Unraveling the immune-pathogenesis of aplastic anemia with imaging mass cytometry
The autoimmune response driving hematopoietic stem and progenitor cell destruction in immune-mediated aplastic anemia (AA) remains incompletely understood. Previously, by studying bone marrow aspirates from AA patients, we identified a disease-specific immune cell network involving T-, B-, and myeloid cells. However, the exact interactions within this network, the interaction with the microenvironment and the chronological events in AA development, remain unclear. Therefore, we used high-dimensional imaging to investigate the spatial orientation of the immune response in AA within its native microenvironment in bone marrow.
An imaging mass cytometric analysis of bone marrow trephine biopsies was performed using a 37-marker panel designed to enable detailed single-cell analyses of both hematopoietic and bone marrow niche cell subpopulations. Paired bone marrow biopsies from AA patients at diagnosis (n=16) and 6 months after start of first-line immunosuppressive therapy with ATGAM and ciclosporin (n=12) were imaged. Additionally, 6 age-matched control bone marrow biopsies were imaged as a reference.
Within the hypocellular bone marrow architecture of AA patients at diagnosis, we identified cellular regions that still contained hematopoietic progenitor cells. These regions could be further subdivided into two main types of immune “hot spots”; Lymphoid-dominant immune hot spots with high densities of proinflammatory lymphocytes, and macrophage-enriched immune hot spots containing enlarged activated macrophages in close proximity to progenitors. These immune hot spots likely represent active sites of hematopoietic stem and progenitor cell destruction. In bone marrow regions depleted of progenitors, terminally differentiated effector cells remained. In patients showing bone marrow recovery 6 months after ATGAM-treatment, most immune hot spots were no longer detectable, underscoring their potential pathogenic role.
Our data indicate that hematopoietic stem and progenitor cell destruction in AA is mediated by coordinated interactions among distinct 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. Collectively, our study visualizes the complex interactions among immune cell subpopulations and reveals, for the first time, the order of events in the immune-mediated pathogenesis of AA. Furthermore, our findings set the stage for future research into the antigenic specificities driving AA pathogenesis.