The landscape of disease: mapping aplastic anemia Free

The development of antibody-based spatial multiplex assays, and the more recent introduction of spatial transcriptomic and proteomic platforms, now allows the characterization and profiling of human tissue samples at unprecedented depth, opening up a new exciting window for a better understanding of human pathology in situ, in both inflammatory and neoplastic conditions. In this issue of Blood, Pool et al¹ give us a first view of the development of immune-mediated aplastic anemia where the pathogenesis still remains insufficiently understood. Using imaging mass cytometry of patients at diagnosis and 6 months after treatment with antithymocyte globulin, the authors found that specific multicellular lymphoid-dominant clusters develop within the bone marrow of affected patients. Subsequently, the cellular component is depleted, resulting ultimately in a hypocellular bone marrow, with only few differentiated effector cells remaining. Pool et al’s findings align with the existing paradigm that aplastic anemia results from immune-mediated bone marrow destruction, with T cells associated with inflammation playing an important role.² 

The authors identified lymphoid-dominant “immune hotspots” with high densities of proinflammatory lymphocytes, and macrophage-enriched hotspots in proximity to progenitors, suggesting that these immune cell populations play an active role in disease development. Furthermore, they show the persistence of terminally differentiated T cells in areas depleted of progenitors. Therefore, their study expands the existing paradigm that aplastic anemia is solely a T-cell–mediated disease and instead suggests that it results from an immune response involving a spectrum of leukocyte populations.³ Although this study is entirely descriptive in nature, the novel possibility of visualizing cell interaction patterns and clusters of cells within the actual patient now offers novel possibilities of investigating diseases such as aplastic anemia and obtaining information that is usually lost with conventional cell isolation techniques or molecular bulk tissue investigations.^4,5 Tissue biopsy investigations are better equipped not only to understand the pathologic “geography” of diseases,⁶ but will also mitigate isolation bias, an often neglected or underestimated issue when isolating cell suspensions, especially in difficult-to-isolate populations.⁷ Also, algorithms that calculate cell interaction scores in isolated cell suspensions, that are now widely used in biomedical research, are often problematic as they do not take into consideration true physical contact between cells, making them prone to bioinformatics artefacts.⁸ Thus, real in situ investigations are necessary to corroborate findings by cell isolates. In this article, the authors use imaging mass cytometry with a panel of 37 metal-conjugated antibodies to profile aplastic anemia bone marrow biopsies from 16 patients, in comparison to 12 paired samples 6 months after treatment with horse antithymocyte globulin that were available for assessing treatment responses, as well as normal controls.

Although the current investigation is still somewhat biased because of the use of a curated list of monoclonal antibodies detecting only a limited number of markers of a preselected panel, future, less supervised analyses using spatial transcriptomics and proteomics might further enhance our spatiotemporal understanding of aplastic anemia. Although spatial-omics investigations are still in their infancy, it is to be expected that future assays will soon be marketed with improved resolution and sensitivity compared with those currently available.⁹ One major impediment of spatial assays in autoimmune diseases is the current lack of kits that can pair individual T and B cells with the respective T- or B-cell receptor, information that would be highly desirable to better characterize mechanisms of autoimmunity, as evidenced by single-cell RNA-sequencing investigations, where such a pairing is now generally possible.¹⁰ 

Taken together, the authors suggest that an active immune response in aplastic anemia that results in hematopoietic stem and progenitor cell destruction is mediated by a diverse spectrum of T, B, and myeloid cells that show close interaction within the bone marrow microenvironment. However, whether these cells are the primary orchestrators of the targeted immune responses through cytokine secretion and cytotoxic attacks remains to be demonstrated functionally.

Conflict-of-interest disclosure: P.M.B. has received personal fees from Almirall, Sanofi, Janssen, Amgen, LEO Pharma, AbbVie, Pfizer, Boehringer Ingelheim, GlaxoSmithKline, Regeneron, Eli Lilly, Celgene, Arena Pharma, Novartis, UCB Pharma, Biotest, and Bristol Myers Squibb. P.M.B. has received research support from Pfizer (grant paid to his institution). P.M.B. is an investigator for Pfizer and AbbVie.