PD-1 Expression On Total and CMV-Specific T Cells In Early Post Transplant Is Associated With Donor Source, T Cell Maturation Profile, and Effectiveness Of CMV Control

T cell inhibitory receptors upregulate in parallel with activating receptors to prevent overwhelming immune response. Excessive or persistent inhibitory receptor expression on T cells, however, has been associated with several pathologic states, including chronic viral infection and malignancy. Programmed death-1 (PD-1) is among the most well studied inhibitory receptors, and PD-1 inhibition has resulted in regression of solid tumor malignancies and improved control of HIV infection.

To examine the role of PD-1 in T cell function post allotransplant we examined PD-1 expression on total and CMV antigen specific T cells on samples collected from 43 patients between 55 and 85 days post cord blood transplant (CBT) (n=13),matched unrelated donor transplant (MURD) (n=12), matched related donor transplant (MRD) (n=18) and compared results to normal adult peripheral blood controls (PBMCs) (n=22), and cord blood controls (CB) (n=11). PD-1 expression was significantly upregulated (p<0.05) on CD4+ T cells following CBT and MURD transplantation as compared to PBMCs and on all donor sources as compared to normal CB. On CD8+ T cells, PD-1 expression was significantly upregulated (p<0.05) following CB and MURD transplants as compared to PBMC. For each donor source, there was no difference in PD1 expression on CD4+ or CD8+ T cells based on conditioning intensity, GVHD prophylaxis, use of prednisone at the time the sample was drawn, or degree of T cell chimerism. Naïve CD4+ T cells were a statistically smaller proportion of total T cells following CB transplants as compared to PBMC and central memory (CM) cells were a statistically greater proportion of total T cells following CB and MURD transplant compared to PBMC. Effector memory terminally differentiated (EMTD)s were a statistically greater portion of total T cells on MRD transplants as compared to CB.

We next assessed PD-1 expression on CMV specific CD8+ T cells from HLA A2 patients using a CMVpp65 peptide (NLVPMVATV) specific A2 tetramer. Among 15 CMV positive patients assessed between 55 and 85 days post-transplant, tetramer+, CD8+ T cells were detectable in 10 patients. In patients who effectively controlled CMV clinically, defined as no reactivation (n=12) or reactivation controlled within 1 week of preemptive therapy (n=2), tetramer+, CD8+ T cell populations consisted primarily of EM/EMTD cells (28.9-98.2% of cells, median 75%, n=8) and PD-1 expression was comparable, though modestly increased, compared to total CD8+ T cell populations. In 2 patients not controlling CMV, tetramer+, CD8+ T cells had a greater proportion of naïve/CM phenotype (45.7-54.6% of cells, median 50%, n=2) and expressed markedly higher PD-1 levels than total CD8+T cells.

To investigate the potential for PD-1 pathway blockade to enhance CMV responses, we performed stimulations with and without PD-L1 antibody blockade on 17 patient samples collected between 55 and 85 days post-transplant. Eleven patients demonstrated T cell proliferation in response to CMV. For nine of 11 patients, proliferative responses in CD4+ T cells were enhanced by PD-L1 antibody blockade and for eight of 11 patients proliferative responses in CD8+ T cells were enhanced by PD-L1 antibody blockade.

Our observations suggest different patterns of T cell immune reconstitution depending on donor source. Similar patterns of immune reconstitution were demonstrated in the case of CMV specific responses. Our data suggests that PD-1 interactions might be manipulated to affect immune responses following transplantation. Further investigation of PD-1 and its interactions with other inhibitory receptors as well as the potential to manipulate PD-1 to enhance GVT and abrogate GVHD are ongoing.