Accelerated antibody-mediated graft loss of rodent pancreatic islets after pretreatment with dexamethasone-treated immature donor dendritic cells

The inhibitory receptor Siglec-E controls antigen-presenting cell activation and T cell-mediated transplant rejection

After transplantation, inflammation and tissue injury release danger signals that activate myeloid cells, driving adaptive immune responses and acute rejection. Current immunosuppressants primarily target T cells but inadequately control innate immunity. Regulatory signals controlling innate responses in transplantation remain elusive. The sialic acid-binding immunoglobulin-like lectin-E (Siglec-E, or SigE) binds sialylated ligands to suppress inflammation. In mouse heart transplants, SigE is up-regulated in graft-infiltrating myeloid cells, including dendritic cells (DCs). SigE deficiency in recipients, but not donors, accelerates acute rejection by enhancing DC activation, nuclear factor κB (NF-κB) signaling, and tumor necrosis factor-α (TNF-α) production, thereby boosting alloreactive T cell responses. Conversely, SigE overexpression on DCs reduces activation by danger signals and their T cell allostimulatory capacity. The human homologs Siglecs-7 and -9 were up-regulated in rejecting allograft biopsies, and their higher expression correlated with improved allograft survival. Thus, SigE/7/9 is a crucial inhibitory receptor controlling antigen-presenting cell activation and T cell-mediated transplant rejection, offering therapeutic potential.

Detection and Monitoring of Regulatory Immune Cells Following Their Adoptive Transfer in Organ Transplantation

Application of cell-based immunotherapy in organ transplantation to minimize the burden of immunosuppressive medication and promote allograft tolerance has expanded significantly over the past decade. Adoptively transferred regulatory immune cells prolong allograft survival and transplant tolerance in pre-clinical models. Many cell products are currently under investigation in early phase human clinical trials designed to assess feasibility and safety. Despite rapid advances in manufacturing practices, defining the appropriate protocol that will optimize in vivo conditions for tolerance induction remains a major challenge and depends heavily on understanding the fate, biodistribution, functional stability and longevity of the cell product after administration. This review focuses on in vivo detection and monitoring of various regulatory immune cell types administered for allograft tolerance induction in both pre-clinical animal models and early human clinical trials. We discuss the current status of various non-invasive methods for tracking regulatory cell products in the context of organ transplantation and implications for enhanced understanding of the therapeutic potential of cell-based therapy in the broad context of control of immune-mediated inflammatory disorders.

Apoptotic cell-based therapies for promoting transplantation tolerance

PURPOSE OF REVIEW

This article is aimed to provide readers with an updated review on the applicability, efficacy, and challenges of employing donor apoptotic cell-based therapies to promote transplantation tolerance in various experimental and clinical settings.

RECENT FINDINGS

Recently, donor apoptotic cell-based therapies have been employed in various models of cell (including pancreatic islets and bone marrow hematopoietic stem cells) and solid organ (heart and kidney) transplantation to promote donor-specific tolerance. Published data, thus far, have revealed a high potential of this approach in inducing robust transplantation tolerance. Recent clinical trials have also underscored the safety and potential efficacy of this approach in alleviating graft-versus-host disease (GVHD) in bone marrow transplantation (BMT). Host factors including prior allo-sensitization and opportunistic infections pose major obstacles in establishing transplantation tolerance employing this strategy. However, emerging data provide strategies for overcoming such obstacles in these clinically relevant settings.

SUMMARY

Donor apoptotic cell therapy is an emerging strategy in promoting transplantation tolerance, with recent data emphasizing its efficacy and applicability for transplantation tolerance in the clinic.

March1-dependent modulation of donor MHC II on CD103+ dendritic cells mitigates alloimmunity

In transplantation, donor dendritic cells (do-DCs) initiate the alloimmune response either by direct interaction with host T cells or by transferring intact donor MHC to host DCs. However, how do-DCs can be targeted for improving allograft survival is still unclear. Here we show CD103+ DCs are the major do-DC subset involved in the acute rejection of murine skin transplants. In the absence of CD103+ do-DCs, less donor MHC-II is carried to host lymph nodes, fewer allogenic T cells are primed and allograft survival is prolonged. Incubation of skin grafts with the anti-inflammatory mycobacterial protein DnaK reduces donor MHC-II on CD103+DCs and prolongs graft survival. This effect is mediated through IL-10-induced March1, which ubiquitinates and decreases MHC-II levels. Importantly, in vitro pre-treatment of human DCs with DnaK reduces their ability to prime alloreactive T cells. Our findings demonstrate a novel therapeutic approach to dampen alloimmunity by targeting donor MHC-II on CD103+DCs.

Harnessing Apoptotic Cells for Transplantation Tolerance: Current Status and Future Perspectives

PURPOSE OF REVIEW

The use of donor apoptotic cells is an emerging therapy for inducing transplantation tolerance. In this review, we will discuss current understanding of mechanisms of this approach, as well as crucial aspects necessary for successful translation of this approach to clinical transplantation.

RECENT FINDINGS

Transplantation tolerance by donor apoptotic cells is mediated by their homeostatic interaction with recipient phagocytes, and subsequent expansion of suppressor cell populations as well as inhibition of effector T cells via deletion and anergy. To ensure their tolerogenicity, it is critical to procure non-stressed donor cells, and to induce and arrest their apoptosis at the appropriate stage prior to their administration. Equally important is the monitoring of dynamics of recipient immunological status, and its influences on tolerance efficacy and longevity. Emerging concepts and technologies may significantly streamline tolerogen manufacture and delivery of this approach, and smooth its transition to clinical application.

SUMMARY

Hijacking homeostatic clearance of donor apoptotic cells is a promising strategy for transplantation tolerance. Timing is now mature for concerted efforts for transitioning this strategy to clinical transplantation.

Orchestration of transplantation tolerance by regulatory dendritic cell therapy or in-situ targeting of dendritic cells

PURPOSE OF REVIEW

Extensive research in murine transplant models over the past two decades has convincingly demonstrated the ability of regulatory dendritic cells (DCregs) to promote long-term allograft survival. We review important considerations regarding the source of therapeutic DCregs (donor or recipient) and their mode of action, in-situ targeting of DCregs, and optimal therapeutic regimens to promote DCreg function.

RECENT FINDINGS

Recent studies have defined protocols and mechanisms whereby ex-vivo-generated DCregs of donor or recipient origin subvert allogeneic T-cell responses and promote long-term organ transplant survival. Particular interest has focused on how donor antigen is acquired, processed and presented by autologous dendritic cells, on the stability of DCregs, and on in-situ targeting of dendritic cells to promote their tolerogenic function. New evidence of the therapeutic efficacy of DCregs in a clinically relevant nonhuman primate organ transplant model and production of clinical grade DCregs support early evaluation of DCreg therapy in human graft recipients.

SUMMARY

We discuss strategies currently used to promote dendritic cell tolerogenicity, including DCreg therapy and in-situ targeting of dendritic cells, with a view to improved understanding of underlying mechanisms and identification of the most promising strategies for therapeutic application.

Cotransfection with IL-10 and TGF-β1 into immature dendritic cells enhances immune tolerance in a rat liver transplantation model

Dendritic cells transfected with interleukin (IL)-10 and transforming growth factor-β1 (TGF-β1) enhance T cell immunity and tolerance. However, no quantitative studies have investigated the suppressive functions of immature dendritic cells (imDC) cotransfected with IL-10 and TGF-β1. The effects of imDC cotransfected with IL-10 and TGF-β1 (IL-10-TGF-β1-imDC) on immune tolerance induction in a rat transplantation model were investigated. In addition, effects of IL-10-TGF-β1-imDC relative to IL-10-transfected imDC (IL-10-imDC) and TGF-β1-transfected imDC (TGF-β1-imDC) were compared. The infusion of IL-10-TGF-β1-imDC into recipients prolonged liver graft survival, which was sustained for >90 days. IL-12 serum levels decreased, whereas alanine transaminase and total bilirubin slightly increased in rats infused with IL-10-TGF-β1-imDC compared with the IL-10-imDC and TGF-β1-imDC groups. Furthermore, a higher percentage of terminal transferase-mediated UTP nick end-labeling-positive cells was observed, and histological analysis of the allografts indicated a rejection activity index of mild acute rejection. Our results suggest infusion of IL-10 and TGF-β1 cotransfected imDC induces alloantigen-specific T cell hyporesponsiveness, inhibits antigen-specific immunological responses to liver allografts, prolongs liver allograft survival, and enhances the immune tolerance. This approach may provide a promising alternative for enhancing donor-specific tolerance during liver transplantation.

Survival of porcine fibroblasts enhanced by human FasL and dexamethasone-treated human dendritic cells in vitro

Cell-mediated and acute vascular rejections remain to be one of the primary hurdles to achieve successful xenotransplantation. Fas ligand is known to be an important molecule for the formation of 'immune-privileged' condition and dendritic cells treated with dexamethasone (Dex-DCs) acting like tolerogenic DCs (tDCs) which are known to protect transplanted cells and organs from unwanted immune responses. The present study investigated the possibility that porcine fibroblasts expressing human Fas ligand (PhF) together with human Dex-DCs could induce prolonged survival of porcine fibroblasts in vitro. PhF was collected from an ear of human Fas ligand transgenic porcine and cell-line was established by MGEM Inc. PhF labeled with CFSE co-cultured with human peripheral blood mononuclear cells (hPBMCs) were examined with respect to induction of tolerance and cell death when co-cultured with Dex-DCs for 3days. PhF induced the apoptosis in hPBMCs, especially CD4(+) T cells. Dex-DCs showed significant (P<0.05) reduction on the expression of CD80, CD86 and MHC class I/II, and the secretion of IL-12p70, TNF-α and IL-10, but increase of latency-associated peptide (LAP). Survival of PhF was significantly higher than that of WT and it was increased in the presence of Dex-DCs when compared to the other DCs (i.e.,DCs, LPS-treated DCs and LPS/Dex-treated DCs) in vitro. Survival of PhF did not change by co-culture with Dex-DCs due to apoptotic cell death of Dex-DCs. Dex-DCs reduced the death of porcine fibroblasts and, at the same time, PhF induced the apoptosis from hPBMCs, but it was not synergistic.

Regulatory dendritic cells for immunotherapy in immunologic diseases

We recognize well the abilities of dendritic cells to activate effector T cell (Teff cell) responses to an array of antigens and think of these cells in this context as pre-eminent antigen-presenting cells, but dendritic cells are also critical to the induction of immunologic tolerance. Herein, we review our knowledge on the different kinds of tolerogenic or regulatory dendritic cells that are present or can be induced in experimental settings and humans, how they operate, and the diseases in which they are effective, from allergic to autoimmune diseases and transplant tolerance. The primary conclusions that arise from these cumulative studies clearly indicate that the agent(s) used to induce the tolerogenic phenotype and the status of the dendritic cell at the time of induction influence not only the phenotype of the dendritic cell, but also that of the regulatory T cell responses that they in turn mobilize. For example, while many, if not most, types of induced regulatory dendritic cells lead CD4⁺ naïve or Teff cells to adopt a CD25⁺Foxp3⁺ Treg phenotype, exposure of Langerhans cells or dermal dendritic cells to vitamin D leads in one case to the downstream induction of CD25⁺Foxp3⁺ regulatory T cell responses, while in the other to Foxp3⁻ type 1 regulatory T cells (Tr1) responses. Similarly, exposure of human immature versus semi-mature dendritic cells to IL-10 leads to distinct regulatory T cell outcomes. Thus, it should be possible to shape our dendritic cell immunotherapy approaches for selective induction of different types of T cell tolerance or to simultaneously induce multiple types of regulatory T cell responses. This may prove to be an important option as we target diseases in different anatomic compartments or with divergent pathologies in the clinic. Finally, we provide an overview of the use and potential use of these cells clinically, highlighting their potential as tools in an array of settings.

Donor bone marrow-derived dendritic cells prolong corneal allograft survival and promote an intragraft immunoregulatory milieu

Investigations into cell therapies for application in organ transplantation have grown. Here, we describe the ex vivo generation of donor bone marrow-derived dendritic cells (BMDCs) and glucocorticoid-treated BMDCs with potent immunomodulatory properties for application in allogeneic transplantation. BMDCs were treated with dexamethasone (Dexa) to induce an immature, maturation-resistant phenotype. BMDC and Dexa BMDC phenotype, antigen presenting cell function, and immunomodulatory properties were fully characterized. Both populations display significant immunomodulatory properties, including, but not limited to, a significant increase in mRNA expression of programmed death-ligand 1 and indoleamine 2,3-dioxygenase. BMDCs and Dexa BMDCs display a profound impaired capacity to stimulate allogeneic lymphocytes. Moreover, in a fully MHC I/II mismatched rat corneal transplantation model, injection of donor-derived, untreated BMDC or Dexa BMDCs (1 × 10(6) cells, day -7) significantly prolonged corneal allograft survival without the need for additional immunosuppression. Although neovascularization was not reduced and evidence of donor-specific alloantibody response was detected, a significant reduction in allograft cellular infiltration combined with a significant increase in the ratio of intragraft FoxP3-expressing regulatory cells was observed. Our comprehensive analysis demonstrates the novel cellular therapeutic approach and significant effect of donor-derived, untreated BMDCs and Dexa BMDCs in preventing corneal allograft rejection.

Tolerogenic Donor-Derived Dendritic Cells Risk Sensitization In Vivo owing to Processing and Presentation by Recipient APCs

Modification of allogeneic dendritic cells (DCs) through drug treatment results in DCs with in vitro hallmarks of tolerogenicity. Despite these observations, using murine MHC-mismatched skin and heart transplant models, donor-derived drug-modified DCs not only failed to induce tolerance but also accelerated graft rejection. The latter was inhibited by injecting the recipient with anti-CD8 Ab, which removed both CD8(+) T cells and CD8(+) DCs. The discrepancy between in vitro and in vivo data could be explained, partly, by the presentation of drug-modified donor DC MHC alloantigens by recipient APCs and activation of recipient T cells with indirect allospecificity, leading to the induction of alloantibodies. Furthermore, allogeneic MHC molecules expressed by drug-treated DCs were rapidly processed and presented in peptide form by recipient APCs in vivo within hours of DC injection. Using TCR-transgenic T cells, Ag presentation of injected OVA-pulsed DCs was detectable for ≤ 3 d, whereas indirect presentation of MHC alloantigen by recipient APCs led to activation of T cells within 14 h and was partially inhibited by reducing the numbers of CD8(+) DCs in vivo. In support of this observation when mice lacking CD8(+) DCs were pretreated with drug-modified DCs prior to transplantation, skin graft rejection kinetics were similar to those in non-DC-treated controls. Of interest, when the same mice were treated with anti-CD40L blockade plus drug-modified DCs, skin graft survival was prolonged, suggesting endogenous DCs were responsible for T cell priming. Altogether, these findings highlight the risks and limitations of negative vaccination using alloantigen-bearing "tolerogenic" DCs.