Coevolving JAK2^V617F+relapsed AML and donor T cells with PD-1 blockade after stem cell transplantation: An index case

Relapse of myeloproliferative neoplasms (MPNs) after allogeneic hematopoietic stem cell transplantation (HSCT) is associatedwith poor outcomes, as therapeutic approaches to reinstate effective graft-versus-leukemia (GVL) responses remain suboptimal. Immune escape through overexpression of PD-L1 in JAK2V617F-mutated MPN provides a rationale for therapeutic PD-1 blockade, and indeed, clinical activity of nivolumab in relapsed MPN post-HSCT has been observed. Elucidation of the features of response following PD-1 blockade in such patients could inform novel therapeutic concepts that enhance GVL. Here, we report an integrated highdimensional analysis using single-cell RNA sequencing, T-cell receptor sequencing, cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq), and assay for transposaseaccessible chromatin using sequencing (scATAC-seq), together with mass cytometry, in peripheral bloodmononuclear cells collected at 6 timepoints before, during, and after transient response to PD-1 blockade from an index case of relapsed MPN following HSCT. Before nivolumab infusion, acute myeloid leukemia (AML) blasts demonstrated high expression of chemokines, and T cells were characterized by expression of interferon-response genes. This baseline inflammatory signature disappeared after nivolumab infusion. Clinical response was characterized by transient expansion of a polyclonal CD41 T-cell population and contraction of an AML subpopulation that exhibited megakaryocytic features and elevated PD-L1 expression. At relapse, the proportion of the AML subpopulationwith progenitor-like features progressively increased, suggesting coevolution of AML blasts and donor-derived T cells. We thus demonstrate how single-cell technologies can provide complementary insight into cellular mechanisms underlying response to PD-1 blockade, motivating future longitudinal high-dimensional single-cell studies of GVL responses in relapsed myeloid disease.