Programmed death-1 (PD-1), an inhibitory receptor up-regulated on activated T cells, has been shown to play a critical immunoregulatory role in peripheral tolerance, but its role in alloimmune responses is poorly understood. Using a novel alloreactive TCR-transgenic model system, we examined the functions of this pathway in the regulation of alloreactive CD4⁺ T cell responses in vivo. PD-L1, but not PD-1 or PD-L2, blockade accelerated MHC class II-mismatched skin graft (bm12 (I-A^bm12) into B6 (I-A^b)) rejection in a similar manner to CTLA-4 blockade. In an adoptive transfer model system using the recently described anti-bm12 (ABM) TCR-transgenic mice directly reactive to I-A^bm12, PD-1 and PD-L1 blockade enhanced T cell proliferation early in the immune response. In contrast, at a later time point preceding accelerated allograft rejection, only PD-L1 blockade enhanced T cell proliferation. In addition, PD-L1 blockade enhanced alloreactive Th1 cell differentiation. Apoptosis of alloantigen-specific T cells was inhibited significantly by PD-L1 but not PD-1 blockade, indicating that PD-1 may not be the receptor for the apoptotic effect of the PD-L1-signaling pathway. Interestingly, the effect of PD-L1 blockade was dependent on the presence of CD4⁺CD25⁺ regulatory T cells in vivo. These data demonstrate a critical role for the PD-1 pathway, particularly PD-1/PD-L1 interactions, in the regulation of alloimmune responses in vivo.

BACKGROUND: Graft patency is the postulated mechanism for the benefits of coronary artery bypass grafting (CABG). However, systematic graft imaging assessment after CABG is rare, and there is a lack of contemporary data on the factors associated with graft failure and on the association between graft failure and clinical events after CABG. METHODS: We pooled individual patient data from randomized clinical trials with systematic CABG graft imaging to assess the incidence of graft failure and its association with clinical risk factors. The primary outcome was the composite of myocardial infarction or repeat revascularization occurring after CABG and before imaging. A 2-stage meta-analytic approach was used to evaluate the association between graft failure and the primary outcome. We also assessed the association between graft failure and myocardial infarction, repeat revascularization, or all-cause death occurring after imaging. RESULTS: Seven trials were included comprising 4413 patients (mean age, 64.4±9.1 years; 777 [17.6%] women; 3636 [82.4%] men) and 13 163 grafts (8740 saphenous vein grafts and 4423 arterial grafts). The median time to imaging was 1.02 years (interquartile range [IQR], 1.00–1.03). Graft failure occurred in 1487 (33.7%) patients and in 2190 (16.6%) grafts. Age (adjusted odds ratio [aOR], 1.08 [per 10-year increment] [95% CI, 1.01–1.15]; P =0.03), female sex (aOR, 1.27 [95% CI, 1.08–1.50]; P =0.004), and smoking (aOR, 1.20 [95% CI, 1.04–1.38]; P =0.01) were independently associated with graft failure, whereas statins were associated with a protective effect (aOR, 0.74 [95% CI, 0.63–0.88]; P <0.001). Graft failure was associated with an increased risk of myocardial infarction or repeat revascularization occurring between CABG and imaging assessment (8.0% in patients with graft failure versus 1.7% in patients without graft failure; aOR, 3.98 [95% CI, 3.54–4.47]; P <0.001). Graft failure was also associated with an increased risk of myocardial infarction or repeat revascularization occurring after imaging (7.8% versus 2.0%; aOR, 2.59 [95% CI, 1.86–3.62]; P <0.001). All-cause death after imaging occurred more frequently in patients with graft failure compared with patients without graft failure (11.0% versus 2.1%; aOR, 2.79 [95% CI, 2.01–3.89]; P <0.001). CONCLUSIONS: In contemporary practice, graft failure remains common among patients undergoing CABG and is strongly associated with adverse cardiac events.

CD4⁺CD25⁺ regulatory T cells (T_Regs) are critical for the acquisition of peripheral allograft tolerance. However, it is unclear whether T_Regs are capable of mediating alloantigen-specific suppressive effects and, hence, contributing to the specificity of the tolerant state. In the current report we have used the ABM TCR transgenic (Tg) system, a C57BL/6-derived strain in which CD4⁺ T cells directly recognize the allogeneic MHC-II molecule I-A^bm12, to assess the capacity of T_Regs to mediate allospecific effects. In these mice, 5–6% of Tg CD4⁺ T cells exhibit conventional markers of the T_Reg phenotype. ABM T_Regs are more effective than wild-type polyclonal T_Regs at suppressing effector immune responses directed against I-A^bm12 alloantigen both in vitro and in vivo. In contrast, they are incapable of suppressing responses directed against third-party alloantigens unless these are expressed in the same allograft as I-A^bm12. Taken together, our results indicate that in transplantation, T_Reg function is dependent on TCR stimulation, providing definitive evidence for their specificity in the regulation of alloimmune responses.

The inability to reproducibly induce robust and durable transplant tolerance using CD28-B7 pathway blockade is in part related to the persistence of alloreactive effector/memory CD8⁺ T cells that are less dependent on this pathway for their cellular activation. We studied the role of the novel T cell costimulatory pathway, CD27-CD70, in alloimmunity in the presence and absence of CD28-B7 signaling. CD70 blockade prolonged survival of fully mismatched vascularized cardiac allografts in wild-type murine recipients, and in CD28-deficient mice induced long-term survival while significantly preventing the development of chronic allograft vasculopathy. CD70 blockade had little effect on CD4⁺ T cell function but prevented CD8⁺ T cell-mediated rejection, inhibited the proliferation and activation of effector CD8⁺ T cells, and diminished the expansion of effector and memory CD8⁺ T cells in vivo. Thus, the CD27-CD70 pathway is critical for CD28-independent effector/memory CD8⁺ alloreactive T cell activation in vivo. These novel findings have important implications for the development of transplantation tolerance-inducing strategies in primates and humans, in which CD8⁺ T cell depletion is currently mandatory.