Abstract 6869: Paired single-cell B-cell transcriptomics and receptor sequencing reveal activation states and clonal signatures that characterize B cell immunity in acute myeloid leukemia

Abstract Acute myeloid leukemia (AML) is the second most common adult leukemia with a five-year survival rate around 30%. For decades, AML treatment has centered around intensive chemotherapy frequently followed by allogeneic stem cell transplant (Allo-SCT). The power of Allo-SCT to induce lasting remission has been attributed to a potent graft-versus-leukemia effect and has inspired efforts to stimulate host immunity against AML via immune checkpoint blockade (ICB). Thus far, these studies have yielded mixed results. While tumor-infiltrating T-cells have long been the focus of immunotherapy investigations, a growing body of data points to a central role of B-cells in mediating anti-tumor immunity, and a comprehensive assessment of B lymphocytes within the AML immune microenvironment has never been carried out with single cell clonal resolution. We performed paired single-cell transcriptomic and B cell receptor (BCR) analysis on 52 bone marrow aspirate samples. These samples included 6 from healthy bone marrow donors (normal), 24 from newly diagnosed AML patients (NewlyDx), and 22 from 8 relapsed or refractory AML patients (RelRef), who underwent assessment both before and after ICB with azacitidine/nivolumab. We first used transcriptomic data to delineate canonical B-cell subpopulations and observed a reduction in nascent B cells and an expansion of differentiated B cells in AML patients (RelRef>NewlyDx). Overlaying BCR sequencing and clonotypic analysis revealed clonal expansion, low clonotypic diversity, and extensive somatic hypermutation in the RelRef samples, suggesting antigen-driven affinity maturation within the tumor microenvironment. Furthermore, we identified robust AP-1 expression (a marker of activation) in B-cells from NewlyDx patients and a marked loss of AP-1 in RelRef patients, suggesting that B-cell exhaustion may be a key immunologic feature of AML relapse. Importantly, we observed that AP-1 activity is restored and that specific B-cell clones expand after ICB, but only among patients who clinically respond to ICB. In line with these observations, we also noted the clonal expansion of AML-associated plasma cells and loss of clonotypic diversity among ICB clinical responders, pointing to an antitumor role for these clones. Finally, we characterized atypical memory B cells which demonstrate high antigen presentation activity, close interaction with AML cells and are associated with unfavorable clinical outcomes, suggesting that they play a direct role in aiding AML survival and immune escape. Collectively, our findings establish the dynamic landscape of AML-associated B-cells while identifying specific B-cell subpopulations and B-cell-AML interactions that may be targeted by the next generation of AML-directed immunotherapies. Citation Format: Shengnan Guo, Gopi S. Mohan, Bofei Wang, Tianhao Li, Naval Daver, Yuting Zhao, Patrick K. Reville, Dapeng Hao, Hussein A. Abbas. Paired single-cell B-cell transcriptomics and receptor sequencing reveal activation states and clonal signatures that characterize B cell immunity in acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6869.