LAUSR

Background Heart transplantation is the only definitive therapeutic intervention for end-stage heart failure. The complex mechanisms regulating the alloresponse are not entirely understood. PD-L1 is a co-inhibitory molecule able to induce T cell anergy. We hypothesize that endothelial PD-L1 modulates alloresponses in heart transplantation. Methods Using multiparameter flow cytometry we characterized stromal cells and leukocytes in endomyocardial biopsies (EMB) and peripheral blood of patients post orthotopic heart transplantation (n=21) ISHLT grade 0-1R. Controls were autopsy specimens without cardiac pathologies. Results Most of the patients (n=17) had decreased endothelial PD-L1 expression as compared to controls, 2 patients had preserved endothelial PD-L1, and 2 had over-expression. PD-L1 expression was independent of the time from transplant, immunosuppressive regimen and concentration of calcineurin inhibitor. The percentage of infiltrating CD8 + lymphocytes had an inverse exponential correlation with endothelial PD-L1 expression (r 2 =0.64, Fig 1A). We categorized patients in 2 groups: those with previous ISHLT grade 2R cellular rejection (n=10) and those with stable 0-1R scores since transplantation. A Receiver Operating Characteristic (ROC) analysis showed that the proportion of PD-L1 + endothelial cells could significantly detect history of 2R rejection, as ≤16.25% PD-L1 + endothelial cells retrospectively discriminated patients with history of 2R rejection episodes with 90% sensitivity, 78% specificity and 3.3 likelihood ratio (Fig 1B). Furthermore, the proportion of exhausted CD8 T cells in peripheral blood (CD45RA neg PD1 + ) was proportional to heart endothelial PD-L1 expression (p 2 =0.45) and inverse exponentially correlated with graft infiltrating CD8 T cells (r 2 =0.52). Conclusions Critical loss of graft endothelial PD-L1 is strongly associated with history of moderate-severe acute rejection episodes. These findings suggest that endothelial PD-L1 and its peripheral correlate, exhausted CD8 T cells, might become a useful biomarker in heart transplantation that could allow profiling of patients and personalize therapeutic and follow-up strategies.