Despite efficient intrathymic negative selection of host-reactive T cells, autoimmune disease may develop in porcine thymus-grafted athymic mice: evidence for failure of regulatory mechanisms suppressing autoimmunity

Developments in kidney xenotransplantation

The search for kidney xenografts that are appropriate for patients with end-stage renal disease has been ongoing since the beginning of the last century. The major cause of xenograft loss is hyperacute and acute rejection, and this has almost been overcome via scientific progress. The success of two pre-clinical trials of α1,3-galactosyltransferase gene-knockout porcine kidneys in brain-dead patients in 2021 triggered research enthusiasm for kidney xenotransplantation. This minireview summarizes key issues from an immunological perspective: the discovery of key xenoantigens, investigations into key co-stimulatory signal inhibition, gene-editing technology, and immune tolerance induction. Further developments in immunology, particularly immunometabolism, might help promote the long-term outcomes of kidney xenografts.

Role of the thymus in spontaneous development of a multi-organ autoimmune disease in human immune system mice

We evaluated the role of the thymus in development of multi-organ autoimmunity in human immune system (HIS) mice. T cells were essential for disease development and the same T cell clones with varying phenotypes infiltrated multiple tissues. De novo-generated hematopoietic stem cell (HSC)-derived T cells were the major disease drivers, though thymocytes pre-existing in grafted human thymi contributed if not first depleted. HIS mice with a native mouse thymus developed disease earlier than thymectomized mice with a thymocyte-depleted human thymus graft. Defective structure in the native mouse thymus was associated with impaired negative selection of thymocytes expressing a transgenic TCR recognizing a self-antigen. Disease developed without direct recognition of antigens on recipient mouse MHC. While human thymus grafts had normal structure and negative selection, failure to tolerize human T cells recognizing mouse antigens presented on HLA molecules may explain eventual disease development. These new insights have implications for human autoimmunity and suggest methods of avoiding autoimmunity in next-generation HIS mice.

Reduced positive selection of a human TCR in a swine thymus using a humanized mouse model for xenotolerance induction

BACKGROUND

Tolerance-inducing approaches to xenotransplantation would be optimal and may be necessary for long-term survival of transplanted pig organs in human patients. The ideal approach would generate donor-specific unresponsiveness to the pig organ without suppressing the patient's normal immune function. Porcine thymus transplantation has shown efficacy in promoting xenotolerance in humanized mice and large animal models. However, murine studies demonstrate that T cells selected in a swine thymus are positively selected only by swine thymic epithelial cells, and therefore, cells expressing human HLA-restricted TCRs may not be selected efficiently in a transplanted pig thymus. This may lead to suboptimal patient immune function.

METHODS

To assess human thymocyte selection in a pig thymus, we used a TCR transgenic humanized mouse model to study positive selection of cells expressing the MART1 TCR, a well-characterized human HLA-A2-restricted TCR, in a grafted pig thymus.

RESULTS

Positive selection of T cells expressing the MART1 TCR was inefficient in both a non-selecting human HLA-A2- or swine thymus compared with an HLA-A2+ thymus. Additionally, CD8 MART1 TCRbright T cells were detected in the spleens of mice transplanted with HLA-A2+ thymi but were significantly reduced in the spleens of mice transplanted with swine or HLA-A2- thymi. [Correction added on October 15, 2019, after first online publication: The missing superscript values +, -, and bright have been included in the Results section.] CONCLUSIONS: Positive selection of cells expressing a human-restricted TCR in a transplanted pig thymus is inefficient, suggesting that modifications to improve positive selection of cells expressing human-restricted TCRs in a pig thymus may be necessary to support development of a protective human T-cell pool in future patients.

Preparation of hybrid porcine thymus containing non-human primate thymic epithelial cells in miniature swine

BACKGROUND

We have achieved greater than a 6-month survival of a life-supporting kidney co-transplanted with a vascularized thymic graft into non-human primates (NHPs). Although we have achieved pig-specific unresponsiveness in vitro, immunosuppression was not able to be fully weaned. Studies in mice and humanized mice suggest that a hybrid pig thymus (Hyb-thy)-containing host thymic epithelial cells (TECs) can optimize intra-thymic selection, achieving xenograft tolerance with improved reconstitution of T-cell function.

METHODS

We have tested the feasibility of the preparation of a Hyb-thy that contains NHP TECs in the donor thymic grafts. We first prepared the Hyb-thy in the donor pigs 2-3 weeks before xeno-Tx. We performed six cases of Hyb-thy preparation in six juvenile miniature swine. Two pigs received non-manipulated cynomolgus monkey thymic cells that were isolated from an excised atrophic thymus via injection into their thymic lobes (Group 1). The remaining four received thymic cells that were isolated from non-atrophic thymic glands (Groups 2 and 3). Pigs in Group 2 received unmanipulated thymic cells in one thymic lobe, as well as CD2-positive cell-depleted TEC-enriched cells in the contralateral lobe. Pigs in Group 3 received TEC-enriched cells alone.

RESULTS

All thymus-injected pigs received tacrolimus and rapamycin until endpoint (POD16). We detected cynomolgus monkey TEC networks in pig thymus from Groups 1 and 3, while pigs in Group 2 rejected the thymic cells. We demonstrated the preparation of Hyb-thy in pigs using tacrolimus plus rapamycin therapy.

CONCLUSIONS

Our results suggest that the enrichment of TEC from the excised NHP thymus facilitated NHP TEC engraftment in pig thymus.

IXA Honorary Member Lecture, 2017: The long and winding road to tolerance

The last 15 years or so have seen exciting progress in xenotransplantation, with porcine organ grafts surviving months or even years in non-human primates. These advances reflect the application of new scientific knowledge, improved immunosuppressive agents, and genetic engineering. The field has recently enjoyed a renaissance of interest and hope, largely due to the exponential increase in our capacity to genetically engineer porcine source animals. However, immune responses to xenografts are very powerful and widespread clinical application of xenotransplantation will depend on the ability to suppress these immune responses while preserving the capacity to protect both the recipient and the graft from infectious microorganisms. Our work over the last three decades has aimed to engineer the immune system of the recipient in a manner that achieves specific tolerance to the xenogeneic donor while preserving otherwise normal immune function. Important proofs of principle have been obtained, first in rodents, and later in human immune systems in "humanized mice" and finally in non-human primates, demonstrating the capacity and potential synergy of mixed xenogeneic chimerism and xenogeneic thymic transplantation in tolerizing multiple arms of the immune system. Considering the fact that clinical tolerance has recently been achieved for allografts and the even greater importance of avoiding excessive immunosuppression for xenografts, it is my belief that it is both possible and imperative that we likewise achieve xenograft tolerance. I expect this to be accomplished through the availability of targeted approaches to recipient immune conditioning, understanding of immunological mechanisms of tolerance, advanced knowledge of physiological incompatibilities, and the availability of inbred miniature swine with optimized use of genetic engineering.

Transplant Tolerance: Current Insights and Strategies for Long-Term Survival of Xenografts

Xenotransplantation is an attractive solution to the problem of allograft shortage. However, transplants across discordant species barriers are subject to vigorous immunologic and pathobiologic hurdles, some of which might be overcome with the induction of immunologic tolerance. Several strategies have been designed to induce tolerance to a xenograft at both the central (including induction of mixed chimerism and thymic transplantation) and peripheral (including adoptive transfer of regulatory cells and blocking T cell costimulation) levels. Currently, xenograft tolerance has been well-established in rodent models, but these protocols have not yet achieved similar success in nonhuman primates. This review will discuss the major barriers that impede the establishment of immunological tolerance across xenogeneic barriers and the potential solution to these challenges, and provide a perspective on the future of the development of novel tolerance-inducing strategies.

Tolerance in xenotransplantation

PURPOSE OF REVIEW

This review describes recent progress in tolerance-inducing strategies across xenogeneic immunological barriers as well as the potential benefit of a tolerance strategy for islets and kidney xenotransplantation.

RECENT FINDINGS

Using advanced gene editing technologies, xenotransplantation from multitransgenic alpha-1,3-galactosyltransferase knockout pigs has demonstrated marked prolongation of renal xenograft survival, ranging from days to greater than several months for life-supporting kidneys, and more than 2 years in a heterotopic nonlife-supporting cardiac xenograft model. Continuous administration of multiple immunosuppressive drugs has been required and attempts to taper immunosuppression have been unsuccessful. It appears likely that low levels of T cell dependent antibodies and activation of innate responses are responsible for xenograft loss. Mixed chimerism and thymic transplantation approaches have achieved xenogeneic tolerance in pig-to-mouse models and both have recently been extended to pig-to-baboon models. Encouraging results have been reported, including persistence of macrochimerism, prolonged pig skin graft survival, donor-specific unresponsiveness in vitro and detection of recent T cell emigrants in vivo.

SUMMARY

Although tolerance induction in vivo has not yet been achieved in pig-to-baboon models, recent results are encouraging that this goal will be attainable through genetic engineering of porcine donors.

Xenotransplantation: immunological hurdles and progress toward tolerance

The discrepancy between organ need and organ availability represents one of the major limitations in the field of transplantation. One possible solution to this problem is xenotransplantation. Research in this field has identified several obstacles that have so far prevented the successful development of clinical xenotransplantation protocols. The main immunologic barriers include strong T-cell and B-cell responses to solid organ and cellular xenografts. In addition, components of the innate immune system can mediate xenograft rejection. Here, we review these immunologic and physiologic barriers and describe some of the strategies that we and others have developed to overcome them. We also describe the development of two strategies to induce tolerance across the xenogeneic barrier, namely thymus transplantation and mixed chimerism, from their inception in rodent models through their current progress in preclinical large animal models. We believe that the addition of further beneficial transgenes to Gal knockout swine, combined with new therapies such as Treg administration, will allow for successful clinical application of xenotransplantation.

Xenograft tolerance and immune function of human T cells developing in pig thymus xenografts

Transplantation of xenogeneic thymus tissue allows xenograft tolerance induction in the highly disparate pig-to-mouse model. Fetal swine thymus (SW THY) can support the generation of a diverse human T cell repertoire that is tolerant of the pig in vitro. We demonstrate that SW THY generates all human T cell subsets, including regulatory T cells (Tregs), in similar numbers as fetal human thymus (HU THY) grafts in immunodeficient mice receiving the same human CD34(+) cells. Peripheral T cells are specifically tolerant to the mouse and to the human and porcine donors, with robust responses to nondonor human and pig Ags. Specific tolerance is observed to pig skin grafts sharing the THY donor MHC. SW THY-generated peripheral Tregs show similar function, but include lower percentages of naive-type Tregs compared with HU THY-generated Tregs. Tregs contribute to donor-pig specific tolerance. Peripheral human T cells generated in SW THY exhibit reduced proportions of CD8(+) T cells and reduced lymphopenia-driven proliferation and memory-type conversion, accelerated decay of memory-type cells, and reduced responses to protein Ags. Thus, SW thymus transplantation is a powerful xenotolerance approach for human T cells. However, immune function may be further enhanced by strategies to permit positive selection by autologous HLA molecules.

Toward development and production of human T cells in swine for potential use in adoptive T cell immunotherapy

Immunotherapy and vaccination for cancer or infection are generally approached by administration of antigen or stimulation of antigen-presenting cells or both. These measures may fail if the treated individual lacks T cells specific for the immunogen(s). We tested another strategy-the generation of new T cells from hematopoietic stem cells that might be used for adoptive immunotherapy. To test this concept, we introduced T cell-depleted human bone marrow cells into fetal swine and tested the swine for human T cells at various times after birth. Human T cells were detected in the thymus and blood of the treated swine. These cells were generated de novo as they contained human T cell receptor excision circles not present in the T cell-depleted bone marrow. The human T cells were highly diverse and included novel specificities capable of responding to antigen presented by human antigen-presenting cells. Our findings constitute a first step in a new promising approach to immunotherapy in which tumor- or virus-specific T cell clones lacking in an individual might be generated in a surrogate host from hematopoietic stem cells of the individual to be treated.

Abnormal regulatory and effector T cell function predispose to autoimmunity following xenogeneic thymic transplantation

Porcine thymus grafts support robust murine and human thymopoiesis, generating a diverse T cell repertoire that is deleted of donor and host-reactive cells, achieving specific xenograft tolerance. Positive selection is mediated exclusively by the xenogeneic thymic MHC. Although thymectomized, T cell-depleted normal mice usually remain healthy following xenogeneic thymic transplantation, thymus-grafted congenitally athymic mice frequently develop multiorgan autoimmunity. We investigated the etiology of this syndrome by adoptively transferring lymphocyte populations from fetal pig thymus-grafted BALB/c nude mice to secondary BALB/c nude recipients. Fetal pig thymus-grafted nude mice generated normal numbers of CD25(+)Foxp3(+)CD4 T cells, but these cells lacked the capacity to block autoimmunity. Moreover, thymocytes and peripheral CD4(+)CD25(-) cells from fetal pig thymus-grafted nude mice, but not those from normal mice, induced autoimmunity in nude recipients. Injection of thymic epithelial cells from normal BALB/c mice into fetal pig thymus grafts reduced autoimmunity and enhanced regulatory function of splenocytes. Our data implicate abnormalities in postthymic maturation, expansion, and/or survival of T cells positively selected by a xenogeneic MHC, as well as incomplete intrathymic deletion of thymocytes recognizing host tissue-specific Ags, in autoimmune pathogenesis. Regulatory cell function is enhanced and negative selection of host-specific thymocytes may potentially also be improved by coimplantation of recipient thymic epithelial cells in the thymus xenograft.

Achieving tolerance in pig-to-primate xenotransplantation: reality or fantasy

Because the immunologic differences between species are far greater than those within species, it is likely that the amount of immunosuppression that would be required for successful xenografting would be so much greater than that now used for allografting, that the side-effects and complications would be unacceptable. Tolerance approaches to xenotransplantation would overcome this concern. Studies in humanized mouse models have demonstrated that human T cells can be tolerized to porcine xenografts, providing important proofs of principle of the potential feasibility of pig-to-primate xenograft tolerance. The results available from studies of pig-to-primate xenotransplantation to date have demonstrated that while chronic immunosuppressive drugs have not completely avoided either T cell responses or humoral rejection, approaches directed toward tolerance induction have been encouraging with regard to avoiding immunization at both of these levels.

Comparison of human T cell repertoire generated in xenogeneic porcine and human thymus grafts

BACKGROUND

Xenogeneic thymus transplantation is an effective approach to achieving T cell tolerance across highly disparate xenogeneic species barriers. We have previously demonstrated that phenotypically normal, specifically tolerant human T cells are generated in porcine thymic grafts. In this study, we assessed the diversity of the human T cell repertoire generated in porcine thymic xenografts. We also examined the ability of porcine thymus grafts to coexist with human thymus grafts.

METHODS

Fetal swine (SW) or human (HU) thymus with human fetal liver fragments were transplanted under the kidney capsule of 3Gy irradiated NOD/SCID mouse recipients. Thymus tissue was harvested approximately 16 weeks posttransplant for analysis of mixed lymphocyte reactions and spectratyping of human CD4 and CD8 single positive thymocytes.

RESULTS

T cell receptor beta genes of human CD4 and CD8 single positive cells developing in HU and SW thymus grafts showed similar, normal CDR3 length distributions. Human T cells developing in SW thymus grafts showed specific unresponsiveness to the major histocompatibility complex of the donor swine in mixed leukocyte reaction assays. In two of three animals receiving SW and HU thymus grafts under opposite kidney capsules, both grafts functioned. In animals with surviving SW grafts, thymocytes from the SW but not the HU grafts showed specific unresponsiveness to the SW donor.

CONCLUSION

Swine thymus grafts support generation of human T cells with a diverse T cell receptor repertoire. Human thymocytes in human thymus grafts are not tolerized by the presence of an additional porcine thymus, but tolerance might be achieved by postthymic encounter with porcine antigens.

Occurrence of autoimmunity after xenothymus transplantation in T-cell-deficient mice depends on the thymus transplant technique

Xenothymus transplantation under the kidney capsule in athymic rodents frequently leads to multiorgan autoimmunity. Herein, we explore whether this is an intrinsic risk of xenothymus grafting or whether it depends on the transplant technique. We developed a new technique of "venous pouch" thymus grafting (heart-xenothymus) and compared this with the conventional kidney subcapsular technique (kidney-xenothymus) in a rat-into-nude-mouse model. Whereas lethal autoimmunity developed in 90% of kidney-xenothymus recipients, all heart-xenothymus grafted mice remained completely healthy. Autoimmunity in heart-xenothymus recipients was absent despite a significantly improved T-cell generation and was associated with significantly higher CD4+CD25+ T-cell frequencies and CD4+CD25+ cell Foxp3 mRNA levels than those observed in kidney-xenothymus recipients. In conclusion, we describe a novel vascular pouch technique of xenothymus transplantation that prevents the development of autoimmunity in nude mice. Our data further suggest that prevention of autoimmunity is related to a superior development of regulatory T-cells.

Xenotransplantation: current status and a perspective on the future

Xenotransplantation using pigs as the transplant source has the potential to resolve the severe shortage of human organ donors. Although the development of relatively non-toxic immunosuppressive or tolerance-inducing regimens will be required to justify clinical trials using pig organs, recent advances in our understanding of the biology of xenograft rejection and zoonotic infections, and the generation of alpha1,3-galactosyltransferase-deficient pigs have moved this approach closer to clinical application. This Review highlights the major obstacles impeding the translation of xenotransplantation into clinical therapies and the potential solutions, providing a perspective on the future of clinical xenotransplantation.

A Potential Side Effect of Cyclosporin A: Inhibition of CD4+CD25+ Regulatory T Cells in Mice

BACKGROUND

CD4CD25 regulatory T (Treg) cells are essential for the induction and maintenance of immunologic self-tolerance as well as transplant tolerance. The effects of cyclosporin A (CsA), a widely used immunosuppressive agent, on CD4CD25Treg cells in mice were investigated.

METHODS

Balb/c mice were injected with CsA or control solution for one month. The levels, phenotype, and function of CD4CD25Treg cells in these mice were then assayed.

RESULTS

The percentages and total cell numbers of CD4CD25Treg cells in the peripheral blood and spleen were significantly reduced after the treatment with CsA. The total numbers of CD4CD25Treg cells in the thymus of CsA-treated mice were markedly reduced as compared to the control mice. However, the percentage of CD4CD25Treg cells in the thymus of CsA-treated mice was markedly enhanced. More CD4CD25Treg cells expressing high levels of CD44 and CD45RB, and less CD4CD25Treg cells expressing CD62L were observed in CsA-treated mice, compared with the control mice. CD4CD25Treg cells expressed slightly lower levels of Foxp3 in CsA-treated mice. Furthermore, CsA markedly impaired the immunosuppressive function of CD4CD25Treg cells.

CONCLUSIONS

CsA significantly impaired the development and function of CD4CD25Treg cells. The present studies suggest that CsA may block the potential induction of immune tolerance and increase the susceptibility to develop autoimmune diseases while preventing graft rejection.

Presence of functional mouse regulatory CD4+CD25+T cells in xenogeneic neonatal porcine thymus-grafted athymic mice

Xenotransplantation with porcine thymus is emerging as a possible means to reconstitute host cellular immunity and to induce immune tolerance in rodents and large animals. However, the presence of regulatory T cells (Treg cells) in this model needs to be determined. We herein demonstrated that efficient repopulation of mouse CD4+CD25+Treg cells was achieved in Balb/c nude mice by grafting neonatal porcine thymic tissue (NP THY). Mouse CD4+CD25+T cells expressed normal levels of Foxp3 in NP THY-grafted nude mice. Furthermore, these CD4+CD25+Treg cells showed significant inhibitory effects on the cell proliferation or interleukin-2 products of syngeneic T cells to alloantigens, Con A or a peptide antigen, although the potent immunosuppressive function might be lower than CD4+CD25+Treg cells in Balb/c mice. CD4+CD25+T cells in NP THY-grafted nude mice showed significantly stronger inhibition on the response to donor porcine antigens of CD4+CD25(-)T cells than CD4+CD25+Treg cells in Balb/c mice. Both CD4+CD25+Treg cells in NP THY-grafted nude and Balb/c mice prevented the development of autoimmune disease mediated by syngeneic CD4+CD25(-)T cells in a similar efficient way in the secondary recipients. These findings provide evidence for the potential involvement of CD4+CD25+Treg cells in keeping self-tolerance and transplant tolerance in this xeno-thymus transplantation model.

Reconstitution of a functional human immune system in immunodeficient mice through combined human fetal thymus/liver and CD34+ cell transplantation

Studies of the human immune system have been limited by the lack of an appropriate in vivo model. For this reason, efforts have been made to develop murine models with a functional human immune system. We report here that cotransplantation of human fetal thymus/liver tissues and CD34(+) hematopoietic stem/progenitor cells led to the development of sustained human hematopoiesis and a functional human immune system in immunodeficient NOD/SCID mice. The humanized mice showed systemic repopulation with a comprehensive array of human lymphohematopoietic cells, including T cells, B cells, and dendritic cells, and the formation of secondary lymphoid organs. Furthermore, these mice produce high levels of human IgM and IgG antibodies and mediate strong immune responses in vivo as demonstrated by skin xenograft rejection. Thus, the humanized NOD/SCID mice described in this paper provide a powerful model system to study human immune function.

Host thymectomy and cyclosporine lead to unstable skin graft tolerance after class I mismatched allogeneic neonatal thymic transplantation in mice

BACKGROUND

Our laboratory has demonstrated that xenogeneic porcine thymus tissue grafted in thymectomized (ATX) and T cell-depleted mice induces donor-specific tolerance. Recipient thymectomy is essential for the success of tolerance induction. In contrast, studies in pigs grafted with non-vascularized allogeneic class I mismatched thymus tissue under the cover of CyA have shown that removal of host thymus is detrimental to thymic graft survival. To determine the requirements for nonvascularized allogeneic class I-mismatched thymic engraftment in mice, we performed thymic allotransplantation under the cover of CyA.

MATERIALS AND METHODS

Euthymic and ATX B10.MBR mice received class I mismatched B10.AKM neonatal mouse thymus (NMTHY) tissue under the kidney capsule with or without a short course of CyA. The grafts were allowed to engraft for two and a half months before exploratory laparotomy was performed to evaluate them. Three months after the thymic transplant, mice were challenged with donor-specific skin grafts to assess tolerance. One month after donor-specific skin grafting, they received third party B10.BR skin grafts. Cellular anti-donor immune responses were studied at the time of euthanasia.

RESULTS

CyA-treated ATX and euthymic control mice showed good engraftment of the allogeneic thymic tissue at the time of exploratory laparotomy, whereas non-CyA-treated ATX and euthymic controls had rejected the grafts. The CyA-treated ATX B10.MBR mice accepted donor-specific skin grafts, but rejected them following a challenge with third party B10.BR skin grafts. Untreated ATX and euthymic mice and 6 of 7 CyA-treated euthymic mice rejected donor skin within 15 days. Mixed lymphocyte reactions did not show an increased anti-donor response, but CML clearly showed sensitization and increased killing activity against donor-type targets in these mice.

CONCLUSION

Allogeneic thymic transplantation across a class I MHC barrier under the cover of CyA induces a metastable state of tolerance in mice. To achieve this state, ATX of the recipient is required.