T cells primed by Leishmania major infection cross-react with alloantigens and alter the course of allograft rejection

Potential and limitations of regulatory T-cell therapy in solid organ transplantation

Over the past few years, the therapeutic potential of Treg has been highlighted in the field of autoimmune diseases and after allogeneic transplantation. The first hurdle for the therapeutic use of Treg is their insufficient numbers in non-manipulated individuals, in particular when facing strong immune activation and expanding effector cells, such as in response to an allograft. Here we review current approaches being explored for Treg expansion in the perspective of clinical therapeutic protocols. We describe different Treg subsets that could be suitable for clinical application, as well as discuss factors such as the required dose of Treg, their antigen-specificity and in vivo stability, that have to be considered for optimal Treg-based immunotherapy in transplantation. Since Treg may not be sufficient as stand-alone therapy for solid organ transplantation in humans, we draw attention to possible hurdles and combination therapy with immunomodulatory drugs that could possibly improve the in vivo efficacy of Treg.

Memory T Cells in Transplantation

Following infections and environmental exposures, memory T cells are generated that provide long-term protective immunity. Compared to their naïve T cell counterparts, memory T cells possess unique characteristics that endow them with the ability to quickly and robustly respond to foreign antigens. While such memory T cells are beneficial in protecting their hosts from recurrent infection, memory cells reactive to donor antigens pose a major barrier to successful transplantation and tolerance induction. Significant progress has been made over the past several decades contributing to our understanding of memory T cell generation, their distinct biology, and their detrimental impact in clinical and animal models of transplantation. This review focuses on the unique features which make memory T cells relevant to the transplant community and discusses potential therapies targeting memory T cells which may ameliorate allograft rejection.

Transplantation tolerance: from theory to clinic

Tolerance induction and alloreactivity can be applied to the clinic for the transplantation of solid organs and in the treatment of human cancers respectively. Hematopoietic chimerism, the stable coexistence of host and donor blood cells, guarantees that a solid organ from the same donor will be tolerated without a requirement for maintenance immunosuppression, and it also serves as a platform for the adoptive immunotherapy of hematologic malignancies using donor lymphocyte infusions. This review focuses on clinically relevant methods for inducing hematopoietic chimerism and transplantation tolerance, with a special emphasis on reduced intensity transplantation conditioning and high dose, post-transplantation cyclophosphamide to prevent graft rejection and graft-versus-host disease (GVHD). Reduced intensity transplantation regimens permit a transient cooperation between donor and host immune systems to eradicate malignancy without producing GVHD. Their favorable toxicity profile also enables the application of allogeneic stem cell transplantation to treat non-malignant disorders of hematopoiesis and to induce tolerance for solid organ transplantation.

Microbes and allogeneic transplantation

Microbial products can be recognized by pattern recognition receptors expressed by immune and parenchymal cells and drive innate immunity that can in turn shape adaptive immune responses to microbial and transplant antigens. In transplanted patients, the signals and their downstream inflammatory cytokines elicited in response to infections can modulate ongoing alloimmune responses and modify the fate of transplanted organs. In recent years, it has become apparent that microbial signals can be generated not only by active pathogenic infections but also by commensal microbiota, thus opening a new field of research into the interplay between the microbiota and the immune system in homeostasis and disease. The wide use of antibiotics and immunosuppressive drugs in transplanted patients can have dramatic consequences on the microbiota that can in turn shape immune responses and perhaps alloresponses, whereas the ongoing immune responses can in turn affect the commensal or pathogenic microorganisms in a feed-forward circle. Here, we discuss known and hypothesized mechanisms for how infections or microbiota-derived signals may affect local or systemic alloimmunity and briefly review data on downstream effects of antibiotics and vaccinations.

Effects of preexisting autoimmunity on heart graft prolongation after donor-specific transfusion and anti-CD154

BACKGROUND

Alloreactive memory T cells prevent costimulatory blockade-induced heart graft survival in mice, but whether and how preexisting autoreactive T cells affect solid-organ transplants under these conditions is unknown.

METHODS

We tested the impact of preexisting cardiac myosin (CM)-specific immunity on murine heart transplant recipients treated with donor-specific transfusion (DST) plus anti-CD154 monoclonal antibody MR1.

RESULTS

Preimmunization with CM but not control ovalbumin abrogated the graft prolonging effects of DST/MR1, whether administered 2 weeks or more than 6 weeks before transplantation. Adoptive transfer of spleen cells from CM-immunized mice into naïve recipients had similar effects. CM-specific immunity did not cross-react with donor antigens and preimmunization with CM had no impact on the survival or histology of DST/MR1-treated syngeneic heart grafts, the latter indicating that persistent autoimmunity is insufficient to cause rejection in the context of costimulatory blockade. We observed that the CM preimmunized mice produced higher frequencies of donor-reactive T cells with higher ratios of CD8/CD4Foxp3 cells, suggesting that the autoreactive memory T cells provide help for activation of alloreactive T cells despite the costimulatory blockade.

CONCLUSIONS

These mechanistic insights linking autoimmunity and alloimmunity in a model of murine heart transplantation have clinical relevance to the known association between autoimmunity and an elevated risk of acute and chronic heart transplant injury in humans.

Memory T cells in transplantation - progress and challenges

PURPOSE OF REVIEW

Memory T cells present a different set of challenges to transplant patients; they are needed for protection against invading pathogens, especially under conditions of immunosuppression. But their presence also threatens transplant survival, as some of them are alloreactive. Efforts to resolve this paradox will be critical in the induction of transplant tolerance.

RECENT FINDINGS

There has been significant progress made in the past few years in the areas of population diversity of memory T cells, metabolic control of their induction, and mechanisms and pathways involved in memory cell exhaustion. Multiple targets on memory T cells have been identified, some of which are under vigorous testing in various transplant models.

SUMMARY

Memory T cells are both friends and foes to transplant patients, and tolerance strategies should selectively target alloreactive memory T cells and leave other memory cells unaltered. This situation remains a major challenge in the clinic.

Effects of cellular sensitization and donor age on acute rejection and graft function after deceased-donor kidney transplantation

BACKGROUND

Allografts from older donors may be more immunogenic than those from younger donors. Pretransplantation cellular sensitization may interact with advanced donor age to increase the risk of immune injury after deceased-donor kidney transplantation.

METHODS

The outcomes of 118 consecutive deceased-donor kidney transplant recipients with available pretransplantation donor-stimulated enzyme-linked immunosorbent spot (ELISPOT) assays for interferon gamma were analyzed retrospectively to determine the impact of cellular sensitization and other clinical variables, including donor age, on the incidence of acute rejection (AR) in the first year after deceased-donor transplantation and on estimated glomerular filtration rate 12 months after transplantation.

RESULTS

The incidence of AR was higher in patients with positive pretransplantation ELISPOT assays versus those with negative assays (36% vs. 14%, P=0.009). Logistic regression indicated that the combination of donor age 50 years or older and a positive pretransplantation ELISPOT assay was more strongly associated with AR (odds ratio, 12.1; confidence interval, 1.1-133) than either variable alone. Estimated glomerular filtration 12 months after transplantation was highest in ELISPOT-negative patients receiving kidneys from donors younger than 50 years and lowest in ELISPOT-positive recipients with donors 50 years or older.

CONCLUSION

The combination of advanced donor age and pretransplantation cellular sensitization increases the risk of AR and poor graft function after deceased-donor kidney transplantation beyond the risk associated with each factor alone.

Inhibition of recall responses through complementary therapies targeting CD8+ T-cell- and alloantibody-dependent allocytotoxicity in sensitized transplant recipients

Allospecific T memory cell responses in transplant recipients arise from environmental exposure to previous transplantation or cross-reactive heterologous immunity. Unfortunately, these memory responses pose a significant barrier to the survival of transplanted tissue. We have previously reported that concurrent inhibition of CD154 and LFA-1 suppresses primary CD8-dependent rejection responses that are not controlled by conventional immunosuppressive strategies. We hypothesized that CD154- and LFA-1-mediated inhibition, by targeting activation as well as effector functions, may also be efficacious for the control of alloreactive CD8+ T-cell responses in sensitized hosts. We found that treatment with anti-LFA-1 mAb alone enhanced transplant survival and reduced CD8-mediated cytotoxicity in sensitized CD4 KO recipients. However, treatment with anti-CD154 mAb alone did not have an effect. Notably, when both CD4- and CD8-dependent rejection pathways are operative (wild-type sensitized recipients), LFA-1 significantly inhibited CD8-mediated in vivo allocytotoxicity but did not correspond with enhanced hepatocyte survival. We hypothesized that this was due to alloantibody-mediated rejection. When anti-LFA-1 mAb treatment was combined with macrophage depletion, which we have previously reported impairs alloantibody-mediated parenchymal cell damage, in vivo cytotoxic effector function was significantly decreased and was accompanied by significant enhancement of hepatocyte survival in sensitized wild-type recipients. Therefore, LFA-1 is a potent therapeutic target for reduction of CD8-mediated cytotoxicity in sensitized transplant recipients and can be combined with other treatments that target non-CD8-mediated recall alloimmunity.

T-cell activation and transplantation tolerance

Transplantation of allogeneic or "nonself" tissues stimulates a robust immune response leading to graft rejection, and therefore, most recipients of allogeneic organ transplants require the lifelong use of immune suppressive agents. Excellent outcomes notwithstanding, contemporary immunosuppressive medications are toxic, are often not taken by patients, and pose long-term risks of infection and malignancy. The ultimate goal in transplantation is to develop new treatments that will supplant the need for general immunosuppression. Here, we will describe the development and application of costimulation blockade to induce transplantation tolerance and discuss how the diverse array of signals that act on T cells will determine the balance between graft survival and rejection.

The impact of infection and tissue damage in solid-organ transplantation

Investigations over the past two decades are revealing complexities in the regulation of the innate immune response, and how this response, in turn, controls adaptive immunity. Microbial exposure, infections and tissue damage that accompany solid-organ transplantation result in the release of pathogen- and damage-associated molecular patterns, as well as pathogen- or allograft-derived antigens. Here, we review these triggers of innate and adaptive immunity, and discuss emerging paradigms of the many ways in which infections and tissue damage might directly or indirectly affect alloreactivity and the outcome of transplanted allografts.

Dissecting the T Cell Response: Proliferation Assays vs. Cytokine Signatures by ELISPOT

Chronic allograft rejection is in part mediated by host T cells that recognize allogeneic antigens on transplanted tissue. One factor that determines the outcome of a T cell response is clonal size, while another is the effector quality. Studies of alloimmune predictors of transplant graft survival have most commonly focused on only one measure of the alloimmune response. Because differing qualities and frequencies of the allospecific T cell response may provide distinctly different information we analyzed the relationship between frequency of soluble antigen and allo-antigen specific memory IFN-g secreting CD4 and CD8 T cells, their ability to secrete IL-2, and their proliferative capacity, while accounting for cognate and bystander proliferation. The results show proliferative responses primarily reflect on IL-2 production by antigen-specific T cells, and that proliferating cells in such assays entail a considerable fraction of bystander cells. On the other hand, proliferation (and IL-2 production) did not reflect on the frequency of IFN-γ producing memory cells, a finding particularly accentuated in the CD8 T cell compartment. These data provide rationale for considering both frequency and effector function of pre-transplant T cell reactivity when analyzing immune predictors of graft rejection.

The use of donor-derived veto cells in hematopoietic stem cell transplantation

The induction of immune tolerance by specific agents, as opposed to general immune suppression, is a most desirable goal in transplantation biology. One approach to attain this goal is afforded by the use of donor-derived cells endowed with veto activity, which is the ability of a cell to specifically suppress only T cells directed against its antigens. A megadose of purified veto CD34(+) hematopoietic stem cells is already used in patients to allow hematopoietic stem cells transplantation (HSCT) across major genetic barriers, while avoiding severe graft versus host disease (GVHD). However, allowing engraftment of such T cell-depleted HSCT under safer reduced intensity conditioning (RIC) protocols still remains a challenge. Therefore, combining megadose of CD34(+) HSCT with other GVHD-depleted veto cells could enable facilitation of engraftment of HSCT under RIC without the adverse complication of GVHD. This approach might provide a safer modality for enabling engraftment of HSCT, enabling its application in elderly patients who cannot tolerate intensive protocols and to a variety of patients with non-malignant disorders, associated with longer life expectancy, in whom the use of a high risk conditioning cannot be considered.

Heterogeneity within T Cell Memory: Implications for Transplant Tolerance

Adaptive immunity in both mouse and man results in the generation of immunological memory. Memory T cells are both friend and foe to transplant recipients, as they are intimately involved and in many cases absolutely required for the maintenance of protective immunity in the face immunosuppression, yet from the evidence presented herein they clearly constitute a formidable barrier for the successful implementation of tolerance induction strategies in transplantation. This review describes the experimental evidence demonstrating the increased resistance of memory T cells to many distinct tolerance induction strategies, and outlines recent advances in our knowledge of the ways in which alloreactive memory T cells arise in previously untransplanted individuals. Understanding the impact of alloreactive memory T cell specificity, frequency, and quality might allow for better donor selection in order to minimize the donor-reactive memory T cell barrier in an individual transplant recipient, thus allowing stratification of relative risk of alloreactive memory T cell mediated rejection, and conversely increase the likelihood of successful establishment of tolerance. However, further research into the molecular and cellular pathways involved in alloreactive memory T cell-mediated rejection is required in order to design new strategies to overcome the memory T cell barrier, without critically impairing protective immunity.

Contributions of direct and indirect alloresponses to chronic rejection of kidney allografts in nonhuman primates

The relative contribution of direct and indirect allorecognition pathways to chronic rejection of allogeneic organ transplants in primates remains unclear. In this study, we evaluated T and B cell alloresponses in cynomolgus monkeys that had received combined kidney/bone marrow allografts and myeloablative immunosuppressive treatments. We measured donor-specific direct and indirect T cell responses and alloantibody production in monkeys (n = 5) that did not reject their transplant acutely but developed chronic humoral rejection (CHR) and in tolerant recipients (n = 4) that never displayed signs of CHR. All CHR recipients exhibited high levels of anti-donor Abs and mounted potent direct T cell alloresponses in vitro. Such direct alloreactivity could be detected for more than 1 y after transplantation. In contrast, only two of five monkeys with CHR had a detectable indirect alloresponse. No indirect alloresponse by T cells and no alloantibody responses were found in any of the tolerant monkeys. Only one of four tolerant monkeys displayed a direct T cell alloresponse. These observations indicate that direct T cell alloresponses can be sustained for prolonged periods posttransplantation and result in alloantibody production and chronic rejection of kidney transplants, even in the absence of detectable indirect alloreactivity.

Transplantation tolerance: memories that haunt us

Transplant tolerance, which allows grafts--allogeneic cells, tissues, or organs--to be accepted without host immunosuppression, can be achieved in mice but not in primates. In this issue of Science Translational Medicine, Nadazdin et al. report that a high pretransplant frequency of graft-reactive memory T cells may inhibit the induction of transplant tolerance in nonhuman primates and lead to transplant rejection. Knowing the frequency of allograft-specific memory T cells in potential transplant recipients could aid clinical decision-making by guiding selection of the antigenic profile of the donor organ or by influencing the type of tolerance-induction protocol pursued.

Host alloreactive memory T cells influence tolerance to kidney allografts in nonhuman primates

Transplant tolerance, defined as indefinite allograft survival without immunosuppression, has been regularly achieved in laboratory mice but not in nonhuman primates or humans. In contrast to laboratory mice, primates regularly have high frequencies of alloreactive memory T cells (TMEMs) before transplantation. These TMEMs are poorly sensitive to conventional immunosuppression and costimulation blockade, and the presence of donor-reactive TMEMs in primates may account for their resistance to transplant tolerance protocols that have proven consistently effective in mice. We measured the frequencies of anti-donor TMEMs before and after transplantation in a series of rejecting and tolerant monkeys that underwent nonmyeloablative conditioning, short-term immunosuppression, and combined allogeneic kidney/cell transplantation. Transplants were acutely rejected in all the monkeys with high numbers of donor-specific TMEMs before transplantation. In contrast, long-term survival was observed in the recipients harboring lower frequencies of anti-donor TMEMs before transplantation. Similar amounts of TMEM homeostatic expansion were recorded in all transplanted monkeys upon hematopoietic reconstitution; however, only the tolerant monkeys had no expansion or activation of donor-reactive TMEMs after transplantation. These results indicate that the presence of high frequencies of host donor-reactive TMEMs before transplantation impairs tolerance induction to kidney allografts in this nonhuman primate model. Indeed, recipients harboring a low anamnestic reactivity to their donor before transplantation were successfully rendered tolerant via infusion of donor cells and short-term immunosuppression. This suggests that selection of allogeneic donors with low memory responses in recipients may be essential to successful transplant tolerance induction in patients.

Clinical transplantation tolerance

Transplantation is the treatment of choice for many if not most causes of end-stage organ failure. Over 20,000 organ transplant procedures were performed in the USA in 2009 to treat patients with failed or failing kidneys, livers, hearts, lungs, and intestines, and there remain 85,000 individuals waiting on the transplant list. Currently, in the USA, there are over 170,000 individuals living with a transplanted organ. Virtually, all of these individuals receive maintenance immunosuppression in an attempt to maximize the function and survival of the transplanted organ. However, it is clear that the long-term use of immunosuppressive agents is associated with an extensive list of undesirable side effects that have the potential to limit the survival of the patient and transplanted organ as well as to compromise quality of life. Although the ability to induce reproducibly a state of robust, stable tolerance would address this problem, tolerance remains an infrequent event in clinical transplantation that is largely a consequence of chance. Factors limiting the broader investigation of clinical transplantation tolerance include the lack of therapeutic regimens known to favor tolerance in humans, the lack of validated assays or biomarkers predictive of tolerance, and concerns about the safety and ethics of complete withdrawal of immunosuppression given the very good results achievable with current immunosuppression. Despite these barriers, a number of investigators have continued to conduct well-designed and carefully supervised studies with the long-term goal of making clinical transplantation tolerance more feasible. The aim of this review is to summarize the status of these studies.

Limiting the amount and duration of antigen exposure during priming increases memory T cell requirement for costimulation during recall

Donor-reactive memory T cells (Tmem) can play an important role in mediating graft rejection after transplantation. Transplant recipients acquire donor-reactive Tmem not only through prior sensitization with alloantigens but also through previous exposure to environmental pathogens that are cross-reactive with allogeneic peptide-MHC complexes. Current dogma suggests that most, if not all, Tmem responses are independent of the requirement for CD28 and/or CD154/CD40-mediated costimulation to mount a recall response. However, heterogeneity among Tmem is increasingly being appreciated, and one important factor known to impact the function and phenotype of Ag-specific T cell responses is the amount/duration of Ag exposure. Importantly, the impact of Ag exposure on development of costimulation independence is currently unknown. In this study, we interrogated the effect of decreased Ag amount/duration during priming on the ability of donor-reactive Tmem to mediate costimulation blockade-resistant rejection during a recall response after transplantation in a murine model. Recipients possessing donor-reactive Tmem responses that were generated under conditions of reduced Ag exposure exhibited similar frequencies of Ag-specific T cells at day 30 postinfection, but, strikingly, failed to mediate costimulation blockade-resistant rejection after challenge with an OVA-expressing skin graft. Thus, these data demonstrate the amount/duration of Ag exposure is a critical factor in determining Tmem's relative requirement for costimulation during the recall response after transplantation.

Selective targeting of human alloresponsive CD8+ effector memory T cells based on CD2 expression

Costimulation blockade (CoB), specifically CD28/B7 inhibition with belatacept, is an emerging clinical replacement for calcineurin inhibitor-based immunosuppression in allotransplantation. However, there is accumulating evidence that belatacept incompletely controls alloreactive T cells that lose CD28 expression during terminal differentiation. We have recently shown that the CD2-specific fusion protein alefacept controls costimulation blockade-resistant allograft rejection in nonhuman primates. Here, we have investigated the relationship between human alloreactive T cells, costimulation blockade sensitivity and CD2 expression to determine whether these findings warrant potential clinical translation. Using polychromatic flow cytometry, we found that CD8(+) effector memory T cells are distinctly high CD2 and low CD28 expressors. Alloresponsive CD8(+) CD2(hi) CD28(-) T cells contained the highest proportion of cells with polyfunctional cytokine (IFNγ, TNF and IL-2) and cytotoxic effector molecule (CD107a and granzyme B) expression capability. Treatment with belatacept in vitro incompletely attenuated allospecific proliferation, but alefacept inhibited belatacept-resistant proliferation. These results suggest that highly alloreactive effector T cells exert their late stage functions without reliance on ongoing CD28/B7 costimulation. Their high CD2 expression increases their susceptibility to alefacept. These studies combined with in vivo nonhuman primate data provide a rationale for translation of an immunosuppression regimen pairing alefacept and belatacept to human renal transplantation.

Overcoming the memory barrier in tolerance induction: molecular mimicry and functional heterogeneity among pathogen-specific T-cell populations

PURPOSE OF REVIEW

This review highlights recent advances in our understanding of the frequency and nature of alloreactivity among memory T-cell populations, and discusses recent successes in experimentally targeting these populations in order to prolong graft survival.

RECENT FINDINGS

Recent studies suggest that not only is alloreactivity present within peripheral T-cell compartments of normal healthy individuals, but cross-reactivity between viral-specific T cells and allotropes may in fact be a very common occurrence. Furthermore, this cross-reactivity functions at the level of molecular mimicry of T-cell receptor recognition. Therapeutics that specifically target cell surface molecules or effector pathways used by memory T cells to mediate graft rejection will likely be required in order to attenuate the donor-reactive memory T-cell response during transplantation.

SUMMARY

A major challenge facing the field over the next decade is to define the heterogeneity that exists within memory T-cell populations that impacts graft survival. Understanding the functional and phenotypic differences that modify the memory T-cell barrier to tolerance induction might allow a strategy in which strength of immunosuppression could be tailored to fit the immunological history of a given transplant recipient in order to minimize nonimmune toxicities, maximize protective immunity, and prolong graft survival.

25-OH-vitamin D deficiency and cellular alloimmunity as measured by panel of reactive T cell testing in dialysis patients

Primed antidonor alloreactive T cells are detrimental to transplant outcome, but factors that impact the strength of this immune response prior to transplantation are unknown. We tested peripheral blood mononuclear cells from dialysis patients, against panels of allogeneic, primary B-cell lines in a newly standardized IFNγ ELISPOT panel of reactive T cell (PRT) assay. Results were correlated with known alloantibody-sensitizing events and other clinical parameters. As 25-OH-vitamin D deficiency is associated with enhanced cellular immunity, is common in dialysis patients and is correctable, we assessed the relationship between serum 25-OH-vitamin D and the PRT. Using independent test and validation cohorts we found that low serum levels of 25-OH-vitamin D (<26 ng/mL) correlated with high-PRT values (in the upper 50th percentile, OR 0.02, p = 0.01) independent of age, sex, race, previous transplant, transfusion, pregnancy, time on dialysis, panel of reactive antibody, iPTH, and treatment with 1,25-OH-vitamin D. The data provide a potential mechanism for the possible relationship between vitamin D deficiency and poor posttransplant outcome, and support studies to test the impact of 25-OH-vitamin D repletion on alloimmunity and allograft injury in kidney transplant candidates.

Allografts stimulate cross-reactive virus-specific memory CD8 T cells with private specificity

Viral infections have been associated with the rejection of transplanted allografts in humans and mice, and the induction of tolerance to allogeneic tissues in mice is abrogated by an ongoing viral infection and inhibited in virus-immune mice. One proposed mechanism for this 'heterologous immunity' is the induction of alloreactive T cell responses that cross-react with virus-derived antigens. These cross-reactive CD8 T cells are generated during acute viral infection and survive into memory, but their ability to partake in the immune response to allografts in vivo is not known. We show here that cross-reactive, virus-specific memory CD8 T cells from mice infected with LCMV proliferated in response to allografts. CD8 T cells specific to several LCMV epitopes proliferated in response to alloantigens, with the magnitude and hierarchy of epitope-specific responses varying with the private specificities of the host memory T cell repertoire, as shown by adoptive transfer studies. Last, we show that purified LCMV-specific CD8 T cells rejected skin allografts in SCID mice. These findings therefore implicate a potential role for heterologous immunity in virus-induced allograft rejection.

Cutting edge: Rapamycin augments pathogen-specific but not graft-reactive CD8+ T cell responses

Recent evidence demonstrating that exposure to rapamycin during viral infection increased the quantity and quality of Ag-specific T cells poses an intriguing paradox, because rapamycin is used in transplantation to dampen, rather than enhance, donor-reactive T cell responses. In this report, we compared the effects of rapamycin on the Ag-specific T cell response to a bacterial infection versus a transplant. Using a transgenic system in which the Ag and the responding T cell population were identical in both cases, we observed that treatment with rapamycin augmented the Ag-specific T cell response to a pathogen, whereas it failed to do so when the Ag was presented in the context of a transplant. These results suggest that the environment in which an Ag is presented alters the influence of rapamycin on Ag-specific T cell expansion and highlights a fundamental difference between Ag presented by an infectious agent as compared with an allograft.

Phenotype, distribution and alloreactive properties of memory T cells from cynomolgus monkeys

The high frequency of memory T cells present in primates is thought to represent a major barrier to tolerance induction in transplantation. Therefore, it is crucial to characterize these memory T cells and determine their functional properties. High numbers of memory T cells were detected in peripheral blood and all lymphoid tissues except lymph nodes, which were essentially the site of naïve T cells. The majority of CD8(+) memory T cells were effector memory cells located in the blood and bone marrow while most CD4(+) memory T cells were central memory cells present in the spleen. Next, memory T cells from over 100 monkeys were tested for their response to alloantigens by ELISPOT. Memory alloreactivity mediated via direct but not indirect allorecognition was detected in all animals. The frequency of allospecific memory T cells varied dramatically depending upon the nature of the responder/stimulator monkey combination tested. MHC gene matching was generally associated with a low-memory alloreactivity. Nevertheless, low anamnestic alloresponses were also found in a significant number of fully MHC-mismatched monkey combinations. These results show that selected donor/recipient combinations displaying a low memory alloresponsiveness can be found. These combinations may be more favorable for transplant tolerance induction.

Recent progress and new perspectives in studying T cell responses to allografts

Studies in the past decade advanced our understanding of the development, execution and regulation of T-cell-mediated allograft rejection. This review outlines recent progress and focuses on three major areas of investigation that are likely to guide the development of graft-prolonging therapies in the future. The discussed topics include the contribution of recently discovered molecules to the activation and functions of alloreactive T cells, the emerging problem of alloreactive memory T cells and recently gained insights into the old question of transplantation tolerance.

Differential requirement of CD27 costimulatory signaling for naïve versus alloantigen-primed effector/memory CD8+ T cells

CD8(+) memory T cells endanger allograft survival by causing acute and chronic rejection and prevent tolerance induction. We explored the role of CD27:CD70 T-cell costimulatory pathway in alloreactive CD8(+)/CD4(+) T-cell activation. CD27-deficient (CD27(-/-)) and wild-type (WT) B6 mice rejected BALB/c cardiac allografts at similar tempo, with or without depletion of CD4(+) or CD8(+) T cells, suggesting that CD27 is not essential during primary T-cell alloimmune responses. To dissect the role of CD27 in primed effector and memory alloreactive T cells, CD27(-/-) or WT mice were challenged with BALB/c hearts either 10 or 40 days after sensitization with donor-type skin grafts. Compared to WT controls, allograft survival was prolonged in day 40- but not day 10-sensitized CD27(-/-) recipients. Improved allograft survival was accompanied by diminished secondary responsiveness of memory CD8(+) T cells, which resulted from deficiency in memory formation rather than their lack of secondary expansion. Chronic allograft vasculopathy and fibrosis were diminished in CD27(-/-) recipients of class I- but not class II-mismatched hearts as compared to WT controls. These data establish a novel role for CD27 as an important costimulatory molecule for alloreactive CD8(+) memory T cells in acute and chronic allograft rejection.

Effects of influenza immunization on humoral and cellular alloreactivity in humans

BACKGROUND

Alloreactive T cells and anti-human leukocyte antigen antibodies mediate transplant injury. Environmental exposures, including vaccinations, may activate the alloimmune repertoire leading to accelerated allograft injury. To test whether vaccination impacts human alloimmunity, we analyzed humoral and cellular immune reactivity in subjects undergoing influenza vaccination.

METHODS

We serially obtained blood samples from 30 healthy subjects and 8 kidney and 9 lung transplant recipients who received influenza vaccination, and from 20 healthy unvaccinated controls. We measured cellular and humoral anti-influenza responses, anti-human leukocyte antigen antibodies, and alloreactive T-cell immunity (interferon-gamma ELISPOT) at 0, 2, 4, and 12 weeks after vaccination.

RESULTS

Vaccination induced influenza-reactive humoral and cellular responses in control subjects and in transplant recipients. Only two of 30 vaccinated volunteers developed new alloantibodies, but none of the transplant patients. Vaccination also specifically and significantly augmented cellular alloimmunity based on reactivity to a panel of stimulators in both healthy subjects and in transplant recipients within 4 weeks of vaccination. The enhanced cellular alloresponse waned toward prevaccine levels by week 12.

CONCLUSION

Our findings newly demonstrate that influenza vaccination can have a significant impact on the potency of the alloimmune repertoire. Because the strength of the alloresponse influences long-term graft function, our results suggest that further investigation of alloimmune monitoring after vaccination is needed.

Cross-immune tolerance: conception and its potential significance on transplantation tolerance

The diversity of alloreactive T cells in graft rejection and the presence of extensive crossreactivity among alloreactive T cells indicate that the induction of transplantation tolerance may fundamentally alter the size of host T-cell repertoire involved in protective immunity and immune surveillance, especially those that are crossreactive to conventional antigens. We herein highlight the crossreactive nature of alloreactive T cells and the potential risks of altered T-cell repertoire associated with the induction of transplantation tolerance. The possibility that T-cell tolerance to one set of antigens results in their tolerance to other unrelated antigens due to T-cell crossreactivity and/or heterogeneity is defined as 'cross-immune tolerance'. The definition and significance of this concept were discussed in details.

ADAP deficiency combined with costimulation blockade synergistically protects intestinal allografts

Adhesion and degranulation promoting adapter protein (ADAP) plays an important role in T cell activation. ADAP deficiency was recently found to prolong heart graft survival in mice. We investigated the role of ADAP in intestinal transplantation and the synergistic effect of ADAP deficiency and Costimulation blockade (CB). T cell proliferation and cytotoxic T lymphocyte (CTL) activity were determined. MHC mismatched intestinal allografts was transplanted heterotopically. Anti-CD40L antibody was applied to the recipient. Upon stimulation with allogenic dendritic cells (DC), ADAP-deficient (ADAP-/-) T cells displayed impaired proliferative responses compared with that of wild-type (WT) T cells. In contrast, the CTL activity in ADAP-/- mice was comparable with that of WT mice. Rejection of intestinal allografts was ameliorated, but not prevented in ADAP-/- mice. Although CB alone was not sufficient to mitigate the rejection, the combination of CB and ADAP deficiency profoundly inhibited rejection. This was accompanied by less infiltration and activation of host lymphocytes in the gut-associated lymphoid tissue of intestinal allografts. ADAP deficiency combined with CB protected the intestinal allografts synergistically. ADAP could be a novel target in the induction phase of the immune responses in organ transplantation.

Donor-reactive T-cell stimulation history and precursor frequency: barriers to tolerance induction

Blockade of T-cell costimulatory pathways represents a potent and specific method of preventing naïve antidonor T-cell responses after transplantation in mouse, monkey, and man. However, numerous studies have shown that the presence of donor-reactive memory T cells in the recipient poses a sometimes insurmountable barrier to long-term graft survival and tolerance induction. Here, we discuss the ways in which donor-reactive memory T cells may arise from environmental exposure to pathogens. Pathogen-specific memory T cells, by virtue of the inherent degeneracy of T-cell receptor recognition of peptide:major histocompatibility complex ligands, may exhibit cross reactivity with allogeneic peptide:major histocompatibility complexes and thereby mediate graft rejection. From the recent explosion in knowledge of the heterogeneity of memory T-cell resulting from variations in frequency and duration of antigen exposure, cytokine milieu, site of priming, and a host of other factors, it is becoming increasingly well appreciated that different memory T-cell populations may exhibit differential susceptibilities to tolerance induction. Thus, the immune history of a transplant recipient and frequencies of donor-cross-reactive memory T cells within the various compartments may dictate the likelihood of success or failure of tolerance induction.

Antagonistic effect of toll-like receptor signaling and bacterial infections on transplantation tolerance

The induction of donor-specific tolerance remains a major goal in the field of transplantation immunology. Therapies that target costimulatory molecules can induce tolerance to heart and pancreatic islet allografts in mouse models, but fail to do so after transplantation of skin or intestinal allografts. We have proposed that organs colonized by commensal bacteria such as skin, lung, and intestine may be resistant to such therapies as a result of bacterial translocation at the time of transplantation, which may promote antigen-presenting cell maturation and the production of proinflammatory cytokines, consequently enhancing responses of alloreactive T cells. Our results indicate that the inability to sense signaling by most toll-like receptors (TLRs), as well as by interleukin-1R and -18R, as a result of genetic ablation of myeloid differentiation factor 88 promotes the acceptance of skin allografts. Conversely, TLR signals and infections by a model bacterium, Listeria monocytogenes (LM), at the time of transplantation can prevent the induction of transplantation tolerance. The effects of the TLR9 agonist CpG are myeloid differentiation factor 88-dependent, whereas the prorejection capacity of LM depends on the intracellular sensing of LM and the production of type I interferon. Therefore, transiently targeting these innate, proinflammatory pathways may have therapeutic value to promote transplantation tolerance.

Toll-like receptors (TLRs) in transplantation

TLRs have been extensively studied over the past decade for their ability to recognize microbial molecular patterns and activate innate immune cells to fight infections. They have also been described to provide a link between innate and adaptive immunity, as TLR signals also enhance the antigen presenting capacity of innate immune cells to T cells. In recent years, a contribution of TLR pathways to immune responses elicited by ischemia/reperfusion injury (IRI), allografts and xenografts has been uncovered, although the ligands that bind TLRs in these settings remain to be revealed. Such research has the potential to identify novel therapeutic targets that may facilitate allograft acceptance. In this review, we will summarize the results published to date on the role of TLRs in experimental and clinical transplantation.

Central memory CD8+ T cells induce graft-versus-host disease and mediate graft-versus-leukemia

In allogeneic hemopoietic stem cell transplantation, mature donor alphabeta T cells in the allograft promote T cell reconstitution in the recipient and mediate the graft-vs-leukemia (GVL) effect. Unfortunately, donor T cells can attack nonmalignant host tissues and cause graft-vs-host disease (GVHD). It has previously been shown that effector memory T cells not primed to alloantigen do not cause GVHD yet transfer functional T cell memory and mediate GVL. Recently, central memory T cells (T(CM)) have also been reported to not cause GVHD. In contrast, in this study, we demonstrate that purified CD8(+) T(CM) not specifically primed to alloantigens mediate GVHD in the MHC-mismatched C57BL/6 (B6)-->BALB/c and the MHC-matched, multiple minor histocompatibility Ag-mismatched C3H.SW-->B6 strain pairings. CD8(+) T(CM) and naive T cells (T(N)) caused similar histological disease in liver, skin, and bowel. B6 CD8(+) T(CM) and T(N) similarly expanded in BALB/c recipients, and the majority of their progeny produced IFN-gamma upon restimulation. However, in both models, CD8(+) T(CM) induced milder clinical GVHD than did CD8(+) T(N). Nonetheless, CD8(+) T(CM) and T(N) were similarly potent mediators of GVL against a mouse model of chronic-phase chronic myelogenous leukemia. Thus, in contrast to what was previously thought, CD8(+) T(CM) are capable of inducing GVHD and are substantially different from T(EM) but only subtly so from T(N).

Translating costimulation blockade to the clinic: lessons learned from three pathways

SUMMARY

As the recognition that costimulatory signals are critical for optimal T-cell activation, proliferation, and differentiation, there has been an explosion in the study of costimulatory molecules and their roles in enhancing anti-donor T-cell responses following transplantation. Here, we focus on the bench-to-beside translation of blocking agents designed to target three critical costimulatory pathways: the CD28/CD80/CD86 pathway, the CD154/CD40 pathway, and the lymphocyte function associated antigen-1/intercellular adhesion molecule pathway. While blockade of each of these pathways proved promising in inhibiting donor-reactive T-cell responses and promoting long-term graft survival in murine models of transplantation, the progression of development of therapeutic agents to block these pathways has each taken a slightly different course. Both logistical and biological pitfalls have accompanied the translation of blockers of all three pathways into clinically applicable therapies, and the development of costimulatory blockade as a substitute for current standard-of-care calcineurin inhibitors has by no means reached completion. Collaboration between both the basic and clinical arenas will further propel the development of costimulation blockers currently in the pipeline, as well as of novel methods to target these critical pathways during transplantation.

The early course of kidney allograft rejection: defining the time when rejection begins

We studied the early events in mouse kidney allografts and isografts to define when allorecognition begins and when alloimmune tissue injury begins. Allografts but not isografts showed T-cell infiltration in perivascular areas from day 1, but tubulitis and arteritis did not develop until day 7. Flow cytometry confirmed the early allospecific CD3(+)CD8(+) T-cell infiltrate. At day 1, both allografts and isografts showed extensive transcriptome changes, reflecting the response to surgery, but only allografts showed expression of interferon-gamma (IFN-gamma)-inducible transcripts and T-cell-associated transcripts. Although the number of CD68(+) myeloid cell numbers did not increase in day 1 isografts or allografts, mRNA expression for myeloid markers was increased in isografts and allografts, suggesting activation of resident cells of the macrophage-dendritic cell series (MMDCs) in response to injury, followed by increased CD68(+) cell numbers from day 2. By day 3, an interstitial T-cell and MMDC infiltrate was established in allografts, corresponding with the emergence of allospecific tissue injury, as reflected by decreased parenchymal transcripts. Thus, in renal allografts, allorecognition by T cells occurs in perivascular sites by day 1, but alloimmune parenchymal damage begins at day 3, coinciding with the emergence of the interstitial T-cell-MMDC infiltrate.

Tolerance-inducing immunosuppressive strategies in clinical transplantation: an overview

The significant development of immunosuppressive drug therapies within the past 20 years has had a major impact on the outcome of clinical solid organ transplantation, mainly by decreasing the incidence of acute rejection episodes and improving short-term patient and graft survival. However, long-term results remain relatively disappointing because of chronic allograft dysfunction and patient morbidity or mortality, which is often related to the adverse effects of immunosuppressive treatment. Thus, the induction of specific immunological tolerance of the recipient towards the allograft remains an important objective in transplantation. In this article, we first briefly describe the mechanisms of allograft rejection and immune tolerance. We then review in detail current tolerogenic strategies that could promote central or peripheral tolerance, highlighting the promises as well as the remaining challenges in clinical transplantation. The induction of haematopoietic mixed chimerism could be an approach to induce robust central tolerance, and we describe recent encouraging reports of end-stage kidney disease patients, without concomitant malignancy, who have undergone combined bone marrow and kidney transplantation. We discuss current studies suggesting that, while promoting peripheral transplantation tolerance in preclinical models, induction protocols based on lymphocyte depletion (polyclonal antithymocyte globulins, alemtuzumab) or co-stimulatory blockade (belatacept) should, at the current stage, be considered more as drug-minimization rather than tolerance-inducing strategies. Thus, a better understanding of the mechanisms that promote peripheral tolerance has led to newer approaches and the investigation of individualized donor-specific cellular therapies based on manipulated recipient regulatory T cells.

An antibody combination that targets activated T cells extends graft survival in sensitized recipients

Memory T cells are the very essence of adaptive immunity with their rapid and efficient response to antigen rechallenge and long-term persistence. However, it is becoming increasingly evident that when primed with self or transplanted tissue, these cells play a key role in causing and perpetuating tissue damage. Furthermore, current treatments, which efficiently control the naive response, have limited effects on primed T cells. We have used a treatment based on a combination of antibodies specific for molecules expressed by activated T lymphocytes to selectively remove these cells. This approach, which we termed multi-hit therapy, leads to cumulative binding of antibodies to the target T cells and a striking prolongation of skin graft survival in presensitized recipients in a stringent skin transplant model. The findings are consistent with the depletion of graft-specific CD4+ and CD8+ T cells, although other modes of action, such as T-cell regulation and altered migration could play a role. In conclusion, our therapeutic strategy controls primed T cells which are a major driving force in the pathology of many autoimmune diseases and in transplant rejection.

Differential effects of donor-specific alloantibody

Alloantigen exposure typically provokes an adaptive immune response that can foster rejection of transplanted organs, and these responses present the most formidable biological barrier to kidney transplantation. Although most cellular alloimmune responses can be therapeutically controlled with T-cell-specific immunosuppressants, humoral alloimmune responses remain relatively untamed. Importantly, humoral immunity, typically manifesting as allospecific antibody production, is increasingly recognized for its variable appearance after kidney transplantation. Indeed, the appearance of alloantibody can herald the onset of rapid and destructive antibody-mediated rejection or have no demonstrable acute effects. The factors determining the end result of alloantibody formation remain poorly understood. This review will discuss the breadth of alloantibody responses seen in clinical kidney transplantation and provide an overview of potential factors explaining the phenotypic variability associated with humoral alloimmunity. We propose several avenues ripe for future investigation including the influence of innate immune components and the potential influence of heterologous immune responses in determining the ultimate clinical import of an alloantibody response.

Expansion of effector memory TCR Vbeta4+ CD8+ T cells is associated with latent infection-mediated resistance to transplantation tolerance

Therapies that control largely T cell-dependent allograft rejection in humans also possess the undesirable effect of impairing T cell function, leaving transplant recipients susceptible to opportunistic viruses. Prime among these opportunists are the ubiquitous herpesviruses. To date, studies are lacking that address the effect of viruses that establish a true latent state on allograft tolerance or the effect of tolerance protocols on the immune control of latent viruses. By using a mixed chimerism-based tolerance-induction protocol, we found that mice undergoing latent infection with gammaHV68, a murine gamma-herpesvirus closely related to human gamma-herpesviruses such as EBV and Kaposi's sarcoma-associated herpesvirus, significantly resist tolerance to allografts. Limiting the degree of virus reactivation or innate immune response did not reconstitute chimerism in latently infected mice. However, gammaHV68-infected mice showed increased frequency of CD8+ T cell alloreactivity and, interestingly, expansion of virus-induced, alloreactive, "effector/effector memory" TCR Vbeta4+CD8+ T cells driven by the gammaHV68-M1 gene was associated with resistance to tolerance induction in studies using gammaHV68-M1 mutant virus. These results define the viral gene and immune cell types involved in latent infection-mediated resistance to allograft tolerance and underscore the influence of latent herpesviruses on allograft survival.

Critical role of CD4 help in CD154 blockade-resistant memory CD8 T cell activation and allograft rejection in sensitized recipients

Allograft rejection in sensitized recipients remains the major problem in clinical organ transplantation. We have developed a donor-type skin-sensitized mouse cardiac allograft model (BALB/c-->C57BL/6) in which both rejection (<5 days) and alloreactive CD8 activation are resistant to CD154 blockade. First, we attempted to elucidate why CD154 blockade fails to protect cardiac grafts in sensitized recipients. The gene array analysis has revealed that treatment with anti-CD154 mAb (MR1) had distinctive impact on host immunity in naive vs sensitized animals. Unlike in naive counterparts, host sensitization mitigated the impact of CD154 blockade on critical immune signaling pathways. Indeed, we identified 3234 genes in cardiac grafts that were down-regulated by MR1 in naive (at least 5-fold), but remained unaffected in sensitized hosts. Moreover, MR1 treatment failed to prevent accumulation of CD4 T cells in cardiac allografts of sensitized recipients. Then, to determine the role of CD4 help in CD154 blockade-resistant immune response, we used CD4-depleting and CD4-blocking Ab, in conjunction with MR1 treatment. Our data revealed that CD154 blockade-resistant CD8 activation in sensitized mice was dependent on CD4 T cells. In the absence of CD4 help, CD154 blockade prevented differentiation of alloreactive CD8 T cells into CTL effector/memory cells and abrogated acute rejection (cardiac graft survival for >30 days), paralleled by selective target gene depression at the graft site. These results provide the rationale to probe potential synergy of adjunctive therapy targeting CD4 and CD154 to overcome graft rejection in sensitized recipients.

Prevention of allograft tolerance by bacterial infection with Listeria monocytogenes

Exposure to certain viruses and parasites has been shown to prevent the induction of transplantation tolerance in mice via the generation of cross-reactive memory T cell responses or the induction of bystander activation. Bacterial infections are common in the perioperative period of solid organ allograft recipients in the clinic, and correlations between bacterial infections and acute allograft rejection have been reported. However, whether bacterial infections at the time of transplantation have any effect on the generation of transplantation tolerance remains to be established. We used the Gram-positive intracellular bacterium Listeria monocytogenes (LM) as a model pathogen because its effects on immune responses are well described. Perioperative LM infection prevented cardiac and skin allograft acceptance induced by anti-CD154 and donor-specific transfusion in mice. LM-mediated rejection was not due to the generation of cross-reactive T cells and was largely independent of signaling via MyD88, an adaptor for most TLRs, IL-1, and IL-18. Instead, transplant rejection following LM infection was dependent on the expression of the phagosome-lysing pore former listeriolysin O and on type I IFN receptor signaling. Our results indicate that bacterial exposure at the time of transplantation can antagonize tolerogenic regimens by enhancing alloantigen-specific immune responses independently of the generation of cross-reactive memory T cells.

ICOS-Dependent and -independent functions of memory CD4 T cells in allograft rejection

Donor-reactive memory T cells undermine the survival of transplanted organs through multiple pathways. We have previously reported that memory CD4 T cells resist treatment with anti-CD154 antibody and donor-specific transfusion (DST/MR1) and promote cardiac allograft rejection via generation of effector CD4 T cells and alloantibody. We hypothesized that the helper functions of memory CD4 T cells are independent of T-cell costimulation through CD154 but instead are regulated by alternative costimulatory pathways. This study investigated how blocking ICOS/B7RP-1 interactions affects functions of donor-reactive memory CD4 T cells. Treatment with blocking anti-ICOS mAb synergized with DST/MR1 and prolonged mouse cardiac allograft survival despite the presence of donor-reactive memory CD4 T cells. While blocking ICOS did not diminish the expansion of preexisting memory CD4 T cells or the induction of allospecific effector T cells, it did inhibit recruitment of the activated memory and effector T cells into the graft. In addition, anti-ICOS mAb treatment in combination with DST/MR1 prevented help provided by memory CD4 T cells for production of donor-specific IgG antibody. These results demonstrate the potential efficacy of ICOS blockade in sensitized transplant patients and provide the foundation for rational use of ICOS blockade in combination with other graft-prolonging strategies.

Provision of antifungal immunity and concomitant alloantigen tolerization by conditioned dendritic cells in experimental hematopoietic transplantation

FoxP3(+) regulatory T (Treg) cells are important mediators of peripheral tolerance, and deficiency of this population is associated with autoimmune inflammation and onset of acute lethal graft-vs.-host disease in transplantation. Type I IFN-producing plasmacytoid dendritic cells (pDC) are implicated in the induction and maintenance of tolerance and contribute to engraftment facilitation and prevention of graft-vs.-host disease after allogeneic hematopoietic stem cells transplantation (HSCT). Because host DC function is impaired during the immediate period post-transplant, the administration of donor DC may be useful for the educational program of recovering T cells. Distinct DC subsets could be derived from bone marrow (murine) or peripheral CD14(+) cell (human) cultures in the presence of either GM-CSF/IL-4 (myeloid DC) or FLT3-ligand (mainly pDC). The ability of either DC subset to induce Th1/Treg cell priming against Aspergillus fumigatus as well as the relative contribution of murine DC subsets to antifungal priming upon adoptive transfer in hematopoietic transplanted mice with aspergillosis is not known. We found specialization and complementarity in priming and tolerization by the different DC subsets, with FL-DC fulfilling the requirement for (i) Th1/Treg antifungal priming; ii) tolerization toward alloantigens and (iii) diversion from alloantigen-specific to antigen-specific T cell responses in the presence of donor T lymphocytes. Interestingly, thymosin alpha1 (Talpha1), known to modulate human pDC functions trough TLR9, affects mobilization and tolerization of pDC by activating the indoleamine 2,3-dioxygenase-dependent pathway, and this resulted in Treg development and tolerization. Thus, transplantation tolerance and concomitant pathogen clearance could be achieved through the therapeutic induction of antigen-specific Treg cells via instructive immunotherapy with pathogen- or TLR-conditioned donor DC.

Allograft rejection mediated by memory T cells is resistant to regulation

Alloreactive memory T cells may be refractory to many of the tolerance-inducing strategies that are effective against naive T cells and thus present a significant barrier to long-term allograft survival. Because CD4(+)CD25(+) regulatory T cells (Tregs) are critical elements of many approaches to successful induction/maintenance of transplantation tolerance, we used MHC class I and II alloreactive TCR-transgenic models to explore the ability of antigen-specific Tregs to control antigen-specific memory T cell responses. Upon coadoptive transfer into RAG-1(-/-) mice, we found that Tregs effectively suppressed the ability of naive T cells to reject skin grafts, but neither antigen-unprimed nor antigen-primed Tregs suppressed rejection by memory T cells. Interestingly, different mechanisms appeared to be active in the ability of Tregs to control naive T cell-mediated graft rejection in the class II versus class I alloreactive models. In the former case, we observed decreased early expansion of effector cells in lymphoid tissue. In contrast, in the class I model, an effect of Tregs on early proliferation and expansion was not observed. However, at a late time point, significant differences in cell numbers were seen, suggesting effects on responding T cell survival. Overall, these data indicate that the relative resistance of both CD4(+) and CD8(+) alloreactive memory T cells to regulation may mediate resistance to tolerance induction seen in hosts with preexisting alloantigen-specific immunity and further indicate the multiplicity of mechanisms by which Tregs may control alloimmune responses in vivo.

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.

Transplantation tolerance: lessons from experimental rodent models

Immunological tolerance or functional unresponsiveness to a transplant is arguably the only approach that is likely to provide long-term graft survival without the problems associated with life-long global immunosuppression. Over the past 50 years, rodent models have become an invaluable tool for elucidating the mechanisms of tolerance to alloantigens. Importantly, rodent models can be adapted to ensure that they reflect more accurately the immune status of human transplant recipients. More recently, the development of genetically modified mice has enabled specific insights into the cellular and molecular mechanisms that play a key role in both the induction and maintenance of tolerance to be obtained and more complex questions to be addressed. This review highlights strategies designed to induce alloantigen specific immunological unresponsiveness leading to transplantation tolerance that have been developed through the use of experimental models.