Generation of inositol phosphates during triggering of cytotoxicity in human natural killer and lymphokine-activated killer cells

G Protein-coupled Receptor Kinase and β-Arrestin-mediated Desensitization of the Angiotensin II Type 1A Receptor Elucidated by Diacylglycerol Dynamics

Receptor desensitization progressively limits responsiveness of cells to chronically applied stimuli. Desensitization in the continuous presence of agonist has been difficult to study with available assay methods. Here, we used a fluorescence resonance energy transfer-based live cell assay for the second messenger diacylglycerol to measure desensitization of a model seven-transmembrane receptor, the Gq-coupled angiotensin II type 1(A) receptor, expressed in human embryonic kidney 293 cells. In response to angiotensin II, we observed a transient diacylglycerol response reflecting activation and complete desensitization of the receptor within 2-5 min. By utilizing a variety of approaches including graded tetracycline-inducible receptor expression, mutated receptors, and overexpression or short interfering RNA-mediated silencing of putative components of the cellular desensitization machinery, we conclude that the rate and extent of receptor desensitization are critically determined by the following: receptor concentration in the plasma membrane; the presence of phosphorylation sites on the carboxyl terminus of the receptor; kinase activity of G protein-coupled receptor kinase 2, but not of G protein-coupled receptor kinases 3, 5, or 6; and stoichiometric expression of beta-arrestin. The findings introduce the use of the biosensor diacylglycerol reporter as a powerful means for studying Gq-coupled receptor desensitization and document that, at the levels of receptor overexpression commonly used in such studies, the properties of the desensitization process are markedly perturbed and do not reflect normal cellular physiology.

c-Myc and E1A induced cellular sensitivity to activated NK cells involves cytotoxic granules as death effectors

The contact of natural killer (NK) cells with foreign cells and with certain virus-infected or tumor cells triggers the cytolytic machinery of NK cells. This triggering leads to exocytosis of the cytotoxic NK cell granules. The oncoproteins c-Myc and E1A render cells vulnerable to NK cell mediated cytolysis yet the mechanisms of sensitization are not well understood. In a model where foreign cells (rat fibroblasts) were cocultured with human IL-2 activated NK cells, we observed that NK cells were capable of efficiently killing their targets only if the cells overexpressed the oncogene c-Myc or E1A. Both the parental and the oncogene expressing fibroblasts similarly triggered phosphoinositide hydrolysis in the bound NK cells, demonstrating that NK cells were cytolytically activated in contact with both resistant parental and oncogene expressing sensitive target fibroblasts. The cell death was independent of wild-type p53 and was not inhibited by an anti-apoptotic protein EIB19K. These results provided evidence that c-Myc and E1A activated the NK cell induced cytolysis at a post-triggering stage of NK cell-target cell interaction. In consistence, the c-Myc and E1A overexpressing fibroblasts were more sensitive to the cytolytic effects of isolated NK cell-derived granules than parental cells. The data indicate that oncogenes activate the cytotoxicity of NK cell granules. This mechanism can have a role in directing the cytolytic action of NK cells towards the virus-infected and cancer cells.

Signal transduction during NK cell activation: balancing opposing forces

Significant progress has been made in our understanding of the basic signaling mechanisms regulating NK cell activation. Advances have been fueled in part by the molecular characterization of specific activating receptors (e.g., the Fc gamma RIII multi-subunit complex) and inhibitory receptors (e.g., novel MHC-recognizing inhibitory receptors). However, certain aspects of these analyses are complicated by the heterogeneous nature of the receptor-ligand interactions utilized during the development of a cytotoxic response. Future advances will depend in part on the further molecular characterization of the involved receptors and second messengers and on the development of experimental models for genetically manipulating the signaling elements. It will remain important to understand both activating and inhibitory signaling pathways as the emerging theme is that the balance of these two opposing forces determines the functional outcome of an NK cells interaction with its target.

Functional role for Syk tyrosine kinase in natural killer cell-mediated natural cytotoxicity

Natural killer (NK) cells are named based on their natural cytotoxic activity against a variety of target cells. However, the mechanisms by which sensitive targets activate killing have been difficult to study due to the lack of a prototypic NK cell triggering receptor. Pharmacologic evidence has implicated protein tyrosine kinases (PTKs) in natural killing; however, Lck-deficient, Fyn-deficient, and ZAP-70-deficient mice do not exhibit defects in natural killing despite demonstrable defects in T cell function. This discrepancy implies the involvement of other tyrosine kinases. Here, using combined biochemical, pharmacologic, and genetic approaches, we demonstrate a central role for the PTK Syk in natural cytotoxicity. Biochemical analyses indicate that Syk is tyrosine phosphorylated after stimulation with a panel of NK-sensitive target cells. Pharmacologic exposure to piceatannol, a known Syk family kinase inhibitor, inhibits natural cytotoxicity. In addition, gene transfer of dominant-negative forms of Syk to NK cells inhibits natural cytotoxicity. Furthermore, sensitive targets that are rendered NK-resistant by major histocompatibility complex (MHC) class I transfection no longer activate Syk. These data suggest that Syk activation is an early and requisite signaling event in the development of natural cytotoxicity directed against a variety of cellular targets.

Cytokine-induced cytotoxic function expressed by lymphocytes of the innate immune system: distinguishing characteristics of NK and LAK based on functional and molecular markers

Several molecular events are now identifiable during the activation, recognition, and killing by natural killer (NK) cells that are distinct from those differentiated in response to cytokines during the generation of lymphokine-activated killer (LAK) cells or during lymphocyte proliferation. Because LAK and MHC-unrestricted killing activities also include the prototypic NK targets as part of their broad recognition spectra, accurate identification of the complete function being studied is critical. In many publications, past and present, only NK-sensitive target cells were used (K562, Molt-4, others), and, therefore, the results do not necessarily indicate whether the effectors are NK or have differentiated into LAK cells. Such a consideration becomes critical when the effectors are grown in interleukin-2 (IL-2), and an attempt is made to define receptor recognition, signal transduction pathways, and specificity at the molecular level. In some instances, effector cells are likely to have stopped, therefore merely expressing NK activity, and have also acquired LAK function. The identified receptors may not have been unique to NK cells or NK function. Not until the targets employed are also confirmed to be NK sensitive, and the effectors do not kill NK-resistant targets can the results of molecular studies be proposed to represent aspects unique to NK. Reports of the use of IL-2-expanded NK clones are most likely providing data concerning the biology of LAK and not of classic NK. The classic NK activity surveying our blood apparently performs an important function, including the ability to respond rapidly to certain cytokines and to acquire additional functions and receptors for use in destroying a vast array of target cells. It is critical for scientists to appreciate the functional distinctions and to identify the molecules and pathways unique to each of these curious cytolytic systems.

Protein tyrosine kinase inhibition blocks granule exocytosis and cytolytic function of lymphokine-activated killer cells

Short-term pretreatment of human lymphokine-activated killer cells (LAK) with the protein tyrosine kinase-specific inhibitor Herbimycin A (Herb A) blocked cytotoxic function against the NK-resistant (LAK-sensitive) tumor targets, SK-Mel-1 (human melanoma) and Daudi (human lymphoma). Greater than 50% inhibition of LAK activity was observed after a 2.5-h pretreatment with 0.125 microgram/ml (ca. 0.2 microM) Herb A. Inhibition of LAK occurred over a time interval in which LAK were not dependent upon IL-2 for maintenance of killing function, supporting the conclusion that the drug interfered with mobilization of cytotoxic function. Conjugate formation between LAK and tumor targets was unaffected by Herb A, indicating that inhibition was occurring at a post-binding step. Granule exocytosis as measured by BLT-esterase release was detected from LAK after coincubation with tumor targets, and was inhibited by Herb A pretreatment. The majority of LAK killing was dependent upon extracellular calcium, supporting the hypothesis that granule exocytosis rather than Fas ligand was the principal pathway leading to target cell death. The data suggest that protein tyrosine kinases play a pivotal role in LAK cytolytic function by regulating granule exocytosis.

Activation of natural killer cell migration by leukocyte integrin-binding peptide from intracellular adhesion molecule-2 (ICAM-2)

Intracellular adhesion molecule-2 (ICAM-2), one of the ligands of CD11a/CD18 (LFA-1), is mainly expressed on endothelial and hematopoietic cells. The biological significance of ICAM-2 has remained unclear. Previous findings have shown that a peptide from ICAM-2, spanning residues 21-42 from the first immunoglobulin domain, enhances natural killer (NK) cell cytotoxicity and induces T cell aggregation. We have now studied the effect of the same ICAM-2 peptide on NK cell migration in the Boyden chamber assay. The peptide significantly increased NK cell migration up to 215 +/- 21%, as compared to migration of control cells (100%), and the induction was inhibited by anti-CD11a monoclonal antibodies. The ICAM-2 peptide also induced polymerization of F-actin at the leading edge of migratory NK cells. Cross-linking of CD11a/CD18 receptors with anti-CD11a or anti-CD18 monoclonal antibodies and secondary antibodies resulted in receptor recycling, increased migration, and actin polymerization, but led to slight inhibition of cytotoxicity. The ICAM-2 peptide did not induce such a receptor recycling. Phosphotyrosine immunoblotting experiments showed that the ICAM-2 peptide increased the phosphorylation of 150- and 35-kDa proteins. During cross-linking with antibodies, only the 150-kDa protein showed increased phosphorylation. The results show that depending on the type of CD11a/CD18 receptor ligation different kinds of signals are transduced in NK cells. These signals may either trigger only locomotion, or both locomotion and cytotoxicity. Based on these findings, a major function for ICAM-2 on endothelium may be triggering of migration of adhering leukocytes.

Triggering of human natural killer cells through CD16 induces tyrosine phosphorylation of the p72syk kinase

Activation of protein tyrosine kinases (PTK) is an initial and obligatory event for the triggering of human natural killer (NK) cells to cytotoxicity. Of the different PTK detected in NK cells, only p56lck has previously been shown to participate in NK cell activation. Here we present evidence that another PTK, p72syk, is involved in activation of NK cells. Stimulation with a monoclonal antibody to to the Fc gamma RIII receptor (CD16) induced an increased tyrosine phosphorylation of p72syk. This phosphorylation correlated with an increased tyrosine kinase activity of p72syk towards a synthetic peptide substrate. A severalfold increase in the catalytic activity of p72syk was also seen after treatment of NK cells with an inhibitor of phosphotyrosine phosphatases, pervanadate. We conclude that triggering of the cytotoxic response in NK cells is associated with activation of p72syk.

Flow cytometric measurement of [Ca2+]i and pHi in conjugated natural killer cells and K562 target cells during the cytotoxic process

We describe a flow cytometric assay that enables one to follow conjugate formation between cytotoxic cells and their target cells during the cytotoxic process. In addition, the internal calcium concentration ([Ca2+]i) and internal pH (pHi) of the conjugated cells can be monitored and directly compared to the nonconjugated cells. This is achieved by labeling one cell type with the Ca(2+)-specific dye Fluo-3, while the other cell type is labeled with the pH-sensitive dye SNARF-1. As these fluorochromes have different emission spectra, events positive for both fluorochromes are identified as conjugates. The results show that the conjugates can be clearly distinguished from single cytotoxic cells [natural killer (NK) cells] and target cells [K562 cells, (TC)]. Upon binding, [Ca2+]i is increased in the NK cells as well as in the TC. In conjugated NK cells this increase of [Ca2+]i is temperature dependent and is followed by a decrease to a normal [Ca2+]i value later on. The [Ca2+]i in NK cells increases in 2 steps, which may be related to the binding--and lethal hit phase. Upon conjugate formation, NK cells show a slight increase in pHi (0.2-0.3 pH units). TC do not reveal a significant change in pHi.

Inhibition of interleukin-2-stimulated enhancement of human natural killer (NK) cell activity by carbaryl, an anticholinesterase insecticide

The potency of the anticholinesterase (antiCHE) insecticides as serine hydrolase inhibitors, and evidence for serine hydrolase activity in interleukin-2 (IL2) signalling suggest that the natural killer (NK) cell may be a target for dysregulation by antiCHE insecticides. NK cells are large granular lymphocytes (LGL) that respond to IL2 by proliferating and increasing their cytolytic efficiency. In the present study, we assessed the effects of carbaryl (CA, an antiCHE insecticide) and alpha-naphthol (NA, the major metabolite of CA) on both target cell killing per se and IL2 enhancement of target cell killing by human NK cells. Human LGL, collected from the peripheral blood of normal donors, were cultured for 4 days with human recombinant IL2 (HRIL2), then assayed by a 51Chromium (51Cr) release assay for lytic activity against human K562 cells. When added at the beginning of the culture period, CA inhibited enhancement of cytolytic efficiency in a concentration-dependent manner; at concentrations (0.5 and 5.0 microM) compatible with no cholinergic toxicity. Reduction of the effector/target cell (E/T) ratio in the 51Cr release assay markedly enhanced the observed inhibition by CA. In one experiment, inhibition increased from 6% to 20%, 17% to 35%, and 53% to 73% at 0.5, 5.0, and 50 microM CA, respectively, when E/T was reduced from 10:1 to 2.5:1. This result is consistent with reduced cytolytic efficiency of individual NK cells exposed to CA. NA had no effect at 0.5 or 5.0 microM but caused some inhibition at 50 microM. Neither CA nor NA produced LGL death. When CA or NA was added directly to the 51Cr release assay, inhibition was not observed. The mechanism of inhibition of IL2-stimulated enhancement of target cell killing is not yet known, however, the results are consistent with impairment of IL2 signalling, by CA.

Resolution of adhesion- and activation-associated components of monoclonal antibody-dependent human NK cell-mediated cytotoxicity

Flow cytometry was used to investigate two functional parameters of human natural-killer (NK) cell-mediated antibody-dependent cellular cytotoxicity (ADCC): (i) the frequency of NK cells which formed conjugates (NKC) with autologous monoclonal antibody (mAb)-coated lymphocyte target cells, a measure of the avidity of CD16-dependent cell-cell adhesion, and (ii) the rise in the intracellular concentration of ionized calcium ([Ca2+]i) elicited in NKC by contact with target cells, a measure of CD16-dependent NK cell activation. For each of four rat IgG2b mAb directed against target cell antigens CDw52, CD5, CD45, and class I HLA, there existed quantitatively similar relationships between ADCC and rise in NKC[Ca2+]i but significant inter-mAb differences with respect to the ADCC vs the NKC frequency relationship. Cytolytic efficiencies of mAb appeared to be determined at the level of the NK cell, dependent upon CD16 and LFA-1, but restricted with respect to quantitative levels of NKC[Ca2+]i. In concert with this notion, targets coated with an IgG1 isotype-switch variant alpha CDw52 mAb promoted significant conjugate formation but failed to elicit a rise in NKC[Ca2+]i or ADCC. Thus, Fc regions of antibodies make contacts with NK cell CD16 which may strengthen cell-cell adhesion without eliciting an activation stimulus, a finding which supports a complexity of CD16 functional regulation of probable significance in the clinical consequences of antibody responses or therapeutic mAb manipulations.

Activation of natural killer cells via the Fc gamma RIII (CD16) requires initial tyrosine phosphorylation

Triggering of the Fc gamma RIII (CD16) on natural killer (NK) cells by monoclonal antibodies or antibody-coated target cells stimulates a rapid phospholipase C (PLC)-mediated hydrolysis of inositol phospholipids and results in subsequent delivery of the lytic hit. The role of initial tyrosine phosphorylation in these events was investigated with a tyrosine protein kinase (TPK) inhibitor, genistein. At doses that inhibited CD16-triggered tyrosine phosphorylation of substrates in intact cells, genistein did not influence serine/threonine phosphorylation or target cell binding but prevented PLC activation, cell-mediated cytotoxicity and antibody-dependent cellular cytotoxicity. These findings indicate that tyrosine phosphorylation is an early and critical event during receptor-mediated activation of the lytic machinery.

Cytotoxicity of human natural killer (NK) cell subsets for Plasmodium falciparum erythrocytic schizonts: stimulation by cytokines and inhibition by neomycin

Quantification of human peripheral blood NK subsets has been made in a group of Kenyan adults and children with acute P. falciparum malaria. Results were compared with data obtained from three age- and sex-matched control cohorts: parasitaemic but asymptomatic children; aparasitaemic children and adults; and adult Caucasians with no previous history of malaria. Separated NK subsets were tested in vitro for cytotoxicity to erythrocytic schizonts of P. falciparum in the presence and absence of cytokines. There was a statistically significant quantitative and qualitative depression of the CD3-CD56+ subset in patients with acute malaria and this was accompanied by an expansion of the 'non-functional' CD3-CD57+CD16-CD56- subset. Both CD3-CD16+ and CD3-CD56+ NK cells from all patients and donors lysed schizonts, and this cytotoxicity was enhanced by the addition of recombinant interferon-alpha and/or IL-2, notably with the CD3-CD56+ subset. Interestingly, asymptomatic donors had the highest levels of CD3-CD56+ NK cells, which also demonstrated an enhanced response to cytokine stimulation. Cytotoxicity to schizonts was accompanied by the release of soluble NK cell lytic factors. Neomycin suppressed cytotoxicity in a dose-dependent manner, indicating that the lysis of schizonts by NK cells involves phospholipase C-mediated phosphoinositide metabolism. Our findings define a role for NK cells in immunity to malaria through the lysis of infected erythrocytes as a first-line defence against the parasite.

Inhibition of CD3-induced Ca2+ signals in Jurkat T-cells by myristic acid

Stimulation of T lymphocytes with antibodies against the T cell receptor/CD3 complex induces within seconds a rise in the concentration of intracellular free Ca2+. Here we show that treatment with 20 microM free myristic acid completely inhibits this Ca2+ signal and the cellular proliferation in Jurkat T cells. Also lauric acid inhibited cell growth while its blocking effect on the Ca2+ signal was weaker than that of myristic acid. Other saturated free fatty acids were inactive. The inhibitory effect of myristic acid could be reversed by the addition of fatty acid free albumin, which will bind the fatty acid. Myristic acid, but not its methyl ester, inhibited both the anti-CD3-induced Ca2+ influx across the cell membrane and Ca2+ release from intracellular stores, but not the formation of inositol phosphates. In contrast, thapsigargin-induced release of Ca2+ from the same intracellular stores was unaffected by myristic acid. Thus, myristic acid specifically blocks T cell antigen receptor-CD3 induced Ca2+ mobilization in T cells.

Molecular cloning of gp42, a cell-surface molecule that is selectively induced on rat natural killer cells by interleukin 2: glycolipid membrane anchoring and capacity for transmembrane signaling

We have previously shown that in vitro culture of rat natural killer (NK) cells in high concentrations of recombinant interleukin 2 (rIL-2) leads to the expression of a surface glycoprotein with a molecular mass of approximately 42 kD. This glycoprotein, gp42, is not induced on other lymphocytes and thus provides a lineage-specific marker for rIL-2-activated NK cells. We here present the nucleotide sequence for gp42 cDNA. The open reading frame encodes 233 amino acids with three potential sites for N-linked glycosylation. The deduced amino acid sequence lacks an apparent transmembrane domain and instead contains a hydrophobic COOH terminus that is characteristic of glycosylphosphatidylinositol (GPI)-anchored surface proteins. Consistent with this, gp42 is cleaved from the NK-like cell line, RNK-16, by phosphatidylinositol-specific phospholipase C (PI-PLC), as is gp42 expressed on CHO cells that have been transformed with gp42 cDNA. On rIL-2-activated NK cells, gp42 is resistant to PI-PLC, though our studies suggest that gp42 on these cells is still expressed as a GPI-anchored molecule. Antibody to gp42 stimulates in RNK-16 cells an increase in inositol phosphates and in intracellular calciu, signals that are associated with the activation of lymphocytes, including NK cells. rIL-2-activated NK cells, however, lack this response to gp42 as well as to other stimuli. Thus, gp42, the only NK-specific activation antigen, is a GPI-anchored surface molecule with the capacity to stimulate transmembrane signaling.

Phorbol 12-myristate 13-acetate (TPA) blocks CD3-mediated Ca2+ mobilization in Jurkat T cells independently of protein kinase C activation

Stimulation of Jurkat T cells with antibodies against the T cell receptor/CD3 complex induces a rise in the intracellular concentration of Ca2+ within seconds. The inositol phosphate-dependent Ca2+ mobilization induced by OKT3 was completely abrogated when Jurkat cells were pretreated for 1 min with the phorbol 12-myristate 13-acetate TPA (10nM), a concentration which activates protein kinase C (PKC). The effects of TPA on the Ca2+ fluxes were insensitive to treatment of the cells with known PKC inhibitors (H-7 and staurosporin) under conditions where the PKC-mediated phosphorylation was blocked. Furthermore, another activator of PKC, mezerein, inhibited the Ca2+ signal induced by OKT3. This inhibition, however, could completely be reversed by pretreatment with H-7 or staurosporine. We conclude that the TPA-mediated inhibition of Ca2+ fluxes in Jurkat T cells largely acts through a PKC-independent pathway.

Pathogenesis of the natural killer cell deficiency in AIDS

Deficiency in natural killer (NK) cell activity is a common feature of acquired immune deficiency syndrome (AIDS). This is part of a general immune dysfunction in AIDS and may lead to progression of the disease, since NK cells are known to be involved in protection against tumors and against viral infections. The lack of immunological surveillance by NK cells of the growth of pathogens that activate the HIV-1 tat infectivity gene may also favor progression to AIDS. The pathogenesis of NK cell deficiency in AIDS is not known. Previous studies have shown that NK cells from AIDS patients are able to bind but not to lyse the target cell line K562. This results from an inability to rearrange the cytoskeleton microtubular (MT) system and to release the natural killer cytotoxic factor (NKCF). This report by Maria Caterina Sirianni and colleagues evaluates the possible mechanisms leading to this NK cell deficiency.