Characterization of CD123 expression by mast cells in systemic mastocytosis with multicolor flow cytometry

Systemic mastocytosis (SM) is a clonal mast cell (MC) disorder characterized by aberrant immunophenotypes, including expression of CD25, CD2, and occasionally CD30. CD123, the α-subunit of the interleukin-3 receptor, is a therapeutic target in hematologic malignancies and has been reported to be expressed on neoplastic MCs by immunohistochemistry (IHC) with prognostic implications. This study aims to characterize CD123 expression in SM by flow cytometry. We retrospectively analyzed 142 bone marrow samples from 79 SM patients (81 diagnostic samples) and 25 controls with normal MC immunophenotype. Flow cytometry was performed using a clinically validated 9-color mast cell tube which included CD123. Data collected included SM subtype, clinical and laboratory features, MC burden, and marker expression. Statistical analyses were performed in R. CD123 was expressed on MCs in 91% of SM cases (ISM 92%, SM-AHN 94%, SSM 100%, ASM 100%, MCL 50%). Median percentage of MCs positive for CD123 was 53.9% (IQR 8.1-83.4). Compared to prior IHC data (overall 64% positivity), flow cytometry demonstrated more cases with CD123 expression by MCs. No significant correlations were observed between CD123 expression and serum tryptase, KIT D816V allele burden, or MC burden. CD123 is frequently expressed on neoplastic MCs in SM by flow cytometry, across all subtypes. These findings support further investigation of CD123 as a therapeutic target and warrant correlation with IHC and clinical outcomes in larger cohorts.