Phosphotyrosines in the killer cell inhibitory receptor motif of NKB1 are required for negative signaling and for association with protein tyrosine phosphatase 1C

Association of DAP12 with activating CD94/NKG2C NK cell receptors

While the inhibitory NK cell receptors for MHC class I express immunoreceptor tyrosine-based inhibitory motifs that recruit intracellular tyrosine phosphatases and prevent NK cell effector function, the activating NK cell receptors lack intrinsic sequences required for cellular stimulation. CD94/NKG2C, an activating NK cell receptor of the C-type lectin superfamily that binds to HLA-E, noncovalently associates with DAP12, a membrane receptor containing an immunoreceptor tyrosine-based activating motif. Efficient expression of CD94/NKG2C on the cell surface requires the presence of DAP12, and charged residues in the transmembrane domains of DAP12 and NKG2C are necessary for this interaction. These results provide a molecular basis for the assembly of NK cell receptors for MHC class I involved in cellular activation and inhibition.

Control of lytic function by mitogen-activated protein kinase/extracellular regulatory kinase 2 (ERK2) in a human natural killer cell line: identification of perforin and granzyme B mobilization by functional ERK2

The signal pathways that control effector function in human natural killer (NK) cells are little known. In this study, we have identified the critical role of the mitogen-activated protein kinase (MAPK) pathway in NK lysis of tumor cells, and this pathway may involve the mobilization of granule components in NK cells upon interaction with sensitive tumor target cells. Evidence was provided by biological, biochemical, and gene transfection methods. NK cell binding to tumor cells for 5 min was sufficient to maximally activate MAPK/extracellular signal-regulatory kinase 2 (ERK2), demonstrated by its tyrosine phosphorylation and by its ability to function as an efficient kinase for myelin basic protein. MAPK activation was achieved in NK cells only after contact with NK-sensitive but not NK-resistant target cells. In immunocytochemical studies, cytoplasmic perforin and granzyme B were both maximally redirected towards the tumor contact zone within 5 min of NK cell contact with tumor cells. A specific MAPK pathway inhibitor, PD098059, could block not only MAPK activation but also redistribution of perforin/granzyme B in NK cells, which occur upon target ligation. PD098059 also interfered with NK lysis of tumor cells in a 5-h 51Cr-release assay, but had no ability to block NK cell proliferation. Transient transfection studies with wild-type and dominant-negative MAPK/ERK2 genes confirmed the importance of MAPK in NK cell lysis. These results document a pivotal role of MAPK in NK effector function, possibly by its control of movement of lytic granules, and clearly define MAPK involvement in a functional pathway unlinked to cell growth or differentiation.

Negative regulation of T cell activation

T cell activation is negatively regulated by cytotoxic T lymphocyte antigen 4 (CTLA-4) and the killer cell inhibitory receptors. Endocytosis and signaling of CTLA-4 are regulated by tyrosine phosphorylation. While T cell activation is mediated by phosphorylation of immunoreceptor tyrosine-based activation motifs and tyrosine kinases, inhibitory signals are delivered by tyrosine phosphatases. Unresponsiveness is also induced by modulation of signaling components of the T cell receptor complex.

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.

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.

Natural killer activating receptors trigger interferon gamma secretion from T cells and natural killer cells

Proliferation of human CD4+ alphabeta T cells expressing a natural killer cell activating receptor (NKAR) has been shown to be enhanced, particularly in response to low doses of antigen, if the target cells present appropriate human class I major histocompatibility complex (MHC) molecules. Here, we show that NKAR also enhance proliferation and killing of target cells by subsets of CD8+ alphabeta and CD8+ gammadelta T cells, as well as by NK cells. Strikingly, interferon gamma secretion from all of these types of lymphocytes was markedly increased by interaction of the NKAR with their MHC class I ligands, independently of enhancement of proliferation. Thus, the recognition of class I MHC molecules by NKAR on both T cells and NK cells may provide a regulatory mechanism that affects immune responses through the secretion of interferon gamma and possibly other cytokines. It represents a signal for cytokine secretion alternative and/or augmentative to that through the T cell receptor.

Participation of the CD94 Receptor Complex in Costimulation of Human Natural Killer Cells

Optimal proliferation and expansion of human NK cells require mitogenic cytokines together with cell contact-dependent costimulation. Production of mAb that can modulate human NK cell proliferation yielded NKH3, which recognizes the CD94 Ag. NKH3 immunoprecipitates contain ∼70-kDa heterodimeric complexes consisting of a ∼25-kDa glycoprotein and ∼40- to 45-kDa molecules. Analysis by two-dimensional isoelectric focusing/SDS-PAGE suggests that several different 40- to 45-kDa species are present in the CD94 receptor complex in human NK cells. NKH3 reacted with essentially all resting NK cells, although CD94 is expressed at higher levels on the CD56bright (i.e., high level of CD56) CD16dim/neg (i.e., low level of or absent CD16) subpopulation than on the more abundant CD56dimCD16bright NK cell subset. Moreover, the Z199 mAb, which appears to recognize NKG2-A species that can form heterodimers with CD94, stained virtually all CD56bright NK cells, but only a subset of CD56dim NK cells. Ligation of CD94 augmented the proliferation of CD56bright NK cells in response to IL-2 or IL-15 by as much as 10-fold. Secretion of IFN-γ by CD56bright NK cells stimulated with IL-2 or IL-15 was also enhanced up to 10-fold after CD94 ligation. CD94 mAb did not consistently costimulate proliferation of or IFN-γ production by CD56dim NK cells cultured with IL-2 or IL-15. In contrast, irradiated K562 cells costimulated proliferation of both CD56bright and CD56dim NK cells. These results indicate that CD56bright and CD56dim NK cells can be costimulated through different receptors, which may allow these distinct NK cell subsets to be independently regulated in vivo.

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.

Inhibition of antigen-induced T cell response and antibody-induced NK cell cytotoxicity by NKG2A: association of NKG2A with SHP-1 and SHP-2 protein-tyrosine phosphatases

Subsets of T and natural killer (NK) lymphocytes express the CD94-NKG2A heterodimer, a receptor for major histocompatibility complex class I molecules. We show here that engagement of the CD94-NKG2A heterodimer inhibits both antigen-driven tumor necrosis factor (TNF) release and cytotoxicity on melanoma-specific human T cell clones. Similarly, CD16-mediated NK cell cytotoxicity is extinguished by cross-linking of the CD94-NKG2A heterodimer. Combining in vivo and in vitro analysis, we report that both I/VxYxxL immunoreceptor tyrosine-based inhibition motifs (ITIM) present in the NKG2A intracytoplasmic domain associate upon tyrosine phosphorylation with the protein tyrosine phosphatases SHP-1 and SHP-2, but not with the polyinositol phosphatase SHIP Determination of the dissociation constant, using surface plasmon resonance analysis, indicates that NKG2A phospho-ITIM interact directly with the SH2 domains of SHP-1 and SHP-2 with a high affinity. Engagement of the CD94-NKG2A heterodimer therefore appears as a protein-tyrosine phosphatase-based strategy that negatively regulates both antigen-induced T cell response and antibody-induced NK cell cytotoxicity. Our results suggest that this inhibitory pathway sets the threshold of T and NK cell activation.

The activating form of CD94 receptor complex: CD94 covalently associates with the Kp39 protein that represents the product of the NKG2-C gene

Inhibitory receptor complexes formed by CD94 and NKG2-A (Kp43) molecules have been implicated in HLA class I recognition by human natural killer (NK) cells. Additional forms of CD94 receptors have recently been described in NK cells characterized by the lack of NKG2-A expression. These CD94 receptors were shown to display activating functions. Immunoprecipitation with anti-CD94 monoclonal antibodies (mAb) led to the identification, in these cells, of a 39-kDa (Kp39) molecule that was originally believed to represent an activating isoform of the CD94 molecules. In the present study we show that the Kp39 molecule is covalently associated with CD94 and displays a protein backbone (26 kDa) similar to that of NKG2-A (Kp43) glycoproteins. Peptide mapping analysis indicates that Kp39 and NKG2-A glycoproteins belong to the same molecular family. A novel NKG2-specific mAb (termed P25) has been generated that specifically reacts with both NKG2-A and NKG2-C molecules, but fails to recognize NKG2-E molecules. Analysis of polyclonal and clonal NK cells shows that P25 mAb reacts with all NKG2-A+ cells and with a fraction of CD94+ cells lacking the expression of NKG2-A. These data indicate that NKG2-C molecules are indeed expressed only in a subset of cells lacking the expression of NKG2-A. The CD94-associated Kp39 molecule can be detected only in NKG2-A- P25+ cells, i.e. cells expressing NKG2-C molecules. Indeed, reverse transcription-polymerase chain reaction analysis performed on a large panel of NK clones indicates that NKG2-A- P25+ NK clones express the NKG2-C transcript. Notably, the cytolytic activity of these clones can be triggered by the P25 mAb in redirected killing analysis. Finally, biochemical analysis of COS7 cells cotransfected with CD94 and NKG2-C demonstrates the identity between Kp39 and NKG2-C molecules. Altogether, our data demonstrate that NKG2-C molecules associate with CD94 to form an activating receptor complex in a subset of human NK cells.

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.

A common inhibitory receptor for major histocompatibility complex class I molecules on human lymphoid and myelomonocytic cells

Natural killer (NK) cell-mediated lysis is negatively regulated by killer cell inhibitory receptors specific for major histocompatibility complex (MHC) class I molecules. In this study, we characterize a novel inhibitory MHC class I receptor of the immunoglobulin-superfamily, expressed not only by subsets of NK and T cells, but also by B cells, monocytes, macrophages, and dendritic cells. This receptor, called Ig-like transcript (ILT)2, binds MHC class I molecules and delivers a negative signal that inhibits killing by NK and T cells, as well as Ca2+ mobilization in B cells and myelomonocytic cells triggered through the B cell antigen receptor and human histocompatibility leukocyte antigens (HLA)-DR, respectively. In addition, myelomonocytic cells express receptors homologous to ILT2, which are characterized by extensive polymorphism and might recognize distinct HLA class I molecules. These results suggest that diverse leukocyte lineages have adopted recognition of self-MHC class I molecules as a common strategy to control cellular activation during an immune response.

HLA class I specific inhibitory receptors

All human natural killer cells and some memory T cells express HLA class I receptors, so-called natural killer cell receptors (NKRs), a receptor class that in the past few years has been shown to include several members of the immunoglobulin superfamily and the C-type lectin CD94-NKG2A complex. NKR ligand mediated cross-linking leads to the recruitment and activation of a tyrosine phosphatase involved in downregulating the phosphorylation of effector molecules involved in cell triggering. Thus, NKR engagement leads to the inhibition of different NK and T cell functions.

Negative signaling pathways of the killer cell inhibitory receptor and Fc gamma RIIb1 require distinct phosphatases

Inhibition of natural killer (NK) cells by the killer cell inhibitory receptor (KIR) involves recruitment of the tyrosine phosphatase SHP-1 by KIR and is prevented by expression of a dominant negative SHP-1 mutant. Another inhibitory receptor, the low affinity Fc receptor for immunoglobulin G (IgG) (Fc gamma RIIb1), has been shown to bind SHP-1 when cocross-linked with the antigen receptor on B cells (BCR). However, coligation of Fc gamma RIIb1 with BCR and with Fc epsilon RI on mast cells leads to recruitment of the inositol 5' phosphatase SHIP and to inhibition of mast cells from SHP-1-deficient mice. In this study, we evaluated the ability of these two inhibitory receptors to block target cell lysis by NK cells, and the contribution of SHP-1 and SHIP to inhibition. Recombinant vaccinia viruses encoding chimeric receptors and dominant negative mutants of SHP-1 and SHIP were used for expression in mouse and human NK cells. When the KIR cytoplasmic tail was replaced by that of Fc gamma RIIb1, recognition of HLA class I on target cells by the extracellular domain resulted in inhibition. A dominant negative mutant of SHP-1 reverted the inhibition mediated by the KIR cytoplasmic tail but not that mediated by Fc gamma RIIb1. In contrast, a dominant negative mutant of SHIP reverted only the inhibition mediated by the Fc gamma RIIb1 tail, providing functional evidence that SHIP plays a role in the Fc gamma RIIb1-mediated negative signal. These data demonstrate that inhibition of NK cells by KIR involves primarily the tyrosine phosphatase SHP-1, whereas inhibition mediated by Fc gamma RIIb1 requires the inositol phosphatase SHIP.

Differential association of phosphatases with hematopoietic co-receptors bearing immunoreceptor tyrosine-based inhibition motifs

A novel family of inhibitory co-receptors has been recently defined according to the presence in their intracytoplasmic domain of immunoreceptor tyrosine-based inhibition motifs (ITIM). In particular, this family includes a low-affinity receptor for IgG, Fc gammaRIIB, which is widely expressed on hematopoietic cells, as well as killer cell inhibitory receptors (KIR) for major histocompatibility complex (MHC) class I proteins, expressed on both T and natural killer (NK) lymphocytes. Fc gammaRIIB and KIR inhibitory function depends upon the tyrosine phosphorylation of their respective ITIM. Phosphorylated Fc gammaRIIB and KIR ITIM bind the tandem SH2 tyrosine phosphatases, SHP-1 and SHP-2. Recently, Fc gammaRIIB has been shown to associate with a polyphosphate inositol 5-phosphatase, SHIP, which appears to be involved in its inhibitory function. Using cell lysate adsorption to phosphorylated ITIM peptides and surface plasmon resonance, we demonstrate here that, in contrast to Fc gammaRIIB, KIR (CD158b: p58.2) do not bind to SHIP, and only recruit SHP-1 and SHP-2. In addition, we show that point mutation of the amino acid residue in position tyrosine-2 of Fc gammaRIIB and KIR ITIM abolihes their binding to SHP-1 and SHP-2, but leaves intact the association of SHIP with Fc gammaRIIB ITIM. These data contribute to the structural definition of ITIM and document a differential recruitment of phosphatases by distinct ITIM. These findings also reveal that diverse strategies of inhibition are used by distinct members of the ITIM-bearing co-receptor family.

LAIR-1, a novel inhibitory receptor expressed on human mononuclear leukocytes

In the present study, we describe a novel inhibitory receptor, leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1), that is constitutively expressed on the majority of human peripheral blood mononuclear leukocytes. LAIR-1 is a 32 kDa transmembrane glycoprotein with a single immunoglobulin-like domain and a cytoplasmic tail containing two immune receptor tyrosine-based inhibitory motifs. LAIR-1 recruits SHP-1 and SHP-2 phosphatases upon activation, and cross-linking of the LAIR-1 antigen on natural killer (NK) cells results in strong inhibition of NK cell-mediated cytotoxicity. Although it is structurally related to human killer cell inhibitory receptors, LAIR-1 does not appear to recognize human leukocyte antigen (HLA) class I molecules and thus represents a novel HLA class I-independent mechanism of NK cell regulation.

A novel immunoglobulin superfamily receptor for cellular and viral MHC class I molecules

The human cytomegalovirus UL18 gene product is a homolog of cellular major histocompatibility (MHC) class I antigens. UL18 has been proposed to protect virus-infected cells against natural killer (NK) cell cytotoxicity by engaging NK cell killer inhibitory receptors (KIR) for MHC class I. UL18 binds to a novel immunoglobulin superfamily glycoprotein, designated Leukocyte Immunoglobulin-like Receptor (LIR-1). This protein is distinct from, but related to, known KIRs and binds cellular MHC class I antigens. The cytoplasmic domain of LIR-1 contains four putative immunoreceptor tyrosine-based inhibitory motifs. Upon tyrosine phosphorylation, LIR-1 associates with the tyrosine phosphatase SHP-1. In contrast to KIRs, LIR-1 is expressed predominantly on monocytic and B lymphoid cell types, suggesting a distinct biological function.

Deletion of SHIP or SHP-1 reveals two distinct pathways for inhibitory signaling

Two signaling molecules have been implicated in the modulation of immune receptor activation by inhibitory coreceptors: an inositol polyphosphate 5'-phosphatase, SHIP, and a tyrosine phosphatase, SHP-1. To address the necessity, interaction, or redundancy of these signaling molecules, we have generated SHP-1- or SHIP-deficient B cell lines and determined their ability to mediate inhibitory signaling. Two distinct classes of inhibitory responses are defined, mediated by the selective recruitment of SHP-1 or SHIP. The Fc gammaRIIB class of inhibitory signaling is dependent on SHIP and not SHP-1; conversely, the KIR class requires SHP-1 and not SHIP. The consequence of this selective recruitment by inhibitory receptor engagement is seen in BCR-triggered apoptosis. SHP-1-mediated inhibitory signaling blocks apoptosis, while SHIP recruitment attenuates a proapoptotic signal initiated by Fc gammaRIIB.

Natural killer cell acceptance of H-2 mismatch bone marrow grafts in transgenic mice expressing HLA-Cw3 specific killer cell inhibitory receptor

Natural killer (NK) cells express killer cell inhibitory receptors (KIRs) for major histocompatibility complex class I molecules. Engagement of these surface receptors inhibits NK cell cytotoxic programs. KIR can also be expressed on T cell subsets, and their engagement similarly results in inhibition of effector functions initiated by the CD3/T cell receptor complex. KIR genes belong to two distinct families: the immunoglobulin superfamily (IgSF KIRs) and dimeric C2 lectins (lectin-like KIRs). Whereas both IgSF (p58: CD158, p70, and p140) and lectin-like KIRs (CD94/NKG2A heterodimers) have been found in human, only lectin-like KIRs (all members of the Ly-49 family) have been described in the mouse. We have generated transgenic mice expressing an IgSF KIR, CD158b (p58.2), which recognizes HLA-Cw3. Our data show that CD158b is necessary and sufficient to confer specificity to NK cells, as well as to modulate T cell activation programs in vitro. In addition, we did not detect any adaptation of CD158b cell surface expression to that of HLA class I ligands in the CD158b x HLA-Cw3 double transgenic mice, in contrast to observations with Ly-49 in the mouse. Therefore, distinct strategies of selection/calibration appear to be used by IgSF and lectin-like KIRs. Finally, the transgenic expression of CD158b KIR prevents the in vivo rejection of H-2 mismatch bone marrow grafts, which express the cognate major histocompatibility class I HLA-Cw3 allele, demonstrating for the first time the in vivo implication of human IgSF KIRs in the negative regulation of NK cell function.

Inhibitory receptors, ITIM sequences and phosphatases

A diverse group of inhibitory receptors, including FcgammaRII, killer cell inhibitory receptors, and B22, shares an immunoreceptor tyrosine-based inhibition motif (ITIM). Recent studies have shown that this motif, when phosphorylated on tyrosine, forms a docking site for the Src homology 2 recognition domains of the protein tyrosine phosphatase SHP-1 and the inositol 5-phosphatase SHIP. A similar motif in cytotoxic T-lymphocyte antigen-4 recruits the related tyrosine phosphatase SHP-2. These three enzymes act to inhibit signaling cascades resulting from ligation of the BCR, TCR, FcgammaRIII, and FcepsilonRI, although the relative importance of the tyrosine phosphatases and the inositol phosphatase differs depending on the cell type.

Role of phosphatases in lymphocyte activation

Many lymphocyte signaling pathways are regulated by protein tyrosyl phosphorylation, which is controlled by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Substantial progress has been made in defining the functions of lymphocyte PTPs. Individual PTPs can enhance or diminish cell signaling levels. The transmembrane PTP CD45 is a key positive element in multiple lymphocyte signaling pathways in vivo. New insights into the function of individual CD45 isoforms have emerged. Anti-CD45 antibodies with potent immunosuppressant activity have been identified, suggesting that CD45 may be a propitious target for drug design. Progress has also been made in elucidating the function and targets of specific nontransmembrane PTPs, particularly those with Src homology 2 domains.

Genomic organization and allelic polymorphism of the human killer cell inhibitory receptor gene KIR103

Killer cell inhibitory receptors (KIR) belong to the immunoglobulin superfamily of molecules and are expressed on natural killer (NK) cells. The KIRs of the p58/p50 family have two immunoglobulin domains and are ligands for HLA-Cw antigens, whereas the p70/p70 delta family has three immunoglobulin domains and comprises ligands for HLA-B antigens and possibly some HLA-A antigens. Members of a third KIR family, KIR103, have two immunoglobulin domains but have highest nucleotide sequence homology to the p70 family. The ligands for KIR103 on target cells are currently unknown. We here report the complete genomic organization of KIR103. It spans about 12 kb of DNA and consists of eight exons of which exon 1 and exon 2 encode the leader sequence. Exon 3 encodes the first immunoglobulin domain (gamma 1), and exon 4 encodes the main part of the second immunoglobulin domain (gamma 3), which also contains sequences contributed by exon 5 and exon 6. Exon 6 encodes the transmembrane domain, whereas exons 7 and 8 encode most of the cytoplasmic domain. KIR103 is polymorphic, and two alleles, 103AS and 103LP, are defined in this study. Additional full-length cDNA clones for KIR103 have been isolated and are shown to be formed by alternative mRNA splicing with exon skipping. Some of these truncated KIR103 cDNA could encode shorter transmembrane molecules, whereas others lack the transmembrane domain and are candidate genes for secreted KIR products. KIR103 is localized to the KIR genetic region on chromosome 19q13.4.

A novel inhibitory receptor (ILT3) expressed on monocytes, macrophages, and dendritic cells involved in antigen processing

Immunoglobulin-like transcript (ILT) 3 is a novel cell surface molecule of the immunoglobulin superfamily, which is selectively expressed by myeloid antigen presenting cells (APCs) such as monocytes, macrophages, and dendritic cells. The cytoplasmic region of ILT3 contains putative immunoreceptor tyrosine-based inhibitory motifs that suggest an inhibitory function of ILT3. Indeed, co-ligation of ILT3 to stimulatory receptors expressed by APCs results in a dramatic blunting of the increased [Ca2+]i and tyrosine phosphorylation triggered by these receptors. Signal extinction involves SH2-containing protein tyrosine phosphatase 1, which is recruited by ILT3 upon cross-linking. ILT3 can also function in antigen capture and presentation. It is efficiently internalized upon cross-linking, and delivers its ligand to an intracellular compartment where it is processed and presented to T cells. Thus, ILT3 is a novel inhibitory receptor that can negatively regulate activation of APCs and can be used by APCs for antigen uptake.

A novel phosphotyrosine motif with a critical amino acid at position -2 for the SH2 domain-mediated activation of the tyrosine phosphatase SHP-1

SHP-1 is a protein-tyrosine phosphatase associated with inhibition of activation pathways in hematopoietic cells. The catalytic activity of SHP-1 is regulated by its two SH2 (Src homology 2) domains; phosphotyrosine peptides that bind to the SH2 domains activate SHP-1. The consensus sequence (I/V)XYXX(L/V) is present in the cytoplasmic tails of several lymphocyte receptors that interact with the second SH2 domain of SHP-1. In several of these receptors, there are two or three occurrences of the motif. Here we show that the conserved hydrophobic amino acid preceding the phosphotyrosine is critical for binding to and activation of SHP-1 by peptides corresponding to sequences from killer cell inhibitory receptors. The interaction of most SH2 domains with phosphopeptides requires only the phosphotyrosine and the three residues downstream of the tyrosine. In contrast, the shortest peptide able to bind or activate SHP-1 also included the two residues upstream of the phosphotyrosine. A biphosphopeptide corresponding to the cytoplasmic tail of a killer cell inhibitory receptor with the potential to interact simultaneously with both SH2 domains of SHP-1 was the most potent activator of SHP-1. The hydrophobic residue upstream of the tyrosine was also critical in the context of the biphosphopeptide. The contribution of a hydrophobic amino acid two residues upstream of the tyrosine in the SHP-1-binding motif may be an important feature that distinguishes inhibitory receptors from those that provide activation signals.

A novel pair of immunoglobulin-like receptors expressed by B cells and myeloid cells

An Fcalpha receptor probe of human origin was used to identify novel members of the Ig gene superfamily in mice. Paired Ig-like receptors, named PIR-A and PIR-B, are predicted from sequence analysis of the cDNAs isolated from a mouse splenic library. Both type I transmembrane proteins possess similar ectodomains with six Ig-like loops, but have different transmembrane and cytoplasmic regions. The predicted PIR-A protein has a short cytoplasmic tail and a charged Arg residue in the transmembrane region that, by analogy with the FcalphaR relative, suggests the potential for association with an additional transmembrane protein to form a signal transducing unit. In contrast, the PIR-B protein has an uncharged transmembrane region and a long cytoplasmic tail containing four potential immunoreceptor tyrosine-based inhibitory motifs. These features are shared by the related killer inhibitory receptors. PIR-A proteins appear to be highly variable, in that predicted peptide sequences differ for seven randomly selected PIR-A clones, whereas PIR-B cDNA clones are invariant. Southern blot analysis with PIR-B and PIR-A-specific probes suggests only one PIR-B gene and multiple PIR-A genes. The PIR-A and PIR-B genes are expressed in B lymphocytes and myeloid lineage cells, wherein both are expressed simultaneously. The characteristics of the highly-conserved PIR-A and PIR-B genes and their coordinate cellular expression suggest a potential regulatory role in humoral, inflammatory, and allergic responses.

The role of phosphotyrosine phosphatases in haematopoietic cell signal transduction

Phosphotyrosine phosphatases (PTPases) are the enzymes which remove phosphate groups from protein tyrosine residues. An enormous number of phosphatases have been cloned and sequenced during the past decade, many of which are expressed in haematopoietic cells. This review focuses on the biochemistry and cell biology of three phosphatases, the transmembrane CD45 and the cytosolic SH2-domain-containing PTPases SHP-1 and SHP-2, to illustrate the diverse ways in which PTPases regulate receptor signal transduction. The involvement of these and other PTPases has been demonstrated in haematopoietic cell development, apoptosis, activation and nonresponsiveness. A common theme in the actions of many haematopoietic cell PTPases is the way in which they modulate the thresholds for receptor signalling, thereby regulating critical events in the positive and negative selection of lymphocytes. There is growing interest in haematopoietic PTPases and their associated regulatory proteins as targets for pharmaceutical intervention and in the involvement of these enzymes in human disease.

Self and viral peptides can initiate lysis by autologous natural killer cells

Natural killer (NK) cells are inhibited by specific allotypes of class I major histocompatibility complex ligands recognized by polymorphic inhibitory receptors (e.g., NKIR1 and NKIR2). NK1- and NK2-specific clones recognize two groups of HLA-C allotypes that are distinguished by a dimorphism at residue 80 in the alpha1 helix (alphaLys-80 and alphaAsn-80, respectively). "Empty" HLA-Cw7 expressed in peptide transporter-deficient cells and HLA-Cw7 loaded with several peptides each functioned as inhibitory ligands for NK2 lines and clones. However, loading of HLA-Cw7 with two other peptides derived from glutamic acid decarboxylase or coxsackie virus (each of which has been associated with autoimmune diabetes mellitus) abrogated this inhibitory recognition. Both peptides contained Lys at P8 of the epitope. Substitution of P8 with Ala or two other basic amino acids, His and Arg, resulted in peptides that were inhibitory, as were peptides with P8 Val, Glu, or Asn. The manner in which a Lys at P8 might affect recognition is discussed, together with a hypothesis for a novel mechanism by which an autoimmune disease might be initiated.

Reconstituted killer cell inhibitory receptors for major histocompatibility complex class I molecules control mast cell activation induced via immunoreceptor tyrosine-based activation motifs

Natural killer and T cells express at their surface, members of a multigenic family of killer cell inhibitory receptors (KIR) for major histocompatibility complex Class I molecules. KIR engagement leads to the inhibition of natural killer and T cell activation programs. We investigated here the functional reconstitution of KIR in a non-lymphoid cell type. Using stable transfection in the RBL-2H3 mast cell line, we demonstrated that (i) KIR can inhibit signals induced by FcepsilonRIgamma or CD3zeta polypeptides that bear immunoreceptor tyrosine-based activation motifs; (ii) two distinct immunoreceptor tyrosine-based inhibition motifs-bearing receptors, i.e. KIR and FcgammaRIIB, use distinct inhibitory pathways since KIR engagement inhibits the intracellular Ca2+ release from endoplasmic reticulum stores, in contrast to FcgammaRIIB, which only inhibits extracellular Ca2+ entry; (iii) KIR require co-ligation with an immunoreceptor tyrosine-based activation motif-dependent receptor to mediate their inhibitory function. This latter finding is central to the mechanism by which KIR selectively inhibit only the activatory receptors in close vicinity. Taken together our observations also contribute to define and extend the family of immunoreceptor tyrosine-based inhibition motif-bearing receptors involved in the negative control of cell activation.

T cell antigen receptor signal transduction

The T cell antigen receptor (TCR) initiates signal transduction by activating multiple cytoplasmic protein tyrosine kinases (PTKs). Considerable progress in the field of TCR signal transduction has been made in three areas recently: first, in understanding the structure and function of the PTK ZAP-70; second, in the elucidation of the function of the substrates and pathways downstream of the PTKs; and third, in the identification of molecules that negatively regulate TCR signalling.

Human uterine NK cells have a similar repertoire of killer inhibitory and activatory receptors to those found in blood, as demonstrated by RT-PCR and sequencing

The expression of natural killer (NK) cell receptors specific for HLA class I molecules has been studied in CD56bright, CD3- NK cells isolated from the pregnant uterine mucosa, the decidua. RT-PCR was performed on cDNA from uterine NK cells with primers designed to amplify members of the killer inhibitory receptor (KIR)/killer activatory receptor (KAR) gene family. Sequencing of the PCR products revealed that uterine NK cells express KIR/KAR which have two or three extracellular immunoglobulin superfamily (Ig-SF) domains. NK receptors for both groups of HLA-C alleles were found. KIR, characterised by a long cytoplasmic tail containing the immune receptor tyrosine-based inhibitory motif (ITIM), and KAR, characterised by a short cytoplasmic domain with a transmembrane region containing a charged lysine, were both identified. Different individuals appear to have a distinct but overlapping repertoire of KIR/KAR. No new members of this NK receptor gene family were identified in the uterine CD56bright NK cells. Similar findings were obtained from non-pregnant endometrial tissues representative of different stages of the menstrual cycle. Immunohistology confirmed that the KIR protein products were expressed by decidual NK cells. These results reveal that NK receptors for trophoblast HLA class I molecules are present in maternal uterine NK cells. Fetal trophoblast cells infiltrating the decidua express HLA-G and HLA-C gene products. This suggests that maternal recognition of the fetus may be mediated by an NK allorecognition system.

Protein tyrosine phosphatases in signal transduction

Protein-tyrosyl phosphorylation, regulated by protein tyrosine kinases and protein tyrosine phosphatases (PTPs), is a key cellular control mechanism. Until recently, little was known about PTPs. However, the past two years have witnessed an explosion of information about PTP structure, regulation and function. Crystal structures of several PTPs have provided insights into enzymatic mechanisms and regulation and suggested the design of 'substrate-trapping' mutants. Candidate homophilic and heterophilic ligands for transmembrane PTPs have been identified, and roles for transmembrane PTPs in regulating cell-cell interactions have been suggested. Finally, progress has been made in understanding signaling by Src homology 2 domain containing PTPs and PTPs controlling yeast osmoregulatory pathways.

The binding site of NK receptors on HLA-C molecules

The protection of cells expressing class I HLA molecules from NK lysis is mediated by natural killer cell inhibitory receptors (NKIR). Using site-directed mutagenesis, residues on HLA-C that determine the locus specificity (alphaVal-76), allotype group specificity (a dimorphism alphaAsn-80/Lys-80), and affinity of NKIR binding (a second pair of dimorphisms, alphaAla-73, Asp-90 or alphaThr-73, Ala-90) have been identified. Thus the "footprint" of the NKIR on the alpha1 helix of the class I MHC molecule HLA-C and its associated beta strands are similar in position to the site occupied by superantigens on and behind the alpha1 helix of the class II MHC molecule HLA-DR1, but further toward its C-terminus. The intermediate affinity binding of NKIR to HLA-C, determined by alpha73 and alpha90, has an essential role in preventing cross-reactivity and ensuring the availability of NK cells for immunosurveillance; low affinity and high affinity mutants are both physiologically impaired.

Cloning of novel immunoglobulin superfamily receptors expressed on human myeloid and lymphoid cells: structural evidence for new stimulatory and inhibitory pathways

We have identified two novel human cDNA encoding transmembrane proteins of the immunoglobulin superfamily (IgSF). The two cDNA, called immunoglobulin-like transcripts 1 and 2 (ILT1 and ILT2), are expressed in myeloid and lymphoid cells and are homologous to bovine Fc gamma2R, human killer cell inhibitory receptors (KIR), human Fc alphaR, and mouse gp49. Furthermore, ILT1 and ILT2 are encoded on chromosome 19, as are Fc alphaR and KIR. While the ILT1 and ILT2 extracellular domains are homologous, the transmembrane and cytoplasmic domains differ substantially. ILT1 has an arginine within the transmembrane region, followed by a short cytoplasmic tail, similar to human Fc alphaRI and bovine Fc gamma2R. ILT2 has a long cytoplasmic tail, which contains two YxxV and two YxxL pairs similar to the immunoreceptor tyrosine-based inhibitory motifs in KIR that are known to bind the phosphotyrosine phosphatase SHP-1. These cytoplasmic features suggest that ILT1 and ILT2 may mediate novel transmembrane signals by which myeloid and lymphoid cell responses can be either activated or inhibited.

NKG2A complexed with CD94 defines a novel inhibitory natural killer cell receptor

CD94 is a C-type lectin expressed by natural killer (NK) cells and a subset of T cells. Blocking studies using anti-CD94 mAbs have suggested that it is a receptor for human leukocyte antigen class I molecules. CD94 has recently been shown to be a 26-kD protein covalently associated with an unidentified 43-kD protein(s). This report shows that NKG2A, a 43-kD protein, is covalently associated with CD94 on the surface of NK cells. Cell surface expression of NKG2A is dependent on the association with CD94 as glycosylation patterns characteristic of mature proteins are found only in NKG2A that is associated with CD94. Analysis of NK cell clones showed that NKG2A was expressed in all NK cell clones whose CD16-dependent killing was inhibited by cross-linking CD94. The induction of an inhibitory signal is consistent with the presence of two immunoreceptor tyrosine-based inhibitory motifs (V/LXYXXL) on the cytoplasmic domain of NKG2A. Similar motifs are found on Ly49 and killer cell inhibitory receptors, which also transmit negative signals to NK cells.

Mouse Ly-49A interrupts early signaling events in natural killer cell cytotoxicity and functionally associates with the SHP-1 tyrosine phosphatase

The lytic activity of natural killer (NK) cells is inhibited by the expression of class I major histocompatibility complex (MHC) antigens on target cells. In murine NK cells, Ly-49A mediates inhibition of cytotoxicity in response to the class I MHC antigen H-2Dd. In this report, we studied the function of mouse Ly-49A in both the rat NK cell tumor line, RNK-16, transfected with Ly-49A cDNA, and in primary NK cells. We show that ligation of Ly-49A by H-2Dd inhibits early signaling events during target cell stimulation, including polyphosphoinositide turnover and tyrosine phosphorylation. We also show that Ly-49A directly associates with the cytoplasmic tyrosine phosphatase SHP-1, and that Ly-49A function is impaired in NK cells from SHP-1 mutant viable motheaten mice and from SHP-1-deficient motheaten mice. Finally, we demonstrate that mutational substitution of the tyrosine within the proposed SHP-1 binding motif in Ly-49A completely abrogates inhibition of NK cell cytotoxicity through this receptor. These results demonstrate that Ly-49A interrupts early activating signals in NK cells, and that SHP-1 is an important mediator of Ly-49A function.

The natural killer gene complex: a genetic basis for understanding natural killer cell function and innate immunity

The natural killer gene complex encodes proteins, some of which are structurally unrelated, that impact on NK-cell function. Detailed analyses have indicated that these molecules are involved in NK-cell recognition, activation, and inhibition. The importance of this genomic region is highlighted by studies indicating that NKC-associated genes significantly influence NK cell-mediated innate host defense against life-threatening pathogens and that the NKC is conserved among diverse species. Thus, further elucidation of the NKC and its gene products will provide a genetic basis for understanding innate immunity and NK-cell activity at the molecular level.