islet graft survival was significantly prolonged in B-cell-depleted mice compared with control groups in transplants of islets from BALB/c into C57BL/6 (mean survival time [MST]: 16.5 vs. 12.0 days; P = 0.004), from NOD.SCID into NOD (MST: 23.5 vs. 14.0 days; P = 0.03), and from BALB/c into NOD (MST: 12.0 vs. 5.5 days; P = 0.003). In the BALB/c into B-cell-deficient mice model, islet survival was prolonged as well (MST: μMT = 32.5 vs. WT = 14 days; P = 0.002). Pathology revealed reduced CD3(+) cell islet infiltration and confirmed the absence of B cells in treated mice. Mechanistically, effector T cells were reduced in number, concomitant with a peripheral Th2 profile skewing and ex vivo recipient hyporesponsiveness toward donor-derived antigen as well as islet autoantigens. Finally, an anti-CD22/cal and CTLA4-Ig-based combination therapy displayed remarkable prolongation of graft survival in the stringent model of islet transplantation (BALB/c into NOD). Anti-CD22/cal-mediated B-cell depletion promotes the reduction of the anti-islet immune response in various models of islet transplantation.

The role of B cells in solid organ transplantation

The role of antibodies in chronic injury to organ transplants has been suggested for many years, but recently emphasized by new data. We have observed that when immunosuppressive potency decreases either by intentional weaning of maintenance agents or due to homeostatic repopulation after immune cell depletion, the threshold of B cell activation may be lowered. In human transplant recipients the result may be donor-specific antibody, C4d+ injury, and chronic rejection. This scenario has precise parallels in a rhesus monkey renal allograft model in which T cells are depleted with CD3 immunotoxin, or in a CD52-T cell transgenic mouse model using alemtuzumab to deplete T cells. Such animal models may be useful for the testing of therapeutic strategies to prevent DSA. We agree with others who suggest that weaning of immunosuppression may place transplant recipients at risk of chronic antibody-mediated rejection, and that strategies to prevent this scenario are needed if we are to improve long-term graft and patient outcomes in transplantation. We believe that animal models will play a crucial role in defining the pathophysiology of antibody-mediated rejection and in developing effective therapies to prevent graft injury. Two such animal models are described herein.