Induction Methods, Experimental Design, and Functional Evaluation of In Vitro T-Cell Activation

In vitro T-cell activation commonly uses CD3/CD28 antibodies or PMA/Ionomycin stimulation to analyze TCR signaling, cell proliferation, metabolic reprogramming, cytokine secretion, and effector differentiation.

Keywords: T-cell activation; TCR-CD3; CD28; PMA; Ionomycin; cytokines; flow cytometry; T-cell exhaustion

1 Signaling Mechanisms of T-Cell Activation

1.1 Three Types of Signals Involved in T-Cell Activation

(1) TCR-CD3 Signaling

The T-cell receptor (TCR) recognizes peptide-MHC complexes on the surface of antigen-presenting cells, while the CD3 complex transmits the signal into the cell. Following aggregation of the TCR-CD3 complex, LCK phosphorylates immunoreceptor tyrosine-based activation motifs within CD3 and the ζ chain, promoting ZAP70 recruitment and activation, followed by formation of a signaling complex composed of LAT, SLP-76, and associated signaling proteins.

(2) CD28 Costimulatory Signaling

CD28 binds CD80 or CD86 on the surface of antigen-presenting cells and enhances PI3K-AKT-mTOR and NF-κB signaling, thereby promoting IL-2 transcription, glucose uptake, cell survival, and clonal expansion. In the absence of effective costimulation, TCR signaling may induce a hyporesponsive state in T cells, making sustained proliferation and complete effector function difficult to achieve.

(3) Cytokine Signaling

Activated T cells secrete IL-2, which activates JAK-STAT5 signaling through the IL-2 receptor and promotes cell-cycle entry and clonal expansion. Cytokines such as IL-12, IL-4, IL-6, TGF-β, and IL-21 further influence the differentiation of CD4⁺ T-cell subsets and the effector function of CD8⁺ T cells.

Figure 1 Three-signal model of T cell activation

1.2 Coordinated Activation of TCR Downstream Signaling

(1) Ca²⁺-Calcineurin-NFAT Pathway

PLCγ1 catalyzes the conversion of PIP₂ into IP₃ and DAG. IP₃ promotes Ca²⁺ release from the endoplasmic reticulum and induces store-operated calcium entry across the plasma membrane. After cytosolic Ca²⁺ increases, calmodulin activates Calcineurin, which dephosphorylates NFAT and promotes its translocation into the nucleus.

(2) DAG-PKCθ-NF-κB Pathway

DAG recruits and activates PKCθ, which further promotes formation of the CARD11, BCL10, and MALT1 signaling complex and ultimately induces NF-κB nuclear translocation, thereby participating in the expression of genes related to IL-2, cell survival, and inflammation.

(3) RAS-MAPK-AP-1 Pathway

DAG can also activate RasGRP and initiate the RAS-RAF-MEK-ERK cascade, promoting the expression of transcription factors such as FOS and JUN and the formation of AP-1. NFAT, NF-κB, and AP-1 jointly determine the transcription of multiple T-cell activation genes.

1.3 Metabolic Reprogramming After Activation

Resting T cells mainly rely on oxidative phosphorylation to maintain basal energy supply. After activation of TCR and CD28 signaling, T cells increase glucose uptake, glycolysis, glutamine utilization, and lipid synthesis, thereby providing energy and biosynthetic substrates for increased cell size, protein synthesis, and rapid division. AKT-mTOR signaling promotes glucose-transporter expression and anabolic metabolism during this process, whereas AMPK limits excessive biosynthesis and maintains metabolic homeostasis under conditions of energy insufficiency.

1.4 Proliferation and Differentiation After Activation

Activated T cells enter the cell cycle and undergo clonal expansion. CD4⁺ T cells can differentiate into Th1, Th2, Th17, Tfh, and other subsets, whereas CD8⁺ T cells can differentiate into cytotoxic T cells and kill target cells through perforin and granzymes. After antigen clearance, most effector cells undergo contraction, while a proportion develops into memory T cells.

2 Methods for In Vitro T-Cell Activation

2.1 CD3/CD28 Antibody Stimulation

Anti-CD3ε antibodies simulate TCR signaling by crosslinking the CD3 complex, whereas anti-CD28 antibodies provide costimulatory signaling. The two antibodies can be coated onto culture plates or conjugated to magnetic beads. Plate-bound antibodies are suitable for analyzing antibody concentration and signal strength, whereas CD3/CD28 magnetic beads are suitable for sustained stimulation and large-scale cell expansion.

CD3/CD28 antibody stimulation preserves proximal TCR signaling involving LCK, ZAP70, LAT, PLCγ1, and related molecules and is suitable for studies of T-cell proliferation, differentiation, metabolic changes, gene expression, and CAR-T-cell preparation. This method produces polyclonal stimulation and does not provide the antigen specificity of a defined peptide-MHC complex.

2.2 PMA/Ionomycin Stimulation

PMA mimics DAG and activates PKC- and RasGRP-associated signaling, whereas Ionomycin increases cytosolic Ca²⁺ concentration and activates the Calcineurin-NFAT pathway. Their combined use bypasses proximal TCR-CD3 signaling and directly promotes activation of NF-κB, AP-1, and NFAT.

PMA/Ionomycin is suitable for inducing cytokine expression and can serve as a strong-stimulation positive control for NFAT-, NF-κB-, and AP-1-associated responses. Because this method bypasses proximal TCR-CD3 signaling and induces signaling and transcriptional programs distinct from those produced by CD3/CD28 stimulation, it should not be used to assess the overall maximum response capacity of T cells or to analyze proximal signaling involving CD3, LCK, ZAP70, and LAT. High concentrations or prolonged stimulation may also cause cell death, receptor internalization, and nonphysiological transcriptional responses.

Table 1 Comparison of CD3/CD28 Antibody and PMA/Ionomycin Stimulation

Comparison Item	CD3/CD28 Antibody Stimulation	PMA/Ionomycin Stimulation
Level of action	TCR-CD3 and CD28 receptors	PKC, RasGRP, and Ca²⁺ signaling
Antigen specificity	None; polyclonal activation	None; receptor-independent stimulation
Proximal TCR signaling	Preserved	Bypassed
Major transcriptional pathways	NFAT, NF-κB, and AP-1	NFAT, NF-κB, and AP-1
Activation speed	Relatively slow	Rapid
Major applications	Proliferation, differentiation, metabolism, and cell expansion	Cytokine induction and strong-stimulation positive control
Major limitations	Affected by antibody clone, coating, and crosslinking method	Strongly nonspecific; excessive stimulation may reduce cell viability

3 Grouping and Detection Design for T-Cell Activation Experiments

3.1 Basic Experimental Groups

(1) Unstimulated Control Group

Cells are cultured only in complete medium to determine baseline activation-marker expression, cytokine levels, and cell-death levels.

(2) CD3 Stimulation Group

Only anti-CD3 antibody is added to analyze the effect of TCR signaling in the absence of CD28 costimulation.

(3) CD28 Stimulation Group

Only anti-CD28 antibody is added to determine whether the antibody has independent agonistic activity and to exclude nonspecific crosslinking effects.

(4) Combined CD3/CD28 Stimulation Group

TCR and costimulatory signals are provided simultaneously to induce complete T-cell activation and proliferation.

(5) PMA/Ionomycin Stimulation Group

This group serves as a strong receptor-independent stimulation group for evaluating NFAT-, NF-κB-, and AP-1-associated responses and cytokine-production capacity, but it cannot be used to determine the integrity of all downstream TCR signaling pathways.

3.2 Detection Time Points

(1) Early Signaling Detection

Phosphorylation of LCK, ZAP70, LAT, ERK, and AKT, changes in Ca²⁺, and NFAT nuclear translocation can be detected from several minutes to several hours after stimulation. This stage is suitable for studying signal initiation and is not suitable for evaluating activation solely on the basis of cell proliferation.

(2) Early Phenotypic Detection

CD69, CD154, and certain cytokines can be measured from several hours to 24 hours after stimulation. CD25 generally increases gradually after activation and can be used to evaluate IL-2 responsiveness and preparation for proliferation.

(3) Proliferation and Effector Detection

Cell division, cell number, CD25, ICOS, CD38, HLA-DR, and effector cytokines can be measured 48-96 hours after stimulation. When the culture period is extended, cell viability and culture-medium nutritional status should also be monitored.

(4) Sustained-Stimulation Detection

After sustained or repeated stimulation, inhibitory receptors including PD-1, TIM-3, LAG-3, and CTLA-4 can be detected, and the presence of dysfunction should be determined in combination with cytokine production, proliferation, and cytotoxic function.

3.3 Essential Experimental Controls

Control Type	Setup	Major Function
Unstimulated control	Complete medium only	Determines baseline expression and background signals
Single-stimulation control	CD3 or CD28 treatment alone	Distinguishes TCR and costimulatory effects
Strong-stimulation control	PMA/Ionomycin treatment	Evaluates downstream signaling and cytokine-production capacity
Viability-staining control	Use of a viability dye	Excludes nonspecific signals caused by dead cells
Single-staining control	Separate staining with each fluorescent antibody	Establishes the flow-cytometry compensation matrix
Fluorescence-minus-one control	Omission of one target antibody	Determines gating positions for weakly positive markers
Isotype control	Matched antibody isotype and fluorophore	Assists in evaluating nonspecific binding

4 CD3/CD28 Antibody-Induced T-Cell Activation Experiment

4.1 Cell Isolation

(1) Collect the spleen from a C57BL/6 mouse, mechanically dissociate it to prepare a single-cell suspension, and pass the suspension through a cell strainer to remove tissue debris.

(2) Treat the cell suspension with red blood cell lysis buffer, collect the leukocytes, and resuspend them.

4.2 T-Cell Isolation and Culture

(1) Use a mouse CD3 or CD8 cell isolation kit to obtain the target T cells by magnetic separation.

(2) Determine cell purity by flow cytometry and ensure that the purity of the target population is greater than 95%.

(3) Culture the cells in RPMI 1640 complete medium containing 2 mM Glutamine, 10% FBS, and Penicillin-Streptomycin at 37 °C with 5% CO₂.

4.3 Antibody Stimulation

(1) Coat a 96-well plate with anti-CD3ε antibody clone 145-2C11 and anti-CD28 antibody clone PV-1 at concentrations of 3 µg/mL and 1 µg/mL, respectively.

(2) Seed 1 × 10⁵ cells per well in 200 μL of complete medium.

(3) Incubate for 24 hours at 37 °C with 5% CO₂.

(4) Collect the cells, wash them with PBS, and assess T-cell activation by flow cytometry.

4.4 CD3/CD28 Magnetic-Bead Stimulation

(1) Adjust the T-cell density as required and resuspend the cells in complete medium.

(2) Add CD3/CD28 activation beads at a bead-to-cell ratio of 1:1.

(3) Culture the cells at 37 °C with 5% CO₂.

(4) After completion of the assay, remove the beads using a magnetic rack.

The continued presence of magnetic beads can prolong receptor stimulation and is suitable for cell expansion, but it cannot directly replace plate-bound antibodies in all signaling studies.

5 PMA/Ionomycin-Induced T-Cell Activation Experiment

5.1 Cell Preparation

Cell isolation, purification, and culture procedures are the same as those described in Sections 4.1 and 4.2.

5.2 PMA and Ionomycin Stimulation

(1) Seed 2 × 10⁵ cells per well in a 96-well plate with 200 μL of complete medium per well.

(2) Add PMA at final concentrations of 50, 100, and 500 ng/mL.

(3) Add Ionomycin to each treatment group at a final concentration of 100 ng/mL.

(4) Add complete medium only to the blank control group.

(5) Culture the cells for 24 hours at 37 °C with 5% CO₂.

(6) Collect the cells and wash them with PBS.

(7) Label dead cells using a fixable viability dye and assess T-cell activation by flow cytometry.

5.3 Evaluation of Stimulation Results

Increasing the PMA concentration may enhance cytokine expression but may also increase cell death and stress responses. Cell viability, CD69 or CD25 expression, and cytokine levels should be measured simultaneously in each concentration group to distinguish effective activation from excessive stimulation.

6 Detection and Interpretation of T-Cell Activation

6.1 Cell Viability and Sample Purity

Cell number, viability, and T-cell purity should be assessed before the activation experiment. An increased proportion of dead cells can increase nonspecific antibody binding and interfere with cytokine measurements. During flow-cytometric analysis, debris, doublets, and dead cells should first be excluded before analyzing the CD3⁺ T-cell population.

6.2 Activation Markers

CD69 can be used to evaluate early activation; CD25 reflects IL-2 responsiveness and preparation for proliferation; CD154 reflects the helper function of activated CD4⁺ T cells; ICOS reflects costimulatory and effector-differentiation status; and CD38 and HLA-DR can be used to evaluate sustained or relatively strong immune activation. CD127 may decrease temporarily after activation and should be interpreted according to culture duration and cell-differentiation status.

6.3 Cell Proliferation

Cell proliferation can be evaluated using cell counting, proliferation-dye dilution, DNA-synthesis assays, or cell-cycle analysis. Proliferation dyes can indicate the number of cell divisions, but viability staining should also be used to exclude signal changes caused by dead cells.

6.4 Cytokine Function

IL-2 reflects early activation and autocrine proliferative signaling; IFN-γ and TNF-α reflect the effector functions of Th1 cells and cytotoxic T cells; and IL-17 reflects Th17-associated differentiation. Cytokines in culture supernatants can be measured by ELISA, whereas cytokine production at the single-cell level can be analyzed by intracellular cytokine staining.

6.5 Cytotoxic Function

The cytotoxic function of CD8⁺ T cells can be evaluated by measuring perforin, Granzyme B, CD107a degranulation, and target-cell killing. Increased perforin and granzyme expression does not necessarily indicate that killing has occurred and should be interpreted together with degranulation or target-cell death results.

6.6 Sustained Stimulation and Functional Exhaustion

Sustained stimulation may be accompanied by increased expression of PD-1, TIM-3, LAG-3, and CTLA-4, but these molecules can also be transiently upregulated during normal activation. Functional exhaustion should be characterized by co-expression of multiple inhibitory receptors together with reduced proliferation, decreased cytokine secretion, or impaired target-cell killing.

Table 2 Common Indicators for Evaluating T-Cell Activation and Sustained Stimulation

Evaluation Category	Representative Indicators	Interpretation
Cell viability	Viability dyes and cell counting	Excludes stimulation toxicity and dead-cell interference
T-cell identification	CD3, CD4, and CD8	Defines the analyzed cell population
Early activation	CD69 and CD154	Reflects early activation after stimulation
Activation and proliferation readiness	CD25 and ICOS	Reflects IL-2 responsiveness and costimulatory status
Sustained activation	CD38 and HLA-DR	Reflects sustained or relatively strong immune activation
Cell proliferation	Proliferation dyes, DNA synthesis, and cell counting	Evaluates clonal expansion
Cytokine function	IL-2, IFN-γ, TNF-α, and IL-17	Evaluates effector function and subset differentiation
Cytotoxic function	Perforin, Granzyme B, and CD107a	Evaluates CD8⁺ T-cell killing-related function
Inhibitory phenotype	PD-1, TIM-3, LAG-3, and CTLA-4	Must be interpreted together with functional decline
Survival and memory association	CD127	Must be interpreted according to activation duration and differentiation status

7 Common Experimental Problems and Analysis of Their Causes

7.1 Low Expression of Activation Markers

Insufficient antibody coating, antibody inactivation, low cell purity, reduced cell viability, or inappropriate stimulation duration can all cause inadequate upregulation of CD69 and CD25. Antibody-storage conditions, coating procedures, cell density, and culture-medium status should be examined, and a PMA/Ionomycin strong-stimulation control should be included to determine whether the cells retain response capacity.

7.2 Increased Cell Death

Excessive PMA or Ionomycin concentration, prolonged stimulation, inappropriate cell density, excessive red blood cell lysis, and insufficient nutrients in the culture medium can all increase cell death. Viability and activation indicators should be compared simultaneously across different doses and stimulation durations.

7.3 Increased CD69 Without Cell Proliferation

Early activation signals can induce CD69 expression, but sustained proliferation also requires CD28 costimulation, IL-2 signaling, and sufficient nutrient availability. When only CD3 stimulation is provided or culture conditions are inadequate, CD69 may increase while cell expansion remains limited.

7.4 Increased Cytokine Levels Without Clear Changes in Surface Markers

PMA/Ionomycin can directly activate intracellular transcriptional pathways and may therefore markedly increase cytokine expression without fully reproducing receptor-dependent changes in surface markers. Such results should be interpreted as downstream functional activation rather than as a complete TCR-CD28 activation process.

7.5 Increased Expression of Inhibitory Receptors

Increased PD-1, TIM-3, LAG-3, or CTLA-4 expression may reflect normal negative feedback, sustained activation, or functional exhaustion. Cell proliferation, IL-2, IFN-γ, TNF-α, and cytotoxic function should be further evaluated, and conclusions should not be based on expression of a single receptor.

8 Products Related to T-Cell Activation, Culture, and Functional Detection

8.1 Products for T-Cell Isolation, Stimulation, and Culture

Catalog #	Description	Grade & Purity	Major Target	Main Research Application
H1522404	Human CD3+T Cell Sorting Kit (Negative Selection)	BioReagent	Human CD3⁺ T cells	Negative isolation of human CD3⁺ T cells from PBMCs and other samples
M1522398	Mouse CD3⁺ T Cell Isolation Kit (Negative Isolation)	BioReagent	Mouse CD3⁺ T cells	Negative isolation of CD3⁺ T cells from mouse spleen or lymphoid tissues
A1522429	Human CD3/CD28 T Cell Activation Magnetic Beads	BioReagent, 1 × 10⁸ beads/mL	CD3 and CD28 on human T cells	Human T-cell activation, expansion, and CAR-T-cell preparation
M1522427	Mouse CD3/CD28 T Cell Activation Magnetic Beads	1 × 10⁸ beads/mL	CD3 and CD28 on mouse T cells	Mouse T-cell activation and expansion
Ab094441	CD3 epsilon Syrian Hamster mAb	Carrier-free, low endotoxin, azide-free, validated, PBS only, ≥95% (SDS-PAGE and HPLC), see COA	CD3ε	TCR-CD3 signaling stimulation and receptor crosslinking
Ab094395	CD28 Armenian Hamster mAb	Carrier-free, ExactAb™, azide-free, validated, PBS only, see COA	CD28	CD28 costimulation and T-cell activation
I133497	Ionomycin (Calcium salt)	≥98% (HPLC)	Intracellular Ca²⁺ and the Calcineurin-NFAT pathway	Calcium-signal activation and combined PMA/Ionomycin stimulation
rp147476	Recombinant Human IL-2 Protein	Carrier-free, biologically active, ActiBioPure™, azide-free, high performance, His tag, ≥95% (SDS-PAGE)	Human IL-2 receptor	Human T-cell survival, proliferation, and expansion
rp154174	Recombinant Mouse IL-2 Protein	Animal-origin-free, carrier-free, biologically active, ActiBioPure™, azide-free, high performance, His tag, ≥98% (SDS-PAGE)	Mouse IL-2 receptor	Mouse T-cell survival, proliferation, and expansion

8.2 Antibodies for Phenotypic Detection of T-Cell Activation and Sustained Stimulation

Catalog #	Description	Grade & Purity	Major Target	Main Research Application
Ab179704	CD3 Mouse mAb (FITC)	ExactAb™, azide-free, validated, Ex: 498 nm, Em: 517 nm, 5 μL/test	CD3	Flow-cytometric identification of CD3⁺ T cells
Ab185381	CD28 Mouse mAb (APC)	ExactAb™, validated, Ex: 650 nm, Em: 660 nm, 5 μL/test	CD28	Detection of CD28 expression and costimulatory phenotype
Ab179011	CD25 Rat mAb (PE)	ExactAb™, azide-free, validated, Ex: 565 nm, Em: 575 nm, 5 μL/test	CD25/IL-2Rα	Detection of T-cell activation and IL-2 responsiveness
Ab178792	CD38 Mouse mAb (APC)	ExactAb™, azide-free, validated, Ex: 650 nm, Em: 660 nm, 5 μL/test	CD38	Detection of sustained or relatively strong immune activation
Ab184636	HLA-DR Mouse mAb (FITC)	Validated, Ex: 498 nm, Em: 517 nm, 5 μL/test	HLA-DR	Detection of sustained activation of human T cells
Ab132316	CD154/CD40L Mouse mAb	Carrier-free, azide-free, validated, PBS only, ≥95% (SDS-PAGE), see COA	CD154/CD40L	Detection of activated CD4⁺ T cells
Ab109267	Recombinant ICOS Antibody	Recombinant, ExactAb™, validated, 3.0 mg/mL	ICOS	Detection of ICOS expression and costimulatory status
Ab112575	LAG-3/CD223 Mouse mAb	Carrier-free, ExactAb™, validated, see COA	LAG-3/CD223	Detection of inhibitory phenotypes after sustained stimulation

8.3 Assay Kits Related to T-Cell Activation and Sustained Stimulation

Catalog #	Description	Grade & Purity	Major Target	Main Research Application
EJ1514358	Human Interleukin 2 (IL-2) ELISA Kit	BioReagent	Human IL-2	Detection of IL-2 in human T-cell culture supernatants
EJ1512916	Mouse Interleukin 2 (IL-2) ELISA Kit	BioReagent	Mouse IL-2	Detection of IL-2 after mouse T-cell stimulation
EJ1514373	Human Interleukin 2 Receptor Alpha (IL-2Rα/CD25) ELISA Kit	BioReagent	Human IL-2Rα/CD25	Quantitative detection of IL-2Rα/CD25 levels in samples as an auxiliary indicator of T-cell activation
EJ1513646	Human Inducible T-Cell Co Stimulator (ICOS) ELISA Kit	BioReagent	Human ICOS	Quantitative detection of ICOS levels in samples as an auxiliary measure in studies of T-cell costimulation
EJ1513459	Human Cluster Of Differentiation 38 (CD38) ELISA Kit	BioReagent	Human CD38	Detection of CD38 levels associated with sustained immune activation
EJ1513666	Human T-cell Immunoglobulin And Mucin Domain-containing Protein 3 (TIM-3) ELISA Kit	BioReagent	Human TIM-3	Detection of TIM-3 levels associated with sustained stimulation
EJ1512565	Mouse T-cell Immunoglobulin Mucin Receptor 3 (TIM-3) ELISA Kit	BioReagent	Mouse TIM-3	Detection of mouse TIM-3 levels
EJ1513942	Human Ble Co-stimulatory Molecule 4(sCTLA-4) ELISA Kit	BioReagent	Human soluble CTLA-4	Detection of immunosuppressive regulatory-factor levels

CD3/CD28 antibody stimulation is suitable for T-cell activation and expansion, whereas PMA/Ionomycin is suitable for cytokine induction and strong-stimulation controls. Activation or exhaustion status should be determined by integrating cell viability, surface markers, proliferation, cytokine production, and cytotoxic function.

References

[1] De Biasi S, et al. Marked T Cell Activation, Senescence, Exhaustion and Skewing Towards TH17 in Patients With COVID-19 Pneumonia. Nat Commun. 2020;11(1):3434.

[2] Rha MS, et al. Activation or Exhaustion of CD8+ T Cells in Patients With COVID-19. Cell Mol Immunol. 2021;18(10):2325-2333.

[3] Ozkazanc D, et al. Functional Exhaustion of CD4+ T Cells Induced by Co-Stimulatory Signals From Myeloid Leukaemia Cells. Immunology. 2016;149(4):460-471.

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