Issue highlights—July 2023

Diagnostic techniques within hematology are becoming more sensitive and measurable residual disease (MRD) is gaining importance as a result—for both diagnosticians and patients. MRD measurement is increasingly used as a study endpoint in clinical trials and also in routine diagnostics—and as a strong predictor of treatment outcome. Currently, more sensitive and specific methods of measurement are in development. Besides molecular assays, flow cytometry is one of the most frequently used methods for MRD assessment due to its shorter turnaround time, its cost-effectiveness, and broader applicability.

In this issue, several researchers have delved into various aspects of flow cytometry techniques being used for MRD detection within hematological malignancies. Gao et al. introduced a single tube flow cytometry assay with high sensitivity for monitoring MRD in B-lymphoblastic leukemia/lymphoma (B-ALL), independent of specific surface antigen expression like CD19 and CD22. As the authors point out, the development of such assays has become relevant due to the emergence of targeted anti-CD19 and anti-CD22 therapies (Gao, Chen, et al., 2023; Gao, Liu, et al., 2023). Targeted immunotherapy demonstrated encouraging results in recent years but induces significant changes in the phenotype of leukemic blasts concurrently. Therefore, alternative gating strategies have gained importance and potential CD19 substitutes have been proposed (Chen et al., 2023; Mikhailova et al., 2022).

The development of alternative gating strategies is also relevant independently of targeted therapies, as shown by the authors of the next article. In a rare case study, Ramalingam et al. reported a patient with CD19-negative diffuse large B-cell lymphoma (DLBCL). Since CD19 is currently the primary gating marker for B cell neoplasms, its absence may lead to erroneous results and potentially affect therapeutic strategies, so the authors (Ramalingam et al., 2022). Challenges for flow cytometry approaches have been studied before (Gao, Chen, et al., 2023; Gao, Liu, et al., 2023; Huang et al., 2023; Martig & Fromm, 2022).

In MRD detection, the avoidance of false positives is crucial as shown by Zhou et al. The authors discussed the pitfalls in MRD detection in B-ALL following targeted immunotherapy, describing the presence of two CD22-positive non-neoplastic cell populations. One progenitor population of uncertain lineage and one mature B-cell population were both immunophenotypic mimics of B-ALL. Zhou et al. concluded that an understanding of these normal cell populations is essential to avoid misinterpretation in MRD assessments and CD19-independent gating strategies, including CD22 and CD24, are key (Zhou et al., 2022). Optimizing MRD measurement is at the forefront of numerous studies and alternative antigens (besides CD22 and CD24 also iCD79a) or radar plots and a flow cytometry scoring system for predicting ZNF384 rearrangements have been suggested (Mikhailova et al., 2022; Shopsowitz et al., 2022; Wang et al., 2022).

As the next article depicts, MRD measurement and its optimization plays a significant role not only in lymphoid but also in myeloid neoplasms. Since MRD analysis is not yet included in routine diagnostics in the United Kingdom, McMillan et al. adapted and validated a multicolor flow cytometry (MCF) assay for multiple myeloma (MM) MRD, allowing widespread usage of this assay in smaller laboratories for real-world practice (McMillan et al., 2022). Achieving MRD negativity in MM is crucial for improved patient outcomes. In newly diagnosed and relapsed MM, a high percentage of BCMA-positive abnormal plasma cells was found, supporting its potential as a target for CAR-T cell and monoclonal-antibody therapies (Sriram et al., 2022). Aberrancies in polyclonal plasma cells and immunomodulation in neoplastic plasma cells post-therapy were identified in another study, underscoring the importance of validating MRD assays under normal and reactive conditions (Das et al., 2022). A consensus protocol has been proposed to reduce inter-laboratory variation in MM MRD reporting, leading to improved consistency. This harmonized approach makes MM MRD a potential surrogate clinical endpoint for assessing progression-free and overall survival in clinical trials (Soh et al., 2022).

Hsu et al. investigated the impact of Down syndrome-specific non-malignant hematopoietic regeneration on MRD detection in myeloid leukemia associated with Down syndrome (ML-DS). The presence of DS-specific myeloid progenitors in the bone marrow can complicate MRD interpretations in ML-DS patients. Therefore, its awareness is essential to the MRD detection, the authors postulate (Hsu et al., 2023).

The last article in this issue, Jurado et al. focused on optimizing the monocyte gating strategy for diagnosing chronic myelomonocytic leukemia (CMML). They proposed a 10-color tube and computational analysis to improve the reproducibility of monocyte subset quantification (Jurado et al., 2022). Further studies highlight the robustness of the “monocyte assay” for CMML diagnosis, the complexities in distinguishing CMML from blastic plasmacytoid dendritic cell neoplasm (BPDCN), and the potential of immunophenotypic analysis in differentiating CMML from reactive monocytosis (Espasa et al., 2021; Feng et al., 2018; Wagner-Ballon et al., 2023).

In conclusion, the highlighted articles in this issue contribute valuable insights into the ongoing efforts to develop sensitive diagnostic tools for MRD detection, address potential diagnostic challenges, and implement strategies to enhance the accuracy of MRD assessments in various hematological malignancies. These advancements hold great promise for improving patient outcomes and guiding treatment decisions in the field of hematology.