*Purpose: PD-1 is a T cell co-inhibitory molecule involved in T cell exhaustion and antigen specific hypo-reactivity. Given the role of PD-1 signaling in the development of self-tolerance and tumor immune evasion, we sought to elucidate its contribution to engraftment of donor hematopoietic cells (HCs) and establishment of mixed chimerism-based operational tolerance in a nonhuman primate model.

*Methods: As shown in Figure 1A, a non-myeloablative, helical tomotherapy-based total lymphoid irradiation, ATG, and Belatacept conditioning regimen was applied to a 3-5 MHC antigen mismatched rhesus macaque kidney + HC transplant model. The maintenance immunosuppression regimen included MMF, sirolimus, and tacrolimus tapers. The experimental cohort consisted of 8 animals. Flow cytometry was utilized for cellular immunophenotyping and PD-1 analysis. Mixed lymphocyte reactions (MLRs) were performed in the presence of a monoclonal anti-PD-1 blocking agent for in vitro functional assessment.

*Results: Mixed chimerism was achieved in 37.5% (3/8) of recipients. None of the chimeric animals developed evidence of alloreactivity. In contrast, 4 of 5 non-chimeric macaques succumbed to antibody- and cellular-mediated rejection. There was an upregulation of PD-1 expression on the surface of emerging host CD8+ T cells in chimeric animals, represented by a 4-fold increase in percentage of PD-1^hi cells compared to non-chimeric recipients at 30 days post-infusion (Figure 1B; P=0.01). This effect was driven by a 4.5-fold differential of PD-1^hi expression among CD8+ T effector memory cells, which was not observed in the central memory or naive subsets (Figure 1C; P=0.02). Importantly, CD8+ T cells were the predominant phenotype among the recovering immune system in both groups (P=0.001). Conversely the CD4+ phenotype, which represented the minority of circulating T cells, exhibited a similar relative frequency of PD-1^hi expression irrespective of chimerism (Figure 1B). Furthermore, a 46% reduction in CD8+ PD-1^hi expression was observed during episodes of acute rejection in the non-chimeric cohort (P=0.01). These findings coincided with in vitro studies utilizing PD-1 blockade in a post-induction allogenic MLR, which yielded a 3-fold increase in CD8+ T cell proliferation and gamma interferon production compared to non-blockade controls.

*Conclusions: PD-1 expression by CD8+ T cells plays an important role in controlling alloreactivity during engraftment of HCs and homeostatic recovery, thus contributing to the development of mixed-chimerism based operational tolerance.

« Back to 2022 American Transplant Congress