Primed allospecific T cells prevent the effects of costimulatory blockade on prolonged cardiac allograft survival in mice

Combined coinhibitory and costimulatory modulation with anti-BTLA and CTLA4Ig facilitates tolerance in murine islet allografts

Complex interactions between positive and negative cosignaling receptors ultimately determine the fate of the immune response. The recently identified coinhibitory receptor, B and T lymphocyte attenuator (BTLA), contributes to regulation of autoimmune and potentially alloimmune responses. We investigated the role of BTLA in a fully major histocompatibility complex-mismatched mouse islet transplant model. We report that anti-BTLA mAb (6F7) alone does not accelerate graft rejection. Rather, while CTLA4Ig alone improved allograft survival, the addition of anti-BTLA mAb to CTLA4Ig led to indefinite (>100 days) allograft survival. Immediately after treatment with anti-BTLA mAb and CTLA4Ig, islet allografts showed intact islets and insulin production despite a host cellular response, with local accumulation of Foxp3+ cells. We clearly demonstrate that combined therapy with anti-BTLA mAb and CTLA4Ig mice induced donor-specific tolerance, since mice accepted a second donor-specific islet graft without further treatment and rejected third party grafts. CTLA4Ig and anti-BTLA mAb limited the initial in vivo proliferation of CFSE-labeled allogeneic lymphocytes, and anti-BTLA mAb enhanced the proportion of PD-1 expressing T cells while depleting pathogenic BTLA+ lymphocytes. We conclude that targeting the BTLA pathway in conjunction with CTLA4Ig costimulatory blockade may be a useful strategy for promoting immunological tolerance in murine islet allografts.

Defective alloreactive CD8 T cell function and memory response in allograft recipients in the absence of CD4 help

We have shown that alloreactive CD8 T cell activation may proceed via CD4-dependent and CD4-independent pathways, and that CD8 T cell activation in Ag-primed animals is independent of CD154 costimulation. In this report, we further analyzed the activation and function of alloreactive CD8 CTL effectors in CD4 knockout (KO) skin/cardiac allograft recipients. FACS analysis showed that alloreactive CD8 T cells were activated at a significantly reduced level in CD4 KO mice. Importantly, these helpless CD8 T cells failed to develop CD154 blockade resistance following reactivation by the same alloantigen, indicative of defective memory formation. Only transient CD4 help was required, as short-term CD4 blockade at the time of first skin graft challenge only delayed alloreactive CD8 activation, without affecting the CD8 T cell memory response to a second skin graft. Moreover, postoperative CD4 blockade had no effect on alloreactive CD8 activation. Alloreactive CD8 cells generated in the absence of CD4 help exhibited decreased effector responses. Interestingly, intragraft induction of T cell-targeted chemokines early after transplant was also dependent on CD4 help, as the induction kinetics of CXCL9 and CCL5 in CD4 KO recipients was significantly delayed, coupled with similarly delayed infiltration by CD3/CD8 cells. Remarkably, helpless CD8 cells ultimately entering the graft still displayed significantly diminished T cell effector molecules (IFN-gamma, granzyme B). Thus, CD4 help is critical for alloreactive CD8 activation, function, and memory formation.

Human effector memory CD4+ T cells directly recognize allogeneic endothelial cells in vitro and in vivo

The frequency of circulating alloreactive human memory T cells correlates with allograft rejection. Memory T cells may be divided into effector memory (T(EM)) and central memory (T(CM)) cell subsets, but their specific roles in allograft rejection are unknown. We report that CD4+ T(EM) (CD45RO+ CCR7- CD62L-) can be adoptively transferred readily into C.B-17 SCID/bg mice and mediate the destruction of human endothelial cells (EC) in vascularized human skin grafts allogeneic to the T cell donor. In contrast, CD4+ T(CM) (CD45RO+ CCR7+ CD62L+) are inefficiently transferred and do not mediate EC injury. In vitro, CD4+ T(EM) secrete more IFN-gamma within 48 h in response to allogeneic ECs than do T(CM). In contrast, T(EM) and T(CM) secrete comparable amounts of IFN-gamma in response to allogeneic monocytes (Mo). In the same cultures, both T(EM) and T(CM) produce IL-2 and proliferate in response to IFN-gamma-treated allogeneic human EC or Mo, but T(CM) respond more vigorously in both assays. Blockade of LFA-3 strongly inhibits both IL-2 and IFN-gamma secretion by CD4+ T(EM) cultured with allogeneic EC but only minimally inhibits responses to allogeneic Mo. Blockade of CD80 and CD86 strongly inhibits IL-2 but not IFN-gamma production by in response to allogeneic EC or Mo. Transduction of EC to express B7-2 enhances allogeneic T(EM) production of IL-2 but not IFN-gamma. We conclude that human CD4+ T(EM) directly recognize and respond to allogeneic EC in vitro by secreting IFN-gamma and that this response depends on CD2 but not CD28. Consistent with EC activation of effector functions, human CD4+ T(EM) can mediate allogeneic EC injury in vivo.

New insights in CD28-independent allograft rejection

CD28 costimulatory blockade induces tolerance in most murine transplant models but fails to do so in stringent transplant models, such as skin transplantation. The precise immunological mechanisms of CD28-independent rejection remain to be fully defined. Using two novel mouse strains in which both CD28 and either CD4 or CD8 are knocked out (CD4(-/-)CD28(-/-) or CD8(-/-)CD28(-/-) mice), we examined mechanisms of CD28-independent CD4(+) or CD8(+) T-cell-mediated allograft rejection. CD4(-/-)CD28(-/-) and CD8(-/-)CD28(-/) deficient mice rejected fully allogeneic skin allografts at a tempo comparable with that in wild-type mice. Rejection proceeded despite significant reduction in alloreactive T-cell clone sizes suggesting the presence of a subset of T cells harnessing alternate CD28-independent costimulatory pathways. Blockade of CD40-CD154 and CD134-CD134L, but not ICOS-B7h pathways in combination significantly prolonged allograft survival in CD8(-/-)CD28(-/-) recipients and to a lesser extent in CD4(-/-)CD28(-/-) recipients. Prolongation in allograft survival was associated with reduced effector-memory T-cell generation, decreased allospecific Th1 cytokine generation and diminished alloreactive T-cell proliferation in vivo. In aggregate, the data identify these two pathways as critical mediators of CD28-independent rejection by CD4(+) and to a lesser extent CD8(+) T cells, and provide novel mechanistic insights into functions of novel T-cell co-stimulatory pathways in vivo.

Clinical transplantation tolerance: many rivers to cross

Modern immunosuppressive regimens for organ transplantation have resulted in excellent short-term results but less dramatic improvements in long-term outcomes. Moreover, they are associated with significant deleterious effects. One solution that should avoid the adverse drug effects and result in improved graft and patient longevity as well as positively impacting on the organ shortage is the establishment of transplantation tolerance. Ever since the original description of transplantation tolerance in rodent allografts, there have been significant efforts made to translate tolerance-promoting strategies to the clinical arena. However, >50 years later, we are still faced with significant barriers that are preventing such a goal from being widely attained. Nonetheless, pilot clinical tolerance protocols are underway in selected transplant recipients. In this review, we discuss the scientific and nonscientific issues that must be overcome for successful transplantation tolerance to become a clinical reality.

No synergy between ATG induction and costimulation blockade induced kidney allograft survival in rhesus monkeys

BACKGROUND

Costimulation blockade with antibodies directed against human CD40 and CD86 leads to prolonged kidney allograft survival in rhesus monkeys, but fails to induce permanent graft acceptance. We have tested whether costimulation blockade is more effective after peripheral T-cell ablation with antithymocyte globulin (ATG), with the aim to remove already primed autoreactive cells present in the normal repertoire.

METHODS

Rhesus monkeys were transplanted with a mismatched kidney allograft. ATG was given around the time of transplantation (day -1 and 0). Costimulation blockade with anti-CD40+anti-CD86 was given at tapering dosages from day -1 to 56. Cyclosporin A (CsA) was given from day 42 onwards and first rejections occurring after day 42 were treated with prednisone.

RESULTS

We observed accelerated rejection in ATG-treated monkeys, compared to animals receiving only costimulation blockade. The accelerated rejection of the kidney allograft occurred despite the application of rejection therapy with steroids and CsA. Three of the five ATG-treated animals were found seropositive for donor-specific alloantibodies. Early biopsies (day 21) from animals treated with ATG and anti-CD40+anti-CD86 show substantially reduced expression of cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) and forkhead box P3 (FOXP3) in focal infiltrates as compared to animals treated with only costimulation blockade. Furthermore, we observed the rapid reappearance of CD8 T-cells with a memory phenotype (disappearance of naive CD95/CD11a T-cells) in peripheral blood.

CONCLUSION

We conclude that (subtotal) T-cell depletion using ATG does not add to costimulation blockade induced kidney allograft survival.

Memory T cells in allograft rejection

T cell repertoire of many humans contains high frequencies of memory T cells specific for alloantigens. The increasing evidence implicates these cells as a barrier to allograft survival and to the induction of transplantation tolerance. This review discusses several aspects of memory T cell immunobiology pertinent to their role in transplantation.

Costimulation blockade and its possible future use in clinical transplantation

The nonimmune effects of currently used immunosuppressive drugs result in a high incidence of late graft loss due to nephrotoxicity and death of patients. As an immune-specific alternative to conventional immunosuppressants, new biotechnology tools can be used to block the costimulation signals of T-cell activation. Many experimental studies--particularly preclinical studies in nonhuman primates--have focused on blocking the 'classical' B7/CD28 and CD40/CD40L pathways, which are critical in primary T-cell activation. Here, we review the limitations, the recent advances and the first large-scale clinical application of the CTLA4-Ig fusion protein to block the B7/CD28 costimulation pathway. We also focus on new B7/CD28 and tumor necrosis factor (TNF)/TNF-R family costimulatory molecules that can deliver positive or negative costimulation signals regulating the alloimmune response. Strategies that use single agents to block costimulation have often proved to be insufficient. Given the diversity of the different costimulation molecules, future strategies for human transplantation may involve the simultaneous blockade of several selected pathways or the simultaneous use of conventional immunosuppressants.

Preformed antibody, not primed T cells, is the initial and major barrier to bone marrow engraftment in allosensitized recipients

Multiply-transfused individuals are at higher risk for BM rejection. We show that whereas allosensitization resulted in the priming of both cellular and humoral immunity, preformed antibody was the major barrier to engraftment. The generation of cross-reactive alloantibody led to rejection of BM of a different MHC-disparate strain. Imaging studies indicated that antibody-mediated rejection was very rapid (<3 hours) in primed recipients, while T-cell-mediated rejection in nonprimed mice took more than 6 days. Antibody-mediated BM rejection was not due to a defect in BM homing as rejection occurred despite direct intra-BM infusion of donor BM. Rejection was dependent upon host FcR+ cells. BM cells incubated with serum from primed mice were eliminated in nonprimed recipients, indicating that persistent exposure to high-titer antibody was not essential for rejection. High donor engraftment was achieved in a proportion of primed mice by mega-BM cell dose, in vivo T-cell depletion, and high-dose immunoglobulin infusion. The addition of splenectomy to this protocol only modestly added to the efficacy of this combination strategy. These data demonstrate both rapid alloantibody-mediated elimination of BM by host FcR+ cells and priming of host antidonor T cells and suggest a practical strategy to overcome engraftment barriers in primed individuals.

The impact of memory T cells on rejection and the induction of tolerance

Accumulating evidence suggests that alloreactive memory T cells (Tm) may be generated in transplant recipients that have not previously been exposed to alloantigen through mechanisms such as cross-reactivity and homeostatic proliferation. The presence of Tm correlates with both acute and chronic rejection episodes and, furthermore, may be responsible for the failure to induce tolerance in large animal and clinical settings. A clearer understanding of how Tm function and their requirements to mount an effective response to alloantigen will be key to further attempts to translate tolerance induction protocols from the experimental setting to the clinic.

How can we measure immunologic tolerance in humans?

Prevention and treatment of allograft rejection in organ transplant recipients relies primarily on non-antigen-specific immunosuppression, with all its associated potential hazards and costs. Currently, the status of the recipient immune response is measured by monitoring pharmacologic drug levels and clinical/pathologic evaluation of graft function. Development of reliable assays that can measure accurately the status of the immune response not only would help clinicians customize the prescription of immunosuppressive drugs in individual patients but also may allow their complete withdrawal in some patients with immunologic tolerance. Furthermore, these assays would facilitate the safe evaluation of novel tolerogenic regimens. Achieving this goal has proved to be very difficult because it requires both a more in-depth understanding of complex mechanisms of tolerance and also identification of transplant patients with acquired tolerance to an allograft that can be studied. This review discusses the current understanding of tolerance mechanisms and outlines the unique and specific challenges in development of tolerance/monitoring assays in the field of transplantation. In addition, several of the most promising candidate assays are discussed in detail.

Reshaping the past: Strategies for modulating T-cell memory immune responses

Memory T cells are generated following an initial encounter with antigen, persist over the lifetime of an individual, and mediate rapid and robust functional responses upon antigenic recall. While immune memory is generally associated with protective immune response to pathogens, memory T cells can be generated to diverse types of antigens including autoantigens and alloantigens through homologous or crossreactive priming and comprise the majority of circulating T cells during adulthood. Memory T cells can therefore play critical roles in propagating and perpetuating autoimmune disease and in mediating allograft rejection, although the precise pathways for regulation of memory immune responses remain largely undefined. Moreover, evaluating and designing strategies to modulate memory T-cell responses are challenging given the remarkable heterogeneity of memory T cells, with different subsets predominating in lymphoid versus non-lymphoid tissue sites. In this review, we discuss what is presently known regarding the effect of current immunomodulation strategies on the memory T-cell compartment and potential strategies for controlling immunological recall.

The last 5 years of basic science investigation in transplant immunology

Many new insights have been gained over the past 5 years into the mechanisms that regulate immune reactivity to cell and organ transplants. This new knowledge is being applied to the development of innovative experimental strategies that may soon be evaluated in the clinic.

Generation and functional capacity of polyclonal alloantigen-specific memory CD4 T cells

Alloreactive memory T cells can significantly impact graft survival due to their enhanced functional capacities, diverse tissue distribution and resistance to tolerance induction and depletional strategies. However, their role in allograft rejection is not well understood primarily due to the lack of suitable in vivo models. In this study, we use a novel approach to generate long-lived polyclonal alloreactive memory CD4 T cells from adoptive transfer of alloantigen-activated precursors into mouse hosts. We demonstrate that CD25 upregulation is a marker for precursors to alloantigen-specific memory and have created a new mouse model that features an expanded population of polyclonal alloreactive memory T cells that is distinguishable from the naive T-cell population. Furthermore, we show that alloreactive memory T cells exhibit rapid recall effector responses with predominant IFN-gamma and IL-2 production, and mediate vigorous allograft rejection. Interestingly, while we found a heterogeneous distribution of allomemory T cells in lymphoid and nonlymphoid tissues, they were all predominantly of the effector-memory (CD62Llo) phenotype. Our results present a unique model for the generation and tracking of polyclonal allospecific memory CD4 T cells in vivo and reveal insights into the distinct and robust nature of alloreactive T-cell memory.

Accelerated Memory Cell Homeostasis during T Cell Depletion and Approaches to Overcome It

Partial T cell depletion is used in solid organ transplantation as a valuable strategy of peritransplant induction immunosuppression. Using a murine cardiac allograft model, we recently demonstrated that this led to lymphopenia-induced (homeostatic) proliferation among the residual nondepleted lymphocytes. Rather than promoting tolerance, peritransplant T cell-depleting Abs actually resulted in resistance to tolerance induction by costimulatory blockade. In this study we show that memory T cells predominate shortly after subtotal lymphodepletion due to two distinct mechanisms: relative resistance to depletion and enhanced homeostatic proliferation. In contrast, regulatory cells (CD4+ CD25+ Foxp3+) are depleted as efficiently as nonregulatory cells and exhibit reduced homeostatic expansion compared with memory cells. The resistance to tolerance induction seen with subtotal T cell depletion can be overcome in two different ways: first, by the adoptive transfer of additional unprimed regulatory cells at the time of transplant, and second, by the adjunctive use of nondepleting anti-CD4 and anti-CD8 mAbs, which effectively block homeostatic expansion. We conclude that the resistance to tolerance induction seen after subtotal lymphocyte depletion can be attributed to alterations in the balance of naive, memory, and regulatory T cells. These data have clinically relevant implications related to the development of novel strategies to overcome resistance to tolerance.

The challenge of inhibiting alloreactive T-cell memory

Memory T cells specific for donor antigens present a unique challenge in transplantation. In addition to expressing rapid and robust immune responses to a transplanted organ, memory T cells may be resistant to the effects of currently used graft-prolonging therapies. The increasing recognition that alloreactive memory T cells participate in transplant rejection is driving new lines of research focusing on understanding the immunobiology of alloreactive memory T cells and on designing novel therapies to specifically target memory T cells. The purpose of this review is to summarize the effects of existing immunosuppressive drugs and costimulatory blockade on functions of alloreactive memory T cells that undermine allograft survival.

Critical, but conditional, role of OX40 in memory T cell-mediated rejection

Memory T cells can be a significant barrier to the induction of transplant tolerance. However, the molecular pathways that can regulate memory T cell-mediated rejection are poorly defined. In the present study we tested the hypothesis that the novel alternative costimulatory molecules (i.e., ICOS, 4-1BB, OX40, or CD30) may play a critical role in memory T cell activation and memory T cell-mediated rejection. We found that memory T cells, generated by either homeostatic proliferation or donor Ag priming, induced prompt skin allograft rejection regardless of CD28/CD154 blockade. Phenotypic analysis showed that, in contrast to naive T cells, such memory T cells expressed high levels of OX40, 4-1BB, and ICOS on the cell surface. In a skin transplant model in which rejection was mediated by memory T cells, blocking the OX40/OX40 ligand pathway alone did not prolong the skin allograft survival, but blocking OX40 costimulation in combination with CD28/CD154 blockade induced long-term skin allograft survival, and 40% of the recipients accepted their skin allograft for >100 days. In contrast, blocking the ICOS/ICOS ligand and the 4-1BB/4-1BBL pathways alone or combined with CD28/CD154 blockade had no effect in preventing skin allograft rejection. OX40 blockade did not affect the homeostatic proliferation of T cells in vivo, but markedly inhibited the effector functions of memory T cells. Our data demonstrate that memory T cells resisting to CD28/CD154 blockade in transplant rejection are sensitive to OX40 blockade and suggest that OX40 is a key therapeutic target in memory T cell-mediated rejection.

Transplant tolerance: models, concepts and facts

Despite extensive research, our understanding of immunological tolerance to self-antigens is incomplete, and the goal of achieving tolerance to allogeneic transplanted tissue remains elusive. Currently, it is generally believed that the blockade of T cell co-stimulation offers considerable potential for achieving tolerance in the clinical setting. However, the recent finding that CD154-specific antibody may act through the depletion of activated T cells rather than co-stimulation blockade alone highlights the need for a re-evaluation of published data and the role of co-stimulation blockade in transplant tolerance. Activated T cells are programmed to die unless they receive sufficient survival signals in the form of inflammatory and lymphotropic cytokines produced by activated antigen-presenting cells or the T cells themselves. In conditions where the threshold for surviving activation is not reached, for example when a small number of responder T cells are activated in the absence of substantial injury or inflammation, the ensuing death of all activated T cells can result in deletional tolerance. Therefore, we propose that tolerance represents a failure of T cells to survive activation and develop into memory cells. This concept is likely to apply in the transplant setting, where the strength of the alloresponse depends on both the number/phenotype of the recipients' alloreactive T cells and immunogenicity of the transplanted tissue. Hence, in some rodent donor-recipient strain combinations that instigate a weak alloresponse, many treatments that only modestly decrease the alloresponse can achieve tolerance. In contrast, clinical transplantation is characterised by a strong alloresponse and highly immunogenic allografts, and thus, most treatments fail to control allograft rejection, and tolerance is difficult to achieve.

Patients, Pathogens, and Protective Immunity: The Relevance of Virus-Induced Alloreactivity in Transplantation

Successful transplantation requires the establishment of an ongoing state in which there is simultaneous inhibition of the undesired T cell-dependent rejection response and yet retention of the ability to develop effective cell-mediated primary and memory responses to pathogens. The complexity of attaining such a precarious state is underscored by the growing body of evidence that alloreactivity can be profoundly influenced by infections that occur before, concurrent with, or subsequent to an organ transplant. In this review, we explore the growing list of mechanisms that have been identified by which pathogen-host interactions might influence rejection, including the degeneracy of TCR recognition leading to cross-reactive immune responses, the effects of pathogens on innate immune mechanisms, and the potential impact of virally induced lymphopenia.

CD8 T Cells Specific for a Donor-Derived, Self-Restricted Transplant Antigen Are Nonpathogenic Bystanders after Vascularized Heart Transplantation in Mice

CD8 T cell cross-priming, an established mechanism of protective antiviral immunity, was originally discovered during studies involving minor transplantation Ags. It is unclear whether or how cross-primed CD8 T cells, reactive to donor-derived, but recipient class I MHC-restricted epitopes, could injure a fully MHC-disparate, vascularized transplant. To address this question we studied host class I MHC-restricted, male transplantation Ag-reactive T cell responses in female recipients of fully MHC-disparate, male heart transplants. Cross-priming to the immune-dominant determinant HYUtyp occurred at low frequency after heart transplantation. CD8 T cell preactivation through immunization with HYUtyp mixed in CFA did not alter the kinetics of acute rejection. Furthermore, neither HYUtyp immunization nor adoptive transfer of HYUtyp-specific TCR-transgenic T cells affected outcome in 1) a model of chronic rejection in the absence of immunosuppression or 2) a model of allograft acceptance induced by costimulatory blockade. The results support the contention that CD8 T cells reactive to host-restricted, but donor-derived, Ags are highly specific and are nonpathogenic bystanders during rejection of MHC-disparate cardiac allografts.

Lymphoid Sequestration of Alloreactive Memory CD4 T Cells Promotes Cardiac Allograft Survival

Memory T cells specific for donor Ags present a unique challenge in transplantation. In addition to expressing robust immune responses to a transplanted organ, memory T cells may be resistant to the effects of immunosuppressive therapies used to prolong graft survival. In this study, we explore the possibility of controlling deleterious donor-reactive memory CD4 T cells through lymphoid sequestration. We showed that sphingosine 1-phosphate receptor-1 agonist FTY720 induces relocation of circulating memory CD4 T cells into secondary lymphoid organs. Lymphoid sequestration of these donor-reactive memory CD4 T cells prolonged survival of murine heterotopic cardiac allografts and synergizes with conventional costimulatory blockade to further increase graft survival. Despite limited trafficking, memory CD4 T cells remain capable of providing help for the induction of anti-donor CD8 T cell and alloantibody responses. Elimination of antidonor humoral immunity resulted in indefinite allograft survival proving the pathogenicity of alloantibody under these conditions. Overall, this is the first demonstration that FTY720 influences memory CD4 T cell trafficking and attenuates their contribution to allograft rejection. The data have important implications for guiding FTY720 therapy and for designing combinatorial strategies aimed at prolonging allograft survival in sensitized transplant patients with donor-specific memory T cells.

Analysis of the role of negative T cell costimulatory pathways in CD4 and CD8 T cell-mediated alloimmune responses in vivo

Negative costimulatory signals mediated via cell surface molecules such as CTLA-4 and programmed death 1 (PD-1) play a critical role in down-modulating immune responses and maintaining peripheral tolerance. However, their role in alloimmune responses remains unclear. This study examined the role of these inhibitory pathways in regulating CD28-dependent and CD28-independent CD4 and CD8 alloreactive T cells in vivo. CTLA-4 blockade accelerated graft rejection in C57BL/6 wild-type recipients and in a proportion of CD4(-/-) but not CD8(-/-) recipients of BALB/c hearts. The same treatment led to prompt rejection in CD28(-/-) and a smaller proportion of CD4(-/-)CD28(-/-) mice with no effect in CD8(-/-)CD28(-/-) recipients. These results indicate that the CTLA-4:B7 pathway provides a negative signal to alloreactive CD8(+) T cells, particularly in the presence of CD28 costimulation. In contrast, PD-1 blockade led to accelerated rejection of heart allografts only in CD28(-/-) and CD8(-/-)CD28(-/-) recipients. Interestingly, PD-1 ligand (PD-L1) blockade led to accelerated rejection in wild-type mice and in all recipients lacking CD28 costimulation. This effect was accompanied by expansion of IFN-gamma-producing alloreactive T cells and enhanced generation of effector T cells in rejecting allograft recipients. Thus, the PD-1:PD-L1 pathway down-regulates alloreactive CD4 T cells, particularly in the absence of CD28 costimulation. The differential effects of PD-1 vs PD-L1 blockade support the possible existence of a new receptor other than PD-1 for negative signaling through PD-L1. Furthermore, PD-1:PD-L1 pathway can regulate alloimmune responses independent of an intact CD28/CTLA-4:B7 pathway. Harnessing physiological mechanisms that regulate alloimmunity should lead to development of novel strategies to induce durable and reproducible transplantation tolerance.

Heterogeneous Alterations in Human Alloimmunity Associated with Immunization

BACKGROUND

The presence of alloantibodies and/or alloreactive T cells in a patient prior to a transplant can impact graft outcome. Environmental factors, including therapeutic vaccinations, may influence the strength and/or specificity of alloimmunity.

METHODS

To address this issue, we prospectively evaluated the effects of two different immunization protocols in human subjects on cellular alloimmunity using an IFNgamma ELISPOT assay and on alloantibody reactivity by flow cytometric analysis of HLA-coated beads.

RESULTS

Vaccination/immunization was associated with augmentation of cellular and/or humoral alloimmune reactivity in >50% of the test subjects. The effects were heterogeneous in that some detected increases were transient, peaking 30-60 days postimmunization, whereas others persisted for the length of the study. Antibodies reactive to the immunizing agent did not cross react with the detected alloantibodies, suggesting that the augmentation of alloimmune reactivity was most likely due to a nonspecific adjuvant effect from the vaccine.

CONCLUSIONS

Therapeutic vaccinations can alter the strength of cellular and humoral alloimmunity in humans. The results suggest that serial immune monitoring of alloreactivity might be beneficial when immunizations are administered to potential transplant recipients.

Impaired recall of CD8 memory T cells in immunologically privileged tissue

Foreign Ags that enter immunologically privileged sites such as the eye, brain, and testis persist for an extended period of time, whereas the same Ags are rapidly eliminated at conventional sites. Immune privilege, therefore, provides unwanted refuge for pathogens and tumor cells but is beneficial for the survival of allogeneic grafts. In this study, we asked whether memory T cells can eliminate foreign Ags deposited at an immunologically privileged site by studying CD8 memory T cell-mediated rejection of pancreatic islet allografts placed either in the testis (a privileged organ) or under the kidney capsule (a nonprivileged site) of diabetic mice. We found that CD8 memory T cells reject intratesticular grafts at a significantly slower rate than the rejection of intrarenal grafts. Delayed graft rejection in the testis was not due to reduced homing or proliferation of memory T cells but due to their increased apoptosis at that site. Apoptosis was mediated by the combined actions of two TNFR family members that are up-regulated on activated memory T cells, Fas, and CD30. Therefore, memory T cells survey immunologically privileged tissues but are subject to the immunosuppressive mechanisms present at these sites.

Heterologous immunity and homeostatic proliferation as barriers to tolerance

The different threshold of activation for memory T cells compared to that of naïve T cells makes them resistant to immunomodulation, thus representing a barrier to tolerance. Recently it has been demonstrated that homeostatic proliferation and heterologous immunity represent two naturally occurring and distinct processes that can generate memory T cells. Homeostatic proliferation refers to the process by which, in a lymphodeficient host, normal T cells 'spontaneously' proliferate in response to self-MHC-peptide complexes. Heterologous immunity refers to a process in which a response to one or more infectious agents generates effector/memory T cells with cross-reactive specificities. Recent new studies have defined the importance of these processes in transplantation models and implicated strategies to induce transplantation tolerance.

The allograft defines the type of rejection (acute versus chronic) in the face of an established effector immune response

Transplanted organs fail due to either acute or chronic rejection. The prevailing view is that the nature or magnitude of the recipient's immune response to donor Ags determines the type of rejection. In variance with this view, we show in this study that the status of the graft itself plays a dominant role in defining the type of rejection even in the face of an established alloimmune response. Using adoptive transfer mouse models in which the graft is exposed to a constant number of effector lymphocytes, we found that newly transplanted heart allografts are rejected acutely, while healed-in allografts undergo chronic rejection. Acute rejection of healed-in allografts was largely recapitulated by subjecting the grafts to ischemia-reperfusion injury similar to that present in newly transplanted organs. Ischemia-Reperfusion injury altered the outcome of rejection by enhancing the accumulation of effector T cells within the graft. The accumulation of effector T cells in the graft was dependent on the presence of both ischemia-reperfusion injury (inflammation) and alloantigens. These findings demonstrate that the graft plays a dominant role in shaping the outcome of rejection by controlling the trafficking of effector T cells.

Oral tolerization with peptide 336-351 linked to cholera toxin B subunit in preventing relapses of uveitis in Behcet's disease

Behcet's disease (BD) specific peptide (p336-351) was identified within the human 60 kD heat shock protein (HSP60). Oral p336-351 induced uveitis in rats which was prevented by oral tolerization with the peptide linked to recombinant cholera toxin B subunit (CTB). This strategy was adopted in a phase I/II clinical trial by oral administration of p336-351-CTB, 3 times weekly, followed by gradual withdrawal of all immunosuppressive drugs used to control the disease in 8 patients with BD. The patients were monitored by clinical and ophthalmological examination, as well as extensive immunological investigations. Oral administration of p336-351-CTB had no adverse effect and withdrawal of the immunosuppressive drugs showed no relapse of uveitis in 5 of 8 patients or 5 of 6 selected patients who were free of disease activity prior to initiating the tolerization regimen. After tolerization was discontinued, 3 of 5 patients remained free of relapsing uveitis for 10-18 months after cessation of all treatment. Control of uveitis and extra-ocular manifestations of BD was associated with a lack of peptide-specific CD4+ T cell proliferation, a decrease in expression of TH1 type cells (CCR5, CXCR3), IFN-gamma and TNF-alpha production, CCR7+ T cells and costimulatory molecules (CD40 and CD28), as compared with an increase in these parameters in patients in whom uveitis had relapsed. The efficacy of oral peptide-CTB tolerization will need to be confirmed in a phase III trial, but this novel strategy in humans might be applicable generally to autoimmune diseases in which specific antigens have been identified.

In vivo helper functions of alloreactive memory CD4+ T cells remain intact despite donor-specific transfusion and anti-CD40 ligand therapy

Memory T cells have specific properties that are beneficial for rapid and efficient protection from pathogens previously encountered by a host. These same features of memory T cells may be deleterious in the context of a transplanted organ. Consistent with this contention is the accumulating evidence in experimental transplantation that previously sensitized animals are resistant to the effects of costimulatory blockade. Using a model of murine cardiac transplantation, we now demonstrate that alloreactive memory CD4(+) T cells prevent long-term allograft survival induced through donor-specific cell transfusion in combination with anti-CD40 ligand Ab (DST/anti-CD40L). We show that memory donor-reactive CD4(+) T cells responding through the direct or indirect pathways of allorecognition provide help for the induction of antidonor CD8(+) T effector cells and for Ab isotype switching, despite DST/anti-CD40L. The induced pathogenic antidonor immunity functions in multiple ways to subsequently mediate graft destruction. Our findings show that the varied functions of alloreactive memory CD4(+) T cells remain intact despite DST/anti-CD40L-based costimulatory blockade, a finding that will likely have important implications for designing approaches to induce tolerance in human transplant recipients.

In remembrance of things past: memory T cells and transplant rejection

A cardinal feature of the adaptive immune response is its ability to generate long-lived populations of memory T lymphocytes. Memory T cells are specific to the antigen encountered during the primary immune response and react rapidly and vigorously upon re-encounter with the same antigen. Memory T cells that recognize microbial antigens provide the organism with long-lasting protection against potentially fatal infections. On the other hand, memory T cells that recognize donor alloantigens can jeopardize the survival of life-saving organ transplants. We review here the immunobiology of memory T cells and describe their role in the rejection of solid organ allografts.

Memory T cells in transplantation: generation, function, and potential role in rejection

The adaptive immune system is endowed with long-lived memory to recall previous antigen encounters and respond more effectively to them. Memory immune responses are mediated by antigen-specific memory T lymphocytes that exhibit enhanced function compared with naïve T cells that have never encountered antigen. While the generation of memory T cells specific for pathogens is beneficial in providing protective immunity, memory T cells specific for alloantigens can be deleterious to the recipient of a transplanted organ. In graft rejection, memory T cells mediate accelerated, "second-set" rejection and their presence has been associated with increased propensity for early rejection. Recent findings have demonstrated that alloreactive memory T cells can be generated via exposure to alloantigens, as well as stimuli that are cross-reactive with alloantigens, and are therefore likely present in "naïve" individuals. This review focuses on the characteristics of memory T cells which make them of special interest to the transplant community, including differential activation requirements, broad homing properties, and resistance to tolerance induction. The multiple ways in which memory T cells can contribute to early and late graft rejection are discussed, as well as potential targets for combating alloreactive memory to be considered in the future design of tolerance induction strategies.

Antigen location contributes to the pathological features of a transplanted heart graft

Organ-specific injury after transplantation presents with a variety of clinical and pathological phenotypes, yet the factors influencing development of each outcome are poorly understood. Because primed T lymphocytes must re-encounter their antigen within the target organ to engage effector functions, we postulated that the cellular location of antigen within that organ could significantly impact the induced pathology. We challenged female Marilyn CD4 T-cell receptor transgenic mice, in which all T cells are specific for the male minor transplantation antigen, with male heart transplants expressing the relevant peptide: major histocompatibility complex on either graft parenchymal/vascular cells or alternatively, on graft-infiltrating mononuclear cells. The two different graft donors led to equivalent activation of recipient T cells as assessed by frequency, cell surface marker expression, cytokine production, and the ability to traffic to the graft. Nonetheless, if the target antigen was expressed on graft vascular and/or parenchymal cells, the outcome was acute graft destruction. In contrast, if the antigen was expressed only on graft-infiltrating mononuclear cells the same effector T-cell repertoire caused chronic rejection and vasculopathy. This unique result, that target antigen location can influence pathological outcome, has significant implications for understanding the pathogenesis of chronic allograft injury in humans.

Effects of T cell frequency and graft size on transplant outcome in mice

The features that determine whether graft-reactive T lymphocytes develop into effector cells capable of mediating organ destruction are not well understood. To investigate potential factors involved in this process, we first confirmed that female recipient mice acutely rejected minor Ag-disparate male skin, but not heart transplants. Despite this difference in outcome, heart and skin transplantation induced antidonor T cell responses of similar magnitude, specificity, and cytokine profile. The heart-graft-primed T cells transiently infiltrated the graft and ultimately induced the development of chronic transplant vasculopathy. Increasing the frequency of donor-reactive T cells by presensitization or by using TCR (CD8+ antimale)-transgenic recipients did not mediate acute rejection but accelerated the pace and severity of the vasculopathy. Surprisingly, decreasing the tissue mass of the donor heart by 50% resulted in acute rejection of these smaller grafts without increasing the frequency of antidonor effector T cells in the recipients. In complementary studies, placement of one or two male skin grafts on a single recipient did not affect the frequency or cytokine profile of the induced antimale T cell repertoire. Nonetheless, the recipients of single grafts acutely rejected the transplanted skin while the recipients of two skin grafts did not. These results provide new insight into the pathogenesis of transplant vasculopathy and provide an explanation for the difference in outcome between murine skin and heart transplants by highlighting the novel concept that the efficiency of transplant-reactive T cell immunity is heavily influenced by the tissue burden it encounters at the effector stage.

Routes to Transplant Tolerance versus Rejection

The alloimmune response can be divided into specific junctures where critical decisions between tolerance and immunity are made which define the outcome of the transplant. At these "decision nodes" various cytokines direct alloresponsive T cells to develop either a proinflammatory response aimed at graft destruction or an immunoregulatory response facilitating graft acceptance. This review will focus on the role of these cytokines in influencing the progression of an alloimmune response leading ultimately to either allograft survival or rejection.

Homeostatic proliferation is a barrier to transplantation tolerance

Despite the ease of inhibiting immune responses by blockade of T-cell costimulation in naive rodent models, it is difficult to suppress those responses in animals with memory cells. Studies demonstrating the importance of alloreactive T-cell deletion during tolerance induction have promoted use of peritransplant T-cell-depleting therapies in clinical trials. But potentially complicating wide-scale, nonspecific T-cell depletion is the finding that extensive T-cell proliferation can occur under conditions of lymphopenia. This process, termed homeostatic proliferation, may induce acquisition of functional memory T cells. Here, using clinically relevant mouse models of peripheral T-cell depletion, we show that residual nondepleted T cells undergo substantial homeostatic expansion. In this setting, costimulatory blockade neither significantly suppresses homeostatic proliferation nor prevents allograft rejection. In addition, T cells that have completed homeostatic proliferation show dominant resistance to tolerance when adoptively transferred into wild-type recipients, consistent with known properties of memory cells in vivo. These findings establish the importance of homeostatic proliferation in clinically relevant settings, demonstrate the barrier that homeostatic proliferation can present to the induction of transplantation tolerance, and have important implications for transplantation protocols that use partial or complete peripheral T-cell depletion.

Heterologous immunity: an overlooked barrier to tolerance

In less than 50 years the field of organ transplantation has transitioned from an experimental concept to clinical commonplace. Notwithstanding the dramatic improvements in patient and allograft outcomes, chronic rejection and the complications from life-long immunosuppressive therapy remain significant problems. The induction of transplantation tolerance, indefinite allograft acceptance independent of chronic immunosuppressive therapy, remains the ultimate objective in transplantation. Many strategies have achieved tolerance to transplanted tissue in rodents; however, few, if any, have shown equal efficacy when tested in non-human primate transplant models or human patients. A critical distinction between specific pathogen-free mice and primates or human patients is the exposure of the latter to environmental pathogens and the resultant-acquired immune history. Recent data has shown that virally induced, alloreactive immune responses can provide a potent barrier to tolerance. In this review, we discuss one of the most robust methods for tolerance, the induction of hematopoietic chimerism as well as the influence of viral infections on the alloimmune response.

What's new and what's hot in transplantation: basic science ATC 2003

It has been approximately 50 years since the initial descriptions of acquired transplant tolerance, and our understanding of the immune response to a transplanted organ has progressed enormously during the ensuing years. Recent studies have shed new light on the molecular and cellular basis of transplant rejection, have better defined the mechanisms of allograft tolerance with particular emphasis on a role for regulatory T cells, have identified important new hurdles to overcome in order to prolong allograft survival, have brought xenotransplantation closer to becoming a clinical reality, and have led to the development of novel techniques that may permit analysis of immune responses to transplanted organs in vivo.

Long-term survival of intestinal allografts induced by costimulation blockade, busulfan and donor bone marrow infusion

Tolerance-inducing strategies that infuse donor bone marrow cells in conjunction with costimulation blockade have not been applied to intestinal transplantation. Intestines from BALB/c mice were transplanted into C57BL/6 recipients treated with anti-CD40L mAb, CTLA4-Ig, donor bone marrow, and busulfan. The majority of mice transplanted after completion of this regimen developed hematopoietic macrochimerism, although the degree of chimerism varied widely between recipients, and experienced long-term allograft survival. T cells from these mice demonstrated donor-specific hyporesponsiveness in vitro. However, T cells from chimeric mice proliferated to donor alloantigen in vivo. Furthermore, chimeric mice bearing intestinal allografts were capable of rejecting subsequently placed donor-strain skin grafts. These data suggest that although long-term allograft survival occurs in the absence of acute or chronic rejection, recipient mice are not completely unresponsive to donor alloantigens. When intestinal transplantation was performed at the time of initial bone marrow infusion (initiation of the chimerism protocol), most recipients failed to develop chimerism and promptly rejected the intestinal allograft. Although this is the most effective protocol that we have tested using this stringent model of transplantation, our observations suggest that modifications will be necessary before it can be reliably applied to the transplantation of highly immunogeneic organs like the intestine.

Heterologous immunity provides a potent barrier to transplantation tolerance

Many strategies have been proposed to induce tolerance to transplanted tissue in rodents; however, few if any have shown equal efficacy when tested in nonhuman primate transplant models. We hypothesized that a critical distinction between specific pathogen-free mice and nonhuman primates or human patients is their acquired immune history. Here, we show that a heterologous immune response--specifically, virally induced alloreactive memory--is a potent barrier to tolerance induction. A critical threshold of memory T cells is needed to promote rejection, and CD8(+) "central" memory T cells are primarily responsible. Finally, treatment with deoxyspergualin, an inhibitor of NF-kappa B translocation, together with costimulation blockade, synergistically impairs memory T cell activation and promotes antigen-specific tolerance of memory. These data offer a potential explanation for the difficulty encountered when inducing tolerance in nonhuman primates and human patients and provide insight into the signaling pathways essential for memory T cell activation and function.

Cardiac allograft rejection in the absence of macrophage migration inhibitory factor

BACKGROUND

Macrophage migration inhibitory factor (MIF) is a secreted proinflammatory lymphokine essential for elicitation of delayed-type hypersensitivity (DTH) reactions in vivo. We tested whether MIF blockade-absence affected acute or chronic murine cardiac allograft rejection.

METHODS

Wild-type (WT) C57BL/6 (B6) mice underwent transplantation with BALB/c hearts with or without blocking anti-MIF antibody, and MIF knockout (KO) B6 mice underwent transplantation with MIF KO BALB/c hearts. Chronic immune injury was induced in WT and KO recipients using donor-specific transfusion and anti-CD40L antibody.

RESULTS

Unexpectedly, the blockade or genetic absence of MIF did not prolong graft survival even if recipient T-cell cytotoxicity was additionally impaired. The histologic manifestations of acute and chronic immune injury to the allograft were similar between groups.

CONCLUSIONS

MIF is not required for acute or chronic allograft rejection in mice. The findings raise questions about the role of DTH as an important mediator of cardiac allograft injury.

The role of the ICOS-B7h T cell costimulatory pathway in transplantation immunity

Inducible costimulatory molecule (ICOS) plays a pivotal role in T cell activation and Th1/Th2 differentiation. ICOS blockade has disparate effects on immune responses depending on the timing of blockade. Its role in transplantation immunity, however, remains incompletely defined. We used a vascularized mouse cardiac allograft model to explore the role of ICOS signaling at different time points after transplantation, targeting immune initiation (early blockade) or the immune effector phase (delayed blockade). In major histocompatibility-mismatched recipients, ICOS blockade prolonged allograft survival using both protocols but did so more effectively in the delayed-treatment group. By contrast, in minor histocompatibility-mismatched recipients, early blockade accelerated rejection and delayed blockade prolonged graft survival. Alloreactive CD4+ T cell expansion and alloantibody production were suppressed in both treatment groups, whereas only delayed blockade resulted in suppression of effector CD8+ T cell generation. After delayed ICOS blockade, there was a diminished frequency of allospecific IL-10-producing cells and an increased frequency of both IFN-gamma- and IL-4-producing cells. The beneficial effects of ICOS blockade in regulating allograft rejection were seen in the absence of CD28 costimulation but required CD8+ cells, cytotoxic T lymphocyte antigen-4, and an intact signal transducer and activator of transcription-6 pathway. These data define the complex functions of the ICOS-B7h pathway in regulating alloimmune responses in vivo.

Heterologous immunity provides a potent barrier to transplantation tolerance

Many strategies have been proposed to induce tolerance to transplanted tissue in rodents; however, few if any have shown equal efficacy when tested in nonhuman primate transplant models. We hypothesized that a critical distinction between specific pathogen-free mice and nonhuman primates or human patients is their acquired immune history. Here, we show that a heterologous immune response--specifically, virally induced alloreactive memory--is a potent barrier to tolerance induction. A critical threshold of memory T cells is needed to promote rejection, and CD8(+) "central" memory T cells are primarily responsible. Finally, treatment with deoxyspergualin, an inhibitor of NF-kappa B translocation, together with costimulation blockade, synergistically impairs memory T cell activation and promotes antigen-specific tolerance of memory. These data offer a potential explanation for the difficulty encountered when inducing tolerance in nonhuman primates and human patients and provide insight into the signaling pathways essential for memory T cell activation and function.