Tolerance induction in clinical transplantation

Hematopoiesis from Human Embryonic Stem Cells: Overcoming the Immune Barrier in Stem Cell Therapies

The multipotency and proliferative capacity of human embryonic stem cells (hESCs) make them a promising source of stem cells for transplant therapies and of vital importance given the shortage in organ donation. Recent studies suggest some immune privilege associated with hESC-derived tissues. However, the adaptability of the immune system makes it unlikely that fully differentiated tissues will permanently evade immune rejection. One promising solution is to induce a state of immune tolerance to a hESC line using tolerogenic hematopoietic cells derived from it. This could provide acceptance of other differentiated tissues from the same line. However, this approach will require efficient multilineage hematopoiesis from hESCs. This review proposes that more efficient differentiation of hESCs to the tolerogenic cell types required is most likely to occur through applying knowledge gained of the ontogeny of complex regulatory signals used by the embryo for definitive hematopoietic development in vivo. Stepwise formation of mesoderm, induction of definitive hematopoietic stem cells, and the application of factors key to their self-renewal may improve in vitro production both quantitatively and qualitatively.

Rapid deletional peripheral CD8 T cell tolerance induced by allogeneic bone marrow: role of donor class II MHC and B cells

Mixed chimerism and donor-specific tolerance are achieved in mice receiving 3 Gy of total body irradiation and anti-CD154 mAb followed by allogeneic bone marrow (BM) transplantation. In this model, recipient CD4 cells are critically important for CD8 tolerance. To evaluate the role of CD4 cells recognizing donor MHC class II directly, we used class II-deficient donor marrow and were not able to achieve chimerism unless recipient CD8 cells were depleted, indicating that directly alloreactive CD4 cells were necessary for CD8 tolerance. To identify the MHC class II(+) donor cells promoting this tolerance, we used donor BM lacking certain cell populations or used positively selected cell populations. Neither donor CD11c(+) dendritic cells, B cells, T cells, nor donor-derived IL-10 were critical for chimerism induction. Purified donor B cells induced early chimerism and donor-specific cell-mediated lympholysis tolerance in both strain combinations tested. In contrast, positively selected CD11b(+) monocytes/myeloid cells did not induce early chimerism in either strain combination. Donor cell preparations containing B cells were able to induce early deletion of donor-reactive TCR-transgenic 2C CD8 T cells, whereas those devoid of B cells had reduced activity. Thus, induction of stable mixed chimerism depends on the expression of MHC class II on the donor marrow, but no requisite donor cell lineage was identified. Donor BM-derived B cells induced early chimerism, donor-specific cell-mediated lympholysis tolerance, and deletion of donor-reactive CD8 T cells, whereas CD11b(+) cells did not. Thus, BM-derived B cells are potent tolerogenic APCs for alloreactive CD8 cells.

Mesenchymal stem cells enhance the induction of mixed chimerism and tolerance to rat hind-limb allografts after bone marrow transplantation

BACKGROUND

Mixed hematopoietic chimerism via bone marrow transplantation has been shown to induce donor specific tolerance to solid organ allografts, but graft versus host disease (GVHD) remains to be a risk. Composite tissue allografts may need a higher percentage of donor chimerism compared with less immunogenic solid organ allografts. In this study, we investigated the potential of mesenchymal stem cells (MSCs) in the induction of stable and high level chimerism and subsequent donor-specific tolerance to composite tissue allograft without the incidence of graft versus host disease (GVHD).

METHODS

Fully mismatched, 4- to 8-wk-old Brown Norway (RT1(n)) and Lewis (RT1(1)) rats were used as cell or hind-limb donors and recipients, respectively. Recipients received a conditioning regimen consisting of antilymphocyte serum, standard immunosuppressive therapy (rapamycin: 0.2 mg/kg/d; days 0 approximately 130) and 3 Gy total body irradiation, followed by an intraportal co-infusion of allogeneic MSCs and bone marrow cells (day 0); then performing hind-limb allotransplants (30 d after BMT); 100 d after hind-limb transplantations, immunosuppressive therapy was stopped and to observe the survival time of the hind-limb allografts.

RESULTS

When co-infusion of allogeneic MSCs was administered, 14/15 recipients developed stable and high level chimerism. Subsequently, in the donor specific group, the survival time of hind-limb allografts without RAPA were remarkably prolonged compared with other groups. The results of the histopathological evaluation of skin biopsy and mixed lymphocyte cultures confirmed this operational donor specific tolerance. Without MSCs, only 2/7 of the recipients developed stable chimerism, but GVHD occurred in both of them, and the level of the chimerism was much lower. On the other hand, GVHD was not observed in any of the recipients infused with MSCs (0/15).

CONCLUSION

This study indicates a potential use of MSCs for induction of stable and high level mixed hematopoietic chimerism and subsequent donor specific tolerance in clinical composite tissue allotransplantation.

Depleting T-cell subpopulations in organ transplantation

T-cell depletion strategies are an efficient therapy for the treatment of acute rejection after organ transplantation and have been successfully used as induction regimens. Although eliminating whole T cells blocks alloreactivity, this therapy challenges the development of regulatory mechanisms because it depletes regulatory cells and modifies the profile of T cells after homeostatic repopulation. Targeting T-cell subpopulations or selectively activated T cells, without modifying Treg cells, could constitute a pro-tolerogenic approach. However, the perfect molecular target that would be totally specific probably still needs to be identified. In this study, we have reviewed the biological activities of broad or specific T-cell depletion strategies as these contribute to the induction of regulatory cells and tolerance in organ transplantation.

Clinical experience with mixed chimerism to induce transplantation tolerance

Lymphohematopoietic chimerism was first shown to be associated with donor-specific allograft tolerance more than 60 years ago. However, early clinical experience with bone marrow transplantation soon revealed that conventional, myeloablative approaches were far too toxic and the risk of graft-versus-host disease too great to justify using this technology for the purpose of organ allograft tolerance induction in the absence of malignant disease. In this review, we discuss a step-wise approach that has been applied by several centers to establish less toxic approaches to using hematopoietic cell transplantation (HCT) for tolerance induction. These steps include (i) feasibility and efficacy data for tolerance induction in large animal models; (ii) safety data in clinical trials for patients with hematologic malignancies; and (iii) pilot trials of combined HCT and kidney transplantation for tolerance induction. Thus far, only one published trial conducted at the Massachusetts General Hospital in Boston has achieved long-term acceptance of human leukocyte antigen-mismatched kidney allografts without chronic immunosuppressive therapy. Alternative protocols have been successful in large animals, but long-term organ allograft tolerance has not been reported in patients. Thus, proof-of-principle that nonmyeloablative induction of mixed chimerism can be used intentionally to induce organ allograft tolerance has now been achieved. Directions for further research to make this approach applicable for a broader patient population are discussed.

Rapamycin, mycophenolate mofetil, methylprednisolone, and cytotoxic T-lymphocyte-associated antigen 4 immunoglobulin-based conditioning regimen to induce partial tolerance to hind limb allografts without cytoreductive conditioning

BACKGROUND

Composite tissue allograft transplantation may represent the next frontier in the field of reconstructive surgery. However, the main obstacles precluding the routine use of composite tissue allotransplants are rejection and toxicity associated with life-long immunosuppressive therapy. In this study, we investigated a nontoxic immunosuppressant and cytotoxic T-lymphocyte-associated antigen 4 immunoglobulin (CTLA4-Ig)-based protocol to induce donor-specific tolerance to hind limb allografts in rats.

METHODS

Fully mismatched, 4- to 10-week-old Brown Norway (BN, RT1n) and Lewis (RT1) rats were used as cell/organ donors and recipients, respectively. Recipients were treated with CTLA4-Ig (2 mg/kg/d) on days -30, -28, -26, -24, and -22, rapamycin, mycophenolate mofetil, and methylprednisolone (RAPA/MMF/MP) combined therapy (from days -30 to day 100), a single dose of anti-lymphocyte serum (10 mg, on day -30), and donor bone marrow (10 x 10(7) T-cell-depleted cells) transplantation (BMT, on day -30). Thirty days after BMT, chimeric animals received hind limb allotransplantations (on day 0). The RAPA/MMF/MP combined therapy was changed to Cyclosporine (CsA, 8 mg/kg/d) on day 100 and maintained thereafter at this level.

RESULTS

Hematopoietic chimerism of 17.6 +/- 9.5% at day 0, was stable (15.2 +/- 5.6%) at 230 days post-BMT; there was no sign of graft-versus-host disease. Chimeric recipients (Lewis) permanently accepted (>200 days) donor (BN)-specific (RT11, n = 6) hind limbs, yet rapidly rejected (20 +/- 2 days) third-party hind limbs (Wistar Furth [WF]). Lymphocytes of graft-tolerant animals demonstrated hyporesponsiveness in mixed lymphocyte cultures in a donor-specific manner. Tolerant graft histology showed no signs of acute and chronic rejection.

CONCLUSIONS

The immunosuppressant and CTLA4-Ig-based conditioning regimen with donor BMT produced mixed chimerism and induced partial donor-specific tolerance to hind limb allografts.

Immunosuppression enhances oncolytic adenovirus replication and antitumor efficacy in the Syrian hamster model

We recently described an immunocompetent Syrian hamster model for oncolytic adenoviruses (Ads) that permits virus replication in tumor cells as well as some normal tissues. This model allows exploration of interactions between the virus, tumor, normal organs, and host immune system that could not be examined in the immunodeficient or nonpermissive animal models previously used in the oncolytic Ad field. Here we asked whether the immune response to oncolytic Ad enhances or limits antitumor efficacy. We first determined that cyclophosphamide (CP) is a potent immunosuppressive agent in the Syrian hamster and that CP alone had no effect on tumor growth. Importantly, we found that the antitumor efficacy of oncolytic Ads was significantly enhanced in immunosuppressed animals. In animals that received virus therapy plus immunosuppression, significant differences were observed in tumor histology, and in many cases little viable tumor remained. Notably, we also determined that immunosuppression allowed intratumoral virus levels to remain elevated for prolonged periods. Although favorable tumor responses can be achieved in immunocompetent animals, the rate of virus clearance from the tumor may lead to varied antitumor efficacy. Immunosuppression, therefore, allows sustained Ad replication and oncolysis, which leads to substantially improved suppression of tumor growth.

Alloreactive CD8 T cell tolerance requires recipient B cells, dendritic cells, and MHC class II

Allogeneic bone marrow chimerism induces robust systemic tolerance to donor alloantigens. Achievement of chimerism requires avoidance of marrow rejection by pre-existing CD4 and CD8 T cells, either of which can reject fully MHC-mismatched marrow. Both barriers are overcome with a minimal regimen involving anti-CD154 and low dose (3 Gy) total body irradiation, allowing achievement of mixed chimerism and tolerance in mice. CD4 cells are required to prevent marrow rejection by CD8 cells via a novel pathway, wherein recipient CD4 cells interacting with recipient class II MHC tolerize directly alloreactive CD8 cells. We demonstrate a critical role for recipient MHC class II, B cells, and dendritic cells in a pathway culminating in deletional tolerance of peripheral alloreactive CD8 cells.

Long-term control of alloreactive B cell responses by the suppression of T cell help

Alloantibodies can play a key role in acute and chronic allograft rejection. However, relatively little is known of factors that control B cell responses following allograft tolerance induction. Using 3-83 Igi mice expressing an alloreactive BCR, we recently reported that allograft tolerance was associated with the sustained deletion of the alloreactive B cells at the mature, but not the immature, stage. We have now investigated the basis for the long-term control of alloreactive B cell responses in a non-BCR-transgenic model of C57BL/6 cardiac transplantation into BALB/c recipients treated with anti-CD154 and transfusion of donor-specific spleen cells. We demonstrate that the long-term production of alloreactive Abs by alloreactive B cells is actively regulated in tolerant BALB/c mice through the dominant suppression of T cell help. Deletion of CD25(+) cells resulted in a loss of tolerance and an acquisition of the ability to acutely reject allografts. In contrast, the restoration of alloantibody responses required both the deletion of CD25(+) cells and the reconstitution of alloreactive B cells. Collectively, these data suggest that alloreactive B cell responses in this model of tolerance are controlled by dominant suppression of T cell help as well as the deletion of alloreactive B cells in the periphery.

Short-term cyclosporine therapy and cotransplantation of donor splenocytes: effects on graft rejection and survival rates in pigs subjected to renal transplantation

BACKGROUND

Donor-specific allogeneic loading can prolong the survival of solid organ transplants by inducing a state known as acceptance. Several populations of cells are known to be involved in this process, but their exact roles have yet to be defined. The aim of this study was to assess the effects of portal-vein transfusion of donor-specific splenocytes (DST) after short-term cyclosporine A (CyA) therapy in pigs subjected to renal transplantation.

METHODS

Four groups of unrelated swine underwent renal transplantation with removal of the native kidneys. Antirejection protocols consisted in portal-vein DST (3 x 10(8) cells/kg) (Group 2, n = 7); intravenous CyA (9 mg/kg/d) on postoperative days 1-12 (Group 3, n = 14); and DST + CyA (as described above) (Group 4, n = 13). Results (through postoperative day 90) were compared with those obtained in untreated control recipients (Group 1, n = 7).

RESULTS

Compared with animals of Groups 1, 2, and 3, Group 4 recipients presented significantly longer survival (mean: 90 days, P < 0.01 in Kaplan-Meier analysis) and better renal function (P < 0.05). Graft histology revealed preserved parenchyma.

CONCLUSION

The role of spleen cells in the immune response has probably been underestimated. Cotransplantation of donor splenocytes seems to induce a certain degree of acceptance toward the renal allograft. The route of administration (portal-vein infusion in this study) may be crucial for developing favorable mechanisms of recognition.

Development and in vitro characterization of canine CD40-Ig

We recently reported that blockade of the CD40-CD154 ligand interaction with the cross-reacting mouse anti-human CD154 antibody, 5c8, together with donor-specific transfusion led to enhanced but not completely successful engraftment in a canine model of DLA-identical marrow transplantation after 100cGy total body irradiation (TBI). In order to improve the transplantation outcomes, we sought to develop a canine-specific reagent. To that end, we fused the extracellular domain of the canine CD40 with a mouse IgG2a Fc tail and tested the immunosuppressive effectiveness of the fusion protein in mixed leukocyte reactions. The extracellular domain of canine CD40 was fused with the Fc portion of mouse IgG2a in a pcDNA3.1+vector. Dhfr-deficient CHO cells were co-transfected with the CD40-Ig vector and a dhfr-containing vector. Stable, high producing clones were selected under increasing methotrexate concentrations. The fusion protein was purified, tested in mixed leukocyte reactions, and its immunosuppressive effect compared to that of the anti-CD154 antibody 5c8. The transfected cell line produced a CD40-Ig dimer whose identity was confirmed by mass spectroscopy. The purified canine CD40-Ig blocked mixed leukocyte reactions at a concentration of 1nM, which was 10 times more effective than the anti-CD154 antibody. Canine CD40-Ig is more immunosuppressive than the anti-human CD154 antibody 5c8 in canine mixed leukocyte reactions and may be more effective in vivo in a model of marrow transplantation.

Induction of transplantation tolerance by combining non-myeloablative conditioning with delivery of alloantigen by T cells

The observation that bone marrow derived hematopoietic cells are potent inducers of tolerance has generated interest in trying to establish transplantation tolerance by inducing a state of hematopoietic chimerism through allogeneic bone marrow transplantation. However, this approach is associated with serious complications that limit its utility for tolerance induction. Here we describe the development of a novel approach that allows for tolerance induction without the need for an allogeneic bone marrow transplant by combining non-myeloablative host conditioning with delivery of donor alloantigen by adoptively transferred T cells. CBA/Ca mice were administered 2.5 Gy whole body irradiation (WBI). The following day the mice received K(b) disparate T cells from MHC class I transgenic CBK donor mice, as well as rapamycin on days 0-13 and anti-CD40L monoclonal antibody on days 0-5, 8, 11 and 14 relative to T cell transfer. Mice treated using this approach were rendered specifically tolerant to CBK skin allografts through a mechanism involving central and peripheral deletion of alloreactive T cells. These data suggest robust tolerance can be established without the need for bone marrow transplantation using clinically relevant non-myeloablative conditioning combined with antigen delivery by T cells.

Use of donor bone marrow mesenchymal stem cells for treatment of skin allograft rejection in a preclinical rat model

Recent studies indicate that mesenchymal stem cells (MSC) exhibit a degree of immune privilege due to their ability to suppress T cell mediated responses causing tissue rejection; however, the impact of allogeneic MSC in the setting of organ transplantation has been poorly investigated so far. The aim of our study was to evaluate the effect of intravenous donor MSC infusion for clinical tolerance induction in allogeneic skin graft transplantations in rats. MSC were isolated from Wistar rats and administered in Sprague-Dawley rats receiving Wistar skin graft with or without cyclosporine A (CsA). Graft biopsies were performed at day 10 post transplantation in all experimental groups for histological and gene expression studies. Intravenous infusion with donor MSC in CsA-treated transplanted rats resulted in prolongation of skin allograft survival compared to control animals. Unexpectedly, donor MSC infusion in immunocompetent rats resulted in a faster rejection as compared to control group. Cytokine expression analysis at the site of skin graft showed that CsA treatment significantly decreased pro-inflammatory cytokines IFN-gamma and IL-2 and reduced TNF-alpha gene expression; however, the level of TNF-alpha is high in MSC-treated and not immunosuppressed rats. Results of our study in a rat tissue transplantation model demonstrated a possible immunogenic role for donor (allogeneic) MSC, confirming the need of adequate preclinical experimentation before clinical use.

Basics of immune responses in transplantation in preparation for application of composite tissue allografts in plastic and reconstructive surgery: part I

Currently, composite tissue allografts are applied only occasionally as a reconstructive option in the field of plastic and reconstructive surgery. Composite tissue allografts offer a unique potential for coverage of large multitissue defects. However, compared with the relatively homogenous tissue of solid organ transplants, the heterogenicity of tissue components of composite tissue allografts may generate high immunologic responses. Modern immunosuppressive agents significantly improve successful allograft acceptance. However, chronic allograft rejection and immunosuppressive drug toxicity are still major problems in the clinical practice of transplantation. The major goals of transplantation immunology are (1) to develop tolerance to allograft transplants and (2) long-term drug-free survival. A number of experimental protocols were designed to develop tolerance; however, none of them has been proven to induce tolerance in clinical transplantation. In this article, the authors outline the mechanisms of allograft acceptance and rejection and barriers to transplantation tolerance. Novel immunosuppressive protocols are discussed in this review. This basic immunologic knowledge of allograft acceptance and rejection will allow plastic surgeons to apply composite tissue allograft transplants to plastic and reconstructive surgery.

Skin tolerance: in search of the Holy Grail

In 1943, Gibson and Medawar opened the modern era of transplantation research with a paper on the problem of skin allograft rejection. Ten years later Billingham, Brent and Medawar demonstrated that it was possible to induce selective immune acceptance of skin grafts in mice, a state of tolerance. After over six decades, however, the precise mechanism of skin allograft rejection remains still ill-defined. Furthermore, it has not been possible to achieve reliably clinical tolerance allowing the widespread application of skin allotransplantation techniques. The first successful applications of skin allotransplantation have included the hand and face. However, complications from the chronic immunosuppression regimens limit the application of these techniques. Induction of tolerance to skin (and the other tissues in the allograft) would be the most effective way to overcome all these difficulties, but this is yet to be achieved reliably, stimulating some to look for other ways to surmount the current limitations. This paper summarizes alternatives to enlarge the scope of skin allotransplantation techniques, current understanding of mechanisms of skin rejection, and the utility and limitations of animal models used to study skin rejection and tolerance induction. Finally, manipulation strategies to achieve skin tolerance are outlined.

Depletion of CD8 memory T cells for induction of tolerance of a previously transplanted kidney allograft

Heterologous immunologic memory has been considered a potent barrier to tolerance induction in primates. Induction of such tolerance for a previously transplanted organ may be more difficult, because specific memory cells can be induced and activated by a transplanted organ. In the current study, we attempted to induce tolerance to a previously transplanted kidney allograft in nonhuman primates. The conditioning regimen consisted of low dose total body irradiation, thymic irradiation, antithymocyte globulin, and anti-CD154 antibody followed by a brief course of a calcineurin inhibitor. This regimen had been shown to induce mixed chimerism and allograft tolerance when kidney transplantation (KTx) and donor bone marrow transplantation (DBMT) were simultaneously performed. However, the same regimen failed to induce mixed chimerism when delayed DBMT was performed after KTx. We found that significant levels of memory T cells remained after conditioning, despite effective depletion of naïve T cells. By adding humanized anti-CD8 monoclonal antibody (cM-T807), CD8 memory T cells were effectively depleted and these recipients successfully achieved mixed chimerism and tolerance. The current studies provide 'proof of principle' that the mixed chimerism approach can induce renal allograft tolerance, even late after organ transplantation if memory T-cell function is adequately controlled.

Cellular Therapies for Prolongation of Composite Tissue Allograft Transplantation

Complex musculoskeletal defects resulting from cancer, congenital absence, and trauma represent a unique reconstructive challenge. Autologous tissue is often unavailable to reconstruct these deformities. Composite tissue allograft transplantation represents a unique solution for these clinical problems. Face, hand, or limb transplants can be performed in a single procedure. However, the use of chronic nonspecific systemic immunosuppression can lead to side effects such as drug toxicity, opportunistic infections, and malignancies. This article explores various cell-based therapies that represent promising modalities to reduce chronic immunosuppression and alter the risk/benefit ratios for the prospect of composite tissue allograft transplantation.

Intrinsic ability of GM+IL-4 but not Flt3L-induced rat dendritic cells to promote allogeneic T cell hyporesponsiveness

The influence of GM+IL-4 and Flt3 ligand (FL) on phenotype and function of BM-derived DC from Lewis rats was investigated. GM+IL-4-induced DC, despite expression of CD80/CD86, were less stimulatory than FL-induced DC that expressed low CD80/CD86 and were efficient stimulators of allogeneic T cells. GM+IL-4 DC were CD11b+ OX62lo, whereas FL DC were CD11blo OX62+. Following activation, GM+IL-4 DC produced IL-10 and IL-6, but no IL-12p70, and were resistant to further maturation. FL DC produced IL-12p70, IFN-alpha/beta, IL-10 and IL-6 and underwent maturation. Repeated stimulation of T cells with GM+IL-4 DC inhibited proliferation, cytokine production and induced early T cell apoptosis. FL DC-activated T cells produced large amounts of IFN-gamma/IL-10 and exhibited late T cell apoptosis/necrosis. In vivo, GM+IL-4 DC induced alloAg-specific hyporesponsiveness following T cell restimulation. These results demonstrate that GM+IL-4 DC display intrinsic regulatory properties, inducing passive-cell-death in T cells with potential for inactivation/regulation of alloreactive T cells in transplantation.

Frontiers of immunological tolerance

Herein, we succinctly review mechanisms underlying self-tolerance and the roles of dendritic leukocytes (DCs) in T-cell tolerance to self and foreign antigens. We also consider the properties of naturally arising and other populations of regulatory T cells (Treg), together with growing evidence that interplay between DCs and Treg cells can sustain antigen-specific tolerance. B-cell tolerance and the role of hematopoietic cell chimerism in the induction and maintenance of tolerance are also discussed, as is the impact of cosignaling pathway manipulation on tolerance induction. This overview also surveys prospects for technological advances in the monitoring and prediction of tolerance and the application of genomic and proteomic analysis. In addition, we consider potential novel therapeutic targets for promotion of tolerance induction.

Undifferentiated Murine Embryonic Stem Cells Cannot Induce Portal Tolerance but May Possess Immune Privilege Secondary to Reduced Major Histocompatibility Complex Antigen Expression

Induction of donor-specific tolerance using embryonic stem (ES) cells followed by transplantation of ES cell-derived tissues from the same allogeneic strain could theoretically engender successful transplantation without immunosuppression. We sought to induce tolerance using bona fide murine ES cells in immunocompetent mice. ES cells were evaluated for the expression of markers restricted to undifferentiated cells [stage-specific embryonic antigen-1 (SSEA-1) and OCT-4] and the ability to form teratomas in immunodeficient mice. BALB/cByJ mice underwent intraportal inoculation with YC5-EYFP ES cells (129 strain; R1-derived) or saline followed by transplantation with 129X1/SvJ, CBA/J, or BALB/cByJ nonvascularized, neonatal cardiac grafts. Mice were sacrificed at graft failure and underwent histologic evaluation of transplanted grafts and lymphoid organs. ES cells and early differentiated progeny underwent real time (RT)-PCR and fluorescence-activated cell sorting (FACS) analysis to detect major histocompatibility complex (MHC) gene transcription and antigen expression. ES cells expressed markers restricted to undifferentiated cells while maintaining the ability to form teratomas in immunodeficient mice. No prolongation of allograft survival or evidence of lymphoid chimerism was observed in immunocompetent recipient mice despite hepatic teratoma formation. MHC class I, class II, and nonclassical antigens were undetectable on ES cells and early differentiated progeny despite the presence of mRNA transcripts. Class I expression was strongly upregulated upon exposure to gamma-interferon. Intraportal inoculation with murine ES cells does not produce lymphoid chimerism or induce donor-specific unresponsiveness to neonatal cardiac grafts in unmanipulated immunocompetent hosts. However, specific differentiated cell types such as ES cellderived dendritic cells, or alternate routes of ES cell administration, may be effective. ES cells appear to have immune privilege, allowing them to form teratomas in immunocompetent mice.

Porcine UL16-binding protein 1 expressed on the surface of endothelial cells triggers human NK cytotoxicity through NKG2D

Cellular rejection mechanisms, including NK cells, remain a hurdle for successful pig-to-human xenotransplantation. Human anti-pig NK cytotoxicity depends on the activating receptor NKG2D. Porcine UL16-binding protein 1 (pULBP1) and porcine MHC class I chain-related protein 2 (pMIC2) are homologues of the human NKG2D ligands ULBP 1-4 and MICA and B, respectively. Although transcribed in porcine endothelial cells (pEC), it is not known whether pULBP1 and pMIC2 act as functional ligands for human NKG2D. In this study, surface protein expression of pULBP1 was demonstrated by flow cytometry using a novel pULBP1-specific polyclonal Ab and by cellular ELISA using NKG2D-Fc fusion protein. Reciprocally, pULBP1-Fc bound to primary human NK cells, whereas pMIC2-Fc did not. Transient and stable down-regulation of pULBP1 mRNA in pEC using short-interfering RNA oligonucleotide duplexes and short hairpin RNA, respectively, resulted in a partial inhibition of xenogeneic NK cytotoxicity through NKG2D in (51)Cr release assays. In contrast, down-regulation of pMIC2 mRNA did not inhibit NK cytotoxicity. Human NK cytotoxicity against pEC mediated by freshly isolated or IL-2-activated NK cells through NKG2D was completely blocked using anti-pULBP1 polyclonal Ab. In conclusion, this study suggests that pULBP1 is the predominant, if not only, functional porcine ligand for human NKG2D. Thus, the elimination of pULBP1 on porcine tissues represents an attractive target to protect porcine xenografts from human NK cytotoxicity.

Stem cell based therapies to treat muscular dystrophy

Muscular dystrophies comprise a heterogeneous group of neuromuscular disorders, characterized by progressive muscle wasting, for which no satisfactory treatment exists. Multiple stem cell populations, both of adult or embryonic origin, display myogenic potential and have been assayed for their ability to correct the dystrophic phenotype. To date, many of these described methods have failed, underlying the need to identify the mechanisms controlling myogenic potential, homing of donor populations to the musculature, and avoidance of the immune response. Recent results focus on the fresh isolation of satellite cells and the use of multiple growth factors to promote mesangioblast migration, both of which promote muscle regeneration. Throughout this chapter, various stem cell based therapies will be introduced and evaluated based on their potential to treat muscular dystrophy in an effective and efficient manner.

ABO-histo blood group incompatibility in hematopoietic stem cell and solid organ transplantation

In contrast to solid organ transplantation (SOT), ABO-histo blood group incompatibility is of minor importance for hematopoietic stem cell transplantation (HSCT). Patients receiving ABO-incompatible HSCT are at an increased risk for immune-mediated hematological complications including immediate and delayed hemolysis, late red blood cell engraftment and pure red cell aplasia, but seem not to have a worse overall survival or increased transplant-related mortality. This review gives an overview of the immunological mechanisms leading to complications associated with ABO-incompatible HSCT and describes approaches to prevent them. The current organ shortage in SOT stimulates the exploration of new strategies to expand the donor pool including ABO-incompatible SOT and xenotransplantation. Here, we discuss the hypothesis that ABO-incompatible transplantation may be viewed as a human in vivo model for the humoral immune mechanisms of antigen-mismatched transplantation. ABO-incompatible HSCT and SOT provide excellent possibilities to analyze graft accommodation and transplantation tolerance. Understanding the underlying mechanisms of graft survival in ABO-incompatible transplantation may facilitate new strategies to overcome the immunological barriers in SOT and xenotransplantation.

Role of thymus in operational tolerance induction in limb allograft transplant model

BACKGROUND

In this study, we evaluated the role of host thymus in tolerance induction in composite tissue allografts (CTA) across major histocompatibility complex (MHC) barrier during a 7-day alphabeta- T-cell receptor (TCR)/ cyclosporine A (CsA) protocol.

MATERIALS AND METHODS

A total of 62 limb allograft transplants were studied. Euthymic (group A) and thymectomized (group B) Lewis recipients (LEW, RT1(1)) received vascularized hind-limb allografts from hybrid Lewis x Brown-Norway (F1), (LBN, RT1(1+n)) donors. Mixed lymphocyte reaction (MLR) and skin grafting assessed donor-specific tolerance in vitro and in vivo, respectively. Flow cytometry determined the efficacy of immunosuppressive protocols and the presence of donor-specific chimerism. Immunocytochemistry revealed the presence of donor-specific cells in the lymphoid organs of recipients.

RESULTS

Isograft transplants survived indefinitely. For thymectomized rats, the median survival time (MST) of limb allograft in non-treated recipients was 7 days; monotherapy with alphabeta-TCR extended MST to 16 days, and CsA therapy extended it to 30 days. Using the alphabeta-TCR/CsA protocol, the MST of allografts was 51 days. For euthymic rats, the MST of limb allograft in non-treated recipients was 7 days; monotherapy with alphabeta-TCR or CsA extended MST to 13 or 22 days, respectively. Treatment with alphabeta-TCR/CsA resulted in indefinite allografts survival (MST=370 days). MLR and skin grafting confirmed donor-specific tolerance in euthymic recipients. Flow cytometry showed stable chimerism in the euthymic rats and transient chimerism in thymectomized limb recipients. Immunoperoxidase staining revealed the persistence of donor-derived cells in the lymphoid tissues of euthymic recipients.

CONCLUSION

We found that the presence of thymus was imperative for the induction of donor-specific tolerance in rat hind-limb composite tissue allografts using a alphabeta-TCR/CsA protocol.

Attenuation of murine lysosomal storage disease by allogeneic neonatal bone marrow transplantation using costimulatory blockade and donor lymphocyte infusion without myeloablation

Treatment of nonmalignant childhood disorders by bone marrow transplantation (BMT) is limited by toxicity from preparatory regimens and immune consequences associated with engraftment of allogeneic donor cells. Using costimulatory blockade (anti-CD40L mAb and CTLA-4Ig) combined with high-dose BMT in nonablated neonates, we obtained engraftment and established tolerance using both partially MHC mismatched (H2g7 into H2b) and fully mismatched BM (H2s into H2b). Recipients were mucopolysaccharidosis type VII (MPS VII) mice with lysosomal storage disease in order to assess therapeutic outcome. Recipients treated with donor lymphocyte infusion (DLI) amplified microchimerism to full donor. Recipients without DLI maintained long-term engraftment, tolerance, and had extended life spans. DLI increased donor cell mediated replacement of beta-glucuronidase (GUSB) activity in all tissues and maintained clearance of lysosomes better than in non-DLI-treated mice. DLI amplification of partially mismatched BM and fully mismatched BM caused late onset chronic GvHD in 56% and 100% of recipients, respectively.

Soluble HLA-G in heart transplantation: their relationship to rejection episodes and immunosuppressive therapy

The aims of this study were to quantify the level of soluble HLA-G in heart transplant patients, to determine the relationship between the sHLA-G levels and the appearance of acute rejection episodes, and to identify the influence of immunosuppressive therapy on sHLA-G levels. Analysis of sHLA-G, measured by enzyme-linked immunosorbent assay in the transplant patients, revealed the existence of two similarly sized groups of patients. One group displayed a significant increase (p < 0.001) in sHLA-G during the first month after transplantation while the other group maintained low levels of the molecule (0-30 ng/ml) throughout the study. The latter group displayed a high incidence of recurrent severe rejection. A significant increase (p < 0.01) in sHLA-G 2 hours after administration of immunosuppressive treatment (mycophenolate mofetil, cyclosporine A/FK506, corticoids) was found. These results suggest that sHLA-G participates in the induction of certain levels of immunological tolerance in these recipients.

Clinical operational tolerance after kidney transplantation

Induction of allograft-specific tolerance and the detection of a "tolerance" state in recipients under immunosuppression with long-term stable graft function are major challenges in transplantation. Clinical "operational tolerance," defined as stable and acceptable graft function without immunosuppression for years, is a rare event. There is no report on the clinical history of such patients. In this article, we report on the medical history of 10 kidney recipients who display an immunosuppressive drug-free "operational tolerance" for 9.4 +/- 5.2 years. Clinical factors that may favor such a tolerant state are underlined. Firstly, most of the patients interrupted immunosuppression over a long time period (until 4 years), which mimics the procedure of intentional immunosuppression weaning following liver transplantation. Secondly, donor age was younger (median 25 years) than the one of the general transplanted population, suggesting that graft quality is one of the conditions favoring "operational tolerance." Moreover, the "operationally tolerant" recipients may be 'low responders' to blood transfusions (PRA 6 +/- 5.4%, six blood transfusions). We also show that "operational tolerance" occurs in the presence of anti-donor class II antibodies, as assessed in two patients. Finally, two patients degraded their renal function 9 to 13 years after treatment withdrawal, however only one presented histological lesions of chronic rejection.

Ex vivo expanded dendritic cells home to T-cell zones of lymphoid organs and survive in vivo after allogeneic bone marrow transplantation

Little is known about adoptive transfer of allogeneic ex vivo expanded dendritic cells (eDCs). We investigated the trafficking pattern of eDCs in mice after allogeneic bone marrow transplantation by using bioluminescence imaging. eDCs were expanded from bone marrow precursors in the presence of GM-CSF, interleukin-4, and Flt3L and retrovirally transduced to express luciferase (luc) and green fluorescence protein (gfp). Flow cytometry showed polyclonal DC populations after expansion that consisted of CD11c+CD11b+ and CD11c-CD11b+ cells that co-expressed CD40, CD80, CD86, and MHCII. eDCs were functional in mixed lymphocyte reactions and produced tumor necrosis factor-alpha on phytohemagglutinin stimulation. The eDCs were then injected intravenously into BALB/c recipient mice that had received allogeneic bone marrow transplantation 6 weeks previously. On day 1 after transfer, eDCs were detected by bioluminescence imaging throughout the lungs and spleen. In the later course, signals were observed throughout thymus, lower abdomen, and spleen throughout a period of more than 42 days. Immunofluorescence microscopy confirmed CD11c positivity on the gfp+ donor cells, which localized in T-cell zones of mesenteric lymph nodes, Peyer's patches, spleen, and thymus. These findings are important for adoptive immunotherapies because they indicate that eDCs migrate efficiently in vivo and are capable of surviving long term.

Microchimerism maintains deletion of the donor cell-specific CD8+ T cell repertoire

Rare cases of stable allograft acceptance after discontinuation of immunosuppression are often accompanied by macrochimerism (> 1% donor cells in blood) or microchimerism (< 1% donor cells in blood). Here, we have investigated whether persistence of donor cells is the cause or the consequence of long-lasting CTL unresponsiveness. We found that engraftment of splenocytes bearing a single foreign MHC class I-restricted epitope resulted in lifelong donor cell microchimerism and specific CTL unresponsiveness. This status was reversed in a strictly time- and thymus-dependent fashion when the engrafted cells were experimentally removed. The results presented herein show that microchimerism actively maintains CTL unresponsiveness toward a minor histocompatibility antigen by deleting the specific repertoire and thus excluding dominant, T cell extrinsic mechanisms of CTL unresponsiveness independent of systemically persisting donor cell antigen.

Prevention of chronic allograft rejection by Vitamin D receptor agonists

While immunosuppressive drugs now permit a good control of acute allograft rejection, chronic rejection remains an important unmet medical problem. We propose that Vitamin D receptor (VDR) agonists, secosteroid hormones that control cell proliferation and differentiation and exert immunoregulatory activities, in addition to regulate calcium and bone metabolism, have the potential to contribute to the management of chronic allograft rejection. Recent advances in understanding the immunomodulatory and growth-regulating properties of VDR agonists indicate the clinical applicability of these hormones in transplantation, with the aim of facilitating tolerance induction and preventing chronic graft rejection.

The immunobiology of Guillain-Barré syndromes

This presentation highlights aspects of the immunobiology of the Guillain-Barré syndromes (GBS), the world's leading cause of acute autoimmune neuromuscular paralysis. Understanding the key pathophysiological pathways of GBS and developing rational, specific immunotherapies are essential steps towards improving the clinical outcome of this devastating disorder. Much of the research into GBS over the last decade has focused on the forms mediated by anti-ganglioside antibodies, and we have made substantial progress in our understanding in several related areas. Particular highlights include (a) the emerging correlations between anti-ganglioside antibodies and specific clinical phenotypes, notably between anti-GM1/anti-GD1a antibodies and the acute motor axonal variant and anti-GQ1b/anti-GT1a antibodies and the Miller Fisher syndrome; (b) the identification of molecular mimicry between GBS-associated Campylobacter jejuni oligosaccharides and GM1, GD1a, and GT1a gangliosides as a mechanism for anti-ganglioside antibody induction; (c) the development of rodent models of GBS with sensory ataxic or motor phenotypes induced by immunisation with GD1b or GM1 gangliosides, respectively. Our work has particularly studied the motor nerve terminal as a model site of injury, and through combined active and passive immunisation paradigms, we have developed murine neuropathy phenotypes mediated by anti-ganglioside antibodies. This has been achieved through use of glycosyltransferase and complement regulator knock-out mice, both for cloning anti-ganglioside antibodies and inducing disease. Through such studies, we have proven a neuropathogenic role for murine anti-ganglioside antibodies and human GBS-associated antisera and identified several determinants that influence disease expression including (a) the level of immunological tolerance to microbial glycans that mimic self-gangliosides; (b) the ganglioside density in target tissue; (c) the level of complement activation and the neuroprotective effects of endogenous complement regulators; and (d) the role of calcium influx through complement pores in mediating axonal injury. Such studies provide us with clear information on an antibody-mediated pathogenesis model for GBS and should lead to rational therapeutic testing of agents that are potentially suitable for use in humans.

The concept of “partial” clinical tolerance

The status of "partial" tolerance to organ allografts versus the status of complete tolerance is the main topic of this paper. Progress made in immunosuppression, particularly by use of various lymphocyte depleting agents for "induction therapy", seems to favor the subsequent development of T cells with suppressor/regulatory properties. The effective deletion of alloreactive T helper and cytotoxic cells in conjunction with the expansion of antigen-specific suppressor (CD8 + CD28 - FOXP3+) and regulatory (CD4 + CD25+ FOXP3+) T cells creates a milieu in which the graft is well tolerated under an "umbrella" of low dosage immunosuppression. The most effective induction treatment is Campath-1H, although ATGAM at high dosage is also widely used. Total lymphoid irradiation (TLI) is another very effective pretreatment strategy in spite of the risks which are associated with it. The induction of "partial" tolerance is a step in the right direction for exploring strategies that may lead to the induction of complete tolerance. It is safe for the patients and can prolong significantly the function of the graft, preventing the onset of chronic rejection.