Induction Methods, Experimental Design, and Functional Evaluation of In Vitro T-Cell Activation
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
(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 |
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 | |
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 | |
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 | |
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 | |
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 | |
CD28 Armenian Hamster mAb | Carrier-free, ExactAb™, azide-free, validated, PBS only, see COA | CD28 | CD28 costimulation and T-cell activation | |
Ionomycin (Calcium salt) | ≥98% (HPLC) | Intracellular Ca²⁺ and the Calcineurin-NFAT pathway | Calcium-signal activation and combined PMA/Ionomycin stimulation | |
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 | |
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 |
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 | |
CD28 Mouse mAb (APC) | ExactAb™, validated, Ex: 650 nm, Em: 660 nm, 5 μL/test | CD28 | Detection of CD28 expression and costimulatory phenotype | |
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 | |
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 | |
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 | |
CD154/CD40L Mouse mAb | Carrier-free, azide-free, validated, PBS only, ≥95% (SDS-PAGE), see COA | CD154/CD40L | Detection of activated CD4⁺ T cells | |
Recombinant ICOS Antibody | Recombinant, ExactAb™, validated, 3.0 mg/mL | ICOS | Detection of ICOS expression and costimulatory status | |
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 |
Human Interleukin 2 (IL-2) ELISA Kit | BioReagent | Human IL-2 | Detection of IL-2 in human T-cell culture supernatants | |
Mouse Interleukin 2 (IL-2) ELISA Kit | BioReagent | Mouse IL-2 | Detection of IL-2 after mouse T-cell stimulation | |
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 | |
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 | |
Human Cluster Of Differentiation 38 (CD38) ELISA Kit | BioReagent | Human CD38 | Detection of CD38 levels associated with sustained immune activation | |
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 | |
Mouse T-cell Immunoglobulin Mucin Receptor 3 (TIM-3) ELISA Kit | BioReagent | Mouse TIM-3 | Detection of mouse TIM-3 levels | |
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
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[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.