Unlocking the Power of NK Cells: Activation Mechanisms and Cytokine Stimulation

About NK Cells

Natural Killer (NK) cells play a critical role in the innate immune surveillance against cancer and viral infections. They are the "first responders" capable of spontaneously recognizing abnormal cells in the body, swiftly eliminating them through targeted cytotoxic mechanisms, and effectively producing pro-inflammatory cytokines and chemokines to recruit and activate other immune cells to initiate adaptive responses. Therefore, understanding the mechanisms of NK cell activation and identifying methods to stimulate their activation are crucial for the development of anti-tumor and antiviral drugs.

Mechanisms of NK Cell Activation

NK cells are regulated by a dynamic balance between a series of activating and inhibitory receptors. Normal cells express major histocompatibility complex class I molecules (MHC-I), also known as class I HLAs, which trigger inhibitory signaling in NK cells, preventing their attack. However, virus-infected cells or tumor cells often downregulate the expression of MHC-I to evade recognition by CD8+ cytotoxic T lymphocytes (CTLs) that rely on detecting antigen peptides presented by MHC-I. In contrast, the absence of inhibitory receptor ligands and/or upregulation of activating receptor ligands can activate NK cells, leading to their spontaneous attack. Once activated, NK cells induce apoptosis in target cells by releasing a large amount of perforin and granzymes.

Cytokine that Activates NK Cells

IL-2 and IL-15

IL-2 directly stimulates the proliferation and activation of T cells and NK cells. Initially, it was found that stimulating peripheral blood mononuclear cells with IL-2 leads to the expansion of cytotoxic cells, which were named lymphokine-activated killer (LAK) cells. Subsequent studies revealed that the destruction of tumors by LAK cells is primarily mediated by NK cells. Further research found that IL-2 can induce the production of inhibitory regulatory T cells (Tregs), which limit the application of IL-2 in activating NK cells.

IL-15 can effectively substitute IL-2 for the activation of NK cells because IL-2 and IL-15 share common receptor components, namely the β chain and γ chain heterodimer expressed on NK cells. Importantly, IL-15 stimulates NK and CD8+ T cells but not Treg cells. In vitro studies have shown that IL-15 enhances the function of NK cells depleted by the tumor microenvironment (TME). Under hypoxic conditions, treating dysfunctional NK cells from tumor sources with IL-15 can restore mitochondrial integrity, increase granzyme B expression, reduce cell apoptosis, enhance NK cell cytotoxicity, and promote IFNγ production[1].

Due to their strong impact on NK and cytotoxic T cells, various forms of recombinant IL-15 have been designed for clinical use. These include the complex of recombinant IL-15 and IL-15Rα (hetIL-15), which is a heterodimeric complex packaged in extracellular vesicles. Preclinical studies have shown that hetIL-15 can slow tumor growth, increase tumor infiltration by NK cells and CD8+ T cells, promote the secretion of IFNγ and cytotoxic granule components, and enhance the expression of the anti-apoptotic protein BCL-2[2].

IL-12

IL-12 is a heterodimeric pro-inflammatory cytokine primarily produced by antigen-presenting cells, especially dendritic cells (DCs) and macrophages. It stimulates the recruitment, effector functions, and IFNγ production of CD8+ T cells and NK cells. In addition to directly activating NK cells, IL-12 also stimulates the production of IFNγ and IL-2 by T helper 1 (TH1) cells, thereby promoting the production of more IFNγ, perforin, and granzyme B by NK cells[3].

IL-18

IL-18 belongs to the IL-1 cytokine family and is a pro-inflammatory cytokine produced by various cells, including myeloid cells, intestinal epithelial cells, keratinocytes, and endothelial cells. It is produced in the form of pro-IL-18 and is activated via the inflammasome pathway, undergoing cleavage mediated by caspase-1 to form the mature cytokine. It then binds to a heterodimeric receptor composed of IL-18Rα and IL-18Rβ subunits. IL-18, in combination with IL-12, induces the expression of FAS ligand (FASL) in NK cells and enhances the production of IFNγ and TNF[4].

IL-21

IL-21 is produced by TH17 cells and dendritic cells (DC), and it signals through a heterodimeric receptor composed of IL-12Rα and IL-12γc, which shares the γ chain with IL-2R and IL-15R. IL-21 enhances the maturation and proliferation of NK cells by increasing the expression of perforin and granzyme B, as well as upregulating NKp30 and NK cell receptor 2B4, thereby enhancing their cytotoxic activity[5].

Beta LifeScience offers research-grade and GMP-grade cytokine products including IL-2, IL-15, IL-12, IL-18, and IL-21. These cytokines have high activity and low endotoxin levels, facilitating the in vitro culture of NK cells.

Sample Product Data

Recombinant Human IL-15 (Cat. No.:BL-1715NP GMP-Grade)

Recommended Cytokines (For NK cells Activation)

References

[1] Yang Y, et al. Thioredoxin activity confers resistance against oxidative stress in tumor-infiltrating NK cells. J Clin Invest. 2020 Oct
[2] Watson DC, et al. Scalable, cGMP-compatible purification of extracellular vesicles carrying bioactive human heterodimeric IL-15/lactadherin complexes. J Extracell Vesicles. 2018 Feb
[3] Lasek W, et al. Interleukin 12: still a promising candidate for tumor immunotherapy? Cancer Immunol Immunother. 2014 May
[4] Konjević GM, et al. The role of cytokines in the regulation of NK cells in the tumor environment. Cytokine. 2019 May
[5] Skak K, et al. Interleukin-21 activates human natural killer cells and modulates their surface receptor expression. Immunology. 2008 Apr