Flow Cytometry-based Assay for the Monitoring of NK Cell Functions
October 30th, 2016
A simple and reliable method is described here to analyze a set of NK cell functions such as degranulation, cytokine and chemokine production within different NK cell subsets.
The overall goal of this procedure is to monitor NK cell functions using a flow cytometry-based assay. Natural killer cells are crucial for the outcome of various viral infections and malignant diseases. To better monitor NK cell functions, our lab has further developed a flow cytometry-based assay that can be used in both scientific research and clinical evaluations.
We optimized the protocol to permit the analysis of small NK cell sample sizes which is essential for its use in pediatrics and for immunodeficient patients. An additional advantage of this technique is the possibility to measure NK cell functions not only within the whole NK cell population, but also in different NK cell subsets. To isolate the NK cells, begin by mixing the appropriate volume of NK cell isolation cocktail solution with six to 10 milliliters of EDTA peripheral whole blood from a patient by several inversions.
Next, further homogenize the sample on a tube rotator for five minutes at room temperature. Then place the tube into a magnetic separator for 15 minutes under sterile conditions, taking care that the tube label faces toward the back of the magnet to facilitate the visibility of the separation line. At the end of the separation, carefully transfer the supernatant into a new 15 milliliter tube and bring the final volume up to 15 milliliters with complete medium.
Wash the cells by centrifugation and resuspend the pellet in one milliliter of fresh complete medium. Then count the cells and adjust the volume to a concentration of at least two times 10 to the fourth NK cells per 100 microliters in complete medium. To stimulate the isolated NK cells, first gently mix by pipetting at least five times and aliquot 100 microliters of the cells into the appropriate wells of a 96-well V-bottomed plate.
Switch off the light and add anti-CD107a antibody to each well of the NK cells. Then carefully mix an aliquot of K562 tumor cells at a two times 10 to the fourth cells per 100 microliter volume by pipetting at least five times and transfer 100 microliters of the tumor cells to each well planned for the stimulation. Precise pipetting and the preparation of a one-to-one effector target ratio are of key importance for the accurate assessment of NK cell function.
Next, add IL-2 and IL-15 to the stimulation wells and mix all of the wells by pipetting at least five times. Incubate the plate protected from light for three hours at 37 degrees Celsius and 5%carbon dioxide. After the first hour with the light switched off, add freshly prepared protein transport inhibitor solution to each well with careful mixing and return the plate to the incubator.
At the end of incubation, carefully mix the co-cultures and transfer them into their corresponding flow cytometry tubes. Add one milliliter of wash buffer to each tube for centrifugation and resuspend the pellets in 99 microliters of PBS plus one microliter of fixable dead cell marker per sample. Mix the cells by vortexing, followed by a 15-minute incubation at room temperature in the dark.
At the end of the incubation, wash the cells in one milliliter of wash buffer and resuspend the pellets in 84 microliters of wash buffer plus 16 microliters of the appropriate cell surface antibody cocktail. Mix the samples by vortexing and incubate for 20 minutes in the dark at four degrees Celsius. Then wash the cells in one milliliter of wash buffer and resuspend the pellets in 100 microliters of a cold fixation solution.
Mix by vortexing, followed by 10 minutes at four degrees Celsius in the dark. Now wash the cells in one milliliter of wash buffer and resuspend the pellets in 90 microliters of permeabilization buffer. Stain with 10 microliters of the appropriate intracellular antibodies and mix by vortexing.
After 30 minutes in the dark at four degrees Celsius, wash the cells two times in one milliliter of fresh permeabilization buffer. Then resuspend the samples in 400 microliters of fresh permeabilization buffer. Mix well by vortexing and place the cells on ice until their analysis by flow cytometry.
As soon as possible, transfer all tubes into the flow cytometer and start the data acquisition. Here, gating strategies for analyzing the de-granulation and cytokine and chemokine production of the whole NK cell population as well as three different NK cell subsets are illustrated. Unstimulated NK cells from healthy donors produced neither interferon gamma nor MIP-1 beta and do not express CD107a on their surface.
In contrast, NK cell stimulated with tumor cells and cytokines produced significant amounts of intracellular interferon gamma and MIP-1 beta with over 20%of the cells demonstrating de-granulation upon tumor cell interaction as indicated by their CD107a cell surface expression. Once mastered, this technique can be completed in eight hours. While attempting this procedure, it is important to be precise in the plating of the NK cells and K562 tumor cells.
To maintain the selected effector target ratio, the counting should be as accurate as possible. This flexible procedure can be easily modified. For example, NK cells within the PBMC population can be analyzed or different antibodies can be used to check for other markers of interest.
After watching this video, you should have a good understanding of how to purify, stimulate and label NK cells to monitor their function using a flow cytometry-based assay. Don't forget that working with human and target cells is potentially hazardous and that safety precautions such as wearing the appropriate personal protection equipment should always be taken while performing this procedure.
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Overview
This article describes a reliable flow cytometry-based assay to analyze natural killer (NK) cell functions, including degranulation and cytokine production. The method is optimized for small sample sizes, making it suitable for pediatric and immunodeficient patients.
Key Study Components
Area of Science
- Immunology
- Cell Biology
- Flow Cytometry
Background
- Natural killer cells play a vital role in combating viral infections and cancer.
- Monitoring NK cell functions is essential for understanding immune responses.
- Current methods may not be suitable for small sample sizes.
- This study aims to improve the analysis of NK cell subsets.
Purpose of Study
- To develop a flow cytometry-based assay for NK cell function analysis.
- To enable analysis of small NK cell samples.
- To assess NK cell functions in different subsets.
Methods Used
- Flow cytometry-based assay for NK cell analysis.
- Isolation of NK cells from peripheral whole blood.
- Use of NK cell isolation cocktail solution.
- Optimization for pediatric and immunodeficient patient samples.
Main Results
- Successful monitoring of NK cell functions.
- Ability to analyze small sample sizes effectively.
- Insights into NK cell subset functions.
- Potential applications in clinical evaluations.
Conclusions
- The developed assay is reliable for NK cell function analysis.
- It is particularly useful for small sample sizes in vulnerable populations.
- This method enhances understanding of NK cell roles in immunity.
Frequently Asked Questions
Executive Industry Relevance
Accurate assessment of natural killer (NK) cell function is critical for evaluating immune responses in oncology and infectious disease drug development. This flow cytometry-based assay enables quantitative monitoring of NK cell degranulation and cytokine production, supporting mechanistic de-risking of immunomodulatory therapies. The method’s compatibility with small sample sizes enhances its utility in early-phase clinical trials and pediatric populations, where limited biospecimen availability often constrains immune monitoring.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables functional validation of NK cell-engaging biologics by measuring degranulation (CD107a) and intracellular cytokine (IFN-γ, MIP-1β) release as direct readouts of target-mediated immune activation.
- Operational Value: Supports subset-specific analysis (e.g., CD56dim/CD56bright) to clarify mechanistic heterogeneity and de-risk target hypotheses across diverse donor backgrounds.
Screening & Assay Development
- Scientific Value: Provides a standardized, quantitative readout for screening immunomodulatory compounds or bispecific engagers designed to enhance NK cell effector functions.
- Operational Value: Optimized for low-input samples (≥2×10⁴ NK cells/100 µL), enabling assay feasibility in early discovery with limited primary cell yields from preclinical models or cryopreserved clinical samples.
Translational & Preclinical Research
- Scientific Value: Facilitates correlation of NK functional phenotypes with clinical outcomes in preclinical models, supporting biomarker qualification for immune effector activity.
- Operational Value: Enables longitudinal monitoring of NK cell function in PBMC or purified populations across study phases, improving translational continuity from discovery to IND-enabling studies.
Pipeline & Workflow Integration
The assay fits within the immunotherapy discovery continuum, linking early target engagement screening to functional immune monitoring in preclinical and early clinical development, particularly for NK cell-directed therapeutics.
- Discovery Biology: Supports hypothesis testing of immunomodulatory mechanisms by quantifying effector functions (degranulation, cytokine release) in response to tumor cell co-culture or cytokine stimulation.
- Screening: Delivers reproducible, quantitative flow cytometry outputs (e.g., % CD107a+, IFN-γ+ cells) that enable rank-ordering of compound effects on NK cell activation.
- Analytics: Generates multi-parametric data (surface/degranulation + intracellular cytokines) allowing simultaneous assessment of multiple functional endpoints per sample.
- Translational Research: Enables subset-resolved functional profiling to align preclinical NK activity with potential biomarkers of response in early clinical samples.
- Enterprise Reuse: Establishes a reusable platform for immune monitoring across multiple NK cell-targeting programs, reducing redundant assay development.
Operational & Enterprise Impact
- Scientific Value: Increases predictive confidence in NK cell-mediated mechanisms of action by providing direct, quantitative functional readouts.
- Operational Value: Delivers high reproducibility and scalability through standardized stimulation, staining, and gating strategies applicable across sites and operators.
- Strategic Value: Improves go/no-go decision quality by reducing biological false positives in immunomodulator screening through objective functional validation.
- Portfolio Impact: Enables risk-adjusted prioritization of NK cell-engaging candidates based on validated functional potency and subset-specific activity.
Implementation Considerations
- Requires expertise in flow cytometry compensation, gating strategy design, and intracellular cytokine staining protocols.
- Dependent on access to flow cytometers capable of detecting CD107a, IFN-γ, and MIP-1β with appropriate laser/filter configurations.
- Necessitates standardized cell culture conditions (IL-2/IL-15, K562 titration) and strict adherence to effector-to-target ratio (1:1) for assay consistency.
- Requires optimization when adapting to alternative target cells (e.g., patient-derived blanks) or NK cell sources (e.g., cord blood, iPSC-derived).
- Limited by the need for fresh or viability-preserved PBMC samples; cryopreservation may affect baseline NK function and requires validation per sample type.
