IFN-γ ELISpot Assay for Enumerating Antigen-Specific T Cells in Human PBMCs Following Peptide Pool Stimulation

Each condition is plated in technical triplicate (or quadruplicate) wells. Conditions: (1) peptide pool (e.g., CEF or pathogen-specific 15-mers, 1-2 µg/mL/peptide); (2) medium-only negative/background; (3) anti-CD3 (or PMA/ionomycin) positive control; (4) DMSO vehicle control matched to peptide diluent. Input 2-4 x 10^5 PBMCs per well. Plates incubate 18-20 h (overnight) at 37 °C, 5% CO2. Counts are read on an automated ELISpot reader by an operator blinded to plate layout. Plan n = 8-12 donors.

BSL-2 for human PBMC handling in a Class II biosafety cabinet. NBT/BCIP substrate is an irritant; dimethylformamide in some substrate kits is toxic and a reproductive hazard — handle in a fume hood and dispose as hazardous waste. PMA is a tumor promoter — handle with care. PPE: gloves, lab coat, eye protection. Decontaminate cell/liquid waste with 10% bleach; autoclave solids. Follow bloodborne-pathogen procedures.

Negative/background: medium-only wells define spontaneous IFN-γ secretion. Vehicle control: DMSO at the peptide-matched concentration excludes solvent-driven background. Positive control: anti-CD3 or PMA/ionomycin confirms cell viability and IFN-γ secretion competence (expect confluent/too-numerous-to-count spots). A no-cell well checks for nonspecific substrate/antibody background. A known-responder reference PBMC lot (e.g., CMV+ donor with CEF pool) validates assay sensitivity each run.

Medium-only background is typically <10-25 SFU/well; well-prepared PBMCs give <5 SFU. Antigen-specific responses commonly range from 50 to >1000 SFU per 10^6 PBMCs depending on antigen and donor. Positive-control wells are usually saturated/confluent. A response is scored positive if it exceeds background by both a fold (≥2-3x) and an absolute (>5-10 net SFU/well) threshold (DFR or distribution-free resampling rule).

• Pre-coated or self-coated human IFN-γ ELISpot PVDF 96-well plates (capture mAb 1-D1K, 10 µg/mL coat)
• Detection antibody: biotinylated anti-IFN-γ (7-B6-1-biotin), 1 µg/mL
• Streptavidin-ALP (alkaline phosphatase) conjugate, 1:1000
• BCIP/NBT-plus substrate (precipitating chromogen)
• Human PBMCs, 2-4 x 10^5/well, ≥90% viability
• Peptide pool (15-mer overlapping or CEF control pool), 1-2 µg/mL final; DMSO vehicle matched
• Anti-CD3 (clone CD3-2, 0.1 µg/mL) or PMA (50 ng/mL)/ionomycin (1 µg/mL) positive control
• AIM-V or RPMI 1640 + 10% human AB serum, 1% pen/strep (serum-free or low-serum preferred for low background)
• PBS; PBS-0.05% Tween wash buffer; 70% ethanol for PVDF pre-wetting

To quantify the frequency of antigen-specific, IFN-γ-producing T cells in human PBMCs by capturing secreted IFN-γ on a membrane-bound capture antibody and visualizing individual responding cells as discrete spots. The readout — spot-forming units per 10^6 input cells — provides a sensitive (detection limit ~1 in 10^5-10^6) measure of functional, antigen-specific effector T-cell frequency for vaccine immunogenicity or infection-recall studies.

1. If self-coating, coat plates overnight at 4 °C with capture antibody; pre-wet PVDF with 15 µL 35% ethanol for 1 min, then wash, before coating.
2. Wash coated plates 4x with sterile PBS; block with complete medium 1-2 h at 37 °C.
3. Discard block; add 50 µL stimulus (peptide, anti-CD3, DMSO, or medium) per well.
4. Add 100 µL PBMC suspension (2-4 x 10^5 cells) per well; final 150 µL.
5. Incubate 18-20 h at 37 °C, 5% CO2 — do NOT disturb or move the plate (vibration causes streaking).
6. Discard cells; wash 5x with PBS-0.05% Tween.
7. Add biotinylated detection antibody (1 µg/mL, 100 µL), 2 h at room temperature.
8. Wash 5x; add streptavidin-ALP (1:1000, 100 µL), 1 h at room temperature.
9. Wash 5x; add filtered BCIP/NBT substrate, develop 5-15 min until spots appear; stop by washing with tap water.
10. Air-dry plates in the dark; read on an automated ELISpot reader.

Independent: stimulation condition (peptide vs. medium vs. vehicle vs. polyclonal). Dependent: spot-forming units per well and per 10^6 PBMCs, and spot size/intensity. Controlled: PBMC input number, peptide concentration, overnight incubation time (18-20 h), serum lot/type, plate coating, wash stringency, substrate development time, and reader spot-size/intensity gating thresholds.

PBMCs from antigen-experienced donors stimulated with a cognate peptide pool will produce a significantly higher number of IFN-γ spot-forming units than unstimulated (medium-only) wells, and this antigen-specific response will exceed the assay's positivity threshold (e.g., ≥3x mean background and >5 SFU above background per well).

Read all wells with consistent reader settings (spot size, intensity, gradient). Average replicate wells, subtract mean background (medium-only), and normalize to SFU per 10^6 PBMCs given the input cell number. Flag wells with confluence, debris, or saturation. Apply a predefined positivity rule (e.g., distribution-free resampling, DFR2x). Export per-donor net SFU/10^6 for group comparison.

1. High background everywhere: cells over-rested or serum-driven activation — use serum-free AIM-V or pre-screened low-background human AB serum and reduce input number. 2. Streaky/smeared spots: plate disturbed during incubation — keep the plate motionless on a level shelf, away from incubator fans. 3. No spots in positive control: cells dead or detection chain failed — check viability, verify streptavidin-ALP and substrate freshness. 4. Spots too faint to count: under-developed substrate — extend development a few minutes, ensure correct pH. 5. Edge-well artifacts: evaporation — use a humidified incubator and avoid outermost wells or fill them with PBS.

Use paired non-parametric tests (Wilcoxon signed-rank) to compare antigen vs. medium within donors since spot counts are typically right-skewed; for ≥3 conditions use Friedman with Dunn's correction. Across treatment groups use Mann-Whitney or Kruskal-Wallis. Apply Benjamini-Hochberg FDR if testing multiple peptide pools. Alpha = 0.05. With n = 10 donors and a large within-donor effect, power exceeds 0.8. Report median and interquartile range.