Mega-scale single-cell profiling reveals novel biomarkers associated with acute GvHD after allogeneic hematopoietic stem cell transplantation

Alloreactive T cells mediate graft-versus-leukemia (GvL) reactions and acute graft-versus-host disease (aGvHD) in AML patients following allogeneic hematopoietic stem cell transplantation. To investigate biomarkers that identify alloreactive T cells associated with either beneficial GvL or detrimental aGvHD, we collected graft samples and two post-transplant follow-up blood samples (day 30 and day 100) of ten AML patients undergoing hematopoietic stem cell transplantation and profiled over 777,000 CD45+ leukocytes in total by combinatorial barcoding-based mega-scale single-cell RNA sequencing. Using immune receptor sequences as intrinsic clonal barcodes, we observed that especially CD8+ graft-derived T cells persisted and displayed enhanced proliferation, clonal expansion, and likely alloreactivity. Notably, patient-derived peripheral leukocytes that survived the conditioning, as identified by sex-chromosome-related genes, were primarily CD4+ T helper cells. MDGA1 expression on T cells and NK cells emerged as a novel biomarker potentially associated with aGvHD. Additionally, we observed a significant deficiency of ADGRG1 expression, a marker of alloreactive cytotoxic T cells, by αβ and γδ T cells from relapsed patients. In conclusion, mega-scale single-cell monitoring of graft and hematopoietic immune cell reconstitution allowed us to demonstrate that MDGA1 and ADGRG1 may function as complementary biomarkers expressed by distinct circulating T cells that are associated with divergent outcomes in AML patients, enabling precise risk stratification of alloHSCT outcomes and presenting potential therapeutic targets.