Prolonged survival of rat islet and skin xenografts in mice treated with donor splenocytes and anti-CD154 monoclonal antibody

Immunological challenges associated with artificial skin grafts: available solutions and stem cells in future design of synthetic skin

The repair or replacement of damaged skins is still an important, challenging public health problem. Immune acceptance and long-term survival of skin grafts represent the major problem to overcome in grafting given that in most situations autografts cannot be used. The emergence of artificial skin substitutes provides alternative treatment with the capacity to reduce the dependency on the increasing demand of cadaver skin grafts. Over the years, considerable research efforts have focused on strategies for skin repair or permanent skin graft transplantations. Available skin substitutes include pre- or post-transplantation treatments of donor cells, stem cell-based therapies, and skin equivalents composed of bio-engineered acellular or cellular skin substitutes. However, skin substitutes are still prone to immunological rejection, and as such, there is currently no skin substitute available to overcome this phenomenon. This review focuses on the mechanisms of skin rejection and tolerance induction and outlines in detail current available strategies and alternatives that may allow achieving full-thickness skin replacement and repair.

Adenovirus-Mediated CTLA4Ig or CD40Ig Gene Transfer Delays Pancreatic Islet Rejection in a Rat-to-Mouse Xenotransplantation Model after Systemic but Not Local Expression

Transient costimulation signal blockade of either CD28/CD80–86 interactions and/or CD40/CD154 interactions can prevent islet rejection in some models of both allo- and xenotransplantation. We have used adenoviruses coding for CTLA4Ig or CD40Ig and compared the efficacy of genetic modification of islets to systemic production through either intramuscular (IM) or intravenous (IV) injection of these vectors in a rat-to-mouse islet transplantation model. When gene transfer was performed into islets, a high level of primary nonfunction was induced. Furthermore, transduced functional grafts were rejected with the same kinetics as nontransduced islets. In contrast, IM AdCTLA4Ig and IV AdCD40Ig significantly delayed rejection (mean survival time of 54 ± 26.9 and 67.6 ± 44.9 days, respectively, vs. 24.3 ± 9.7 days for unmodified islets, p < 0.05). Combination of ex vivo AdCTLA4Ig islet transduction and IV AdCD40Ig did not improve graft survival further. In conclusion, islet graft transduction with adenoviruses coding for costimulation inhibitors resulted in local expression with low serum concentrations of CTLA4Ig or CD40Ig and was unable to protect islet xenografts from rejection. In contrast, IM or IV gene transfer resulted in high serum concentrations of these molecules and was highly efficient in prolonging xenograft survival. These results contrast with the efficacy of AdCTLA4Ig we observed in a rat islet allotransplantation model and suggest that islet xenograft rejection might be more difficult to control.

Allospecific CD4(+) T cells retain effector function and are actively regulated by Treg cells in the context of transplantation tolerance

Although donor-specific transfusion (DST) plus CD154 blockade represents a robust protocol for inducing transplantation tolerance, the underlying mechanisms are incompletely understood. In a murine T-cell adoptive transfer model, we have visualized alloantigen-specific, TCR-transgenic for H2-A(b) /H2-K(d) 54-68 epitope (TCR75) CD4(+) T cells with indirect allospecificity during the course of tolerance induction. Three main observations were made. First, although the majority of TCR75 CD4(+) T cells were deleted following DST plus CD154 blockade, the surviving TCR75 CD4(+) T cells were capable of making IL-2, upregulating CD44, and undergoing cell division, suggesting that they were functionally active. Indeed, residual TCR75 CD4(+) T cells reisolated from the primary recipients given DST plus CD154 blockade were fully capable of rejecting allografts upon secondary transfer. Second, in tolerant mice, TCR75 CD4(+) T cells were not induced to express Foxp3 in the graft-draining lymph node. TCR75 CD4(+) T cells were also absent in accepted graft tissues in which endogenous Treg cells were enriched. Finally, DST plus CD154 blockade resulted in an abortive expansion of TCR75 CD4(+) T cells, a process that required the presence of endogenous Treg cells. Collectively, surviving TCR75 CD4(+) T cells are immunocompetent but kept in check by an endogenous immunosuppressive network induced by DST plus CD154 blockade.

Antigen-specific induced Foxp3+ regulatory T cells are generated following CD40/CD154 blockade

Blockade of the CD40/CD154 pathway potently attenuates T-cell responses in models of autoimmunity, inflammation, and transplantation. Indeed, CD40 pathway blockade remains one of the most powerful methods of prolonging graft survival in models of transplantation. But despite this effectiveness, the cellular and molecular mechanisms underlying the protective effects of CD40 pathway blockade are incompletely understood. Furthermore, the relative contributions of deletion, anergy, and regulation have not been measured in a model in which donor-reactive CD4(+) and CD8(+) T-cell responses can be assessed simultaneously. To investigate the impact of CD40/CD154 pathway blockade on graft-specific T-cell responses, a transgenic mouse model was used in which recipients containing ovalbumin-specific CD4(+) and CD8(+) TCR transgenic T cells were grafted with skin expressing ovalbumin in the presence or absence of anti-CD154 and donor-specific transfusion. The results indicated that CD154 blockade altered the kinetics of donor-reactive CD8(+) T-cell expansion, delaying differentiation into IFN-γ(+) TNF(+) multifunctional cytokine producers. The eventual differentiation of cytokine-producing effectors in tolerant animals coincided with the emergence of an antigen-specific CD4(+) CD25(hi) Foxp3(+) T-cell population, which did not arise from endogenous natural T(reg) but rather were peripherally generated from naïve Foxp3(-) precursors.

In vitro suppression of xenoimmune-mediated macrophage activation by human CD4+CD25+ regulatory T cells

BACKGROUND

Macrophages are important effector cells in T cell-mediated xenograft rejection. The aim of this study was to determine whether CD4+CD25+ regulatory T cells (Tregs) were capable of suppressing macrophage activation in vitro.

METHODS

Porcine cell or xenoantigen-primed human peripheral blood mononuclear cells, CD4+ T cell-depleted peripheral blood mononuclear cells, or CD14+ macrophages plus autologous CD4+CD25- T cells were cultured with or without expanded autologous Tregs. Transwell cultures were used to separate the various components to determine the need for cell-cell contact.

RESULTS

Pig cell primed CD14+ macrophages required the presence of CD4+CD25- T cells for activation and increased expression of CD40, interleukin-12, and tumor necrosis factor-alpha. This up-regulated expression of macrophage activation markers was reduced substantially in the presence of autologous Tregs. Coculture with Tregs did not alter macrophage viability but reduced the capacity of macrophages to stimulate proliferation of responder T cells. Tregs required direct contact with CD4+CD25- T cells to inhibit macrophage activation but activated macrophage phenotype was not altered by separating the stimulated human peripheral blood mononuclear cells or CD14+ macrophages from Tregs in a transwell system. Macrophages did not require direct cell contact with porcine stimulator cells for full activation by CD4+CD25- T cells.

CONCLUSIONS

Human Tregs were able to suppress xenoantigen-primed and CD4+ T-cell-mediated macrophage activation and antigen-presenting cell function. However, Tregs had no direct effect on macrophages in vitro.

Application of cyclophosphamide-induced tolerance in alpha1,3-galactosyltransferase knockout mice presensitized with Gal alpha 1-3Gal beta-4-GlcNAc antigens

PURPOSE

Hyperacute rejection (HAR) mediated by the natural antibody (nAb) against Gal alpha 1-3Gal beta-4-GlcNAc (alpha Gal) is the major obstacle in xenogeneic organ transplantation. Previously, we reported the acceptance of donor heart grafts in anti-alpha Gal nAb-producing galactosyltransferase knockout (GalT KO) mice after cyclophosphamide (CP)-induced tolerance conditioning. In the present study, we applied our tolerance induction conditioning in presensitized recipient mice.

METHODS

GalT KO (alpha Gal(-/-), H-2(b/d)) recipient mice were presensitized with alpha Gal(+) rabbit red blood cells (RRBCs). Presensitized or nonsensitized recipient mice were treated with CP-induced tolerance conditioning, consisting of AKR (alpha Gal(+/+), H-2(k)) spleen cells (SC), CP, busulfan (BU), and AKR bone marrow cells (BMC). We assessed the survival of donor hearts and skin grafts and analyzed the production of anti-alpha Gal Abs by flow cytometry.

RESULTS

Donor mixed chimerism was achieved in the presensitized GalT KO mice treated with CP-induced tolerance conditioning. In parallel with the disappearance of anti-alpha Gal Abs, permanent acceptance of donor heart grafts and skin grafts was observed in presensitized and GalT KO mice treated with CP-induced tolerance conditioning.

CONCLUSIONS

Both B-cell and T-cell tolerance was achieved in the presence of a higher titer of anti-alpha Gal Abs after treatment with CP-induced tolerance conditioning.

Mechanisms underlying B-cell tolerance induction by antigen-immunoglobulin G gene transfer

Previous studies on the mechanisms underlying tolerance induction in diabetes have mainly focused on T cells, however B cells also have an important role in diabetes. Based on our previous studies that splenocytes, transduced with glutamic acid decarboxylase (GAD) 65 fused to immunoglobulin (Ig) G carrier, reduced antibody-mediated response in non-obese diabetic (NOD) mice, here we examined the mechanisms underlying B-cell tolerance in this system. We found that GAD-IgG-transduced splenocytes did not reduce CD40 expression on B-cells in NOD mice, but they did downregulate CD40 ligand (CD40L) expression. Furthermore, anti-CD40L injection reduced autoantibody levels in NOD mice and in vitro experiments demonstrated that CD40L blockade reduced the antigenpresenting capability of B-cells. In conclusion, the results of this study suggest that downregulation of CD40L may be one mechanism underlying the induction of B-cell tolerance in GAD-IgG-treated NOD mice.

Chemokine and toll-like receptor signaling in macrophage mediated islet xenograft rejection

BACKGROUND

Adoptive transfer of antigen-primed T-cell-activated macrophages into NOD-SCID mice within 14 days of foetal porcine pancreatic fragment (FPP) or foetal porcine skin (FPS) transplantation had been shown to cause xenograft rejection. In the present study, it was proposed that signaling between the graft and macrophages promoted specific graft recognition and destruction in this setting.

METHODS

Exogenous macrophages isolated from rejecting FPP xenografts were transferred to NOD-SCID FPP recipients and tracked by Ly5.1 surface antigen or via CSFE staining. Monocyte chemoattractant protein-1 (MCP-1), macrophage inflammatory protein-1alpha (MIP-1alpha), macrophage inflammatory protein-1beta (MIP-1beta), regulated upon activation, normal T-cell expressed and secreted (RANTES), chemokine (C-C motif) receptor 2 (CCR2), chemokine (C-C motif) receptor 5 (CCR5), toll-like receptors (TLRs) (1-9) and gene expression in transplanted FPP xenografts was evaluated by real-time polymerase chain reaction. Gene expression of CCR2, CCR5 and TLRs was also analyzed in pooled samples of activated and non-activated macrophages.

RESULTS

Exogenous macrophages were shown to track to and reject recently transplanted but not established FPP xenografts. Gene expression for MCP-1, RANTES, MIP-1alpha and MIP-1beta was at least 3-fold greater in recently transplanted compared with established xenografts (P < 0.05), and CCR2 and CCR5 gene expression was 10-fold greater in activated compared non-activated macrophages, suggesting that graft-mediated pro-inflammatory signals were important for macrophage recruitment. Specific graft recognition by macrophages may involve TLR signaling as macrophages exposed to porcine islets had higher levels of TLR gene expression compared with those exposed to allografts regardless of the level of activation.

CONCLUSION

Xenografts provide additional activation signals to macrophages that are not seen following allotransplantation. This study identifies chemokines and TLR as important signals in macrophage-mediated recognition and rejection of islet xenografts.

Beneficial effects of simultaneous treatment with 15-deoxyspergualin and monoclonal antibodies to CD45RB and CD154 on murine islet transplantation recipients

BACKGROUND

Treatment of transplant recipients with either 15-deoxyspergualin (DSG) or monoclonal antibodies (mAbs) to T-cell proteins CD45RB and CD154 (a two-signal blockade) has been shown to prolong islet graft survival. Therefore, we investigated the combined effect of DSG, anti-CD45RB, and anti-CD154 in murine islet model.

METHODS

Chemically induced diabetic C57BL/6 mice underwent allografting with islets from BALB/c mice or xenografting with rat islets. After transplantation, they were treated with either DSG, the two-signal blockade, or both (the triple treatment). The tolerogenic effects of the posttransplant treatments were measured with an intraperitoneal glucose tolerance test (IPGTT), immunohistology, enzyme-linked immunosorbent assays, and flow cytometry.

RESULTS

Blood glucose profiles measured after glucose challenges were improved in all islet recipients. Enhancement of xenograft survival in triple-treated groups was not statistically significant (P = 0.08), compared to graft survival in group received only the two-signal blockade. However, 15 days after transplantation, xenografts in the triple-treated group showed a significant decrease in the proportion of CD4, CD8, and CD4CD45RB T-cells, and in the expression of interleukin-10 and interferon-gamma, relative to grafts in the other treatment groups. In addition, reduced infiltration of the xenografts by CD3 T-cells was observed in groups that had received either the two-signal blockade or the triple treatment. With long-term (>248 days) xenografts, only those in the triple-treated group were free of inflammatory infiltrates. These grafts also exhibited larger islet clusters and contained more insulin- and glucagon-positive cells, relative to grafts in the other treatment groups.

CONCLUSION

Triple treatment has a beneficial effect in murine islet xenotransplantation.

Anti-CD40L Monoclonal Antibodies Can Replace Anti-CD4 Monoclonal Antibodies for the Nonmyeloablative Induction of Mixed Xenogeneic Chimerism

BACKGROUND

We have previously demonstrated that xenogeneic bone marrow engraftment and donor-specific tolerance can be induced in mice receiving anti-CD4, -CD8, -Thy-1.2, and -NK1.1 monoclonal antibodies (mAbs) on Days -6 and -1, 3 Gy total body irradiation (TBI), and 7 Gy thymic irradiation on Day 0, followed by injection of T-cell depleted (TCD) rat bone marrow cells. We have recently demonstrated that anti-CD40L mAb treatment is sufficient to completely overcome CD4 cell-mediated resistance to allogeneic marrow engraftment and rapidly induce CD4 cell tolerance in an allogeneic combination.

METHODS

We investigated the ability of anti-CD40L mAb to promote mixed xenogeneic chimerism and donor-specific tolerance in B6 mice receiving anti-CD8, -Thy1.2 and -NK1.1 mAbs and 3 Gy TBI followed by TCD bone marrow transplantation (BMT) from F344 rats.

RESULTS

Administration of anti-CD4 mAb in this model could be completely replaced by one injection of anti-CD40L mAb. Evidence for deletional tolerance was obtained in mixed chimeras prepared with this anti-CD40L-based regimen. However, anti-NK1.1 and anti-Thy1.2 mAb could not be replaced by anti-CD40L mAb.

CONCLUSIONS

These results demonstrate that anti-CD40L in combination with xenogeneic BMT can tolerize preexisting peripheral and intrathymic CD4 cells to xenoantigens. However, anti-CD40L does not prevent NK cell and/or gammaDelta cell-mediated rejection of xenogeneic bone marrow.

Biological and Biomaterial Approaches for Improved Islet Transplantation

Islet transplantation may be used to treat type I diabetes. Despite tremendous progress in islet isolation, culture, and preservation, the clinical use of this modality of treatment is limited due to post-transplantation challenges to the islets such as the failure to revascularize and immune destruction of the islet graft. In addition, the need for lifelong strong immunosuppressing agents restricts the use of this option to a limited subset of patients, which is further restricted by the unmet need for large numbers of islets. Inadequate islet supply issues are being addressed by regeneration therapy and xenotransplantation. Various strategies are being tried to prevent beta-cell death, including immunoisolation using semipermeable biocompatible polymeric capsules and induction of immune tolerance. Genetic modification of islets promises to complement all these strategies toward the success of islet transplantation. Furthermore, synergistic application of more than one strategy is required for improving the success of islet transplantation. This review will critically address various insights developed in each individual strategy and for multipronged approaches, which will be helpful in achieving better outcomes.

Anti-mouse CD154 antibody treatment facilitates generation of mixed xenogeneic rat hematopoietic chimerism, prevents wasting disease and prolongs xenograft survival in mice

BACKGROUND

The induction of xenogeneic hematopoietic chimerism is an attractive approach for overcoming the host response to xenografts, but establishing xenogeneic chimerism requires severe myeloablative conditioning of the recipient. The goal of this study was to determine if co-stimulation blockade would facilitate chimerism and xenograft tolerance in irradiation-conditioned concordant recipients.

METHODS

Wistar Furth rat bone marrow (BM) cells were injected into irradiation-conditioned C57BL/6 mice with or without co-administration of anti-mouse CD154 monoclonal antibody (mAb). Chimerism was quantified by flow cytometry, and mice were transplanted with WF rat skin and islet xenografts.

RESULTS

Blockade of CD40-CD154 interaction facilitated establishment of xenogeneic chimerism in mice conditioned with 600 cGy irradiation. Anti-CD154 mAb was not required for establishment of chimerism in mice treated with 700 cGy. However, mice irradiated with 700 cGy but not treated with anti-CD154 mAb developed a "graft-versus-host disease (GVHD)-like" wasting syndrome and died, irrespective of their development of chimerism. Xenogeneic chimeras established with irradiation and anti-CD154 mAb treatment exhibited prolonged skin and, in many cases, permanent islet xenograft survival. Chimerism was unstable and eventually lost in most recipients. Skin xenografts were rejected even in mice that remained chimeric, whereas most islet xenografts survived to the end of the observation period.

CONCLUSIONS

Blockade of host CD40-CD154 interaction facilitates the establishment of xenogeneic chimerism and prevents wasting disease and death. Chimerism permits prolonged xenograft survival, but chimerism generated in this way is unstable over time. Skin xenografts are eventually rejected, whereas most islet xenografts survive long term and perhaps permanently.

Analysis of the underlying cellular mechanisms of anti-CD154-induced graft tolerance: the interplay of clonal anergy and immune regulation

Although it has been shown that CD4(+)CD25(+) regulatory T cells (T(reg)) contribute to long-term graft acceptance, their impact on the effector compartment and the mechanism by which they exert suppression in vivo remain unresolved. Using a CD4(+) TCR transgenic model for graft tolerance, we have unveiled the independent contributions of anergy and active suppression to the fate of immune and tolerant alloreactive T cells in vivo. First, it is shown that anti-CD154-induced tolerance resulted in the abortive expansion of the alloreactive, effector T cell pool. Second, commensurate with reduced expansion, there was a loss of cytokine production, activation marker expression, and absence of memory T cell markers. All these parameters defined the tolerant alloreactive T cells and correlated with the inability to mediate graft rejection. Third, the tolerant alloreactive T cell phenotype that is induced by CD154 was reversed by the in vivo depletion of T(reg). Reversal of the tolerant phenotype was followed by rapid rejection of the allograft. Fourth, in addition to T(reg) depletion, costimulation of the tolerant alloreactive T cells or activation of the APC compartment also reverted alloreactive T cell tolerance and restored an activated phenotype. Finally, it is shown that the suppression is long-lived, and in the absence of anti-CD154 and donor-specific transfusion, these T(reg) can chronically suppress effector cell responses, allowing long-lived graft acceptance.

Therapeutic peptidomimetic strategies for autoimmune diseases: costimulation blockade*

Cognate interactions between immune effector cells and antigen-presenting cells (APCs) govern immune responses. Specific signals occur between the T-cell receptor peptide and APCs and nonspecific signals between pairs of costimulatory molecules. Costimulation signals are required for full T-cell activation and are assumed to regulate T-cell responses as well as other aspects of the immune system. As new discoveries are made, it is becoming clear how important these costimulation interactions are for immune responses. Costimulation requirements for T-cell regulation have been extensively studied as a way to control many autoimmune diseases and downregulate inflammatory reactions. The CD28:B7 and the CD40:CD40L families of molecules are considered to be critical costimulatory molecules and have been studied extensively. Blocking the interaction between these molecules results in a state of immune unresponsiveness termed 'anergy'. Several different strategies for blockade of these interactions are explored including monoclonal antibodies (mAbs), Fab fragments, chimeric, and/or fusion proteins. We developed novel, immune-specific approaches that interfere with these interactions. Using experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis mediated by central nervous system (CNS)-specific T-cells, we developed a multi-targeted approach that utilizes peptides for blockade of costimulatory molecules. We designed blocking peptide mimics that retain the functional binding area of the parent protein while reducing the overall size and are thus capable of blocking signal transduction. In this paper, we review the role of costimulatory molecules in autoimmune diseases, two of the most well-studied costimulatory pathways (CD28/CTLA-4:B7 and CD40:CD40L), and the advantages of peptidomimetic approaches. We present data showing the ability of peptide mimics of costimulatory molecules to suppress autoimmune disease and propose a mechanism for disease suppression.

Inhibition of cellular immune responses to encapsulated porcine islet xenografts by simultaneous blockade of two different costimulatory pathways

BACKGROUND

Transplantation of human islets has been successful clinically. Since human islets are scarce, we are studying microencapsulated porcine islet xenografts in nonobese diabetic (NOD) mice. We have evaluated the cellular immune response in NOD mice with and without dual costimulatory blockade.

METHODS

Alginate-poly-L-lysine-encapsulated adult porcine islets were transplanted i.p. in untreated diabetic NODs and NODs treated with CTLA4-Ig to block CD28/B7 and with anti-CD154 mAb to inhibit CD40/CD40-ligand interactions. Groups of mice were sacrificed on subsequent days; microcapsules were evaluated by histology; peritoneal cells were analyzed by FACS; and peritoneal cytokines were quantified by ELISA. Controls included immunoincompetent NOD-Scids and diabetic NODs given sham surgery or empty microcapsules.

RESULTS

Within 20 days, encapsulated porcine islets induced accumulation of large numbers of macrophages, eosinophils, and significant numbers of CD4 and CD8 T cells at the graft site, and all grafts were rejected. During rejection, IFNgamma, IL-12 and IL-5 were significantly elevated over sham-operated controls, whereas IL-2, TNFalpha, IL-4, IL-6, IL-10, IL-1beta and TGFbeta were unchanged. Treatment with CTLA4-Ig and anti-CD154 prevented graft destruction in all animals during the 26 days of the experiment, dramatically inhibited recruitment of host inflammatory cells, and inhibited peritoneal IFNgamma and IL-5 concentrations while delaying IL-12 production.

CONCLUSIONS

When two different pathways of T cell costimulation were blocked, T cell-dependent inflammatory responses were inhibited, and survival of encapsulated islet xenografts was significantly prolonged. These findings suggest synergy between encapsulation of donor islets and simultaneous blockade of two host costimulatory pathways in prolonging xenoislet transplant survival.

The efficacy of CD40 ligand blockade in discordant pig-to-rat islet xenotransplantation is correlated with an immunosuppressive effect of immunoglobulin

BACKGROUND

The authors' aim was to evaluate the efficacy of immunosuppression with monoclonal anti-CD40 ligand antibodies (aCD40L) or nonspecific polyclonal intravenous immunoglobulin (IVIG) in the pig-to-rat islet xenotransplantation model.

METHODS

Fetal porcine islet-like cell clusters were transplanted under the kidney capsule of nondiabetic rats. All antibodies were administered alone or in combination with cyclosporine A (CsA). In addition, some animals were administered antibodies plus tacrolimus (TAC) or sirolimus (SIR). Twelve days after transplantation, islet xenograft survival and rejection were evaluated using immunohistochemistry.

RESULTS

aCD40L plus CsA had a pronounced inhibitory effect on islet xenograft rejection for up to 12 days after transplantation. Unexpectedly, treatment with a monoclonal control antibody (anti-keyhole limpet hemocyanin [aKLH]) plus CsA had a similar inhibitory effect. Furthermore, a similar inhibition of islet xenograft rejection was observed also in animals administered IVIG plus CsA. Monotherapy with aCD40L, aKLH, IVIG, or CsA had no effect on the rejection process. Also, when aCD40L or aKLH was administered together with TAC, islet xenograft rejection was inhibited. There was no marked difference compared with rats treated with aCD40L or aKLH and CsA. Immunosuppression with aCD40L or aKLH in combination with SIR also inhibited pig-to-rat islet xenograft rejection, but the protective effect was not as pronounced.

CONCLUSIONS

Immunosuppression with high doses of antibodies, monoclonal or polyclonal, in combination with CsA or TAC inhibits pig-to-rat islet xenograft rejection. No specific effect of co-stimulatory blockade with aCD40L could be observed. Instead, the results indicate a nonspecific immunosuppressive effect of high doses of antibodies in this model.

Indefinite survival of neonatal porcine islet xenografts by simultaneous targeting of LFA-1 and CD154 or CD45RB

A variety of transient therapies directed against molecules involved in T-cell activation and function result in long-term islet allograft survival. However, there are relatively few examples of durable islet xenograft survival using similar short-term approaches, especially regarding highly phylogenetically disparate xenograft donors. Previous studies demonstrate that combined anti-lymphocyte function-associated antigen-1 (LFA-1) plus anti-CD154 therapy results in a robust form of islet allograft tolerance not observed with either individual monotherapy. Thus, the aim of this study was to determine whether the perturbation of anti-LFA-1, either alone or in combination with targeting CD154 or CD45RB, would promote neonatal porcine islet (NPI) xenograft survival in mice. NPI xenografts are rapidly rejected in wild-type C57BL/6 mice but reproducibly mature and restore durable euglycemia in diabetic, immune-deficient C57BL/6 rag-1(-/-) recipients. A short course of individual anti-LFA-1, anti-CD154, or anti-CD45RB therapy resulted in long-term (>100 days) survival in a moderate proportion of C57BL/6 recipients. However, simultaneous treatment with anti-LFA-1 plus either anti-CD154 or anti-CD45RB therapy could achieve indefinite xenograft function in the majority of recipient animals. Importantly, prolongation of islet xenograft survival using combined anti-LFA-1/anti-CD154 therapy was associated with little mononuclear cell infiltration and greatly reduced anti-porcine antibody levels. Taken together, results indicate that therapies simultaneously targeting differing pathways impacting T-cell function can show marked efficacy for inducing long-term xenograft survival and produce a prolonged state of host hyporeactivity in vivo.

Autoimmune diabetes and resistance to xenograft transplantation tolerance in NOD mice

Costimulation blockade induces prolonged rat islet and skin xenograft survival in C57BL/6 mice. Nonobese diabetic (NOD) mice, which are used to model human autoimmune diabetes, are resistant to costimulation blockade-induced allograft tolerance. We tested the hypothesis that NOD mice would also be resistant to costimulation blockade-induced rat xenograft tolerance. We report that rat islet xenograft survival is short in spontaneously diabetic NOD mice treated with a tolerizing regimen of donor-specific transfusion and anti-CD154 antibody. Rat islet xenograft survival is only marginally longer in chemically diabetic NOD mice treated with costimulation blockade but is prolonged further in NOD Idd congenic mice bearing C57-derived chromosome 3 loci. Reciprocally, the presence of NOD-derived chromosome 3 loci shortens islet xenograft survival in tolerized C57BL/6 mice. Islet xenograft survival is longer in tolerized NOD.CD4a(-/-) and (NOD x C57BL/6)F1 mice than in NOD mice but still much shorter than in C57BL/6 mice. Skin xenograft survival in (NOD x C57BL/6)F1 mice treated with costimulation blockade is short, suggesting a strong genetic resistance to skin xenograft tolerance induction. We conclude that the resistance of NOD mice to xenograft tolerance induction involves some mechanisms that also participate in the expression of autoimmunity and other mechanisms that are distinct.

CD154 is a negative regulator of autoaggressive CD8+ T cells in type 1 diabetes

TNF/CD80 mice, a CD8(+) T cell-mediated model for type 1 diabetes, transgenically express tumor necrosis factor alpha (TNF-alpha) and the costimulatory molecule CD80 in their pancreatic islets. Here we show that these molecules bypass the need for CD40-CD154 costimulatory interactions in activation of CD8(+) T cells, allowing us to determine the role of CD40-CD154 signals in regulation of autoaggressive CD8(+) T cells after their in vivo priming. TNF/CD80 CD154-deficient mice rapidly develop diabetes, whereas CD154-sufficient mice do not. This finding correlates with the decreased numbers of CD4(+)CD25(+) T regulatory (T(R)) cells in the islets and pancreatic lymph nodes, in comparison to disease-protected CD154-sufficient mice. Administration of a CD40 agonistic antibody induces a systemic and tissue-specific increase in T(R) cells. However, this increase fails to delay diabetes development in the absence of CD154. Adoptive transfer studies show that CD8(+) T cells from TNF/CD80 CD154-deficient, but not CD154-sufficient, mice are resistant to regulation in vivo. This study provides evidence that CD40-transduced signals initiate T(R) cell increase in vivo and that CD154-transduced signals sensitize autoaggressive CD8(+) T cells to suppression.

PROLONGED SURVIVAL OF NEONATAL PORCINE ISLET XENOGRAFTS IN MICE TREATED WITH A DONOR-SPECIFIC TRANSFUSION AND ANTI-CD154 ANTIBODY1

BACKGROUND

Combined treatment with a single donor-specific transfusion (DST) and a brief course of anti-mouse CD154 monoclonal antibody (mAb) to induce co-stimulation blockade leads to long-term murine islet allograft survival. The authors hypothesized that this protocol could also induce long-term survival of neonatal porcine islet cell clusters (NPCC) in chemically diabetic immunocompetent mice and allow their differentiation into functional insulin-producing cells.

METHODS

Pancreata from 1- to 3-day-old pigs were collagenase digested and cultured for 8 days. NPCC were recovered and transplanted into the renal subcapsular space. Recipients included chemically diabetic nonobese diabetic (NOD)-scid and C57BL/6 mice that were otherwise untreated, treated with anti-CD154 mAb alone, or treated with DST plus anti-CD154 mAb. Plasma glucose concentration and body weight were measured, and xenografts were examined histologically.

RESULTS

NPCC fully differentiated and restored normoglycemia in four of five diabetic NOD-scid recipients but were uniformly rejected by diabetic C57BL/6 recipients. Anti-CD154 mAb monotherapy restored normoglycemia in 4 of 10 (40%) NPCC-engrafted, chemically diabetic C57BL/6 mice, but combined treatment with DST and anti-CD154 mAb restored normoglycemia in 12 of 13 (92%) recipients. Reversal of diabetes required 5 to 12 weeks. Surviving grafts were essentially free of inflammatory infiltrates 15 weeks after transplantation.

CONCLUSIONS

Combination therapy with a single DST and a brief course of anti-mouse CD154 mAb without maintenance immunosuppression permits survival and differentiation of NPCC in diabetic C57BL/6 mice. Successful grafts were associated with durable restoration of normoglycemia and the absence of graft inflammation.

Co-stimulatory molecules in islet xenotransplantation: CTLA4Ig treatment in CD40 ligand-deficient mice

Previous work has demonstrated that short-term systemic administration of cytotoxic T lymphocyte antigen-4 (CTLA-4) Ig blocks human pancreatic islet xenograft rejection in mice and induces long-term, donor-specific tolerance, whereas studies on pig pancreatic islet rejection in mice have failed to demonstrate a role for CTLA4Ig in preventing rejection. Treatment with anti-CD40 ligand (L) monoclonal antibodies alone is somewhat effective in prolonging the survival of islet xenografts, but ineffective when applied to skin xenografts. However, simultaneous blockade of the CD28 and CD40 co-stimulatory pathways prolongs the survival of pig skin on recipient mice. To evaluate the role of CD28 and CD40 co-stimulatory pathways in pig islet-like cell cluster (ICC) xenograft rejection in mice, CD40L-deficient mice transplanted with fetal porcine ICCs were given posttransplant treatment with human (h) CTLA4Ig or a human IgG1 chimeric mAb (hL6). Xenografts were evaluated 6 or 12 days after transplantation. Fetal porcine ICC xenografts were protected from rejection in hCTLA4Ig-treated CD40L-deficient mice, whereas xenograft rejection persisted in untreated CD40L-deficient mice. Simultaneous blockade of the CD28 and CD40 co-stimulatory pathways is mandatory to inhibit ICC xenograft rejection in the pig-to-mouse model, because the CD28 and CD40 co-stimulatory pathways seem capable of efficiently substituting for one another.

A CD200FC immunoadhesin prolongs rat islet xenograft survival in mice

BACKGROUND

A solubilized form of the CD200 molecule, CD200Fc, has been shown to suppress allograft rejection and development of collagen-induced arthritis in mice. We investigated whether the same molecule could prolong survival of rat islet xenografts.

METHODS

Streptozocin-treated mice, receiving injections with anti-asialo-GM1 antibody, received rat islets ( approximately 400/mouse) under the kidney capsule or injected into the portal vein, along with rapamycin treatment. Thereafter mice received injections of CD200Fc (10 microg/mouse/injection) or control mouse IgG2. Blood glucose was monitored daily. Some mice received additional injections of anti-CD200/-CD200R monoclonal antibodies.

RESULTS

Portal vein delivery of islets led to more extended resolution of diabetes than did transplantation under the kidney capsule. CD200Fc further prolonged survival in either case, an effect abolished by anti-CD200 or F(ab')2 anti-CD200R mAbs, but not by whole anti-CD200R (anti-CD200R Ig). Spleen cells taken from CD200Fc-treated mice showed polarization to type-2 cytokine production (interleukin-4, interleukin-10) on restimulation with rat splenocytes in culture, in comparison to cells from control mice (type-1 cytokines, interlulin-2, interferon-gamma).

CONCLUSION

CD200:CD200R interactions are important in regulating rat islet xenograft survival.

Costimulatory blockade prevents early rejection, promotes lymphocyte apoptosis, and inhibits the upregulation of intragraft interleukin-6 in an orthotopic liver transplant model in the rat

Costimulatory pathways have a pivotal role in the T-cell response to alloantigen. The role of costimulatory blockade with anti-CD154 in orthotopic liver transplantation (OLT) has not been examined previously. This study aims to investigate effects of anti-CD154 and CTLA4-immunoglobulin (Ig) in the early post-OLT period using a major histocompatibility complex-disparate fully arterialized OLT model in the rat. Lewis rats underwent OLT with Dark Agouti liver allografts. Recipients were randomized to receive (1) isotype control, (2) anti-CD154, (3) CTLA4-Ig, or (4) cyclosporine A (CyA). Rats were killed day 8, and specimens were obtained for histological examination, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling, immunohistochemistry, and quantitative reverse-transcriptase polymerase chain reaction. An additional five transplant recipients were treated with anti-CD154 for 14 days postoperatively to assess long-term allograft survival. All isotype control animals died on or before day 6 of acute rejection. Apart from four deaths caused by nonimmunologic causes, all treated recipients survived to day 8. The median survival of rats treated for 14 days with anti-CD154 was greater than 150 days. Serum aspartate aminotransferase and bilirubin levels normalized by day 3 in the CyA group and day 5 in transplant recipients treated with costimulatory blockade. Histologically, there was no difference between isotype controls and CTLA4-Ig-treated animals, whereas anti-CD154-treated transplant recipients had a lower Banff score. CD4+ and CD8+ T-cell infiltrates were prominent in transplant recipients treated with costimulatory blockade. Intragraft analysis showed an increase in lymphocyte apoptosis, Fas ligand messenger RNA expression, and reduction in interleukin-6 gene expression in transplant recipients treated with costimulatory blockade. Costimulatory blockade did not alter intragraft gene expression of other mediators of T-cell priming, differentiation, and effector function compared with isotype control animals. In conclusion, costimulatory blockade prevented acute rejection, enabled long-term survival, and increased intragraft lymphocyte apoptosis in a high-responding rat OLT model.

Co-stimulatory blockade and tolerance induction in transplantation

Recipients of organ and tissue transplants require lifelong immunosuppression to prevent rejection. Better understanding of the processes culminating in allograft rejection has led to novel approaches to modulating the immune response. Co-stimulatory signals between antigen-presenting and -responding cells are essential for a normal alloimmune response, and blockade of these pathways during initial graft-host interaction may be used to ameliorate or prevent a destructive response from proceeding. A large number of experimental studies now support this concept, and early clinical trials have been initiated. Despite some early difficulties and many unanswered questions, co-stimulatory blockade has major potential as a future immune-modulating mechanism for use in clinical transplantation.

Prolonged islet graft survival in NOD mice by blockade of the CD40-CD154 pathway of T-cell costimulation

Allorejection and recurrence of autoimmunity are the major barriers to transplantation of islets of Langerhans for the cure of type 1 diabetes in humans. CD40-CD154 (CD40 ligand) interaction blockade by the use of anti-CD154 monoclonal antibody (mAb) has shown efficacy in preventing allorejection in several models of organ and cell transplantation. Here we report the beneficial effect of the chronic administration of a hamster anti-murine CD154 mAb, MR1, in prolonging islet graft survival in NOD mice. We explored the transplantation of C57BL/6 islets into spontaneously diabetic NOD mice, a combination in which both allogeneic and autoimmune components are implicated in graft loss. Recipients were treated either with an irrelevant control antibody or with MR1. MR1 administration was effective in prolonging allograft survival, but did not provide permanent protection from diabetes recurrence. The autoimmune component of graft loss was studied in spontaneously diabetic NOD mice that received syngeneic islets from young male NOD mice. In this combination, a less dramatic yet substantial delay in diabetes recurrence was observed in the MR1-treated recipients when compared with the control group. Finally, the allogeneic component was explored by transplanting C57BL/6 islets into chemically induced diabetic male NOD mice. In this setting, long-term graft survival (>100 days) was achieved in MR1-treated mice, whereas control recipients rejected their grafts within 25 days. In conclusion, chronic blockade of CD154 results in permanent protection from allorejection and significantly delays recurrence of diabetes in NOD mice.

Rat xenograft survival in mice treated with donor-specific transfusion and anti-CD154 antibody is enhanced by elimination of host CD4+ cells

BACKGROUND

Treatment with a donor-specific transfusion (DST) and a brief course of anti-mouse CD154 (anti-CD40-ligand) monoclonal antibody (mAb) prolongs the survival of both allografts and rat xenografts in mice. The mechanism by which allograft survival is prolonged is incompletely understood, but depends in part on the presence of CD4+ cells and the deletion of alloreactive CD8+ T cells. Less is known about the mechanism by which this protocol prolongs xenograft survival.

METHODS

We measured rat islet and skin xenograft survival in euthymic and thymectomized mice treated with combinations of DST, anti-CD154 mAb, anti-CD4 mAb, and anti-CD8 mAb. Recipients included C57BL/6, C57BL/6-scid, C57BL/6-CD4null, and C57BL/6-CD8null mice.

RESULTS

Pretreatment with a depleting anti-CD4 mAb markedly prolonged the survival of both skin and islet xenografts in mice given DST plus anti-CD154 mAb. Comparable prolongation of xenograft survival was obtained in C57BL/6-CD4null recipients treated with DST and anti-CD154 mAb. In contrast, anti-CD8 mAb did not prolong the survival of either islet or skin xenografts in mice treated with DST and anti-CD154 mAb. Thymectomy did not influence xenograft survival in any treatment group. Adoptive transfer of splenocytes from C57BL/6-CD4null recipients treated with DST and anti-CD154 mAb and bearing long-term skin xenografts revealed the presence of residual xenoreactive cells.

CONCLUSIONS

These data suggest that treatment with DST and anti-CD154 mAb induces a state of "functional" transplantation tolerance. They also support the hypothesis that both the induction and maintenance of graft survival based on this protocol depend on different cellular mechanisms in allogeneic and xenogeneic model systems.

Pancreatic islet xenograft tolerance after short-term costimulation blockade is associated with increased CD4+ T cell apoptosis but not immune deviation

BACKGROUND

Our purpose was to determine if short-term inhibition of the CD40/CD40L and CD28/B7 costimulatory pathways was capable of inducing specific unresponsiveness to pancreatic islet xenografts and to ascertain the mechanism of tolerance induction.

METHODS

Diabetic B6AF1 mice were transplanted with Wistar or DA rat islets and were treated short term with CTLA4-Fc and anti-CD40L mAb (MR1).

RESULTS

Coadministration of CTLA4-Fc with MR1, resulted in indefinite rat islet xenograft survival in mice. Tolerance was species but not strain specific as long-term surviving recipients rejected third party BALB/c islet allografts but accepted a second rat islet xenograft from the same or different donor strain. Tolerance induction was associated with a large leukocyte infiltrate that did not exhibit features of immune deviation as intragraft T cell-specific cytokine gene expression was globally reduced. In particular, interleukin-4 gene expression was markedly suppressed. There was a complete inhibition of anti-donor IgG, IgG1, and IgM antibody in the serum of CTLA4-Fc/MR1- treated animals. Tolerance induction was associated with increased CD4+ T cell apoptosis as there was an increased proportion of annexin-V staining and Fas expressing CD4+ T cells and a decrease in CD4+ T cell Bcl-2 expression in the grafts and draining lymph nodes of CTLA4-Fc/MR1-treated recipients.

CONCLUSION

Combined costimulatory blockade was capable of producing tolerance to pancreatic islet xenografts. The induction of this tolerant state was associated with increased T cell apoptosis, whereas the maintenance phase of tolerance was associated with the accumulation of a large number of inactive lymphocytes within the graft.

Induction of immunologic tolerance for transplantation

In the second half of the 20th century, the transplantation of replacement organs and tissues to cure disease has become a clinical reality. Success has been achieved as a direct result of progress in understanding the cellular and molecular biology of the immune system. This understanding has led to the development of immunosuppressive pharmaceuticals that are part of nearly every transplantation procedure. All such drugs are toxic to some degree, however, and their chronic use, mandatory in transplantation, predisposes the patient to the development of infection and cancer. In addition, many of them may have deleterious long-term effects on the function of grafts. New immunosuppressive agents are constantly under development, but organ transplantation remains a therapy that requires patients to choose between the risks of their primary illness and its treatment on the one hand, and the risks of life-long systemic immunosuppression on the other. Alternatives to immunosuppression include modulation of donor grafts to reduce immunogenicity, removal of passenger leukocytes, transplantation into immunologically privileged sites like the testis or thymus, encapsulation of tissue, and the induction of a state of immunologic tolerance. It is the last of these alternatives that has, perhaps, the most promise and most generic applicability as a future therapy. Recent reports documenting long-term graft survival in the absence of immunosuppression suggest that tolerance-based therapies may soon become a clinical reality. Of particular interest to our laboratory are transplantation strategies that focus on the induction of donor-specific T-cell unresponsiveness. The basic biology, protocols, experimental outcomes, and clinical implications of tolerance-based transplantation are the focus of this review.