Thomas G. Markees

Long-term survival of skin allografts induced by donor splenocytes and anti-CD154 antibody in thymectomized mice requires CD4(+) T cells, interferon-gamma, and CTLA4.

Treatment of C57BL/6 mice with one transfusion of BALB/c spleen cells and anti-CD154 (anti-CD40-ligand) antibody permits BALB/c islet grafts to survive indefinitely and BALB/c skin grafts to survive for approximately 50 d without further intervention. The protocol induces long-term allograft survival, but the mechanism is unknown. We now report: (a) addition of thymectomy to the protocol permitted skin allografts to survive for > 100 d, suggesting that graft rejection in euthymic mice results from thymic export of alloreactive T cells. (b) Clonal deletion is not the mechanism of underlying long-term graft survival, as recipient thymectomized mice were immunocompetent and harbor alloreactive T cells. (c) Induction of skin allograft acceptance initially depended on the presence of IFN-gamma, CTLA4, and CD4(+) T cells. Addition of anti-CTLA4 or anti-IFN-gamma mAb to the protocol was associated with prompt graft rejection, whereas anti-IL-4 mAb had no effect. The role of IFN-gamma was confirmed using knockout mice. (d) Graft survival was associated with the absence of IFN-gamma in the graft. (e) Long-term graft maintenance required the continued presence of CD4(+) T cells. The results suggest that, with modification, our short-term protocol may yield a procedure for the induction of long-term graft survival without prolonged immunosuppression.

Treatment of Allograft Recipients with Donor-Specific Transfusion and Anti-CD154 Antibody Leads to Deletion of Alloreactive CD8+ T Cells and Prolonged Graft Survival in a CTLA4-Dependent Manner

A two-element protocol consisting of one donor-specific transfusion (DST) plus a brief course of anti-CD154 mAb greatly prolongs the survival of murine islet, skin, and cardiac allografts. To study the mechanism of allograft survival, we determined the fate of tracer populations of alloreactive transgenic CD8+ T cells in a normal microenvironment. We observed that DST plus anti-CD154 mAb prolonged allograft survival and deleted alloreactive transgenic CD8+ T cells. Neither component alone did so. Skin allograft survival was also prolonged in normal recipients treated with anti-CD154 mAb plus a depleting anti-CD8 mAb and in C57BL/6-CD8 knockout mice treated with anti-CD154 mAb monotherapy. We conclude that, in the presence of anti-CD154 mAb, DST leads to an allotolerant state, in part by deleting alloreactive CD8+ T cells. Consistent with this conclusion, blockade of CTLA4, which is known to abrogate the effects of DST and anti-CD154 mAb, prevented the deletion of alloreactive transgenic CD8+ T cells. These results document for the first time that peripheral deletion of alloantigen-specific CD8+ T cells is an important mechanism through which allograft survival can be prolonged by costimulatory blockade. We propose a unifying mechanism to explain allograft prolongation by DST and blockade of costimulation.

Direct Visualization of Cross-Reactive Effector and Memory Allo-Specific CD8 T Cells Generated in Response to Viral Infections

CD8 T cell cross-reactivity between heterologous viruses has been shown to provide protective immunity, induce immunopathology, influence the immunodominance of epitope-specific T cell responses, and shape the overall memory population. Virus infections also induce cross-reactive allo-specific CTL responses. In this study, we quantified the allo-specific CD8 T cells elicited by infection of C57BL/6 (B6) mice with lymphocytic choriomeningitis virus (LCMV). Cross-reactive LCMV-specific CD8 T cells were directly visualized using LCMV peptide-charged MHC tetramers to costain T cells that were stimulated to produce intracellular IFN-γ in response to allogeneic target cells. The cross-reactivity between T cells specific for LCMV and allogeneic Ags was broad-based, in that it involved multiple LCMV-derived peptides, but there were distinctive patterns of reactivity against allogeneic cells with different haplotypes. Experiments indicated that this cross-reactivity was not due to the expression of two TCR per cell, and that the patterns of allo-reactivity changed during sequential infection with heterologous viruses. The allo-specific CD8 T cells generated by LCMV infection were maintained at relatively high frequencies in the memory pool, indicating that memory allo-specific CD8 T cell populations can arise as a consequence of viral infections. Mice previously infected with LCMV and harboring allo-specific memory T cells were refractory to the induction of tolerance to allogeneic skin grafts.

Virus-Induced Abrogation of Transplantation Tolerance Induced by Donor-Specific Transfusion and Anti-CD154 Antibody

ABSTRACT Treatment with a 2-week course of anti-CD154 antibody and a single transfusion of donor leukocytes (a donor-specific transfusion or DST) permits skin allografts to survive for >100 days in thymectomized mice. As clinical trials of this methodology in humans are contemplated, concern has been expressed that viral infection of graft recipients may disrupt tolerance to the allograft. We report that acute infection with lymphocytic choriomeningitis virus (LCMV) induced allograft rejection in mice treated with DST and anti-CD154 antibody if inoculated shortly after transplantation. Isografts resisted LCMV-induced rejection, and the interferon-inducing agent polyinosinic:polycytidylic acid did not induce allograft rejection, suggesting that the effect of LCMV is not simply a consequence of nonspecific inflammation. Administration of anti-CD8 antibody to engrafted mice delayed LCMV-induced allograft rejection. Pichinde virus also induced acute allograft rejection, but murine cytomegalovirus and vaccinia virus (VV) did not. Injection of LCMV ∼50 days after tolerance induction and transplantation had minimal effect on subsequent allograft survival. Treatment with DST and anti-CD154 antibody did not interfere with clearance of LCMV, but a normally nonlethal high dose of VV during tolerance induction and transplantation killed graft recipients. We conclude that DST and anti-CD154 antibody induce a tolerant state that can be broken shortly after transplantation by certain viral infections. Clinical application of transplantation tolerance protocols may require patient isolation to facilitate the procedure and to protect recipients.

TLR Agonists Abrogate Costimulation Blockade-Induced Prolongation of Skin Allografts

Costimulation blockade protocols are effective in prolonging allograft survival in animal models and are entering clinical trials, but how environmental perturbants affect graft survival remains largely unstudied. We used a costimulation blockade protocol consisting of a donor-specific transfusion and anti-CD154 mAb to address this question. We observed that lymphocytic choriomeningitis virus infection at the time of donor-specific transfusion and anti-CD154 mAb shortens allograft survival. Lymphocytic choriomeningitis virus 1) activates innate immunity, 2) induces allo-cross-reactive T cells, and 3) generates virus-specific responses, all of which may adversely affect allograft survival. To investigate the role of innate immunity, mice given costimulation blockade and skin allografts were coinjected with TLR2 (Pam3Cys), TLR3 (polyinosinic:polycytidylic acid), TLR4 (LPS), or TLR9 (CpG) agonists. Costimulation blockade prolonged skin allograft survival that was shortened after coinjection by TLR agonists. To investigate underlying mechanisms, we used “synchimeric” mice which circulate trace populations of anti-H2b transgenic alloreactive CD8+ T cells. In synchimeric mice treated with costimulation blockade, coadministration of all four TLR agonists prevented deletion of alloreactive CD8+ T cells and shortened skin allograft survival. These alloreactive CD8+ T cells 1) expressed the proliferation marker Ki-67, 2) up-regulated CD44, and 3) failed to undergo apoptosis. B6.TNFR2−/− and B6.IL-12R−/− mice treated with costimulation blockade plus LPS also exhibited short skin allograft survival whereas similarly treated B6.CD8α−/− and TLR4−/− mice exhibited prolonged allograft survival. We conclude that TLR signaling abrogates the effects of costimulation blockade by preventing alloreactive CD8+ T cell apoptosis through a mechanism not dependent on TNFR2 or IL-12R signaling.

Skin Allograft Maintenance in a New Synchimeric Model System of Tolerance

Treatment of mice with a single donor-specific transfusion plus a brief course of anti-CD154 mAb uniformly induces donor-specific transplantation tolerance characterized by the deletion of alloreactive CD8+ T cells. Survival of islet allografts in treated mice is permanent, but skin grafts eventually fail unless recipients are thymectomized. To analyze the mechanisms underlying tolerance induction, maintenance, and failure in euthymic mice we created a new analytical system based on allo-TCR-transgenic hemopoietic chimeric graft recipients. Chimeras were CBA (H-2k) mice engrafted with small numbers of syngeneic TCR-transgenic KB5 bone marrow cells. These mice subsequently circulated a self-renewing trace population of anti-H-2b-alloreactive CD8+ T cells maturing in a normal microenvironment. With this system, we studied the maintenance of H-2b allografts in tolerized mice. We documented that alloreactive CD8+ T cells deleted during tolerance induction slowly returned toward pretreatment levels. Skin allograft rejection in this system occurred in the context of 1) increasing numbers of alloreactive CD8+ cells; 2) a decline in anti-CD154 mAb concentration to levels too low to inhibit costimulatory functions; and 3) activation of the alloreactive CD8+ T cells during graft rejection following deliberate depletion of regulatory CD4+ T cells. Rejection of healed-in allografts in tolerized mice appears to be a dynamic process dependent on the level of residual costimulation blockade, CD4+ regulatory cells, and activated alloreactive CD8+ thymic emigrants that have repopulated the periphery after tolerization.

Type 1 IFN Mediates Cross-Talk between Innate and Adaptive Immunity That Abrogates Transplantation Tolerance

TLR activation of innate immunity prevents the induction of transplantation tolerance and shortens skin allograft survival in mice treated with costimulation blockade. The mechanism by which TLR signaling mediates this effect has not been clear. We now report that administration of the TLR agonists LPS (TLR4) or polyinosinic:polycytidylic acid (TLR3) to mice treated with costimulation blockade prevents alloreactive CD8+ T cell deletion, primes alloreactive CTLs, and shortens allograft survival. The TLR4- and MyD88-dependent pathways are required for LPS to shorten allograft survival, whereas polyinosinic:polycytidylic acid mediates its effects through a TLR3-independent pathway. These effects are all mediated by signaling through the type 1 IFN (IFN-αβ) receptor. Administration of IFN-β recapitulates the detrimental effects of TLR agonists on transplantation tolerance. We conclude that the type 1 IFN generated as part of an innate immune response to TLR activation can in turn activate adaptive immune responses that abrogate transplantation tolerance. Blocking of type 1 IFN-dependent pathways in patients may improve allograft survival in the presence of exogenous TLR ligands.

Genetic Disassociation of Autoimmunity and Resistance to Costimulation Blockade-Induced Transplantation Tolerance in Nonobese Diabetic Mice

Curing type 1 diabetes by islet transplantation requires overcoming both allorejection and recurrent autoimmunity. This has been achieved with systemic immunosuppression, but tolerance induction would be preferable. Most islet allotransplant tolerance induction protocols have been tested in nonobese diabetic (NOD) mice, and most have failed. Failure has been attributed to the underlying autoimmunity, assuming that autoimmunity and resistance to transplantation tolerance have a common basis. Out of concern that NOD biology could be misleading in this regard, we tested the hypothesis that autoimmunity and resistance to transplantation tolerance in NOD mice are distinct phenotypes. Unexpectedly, we observed that (NOD × C57BL/6)F1 mice, which have no diabetes, nonetheless resist prolongation of skin allografts by costimulation blockade. Further analyses revealed that the F1 mice shared the dendritic cell maturation defects and abnormal CD4+ T cell responses of the NOD but had lost its defects in macrophage maturation and NK cell activity. We conclude that resistance to allograft tolerance induction in the NOD mouse is not a direct consequence of overt autoimmunity and that autoimmunity and resistance to costimulation blockade-induced transplantation tolerance phenotypes in NOD mice can be dissociated genetically. The outcomes of tolerance induction protocols tested in NOD mice may not accurately predict outcomes in human subjects.

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

Treatment of C57BL/6 mice with one transfusion of BALB/c spleen cells and a brief course of anti-CD154 (anti-CD40 ligand) antibody permits BALB/c islet grafts to survive indefinitely and BALB/c skin grafts to survive for ∼50 days without further intervention. We now report adaptation of this protocol to the transplantation of islet and skin xenografts. We observed prolonged survival of rat islet xenografts in mice treated with donor-specific spleen cell transfusion and anti-CD154 monoclonal antibody (mAb). Challenge islet xenografts placed on these animals indicated that graft acceptance was species-specific but not strain specific. Spleen cells from recipients bearing intact grafts led to rejection of rat islet xenografts in scid mice, suggesting that graft acceptance was not due to complete clonal deletion of xenoreactive cells. We also observed prolonged survival of rat skin xenografts in mice treated with donor-specific transfusion and anti-CD154 mAb. Prolonged survival of skin xenografts was also species specific. We conclude that treatment with appropriately timed donor-specific transfusion and anti-CD154 mAb induces durable survival of both islet and skin xenografts in mice. Because this procedure is targeted directly at CD154, a co-activation molecule expressed predominantly by activated CD4+ T-cells, the results suggest that CD4+ cells have a major role in the cellular immune response to xenografts.

Viral Abrogation of Stem Cell Transplantation Tolerance Causes Graft Rejection and Host Death by Different Mechanisms

Tolerance-based stem cell transplantation using sublethal conditioning is being considered for the treatment of human disease, but safety and efficacy remain to be established. We have shown that mouse bone marrow recipients treated with sublethal irradiation plus transient blockade of the CD40-CD154 costimulatory pathway develop permanent hematopoietic chimerism across allogeneic barriers. We now report that infection with lymphocytic choriomeningitis virus at the time of transplantation prevented engraftment of allogeneic, but not syngeneic, bone marrow in similarly treated mice. Infected allograft recipients also failed to clear the virus and died. Postmortem study revealed hypoplastic bone marrow and spleens. The cause of death was virus-induced IFN-αβ. The rejection of allogeneic bone marrow was mediated by a radioresistant CD8+TCR-αβ+NK1.1− T cell population. We conclude that a noncytopathic viral infection at the time of transplantation can prevent engraftment of allogeneic bone marrow and result in the death of sublethally irradiated mice treated with costimulation blockade. Clinical application of stem cell transplantation protocols based on costimulation blockade and tolerance induction may require patient isolation to facilitate the procedure and to protect recipients.