ReviewActivation of NK cell cytotoxicity
Introduction
Natural killer (NK) cells are specialized innate lymphocytes capable of responding to virus-infected and tumor cells. Unlike cytotoxic T lymphocytes (CTL), NK cells do not require antigen-specific recognition to kill target cells, and as such are capable of limiting viral infection prior to the induction of adaptive immune responses. Compelling evidence for a critical role for NK cells in limiting viral infection has been provided by studies with herpesviruses, such as cytomegalovirus (CMV), herpes simplex virus (HSV) and Epstein-Barr virus (EBV), as well as HIV (Biron et al., 1999). NK cells also exhibit spontaneous cytotoxicity against major histocompatibility complex (MHC) class I-deficient target cells, and in particular they participate in the innate immune responses against transformed cells and tumor metastases in vivo (Smyth et al., 1999b, Smyth et al., 2002). The effector functions of NK cells, including cytotoxicity and the capacity to produce a variety of cytokines (including IFN-γ) following activation, are of pathophysiological importance. Granule exocytosis is the major mechanism of killing used by NK cells, however, these cells also express members of the tumor necrosis factor (TNF) superfamily, the role of which has recently begun to be elucidated. NK cell function is tightly regulated by a balance between positive and negative signals provided by a diverse array of cell surface receptors (Cerwenka and Lanier, 2001). Activation requires the action of pro-inflammatory cytokines in combination with differential engagement of cell surface receptors. In particular, cytokines such as IL-12, IL-15, IL-18, IL-21 and IFN-αβ can induce NK cell proliferation, as well as promoting NK cell cytotoxicity and/or production of IFN-γ (Biron et al., 1999). NK cells are inhibited by receptors that recognize MHC class I molecules, and thus healthy cells expressing normal levels of MHC class I are generally protected from NK cell-mediated lysis. Virus-infected and malignant cells may express reduced levels of MHC class I molecules and thus, provided that they also express appropriate ligands, they become vulnerable to NK cell attack. Thus triggering of activating receptors, accompanied by reduced signaling through inhibitory receptors, leads to NK cell activation. The activation of NK cells also results from the concerted action of co-stimulatory molecules already well characterized for their function in T cells. In this review we focus on the role and regulation of NK cell cytotoxicity.
Section snippets
NK cell effector functions
The direct killing of cancer or pathogen-infected cells is just one component of the total NK cell response. NK cell production of cytokines, such as IFN-γ, also restricts tumor angiogenesis and stimulates adaptive immunity. IFN-γ, secreted by NK cells, plays a critical role in suppressing pathogen challenge, both to contain the initial infection, and to promote an appropriate adaptive response that may take several days to mature. NK cells mediate cell killing though a variety of mechanisms,
Granule exocytosis – an introduction
The cytotoxic granules of NK cells are complex organelles that combine specialized storage and secretory functions with the general degradative functions of typical lysosomes. Granules package the mediators of a diverse range of cell-death pathways that have evolved to rapidly kill cells harbouring intracellular pathogens. Although pathogens have devised ways to prolong the life of an infected cell by blocking apoptotic cell death, the death mediated by an activated NK cell remains rapid,
Granulysin
Granulysin is a member of the saposin-like protein family that includes amoebapores and NK lysin. It is a cytolytic protein present in the granules of human CTLs and NK cells (Clayberger and Krensky, 2003) and is lytic against both microbes and tumors (Gamen et al., 1998, Stenger et al., 1998). Its structure suggests a potential mechanism of action whereby granulysin functions as a lytic molecule; the positive charges of granulysin appear to orient the molecule towards the negatively charged
Biological relevance of NK cell cytotoxic mechanisms
The relative importance of each of the granule proteins in NK cell cytotoxicity has been evaluated in vivo using gene-targeted mice, and deduced from patients carrying specific mutations in relevant genes. Granule proteins are expressed by most cytotoxic lymphocytes, including NK cells, CTLs, NKT cells and γδ+T cells. For the purpose of this review, we will limit the discussion of granule proteins to their role in NK cell function.
Functional consequences of perforin deficiency
Studies in gene-disrupted mice indicate that perforin is vital for NK cell cytotoxicity (Kagi et al., 1994) and it has an indispensable, but undefined, role in Grz-mediated apoptosis. The mapping of perforin mutations in a lethal, inherited human disorder of immune dysregulation known as familial haemophagocytic lymphohistiocytosis (FHL), also highlights the important role for perforin in human NK cell cytotoxicity (Stepp et al., 1999). A thorough examination of individual mutations in FHL
Functional consequences of Grz and granulysin deficiency
The role of Grz in NK cell cytotoxic responses remains comparatively poorly defined in vivo. Individual Grz show considerable functional redundancy, so that mice that are deficient in one, or even several Grz, have more focal immune deficits than perforin-deficient mice on the same genetic background. The NK cells of mice that lack GrzA (Ebnet et al., 1995) induce morphologically normal apoptosis in target cells in vitro, and those of mice that are deficient in the GrzB cluster (Heusel et al.,
TRAIL
Members of the TNF family of cytokines are expressed by NK cells, and are important mediators of apoptosis that both shape and regulate the immune system. TNF-related apoptosis-inducing ligand (TRAIL), also known as Apo2 ligand, is a type II transmembrane protein belonging to the TNF superfamily. Five receptors for TRAIL have been identified in humans, and two of them, TRAIL-R1 (DR4) and TRAIL-R2 (DR5), are capable of transducing an apoptotic signal (Degli-Esposti, 1999). The other three
Receptor classes – an introduction
When NK cell inhibitory receptors bind to MHC class I molecules, their effector functions (cytotoxicity and cytokine production) are blocked. Inhibitory receptors specific for MHC class I or MHC class I-related molecules can provide protection for target cells that express normal levels of class I molecules on their surface. Three such inhibitory receptor families have been discovered and characterized: the killer cell Ig-like receptors (KIR) in humans, the Ly49 lectin-like receptors in mice
Conclusions
There is still a great deal to learn about the biological importance of various NK cell cytotoxic pathways. In particular, a good understanding of what role Grz play in NK cell function remains to be established. It is still not clear whether Grz have important pro-apoptotic functions in vivo and at present their physiological importance appears restricted to particular viral infections. Perforin and TRAIL have emerged as important effectors in NK cell cytotoxicity against both tumors and
Acknowledgements
We thank Joe Trapani for helpful discussions. Work in our laboratories was supported by a Cancer Research Institute Post-Doctoral Fellowship to YH, an Melbourne Research Scholarship to EC, a Human Frontiers Science Program research grant to MJS and HY, National Health and Medical Research Council of Australia Project Grants to MJS and MAD-E, a Program Grant to MJS, a Dora Lush Postgraduate Award to SEAS and a Research Fellowship to MJS. MAD-E is supported by a Wellcome Trust Overseas Senior
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