CFSE Dilution Assay for Quantifying Antigen-Specific CD4+ T-Cell Proliferation in Human PBMCs

Each donor sample is cultured in technical triplicate wells per condition. Conditions: (1) CFSE-labeled PBMCs + recall antigen (tetanus toxoid 2.5 Lf/mL or CMV pp65 peptide pool 1 µg/mL); (2) unstimulated CFSE-labeled PBMCs (medium only, negative/background); (3) CFSE-labeled PBMCs + anti-CD3/anti-CD28 beads or 5 µg/mL PHA (positive polyclonal control). Plan n = 8-10 donors for population-level inference. Cultures run for 6 days at 37 °C, 5% CO2; a single endpoint harvest on day 6. Operators acquiring flow data are blinded to condition by coded plate maps where feasible. Seed 2 x 10^5 PBMCs per well in 200 µL.

BSL-2 for all human PBMC work; handle in a Class II biosafety cabinet treating blood-derived material as potentially infectious. PPE: lab coat, nitrile gloves, eye protection. CFSE/DMSO is a skin permeant — minimize exposure, handle stocks in the hood. 2-mercaptoethanol is toxic; prepare in the fume hood. Decontaminate all liquid waste with 10% bleach (≥30 min contact) and autoclave solid biohazard waste. Follow institutional bloodborne-pathogen and hepatitis B vaccination policies.

Negative/background: unstimulated medium-only CFSE-labeled cells define the undivided (CFSE-high) peak and spontaneous background proliferation. Positive control: anti-CD3/CD28 beads or PHA drives broad polyclonal division across all CFSE generations, confirming labeling and culture viability. Unstained and single-stain compensation controls (cells + each fluorophore) set spectral compensation. Viability dye control (heat-killed cells) confirms live/dead discrimination. Day-0 CFSE-labeled aliquot establishes the maximum (undivided) fluorescence reference for generation gating.

Unstimulated wells show a single sharp CFSE-high peak with <2-3% divided cells (background). Recall antigen typically drives 2-15% of CD4+ cells into ≥1 division with a clear stepwise CFSE-dilution ladder (each generation ~half-log lower). PHA/anti-CD3/CD28 positive controls show 60-90% divided cells across 5-7 generations. Division index for a strong responder is commonly 0.3-1.5; antigen-specific signal should be statistically above the matched unstimulated control.

• Freshly isolated or thawed human PBMCs (≥90% viability by trypan blue), 2 x 10^5 cells/well
• CFSE cell proliferation dye (e.g., CellTrace CFSE), 5 mM DMSO stock; working 1-2.5 µM in PBS
• RPMI 1640 with L-glutamine, 10% heat-inactivated FBS, 1% penicillin/streptomycin, 1 mM sodium pyruvate, 50 µM 2-mercaptoethanol (complete medium)
• Tetanus toxoid (2.5 Lf/mL final) and/or CMV pp65 overlapping peptide pool (1 µg/mL/peptide)
• Anti-CD3/anti-CD28 activation beads (1:1 bead:cell) or PHA-L (5 µg/mL)
• Fixable Viability Dye eFluor 780 (1:1000)
• Anti-human CD3-BV421, CD4-PE, CD8-APC (titrated, typically 1:50-1:100)
• PBS, pH 7.4; FACS buffer (PBS + 2% FBS + 2 mM EDTA)
• 96-well round-bottom plates; sterile reagent reservoirs

To measure antigen-specific CD4+ T-cell proliferation in human PBMCs by tracking the dilution of the intracellular dye CFSE (carboxyfluorescein succinimidyl ester) over a 6-day in vitro culture following stimulation with a recall antigen (e.g., tetanus toxoid or CMV pp65 peptide pool). The assay yields a precise division index and the frequency of responding (divided) cells within the CD3+CD4+ gate, enabling comparison of antigen-specific memory responses across donors or treatment conditions.

1. Resuspend PBMCs at 1 x 10^7/mL in pre-warmed PBS (no protein, which quenches CFSE).
2. Add CFSE to 1 µM final; mix immediately and incubate 8 min at room temperature in the dark.
3. Quench by adding 5 volumes of cold complete medium; incubate 5 min on ice.
4. Centrifuge 300 x g, 5 min; wash twice in complete medium to remove free dye.
5. Resuspend at 1 x 10^6/mL; seed 200 µL (2 x 10^5 cells) per well in triplicate per condition.
6. Add antigen, polyclonal stimulus, or medium per the design.
7. Incubate 6 days at 37 °C, 5% CO2 undisturbed (no media change).
8. On day 6, harvest cells; wash once in PBS (300 x g, 5 min).
9. Stain with Fixable Viability Dye eFluor 780, 1:1000 in PBS, 15 min at 4 °C in the dark.
10. Wash; surface-stain with CD3-BV421/CD4-PE/CD8-APC in 50 µL FACS buffer, 20 min at 4 °C.
11. Wash twice; resuspend in 200 µL FACS buffer and acquire ≥50,000 live CD3+ events per well.

Independent: stimulation condition (antigen vs. medium vs. polyclonal). Dependent: % divided cells within CD3+CD4+ gate, division index, proliferation index, and number of CFSE generations. Controlled: CFSE concentration and labeling time, cell seeding density, culture duration (6 days), medium lot, incubator CO2/temperature, antibody panel and titers, and flow cytometer voltage settings (tracked by daily QC beads).

PBMCs from donors with prior antigen exposure will contain a measurable population of antigen-specific CD4+ memory T cells that undergo multiple rounds of division upon recall stimulation, producing a stepwise dilution of CFSE fluorescence (each division halving the per-cell signal) that is significantly greater than that of unstimulated (medium-only) control cultures.

Analyze FCS files in FlowJo. Gate: time/singlets (FSC-H vs FSC-A) → live (viability-negative) → lymphocytes (FSC/SSC) → CD3+ → CD4+ (exclude CD8+). Apply the FlowJo Proliferation Modeling tool to the CFSE histogram, anchoring generation 0 to the day-0/unstimulated peak. Extract % divided, division index, proliferation index, and per-generation frequencies. Background-subtract the matched unstimulated well from each antigen-stimulated well for net antigen-specific proliferation.

1. No CFSE-dilution ladder even in PHA control: free dye not washed out or cells over-labeled/toxic — reduce CFSE to 0.5-1 µM, ensure PBS (protein-free) labeling, and wash twice. 2. High background division in unstimulated wells: endotoxin in medium/serum or carryover stimulant — use low-endotoxin FBS and dedicated pipettes. 3. CFSE-high peak too dim to resolve generations: increase labeling concentration or reduce PMT voltage on the FITC/BB515 channel. 4. Low viability at day 6: seeding too dense or 2-ME omitted — confirm 2 x 10^5/well and fresh 2-ME. 5. Poor antigen response across all donors: antigen degraded or wrong dose — titrate antigen and verify with a known-responder donor.

Compare net % divided across conditions with a repeated-measures (paired) design since donors contribute to all conditions. Use paired t-test for two conditions or repeated-measures one-way ANOVA with Tukey's correction for ≥3 conditions; if non-normal (Shapiro-Wilk fail), use Wilcoxon signed-rank or Friedman with Dunn's correction. Set alpha = 0.05. With n = 8-10 donors and an expected paired effect size d ≈ 1.0, power is ~0.8. Report mean ± SEM and individual donor points.