Evaluation of a multiple myeloma synthetic cell mimic as an assay control for flow cytometry

The reliability of clinical flow cytometry assays hinges on the availability of robust assay controls that accurately reflect representative samples. Widely used conventional assay controls, such as cell lines, often fail to replicate the phenotypical and biophysical properties of representative primary samples resulting in gating variability and limited confidence in assay performance as certain markers may not be expressed. To address these limitations, we developed a custom synthetic multiple myeloma (MM) cell mimic using Slingshot Biosciences TruCytes® technology. Currently, the MM1.S cell line can be used as an assay control in a flow cytometry panel to evaluate key markers, B cell maturation antigen (BCMA) and G protein-coupled receptor, class C, group 5, member D (GPRC5D), on plasma cells (PCs) and B cells in bone marrow aspirate, for MM clinical trials. Although MM.1S shows strong expression of the key markers BCMA and GPRC5D, it lacks CD45 and CD19 which are needed for PC and B cell identification. We aimed to engineer a MM cell mimic to simulate the scatter properties and antigen density for our fresh MM bone marrow aspirate sample testing. Representative MM bone marrow mononuclear cells (BMMCs) were reviewed to find scatter properties and representative median fluorescence intensity (MFI) of CD45, CD19, CD38, CD138, BCMA, GPRC5D, CD56, and PD-L1 on PCs and B cells. We engineered the MM cell mimic with one population representing PCs and another population representing B cells, based on MM BMMC samples. The MM cell mimic overcame the lack of CD45 and CD19 expression and the divergent scatter and MFI profiles observed in traditional MM cell lines like MM1.S. Qualification studies demonstrated intra- and inter-assay precision (%CV ≤20%), robust inter-instrument reproducibility, post-reconstitution stability up to 7 days at 4°C, and closed-vial storage stability of up to 13 months at -20°C. In summary, these results establish the MM mimics as an improved and alternate assay control to MM1.S, facilitating longitudinal assay performance monitoring and ongoing quality assurance in clinical flow cytometry.