PF755 PD‐1 PATHWAY IS A KEY IMMUNOSUPPRESSIVE MECHANISM IN PATIENTS AFTER HAPLOIDENTICAL STEM CELL TRANSPLANTATION WITH TCR AB–DEPLETION
Authors: Popova N. , Drokov M. , Davydova Y. , Kapranov N. , Mikhaltsova E. , Koroleva O. , Vasilyeva V. , Konova Z. , Usikova E. , Dmitrova A. , Nareyko M. , Maslikova U. , Starikova O. , Dubnyak D. , Kazachenok A. , Muzalevskii Y. , Kamelskikh D. , Dvirnyk V. , Galtseva I. , Gaponova T. , Maschan M. , Kuzmina L. , Parovichnikova E. , Savchenko V. 
Published: June 2019

Background:Programmed cell death protein 1 (PD‐1) is a surface receptor expressed normally on distinct immune cells. It's well known PD‐1 pathway blockade is a negative immune regulatory mechanism that has detrimental effects on anti‐tumor immunity. Currently some studies have revealed different immune cells overexpressed PD‐1 as a predictable marker of leukemia relapse. On the other hand its assay after allo‐HSCT could be controversial due to alternative GVHD prophylaxis regimens that might affect PD‐1 expression, such as post‐transplant high dose Cyclophosphamide (PT‐Cy) or graft‐manipulated TCR αβ‐depletion are taken place if mismatched or haploidentical donors are used.Aims:To evaluate PD‐1 expression on bone marrow (BM) resident CD8+ memory T‐cell subsets in leukemia patients after allo‐HSCT and its input on immunosuppressive mechanisms of different GVHD prophylaxis regimens after allo‐HSCT.Methods:BM samples were collected from 56 leukemia patients with a median age of 33 years on day +30, +60 and +90 after allo‐HSCT. Detailed patients characteristics are presented in Table 1. Flow cytometry analysis was performed on BD FACS Canto II (Becton Dickinson, USA) to define CD8+ T‐memory subsets with PD‐1 expression: T‐naive and T‐stem cell memory (Tnv+Tscm) –CD45R0‐CCR7+CD28+; T‐central memory (Tcm) ‐ CD45R0+CCR7+CD28+; T‐transitional memory (Ttm) ‐ CD45R0+CCR7‐CD28+; T‐effector memory (Tem) ‐ CD45R0+CCR7‐CD28‐; T‐terminal effector (Tte) ‐ CD45R0‐CCR7‐CD28‐. Sysmex XE‐2100 was used to calculate absolute count of different CD8+ T‐ memory cell subsets with PD‐1. Mann–Whitney U test was used for nonparametric data analysis. A p‐value less than 0.05 was considered as significant.Table 1. Patients characteristics.Results:During all follow‐up period PD‐1 expression remains increased on all CD8+ T‐memory cell subsets after TCR αβ‐depletion compared to PT‐Cy–based and classical immunosuppressive regimen based on Cyclosporine+MMF. On day +30 we observe significantly higher expression of PD‐1 on Tnv+scm, Tcm, Ttm after TCR αβ‐depletion compared to PT‐Cy or classical regimen (p < 0.05). On day +60 PD‐1 is still overexpressed on Tnv+scm after TCR αβ‐depletion compared to PT‐Cy or classical regimen (p = 0.008). Moreover on day +60 and +90 PD‐1 expression is elevated on terminally differentiated CD8+ T cells (Ttm, Tem) after TCR αβ‐depletion comparing PT‐Cy or classical regimen (р<0.05). (Figure 1).imageSummary/Conclusion:According to our data PD‐1 is overexpressed on distinct subsets of BM resident CD8+ T‐memory cells after TCR αβ‐depletion during follow‐up period in first 3 months after allo‐HSCT. Since other immune signaling pathways are inhibited due to the other immunosupressive agents had been used, PD‐1 pathway seems to be a key mechanism of immunological tolerance after haploidentacal allo‐HSCT with TCR αβ‐depletion.image