Signal transduction during natural killer cell activation: inside the mind of a killer

CD38 Triggers Cytotoxic Responses in Activated Human Natural Killer Cells

Receptors used by natural killer (NK) cells to mediate natural cytotoxicity are poorly defined, although it is now clear that a number of adhesion molecules can serve this function. CD38 transduces signals on T- and B-cell lines, and we asked whether it could trigger lytic and secretory responses in human NK cells. By using an anti-CD38 monoclonal antibody in reverse antibody-dependent cellular cytotoxicity experiments, it is shown that CD38 engagement triggers cytotoxic responses by activated NK cells, but not by cytotoxic T lymphocytes or fresh NK cells. Cross-linking with anti-CD38 F(ab′)2 caused activated NK cells to release granzymes and cytokines, but did not trigger an increase in intracellular Ca2+. Fresh NK cells acquired CD38-dependent lytic function during activation with interleukin-2 (IL-2), and inhibitor studies suggested that IL-2 stimulated the de novo expression of proteins that act between CD38 and the lytic machinery in NK cells. The induction of proteins that link commonly expressed adhesion molecules to effector mechanisms could provide a paradigm for pathogen recognition by the innate immune system.

CD38 Triggers Cytotoxic Responses in Activated Human Natural Killer Cells

Receptors used by natural killer (NK) cells to mediate natural cytotoxicity are poorly defined, although it is now clear that a number of adhesion molecules can serve this function. CD38 transduces signals on T- and B-cell lines, and we asked whether it could trigger lytic and secretory responses in human NK cells. By using an anti-CD38 monoclonal antibody in reverse antibody-dependent cellular cytotoxicity experiments, it is shown that CD38 engagement triggers cytotoxic responses by activated NK cells, but not by cytotoxic T lymphocytes or fresh NK cells. Cross-linking with anti-CD38 F(ab′)2 caused activated NK cells to release granzymes and cytokines, but did not trigger an increase in intracellular Ca2+. Fresh NK cells acquired CD38-dependent lytic function during activation with interleukin-2 (IL-2), and inhibitor studies suggested that IL-2 stimulated the de novo expression of proteins that act between CD38 and the lytic machinery in NK cells. The induction of proteins that link commonly expressed adhesion molecules to effector mechanisms could provide a paradigm for pathogen recognition by the innate immune system.

Signaling Through CD43 Induces Natural Killer Cell Activation, Chemokine Release, and PYK-2 Activation

Natural killer (NK) cell activation is the result of a balance between positive and negative signals triggered by specific membrane receptors. We report here the activation of NK cells induced through the transmembrane glycoprotein CD43 (leukosialin, sialophorin). Engagement of CD43 by specific antibodies stimulated the secretion of the chemokines RANTES, macrophage inflammatory protein (MIP)-1, and MIP-1β, which was prevented by treatment of cells with the specific tyrosine kinase inhibitor genistein. Furthermore, signaling through CD43 increased the cytotoxic activity of NK cells and stimulated an increase in the tyrosine kinase activity in antiphosphotyrosine immune complexes of NK cell lysates. PYK-2 was identified among the tyrosine kinase proteins that become activated. Hence, PYK-2 activation was observed after 20 minutes of CD43 stimulation, reached a maximum after 45 to 60 minutes, and decreased to almost basal levels after 120 minutes of treatment. Together, these results demonstrate the role of CD43 as an activation molecule able to transduce positive activation signals in NK cells, including the regulation of chemokine synthesis, killing activity, and tyrosine kinase activation.

Natural killer cell cytotoxicity of breast cancer targets is enhanced by two distinct mechanisms of antibody-dependent cellular cytotoxicity against LFA-3 and HER2/neu

Treatment of advanced breast cancer with autologous stem cell transplantation is limited by a high probability of disease relapse. In clinical trials, interleukin 2 (IL-2) alone can expand natural killer (NK) cells in vivo and increase their cytotoxic activity against breast cancer cell lines, but this increase is modest. Understanding the mechanisms that mediate NK cell lysis of breast cancer targets may lead to improvements of current immunotherapy strategies. NK cells from normal donors or patients receiving subcutaneous IL-2 were tested in cytotoxicity assays against five breast cancer cell lines. The role of adhesion molecules and antibodies that interact through Fc receptors on NK cells was explored. NK cell lysis of breast cancer targets is variable and is partially dependent on recognition through ICAM-1 and CD18. While blocking CD2 slightly decreased cytotoxicity, contrary to expectations, an antibody against CD58 (the ligand for CD2), failed to block killing and instead mediated an increased cytotoxicity that correlated with target density of CD58. The CD58 antibody-enhanced killing was dependent not only on FcRgammaIII but also on CD2 and ICAM-1/CD18. To further elucidate the mechanism of this CD58 antibody-dependent cellular cytotoxicity (ADCC), another antibody was tested. Trastuzumab (Herceptin), a humanized antibody against HER2/neu, mediated potent ADCC against all the HER2/neu positive breast cancer targets. Unlike CD58 antibody-mediated ADCC, Herceptin ADCC was minimally affected by blocking antibodies to CD2 or ICAM-1/CD18, which suggests a different mechanism of action. This study shows that multiple mechanisms are involved in NK cell lysis of breast cancer targets, that none of the targets are inherently resistant to killing, and that two distinct mechanisms of ADCC can target immunotherapy to breast cancer cells.

Tyrosine kinase-dependent ubiquitination of CD16 zeta subunit in human NK cells following receptor engagement

We investigated whether aggregation of the low-affinity immunoglobulin G receptor (CD16) on human NK cells results in receptor ubiquitination. We found that the CD16 zeta subunit becomes ubiquitinated in response to receptor engagement. We then investigated whether protein tyrosine kinase (PTK) activation is required for CD16-mediated receptor ubiquitination. Pretreatment with the PTK inhibitor genistein substantially decreased ligand-induced zeta ubiquitination, suggesting a requirement for PTK activation in receptor ubiquitination. We further analyzed PTK involvement in controlling receptor ubiquitination by using the vaccinia virus expression system. Overexpression of wild-type active lck, but not a kinase-deficient mutant, enhanced both ligand-induced tyrosine phosphorylation and ubiquitination of the CD16 zeta subunit. Taken together, our data demonstrate that CD16 engagement induces zeta chain ubiquitination and strongly suggest a role for lck in regulating this modification.

Ligand binding specificities and signal transduction pathways of Fc gamma receptor IIc isoforms: the CD32 isoforms expressed by human NK cells

We recently reported that human NK cells express, in addition to CD16 [Fcgamma receptor (FcgammaR) IIIA], a second type of FcgammaR, namely CD32 (FcgammaRII). Molecular characterization of CD32 transcripts expressed by highly purified NK cells revealed that they predominantly express products of the FcgammaRIIC gene. Using stable Jurkat transfectants we have analyzed the functional properties of two FcgammaRIIc-specific isoforms isolated from NK cells, namely FcgammaRIIc1 and FcgammaRIIc3, which differ in their cytoplasmic tails. The ligand binding specificity for both murine and human IgG isotypes was found to be similar to that observed for FcgammaRIIb isoforms. Immunoprecipitation studies of FcgammaRIIc isoforms expressed in Jurkat cells revealed a protein of around 40 kDa for FcgammaRIIc1, and a protein of around 32 kDa for FcgammaRIIc3. Signal transduction studies performed on FcgammaRIIc1-expressing Jurkat cells indicated that this molecule is functional, i. e. capable of Ca2+ mobilization and activation of Lck, Zap-70 and Syk protein tyrosine kinases, although the CD3 zeta chain was not found to functionally associate with FcgammaRIIc1. In contrast, FcgammaRIIc3 transfectants showed an impaired ability of this molecule to mobilize Ca2+, but activation of Lck was detected following activation via FcgammaRIIc3. These studies demonstrate the functional activity of FcgammaRIIc isoforms and suggest that the presence of CD32, in addition to CD16, on NK cells may have functional relevance.

Functional CD32 molecules on human NK cells

Human NK cells are large granular lymphocytes that kill neoplastic or virally infected targets using perforin-dependent mechanisms. CD16 or FcgammaRIII is one of the cell surface molecules that can trigger the killing machinery following binding of the Fc portion of IgG to the receptor: a mechanism known as antibody dependent cell-mediated cytotoxicity (ADCC). We have recently shown that some individuals express an additional FcgammaR on their NK cells, CD32 or FcgammaRII. This receptor has now been characterized at the molecular, biochemical and functional level. The present review outlines our findings to date on the features of this novel receptor. These findings suggest that the presence of a functional FcgammaRII on the surface of NK cells could have important clinical consequences in both tumor immunotherapy and autoimmune disease.

Redundant Role of the Syk Protein Tyrosine Kinase in Mouse NK Cell Differentiation

Syk and ZAP-70 subserve nonredundant functions in B and T lymphopoiesis. In the absence of Syk, B cell development is blocked, while T cell development is arrested in the absence of ZAP-70. The receptors and the signaling molecules required for differentiation of NK cells are poorly characterized. Here we investigate the role of the Syk protein tyrosine kinase in NK cell differentiation. Hemopoietic chimeras were generated by reconstituting alymphoid (B−, T−, NK−) recombinase-activating gene-2 × common cytokine receptor γ-chain double-mutant mice with Syk−/− fetal liver cells. The phenotypically mature Syk−/− NK cells that developed in this context were fully competent in natural cytotoxicity and in calibrating functional inhibitory receptors for MHC molecules. Syk-deficient NK cells demonstrated reduced levels of Ab-dependent cellular cytotoxicity. Nevertheless, Syk−/− NK cells could signal through NK1.1 and 2B4 activating receptors and expressed ZAP-70 protein. We conclude that the Syk protein tyrosine kinase is not essential for murine NK cell development, and that compensatory signaling pathways (including those mediated through ZAP-70) may sustain most NK cell functions in the absence of Syk.

Human NK Cells Express Endothelial Nitric Oxide Synthase, and Nitric Oxide Protects Them from Activation-Induced Cell Death by Regulating Expression of TNF-α

Although NO appears important in rodent immune responses, its involvement in the human immune system is unclear. We report that human NK cells express constitutive endothelial NO synthase mRNA and protein, but not detectable levels of inducible NO synthase. They produce NO following activation by coculture with target cells or cross-linking with anti-CD16 mAb, and production is increased in the presence of IL-2. N-monomethyl-l-arginine (l-NMA), a NOS inhibitor, partially inhibited NK cell lysis of four different target cells (<40% inhibition at 500 μM l-NMA), but not granule release following coculture with target cells, or Fas ligand induction following cross-linking with anti-CD16 mAb. However, l-NMA augmented apoptosis of NK cells induced by activation through CD16 ligation or coculture with K562. An NO donor, S-nitroso-N-acetylpenicillamine (SNAP), suppressed apoptosis of NK cells induced by CD16 cross-linking or coculture with target cells, suggesting that endogenous NO production is involved in protection of NK cells from activation-induced apoptosis, thereby maintaining NK activity. SNAP also suppressed, and l-NMA enhanced, expression of TNF-α, reported to be involved in activation-induced NK cell death, in response to CD16 cross-linking. Suppression of anti-CD16-induced apoptosis by SNAP was reversed by the addition of rTNF-α. DNA-binding activity of the transcription factor, NF-AT, which is involved in TNF-α induction upon ligation of CD16, was inhibited by SNAP and enhanced by l-NMA. Our results suggest that down-regulation of TNF-α expression, possibly due to suppression of NF-AT activation, is a mechanism by which endogenous NO protects NK cells from activation-induced apoptosis, and maintains lytic capacity.

NK cytokine secretion and cytotoxicity occur independently of the SLP-76 adaptor protein

The adapter protein SLP-76 is required for T cell development and TCR signal transduction. SLP-76 is also expressed in NK cells, yet splenic populations of NK cells develop normally in SLP-76-deficient mice. We examined the effects of SLP-76 deficiency upon cellular activation through studies of NK function in SLP-76(-/-) mice. This study presents evidence that NK populations in both spleen and liver of SLP-76(-/-) mice remain intact. Natural cytotoxic responses of SLP-76(-/-) splenocytes proceed in a manner comparable to those of wild-type control splenocytes. Similar to controls, SLP-76(-/-) splenocytes exhibit enhanced survival and augmented cytotoxic capacity after in vitro culture with IL-2. IL-2-stimulated SLP-76(-/-) splenocytes also retain normal antibody-dependent cellular cytotoxicity and the ability to secrete IFN-gamma in response to IL-12 stimulation. These results indicate that, unlike events stimulated by TCR engagement, signaling cascades engaged by IL-2 and IL-12 receptors, by Fc gammaRIIIA (which mediates antibody-dependent cellular cytotoxicity), and by natural cytotoxicity-associated receptors on murine NK cells can occur independently of SLP-76.

CD94/NKG2-A Inhibitory Complex Blocks CD16-Triggered Syk and Extracellular Regulated Kinase Activation, Leading to Cytotoxic Function of Human NK Cells

The CD94/NKG2-A complex is the inhibitory receptor for the nonclassical MHC class I molecule HLA-E on human NK cells. Here we studied the molecular mechanisms underlying the inhibitory activity of CD94/NKG2-A on NK cell functions by analyzing its interference on CD16-initiated signaling pathways involved in the control of cytolytic activity. Both tyrosine phosphorylation and activation of Syk kinase together with tyrosine phosphorylation of CD16 receptor ζ subunit are markedly inhibited by the coengagement of CD94/NKG2-A complex. As a downstream consequence, CD94/NKG2-A cross-linking impairs the CD16-induced activation of extracellular regulated kinases (ERKs), a pathway involved in NK cytotoxic function. The block of ERK activation is exerted at an early, PTK-dependent stage in the events leading to p21ras activation, as the CD16-induced tyrosine phosphorylation of Shc adaptor protein and the formation of Shc/Grb-2 complex are abrogated by CD94/NKG2-A simultaneous engagement. Our observations indicate that CD94/NKG2-A inhibits the CD16-triggered activation of two signaling pathways involved in the cytotoxic activity of NK cells. They thus provide molecular evidence to explain the inhibitory function of CD94/NKG2-A receptor on NK effector functions.

Regulation of immune responses through inhibitory receptors

Major histocompatibility complex class I-specific inhibitory receptors on natural killer cells prevent the lysis of healthy autologous cells. The outcome of this negative signal is not anergy or apoptosis of natural killer cells but a transient abortion of activation signals. The natural killer inhibitory receptors fulfill this function by recruiting the tyrosine phosphatase SHP-1 through a cytoplasmic immunoreceptor tyrosine-based inhibition motif. This immunoreceptor tyrosine-based inhibition motif has become the hallmark of a growing family of receptors with inhibitory potential, which are expressed in various cell types such as monocytes, macrophages, dendritic cells, leukocytes, and mast cells. Most of the natural killer inhibitory receptors and two members of a monocyte inhibitory-receptor family bind major histocompatibility complex class I molecules. Ligands for many of the other receptors have yet to be identified. The inhibitory-receptor superfamily appears to regulate many types of immune responses by blocking cellular activation signals.

DAP12: a key accessory protein for relaying signals by natural killer cell receptors

DAP12 is a 12 kDa transmembrane protein recently recognized as a key signal transduction receptor element in Natural Killer (NK) cells. It is a disulfide-linked homodimer that non-covalently associates with several activating receptors expressed on NK cells. Activation signals initiated through DAP12 are predicted to play strategic roles in triggering NK cell cytotoxicity responses toward certain tumor cells and virally infected cells. The cytoplasmic domain of DAP12 contains an Immunoreceptor Tyrosine-based Activation Motif (ITAM). Phosphorylation of ITAM tyrosines mediates associations with protein tyrosine kinases, which is a resonant feature of signalling through these motifs in T and B cell antigen receptors. In addition, its expression in other tissues, including dendritic cells and monocytes, suggests that DAP12 transduces ITAM-mediated activation signals for an extended array of receptors in those cells as well.

Immunoreceptor tyrosine-based inhibition motif-bearing receptors regulate the immunoreceptor tyrosine-based activation motif-induced activation of immune competent cells

ITIM-bearing receptors, a family which only recently has been recognized, play a key role in the regulation of the ITAM-induced activation of immune competent cells. The mechanism of ITM-mediated regulation in various cells was recently clarified. The present review focuses on ITIM bearing membrane proteins that negatively regulate the activation of cells when co-crosslinked with ITAM containing receptors, illustrates the inhibitory processes by the negative regulation of B-, NK-, T-cells and mast cells and summarizes current views on the mechanism of ITIM-mediated inhibition.

High-level variability in the ORF-K1 membrane protein gene at the left end of the Kaposi's sarcoma-associated herpesvirus genome defines four major virus subtypes and multiple variants or clades in different human populations

Infection with Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) or human herpesvirus 8 (HHV8) is common in certain parts of Africa, the Middle East, and the Mediterranean, but is rare elsewhere, except in AIDS patients. Nevertheless, HHV8 DNA is found consistently in nearly all classical, endemic, transplant and AIDS-associated KS lesions as well as in some rare AIDS-associated lymphomas. The concept that HHV8 genomes fall into several distinct subgroups has been confirmed and refined by PCR DNA sequence analysis of the ORF-K1 gene encoding a highly variable glycoprotein related to the immunoglobulin receptor family that maps at the extreme left-hand end of the HHV-8 genome. Among more than 60 different tumor samples from the United States, central Africa, Saudi Arabia, Taiwan, and New Zealand, amino acid substitutions were found at a total of 62% of the 289 amino acid positions. These variations defined four major subtypes and 13 distinct variants or clades similar to those found for the HIV ENV protein. The B and D subtype ORF-K1 proteins differ from the A and C subtypes by 30 and 24%, respectively, whereas A and C differ from each other by 15%. In all cases tested, multiple samples from the same patient were identical. Examples of the B subtype were found almost exclusively in KS patients from Africa or of African heritage, whereas the rare D subtypes were found only in KS patients of Pacific Island heritage. In contrast, C subtypes were found predominantly in classic KS and in iatrogenic and AIDS KS in the Middle East and Asia, whereas U.S. AIDS KS samples were primarily A1, A4, and C3 variants. We conclude that this unusually high diversity, in which 85% of the nucleotide changes lead to amino acid changes, reflects some unknown powerful biological selection process that has been acting preferentially on this early lytic cycle membrane signalling protein. Two distinct levels of ORF-K1 variability are recognizable. Subtype-specific variability indicative of long-term evolutionary divergence is both spread throughout the protein as well as concentrated within two 40-amino-acid extracellular domain variable regions (VR1 and VR2), whereas intratypic variability localizes predominantly within a single 25-amino-acid hypervariable Cys bridge loop and apparently represents much more recent changes that have occurred even within specific clades. In contrast, numerous extracellular domain glycosylation sites and Cys bridge residues as well as the ITAM motif in the cytoplasmic domain are fully conserved. Overall, we suggest that rather than being a newly acquired human pathogen, HHV8 is an ancient human virus that is preferentially transmitted in a familial fashion and is difficult to transmit horizontally in the absence of immunosuppression. The division into the four major HHV8 subgroups is probably the result of isolation and founder effects associated with the history of migration of modern human populations out of Africa over the past 35,000 to 60,000 years.

Differential regulation of killer cell Ig-like receptors and CD94 lectin-like dimers on NK and T lymphocytes from HIV-1-infected individuals

NK and T lymphocytes share various cell surface receptors, including NK receptors for MHC class I molecules (NKR). NKR include killer cell Ig-like receptors (KIR) and lectin-like dimers which are composed of the invariant CD94 associated with a variety of NKG2 molecules. The combination of KIR and CD94/NKG2 dimers expressed on NK and T cell subsets defines a repertoire of MHC class I recognition. Engagement of NKR by cognate MHC class I molecules governs T and NK cell activation. We investigated the NKR distribution on NK and T cell subsets from uninfected and HIV-infected individuals, according to the clinical status, the absolute numbers of CD4+ T cells as well as the plasmatic viral load of the patients. We show that the KIR distribution on NK cells is not affected by HIV-1 infection, whereas the absolute numbers of T cells expressing specific KIR members (CD158b, p70) transiently increase in early stages of HIV infection. By contrast, the percentages of NK and T cells which express CD94 dimers increase in parallel with the disease. These results document a differential regulation of KIR and CD94 lectin-like dimers during the course of a chronic viral infection in humans and further suggest that both types of NKR are independently regulated.

Engagement of Natural Cytotoxicity Programs Regulates AP-1 Expression in the NKL Human NK Cell Line

NK cell cytotoxicity is a fast and efficient mechanism of target cell lysis. Using transcription analysis, such as multiplex messenger assays, we show here that natural cytotoxicity exerted by the human NKL cell line correlates with mRNA accumulation of very early activator protein (AP)-1 transcription factor genes such as JunB, FosB and c-Fos. In addition, DNA-binding activities of Jun-Fos heterodimers were observed by electrophoretic mobility shift assays during the course of natural cytotoxicity. Interaction between immunoglobulin-like transcript-2/leukocyte Ig-like receptor 1 on NKL cells and HLA-B27 on target cells leads to an impairment of NKL natural cytotoxicity, which correlates with an absence of JunB, FosB, and c-Fos transcription, as well as an absence of their DNA-binding activity. Our studies thus indicate that, despite the rapidity of NK cell-mediated lysis, AP-1 transcription factor is activated during the early stage of NK cell cytolytic programs and that engagement of NK cell inhibitory receptors for MHC class I molecules impairs the very early activation of AP-1.

Ligation of FcγRII (CD32) Pivotally Regulates Survival of Human Eosinophils

The low-affinity IgG Fc receptor, FcγRII (CD32), mediates various effector functions of lymphoid and myeloid cells and is the major IgG Fc receptor expressed by human eosinophils. We investigated whether FcγRII regulates both cell survival and death of human eosinophils. When cultured in vitro without growth factors, most eosinophils undergo apoptosis within 96 h. Ligation of FcγRII by anti-CD32 mAb in solution inhibited eosinophil apoptosis and prolonged survival in the absence of growth factors. Cross-linking of human IgG bound to FcγRII by anti-human IgG Ab or of unoccupied FcγRII by aggregated human IgG also prolonged eosinophil survival. The enhanced survival with anti-CD32 mAb was inhibited by anti-granulocyte-macrophage-CSF (GM-CSF) mAb, suggesting that autocrine production of GM-CSF by eosinophils mediated survival. In fact, mRNA for GM-CSF was detected in eosinophils cultured with anti-CD32 mAb. In contrast to mAb or ligands in solution, anti-CD32 mAb or human IgG, when immobilized onto tissue culture plates, facilitated eosinophil cell death even in the presence of IL-5. Cell death induced by these immobilized ligands was accompanied by DNA fragmentation and was inhibited when eosinophil β2 integrin was blocked by anti-CD18 mAb, suggesting that β2 integrins play a key role in initiating eosinophil apoptosis. Thus, FcγRII may pivotally regulate both survival and death of eosinophils, depending on the manner of receptor ligation and β2 integrin involvement. Moreover, the FcγRII could provide a novel mechanism to control the number of eosinophils at inflammation sites in human diseases.

Role for the Rac1 Exchange Factor Vav in the Signaling Pathways Leading to NK Cell Cytotoxicity

Here we investigate the activation of and a possible role for the hematopoietic Rac1 exchange factor, Vav, in the signaling mechanisms leading to NK cell-mediated cytotoxicity. Our data show that direct contact of NK cells with a panel of sensitive tumor targets leads to a rapid and transient tyrosine phosphorylation of Vav and to its association with tyrosine-phosphorylated Syk. Vav tyrosine phosphorylation is also observed following the activation of NK cells through the low-affinity Fc receptor for IgG (FcγRIII). In addition, we demonstrate that both direct and Ab-mediated NK cell binding to target cells result in the activation of nucleotide exchange on endogenous Rac1. Furthermore, Vav antisense oligodeoxynucleotide treatment leads to an impairment of NK cytotoxicity, with FcγRIII-mediated killing being more sensitive to the abrogation of Vav expression. These results provide new insight into the signaling pathways leading to cytotoxic effector function and define a role for Vav in the activation of NK cell-mediated killing.

Characterization of the interaction between the herpes simplex virus type I Fc receptor and immunoglobulin G

Herpes simplex virus type I (HSV-1) virions and HSV-1-infected cells bind to human immunoglobulin G (hIgG) via its Fc region. A complex of two surface glycoproteins encoded by HSV-1, gE and gI, is responsible for Fc binding. We have co-expressed soluble truncated forms of gE and gI in Chinese hamster ovary cells. Soluble gE-gI complexes can be purified from transfected cell supernatants using a purification scheme that is based upon the Fc receptor function of gE-gI. Using gel filtration and analytical ultracentrifugation, we determined that soluble gE-gI is a heterodimer composed of one molecule of gE and one molecule of gI and that gE-gI heterodimers bind hIgG with a 1:1 stoichiometry. Biosensor-based studies of the binding of wild type or mutant IgG proteins to soluble gE-gI indicate that histidine 435 at the CH2-CH3 domain interface of IgG is a critical residue for IgG binding to gE-gI. We observe many similarities between the characteristics of IgG binding by gE-gI and by rheumatoid factors and bacterial Fc receptors such as Staphylococcus aureus protein A. These observations support a model for the origin of some rheumatoid factors, in which they represent anti-idiotypic antibodies directed against antibodies to bacterial and viral Fc receptors.

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.

β1 Integrin Cross-Linking Inhibits CD16-Induced Phospholipase D and Secretory Phospholipase A2 Activity and Granule Exocytosis in Human NK cells: Role of Phospholipase D in CD16-Triggered Degranulation

Recent data indicate that integrin-generated signals can modulate different receptor-stimulated cell functions in both a positive (costimulation) and a negative (inhibition) fashion. Here we investigated the ability of β1 integrins, namely α4β1 and α5β1 fibronectin receptors, to modulate CD16-triggered phospholipase activation in human NK cells. β1 integrin simultaneous cross-linking selectively inhibited CD16-induced phospholipase D (PLD) activation, without affecting either phosphatidylinositol-phospholipase C or cytosolic phospholipase A2 (PLA2) enzymatic activity. CD16-induced secretory PLA2 (sPLA2) protein release as well as its enzymatic activity in both cell-associated and soluble forms were also found to be inhibited upon β1 integrin coengagement. The similar effects exerted by specific PLD pharmacological inhibitors (2,3-diphosphoglycerate, ethanol) suggest that in our experimental system, sPLA2 secretion and activation are under the control of a PLD-dependent pathway. By using pharmacological inhibitors (2,3-diphosphoglycerate, wortmannin, ethanol) we also demonstrated that PLD activation is an important step in the CD16-triggered signaling cascade that leads to NK cytotoxic granule exocytosis. Consistent with these findings, fibronectin receptor engagement, by either mAbs or natural ligands, resulted in a selective inhibition of CD16-triggered, but not of PMA/ionomycin-induced, degranulation that was reversed by the exogenous addition of purified PLD from Streptomyces chromofuscus.

Mouse Ly-49D recognizes H-2Dd and activates natural killer cell cytotoxicity

Although activation of natural killer (NK) cytotoxicity is generally inhibited by target major histocompatibility complex (MHC) class I expression, subtle features of NK allorecognition suggest that NK cells possess receptors that are activated by target MHC I. The mouse Ly-49D receptor has been shown to activate NK cytotoxicity, although recognition of MHC class I has not been demonstrated previously. To define Ly-49D-ligand interactions, we transfected the mouse Ly-49D receptor into the rat NK line, RNK-16 (RNK.mLy-49D). As expected, anti- Ly-49D monoclonal antibody 12A8 specifically stimulated redirected lysis of the Fc receptor- bearing rat target YB2/0 by RNK.mLy-49D transfectants. RNK.mLy-49D effectors were tested against YB2/0 targets transfected with the mouse MHC I alleles H-2Dd, Db, Kk, or Kb. RNK.mLy-49D cells lysed YB2/0.Dd targets more efficiently than untransfected YB2/0 or YB2/0 transfected with Db, Kk, or Kb. This augmented lysis of H-2Dd targets was specifically inhibited by F(ab')2 anti-Ly-49D (12A8) and F(ab')2 anti-H-2Dd (34-5-8S). RNK.mLy-49D effectors were also able to specifically lyse Concanavalin A blasts isolated from H-2(d) mice (BALB/c, B10.D2, and DBA/2) but not from H-2(b) or H-2(k) mice. These experiments show that the activating receptor Ly-49D specifically interacts with the MHC I antigen, H-2Dd, demonstrating the existence of alloactivating receptors on murine NK cells.

Tolerance of NK and LAK activity for HLA class I-deficient targets in a TAP1-deficient patient (bare lymphocyte syndrome type I)

NK cells recognize target cells that lack HLA class I molecules and lyse them, according to the 'missing self' hypothesis. It was previously reported that a TAP2-deficient patient with an HLA class I-deficiency, had a normal number of NK cells but that the lymphocytes of this patient had lost their NK activity against K562 cells. In this study, we investigated the HLA class I-recognizing NK receptor expressions and the NK and LAK activities of the lymphocytes of a TAP1-deficient patient. The patient had a normal number of NK cells. Although the lymphocytes showed LAK activity against class I expressing targets following IL-2, IL-12 and IL-15 stimulation for 3 days, neither NK nor LAK activity against targets lacking class I molecules was induced. The NK cells of the patient expressed class I-recognizing NK receptors, although the percentages of such cells were low. However, no differences were observed in the expression levels of inhibitory and activating NK receptors between lymphocytes of the patient and those of healthy controls, suggesting that the modulation of the NK receptor expression is not primarily responsible for this tolerance. These results also suggest that the lymphocytes of the patient are defective in the recognition of class I-deficient target cells in order to promote the induction of self tolerance.

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.

NK1.1 cells express 4-1BB (CDw137) costimulatory molecule and are required for tumor immunity elicited by anti-4-1BB monoclonal antibodies

The 4-1BB (CDw137) T-cell molecule is a member of the TNF receptor family and triggering by either 4-1BB ligand or antibody ligation induces T-cell activation and growth. We have recently demonstrated that administration of anti-4-1BB monoclonal antibodies (mAb) induced the regression of established large tumors in several mouse models by activation of T-cell-mediated immunity. Herein we report that selective depletion of natural killer (NK) cells in mice by the anti-AsialoGM1 or anti-NK1.1 antibodies completely abrogated the antitumor effect of anti-4-1BB mAb. However, it is unlikely that NK1. 1 cells are the effectors of the response because P815 cells are resistant to lysis by NK1.1 cells in vitro. Despite the fact that activated NK1.1 cells express 4-1BB on their surface, redirection of NK1.1 cells by anti-4-1BB mAb or by transfection into P815 cells of the 4-1BB natural ligand did not trigger cytolysis. Our results thus gain further insight into the cellular mechanisms of the antitumor effects of anti-4-1BB mAb and implicate an immunoregulatory rather than effector function of 4-1BB molecule on NK1.1 cells.

DAP12-mediated signal transduction in natural killer cells. A dominant role for the Syk protein-tyrosine kinase

The murine Ly49 family contains nine genes in two subgroups: the inhibitory receptors (Ly49A, B, C, E, F, G2, and I) and the noninhibitory receptors (Ly49D and H). Unlike their inhibitory counterparts, Ly49D and H do not contain immunoreceptor tyrosine-based inhibitory motifs but associate with a recently described co-receptor, DAP12, to transmit positive signals to natural killer (NK) cells. DAP12 is also expressed in myeloid cells, but the receptors coupled to it there are unknown. Here we document the signaling pathways of the Ly49D/DAP12 complex in NK cells. We show that ligation of Ly49D results in 1) tyrosine phosphorylation of several substrates, including phospholipase Cgamma1, Cbl, and p44/p42 mitogen-activated protein kinase, and 2) calcium mobilization. Moreover, we demonstrate that although human DAP12 reportedly binds the SH2 domains of both Syk and Zap-70, ligation of Ly49D leads to activation of Syk but not Zap-70. Consistent with this observation, Ly49D/DAP12-mediated calcium mobilization is blocked by dominant negative Syk but not by catalytically inactive Zap-70. These data demonstrate the dependence of DAP12-coupled receptors on Syk and suggest that the outcome of Ly49D/DAP12 engagement will be regulated by Cbl and culminate in the activation of transcription factors.

2F1 antigen, the mouse homolog of the rat "mast cell function-associated antigen", is a lectin-like type II transmembrane receptor expressed by natural killer cells

Inhibitory lectin-like receptors expressed on the surface of hematopoietic cells are critically involved in regulation of their effector functions. Here we report that a novel mAb specific for mouse NK cells, 2F1, recognizes the mouse homolog of the mast cell function-associated antigen (MAFA), an inhibitory lectin-like transmembrane receptor expressed on rat mast cells. The 2F1 antigen (2F1-Ag) and rat MAFA are structurally highly conserved and contain a cytoplasmic motif similar to the immunoreceptor tyrosine-based inhibitory motif that is presumably utilized for inhibitory signaling. We also identified a human homolog that is closely related to the rodent MAFA/2F1-Ag proteins. Like rat MAFA, 2F1-Ag is probably encoded by a single gene, which exhibits relatively little polymorphism. Strikingly, while rat MAFA is considered a mast cell antigen, we have been unable to detect cell surface expression of 2F1-Ag by mouse mast cell lines, bone marrow-derived mast cells, or peritoneal mast cells. Furthermore, mouse bone marrow-derived mast cells were devoid of 2F1-Ag mRNA. Instead, we find that approximately 40% of mouse NK cells express 2F1-Ag. Thus, MAFA/2F1-Ag may modulate immunological responses on at least two different cell types bridging the specific and innate immune system.

SLP-76 is a direct substrate of SHP-1 recruited to killer cell inhibitory receptors

Activation of immune system cells via antigen-, Fc-, or natural killer cell-triggering-receptor stimulation is aborted by co-engagement of inhibitory receptors. Negative signaling by killer cell inhibitory receptors and related receptors depends on the Src homology 2 (SH2)-containing protein tyrosine phosphatase SHP-1. Using a combination of direct binding and functional assays, we demonstrated that the SH2 domain-containing leukocyte protein 76 (SLP-76) is a specific target for dephosphorylation by SHP-1 in T cells and natural killer cells. Furthermore, we showed that tyrosine-phosphorylated SLP-76 is required for optimal activation of cytotoxic lymphocytes, suggesting that the targeted dephosphorylation of SLP-76 by SHP-1 is an important mechanism for the negative regulation of immune cell activation by inhibitory receptors.

Integrin-mediated ras-extracellular regulated kinase (ERK) signaling regulates interferon gamma production in human natural killer cells

Recent evidence indicates that integrin engagement results in the activation of biochemical signaling events important for regulating different cell functions, such as migration, adhesion, proliferation, differentiation, apoptosis, and specific gene expression. Here, we report that beta1 integrin ligation on human natural killer (NK) cells results in the activation of Ras/mitogen-activated protein kinase pathways. Formation of Shc-growth factor receptor-bound protein 2 (Grb2) and Shc-proline-rich tyrosine kinase 2-Grb2 complexes are the receptor-proximal events accompanying the beta1 integrin-mediated Ras activation. In addition, we demonstrate that ligation of beta1 integrins results in the stimulation of interferon gamma (IFN-gamma) production, which is under the control of extracellular signal-regulated kinase 2 activation. Overall, our data indicate that beta1 integrins, by delivering signals capable of triggering IFN-gamma production, may function as NK-activating receptors.

Differential Transcriptional Regulation of CD161 and a Novel Gene, 197/15a, by IL-2, IL-15, and IL-12 in NK and T Cells

Cytokine-mediated enhancement of spontaneous cytotoxicity depends, at least in part, on modulation of the expression of surface molecules responsible for recognition of target cell structures and triggering or inhibition of the cytotoxic machinery. We previously demonstrated that expression of transcription factors (e.g., Egr-1, JunB, and c-Fos) is differentially regulated by IL-2 and IL-12. Here we show that expression of CD161/NKR-P1A, a molecule involved in triggering cytotoxicity, is specifically up-regulated by IL-12. CD161 transcription, mRNA accumulation, and surface expression are increased by IL-12. Other cytokines sharing the IL-2R β- and/or common γ-chains (i.e., IL-15, IL-4, and IL-7) do not mediate these effects. In an effort to analyze the mechanisms by which IL-2, IL-12, and IL-15 differentially regulate gene transcription, we have isolated a novel gene, 197/15a, the expression of which in NK and T cells is down-regulated by IL-2 and IL-15, up-regulated by IL-12, and not affected by IL-4 and IL-7. IL-2 and IL-15 act, at least in part, repressing 197/15a transcription; their effect on 197/15a mRNA accumulation is partially independent of novel protein synthesis, likely not mediated by JunB, Bcl-2, or Bax, and requires the activity of rapamycin-sensitive molecule(s). The observation that IL-2 and IL-12 differentially modulate CD161 expression suggests the existence of cytokine-specific mechanisms of modulation of spontaneous cytotoxicity based on the regulation of expression of surface molecules involved in target cell recognition and/or triggering of the cytolytic machinery.

The Ets-1 transcription factor is required for the development of natural killer cells in mice

In this report we have investigated the role of the Ets-1 transcription factor in the differentiation of the NK cell lineage in mice. Splenic NK cells express high levels of Ets-1. Ets-1-deficient mice produced by gene targeting developed mature erythrocytes, monocytes, neutrophils, and T and B lymphocytes. However, spleens from the Ets-1-deficient mice contained significantly reduced numbers of natural killer (NK) cells, and splenocytes from these mice lacked detectable cytolytic activity against NK cell targets in vitro. Moreover, unlike wild-type animals, Ets-1-deficient mice developed tumors following subcutaneous injection of NK-susceptible RMA-S cells. These NK cell defects could not be correlated with defects in the expression of IL-12, IL-15, and IL-18 or the IL-2 or IL-15 receptors. Thus, Ets-1 defines a novel transcriptional pathway that is required for the development of the NK cell lineage in mice.

Natural killer cell differentiation: insights from knockout and transgenic mouse models and in vitro systems

In the last few years, the routine development of knockout and transgenic mice and the ease with which rare progenitor populations can be isolated from hematopoietic organs and cultured in vitro has facilitated significant advances in understanding the lineage and development of natural killer (NK) cells. Fluorescence-activated cell sorter analyses have identified a common lymphoid progenitor capable of giving rise to NK, T, and B cells, confirming the lymphoid origin of NK cells. Knockout and transgenic mouse models have pointed to an absolutely critical role for signals sent through the interleukin (IL)-2/15 receptor beta (CD122) chain and common gamma (gamma c) chain for NK development. Such signals are likely relayed inside the cell by the tyrosine kinase Jak3, which associates with gamma c. Recently developed IL-15 and IL-15 receptor alpha knockout mice have pinpointed IL-15 as the mediator of this signal. Other mouse models have indicated an unexpected role for flt3 ligand in early NK-cell development as well as minor roles for stem cell factor and IL-7 in expanding NK-cell progenitor numbers. Finally, in vitro culture systems have proven useful in identifying the point in NK development at which each of these signals is critical.

Natural killer cells and their role in graft rejection

Natural killer cells can weakly resist engraftment of allogeneic bone marrow transplants in mice. Functional studies suggest that natural killer cell tolerance can be induced by bone marrow transplantation. Human natural killer cell inhibitory receptor repertoires differ between individuals, depending on their MHC genotype. This supports the concept that the human natural killer cell repertoire, like that of mice, adapts to the MHC molecules presented in its environment. Natural killer cells play a greater role in rejecting xenogeneic than allogeneic bone marrow and have been implicated in the rejection of xenogeneic solid organ transplants. Natural killer cell inhibitory receptors may have a lower likelihood of cross-reacting with xenogeneic than with allogeneic MHC class I molecules; important glycosylation differences between species may affect the propensity of natural killer cells to kill xenogeneic targets.

LCK-phosphorylated human killer cell-inhibitory receptors recruit and activate phosphatidylinositol 3-kinase

HLA-specific killer cell inhibitory receptors (KIR) are thought to impede natural killer (NK) and T cell activation programs through recruitment of the SH2 domain-containing tyrosine phosphatases, SHP-1 and SHP-2, to their cytoplasmic tails (CYT). To identify other SH2 domain-containing proteins that bind KIR CYT, we used the recently described yeast two-bait interaction trap and a modified version of this system, both of which permit tyrosine phosphorylation of bait proteins. Using these systems, we show that KIR CYT, once phosphorylated by the src-family tyrosine kinase LCK, additionally bind the p85alpha regulatory subunit of phosphatidylinositol (PI) 3-kinase. Furthermore, we show that in an NK cell line, NK3.3, cross-linking of KIR results in recruitment of p85alpha to KIR and activation of PI 3-kinase lipid kinase activity. One consequence of KIR coupling to PI 3-kinase is downstream activation of the antiapoptotic protein kinase AKT. Therefore, in addition to providing negative signals, KIR may also contribute positive signals for NK and T cell growth and/or survival.

HIV-1 Tat Inhibits Human Natural Killer Cell Function by Blocking L-Type Calcium Channels

Herein we show that functional phenylalkylamine-sensitive L-type calcium channels are expressed by human NK cells and are involved in the killing of tumor targets. Blocking of these channels by phenylalkylamine drugs does not affect effector/target cell binding but inhibits the release of serine esterases responsible for cytotoxicity. Interestingly, treatment of NK cells with HIV-1 Tat, which is known to affect several calcium-mediated events in immune cells, impairs their cytotoxic activity. In addition, Tat inhibits the rise in intracellular free calcium concentration upon cross-linking of the adhesion molecule CD11a, engaged during effector/target cell interaction, and the activation molecule CD16. Exogenous Tat does not influence NK-target cell binding but prevents NK cell degranulation. We propose that the molecular structure(s) on NK cells mediating the inhibitory effects HIV-1 Tat belong to L-type calcium channels, based on three lines of evidence: 1) binding of phenylalkylamine derivatives to these channels is cross-inhibited by Tat; 2) L-type calcium channels from NK cell lysates bind to Tat linked to Sepharose columns; 3) the inhibitory effect of HIV-1 Tat on NK cell function is prevented by the agonist of L-type calcium channels, Bay K 8644. Altogether, these results suggest that exogenous Tat is deeply involved in the impairment of NK cell function during HIV-1 infection.

HLA-E-bound peptides influence recognition by inhibitory and triggering CD94/NKG2 receptors: preferential response to an HLA-G-derived nonamer

The HLA-E class Ib molecule constitutes a major ligand for the lectin-like CD94/NKG2 natural killer (NK) cell receptors. Specific HLA class I leader sequence-derived nonapeptides bind to endogenous HLA-E molecules in the HLA-defective cell line 721.221, inducing HLA-E surface expression, and promote CD94/NKG2A-mediated recognition. We compared the ability of NK clones which expressed either inhibitory or activating CD94/NKG2 receptors to recognize HLA-E molecules on the surface of 721.221 cells loaded with a panel of synthetic nonamers derived from the leader sequences of most HLA class I molecules. Our results support the notion that the primary structure of the HLA-E-bound peptides influences CD94/ NKG2-mediated recognition, beyond their ability to stabilize surface HLA-E. Further, CD94/ NKG2A+ NK clones appeared more sensitive to the interaction with most HLA-E-peptide complexes than did effector cells expressing the activating CD94/NKG2C receptor. However, a significant exception to this pattern was HLA-E loaded with the HLA-G-derived nonamer. This complex triggered cytotoxicity very efficiently over a wide range of peptide concentrations, suggesting that the HLA-E/G-nonamer complex interacts with the CD94/NKG2 triggering receptor with a significantly higher affinity. These results raise the possibility that CD94/NKG2-mediated recognition of HLA-E expressed on extravillous cytotrophoblasts plays an important role in maternal-fetal cellular interactions.

Cooperation Between CD44 and LFA-1/CD11a Adhesion Receptors in Lymphokine-Activated Killer Cell Cytotoxicity

IL-2-activated NK cells exhibit cytotoxic activity against a wide variety of tumor cells in a non-MHC-restricted fashion and in the absence of prior sensitization. The molecular mechanisms that regulate the cytotoxicity and attachment of activated killer cells to tumor target cells are not known. We provide genetic evidence in CD44−/− and LFA-1−/− mice that the cell adhesion receptors LFA-1 and CD44 regulate the cytotoxic activity of IL-2-activated NK cells against a variety of different tumor cells. This defect in cytotoxicity was significantly enhanced in mice that carried a double mutation of both CD44 and LFA-1. In vitro differentiation, TNF-α and IFN-γ production, and expression of the cytolytic effector molecules perforin and Fas-L were comparable among IL-2-activated NK cells from LFA-1−/−, CD44−/−, CD44−/−LFA-1−/−, and control mice. However, CD44−/−, LFA-1−/−, and CD44−/−LFA-1−/− IL-2-activated NK cells showed impaired binding and conjugate formation with target cells. We also show that hyaluronic acid is the principal ligand on tumor cells for CD44-mediated cytotoxicity of IL-2-activated NK cells. These results provide the first genetic evidence of the role of adhesion receptors in IL-2-activated NK killing. These data also indicate that distinct adhesion receptors cooperate to mediate binding between effector and target cells required for the initiation of “natural” cytotoxicity.

Natural killer cell receptors

In killing of cellular targets, natural killer cells employ receptors that activate them and receptors specific for MHC class I that inhibit their activation. Progress in understanding the inhibitory receptors has been rapid, and indications are that they fall into two distinct structural types that appear to utilize the same inhibitory signaling cascade; meanwhile, components of the activation cascade are being elucidated, permitting us to integrate the pathways involved.

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).

Specific engagement of the CD94/NKG2-A killer inhibitory receptor by the HLA-E class Ib molecule induces SHP-1 phosphatase recruitment to tyrosine-phosphorylated NKG2-A: evidence for receptor function in heterologous transfectants

It has been recently demonstrated that the CD94/NKG2-A killer inhibitory receptor (KIR) specifically recognizes the HLA-E class Ib molecule. Moreover, the apparent CD94-mediated specific recognition of different HLA class Ia allotypes, transfected into the HLA-defective cell line 721.221, indeed depends on their selective ability to concomitantly stabilize the surface expression of endogenous HLA-E molecules, which confer protection against CD94/NKG2-A+ effector cells. In the present study, we show that a selective engagement of the CD94/NKG2-A inhibitory receptor with a specific monoclonal antibody (mAb) (Z199) was sufficient to induce tyrosine phosphorylation of the NKG2-A subunit and SHP-1 recruitment. These early biochemical events, commonly related to negative signaling pathways, were also detected upon the specific interaction of NK cells with an HLA-E+ 721.221 transfectant (.221-AEH), and were prevented by pre-incubation of .221-AEH with an anti-HLA class I mAb. Furthermore, mAb cross-linking of the CD94/NKG2-A receptor, segregated from other NK-associated molecules by transfection into a rat basophilic leukemia cell line (RBL-2H3), promoted tyrosine phosphorylation of NKG2-A and co-precipitation of SHP-1, together with an inhibition of secretory events triggered via Fc epsilonRI. Remarkably, interaction of CD94/NKG2-A+ RBL cells with the HLA-E+ .221-AEH transfectant specifically induced a detectable association of SHP-1 with NKG2-A, constituting a more formal evidence for the receptor-HLA class I interaction.

The paired Ig-like receptor PIR-B is an inhibitory receptor that recruits the protein-tyrosine phosphatase SHP-1

An emerging family of cell surface inhibitory receptors is characterized by the presence of intracytoplasmic immunoreceptor tyrosine-based inhibition motifs (ITIM). These ITIM-bearing inhibitory receptors, which are typically paired with activating isoforms, associate with Src homology domain 2-containing phosphatases following ITIM tyrosine phosphorylation. Two categories of phosphatases are recruited by the ITIM-bearing receptors: the protein-tyrosine phosphatases, SHP-1 and SHP-2, and the polyphosphate inositol 5-phosphatase, SHIP. The dynamic equilibrium of B cell activation is partially controlled by two well known ITIM-bearing receptors, CD22 and FcgammaRIIB, a low affinity receptor for IgG. We describe here that a murine ITIM-bearing molecule, PIR-B, can also negatively regulate B cell activation. Tyrosine-phosphorylated ITIMs allow PIR-B to associate with SHP-1 but not with SHIP. Engagement of PIR-B thereby initiates a SHP-1-dependent inhibitory pathway that may play an important role in regulating B lymphocyte activation.

NK Cell Natural Cytotoxicity and IFN-γ Production Are Not Always Coordinately Regulated: Engagement of DX9 KIR+ NK Cells by HLA-B7 Variants and Target Cells

DX9 mAb-binding killer cell-inhibitory receptors (KIR) recognize HLA-B molecules that express the Bw4 public serologic epitope. We assessed DX9+ NK cell fine specificity recognition of HLA-B7 variants and HLA-B27 alleles by 51Cr release natural cytotoxicity assays and by flow cytometry and enzyme-linked immunospot (ELISPOT) IFN-γ synthesis and release assays. 721.221 target cell expression of Bw4+ HLA-B27 alleles specifically inhibited DX9+ NK cell natural cytotoxicity and IFN-γ synthesis and release. A triple substitution of HLA-B7 at residues 80, 82, and 83 known to induce expression of the Bw4 serologic epitope also specifically inhibited DX9+ NK cell natural cytotoxicity and IFN-γ responses. Single HLA-B7 amino acid substitution variants were recognized in the same decreasing rank order by DX9+ NK cells and Bw4-reactive mAbs: G83R > R82L > N80T = HLA-B7. Natural cytotoxicity inhibition was reversed by the presence of blocking DX9 mAb. Natural cytotoxicity and IFN-γ production were coordinately regulated by a panel of HLA-B7 variants expressed on 721.221 cells, suggesting that these two effector functions are inhibited by the same KIR-mediated signaling mechanisms. In contrast, some NK cell clones killed 721.221 and K562 target cells equally well but released much more IFN-γ in response to K562 target cells. Differential regulation of natural cytotoxicity and IFN-γ release shows that NK cell effector functions respond to distinct signals.

Signaling through human killer cell activating receptors triggers tyrosine phosphorylation of an associated protein complex

Our understanding of the biology of human natural killer (NK) cells has significantly advanced in recent years upon identification of a family of NK cell-expressed genes that encode killer cell inhibitory receptors (KIR). Individual KIR can selectively bind various HLA class I allotypes and consequently transduce inhibitory signals that block NK cell lysis of ligand-bearing target cells. A distinct subset of related and linked genes express truncated versions of KIR that are otherwise highly homologous in amino acid sequence. Interestingly, these receptors appear to transmit stimulatory signals into NK cells and have been termed killer cell activating receptors (KAR). In this report, we demonstrate that recognition of HLA-Cw3 by the p50 KAR, NKAT8, can potentiate the cytotoxic response of appropriate NK cell clones. Specific cross-linking of this KAR with a monoclonal antibody resulted in intracellular calcium mobilization, protein tyrosine phosphorylation, and phosphorylation of the MAP kinases, ERK1 and ERK2. In addition, we identified a KAR-associated disulfide-linked dimer of a 13-kDa protein that was absent in the Jurkat T cell line and is predicted to participate in these activation signaling events. Upon treatment of NK cells with pervanadate, the disulfide-linked p13 and additional proteins of 25, 30, 37 and 50-95 kDa were identified as KAR-associated tyrosine phosphoproteins. Importantly, p13 was inducibly tyrosine phosphorylated upon cross-linking of NKAT8, which strongly suggests that the associated p13 provides KAR with appropriate cytoplasmic structure to couple with tyrosine kinase-mediated signaling effectors.

Association of tyrosine phosphatases SHP-1 and SHP-2, inositol 5-phosphatase SHIP with gp49B1, and chromosomal assignment of the gene

We have analyzed the molecules participating in the inhibitory function of gp49B1, a murine type I transmembrane glycoprotein expressed on mast cells and natural killer cells, as well as the chromosomal location of its gene. As assessed by SDS-polyacrylamide gel electrophoresis and immunoblot analysis, tyrosine-phosphorylated, but not nonphosphorylated, synthetic peptides matching each of the two immunoreceptor tyrosine-based inhibitory motif (ITIM)-like sequences found in the cytoplasmic portion of gp49B1 associated with the approximately 65-kDa tyrosine phosphatase SHP-1 and approximately 70-kDa SHP-2 derived from RBL-2H3 cells. In addition, the phosphotyrosyl peptide matching the second ITIM-like sequence also bound the approximately 145-kDa inositol polyphosphate 5-phosphatase SHIP. Thus, it has been strongly suggested that the inhibitory nature of gp49B involves the recruitment of SHP-1, SHP-2, and SHIP for the delivery of inhibitory signal to the cell interior upon phosphorylation of tyrosine residues in their ITIMs. The gp49B gene has been found to be in the juxtaposition of its cognate gene, gp49A. The gene pair was shown to locate in the B4 band of mouse chromosome 10. In this region, no conserved linkage homology to human chromosome 19, where the genes for killer cell inhibitory receptors are found, has been identified.