Association of a 70-kDa tyrosine phosphoprotein with the CD16: zeta: gamma complex expressed in human natural killer cells

Engagement of TRAIL triggers degranulation and IFNγ production in human natural killer cells

NK cells utilize a large array of receptors to screen their surroundings for aberrant or virus‐infected cells. Given the vast diversity of receptors expressed on NK cells we seek to identify receptors involved in the recognition of HIV‐1‐infected cells. By combining an unbiased large‐scale screening approach with a functional assay, we identify TRAIL to be associated with NK cell degranulation against HIV‐1‐infected target cells. Further investigating the underlying mechanisms, we demonstrate that TRAIL is able to elicit multiple effector functions in human NK cells independent of receptor‐mediated induction of apoptosis. Direct engagement of TRAIL not only results in degranulation but also IFNγ production. Moreover, TRAIL‐mediated NK cell activation is not limited to its cognate death receptors but also decoy receptor I, adding a new perspective to the perceived regulatory role of decoy receptors in TRAIL‐mediated cytotoxicity. Based on these findings, we propose that TRAIL not only contributes to the anti‐HIV‐1 activity of NK cells but also possesses a multifunctional role beyond receptor‐mediated induction of apoptosis, acting as a regulator for the induction of different effector functions.

Engineering Anti-Tumor Monoclonal Antibodies and Fc Receptors to Enhance ADCC by Human NK Cells

Tumor-targeting monoclonal antibodies (mAbs) are the most widely used and characterized immunotherapy for hematologic and solid tumors. The significance of this therapy is their direct and indirect effects on tumor cells, facilitated by the antibody's antigen-binding fragment (Fab) and fragment crystallizable region (Fc region), respectively. The Fab can modulate the function of cell surface markers on tumor cells in an agonistic or antagonistic manner, whereas the Fc region can be recognized by an Fc receptor (FcR) on leukocytes through which various effector functions, including antibody-dependent cell-mediated cytotoxicity (ADCC), can be elicited. This process is a key cytolytic mechanism of natural killer (NK) cells. These innate lymphocytes in the human body recognize tumor-bound antibodies exclusively by the IgG Fc receptor CD16A (FcγRIIIA). Two allelic versions of CD16A bind IgG with either lower or higher affinity. Cancer patients homozygous for the higher affinity allele of CD16A have been reported to respond significantly better to mAb therapies for various malignancies. These studies revealed that mAb therapy efficacy positively correlates with higher affinity binding to CD16A. Approaches to enhance tumor antigen targeting by NK cells by modifying the Fc portion of antibodies or the FcR on NK cells are the focus of this review.

Profiling of human lymphocytes reveals a specific network of protein kinases modulated by endurance training status

To date, the effects of endurance exercise training on lymphocyte physiology at the kinome level are largely unknown. Therefore, the present study used a highly sensitive peptide-based kinase activity profiling approach to investigate if the basal activity of tyrosine (Tyr) and serine/threonine (Ser/Thr) kinases of human lymphocytes is affected by the aerobic endurance training status. Results revealed that the activity of various tyrosine kinases of the FGFR family and ZAP70 was increased, whereas the activity of multiple Ser/Thr kinases such as IKKα, CaMK4, PKAα, PKCα+δ (among others) was decreased in lymphocytes of endurance trained athletes (ET). Moreover, functional associations between several differentially regulated kinases in ET-derived lymphocytes were demonstrated by phylogenetic mapping and network analysis. Especially, Ser/Thr kinases of the AGC-kinase (protein kinase A, G, and C) family represent exercise-sensitive key components within the lymphocytes kinase network that may mediate the long-term effects of endurance training. Furthermore, KEGG (Kyoto Encyclopedia of Genes and Genomes) and Reactome pathway analysis indicate that Ras as well as intracellular signaling by second messengers were found to be enriched in the ET individuals. Overall, our data suggest that endurance exercise training improves the adaptive immune competence by modulating the activity of multiple protein kinases in human lymphocytes.

NK Cell Hyporesponsiveness: More Is Not Always Better

Natural Killer (NK) cells are a type of cytotoxic lymphocytes that play an important role in the innate immune system. They are of particular interest for their role in elimination of intracellular pathogens, viral infection and tumor cells. As such, numerous strategies are being investigated in order to potentiate their functions. One of these techniques aims at promoting the function of their activating receptors. However, different observations have revealed that providing activation signals could actually be counterproductive and lead to NK cells’ hyporesponsiveness. This phenomenon can occur during the NK cell education process, under pathological conditions, but also after treatment with different agents, including cytokines, that are promising tools to boost NK cell function. In this review, we aim to highlight the different circumstances where NK cells become hyporesponsive and the methods that could be used to restore their functionality.

ZAP-70 Regulates Autoimmune Arthritis via Alterations in T Cell Activation and Apoptosis

T cells play an essential role in the pathogenesis of both human rheumatoid arthritis (RA) and its murine models. A key molecule in T cell activation is ZAP-70, therefore we aimed to investigate the effects of partial ZAP-70 deficiency on the pathogenesis of recombinant human G1(rhG1)-induced arthritis (GIA), a well-established mouse model of RA. Arthritis was induced in BALB/c and ZAP-70^+/- heterozygous mice. Disease progression was monitored using a scoring system and in vivo imaging, antigen-specific proliferation, cytokine and autoantibody production was measured and T cell apoptotic pathways were analyzed. ZAP-70^+/- mice developed a less severe arthritis, as shown by both clinical picture and in vitro parameters (decreased T cell proliferation, cytokine and autoantibody production). The amount of cleaved Caspase-3 increased in arthritic ZAP-70^+/- T cells, with no significant changes in cleaved Caspase-8 and -9 levels; although expression of Bim, Bcl-2 and Cytochrome C showed alterations. Tyrosine phosphorylation was less pronounced in arthritic ZAP-70^+/- T cells and the amount of Cbl-b-a negative regulator of T cell activation-decreased as well. We hypothesize that the less severe disease seen in the partial absence of ZAP-70 might be caused by the decreased T cell activation accompanied by increased apoptosis.

The Natural Cytotoxicity Receptors in Health and Disease

The Natural Cytotoxicity Receptors (NCRs), NKp46, NKp44, and NKp30, were some of the first human activating Natural Killer (NK) cell receptors involved in the non-MHC-restricted recognition of tumor cells to be cloned over 20 years ago. Since this time many host- and pathogen-encoded ligands have been proposed to bind the NCRs and regulate the cytotoxic and cytokine-secreting functions of tissue NK cells. This diverse set of NCR ligands can manifest on the surface of tumor or virus-infected cells or can be secreted extracellularly, suggesting a remarkable NCR polyfunctionality that regulates the activity of NK cells in different tissue compartments during steady state or inflammation. Moreover, the NCRs can also be expressed by other innate and adaptive immune cell subsets under certain tissue conditions potentially conferring NK recognition programs to these cells. Here we review NCR biology in health and disease with particular reference to how this important class of receptors regulates the functions of tissue NK cells as well as confer NK cell recognition patterns to other innate and adaptive lymphocyte subsets. Finally, we highlight how NCR biology is being harnessed for novel therapeutic interventions particularly for enhanced tumor surveillance.

Exploiting NK Cell Surveillance Pathways for Cancer Therapy

Natural killer (NK) cells can evoke potent anti-tumour activity. This function is largely mediated through a battery of specialised cell-surface receptors which probe the tissue microenvironment for changes in surface and secretory phenotypes that may alert to the presence of infection or malignancy. These receptors have the potential to arouse the robust cytotoxic and cytokine-secreting functions of NK cells and so must be tightly regulated to prevent autoimmunity. However, such functions also hold great promise for clinical intervention. In this review, we highlight some of the latest breakthroughs in fundamental NK cell receptor biology that have illuminated our understanding of the molecular strategies NK cells employ to perceive malignant cells from normal healthy cells. Moreover, we highlight how these sophisticated tumour recognition strategies are being harnessed for cancer immunotherapies in the clinic.

Differential Requirements for Src-Family Kinases in SYK or ZAP70-Mediated SLP-76 Phosphorylation in Lymphocytes

In a synthetic biology approach using Schneider (S2) cells, we show that SLP-76 is directly phosphorylated at tyrosines Y113 and Y128 by SYK in the presence of ITAM-containing adapters such as CD3ζ, DAP12, or FcεRγ. This phosphorylation was dependent on at least one functional ITAM and a functional SH2 domain within SYK. Inhibition of Src-kinases by inhibitors PP1 and PP2 did not reduce SLP-76 phosphorylation in S2 cells, suggesting an ITAM and SYK dependent, but Src-kinase independent signaling pathway. This direct ITAM/SYK/SLP-76 signaling pathway therefore differs from previously described ITAM signaling. However, the SYK-family kinase ZAP70 required the additional co-expression of the Src-family kinases Fyn or Lck to efficiently phosphorylate SLP-76 in S2 cells. This difference in Src-family kinase dependency of SYK versus ZAP70-mediated ITAM-based signaling was further demonstrated in human lymphocytes. ITAM signaling in ZAP70-expressing T cells was dependent on the activity of Src-family kinases. In contrast, Src-family kinases were partially dispensable for ITAM signaling in SYK-expressing B cells or in natural killer cells, which express SYK and ZAP70. This demonstrates that SYK can signal using a Src-kinase independent ITAM-based signaling pathway, which may be involved in calibrating the threshold for lymphocyte activation.

Afucosylated antibodies increase activation of FcγRIIIa-dependent signaling components to intensify processes promoting ADCC

Antibody-dependent cellular cytotoxicity (ADCC) is a key mechanism by which therapeutic antibodies mediate their antitumor effects. The absence of fucose on the heavy chain of the antibody increases the affinity between the antibody and FcγRIIIa, which results in increased in vitro and in vivo ADCC compared with the fucosylated form. However, the cellular and molecular mechanisms responsible for increased ADCC are unknown. Through a series of biochemical and cellular studies, we find that human natural killer (NK) cells stimulated with afucosylated antibody exhibit enhanced activation of proximal FcγRIIIa signaling and downstream pathways, as well as enhanced cytoskeletal rearrangement and degranulation, relative to stimulation with fucosylated antibody. Furthermore, analysis of the interaction between human NK cells and targets using a high-throughput microscope-based antibody-dependent cytotoxicity assay shows that afucosylated antibodies increase the number of NK cells capable of killing multiple targets and the rate with which targets are killed. We conclude that the increase in affinity between afucosylated antibodies and FcγRIIIa enhances activation of signaling molecules, promoting cytoskeletal rearrangement and degranulation, which, in turn, potentiates the cytotoxic characteristics of NK cells to increase efficiency of ADCC.

The Treg-specific demethylated region stabilizes Foxp3 expression independently of NF-κB signaling

Regulatory T cells (Tregs) obtain immunosuppressive capacity by the upregulation of forkhead box protein 3 (Foxp3), and persistent expression of this transcription factor is required to maintain their immune regulatory function and ensure immune homeostasis. Stable Foxp3 expression is achieved through epigenetic modification of the Treg-specific demethylated region (TSDR), an evolutionarily conserved non-coding element within the Foxp3 gene locus. Here, we present molecular data suggesting that TSDR enhancer activity is restricted to T cells and cannot be induced in other immune cells such as macrophages or B cells. Since NF-κB signaling has been reported to be instrumental to induce Foxp3 expression during Treg development, we analyzed how NF-κB factors are involved in the molecular regulation of the TSDR. Unexpectedly, we neither observed transcriptional activity of a previously postulated NF-κB binding site within the TSDR nor did the entire TSDR show any transcriptional responsiveness to NF-κB activation at all. Finally, the NF-κB subunit c-Rel revealed to be dispensable for epigenetic imprinting of sustained Foxp3 expression by TSDR demethylation. In conclusion, we show that NF-κB signaling is not substantially involved in TSDR-mediated stabilization of Foxp3 expression in Tregs.

Neutrophils mediate antibody-induced antitumor effects in mice

Tumor engraftment followed by monoclonal antibody (mAb) therapy targeting tumor antigens represents a gold standard for assessing the efficiency of mAbs to eliminate tumor cells. Mouse models have demonstrated that receptors for the Fc portion of immunoglobulin G (FcγRs) are critical determinants of mAb therapeutic efficacy, but the FcγR-expressing cell populations responsible remain elusive. We show that neutrophils are responsible for mAb-induced therapy of both subcutaneous syngeneic melanoma and human breast cancer xenografts. mAb-induced tumor reduction, abolished in neutropenic mice, could be restored in FcγR-deficient hosts upon transfer of FcγR+ neutrophils or upon human FcγRIIA/CD32A transgenic expression. Finally, conditional knockout mice unable to perform FcγR-mediated activation and phagocytosis specifically in neutrophils were resistant to mAb-induced therapy. Our work suggests that neutrophils are necessary and sufficient for mAb-induced therapy of subcutaneous tumors, and represent a new and critical focal point for optimizing mAb-induced immunotherapies that will impact on human cancer treatment.

Requirement and redundancy of the Src family kinases Fyn and Lyn in perforin-dependent killing of Cryptococcus neoformans by NK cells

Natural killer (NK) cells directly recognize and kill fungi, such as the pathogenic fungus Cryptococcus neoformans, via cytolytic mechanisms. However, the precise signaling pathways governing this NK cell microbicidal activity and the implications for fungal recognition are still unknown. Previously, it was reported that NK cell anticryptococcal activity is mediated through a conserved phosphatidylinositol 3-kinase-extracellular signal-regulated kinase 1/2 (PI3K-ERK1/2) pathway. Using YT (a human NK-like cell line) and primary human NK cells, we sought to identify the upstream, receptor-proximal signaling elements that led to fungal cytolysis. We demonstrate that Src family kinases were activated in response to C. neoformans. Furthermore, pharmacologic inhibition with an Src kinase inhibitor blocked C. neoformans-induced downstream activation of PI3K and ERK1/2 and abrogated cryptococcal killing. At the same time, the inhibitor disrupted the polarization of perforin-containing granules toward the NK cell-cryptococcal synapse but had no effect on conjugate formation between the organism and the NK cell. Finally, small interfering RNA (siRNA) double (but not single) knockdown of two Src family kinases, Fyn and Lyn, blocked cryptococcal killing. Together these data demonstrate a mechanism whereby the Src family kinases, Fyn and Lyn, redundantly mediate anticryptococcal activity through the activation of PI3K and ERK1/2, which in turn facilitates killing by inducing the polarization of perforin-containing granules to the NK cell-cryptococcal synapse.

Signal transduction during natural killer cell activation

Understanding of transmembrane signaling during NK-cell activation has greatly expanded during the past few years. The discovery and characterization of novel triggering and inhibitory receptors have revealed the complexity of these processes. This unit focuses on receptor-initiated signaling pathways that modulate NK functions. Establishing the roles of different signaling pathways in NK cells is a crucial step in the design of therapeutic approaches for selective enhancement or suppression of NK-cell activation.

Signal transduction during natural killer cell activation

Natural killer (NK) cells are a subpopulation of lymphocytes that can mediate cytotoxicity of certain tumor cells, virus-infected cells, and normal cells. In addition to their cytotoxic potential, NK cells secrete a variety of cytokines and chemokines that can modulate the function, growth, and differentiation of other immune cells. These different responses are initiated by the interaction of specific NK surface receptors with defined soluble or cell-associated ligands. There are several different types of receptors on the NK cell surface including "triggering" receptors, adhesion molecules, cytokine receptors, and MHC-recognizing killer-cell inhibitory receptors. The functional response of an NK cell is the result of the integration of signals transduced by these different types of receptors. Some of these signaling pathways are similar to other lymphoid cells, but there are also unique features employed by NK cells. This overview focuses on receptor-initiated signaling pathways that modulate NK functions.

Dissecting natural killer cell activation pathways through analysis of genetic mutations in human and mouse

Natural killer (NK) cell cytotoxicity is mediated by multiple germ line-encoded activating receptors that recognize specific ligands expressed by tumor cells and virally infected cells. These activating receptors are opposed by NK inhibitory receptors, which recognize major histocompatibility complex class I molecules on potential targets, raising the threshold for NK cell activation. Once an abnormal cell has been detected, NK cells are the sentinel source of cytolytic mediators, such as granzymes and perforins, as well as interferon-gamma, which can polarize the immune response to a T-helper 1 cell type. Activation signals are transmitted by adhesion-dependent pathways, immunoreceptor tyrosine-based activation motif (ITAM)-dependent pathways, DAP10 ITAM-independent pathways, and by signaling through immunoreceptor tyrosine-based switch motifs. These pathways activate downstream signaling partners to trigger NK cell cytotoxicity. Some of these downstream molecules are unique to the various pathways, and some of these molecules are shared. Because of the complexity of signals involved in NK cell-target cell interaction, the generation of mice with targeted mutations in signaling molecules involved in adhesion, activation, or inhibition is essential for a precise dissection of the mechanisms regulating NK cell effector functions. Here we review recent advances in the genetic analysis of the signaling pathways that mediate NK cell killing.

A robust ratio metric method for analysis of Zap-70 expression in chronic lymphocytic leukemia (CLL)

Since Zap-70 expression in chronic lymphocytic leukemia (CLL) cells correlates with a lack of somatic mutation of the immunoglobulin variable heavy chain (IgVH) genes, it has been proposed as a surrogate marker for disease prognosis. However, published studies of Zap-70 expression have used different commercial antibodies and analytic strategies. This study was undertaken to determine if any strategy was broadly applicable in a clinical flow cytometry laboratory. Expression of Zap-70 was determined in 37 CLL patients using four different commercial antibodies. T, NK, and CLL cells were identified by immunophenotyping along with Zap-70 expression. Data was analyzed in terms of both percent of CLL cells expressing Zap-70 and the ratio of Zap-70 expression in CLL cells compared to that in T + NK cells. Three Zap-70 antibodies showed wide ranges of Zap-70 expression as a percentage of tumor cells, while a fourth gave consistently elevated results. Comparing the percent Zap-70 expression with any two antibodies gave poor correlations (r(2) = 0.45-0.63). Our results indicated that the previous analytical strategies were not reproducible. A ratio metric is proposed, which gave better correlations (r(2) as high as 0.95) and would allow separation of CLL patients with elevated or decreased Zap-70 expression.

The low affinity Fc receptor for IgG functions as an effective cytolytic receptor for self-specific CD8 T cells

We have recently described a population of self-Ag-specific murine CD8(+) T cells with a memory phenotype that use receptors of both the adaptive and innate immune systems in the detection of transformed and infected cells. In this study we show that upon activation with IL-2 with or without Ag, between 10 and 20% of the activated self-specific CD8(+) T cells express the low affinity FcR for IgG. By contrast, all IL-2-activated NK cells express high levels of this FcR. The FcR comprises the FcgammaRIIIalpha and FcRgamma subunits. However, the FcRgamma subunit also associates with the CD3 complex, and this association probably contributes to the low expression of FcR in activated cells. Although the FcR is expressed at a low level on activated self-specific CD8(+) T cells, it functions very efficiently as a cytolytic receptor in ADCC. FcR-dependent killing occurred in the absence of TCR stimulation, but could be augmented by concurrent stimulation of the TCR. In addition to mediating ADCC, engagement of the FcR on self-specific CD8(+) T cells results in the production of both IFN-gamma and TNF-alpha. This is the first report of an activating FcR on self-specific murine CD8(+)alphabeta TCR(+) T cells and establishes the importance of innate immune system receptors in the function of these self-specific CD8(+) T cells.

ZAP-70 expression as a surrogate for immunoglobulin-variable-region mutations in chronic lymphocytic leukemia

BACKGROUND

The mutational status of immunoglobulin heavy-chain variable-region (IgVH) genes in the leukemic cells of chronic lymphocytic leukemia (CLL) is an important prognostic factor in the disease. We investigated whether the expression of ZAP-70 by CLL cells correlated with the IgVH mutational status, disease progression, and survival.

METHODS

The expression of ZAP-70 was analyzed in T-cell and B-cell lines and in peripheral-blood samples from 56 patients with CLL with the use of flow cytometry, Western blotting, and immunohistochemistry. The results were correlated with the IgVH mutational status and clinical outcome.

RESULTS

ZAP-70 was detected by flow-cytometric analysis in cells of T-cell lineage and in leukemic cells from 32 of 56 patients with CLL. In all patients in whom at least 20 percent of the leukemic cells were positive for ZAP-70, IgVH was unmutated, whereas IgVH mutations were found in 21 of 24 patients in whom less than 20 percent of the leukemic cells were positive for ZAP-70 (P<0.001). Concordant results were obtained when ZAP-70 expression was assessed by immunohistochemistry or Western blotting. The level of ZAP-70 expression did not change over time (median, 37 months) in sequential samples from 30 patients with CLL. Patients with Binet stage A CLL who had at least 20 percent ZAP-70-positive leukemic cells had more rapid progression and poorer survival than those with less than 20 percent ZAP-70-positive cells.

CONCLUSIONS

Among patients with CLL, expression of ZAP-70, as detected by flow-cytometric analysis, correlated with IgVH mutational status, disease progression, and survival.

Transmembrane T-cell receptor peptides inhibit B- and natural killer-cell function

A synthetic hydrophobic peptide (core peptide; CP) containing two positively charged amino acids, lysine and arginine was derived from the transmembrane sequence of the T-cell receptor (TCR) alpha chain and has been shown to inhibit T-cell-mediated inflammation. In this study, we investigated the specificity of CP (10 microm) on lymphocyte function and found that it significantly inhibited interleukin-2 production in T cells and natural killer cytotoxicity by 46-58% compared to positive control. CP had no effects on B-cell proliferative responses when used at these concentrations; however, it suppressed B-cell proliferation at higher concentrations (50 microm). Inhibition by CP was not the result of membrane pore formation or cytotoxicity when examined by trypan blue, propidium iodide staining or transmission electron microscopy. CP analogues, with both lysine and arginine replaced by neutral or negatively charged amino acids, or by randomly distributing charges in the peptide sequence, had no effect on lymphocyte function. These results suggest that peptide inhibition is affected by its structure and charge interactions, and may involve common signalling molecules in T, B and natural killer cells. The potential of the immuno-inhibitory effects of CP as a novel anti-inflammatory peptide in therapy should be further explored.

Natural killer cell activation receptors in innate immunity to infection

Natural killer (NK) cells are best known for their capacity to kill tumors but they are also critical in early innate responses to infection, especially herpesviruses. Recent studies indicate that NK cell receptors involved in tumor target specificity are also involved in responses to viral infections.

Regulation of NK cell-mediated cytotoxicity by the adaptor protein 3BP2

Stimulation of lymphocytes through multichain immune recognition receptors activates multiple signaling pathways. Adaptor proteins play an important role in integrating these pathways by their ability to simultaneously bind multiple signaling components. Recently, the 3BP2 adaptor protein has been shown to positively regulate the transcriptional activity of T cells. However, the mechanisms by which signaling components are involved in this regulation remain unclear, as does a potential role for 3BP2 in the regulation of other cellular functions. Here we describe a positive regulatory role for 3BP2 in NK cell-mediated cytotoxicity. We also identify p95(vav) and phospholipase C-gamma isoforms as binding partners of 3BP2. Our results show that tyrosine-183 of 3BP2 is specifically involved in this interaction and that this residue critically influences 3BP2-dependent function. Therefore, 3BP2 regulates NK cell-mediated cytotoxicity by mobilizing key downstream signaling effectors.

Ig-binding receptors on human NK cells as effector and regulatory surface molecules

The receptors on human natural killer 9NK cells which can specifically bind the Fc portion of immunoglobulin molecules (Fc receptors) have been extensively studied. The best known and studied Fc receptor on human NK cells is FcgammaRIIIa. Interactions of NK cells with IgG antibodies via this receptor are well known to induce a signal transduction cascade and lead to antibody-dependent cell-mediated cytotoxicity (ADCC) as well as release of various cytokines. In addition, interactions with monomeric IgG and FcgammaRIIIa have been demonstrated, which result in negative regulation of NK activity and other immunomodulatory effects. Over the past several years, it has also become increasingly appreciated that human NK cells express a variety of other Fc receptors, including FcmuR, which also can mediate effector and immunoregulatory functions. Also, a novel form of FcgammaR has been demonstrated on human NK cells, termed FcgammaRIIc. Recent molecular studies have shown considerable polymorphism in the genes for FcgammaIIc and the functional consequences are being dissected. This appears to include cross-talk between FcgammaRIIIa and at least some forms of FcgammaRIIc, which may have important functional consequences.

Human natural killer cell receptors and signal transduction

Natural killer (NK) cells express numerous receptors, which continually engage with ligands on cell surfaces. Until 1995, only a handful of these receptors were characterized and the molecular basis of NK cell activation was obscure. Recently, considerable advances have been made in characterizing the receptor repertoire on human NK cells. Both activating and inhibitory receptors can transduce positive or negative signals to regulate NK cell cytotoxicity and cytokine release responses. The inhibitory receptors normally predominate in this balance of signals. Certain tumor cells and virally infected cells that lack major histocompatibility complex (MHC) class I molecules, however, can rapidly trigger NK cell activation. The basis of this activation is the loss of negative signals that are normally transmitted by MHC class I-binding inhibitory receptors, and the corresponding domination of activating receptor signals. While ligand specificity for a number of the recently described receptors is still a mystery, their signal transduction properties have begun to be defined. The dynamic crosstalk between these receptors ultimately governs the NK cell activation state. Although the complexities of NK cell signalling are only marginally understood, several overall themes have been defined by characterizing the roles of distinct pathways during NK cell responses.

A "chimeric" C57l-derived Ly49 inhibitory receptor resembling the Ly49D activation receptor

Ly49D is a natural killer (NK) cell activation receptor that is responsible for differential mouse inbred strain-determined lysis of Chinese hamster ovary (CHO) cells. Whereas C57BL/6 NK cells kill CHO, BALB/c-derived NK cells cannot kill because they lack expression of Ly49D. Furthermore, the expression of Ly49D, as detected by monoclonal antibody 4E4, correlates well with CHO lysis by NK cells from different inbred strains. However, one discordant mouse strain was identified; C57L NK cells express the mAb 4E4 epitope but fail to lyse CHO cells. Herein we describe a Ly49 molecule isolated from C57L mice that is recognized by mAb 4E4 (anti-Ly49D). Interestingly, this molecule shares extensive similarity to Ly49D(B6) in its extracellular domain, but its cytoplasmic and transmembrane domains are identical to the inhibitory receptor Ly49A(B6), including a cytoplasmic ITIM. This molecule bears substantial overall homology to the previously cloned Ly49O molecule from 129 mice the serologic reactivity and function of which were undefined. Cytotoxicity experiments revealed that 4E4(+) LAK cells from C57L mice failed to lyse CHO cells and inhibited NK cell function in redirected inhibition assays. MHC class I tetramer staining revealed that the Ly49O(C57L)-bound H-2D(d) and lysis by 4E4(+) C57L LAK cells is inhibited by target H-2D(d). The structural basis for ligand binding was also examined in the context of the recent crystallization of a Ly49A-H-2D(d) complex. Therefore, this apparently "chimeric" Ly49 molecule serologically resembles an NK cell activation receptor but functions as an inhibitory receptor.

Signaling through CD38 induces NK cell activation

Human CD38 is a signal transduction molecule, and, concurrently, an ectoenzyme catalyzing the synthesis and degradation of cyclic ADP-ribose (cADPR), a potent Ca2+ mobilizer. One facet of CD38 that has not yet been addressed is its role in NK cells. To this end, the events triggered by CD38 ligation with agonistic mAb were analyzed on freshly purified human NK cells. Ligation was followed by (i) a significant rise in the intracellular level of Ca2+, (ii) increased expression of HLA class II and CD25, and (iii) tyrosine phosphorylation of discrete cytoplasmic substrates. The phosphorylation cascade involved CD3-zeta and FcepsilonRIgamma chains, zeta-associated protein (ZAP)-70 and the proto-oncogene product c-Cbl. NK effector functions were then analyzed: CD38 signaling was able (iv) to induce release of IFN-gamma and, more prominently, of granulocyte macrophage colony stimulating factor, as assessed by measuring both mRNA and protein products; and, lastly, (v) to induce cytolytic effector functions on target cells after IL-2 activation, as shown both by cytotoxicity assays and ultrastructural changes. The tyrosine-phosphorylated substrates and all the effects mediated by CD38 were similar to those observed following triggering via CD16 (FcgammaRIIIA); moreover, Ca2+ mobilization via CD38 no longer operated in NK-derived cell lines lacking CD16. These results suggest that the activation signals transduced by CD38 in NK cells elicit relevant cellular events. The effects are similar to those elicited via CD16 and possibly rely on common signaling pathways.

Regulation of p38 mitogen-activated protein kinase during NK cell activation

The mitogen-activated protein kinase (MAPK) p38 modulates a variety of cellular functions, including proliferation, differentiation and cell death. However, we report here a novel function for p38, i.e. the regulation of cytotoxic lymphocyte-mediated cytotoxicity. Stimulation of NK cells by either cross-linking of their FcgammaRIII receptors or by binding to NK-sensitive target cells induces the phosphorylation and activation of p38, and also of its upstream regulators MKK3/MKK6. Pharmacologic analyses suggest that Src-family and Syk-family protein tyrosine kinases couple the NK cell surface receptors to p38 activation. The role of p38 in the cytotoxic function of NK cells was tested by treatment of NK cells with the cell-permeable, p38-specific inhibitor SB203580. Interestingly, exposure to the drug reduced both antibody-dependent cellular cytotoxicity and natural cytotoxicity, but maximal inhibitory concentrations resulted in only partial inhibition. Collectively, these results suggest that the p38 MAPK pathway is stimulated during the development of NK cell-mediated cytotoxicity and that efficient killing is influenced by both p38-dependent and p38-independent pathways. More broadly, this study identifies the regulation of cell-mediated killing as a novel role for p38 in cytotoxic lymphocytes.

NK cell activation: distinct stimulatory pathways counterbalancing inhibitory signals

A delicate balance between positive and negative signals regulates NK cell effector function. Activation of NK cells may be initiated by the triggering of multiple adhesion or costimulatory molecules, and can be counterbalanced by inhibitory signals induced by receptors for MHC class I. A common pathway of inhibitory signaling is provided by immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in the cytoplasmic domains of these receptors which mediate the recruitment of SH2 domain-bearing tyrosine phosphate-1 (SHP-1). In contrast to the extensive progress that has been made regarding the negative regulation of NK cell function, our knowledge of the signals that activate NK cells is still poor. Recent studies of the activating receptor complexes have shed new light on the induction of NK cell effector function. Several NK receptors using novel adaptors with immunoreceptor tyrosine-based activation motifs (ITAMs) and with PI 3-kinase recruiting motifs have been implicated in NK cell stimulation.

Linker for activation of T cells (LAT), a novel immunohistochemical marker for T cells, NK cells, mast cells, and megakaryocytes: evaluation in normal and pathological conditions

LAT (linker for activation of T cells) is an integral membrane protein of 36-38 kd that plays an important role in T cell activation. Using a rabbit polyclonal antibody generated against the cytosolic portion of LAT, we investigated the immunohistochemical expression of LAT in normal and pathological hematolymphoid tissues. LAT reacts with human T cells in paraffin sections, including decalcified bone marrow trephines. LAT appears early in T cells at the thymocyte stage and before TdT expression in embryos, and is expressed in peripheral lymphoid tissues, without restriction to any T cell subpopulations. In addition to T cells, natural killer (NK) cells (evaluated with flow cytometry), megakaryocytes and mast cells are also LAT-positive, whereas B cells and other myeloid and monocytic derived cells are negative. Tested on a total of 264 paraffin-embedded tissue biopsies, LAT reacted with the great majority (96.8%) of T/NK-cell neoplasms, covering the full range of T cell maturation. Although antibodies to both LAT and CD3 had a similarly high sensitivity in the staining of T/NK-cell lymphomas, when used in conjunction, they successfully identified a higher number of cases (98.4%). Atypical megakaryocytes from different hematological disorders, as well as mast cells in mastocytosis were also LAT-positive, but all neoplasms of B cell origin, Hodgkin's lymphomas, and several nonlymphoid malignancies were negative. These data indicate that the anti-LAT antibody may be of value to diagnostic histopathologists for the identification of T cell neoplasms.

Human decidualized endometrial T lymphocytes do not substantially down-regulate CD3zeta

PROBLEM

The T-cell antigen receptor (TCR) has been reported to be down-regulated on T-cells in the decidualized endometrium in early pregnancy.

METHOD OF STUDY

The expression of CD3zeta, a component of the TCR complex, has been investigated in human first-trimester decidual T-cells using flow cytometric analysis of permeabilized cells.

RESULTS

Levels of CD3zeta expression were significantly lower in decidual than in peripheral T-cells from non-pregnant women, as assessed by mean fluorescence intensity (4.2 vs. 5.5, logarithmic scale, P < 0.05). However, when decidual and peripheral T-cells from the same subjects were analyzed (n = 10), mean levels of CD3zeta were slightly, but not significantly, lower in decidual than in peripheral T-cells (P > 0.1). CD3zeta was not substantially down-regulated systemically as mean cytoplasmic CD3zeta levels did not differ significantly between peripheral blood T-cells from pregnant women and non-pregnant controls (P > 0.2). CD8+ cells outnumber CD4+ cells in decidua, but neither the proportions of these two T-cell subsets positive for cytoplasmic CD3zeta nor the mean levels of CD3zeta were significantly different.

CONCLUSIONS

These results indicate that human decidual T-cells do not greatly down-regulate CD3zeta, but it is unclear if a small decrease in mean levels may be sufficient to compromise their capacity for activation.

Cutting Edge: A Role for the Adaptor Protein LAT in Human NK Cell-Mediated Cytotoxicity

Stimulation of NK cell-mediated cytotoxicity involves the coupling of proximal Src and Syk family protein tyrosine kinases to downstream effectors. However, the mechanisms linking these second messenger pathways are incompletely understood. Here, we describe a key role for the LAT (p36) adaptor protein in human NK cell activation. LAT is tyrosine phosphorylated upon stimulation of NK cells through FcγRIII receptors and following direct contact with NK-sensitive target cells. This NK stimulation induces the association of LAT with several phosphotyrosine-containing proteins. In addition to the biochemical evidence showing LAT involvement in NK cell activation, a genetic model shows that LAT is required for FcR-dependent phosphorylation of phospholipase C-γ. Furthermore, overexpression of LAT in NK cells leads to increased Ab-dependent cell-mediated cytotoxicity and “natural cytotoxicity,” thus demonstrating a functional role for LAT in NK cells. These data suggest that LAT is an important adaptor protein for the regulation of human NK cell-mediated cytotoxicity.

Quantitative Analysis of the Effect of CD16 Ligation on Human NK Cell Proliferation

CD16 (FcγRIIIA), the low affinity receptor for IgG, is expressed on the majority of human peripheral blood NK cells. Ligation of CD16 with mAb or immune complexes activates NK cell cytotoxicity and cytokine secretion, and stimulates death of activated NK cells by apoptosis. This study uses NK cells labeled with the stable intracytoplasmic fluorescent dye 5- and 6-carboxyfluorescein diacetate succinimidyl ester to provide quantitative data on the effect of CD16 ligation on NK cell division and NK cell survival. When NK cells are cultured with rIL-2 and CD16 is ligated, NK cell division is stimulated, but there also is a substantial loss of NK progenitor cells. When NK cell proliferation is stimulated by coculture with γ-irradiated MM-170 malignant melanoma cells and rIL-2, CD16 ligation enhances entry of NK cells into division. In some cases, CD16 ligation is essential for NK cell proliferation stimulated by MM-170 cells. In these cultures, there is no loss of NK progenitor cells. This study demonstrates that CD16 is an activation receptor for NK cell proliferation, and suggests that cellular costimulation alters the balance between NK cell death and NK cell proliferation stimulated by CD16 ligation.

Dependence of Both Spontaneous and Antibody-Dependent, Granule Exocytosis-Mediated NK Cell Cytotoxicity on Extracellular Signal-Regulated Kinases

Extracellular signal-regulated kinases (ERK, also known as mitogen-activated protein kinases) are serine-threonine kinases transducing signals elicited upon ligand binding to several tyrosine kinase-associated receptors. We have reported that ERK2 phosphorylation and activation follows engagement of the low affinity receptor for the Fc portion of IgG (CD16) on NK cells, and is necessary for CD16-induced TNF-α mRNA expression. Here, we analyzed the involvement of ERK in NK cell-mediated cytotoxicity and IFN-γ expression induced upon stimulation with targets cells, coated or not with Abs. Our data indicate that, as with immune complexes, ERK2 phosphorylation occurs in human primary NK cells upon interaction with target cells sensitive to granule exocytosis-mediated spontaneous cytotoxicity, and that this regulates both target cell- and immune complex-induced cytotoxicity and IFN-γ mRNA expression. A specific inhibitor of mitogen-activated protein kinase kinase reduced both spontaneous and Ab-dependent cytotoxicity in a dose-dependent manner involving, at least in part, inhibition of granule exocytosis without affecting effector/target cell interaction and rearrangement of the cytoskeleton proteins actin and tubulin. Involvement of ERK in the regulation of Ca2+-dependent cell-mediated cytotoxicity was confirmed, using a genetic approach, in primary NK cells infected with a recombinant vaccinia virus encoding an ERK inactive mutant. These data indicate that the biochemical pathways elicited in NK cells upon engagement of receptors responsible for either spontaneous or Ab-dependent recognition of target cells, although distinct, utilize ERK as one of their downstream molecules to regulate effector functions.

Fcgamma receptor-mediated mitogen-activated protein kinase activation in monocytes is independent of Ras

Receptors for the Fc portion of immunoglobulin molecules (FcR) present on leukocyte cell membranes mediate a large number of cellular responses that are very important in host defense, including phagocytosis, cell cytotoxicity, production and secretion of inflammatory mediators, and modulation of the immune response. Cross-linking of FcR with immune complexes leads, first to activation of protein-tyrosine kinases. The molecular events that follow and that transduce signals from these receptors to the nucleus are still poorly defined. We have investigated the signal transduction pathway from Fc receptors that leads to gene activation and production of cytokines in monocytes. Cross-linking of FcR, on the THP-1 monocytic cell line, by immune complexes resulted in both activation of the transcription factor NF-kappaB and interleukin 1 production. These responses were completely blocked by tyrosine kinase inhibitors. In contrast, expression of dominant negative mutants of Ras and Raf-1, in these cells, did not have any effect on FcR-mediated nuclear factor activation, suggesting that the mitogen-activated protein kinase (MAPK) signaling pathway was not used by these receptors. However, MAPK activation was easily detected by in vitro kinase assays, after FcR cross-linking with immune complexes. Using the specific MAPK/extracellular signal-regulated kinase kinase (MAPK kinase) inhibitor PD98059, we found that MAPK activation is necessary for FcR-dependent activation of the nuclear factor NF-kappaB. These results strongly suggest that the signaling pathway from Fc receptors leading to expression of different genes important to leukocyte biology, initiates with tyrosine kinases and requires MAPK activation; but in contrast to other tyrosine kinase receptors, FcR-mediated MAPK activation does not involve Ras and Raf.

The Vav-Rac1 pathway in cytotoxic lymphocytes regulates the generation of cell-mediated killing

The Rac1 guanine nucleotide exchange factor, Vav, is activated in hematopoietic cells in response to a large variety of stimuli. The downstream signaling events derived from Vav have been primarily characterized as leading to transcription or transformation. However, we report here that Vav and Rac1 in natural killer (NK) cells regulate the development of cell-mediated killing. There is a rapid increase in Vav tyrosine phosphorylation during the development of antibody-dependent cellular cytotoxicity and natural killing. In addition, overexpression of Vav, but not of a mutant lacking exchange factor activity, enhances both forms of killing by NK cells. Furthermore, dominant-negative Rac1 inhibits the development of NK cell-mediated cytotoxicity by two mechanisms: (a) conjugate formation between NK cells and target cells is decreased; and (b) those NK cells that do form conjugates have decreased ability to polarize their granules toward the target cell. Therefore, our results suggest that in addition to participating in the regulation of transcription, Vav and Rac1 are pivotal regulators of adhesion, granule exocytosis, and cellular cytotoxicity.

NK cell receptors

NK cells are regulated by opposing signals from receptors that activate and inhibit effector function. While positive stimulation may be initiated by an array of costimulatory receptors, specificity is provided by inhibitory signals transduced by receptors for MHC class I. Three distinct receptor families, Ly49, CD94/NKG2, and KIR, are involved in NK cell recognition of polymorphic MHC class I molecules. A common pathway of inhibitory signaling is provided by ITIM sequences in the cytoplasmic domains of these otherwise structurally diverse receptors. Upon ligand binding and activation, the inhibitory NK cell receptors become tyrosine phosphorylated and recruit tyrosine phosphatases, SHP-1 and possibly SHP-2, resulting in inhibition of NK cell-mediated cytotoxicity and cytokine expression. Recent studies suggest these inhibitory NK cell receptors are members of a larger superfamily containing ITIM sequences, the inhibitory receptor superfamily (IRS).

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.

Syk is required for BCR-mediated activation of p90Rsk, but not p70S6k, via a mitogen-activated protein kinase-independent pathway in B cells

The tyrosine kinases Syk and Lyn are activated in B lymphocytes following antibody induced cross-linking of the B cell receptor for antigen (BCR). It has been suggested that activation of Syk is dependent on Lyn. We tested this hypothesis by comparing the phosphorylation and activation of several downstream effector molecules in parental DT40, DT40Syk- and DT40Lyn- B cells. The phosphorylation and activation of p90Rsk was ablated in Syk-deficient B cells but unaffected in Lyn-deficient B cells while the phosphorylation/activation of Ras GTPase activating protein (Ras GAP) and mitogen activated protein (MAP) kinase required both Syk and Lyn. Thus, these data indicate that Syk can be activated in the absence of Lyn after BCR cross-linking and results in the activation of p90Rsk via a MAP kinase-independent pathway in DT40Lyn- cells. We also demonstrated that BCR mediates the activation of p70S6k. However, activation of p70S6k in DT40Syk- and DT40Lyn- cells was comparable with that observed in parental cells. Thus, either Syk or Lyn may be sufficient for activation of p70S6k, or activation of p70S6k occurs independently of both Syk and Lyn. The kinase activity of Syk was required for the phosphorylation/activation of each of these downstream effector molecules but only the phosphorylation of Ras GAP was affected in cells expressing a mutant of Syk in which tyrosines 525 and 526 were substituted to phenlyalanines.

Arousal and inhibition of human NK cells

NK cells express receptors for polymorphic MHC class I molecules that inhibit killing of potential target cells bearing appropriate class I allotypes. Here we review the membrane receptors on human NK cells that are known to initiate cell-mediated cytotoxicity and demonstrate regulation of these responses by the killer cell inhibitory (KIR) receptors.

Signal transduction by human NK cell MHC-recognizing receptors

Cells may be protected from natural killer (NK)-cell-mediated killing by the expression of specific MHC class I complexes. This protective effect is due to the expression on NK cells of MHC class I-recognizing receptors which, upon ligation, transduce potent inhibitory signals into the NK cells. The molecular signalling mechanisms employed by the human NK-cell MHC-recognizing killer cell inhibitory receptors (KIR) and CD94 are the focus of this review. A sequential model of KIR signalling involving lck-dependent tyrosine phosphorylation of KIR and subsequent association of KIR with the SH2-containing tyrosine phosphatase, SHP-1, is presented. We explore how engagement of either KIR or CD94 modulates the protein tyrosine kinase-dependent biochemical signals responsible for activation of NK-cell cytotoxic function. Additionally, we discuss models of inhibitory signalling proposed for each of the lymphocyte lineages, emphasizing that disparate molecular mechanisms may be utilized by cells to produce similar biological responses.

Differential Activation of the Tyrosine Kinases ZAP-70 and Syk After FcγRI Stimulation

Engagement of the high-affinity IgG Fc receptor (FcγRI) activates a signal transduction pathway involving tyrosine phosphorylation of associated kinases. We compared the activation of the related protein tyrosine kinases (PTKs), Syk and ZAP-70, in FcγRI-mediated signaling. Cross-linking of the FcγRI multimeric receptor in monocytic cells results in tyrosine phosphorylation of the FcεRIγ subunit and association of Syk with this complex. We stably introduced ZAP-70 via a retroviral vector into two monocytic cell lines, U937 and THP-1, which normally do not express ZAP-70. Neither Syk nor MAP kinase activation was affected by the presence of ZAP-70. Although transduced ZAP-70 had in vitro kinase activity and associated with FcεRIγ after receptor aggregation, it was not tyrosine phosphorylated. In contrast, both ZAP-70 and Syk were phosphorylated in a T-cell line in which their respective levels of expression were similar to those detected in U937/ZAP-70 cells. Therefore, these results suggest that requirements for Syk and ZAP-70 phosphorylation are distinct in a monocytic cell context.

Identification of the site in the Syk protein tyrosine kinase that binds the SH2 domain of Lck

The Syk protein tyrosine kinase (PTK) is expressed in many hematopoietic cells and is involved in signaling from various receptors for antigen and Fc portions of IgG and IgE. Upon cross-linking of these receptors, Syk is rapidly phosphorylated on tyrosine residues and enzymatically activated. We and others have found that the Lck kinase, a member of the Src family of PTKs, binds through its Src homology (SH) 2 domain to tyrosine phosphorylated Syk and to the related Zap kinase. Here we report that this interaction is direct and identify the two tandem tyrosines at the autophosphorylation site of Syk, Tyr518, and Tyr519, as the binding site for the SH2 domain of Lck. Mutation of either or both tyrosines to phenylalanines abrogated binding, while mutation of a second repetition of the motif at Tyr539 and Tyr540, or of the three tyrosines in the C terminus of Syk, did not. The SH2 domain of Lck bound the autophosphorylation site only when both Tyr518 and Tyr519 were phosphorylated. In intact cells the binding of the SH2 domain of Lck correlated with the ability of Syk to induce tyrosine phosphorylation of cellular proteins.

The cytoplasmic tail of FcgammaRIIIAalpha is involved in signaling by the low affinity receptor for immunoglobulin G

The low affinity receptor for IgG, FcgammaRIIIA, is a multimeric receptor composed of the ligand binding subunit FcgammaRIIIAalpha (CD16) in association with the signal-transducing subunits zeta or gamma. Previous studies suggested that the cytoplasmic tail of FcgammaRIIIAalpha was not required for FcgammaRIIIAalpha-zeta association or signaling by FcgammaRIIIA. However, in these studies, the truncated FcgammaRIIIAalpha chains still expressed the four most membrane-proximal amino acids of the cytoplasmic tail (amino acids 230-233). By successive truncations from the C terminus of FcgammaRIIIAalpha, we have studied the role played by the membrane-proximal amino acids of the cytoplasmic tail of FcgammaRIIIAalpha in (i) FcgammaRIIIA expression, (ii) FcgammaRIIIAalpha-zeta association, and (iii) signal transduction. We provide evidence that this region is not required for FcgammaRIIIA expression or FcgammaRIIIAalpha-zeta association. However, signaling by FcgammaRIIIA is strictly dependent on the membrane-proximal amino acids in the cytoplasmic tail of FcgammaRIIIAalpha. Thus, total deletion of the cytoplasmic tail of FcgammaRIIIAalpha results in a severely impaired tyrosine phosphorylation of phospholipase C-gamma1, zap, and syk and rise in intracellular free Ca2+ following receptor ligation with specific anti-CD16 monoclonal antibody or Ig-anti-Ig complexes, suggesting that FcgammaRIIIAalpha-zeta association per se is not sufficient to establish the signal function of FcgammaRIIIA. In conclusion, the present findings demonstrate that the most membrane-proximal amino acids of the FcgammaRIIIAalpha cytoplasmic tail play a critical role in ligand-induced signal transduction by the FcgammaRIIIAalpha-zeta complex.

Vav in natural killer cells is tyrosine phosphorylated upon cross-linking of Fc gamma RIIIA and is constitutively associated with a serine/threonine kinase

Cross-linking of Fc gamma RIIIA (CD16) receptor on natural killer (NK) cells induces receptor-associated tyrosine kinase activation and tyrosine phosphorylation of numerous intracellular proteins, including phospholipase C (PLC)-gamma 1, PLC-gamma 2 and the associated zeta chain. Here we report that Vav, a proto-oncogene, also became tyrosine phosphorylated upon stimulation of CD16 in interleukin 2-activated NK cells (LAK-NK) as well as in an NK cell line, NK3.3. In addition, we observed that in LAK-NK cells, Vav was associated with a 70 kDa protein that also became tyrosine phosphorylated upon CD16 cross-linking. The association of this 70 kDa protein with Vav was disrupted by ionic detergent treatment. Tyrosine phosphorylation of Vav was inhibited by herbimycin A, a specific tyrosine kinase inhibitor. In vitro kinase assays with Vav immunoprecipitates derived from NK3.3 cells or LAK-NK cells resulted in the appearance of a phosphorylated 58 kDa protein, suggesting the presence of a kinase within the Vav immunoprecipitates. Cross-linking of CD16 did not enhance this Vav-associated kinase activity. Phosphoamino acid analysis of the 58 kDa protein revealed that it was phosphorylated only on serine and threonine residues, indicating that an unidentified serine/threonine kinase is constitutively associated with Vav. These observations suggest that the downstream signalling events regulated by Vav and its associated proteins are complex involving both tyrosine kinases as well as the yet unidentified serine/threonine kinase in NK cells.

Fc gamma R-dependent mitogen-activated protein kinase activation in leukocytes: a common signal transduction event necessary for expression of TNF-alpha and early activation genes

Cross-linking the receptors for the Fc domain of IgG (Fc gamma R) on leukocytes induces activation of protein tyrosine kinases. The intermediary molecules that transduce to the nucleus the signals leading to induction of the diverse biological responses mediated by these receptors are not clearly identified. We have investigated whether mitogen-activated protein kinases (MAPK) are involved in transmembrane signaling via the three Fc gamma R present on monocytic, polymorphonuclear, and natural killer (NK) cells. Our results indicate that occupancy of Fc gamma RI and Fc gamma RII on the monocytic cell line THP-I and on polymorphonuclear leukocytes (PMN) induces, transiently and with fast kinetics, MAPK phosphorylation, as indicated by decreased electrophoretic mobility in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and increased amounts of the proteins in antiphosphotyrosine antibody immunoprecipitates. This, associated with increased enzymatic activity, also occurs upon stimulation of the transmembrane isoform of CD16 (Fc gamma RIIIA) in NK cells and in a T cell line expressing transfected Fc gamma RIIIA alpha ligand-binding chain in association with zeta, but not upon stimulation of the glycosil-phosphatidylinositol-anchored Fc gamma RIIIB on PMN. Using the specific MAP kinase kinase inhibitor-PD 098059, we show that activation of MAPK is necessary for the Fc gamma R-dependent induction of c-fos and tumor necrosis factor alpha mRNA expression in monocytes and NK cells. These results underscore the role of MAPK as signal-transducing molecules controlling the expression of different genes relevant to leukocyte biology upon Fc gamma R stimulation.

Hydrogen peroxide secreted by tumor-derived macrophages down-modulates signal-transducing zeta molecules and inhibits tumor-specific T cell-and natural killer cell-mediated cytotoxicity

Although alterations in CD3-associated signal-transducing molecules in tumor-infiltrating T cells of patients with advanced cancer have been previously described, the mechanism behind these changes is not known. We demonstrate that macrophages isolated from metastatic lymph nodes of patients with malignant melanoma down-regulate levels of CD3 zeta in autologous peripheral blood T cells. Lipopolysaccharide (LPS)- or phorbol 12-myristate 13-acetate (PMA)-stimulated monocytes derived from peripheral blood of healthy donors also induced decreased expression of CD3 and CD16-associated zeta chains similar to that observed in T cells and natural killer (NK) cells of patients with advanced cancer. Co-culture with activated monocytes impaired Ca2+ mobilization in peripheral blood derived-T cells when stimulated with monoclonal antibodies to CD3 and also strongly inhibited melanoma-specific cytotoxic T lymphocyte (CTL) activity and NK activity. The presence of catalase, a scavenger of H2O2, during co-culture almost totally abrogated the inhibitory effect of activated monocytes on melanoma-specific CTL lines and on NK cells. Pre-treatment of CTL or NK cells with nontoxic concentrations (1 x 10(-5) M) of H2O2 also severely reduced their cytotoxic activity which could be prevented by catalase. The decrease in CD3 zeta and in CD16 zeta expression, induced by macrophages isolated from metastatic lymph nodes or by LPS-stimulated monocytes, was also prevented by catalase when maintained throughout the co-culture period. The possibility that monocyte/macrophage-derived reactive oxygen metabolites contribute directly to alterations in signal transducing molecules of T cells and NK cells and to the mechanism of immunosuppression in individuals with cancer should be considered.