??Immune Escape after Allogeneic Hematopoietic Cell Transplantation in Pediatric Patients with Acute Myeloid Leukemia

Although allogeneic hematopoietic cell transplantation (allo-HCT) is curative for high-risk pediatric acute myeloid leukemia (AML), disease relapse remains the primary cause of post-transplant mortality. To identify pressures imposed by allo-HCT on AML cells that escape the graft-versus-leukemia effect, we evaluated immune signatures in bone marrow samples from four pediatric patients using a multimodal single-cell approach. Downregulation of MHC class II (MHC-II) expression observed at post-transplant relapse was most profound in progenitor-like blasts and was accompanied by correlative changes in transcriptional regulation. Dysfunction of activated NK cells and CD8+ T cell subsets at the time of relapse was evidenced by the loss of IFN-? response, TNF-a signaling via NF-kB, and IL-2/STAT5 signaling. Clonotype analysis of post-transplant relapse samples revealed expansion of dysfunctional T cells and enrichment of T regulatory and T helper cells. Using novel computational methods, our results illustrate a diverse immune-related transcriptional signature in post-transplantation relapses not previously reported in pediatric AML. Overall design: scRNA/TCR sequencing with feature barcoding