2B4 (CD244)-mediated activation of cytotoxicity and IFN-gamma release in human NK cells involves distinct pathways

Estrogen induction of the granzyme B inhibitor, proteinase inhibitor 9, protects cells against apoptosis mediated by cytotoxic T lymphocytes and natural killer cells

Exposure to estrogens is associated with an increased risk of developing breast, cervical, and liver cancer. Estrogens strongly induce the human granzyme B inhibitor, proteinase inhibitor 9 (PI-9). Because cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells use the granzyme pathway to induce apoptosis of target cells, we tested the ability of activated CTLs and the human NK cell line, YT cells, to lyse human liver cells. Estrogen induction of PI-9 protected the liver cells against CTL and NK cell-mediated, granzyme-dependent, apoptosis. Knockdown of PI-9 by RNA interference blocked the protective effect of estrogen. This work demonstrates that estrogens can act on target cells to control their destruction by immune system cells and shows that induction of PI-9 expression can inhibit both CTL and NK cell-mediated apoptosis. Estrogen induction of PI-9 may reduce the ability of cytolytic lymphocytes-mediated immune surveillance to destroy newly transformed cells, possibly providing a novel mechanism for an estrogen-mediated increase in tumor incidence.

The CD2 family of natural killer cell receptors

The CD2 family of receptors is evolutionarily conserved and widely expressed on cells within the hematopoietic compartment. In recent years several new members have been identified with important roles in the immune system. CD2 family members regulate natural killer (NK) cell lytic activity and inflammatory cytokine production when engaged by ligands on tumor cells. Furthermore, a subfamily of CD2 receptors, the CD 150-like molecules, has been implicated in the pathogenesis of X-linked lymphoproliferative disease (XLP). Many of these receptors have now been shown to bind homophilically or heterophilically to other molecules within the family. With these discoveries a novel mechanism for lymphocyte regulation has emerged: CD2 family members on NK cells engage ligands on neighboring NK cells, leading to NK cell stimulation. Moreover, heterotypic stimulatory interactions between NK cells and other leukocytes also occur. In this manner, CD2 family members may provide interlymphocyte communication that maintains organization within the hematopoietic compartment and amplifies immune responses. This review discusses these multiple roles for CD2 family members, focusing specifically on the regulation of NK cells.

Strategies of natural killer cell recognition and signaling

The participation of natural killer (NK) cells in multiple aspects of innate and adaptive immune responses is supported by the wide array of stimulatory and inhibitory receptors they bear. Here we review the receptor-ligand interactions and subsequent signaling events that culminate in NK effector responses. Whereas some receptor-ligand interactions result in activation of both NK cytotoxicity and cytokine production, others have more subtle effects, selectively activating only one pathway or having distinct context-dependent effects. Recent approaches offer ways to unravel how the integration of complex signaling networks directs the NK response.

Modulation of natural killer (NK) receptors on NK (CD3-/CD56+), T (CD3+/CD56-) and NKT-like (CD3+/CD56+) cells after heart transplantation

BACKGROUND

After undergoing heart transplantation and the subsequent compulsive immunosuppressive treatments, patients are at risk of rejection episodes, infectious complications or cancer development. Thus, it is probable that the various subsets of peripheral cytotoxic lymphocytes are modulated in such patients. This area of study can now be investigated by examining the numerous recently described natural killer (NK)-cell-related surface receptors.

METHODS

A prospective cohort of 60 heart transplant recipients and 60 controls was studied. The partitioning of lymphocyte subsets, especially NK (CD3-/CD56+), T (CD3+/CD56-) and NKT-like (CD3+/CD56+) cells, was compared in both groups using multi-parametric flow cytometry. Moreover, expression of a series of seven NK-related receptors was compared on the three subsets defined by CD56 expression.

RESULTS

A significant increase in NK-cell levels was observed in transplanted patients, as compared with controls, whereas T and NKT-like cells were in similar proportions in both groups. Two NK-related receptors showed significantly different levels of expression in heart transplant recipients: the cytotoxic effector, CD244, which was in a significantly increased proportion on T and NKT-like cells; and the activating receptor, CD161, which was expressed significantly less on NK and NKT-like cells, but more on T cells.

CONCLUSIONS

These findings indicate that cytotoxic NK-related cells, increased in proportion, also display increased levels of activity-associated markers in heart transplant recipients. Viral infection or the immunosuppressive regimen could be responsible for the modulation of regulatory receptors on NK and NKT-like cells in heart transplant recipients.

Signal transduction in natural killer cells

Tolerance of natural killer (NK) cells toward normal cells is mediated through their expression of inhibitory receptors that detect the normal expression of self in the form of class I major histocompatibility complex (MHC-I) molecules on target cells. These MHC-I-binding inhibitory receptors recruit tyrosine phosphatases, which are believed to counteract activating receptor-stimulated tyrosine kinases. The perpetual balance between signals derived from inhibitory and activating receptors controls NK cell responsiveness and provides an interesting paradigm of signaling cross talk. This review summarizes our knowledge of the intracellular mechanisms by which cell surface receptors influence biological responses by NK cells. Special emphasis focuses on the dynamic signaling events at the NK immune synapse and the unique signaling characteristics of specific receptors, such as NKG2D, 2B4, and KIR2DL4.

The adaptor protein 3BP2 binds human CD244 and links this receptor to Vav signaling, ERK activation, and NK cell killing

Adaptor proteins, molecules that mediate intermolecular interactions, are crucial for cellular activation. The adaptor 3BP2 has been shown to positively regulate NK cell-mediated cytotoxicity. In this study we present evidence for a physical interaction between 3BP2 and the CD244 receptor. CD244, a member of the CD150 family, is a cell surface protein expressed on NK, CD8+ T, and myeloid cells. CD244 interacts via its Src homology 2 domain with the X-linked lymphoproliferative disease gene product signaling lymphocytic activation molecule-associated protein (SAP)/SH2 domain protein 1A. 3BP2 interacts with human but not murine CD244. CD244-3BP2 interaction was direct and regulated by phosphorylation, as shown by a three-hybrid analysis in yeast and NK cells. Tyr337 on CD244, part of a consensus motif for SAP/SH2 domain protein 1A binding, was critical for the 3BP2 interaction. Although mutation of Tyr337 to phenylalanine abrogated human 3BP2 binding, we still observed SAP association, indicating that this motif is not essential for SAP recruitment. CD244 ligation induced 3BP2 phosphorylation and Vav-1 recruitment. Overexpression of 3BP2 led to an increase in the magnitude and duration of ERK activation, after CD244 triggering. This enhancement was concomitant with an increase in cytotoxicity due to CD244 ligation. However, no differences in IFN-gamma secretion were found when normal and 3BP2-transfected cells were compared. These results indicate that CD244-3BP2 association regulates cytolytic function but not IFN-gamma release, reinforcing the hypothesis that, in humans, CD244-mediated cytotoxicity and IFN-gamma release involve distinct NK pathways.

Insights into seven and single transmembrane-spanning domain receptors and their signaling pathways in human natural killer cells

Human natural killer (NK) cells are important cells of the innate immune system. These cells perform two prominent functions: the first is recognizing and destroying virally infected cells and transformed cells; the second is secreting various cytokines that shape up the innate and adaptive immune re-sponses. For these cells to perform these activities, they express different sets of receptors. The receptors used by NK cells to extravasate into sites of injury belong to the seven transmembrane (7TM) family of receptors, which characteristically bind heterotrimeric G proteins. These receptors allow NK cells to sense the chemotactic gradients and activate second messengers, which aid NK cells in polarizing and migrating toward the sites of injured tissues. In addition, these receptors determine how and why human resting NK cells are mainly found in the bloodstream, whereas activated NK cells extravasate into inflammatory sites. Receptors for chemokines and lysophospholipids belong to the 7TM family. On the other hand, NK cells recognize invading or transformed cells through another set of receptors that belong to the single transmembrane-spanning domain family. These receptors are either inhibitory or activating. Inhibitory receptors contain the immune receptor tyrosine-based inhibitory motif, and activating receptors belong to either those that associate with adaptor molecules containing the immune receptor tyrosine-based activating motif (ITAM) or those that associate with adaptor molecules containing motifs other than ITAM. This article will describe the nature of these receptors and examine the intracellular signaling pathways induced in NK cells after ligating both types of receptors. These pathways are crucial for NK cell biology, development, and functions.

Distinctive Lack of CD48 Expression in Subsets of Human Dendritic Cells Tunes NK Cell Activation

CD48 is a glycosyl phosphatidylinositol anchor protein known to be virtually expressed by all human leukocytes. Its ligand, 2B4, is a signaling lymphocyte activation molecule-related receptor involved in NK cell activation. Because dendritic cells (DCs) are strong inducers of NK cell functions, we analyzed the expression of CD48 in different human DC subsets. We observed that monocytes differentiating in DCs promptly down-regulate CD48. Similarly, DCs isolated from inflamed lymph nodes generally do not express CD48. Plasmocytoid DCs do not express CD48 either, whereas myeloid DCs harbored in blood, bone marrow, and thymus express it. In addition, we showed that CD48 expression in DCs affects NK cell functions during NK/DC cross-talk, because NK cells obtained from normal donors and from X-linked lymphoproliferative disease patients are, respectively, triggered or inhibited by DCs expressing surface CD48. Remarkably, IFN-gamma production by lymph node NK cells, in contrast to blood NK cells, can be negatively modulated by 2B4/CD48 interactions, indicating a 2B4 inhibitory pathway in lymph node NK cells. Therefore, the CD48 deficiency of DCs harbored in inflamed lymph nodes that we report in this study might be relevant to successfully activate lymph node NK cells in the early phase of the immune response. Our results show that distinct subsets of human DCs, differently from all other mononuclear hemopoietic cells, specifically do not express CD48. Moreover, the expression of CD48 depends on the anatomic location of DCs and might be related to the tissue-specific 2B4 function (activating or inhibitory) of the NK cells with which they interact.

Role for glycogen synthase kinase-3 in NK cell cytotoxicity and X-linked lymphoproliferative disease

NK cells from individuals with X-linked lymphoproliferative (XLP) disease exhibit functional defects when stimulated through the NK receptor, 2B4 (CD244). These defects are likely a consequence of aberrant intracellular signaling initiated by mutations of the adaptor molecule SLAM-associated protein. In this report, we show that NK cells from individuals with XLP but not healthy individuals fail to phosphorylate and thereby inactivate glycogen synthase kinase-3 (GSK-3) following 2B4 stimulation. Lack of GSK-3 phosphorylation prevented the accumulation of the transcriptional coactivator beta-catenin in the cytoplasm and its subsequent translocation to the nucleus. Potential signaling pathways leading from 2B4 stimulation to GSK-3 phosphorylation were also investigated. Ligation of 2B4 resulted in the phosphorylation of the guanine nucleotide exchange factor, Vav-1, and subsequent activation of the GTP-binding protein Rac-1 (but not Ras) and the serine-threonine kinase Raf-1 in healthy but not XLP-derived NK cells. In addition, the activity of MEK-2 (but not MEK-1) was up-regulated, and Erk1/2 was phosphorylated in normal NK cells but not those from an individual with XLP suggesting that these proteins relay SLAM-associated protein-dependent signals from 2B4. Finally, inactivation of GSK-3 using a specific inhibitor of GSK-3beta increased the cytotoxicity and cytokine secretion of both healthy and XLP NK cells. These data indicate that the signaling of 2B4 in NK cells is mediated by GSK-3 and beta-catenin, possibly through a signal transduction pathway that involves Vav-1, Rac-1, Raf-1, MEK-2, and Erk1/2 and that this pathway is aberrant in individuals with XLP.

Identification of Grb2 As a Novel Binding Partner of the Signaling Lymphocytic Activation Molecule-Associated Protein Binding Receptor CD229

Ag recognition by the TCR determines the subsequent fate of the T cell and is regulated by the involvement of other cell surface molecules, termed coreceptors. CD229 is a lymphocyte cell surface molecule that belongs to the CD150 family of receptors. Upon tyrosine phosphorylation, CD229 recruits various signaling molecules to the membrane. One of these molecules is the signaling lymphocytic activation molecule-associated protein, of which a deficiency leads to the X-linked lymphoproliferative syndrome. We report that CD229 interacts in a phosphorylation-dependent manner with Grb2. We mapped this interaction showing that the Src homology 2 domain of Grb2 and the tyrosine residue Y606 in CD229 are required for CD229-Grb2 complex formation. The Grb2 motif in the cytoplasmic tail of CD229 is distinct and independent from the two tyrosines required for efficient signaling lymphocytic activation molecule-associated protein recruitment. CD229, but not other members of the CD150 family, directly bound Grb2. We also demonstrate that CD229 precipitates with Grb2 in T lymphocytes after pervanadate treatment, as well as CD229 or TCR ligation. Interestingly, the CD229 mutant lacking the Grb2 binding site is not internalized after CD229 engagement with specific Abs. Moreover, a dominant negative form of Grb2 (containing only Src homology 2 domain) impaired CD229 endocytosis. Unexpectedly, Erk phosphorylation was partially inhibited after activation of CD229 plus CD3. Consistent with this, CD229 ligation partially inhibited TCR signaling in peripheral blood cells and CD229-Jurkat cells transfected with the 3XNFAT-luciferase reporter construct. Altogether, the data suggest a model whereby CD229 ligation attenuates TCR signaling and Grb2 recruitment to CD229 controls its rate of internalization.

Targeted disruption of the 2B4 gene in mice reveals an in vivo role of 2B4 (CD244) in the rejection of B16 melanoma cells

Murine 2B4 (CD244) is a cell surface receptor expressed on all NK cells, gammadelta-T cells, a subset of CD8(+) T cells, and all CD14(+) monocytes. 2B4 binds to CD48 with high affinity, and cross-linking 2B4 with anti-2B4 Ab in vitro causes activation of NK cells. To study its physiological role, we have generated, by gene targeting, mice deficient in the expression of this cell surface molecule. The expression of lymphoid cell surface markers on PBMC and splenocytes of mice homozygous for the mutation in 2B4 (2B4(-/-)) is identical to that in wild-type mice. However, thymocytes from female 2B4(-/-) mice, but not male 2B4(-/-) mice, have an increase in the immature CD4(-)/CD8(-) population. To investigate the in vivo role of 2B4, wild-type and 2B4(-/-) mice were injected with CD48(+) and CD48(-) metastatic B16 melanoma cells. Wild-type mice rejected CD48(+) melanoma poorly compared with CD48(-) tumor cells, suggesting that ligation of 2B4 by CD48 on melanoma cells is inhibitory. In keeping with this, male 2B4(-/-) mice showed enhanced ability to reject CD48(+) melanoma cells. However, female 2B4(-/-) mice poorly rejected both CD48(+) and CD48(-) melanoma cells, revealing a gender-specific and CD48-independent defect in mice lacking 2B4. In vitro and in vivo experiments reveal a complex role of NK cells in gender specificity.

Anatomy of a murder—signal transduction pathways leading to activation of natural killer cells

Natural killer (NK) cells control the early phases of viral infections, modulate antigen-specific immune responses, and participate in the rejection of tumours and bone marrow grafts. A fine balance between inhibitory and activating receptors tightly regulates NK cell activation. Biochemical studies in human cell lines and primary cells have revealed some of the activating NK cell signalling pathways, however animal models are instrumental to understand the physiological implications of these findings for immune responses in vivo. Gene targeting in mice and biochemical studies in cells are helping to dissect out the various signal transduction pathways that control NK cell activation. A clearer view of these pathways may eventually help designing more effective immune therapies based on the use of NK cells.

2B4 (CD244) is expressed and functional on human eosinophils

Eosinophils are present in parasitic, allergic, various immunological, and malignant disorders as well as in a variety of idiopathic hypereosinophilic syndromes. However, their exact role in some of these conditions remains elusive. They can be activated both in vivo and in vitro by various agonists, such as Igs, lipid mediators, and cytokines. By phenotyping the surface of the eosinophils, it may be possible to better define their function(s) in different pathophysiological settings. In the present work we screened eosinophils with a panel of Abs recognizing CD2 subfamily receptors usually present on a number of hemopoietic cells. We have demonstrated that human peripheral blood eosinophils, but not basophils or neutrophils, express NTB-A. In addition eosinophils express 2B4, CD84, CD58, and CD48, but not signaling lymphocytic activation molecule or CD2, on their surface (FACS). Cross-linking of 2B4 on eosinophils elicited a significant release of eosinophil peroxidase (30 min), IFN-gamma, and IL-4 (18 h). Moreover, activation of eosinophils via 2B4 induced eosinophil-mediated cytotoxicity toward two malignant cell lines, i.e., mouse mastocytoma P815 and EBV-infected 721.221 B cell lines. Cross-linking of 2B4 on the surface of eosinophils or pervenadate treatment elicited ERK and tyrosine phosphorylation, respectively. Furthermore, we showed that eosinophils express slam-associated protein. The demonstration that human eosinophils express a functional 2B4 receptor indicates a broader role for these cells in health and disease.

The SAP family of adaptors in immune regulation

SAP and EAT-2 define a new class of adaptor proteins composed almost exclusively of a Src homology 2 (SH2) domain. By way of their SH2 domain, SAP-like adaptors interact with tyrosine-based motifs in the cytoplasmic region of SLAM-related receptors, a family of immune cell-specific molecules involved in immunoregulation. Recent findings indicate that SAP is required for the functions of SLAM family receptors, as a consequence of its ability to promote recruitment of Src-related protein tyrosine kinase FynT and allow SLAM-related receptors to transduce tyrosine phosphorylation signals. SAP is mutated in X-linked lymphoproliferative (XLP) syndrome, a rare inherited human disease characterized by an deregulated immune response to Epstein-Barr virus infection. Several lines of evidence indicate that defects in the activities of SLAM-related receptors caused by SAP deficiency account for the immune dysfunctions associated with XLP.

Regulation of FasL expression in natural killer cells

Fas ligand (FasL)-mediated cytotoxicity is initiated in natural killer (NK) cells through ligation of their activating receptors. The CD16 receptor has been shown to induce FasL expression and cytotoxicity in NK cells. In this study, we made the novel observation that FasL expression was upregulated in NKL cells stimulated through 2B4 and LFA-1 activating receptors, implying a role for FasL-mediated cytotoxicity early in the immune response. Coligation with CD94/NKG2A human leukocyte antigen (HLA) class I inhibitory receptor did not block the induced FasL expression; therefore, these opposing pathways appear to function independently. We also showed, however, that FasL-mediated cytotoxicity was downregulated in CD94/NKG2A-expressing LAK cells in response to the HLA-E ligand, suggesting a mechanism by which aberrant cells expressing class I may evade FasL-mediated cytotoxicity. Thus we show for the first time that 2B4, LFA-1, and CD94/NKG2A receptors are involved in modulating FasL expression and, therefore, cytotoxicity mediated by NK cells.

Dynamic redistribution of the activating 2B4/SAP complex at the cytotoxic NK cell immune synapse

The 2B4 molecule (CD244) has been described as a coreceptor in human NK cell activation. However, the behavior of 2B4 during the cytotoxic NK cell immune synapse (NK-IS) formation remains undetermined. In this study, we demonstrate the redistribution of 2B4 and the signaling adaptor molecule, signaling lymphocyte activation molecule-associated protein (SAP), to the cytotoxic NK-IS upon formation of conjugates between resting NK cells and EBV-infected 721.221 human cells. Confocal microscopy showed that 2B4 localized at the central supramolecular activation cluster, surrounded by a peripheral supramolecular activation cluster containing talin within NK cell and ICAM-1 on target cells. Videomicroscopy studies with 2B4-GFP-transfected NK cells revealed that 2B4 redistributed to cytotoxic NK-IS as soon as the cell contact occurred. Simultaneously, a SAP-GFP also clustered at the contact site, where it remained during the interaction period. The 2B4 molecular clusters remained bound to the target cell even after NK cell detachment. These results underscore the function of 2B4 as an adhesion molecule and suggest a relevant role in the initial binding, scanning of target cells, and formation of cytotoxic NK-IS. Finally, these findings are indicative of an important role of the activating 2B4/signaling lymphocyte activation molecule-associated protein complex during the recognition of EBV-infected cells.

Stabilization of IFN-gamma mRNA by MAPK p38 in IL-12- and IL-18-stimulated human NK cells

The rapid induction of interferon-gamma (IFN-gamma) by innate cytokines such as interleukin 12 (IL-12) and IL-18 is critical for immunity against infectious pathogens. We investigated the molecular mechanisms underlying this response. IL-12 and IL-18 rapidly and synergistically induced the secretion of IFN-gamma by freshly purified human peripheral blood lymphocytes. At early time points, IFN-gamma was expressed almost exclusively by natural killer cells and in both CD56bright and CD56dim subpopulations. Mitogen-activated protein kinase p38 was activated strongly by IL-18 and weakly by IL-12 in natural killer cells but was not activated by either cytokine in T cells. The expression of IFN-gamma mRNA and protein was dose-dependently blocked by SB203580, a specific inhibitor of mitogen-activated protein kinase p38, which also caused a dramatic destabilization of IFN-gamma mRNA. The 3' untranslated region (UTR) of IFN-gamma mRNA conferred p38 responsiveness to a heterologous reporter mRNA. Therefore, the synergistic induction of IFN-gamma by IL-12 and IL-18 in natural killer cells is mediated at least in part by p38-dependent and 3' UTR-mediated stabilization of IFN-gamma mRNA.

Differential expression of SAP and EAT-2-binding leukocyte cell-surface molecules CD84, CD150 (SLAM), CD229 (Ly9) and CD244 (2B4)

The CD150 (SLAM) family consists of nine leukocyte cell-surface proteins involved in lymphocyte activation that belong to the immunoglobulin (Ig) superfamily. Six members of this family--CD84, CD150 (SLAM), CD229 (Ly9), CD244 (2B4), NTB-A, and CS1--associate with adapter proteins--SLAM-associated protein (SAP) and EAT-2. SAP is a short intracellular molecule that is mutated in humans with X-linked lymphoproliferative disease. Flow cytometric analysis of the expression of CD84, CD150, CD229, and CD244 cell-surface receptors on several leukocyte and lymphocyte subsets was performed. CD84 and CD150 were present on thymocytes, mature T cells and antigen-presenting cells. The expression of CD84 and CD150 was high on memory T cells. CD150 expression was strongly up-regulated after cell activation. In contrast to CD84, CD150 was absent on resting monocytes and immature dendritic cells (DCs). CD229 presented a pattern of expression restricted to lymphocytes. CD244 was preferentially expressed on natural killer cells, CD8(+) effector cells, resting monocytes, basophils, and eosinophils. We describe a broader distribution of CD84, CD150, CD229, and CD244 than previously reported and show that they are differentially expressed on hematopoietic cells. The heterogeneous expression of these receptors indicates that these molecules may play non-redundant functions in the regulation of both innate and adaptive immune responses.

2B4 Acts As a Non–Major Histocompatibility Complex Binding Inhibitory Receptor on Mouse Natural Killer Cells

Natural killer (NK) cells are critical in the immune response to tumor cells, virally infected cells, and bone marrow allografts. 2B4 (CD244) is expressed on all NK cells and the ligand for 2B4, CD48, is expressed on hematopoietic cells. Cross-linking 2B4 on NK cells with anti-2B4 monoclonal antibody leads to NK cell activation in vitro. Therefore, 2B4 is considered to be an activating receptor. Surprisingly, we have found, using antibody-blocking and 2B4-deficient NK cells, that NK lysis of CD48⁺ tumor and allogeneic targets is inhibited by 2B4 ligation. Interferon γ production by NK cells is also inhibited. Using a peritoneal tumor clearance assay, it was found that 2B4^−/− mice have increased clearance of CD48⁺ tumor cells in vivo. Retroviral transduction of 2B4 was sufficient to restore inhibition in 2B4^−/− primary NK cells. It was found that although mature NK cells express SH2D1A, in vitro–derived NK cells do not. However, both populations are inhibited by 2B4 ligation. This indicates that 2B4 inhibitory signaling occurs regardless of the presence of SH2D1A. These findings reveal a novel role for 2B4 as a non–major histocompatibility complex binding negative regulator of NK cells.

NK cells regulate CD8+ T cell effector function in response to an intracellular pathogen

We studied the role of NK cells in regulating human CD8+ T cell effector function against mononuclear phagocytes infected with the intracellular pathogen Mycobacterium tuberculosis. Depletion of NK cells from PBMC of healthy tuberculin reactors reduced the frequency of M. tuberculosis-responsive CD8+IFN-gamma+ cells and decreased their capacity to lyse M. tuberculosis-infected monocytes. The frequency of CD8+ IFN-gamma+ cells was restored by soluble factors produced by activated NK cells and was dependent on IFN-gamma, IL-15, and IL-18. M. tuberculosis-activated NK cells produced IFN-gamma, activated NK cells stimulated infected monocytes to produce IL-15 and IL-18, and production of IL-15 and IL-18 were inhibited by anti-IFN-gamma. These findings suggest that NK cells maintain the frequency of M. tuberculosis-responsive CD8+IFN-gamma+ T cells by producing IFN-gamma, which elicits secretion of IL-15 and IL-18 by monocytes. These monokines in turn favor expansion of Tc1 CD8+ T cells. The capacity of NK cells to prime CD8+ T cells to lyse M. tuberculosis-infected target cells required cell-cell contact between NK cells and infected monocytes and depended on interactions between the CD40 ligand on NK cells and CD40 on infected monocytes. NK cells link the innate and the adaptive immune responses by optimizing the capacity of CD8+ T cells to produce IFN-gamma and to lyse infected cells, functions that are critical for protective immunity against M. tuberculosis and other intracellular pathogens.

Transmembrane adaptor proteins in membrane microdomains: important regulators of immunoreceptor signaling

Membrane microdomains enriched in glycosphingolipids, cholesterol, glycosylphosphatidylinositol-anchored proteins and Src-family kinases (lipid rafts, GEMs) appear to play many important roles, especially in immunoreceptor signaling. Most transmembrane proteins are excluded from these specialized areas of membranes, notable exceptions being several palmitoylated proteins such as the T cell coreceptors CD4 and CD8, and several recently described transmembrane adaptor proteins, LAT, non-T cell activation linker (NTAL)/linker for activation of B cells (LAB), phosphoprotein associated with GEMs (PAG)/Csk-binding protein (Cbp) and LIME. All these molecules possess a very short N-terminal extracellular peptide (4-17 amino acids), transmembrane segment followed by a palmitoylation motif (CxxC) and cytoplasmic domain containing up to 10 tyrosine residues potentially phosphorylated by the Src- or Syk-family kinases. Tyrosine-phosphorylated transmembrane adaptors bind (directly via SH2 domains or indirectly) other signaling molecules such as several cytoplasmic adaptors and enzymes. LAT is indispensable for TCR signaling (and participates also at signal transduction initiated by some other receptors), NTAL/LAB appears to play a LAT-like role in signaling initiated by BCR and some Fc-receptors; PAG/Cbp cooperates with Csk, the cytoplasmic tyrosine kinase negatively regulating Src-family kinases. Additional transmembrane adaptors exist (TRIM, SIT, LAX) that are however not palmitoylated and therefore excluded from the lipid rafts; structurally and functionally, the zeta-chain family proteins tightly associated with immunoreceptors and activating NK-receptors may be also considered as transmembrane adaptors.

The SAP and SLAM families in immune responses and X-linked lymphoproliferative disease

SAP (signalling lymphocytic activation molecule (SLAM)-associated protein) is a T- and natural killer (NK)-cell-specific protein containing a single SH2 domain encoded by a gene that is defective or absent in patients with X-linked lymphoproliferative syndrome (XLP). The SH2 domain of SAP binds with high affinity to the cytoplasmic tail of the haematopoietic cell-surface glycoprotein SLAM and five related receptors. SAP regulates signal transduction of the SLAM-family receptors by recruiting SRC kinases. Similarly, the SAP-related proteins EAT2A and EAT2B are thought to control signal transduction that is initiated by SLAM-related receptors in professional antigen-presenting cells. In this review, we discuss recent findings on the structure and function of proteins of the SAP and SLAM families.

In vivo antitumor activity of NKT cells activated by the combination of IL-12 and IL-18

Interleukin-12 and IL-18 have been demonstrated to potentiate innate immunity in a variety of experimental tumor models, but the functional roles of NK and/or NKT cells and their mechanism of action in these models have not been fully addressed. Through adoptive transfer of NKT cells activated in vitro with a combination of IL-12 plus IL-18 (IL-12/IL-18 NKT) into syngeneic animals, we demonstrate in this study that IL-12/IL-18 NKT cells are essential and collaborate with the host's own NK cells in natural host immunity against the growth of ALC and MC57X syngeneic tumors. The relative roles of the adoptively transferred IL-12/IL-18 NKT cells and endogenous NK cells in host protection were first shown in normal C57BL/6 (B6) mice treated with anti-asialo GM1 Ab that selectively depletes NK cells; second, in B6.TCRJalpha281(-/-) mice specifically deficient for NKT cells; and third, in B6.scid mice that also lack NKT cells. Furthermore, by injecting normal B6 mice with anti-IL-2 and/or anti-IFN-gamma mAb, we could demonstrate that effective innate immunity against both types of syngeneic tumors was dependent on the production of IL-2 and IFN-gamma by the adoptively transferred NKT cells. In vitro studies confirmed both the secretion of IL-2 and IFN-gamma by the IL-12/IL-18-activated NKT cells and their collaborative role with NK cells for lysis of ALC and MC57X syngeneic tumor targets. This is the first description of an antitumor function of IL-12/IL-18 NKT cells adoptively transferred into syngeneic hosts that provides the basis for a new modality in the cellular immunotherapy of cancer.

Functional requirements for interactions between CD84 and Src homology 2 domain-containing proteins and their contribution to human T cell activation

Cell surface receptors belonging to the CD2 subset of the Ig superfamily of molecules include CD2, CD48, CD58, 2B4, signaling lymphocytic activation molecule (SLAM), Ly9, CD84, and the recently identified molecules NTB-A/Ly108/SLAM family (SF) 2000, CD84H-1/SF2001, B lymphocyte activator macrophage expressed (BLAME), and CRACC (CD2-like receptor-activating cytotoxic cells)/CS-1. Some of these receptors, such as CD2, SLAM, 2B4, CRACC, and NTB-A, contribute to the activation and effector function of T cells and NK cells. Signaling pathways elicited via some of these receptors are believed to involve the Src homology 2 (SH2) domain-containing cytoplasmic adaptor protein SLAM-associated protein (SAP), as it is recruited to SLAM, 2B4, CD84, NTB-A, and Ly-9. Importantly, mutations in SAP cause the inherited human immunodeficiency X-linked lymphoproliferative syndrome (XLP), suggesting that XLP may result from perturbed signaling via one or more of these SAP-associating receptors. We have now studied the requirements for SAP recruitment to CD84 and lymphocyte activation elicited following ligation of CD84 on primary and transformed human T cells. CD84 was found to be rapidly tyrosine phosphorylated following receptor ligation on activated T cells, an event that involved the Src kinase Lck. Phosphorylation of CD84 was indispensable for the recruitment of SAP, which was mediated by Y(262) within the cytoplasmic domain of CD84 and by R(32) within the SH2 domain of SAP. Furthermore, ligating CD84 enhanced the proliferation of anti-CD3 mAb-stimulated human T cells. Strikingly, this effect was also apparent in SAP-deficient T cells obtained from patients with XLP. These results reveal a novel function of CD84 on human lymphocytes and suggest that CD84 can activate human T cells via a SAP-independent mechanism.

Protein kinase C is involved in 2B4 (CD244)-mediated cytotoxicity and AP-1 activation in natural killer cells

2B4 (CD244) is a member of the CD2 subset of the immunoglobulin superfamily and functions as a triggering molecule on natural killer (NK) cells. Previously, we have found that 2B4-mediated activation of NK cells involves complex interactions involving LAT, Ras, Raf, ERK and p38 and that cytolytic function and cytokine production may be regulated by distinct pathways. Here we assessed the role of protein kinase C (PKC) in 2B4-mediated cytotoxicity of YT cells, a human NK cell line. Our data indicate that PKC-delta is activated upon stimulation with monoclonal antibody against 2B4. Treatment with the PKC inhibitor, bisindolylmaleimide I (Gö6850), of YT cells or YT cells depleted of Ca2+-dependent isoforms of PKC prior to 2B4 stimulation, resulted in inhibition of natural cytotoxicity and redirected antibody-dependent cellular cytotoxicity. However, inhibition of PKC failed to block 2B4 stimulation of interferon-gamma secretion as opposed to pretreatment with LY294002, a phosphoinositide 3-kinase inhibitor. We also examined the effect of phorbol 12-myristate 13-acetate (PMA) induction on 2B4 gene transcription. PMA induction resulted in a more than two-fold increase of 2B4 transcription. However, when we introduced a three-base substitution mutation to disrupt the activator protein-1 binding site at (-106 to -100) in the 2B4 promoter, we found complete loss of transcriptional activity, including the two-fold increase due to PMA induction of PKC. The present study indicated that PKC may play an important role in 2B4 signalling and activator protein-1 activation.

NKG2D triggers cytotoxicity in mouse NK cells lacking DAP12 or Syk family kinases

In activated mouse natural killer (NK) cells, the NKG2D receptor associates with two intracellular adaptors, DAP10 and DAP12, which trigger phosphatidyl inositol 3 kinase (PI3K) and Syk family protein tyrosine kinases, respectively. Here we show that cytotoxicity, but not cytokine production, is triggered by NKG2D in activated NK cells lacking either DAP12 or the Syk family members Syk and ZAP70. Inhibition of PI3K blocks this cytotoxicity, suggesting that the DAP10-PI3K pathway is sufficient to initiate NKG2D-mediated killing of target cells. Our results highlight signaling divergence in the effector functions of NKG2D and indicate that alternative associations between a receptor and its adaptors may provide a single receptor with a dual 'on-switch', giving mouse NK cells more choices through which to trigger cytotoxicity.

Natural killer cell receptor signaling

Natural killer (NK) cell immune responses are regulated by a balance of activating and inhibitory signals transmitted by cell surface receptors. Immunoreceptor tyrosine-based inhibition motifs in the cytoplasmic domains of inhibitory NK receptors recruit tyrosine or lipid phosphatases, which modulate the activation signals transmitted by receptors linked to the Syk and ZAP70 tyrosine kinases and phosphatidylinositol-3 kinases. In addition, recent studies of gene-deficient animals, in particular Syk and ZAP70 double-deficient mice, suggest that NK cells possess a robust and potentially redundant receptor system to ensure their development and function.

The gp49B1 inhibitory receptor regulates the IFN-gamma responses of T cells and NK cells

The magnitude and diversity of Ag-specific T cell effector activity have been proposed to be controlled by an integration of positive signals transduced by the TCR and negative signals originating from inhibitory cell surface molecules. Although the lectin family of NK cell-associated inhibitory receptors has been reported to regulate the function of murine CTLs, gp49B1, the Ig superfamily member is not known to be expressed on T cells. Moreover, the consequences of the lack of an endogenously expressed NK cell-associated inhibitory receptor on T cell functions are not known. We report that gp49B1 is expressed by nearly all activated CD8 and CD4 T cells in addition to NK cells during an immune response to viral, bacterial, or tumor challenge. Kinetics of gp49B1 expression parallel functional capability and subside in the memory phase. Following vaccinia viral infection, IFN-gamma production by both subsets of T cells and NK cells is enhanced in gp49B1-deficient mice compared with gp49B1(+/+) mice. The stimulation threshold for IFN-gamma production is also lower in gp49B1-deficient T cells. In contrast, no significant differences were observed in the cytotoxic responses. We conclude that gp49B1 is a unique inhibitory receptor that is induced in multiple lineages of innate and adaptive immune cells during an infection and controls their IFN-gamma, but not cytotoxic responses.

Molecular and immunological basis of X-linked lymphoproliferative disease

X-linked lymphoproliferative (XLP) disease is a human immune dysfunction characterized primarily by an inappropriate response to Epstein-Barr virus infection. In 1998, it was discovered that XLP is caused by inactivating mutations in the SAP/SH2D1A/DSHP gene. This gene codes for an immune cell-specific polypeptide termed SAP (SLAM-associated protein) that is composed almost exclusively of an Src homology 2 (SH2) domain. By way of its SH2 domain, SAP interacts with tyrosine-based motifs located in the cytoplasmic region of members of the SLAM (signaling lymphocyte activation molecule) family of receptors. Recent findings indicate that SAP is required for the function of SLAM-related receptors, as a consequence of its capacity to promote the recruitment and activation of the Src-related protein tyrosine kinase FynT, thereby allowing SLAM receptor-mediated protein tyrosine phosphorylation signals in immune cells. Functional and genetic analyses suggest that the phenotype associated with XLP is caused in large part by defects in the functions of SLAM-related receptors due to SAP deficiency.

Natural killer cell inhibitory receptors block actin cytoskeleton-dependent recruitment of 2B4 (CD244) to lipid rafts

A dynamic balance of positive and negative signals regulates target cell lysis by natural killer (NK) cells upon engagement of a variety of different activation receptors and of inhibitory receptors that recruit the tyrosine phosphatase SHP-1. However, the step at which activation signals are blocked by SHP-1 is not known. We have been using activation receptor 2B4 (CD244) to study the influence of inhibitory receptors on NK cell activation. Engagement of inhibitory receptors by HLA class I on target cells blocks phosphorylation of 2B4, placing the inhibitory step at the level, or upstream of 2B4 phosphorylation. Here we show that phosphorylated 2B4, after engagement with either antibodies or target cells that express the 2B4 ligand, is found exclusively in a detergent-resistant membrane fraction that contains lipid rafts. Integrity of lipid rafts was essential for phosphorylation and activating function of 2B4. Coengagement of inhibitory receptors blocked 2B4 phosphorylation and 2B4 association with detergent-resistant membranes, indicating that inhibitory receptors function upstream of raft-dependent signals. Recruitment of 2B4 into detergent-resistant membrane fractions and 2B4 phosphorylation were dependent on actin polymerization. Blocking actin cytoskeleton-dependent raft recruitment of different receptors may be a general mechanism by which inhibitory receptors control NK cell activation.

The dual-function CD150 receptor subfamily: the viral attraction

The CD150 subfamily within the CD2 family is a growing group of dual-function receptors that have within their cytoplasmic tails a characteristic signaling motif. The ITSM (immunoreceptor tyrosine-based switch motif) enables these receptors to bind to and be regulated by small SH2 domain adaptor proteins, including SH2D1A (SH2-containing adaptor protein SH2 domain protein 1A) and EAT-2 (EWS-activated transcript 2). A major signaling pathway through the prototypic receptor in this subfamily, CD150, leads to the activation of interferon-gamma, a key cytokine for viral immunity. As a result, many viruses have designed strategies to usurp or alter CD150 functions. Measles virus uses CD150 as a receptor and Molluscum contagiosum virus encodes proteins that are homologous to CD150. Thus, viruses use CD150 subfamily receptors to create a favorable environment to elude detection and destruction. Understanding the CD150 subfamily may lead to new strategies for vaccine development and antiviral therapies.

Natural killer cell activation in mice and men: different triggers for similar weapons?

The signaling pathways that regulate B and T lymphocytes are remarkably conserved between humans and mice. However, recent evidence suggests that the pathways regulating natural killer (NK) cell activation may actually differ between these two species. We discuss the controversies in the field and propose that this divergence could be deceptive: despite some clear differences between human and mouse NK cell receptors, the many ways of activating NK cells and their functions may well be conserved.

2B4 is constitutively associated with linker for the activation of T cells in glycolipid-enriched microdomains: properties required for 2B4 lytic function

2B4 is a receptor belonging to the Ig superfamily and is found on all murine NK cells as well as a small subset of T cells. Previous studies have found that cross-linking of the 2B4 receptor results in both increased cytotoxicity and IFN-gamma secretion. We have discovered that 2B4 from transfected NK and T cell lines, as well as from primary murine cells, coimmunoprecipitates with the phosphoprotein linker for the activation of T cells (LAT), which is essential for TCR-mediated signaling. This association is independent of both 2B4 phosphorylation and the cytoplasmic tail of 2B4. We have found that, along with LAT, 2B4 is constitutively located in glycolipid-enriched microdomains of the plasma membrane. In fact, 2B4 appears to associate with LAT only when it localizes to glycolipid-enriched microdomains. This localization of 2B4 occurs due to a CxC cysteine motif found in the transmembrane region, as determined by mutagenesis studies. 2B4-mediated cytotoxicity is defective in the absence of LAT, indicating that LAT is a required intermediate for 2B4 signal transduction. However, we have also shown that LAT association alone is not sufficient for maximal 2B4 activation.

SHP-1- and phosphotyrosine-independent inhibitory signaling by a killer cell Ig-like receptor cytoplasmic domain in human NK cells

Killer cell Ig-like receptors (KIR) are MHC class I-binding immunoreceptors that can suppress activation of human NK cells through recruitment of the Src homology 2-containing protein tyrosine phosphatase-1 (SHP-1) to two immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in their cytoplasmic domains. KIR2DL4 (2DL4; CD158d) is a structurally distinct member of the KIR family, which is expressed on most, if not all, human NK cells. 2DL4 contains only one ITIM in its cytoplasmic domain and an arginine in its transmembrane region, suggesting both inhibitory and activating functions. While 2DL4 can activate IFN-gamma production, dependent upon the transmembrane arginine, the function of the single ITIM of 2DL4 remains unknown. In this study, tandem ITIMs of KIR3DL1 (3DL1) and the single ITIM of 2DL4 were directly compared in functional and biochemical assays. Using a retroviral transduction method, we show in human NK cell lines that 1) the single ITIM of 2DL4 efficiently inhibits natural cytotoxicity responses; 2) the phosphorylated single ITIM recruits SHP-2 protein tyrosine phosphatase, but not SHP-1 in NK cells; 3) expression of dominant-negative SHP-1 does not block the ability of 2DL4 to inhibit natural cytotoxicity; 4) surprisingly, mutation of the tyrosine within the single ITIM does not completely abolish inhibitory function; and 5) this correlates with weak SHP-2 binding to the mutant ITIM of 2DL4 in NK cells and a corresponding nonphosphorylated ITIM peptide in vitro. These results reveal new aspects of the KIR-inhibitory pathway in human NK cells, which are SHP-1 and phosphotyrosine independent.