Receptor glycosylation regulates Ly-49 binding to MHC class I

Murine NK cells express the Ly-49 family of class I MHC-binding receptors that control their ability to lyse tumor or virally infected host target cells. X-ray crystallography studies have identified two predominant contact sites (sites 1 and 2) that are involved in the binding of the inhibitory receptor, Ly-49A, to H-2D(d). Ly-49G2 (inhibitory) and Ly-49D (activating) are highly homologous to Ly-49A and also recognize H-2D(d). However, the binding of Ly-49D and G(2) to H-2D(d) is of lower affinity than Ly-49A. All Ly-49s contain N-glycosylation motifs; however, the importance of receptor glycosylation in Ly-49-class I interactions has not been determined. Ly-49D and G(2) contain a glycosylation motif (NTT (221-223)), absent in Ly-49A, adjacent to one of the proposed binding sites for H-2D(d) (site 2). The presence of a complex carbohydrate group at this critical site could interfere with class I binding. In this study, we are able to demonstrate for the first time that Ly-49D binds H-2D(d) in the presence of mouse beta(2)-microglobulin. We also demonstrate that glycosylation of the NTT (221-23) motif of Ly-49D inteferes with recognition of H-2D(d). Alteration of the Ly-49D-NTT (221-23) motif to abolish glycosylation at this site resulted in enhanced H-2D(d) binding and receptor activation. Furthermore, glycosylation of Ly-49G2 at NTT (221-23) also reduces receptor binding to H-2D(d) tetramers. Therefore, the addition of complex carbohydrates to the Ly-49 family of receptors may represent a mechanism by which NK cells regulate affinity for host class I ligands.

Dissociation of NKT Stimulation, Cytokine Induction, and NK Activation In Vivo by the Use of Distinct TCR-Binding Ceramides

NKT and NK cells are important immune regulatory cells. The only efficient means to selectively stimulate NKT cells in vivo is alpha-galactosylceramide (alphaGalCer). However, alphaGalCer effectively stimulates and then diminishes the number of detectable NKT cells. It also exhibits a potent, indirect ability to activate NK cells. We have now discovered another ceramide compound, beta-galactosylceramide (betaGalCer) (C12), that efficiently diminishes the number of detectable mouse NKT cells in vivo without inducing significant cytokine expression or activation of NK cells. Binding studies using CD1d tetramers loaded with betaGalCer (C12) demonstrated significant but lower intensity binding to NKT cells when compared with alphaGalCer, but both ceramides were equally efficient in reducing the number of NKT cells. However, betaGalCer (C12), in contrast to alphaGalCer, failed to increase NK cell size, number, and cytolytic activity. Also in contrast to alphaGalCer, betaGalCer (C12) is a poor inducer of IFN-gamma, TNF-alpha, GM-CSF, and IL-4 gene expression. These qualitative differences in NKT perturbation/NK activation have important implications for delineating the unique in vivo roles of NKT vs NK cells. Thus, alphaGalCer (which triggers NKT cells and activates NK cells) efficiently increases the resistance to allogeneic bone marrow transplantation while betaGalCer (C12) (which triggers NKT cells but does not activate NK cells) fails to enhance bone marrow graft rejection. Our results show betaGalCer (C12) can effectively discriminate between NKT- and NK-mediated responses in vivo. These results indicate the use of different TCR-binding ceramides can provide a unique approach for understanding the intricate immunoregulatory contributions of these two cell types.

NK-cell purging of leukemia: superior antitumor effects of NK cells H2 allogeneic to the tumor and augmentation with inhibitory receptor blockade

Natural killer (NK) cells are composed of subsets characterized by the expression of inhibitory or activating receptors, or both, specific for different major histocompatibility complex (MHC) class I determinants. We have previously shown that inhibitory receptor blockade of syngeneic NK cells was an effective means of ex vivo purging of leukemia-contaminated bone marrow and that the transplantation of mice with the purged bone marrow cells (BMCs) resulted in long-term, relapse-free survival. We have extended the investigation to assess the antitumor effects mediated by NK cells H2-allogeneic to tumor cells. We demonstrate that various tumor cell lines are more susceptible to lysis by H2-allogeneic NK cells than by syngeneic NK cells in vitro even though comparable percentages of Ly49 NK cells were present. Using allogeneic NK cells to purge leukemia-contaminating BMCs before transplantation resulted in a higher proportion of mice with long-term survival than using syngeneic NK cells. Allogeneic NK cells did not suppress hematopoietic reconstitution as measured by granulocyte/monocyte-colony-forming unit (CFU-GM), complete blood count (CBC), and donor chimerism at various days after transplantation. Inhibitory receptor blockade of allogeneic NK cells also significantly increased these antitumor effects at lower NK/tumor ratios compared with those of syngeneic NK cells. These results demonstrate that H2-allogeneic NK cells mediate more potent antitumor effects than syngeneic NK cells without adverse hematologic effects and thus may be useful in cancer therapy.

Expression of IFN-gamma upon triggering of activating Ly49D NK receptors in vitro and in vivo: costimulation with IL-12 or IL-18 overrides inhibitory receptors

NK cells can express both activating and inhibitory Ly49 receptors on their cell surface. When cells expressing both receptors are presented with a ligand, inhibition dominates the functional outcome. In this report we demonstrate that costimulation of the activating Ly49D murine NK cell receptor with IL-12 or IL-18 is capable of over-riding the inhibitory Ly49G2 receptor blockade for cytokine production both in vitro and in vivo. This synergy is mediated by and dependent upon Ly49D-expressing NK cells and results in significant systemic expression of IFN-gamma. This would place NK cells and their activating Ly-49 receptors as important initiators of microbial, antiviral, and antitumor immunity and provide a mechanism for the release of activating Ly49 receptors from inhibitory receptor blockade.

Increased bone marrow allograft rejection by depletion of NK cells expressing inhibitory Ly49 NK receptors for donor class I antigens

Natural killer (NK) cells are the major effectors of acute rejection of incompatible bone marrow cell (BMC) grafts in lethally irradiated mice. The immunogenetics of BMC rejection are largely controlled by the coexpression (or not) of inhibitory and stimulatory Ly49 receptors whose ligands are class I major histocompatibility complex (MHC) molecules. The majority of the BMC rejection studies involved low numbers of BMCs that were resisted by host NK cells. In the present study, larger numbers of BMCs were given in which rejection was not detected and the role of different Ly49 NK subsets not presumably involved in the rejection of a particular BMC haplotype was examined. Surprisingly, the data show that the removal of NK cell subsets expressing Ly49 inhibitory receptors for donor class I antigens, which would be predicted to have no effect on the BMC rejection capability, resulted in the marked rejection of BMCs where no resistance was normally seen. These results extend the "missing self" hypothesis to suggest that NK Ly49 inhibitory receptors can both inhibit activation and killing by those cells, but also can in some way influence the function of NK cells that do not express that inhibitory receptor in a cell-cell interaction. This suggests that caution must be exercised before removal of host NK cell subset is applied clinically because enhanced BMC rejection may result. Altering the balance of Ly49 NK subsets may also affect other in vivo activities of these cells.

Aberrant DAP12 signaling in the 129 strain of mice: implications for the analysis of gene-targeted mice

NK cells are implicated in antiviral responses, bone marrow transplantation and tumor immunosurveillance. Their function is controlled, in part, through the Ly49 family of class I binding receptors. Inhibitory Ly49s suppress signaling, while activating Ly49s (i.e., Ly49D) activate NK cells via the DAP12 signaling chain. Activating Ly49 signaling has been studied primarily in C57BL/6 mice, however, 129 substrains are commonly used in gene-targeting experiments. In this study, we show that in contrast to C57BL/6 NK cells, cross-linking of DAP12-coupled receptors in 129/J mice induces phosphorylation of DAP12 but not calcium mobilization or cytokine production. Consistent with poor-activating Ly49 function, 129/J mice reject bone marrow less efficiently than C57BL/6 mice. Sequence analysis of receptors and DAP12 suggests no structural basis for inactivity, and both the 129/J and C57BL/6 receptors demonstrate normal function in a reconstituted receptor system. Most importantly, reconstitution of Ly49D in 129/J NK cells demonstrated that the signaling deficit is within the NK cells themselves. These unexpected findings bring into question any NK analysis of 129/J, 129Sv, or gene-targeted mice derived from these strains before complete backcrossing, and provide a possible explanation for the differences observed in the immune response of 129 mice in a variety of models.

IL-2 and IL-12 alter NK cell responsiveness to IFN-gamma-inducible protein 10 by down-regulating CXCR3 expression

Cytokine treatment of NK cells results in alterations in multiple cellular responses that include cytotoxicity, cytokine production, proliferation, and chemotaxis. To understand the molecular mechanisms underlying these responses, microarray analysis was performed and the resulting gene expression patterns were compared between unstimulated, IL-2, IL-2 plus IL-12, and IL-2 plus IL-18-stimulated NK92 cells. RNase protection assays and RT-PCR confirmed microarray predictions for changes in mRNA expression for nine genes involved in cell cycle progression, signal transduction, transcriptional activation, and chemotaxis. Multiprobe RNase protection assay also detected changes in the expression of CCR2 mRNA, a gene that was not imprinted on the microarray. We subsequently expanded our search for other chemokine receptor genes absent from the microarray and found an IL-2- and IL-12-dependent decrease in CXCR3 receptor mRNA expression in NK92 cells. A detailed analysis of CXCR3 expression in primary NK cells revealed that an IL-2 and an IL-12 together significantly decreased the CXCR3 receptor mRNA and receptor surface expression by 6 and 24 h of treatment, respectively. This decrease in receptor expression was associated with a significant reduction in chemotaxis in the presence of IFN-gamma-inducible protein-10. The decline in CXCR3 mRNA was due to transcriptional and posttranscriptional mechanisms as the addition of actinomycin D to IL-2- and IL-12-treated NK92 slightly altered the half-life of the CXCR3 mRNA. Collectively, these data suggest that IL-2 and IL-12 directly affect NK cell migratory ability by rapid and direct down-regulation of chemokine receptor mRNA expression.

Natural cytotoxicity uncoupled from the Syk and ZAP-70 intracellular kinases

The intracellular signals that trigger natural cytotoxicity have not been clearly determined. The Syk and ZAP-70 tyrosine kinases are essential for cellular activation initiated by B and T cell antigen receptors and may drive natural killer (NK) cell cytotoxicity via receptors bearing immunoreceptor tyrosine-based activation motifs (ITAMs). However, we found that, unlike B and T cells, NK cells developed in Syk-/-ZAP-70-/- mice and, despite their nonfunctional ITAMs, lysed various tumor targets in vitro and eliminated tumor cells in vivo, including those without NKG2D ligands. The simultaneous inhibition of phosphatidyl inositol 3 kinase and Src kinases abrogated the cytolytic activity of Syk-/-ZAP-70-/- NK cells and strongly reduced that of wild-type NK cells. This suggests that distinct and redundant signaling pathways act synergistically to trigger natural cytotoxicity.

Impaired NK cell development in an IFN-gamma transgenic mouse: aberrantly expressed IFN-gamma enhances hematopoietic stem cell apoptosis and affects NK cell differentiation

Aberrant expression of IFN-gamma has been demonstrated to cause a wide variety of alterations in cell function and development. Previously we reported that constitutive expression of IFN-gamma in bone marrow (BM) and thymus results in a total absence of B cells and a substantial decrease in the number of hematopoietic progenitor cells. In this study, we demonstrate a severe deficiency of NK1.1(+)CD3(-) cells in this transgenic mouse model. Compared with normal control littermates, we found a pronounced reduction of NK cells in IFN-gamma transgenic mouse spleen and liver despite maintenance of normal function. In addition, we observed a reduced number of BM cells in the IFN-gamma transgenic mouse despite normal expression of hematopoietic growth factors in the BM. Interestingly, these cells were less responsive to stem cell factor (SCF) despite c-kit expression on hematopoietic stem cells (HSCs). We observed that addition of exogenous IFN-gamma inhibited proliferation of HSCs and differentiation of NK precursors from HSCs in normal mice in response to SCF, IL-7, fms-like tyrosine kinase 3 ligand, and IL-15. Furthermore, we found that HSCs express the IFN-gammaRalpha subunit and undergo apoptosis in response to exogenous IFN-gamma. Thus, we have demonstrated the occurrence of a severe deficiency of NK cells and lower numbers of BM cells in an IFN-gamma transgenic mouse model. Furthermore, because exogenous IFN-gamma affects the responsiveness to hematopoietic growth factors such as SCF in vitro, our results indicate that chronic expression of IFN-gamma in vivo leads to widespread immune system defects, including alterations in NK cell differentiation.

Human B cell activation by autologous NK cells is regulated by CD40-CD40 ligand interaction: role of memory B cells and CD5+ B cells

NK cells are a subpopulation of lymphocytes characterized primarily by their cytolytic activity. They are recognized as an important component of the immune response against virus infection and tumors. In addition to their cytolytic activity, NK cells also participate either directly or indirectly in the regulation of the ongoing Ab response. More recently, it has been suggested that NK cells have an important role in the outcome of autoimmune diseases. Here, we demonstrate that human NK cells can induce autologous resting B cells to synthesize Ig, including switching to IgG and IgA, reminiscent of a secondary Ab response. B cell activation by the NK cell is contact-dependent and rapid, suggesting an autocrine B cell-regulated process. This NK cell function is T cell-independent, requires an active cytoplasmic membrane, and is blocked by anti-CD40 ligand (anti-CD154) or CD40-mIg fusion protein, indicating a critical role for CD40-CD40 ligand interaction. Depletion studies also demonstrate that CD5+ B cells (autoreactive B-1 cells) and a heterogeneous population of CD27+ memory B cells play a critical role in the Ig response induced by NK cells. The existence of this novel mechanism of B cell activation has important implications in innate immunity, B cell-mediated autoimmunity, and B cell neoplasia.

The ever-expanding Ly49 gene family: repertoire and signaling

The mouse lectin-related Ly49 family and the human killer cell Ig-like receptor (KIR) family represent structurally distinct, yet functionally analogous, class I MHC receptors that are expressed on natural killer cells and some T cells. The functional similarity of these two families has been borne out by the demonstration of identical signal transduction pathways associated with each receptor family. The Ly49 family therefore provides a useful model system to study the role of this dass of receptors in the regulation of the immune system. Recent data relating to the Ly49 repertoire in several mouse strains has revealed an additional evolutionary parallel between KIR and Ly49 receptor families. There is now an appreciation of the variation in the number and type of Ly49s expressed in different mouse strains, similar to the previously demonstrated differences in the number of KIR genes found in humans. This review summarizes the current members of the Ly49 gene family, their MHC class I recognition and associated signal transduction pathways.

Activating Ly-49 NK receptors: central role in cytokine and chemokine production

In an attempt to understand potential novel functions of receptors in vivo, we evaluated gene expression after cross-linking the activating Ly-49D mouse NK receptor. Gene expression was evaluated using a mouse GEM 2 microarray chip (Incyte Genomics, St. Louis, MO). Each chip displays a total of 8734 elements. The strongly induced genes fell into two categories: 1) soluble factors and 2) apoptotic genes. The majority of the strongly induced mRNAs as analyzed by microarray hybridization were chemokine genes. RNase protection assays and chemokine protein production analysis validated the microarray results, as cross-linking the Ly-49D mouse NK receptor induced high levels of IFN-gamma, lymphotactin, macrophage-inflammatory protein (MIP)1alpha, and MIP1beta. This gene expression was specific because other chemokines were not induced by anti-Ly-49D receptors. In addition, a series of pharmacological inhibitors were used to identify the key signaling pathways involved in the cellular response. The primary Ly-49D signaling for IFN-gamma production is predominantly mediated through Src kinase pathways involving membrane proximal events, whereas MIP1alpha and MIP1beta gene induction is more complex and may involve multiple biochemical pathways. Thus, we conclude that a primary role for the activating NK receptors in vivo may be to trigger soluble factor production and regulation of the immune response. This would place NK cells and their activating Ly-49 receptors as important initiators of microbial immunity and key elements of the innate immune system.

Class I MHC-binding characteristics of the 129/J Ly49 repertoire

The Ly49 family of NK cell receptors and its MHC-binding characteristics have only been well characterized in C57BL/6 (B6) mice. Previous studies have shown that 129/J mice express unique Ly49 genes that are not found in the B6 strain. Screening of a 129/J cDNA library led to the discovery of 10 distinct full-length Ly49-related coding sequences (Ly49e, g, i, o, p, r, s, t, u, and v). Although 129/J mice share identical class I MHC (K(b) and D(b)) transcripts with B6 mice, only one Ly49 is identical in the two strains (Ly49E). In addition to the previously characterized Ly49P, two new activating Ly49 proteins were discovered, Ly49R and U. The MHC specificity of the total 129/J Ly49 repertoire was evaluated with soluble class I MHC tetramers and found to be distinct compared with the B6 Ly49 repertoire. Ly49V bound to many types of class I MHC, suggesting that Ly49V(+) NK cells may monitor host cells for a global down-regulation in MHC levels. An activating receptor, Ly49R, was shown to bind soluble class I molecules to a moderate degree, a result not previously observed for other activating Ly49 proteins. Furthermore, tetramer-binding results were confirmed functionally with cytotoxicity assays using sorted 129/J NK cells. This study shows that the Ly49 repertoire and its MHC-binding characteristics can be very different among inbred mouse strains. Ly49 divergence should be considered when using 129-derived embryonic stem cells for the production of gene-targeted mice, especially when an immune or NK-derived phenotype is under scrutiny.

Identification of the Ly49L protein: evidence for activating counterparts to inhibitory Ly49 proteins

Previous studies have indicated that NK cells from different strains of inbred mice may express distinct Ly49 repertoires. Screening of NK cells from the CBA/J mouse for inhibitory and activating Ly49s revealed a novel DAP12-associated receptor that was immunoprecipitated with the Ly49G-specific mAb 4D11. Degenerate primers were designed to amplify and clone Ly49 cDNAs from CBA/J NK cells. A novel activating Ly49 cDNA was identified, which bears strong homology to the partially sequenced Ly49l gene found in C57BL/6 mice. Transfection of Ly49l into a DAP12+ cell line and subsequent immunoprecipitation experiments showed that Ly49L is likely the activating Ly49 detected by the 4DD11 antibody in CBA/J NK cells. Antibody-mediated cross-linking of Ly49L induced DAP12 phosphorylation, providing evidence that Ly49L is a functional activating receptor. Comparison of the extracellular domains of Ly49 family members indicates that all known activating members have an inhibitory counterpart with a highly related extracellular region.

Activating Ly-49D NK receptors: expression and function in relation to ontogeny and Ly-49 inhibitor receptors

Developmental changes in the repertoire of activating Ly-49 family members have not been examined previously. In the present study, we have examined the expression and function of the activating Ly-49s (D and H) from birth through 8 weeks of age. We demonstrate that 1) activating Ly-49s are expressed early, 2) their expression intensity is not different from adult NK cells, and 3) activating receptors are functional. Examination of the inhibitory Ly-49s also demonstrated functional capacity immediately upon expression. To examine the kinetics of expression of the repertoire of activating Ly-49 members, we utilized five- and six-color flow cytometric analyses of NK cells from birth through adulthood. Previous studies examining the inhibitory Ly-49 repertoire have proposed that expression is regulated by the product rule. Our results indicated that Ly-49D, which recognizes H-2Dd, had a discordantly high coexpression of the inhibitory Ly-49s that recognized H-2Dd (Ly-49A and Ly-49G2). The product rule of Ly-49 expression does not explain the coexpression of selected activating and inhibitory receptors. This high level of coexpression of H-2Dd recognizing activating and inhibitory Ly-49s suggests an in vivo selection or regulated coexpression.

In vivo administration of IL-18 can induce IgE production through Th2 cytokine induction and up-regulation of CD40 ligand (CD154) expression on CD4+ T cells

IL-18 is considered to be a strong cofactor for CD4+ T helper 1 (Th1) cell induction. We have recently reported that IL-18 can induce IL-13 production in both NK cells and T cells in synergy with IL-2 but not IL-12, suggesting IL-18 can induce Th1 and Th2 cytokines when accompanied by the appropriate first signals for T cells. We have now found that IL-18 can act as a cofactor to induce IL-4, IL-10 and IL-13 as well as IFN-gamma production in T cells in the presence of anti-CD3 monoclonal antibodies (mAb). IL-18 can rapidly induce CD40 ligand (CD154) mRNA and surface expression on CD4+ but not CD8+ T cells. The administration of IL-18 alone in vivo significantly increased serum IgE levels in C57BL/6 (B6) and B6 IL-4 knockout mice. Furthermore, the administration of IL-18 plus IL-2 induced approximately 70-fold and 10-fold higher serum levels of IgE and IgG1 than seen in control B6 mice, respectively. IgE and IgG1 induction in B6 mice by administration of IL-18 plus IL-2 was eliminated by the pretreatment of mice with anti-CD4 or anti-CD154, but not anti-CD8 or anti-NK1.1 mAb. These results suggest that IL-18 can induce Th2 cytokines and CD154 expression, and can contribute to CD4+ T cell-dependent, IL-4-independent IgE production.

The virus-specific and allospecific cytotoxic T-lymphocyte response to lymphocytic choriomeningitis virus is modified in a subpopulation of CD8(+) T cells coexpressing the inhibitory major histocompatibility complex class I receptor Ly49G2

The role of negatively signaling NK cell receptors of the Ly49 family on the specificity of the acute CD8(+) cytotoxic T-lymphocyte (CTL) response was investigated in lymphocytic choriomeningitis virus (LCMV)-infected C57BL/6 mice. Activated CD8(+) T cells coexpressing Ly49G2 expanded during LCMV infection, and T-cell receptor analyses by flow cytometry and CDR3 spectratyping revealed a unique polyclonal T-cell population in the Ly49G2(+) fraction. These cells lysed syngeneic targets infected with LCMV or coated with two of three LCMV immunodominant peptides examined. Transfection of these sensitive targets with H2D(d), a ligand for Ly49G2, inhibited lysis. This was reversed by antibody to Ly49G2, indicating effective negative signaling. LCMV characteristically induces an anti-H2(d) allospecific T-cell response that includes T-cell clones cross-reactive between allogeneic and LCMV-infected syngeneic targets. The CD8(+) Ly49G2(+) population mediated no allospecific killing, nor was any NK-like killing observed against YAC-1 cells. This study shows that CD8(+) Ly49G2(+) cells participate in the virus-induced CTL response but lyse a more restricted range of targets than the rest of the virus-induced CTL population.

In vivo administration of IL-18 can induce IgE production through Th2 cytokine induction and up-regulation of CD40 ligand (CD154) expression on CD4+ T cells

IL-18 is considered to be a strong cofactor for CD4+ T helper 1 (Th1) cell induction. We have recently reported that IL-18 can induce IL-13 production in both NK cells and T cells in synergy with IL-2 but not IL-12, suggesting IL-18 can induce Th1 and Th2 cytokines when accompanied by the appropriate first signals for T cells. We have now found that IL-18 can act as a cofactor to induce IL-4, IL-10 and IL-13 as well as IFN-gamma production in T cells in the presence of anti-CD3 monoclonal antibodies (mAb). IL-18 can rapidly induce CD40 ligand (CD154) mRNA and surface expression on CD4+ but not CD8+ T cells. The administration of IL-18 alone in vivo significantly increased serum IgE levels in C57BL/6 (B6) and B6 IL-4 knockout mice. Furthermore, the administration of IL-18 plus IL-2 induced approximately 70-fold and 10-fold higher serum levels of IgE and IgG1 than seen in control B6 mice, respectively. IgE and IgG1 induction in B6 mice by administration of IL-18 plus IL-2 was eliminated by the pretreatment of mice with anti-CD4 or anti-CD154, but not anti-CD8 or anti-NK1.1 mAb. These results suggest that IL-18 can induce Th2 cytokines and CD154 expression, and can contribute to CD4+ T cell-dependent, IL-4-independent IgE production.

Synergistic effects of in vivo depletion of Ly-49A and Ly-49G2 natural killer cell subsets in the rejection of H2(b) bone marrow cell allografts

Subsets of murine natural killer (NK) cells exist that express the Ly-49 family of molecules that recognize different major histocompatibility complex (MHC) determinants. Bone marrow transplantation studies were performed to examine the in vivo functions of 2 of these subsets. Subsets of Ly-49A and Ly-49G2 NK share specificity for the same MHC class 1 ligand, D(d), binding of which results in an inhibitory signal to the NK cell but allows them to lyse H2(b) targets in vitro. We therefore examined the ability of these subsets to reject H2(b) bone marrow cell allografts in lethally irradiated mice. Surprisingly, depletion of Ly-49A(+) NK cells in BALB/c or B10.D2 mice (both H2(d)) had no effect on the rejection of H2(b) BMC. However, Ly-49A depletion did partially abrogate the ability of B10.BR (H2(k)) mice to reject H2(b) allografts. Although depletion of either Ly-49A(+) or Ly-49G2(+) NK cells alone had no effect on the ability of B10.D2 mice to reject H2(b) BMC, depletion of both subsets dramatically and synergistically abrogated rejection. Studies with various B10 congenic mice and their F(1) hybrids indicate that this synergy between Ly49A and Ly4G2 depletion occurs in every instance. Thus, Ly-49A(+) NK cells appear to play a role in the rejection H2(b) bone marrow allografts, but, in most strains of mice studied, Ly-49G2(+) NK cells must also be eliminated. The putative roles of these NK cell subsets in clinical transplantation remains to be elucidated. (Blood. 2000;95:3840-3844)

IL-2 signaling in human monocytes involves the phosphorylation and activation of p59hck

The activating properties of IL-2 and the structure of the IL-2R on human monocytes are well characterized. However, relatively little is known about the biochemical mechanisms involved in IL-2 signal transduction in these cells. We investigated the role of protein tyrosine kinases (PTKs) in the activation of monocytes by IL-2. Incubation of monocytes with the PTK inhibitor herbimycin A (HA) resulted in the dose-dependent suppression of IL-2-induced monocyte tumoricidal activity. This inhibition was rather potent, as a concentration of HA as low as 0.5 microM caused a complete abrogation of cytolytic activity. Furthermore, HA markedly suppressed the ability of IL-2 to induce IL-1 beta, TNF-alpha, IL-6, and IL-8 mRNA expression and protein secretion by monocytes. Anti-phosphotyrosine immunoblotting demonstrated that IL-2 induced a rapid and time-dependent increase in tyrosine phosphorylation of several cellular proteins of molecular masses ranging from 35 to 180 kDa. Interestingly, IL-2 caused a significant up-regulation of the constitutive levels of hck PTK mRNA and protein relative to medium-treated cells as well as an increase in p59hck tyrosine phosphorylation. Finally, we demonstrated by in vitro kinase assay that the specific activity of p59hck PTK was also induced by IL-2 in monocytes. Thus, these data show that the activation of PTKs is required for the triggering of monocyte effector and secretory functions by IL-2 and strongly suggest that p59hck is a key participant in IL-2 signaling in human monocytes.

Interaction of Ly-49D+ NK cells with H-2Dd target cells leads to Dap-12 phosphorylation and IFN-gamma secretion

Murine Ly-49D augments NK cell function upon recognition of target cells expressing H-2Dd. Ly-49D activation is mediated by the immunoreceptor tyrosine-based activation motif-containing signaling moiety Dap-12. In this report we demonstrate that Ly-49D receptor ligation can lead to the rapid and potent secretion of IFN-gamma. Cytokine secretion can be induced from Ly-49D+ NK cells after receptor ligation with Ab or after interaction with target cells expressing their H-2Dd ligand. Consistent with the dominant inhibitory function of Ly-49G, NK cells coexpressing Ly-49D and Ly-49G show a profound reduction in IFN-gamma secretion after interaction with targets expressing their common ligand, H-2Dd. Importantly, we are able to demonstrate for the first time that effector/target cell interactions using Ly-49D+ NK cells and H-2Dd targets result in the rapid phosphorylation of Dap-12. However, Dap-12 is not phosphorylated when Ly-49D+ NK cells coexpress the inhibitory receptor, Ly-49G. These studies are novel in describing Ly-49 activation vs inhibition, where two Ly-49 receptors recognize the same class I ligand, with the dominant inhibitory receptor down-regulating phosphorylation of Dap-12, cytokine secretion, and cytotoxicity in NK cells.

Structure/function relationship of activating Ly-49D and inhibitory Ly-49G2 NK receptors

Murine NK cells express Ly-49 family receptors capable of either inhibiting or activating lytic function. The overlapping patterns of expression of the various receptors have complicated their precise biochemical characterization. Here we describe the use of the Jurkat T cell line as the model for the study of Ly-49s. We demonstrate that Ly-49D is capable of delivering activation signals to Jurkat T cells even in the absence of the recently described Ly-49D-associated chain, DAP-12. Ly-49D signaling in Jurkat leads to tyrosine phosphorylation of TCRzeta and requires Syk/Zap70 family kinases and arginine 54 of Ly-49D, suggesting that Ly-49D signals via association with TCRzeta. Coexpression studies in 293-T cells confirmed the ability of Ly-49D to associate with TCRzeta. In addition, we have used this model to study the functional interactions between an inhibitory Ly-49 (Ly-49G2) and an activating Ly-49 (Ly-49D). Ly-49G2 blocks activation mediated by Ly-49D in an immunoreceptor tyrosine-based inhibitory motif (ITIM)-dependent manner. In contrast, Ly-49G2 was incapable of inhibiting activation by the TCR even though human killer cell inhibitory receptor (KIR) (KIR3DL2(GL183)) effectively inhibits TCR. Both the ability of Ly-49G2 to block Ly-49D activation and the failure of Ly-49G2 to inhibit TCR signaling were confirmed in primary murine NK cells and NK/T cells, respectively. These data demonstrate the dominant effects of the inhibitory receptors over those that activate and suggest an inability of the Ly-49 type II inhibitory receptors to efficiently inhibit type I transmembrane receptor signaling in T cells and NK cells.

Structure/Function Relationship of Activating Ly-49D and Inhibitory Ly-49G2 NK Receptors

Murine NK cells express Ly-49 family receptors capable of either inhibiting or activating lytic function. The overlapping patterns of expression of the various receptors have complicated their precise biochemical characterization. Here we describe the use of the Jurkat T cell line as the model for the study of Ly-49s. We demonstrate that Ly-49D is capable of delivering activation signals to Jurkat T cells even in the absence of the recently described Ly-49D-associated chain, DAP-12. Ly-49D signaling in Jurkat leads to tyrosine phosphorylation of TCRζ and requires Syk/Zap70 family kinases and arginine 54 of Ly-49D, suggesting that Ly-49D signals via association with TCRζ. Coexpression studies in 293-T cells confirmed the ability of Ly-49D to associate with TCRζ. In addition, we have used this model to study the functional interactions between an inhibitory Ly-49 (Ly-49G2) and an activating Ly-49 (Ly-49D). Ly-49G2 blocks activation mediated by Ly-49D in an immunoreceptor tyrosine-based inhibitory motif (ITIM)-dependent manner. In contrast, Ly-49G2 was incapable of inhibiting activation by the TCR even though human killer cell inhibitory receptor (KIR) (KIR3DL2(GL183)) effectively inhibits TCR. Both the ability of Ly-49G2 to block Ly-49D activation and the failure of Ly-49G2 to inhibit TCR signaling were confirmed in primary murine NK cells and NK/T cells, respectively. These data demonstrate the dominant effects of the inhibitory receptors over those that activate and suggest an inability of the Ly-49 type II inhibitory receptors to efficiently inhibit type I transmembrane receptor signaling in T cells and NK cells.