Does serum-free medium for NK cells still require additional supplementation with IL-2 and serum replacement?

Most NK serum-free basal media do not contain sufficient IL-2, or the concentration is maintained only for a short period, so supplementation is required during the mid-to-late expansion phase. We add IL-2 at 500-1000 IU/mL from day 0, and during medium replenishment every 2-3 days, we top up to the target concentration based on the newly added volume. IL-2 activity and manufacturer variability are substantial; it is best to use recombinant human IL-2 labeled in international units and avoid low-quality products containing carrier proteins.

When supplementing factors such as IL-2, pay attention to aliquoting and storage. Repeated freeze-thaw cycles will reduce IL-2 activity. It is recommended to aliquot according to single-use amounts and store at -80°C, then thaw at 4°C before use. When supplementing the medium, first calculate the newly added volume, then add the corresponding factors, avoiding dropping the factors directly onto the cell surface. For working solution preparation, use sterile PBS or culture medium. Regarding filtration, follow the manufacturer's instructions and do not filter through a 0.22 μm filter on your own, as some factors will be lost due to adsorption.

Serum replacement is not necessarily required. Serum-free NK culture media usually already contain albumin, transferrin, insulin, etc., but some people add 2-5% autologous plasma or platelet lysate to enhance expansion. This introduces batch-to-batch variability, and unless expansion is particularly poor, it is not recommended. We have conducted comparisons: adding autologous plasma accelerates early expansion, but purity declines in the later stages, with a slightly lower CD3-CD56+ ratio.

A starting amount of 5×10^6 PBMCs is suitable for small-scale preliminary experiments. It is recommended to set up several factor gradients: IL-2 alone at 500 and 1000 IU/mL; IL-2 + IL-15; IL-2 + IL-15 + IL-21. Use a seeding density of 1-2×10^6/ml and try a 24-well plate first. Count and replenish medium every 2 days, and check the expansion fold and CD56 flow cytometry around day 14 of culture. 100 ml is generally enough for 6-10 wells, and repeated opening of the cap should be avoided to prevent contamination.

When the instructions are not clear, do not guess—contact the manufacturer's technical support directly for the cytokine supplementation protocol. There are significant differences between brands: some media come with IL-2, IL-15, IL-18, etc., while others use only basal medium and require separate supplementation. Small 100 ml packaging is often available, but supply depends on the model; confirm stock and shelf life before ordering. Cold-chain shipping generally takes 3–7 days to arrive, and in summer, ice packs and a thermometer are required.