Impaired natural killing of MHC class I-deficient targets by NK cells expressing a catalytically inactive form of SHP-1

The diacylglycerol kinase ζ inhibitor ASP1570 augments natural killer cell function

The enhancement of T cell and NK cell function is an immunotherapeutic strategy for combating cancer. Antibodies that block inhibitory receptors, such as PD-1 and CTLA4, augment T cell function and have been successful in curing patients with some types of cancer. As an alternative approach to targeting specific inhibitory receptors by antibodies, small molecule drugs that inhibit negative regulators of T cell activation have been sought. One potential pharmacological target is diacylglycerol (DAG) kinase (DGK)ζ, which is an enzyme that acts as a negative regulator of DAG by phosphorylating DAG and converting it into phosphatidic acid. DAG-mediated signaling is critical for T cell activation through its T cell receptor and NK cell activation downstream of a variety of activating receptors. Thus, DGKζ-deficient T cells and NK cells display increased function upon activating receptor engagement. Moreover, treatment with the DGKζ-selective inhibitor ASP1570 augments T cell function. In this study, we sought to test whether the acute inhibition of DGKζ by ASP1570 augments NK cell function. We find that ASP1570 enhances DAG-mediated signaling in immunoreceptor-stimulated NK cells. Accordingly, ASP1570 treatment enhanced IFNγ production and degranulation of immunoreceptor-activated NK cells in vitro and NK cell-mediated tumor clearance in vivo. Thus, ASP1570 enhances both T and NK cell function, which could possibly induce more durable anti-tumor responses for immunotherapy.

SHP-1 localization to the activating immune synapse promotes NK cell tolerance in MHC class I deficiency

Natural killer (NK) cells recognize virally infected cells and tumors. NK cell function depends on balanced signaling from activating receptors, recognizing products from tumors or viruses, and inhibitory receptors (such as KIR/Ly49), which recognize major histocompatibility complex class I (MHC-I) molecules. KIR/Ly49 signaling preserves tolerance to self but also conveys reactivity toward MHC-I-low target cells in a process known as NK cell education. Here, we found that NK cell tolerance and education were determined by the subcellular localization of the tyrosine phosphatase SHP-1. In mice lacking MHC-I molecules, uneducated, self-tolerant Ly49A⁺ NK cells showed accumulation of SHP-1 in the activating immune synapse, where it colocalized with F-actin and the signaling adaptor protein SLP-76. Education of Ly49A⁺ NK cells by the MHC-I molecule H2D^d led to reduced synaptic accumulation of SHP-1, accompanied by augmented signaling from activating receptors. Education was also linked to reduced transcription of Ptpn6, which encodes SHP-1. Moreover, synaptic SHP-1 accumulation was reduced in NK cells carrying the H2D^d-educated receptor Ly49G2 but not in those carrying the noneducating receptor Ly49I. Colocalization of Ly49A and SHP-1 outside of the synapse was more frequent in educated compared with uneducated NK cells, suggesting a role for Ly49A in preventing synaptic SHP-1 accumulation in NK cell education. Thus, distinct patterning of SHP-1 in the activating NK cell synapse may determine NK cell tolerance.

Combinatorial Expression of NK Cell Receptors Governs Cell Subset Reactivity and Effector Functions but Not Tumor Specificity

NK cell receptors allow NK cells to recognize targets such as tumor cells. Many of them are expressed on a subset of NK cells, independently of each other, which creates a vast diversity of receptor combinations. Whether these combinations influence NK cell antitumor responses is not well understood. We addressed this question in the C57BL/6 mouse model and analyzed the individual effector response of 444 mouse NK cell subsets, defined by combinations of 12 receptors, against tumor cell lines originating from different tissues and mouse strains. We found a wide range of reactivity among NK subsets, but the same hierarchy of responses was observed for the different tumor types, showing that the repertoire of NK cell receptors does not encode for different tumor specificities but for different intrinsic reactivities. The coexpression of CD27, NKG2A, and DNAM-1 identified subsets with relative cytotoxic specialization, whereas reciprocally, CD11b and KLRG1 defined the best IFN-γ producers. The expression of educating receptors Ly49C, Ly49I, and NKG2A was also strongly correlated with IFN-γ production, but this effect was suppressed by unengaged receptors Ly49A, Ly49F, and Ly49G2. Finally, IL-15 coordinated NK cell effector functions, but education and unbound inhibitory receptors retained some influence on their response. Collectively, these data refine our understanding of the mechanisms governing NK cell reactivity, which could help design new NK cell therapy protocols.

Dynamic variability in SHP-1 abundance determines natural killer cell responsiveness

[Figure: see text].

CD155: A Key Receptor Playing Diversified Roles

Cluster of differentiation (CD155), formerly identified as poliovirus receptor (PVR) and later as immunoglobulin molecule involved in cell adhesion, proliferation, invasion and migration. It is a surface protein expressed mostly on normal and transformed malignant cells. The expression of the receptor varies based on the origin of tissue. The expression of the protein is determined by factors involved in sonic hedgehog pathway, Ras-MEK-ERK pathway and during stress conditions like DNA damage response. The protein uses alternate splicing mechanism, producing four isoforms - two being soluble (CD155β and CD155γ) and two being transmembrane protein (CD155α and CD155δ). Apart from being a viral receptor, researchers have identified CD155 having important roles in cancer research and cell signaling field. The receptor is recognized as biomarker for identifying cancerous tissue. The receptor interacts with molecules involved in cells defense mechanism. The immune-surveillance role of CD155 is being deciphered to understand the mechanistic approach it utilizes as onco-immunologic molecule. CD155 is a non-MHC-I ligand which helps in identifying non-self to NK cells via an inhibitory TIGIT ligand. The TIGIT-CD155 pathway is a novel MHC-I-independent education mechanism for cell tolerance and activation of NK cell. The receptor also has a role in metastasis of cancer and trans endothelial mechanism. In this review, authors discuss the virus-host interaction that occurs via single transmembrane receptor, the poliovirus infection pathway, which is being exploited as therapeutic pathway. The oncolytic virotherapy is now promising way for curing cancer.

NK Cell Development in Times of Innate Lymphoid Cell Diversity

After being described in the 1970s as cytotoxic cells that do not require MHC-dependent pre-activation, natural killer (NK) cells remained the sole member of innate lymphocytes for decades until lymphoid tissue-inducer cells in the 1990s and helper-like innate lymphoid lineages from 2008 onward completed the picture of innate lymphoid cell (ILC) diversity. Since some of the ILC members, such as ILC1s and CCR6^- ILC3s, share specific markers previously used to identify NK cells, these findings provoked the question of how to delineate the development of NK cell and helper-like ILCs and how to properly identify and genetically interfere with NK cells. The description of eomesodermin (EOMES) as a lineage-specifying transcription factor of NK cells provided a candidate that may serve as a selective marker for the genetic targeting and identification of NK cells. Unlike helper-like ILCs, NK cell activation is, to a large degree, regulated by the engagement of activating and inhibitory surface receptors. NK cell research has revealed some elegant mechanisms of immunosurveillance, coined "missing-self" and "induced-self" recognition, thus complementing "non-self recognition", which is predominantly utilized by adaptive lymphocytes and myeloid cells. Notably, the balance of activating and inhibitory signals perceived by surface receptors can be therapeutically harnessed for anti-tumor immunity mediated by NK cells. This review aims to summarize the similarities and the differences in development, function, localization, and phenotype of NK cells and helper-like ILCs, with the purpose to highlight the unique feature of NK cell development and regulation.

Natural Killer Cells: Tumor Surveillance and Signaling

Natural killer (NK) cells play a pivotal role in cancer immunotherapy due to their innate ability to detect and kill tumorigenic cells. The decision to kill is determined by the expression of a myriad of activating and inhibitory receptors on the NK cell surface. Cell-to-cell engagement results in either self-tolerance or a cytotoxic response, governed by a fine balance between the signaling cascades downstream of the activating and inhibitory receptors. To evade a cytotoxic immune response, tumor cells can modulate the surface expression of receptor ligands and additionally, alter the conditions in the tumor microenvironment (TME), tilting the scales toward a suppressed cytotoxic NK response. To fully harness the killing power of NK cells for clinical benefit, we need to understand what defines the threshold for activation and what is required to break tolerance. This review will focus on the intracellular signaling pathways activated or suppressed in NK cells and the roles signaling intermediates play during an NK cytotoxic response.

Synergized regulation of NK cell education by NKG2A and specific Ly49 family members

Mice lacking MHC class-I (MHC-I) display severe defects in natural killer (NK) cell functional maturation, a process designated as “education”. Whether self-MHC-I specific Ly49 family receptors and NKG2A, which are closely linked within the NK gene complex (NKC) locus, are essential for NK cell education is still unclear. Here we show, using CRISPR/Cas9-mediated gene deletion, that mice lacking all members of the Ly49 family exhibit a moderate defect in NK cell activity, while mice lacking only two inhibitory Ly49 members, Ly49C and Ly49I, have comparable phenotypes. Furthermore, the deficiency of NKG2A, which recognizes non-classical MHC-Ib molecules, mildly impairs NK cell function. Notably, the combined deletion of NKG2A and the Ly49 family severely compromises the ability of NK cells to mediate “missing-self” and “induced-self” recognition. Therefore, our data provide genetic evidence supporting that NKG2A and the inhibitory members of Ly49 family receptors synergize to regulate NK cell education.

MHC-I-induced signalling of various natural killer (NK) inhibitory receptors is critical for regulation NK cell education, but clear genetic evidence is still lacking. Here the authors generate multiple lines of mice differentially deficient in Ly49 family and/or NKG2A NK receptors, and find that self-MHCI specific Ly49 members and NKG2A synergize to regulate NK education.

SHP-2 in Lymphocytes' Cytokine and Inhibitory Receptor Signaling

Somewhat counterintuitively, the tyrosine phosphatase SHP-2 (SH2 domain-containing protein tyrosine phosphatase-2) is crucial for the activation of extracellular signal-regulated kinase (ERK) downstream of various growth factor receptors, thereby exerting essential developmental functions. This phosphatase also deploys proto-oncogenic functions and specific inhibitors have recently been developed. With respect to the immune system, the role of SHP-2 in the signaling of cytokines relevant for myelopoiesis and myeloid malignancies has been intensively studied. The function of this phosphatase downstream of cytokines important for lymphocytes is less understood, though multiple lines of evidence suggest its importance. In addition, SHP-2 has been proposed to mediate the suppressive effects of inhibitory receptors (IRs) that sustain a dysfunctional state in anticancer T cells. Molecules involved in IR signaling are of potential pharmaceutical interest as blockade of these inhibitory circuits leads to remarkable clinical benefit. Here, we discuss the dichotomy in the functions ascribed to SHP-2 downstream of cytokine receptors and IRs, with a focus on T and NK lymphocytes. Further, we highlight the importance of broadening our understanding of SHP-2′s relevance in lymphocytes, an essential step to inform on side effects and unanticipated benefits of its therapeutic blockade.

Shp-2 is critical for ERK and metabolic engagement downstream of IL-15 receptor in NK cells

The phosphatase Shp-2 was implicated in NK cell development and functions due to its interaction with NK inhibitory receptors, but its exact role in NK cells is still unclear. Here we show, using mice conditionally deficient for Shp-2 in the NK lineage, that NK cell development and responsiveness are largely unaffected. Instead, we find that Shp-2 serves mainly to enforce NK cell responses to activation by IL-15 and IL-2. Shp-2-deficient NK cells have reduced proliferation and survival when treated with high dose IL-15 or IL-2. Mechanistically, Shp-2 deficiency hampers acute IL-15 stimulation-induced raise in glycolytic and respiration rates, and causes a dramatic defect in ERK activation. Moreover, inhibition of the ERK and mTOR cascades largely phenocopies the defect observed in the absence of Shp-2. Together, our data reveal a critical function of Shp-2 as a molecular nexus bridging acute IL-15 signaling with downstream metabolic burst and NK cell expansion.

Contribution of inhibitory receptor TIGIT to NK cell education

Engagement of inhibitory receptors by cognate host MHC-I molecules triggers NK cell education, resulting in functional maturation and allowing NK cells to sense missing-self. However, NK cells also express inhibitory receptors for non-MHC-I ligands and their role in NK cell education is poorly understood. TIGIT is a recently identified inhibitory receptor that recognizes a non-MHC-I ligand CD155. Here, we demonstrated that TIGIT⁺ NK cells from wild-type mice exerted augmented responsiveness to various stimuli, including targets that lacked expression of CD155 ligand. TIGIT⁺ NK cells derived from CD155-deficient hosts, however, exhibited functional impairment, indicating that the engagement of TIGIT receptor by host CD155 promoted NK cell functional maturation. Furthermore, TIGIT deficiency impaired NK cell-mediated missing-self recognition and rejection of CD155^- targets, such as allogenic splenocytes and certain tumor cells, in an MHC-I-independent and CD226-unrelated manner. Thus, TIGIT-CD155 pathway is also involved in the acquisition of optimal NK cell effector function, representing a novel MHC-I-independent education mechanism for NK cell tolerance and activation.

The Immunomodulatory Functions of Diacylglycerol Kinase ζ

The generation of diacylglycerol (DAG) is critical for promoting immune cell activation, regulation, and function. Diacylglycerol kinase ζ (DGKζ) serves as an important negative regulator of DAG by enzymatically converting DAG into phosphatidic acid (PA) to shut down DAG-mediated signaling. Consequently, the loss of DGKζ increases DAG levels and the duration of DAG-mediated signaling. However, while the enhancement of DAG signaling is thought to augment immune cell function, the loss of DGKζ can result in both immunoactivation and immunomodulation depending on the cell type and function. In this review, we discuss how different immune cell functions can be selectively modulated by DGKζ. Furthermore, we consider how targeting DGKζ can be potentially beneficial for the resolution of human diseases by either promoting immune responses important for protection against infection or cancer or dampening immune responses in immunopathologic conditions such as allergy and septic shock.

Activating Receptor Signals Drive Receptor Diversity in Developing Natural Killer Cells

It has recently been appreciated that NK cells exhibit many features reminiscent of adaptive immune cells. Considerable heterogeneity exists with respect to the ligand specificity of individual NK cells and as such, a subset of NK cells can respond, expand, and differentiate into memory-like cells in a ligand-specific manner. MHC I-binding inhibitory receptors, including those belonging to the Ly49 and KIR families, are expressed in a variegated manner, which creates ligand-specific diversity within the NK cell pool. However, how NK cells determine which inhibitory receptors to express on their cell surface during a narrow window of development is largely unknown. In this manuscript, we demonstrate that signals from activating receptors are critical for induction of Ly49 and KIR receptors during NK cell development; activating receptor-derived signals increased the probability of the Ly49 bidirectional Pro1 promoter to transcribe in the forward versus the reverse direction, leading to stable expression of Ly49 receptors in mature NK cells. Our data support a model where the balance of activating and inhibitory receptor signaling in NK cells selects for the induction of appropriate inhibitory receptors during development, which NK cells use to create a diverse pool of ligand-specific NK cells.

Diacylglycerol Kinase ζ Is a Target To Enhance NK Cell Function

Enhancement of NK cell function could be beneficial in treatment of a variety of tumors and infections. However, efforts to improve NK cell function by disrupting negative regulators that target proximal signaling pathways paradoxically results in hyporesponsive rather than hyperresponsive NK cells. In this study, we demonstrate that genetic deletion of diacylglycerol kinase (DGK)ζ, a negative regulator of diacylglycerol-mediated signaling, has the desired effect of enhancing NK cell function due to its distal position in the activating receptor-mediated signaling cascade. Upon stimulation through multiple activating receptors, NK cells from mice lacking DGKζ display increased cytokine production and degranulation in an ERK-dependent manner. Additionally, they have improved cytotoxic functions against tumor cell lines. The enhancement of NK cell function by DGKζ deficiency is NK cell-intrinsic and developmentally independent. Importantly, DGKζ deficiency does not affect inhibitory NK cell receptor expression or function. Thus, DGKζ knockout mice display improved missing self recognition, as evidenced by enhanced rejection of a TAP-deficient tumor in vivo. We propose that enzymes that negatively regulate distal activating receptor signaling pathways such as DGKζ represent novel targets for augmenting the therapeutic potential of NK cells.

NK cell education via nonclassical MHC and non-MHC ligands

Natural killer (NK) cell education, a process for achieving functional maturation and self-tolerance, has been previously defined by the interaction between self-major histocompatibility complex class I (MHC-I) molecules and their specific inhibitory receptors. Over the past several years, growing evidence has highlighted the important roles of nonclassical MHC-I and non-MHC-I molecules in NK cell education. Herein, we review the current knowledge of NK cell education, with a particular focus on nonclassical MHC-I- and non-MHC-I-dependent education, and compare them with the classical MHC-I-dependent education theory. In addition, we update and extend this theory by presenting the 'Confining Model', discussing cis and trans characteristics, reassessing quantity and quality control, and elucidating the redundancy of NK cell education in tumor and virus infection.

SHP-1-mediated inhibitory signals promote responsiveness and anti-tumour functions of natural killer cells

Natural killer (NK) cells are cytotoxic innate lymphoid cells that are involved in immune defense. NK cell reactivity is controlled in part by MHC class I recognition by inhibitory receptors, but the underlying molecular mechanisms remain undefined. Using a mouse model of conditional deletion in NK cells, we show here that the protein tyrosine phosphatase SHP-1 is essential for the inhibitory function of NK cell MHC class I receptors. In the absence of SHP-1, NK cells are hyporesponsive to tumour cells in vitro and their early Ca(2+) signals are compromised. Mice without SHP-1 in NK cells are unable to reject MHC class I-deficient transplants and to control tumours in vivo. Thus, the inhibitory activity of SHP-1 is needed for setting the threshold of NK cell reactivity.

Adaptations of Natural Killer Cells to Self-MHC Class I

Natural Killer (NK) cells use germ line encoded receptors to detect diseased host cells. Despite the invariant recognition structures, NK cells have a significant ability to adapt to their surroundings, such as the presence or absence of MHC class I molecules. It has been assumed that this adaptation occurs during NK cell development, but recent findings show that mature NK cells can also adapt to the presence or absence of MHC class I molecules. Here, we summarize how NK cells adjust to changes in the expression of MHC class I molecules. We propose an extension of existing models, in which MHC class I recognition during NK cell development sequentially instructs and maintains NK cell function. The elucidation of the molecular basis of the two effects may identify ways to improve the fitness of NK cells and to prevent the loss of NK cell function due to persistent alterations in their environment.

Know Thyself: NK-Cell Inhibitory Receptors Prompt Self-Tolerance, Education, and Viral Control

Natural killer (NK) cells provide essential protection against viral infections. One of the defining features of this lymphocyte population is the expression of a wide array of variable cell surface stimulatory and inhibitory NK receptors (sNKR and iNKR, respectively). The iNKR are particularly important in terms of NK-cell education. As receptors specific for MHC class I (MHC I) molecules, they are responsible for self-tolerance and adjusting NK-cell reactivity based on the expression level of self-MHC I. The end result of this education is twofold: (1) inhibitory signaling tunes the functional capacity of the NK cell, endowing greater potency with greater education, and (2) education on self allows the NK cell to detect aberrations in MHC I expression, a common occurrence during many viral infections. Many studies have indicated an important role for iNKR and MHC I in disease, making these receptors attractive targets for manipulating NK-cell reactivity in the clinic. A greater understanding of iNKR and their ability to regulate NK cells will provide a basis for future attempts at translating their potential utility into benefits for human health.

The ly49 gene family. A brief guide to the nomenclature, genetics, and role in intracellular infection

Understanding the Ly49 gene family can be challenging in terms of nomenclature and genetic organization. The Ly49 gene family has two major gene nomenclature systems, Ly49 and Killer Cell Lectin-like Receptor subfamily A (klra). Mice from different strains have varying numbers of these genes with strain specific allelic variants, duplications, deletions, and pseudogene sequences. Some members activate NK lymphocytes, invariant NKT (iNKT) lymphocytes and γδ T lymphocytes while others inhibit killing activity. One family member, Ly49Q, is expressed only on myeloid cells and is not found on NK, iNKT, or γδ T cells. There is growing evidence that these receptors may regulate not just the immune response to viruses, but other intracellular pathogens as well. Thus, this review’s primary goal is to provide a guide for researchers first encountering the Ly49 gene family and a foundation for future studies on the role that these gene products play in the immune response, particularly the response to intracellular viral and bacterial pathogens.

Functional education of invariant NKT cells by dendritic cell tuning of SHP-1

Invariant NKT (iNKT) cells play key roles in host defense by recognizing lipid Ags presented by CD1d. iNKT cells are activated by bacterial-derived lipids and are also strongly autoreactive toward self-lipids. iNKT cell responsiveness must be regulated to maintain effective host defense while preventing uncontrolled stimulation and potential autoimmunity. CD1d-expressing thymocytes support iNKT cell development, but thymocyte-restricted expression of CD1d gives rise to Ag hyperresponsive iNKT cells. We hypothesized that iNKT cells require functional education by CD1d(+) cells other than thymocytes to set their correct responsiveness. In mice that expressed CD1d only on thymocytes, hyperresponsive iNKT cells in the periphery expressed significantly reduced levels of tyrosine phosphatase SHP-1, a negative regulator of TCR signaling. Accordingly, heterozygous SHP-1 mutant mice displaying reduced SHP-1 expression developed a comparable population of Ag hyperresponsive iNKT cells. Restoring nonthymocyte CD1d expression in transgenic mice normalized SHP-1 expression and iNKT cell reactivity. Radiation chimeras revealed that CD1d(+) dendritic cells supported iNKT cell upregulation of SHP-1 and decreased responsiveness after thymic emigration. Hence, dendritic cells functionally educate iNKT cells by tuning SHP-1 expression to limit reactivity.

Inositol tetrakisphosphate limits NK cell effector functions by controlling PI3K signaling

Natural killer (NK) cells have important functions in cancer immunosurveillance, BM allograft rejection, fighting infections, tissue homeostasis, and reproduction. NK cell-based therapies are promising treatments for blood cancers. Overcoming their currently limited efficacy requires a better understanding of the molecular mechanisms controlling NK cell development and dampening their effector functions. NK cells recognize the loss of self-antigens or up-regulation of stress-induced ligands on pathogen-infected or tumor cells through invariant NK cell receptors (NKRs), and then kill such stressed cells. Two second-messenger pathways downstream of NKRs are required for NK cell maturation and effector responses: PIP(3) generation by PI3K and generation of diacylglycerol and IP(3) by phospholipase-Cγ (PLCγ). In the present study, we identify a novel role for the phosphorylated IP(3) metabolite inositol (1,3,4,5)tetrakisphosphate (IP(4)) in NK cells. IP(4) promotes NK cell terminal differentiation and acquisition of a mature NKR repertoire. However, in mature NK cells, IP(4) limits NKR-induced IFNγ secretion, granule exocytosis, and target-cell killing, in part by inhibiting the PIP(3) effector-kinase Akt. This identifies IP(4) as an important novel regulator of NK cell development and function and expands our understanding of the therapeutically important mechanisms dampening NK cell responses. Our results further suggest that PI3K regulation by soluble IP(4) is a broadly important signaling paradigm.

SHP-1 phosphatase is a critical regulator in preventing natural killer cell self-killing

Balance of signals generated from the engaged activating and inhibitory surface receptors regulates mature NK cell activities. The inhibitory receptors signal through immunoreceptor tyrosine based inhibitory motifs (ITIM), and recruit phosphatases such as SHP-1 to inhibit NK cell activation. To directly examine the importance of SHP-1 in regulating activities and cell fate of mature NK cells, we used our established lentiviral-based engineering protocol to knock down the SHP-1 protein expression in primary C57BL/6NCrl cells. Gene silencing of the SHP-1 in primary NK cells abrogated the ability of ITIM-containing NK inhibitory receptors to suppress the activation signals induced by NK1.1 activating receptors. We followed the fates of stably transduced SHP-1 silenced primary NK cells over a longer period of time in IL-2 containing cultures. We observed an impaired IL-2 induced proliferation in the SHP-1 knockdown NK cells. More interestingly, these "de-regulated" SHP-1 knockdown NK cells mediated specific self-killing in a real-time live cell microscopic imaging system we developed to study NK cell cytotoxicity in vitro. Selective target recognition of the SHP-1 knockdown NK cells revealed also possible involvement of the SHP-1 phosphatase in regulating other NK functions in mature NK cells.

Current perspectives of natural killer cell education by MHC class I molecules

From the early days of natural killer (NK) cell research, it was clear that MHC genes controlled the specificity of mouse NK cell-dependent responses, such as the ability to reject transplanted allogeneic bone marrow and to kill tumour cells. Although several mechanisms that are involved in this 'education' process have been clarified, most of the mechanisms have still to be identified. Here, we review the current understanding of the processes that are involved in NK cell education, including how the host MHC class I molecules regulate responsiveness and receptor repertoire formation in NK cells and the signalling pathways that are involved.

Tolerance and reactivity of NK cells: two sides of the same coin?

NK cells can kill transformed, infected and stressed cells while most normal cells are spared. NK cells are activated by various endogenous self-ligands, some of which are actually expressed by normal cells. Thus, NK cells are inherently self-reactive and consequently, potentially auto-aggressive. How these cells are prevented from attacking normal cells while ensuring reactivity to diseased cells is a major unresolved question for NK-cell biologists.

Natural killer cell tolerance licensing and other mechanisms

Armed with potent cytotoxic and immunostimulatory effector functions, natural killer (NK) cells have the potential to cause significant damage to normal self cells unless controlled by self-tolerance mechanisms. NK cells identify and attack target cells based on integration of signals from activation and inhibitory receptors, whose ligands exhibit complex expression and/or binding patterns. Preservation of NK cell self-tolerance must therefore go beyond mere engagement of inhibitory receptors during effector functions. Herein, we review recent work that has uncovered a number of mechanisms to ensure self-tolerance of NK cells. For example, licensing of NK cells allows only NK cells that can engage self-MHC to become functionally competent, or licensed. The molecular mechanism of this phenomenon appears to require signaling by receptors that were originally identified in effector inhibition. However, the nature of the signaling event has not yet been defined, but new interpretations of several published experiments provide valuable clues. In addition, several other cell-intrinsic and -extrinsic mechanisms of NK cell tolerance are discussed, including activation receptor cooperation and synergy, cytokine stimulation, and the opposing roles of accessory and regulatory cells. Finally, NK cell tolerance is discussed as it relates to the clinic, such as KIR-HLA disease associations, tumor immunotherapy, and fetal tolerance.

MHC variant peptide-mediated anergy of encephalitogenic T cells requires SHP-1

Our lab has demonstrated that encephalitogenic T cells can be effectively anergized by treatment with MHC variant peptides, which are analogues of immunogenic peptides containing an amino acid substitution at an MHC anchor residue. The MHC variant peptide of myelin oligodendrocyte glycoprotein (MOG)(35-55) proves an effective treatment as it does not induce symptoms of experimental autoimmune encephalomyelitis and fails to recruit macrophages or MOG(35-55)-specific T cells to the CNS. In this study, we sought to characterize the signaling pathways required for the induction of anergy by building upon the observations identifying the tyrosine phosphatase SHP-1 as a critical regulator of T cell responsiveness. Motheaten viable heterozygous mice, which contain a mutation in the SHP-1 gene resulting in a reduction in functional SHP-1, were challenged with MOG(35-55) or the MOG(35-55) MHC variant 45D. These mice display symptoms of experimental autoimmune encephalomyelitis upon immunization with MHC variant peptide and have significant CNS infiltration of tetramer-positive CD4(+) cells and macrophages, unlike B6 mice challenged with the variant peptide. The effects of SHP-1 are directly on the T cell as Motheaten viable heterozygous mice autoreactive T cells are not anergized in vitro. Lastly, we demonstrate no distinguishable difference in the initial interaction between the TCR and agonist or MHC variant. Rather, an unstable interaction between peptide and MHC attenuates the T cell response, seen in a decreased half-life relative to MOG(35-55). These results identify SHP-1 as a mediator of T cell anergy induced by destabilized peptide:MHC complexes.

Regulation of NK cell responsiveness to achieve self-tolerance and maximal responses to diseased target cells

Inhibitory receptors specific for major histocompatibility complex (MHC) class I molecules govern the capacity of natural killer (NK) cells to attack class I-deficient cells ('missing-self recognition'). These receptors are expressed stochastically, such that the panel of expressed receptors varies between NK cells. This review addresses how the activity of NK cells is coordinated in the face of this variation to achieve a repertoire that is self-tolerant and optimally reactive with diseased cells. Recent studies show that NK cells arise in normal animals or humans that lack any known inhibitory receptors specific for self-MHC class I. These NK cells exhibit self-tolerance and exhibit functional hyporesponsiveness to stimulation through various activating receptors. Evidence suggests that hyporesponsiveness is induced because these NK cells cannot engage inhibitory MHC class I molecules and are therefore persistently over-stimulated by normal cells in the environment. Finally, we discuss evidence that hyporesponsiveness is a quantitative trait that varies depending on the balance of signals encountered by developing NK cells. Thus, a tuning process determines the functional set-point of NK cells, providing a basis for discriminating self from missing-self, and at the same time endowing each NK cell with the highest inherent responsiveness compatible with self-tolerance.

Enhanced NK-cell development and function in BCAP-deficient mice

In B lymphocytes, the B-cell adaptor for phosphatidylinositol 3-kinase (BCAP) facilitates signaling from the antigen receptor. Mice lacking BCAP have a predominantly immature pool of B cells with impaired immune function and increased susceptibility to apoptosis. Unexpectedly, we have found that natural killer (NK) cells from BCAP-deficient mice are more mature, more long-lived, more resistant to apoptosis, and exhibit enhanced functional activity compared with NK cells from wild-type mice. Surprisingly, these effects are evident despite a severe impairment of the immunoreceptor tyrosine-based activation motif-mediated Akt signaling pathway. The seemingly paradoxical phenotype reveals inherent differences in the signals controlling the final maturation of B cells and NK cells, which depend on positive and negative signals, respectively. Both enhanced interferon-gamma responses and augmented maturation of NK cells in BCAP-deficient mice are independent of available MHC class I ligands. Our data support a model in which blunting of BCAP-mediated activation signaling in developing NK cells promotes functionality, terminal maturation, and long-term survival.

NK cytotoxicity mediated by CD16 but not by NKp30 is functional in Griscelli syndrome

Griscelli syndrome (GS) type 2 is an autosomal recessive disorder represented by pigment dilution and impaired cytotoxic T lymphocyte (CTL) activity. NK activity has been scarcely investigated in GS patients. Here, we describe a new patient, possessing a hemophagocytic syndrome with a homozygous Q118X nonsense RAB27A mutation. Single specific primer–polymerase chain reaction (SSP-PCR) was developed based on this mutation and is currently used in prenatal genetic analysis. As expected, CTLs in the patient are not functional and NK cytotoxicity against K562 or 721.221 cells is diminished. Surprisingly, however, we demonstrate that CD16-mediated killing is intact in this patient and is therefore RAB27A independent, whereas NKp30-mediated killing is impaired and is therefore RAB27A dependent. We further analyzed the signaling pathways of these 2 receptors and demonstrated phosphorylation of Vav1 after CD16 activation but not after NKp30 engagement. Thus, we identify a novel homozygous mutation in the RAB27A gene of a new GS patient, observe for the first time that some activating NK receptors function in GS patients, and demonstrate a functional dichotomy in the killing mediated by these human NK-activating receptors.

NK cell regulation by SLAM family receptors and SAP-related adapters

Signaling lymphocytic activating molecule (SLAM) family receptors and SLAM-associated protein (SAP)-related adapters play several important roles in the immune system. Natural killer (NK) cells express at least three members of the SLAM family. They are 2B4, NK, T- and B-cell antigen (NTB-A), and CD2-like receptor-activating cytotoxic cells (CRACC), which recognize their respective ligands CD48, NTB-A, and CRACC on target cells and possibly on other NK cells. In mature human NK cells, SLAM family receptors appear to have activating functions. In mature mouse NK cells, however, the only available information is for 2B4, which reportedly has the capacity to either stimulate or inhibit NK cell activation. The ability of SLAM family receptors to regulate NK cell functions seems to be largely dependent on their capacity to associate, by way of their cytoplasmic domain, with members of the SAP family of adapters, including SAP, Ewing's sarcoma-activated transcript-2 (EAT-2), and EAT-2-related transducer (ERT). By binding to SAP, SLAM family receptors are coupled to the Src kinase FynT, thereby evoking protein tyrosine phosphorylation signals. In human NK cells, SAP is likely to be crucial for the activating function of 2B4 and NTB-A but not of CRACC and also crucial for the activating function of 2B4 in mouse NK cells. EAT-2. SAP is ERT link SLAM family receptors to distinct, albeit poorly understood, signals. These two SAP-related adapters may be implicated in the inhibitory function of 2B4 observed in mouse NK cells. While much work remains to be carried out to fully understand the roles and mechanisms of action of the SLAM and SAP families in human and mouse NK cells, the published findings clearly establish that these molecules have important functions in NK cell biology.

Licensing of natural killer cells by self-major histocompatibility complex class I

Natural killer (NK) cells have potent capacities to immediately kill cellular targets and produce cytokines that may potentially damage normal self-tissues unless they are kept in check. Such tolerance mechanisms are incompletely understood. Here we discuss recent studies suggesting that NK cells undergo a host major histocompatibility complex (MHC) class I-dependent functional maturation process, termed 'licensing'. Ironically, licensing directly involves inhibitory receptors that recognize target cell MHC class I molecules and block activation of NK cells in effector responses. This process results in two types of tolerant NK cells: functionally competent (licensed) NK cells, whose effector responses are inhibited by self-MHC class I molecules through the same receptors that conferred licensing, and functionally incompetent (unlicensed) NK cells.

Self-tolerance of natural killer cells

Natural killer (NK) cells, similar to other lymphocytes, acquire tolerance to self. This means that NK cells have the potential to attack normal self cells but that there are mechanisms to ensure that this does not usually occur. Self-tolerance is acquired by NK cells during their development, but the underlying molecular and cellular mechanisms remain poorly understood. Recent studies have produced important new information about NK-cell self-tolerance. Here, we review the evidence for and against possible mechanisms of NK-cell self-tolerance, with an emphasis on the role of MHC-specific receptors.

How do natural killer cells find self to achieve tolerance?

Natural killer (NK) cells provide innate defense against tumors and infections by virtue of potent capacities to immediately kill cellular targets and produce cytokines. These effector functions may potentially damage normal self-tissues unless they are kept in check by tolerance mechanisms that need clarification. Here, we discuss recent studies indicating that the NK cells acquire functional competence directly through engagement of their MHC-specific receptors by self-MHC. Ironically, these receptors were first identified in terms of recognizing target cell MHC class I molecules and inhibiting NK cells in effector responses. Other studies of NK cell tolerance are also discussed. Although these studies begin to clarify the means by which NK cell tolerance is achieved, much more investigation is needed because NK cell tolerance is relevant to clinical observations in patients with infections and cancer.

Soluble branched (1,4)-beta-D-glucans from Acetobacter species enhance antitumor activities against MHC class I-negative and -positive malignant melanoma through augmented NK activity and cytotoxic T-cell response

We previously found that an extracellular polysaccharide, AC-1, produced by Acetobacter polysaccharogenes composed of (1,4)-beta-D-glucan with branches of glucosyl residues showed a strong activity to induce production of interleukin (IL)-12 p40 and tumor necrosis factor-alpha by macrophage cell lines in vitro via Toll-like receptor-4 signaling. In the present study, we examined the effects of oral administration of AC-1 on protection against 2 types of murine B16 melanoma lines, major histocompatibility complex (MHC) class I-negative B16L and MHC class I gene-transfected B16K(b) cells. Mice were inoculated subcutaneously with B16L or B16K(b) cells on day 0 and administrated intragastrically with AC-1 or PBS once every 5 days from 1 day before tumor inoculation. The tumor growth was severely retarded in AC-1-treated mice after subcutaneous inoculation with B16L or B16K(b) cells. The AC-1-treated mice showed augmented natural killer (NK) cell activity against B16L cells, and in vivo depletion of NK cells by antiasialoGM1 antibody (Ab) treatment abrogated the antitumor activity in AC-1-treated mice. On the other hand, AC-1-treated mice inoculated with B16K(b) cells developed a significantly higher level of cytotoxic T-lymphocyte response against B16K(b) cells, and in vivo depletion of CD8(+) T cells by anti-CD8 mAb treatment abrogated the antitumor activity. Thus, AC-1 augmented antitumor activity against different tumors via augmentation of different antitumor mechanisms. These results suggest a possible prophylactic application of AC-1 for human neoplasms irrespective of expression levels of their MHC class I molecules.

Natural killer cell education in mice with single or multiple major histocompatibility complex class I molecules

The ability of murine NK cells to reject cells lacking self MHC class I expression results from an in vivo education process. To study the impact of individual MHC class I alleles on this process, we generated mice expressing single MHC class I alleles (K^b, D^b, D^d, or L^d) or combinations of two or more alleles. All single MHC class I mice rejected MHC class I–deficient cells in an NK cell–dependent way. Expression of K^b or D^d conveyed strong rejection of MHC class I–deficient cells, whereas the expression of D^b or L^d resulted in weaker responses. The educating impact of weak ligands (D^b and L^d) was further attenuated by the introduction of additional MHC class I alleles, whereas strong ligands (K^b and D^d) maintained their educating impact under such conditions. An analysis of activating and inhibitory receptors in single MHC class I mice suggested that the educating impact of a given MHC class I molecule was controlled both by the number of NK cells affected and by the strength of each MHC class I–Ly49 receptor interaction, indicating that NK cell education may be regulated by a combination of qualitative and quantitative events.

Ly49Q, a member of the Ly49 family that is selectively expressed on myeloid lineage cells and involved in regulation of cytoskeletal architecture

Here, we identified and characterized a Ly49 family member, designated as Ly49Q. The Ly49q gene encodes a 273-aa protein with an immunoreceptor tyrosine-based inhibitory motif (ITIM) at the N terminus of its cytoplasmic domain. We show that the ITIM of Ly49Q can recruit SHP-2 and SHP-1 in a tyrosine phosphorylation-dependent manner. In contrast to other known members of the Ly49 family, Ly49Q was found not to be expressed on NK1.1(+) cells, but instead was detectable on virtually all Gr-1(+) cells, such as myeloid precursors in bone marrow. Monocytes/macrophages also expressed low levels of Ly49Q, and the expression was enhanced by the treatment of cells with IFN-gamma. Treatment of activated macrophages with anti-Ly49Q mAb induced rapid formation of polarized actin structures, showing filopodia-like structure on one side and lamellipodial-like structure on the other side. A panel of proteins became tyrosine-phosphorylated in myeloid cells when treated with the mAb. Induction of the phosphorylation depends on the ITIM of Ly49Q. Thus, Ly49Q has unique features different from other known Ly49 family members and appears to be involved in regulation of cytoskeletal architecture of macrophages through ITIM-mediated signaling.

Decreased expression of Src homology 2 domain-containing protein tyrosine phosphatase 1 reduces T cell activation threshold but not the severity of experimental autoimmune myasthenia gravis

Myasthenia gravis (MG) and its murine model experimental autoimmune myasthenia gravis (EAMG) are T cell-dependent, antibody-mediated autoimmune diseases. Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP-1) is a cytosolic tyrosine phosphatase that is involved in regulating the T cell activation cascade from signals initiated through the TCR. To study the role of SHP-1 in EAMG pathogenesis, we immunized C57BL/6 (B6) mice heterozygous for deletion of the SHP-1 gene (me(v+/-)) and their littermate wild type B6 mice with torpedo acetylcholine receptor (TAChR). T cell proliferation and IFNgamma production were significantly increased in B6.me(v+/-) mice after immunization with AChR compared to that of wild type littermates. However, clinical incidence and severity of the disease were not changed. There also were no significant differences in AChR-specific antibodies produced between wild type and me(v+/-) mice. These data suggest that deficiency in SHP-1 expression does decrease the activation threshold of autoreactive T cells in EAMG, but the increased frequency of autoreactive T cells does not aggravate EAMG in terms of clinical score, incidence, or antibody titers.

Visualization of signaling pathways and cortical cytoskeleton in cytolytic and noncytolytic natural killer cell immune synapses

Recent applications of imaging approaches and other methods of cell biology have provided high-resolution visualization of the location of fluorescent proteins in living and fixed cells during cell-cell interactions between lymphocytes, antigen presenting cells and target cells. We review the composition and dynamics of molecular and cytoskeletal events occurring during natural killer cell interactions with susceptible and nonsusceptible target cells. The natural killer cell immune synapse and the concomitant changes in cytoskeletal components and cytoplasmic organelles are described. The findings are compared with the observations made in T helper cells and cytotoxic T cells. It is concluded that the cytolytic immune synapses display spatial-temporal dynamics that are accelerated as compared with T helper cells. In addition, the cytolytic conjugates have unique characteristics relating to their effector function. Furthermore, the natural killer cell immune synapses in cytolytic and noncytolytic interactions are distinctly different and display patterns consistent with characteristic signaling pathways identified in biochemical studies of disrupted cells. The precise relationship between different stages of the natural killer cell immune synapse formation and progression in signal transduction pathways is yet to be established.

Expression of p58.2 or CD94/NKG2A inhibitory receptors in an NK-like cell line, YTINDY, leads to HLA Class I-mediated inhibition of cytotoxicity in the p58.2- but not the CD94/NKG2A-expressing transfectant

Natural killer cytotoxicity is down-regulated by HLA Class I-specific inhibitory receptors classified as killer inhibitory receptors (KIRs) or C-type lectins. The regulation of their inhibitory signaling pathways is not completely understood. The YTINDY NK-like cell line was transfected to express p58.2 KIR (YT/C143 transfectant) or CD94/NKG2A C-type lectin (YT/CD94 transfectant); and YT/C143, but not YT/CD94, cytotoxicity was down-regulated by Class I. YT/C143 and YT/CD94 expressed equally low p56(lck) levels, suggesting that p56(lck) is not absolutely required for p58.2 signaling but may be required for CD94/NKG2A signaling. Lower SHP-1 levels and activity were observed in YT/CD94 compared to YT/C143. However, increasing SHP-1 to equivalent levels in YT/C143 did not restore inhibition in YT/CD94. Our results suggest that the combination of low p56(lck) and SHP-1 levels may be responsible for the absent inhibitory signal in YT/CD94. In addition, the possible expression of CD94/NKG2C activating receptor may override inhibitory signals transduced through CD94/NKG2A.

Deficient natural killer cell cytotoxicity in patients with IKK-gamma/NEMO mutations

NF-kappaB essential modifier (NEMO), also known as IKK-gamma, is a member of the I-kappaB kinase complex responsible for phosphorylating I-kappaB, allowing the release and activation of NF-kappaB. Boys with an expressed NEMO mutation have an X-linked syndrome characterized by hypohidrotic ectodermal dysplasia with immune deficiency (HED-ID). The immunophenotype resulting from NEMO mutation is highly variable, with deficits in both T and B cell responses. We evaluated three patients with NEMO mutations (L153R, Q403X, and C417R) and HED-ID who had evidence of defective CD40 signaling. All three patients had normal percentages of peripheral blood NK cells, but impaired NK cell cytotoxic activity. This was not due to a generalized defect in cytotoxicity because antibody-dependent cellular cytotoxicity was intact. This abnormality was partially reversed by in vitro addition of IL-2, which was also able to induce NF-kappaB activation. In one patient with recurrent cytomegalovirus infections, administration of IL-2 partially corrected the NK cell killing deficit. These data suggest that NEMO participates in signaling pathways leading to NK cell cytotoxicity and that IL-2 can activate NF-kappaB and partially overcome the NK cell defect in patients with NEMO mutations.

Overexpression of interleukin-15 in vivo enhances antitumor activity against MHC class I-negative and -positive malignant melanoma through augmented NK activity and cytotoxic T-cell response

Interleukin (IL)-15, a pleiotropic cytokine, is involved in the development and maintenance of NK cells and memory CD8+ T cells. We examined the effects of in vivo overexpression of IL-15 on protection against 2 types of murine B16 melanoma lines, MHC class I-negative B16.44 and MHC class I-positive B16F10 cells, using IL-15 transgenic (Tg) mice that we have recently constructed. The tumor growth was severely retarded in IL-15 Tg mice after subcutaneous (s.c.) inoculation with B16.44 or B16F10 cells. IL-15 Tg mice showed an augmented NK cell activity against B16.44 cells, and in vivo depletion of NK cells by anti-asialoGM1 Ab treatment abrogated the antitumor activity in IL-15 Tg mice. On the other hand, IL-15 Tg mice inoculated with B16F10 cells developed a significant level of CTL response against B16F10 cells, and in vivo depletion of CD8+ T cells by anti-CD8 MAb treatment abrogated the antitumor activity. Thus, overexpression of IL-15 augmented antitumor activity against different tumors via augmentation of different antitumor mechanisms. These results suggest a possible therapeutic application of IL-15 for human neoplasms expressing a wide range of MHC class molecules.

Expression of the tyrosine phosphatase SRC homology 2 domain-containing protein tyrosine phosphatase 1 determines T cell activation threshold and severity of experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is a CD4 Th1-mediated inflammatory demyelinating disorder of the CNS and a well-established animal model for multiple sclerosis. Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP-1) is a cytosolic tyrosine phosphatase that is involved in regulating the T cell activation cascade from signals initiated through the TCR. To study the role of SHP-1 in EAE pathogenesis, we immunized B10.PL mice heterozygous for deletion of the SHP-1 gene (me(v+/-)) and B10.PL wild-type mice with the immunodominant epitope of myelin basic protein (MBP Ac1-11). T cell proliferation and IFN-gamma production were significantly increased in me(v+/-) mice after immunization with MBP Ac1-11. The frequency of MBP Ac1-11-specific CD4 T cells, analyzed by staining with fluorescently labeled tetramers (MBP1-11[4Y]: I-A(u) complexes), was increased in the draining lymph node cells of me(v+/-) mice compared with wild-type mice. In addition, me(v+/-) mice developed a more severe course of EAE with epitope spreading to proteolipid protein peptide 43-64. Finally, expansion of MBP Ac1-11-specific T cells in response to Ag was enhanced in me(v+/-) T cells, particularly at lower Ag concentrations. These data demonstrate that the level of SHP-1 plays an important role in regulating the activation threshold of autoreactive T cells.

Spatial organization of signal transduction molecules in the NK cell immune synapses during MHC class I-regulated noncytolytic and cytolytic interactions

The cytolytic activity of NK cells is tightly regulated by inhibitory receptors specific for MHC class I Ags. We have investigated the composition of signal transduction molecules in the supramolecular activation clusters in the MHC class I-regulated cytolytic and noncytolytic NK cell immune synapses. KIR2DL3-positive NK clones that are specifically inhibited in their cytotoxicity by HLA-Cw*0304 and polyclonal human NK cells were used for conjugate formation with target cells that are either protected or are susceptible to NK cell-mediated cytotoxicity. Polarization of talin, microtubule-organizing center, and lysosomes occurred only during cytolytic interactions. The NK immune synapses were analyzed by three-dimensional immunofluorescence microscopy, which showed two distinctly different synaptic organizations in NK cells during cytolytic and noncytolytic interactions. The center of a cytolytic synapse with MHC class I-deficient target is comprised of a complex of signaling molecules including Src homology (SH)2-containing protein tyrosine phosphatase-1 (SHP-1). Closely related molecules with overlapping functions, such as the Syk kinases, SYK, and ZAP-70, and adaptor molecules, SH2 domain-containing leukocyte protein of 76 kDa and B cell linker protein, are expressed in activated NK cells and are all recruited to the center of the cytolytic synapse. In contrast, the noncytolytic synapse contains SHP-1, but is lacking other components of the central supramolecular activation cluster. These findings indicate a functional role for SHP-1 in both the cytolytic and noncytolytic interactions. We also demonstrate, in three-cell conjugates, that a single NK cell forms a cytolytic synapse with a susceptible target cell in the presence of both susceptible and nonsusceptible target cells.

Regulation of the natural killer cell receptor repertoire

Natural killer cells express inhibitory receptors specific for MHC class I proteins and stimulatory receptors with diverse specificities. The MHC-specific receptors discriminate among different MHC class I alleles and are expressed in a variegated, overlapping fashion, such that each NK cell expresses several inhibitory and stimulatory receptors. Evidence suggests that individual developing NK cells initiate expression of inhibitory receptor genes in a sequential, cumulative, and stochastic fashion. Superimposed on the receptor acquisition process are multiple education mechanisms, which act to coordinate the stimulatory and inhibitory specificities of developing NK cells. One process influences the complement of receptors expressed by individual NK cells. Other mechanisms may prevent NK cell autoaggression even when the developing NK cell fails to express self-MHC-specific inhibitory receptors. Together, these mechanisms ensure a self-tolerant and maximally discriminating NK cell population. Like NK cells, a fraction of memory phenotype CD8(+) T cells, as well as other T cell subsets, express inhibitory class I--specific receptors in a variegated, overlapping fashion. The characteristics of these cells suggest that inhibitory receptor expression may be a response to prior antigenic stimulation as well as to poorly defined additional signals. A unifying hypothesis is that both NK cells and certain T cell subsets initiate expression of inhibitory receptors in response to stimulation.