Imaging mass cytometry reveals the order of events in the pathogenesis of immune-mediated aplastic anemia

The autoimmune response driving hematopoietic stem and progenitor cell (HSPC) destruction in immune-mediated aplastic anemia (AA) remains incompletely understood. We previously identified a disease-specific immune cell network involving T, B, and myeloid cells. However, the interactions within this network, the interaction with the microenvironment, and the chronological events in AA development, remain unclear. In this study, we aimed to characterize the changes occurring during disease development and to define the interactions between potential autoreactive cells and their target. Using imaging mass cytometry, we analyzed bone marrow (BM) biopsies from patients with AA at diagnosis and after treatment with horse-derived anti-thymocyte globulin (hATG), and 6 controls. Within the hypocellular BM architecture, we identified lymphoid-dominant "immune hot spots" with high densities of proinflammatory lymphocytes, and macrophage-enriched hot spots that additionally contained activated macrophages in proximity to progenitors. These immune hot spots potentially represent sites in which the active immune response resulting in HSPC destruction takes place. In BM regions depleted of progenitors, effector cells with a differentiated phenotype remain. Our data indicate that HSPC destruction in AA is mediated by coordinated interactions among specific immune cell subpopulations. As the immune response progresses and HSPCs are depleted, the immune composition shifts, with activated T and B cells differentiating into terminally differentiated T cells and plasma cells. In patients with normalizing BM after hATG treatment, most immune hot spots were depleted, underscoring their potential pathogenic role. Collectively, our study visualizes the complex interactions among immune cell subpopulations and reveals, to our knowledge, for the first time, the order of events in the immune-mediated pathogenesis of AA.