Peri‐graft porcine‐specific CD4 + FoxP3 + regulatory T cells by CD40‐CD154 blockade prevented the rejection of porcine islet graft in diabetic mice

Pancreatic islet transplantation: current advances and challenges

Diabetes is a prevalent chronic disease that traditionally requires severe reliance on medication for treatment. Oral medication and exogenous insulin can only temporarily maintain blood glucose levels and do not cure the disease. Most patients need life-long injections of exogenous insulin. In recent years, advances in islet transplantation have significantly advanced the treatment of diabetes, allowing patients to discontinue exogenous insulin and avoid complications.Long-term follow-up results from recent reports on islet transplantation suggest that they provide significant therapeutic benefit although patients still require immunotherapy, suggesting the importance of future transplantation strategies. Although organ shortage remains the primary obstacle for the development of islet transplantation, new sources of islet cells, such as stem cells and porcine islet cells, have been proposed, and are gradually being incorporated into clinical research. Further research on new transplantation sites, such as the subcutaneous space and mesenteric fat, may eventually replace the traditional portal vein intra-islet cell infusion. Additionally, the immunological rejection reaction in islet transplantation will be resolved through the combined application of immunosuppressant agents, islet encapsulation technology, and the most promising mesenchymal stem cells/regulatory T cell and islet cell combined transplantation cell therapy. This review summarizes the progress achieved in islet transplantation, and discusses the research progress and potential solutions to the challenges faced.

CD27-Expressing Xenoantigen-Expanded Human Regulatory T Cells Are Efficient in Suppressing Xenogeneic Immune Response

Clinically, xenotransplantation often leads to T-cell-mediated graft rejection. Immunosuppressive agents including polyclonal regulatory T cells (poly-Tregs) promote global immunosuppression, resulting in serious infections and malignancies in patients. Xenoantigen-expanded Tregs (xeno-Tregs) have become a promising immune therapy strategy to protect xenografts with fewer side effects. In this study, we aimed to identify an efficient and stable subset of xeno-Tregs. We enriched CD27⁺ xeno-Tregs using cell sorting and evaluated their suppressive functions and stability in vitro via mixed lymphocyte reaction (MLR), real-time polymerase chain reaction, inflammatory induction assay, and Western blotting. A STAT5 inhibitor was used to investigate the relationship between the function and stability of CD27⁺ xeno-Tregs and the JAK3-STAT5 signaling pathway. A humanized xenotransplanted mouse model was used to evaluate the function of CD27⁺ xeno-Tregs in vivo. Our results show that CD27⁺ xeno-Tregs express higher levels of Foxp3, cytotoxic T-lymphocyte antigen-4 (CTLA4), and Helios and lower levels of interleukin-17 (IL-17) than their CD27^- counterparts. In addition, CD27⁺ xeno-Tregs showed enhanced suppressive function in xeno-MLR at ratios of 1:4 and 1:16 of Tregs:responder cells. Under inflammatory conditions, a lower percentage of CD27⁺ xeno-Tregs secretes IL-17 and interferon-γ (IFN-γ). CD27⁺ xeno-Tregs demonstrated an upregulated JAK3-STAT5 pathway compared with that of CD27^- xeno-Tregs and showed decreased Foxp3, Helios, and CTLA4 expression after addition of STAT5 inhibitor. Mice that received porcine skin grafts showed a normal tissue phenotype and less leukocyte infiltration after reconstitution with CD27⁺ xeno-Tregs. Taken together, these data indicate that CD27⁺ xeno-Tregs may suppress immune responses in a xenoantigen-specific manner, which might be related to the activation of the JAK3-STAT5 signaling pathway.

In vivo induction of regulatory T cells by anti-CD45RB antibody causes transferable tolerance to discordant human xenogenic islets

BACKGROUND

The treatment of diabetes by islet cell transplantation has become an accepted therapy, with transplantation of xenogeneic islet cells an attractive alternative to the problem. Previous studies in mice have demonstrated that anti-CD45RB induce immune tolerance in human pancreatic islet cells. The current study was to define the mechanism of action of anti-CD45RB induced nonspecific immune tolerance to heteroantigens.

METHODS

A total of 1500 IEQ human islets were transplanted to diabetic B6μMT-/- mice, B6 mice, and μMT-/- diabetic mice undergoing thymectomy. These mice were treated short-term with doses of anti-CD45RB. CD4+Foxp3+Tregs were detected in the blood, peripheral lymphatic organs by flow cytometry, and immunohistochemistry. In addition, anti-CD25 mAb was administered to tolerant human islet cells B6μMT-/-mice. Mice then were transplanted with other human islet cells and received CD4+CD25+Tregs isolated from tolerant human islets mice to observe islet destruction.

RESULTS

Anti-CD45RB treatment-induced tolerance to islets in both immunocompetent and B-cell-deficient mice (μMT-/- mice) by processes that were dependent on CD25+ Tregs, but not B cells. Anti-CD45RB treatment increased the number of CD4+Foxp3+Tregs cells. Anti-CD45RB treatment-induced immune tolerance that was antigen nonspecific, with Tregs playing an important role. Anti-CD45RB treatment-induced tolerance generated Tregs that could be transferred to another individual to manifest nonspecific immune tolerance.

CONCLUSION

The results of the experiment suggest that anti-CD45RB induced tolerance to human islet xenografts is mediated by the proliferation of Tregs. These tolerogenic Tregs can be transferred to other mice and induce nonspecific immune tolerance.

Progress in islet xenotransplantation: Immunologic barriers, advances in gene editing, and tolerance induction strategies for xenogeneic islets in pig-to-primate transplantation

Islet transplantation has emerged as a curative therapy for diabetes in select patients but remains rare due to shortage of suitable donor pancreases. Islet transplantation using porcine islets has long been proposed as a solution to this organ shortage. There have already been several small clinical trials using porcine islets in humans, but results have been mixed and further trials limited by calls for more rigorous pre-clinical data. Recent progress in heart and kidney xenograft transplant, including three studies of pig-to-human xenograft transplant, have recaptured popular imagination and renewed interest in clinical islet xenotransplantation. This review outlines immunologic barriers to islet transplantation, summarizes current strategies to overcome these barriers with a particular focus on approaches to induce tolerance, and describes an innovative strategy for treatment of diabetic nephropathy with composite islet-kidney transplantation.

Current status of xenotransplantation research and the strategies for preventing xenograft rejection

Transplantation is often the last resort for end-stage organ failures, e.g., kidney, liver, heart, lung, and pancreas. The shortage of donor organs is the main limiting factor for successful transplantation in humans. Except living donations, other alternatives are needed, e.g., xenotransplantation of pig organs. However, immune rejection remains the major challenge to overcome in xenotransplantation. There are three different xenogeneic types of rejections, based on the responses and mechanisms involved. It includes hyperacute rejection (HAR), delayed xenograft rejection (DXR) and chronic rejection. DXR, sometimes involves acute humoral xenograft rejection (AHR) and cellular xenograft rejection (CXR), which cannot be strictly distinguished from each other in pathological process. In this review, we comprehensively discussed the mechanism of these immunological rejections and summarized the strategies for preventing them, such as generation of gene knock out donors by different genome editing tools and the use of immunosuppressive regimens. We also addressed organ-specific barriers and challenges needed to pave the way for clinical xenotransplantation. Taken together, this information will benefit the current immunological research in the field of xenotransplantation.

Cellular Immune Responses in Islet Xenograft Rejection

Porcine islets surviving the acute injury caused by humoral rejection and IBMIR will be subjected to cellular xenograft rejection, which is predominately mediated by CD4⁺ T cells and is characterised by significant infiltration of macrophages, B cells and T cells (CD4⁺ and CD8⁺). Overall, the response is different compared to the alloimmune response and more difficult to suppress. Activation of CD4⁺ T cells is both by direct and indirect antigen presentation. After activation they recruit macrophages and direct B cell responses. Although they are less important than CD4⁺ T cells in islet xenograft rejection, macrophages are believed to be a major effector cell in this response. Rodent studies have shown that xenoantigen-primed and CD4⁺ T cell-activated macrophages were capable of recognition and rejection of pancreatic islet xenografts, and they destroyed a graft via the secretion of various proinflammatory mediators, including TNF-α, reactive oxygen and nitrogen species, and complement factors. B cells are an important mediator of islet xenograft rejection via xenoantigen presentation, priming effector T cells and producing xenospecific antibodies. Depletion and/or inhibition of B cells combined with suppressing T cells has been suggested as a promising strategy for induction of xeno-donor-specific T- and B-cell tolerance in islet xenotransplantation. Thus, strategies that expand the influence of regulatory T cells and inhibit and/or reduce macrophage and B cell responses are required for use in combination with clinical applicable immunosuppressive agents to achieve effective suppression of the T cell-initiated xenograft response.

CTLA4Ig/VISTAIg combination therapy selectively induces CD4+ T cell-mediated immune tolerance by targeting the SOCS1 signaling pathway in porcine islet xenotransplantation

T cell inhibitory receptors can regulate the proliferation or function of T cells by binding to their ligands and present a unique opportunity to manage destructive immune responses during porcine islet xenotransplantation. We applied ex vivo porcine islet xenotransplantation and in vitro mixed lymphocyte-islet reaction models to assess immune checkpoint receptor expression profiles in recipient T cells, investigate whether CTLA4 or VISTA immunoglobulin (Ig) combination therapy alone could suppress porcine islet xenograft rejection and further analyze its potential immune tolerance mechanism. Recipient T cells expressed moderate to high levels of CTLA4, PD-1, TIGIT, and VISTA, and the frequency of CTLA4⁺ CD4⁺ , TIGIT⁺ CD4⁺ , VISTA⁺ CD4⁺ and VISTA⁺ CD8⁺ T cells was positively correlated with porcine islet xenograft survival time in xenotransplant recipients. Combined treatment with CTLA4Ig and VISTAIg selectively inhibited recipient CD4⁺ T cell hyperresponsiveness and proinflammatory cytokine production and significantly delayed xenograft rejection. SOCS1 deficiency in CD4⁺ T cells stimulated by xenogeneic islets facilitated hyperresponsiveness and abolished the suppressive effect of combination therapy on recipient T cell-mediated porcine islet damage in vivo and in vitro. Further mechanistic studies revealed that combined treatment significantly induced SOCS1 expression and inhibited the Jak-STAT signaling pathway in wild-type recipient CD4⁺ T cells stimulated by xenogeneic islets, whereas SOCS1 deficiency resulted in Jak-STAT signaling pathway activation in recipient CD4⁺ T cells. We demonstrated a major role for CTLA4 and VISTA as key targets in CD4⁺ T cell hyperresponsiveness and porcine islet xenograft rejection. The selective inhibition of CD4⁺ T cell immunity by CTLA4Ig/VISTAIg is based on SOCS1-dependent signaling.

Neonatal Pig Sertoli Cells Survive Xenotransplantation by Creating an Immune Modulatory Environment Involving CD4 and CD8 Regulatory T Cells

The acute cell-mediated immune response presents a significant barrier to xenotransplantation. Immune-privileged Sertoli cells (SC) can prolong the survival of co-transplanted cells including xenogeneic islets, hepatocytes, and neurons by protecting them from immune rejection. Additionally, SC survive as allo- and xenografts without the use of any immunosuppressive drugs suggesting elucidating the survival mechanism(s) of SC could be used to improve survival of xenografts. In this study, the survival and immune response generated toward neonatal pig SC (NPSC) or neonatal pig islets (NPI), nonimmune-privileged controls, was compared after xenotransplantation into naïve Lewis rats without immune suppression. The NPSC survived throughout the study, while NPI were rejected within 9 days. Analysis of the grafts revealed that macrophages and T cells were the main immune cells infiltrating the NPSC and NPI grafts. Further characterization of the T cells within the grafts indicated that the NPSC grafts contained significantly more cluster of differentiation 4 (CD4) and cluster of differentiation 8 (CD8) regulatory T cells (Tregs) at early time points than the NPI grafts. Additionally, the presence of increased amounts of interleukin 10 (IL-10) and transforming growth factor (TGF) β and decreased levels of tumor necrosis factor (TNF) α and apoptosis in the NPSC grafts compared to NPI grafts suggests the presence of regulatory immune cells in the NPSC grafts. The NPSC expressed several immunoregulatory factors such as TGFβ, thrombospondin-1 (THBS1), indoleamine-pyrrole 2,3-dioxygenase, and galectin-1, which could promote the recruitment of these regulatory immune cells to the NPSC grafts. In contrast, NPI grafts had fewer Tregs and increased apoptosis and inflammation (increased TNFα, decreased IL-10 and TGFβ) suggestive of cytotoxic immune cells that contribute to their early rejection. Collectively, our data suggest that a regulatory graft environment with regulatory immune cells including CD4 and CD8 Tregs in NPSC grafts could be attributed to the prolonged survival of the NPSC xenografts.

Vitamin D3 combined with antibody agents suppresses alloreactive memory T-cell responses to induce heart allograft long-term survival

BACKGROUND

The pre-stored memory T cells in organ transplant patient carry a high risk of allograft rejection. The current study aimed to determine whether the allogenic response of adoptively transferred memory T cells in mice was suppressed by vitamin D3 monotherapy alone or in combination with monoclonal antibody treatment.

METHODS

Prior to vascularized heterotopic heart transplantation, naïve C57BL/6 mice were primed with memory T cells. Recipient mice were administered vitamin D3 alone or in combination with monoclonal antibodies (anti-CD40L/ anti-LFA-1). Memory T cells and CD4⁺ forkhead box P3⁺ T cells in recipient spleens were measured using flow cytometry. Additionally, the expression of cytokines was measured by ELISA and quantitative PCR. Inflammatory factors in the grafts were identified by hematoxylin and eosin staining.

RESULTS

Vitamin D3 in conjunction with anti-CD40L/ anti-LFA-1 antibodies were administered according to the median survival time from 6.5 to 80 days. The results revealed that grafts were protected through the prevention of inflammatory cell infiltration. Combined treatment decreased the mRNA levels of IL-2, IFN-γ and IL-10 and increased the mRNA levels of IL-4, Foxp3 and TGF-β in the allograft. Rejection was suppressed by a reduction of CD4⁺CD44^high CD62L⁺ and CD8⁺ CD44^high CD62L⁺ memory T cells, the induction of regulatory T cells in the recipient spleen and a reduction of serum IL-2, IFN-γ and IL-10 levels.

CONCLUSION

Vitamin D3 efficiently protected allografts from memory T-cell allo-responses when combined with anti-CD40L/anti-LFA-1 antibodies therapy.

Tregs and Mixed Chimerism as Approaches for Tolerance Induction in Islet Transplantation

Pancreatic islet transplantation is a promising method for the treatment of type 1 and type 3 diabetes whereby replacement of islets may be curative. However, long-term treatment with immunosuppressive drugs (ISDs) remains essential for islet graft survival. Current ISD regimens carry significant side-effects for transplant recipients, and are also toxic to the transplanted islets. Pre-clinical efforts to induce immune tolerance to islet allografts identify ways in which the recipient immune system may be reeducated to induce a sustained transplant tolerance and even overcome autoimmune islet destruction. The goal of these efforts is to induce tolerance to transplanted islets with minimal to no long-term immunosuppression. Two most promising cell-based therapeutic strategies for inducing immune tolerance include T regulatory cells (Tregs) and donor and recipient hematopoietic mixed chimerism. Here, we review preclinical studies which utilize Tregs for tolerance induction in islet transplantation. We also review myeloablative and non-myeloablative hematopoietic stem cell transplantation (HSCT) strategies in preclinical and clinical studies to induce sustained mixed chimerism and allograft tolerance, in particular in islet transplantation. Since Tregs play a critical role in the establishment of mixed chimerism, it follows that the combination of Treg and HSCT may be synergistic. Since the success of the Edmonton protocol, the feasibility of clinical islet transplantation has been established and nascent clinical trials testing immune tolerance strategies using Tregs and/or hematopoietic mixed chimerism are underway or being formulated.

The blockade of cytoplasmic HMGB1 modulates the autophagy/apoptosis checkpoint in stressed islet beta cells

High mobility group (HMGB1) is an alarmin known to be harmful to pancreatic beta cells and associated with diabetes mellitus pathogenesis and pancreatic islet graft failure. It has been long thought that the suppression of HMGB1 molecule is beneficial to the beta cells. However, recent studies have indicated that cytoplasmic HMGB1 (cHMGB1) could function as a modulator to relieve cells from apoptotic stress by autophagy induction. Particularly, pancreatic beta cells have been known to utilize the autophagy-to-apoptosis switch when exposed to hypoxia or lipotoxicity. This study aimed to investigate the beta cells under hypoxic and lipotoxic stress while utilizing a small molecule inhibitor of HMGB1, inflachromene (ICM) which can suppress cHMGB1 accumulation. It was revealed that under cellular stress, blockade of cHMGB1 accumulation decreased the viability of islet grafts, primary islets and MIN6 cells. MIN6 cells under cHMGB1 blockade along with lipotoxic stress showed decreased autophagic flux and increased apoptosis. Moreover, cHMGB1 blockade in HFD-fed mice produced unfavorable outcomes on their glucose tolerance. In sum, these results suggested the role of cHMGB1 within beta cell autophagy/apoptosis checkpoint. Given the importance of autophagy in beta cells under apoptotic stresses, this study might provide further insights regarding HMGB1 and diabetes.

Progress towards xenogenic tolerance

PURPOSE OF REVIEW

To describe the most recent progress towards tolerance in xenotransplantation.

RECENT FINDINGS

Mixed chimerism and thymic transplantation have been used to promote tolerance in xenotransplantation models. Intra-bone bone marrow transplantation is a recent advance for mixed chimerism, which promotes longer lasting chimerism and early graft function of subsequent organ transplantation. The hybrid thymus, an advancement to the vascularized thymokidney and vascularized thymic lobe, is being developed to allow for both donor and recipient T-cell selection in the chimeric thymus, encouraging tolerance to self and donor while maintaining appropriate immune function. Regulatory T cells show promise to promote tolerance by suppressing effector T cells and by supporting mixed chimerism. Monoclonal antibodies such as anti-CD2 may promote tolerance through suppression of CD2+ effector and memory T cells whereas Tregs, which express lower numbers of CD2, are relatively spared and might be used to promote tolerance.

SUMMARY

These findings contribute major advances to tolerance in xenotransplantation. A combination of many of these mechanisms will likely be needed to have long-term tolerance maintained without the use of immunosuppression.

CD4+ /CD8+ T-cell ratio correlates with the graft fate in pig-to-non-human primate islet xenotransplantation

BACKGROUND

Xenogeneic islet transplantation using porcine pancreata has been a promising option for substituting human islet transplantation. Moreover, recent advances in pre-clinical results have put islet xenotransplantation closer to the possibility of clinical application. While preparing for the era of clinical xenotransplantation, developing non-invasive immune monitoring method which could predict the graft fate could benefit the patient. However, there are few reports showing predictive immune parameters associated with the fate of the graft in islet xenotransplantation.

METHODS

The absolute number and ratio of T-cell subsets have been measured via flow cytometry from the peripheral blood of 16 rhesus monkeys before and after porcine islet xenotransplantation. The correlation between the graft survival and the absolute number or ratio of T cells was retrospectively analyzed.

RESULTS

The ratio of CD4⁺ versus CD8⁺ T cells was significantly reduced due to CD8⁺ effector memory cells' increase. Correlation analyses revealed that CD4⁺ /CD8⁺ , CD4⁺ /CD8⁺ naïve, CD4⁺ naïve/CD8⁺ naïve, and CD4⁺ central memory/CD8⁺ naïve cell ratios negatively correlated with the duration of graft survival. Conversely, further analyses discovered strong, positive correlation of CD4⁺ /CD8⁺ cell ratios within the early graft-rejected monkeys (≤60 days).

CONCLUSIONS

This retrospective study demonstrated that CD4⁺ /CD8⁺ ratios correlated with graft survival, especially in recipients which rejected the graft in early post-transplantation periods. CD4⁺ /CD8⁺ ratios could be used as a surrogate marker to predict the graft fate in pig-to-NHP islet xenotransplantation.

A combination regimen of low-dose bortezomib and rapamycin prolonged the graft survival in a murine allogeneic islet transplantation model

As the first FDA-approved proteasome inhibitor drug, bortezomib has been used for the treatment of multiple myeloma and lymphoma. However, its effects alone or in combination with other immunosuppressants on allogeneic islet transplantation have not been reported so far. In this study, we showed that the short-term combination treatment of low-dose bortezomib and rapamycin significantly prolonged the survival of islet allografts. Short-term treatment of low-dose (0.05 mg/kg or 0.1 mg/kg) bortezomib reduced the MHC class II expression in dendritic cells (DCs) of alloantigen-sensitized mice, and prolonged the islet allograft survival for up to 50 days in diabetic mice. Notably, when bortezomib was combined with rapamycin, it induced islet-specific immunological tolerance which allowed the acceptance of a second graft without additional immunosuppression. This regimen dramatically reduced the alloantigen-specific IFN-γ-producing T cells in the spleen, and increased regulatory T cells both at the graft site and in the spleen. Therefore, we propose that short-term treatment of low-dose bortezomib and rapamycin could be a new tolerance-promoting immunosuppressive regimen for allogeneic islet transplantation.