Peripheral mechanisms of self-tolerance often depend on the quiescent state of the immune system. To what degree such mechanisms can be engaged in the enhancement of allograft survival is unclear. To examine the role of the PD-1 pathway in the maintenance of graft survival following blockade of costimulatory pathways, we used a single-Ag mismatch model of graft rejection where we could track the donor-specific cells as they developed endogenously and emerged from the thymus. We found that graft-specific T cells arising under physiologic developmental conditions at low frequency were actively deleted at the time of transplantation under combined CD28/CD40L blockade. However, this deletion was incomplete, and donor-specific cells that failed to undergo deletion up-regulated expression of PD-1. Furthermore, blockade of PD-1 signaling on these cells via in vivo treatment with anti-PD-1 mAb resulted in rapid expansion of donor-specific T cells and graft loss. These results suggest that the PD-1 pathway was engaged in the continued regulation of the low-frequency graft-specific immune response and thus in maintenance of graft survival.
Following transplantation of tissues from allogeneic hosts, large numbers of recipient T cells are activated, clonally expand, and differentiate into effectors, which mediate rapid rejection. To generate effective responses, T cells must receive signals delivered via the TCR but also require costimulatory signals (1). The concept of blocking costimulatory pathways to prevent rejection or promote tolerance has captivated the interest of the transplant research community and shows great promise in clinical trials. Given the explosion of knowledge of the many related costimulatory receptors and ligands, there are many new opportunities for investigation. Yet, two of the originally discovered pathways, CD28 and CD40, remain among the most pivotal identified for the execution of rejection. Short-term blockade of the CD28 pathway with the CTLA4-Ig fusion protein and/or the CD40 pathway with mAbs against CD154 or CD40 can potently inhibit rejection and induce long-term graft acceptance in many rodent and nonhuman primate transplant models (2, 3, 4, 5, 6, 7, 8, 9, 10, 11). Although the mechanisms by which short-term blockade of these pathways results in peripheral tolerance remain incompletely defined, there is evidence that both peripheral deletion of donor-reactive T cells and the development of regulatory mechanisms can be involved. However, relatively few studies have directly tracked the functional status and fate of donor-reactive T cells during tolerance induction in vivo.
Recent studies in viral infection and transplant models have shown that PD-1 (CD279), another member of the CD28 family of costimulatory molecules, can play an important role in peripheral tolerance induction and the exhaustion of Ag-specific T cells. PD-1 is a negative regulator of T cell function that is inducibly expressed on CD4+ T cells, CD8+ T cells, NK cells, B cells, and activated monocytes (12). In T cells, its expression is up-regulated early following TCR signaling but it has been shown by a number of investigators to be expressed on tolerant and exhausted cells in models of autoimmunity and chronic viral infection, respectively (13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23). Ligation by either PD-L1 or PD-L2 transduces a negative signal when transmitted simultaneously with TCR signaling, but does not produce this effect in the absence of TCR engagement (12). Therefore, PD-1 serves to down-modulate T cell activation only in the presence of Ag. This negative regulatory function is accomplished through the cytoplasmic domain of the molecule, which recruits intracellular phosphatases SHP-1 and SHP-2 (12). The recruitment of these phosphatases to the membrane leads to the dephosphorylation of several key signaling intermediates and thus the attenuation of TCR-mediated signals.
The role of PD-1 as a negative regulator of T cell responses has been investigated in models of chronic viral infection, autoimmunity, and, most recently, transplantation. Studies from chronic lymphocytic choriomeningitis virus infection in mice revealed that functionally exhausted Ag-specific T cells expressed high levels of surface PD-1, and that inhibiting signaling through this receptor reversed the exhausted phenotype and restored production of IFN-γ and cytolytic function (13). Moreover, blockade of PD-1 or PD-L1 in vivo not only restored T cell function but also led to viral clearance. Recent reports examining HIV-, hepatitis C virus-, and hepatitis B virus-infected human patients revealed high levels of PD-1 expression on Ag-specific T cells during the course of chronic viral infection (15, 16, 17, 18, 19) and that blockade of PD-1 could restore the function of viral reactive T cells in vitro (16, 17, 18), suggesting that PD-1 may be a potential therapeutic target for chronic viral infections. Studies in murine autoimmune models have provided further evidence that PD-1 is a critical component of peripheral tolerance. For example, genetic silencing or blockade of the PD-1/PD-L1 pathway was shown to accelerate progression and severity of both autoimmune diabetes and experimental autoimmune encephalomyelitis (21, 22, 24), and to reverse established tolerance in the NOD model of type 1 diabetes by promoting expansion and cytokine production by Ag-specific T cells (20).
Recent studies have examined the role of the PD-1 pathway in transplantation tolerance, primarily by blocking the pathway and demonstrating acceleration of rejection of murine skin or cardiac allografts or worsening of graft-vs-host disease (25, 26, 27, 28). The generation of a PD-L1 fusion protein allowed supraphysiological stimulation of PD-1-expressing T cells and has been shown to promote allograft survival in murine models of cardiac, islet, and corneal transplantation (29, 30, 31). Recently, Najafian and colleagues (27) demonstrated that, although dispensable for the induction of mixed allogeneic chimerism, the PD-1 pathway is critical for the maintenance of peripheral tolerance in a murine cardiac allograft model (32). However, precise analysis of the level and kinetics of PD-1 expression on donor-reactive T cells, and their functional phenotype before and after PD-1 blockade, has been hampered by the lack of suitable models in which graft-specific T cells can be tracked throughout the course of the anti-donor immune response. An important study by Sayegh and colleagues (27) made use of CD4+ TCR transgenic (tg)2 cells specific for I-Abm12 and thus was able to demonstrate increased Ag-specific T cell proliferation, decreased apoptosis, and Th1 differentiation following PD-L1 blockade. This study highlights the distinct advantages and opportunities in using TCR tg model systems to explore the impact of various costimulatory pathways, be they positive or negative, on transplantation tolerance.
To explore the role of PD-1-dependent mechanisms in the maintenance of transplantation tolerance, we designed a model system in which readily trackable numbers of donor-reactive T cells were continuously produced in vivo. This system afforded the number, phenotype, and functional properties of graft-specific CD8+ T cells to be systematically tracked over the course of several weeks during transplant acceptance or rejection. As in our previous studies using an adoptive transfer approach (33, 34), we observed that when the initial precursor frequency of donor-reactive T cells is high, recipient mice were refractory to tolerance induction using transient CD28/CD40 blockade. However, when the initial precursor frequency of donor-reactive T cells was low, long-term graft acceptance was uniformly achieved. Moreover, we observed that while transient costimulation blockade led to the deletion of a majority of graft-specific T cells when stimulated at low frequency, a population of PD-1+ donor-reactive T cells persisted in recipients bearing surviving donor skin grafts. Importantly, interruption of the PD-1 pathway with either anti-PD-1 or anti-PD-L1 mAbs led to a rapid expansion of graft-specific T cells concurrent with their re-acquisition of effector function and the precipitation of graft rejection. Thus, our data demonstrate the synergism of blocking positive costimulatory pathways and signaling through negative regulatory pathways in maintaining allograft survival in the face of newly emerging low-frequency thymic emigrants. We conclude that the PD-1 pathway plays a pivotal role in maintaining peripheral tolerance by actively suppressing the proliferation and effector function of low frequencies of remaining donor-specific CD8+ T cells.
Materials and Methods
Adult 6- to 8-wk-old B6.SJL-PtprcaPep3b/BoyJ (CD45.1+) mice were purchased from The Jackson Laboratory. OT-I Rag−/− TCR tg mice were purchased from Jackson ImmonResearch Laboratories and were bred to Thy1.1+ B6 mice at Emory University. mOVA.B6 mice (35) were a gift of Dr. Marc Jenkins (University of Minnesota, Minneapolis, MN) and were maintained at Emory University Division of Animal Resources. All experiments were conducted in accordance with institutional oversight and guidelines for animal care and use.
Full thickness skin grafts (∼1 cm2) were transplanted on the dorsal thorax of recipient mice and secured with a Band-Aid (Johnson & Johnson) for 5 days. Graft survival was then followed by daily visual inspection. Rejection was defined as the complete loss of viable epidermal graft tissue.
OT-I B6 chimera generation
OT-I B6 chimeras were generated by pretreating recipient mice on day −1 with 15–20 mg/kg busulfan (Busulfex; Orphan Medical). On day 0, bone marrow from syngeneic and OT-I (Rag−/−, Thy1.1+, CD45.2+) TCR tg donors was harvested and mixed at a ratio of 4:1 or 5:1 before i.v. injection of 15–20 × 106 cells/recipient. A single 500-mg dose of each hamster anti-murine CD40L Ab (MR-1) and human CTLA4.Ig were included at day 0 to prevent anti-CD45.2 responses (36). OT-I donor chimerism (Thy1.1+, CD8+, Vα2+, and CD45.2+) was monitored in peripheral blood.
Ab was administered i.p. Where indicated, recipients of skin grafts received treatment with 500 μg each of hamster anti-mouse CD40L mAb (MR-1; Bioexpress) and human CTLA-4 Ig (Bristol-Meyers Squibb) administered i.p. on the day of transplantation (day 0) as well as on posttransplant days 2, 4, and 6. Rat anti-mouse PD-L1 (10F.9G2) or rat IgG2b isotype control was given at 200 mg on days 0, 3, 6, and 9 (13). A total of 500 mg of hamster anti-mouse aPD-1 (J43) (37) or control hamster Ig on day 0, then 250 mg on days 2, 4, 6, and 8. Anti-CD25 (PC.61) (Bioexpress) was administered at 500 μg/mouse/day on days 0, 2, 4, and 6.
Surface staining, intracellular staining, and CFSE analysis were conducted as previously described (38) using murine conjugated mAbs CD8 allophycocyanin/Alexa405 (5H10) (Caltag Laboratories), Thy1.1 PerCP, CD45.2 FITC/allophycocyanin-Alexa750, Vα2 PE, PD-1 FITC (J43), CTLA-4-PE TNF-α PE (MP6-XT22), and IFN-γ PE-Cy7/PE (XMG1.2) (BD Pharmingen). BrdU staining was conducted with a kit per manufacturer’s instructions (BD Pharmingen). Data acquisition was performed on an LSRII flow cytometer (BD Biosciences) and analyzed using FlowJo software (Treestar).
BrdU labeling for in vivo proliferation
Following PD-1 blockade, mice were pulsed with 1 mg i.p. of BrdU once per day for 2 days, then sacrificed on the third day. BrdU incorporation was detected using the Allophycocyanin-BrdU Flow Kit according to the manufacturer’s protocol (BD Biosciences).
Statistical significance for skin graft survival was determined using the log-rank test. All other statistical comparisons were performed using the Mann-Whitney nonparametric test in Graphpad Prism software.
Transient blockade of the CD28 and CD40 pathway induces long-term skin graft acceptance when the precursor frequency of donor-reactive T cells is low
To study the fate and function of donor-reactive T cells in settings of transplant acceptance or rejection, we used a model in which skin from tg B6 donor mice that ubiquitously express membrane-bound chicken OVA (mOVA) under the control of the β-actin promoter was transplanted to wild-type B6 recipients. mOVA skin grafts transplanted onto B6 mice are readily rejected in a CD8+ and CD4+ T cell-dependent fashion (35). In our experiments, untreated recipients rapidly rejected their grafts (MST 18 days). Short-term blockade of the CD28 pathway via CTLA4-Ig had a minimal effect on skin graft survival (MST 22 days) while anti-CD154 significantly extended survival (MST 189 days). As in other models (39), combined blockade of these pathways induced long-term graft survival (MST > 200 days) (Fig. 1 A).
We considered several nonmutually exclusive mechanisms by which donor-reactive T cells might be controlled. These included peripheral deletion, functional inactivation (via anergy or exhaustion), peripheral regulation, and, finally, a loss of graft immunogenicity resulting in immunologic ignorance. We performed two sets of experiments to address the issue of ignorance. First, we adoptively transferred CFSE-labeled OVA-specific CD8+ OT-I tg T cells into mice with surviving mOVA skin grafts (>60 days) after short-term costimulation blockade. OT-I T cells are specific for OVA257–264 in the context of Kb, an epitope that has been shown to be constitutively presented by mOVA skin (35). We assessed T cell division (CFSE dilution) in the draining lymph node (LN), the contralateral node, and in the spleen 48 h after transfer. Graft-specific T cells underwent a burst of proliferation in the draining node and spleen but not in the nondraining node (Fig. 1,B), demonstrating that donor Ags are presented in an immunostimulatory form in mice bearing surviving grafts. As a second evaluation of the immunogenicity of the long-term surviving grafts, we performed similar adoptive transfer experiments, but assessed the survival of the grafts after transfer of OT-I T cells. Mice with long-term surviving mOVA grafts received either 5 × 106 OT-I T cells, a similar number of P14 TCR-Tg T cells (which recognize a peptide derived from the lymphocytic choriomeningitis virus), or no T cells. Although grafts in mice that received no T cells or irrelevant P14 T cells continued to survive, the mice that received OT-I T cells rapidly rejected their mOVA grafts (Fig. 1 C). These results provided additional evidence that graft survival was not primarily due to immunologic ignorance.
Because the precursor frequency of T cells specific for OVA in wild-type mice is very low (<1/106) and essentially undetectable under tolerogenic conditions, we developed a model in which physiologic frequencies of donor-reactive T cells could be tracked over time. To do this, we created bone marrow chimeric mice. After conditioning with minimally myelosuppressive doses of busulfan, Thy1.2+, CD45.1+ B6 mice received mixed bone marrow from wild-type B6 donors and B6, RAG-deficient, Thy1.1+ CD45.2+ OT-I TCR tg donors (hereafter referred to as OT-I chimeras) to achieve consistent and stable levels of OT-I T cells allowing us to specifically target levels similar to observed allo-reactive frequencies (0.1–10%) (35, 40, 41). The use of the Thy1.1 and CD45.1 congenic markers as well as anti-mouse Vα2 or Kb/SIINFEKL tetramer permitted us four independent cell surface markers to precisely track graft-specific T cells at low frequencies. Six to eight weeks after chimerism induction, the levels of peripheral OT-I T cells stabilized (Fig. 2,A), and at this time mOVA skin grafts were applied. Based on preliminary experiments, mice were grouped according to the frequency of OT-I T cells in peripheral blood. The groups contained mice in which OT-I represented >5, 3–5, or 0.1–3% of the total CD8+ T cell compartment. As expected, in untreated recipients from all three groups the grafts were promptly rejected (not shown, MST 14 days). However, in recipients that received transient blockade of the CD28 and CD40 pathways, graft survival was influenced by the initial frequency of donor-reactive T cells. Recipients harboring a high initial frequency of OT-I T cells (>5% of CD8+ compartment) consistently rejected mOVA skin grafts despite costimulation blockade (MST 14 days). In contrast, mOVA grafts transplanted to animals with lower frequencies (0.1–3%) of OT-I T cells went on to show long-term graft survival after transient costimulation blockade (MST > 200 days). Animals bearing between 3 and 5% OT-I chimerism demonstrated an intermediate phenotype in which the grafts were either rejected within the first 3 wk or went on to long-term survival (Fig. 2 B). These results support previous findings suggesting that graft-specific precursor frequency is an important predictor of costimulation blockade resistant rejection (33, 34).
Long-term graft acceptance is associated with both deletional and nondeletional peripheral mechanisms
We next investigated the fate of graft-specific OT-I T cells under conditions of persistent graft survival. OT-I chimeras that received a syngeneic B6 skin graft, or that received no skin graft (not shown), exhibited no change in peripheral OT-I frequencies. Recipients with high initial frequencies of OT-I T cells rejected mOVA grafts despite costimulation blockade and did not show evidence of OT-I deletion. In contrast, when OT-I chimeras with initial precursor frequencies of OT-I T cells in the 1–3% range were treated with costimulation blockade and received mOVA skin grafts, the number of OT-I T cells in peripheral blood significantly declined over the first 3–4 wk after transplant but were not completely deleted (Fig. 2,C). These findings are consistent with previous observations that transient costimulation blockade can promote peripheral deletion (39, 42, 43, 44) over a period of weeks (38, 45). However, by using newer, more sensitive polychromatic flow cytometric techniques, we observed that a residual population of OT-I T cells could readily be detected (0.35 ± 0.06% of CD8+ compartment) >80 days posttransplant (Fig. 2,C). Overall donor bone marrow chimerism in these recipients was unchanged, as the level of non-OT-I CD45.2+ (CD8−) donor-derived cells was maintained at ∼1–2% for the term of the experiment, similar to control treatment groups (Fig. 2 D). Therefore, the observed reduction in peripheral OT-I T cells was not due to waning levels of overall donor bone marrow chimerism. Thus, these findings suggest that ongoing peripheral deletion is one mechanism for controlling graft-specific CD8+ T cells.
PD-1 expression is associated with maintained graft survival and functional impairment of donor-reactive CD8+ T cells in low frequency OT-I chimeras treated with costimulation blockade
Given that recipients with a low frequency of donor-reactive T cell precursors still bore surviving grafts as well as significant numbers of donor-reactive T cells in periphery, we sought to determine the mechanisms controlling the function of this residual population and preventing rejection. First, we performed a phenotypic analysis of the OT-I T cells in recipients possessing long-term surviving mOVA grafts to screen for potentially up-regulated inhibitory molecules known to be associated with exhaustion or anergy in T cells stimulated at initial low frequency, including CTLA-4, B and T lymphocyte attenuator, and PD-1. For comparison, we analyzed OT-I T cells in mice that received a syngeneic B6 skin graft or in mice harboring a high frequency of OT-I T cells that had rejected an mOVA graft despite costimulation blockade (Fig. 3,A). We found a marked increase in the cell surface expression of PD-1 on residual OT-I T cells stimulated at an initial low frequency in the presence of a surviving mOVA graft relative to controls at day 45 posttransplant (Fig. 3,B). These cells also stained positive for Vα2 and with the Kb-SIINFEKL tetramer, demonstrating that the residual T cells are in fact donor-reactive and that TCR expression is not overtly altered (not shown). Additional phenotypic analysis revealed no evidence for increased FoxP3 expression or other regulatory markers such as glucocorticoid-induced tumor necrosis factor receptor, CD25 (not shown), or CTLA-4 (Fig. 3 C).
In vivo PD-1 blockade leads to graft loss and expansion of the donor-reactive T cell population
Because the OT-I chimera model afforded us the opportunity to identify and track graft-specific T cells at low but biologically and clinically relevant frequencies, we were able to assess the functionality of the PD-1-expressing donor-reactive T cells. To examine whether the PD-1 pathway plays an important role in maintaining graft survival, we administered mAbs to interrupt the PD-1 pathway in vivo. OT-I chimeric mice bearing surviving mOVA skin grafts for greater than 3 mo after costimulation blockade were given either a blocking anti-PD-1 (J43) or anti-PD-L1 (10F.9G2) (13, 37, 46). In vivo blockade of the PD-1 receptor or its ligand PD-L1 induced rapid graft rejection in >75% of the OT-I chimeric mice (Fig. 4,A). In contrast, recipients treated with isotype control Abs showed continued graft survival (Fig. 4 A). Furthermore, recipients treated with a depleting anti-CD25 mAb (PC.61) did not undergo rejection of their grafts, which suggests that regulation via CD25+ regulatory T cells (Treg) is not a dominant mechanism in this model of peripheral tolerance.
In addition to its profound effect on graft survival, blockade of the PD-1 pathway led to a change in the functional phenotype of donor-reactive T cells. Treatment with anti-PD-L1 resulted in an increased frequency of donor-reactive T cells in the peripheral blood of recipients at the time of graft rejection (Fig. 4,B). At day 97, after graft rejection had occurred, Thy1.1+CD45.2+CD8+ T cells were analyzed for their surface expression of PD-1. Results showed that treatment with anti-PD-L1, but not isotype control Ab, resulted in the loss of surface expression of PD-1 (Fig. 4 C), suggesting that these cells may have undergone a reversion of their exhausted status.
Tolerized donor-reactive T cells regain proliferative capacity and effector function following PD-1 blockade
To determine whether donor-reactive T cells treated with anti-PD-L1 exhibited increased proliferative potential and effector function on a per-cell basis, we analyzed in vivo cell cycling and ex vivo cytokine production by donor-reactive T cells. First, we observed that following PD-1 blockade, but not after treatment with an isotype control, the graft-specific T cell population exhibited a significant increase in the frequency of BrdU+ cells (Fig. 5,A), demonstrating the re-acquisition of proliferative potential of these cells following the cessation of ligation of PD-1. Secondly, ex vivo restimulation with cognate peptide showed that >50% of donor-reactive OT-I T cells made IFN-γ following in vivo PD-L1 blockade, relative to <15% of donor-reactive cells from mice that received isotype control (Fig. 5,B). Analysis of absolute numbers of these IFN-γ-secreting effectors revealed a >5-fold increase in graft-specific effector cells 8 days after PD-1 blockade over isotype control (Fig. 5 B). In conclusion, these results support the hypothesis that PD-1 expression on graft-specific T cells leads to repressed proliferative capacity as well as effector function.
Previous studies have defined a critical role for donor-reactive T cell precursor frequency in susceptibility to tolerance induction via blockade of the CD28 and CD40 costimulatory pathways (33, 34). However, the mechanisms by which donor-reactive T cells stimulated at low frequency become susceptible to tolerance induction are not well understood. To explore the mechanisms underlying the induction and maintenance of transplantation tolerance at low frequency, we used a model system in which tg donor-reactive T cells were continuously produced in vivo. This system allowed us to systematically track the number, phenotype, and functional properties of endogenously generated graft-specific CD8+ T cells over the course of graft acceptance or rejection to more closely investigate the mechanisms of tolerance induction at low precursor frequency. Using this system, we confirmed our previous observation that when the initial precursor frequency of donor-reactive T cells was high (>4%), recipient mice were refractory to tolerance induction using transient CD28/CD40 blockade. However, when the initial precursor frequency of donor-reactive T cells was low (<3%), long-term graft acceptance was achieved.
The observed long-term graft survival in low frequency OT-I chimeras was somewhat unexpected given the transient course of Ab therapy and the expected influx of newly emergent OT-I T cells into the periphery, which are derived from the bone marrow of OT-I chimeric mice. This is in contrast to the situation observed in adoptive transfer experiments, wherein all of the donor-reactive T cells are present and can be tolerized during the initial Ab-administration period. Iwakoshi et al. (43) used a similar TCR tg synchimeric system with an anti-Kb TCR tg (KB5) and found that as anti-CD40L Ab levels waned graft rejection ensued, suggesting that persistent treatment was necessary for graft acceptance in this system. Interestingly, the level of TCR tg T cell chimerism in this model was approximately twice the level observed in our low frequency OT-I chimeras, further suggesting that low donor-reactive precursor frequency is critical for the ability of peripheral tolerance to be maintained in the absence of active costimulation blockade. It should be noted that the half-life of hamster anti-CD154 (MR-1) has been calculated as ∼12 days (47, 48) and the human Ig-fusion protein CTLA-4.Ig is estimated to be less than 4 days in mice (49, 50). This is clinically relevant in that patients that are weaned off of immunosuppression often experience episodes of acute rejection. Our results suggest that patients with organs with a high degree of MHC matching, and thus lower precursor frequencies of allo-reactive T cells potentially emerging from the thymus, may be more likely to experience continued graft survival via mechanisms of peripheral tolerance (such as increased PD-1 expression) following discontinuation of immunosuppressive therapy.
The mechanisms underlying the observation that low-frequency OT-I chimeras could be tolerized by CD28/CD40 pathway blockade were 2-fold: first, transient costimulation blockade led to the deletion of a majority of graft-specific T cells when stimulated at low frequency, and second, a detectable population of functionally quiescent PD-1+ donor-reactive T cells persisted in recipients bearing surviving donor skin grafts. The observed deletion of graft-specific cells stimulated at low frequency in the presence of costimulation blockade is consistent with our previous report demonstrating that although low-frequency donor-reactive T cells were capable of dividing in the presence of costimulation blockade, they exhibited decreased accumulation and increased expression of markers of apoptosis, suggestive of enhanced cell death under these conditions (33). We speculate that low-frequency T cells may exhibit increased death rates in the presence of costimulatory blockade because they may be required to undergo more rounds of division on a per cell basis to achieve a threshold quantity of effectors necessary to precipitate graft rejection (33).
We addressed the role of classical Tregs in maintaining tolerance in this model by depleting CD25+ cells from the periphery (Fig. 4,A) and by adoptively transferring naive OT-I T cells into recipients with long-term surviving grafts to test for the presence of functional regulation (Fig. 1 C). Evidence of regulation was not observed in either of these experiments. However, potential caveats include the fact that a number of CD25− Trge populations have recently been described (51, 52); thus, the possibility that both CD25-negative Tregs and PD-1-dependent mechanisms are required for tolerance induction cannot be ruled out. Interestingly, Najafian and colleagues (26) recently described a population of CD8+ PD-1+ Treg that were capable of inhibiting CD4+ but not CD8+ allospecific responses (52). These cells required PD-1 for their suppressive function. Although we did not observe suppression of naive CD8+ T cells in the adoptive transfer model, the possibility remains that the CD8+ PD-1+ cells may possess regulatory function under different conditions or toward other cell populations.
As stated above, the second mechanism we observed underlying the survival of skin grafts in recipients with a low frequency of donor-reactive T cells was a detectable population of functionally inactive PD-1+ donor-reactive T cells that persisted in recipients bearing surviving donor skin grafts. The up-regulation of PD-1 on the surface of Ag-specific T cells is purported to be linked to the degree of Ag exposure of the cells (13, 22). Thus, we speculate that cells stimulated at low frequency exhibit increased PD-1 expression due to increased exposure to donor Ag:MHC complexes. This increased antigenic exposure may arise from the fact that low frequency donor-reactive T cells would encounter reduced competition for Ag. Indeed, we observed that after rejection occurred in recipients treated with anti-PD-L1 and Ag had been eliminated, surface PD-1 expression was down-regulated on the OT-I population. This finding further suggests that PD-1 expression is exquisitely dependent on the presence of Ag, as has been suggested in studies in models of autoimmunity and viral pathology (13, 22).
The ability to track donor-reactive T cells in this model allowed us to further demonstrate that interruption of the PD-1 pathway with either anti-PD-1 or anti-PD-L1 mAbs not only precipitated graft loss but also led to a rapid expansion of the graft-specific T cell population concurrent with their re-acquisition of effector function. These data suggest that expression of PD-1 is critically important for the attenuation of signals through the TCR that would otherwise allow these cells to respond once the costimulation blockade had waned. Our results suggest that perturbation of the provision of PD-1 signals allows the T cell to respond, presumably with the provision of sufficient costimulation. Therefore, our data highlight the ability of blocking positive costimulatory pathways and engaging negative regulatory pathways to synergize in maintaining allograft survival. We conclude that the PD-1 pathway plays a pivotal role in maintaining peripheral tolerance by actively suppressing the proliferation and effector function of low frequencies of remaining donor-specific CD8+ T cells.
Based on the data presented here, we conclude that PD-1-dependent cell intrinsic mechanisms of peripheral tolerance are indeed active in maintaining allograft survival in much the same way T cells specific for nominal self-Ags are protected to prevent autoimmunity, and virus-specific T cells can be silenced to limit immunopathology in the setting of chronic viral infection. Recent work has suggested that blockade of PD-1 could be beneficial in control of persistent pathogens; however, this and other work in autoimmunity models suggest caution before clinical trials are begun. Clinically, for transplant recipients, monitoring of PD-1 expression could be a biomarker for a tolerogenic state. Conceivably, this could facilitate studies of treatment withdrawal for patients expressing high levels of PD-1 on donor-reactive peripheral T cells. Additionally, reagents designed to specifically engage this negative regulatory pathway both peri-operatively as well as in the maintenance phase after transplantation might aid in promoting graft survival.
The authors have no financial conflict of interest.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Abbreviations used in this paper: tg, transgenic; mOVA, membrane-bound chicken OVA; Treg, regulatory T cell; LN, lymph node.