Issue highlights—May 2023

In the era of personalized medicine, the list of targeted therapeutic drugs has been greatly expanded, many of which are employed to treat hematolymphoid neoplasms. These include drugs specifically targeting immune checkpoint signaling epitopes such as programmed cell death 1 (PD1) and its ligand PDL1 to inhibit T-cell activation. Alternatively, these drugs might target the surface antigens expressed by tumor cells such as CD19 and CD22 in B-cell neoplasms. Of the latter, targeted immunotherapies include monoclonal antibodies with or without drug conjugates, bispecific T-cell engagers, or chimeric antigen receptor (CAR) T-cell therapy. The binding sites of these therapeutic antibodies are often identical or in proximity with the binding sites of the diagnostic antibodies. Therefore, use of these therapies pose great challenges for clinical cytometry labs in the assessment of post-treatment samples, especially in the detection of measurable/minimal residual disease (MRD). In this issue, Chen, Gao, et al. (2023) provided an overview of MRD detection in B-lymphoblastic leukemia/lymphoma in the era of immunotherapy, and Gao et al. (2023) focused their review on the impact of targeted therapy on mature B- and plasma cell neoplasms utilizing flow cytometry assessments. In both reviews, the authors illustrated the challenges, identified the problems, and provided a list of available options and solutions. For each of the above-mentioned disease categories, optimal gating and analysis strategies were illustrated with literature review and inputs from the authors' experience and insights. The utility and interpretation of additional B-cell markers other than CD19 and CD20 for mature or immature B-cell neoplasms such as CD22, CD24, and cCD79a (Mikhailova et al., 2022) were studied as well as VS38, CD229, and CD319 (Pojero et al., 2016; Soh et al., 2021) for plasma cell neoplasms. The authors recommended that the flow cytometry assays in the era of targeted therapies must contain significant redundancy in the antibody panels allowing the detection of the cells of interest and additionally should be ready to utilize several gating strategies for accurate and consistent population identification.

Neoplastic mature B-cells often show restricted kappa or lambda light chain expression; however, light chain expression may be absent in around 5%–10% of mature B-cell lymphoma (Li et al., 2002). It is suggested that mature B-cells lacking surface light chain expression can be used as a surrogate marker to diagnose mature B-cell lymphomas. Huang et al. (2023) reported a series of 89 cases of surface light chain negative B-cell lymphoma which consisted primarily of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL). Interestingly, the authors also reported no detectable light chain expression in normal/reactive mature B cells collected from body fluids and cystic fluids from 14 patients with no clinical and pathological evidence of lymphoma/leukemia. Body fluids are often proteaceous; thus additional washing prior to kappa/lambda staining is often required to obtain adequate light chain staining and caution is needed in the interpretation of light chain expression in B-cells collected from body fluids. It is known that out of the context of chronic myeloid leukemia (CML) (Soma et al., 2016), lymphoblastic crisis is extremely rare in other chronic myeloid neoplasms, such as myelodysplastic syndrome (MDS) (Xie et al., 2019) and Philadelphia negative myeloproliferative neoplasms (MPN). Chan et al. (2023) reported the detection of a low level of abnormal B-lymphoblasts ranging from 0.012% to 3.6% in 9 out of 1262 MDS or Ph-negative MPN. Cell sorting coupled with next-generational sequencing showed that the abnormal B- lymphoblasts in myeloid neoplasms were often, but not always, clonally related to the myeloid compartment, and the molecular genetic findings suggested a mutant multipotent progenitor cell with different lineage output that might be determined by the specific mutations and the particular cell stage where the mutations occurred. Importantly, unlike CML, the presence of abnormal B-lymphoblasts in MDS and Ph-neg MPNs did not necessarily indicate blast crisis, and therapy for the underlying myeloid neoplasms with close monitoring is suggested to be a reasonable approach.

The last article in this issue (Chen, Zhang, et al., 2023) studied mucosal-associated invariant T cells (MAIT) (Godfrey et al., 2019) in aplastic anemia (AA), a type of bone marrow failure directly linked to abnormal immune responses. MAIT cells exhibit an effector-memory phenotype and express several NK receptors such as NKG2D and CD161 and play an important role in the development of autoimmune diseases. The authors showed an increased frequency of MAIT cells with an activation and effector function-related cell surface marker profile in patients with aplastic anemia. The specific property of NKG2D expression on MAIT cells suggests that these cells may have a distinct mechanism involved in AA immune pathogenesis.