Real-time Monitoring of Mitochondrial Respiration in Cytokine-differentiated Human Primary T Cells

Mitochondrial metabolism is an essential regulator of T cell durability and memory. Measuring mitochondrial respiration therefore gives important insights into the properties of T cells. The Seahorse machine can measure various elements of mitochondrial respiration, in real time, in live human primary T lymphocytes, without the need for any labeling.

This method is very suitable for monitoring T cell fitness and memory potential, which are important predictors of cancer immunotherapy success. To begin, wash CD3/CD28 beads by transferring 12.5 microliters of beads per million cells to a microcentrifuge tube and adding 12.5 microliters of PBS per 12.5 microliters of the beads in the tube. Then, place the microcentrifuge tube on a suitable magnet for one minute, discard the buffer, and resuspend the beads in the original volume of T cell medium.

Next, add 12.5 microliters of beads per million cells at a one to two beads-to-cells ratio, and divide the cells into two conditions with around five million cells in each. Then, add the correct volume of cytokines to each condition, and transfer the cells to multi-well plates. Incubate the cells for three days at 37 degree Celsius and 5%carbon dioxide.

After three days of incubation, prepare fresh T cell medium with double the concentration of cytokines. Resuspend and split the cells by transferring half the volume from each well into a new well. Then, add the same volume of freshly prepared T cell medium to each well.

For the extracellular flux assay, prepare coating solution containing sodium bicarbonate, Cell-Tak, and sodium hydroxide. Then, open a fresh XF cell culture plate, and add 12 microliters of the freshly prepared coating solution to each well, ensuring even distribution of coating solution at the bottom of all wells. Incubate the plate at room temperature with the lid on for 30 minutes.

Then, discard remaining liquid solution from all the wells. Wash the plate with 200 microliters of sterile water, and discard the liquid. Next, wash the plate with 200 microliters of cell culture-grade sterile PBS.

After discarding the liquid, allow the plate to dry for 30 minutes at room temperature. To plate T cells in the XF cell culture plate, prepare 50 milliliters of assay media by mixing suitable XF RPMI media with glucose, pyruvate, and glutamine according to the experimental setup. Heat the prepared media to 37 degree Celsius in a non-carbon dioxide regulated incubator, and set the pH to 7.4.

Ensure enough media for planting cells and preparing the oligomycin and FCCP solutions. Next, design a plate layout with an increasing number of cells per well for the optimization or assay run. Use four wells filled with media and injected with media for background measurements.

After counting the T cells prepared earlier, pipette an appropriate number of cells to each well of the previously coated XF cell culture plate according to the plate layout. To adhere the T cells to the coated surface, centrifuge the XF cell culture plate. Then, wash the cells with 200 microliters of assay media.

Discard the media, and add 180 microliters of fresh assay media. After ensuring that the cells are attached and evenly distributed across the well surface, incubate the XF cell culture plate at 37 degrees Celsius in the non-carbon dioxide regulated heating cabinet for 30 minutes. For the optimization run, prepare working solutions of oligomycin and FCCP in assay media.

Then, load the working solutions of either oligomycin or FCCP into the injection ports of the sensor cartridge. Ensure that no injection ports contain only air. Fill all empty ports with assay media.

Then, remove potential bubbles in the injection ports by gently knocking the edges of the plate on the table. For the assay run, prepare a 20-micromolar antimycin A solution. Then, according to the plate layout, load 20 microliters of either oligomycin or FCCP into injection port A of the sensor cartridge, and add 22 microliters of antimycin A to injection port B of all wells.

In a representative oligomycin optimization run, a plateau in oxygen consumption rate, or OCR, is reached when the accumulative concentration reaches one micromole. From this concentration onward, OCR was not reduced further. For cells treated with increasing concentrations of the uncoupler FCCP, OCR levels increased up to 0.2 micromolar FCCP and then reached a plateau, indicating that full uncoupling was obtained.

In a representative cell concentration optimization run, the initial OCR for a run with 200, 000 cells is approximately half of that, with 400, 000 cells. After FCCP treatment, maximal OCR is 61.6 picomoles per minute for 200, 000 cells and 190.4 picomoles per minute for 400, 000 cells. Following oligomycin treatment, the OCR in the run with 200, 000 cells collapses into single digits and is lower than that in the run with 400, 000 cells.

Investigating the effect of cytokines interleukin-2 and interleukin-15 on the metabolism of T cells revealed that interleukin-15 cultured cells possessed higher maximal respiration and spare respiratory capacity. However, basal respiration and ATP production were not affected. Studying mitochondrial metabolism in human T cells gives important clues on their durability and survival potential.

This results in increased knowledge of T cell fitness and can ultimately improve cancer immunotherapy response rates.