Alloantibody production is regulated by CD4+ T cells' alloreactive pathway, rather than precursor frequency or Th1/Th2 differentiation

Pathways of Antigen Recognition by T Cells in Allograft Rejection

The adaptive immune response leading to the rejection of allogeneic transplants is initiated and orchestrated by recipient T cells recognizing donor antigens. T-cell allorecognition is mediated via 3 distinct mechanisms: the direct pathway in which T cells recognize allogeneic major histocompatibility complex (MHC) molecules on donor cells, the indirect pathway through which T cells interact with donor peptides bound with self-MHC molecules on recipient antigen-presenting cells, and the recently described semidirect pathway whereby T cells recognize donor MHC proteins on recipient antigen-presenting cells. In this article, we present a description of each of these allorecognition pathways and discuss their role in acute and chronic rejection of allogeneic transplants.

Designing biomaterials for the modulation of allogeneic and autoimmune responses to cellular implants in Type 1 Diabetes

The effective suppression of adaptive immune responses is essential for the success of allogeneic cell therapies. In islet transplantation for Type 1 Diabetes, pre-existing autoimmunity provides an additional hurdle, as memory autoimmune T cells mediate both an autoantigen-specific attack on the donor beta cells and an alloantigen-specific attack on the donor graft cells. Immunosuppressive agents used for islet transplantation are generally successful in suppressing alloimmune responses, but dramatically hinder the widespread adoption of this therapeutic approach and fail to control memory T cell populations, which leaves the graft vulnerable to destruction. In this review, we highlight the capacity of biomaterials to provide local and nuanced instruction to suppress or alter immune pathways activated in response to an allogeneic islet transplant. Biomaterial immunoisolation is a common approach employed to block direct antigen recognition and downstream cell-mediated graft destruction; however, immunoisolation alone still permits shed donor antigens to escape into the host environment, resulting in indirect antigen recognition, immune cell activation, and the creation of a toxic graft site. Designing materials to decrease antigen escape, improve cell viability, and increase material compatibility are all approaches that can decrease the local release of antigen and danger signals into the implant microenvironment. Implant materials can be further enhanced through the local delivery of anti-inflammatory, suppressive, chemotactic, and/or tolerogenic agents, which serve to control both the innate and adaptive immune responses to the implant with a benefit of reduced systemic effects. Lessons learned from understanding how to manipulate allogeneic and autogenic immune responses to pancreatic islets can also be applied to other cell therapies to improve their efficacy and duration. STATEMENT OF SIGNIFICANCE: This review explores key immunologic concepts and critical pathways mediating graft rejection in Type 1 Diabetes, which can instruct the future purposeful design of immunomodulatory biomaterials for cell therapy. A summary of immunological pathways initiated following cellular implantation, as well as current systemic immunomodulatory agents used, is provided. We then outline the potential of biomaterials to modulate these responses. The capacity of polymeric encapsulation to block some powerful rejection pathways is covered. We also highlight the role of cellular health and biocompatibility in mitigating immune responses. Finally, we review the use of bioactive materials to proactively modulate local immune responses, focusing on key concepts of anti-inflammatory, suppressive, and tolerogenic agents.

PIRCHE-II: an algorithm to predict indirectly recognizable HLA epitopes in solid organ transplantation

Human leukocyte antigen (HLA) mismatches between donors and recipients may lead to alloreactivity after solid organ transplantation. Over the last few decades, our knowledge of the complexity of the HLA system has dramatically increased, as numerous new HLA alleles have been identified. As a result, the likelihood of alloreactive responses towards HLA mismatches after solid organ transplantation cannot easily be assessed. Algorithms are promising solutions to estimate the risk for alloreactivity after solid organ transplantation. In this review, we show that the recently developed PIRCHE-II (Predicted Indirectly ReCognizable HLA Epitopes) algorithm can be used to minimize alloreactivity towards HLA mismatches. Together with the use of other algorithms and simulation approaches, the PIRCHE-II algorithm aims for a better estimated alloreactive risk for individual patients and eventually an improved graft survival after solid organ transplantation.

State of the Art: Role of the Dendritic Cell in Induction of Allograft Tolerance

Despite decades of research, the induction and maintenance of long-term allograft tolerance without immunosuppression remains an elusive goal in the field of solid organ and cell transplantation. Immunosuppressive medications frequently prevent or minimize acute cellular rejection but have failed to halt antidonor antibody production and chronic organ rejection. Past efforts aimed at promoting lasting allograft tolerance have focused primarily on peripheral T-cell depletion, augmentation of regulatory T cells, or induction via simultaneous hematopoietic stem cell transplantation and facilitation of donor chimerism. So far, none of these methods have led to consistently safe, feasible and long lasting donor organ acceptance. Over the course of the past 4 decades, the study of a unique population of antigen-presenting cells known as dendritic cells has shown promise for breaking new ground in achieving indefinite allograft survival without immunosuppression and its associated adverse effects. In this review, we discuss the discovery and early investigations of dendritic cells and chronicle some of the key studies demonstrating their role in transplantation, particularly in indirect allorecognition, the immunologic pathway thought to drive chronic rejection and perhaps tolerance induction.

Immunological challenges associated with artificial skin grafts: available solutions and stem cells in future design of synthetic skin

The repair or replacement of damaged skins is still an important, challenging public health problem. Immune acceptance and long-term survival of skin grafts represent the major problem to overcome in grafting given that in most situations autografts cannot be used. The emergence of artificial skin substitutes provides alternative treatment with the capacity to reduce the dependency on the increasing demand of cadaver skin grafts. Over the years, considerable research efforts have focused on strategies for skin repair or permanent skin graft transplantations. Available skin substitutes include pre- or post-transplantation treatments of donor cells, stem cell-based therapies, and skin equivalents composed of bio-engineered acellular or cellular skin substitutes. However, skin substitutes are still prone to immunological rejection, and as such, there is currently no skin substitute available to overcome this phenomenon. This review focuses on the mechanisms of skin rejection and tolerance induction and outlines in detail current available strategies and alternatives that may allow achieving full-thickness skin replacement and repair.

Transforming Growth Factor-β2 Downregulates Major Histocompatibility Complex (MHC) I and MHC II Surface Expression on Equine Bone Marrow-Derived Mesenchymal Stem Cells Without Altering Other Phenotypic Cell Surface Markers

Allogeneic mesenchymal stem cells (MSCs) are a promising cell source for treating musculoskeletal injuries in horses. Effective and safe allogeneic therapy may be hindered, however, by recipient immune recognition and rejection of major histocompatibility complex (MHC)-mismatched MSCs. Development of strategies to prevent immune rejection of MHC-mismatched MSCs in vivo is necessary to enhance cell survival and potentially increase the efficacy and safety of allogeneic MSC therapy. The purposes of this study were to evaluate if transforming growth factor-β2 (TGF-β2) downregulated MHC expression on equine MSCs and to determine if TGF-β2 treatment altered the phenotype of MSCs. Equine bone marrow-derived MSCs from 12 horses were treated with 1, 5, or 10 ng/ml TGF-β2 from initial isolation until MHC expression analysis. TGF-β2-treated MSCs had reduced MHC I and MHC II surface expression compared to untreated controls. TGF-β2 treatment also partially blocked IFN-γ-induced upregulation of MHC I and MHC II. Constitutive and IFN-γ-induced MHC I and MHC II expression on equine MSCs was dynamic and highly variable, and the effect of TGF-β2 was significantly dependent on the donor animal and baseline MHC expression. TGF-β2 treatment did not appear to change morphology, surface marker expression, MSC viability, or secretion of TGF-β1, but did significantly increase the number of cells obtained from culture. These results indicate that TGF-β2 treatment has promise for regulating MHC expression on MSCs to facilitate allogeneic therapy, but further work is needed to maintain MHC stability when exposed to an inflammatory stimulus.

CD40-independent help by memory CD4 T cells induces pathogenic alloantibody but does not lead to long-lasting humoral immunity

CD40/CD154 interactions are essential for productive antibody responses to T-dependent antigens. Memory CD4 T cells express accelerated helper functions and are less dependent on costimulation when compared with naïve T cells. Here, we report that donor-reactive memory CD4 T cells can deliver help to CD40-deficient B cells and induce high titers of IgG alloantibodies that contribute to heart allograft rejection in CD40-/- heart recipients. While cognate interactions between memory helper T and B cells are crucial for CD40-independent help, this process is not accompanied by germinal center formation and occurs despite inducible costimulatory blockade. Consistent with the extrafollicular nature of T/B cell interactions, CD40-independent help fails to maintain stable levels of serum alloantibody and induce differentiation of long-lived plasma cells and memory B cells. In summary, our data suggest that while CD40-independent help by memory CD4 T cells is sufficient to induce high levels of pathogenic alloantibody, it does not sustain long-lasting anti-donor humoral immunity and B cell memory responses. This information may guide the future use of CD40/CD154 targeting therapies in transplant recipients containing donor-reactive memory T cells.

Germinal center alloantibody responses are mediated exclusively by indirect-pathway CD4 T follicular helper cells

The durable alloantibody responses that develop in organ transplant patients indicate long-lived plasma cell output from T-dependent germinal centers (GCs), but which of the two pathways of CD4 T cell allorecognition is responsible for generating allospecific T follicular helper cells remains unclear. This was addressed by reconstituting T cell-deficient mice with monoclonal populations of TCR-transgenic CD4 T cells that recognized alloantigen only as conformationally intact protein (direct pathway) or only as self-restricted allopeptide (indirect pathway) and then assessing the alloantibody response to a heart graft. Recipients reconstituted with indirect-pathway CD4 T cells developed long-lasting IgG alloantibody responses, with splenic GCs and allospecific bone marrow plasma cells readily detectable 50 d after heart transplantation. Differentiation of the transferred CD4 T cells into T follicular helper cells was confirmed by follicular localization and by acquisition of signature phenotype. In contrast, IgG alloantibody was not detectable in recipient mice reconstituted with direct-pathway CD4 T cells. Neither prolongation of the response by preventing NK cell killing of donor dendritic cells nor prior immunization to develop CD4 T cell memory altered the inability of the direct pathway to provide allospecific B cell help. CD4 T cell help for GC alloantibody responses is provided exclusively via the indirect-allorecognition pathway.

Targeting MHC class I monomers to dendritic cells inhibits the indirect pathway of allorecognition and the production of IgG alloantibodies leading to long-term allograft survival

T cell depletion strategies are an efficient therapy for the treatment of acute rejections and are an essential part of tolerance induction protocols in various animal models; however, they are usually nonselective and cause wholesale T cell depletion leaving the individual in a severely immunocompromised state. So far it has been difficult to selectively delete alloreactive T cells because the majority of protocols either delete all T cells, subsets of T cells, or subpopulations of T cells expressing certain activation markers, ignoring the Ag specificity of the TCR. We have developed a model in which we were able to selectively deplete alloreactive T cells with an indirect specificity by targeting intact MHC molecules to quiescent dendritic cells using 33D1 as the targeting Ab. This strategy enabled us to inhibit the indirect alloresponse against MHC-mismatched skin grafts and hence the generation of IgG alloantibodies, which depends on indirectly activated T cells. In combination with the temporary abrogation of the direct alloresponse, we were able to induce indefinite skin graft survival. Importantly, the targeting strategy had no detrimental effect on CD4(+)CD25(+)FoxP3(+) T cells, which could potentially be used as an adjunctive cellular therapy. Transplantation tolerance depends on the right balance between depletion and regulation. For the former this approach may be a useful tool in the development of future tolerance induction protocols in non-sensitized patients.

Preferential priming of alloreactive T cells with indirect reactivity

The relative contributions of the direct and indirect pathways in alloimmune responses have not been fully elucidated. We report a novel murine TCR transgenic system that can simultaneously track the CD4-direct (CD4-d), CD4-indirect (CD4-i) and CD8-direct (CD8-d) pathways after transplantation. Using this system, we have observed a profoundly greater proliferation of CD4-i T cells relative to CD4-d and CD8-d T cells after transplantation. Furthermore, a much larger proportion of CD4-i T cells attain an effector phenotype. We also analyzed endogenous, wild-type T cells using enzyme-linked immunospot analysis. In naïve mice, T cells with indirect reactivity were undetectable, but T cells with direct reactivity were abundant. However, 10 days after skin or heterotopic heart transplantation, CD4-i T cells comprised approximately 10% of the CD4+ response. Consistent with increased priming of the CD4-i pathway, we observed that the CD4-i T cells were further enriched in the effector cells migrating to the allograft and in memory-like T cells persisting after rejection. Thus, priming of the CD4-i pathway is favored after transplantation, allowing a rare population to rapidly become a major component of the CD4+ T-cell response in acute allograft rejection. The generalizability of this observation to other models remains to be determined.

Activated effector and memory T cells contribute to circulating sCD30: potential marker for islet allograft rejection

T-cell activation up-regulates CD30 resulting in an increase in serum soluble CD30 (sCD30). CD4+ T cells, a major source for sCD30, play a significant role in the pathogenesis of rejection. In this study, sCD30 was measured pre- and posttransplant in mouse islet allograft models and human islet allograft recipients. sCD30 was measured by ELISA in diabetic C57BL/6, CD4Knockout (KO) and CD8KO islet allograft recipients. sCD30 increased significantly prior to rejection (1.8 +/- 1 days) in 80% of allograft recipients. Sensitization with donor splenocytes, or a second graft, further increased sCD30 (282.5 +/- 53.5 for the rejecting first graft vs. 374.6 +/- 129 for the rejecting second graft) prior to rejection suggesting memory CD4+ T cells contribute to sCD30. CD4KO failed to reject islet allograft and did not demonstrate sCD30 increase. CD8KO showed elevated (227 +/- 107) sCD30 (1 day) prior to rejection. High pretransplant sCD30 (>20 U/ml) correlated with poor outcome in human islet allograft recipients. Further, increase in sCD30 posttransplant preceded (3-4 months) loss of islet function. We conclude that sCD30 is released from activated CD4 T cells prior to islet allograft rejection and monitoring sCD30 can be a valuable adjunct in the follow-up of islet transplant recipients.

Indirect recognition of T-cell epitopes derived from the alpha 3 and transmembrane domain of HLA-A2

Indirect allorecognition has been implicated in the mechanism of chronic rejection and alloantibody formation but precise definition of the epitopes involved has been limited. We have undertaken a detailed assessment of the antigenic properties of peptides derived from HLA-A2. Candidate epitopes were identified in vitro by assessment of MHC class II binding. The immune response to these epitopes was determined in patients awaiting a renal transplant by the assessment of PBMC activation using gamma-interferon ELISPOT. Twenty-two of fifty-five patients responded to peptides from HLA-A2 and this was associated with but not confined to those who had made antibody to HLA-A2 (14/18). Nineteen of twenty-two patients responded to peptides derived from the hypervariable alpha1 and alpha2 domains and 18/22 responded to peptides from the alpha 3 and transmembrane domain, the sequences of which show little polymorphism. In six patients, the sequence of these peptides was identical to self, that is, the response was autoimmune. The finding of indirect epitopes derived from regions of MHC class I that exhibit little polymorphism provides a novel perspective on the immune response to alloantigen and has potential implications for the development of specific therapies.

I-Ep-Bound Self-Peptides: Identification, Characterization, and Role in Alloreactivity

T cell recognition of peptide/allogeneic MHC complexes is a major cause of transplant rejection. Both the presented self-peptides and the MHC molecules are involved; however, the molecular basis for alloreactivity and the contribution of self-peptides are still poorly defined. The murine 2.102 T cell is specific for hemoglobin(64-76)/I-Ek and is alloreactive to I-Ep. The natural self-peptide/I-Ep complex recognized by 2.102 remains unknown. In this study, we characterized the peptides that are naturally processed and presented by I-Ep and used this information to define the binding motif for the murine I-Ep class II molecule. Interestingly, we found that the P9 anchor residue preferred by I-Ep is quite distinct from the residues preferred by other I-E molecules, although the P1 anchor residue is conserved. A degree of specificity for the alloresponse was shown by the lack of stimulation of 2.102 T cells by 19 different identified self-peptides. The binding motif was used to search the mouse genome for candidate 2.102 reactive allopeptides that contain strong P1 and P9 anchor residues and possess previously identified allowable TCR contact residues. Two potential allopeptides were identified, but only one of these peptides, G protein-coupled receptor 128, was able to stimulate 2.102 T cells. Thus, the G protein-coupled receptor 128 peptide represents a candidate allopeptide that is specifically recognized by 2.102 T cells bound to I-Ep and was identified using bioinformatics. These studies highlight the specific involvement of self-peptides in alloreactivity.

Exhaustive Depletion of Graft Resident Dendritic Cells: Marginally Delayed Rejection but Strong Alteration of Graft Infiltration

BACKGROUND

Donor dendritic cells (DDC) are believed to sustain direct recognition leading to acute allograft rejection. However, DDC are also required for tolerance induction in various models.

METHODS

We studied the effect of DDC depletion on major histocompatibility complex (MHC) mismatched rat heart allografts in a strain combination characterized by a DDC-dependant tolerance induction. Grafts were depleted of DDC either by pretreating donors with cyclophosphamide (CyP) or by being parked in an intermediate recipient treated with cyclosporine A (CsA).

RESULTS

CyP depleted 95% of resident DC and no specific donor MHC class II staining was observed in parked grafts. Parked grafts survived significantly but only moderately longer than untreated grafts (10.8+/-1.9 days vs. 6.5+/-0.5 days; P<0.05). Compared to unmodified grafts, on day 5 after transplantation, the magnitude of the graft infiltrate was dramatically decreased in DDC-depleted grafts, with IgG deposition within the grafts at the time of rejection. In parallel, the cytokine transcript levels were also lower in these grafts on day 5, but reached levels similar to those of unmodified grafts by day 7, indicating a delayed pattern of rejection.

CONCLUSIONS

Taken collectively, these data suggest that DDC depletion has a greater effect on the capacity of tolerance induction than the rejection process.

Alloreactive T Cell Responses and Acute Rejection of Single Class II MHC-Disparate Heart Allografts Are under Strict Regulation by CD4+CD25+ T Cells

Skin but not vascularized cardiac allografts from B6.H-2bm12 mice are acutely rejected by C57BL/6 recipients in response to the single class II MHC disparity. The underlying mechanisms preventing acute rejection of B6.H-2bm12 heart allografts by C57BL/6 recipients were investigated. B6.H-2bm12 heart allografts induced low levels of alloreactive effector T cell priming in C57BL/6 recipients, and this priming was accompanied by low-level cellular infiltration into the allograft that quickly resolved. Recipients with long-term-surviving heart allografts were unable to reject B6.H-2bm12 skin allografts, suggesting potential down-regulatory mechanisms induced by the cardiac allografts. Depletion of CD25+ cells from C57BL/6 recipients resulted in 15-fold increases in alloreactive T cell priming and in acute rejection of B6.H-2bm12 heart grafts. Similarly, reconstitution of B6.Rag(-/-) recipients with wild-type C57BL/6 splenocytes resulted in acute rejection of B6.H-2bm12 heart grafts only if CD25+ cells were depleted. These results indicate that acute rejection of single class II MHC-disparate B6.H-2bm12 heart allografts by C57BL/6 recipients is inhibited by the emergence of CD25+ regulatory cells that restrict the clonal expansion of alloreactive T cells.

Frontline: Peripheral priming of alloreactive T cells by the direct pathway of allorecognition

Recent studies, though controversial, have suggested that secondary lymphoid organs may not constitute an essential site for the initiation of immune responses to transplant antigens. However, this issue has never been examined in the context of direct and indirect allorecognition. Here, we characterized immune responses arising in draining lymph nodes and skin allografts, in a murine model based on a single T cell clonotype where these two pathways can be independently studied. In this model, graft rejection by the direct or the indirect pathway occurred with similar kinetics, although initiation of the alloreactive responses was clearly different. During indirect responses, expansion and activation of alloreactive T cells were first observed in draining lymph nodes, at day 7 post-transplant, and graft-infiltrating T cells were observed later, at day 11. In striking contrast, directly activated alloreactive T cells were detected at an early stage inside the graft, and only later in the draining lymph nodes, after skin allograft rejection was almost completed. These results suggest that sensitization of naive T cells through the direct pathway could take place outside secondary lymphoid organs.