Recognition of class I major histocompatibility complex molecules by Ly-49: specificities and domain interactions

Ly49G, but not Ly49C/I, is dispensable for diverse antigen-specific memory NK cell responses in H-2d and H-2b mice

Immunological memory is a hallmark of the adaptive immune system. However, considerable evidence indicates that the natural killer (NK) cell subset of innate lymphoid cells also mediates specific memory responses to diverse antigens, including peptides. Even though the existence of NK cell memory is established, the mechanism behind NK cell adaptive responses is yet to be elucidated. Previously, we observed that the Ly49 family of class-I MHC receptors in mice are critical for the formation of adaptive NK cell memory responses. To define the nature of Ly49 involvement in NK cell memory responses, we investigated the contribution of individual Ly49 receptors and their defined class-I MHC ligands. We determined that the Ly49 requirement for the generation of NK memory responses is not uniform. Specifically, Ly49C and/or Ly49I proteins are indispensable for the adaptive NK cell responses as assessed by contact hypersensitivity recall responses to haptens and peptides, in H-2b and H-2d MHC backgrounds. In contrast, the highly expressed inhibitory receptor, Ly49G, did not appear to play any role in NK cell memory responses as determined using antibody-mediated subset depletion and gene-deficient mouse models, even in strains containing known ligands for Ly49G. These findings point to a unique role for Ly49C/I in adaptive NK cell antigen recognition and provide further insight into the mechanism behind adaptive NK cell responses.

Extrinsic and intrinsic drivers of natural killer cell clonality

Clonal expansion of antigen-specific lymphocytes is the fundamental mechanism enabling potent adaptive immune responses and the generation of immune memory. Accompanied by pronounced epigenetic remodeling, the massive proliferation of individual cells generates a critical mass of effectors for the control of acute infections, as well as a pool of memory cells protecting against future pathogen encounters. Classically associated with the adaptive immune system, recent work has demonstrated that innate immune memory to human cytomegalovirus (CMV) infection is stably maintained as large clonal expansions of natural killer (NK) cells, raising questions on the mechanisms for clonal selection and expansion in the absence of re-arranged antigen receptors. Here, we discuss clonal NK cell memory in the context of the mechanisms underlying clonal competition of adaptive lymphocytes and propose alternative selection mechanisms that might decide on the clonal success of their innate counterparts. We propose that the integration of external cues with cell-intrinsic sources of heterogeneity, such as variegated receptor expression, transcriptional states, and somatic variants, compose a bottleneck for clonal selection, contributing to the large size of memory NK cell clones.

Natural killer cell licensing in mice with inducible expression of MHC class I

Mouse natural killer (NK) cells acquire effector function by an education process termed "licensing" mediated by inhibitory Ly49 receptors which recognize self-MHC class I. Ly49 receptors can bind to MHC class I on targets (in trans) and also to MHC class I on the NK-cell surface (in cis). Which of these interactions regulates NK-cell licensing is not yet clear. Moreover, there are no clear phenotypic differences between licensed and unlicensed NK cells, perhaps because of the previously limited ability to study NK cells with synchronized licensing. Here, we produced MHC class I-deficient mice with inducible MHC class I consisting of a single-chain trimer (SCT), ovalbumin peptide-β2 microgloblin-H2K(b) (SCT-K(b)). Only NK cells with a Ly49 receptor with specificity for SCT-K(b) were licensed after MHC class I induction. NK cells were localized consistently in red pulp of the spleen during induced NK-cell licensing, and there were no differences in maturation or activation markers on recently licensed NK cells. Although MHC class I-deficient NK cells were licensed in hosts following SCT-K(b) induction, NK cells were not licensed after induced SCT-K(b) expression on NK cells themselves in MHC class I-deficient hosts. Furthermore, hematopoietic cells with induced SCT-K(b) licensed NK cells more efficiently than stromal cells. These data indicate that trans interaction with MHC class I on hematopoietic cells regulates NK-cell licensing, which is not associated with other obvious phenotypic changes.

Recognition of the nonclassical MHC class I molecule H2-M3 by the receptor Ly49A regulates the licensing and activation of NK cells

The development and function of natural killer (NK) cells is regulated by the interaction of inhibitory receptors of the Ly49 family with distinct peptide-laden major histocompatibility complex (MHC) class I molecules, although whether the Ly49 family is able bind to other MHC class I-like molecules is unclear. Here we found that the prototypic inhibitory receptor Ly49A bound the highly conserved nonclassical MHC class I molecule H2-M3 with an affinity similar to its affinity for H-2D(d). The specific recognition of H2-M3 by Ly49A regulated the 'licensing' of NK cells and mediated 'missing-self' recognition of H2-M3-deficient bone marrow. Host peptide-H2-M3 was required for optimal NK cell activity against experimental metastases and carcinogenesis. Thus, nonclassical MHC class I molecules can act as cognate ligands for Ly49 molecules. Our results provide insight into the various mechanisms that lead to NK cell tolerance.

The transcription factors T-bet and Eomes control key checkpoints of natural killer cell maturation

Natural killer (NK) cells play critical roles defending against tumors and pathogens. We show that mice lacking both transcription factors Eomesodermin (Eomes) and T-bet failed to develop NK cells. Developmental stability of immature NK cells constitutively expressing the death ligand TRAIL depended on T-bet. Conversely, maturation characterized by loss of constitutive TRAIL expression and induction of Ly49 receptor diversity and integrin CD49b (DX5(+)) required Eomes. Mature NK cells from which Eomes was deleted reverted to phenotypic immaturity if T-bet was present or downregulated NK lineage antigens if T-bet was absent, despite retaining expression of Ly49 receptors. Fetal and adult hepatic hematopoiesis restricted Eomes expression and limited NK development to the T-bet-dependent, immature stage, whereas medullary hematopoiesis permitted Eomes-dependent NK maturation in adult mice. These findings reveal two sequential, genetically separable checkpoints of NK cell maturation, the progression of which is metered largely by the anatomic localization of hematopoiesis.

Cytomegalovirus immunoevasin reveals the physiological role of "missing self" recognition in natural killer cell dependent virus control in vivo

Cytomegaloviruses (CMVs) are renowned for interfering with the immune system of their hosts. To sidestep antigen presentation and destruction by CD8(+) T cells, these viruses reduce expression of major histocompatibility complex class I (MHC I) molecules. However, this process sensitizes the virus-infected cells to natural killer (NK) cell-mediated killing via the "missing self" axis. Mouse cytomegalovirus (MCMV) uses m152 and m06 encoded proteins to inhibit surface expression of MHC I molecules. In addition, it encodes another protein, m04, which forms complexes with MHC I and escorts them to the cell surface. This mechanism is believed to prevent NK cell activation and killing by restoring the "self" signature and allowing the engagement of inhibitory Ly49 receptors on NK cells. Here we show that MCMV lacking m04 was attenuated in an NK cell- and MHC I-dependent manner. NK cell-mediated control of the infection was dependent on the presence of NK cell subsets expressing different inhibitory Ly49 receptors. In addition to providing evidence for immunoevasion strategies used by CMVs to avoid NK cell control via the missing-self pathway, our study is the first to demonstrate that missing self-dependent NK cell activation is biologically relevant in the protection against viral infection in vivo.

Inhibitory Ly49 receptors on mouse natural killer cells

The Ly49 receptors, which are expressed in a stochastic manner on subsets of murine natural killer (NK) cells, T cells, and other cells, are encoded by the Klra gene family and include receptors with either inhibitory or activating function. All of the inhibitory Ly49 receptors are characterized by an immunoreceptor tyrosine-based inhibitory motif in their cytoplasmic domain, which upon phosphorylation recruits tyrosine or lipid phosphatases to dampen signals transmitted through other activating receptors. Most of the inhibitory Ly49 receptors recognize polymorphic epitopes on major histocompatibility complex (MHC) class I proteins as ligands. Here, we review the polymorphism, ligand specificity, and signaling capacity of the inhibitory Ly49 receptors and discuss how these molecules regulate NK cell development and function.

Distinct conformations of Ly49 natural killer cell receptors mediate MHC class I recognition in trans and cis

Certain cell-surface receptors engage ligands expressed on juxtaposed cells and ligands on the same cell. The structural basis for trans versus cis binding is not known. Here, we showed that Ly49 natural killer (NK) cell receptors bound two MHC class I (MHC-I) molecules in trans when the two ligand-binding domains were backfolded onto the long stalk region. In contrast, dissociation of the ligand-binding domains from the stalk and their reorientation relative to the NK cell membrane allowed monovalent binding of MHC-I in cis. The distinct conformations (backfolded and extended) define the structural basis for cis-trans binding by Ly49 receptors and explain the divergent functional consequences of cis versus trans interactions. Further analyses identified specific stalk segments that were not required for MHC-I binding in trans but were essential for inhibitory receptor function. These data identify multiple distinct roles of stalk regions for receptor function.

Probing natural killer cell education by Ly49 receptor expression analysis and computational modelling in single MHC class I mice

Murine natural killer (NK) cells express inhibitory Ly49 receptors for MHC class I molecules, which allows for “missing self” recognition of cells that downregulate MHC class I expression. During murine NK cell development, host MHC class I molecules impose an “educating impact” on the NK cell pool. As a result, mice with different MHC class I expression display different frequency distributions of Ly49 receptor combinations on NK cells. Two models have been put forward to explain this impact. The two-step selection model proposes a stochastic Ly49 receptor expression followed by selection for NK cells expressing appropriate receptor combinations. The sequential model, on the other hand, proposes that each NK cell sequentially expresses Ly49 receptors until an interaction of sufficient magnitude with self-class I MHC is reached for the NK cell to mature. With the aim to clarify which one of these models is most likely to reflect the actual biological process, we simulated the two educational schemes by mathematical modelling, and fitted the results to Ly49 expression patterns, which were analyzed in mice expressing single MHC class I molecules. Our results favour the two-step selection model over the sequential model. Furthermore, the MHC class I environment favoured maturation of NK cells expressing one or a few self receptors, suggesting a possible step of positive selection in NK cell education. Based on the predicted Ly49 binding preferences revealed by the model, we also propose, that Ly49 receptors are more promiscuous than previously thought in their interactions with MHC class I molecules, which was supported by functional studies of NK cell subsets expressing individual Ly49 receptors.

Gammadelta T lymphocyte homeostasis is negatively regulated by beta2-microglobulin

Successful application of gammadelta T cells in adoptive cell therapies depends upon our ability to maintain these cells in vivo. Using an adoptive transfer model to study lymphopenia-induced homeostatic expansion, we show that CD8(+) and NK1.1(+) gammadelta T cell subsets are differentially regulated. While CD8(+) gammadelta T cells have an early and sustained advantage following transfer into TCRbeta(-/-)/delta(-/-) mice, NK1.1(+) gammadelta T cells proliferate slowly and are maintained at low numbers. The advantage of the CD8(+) subset could not be explained by increased bcl-2 or cytokine receptor expression but did correlate with Vgamma4(+) and Vdelta5(+) expression. Despite the role of CD8 in MHC class I recognition by alphabeta T cells, beta(2)-microglobulin (beta(2)m)-associated MHC class I molecules were not required for CD8(+) gammadelta T cell homeostatic expansion. Surprisingly, all gammadelta T cells, including the CD8(+) subset, exhibited enhanced proliferation following adoptive transfer into Rag1(-/-)/beta(2)m(-/-) compared with Rag1(-/-) recipients. This effect was most notable for the NK1.1(+) subset, which expresses high levels of NKG2A/CD94 and Ly49. Although expression of these inhibitory receptors correlated with poor homeostatic expansion in the presence of beta(2)m, gammadelta T cell homeostatic proliferation in TCRbeta(-/-)/delta(-/-) mice was not altered in the presence of Ly49C/I- and NKG2-blocking Abs. While the mechanism by which beta(2)m negatively regulates gammadelta T cell homeostasis remains to be determined, this observation is unique to gammadelta T cells and confirms that multiple mechanisms are in place to maintain strict regulation of both the size and the composition of the gammadelta T cell pool.

NK gene complex dynamics and selection for NK cell receptors

Natural killer (NK) cells play important roles in innate defense against infectious agents particularly viruses and also tumors. They mediate their effects through direct cytolysis, release of cytokines and regulation of subsequent adaptive immune responses. NK cells are equipped with sophisticated arrays of inhibitory and activation receptors that regulate their function. In this review we illustrate some of the major evolutionary relationships between NK cell receptors among different animal species and what some of the major mechanisms are that give rise to this diversity in receptor families, including the potential roles of pathogens such as viruses in driving receptor evolution.

Molecular architecture of the major histocompatibility complex class I-binding site of Ly49 natural killer cell receptors

Natural killer (NK) cells play a vital role in the detection and destruction of virally infected and tumor cells during innate immune responses. The highly polymorphic Ly49 family of NK receptors regulates NK cell function by sensing major histocompatibility complex class I (MHC-I) molecules on target cells. Despite the determination of two Ly49-MHC-I complex structures, the molecular features of Ly49 receptors that confer specificity for particular MHC-I alleles have not been identified. To understand the functional architecture of Ly49-binding sites, we determined the crystal structures of Ly49C and Ly49G and completed refinement of the Ly49C-H-2K(b) complex. This information, combined with mutational analysis of Ly49A, permitted a structure-based classification of Ly49s that we used to dissect the binding site into three distinct regions, each having different roles in MHC recognition. One region, located at the center of the binding site, has a similar structure across the Ly49 family and mediates conserved interactions with MHC-I that contribute most to binding. However, the preference of individual Ly49s for particular MHC-I molecules is governed by two regions that flank the central region and are structurally more variable. One of the flanking regions divides Ly49s into those that recognize both H-2D and H-2K versus only H-2D ligands, whereas the other discriminates among H-2D or H-2K alleles. The modular design of Ly49-binding sites provides a framework for predicting the MHC-binding specificity of Ly49s that have not been characterized experimentally.

Ly49G2 receptor blockade reduces tumor burden in a leukemia model but not in a solid tumor model

BACKGROUND

NK cell activity is regulated in part by inhibitory receptors that bind to MHC class I molecules. It is possible to enhance NK cell cytotoxicity against tumor cells by preventing the interaction of these inhibitory receptors with their MHC class I ligands.

RESULTS

In this study, we determined that Ly49G2 is an inhibitory receptor in AKR mice for self-MHC class I, and AKR Ly49G2 has an identical sequence to BALB/c Ly49G2. Blockade of Ly49G2 receptors in vivo resulted in decreased growth of BW-Sp3 lymphoma cells when the tumor cells were given i.v. but not when the tumor cells were inoculated into the flank forming a solid tumor. However, NK cells were involved in inhibiting the growth of BW-Sp3 tumor cells in the flank.

CONCLUSION

These data demonstrate that the effectiveness of inhibitory receptor blockade depends upon the tissue location of the tumor cells.

Distinctive interactions at multiple site 2 subsites by allele-specific rat and mouse ly49 determine functional binding and class I MHC specificity

Rodent Ly49 exhibit allele-specific MHC I recognition, yet the interaction site, site 2, encompassing the area below the MHC peptide-binding groove, the alpha3 domain, and associated beta(2) microglobulin, is highly conserved among rat and mouse MHC I alleles. We previously demonstrated that allele-specific Ly49 recognition can be affected by polymorphisms specifically in the peptide anchor-binding and supertype-defining B pocket of MHC I, possibly through differential conformations assumed by solvent-exposed interaction residues when articulating with this pocket. Through mutagenesis of RT1-A1(c) and H-2D(d), we map for the first time the interaction site(s) on rat MHC I mediating rat Ly49i2 recognition and the previously unexamined Ly49G(BALB/c) interaction with H-2D(d). We demonstrate that rat Ly49i2 and mouse Ly49G use both unique and common interactions at three MHC I H chain subsites to mediate functional binding and allele-specific recognition. We find that the F subsite, formed by solvent-exposed residues below the more conserved C-terminal anchor residue-binding F pocket, acts as an anchoring location for both Ly49i2 and Ly49G, whereas these receptors exhibit distinctive reliance on solvent-exposed residues articulating with the polymorphic anchor-binding and supertype-defining pocket(s) at subsite B, as well as on interaction residues at subsite C in the MHC I alpha3 domain. Our findings, combined with previous Ly49A/H-2D(d) and Ly49C/H-2K(b) cocrystal data, suggest how allele-specific MHC I conformations and Ly49 polymorphisms may affect Ly49 placement on MHC I ligands and residue usage at site 2, thereby mediating allele-specific recognition at the highly conserved MHC I interface.

Interactions of Ly49 family receptors with MHC class I ligands in trans and cis

The Ly49A NK cell receptor interacts with MHC class I (MHC-I) molecules on target cells and negatively regulates NK cell-mediated target cell lysis. We have recently shown that the MHC-I ligand-binding capacity of the Ly49A NK cell receptor is controlled by the NK cells' own MHC-I. To see whether this property was unique to Ly49A, we have investigated the binding of soluble MHC-I multimers to the Ly49 family receptors expressed in MHC-I-deficient and -sufficient C57BL/6 mice. In this study, we confirm the binding of classical MHC-I to the inhibitory Ly49A, C and I receptors, and demonstrate that detectable MHC-I binding to MHC-I-deficient NK cells is exclusively mediated by these three receptors. We did not detect significant multimer binding to stably transfected or NK cell-expressed Ly49D, E, F, G, and H receptors. Yet, we identified the more distantly related Ly49B and Ly49Q, which are not expressed by NK cells, as two novel MHC-I receptors in mice. Furthermore, we show using MHC-I-sufficient mice that the NK cells' own MHC-I significantly masks the Ly49A and Ly49C, but not the Ly49I receptor. Nevertheless, Ly49I was partly masked on transfected tumor cells, suggesting that the structure of Ly49I is compatible in principal with cis binding of MHC-I. Finally, masking of Ly49Q by cis MHC-I was minor, whereas masking of Ly49B was not detected. These data significantly extend the MHC-I specificity of Ly49 family receptors and show that the accessibility of most, but not all, MHC-I-binding Ly49 receptors is modulated by the expression of MHC-I in cis.

Stable masking by H-2Dd cis ligand limits Ly49A relocalization to the site of NK cell/target cell contact

Ly49A is an inhibitory receptor, which counteracts natural killer (NK) cell activation on the engagement with H-2D(d) (D(d)) MHC class I molecules (MHC-I) on target cells. In addition to binding D(d) on apposed membranes, Ly49A interacts with D(d) ligand expressed in the plane of the NK cells' membrane. Indeed, multivalent, soluble MHC-I ligand binds inefficiently to Ly49A unless the NK cells' D(d) complexes are destroyed. However, it is not known whether masked Ly49A remains constitutively associated with cis D(d) also during target cell interaction. Alternatively, it is possible that Ly49A has to be unmasked to significantly interact with its ligand on target cells. These two scenarios suggest distinct roles of Ly49A/D(d) cis interaction for NK cell function. Here, we show that Ly49A contributes to target cell adhesion and efficiently accumulates at synapses with D(d)-expressing target cells when NK cells themselves lack D(d). When NK cells express D(d), Ly49A no longer contributes to adhesion, and ligand-driven recruitment to the cellular contact site is strongly reduced. The destruction of D(d) complexes on NK cells, which unmasks Ly49A, is necessary and sufficient to restore Ly49A adhesive function and recruitment to the synapse. Thus, cis D(d) continuously sequesters a considerable fraction of Ly49A receptors, preventing efficient Ly49A recruitment to the synapse with D(d)+ target cells. The reduced number of Ly49A receptors that can functionally interact with D(d) on target cells explains the modest inhibitory capacity of Ly49A in D(d) NK cells. This property renders Ly49A NK cells more sensitive to react to diseased host cells.

Cross-species dependence of Ly49 recognition on the supertype defining B-pocket of a class I MHC molecule

Ly49 recognition of MHC class I (MHC I) can be allele specific. However, the site of interaction on MHC I consists of highly conserved solvent-exposed amino acids, leaving it unclear how allele specificity occurs. In examining the specificity of mouse and rat Ly49, we noticed that MHC I ligands for mouse Ly49G and W, and the rat Ly49i2, typically share the HLA-B7 supertype, defined by a B-pocket that prefers a proline at position 2 in bound peptides. Through mutagenesis, we show that the supertype-defining B-pocket of RT1-A1(c) controls its allele-specific recognition by the syngeneic rat Ly49i2 inhibitory receptor and xenogeneic mouse inhibitory Ly49G and activating Ly49W receptors. Single amino acid substitutions in the B-pocket that did not prevent peptide binding disrupted Ly49 recognition. In contrast, single mutations in other regions of the peptide-binding groove had no effect. We provide a model whereby the B-pocket dictates the conformation of conserved residues at the Ly49 interaction site below, defining Ly49 allele specificity for MHC I. Therefore, at least some Ly49 may recognize supertypes, detectable even across species, and are sensitive to polymorphisms in the supertype-defining B-pocket. This would ensure that expression of specific MHC I supertypes capable of Ag presentation to T cells is sensed by NK cells, and if lacking, targets a cell for elimination, suggesting a supertype-mediated link between innate and adaptive immunity.

SH-7, a new synthesized shikonin derivative, exerting its potent antitumor activities as a topoisomerase inhibitor

1-(1,4-dihydro-5,8-dihydroxy-1,4-dioxonaphthalen-2-yl)-4-methylpent-3-enylfuran-2-caroxylate (SH-7), a new naphthoquinone compound, derived from shikonin, exhibited obvious inhibitory actions on topoisomerase II (Topo II) and topoisomerase I (Topo I), which were stronger than its mother compound shikonin. Notably, the SH-7's inhibitory potency on Topo II was much stronger than that on Topo I. In addition, SH-7 significantly stabilized Topo II-DNA cleavable complex and elevated the expression of phosphorylated-H2AX. The in vitro cell-based investigation demonstrated that SH-7 displayed wide cytotoxicity in diversified cancer cell lines with the mean IC(50) value of 7.75 microM. One important finding is SH-7 displayed significant cytotoxicity in the 3 MDR cell lines, with an average IC(50) value nearly equivalent to that of the corresponding parental cell lines. The average resistance factor (RF) of SH-7 was 1.74, which was much lower than those of reference drugs VP-16 (RF 145.92), ADR (RF 105.97) and VCR (RF 197.39). Further studies illustrated that SH-7 had the marked apoptosis-inducing function on leukemia HL-60 cells, which was validated to be of mitochondria-dependence. The in vivo experiments showed that SH-7 had inhibitory effects on S-180 sarcoma implanted to mice, SMMC-7721, BEL-7402 human hepatocellular carcinoma and PC-3 human prostate cancer implanted to nude mice. Taken together, these results suggest that SH-7 induces DSBs as a Topo II inhibitor, which was crucial to activate the apoptotic process, and subsequently accounts for its both in vitro and in vivo antitumor activities. The well-defined Topo II inhibitory activity, antitumor effects particularly with its obvious anti-MDR action, better solubility and less toxicity make SH-7 as a potential antitumor drug candidate for further research and development.

Anti-fibrotic activity of NK cells in experimental liver injury through killing of activated HSC

BACKGROUND/AIMS

We have investigated the role of natural killer (NK) cells in hepatic fibrogenesis. Mouse NK cells express both inhibitory/activating-killing-immunoglobulin-related-receptors (iKIR/aKIR) specific for Class-I-molecules.

METHODS

Hepatic fibrosis induced by carbon-tetrachloride (CCl4) was compared between wild-type (WT) male-BALBc; combined-immunodeficiency (SCID, lacking B/T-cells); and SCID-BEIGE-mice (lacking B/T/NK cells), and naive mice.

RESULTS

Hepatic fibrosis significantly increased in all CCl4-treated groups. SCID-BEIGE mice had more fibrosis than SCID-mice (P<0.0001) as assessed by morphometry of sirius-red stained tissue sections. Following fibrosis, hepatic NK cells significantly decreased, the aKIR:iKIR-ratio significantly increased while Class-I expression on HSC decreased (P<0.001). Both freshly isolated and in situ HSC displayed a significant increase in cellular apoptosis following fibrosis induction. Confocal microscopy demonstrated the direct adhesion of NK cells to HSC in mouse liver sections and in vitro human NK/HSC co-culture. In human HSC there was decreased Class-I expression and increased apoptosis as well, which was further increased following blocking of either HSC-related Class-I or NK-related killer inhibitory receptors. Apoptosis was inhibited by pre-incubation of NK cells with the granzyme inhibitor 3,4-dichloroisocoumarin.

CONCLUSIONS

During liver injury, NK cells have an anti-fibrotic activity at least in part through stimulation of HSC killing.

A Role for DNA Hypomethylation and Histone Acetylation in Maintaining Allele-Specific Expression of Mouse NKG2A in Developing and Mature NK Cells

The repertoire of receptors that is expressed by NK cells is critical for their ability to kill virally infected or transformed cells. However, the molecular mechanisms that determine whether and when NK receptor genes are transcribed during hemopoiesis remain unclear. In this study, we show that hypomethylation of a CpG-rich region in the mouse NKG2A gene is associated with transcription of NKG2A in ex vivo NK cells and NK cell lines. This observation was extended to various developmental stages of NK cells sorted from bone marrow, in which we demonstrate that the CpGs are methylated in the NKG2A-negative stages (hemopoietic stem cells, NK progenitors, and NKG2A-negative NK cells), and hypomethylated specifically in the NKG2A-positive NK cells. Furthermore, we provide evidence that DNA methylation is important in maintaining the allele-specific expression of NKG2A. Finally, we show that acetylated histones are associated with the CpG-rich region in NKG2A positive, but not negative, cell lines, and that treatment with the histone deacetylase inhibitor trichostatin A alone is sufficient to induce NKG2A expression. Treatment with the methyltransferase inhibitor 5-azacytidine only is insufficient to induce transcription, but cotreatment with both drugs resulted in a significantly greater induction, suggesting a cooperative role for DNA methylation and histone acetylation status in regulating gene expression. These results enhance our understanding of the formation and maintenance of NK receptor repertoires in developing and mature NK cells.

NK Cell Receptors as Tools in Cancer Immunotherapy

Natural killer (NK) cells were identified 30 years ago based on their ability to "spontaneously" kill tumor cells. The basis for NK cell recognition and activation is due to a variety of receptors that bind to specific ligands on tumor cells and normal cells. Some of these receptors have the ability to inhibit NK cell function, and other receptors activate NK cell function. Therapeutic strategies for cancer therapy are being developed based on preventing NK cell inhibition or using NK cell receptors to activate NK cells or T cells. There are intriguing clinical data from studies of bone marrow transplantation that support the idea that preventing NK cell inhibition by human leukocyte antigen (HLA) class I molecules can be a means to promote graft-versus-leukemia (GvL) effects and limit graft-versus-host disease (GvHD) in acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) patients. Experimental findings also support the blockade of NK cell inhibitory receptors as a way to protect against leukemia relapse. It may be possible to use our knowledge of NK cell activating receptors and their ligands to immunize patients with modified tumor cells to promote beneficial NK cell responses and development of host antitumor cytotoxic T lymphocytes (CTLs). Finally, new data support the idea of using modified NK cell receptors as a means to target patients' T cells against their own tumor cells and induce long-term immunity against them. Tumors are essentially tissues that have overcome normal regulation mechanisms, and therefore the ability to distinguish normal cells from abnormal cells is a key part of selectively attacking tumor cells. NK cells have various receptor systems designed to recognize infected and abnormal cells. Understanding NK cell receptors and their recognition mechanisms provides new tools for the development of immunotherapies against cancer.

Influence of xenogeneic beta2-microglobulin on functional recognition of H-2Kb by the NK cell inhibitory receptor Ly49C

NK cells maintain self-tolerance through expression of inhibitory receptors that bind MHC class I (MHC-I) molecules. MHC-I can exist on the cell surface in several different forms, including "peptide-receptive" or PR-MHC-I that can bind exogenous peptide. PR-MHC-I molecules are short lived and, for H-2K(b), comprise approximately 10% of total MHC-I. In the present study, we confirm that signaling through the mouse NK inhibitory receptor Ly49C requires the presence of PR-K(b) and that this signaling is prevented when PR-K(b) is ablated by pulsing with a peptide that can bind to it with high affinity. Although crystallographic data indicate that Ly49C can engage H-2K(b) loaded with high-affinity peptide, our data suggest that this interaction does not generate an inhibitory signal. We also show that no signaling occurs when the PR-K(b) complex has mouse beta(2)-microglobulin (beta(2)m) replaced with human beta(2)m, although replacement with bovine beta(2)m has no effect. Furthermore, we show that beta(2)m exchange occurs preferentially in the PR-K(b) component of total H-2K(b). These conclusions were reached in studies modulating the sensitivity to lysis of both NK-resistant syngeneic lymphoblasts and NK-sensitive RMA-S tumor cells. We also show, using an in vivo model of lymphocyte recirculation, that engrafted lymphocytes are unable to survive NK attack when otherwise syngeneic lymphocytes express human beta(2)m. These findings suggest a qualitative extension of the "missing self" hypothesis to include NK inhibitory receptors that are restricted to the recognition of unstable forms of MHC-I, thus enabling NK cells to respond more quickly to events that decrease MHC-I synthesis.

An innately interesting decade of research in immunology

"Nature has provided, in the white corpuscles as you call them-in the phagocytes as we call them-a natural means of devouring and destroying all disease germs. There is at bottom only one genuinely scientific treatment for all diseases, and that is to stimulate the phagocytes." So opined B.B. in G.B. Shaw's The Doctor's Dilemma in a dramatic restatement of a key portion of Ilya Metchnikoff's Nobel Prize address: "Whenever the organism enjoys immunity, the introduction of infectious microbes is followed by the accumulation of mobile cells, of white corpuscles of the blood in particular which absorb the microbes and destroy them. The white corpuscles and the other cells capable of doing this have been designated 'phagocytes,' (i.e., devouring cells) and the whole function that ensures immunity has been given the name of 'phagocytosis'". Based on these insights into the foundation of resistance to infectious disease, Metchnikoff was awarded the 1908 Nobel Prize in Physiology or Medicine together with Paul Ehrlich (Fig. 1). Although both were cited for discoveries in immunity, the contributions of the two men seem worlds apart. Ehrlich's studies did not deal with generic responses to infection, but rather with the highly specific nature of antibodies and their relationship to the cells producing them: "As the cell receptor is obviously preformed, and the artificially produced antitoxin only the consequence, i.e. secondary, one can hardly fail to assume that the antitoxin is nothing else but discharged components of the cell, namely receptors discharged in excess". But biological systems are just that-systems-and the parts need to work together. And so we arrive, a century later, at an appreciation for just how intimately related these two seemingly disparate aspects of host defense really are.

Carbohydrate and Non-Carbohydrate Ligands for the C-Type Lectin-Like Receptors of Natural Killer Cells. A Review

The superfamily of C-type animal lectins is defined by a sequence motif of the carbohydrate- recognition domains (CRDs) and comprises seven groups of molecules. The soluble proteins are group I proteoglycans, group III collectins, and group VII containing the isolated CRDs. Type I membrane proteins include group IV selectins and group VI macrophage receptors and related molecules. Type II membrane proteins are group II hepatic lectins and group V natural killer cell receptors. The latter group has recently attracted considerable attention of the biomedical community. These receptors are arranged at the surface of lymphocytes as homo- or heterodimers composed of two polypeptides consisting of N-terminal peptide tails responsible for signaling, transmembrane domain, neck regions of varying length, and C-terminal lectin-like domains (CTLDs). Since this group is evolutionarily most distant from the rest of C-type animal lectins, the sequence of the C-terminal ligand-binding domain has diversified to accommodate other ligands than calcium or carbohydrates. These domains are referred to as natural killer domains (NKDs) forming a large percentage of CTLDs in vertebrates. Here are summarized the data indicating that calcium, carbohydrates, peptides, and large proteins such as major histocompatibility complex (MHC) class I can all be ligands for NKDs. The wide range of ligands that can be recognized by NKDs includes some new, unexpected compounds such as signal peptide-derived fragments, heat shock proteins, or oxidized lipids. The biological importance of this extended range of recognition abilities is also discussed. A review with 134 references.

Subset classification of mouse uterine natural killer cells by DBA lectin reactivity

Uterine Natural Killer (uNK) cells are a transient lymphocyte population found in the pregnant uteri of human and rodents. The pregnant uterine environment appears to influence migration, differentiation and suppression of the cytolytic activation of uNK cells but the mechanisms involved in these processes are not well understood. Similarities to circulating NK (cNK) cells are limited. The present study sought to discrimate uNK cells from cNK cells in mice by identification of a unique uNK cell marker. Dolichos biflorus (DBA) lectin, which has high selectivity for glycoconjugates containing N-acetyl D-galactosomine in the terminal position, reacted with the plasma membranes of mouse uNK cells. DBA lectin did not react with other uterine lymphocytes or with cNK cell surfaces in Swiss, CBA-J, C57BL/6, SJL, BALB/c, DBA-2 mice strains. DBA lectin staining was useful for both light and electron microscopy and distinguished 4 uNK cell subtypes that appear to be stages of differentiation. Quantitative evaluation of these 4 uNK cell subtypes over early to late gestational times showed dynamic changes between immature and mature forms in different compartments of the implantation sites and indicated the occurrence of microdomains in the uterus capable of controlling uNK cell proliferation and differentiation. This is the first report showing mouse uNK cells expressing specific molecules not found in other NK cells. Use of this reagent should enhance studies of earlier, non-granulated forms of uNK cells and provide new strategies for purification of mouse uNK cells for functional and molecular studies.

Receptor/ligand avidity determines the capacity of Ly49 inhibitory receptors to interfere with T-cell receptor-mediated activation

The specificity and the relative affinity of many Ly49 receptors for major histocompatibility complex class I ligands have been studied in detail in various adhesion and binding assays. However, how the level of cell surface expression of a given Ly49 receptor and its ligand affinity influence the strength of the inhibition signal is not well documented. To address this issue, we developed a series of human Jurkat T-cell transfectants expressing the whole range of Ly49A and Ly49C levels found in vivo on natural killer and T cells and evaluated their capacity to alter superantigen-induced NF-AT activation and interleukin-2 production. We show that the strength of the inhibition induced by Ly49A/H-2Dd interaction correlates with Ly49A density up to a certain level after which increasing expression does not further inhibit significantly the T-cell receptor-induced activation. This system also represents a valuable tool for the determination of the relative strength of the inhibitory signals of Ly49 receptors following their interactions with different ligands. Even at high levels of expression there was no evidence that engagement of Ly49A with H-2b class I molecules provided an inhibitory signal. Moreover, we showed that functional inhibitory interactions of Ly49C with H-2b class I molecules were only the result of H-2Kb and that H-2d represent lower affinity ligands for Ly49C than H-2b. Therefore, depending on the relative affinity of Ly49 receptors for their ligands, the modulation of their expression level will be determinant for the functional outcome of activated T cells.

Immune functions encoded by the natural killer gene complex

There has been marked progress in our understanding of the role of natural killer (NK) cells in immune responses, mainly due to the identification of NK-cell receptors and their ligands. The genes encoding many NK-cell receptors are located in the NK-gene complex (NKC). Here, we review the properties of NKC-encoded receptors, and provide a genomic and conceptual framework for an insight into NK-cell function and biology.

B6 Strain Ly49I Inhibitory Gene Expression on T Cells in FVB.Ly49IB6 Transgenic Mice Fails to Prevent Normal T Cell Functions

Inhibitory Ly49 receptors expressed on NK cells provide a mechanism for tolerance to normal self tissues. The immunoregulatory tyrosine-based inhibitory motifs present in some Ly49s are able to transmit an inhibitory signal upon ligation by MHC class I ligands. In our system, as well as others, mice transgenic for inhibitory Ly49 receptors express these receptors on both NK and T cells. FVB (H2(q)) mice transgenic for the B6 strain Ly49I (Ly49I(B6)) express the inhibitory Ly49 receptor on the surface of both T and NK cells. Although Ly49I functions to prevent NK-mediated rejection of H2(b) donor bone marrow cells in this transgenic mouse strain, the T cells do not appear to be affected by the expression of the Ly49I transgene. FVB.Ly49I T cells have normal proliferative capabilities both in vitro and in vivo in response to the Ly49I ligand, H2(b). In vivo functional T cell assays were also done, showing that transgenic T cells were not functionally affected. T cells in these mice also appear to undergo normal T cell development and activation. Only upon stimulation with suboptimal doses of anti-CD3 in the presence of anti-Ly49I is T cell proliferation inhibited. These data are in contrast with findings in Ly49A, and Ly49G2 receptor transgenic models. Perhaps Ly49I-H2(b) interactions are weaker or of lower avidity than Ly49A-H-2D(d) interactions, especially in T cells.

Crystal structure of the Ly49I natural killer cell receptor reveals variability in dimerization mode within the Ly49 family

Natural killer (NK) cells play a crucial role in the detection and destruction of virally infected and tumor cells during innate immune responses. The cytolytic activity of NK cells is regulated through a balance of inhibitory and stimulatory signals delivered by NK receptors that recognize classical major histocompatabilty complex class I (MHC-I) molecules, or MHC-I homologs such as MICA, on target cells. The Ly49 family of NK receptors (Ly49A through W), which includes both inhibitory and activating receptors, are homodimeric type II transmembrane glycoproteins, with each subunit composed of a C-type lectin-like domain tethered to the membrane by a stalk region. We have determined the crystal structure, at 3.0 A resolution, of the murine inhibitory NK receptor Ly49I. The Ly49I monomer adopts a fold similar to that of other C-type lectin-like NK receptors, including Ly49A, NKG2D and CD69. However, the Ly49I monomers associate in a manner distinct from that of these other NK receptors, forming a more open dimer. As a result, the putative MHC-binding surfaces of the Ly49I dimer are spatially more distant than the corresponding surfaces of Ly49A or NKG2D. These structural differences probably reflect the fundamentally different ways in which Ly49 and NKG2D receptors recognize their respective ligands: whereas the single MICA binding site of NKG2D is formed by the precise juxtaposition of two monomers, each Ly49 monomer contains an independent binding site for MHC-I. Hence, the structural constraints on dimerization geometry may be relatively relaxed within the Ly49 family. Such variability may enable certain Ly49 receptors, like Ly49I, to bind MHC-I molecules bivalently, thereby stabilizing receptor-ligand interactions and enhancing signal transmission to the NK cell.

Structure and function of natural killer cell receptors: multiple molecular solutions to self, nonself discrimination

In contrast to T cell receptors, signal transducing cell surface membrane molecules involved in the regulation of responses by cells of the innate immune system employ structures that are encoded in the genome rather than generated by somatic recombination and that recognize either classical MHC-I molecules or their structural relatives (such as MICA, RAE-1, or H-60). Considerable progress has recently been made in our understanding of molecular recognition by such molecules based on the determination of their three-dimensional structure, either in isolation or in complex with their MHC-I ligands. Those best studied are the receptors that are expressed on natural killer (NK) cells, but others are found on populations of T cells and other hematopoietic cells. These molecules fall into two major structural classes, those of the immunoglobulin superfamily (KIRs and LIRs) and of the C-type lectin-like family (Ly49, NKG2D, and CD94/NKG2). Here we summarize, in a functional context, the structures of the murine and human molecules that have recently been determined, with emphasis on how they bind different regions of their MHC-I ligands, and how this allows the discrimination of tumor or virus-infected cells from normal cells of the host.

NK cell inhibitory receptor Ly-49C residues involved in MHC class I binding

Mouse NK cells express Ly-49 receptors specific for classical MHC class I molecules. Several of the Ly-49 receptors have been characterized in terms of function and ligand specificity. However, the only Ly-49 receptor-ligand interaction previously described in detail is that between Ly-49A and H-2D(d), as studied by point mutations in the ligand and the crystal structure of the co-complex of these molecules. It is not known whether other Ly-49 receptors bind MHC class I in a similar manner as Ly-49A. Here we have studied the effect of mutations in Ly-49C on binding to the MHC class I molecules H-2K(b), H-2D(b), and H-2D(d). The MHC class I molecules were used as soluble tetramers to stain transiently transfected 293T cells expressing the mutated Ly-49C receptors. Three of nine mutations in Ly-49C led to loss of MHC class I binding. The three Ly-49C mutations that affected MHC binding correspond to Ly-49A residues that are in contact or close to H-2D(d) in the co-crystal, demonstrating that MHC class I binding by Ly-49C is dependent on residues in the same area as that used by Ly-49A for ligand contacts.

Binding of the natural killer cell inhibitory receptor Ly49A to its major histocompatibility complex class I ligand. Crucial contacts include both H-2Dd AND beta 2-microglobulin

Ly49A, an inhibitory C-type lectin-like mouse natural killer cell receptor, functions through interaction with the major histocompatibility complex class I molecule, H-2D(d). The x-ray crystal structure of the Ly49A.H-2D(d) complex revealed that homodimeric Ly49A interacts at two distinct sites of H-2D(d): Site 1, spanning one side of the alpha1 and alpha2 helices, and Site 2, involving the alpha1, alpha2, alpha3, and beta(2)m domains. Mutants of Ly49A, H-2D(d), and beta(2)-microglobulin at intermolecular contacts and the Ly49A dimer interface were examined for binding affinity and kinetics. Although mutations at Site 1 had little affect, several at Site 2 and at the dimer interface hampered the Ly49A.H-2D(d) interaction, with no effect on gross structure or T cell receptor interaction. The region surrounding the most critical residues (in H-2D(d), Asp(122); in Ly49A, Asp(229), Ser(236), Thr(238), Arg(239), and Asp(241); and in beta(2)-microglobulin, Gln(29) and Lys(58)) of the Ly49A.H-2D(d) interface at Site 2 includes a network of water molecules, suggesting a molecular basis for allelic specificity in natural killer cell recognition.

H-2D ligand expression by Ly49A+ natural killer (NK) cells precludes ligand uptake from environmental cells: implications for NK cell function

To study the adaptation of natural killer (NK) cells to their major histocompatibility complex (MHC) class I environment we have established a novel mouse model with mosaic expression of H-2D(d) using a Cre/loxP system. In these mice, we noticed that NK cells expressing the inhibitory receptor for D(d), Ly49A, were specifically underrepresented among cells with low D(d) levels. That was due to the acquisition of D(d) molecules by the Ly49A+ NK cells that have lost their D(d) transgene. The uptake of H-2D molecules via the Ly49A receptor was restricted to strong ligands of Ly49A. Surprisingly, when Ly49A+ NK cells were D(d+), uptake of the alternative ligand D(k) was not detectable. Similarly, one anti-Ly49A mAb (A1) bound inefficiently when Ly49A was expressed on D(d+) NK cells. Concomitantly, functional assays demonstrated a reduced capacity of Ly49A to inhibit H-2(b)D(d) as compared with H-2(b) NK cells, rendering Ly49A+ NK cells in D(d+) mice particularly reactive. Minor reductions of D(d) levels and/or increases of activating ligands on environmental cells may thus suffice to abrogate Ly49A-mediated NK cell inhibition. The mechanistic explanation for all these phenomena is likely the partial masking of Ly49A by D(d) on the same cell via a lateral binding site in the H-2D(d) molecule.

H-2 allele specificity of the NK cell C-type lectin-like MHC class I receptor Ly49A visualized by soluble Ly49A tetramer

Ly49A is a C-type lectin-like receptor on NK cells that recognizes MHC class I ligands, H-2D(d) and D(k). The engagement of Ly49A with the ligands inhibits activation of NK cells and protects target cells from lysis by NK cells. Here we express the extracellular region of Ly49A with an N-terminal biotinylation tag in Escherichia coli to obtain soluble Ly49A (sLy49A) after refolding. sLy49A is indistinguishable from native Ly49A expressed on NK cells serologically and in the ability to specifically bind H-2D(d) after tetramerization with R-phycoerythrin-coupled streptavidin. The fluorescently labeled tetramer of sLy49A is applied to explore MHC class I haplotype specificity of Ly49A. We demonstrate the hierarchical reactivity of Ly49A with H-2 of various alleles in the order of d > k, r > p > v > q > s > z. Reactivity of sLy49A tetramer to spleen lymphocytes from B10.QBR mice (H-2K(b), I(b), D(q), Qa-1/Tla(b)) but not from C57BL/10 mice (H-2(b)) identifies H-2D(q) and L(q) as candidates for a Ly49A ligand. Binding of sLy49A tetramer to H-2D(q)- or L(q)-transfected cell lines demonstrates that the two highly related MHC class I molecules, H-2D(q) and L(q), are ligands for Ly49A. sLy49A tetramer staining also demonstrates preferential expression of Ly49A ligand on a subset of B cells in P/J mice. These results provide the basis to examine the molecular mechanism by which Ly49A discriminates polymorphic MHC class I molecules.

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

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

Expression of Ly49E and CD94/NKG2 on fetal and adult NK cells

Murine NK cells express inhibitory receptors belonging to the Ly49 and CD94/NKG2 family. Ly49E and CD94 are the only NK cell receptor transcripts detectable in fetal NK cells. Still unproved is the surface expression of Ly49E on NK cells. Here we generated two novel mAbs, a mAb recognizing Ly49E with cross-reactivity to Ly49C, and a mAb against NKG2A/C/E. Ly49E was immunoprecipitated as a disulfide-linked homodimer with 46-kDa subunits. Removal of N-linked carbohydrates revealed a 31-kDa protein backbone. NKG2A was immunoprecipitated as a 38-kDa protein. Although the frequency of fetal NK cells expressing Ly49E was higher than 25%, it decreased drastically from 2 wk after birth. Phenotypic analysis showed that approximately 90% of fetal NK cells and approximately 50% of adult NK cells express high levels of CD94/NKG2. The remaining 50% of adult NK cells expressed low surface levels of CD94/NKG2. Expression of Ly49E and CD94/NKG2 was not restricted to NK cells, but was also observed on NK T and memory T cells. Functional analysis showed that sorted Ly49E(+) and CD94/NKG2(+) fetal NK cells could discriminate between MHC class I-positive and MHC class I-negative tumor cells. We also demonstrated that Ly49E becomes phosphorylated following pervanadate stimulation of fetal NK cells. The expression levels of Ly49E and CD94/NKG2 were similar in wild-type compared with beta(2)-microglobulin(-/-) mice. In conclusion, generation of mAbs against Ly49E and NKG2 extended the phenotypic and functional characterization of NK cells.

Cumulative inhibition of NK cells and T cells resulting from engagement of multiple inhibitory Ly49 receptors

Inhibitory receptors specific for MHC class I molecules are expressed on partially overlapping subpopulations of NK cells and memory T cells. A central question pertinent to NK cell development and function is how the combinatorial expression of different receptors with distinct class I specificities affects functional recognition. We therefore studied the quantitative effects resulting from class I engagement of multiple inhibitory Ly49 receptors. We used a transgenic mouse model in which all NK cells and T cells express two different Ly49 receptors with shared class I specificity. Comparisons of cells from these mice with cells from single transgenic mice and wild-type mice revealed that Ly49 receptors cumulatively inhibit lymphocyte effector functions. Multiple Ly49 interactions also had a cumulative impact on NK cell development. The findings suggest that the interactions of inhibitory receptors with class I are interpreted quantitatively rather than as on/off switches. They have intriguing implications concerning NK cell tolerance and reactivity toward cells with extinguished expression of a limited number of class I molecules.

Expression of natural killer receptor alleles at different Ly49 loci occurs independently and is regulated by major histocompatibility complex class I molecules

Ly49 receptor genes are expressed by subsets of natural killer (NK) cells in an overlapping fashion, accounting for the capacity of NK subsets to attack host cells that have selectively downregulated self-major histocompatibility complex (MHC) class I molecules. It was shown previously that most NK cells express only one or the other allele of a given Ly49 gene, while a smaller population expresses both alleles. However, the methods used to detect monoallelic and biallelic cells were nonquantitative. Here, new allele-specific antibodies were used to provide the first quantitative examination of biallelic and monoallelic expression of Ly49A and Ly49G2. The results demonstrate conclusively that most Ly49A(+) and Ly49G2(+) NK cells express the corresponding gene in a monoallelic fashion, with a smaller subset expressing both alleles. Unexpectedly, biallelic Ly49A(+) NK cells were more numerous than predicted by completely independent allelic expression, suggesting some heterogeneity among NK progenitors in the potential to express a given Ly49 gene. The data also show that cells expressing one allele of Ly49G2 may express Ly49A from the same or opposite chromosome with equal likelihood, indicating that the expressed allele is chosen independently for different Ly49 genes. Finally, the data demonstrate that biallelic expression of Ly49A or Ly49G2 occurs least frequently in mice that express ligands for these receptors (H-2(d) mice), and most frequently in class I-deficient mice. Thus, biallelic expression of Ly49 genes is regulated by interactions of NK cell progenitors with MHC class I molecules.

Divergent and convergent evolution of NK-cell receptors

Natural killer (NK)-cell receptors specific for major histocompatibility complex (MHC) class I molecules have been identified in humans and mice. Some of the most important receptors are structurally unrelated in the two species: the murine Ly-49 receptors are C-type lectins, while human killer-cell inhibitory receptors (KIRs) belong to the immunoglobulin superfamily. Here, Roland Barten and colleagues describe the divergent and convergent evolution of NK-cell receptors.

Comparative ability of Qdm/Qa-1b, kb, and Db to protect class Ilow cells from NK-mediated lysis in vivo

The class Ib molecule Qa-1(b) binds the class Ia leader peptide, Qdm, which reacts with CD94/NKG2R on NK cells. We have generated a gene that encodes the Qdm peptide covalently attached to ss(2)-microglobulin (ss(2)M) by a flexible linker (Qa-1 determinant modifier (Qdm)-ss(2)M). When this construct is expressed in TAP-2(-) or ss(2)M(-) cells, it allows for the expression of a Qdm-ss(2)M protein that associates with Qa-1(b) to generate the Qdm epitope, as detected by Qdm/Qa-1(b)-specific CTL. To test the biological significance of expression of this engineered molecule, we injected TAP-2(-) RMAS-Qdm-ss(2)M cells into C57BL/6 mice and measured their NK cell-mediated clearance from the lungs at 2 h. RMAS cells transfected with Qdm-ss(2)M were resistant to lung clearance, similar to RMA cells or RMAS cells in anti-asialo-GM(1)-treated mice, while untransfected or ss(2)M-transfected RMAS cells were rapidly cleared. Further, pulsing RMAS cells with either Qdm, a K(b)-, or D(b)-binding peptide showed equivalent protection from clearance, indicating that a single class Ia or Ib molecule can afford complete protection from NK cells in this system. In contrast, injection of RMAS cells into DBA/2 animals, which express low levels of receptors for Qdm/Qa-1(b), resulted in protection from lung clearance if pulsed with a K(b)- or D(b)-binding peptide, but not the Qa-1(b)-binding peptide, Qdm.

Reconstitution of bacterial expressed human CD94: the importance of the stem region for dimer formation

Human CD94 is a subunit of the disulfide-linked, heterodimeric natural killer (NK) cell surface receptor CD94/NKG2. This receptor, a member of the C-type lectin superfamily, participates in regulating NK cell directed lysis through interaction with the major histocompatibility antigen HLA-E. Two forms of CD94 were expressed using a bacterial expression system and refolded in vitro. One form, residues 34-179, designated S34, corresponds to the entire extracellular region of the receptor, including a 23-residue stem region, and the other, residues 51-179, designated E51, corresponds only to the putative carbohydrate recognition domain of the receptor. The refolded full-length S34 protein existed as a noncovalent dimer initially but formed an interchain disulfide bond upon storage for several months. In contrast, the stemless construct, E51, existed largely as a monomeric form. The stem region of S34, residues 34-56, is sensitive to proteolysis and its absence results in dissociation of the dimer. This suggests that the residues in the stem region of CD94 help to stabilize the dimeric conformation.

IFN-γ Production and Cytotoxicity of IL-2-Activated Murine NK Cells Are Differentially Regulated by MHC Class I Molecules

Activation of NK cells by target cells leads to cytotoxicity as well as production of various cytokines including IFN-γ. MHC class I molecules on target cells regulate NK cytotoxicity. However, little is known about the regulation of IFN-γ production by NK cells. We examined the production of IFN-γ in individual murine NK cells stimulated with tumor cell lines by flow cytometric analysis of intracellular IFN-γ. Among several tumor lines tested, the rat basophilic leukemia line RBL-1 induced particularly high level of IFN-γ production in IL-2-activated NK cells, whereas other lines, including the prototypic NK target YAC-1, induced very low or no IFN-γ production. Transfection of murine classical MHC class I molecules into RBL-1 cells substantially inhibited IFN-γ production. This inhibition of IFN-γ production by MHC class I was independent of Ly-49 or CD94/NKG2A expression on NK cells. These results indicate that some target cells directly stimulate IL-2-activated NK cells and induce IFN-γ production, but the requirements for the induction of IFN-γ production seem different from those for NK cytotoxicity. Furthermore, similar to NK cytotoxicity, induction of IFN-γ production is inhibited by MHC class I on stimulating cells. However, the MHC class I-specific receptors inhibiting IFN-γ production are different from those for NK cytotoxicity.

Crystal structure of a lectin-like natural killer cell receptor bound to its MHC class I ligand

Natural killer (NK) cell function is regulated by NK receptors that interact with MHC class I (MHC-I) molecules on target cells. The murine NK receptor Ly49A inhibits NK cell activity by interacting with H-2D(d) through its C-type-lectin-like NK receptor domain. Here we report the crystal structure of the complex between the Ly49A NK receptor domain and unglycosylated H-2D(d). The Ly49A dimer interacts extensively with two H-2D(d) molecules at distinct sites. At one interface, a single Ly49A subunit contacts one side of the MHC-I peptide-binding platform, presenting an open cavity towards the conserved glycosylation site on the H-2D(d) alpha2 domain. At a second, larger interface, the Ly49A dimer binds in a region overlapping the CD8-binding site. The smaller interface probably represents the interaction between Ly49A on the NK cell and MHC-I on the target cell, whereas the larger one suggests an interaction between Ly49A and MHC-I on the NK cell itself. Both Ly49A binding sites on MHC-I are spatially distinct from that of the T-cell receptor.

Altered Donor and Recipient Ly49+ NK Cell Subsets in Allogeneic H-2d → H-2b and H-2b → H-2d Bone Marrow Chimeras

NK cells reject non-self hematopoietic bone marrow (BM) grafts via Ly49 receptor-mediated MHC class I-specific recognition and calibration of receptor expression levels. In this paper we investigated how Ly49+ subset frequencies were regulated dependent on MHC class I expression. The development of donor and host Ly49A+ (recognizes H-2Dd and H-2Dk ligands) and Ly49C/I+ (Ly49CBALB/c recognizes H-2Kb, H-2Kd, and H-2Dd, and Ly49CB6 recognizes only H-2Kb) NK cell frequencies were monitored for 120 days in murine-mixed allogeneic BM chimeras. C57BL/6 (H-2b) BM was transplanted into BALB/c (H-2d) mice and vice versa. Peripheral NK cell populations were examined every 5 days. Chimerism was found to be stable with 80–90% donor NK cells. In contrast to syngeneic controls reexpressing pretransplant patterns, donor and host NK cells revealed new and mainly reduced subset frequencies 55 days after allogeneic transplantation. Recipient NK cells acquired these later than donor NK cells. In H-2d → H-2b chimeras Ly49A+, Ly49C/I+, and Ly49A+/Ly49C/I+ proportions were mainly diminished upon interaction with cognate ligands. Also in H-2b → H-2d chimeras, Ly49A+ and Ly49A+/Ly49C/I+ subsets were reduced, but there was a transient normalization of Ly49C/I+ proportions in the noncognate host. After 120 days all subsets were reduced. Therefore, down-regulation of developing Ly49A+ and Ly49C/I+ chimeric NK cell frequencies by cognate ligands within 7–8 wk after BM transplantation may be important for successful engraftment.

Impaired Anti-Viral T Cell Responses Due to Expression of the LY49A Inhibitory Receptor

Inhibitory receptors specific for alleles of MHC class I proteins play an important role in determining the reactivity and specificity of NK cells. To determine whether these receptors are also able to regulate T cell functions, we have studied anti-viral immune responses in mice transgenic for a class I-specific inhibitory receptor, Ly49A. Although nontransgenic mice express Ly49A primarily on NK cells and some T cells, the Ly49A transgenic mice express Ly49A on all lymphocytes, including T cells. We have assessed the activation, expansion, cytokine production, and cytotoxic activity of CD8 T cells in both transgenic and nontransgenic mice following infection with lymphocytic choriomeningitis virus. As expected, nontransgenic mice made a potent virus-specific CD8 T cell response following virus infection. However, as measured in cytolysis assays and by cytokine production, virus-specific CD8 T cell activity was reduced in Ly49A transgenic mice. This inhibition was largely, but not always exclusively, dependent upon the presence, either in vivo or in vitro, of the Ly49A ligand, H-2Dd. Strikingly Ly49A transgenic mice have reduced capacity to control infection with the virulent lymphocytic choriomeningitis virus variant clone 13. Overall, these studies demonstrate that expression of killer inhibitory receptors can modulate anti-viral T cell responses in vivo and in vitro.

Ly-49CB6 NK Inhibitory Receptor Recognizes Peptide-Receptive H-2Kb 1

NK-mediated cytotoxicity involves two families of receptors: activating receptors that trigger lysis of the target cells being recognized and inhibitory receptors specific primarily for MHC I on the target cell surface that can override the activating signal. MHC I molecules on the cell surface can be classified into molecules made stable by the binding of peptide with high affinity or unstable molecules potentially capable of binding high affinity peptide (hence, peptide receptive) and being converted into stable molecules. It has been previously shown that the Ly-49A inhibitory receptor recognizes stable Dd molecules. We show in this study that the inhibitory receptor Ly-49CB6 recognizes peptide-receptive Kb molecules, but does not recognize Kb molecules once they have bound high affinity peptide.

Assessment of Cmv1 candidates by genetic mapping and in vivo antibody depletion of NK cell subsets

The mouse chromosome 6 locus Cmv1 controls resistance to infection with murine cytomegalovirus (MCMV). We have previously shown that Cmv1 is tightly linked to members of the NK gene complex (NKC) including the Ly49 gene family. To assess the candidacy of individual NKC members for the resistance locus, first we followed the co-segregation of Cd94, Nkg2d, and the well-characterized Ly49a, Ly49c and Ly49g genes with respect to Cmv1 in pre-existing panels of intraspecific backcross mice. Gene order and intergene distances (in cM) were: centromere-Cd94/Nkg2d-(0.05)-Ly49a/Ly49c/Ly49 g/Cmv1-(0. 3)-Prp/Kap/D6Mit13/111/219. This result excludes Cd94 and Nkg2d as candidates whereas it localizes the Ly49 genes within the minimal genetic interval for Cmv1. Second, we monitored the cell surface expression of individual Ly49 receptors in MCMV-infected mice over 2 weeks. The proportion of Ly49C(+) and Ly49C/I(+) cells decreased, the proportion of Ly49A(+) and Ly49G2(+) remained constant, and the cell surface density of Ly49G2 increased during infection, suggesting that NK cell subsets might have different roles in the regulation of MCMV infection. Third, we performed in vivo antibody depletion of specific NK cell subsets. Depletion with single antibodies did not affect the resistant phenotype suggesting that Ly49A(+), Ly49C(+), Ly49G2(+) and Ly49C/I(+) populations are not substantial players in MCMV resistance, and arguing for exclusion of the respective genes as candidates for Cmv1. In contrast, mice depleted with combined antibodies showed an intermediate phenotype. Whether residual NK cells, post-depletion, belong to a particular subset expressing another Ly49 receptor, or a molecule encoded by a yet to be identified gene of the NKC, is discussed.