Poster Session I: Stem cell transplantation - Clinical
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.
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.
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.
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 (p<0.05). (Figure 1).
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.