A balance between positive and negative signals in cytotoxic lymphocytes regulates the polarization of lipid rafts during the development of cell-mediated killing

Assessment of membrane lipid state at the natural killer cell immunological synapse

The plasma membrane is a fluid structure that protects cells as one of their first barriers and actively participates in numerous biological processes in many ways including through distinct membrane sub-regions. For immunological cells, highly organized sub-compartments of plasma membranes are vital for them to sense and react to environmental changes. This includes a varying spectrum of lipid ordering in the plasma membrane which signifies or enables cellular functions. Thus, comprehensive analyses of the plasma membrane can facilitate understanding of important cell biological elements which include insights into immune cells. Here, we describe two methods that can be used to assess membrane lipid state at the natural killer cell immunological synapse via high-resolution live cell imaging techniques.

Degranulation enhances presynaptic membrane packing, which protects NK cells from perforin-mediated autolysis

Natural killer (NK) cells kill a target cell by secreting perforin into the lytic immunological synapse, a specialized interface formed between the NK cell and its target. Perforin creates pores in target cell membranes allowing delivery of proapoptotic enzymes. Despite the fact that secreted perforin is in close range to both the NK and target cell membranes, the NK cell typically survives while the target cell does not. How NK cells preferentially avoid death during the secretion of perforin via the degranulation of their perforin-containing organelles (lytic granules) is perplexing. Here, we demonstrate that NK cells are protected from perforin-mediated autolysis by densely packed and highly ordered presynaptic lipid membranes, which increase packing upon synapse formation. When treated with 7-ketocholesterol, lipid packing is reduced in NK cells making them susceptible to perforin-mediated lysis after degranulation. Using high-resolution imaging and lipidomics, we identified lytic granules themselves as having endogenously densely packed lipid membranes. During degranulation, lytic granule–cell membrane fusion thereby further augments presynaptic membrane packing, enhancing membrane protection at the specific sites where NK cells would face maximum concentrations of secreted perforin. Additionally, we found that an aggressive breast cancer cell line is perforin resistant and evades NK cell–mediated killing owing to a densely packed postsynaptic membrane. By disrupting membrane packing, these cells were switched to an NK-susceptible state, which could suggest strategies for improving cytotoxic cell-based cancer therapies. Thus, lipid membranes serve an unexpected role in NK cell functionality protecting them from autolysis, while degranulation allows for the inherent lytic granule membrane properties to create local ordered lipid “shields” against self-destruction.

Degranulation enhances presynaptic membrane packing, which protects NK cells from perforin-mediated autolysis

Natural killer (NK) cells kill a target cell by secreting perforin into the lytic immunological synapse, a specialized interface formed between the NK cell and its target. Perforin creates pores in target cell membranes allowing delivery of proapoptotic enzymes. Despite the fact that secreted perforin is in close range to both the NK and target cell membranes, the NK cell typically survives while the target cell does not. How NK cells preferentially avoid death during the secretion of perforin via the degranulation of their perforin-containing organelles (lytic granules) is perplexing. Here, we demonstrate that NK cells are protected from perforin-mediated autolysis by densely packed and highly ordered presynaptic lipid membranes, which increase packing upon synapse formation. When treated with 7-ketocholesterol, lipid packing is reduced in NK cells making them susceptible to perforin-mediated lysis after degranulation. Using high-resolution imaging and lipidomics, we identified lytic granules themselves as having endogenously densely packed lipid membranes. During degranulation, lytic granule-cell membrane fusion thereby further augments presynaptic membrane packing, enhancing membrane protection at the specific sites where NK cells would face maximum concentrations of secreted perforin. Additionally, we found that an aggressive breast cancer cell line is perforin resistant and evades NK cell-mediated killing owing to a densely packed postsynaptic membrane. By disrupting membrane packing, these cells were switched to an NK-susceptible state, which could suggest strategies for improving cytotoxic cell-based cancer therapies. Thus, lipid membranes serve an unexpected role in NK cell functionality protecting them from autolysis, while degranulation allows for the inherent lytic granule membrane properties to create local ordered lipid "shields" against self-destruction.

Degranulation enhances presynaptic membrane packing, which protects NK cells from perforin-mediated autolysis

Natural killer (NK) cells kill a target cell by secreting perforin into the lytic immunological synapse, a specialized interface formed between the NK cell and its target. Perforin creates pores in target cell membranes allowing delivery of proapoptotic enzymes. Despite the fact that secreted perforin is in close range to both the NK and target cell membranes, the NK cell typically survives while the target cell does not. How NK cells preferentially avoid death during the secretion of perforin via the degranulation of their perforin-containing organelles (lytic granules) is perplexing. Here, we demonstrate that NK cells are protected from perforin-mediated autolysis by densely packed and highly ordered presynaptic lipid membranes, which increase packing upon synapse formation. When treated with 7-ketocholesterol, lipid packing is reduced in NK cells making them susceptible to perforin-mediated lysis after degranulation. Using high-resolution imaging and lipidomics, we identified lytic granules themselves as having endogenously densely packed lipid membranes. During degranulation, lytic granule-cell membrane fusion thereby further augments presynaptic membrane packing, enhancing membrane protection at the specific sites where NK cells would face maximum concentrations of secreted perforin. Additionally, we found that an aggressive breast cancer cell line is perforin resistant and evades NK cell-mediated killing owing to a densely packed postsynaptic membrane. By disrupting membrane packing, these cells were switched to an NK-susceptible state, which could suggest strategies for improving cytotoxic cell-based cancer therapies. Thus, lipid membranes serve an unexpected role in NK cell functionality protecting them from autolysis, while degranulation allows for the inherent lytic granule membrane properties to create local ordered lipid "shields" against self-destruction.

Escape of tumor cells from the NK cell cytotoxic activity

In recent years, NK cells, initially identified as potent cytotoxic effector cells, have revealed an unexpected complexity, both at phenotypic and functional levels. The discovery of different NK cell subsets, characterized by distinct gene expression and phenotypes, was combined with the characterization of the diverse functions NK cells can exert, not only as circulating cells, but also as cells localized or recruited in lymphoid organs and in multiple tissues. Besides the elimination of tumor and virus-infected cells, these functions include the production of cytokines and chemokines, the regulation of innate and adaptive immune cells, the influence on tissue homeostasis. In addition, NK cells display a remarkable functional plasticity, being able to adapt to the environment and to develop a kind of memory. Nevertheless, the powerful cytotoxic activity of NK cells remains one of their most relevant properties, particularly in the antitumor response. In this review, the process of tumor cell recognition and killing mediated by NK cells, starting from the generation of cytolytic granules and recognition of target cell, to the establishment of the NK cell immunological synapse, the release of cytotoxic molecules, and consequent tumor cell death is described. Next, the review focuses on the heterogeneous mechanisms, either intrinsic to tumors or induced by the tumor microenvironment, by which cancer cells can escape the NK cell-mediated attack.

Impact of Plasma Membrane Domains on IgG Fc Receptor Function

Lipid cell membranes not only represent the physical boundaries of cells. They also actively participate in many cellular processes. This contribution is facilitated by highly complex mixtures of different lipids and incorporation of various membrane proteins. One group of membrane-associated receptors are Fc receptors (FcRs). These cell-surface receptors are crucial for the activity of most immune cells as they bind immunoglobulins such as immunoglobulin G (IgG). Based on distinct mechanisms of IgG binding, two classes of Fc receptors are now recognized: the canonical type I FcγRs and select C-type lectin receptors newly referred to as type II FcRs. Upon IgG immune complex induced cross-linking, these receptors are known to induce a multitude of cellular effector responses in a cell-type dependent manner, including internalization, antigen processing, and presentation as well as production of cytokines. The response is also determined by specific intracellular signaling domains, allowing FcRs to either positively or negatively modulate immune cell activity. Expression of cell-type specific combinations and numbers of receptors therefore ultimately sets a threshold for induction of effector responses. Mechanistically, receptor cross-linking and localization to lipid rafts, i.e., organized membrane microdomains enriched in intracellular signaling proteins, were proposed as major determinants of initial FcR activation. Given that immune cell membranes might also vary in their lipid compositions, it is reasonable to speculate, that the cell membrane and especially lipid rafts serve as an additional regulator of FcR activity. In this article, we aim to summarize the current knowledge on the interplay of lipid rafts and IgG binding FcRs with a focus on the plasma membrane composition and receptor localization in immune cells, the proposed mechanisms underlying this localization and consequences for FcR function with respect to their immunoregulatory capacity.

High Serum Levels of Cholesterol Increase Antitumor Functions of Nature Killer Cells and Reduce Growth of Liver Tumors in Mice

BACKGROUND AND AIMS

The relationship between serum cholesterol level and development of hepatocellular carcinoma (HCC) remains unclear. We investigated the effects of serum cholesterol level on development of liver tumors in mice.

METHODS

We performed studies with C57BL/6J mice, mice with disruption of the low-density lipoprotein receptor gene (Ldlr^-/-mice), and mice with conditional deletion of nature killer (NK) cells (NK^dele mice). Some C57BL/6J and NK^dele mice were given injections of diethylinitrosamine to induce liver tumor formation. Mice were placed on a normal diet (ND) or high-cholesterol diet (HCD) to induce high serum levels of cholesterol. We also studied mice with homozygous disruption of ApoE (ApoE^-/- mice), which spontaneously develop high serum cholesterol. C57BL/6J and NK^dele mice on the ND or HCD were implanted with Hep1-6 (mouse hepatoma) cells and growth of xenograft tumors and lung metastases were monitored. Blood samples were collected from mice and analyzed by biochemistry and flow cytometry; liver and tumor tissues were collected and analyzed by histology, immunohistochemistry, and RNA-sequencing analysis. NK cells were isolated from mice and analyzed for cholesterol content, lipid raft formation, immune signaling, and changes in functions. We obtained matched tumor tissues and blood samples from 30 patients with HCC and blood samples from 40 healthy volunteers; levels of cholesterol and cytotoxicity of NK cells were measured.

RESULTS

C57BL/6J mice on HCD and ApoE^-/- mice with high serum levels of cholesterol developed fewer and smaller liver tumors and lung metastases after diethylinitrosamine injection or implantation of Hep1-6 cells than mice on ND. Liver tumors from HCD-fed mice and ApoE^-/- mice had increased numbers of NK cells compared to tumors from ND-fed mice. NK^dele mice or mice with antibody-based depletion for NK cells showed similar tumor number and size in ND and HCD groups after diethylinitrosamine injection or implantation of Hep1-6 cells. NK cells isolated from C57BL/6J mice fed with HCD had increased expression of NK cell-activating receptors (natural cytotoxicity triggering receptor 1 and natural killer group 2, member D), markers of effector function (granzyme B and perforin), and cytokines and chemokines compared with NK cells from mice on ND; these NK cells also had enhanced cytotoxic activity against mouse hepatoma cells, accumulated cholesterol, increased lipid raft formation, and immune signaling activation. NK cells isolated from HCD-fed Ldlr^-/- mice did not have increased cholesterol content or cytotoxic activity against mouse hepatoma cells compared with ND-fed Ldlr^-/- mice. Serum levels of cholesterol correlated with number and activity of NK cells isolated from human HCCs.

CONCLUSIONS

Mice with increased serum levels of cholesterol due to an HCD or genetic disruption of ApoE develop fewer and smaller tumors after injection of hepatoma cells or a chemical carcinogen. We found cholesterol to accumulate in NK cells and activate their effector functions against hepatoma cells. Strategies to increase cholesterol uptake by NK cells can be developed for treatment of HCC.

Human Natural Killer Receptors, Co-Receptors, and Their Ligands

In the last 20 years, the study of human natural killer (NK) cells has moved from the first molecular characterizations of very few receptor molecules to the identification of a plethora of receptors displaying surprisingly divergent functions. We have contributed to the description of inhibitory receptors and their signaling pathways, important in fine regulation in many cell types, but unknown until their discovery in the NK cells. Inhibitory function is central to regulating NK-mediated cytolysis, with different molecular structures evolving during speciation to assure its persistence. More recently, it has become possible to characterize the NK triggering receptors mediating natural cytotoxicity, unveiling the existence of a network of cellular interactions between effectors of both natural and adaptive immunity. This unit reviews the contemporary history of molecular studies of receptors and ligands involved in NK cell function, characterizing the ligands of the triggering receptor and the mechanisms for finely regulating their expression in pathogen-infected or tumor cells. © 2018 by John Wiley & Sons, Inc.

Evidence for host genetic regulation of altered lipid metabolism in experimental toxoplasmosis supported with gene data mining results

Toxoplasma gondii is one of the most successful parasites on Earth, infecting a wide array of mammals including one third of the global human population. The obligate intracellular protozoon is not capable of synthesizing cholesterol (Chl), and thus depends on uptake of host Chl for its own development. To explore the genetic regulation of previously observed lipid metabolism alterations during acute murine T. gondii infection, we here assessed total Chl and its fractions in serum and selected tissues at the pathophysiological and molecular level, and integrated the observed gene expression of selected molecules relevant for Chl metabolism, including its biosynthetic and export KEGG pathways, with the results of published transcriptomes obtained in similar murine models of T. gondii infection. The serum lipid status as well as the transcript levels of relevant genes in the brain and the liver were assessed in experimental models of acute and chronic toxoplasmosis in wild-type mice. The results showed that acute infection was associated with a decrease in Chl content in both the liver and periphery (brain, peripheral lymphocytes), and a decrease in Chl reverse transport. In contrast, in chronic infection, a return to normal levels of Chl metabolism has been noted. These changes corresponded to the brain and liver gene expression results as well as to data obtained via mining. We propose that the observed changes in Chl metabolism are part of the host defense response. Further insight into the lipid metabolism in T. gondii infection may provide novel targets for therapeutic agents.

Enhancement of Lytic Activity by Leptin Is Independent From Lipid Rafts in Murine Primary Splenocytes

Leptin, a pleiotropic adipokine, is known as a regulator of food intake, but it is also involved in inflammation, immunity, cell proliferation, and survival. Leptin receptor is integrated inside cholesterol-rich microdomains called lipid rafts, which, if disrupted or destroyed, could lead to a perturbation of lytic mechanism. Previous studies also reported that leptin could induce membrane remodeling. In this context, we studied the effect of membrane remodeling in lytic activity modulation induced by leptin. Thus, primary mouse splenocytes were incubated with methyl-β-cyclodextrin (β-MCD), a lipid rafts disrupting agent, cholesterol, a major component of cell membranes, or ursodeoxycholic acid (UDCA), a membrane stabilizer agent for 1 h. These treatments were followed by splenocyte incubation with leptin (absence, 10 and 100 ng/ml). Unlike β-MCD or cholesterol, UDCA was able to block leptin lytic induction. This result suggests that leptin increased the lytic activity of primary spleen cells against syngenic EO771 mammary cancer cells independently from lipid rafts but may involve membrane fluidity. Furthermore, natural killer cells were shown to be involved in the splenocyte lytic activity. To our knowledge it is the first publication in primary culture that provides the link between leptin lytic modulation and membrane remodeling. J. Cell. Physiol. 232: 101-109, 2017. © 2016 Wiley Periodicals, Inc.

Ras-related C3 Botulinum Toxin Substrate (Rac) and Src Family Kinases (SFK) Are Proximal and Essential for Phosphatidylinositol 3-Kinase (PI3K) Activation in Natural Killer (NK) Cell-mediated Direct Cytotoxicity against Cryptococcus neoformans

The activity of Rac in leukocytes is essential for immunity. However, its role in NK cell-mediated anti-microbial signaling remains unclear. In this study, we investigated the role of Rac in NK cell mediated anti-cryptococcal killing. We found thatCryptococcus neoformansindependently activates both Rac and SFK pathways in NK cells, and unlike in tumor killing,Cryptococcusinitiated a novel Rac → PI3K → Erk cytotoxicity cascade. Remarkably, Rac was not required for conjugate formation, despite its essential role in NK cytotoxicity againstC. neoformans Taken together, our data show that, unlike observations with tumor cells, NK cells use a novel Rac cytotoxicity pathway in conjunction with SFK, to killC. neoformans.

Elastic Membrane Deformations Govern Interleaflet Coupling of Lipid-Ordered Domains

The mechanism responsible for domain registration in two membrane leaflets has thus far remained enigmatic. Using continuum elasticity theory, we show that minimum line tension is achieved along the rim between thicker (ordered) and thinner (disordered) domains by shifting the rims in opposing leaflets by a few nanometers relative to each other. Increasing surface tension yields an increase in line tension, resulting in larger domains. Because domain registration is driven by lipid deformation energy, it does not require special lipid components or interactions at the membrane midplane.

Dynamics of natural killer cell receptor revealed by quantitative analysis of photoswitchable protein

Natural Killer (NK) cell activation is dynamically regulated by numerous activating and inhibitory surface receptors that accumulate at the immune synapse. Quantitative analysis of receptor dynamics has been limited by methodologies that rely on indirect measurements such as fluorescence recovery after photobleaching. Here, we report an apparently novel approach to study how proteins traffic to and from the immune synapse using NK cell receptors tagged with the photoswitchable fluorescent protein tdEosFP, which can be irreversibly photoswitched from a green to red fluorescent state by ultraviolet light. Thus, after a localized switching event, the movement of the photoswitched molecules can be temporally and spatially resolved by monitoring fluorescence in two regions of interest. By comparing images with mathematical models, we evaluated the diffusion coefficient of the receptor KIR2DL1 (0.23 ± 0.06 μm(2) s(-1)) and assessed how synapse formation affects receptor dynamics. Our data conclude that the inhibitory NK cell receptor KIR2DL1 is continually trafficked into the synapse, and remains surprisingly stable there. Unexpectedly, however, in NK cells forming synapses with multiple target cells simultaneously, KIR2DL1 at one synapse can relocate to another synapse. Thus, our results reveal a previously undetected intersynaptic exchange of protein.

L-plastin is involved in NKG2D recruitment into lipid rafts and NKG2D-mediated NK cell migration

Membrane rafts are microdomains of the plasma membrane that have multiple biological functions. The involvement of these structures in the biology of T cells, namely in signal transduction by the TCR, has been widely studied. However, the role of membrane rafts in immunoreceptor signaling in NK cells is less well known. We studied the distribution of the activating NKG2D receptor in lipid rafts by isolating DRMs in a sucrose density gradient or by raft fractionation by β-OG-selective solubility in the NKL cell line. We found that the NKG2D-DAP10 complex and pVav are recruited into rafts upon receptor stimulation. Qualitative proteomic analysis of these fractions showed that the actin cytoskeleton is involved in this process. In particular, we found that the actin-bundling protein L-plastin plays an important role in the clustering of NKG2D into lipid rafts. Moreover, coengagement of the inhibitory receptor NKG2A partially disrupted NKG2D recruitment into rafts. Furthermore, we demonstrated that L-plastin participates in NKG2D-mediated inhibition of NK cell chemotaxis.

How to trigger a killer: modulation of natural killer cell reactivity on many levels

The functions of Natural Killer (NK) cells are regulated by a highly redundant set of germline-encoded surface receptors that can inhibit or activate NK cell activities. NK cells can be activated by cytokines or through the interaction with transformed or infected cells. This typically results in the production of cytokines, chemokines, and the induction of cellular cytotoxicity. However, the reactivity of NK cells is modulated on various levels and shaped by processes such as development, education, priming, exposure to antigens and cytokines, and the formation of memory-like phenotypes. Here, I will summarize our current understanding of these processes and describe how they influence NK cell reactivity on a molecular level.

Understanding natural killer cell regulation by mathematical approaches

The activity of natural killer (NK) cells is regulated by various processes including education/licensing, priming, integration of positive and negative signals through an array of activating and inhibitory receptors, and the development of memory-like functionality. These processes are often very complex due to the large number of different receptors and signaling pathways involved. Understanding these complex mechanisms is therefore a challenge, but is critical for understanding NK cell regulation. Mathematical approaches can facilitate the analysis and understanding of complex systems. Therefore, they may be instrumental for studies in NK cell biology. Here we provide a review of the different mathematical approaches to the analysis of NK cell signal integration, activation, proliferation, and the acquisition of inhibitory receptors. These studies show how mathematical methods can aid the analysis of NK cell regulation.

Illuminating the dynamics of signal integration in Natural Killer cells

Natural Killer (NK) cell responses are shaped by the integration of signals transduced from multiple activating and inhibitory receptors at their surface. Biochemical and genetic approaches have identified most of the key proteins involved in signal integration but a major challenge remains in understanding how the spatial and temporal dynamics of their interactions lead to NK cells responding appropriately when encountering ligands on target cells. Well over a decade of research using fluorescence microscopy has revealed much about the architecture of the NK cell immune synapse - the structured interface between NK cells and target cells - and how it varies when inhibition or activation is the outcome of signal integration. However, key questions - such as the proximity of individual activating and inhibitory receptors - have remained unanswered because the resolution of optical microscopy has been insufficient, being limited by diffraction. Recent developments in fluorescence microscopy have broken this limit, seeding new opportunities for studying the nanometer-scale organization of the NK cell immune synapse. Here, we discuss how these new technologies, super-resolution imaging and other novel light-based methods, can illuminate our understanding of NK cell biology.

Human NKG2D-ligands: cell biology strategies to ensure immune recognition

Immune recognition mediated by the activating receptor NKG2D plays an important role for the elimination of stressed cells, including tumors and virus-infected cells. On the other hand, the ligands for NKG2D can also be shed into the sera of cancer patients where they weaken the immune response by downmodulating the receptor on effector cells, mainly NK and T cells. Although both families of NKG2D-ligands, major histocompatibility complex class I-related chain (MIC) A/B and UL16 binding proteins (ULBPs), are related to MHC molecules and their expression is increased after stress, many differences are observed in terms of their biochemical properties and cell trafficking. In this paper, we summarize the variety of NKG2D-ligands and propose that selection pressure has driven evolution of diversity in their trafficking and shedding, but not receptor binding affinity. However, it is also possible to identify functional properties common to individual ULBP molecules and MICA/B alleles, but not generally conserved within the MIC or ULBP families. These characteristics likely represent examples of convergent evolution for efficient immune recognition, but are also attractive targets for pathogen immune evasion strategies. Categorization of NKG2D-ligands according to their biological features, rather than their genetic family, may help to achieve a better understanding of NKG2D-ligand association with disease.

Signaling pathways in aged T cells - a reflection of T cell differentiation, cell senescence and host environment

With increasing age, the ability of the immune system to protect against new antigenic challenges or to control chronic infections erodes. Decline in thymic function and cumulating antigenic experiences of acute and chronic infections threaten T cell homeostasis, but insufficiently explain the failing immune competence and the increased susceptibility for autoimmunity. Alterations in signaling pathways in the aging T cells account for many of the age-related defects. Signaling threshold calibrations seen with aging frequently built on mechanisms that are operational in T cell development and T cell differentiation or are adaptations to the changing environment in the aging host. Age-related changes in transcription of receptors and signaling molecules shift the balance towards inhibitory pathways, most dominantly seen in CD8 T cells and to a lesser degree in CD4 T cells. Prominent examples are the expression of negative regulatory receptors of the CD28 and the TNF receptor superfamilies as well the expression of various cytoplasmic and nuclear dual-specific phosphatases.

PIP2-dependent regulation of Munc13-4 endocytic recycling: impact on the cytolytic secretory pathway

Cytotoxic lymphocytes clear infected and transformed cells by releasing the content of lytic granules at cytolytic synapses, and the ability of cytolytic effectors to kill in an iterative manner has been documented previously. Although bidirectional trafficking of cytolytic machinery components along the endosomal pathway has begun to be elucidated, the molecular mechanisms coordinating granule retrieval remain completely unexplored. In the present study, we focus on the lytic granule priming factor Munc13-4, the mutation of which in familial hemophagocytic lymphohistiocytosis type 3 results in a profound defect of cytotoxic function. We addressed the role of phosphatidylinositol (4,5)-bisphosphate (PIP2) in the regulation of Munc13-4 compartmentalization. We observed that in human natural killer cells, PIP2 is highly enriched in membrane rafts. Granule secretion triggering induces a transient Munc13-4 raft recruitment, followed by AP-2/clathrin-dependent internalization. Phosphatidylinositol 4-phosphate 5-kinase (PIP5K) γ gene silencing leads to the impairment of granule secretion associated with increased levels of raft-associated Munc13-4, which is attributable to a defect in AP-2 membrane recruitment. In such conditions, the ability to subsequently kill multiple targets was significantly impaired. These observations indicate that Munc13-4 reinternalization is required for the maintenance of an intracellular pool that is functional to guarantee the serial killing potential.

Finding Balance: T cell Regulatory Receptor Expression during Aging

Aging is associated with a variety of changes to immune responsiveness. Reduced protection against infection, reduced responses to vaccination and increased risk of autoimmunity are all hallmarks of advanced age. Here we consider how changes in the expression of regulatory receptors on the T cell surface contribute to altered immunity during aging.

Simulations of the NK cell immune synapse reveal that activation thresholds can be established by inhibitory receptors acting locally

NK cell activation is regulated by a balance between activating and inhibitory signals. To address the question of how these signals are spatially integrated, we created a computer simulation of activating and inhibitory NK cell immunological synapse (NKIS) assembly, implementing either a "quantity-based" inhibition model or a "distance-based" inhibition model. The simulations mimicked the observed molecule distributions in inhibitory and activating NKIS and yielded several new insights. First, the total signal is highly influenced by activating complex dissociation rates but not by adhesion and inhibitory complex dissociation rates. Second, concerted motion of receptors in clusters significantly accelerates NKIS maturation. Third, when the potential of a cis interaction between Ly49 receptors and MHC class I on murine NK cells was added to the model, the integrated signal as a function of receptor and ligand numbers was only slightly increased, at least up to the level of 50% cis-bound Ly49 receptors reached in the model. Fourth, and perhaps most importantly, the integrated signal behavior obtained when using the distance-based inhibition signal model was closer to the experimentally observed behavior, with an inhibition radius of the order 3-10 molecules. Microscopy to visualize Vav activation in NK cells on micropatterned surfaces of activating and inhibitory strips revealed that Vav is only locally activated where activating receptors are ligated within a single NK cell contact. Taken together, these data are consistent with a model in which inhibitory receptors act locally; that is, that every bound inhibitory receptor acts on activating receptors within a certain radius around it.

The human NKG2D ligand ULBP2 can be expressed at the cell surface with or without a GPI anchor and both forms can activate NK cells

The activating immune receptor NKG2D binds to several stress-induced ligands that are structurally different. MHC-class-I-related chain (MIC) A/B molecules have a transmembrane domain, whereas most UL16 binding proteins (ULBPs) are glycosylphosphatidylinositol (GPI)-linked molecules. The significance of this variability in membrane anchors is unclear. Here, we demonstrate that ULBP2, but not ULBP1 or ULBP3, can reach the cell surface without the GPI modification. Several proteins are expressed at the cell surface as both transmembrane and GPI-linked molecules, either via alternative splicing or by the expression of linked genes. However, to our knowledge, ULBP2 is the first single mammalian cDNA that can be expressed as either a transmembrane or a GPI-anchored protein. The rate of maturation and the levels of cell surface expression of the non-GPI-linked form were lower than those of the GPI-linked ULBP2. Nonetheless, non-GPI ULBP2 was recognised by NKG2D and triggered NK cell cytotoxicity. These data show that differences in membrane attachment by NKG2D ligands are more important for regulation of their surface expression than for cytotoxic recognition by NKG2D and emphasise that detailed characterisation of the cell biology of individual NKG2D ligands will be necessary to allow targeted modulation of this system.

Sphingomyelin metabolism in erythrocyte membrane in asthma

BACKGROUND

Sphingomyelin (SM), a major lipid constituent of outer leaflet of plasma membranes, with cholesterol, constitutes microdomains, which are termed as lipid rafts. These rafts provide support to proteins, receptors, enzymes, and so on and organize and orient them to conduct cellular functions including transmembrane signaling to substances in external milieu. The SM contents are regulated by its metabolism, changes in which may affect the composition of lipid rafts and cell response to the triggers of asthma which may lead to the pathophysiology. For studying changes in membranes, erythrocytes, which contain lipid rafts, are considered to be the best cell type. Hence, this study was conducted on plasma membrane of erythrocytes of asthmatic patients.

OBJECTIVE

The objective is to understand the changes in SM metabolism in asthma.

METHODS

The study included 50 subjects (25 asthmatics and 25 healthy subjects). Erythrocytes were isolated from the peripheral blood and membrane prepared. This was followed by determination of total cholesterol, phospholipids, SM, and sphingomyelinase activity. P < .05 was considered significant.

RESULTS AND CONCLUSIONS

In asthmatics, there was a significant decrease in cholesterol contents (p < .05), decrease in total phospholipid contents (p < .005), increase in SM (p < .01), decrease in cholesterol: SM ratio (p < .001) and increase in sphingomyelinase activity (p < .001) in erythrocyte membranes. We conclude that in asthma, the increase in SM contents is associated with increased sphingomyelinase activity which shows an imbalance in SM metabolism, directed toward its accumulation. The ratio of cholesterol to SM, critical for maintenance of lipid rafts, was significantly lower in asthmatics. This indicates changes in structure of lipid rafts which may lead to the pathophysiology and development of asthma. Regulation of SM metabolism may help in disease regulation and its control.

PD-1 expression on peripheral CD8+ TEM/TEMRA subsets closely correlated with HCV viral load in chronic hepatitis C patients

BACKGROUND

Tight correlation between host circulating CD8+ T cell-mediated immune response and control of viral replication is classical characteristic of long-term HCV infection. CD8+ T cell maturation/activation markers are expected to be associated with viral replication and disease progression in chronic HCV infection. The aim of the present study was to explore novel markers on CD8+ T cells with ability to evaluate HCV viral replication and disease progression.

METHODS

PBMCs were isolated from 37 chronic HCV-infected patients and 17 healthy controls. Distributed pattern of CD8+ T cells subsets and expression of PD-1, CD38, HLA-DR and CD127 were analyzed by flow cytometry. The correlation between expression of surface markers and HCV viral load or ALT was studied.

RESULTS

Declined naïve and increased TEMRA CD8+ T subsets were found in HCV-infected individuals compared with healthy controls. Percentage and MFI of PD-1, CD38 and HLA-DR on all CD8+ T cell subsets were higher in HCV-infected patients than healthy controls. In contrast, CD127 expression on CD8+ TCM showed an opposite trend as PD-1, CD38 and HLA-DR did. In chronic HCV infection, MFI of PD-1 on CD8+ TEM (p < 0.0001) and TEMRA (p = 0.0015) was positively correlated with HCV viral load while HLA-DR expression on non-naive CD8+ T cell subsets (p < 0.05) was negatively correlated with HCV viral load.

CONCLUSION

PD-1 level on peripheral CD8+ TEM/TEMRA was highly correlated with HCV viral load in chronic HCV-infected patients, which made PD-1 a novel indicator to evaluate HCV replication and disease progression in chronic hepatitis C patients.

IL-2-activated haploidentical NK cells restore NKG2D-mediated NK-cell cytotoxicity in neuroblastoma patients by scavenging of plasma MICA

NK group 2D (NKG2D)-expressing NK cells exhibit cytolytic activity against various tumors after recognition of the cellular ligand MHC class I chain-related gene A (MICA). However, release of soluble MICA (sMICA) compromises NKG2D-dependent NK-cell cytotoxicity leading to tumor escape from immunosurveillance. Although some molecular details of the NKG2D-MICA interaction have been elucidated, its impact for donor NK (dNK) cell-based therapy of solid tumors has not been studied. Within an ongoing phase I/II trial, we used allogeneic IL-2 activated dNK cells after haploidentical stem cell transplantation for immunotherapy of patients with high-risk stage IV neuroblastoma. NKG2D levels on activated dNK cells increased strongly when compared with freshly isolated dNK cells and correlated with enhanced NK-cell cytotoxicity. Most importantly, elevated sMICA levels in patients plasma correlated significantly with impaired dNK-cell-mediated cytotoxicity. This effect could be reversed by high-dose infusion of activated dNK cells, which display high levels of surface NKG2D. Our data suggest that the provided excess of NKG2D leads to clearance of sMICA and preserves cytotoxicity of dNK cells via non-occupied NKG2D. In conclusion, our results identify this tumor immune escape mechanism as a target to improve immunotherapy of neuroblastoma and presumably other tumors.

Global network analysis of lipid-raft-related proteins reveals their centrality in the network and their roles in multiple biological processes

Lipid rafts are specialized cholesterol-enriched microdomains in the cell membrane. They have been known as a platform for protein-protein interactions and to take part in multiple biological processes. Nevertheless, how lipid rafts influence protein properties at the proteomic level is still an open question for researchers using traditional biochemical approaches. Here, by annotating the lipid raft localization of proteins in human protein-protein interaction networks, we performed a systematic analysis of the function of proteins related to lipid rafts. Our results demonstrated that lipid raft proteins and their interactions were critical for the structure and stability of the whole network, and that the interactions between them were significantly enriched. Furthermore, for each protein in the network, we calculated its "lipid raft dependency (LRD)," which indicates how close it is topologically associated with lipid rafts, and we then uncovered the connection between LRD and protein functions. Proteins with high LRD tended to be essential for mammalian development, and malfunction of these proteins was inclined to cause human diseases. Coordinated with their neighbors, high-LRD proteins participated in multiple biological processes and targeted many pathways in diseases pathogenesis. High-LRD proteins were also found to have tissue specificity of expression. In summary, our network-based analysis denotes that lipid raft proteins have higher centrality in the network, and that lipid-raft-related proteins have multiple functions and are probably concerned with many biological processes in disease development.

Inhibitory and regulatory immune synapses

Cell contact-dependent inhibition and regulation of immune responses play an essential role in balancing the need for rapid and efficient responses to a wide variety of pathological challenges, while at the same time maintaining self-tolerance. Much attention has been given to immune synapses that lead to the activation of, for example, cell-mediated cytotoxicity, and here we compare the supramolecular dynamics of synapses that lead to inhibition or regulatory functions. We focus on natural killer cells where such different synapses have been best studied. An emergent principle is that inhibition or regulatory responses are commonly achieved by selective recruitment of signalling proteins to the synapse and exclusion of membrane-proximal intracellular proteins needed for activation. We also discuss evidence that an inhibitory synapse triggers or maintains effector cells in a migratory configuration, which serves to break the synapse before the steps needed for effector cell activation can be completed. This model implies that the concept of kinetic-proofreading, previously used to describe activation of individual T-cell receptors, can also apply in determining the outcome of intercellular conjugation.

Significant functional heterogeneity among KIR2DL1 alleles and a pivotal role of arginine 245

Killer immunoglobulin-like receptors (KIRs) play an essential role in the regulation of natural killer cell functions. KIR genes are highly polymorphic in nature, showing both haplotypic and allelic variations among people. We demonstrated in both in vitro and in vivo models a significant heterogeneity in function among different KIR2DL1 alleles, including their ability to inhibit YT-Indy cells from degranulation, interferon gamma production, and cytotoxicity against target cells expressing the HLA-Cw6 ligand. Subsequent experiments showed that the molecular determinant was an arginine residue at position 245 (R245) in its transmembrane domain that mechanistically affects both the efficiency of inhibitory signaling and durability of surface expression. Specifically, in comparison with R245-negative alleles, KIR2DL1 that included R245 recruited more Src-homology-2 domain-containing protein tyrosine phosphatase 2 and beta-arrestin 2, showed higher inhibition of lipid raft polarization at immune synapse, and had less down-regulation of cell-surface expression upon interaction with its ligand. Thus, our findings provide novel insights into the molecular determinant of KIR2DL1 and conceivably a fundamental understanding of KIR2DL1 allelic polymorphism in human disease susceptibility, transplant outcome, and donor selection.

Endocytosis and intracellular trafficking of human natural killer cell receptors

Natural killer (NK) cells play a vital role in the defense against viral infections and tumor development. NK cell function is primarily regulated by the sum of signals from a broad array of activation and inhibitory receptors. Key to generating the input level of either activating or inhibitory signals is the maintenance of receptor expression levels on the cell surface. Although the mechanisms of endocytosis and trafficking for some cell surface receptors, such as transferrin receptor and certain immune receptors, are very well known, that is not the situation for receptors expressed by NK cells. Recent studies have uncovered that endocytosis and trafficking routes characteristic for specific activation and inhibitory receptors can regulate the functional responses of NK cells. In this review, we summarize the current knowledge of receptor endocytosis and trafficking, and integrate this with our current understanding of NK cell receptor trafficking.

Human natural killer receptors, co-receptors, and their ligands

In the last 20 years, the study of human natural killer (NK) cells has moved from the first molecular characterizations of very few receptor molecules to the identification of a plethora of receptors displaying surprisingly divergent functions. Our laboratory has contributed to the description of inhibitory receptors and their signaling pathways, important in fine regulation in many cell types, but unknown until their discovery in the NK cells. Inhibitory function is central to regulating NK-mediated cytolysis, with different molecular structures evolving during speciation to assure its persistence. Only in the last ten years has it become possible to characterize the NK triggering receptors mediating natural cytotoxicity, leading to an appreciation of the existence of a cellular interaction network between effectors of both natural and adaptive immunity. This report reviews the contemporary history of molecular studies of receptors and ligands involved in NK cell function, characterizing the ligands of the triggering receptor and the mechanisms for finely regulating their expression in pathogen-infected or tumor cells.

Formation and function of the lytic NK-cell immunological synapse

The natural killer (NK)-cell immunological synapse is the dynamic interface formed between an NK cell and its target cell. Formation of the NK-cell immunological synapse involves several distinct stages, from the initiation of contact with a target cell to the directed delivery of lytic-granule contents for target-cell lysis. Progression through the individual stages is regulated, and this tight regulation underlies the precision with which NK cells select and kill susceptible target cells (including virally infected cells and cancerous cells) that they encounter during their routine surveillance of the body.

Dendritic cell and natural killer cell cross-talk: a pivotal role of CX3CL1 in NK cytoskeleton organization and activation

Initial molecular events leading to natural killer lymphocyte (NK) and dendritic cell (DC) interactions are largely unknown. Here, the role of CX3CL1 (fractalkine), a chemokine expressed on mature dendritic cells (mDCs) has been investigated. We show that CX3CL1 promotes NK activation by mDCs. After blocking of CX3CL1 by antibody, no activation occurred but major histocompatibility complex (MHC) class I neutralization restored DC-mediated NK activation, suggesting an interaction between CX3CL1 signaling and the functioning of inhibitory KIR. Then the YTS NK cell line, in which the inhibitory receptor KIR2DL1 had been introduced, was used. The presence of KIR2DL1 did not decrease YTS activation by HLA-Cw4 DC when CX3CL1 was functional. In contrast, CX3CL1 neutralization led to killer cell immunoglobulin-like receptor (KIR) phosphorylation and SHP-1 recruitment in YTS(KIR2DL1) cultured with HLA-Cw4 mDCs. Moreover, CX3CL1 neutralization promoted dispersion of lipid rafts and the formation of a multiprotein complex required for cytoskeletal rearrangements in YTS NK cells. These findings point to a pivotal role of CX3CL1 in the activation of resting NK cells by mature DCs.

The killer's kiss: the many functions of NK cell immunological synapses

Natural killer (NK) cells comprise a subset of lymphocytes involved in protection against microbial pathogens and tumors. NK cells recognize host cells that are missing MHC class I molecules and eliminate them through localized delivery of lytic granules. The majority of NK cell effector functions require direct cell-to-cell contact. Binding to a target cell is accompanied by creation of complex structures at the cell-cell interface known as immunological synapses. Recent studies have contributed immensely to the characterization of several types of NK cell immunological synapses and understanding of the variety of processes originating at this intriguing place. The emerging picture illustrates NK cell immune synapses as the sites of highly complex regulation of NK cell activity.

Colocalization of the IL-12 receptor and FcgammaRIIIa to natural killer cell lipid rafts leads to activation of ERK and enhanced production of interferon-gamma

Natural killer (NK) cells express an activating receptor for the Fc portion of IgG (FcgammaRIIIa) that mediates interferon (IFN)-gamma production in response to antibody (Ab)-coated targets. We have previously demonstrated that NK cells activated with interleukin-12 (IL-12) in the presence of immobilized IgG secrete 10-fold or more higher levels of IFN-gamma as compared with stimulation with either agent alone. We examined the intracellular signaling pathways responsible for this synergistic IFN-gamma production. NK cells costimulated via the FcR and the IL-12 receptor (IL-12R) exhibited enhanced levels of activated STAT4 and Syk as compared with NK cells stimulated through either receptor alone. Extracellular signal-regulated kinase (ERK) was also synergistically activated under these conditions. Studies with specific chemical inhibitors revealed that the activation of ERK was dependent on the activation of PI3-K, whose activation was dependent on Syk, and that sequential activation of these molecules was required for NK cell IFN-gamma production in response to FcR and IL-12 stimulation. Retroviral transfection of ERK1 into primary human NK cells substantially increased IFN-gamma production in response to immobilized IgG and IL-12, while transfection of human NK cells with a dominant-negative ERK1 abrogated IFN-gamma production. Confocal microscopy and cellular fractionation experiments revealed that FcgammaRIIIa and the IL-12R colocalized to areas of lipid raft microdomains in response to costimulation with IgG and IL-12. Chemical disruption of lipid rafts inhibited ERK signaling in response to costimulation and significantly inhibited IFN-gamma production. These data suggest that dual recruitment of FcgammaRIIIa and the IL-12R to lipid raft microdomains allows for enhanced activation of downstream signaling events that lead to IFN-gamma production.

Functional role of phosphatidylcholine-specific phospholipase C in regulating CD16 membrane expression in natural killer cells

CD16, the low-affinity FcIgG receptor (FcgammaRIIIA), is predominantly expressed in human NK cells. Our recent findings indicate that CD16 expression on the outer membrane surface of NK cells is correlated with the membrane expression of phosphatidylcholine-specific phospholipase C (PC-PLC). In the present study we analyzed the trafficking of CD16 from the plasma membrane to cytoplasmic regions, after stimulation with specific mAb. The CD16 receptor is internalized, likely degraded and newly synthesized; its endocytosis is independent of ATP, but requires an integral and functional actin cytoskeleton. Antibody-mediated CD16 cross-linking results in an approximately twofold increase in PC-PLC enzymatic activity within 10 min. Analysis of PC-PLC and CD16 distribution in NK cell plasma membrane demonstrates that the proteins are physically associated and partially accumulated in lipid rafts. Pre-incubation of NK cells with a PC-PLC inhibitor, D609, causes a dramatic decrease both in CD16 receptor and PC-PLC enzyme expression on the plasma membrane. Interestingly, among phenotype PBL markers, only CD16 is strongly down-modulated by D609 treatment. CD16-mediated cytotoxicity is also reduced after D609 incubation. Taken together, these data suggest that the PC-PLC enzyme could play an important role in regulating CD16 membrane expression, the CD16-mediated cytolytic mechanism and CD16-triggered signal transduction.

Enhancement of natural killer (NK) cell cytotoxicity by fever-range thermal stress is dependent on NKG2D function and is associated with plasma membrane NKG2D clustering and increased expression of MICA on target cells

Circulating NK cells normally experience temperature gradients as they move about the body, but the onset of inflammation can expose them and their targets to febrile temperatures for several hours. We found that exposure of human peripheral blood NK cells and target cells to fever-range temperatures significantly enhances lysis of Colo205 target cells. A similar effect was not observed when NK cell lines or IL-2-activated peripheral blood NK cells were used as effectors, indicating that thermal sensitivity of effectors is maturation or activation state-dependent. Use of blocking antibodies revealed that this effect is also dependent on the function of the activating receptor NKG2D and its ligand MHC class I-related chain A (MICA). On NK cells, it was observed that thermal exposure does not affect the total level of NKG2D surface expression, but does result in its distinct clustering, identical to that which occurs following IL-2-induced activation. On tumor target cells, a similar, mild temperature elevation results in transcriptional up-regulation of MICA in a manner that correlates with increased sensitivity to cytolysis. Overall, these data reveal that NK cells possess thermally responsive regulatory elements, which facilitate their ability to capitalize on reciprocal, stress-induced changes simultaneously occurring on target cells during inflammation and fever.

Mechanistic insight into lymphocyte activation through quantitative imaging and theoretical modelling

Increasingly, it is apparent that in order to understand the complexity of immunosurveillance at the cell-cell junction, quantitative analysis at the single cell level is necessary. The visualisation of the large-scale rearrangement of proteins characterising what is known as the immunological synapse (IS) was an important discovery shaping our understanding of the events occurring during immune recognition. The use of supported planar bilayers and geometrically designed substrates combined with advanced imaging techniques such as total internal reflection fluorescence (TIRF) and Förster resonance energy transfer (FRET) has provided insight into the spatio-temporal dynamics of receptor signalling and the role of receptor trafficking in regulating cell signalling. Theoretical modelling will play a key role in the integration of such quantitative data providing mechanistic insight into lymphocyte activation.

Inhibitory receptor signals suppress ligation-induced recruitment of NKG2D to GM1-rich membrane domains at the human NK cell immune synapse

NKG2D is an activating receptor expressed on all human NK cells and a subset of T cells. In cytolytic conjugates between NK cells and target cells expressing its ligand MHC class I chain-related gene A, NKG2D accumulates at the immunological synapse with GM1-rich microdomains. Furthermore, NKG2D is specifically recruited to detergent-resistant membrane fractions upon ligation. However, in the presence of a strong inhibitory stimulus, NKG2D-mediated cytotoxicity can be intercepted, and recruitment of NKG2D to the immunological synapse and detergent-resistant membrane fractions is blocked. Also, downstream phosphorylation of Vav-1 triggered by NKG2D ligation is circumvented by coengaging inhibitory receptors. Thus, we propose that one way in which inhibitory signaling can control NKG2D-mediated activation is by blocking its recruitment to GM1-rich membrane domains. The accumulation of activating NK cell receptors in GM1-rich microdomains may provide the necessary platform from which stimulatory signals can proceed.

Stable masking by H-2Dd cis ligand limits Ly49A relocalization to the site of NK cell/target cell contact

Ly49A is an inhibitory receptor, which counteracts natural killer (NK) cell activation on the engagement with H-2D(d) (D(d)) MHC class I molecules (MHC-I) on target cells. In addition to binding D(d) on apposed membranes, Ly49A interacts with D(d) ligand expressed in the plane of the NK cells' membrane. Indeed, multivalent, soluble MHC-I ligand binds inefficiently to Ly49A unless the NK cells' D(d) complexes are destroyed. However, it is not known whether masked Ly49A remains constitutively associated with cis D(d) also during target cell interaction. Alternatively, it is possible that Ly49A has to be unmasked to significantly interact with its ligand on target cells. These two scenarios suggest distinct roles of Ly49A/D(d) cis interaction for NK cell function. Here, we show that Ly49A contributes to target cell adhesion and efficiently accumulates at synapses with D(d)-expressing target cells when NK cells themselves lack D(d). When NK cells express D(d), Ly49A no longer contributes to adhesion, and ligand-driven recruitment to the cellular contact site is strongly reduced. The destruction of D(d) complexes on NK cells, which unmasks Ly49A, is necessary and sufficient to restore Ly49A adhesive function and recruitment to the synapse. Thus, cis D(d) continuously sequesters a considerable fraction of Ly49A receptors, preventing efficient Ly49A recruitment to the synapse with D(d)+ target cells. The reduced number of Ly49A receptors that can functionally interact with D(d) on target cells explains the modest inhibitory capacity of Ly49A in D(d) NK cells. This property renders Ly49A NK cells more sensitive to react to diseased host cells.

The CD94/NKG2 family of receptors: from molecules and cells to clinical relevance

Immune responses must be tightly regulated to avoid hyporesponsiveness on one hand or excessive inflammation and the development of autoimmunity (hyperresponsiveness) on the other hand. This balance is attained through the throttling of activating signals by inhibitory signals that ideally leads to an adequate immune response against an invader without excessive and extended inflammatory signals that promote the development of autoimmunity. The CD94/NKG2 family of receptors is composed of members with activating or inhibitory potential. These receptors are expressed predominantly on NK cells and a subset of CD8+ T cells, and they have been shown to play an important role in regulating responses against infected and tumorigenic cells. In this review, we discuss the current knowledge about this family of receptors, including ligand and receptor interaction, signaling, membrane dynamics, regulation of gene expression and their roles in disease regulation, infections, and cancer, and bone marrow transplantation.

CD94/NKG2A inhibits NK cell activation by disrupting the actin network at the immunological synapse

An adequate immune response is the result of the fine balance between activation and inhibitory signals. The exact means by which inhibitory signals obviate activation signals in immune cells are not totally elucidated. Human CD94/NKG2A is an ITIM-containing inhibitory receptor expressed by NK cells and some CD8+ T cells that recognize HLA-E. We show that the engagement of this receptor prevents NK cell activation by disruption of the actin network and exclusion of lipid rafts at the point of contact with its ligand (inhibitory NK cell immunological synapse, iNKIS). CD94/NKG2A engagement leads to recruitment and activation of src homology 2 domain-bearing tyrosine phosphatase 1. This likely explains the observed dephosphorylation of guanine nucleotide exchange factor and regulator of actin, Vav1, as well as ezrin-radixin-moesin proteins that connect actin filaments to membrane structures. In contrast, NK cell activation by NKG2D induced Vav1 and ezrin-radixin-moesin phosphorylation. Thus, CD94/NKG2A prevents actin-dependent recruitment of raft-associated activation receptors complexes to the activating synapse. This was further substantiated by showing that inhibition of actin polymerization abolished lipid rafts exclusion at the iNKIS, whereas cholesterol depletion had no effect on actin disruption at the iNKIS. These data indicate that the lipid rafts exclusion at the iNKIS is an active process which requires an intact cytoskeleton to maintain lipid rafts outside the inhibitory synapse. The net effect is to maintain an inhibitory state in the proximity of the iNKIS, while allowing the formation of activation synapse at distal points within the same NK cell.

Microclusters of inhibitory killer immunoglobulin-like receptor signaling at natural killer cell immunological synapses

We report the supramolecular organization of killer Ig–like receptor (KIR) phosphorylation using a technique applicable to imaging phosphorylation of any green fluorescent protein–tagged receptor at an intercellular contact or immune synapse. Specifically, we use fluorescence lifetime imaging (FLIM) to report Förster resonance energy transfer (FRET) between GFP-tagged KIR2DL1 and a Cy3-tagged generic anti-phosphotyrosine monoclonal antibody. Visualization of KIR phosphorylation in natural killer (NK) cells contacting target cells expressing cognate major histocompatibility complex class I proteins revealed that inhibitory signaling is spatially restricted to the immune synapse. This explains how NK cells respond appropriately when simultaneously surveying susceptible and resistant target cells. More surprising, phosphorylated KIR was confined to microclusters within the aggregate of KIR, contrary to an expected homogeneous distribution of KIR signaling across the immune synapse. Also, yellow fluorescent protein–tagged Lck, a kinase important for KIR phosphorylation, accumulated in a multifocal distribution at inhibitory synapses. Spatial confinement of receptor phosphorylation within the immune synapse may be critical to how activating and inhibitory signals are integrated in NK cells.

T Cell Recognition and Killing of Vascular Smooth Muscle Cells in Acute Coronary Syndrome

Loss of vascular smooth muscle cells (VSMCs) has been proposed to destabilize the atherosclerotic plaque and contribute to plaque rupture, superimposed thrombosis, and acute coronary syndromes (ACSs). We examined whether VSMCs are susceptible to T cell-induced apoptosis and found that CD4 T cells are highly effective in establishing cell-cell contact with VSMCs and triggering apoptotic death. Visualization of the T cell-VSMC contact zone on the single-cell level revealed that both patient-derived and control CD4 T cells reorganized their cell membrane to assemble an immunologic synapse with the VSMCs. Within 4 to 10 minutes, the membrane proximal signaling molecule ZAP-70 was recruited and phosphorylated. However, only patient-derived CD4 T cells sustained an intact immunologic synapse beyond 10 minutes and generated intracellular calcium signals. CD4 T cells that maintained a synaptic contact and appeared to be responsible for VSMC apoptosis accounted for approximately 20% of the circulating memory T cell population in ACS patients and were rare in the blood of age-matched controls. CD4 T cells from ACS patients were also hyperresponsive to T cell receptor-mediated stimulation when triggered by a superantigen and non-VSMC target cells. Lowered setting of the T cell activation threshold, attributable to excessive amplification of proximal CD3-mediated signals, may contribute to CD4 T cell-mediated killing of VSMCs and promote plaque instability.

CD48 Stimulation by 2B4 (CD244)-Expressing Targets Activates Human NK Cells

Human NK cells can be activated by a variety of different cell surface receptors. Members of the SLAM-related receptors (SRR) are important modulators of NK cell activity. One interesting feature of the SRR is their homophilic interaction, combining receptor and ligand in the same molecule. Therefore, SRR cannot only function as activating NK cell receptors, but also as activating NK cell ligands. 2B4 (CD244) is the only SRR that does not show homophilic interaction. Instead, 2B4 is activated by binding to CD48, a GPI-anchored surface molecule that is widely expressed in the hemopoietic system. In this study, we show that 2B4 also can function as an activating NK cell ligand. 2B4-expressing target cells can efficiently stimulate NK cell cytotoxicity and IFN-gamma production. Using soluble receptor fusion proteins and SRR-transfected cells, we show that 2B4 does not bind to any other SRR expressed on NK cells, but only interacts with CD48. Lysis of 2B4-expressing target cells can be blocked by anti-CD48 Abs and triggering of CD48 in a redirected lysis assay can stimulate NK cell cytotoxicity. This demonstrates that 2B4 can stimulate NK cell cytotoxicity and cytokine production by interacting with NK cell expressed CD48 and adds CD48 to the growing number of activating NK cell receptors.

Cholesterol and sphingolipids as lipid organizers of the immune cells’ plasma membrane: Their impact on the functions of MHC molecules, effector T-lymphocytes and T-cell death

The possible regulatory mechanisms by which glycosphingolipid- and cholesterol-rich membrane microdomains, caveolar and non-caveolar lipid rafts, control the immune response are continuously expanding. In the present overview we will focus on how these membrane-organizing lipids are involved, in collaboration with tetraspanin proteins, in the formation of distinct MHC-I and MHC-II microdomains at the cell surface and will analyze the possible roles of MHC compartmentation in the processes of antigen presentation and regulation of various stages of the cellular immune response. Some basic, lipid raft- and tetraspan mediated mechanisms involved in the formation and function of immunological synapses between various APCs and T-cells will also be discussed. Finally, a new aspect of immune regulation by sphingolipids will be briefly described, namely how can the death or stress signals, leading to ceramide accumulation, result in raft-associated regulatory platforms controlling cell death or antigen-induced, TCRmediated signaling of T-lymphocytes. The influence of these signals and their cross-talk on the fate (death or survival) of T-cells and the outcome of T-cell response will also be discussed.

Uncoupling of T-cell effector functions by inhibitory killer immunoglobulin-like receptors

Killer immunoglobulin-like receptors (KIRs) are a family of regulatory cell-surface molecules expressed on natural killer (NK) cells and memory T-cell subsets. Their ability to prevent the formation of an activation platform and to inhibit NK cell activation is the basis of the missing self model of NK cell function. The benefits of KIR expression for T-cell biology are unclear. We studied how KIR2DL2 regulates T-cell function. Engagement of KIR2DL2 by the ligand human leukocyte antigen (HLA)-Cw3 did not affect conjugate formation between CD4(+)KIR2DL2(+) T cells and superantigen-pulsed target cells or the development of mature immune synapses with lipid rafts. KIR2DL2 and the corresponding HLA-C ligand were initially recruited to the peripheral supramolecular activation cluster (pSMAC). Consequently, KIR2DL2 engagement did not inhibit the phosphorylation of early signaling proteins and T-cell-receptor (TCR)-mediated cytotoxicity or granule exocytosis. After 15-30 minutes, KIR2DL2 moved to the central supramolecular activation cluster (cSMAC), colocalizing with CD3. TCR synapses dissociated, and phosphorylated phospholipase C (PLC)-gamma1, Vav1, and extracellular signal-regulated kinase 1/2 (ERK1/2) were reduced 90 minutes after stimulation. Gene array studies documented that the inhibition of late signaling events by KIR2DL2 affected transcriptional gene activation. We propose that KIRs on memory T cells operate to uncouple effector functions by modifying the transcriptional profile while leaving granule exocytosis unabated.