ALBERT

Description: Abstract 1773 T cell exhaustion is a state of T cell dysfunction in response to chronic antigen exposure, marked by impaired effector function and the continued expression of inhibitory receptors such as Programmed Death 1 (PD-1) (Wherry EJ. Nat Immunol. 2011 Jun;12(6):492–9.). Because tumour growth in chronic lymphocytic leukaemia (CLL) occurs over a long period of time, we hypothesized that the continued exposure of T cells to a CLL-derived antigen could also lead to a state of T cell exhaustion. We therefore investigated whether T cell exhaustion is induced in CLL by using the Eμ-TCL1 transgenic (tcl1tg) tumour transfer mouse model for this disease (Hofbauer JP, et al. Leukemia. 2011 Sep;25(9):1452-8) and by analyzing primary samples from CLL patients. We found that the number of PD-1+ T cells was increased in both CD4+ and CD8+ populations and in all lymphoid compartments examined of the Eμ-TCL1 transgenic (tcl1tg) tumour recipient mice, but not in recipient mice receiving wildtype (WT) splenocytes showing that leukemic mice have an increased number of T cells displaying an exhausted phenotype that is induced by the presence of CLL cells. We next assessed the expression of the ligands for PD-1 on the surface of murine CLL cells. Peripheral CLL tumour cells showed only a modest increase in PD-L1 expression as compared to WT B cells. However, lymph node and spleen residing tumour cells showed a marked increase in PD-L1 expression, which suggests a microenvironment-induced upregulation of PD-L1 on tumour cells, e.g. by their close contact to accessory cells. To validate our results on primary human CLL samples, we collected peripheral blood from 89 unselected CLL patients and 18 healthy donors and observed an increase in surface expression of PD-1 on the CD4+ and CD8+ T cell populations. While the percentage of PD-1+ CD4+ T cells in chemonaive patients was comparable to healthy donors, chemotherapy drastically increased the number of PD-1-expressing CD4+ T cells (63.81% ±19.75 vs 35.70% ±19.22; p