Induction of peripheral tolerance by intrathymic inoculation of soluble alloantigens: evidence for the role of host antigen-presenting cells and suppressor cell mechanism

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.

Transfer of multiple loci of donor's genes to induce recipient tolerance in organ transplantation

Donor organ rejection remains a significant problem. The present study aimed to assess whether transferring a donor's major histocompatibility complex (MHC) genes to the recipient could mitigate rejection in organ transplantation. Seven loci of MHC genes from donor mice were amplified and ligated into vectors; the vectors either contained one K locus, seven loci or were empty (control). The vectors were subsequently injected into the thymus of recipients (in heterotransplants, recipient rats received the vector containing one K locus), following which donor mouse hearts were transplanted. Following the transplantation of allograft and heterograft, electrocardiosignals were viable for a significantly longer duration in recipient mice and rats receiving the donor histocompatibility-2 complex (H-2)^d genes compared with those in controls, and in mice that received seven vectors compared with those receiving one vector. Mixed lymphocyte cultures containing cells from these recipients proliferated significantly less compared with mixed lymphocyte cultures containing controls. Also, hearts from H-2^d genes-treated recipients demonstrated less lymphocyte infiltration and necrosis compared with the control recipient. The present study concluded that allograft and heterograft rejection may be mitigated by introducing the donor's MHC into the recipient; transferring seven loci has been demonstrated to be more effective than transferring one locus.

The Lymphatic Fluid

This review will highlight our current understanding of the formation, circulation, and immunological role of lymphatic fluid. The formation of the extracellular fluid depends on the net balance between the hydrostatic and osmotic pressure gradients effective in the capillary beds. Lymph originates from the extracellular fluid and its composition combines the ultrafiltrated plasma proteins with the proteome generated by the metabolic activities of each parenchymal tissue. Several analyses have indicated how the lymph composition reflects the organs' physiological and pathological states. The collected lymphatic fluid moves from the capillaries into progressively larger collectors toward the draining lymph node aided by the lymphangion contractility and unidirectional valves, which prevent backflow. The proteomic composition of the lymphatic fluid is reflected in the MHC II peptidome presented by nodal antigen-presenting cells. Taken together, the past few years have generated new interest in the formation, transport, and immunological role of the lymphatic fluid.

Mechanisms of Tolerance Induction by Hematopoietic Chimerism: The Immune Perspective

Hematopoietic chimerism is one of the effective approaches to induce tolerance to donor‐derived tissue and organ grafts without administration of life‐long immunosuppressive therapy. Although experimental efforts to develop such regimens have been ongoing for decades, substantial cumulative toxicity of combined hematopoietic and tissue transplants precludes wide clinical implementation. Tolerance is an active immunological process that includes both peripheral and central mechanisms of mutual education of coresident donor and host immune systems. The major stages include sequential suppression of early alloreactivity, establishment of hematopoietic chimerism and suppressor cells that sustain the state of tolerance, with significant mechanistic and temporal overlap along the tolerization process. Efforts to devise less toxic transplant strategies by reduction of preparatory conditioning focus on modulation rather than deletion of residual host immunity and early reinstitution of regulatory subsets at the central and peripheral levels. Stem Cells Translational Medicine2017;6:700–712

The melting pot of the MHC II peptidome

Recent advances in mass spectrometry technology have facilitated detailed examination of MHC-II immunopeptidomes, for example the repertoires of peptides bound to MHC-II molecules expressed in antigen presenting cells. These studies have deepened our view of MHC-II presentation. Other studies have broadened our view of pathways leading up to peptide loading. Here we review these recent studies in the context of earlier work on conventional and non-conventional MHC-II processing. The message that emerges is that sources of antigen beyond conventional endosomal processing of endocytosed proteins are important for generation of cellular immune responses to pathogens and maintenance of central and peripheral tolerance. The multiplicity of pathways results in a broad MHC II immunopeptidome that conveys the sampled environment to patrolling T cells.

The Dendritic Cell Major Histocompatibility Complex II (MHC II) Peptidome Derives from a Variety of Processing Pathways and Includes Peptides with a Broad Spectrum of HLA-DM Sensitivity

The repertoire of peptides displayed in vivo by MHC II molecules derives from a wide spectrum of proteins produced by different cell types. Although intracellular endosomal processing in dendritic cells and B cells has been characterized for a few antigens, the overall range of processing pathways responsible for generating the MHC II peptidome are currently unclear. To determine the contribution of non-endosomal processing pathways, we eluted and sequenced over 3000 HLA-DR1-bound peptides presented in vivo by dendritic cells. The processing enzymes were identified by reference to a database of experimentally determined cleavage sites and experimentally validated for four epitopes derived from complement 3, collagen II, thymosin β4, and gelsolin. We determined that self-antigens processed by tissue-specific proteases, including complement, matrix metalloproteases, caspases, and granzymes, and carried by lymph, contribute significantly to the MHC II self-peptidome presented by conventional dendritic cells in vivo. Additionally, the presented peptides exhibited a wide spectrum of binding affinity and HLA-DM susceptibility. The results indicate that the HLA-DR1-restricted self-peptidome presented under physiological conditions derives from a variety of processing pathways. Non-endosomal processing enzymes add to the number of epitopes cleaved by cathepsins, altogether generating a wider peptide repertoire. Taken together with HLA-DM-dependent and-independent loading pathways, this ensures that a broad self-peptidome is presented by dendritic cells. This work brings attention to the role of "self-recognition" as a dynamic interaction between dendritic cells and the metabolic/catabolic activities ongoing in every parenchymal organ as part of tissue growth, remodeling, and physiological apoptosis.

The lymph self-antigen repertoire

The lymphatic fluid originates from the interstitial fluid which bathes every parenchymal organ and reflects the “omic” composition of the tissue from which it originates in its physiological or pathological signature. Several recent proteomic analyses have mapped the proteome-degradome and peptidome of this immunologically relevant fluid pointing to the lymph as an important source of tissue-derived self-antigens. A vast array of lymph-circulating peptides have been mapped deriving from a variety of processing pathways including caspases, cathepsins, MMPs, ADAMs, kallikreins, calpains, and granzymes, among others. These self peptides can be directly loaded on circulatory dendritic cells and expand the self-antigenic repertoire available for central and peripheral tolerance.

Adoptive infusion of tolerogenic dendritic cells prolongs the survival of pancreatic islet allografts: a systematic review of 13 mouse and rat studies

Objective

The first Phase I study of autologous tolerogenic dendritic cells (Tol-DCs) in Type 1 diabetes (T1D) patients was recently completed. Pancreatic islet transplantation is an effective therapy for T1D, and infusion of Tol-DCs can control diabetes development while promoting graft survival. In this study, we aim to systematically review islet allograft survival following infusion of Tol-DCs induced by different methods, to better understand the mechanisms that mediate this process.

Methods

We searched PubMed and Embase (from inception to February 29^th, 2012) for relevant publications. Data were extracted and quality was assessed by two independent reviewers. We semiquantitatively analyzed the effects of Tol-DCs on islet allograft survival using mixed leukocyte reaction, Th1/Th2 differentiation, Treg induction, and cytotoxic T lymphocyte activity as mechanisms related-outcomes. We discussed the results with respect to possible mechanisms that promote survival.

Results

Thirteen articles were included. The effects of Tol-DCs induced by five methods on allograft survival were different. Survival by each method was prolonged as follows: allopeptide-pulsed Tol-DCs (42.14±44 days), drug intervention (39 days), mesenchymal stem cell induction (23 days), genetic modification (8.99±4.75 days), and other derivation (2.61±6.98 days). The results indicate that Tol-DC dose and injection influenced graft survival. Single-dose injections of 10⁴ Tol-DCs were the most effective for allograft survival, and multiple injections were not superior. Tol-DCs were also synergistic with immunosuppressive drugs or costimulation inhibitors. Possible mechanisms include donor specific T cell hyporesponsiveness, Th2 differentiation, Treg induction, cytotoxicity against allograft reduction, and chimerism induction.

Conclusions

Tol-DCs induced by five methods prolong MHC mismatched islet allograft survival to different degrees, but allopeptide-pulsed host DCs perform the best. Immunosuppressive or costimulatory blockade are synergistic with Tol-DC on graft survival. Multiple injections are not superior to single injection. Yet more rigorously designed studies with larger sample sizes are still needed in future.

Immune suppression produced by intrathymic inoculation with xenogeneic antigen and whole-body γ-irradiation in a pig-to-monkey heart transplantation model

OBJECTIVE

To validate the use of pig-to-monkey transplantation as an experimental animal model for study of the induction of immune suppression of cardiac xenografts via intrathymic inoculation with xenogeneic spleen cells and whole-body γ-irradiation.

MATERIALS AND METHODS

Donors (Meishan pigs) and recipients (Rhesus monkeys) were randomized into 3 groups. Group 2 was the control group; group 2 received whole-body irradiation, and group 3 received whole-body irradiation and intrathymic inoculation. In each group, survival time of heterotopic heart xenografts in monkeys were observed. A mixed lymphocyte reaction assay with (3)H thymidine was used to examine changes in lymphocyte function in all groups on the day of transplantation. In each group, recipient serum was harvested for analysis of IL-2 and IL-10 concentrations.

RESULTS

Mean (SD) survival time of donor hearts in group 3 was significantly longer (91.1 [22.8] hours) than in group 1 (36.6 [5.8] hours) (P < .01). The results of mixed lymphocyte reaction assay demonstrated a significant reduction in the stimulation index in group 3 compared with groups 1 and 2 (P < .01) when recipient splenocytes responded to the donor stimulator. The IL-2 concentration in recipients in all groups was much higher during rejection of xenografts than before transplantation (P < .01).

CONCLUSION

These results suggest that pretreatment with intrathymic inoculation or whole-body irradiation induces T-cell immunosuppression. The IL-2 concentration is closely correlated with xenograft rejection.

Regulatory dendritic cell therapy in organ transplantation

Dendritic cells (DCs) are uniquely well equipped antigen (Ag)-presenting cells. Their classic function was thought to be that of potent initiators of innate and adaptive immunity to infectious organisms and other Ags (including transplanted organs). Evidence has emerged, however, that DCs have a central and crucial role in determining the fate of immune responses toward either immunity or tolerance. This dichotomous function of DCs, coupled with their remarkable plasticity, renders them attractive therapeutic targets for immune modulation. In transplantation, much recent work has focused on the ability of DCs to silence immune reactivity in an Ag-specific manner in the hope of preventing rejection and diminishing reliance on potentially harmful immunosuppressive agents. Experimental strategies have included in vivo targeting of DCs, as well as ex vivo generation of regulatory (or tolerogenic) DCs with subsequent reinfusion (i.e. cell therapy). Different approaches to 'program' DC toward tolerogenic properties include genetic (transgene insertion), biologic (differential culture conditions, anti-inflammatory cytokine exposure) and pharmacologic manipulation. Recent data suggest a promising role for pharmacologic treatment as a means of generating potent regulatory DCs and have further stimulated speculation regarding their potential clinical application. Herein, we discuss evidence that the potential of regulatory DC therapy is considerable and that there are compelling reasons to evaluate it in the setting of organ transplantation in the near future.

Medium calcium concentration determines keratin intermediate filament density and distribution in immortalized cultured thymic epithelial cells (TECs)

Isolation and culture of thymic epithelial cells (TECs) using conventional primary tissue culture techniques under conditions employing supplemented low calcium medium yielded an immortalized cell line derived from the LDA rat (Lewis [Rt1l] cross DA [Rt1a]) that could be manipulated in vitro. Thymi were harvested from 4-5-day-old neonates, enzymically digested using collagenase (1 mg/ml, 37 degrees C, 1 h) and cultured in low calcium WAJC404A medium containing cholera toxin (20 ng/ml), dexamethasone (10 nM), epidermal growth factor (10 ng/ml), insulin (10 mug/ml), transferrin (10 mug/ml), 2% calf serum, 2.5% Dulbecco's Modified Eagle's Medium (DMEM), and 1% antibiotic/antimycotic. TECs cultured in low calcium displayed round to spindle-shaped morphology, distinct intercellular spaces (even at confluence), and dense reticular-like keratin patterns. In high calcium (0.188 mM), TECs formed cobblestone-like confluent monolayers that were resistant to trypsinization (0.05%) and displayed keratin intermediate filaments concentrated at desmosomal junctions between contiguous cells. Changes in cultured TEC morphology were quantified by an analysis of desmosome/membrane relationships in high and low calcium media. Desmosomes were significantly increased in the high calcium medium. These studies may have value when considering the growth conditions of cultured primary cell lines like TECs.

Adenoviral vectors: development and application

One of the prerequisites for the successful application of gene vaccination and therapy is the development of efficient gene delivery vectors. The rate-limiting nature of vectors was clearly manifested during the first wave of gene therapy testing, resulting in the demand for more effective and suitable vector systems. Adenoviral (Ad) vectors have recently played a central role in the development of gene-vector technology due to their practical advantages and potential applications. A large number of preclinical and clinical studies both have generated an overwhelming amount of data and literature on this vector system. It is the intention of this article to provide a systematic and broad spectrum review of this system, outlining the principle, potential, and limitations, and evaluating the rational development of this delivery approach. Recombinant adenoviruses (Ad), helper cell lines, and related technologies have been developed and applied to many indications owing to progress in virological research, molecular and cellular biology, eukaryotic protein expression, recombinant vaccines, and gene therapy. The technical depth this article covers should be useful to both the experienced researcher and to beginners in this field.

ROLE OF THE THYMUS IN TRANSPLANTATION TOLERANCE IN MINIATURE SWINE: IV. THE THYMUS IS REQUIRED DURING THE INDUCTION PHASE, BUT NOT THE MAINTENANCE PHASE, OF RENAL ALLOGRAFT TOLERANCE1

BACKGROUND

The authors' laboratory previously demonstrated that long-term tolerance to class I-disparate renal allografts in miniature swine can be induced by a short course of cyclosporine A (CsA), and that this stable tolerance is dependent on the presence of an intact thymus. In the present study, the authors have examined the requirement for a thymus during the pretransplant, induction, and maintenance phases of tolerance.

METHODS

Twenty-two miniature swine underwent class I major histocompatibility complex-mismatched renal transplantation, with a 12-day course of CsA. Thymectomies were performed on days -21, 0, +8, +21, and greater than or equal to +42, in relation to the day of transplantation. Historical controls consisted of euthymic and sham-thymectomized recipients.

RESULTS

Euthymic, sham-thymectomized, and day-greater than or equal to +42 thymectomized recipients demonstrated stable renal function and minimal anti-donor cytotoxic T-lymphocyte (CTL) responses. In contrast, day -21 and day 0 thymectomized recipients demonstrated allograft dysfunction, marked cellular infiltrates, with severe vasculitis and glomerular changes, and strong anti-donor CTL responses. Animals thymectomized on days +8 and +21 did not undergo severe rejection, but likewise did not demonstrate a stable clinical course.

CONCLUSIONS

These data indicate that the requirement for thymic function in the induction of rapid and stable tolerance is greatest during the first 8 days and then diminishes over the next 2 weeks posttransplant. Failure of thymectomy to affect the course of tolerance after day +21 suggests that thymic function is not required for the maintenance of tolerance. Understanding the role of the thymus in establishing tolerance may permit the development of tolerance induction strategies, especially for pediatric transplant recipients.

Functional and immunohistochemical evaluation of porcine neonatal islet-like cell clusters

Porcine neonatal islet-like cell clusters (NICCs) may be an attractive source of insulin-producing tissue for xenotransplantation in type I diabetic patients. We examined the functional and immunohistochemical outcome of the islet grafts in vitro during long-term culture and in vivo after transplantation to athymic nude mice. On average we obtained 29,000 NICCs from each pancreas. In a perifusion system, NICCs responded poorly to a glucose challenge alone, but 10 mmol/L arginine elicited a fourfold increase in insulin secretion and 16.7 mmol/L glucose + 10 mmol/L arginine caused a sevenfold increase in insulin section indicating some sensitivity towards glucose. Hormone content as well as the number of hormone-containing cells increased for the first 14 days of culture. When NICCs were stained for hormones, proliferation (Ki67), and duct cells (CK7), some insulin- and glucagon-positive cells co-stained for proliferation. However no co-staining was observed between insulin- and glucagon-positive cells or between hormone-and CK-positive cells. Following transplantation of 2000 NICCs under the renal capsule of diabetic nude mice, BG levels were normalized within an average of 13 weeks. Oral and IP glucose tolerance tests revealed a normal or even faster clearance of a glucose load compared with normal controls. Immunohistochemical examination of the grafts revealed primarily insulin-positive cells. In summary, in vitro, NICCs responded to a challenge including glucose and arginine. There was a potential for expansion of the beta-cell mass of NICCs in vitro as well as in vivo where NICCs eventually may normalize blood glucose of diabetic mice.

Prevention of chronic rejection with immunoregulatory cells induced by intrathymic immune modulation with class I allopeptides

Intrathymic immune modulation with RT1.Aa allopeptides in the PVG.R8-to-PVG.1 U rat strain combination leads to long-term survival of cardiac allografts. This regimen, however, does not induce transplantation tolerance, since most long-surviving allografts undergo chronic rejection. We investigated recipients with chronic rejection for donor-specific immune nonresponsiveness and immunoregulatory cells as possible mechanisms responsible for long-term graft survival. There was a significant reduction in the proliferative response of T cells from long-term allograft recipients to donor alloantigens as compared with that of naïve T cells. Adoptive transfer of splenocytes from intrathymically manipulated primary long-term graft survivors into minimally irradiated secondary hosts resulted in indefinite survival of > 80% of allografts, providing evidence for immunoregulatory cells. Secondary recipients had total absence of donor-reactive cellular and humoral responses. Immunoregulation was also transferable from secondary to tertiary graft recipients. More importantly, there was a significant reduction in the incidence of chronic rejection in secondary hosts (> 85%) and complete prevention of acute and chronic rejection in tertiary hosts. This study demonstrates that intrathymic immunomodulation with class I allopeptides results in the generation of immunoregulatory cells that do not block chronic rejection in primary hosts where they develop, but prevent both acute and chronic allograft rejection when adoptively transferred into secondary and tertiary recipients.

Inhibitory feedback loop between tolerogenic dendritic cells and regulatory T cells in transplant tolerance

An active role of T regulatory cells (Treg) and tolerogenic dendritic cells (Tol-DC) is believed important for the induction and maintenance of transplantation tolerance. However, interactions between these cells remain unclear. We induced donor-specific tolerance in a fully MHC-mismatched murine model of cardiac transplantation by simultaneously targeting T cell and DC function using anti-CD45RB mAb and LF 15-0195, a novel analog of the antirejection drug 15-deoxyspergualin, respectively. Increases in splenic Treg and Tol-DC were observed in tolerant recipients as assessed by an increase in CD4(+)CD25(+) T cells and DC with immature phenotype. Both these cell types exerted suppressive effects in MLR. Tol-DC purified from tolerant recipients incubated with naive T cells induced the generation/expansion of CD4(+)CD25(+) Treg. Furthermore, incubation of Treg isolated from tolerant recipients with DC progenitors resulted in the generation of DC with Tol-DC phenotype. Treg and Tol-DC generated in vitro were functional based on their suppressive activity in vitro. These results are consistent with the notion that tolerance induction is associated with a self-maintaining regulatory loop in which Tol-DC induce the generation of Treg from naive T cells and Treg programs the generation of Tol-DC from DC progenitors.

A central role for peripheral dendritic cells in the induction of acquired thymic tolerance

Despite extensive negative selection in the thymus, numerous clones of self-reactive T cells are normally exported to the periphery. In most instances, autoimmunity is prevented by regulatory T (Tr) cells, many of which are also of recent thymic origin. We have demonstrated recently that natural killer (NK) Tr thymocytes (THYr) can be induced by the injection of antigen into the eye, an immunologically privileged site; and that the intravenous infusion of antigen-presenting cells (APCs) from such animals also induces NKT THYr. Furthermore, we have also observed that some of these APCs migrate to the thymus as CD11c(+) dendritic cells (DCs). Other authors have correlated the migration of DCs to the thymus with the generation of CD4(+)CD25(+) THYr. We therefore propose a novel tolerance induction pathway by which tolerogenic DCs routinely transport antigen (both self and nonself) from the periphery to the thymus, where they positively select THYr. We also propose that the ability of tolerogenic DCs to induce acquired thymic tolerance on demand might have important implications for the immunotherapy of autoimmunity and allotransplantation.

Islet/pancreas transplantation: challenges for pediatrics

Beta cell replacement is a valid alternative to exogenous insulin injections to treat type 1 diabetic patients. The rate of success obtained after whole-pancreas transplantation, performed alone or in combination with kidney, and, as shown recently, by islet transplantation, justifies optimism and sets the stage for a larger clinical application of these approaches. Lifetime immunosuppression, however, required to protect the graft against recurrent autoimmune destruction and allorejection, raises serious doubts about the safety of its employment in children. While it is evident that children may be helped even more than adults by the possibility to correct diabetic metabolic disorders without exogenous insulin, and to lower in a more effective way the chance to develop secondary complications, the drawbacks of the currently used immunosuppressive drugs largely overcome the potential benefits. A great step forward for immediate applicability of transplantation to children involves the optimization of tolerogenic protocols and a better understanding of the concept of immune ignorance. Functional tolerance should be sufficient to entail the absence of immune reactivity against self- and graft antigens, while maintaining immune reactivity against other non-self, non-donor antigens. In addition, novel strategies aimed at utilizing surrogate beta cells obtained from non-islet cells, or by genetic manipulation of beta-cell precursors merit consideration as the use of xenogeneic donors. However, much work is still needed for their safe clinical implementation.

Prolongation of small bowel allograft survival with a sequential therapy consisting of a synthetic MHC class II peptide and temporarily low-dose cyclosporine A

It has been extensively documented the role of the indirect pathway of allorecognition in allograft rejection. However, recent data demonstrate that the manipulation of this pathway could be also sufficient to promote prolongation of allograft survival. In the present study we evaluated the effect of preoperative immunization with the WF-specific MHC class II peptides RT1.D2 and RT1.B2 in combination with low-dose CsA from days 0 to 7 (5 mg/kg/day) and from days 8 to 30 (1 mg/kg/day) after WF small bowel transplantation. Seven days before and on the day of transplantation, LEW recipients were immunized with the two WF MHC class II peptides RT1.B2 and RT1.D2. The CsA monotherapy induced an allograft survival of 49.3 +/- 6.1 days. MHC class II peptide immunization had a limited effect on allograft survival for RT1.D2 (47.1 +/- 3.8 days) and induced prolongation of allograft survival for RT1.B2 (73.6 +/- 34.6 days). This effect seems to be based on the absence or silence of RT1.B2-reactive T cells and rejection seems to be correlated with the presence of RT1.B2-specific T cells in the late phase. Therefore, the combination of RT1.B2 with low-dose CsA shifts the immunological response and protects small bowel allograft rejection.

Proposition of treatment to improve the immune response: possible application to AIDS

The molecular similarity between certain human antigenic determinants with those of HIV has been already described. In this matter, we have previously demonstrated, by a chromatographic method, the cross reactivity of human serum albumin with HIV gp 120. The hypothesis that this similarity could be one of the reasons why the virus escapes to the immune system is presented in this paper and a treatment is proposed to enhance the efficiency of the immune response: it is based on the blocking of the determinants of the self, expressed in the thymus, which are in common with the virus. Repeated injections within the thymus of neutralizing antibodies against the pathogen, obtained from a sufficiently distant animal species and purified by affinity chromatography, would prevent the T cells of the host recognizing these epitopes, common to the host and to the virus, from clonal deletion and would improve the immune response.

Tolerance and pancreatic islet transplantation

Islet transplantation holds renewed promise as a cure for type I diabetes mellitus. Results of recent clinical trials have shown remarkable success, and have reignited universal optimism for this procedure. In spite of this success, the need for life-long immunosuppression of the recipient still limits islet transplantation to patients with poorly controlled diabetes or to those requiring kidney transplantation. It is obvious that the achievement of immunological tolerance would broaden the indication for islet transplantation to a much larger cohort of patients with type I diabetes mellitus, most likely preventing long-term complications and contributing to a much improved quality of life. Increased understanding of the basic mechanisms of tolerance induction has resulted in the implementation of numerous experimental approaches to achieve long-term survival of islet grafts in the absence of chronic immunosuppression. In this brief review we will attempt to summarize the current status of research and knowledge.

What is the role of regulatory T cells in transplantation tolerance?

There has been considerable recent progress in the characterization of the regulatory T cells that mediate tolerance in a number of transplantation models. The conditions that facilitate the generation of regulatory T cells point to the thymus, the nature of immune suppression and the dose of immunosuppressive agent(s) being important. Putative mechanisms of immune regulation by regulatory T cells, particularly in the 'infectious' tolerance pathway, include Th2-type cytokines (IL-4, IL-10 and transforming growth factor beta) that may play a direct role as an indispensable requirement or may contribute indirectly as a favorable milieu for acquisition of tolerance. Anergic T cells may suppress immune responses via either cytokine competition or antigen-presenting cells. Models of autoimmune disease, in which regulatory T cells were shown to represent a distinct thymus-derived T cell subset, also suggest the role of antigen-presenting cells in mediating immune suppression. Progress has also been made in generating and characterizing regulatory T cells in vitro.

Dendritic cells as regulators of immune reactivity: implications for transplantation

Dendritic cells are now regarded not only as the initiators but also as regulators of immune responses. They are potentially powerful tools for the therapeutic manipulation of immune reactivity in cancer, infectious disease, and allograft rejection. We provide a brief overview of the properties of dendritic cells, with emphasis on recently acquired information, then focus attention on their capacity to modulate immune reactivity, and its relevance to transplantation.

Regulatory T cells maintain peripheral tolerance to islet allografts induced by intrathymic injection of MHC class I allopeptides

Although transplantation remains the treatment of choice for diabetes mellitus, immunological rejection of allografts continues to be a major problem. The search for strategies to prevent graft rejection led us to examine if the fate of developing T cells may be influenced by the presence of allo MHC class I peptides in the thymus because T cell receptor-MHC class I/self-peptide interaction regulates thymocyte development. We studied the effects of intrathymic (IT) injection of a short segment of a synthetic immunogenic MHC class I peptide (peptide 2, residues 67-85) of the hypervariable domain of RT1.A derived from WAG rat (RT1U) on islet graft survival in the WF(RT1U)-to-ACI combination. Adult diabetic male recipients were treated with IT injection of a single WAG-derived MHC class I peptide 7 days before intraportal islet transplantation. Long-term unresponsive islet recipients were examined for the development of alloantigen (Ag)-specific regulatory cells. The results showed that while IT injection of 150 microg peptide 2 on day -7 did not prolong graft survival in naive recipients [median survival time (MST) of 14.0 days vs. 9.6 in controls], IT injection of 300 or 600 microg peptide 2 led to normoglycemia and permanent islet survival (> 200 days) in 4/6 and 3/5 STZ-induced diabetic ACI recipients, respectively. IT injection of 150, 300, or 600 microg peptide 2 combined with 0.5 antilymphocyte serum (ALS) immunosuppression on day -7 led to 100% permanent islet allograft survival (> 200 days) compared to MST of 15.0 +/- 2.3 days in ALS alone-treated controls. Similarly prepared animals rejected third-party Brown Norway (BN) islets in an acute fashion, thus demonstrating donor specificity. Intravenous injection of 300 microg peptide 2 combined with 0.5 ml ALS did not prolong islet allograft survival. The long-term unresponsive islet allograft recipients challenged with second set grafts accepted permanently 100% donor-type cardiac allografts while rejecting third-party (BN) hearts without rejecting the primary Wistar Furth (WF) islets. In analyzing the underlying mechanisms of acquired systemic tolerance, we found no suppressor/regulatory cells in adoptive transfer studies in tolerant animals at 30 days after IT injection of allopeptides. In contrast, adoptive transfer of 5 x 10(7) unseparated spleen cells from tolerant animals at 60 and 100 days after islet transplantation into lightly irradiated [200 rad total body irradiation (TBI)] ACI recipients led to donor-specific permanent islet graft survival in 2/3 and 4/5 secondary recipients, respectively, compared to an MST of 13.8 days in lightly irradiated ACI given unmodified syngeneic spleen cells. In addition, adoptive transfer of 2 x 10(7) purified T cells obtained from long-term functioning islet recipients led to permanent donor-specific islet survival in secondary recipients. The finding that IT injection of a short segment of a synthetic immunodominant MHC class I peptide derived from WAG that shares the RT1.A(U) domain with the graft donor is capable of inducing acquired systemic tolerance to WF islets suggests that linked recognition or epitope suppression may be involved in the induction of unresponsiveness. Generation of peripheral Ag-specific regulatory cells that suppress Ag-specific alloreactive T cells is, in part, responsible for the maintenance of tolerance in this model.

Comparative studies of specific acquired systemic tolerance induced by intrathymic inoculation of a single synthetic Wistar-Furth (RT1U) allo-MHC class I (RT1.AU) peptide or WAG (RT1U)-derived class I peptide

BACKGROUND

Because T cell receptor-MHC class I/self-peptide interactions regulate T-cell development, the presence of MHC allopeptides in the thymus may influence T-cell tolerance to alloantigens. This hypothesis is supported by our most recent finding that intrathymic (IT) inoculation of nonimmunogenic synthetic peptides derived from "WAG" RT1.A induces tolerance to cardiac allografts in the Wistar-Furth (WF)-to-ACI model. To evaluate whether in vivo immunogenicity of MHC peptides is relevant to tolerance induction and to examine the effect of peptide specificity, we compared the effects on graft survival of well-defined, strain-specific immunogenic WF MHC class I peptides (RT1.AU) with closely related but non-strain-specific class I peptides derived from WAG (RT1U).

METHODS

In vivo immunization of seven MHC class I peptides synthesized from RT1.AU sequences showed that two (u-5 and u-7) were immunogenic, whereas five others were not immunogenic in ACI recipients. We then examined the effects on cardiac allograft survival in the WF-to-ACI model of the two immunogenic RT1.AU peptides (u-5 and u-7) and three immunogenic WAG-derived peptides (peptides 1, 2, and 5).

RESULTS

A combination of equal amounts (150 microg or 300 microg) of u-5 or u-7 each with 0.5 ml of antilymphocyte serum (ALS) on day -7 led to 60% and 100% permanent graft survival (>150 days), respectively. IT injection of the individual peptides on day -7 showed that only 300 microg of u-5 significantly prolonged graft survival to a median survival time of 17.3 days from 10.5 days in naive recipients. IT injection of 150, 300, and 600 microg of u-5 combined with 0.5 ml of ALS on day -7 led to permanent graft survival (> 150 days) in four of six, nine of nine, and six of six ACI recipients, respectively, compared with a median survival time of 15.4 days in ALS alone-treated controls. In contrast, similar treatments with peptide u-7 with or without 0.5 ml of ALS did not prolong graft survival, thus demonstrating that peptide u-5 alone mediates the observed effects on graft prolongation. A total of 300 microg of u-5 injected IT combined with ALS led to acute rejection of third-party (Lewis) grafts. Intravenous injection of 300 microg of u-5 with ALS also did not prolong WF graft survival in ACI recipients. The long-term unresponsive ACI recipients accepted permanently donor-type (WF) but not third-party (Lewis) second-set cardiac and islet allografts. Similarly, we showed that although IT injection of 600 and 1200 microg of a mixture of immunogenic WAG-derived peptides 1, 2, and 5 combined with 0.5 ml of ALS on day -7 led to permanent WF graft survival in ACI, only IT injection of 300 microg of peptide 2 combined with ALS led to permanent graft survival (>150 days) in four of five animals. To define the underlying mechanisms of tolerance, we examined in vitro the mixed lymphocyte reaction (MLR), cell-mediated lymphocytotoxicity, and cytokine profile of unresponsive recipients. Although the results showed nonspecific T-cell suppression in the MLR at 25 days after transplantation, which correlated with the persistence of ALS immunosuppression, long-term unresponsive animals showed normal MLR to donor and third-party antigens. In contrast, the donor-specific reactive cytotoxic T lymphocytes remained suppressed in short-term and long-term unresponsive rats.

CONCLUSION

Of interest is our finding that IT injection of a short segment of WAG-derived MHC class I peptide induces active acquired tolerance similar to results obtained with the use of pure WF-derived peptide u-5 in the WF-to-ACI rat combination. It is noteworthy that we could not confirm the T helper (Th)1/Th2 paradigm in this model by initial cytokine analysis. Whether induction of tolerance by IT injection of allo-MHC peptides will have clinical usefulness must await results of similar studies in large animals. However, of major interest is the finding that a short segment of RT1.AU represents the tolerogenic

Prolonged allogeneic and xenogeneic microchimerism in unmatched primates without immunosuppression by intrathymic implantation of CD34+ donor marrow cells

Engraftment of stem cell-enriched donor marrow implanted in the thymus of a foreign host might facilitate acceptance of donor-specific organ or tissue grafts. To test this hypothesis, allogeneic and xenogeneic CD34+ marrow cells from unrelated adult male baboons and humans were injected intrathymically in eight infant female baboons, both with and without standard cyclosporine-based immunosuppression. In allogeneic experiments, male (donor) cells, of both T- and B-cell lineages, were detected by PCR in the peripheral blood of all six recipients and persisted for at least 15 months in 2/4 recipients studied longtutudinally. Donor-derived skin grafts survived twice as long as third party grafts in unimmunosuppressed recipients. In xenogeneic protocols, human male (donor) cells were demonstrable for 7 and 15 months, respectively, in two baboon recipients with evidence that implanted human CD34+ cells had produced lymphoid progeny. Survival of donor-specific skin xenografts was prolonged in one of two recipients. These experiments demonstrate that the intrathymic injection of CD34+ marrow cells can result in long-lasting lymphohematopoietic microchimerism in unrelated primates even without immunosuppression and can alter donor-specific skin graft survival.

Split tolerance induced by immunotoxin in a rhesus kidney allograft model

BACKGROUND

Renal allografts were performed in rhesus monkeys using FN18-CRM9, a potent immunotoxin capable of depleting T cells to less than 1% of baseline levels in blood and lymph nodes, as a preparative agent. We have recently reported that animals pretreated with FN18-CRM9 1 week before transplantation without further immunosuppression had prolonged graft survival time compared with control animals, and frequently became tolerant.

METHODS

This report examines the alloimmune responses of recipient monkeys to the donor, including cytotoxic T lymphocyte precursor (CTLp) frequency, mixed lymphocyte response, and antidonor IgG response.

RESULTS

CTLp frequencies declined significantly (P<0.01) after FN18-CRM9 treatment and renal transplantation. This decline in CTLp was initially nonspecific, as CTLp frequencies against third-party animals also declined (P<0.01). The decrease in CTLp was maintained in five of five animals tested 6 months after transplant. However, unresponsiveness was limited to the CTL arm of the immune response as antidonor IgG was detected in four of four animals tested, and the 5-day mixed lymphocyte response stimulation index and relative response were not significantly different before and after transplant. In long-term survivors (>150 days), an increase in anti-third-party CTLp was detected 1 month after grafting with third-party skin. No change was seen in the antidonor CTLp frequency after donor skin grafting, indicating that a specific defect in the antidonor CTL response had developed.

CONCLUSIONS

These data suggest that FN18-CRM9 treatment of rhesus monkeys allows the development of specific down-regulation of antidonor CTL activity in renal allograft recipients.

Dependence of acquired systemic tolerance to rat islet allografts induced by intrathymic soluble alloantigens on host responsiveness, MHC differences, and transient immunosuppression in the high responder recipient

Recent studies suggest that the adult immune system can be manipulated by intrathymic (IT) inoculation of donor Ag to accept cardiac and islet allografts in the low responder rat combination of Lewis-to-WF. We have now extended this study to examine the effect of IT inoculation of soluble protein Ag obtained from 3M KCl extracts of resting T cells combined with transient ALS immunosuppression on islet allograft survival in the high responder combination of WF-to-Lewis. We first confirmed the earlier observation that IT injection of 2 mg soluble Ag on day -7 led to permanent islet graft survival (>200 days) in the Lewis-to-WF rat combination without the use of recipient immunosuppression and found this to be true in the Lewis-to-ACI rat combination. In the high responder combination of WF-to-Lewis, unmodified Lewis rats pretreated with IT inoculation of 2 mg soluble Ag acutely rejected WF and BN islet allografts. IT inoculation of donor Ag combined with 1 ml ALS transient immunosuppression on day -7 led to a modest graft prolongation [24.8+/-10.1 days as compared with 15.2+/-3.6 days in ALS only treated controls]. Intrathymic injection of soluble Ag on day -7 combined with 1 ml ALS on days -7 and 0 relative to allografting resulted in 100% permanent islet graft survival (>200 days) compared with an MST of 20.6+/-2.3 days in ALS only-treated controls. Similar treatment led to acute rejection of 3rd party (BN) grafts, thus demonstrating donor-specificity. In addition, extrathymic inoculation of donor Ag in similarly immunosuppressed animals did not result in islet graft prolongation, once again confirming the importance of the thymus in tolerance induction. To examine them for donor-specific tolerance, long-term unresponsive (>120 days) Lewis recipients of renal subcapsular islets underwent nephrectomy of the islet bearing kidneys and were challenged with intraportal donor- or third party-type islets after becoming diabetic. All the nonimmunosuppressed recipients of donor-type (WF) islets became permanently normoglycemic (>100 days) while the third-party (BN) grafts were promptly rejected, with an MST of 10.6 days. These findings confirm that acquired thymic tolerance induced by IT inoculation of soluble protein Ag in the low to moderate responder rat combinations is reproducible in the high responder combination provided that adequate peritransplant immunosuppression is used. This study suggests that acquired thymic tolerance in the rat model is dependent on host responsiveness to alloantigens, MHC differences between the donor-recipient pair, and the use of transient immunosuppression in the high responder recipient. This model may have potential clinical application in the development of strategies for specific transplantation tolerance.

Acquired systemic tolerance to rat cardiac allografts induced by intrathymic inoculation of synthetic polymorphic MHC class I allopeptides

This study extends the finding that intrathymic (IT) injection of 3M KC1 extracts of T cells induces transplant tolerance to the use of well defined polymorphic MHC class I allopeptides derived from the hypervariable domain of RT1.Au (WF MHC class I). While three of the six synthetic RT1.Au peptides were immunogenic, three others were nonimmunogenic when tested in ACI responders. In our initial studies, we examined the effects of IT injection of a mixture of equal concentrations of the three nonimmunogenic RT1.Au peptides on WF cardiac allograft survival in ACI recipients. The results showed that a single IT injection of 100 and 300 microg class I MHC allopeptides on day -7 relative to cardiac transplant did not significantly prolong graft survival in naive ACI recipients (MST of 9.8, and 12.3 days vs. 10.5 days in controls). In contrast, 600 microg allopeptides injected IT resulted in modest prolongation of graft to an MST of 19.5 days. However, IT injection of 600 microg allopeptides combined with 0.5 ml ALS on day -7 led to permanent acceptance (>200 days) of cardiac allografts in 7/9 ACI recipients compared with survival of 24.2 days in ALS alone treated controls. In contrast, similar treatment led to acute rejection of third party (Lewis) cardiac allografts. Intravenous injection of 600 microg allopeptides combined with ALS did not result in prolonged graft survival (26.8 days). The long-term unresponsive ACI recipients (>100 days) challenged with second-set cardiac grafts accepted permanently donor-type (WF) grafts while rejecting the third party (Lewis) grafts, a finding that confirms acquired systemic tolerance. These findings confirm the role of IT injection of synthetic polymorphic allopeptides in the induction of acquired thymic tolerance and provide the rationale for testing this strategy in large animals and eventually in man.

Induction of central tolerance by intrathymic inoculation of adenoviral antigens into the host thymus permits long-term gene therapy in Gunn rats

Recombinant adenoviruses are highly efficient at transferring foreign genes in vivo. However, duration of gene expression is limited by the host antiviral immune response which precludes expression upon viral readministration. We tested the feasibility of prolonging gene expression by induction of central tolerance to adenoviral antigens in bilirubin-UDP-glucuronosyltransferase-1 (BUGT1)-deficient Gunn rats. Tolerance was induced by intraperitoneal injection of antilymphocyte serum, followed by intrathymic inoculation of one of the following: a recombinant adenovirus (Ad), adenovirus human UDP-glucuronosyltransferase (Ad-hBUGT1) carrying the hBUGT1 gene; a protein extract of the same virus; or viral infected hepatocytes. Controls received intrathymic injections of normal saline. After 12 d all groups were injected intravenously with 5 x 10(9) pfu of either Ad-hBUGT1 or adenovirus beta-galactosidase (Ad-LacZ) (expressing the Escherichia coli beta-galactosidase [LacZ] gene). In all three groups of tolerized rats, hBUGT1 was expressed in the liver after administration of Ad-hBUGT1, with glucuronidation of biliary bilirubin of above 95%. Serum bilirubin levels decreased from 7.2 to 1.8 mg/dl within 1 wk and remained low for 7 wk. Similar findings were observed following repeat injections given on days 45 and 112. In control rats serum bilirubin levels were reduced for only 4 wk, and viral readministration was ineffective. In all tolerized groups, but not in controls, there was a marked inhibition of appearance of neutralizing antibodies and cytotoxic lymphocytes against the recombinant adenovirus. Injection of wild type adenovirus-5 (Ad5) into the tolerized rats elicited a wild type-specific cytotoxic lymphocyte response. This is the first demonstration of Ad-directed long-term correction of an inherited metabolic disease following central tolerization with thymic antigen.