New Products | Slingshot Biosciences: synthetic flow cytometry controls TrueCytesTM®

Flow cytometry controls have long been constrained by the biology they are meant to measure — donor variability, cold-chain dependency, lot inconsistency, and antigen drift over cell passage. Slingshot Biosciences resolves this by replacing biological material entirely with precision-engineered synthetic mimics that replicate cell scatter, surface biochemistry, and fluorescence properties under defined, manufacturable tolerances. The result is a control reagent system that behaves like a cell, but performs like a standard.

Standard flow cytometry controls rely on primary human blood, fixed whole blood, isolated leukocyte fractions, or cell lines. These introduce fundamental reproducibility problems:

• Donor-to-donor variability in antigen density
• Lot-to-lot inconsistency, cold-chain dependency
• Short viable shelf life
• Biosafety considerations with unfixed material

In cell therapy manufacturing specifically, using primary cells as release testing controls introduces additional risk because the control itself is unstable and uncharacterized at the antigen copy-number level. Slingshot's synthetic mimics are purpose-built to eliminate each of these failure points.

TruCytes™ are precision-engineered, synthetic cell mimics that display certified antigen copy numbers per particle. Unlike biological cell lines, which express surface antigens at undefined and passage-variable densities, TruCytes™ provide discrete, calibrated antigen populations for quantitative flow cytometry applications including CAR-T potency testing and inter-laboratory standardization.

Key TruCytes™ Product Lines

• TruCytes™ CD19 Quant — four discrete antigen density populations enabling antibody binding capacity (ABC) measurements directly from PE- or APC-conjugated CD19 antibodies, without prior knowledge of fluorophore-to-protein (F:P) ratios.
• TruCytes™ Potency CD19 Kit — designed as CAR-T effector:target controls for CD19-directed therapies, replacing Raji cells as target surrogates in killing assays.
• TruCytes™ Potency CD20 Evaluation Kit — enables CD20 CAR-T and CD20-targeted therapy (e.g., rituximab) QC with robust, specific CAR-T activation and approximately 2× reduced off-target effects compared to tumor cell lines.
• Single-biomarker TruCytes™ — available for CD2, CD3, CD25, CD30, CD33, CD45, CD117, and EGFR, enabling targeted QC controls across multiple therapeutic contexts.

TruCytes™ vs. Traditional Cell Line Controls

Traditional antigen-positive cell lines like Raji (CD19) or Daudi (CD20) are the current standard, but they carry fundamental confounders in GMP-grade assay settings. Here is how TruCytes™ compare directly:

Advantages

Elimination of Passage-Dependent Antigen Drift
The most critical technical advantage. Cell lines like Raji accumulate genetic and epigenetic changes with passage, causing CD19 surface density to shift unpredictably. This is a recognized confounding variable under GMP frameworks for advanced therapy medicinal products (ATMPs), where potency assays must be reproducible and traceable. TruCytes™ remove this variable entirely by decoupling antigen presentation from biology.

Quantitative Antibody Binding Capacity Without F:P Dependence
Conventional ABC quantification requires antibodies with defined fluorophore-to-protein ratios alongside calibration bead ladders (e.g., BD Quantibrite). TruCytes™ CD19 Quant eliminates this dependency — certified copy numbers are built into the particle, so ABC can be read directly from your standard antibody without additional characterization steps.

GMP-Compatible Inter-Laboratory Standardization
Because TruCytes™ are synthetic and lot-certified, they function as stable reference materials that can anchor potency assay values across multiple manufacturing sites or clinical trial centers. This is particularly relevant under EMA and FDA guidance requiring potency assay comparability across ATMP batch releases.

Tumor-Free Assay Environment
Raji and Daudi are human lymphoma lines. Using them in CAR-T killing assays introduces tumor-derived cytokines and metabolites that can confound effector:target readouts. The TruCytes™ Potency CD20 kit demonstrated approximately 2× reduced off-target cytokine effects compared to tumor cell lines, sharpening the signal-to-noise ratio in functional killing assays.

Workflow and Biosafety Simplification
Cell lines require continuous culture, mycoplasma testing, viability assessment before each assay run, and BSL-2 disposal. TruCytes™ are lyophilized, biohazard-free, and require only PBS reconstitution — directly analogous to the workflow advantages described for FlowCytes, but now applied in a GMP CAR-T QC context.

TruCytes™ represent the most technically significant product line. Conventional antibody titration and assay validation relies on antigen-positive cell lines — Raji for CD19 or Daudi for CD20 — which express antigens at undefined, variable copy numbers that drift over passage number. TruCytes™ present certified antigen copy numbers per particle, enabling quantitative antibody binding capacity measurements, potency assay calibration for CAR-T product characterization, and inter-laboratory assay standardization.

The most recent portfolio expansion, launched in 2024, targets the specific QC demands of CAR-T and broader adoptive cell therapy workflows. Synthetic cells expressing BCMA and CD19 at defined densities are used to characterize CAR product functionality and confirm target binding specificity, replacing unstable antigen-expressing target cell lines. Lymphocyte subset mimics — presenting defined T cell, B cell, and NK cell population ratios — provide reference material for leukapheresis starting material QC and release testing, where current practice relies on donor-matched primary cell aliquots that are both variable and exhaustible. The core regulatory value is that GMP-grade release assays require reference standards with documented, certified characteristics that primary cells fundamentally cannot guarantee.