In Vitro CD4+CD25+ Regulatory T-Cell Suppression Assay Measuring Inhibition of CFSE-Labeled Responder T-Cell Proliferation in Human PBMCs

Treg:Tresp ratios tested: 1:1, 1:2, 1:4, 1:8, plus Tresp-alone (0 Treg, maximal proliferation) and Tresp-alone unstimulated (no stimulus). Each ratio in technical triplicate. Tresp fixed at 2.5 x 10^4/well; Tregs titrated accordingly. Anti-CD3/CD28 stimulation via coated beads (1 bead:2 Tresp) or soluble anti-CD3 + irradiated APCs. A Treg-only control confirms Tregs do not proliferate. Culture 4-5 days. Plan n = 6-8 donors. Operator blinded to ratio at flow acquisition.

BSL-2 for human PBMC isolation and culture in a Class II biosafety cabinet. If irradiated APCs are used, follow institutional irradiator/radiation-safety procedures and dosimetry. 2-mercaptoethanol is toxic — prepare in a fume hood. PPE: gloves, lab coat, eye protection. Decontaminate liquid waste with 10% bleach; autoclave solid biohazard waste. Follow bloodborne-pathogen procedures and dispose of magnetic beads as biohazard.

Maximal proliferation control: Tresp-alone + anti-CD3/CD28 (0% suppression reference). Background control: Tresp-alone, unstimulated (no proliferation). Treg-only control: confirms Tregs are anergic/non-proliferative and do not contaminate the CFSE+ gate. Treg purity control: FoxP3/CD25/CD127 staining on the sorted product. Optional isotype/no-suppressor control with CD4+CD25- 'mock suppressor' cells confirms suppression is Treg-specific, not a cell-crowding artifact (key control to rule out density effects).

Tresp-alone stimulated wells show robust CFSE dilution (60-90% divided). Adding Tregs reduces the divided fraction dose-dependently: typically ≥50% suppression at 1:1, declining to 10-30% at 1:8. Treg-only wells show minimal proliferation. Unstimulated Tresp remain CFSE-high (undivided). A clean dose-response with monotonic decrease in division index across ratios indicates functional, high-quality Tregs.

• Human PBMCs for cell isolation
• CD4+CD25+ Regulatory T Cell Isolation Kit (negative CD4 selection then CD25+ positive selection) or FACS sort of CD4+CD25hiCD127lo
• CellTrace CFSE/Violet proliferation dye, working 1 µM
• Anti-CD3/anti-CD28 Dynabeads (Treg Expander or T-Activator), 1 bead:2 cells; or soluble anti-CD3 (1 µg/mL) + irradiated autologous APCs (3000 rad)
• Complete RPMI 1640 + 10% human AB serum + 1% pen/strep + 50 µM 2-ME + 1 mM sodium pyruvate
• Recombinant human IL-2 (optional, low dose for Treg viability)
• Fixable Viability Dye eFluor 780; CD4-PE, CD25-APC, FoxP3-PE-Cy7 (intracellular, for purity check)
• FACS buffer (PBS + 2% FBS + 2 mM EDTA); FoxP3 Fix/Perm kit
• 96-well round-bottom plates

To determine the functional suppressive capacity of FACS-sorted or bead-isolated human CD4+CD25+CD127lo Tregs by co-culturing them at graded ratios with CFSE-labeled autologous CD4+CD25- responder T cells (Tresp) stimulated with anti-CD3/CD28, and quantifying the percent suppression of responder proliferation by flow cytometry. The assay outputs a suppression dose-response curve and % suppression at defined Treg:Tresp ratios.

1. Isolate CD4+CD25+CD127lo Tregs and CD4+CD25- responders from PBMCs by magnetic kit or FACS sort; verify Treg purity by FoxP3 staining (target ≥80% FoxP3+).
2. Label responder (Tresp) cells with 1 µM CFSE in protein-free PBS, 8 min RT; quench with 5 vol complete medium; wash twice.
3. Seed 2.5 x 10^4 CFSE+ Tresp per well across all conditions.
4. Add unlabeled Tregs at 1:1, 1:2, 1:4, 1:8 ratios; include Tresp-alone (stimulated and unstimulated) and Treg-alone wells.
5. Add anti-CD3/CD28 beads at 1 bead:2 Tresp (or soluble anti-CD3 + irradiated APCs) to all stimulated wells.
6. Top to 200 µL with complete medium; incubate 4-5 days at 37 °C, 5% CO2.
7. Harvest; magnetically remove beads if used.
8. Stain with viability dye (1:1000, 15 min, 4 °C) then CD4-PE/CD25-APC surface stain (20 min, 4 °C).
9. Wash; acquire ≥20,000 live CFSE+ (CD4+) responder events per well on the flow cytometer.
10. Gate on CFSE+ responders and quantify the divided fraction per generation.

Independent: Treg:Tresp ratio (suppressor dose). Dependent: % suppression of responder proliferation (division index reduction), and % divided responders. Controlled: responder cell number (fixed 2.5 x 10^4), stimulation strength (bead:cell ratio), CFSE labeling, Treg purity, culture duration (4-5 days), serum lot, and total well cell number where the mock-suppressor control is used to balance density.

Functional Tregs will inhibit anti-CD3/CD28-driven proliferation of autologous responder T cells in a dose-dependent manner, such that increasing Treg:Tresp ratios (1:1 → 1:8) produce progressively greater suppression of CFSE dilution, with maximal suppression (≥50%) at the 1:1 ratio compared to Tresp-alone control.

In FlowJo, gate live → CD4+ → CFSE+ responders, excluding unlabeled Tregs. Compute the division index (or % divided) for each well using the Proliferation tool anchored to the unstimulated Tresp peak. Calculate % suppression = (1 - [division index of Treg co-culture] / [division index of Tresp-alone stimulated]) x 100 for each ratio. Plot % suppression vs Treg:Tresp ratio per donor.

1. No suppression even at 1:1: poor Treg purity or loss of FoxP3 — re-sort to ≥80% FoxP3+ and avoid over-expansion. 2. Responders fail to proliferate in Tresp-alone: weak stimulation — verify bead:cell ratio and bead viability, or add APCs. 3. Apparent 'suppression' that is actually crowding: include the mock CD25- suppressor control and keep total cell density balanced. 4. High Treg proliferation contaminating the gate: ensure responders are CFSE-labeled and Tregs are not; gate strictly on CFSE+. 5. Low responder viability at harvest: shorten culture to 4 days, add low-dose IL-2 for Treg survival, confirm 2-ME present.

Fit per-donor suppression dose-response and compare % suppression across ratios with repeated-measures one-way ANOVA (or Friedman) with Tukey/Dunn correction. For comparing Treg function between groups (e.g., disease vs healthy) at a fixed ratio, use mixed-effects models or Mann-Whitney with FDR correction. Alpha = 0.05; n = 6-8 donors gives ~0.8 power for a moderate-to-large within-donor effect. Report mean ± SEM with individual donor curves.