Issue Highlights—July 2021

A fundamental challenge of clinical flow cytometry is to distinguish abnormal from normal cells on the basis of their immunophenotypic properties. Several articles in this issue of Cytometry Part B: Clinical Cytometry highlight the protean nature of normal and neoplastic cellular phenotypes, particularly in patients undergoing treatment. For instance, since bright CD38 expression typifies normal and neoplastic plasma cells, CD38 is nearly ubiquitous as a gating reagent in clinical assays (Scott et al., 2019; Soh et al., 2021; Wang & Lin, 2019). However, the expanding clinical use of daratumumab (Mateos et al., 2018), a monoclonal antibody directed against CD38, calls for the characterization of alternative gating reagents in patients with myeloma. The report by Soh and colleagues in this issue suggests that CD319 (SLAMF7) may represent one such alternative reagent, not only in daratumumab-treated patients, but also in patients treated with elotuzumab, which appears to target an epitope of CD319 distinct from those recognized by commercially-available antibody conjugates (Soh et al., 2020). In patients undergoing treatment for B-lymphoblastic leukemia, phenotypic differences between normal and neoplastic B-cell precursors underlie the ability to detect residual disease by flow cytometry (DiGiuseppe & Wood, 2019). Chatterjee and colleagues now demonstrate phenotypic differences between normal Joseph A. DiGiuseppe [Color figure can be viewed at wileyonlinelibrary.com] regenerating B-cell precursors in patients treated for Bor T-lymphoblastic leukemia and those in control samples obtained prior to any treatment (Chatterjee et al., 2021), highlighting the importance of a comprehensive understanding of potential variability in normal B-cell maturation for the evaluation of measurable residual disease (MRD). Normal phenotypes may also be affected by microenvironmental factors, unrelated to treatment; Gadgeel and colleagues suggest in this issue (Gadgeel et al., 2021) that phenotypic maturation patterns in ectopic intrathyroidal thymic tissue differ from those described in normal thymus and thymoma (DiGiuseppe & Wood, 2019). Along these lines, microenvironmental cues have also been suggested to explain the apparent association between extrathymic immature Tcell populations and Castleman disease (Fromm et al., 2020). One must also be alert for unexpected phenotypes in hematolymphoid neoplasms, such as the case of CD45-negative follicular lymphoma reported by Khanlari and colleagues in the current issue (Khanlari et al., 2021). The consistent use of well-designed multicolor panels is one approach to managing the problem of phenotypic variability (and in mitigating the risk of unanticipated pitfalls (Cherian et al., 2019), such as steric hindrance (Matos, 2021); in this issue of the journal, several such multicolor combinations are evaluated. Goshaw and colleagues describe a 14-color, single-tube assay incorporating antigens recommended by the ERIC and ESCCA harmonization project (Rawstron et al., 2018), which enables characterization of CD5+ B-cell lymphoproliferative disorders and detection of MRD in patients treated for chronic lymphocytic leukemia (CLL) (Goshaw et al., 2021). The authors further demonstrate the ability of dimensionality reduction (viSNE) to facilitate MRD detection in CLL, as has been shown previously in B-lymphoblastic leukemia (DiGiuseppe et al., 2015). Gupta and colleagues, also in this issue (Gupta et al., 2021), illustrate the use of another multiparametric data visualization method, the radar plot (Jafari et al., 2018; Violidaki et al., 2020), to facilitate distinction between acute promyelocytic leukemia with PML-RARA and AML with NPM1 mutation, whose phenotypes may overlap (Zhou et al., 2019). Other multicolor antibody panels described and evaluated in this issue are a 10-color screening panel for B-cell neoplasia (Espasa et al., 2021) that includes CD200 (Sorigue et al., 2020), and a 10-color panel for MRD assessment in T-lymphoblastic leukemia (Tembhare et al., 2021). The former panel (Espasa et al., 2021), which makes use DOI: 10.1002/cyto.b. 22027