6.3.2　NK Cell Effector Function

Natural killer cells mediate their immunomodulatory effects through two critical effector functions.First, NK cells are cytotoxic lymphocytes that can directly lyse cells that have undergone a malignant transformation or have become infected with a virus or other intracellular pathogen.The cytolytic function of NK cells can initiate through a variety of processes, including degranulation and death receptor ligation, and is critical for the clearance of diseased and dysfunctional cells.Second, NK cells can produce a variety of inflammatory cytokines in response to activation receptor stimulation as well as inflammatory cytokine-induced activation signaling.These NK cell effector functions are essential components of the immune response and are the primary mechanisms through which NK cells mediate protective immunity [83-86].

The molecular mechanisms that regulate NK cell cytotoxicity have been well described and can be divided into three main processes: (1) target cell recognition, (2) target cell contact and immunological synapse (IS) formation, and (3) NK cell-induced target cell death.Distinct mechanisms have been described for how target cells are recognized by NK cells and how they deem diseased cells appropriate for destruction (Figure 6.8) [77].Once recognized, NK cells directly interact with the target cell of interest through the formation of a lytic IS which facilitates NK cell-induced target cell death through two essential mechanisms [85].

Figure 6.8　Role of a “third signal” in natural killer (NK) cell activation.(A) A brief description of the significant interactions between NK and myeloid cells.NK cells possess inherent abilities to mediate cytotoxicity and produce inflammatory cytokines and chemokines.Myeloid cell-derived cytokines play a central role in regulating the effector functions of NK cells.Interactions between the innate NK cells and the primary arms of the adaptive immunity (T and B cells) are less explored.Stimulation through activation receptors (i.e., NKG2D or Ly49H) help recognize tumor (H60) or infected target cells (murine cytomegalovirus-derived m157).(B) A summary of major soluble factors produced by NK cells and their intended functions.(Please scan the QR code on the Preface to get original color figures.)
Source: Abel A M, Yang C, Thakar M S, Malarkannan S.“Natural killer cells: Development, maturation,and clinical utilization”.FrontiersinImmunology, 2018, 9:1869.(https://www.daowen.com)

The first mechanism involves the activation of death receptors present on the surface of the target cell which initiates the extrinsic apoptotic pathway.These receptors include TNF-related apoptosis-inducing ligand-receptor (TRAIL-R) and Fas (CD95) which are activated by their cognate ligands, Fas ligand (FasL) (CD95L) and TRAIL, present on NK cells.The surface expression of death receptors can be induced on target cells by NK cell-derived IFN-γ, and their activation initiates many pro-apoptotic signaling programs.The death receptor superfamily is characterized by the utilization of a cytoplasmic death domain which enables these receptors to activate the apoptotic machinery including initiator caspases-8 and 10.Initiator caspases promote a cascade of IL1β-converting enzyme (ICE) superfamily proteases, including caspase-3 , and induce mitochondrial damage and cytochrome C release resulting in the formation of the apoptosome.The apoptosome amplifies initiator caspase-mediated substrate cleavage and, along with caspase-3-induced DNA fragmentation via caspase-activated DNase activation, results in cell death via apoptosis [87-89].

The primary mechanism of NK cell-mediated cytotoxicity involves the directed release of lytic molecules to the target cell.NK cells store these molecules in cytolytic granules that are delivered to the target cell through membrane fusion at the IS.This process requires cytoskeletal reorganization events including actin polymerization at the IS as well as polarization of the microtubule organizing center toward the target cell.Polarized lytic granules travel along microtubules and, once at the IS, fuse with the target cell membrane and release enzymes that facilitate that activation of the intrinsic apoptosis program within the target cell [90,91].

Natural killer cells are potent producers of pro-inflammatory and immunosuppressive cytokines.However, the release of inflammatory cytokines is distinct from cytotoxic granule secretion and NK cells utilize activation-induced signaling components to differentially regulate these two functions.Although NK cells can produce a wide-range of cytokines depending on the inflammatory environment, NK cells primarily produce Th1-type cytokines when responding to tumor ligands and intracellular pathogens.These include IFN-γ, TNF, and granulocyte/monocyte colony-stimulating factor (GM-CSF) which facilitate the activation of T cells as well as other innate immune mediators such as DCs, macrophages, and neutrophils.NK cells also produce chemotactic cytokines (chemokines) including CCL3 (MIP-1α), CCl4 (MIP-1β), CCL5 (RANTES),XCL1 (lymphotoxin), and CXCL8 (IL-8) which can attract effector lymphocytes and myeloid cells to inflamed tissues.Transcriptional activation of cytolytic molecules and inflammatory cytokines is a highly regulated process mediated by a variety of transcriptional regulators in NK cells.Many of these transcription factors, such as T-bet, are lineage defining and become activated early in NK cell development.Cytokine-induced activation of transcription factors, such as signal Transducers and Activators of Transcription (STAT) 4 and 5, occurs in response to IL-12 and IL-2 + IL-15 signaling, respectively.NKRs also initiate inflammatory transcriptional programs upon activation.These include the c-Fos and c-Jun heterodimer, AP-1, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), and nuclear factor of activated T cells which bind promotor regions and promote inflammatory cytokine gene transcription [77,92].