B-cell-dependent memory T cells impede nonmyeloablative mixed chimerism induction in presensitized mice

Bioengineered omental transplant site promotes pancreatic islet allografts survival in non-human primates

The transplanting islets to the liver approach suffers from an immediate posttransplant loss of islets of more than 50%, progressive graft dysfunction over time, and precludes recovery of grafts should there be serious complications such as the development of teratomas with grafts that are stem cell-derived islets (SC-islets). The omentum features an attractive extrahepatic alternative site for clinical islet transplantation. We explore an approach in which allogeneic islets are transplanted onto the omentum, which is bioengineered with a plasma-thrombin biodegradable matrix in three diabetic non-human primates (NHPs). Within 1 week posttransplant, each transplanted NHP achieves normoglycemia and insulin independence and remains stable until termination of the experiment. Success was achieved in each case with islets recovered from a single NHP donor. Histology demonstrates robust revascularization and reinnervation of the graft. This preclinical study can inform the development of strategies for β cell replacement including the use of SC-islets or other types of novel cells in clinical settings.

Contrasting effects of B cell depletion on CD4+ and CD8+ memory T cell responses generated after transplantation

Alloreactive memory T cells play a key role in transplantation by accelerating allograft rejection and preventing tolerance induction. Some studies using µMT mice, which are constitutionally devoid of B cells, showed that B cells were required for the generation of memory T cells after allotransplantation. However, whether B cell depletion in normal adult mice has the same effect on memory responses by CD4⁺ and CD8⁺ T cells activated after transplantation has not been thoroughly investigated. In this study, we tested the effect of anti-CD20 antibody-mediated B cell depletion on CD4⁺ and CD8⁺ memory T cell alloresponses after skin transplantation in wild-type mice. We found that B cell depletion prevented the development of memory alloresponses by CD4⁺ T cells but enhanced that of CD8⁺ memory T cells. Next, we tested the influence of B cell depletion on hematopoietic chimerism. In OT-II CD4⁺ anti-OVA TCR transgenic mice sensitized to ovalbumin antigen, B cell depletion also impaired allospecific memory T cell responses and thereby enhanced donor hematopoietic chimerism and T cell deletion after bone marrow transplantation. This study underscores the complexity of the relationships between B and T cells in the generation and reactivation of different memory T cell subsets after transplantation.

Mechanisms of Mixed Chimerism-Based Transplant Tolerance

Immune responses to allografts represent a major barrier in organ transplantation. Immune tolerance to avoid chronic immunosuppression is a critical goal in the field, recently achieved in the clinic by combining bone marrow transplantation (BMT) with kidney transplantation following non-myeloablative conditioning. At high levels of chimerism such protocols can permit central deletional tolerance, but with a significant risk of graft-versus-host (GVH) disease (GVHD). By contrast, transient chimerism-based tolerance is devoid of GVHD risk and appears to initially depend on regulatory T cells (Tregs) followed by gradual, presumably peripheral, clonal deletion of donor-reactive T cells. Here we review recent mechanistic insights into tolerance and the development of more robust and safer protocols for tolerance induction that will be guided by innovative immune monitoring tools.

Macrochimerism in Intestinal Transplantation: Association With Lower Rejection Rates and Multivisceral Transplants, Without GVHD

Blood chimerism has been reported sporadically among visceral transplant recipients, mostly in association with graft-vs-host disease (GVHD). We hypothesized that a higher degree of mixed chimerism would be observed in multivisceral (MVTx) than in isolated intestinal (iITx) and isolated liver transplant (iLTx) recipients, regardless of GVHD. We performed a longitudinal prospective study investigating multilineage blood chimerism with flow cytometry in 5 iITx and 4 MVTx recipients up to one year posttransplant. Although only one iITx patient experienced GVHD, T cell mixed chimerism was detected in 8 out of 9 iITx/MVTx recipients. Chimerism was significantly lower in the four subjects who displayed early moderate to severe rejection. Pre-formed high-titer donor-specific antibodies, bound in vivo to the circulating donor cells, were associated with an accelerated decline in chimerism. Blood chimerism was also studied in 10 iLTx controls. Among nonsensitized patients, MVTx recipients exhibited greater T and B cell chimerism than either iITx or iLTx recipients. Myeloid lineage chimerism was present exclusively among iLTx and MVTx (6/13) recipients, suggesting that its presence required the hepatic allograft. Our study demonstrates, for the first time, frequent T cell chimerism without GVHD following visceral transplantation and a possible relationship with reduced rejection rate in MVTx recipients.

Tolerance of Lung Allografts Achieved in Nonhuman Primates via Mixed Hematopoietic Chimerism

While the induction of transient mixed chimerism has tolerized MHC-mismatched renal grafts in nonhuman primates and patients, this approach has not been successful for more immunogenic organs. Here, we describe a modified delayed-tolerance-induction protocol resulting in three out of four monkeys achieving long-term lung allograft survival without ongoing immunosuppression. Two of the tolerant monkeys displayed stable mixed lymphoid chimerism, and the other showed transient chimerism. Serial biopsies and post-mortem specimens from the tolerant monkeys revealed no signs of chronic rejection. The tolerant recipients also exhibited T cell unresponsiveness and a lack of alloantibody. This is the first report of durable mixed chimerism and successful tolerance induction of MHC-mismatched lungs in primates.

Optimising long-term graft survival: establishing the benefit of targeting B lymphocytes

Kidney transplants do not last for the natural lifespan of most recipients. Of the reasons why transplants fail, damage by the immune system is the commonest cause. Understanding how the immune system recognises transplanted organs has increased significantly in recent years, but there is little insight into how organs are damaged, and no still no way of suppressing immune-mediated damage without exposing patients to the detrimental effects of long-term immunosuppression. In this article, we review the role of antibodies and B cells in immune-mediated damage of kidney transplants, and discuss the potential for manipulation of B cells to improve clinical outcomes.

Desensitization for solid organ and hematopoietic stem cell transplantation

Desensitization protocols are being used worldwide to enable kidney transplantation across immunologic barriers, i.e. antibody to donor HLA or ABO antigens, which were once thought to be absolute contraindications to transplantation. Desensitization protocols are also being applied to permit transplantation of HLA mismatched hematopoietic stem cells to patients with antibody to donor HLA, to enhance the opportunity for transplantation of non-renal organs, and to treat antibody-mediated rejection. Although desensitization for organ transplantation carries an increased risk of antibody-mediated rejection, ultimately these transplants extend and enhance the quality of life for solid organ recipients, and desensitization that permits transplantation of hematopoietic stem cells is life saving for patients with limited donor options. Complex patient factors and variability in treatment protocols have made it difficult to identify, precisely, the mechanisms underlying the downregulation of donor-specific antibodies. The mechanisms underlying desensitization may differ among the various protocols in use, although there are likely to be some common features. However, it is likely that desensitization achieves a sort of immune detente by first reducing the immunologic barrier and then by creating an environment in which an autoregulatory process restricts the immune response to the allograft.

Bcl-2 inhibition to overcome memory cell barriers in transplantation

Memory T cells (Tm) represent a major barrier for immunosuppression and tolerance induction after solid organ transplantation. Taking into consideration the critical role of the intrinsic apoptosis pathway in the generation and maintenance of Tm, we developed a new concept to deplete alloreactive Tm by targeting Bcl-2 proteins. The small-molecule Bcl-2/Bcl-XL inhibitor ABT-737 efficiently induced apoptosis in alloreactive Tm in vitro and in vivo and prolonged skin graft survival in sensitized recipients. A short course of ABT-737 induction therapy prevented Tm-mediated resistance in a donor-specific transfusion model and allowed mixed chimerism induction across Tm barriers. Since Bcl-2 inhibitors yielded encouraging safety results in cancer trials, this novel approach might represent a substantial advance to prevent allograft rejection and induce tolerance in sensitized recipients.