Dendritic Cells of Leukemic Origin (DCleu) Modulate the Expression of Inhibitory Checkpoint Molecules and Their Ligands on T Cells and Blasts in AML Relapse After Allogeneic Stem Cell Transplantation

<b>Background:</b> Allogeneic hematopoietic stem cell transplantation (allo-HCT) is a potentially curative treatment for high-risk acute myeloid leukemia and myelodysplastic neoplasms (AML/MDS). However, AML/MDS relapse post-transplant is driven by immune escape mechanisms, limiting treatment options and contributing to poor prognosis. Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF) and Prostaglandin-E1 (PGE-1) (termed "Kit M") induce dendritic cells of leukemic origin (DC_leu) that have been shown to induce antileukemic immune responses. <b>Methods:</b> Using flow cytometry, we analyzed ICM/ICML-expressing uncultured T-cells/blasts in whole blood (WB) samples from patients with AML/MDS relapse post-allo-HCT who had received salvage treatment. Using Kit M, DC_leu were generated ex vivo. T-cell-enriched mixed-lymphocyte cultures (MLC) were performed for functional assessment of DC/DC_leu to stimulate (leukemia specifically) patients' immune cells. After MLC, ICM/ICML-expressing cells and immune activation were assessed. The experimental results were correlated with patients' clinical responses to salvage treatment. <b>Results</b>: WB samples from 15 patients were analyzed. On average, high frequencies of ICM (CTLA4/PD1)-co-expressing blasts and high frequencies of ICM (CTLA4/PD1/TIGIT/TIM3/2B4)-co-expressing T-cells were found in uncultured WB, compared to the frequencies of healthy ICM-expressing T-cells. Treatment with Kit M induced DC/DC_leu, which, after MLC, downregulated ICM-expressing T-cells and enhanced activated and memory T-cells. High frequencies of ICM-co-expressing uncultured T-cells/blasts correlated negatively with the blast lysis capacity and patients' clinical response to relapse treatment. However, post-MLC, Kit-mediated modulation of ICM-expressing T cells did not correlate with blast lysis, nor with patients' clinical response to treatment. <b>Conclusions:</b> We conclude that Kit M contributes to overcoming impaired antileukemic reactions, independent of the presence of ICM-expressing T-cells in relapsed AML patients after allo-HCT ex vivo.