Alternatives to serum have been developed for the NK-92 cell line, but a broad comparison of SFM solutions for expansion of primary NK cells has not been reported. However, NK cells have remained difficult to culture in SFM, and clinical trials continue to use media with serum-based supplements. Previous studies have validated effective activation and expansion of T cells using Good Manufacturing Practice–grade serum-free media (SFM). Human alternatives, such as human serum, human platelet lysate and human plasma, have recently been suggested as a comparable or superior alternative to FBS, but concerns about supply, lot-to-lot variability and risk of infection remain. The use of FBS not only significantly increases the cost of NK cell therapy, but, most importantly, has several logistical and regulatory concerns, such as lot-to-lot variability and potential infections in cattle. However, activation and propagation of NK cells for therapeutic utility currently require media supplementation with fetal bovine serum (FBS) or human serum, plasma or platelet lysates to support optimal proliferation. NK cells expanded with engineered feeder cells have been delivered by adoptive transfer in several clinical trials, with encouraging results against multiple myeloma and myeloid leukemias, and also associated with decreased BK virus and cytomegalovirus reactivation following transplantation in Acute myeloid leukemia.
To address this limitation, a variety of ex vivo expansion methods have been developed using soluble or membrane-bound (mb) interleukin (IL)-15 or IL-21 along with feeder cells engineered to express 4-1BBL (CD137L) to promote large-scale NK cell propagation, ,. A major limitation of NK cell immunotherapy has been generating a sufficient number of cells required for infusion. NK cells have demonstrated their potential for cancer immunotherapy due to their ability to recognize and kill tumor targets without prior sensitization and the ability to use allogeneic donors without causing graft-versus-host disease. NK cells also secrete cytokines, such as interferon (IFN)–γ and TNF–α, which allow them to activate other immune cells. In contrast, inhibitory receptors, such as CD94, NKG2A and killer immunoglobulin-like receptors (KIR), recognize major histocompatibility complex class I as “self” and suppress NK cell activation, ensuring that healthy cells escape killing. Upon activation by target cells, NK cells kill malignant cells through the release of cytolytic granules such as granzymes and perforin or ligation of the death receptors FasL, tumor necrosis factor (TNF)–α and TNF-related apoptosis-inducing ligand (TRAIL). NK cell recognition of malignant cells is distinct from T cells in that the recognition is major histocompatibility complex–independent and relies instead on an array of activating and inhibitory receptors, including DNAM-1, NKG2D, CD16 and the natural cytotoxicity receptors NKp46, NKp44 and NKp30. Natural killer (NK) cells are highly cytotoxic cells of the innate immune system that recognize and kill virally infected and cancerous cells.
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