Cloning of a mouse homolog of CD94 extends the family of C-type lectins on murine natural killer cells

Viral infection modulates Qa-1b in infected and bystander cells to properly direct NK cell killing

Natural killer (NK) cell activation depends on the signaling balance of activating and inhibitory receptors. CD94 forms inhibitory receptors with NKG2A and activating receptors with NKG2E or NKG2C. We previously demonstrated that CD94-NKG2 on NK cells and its ligand Qa-1b are important for the resistance of C57BL/6 mice to lethal ectromelia virus (ECTV) infection. We now show that NKG2C or NKG2E deficiency does not increase susceptibility to lethal ECTV infection, but overexpression of Qa-1b in infected cells does. We also demonstrate that Qa-1b is down-regulated in infected and up-regulated in bystander inflammatory monocytes and B cells. Moreover, NK cells activated by ECTV infection kill Qa-1b-deficient cells in vitro and in vivo. Thus, during viral infection, recognition of Qa-1b by activating CD94/NKG2 receptors is not critical. Instead, the levels of Qa-1b expression are down-regulated in infected cells but increased in some bystander immune cells to respectively promote or inhibit their killing by activated NK cells.

Human Natural Killer Receptors, Co-Receptors, and Their Ligands

In the last 20 years, the study of human natural killer (NK) cells has moved from the first molecular characterizations of very few receptor molecules to the identification of a plethora of receptors displaying surprisingly divergent functions. We have contributed to the description of inhibitory receptors and their signaling pathways, important in fine regulation in many cell types, but unknown until their discovery in the NK cells. Inhibitory function is central to regulating NK-mediated cytolysis, with different molecular structures evolving during speciation to assure its persistence. More recently, it has become possible to characterize the NK triggering receptors mediating natural cytotoxicity, unveiling the existence of a network of cellular interactions between effectors of both natural and adaptive immunity. This unit reviews the contemporary history of molecular studies of receptors and ligands involved in NK cell function, characterizing the ligands of the triggering receptor and the mechanisms for finely regulating their expression in pathogen-infected or tumor cells. © 2018 by John Wiley & Sons, Inc.

Evidence of functional Cd94 polymorphism in a free-living house mouse population

The CD94 receptor, expressed on natural killer (NK) and CD8+ T cells, is known as a relatively non-polymorphic receptor with orthologues in humans, other primates, cattle, and rodents. In the house mouse (Mus musculus), a single allele is highly conserved among laboratory strains, and reports of allelic variation in lab- or wild-living mice are lacking, except for deficiency in one lab strain (DBA/2J). The non-classical MHC-I molecule Qa-1b is the ligand for mouse CD94/NKG2A, presenting alternative non-americ fragment of leader peptides (Qa-1 determinant modifier (Qdm)) from classical MHC-I molecules. Here, we report a novel allele identified in free-living house mice captured in Norway, living among individuals carrying the canonical Cd94 allele. The novel Cd94^LocA allele encodes 12 amino acid substitutions in the extracellular lectin-like domain. Flow cytometric analysis of primary NK cells and transfected cells indicates that the substitutions prevent binding of CD94 mAb and Qa-1b/Qdm tetramers. Our data further indicate correlation of Cd94 polymorphism with the two major subspecies of house mice in Europe. Together, these findings suggest that the Cd94^LocA/NKG2A heterodimeric receptor is widely expressed among M. musculus subspecies musculus, with ligand-binding properties different from mice of subspecies domesticus, such as the C57BL/6 strain.

MICA and NKG2D: Is There an Impact on Kidney Transplant Outcome?

This paper aims to present an overview of MICA and natural killer group 2 member D (NKG2D) genetic and functional interactions and their impact on kidney transplant outcome. Organ transplantation has gone from what can accurately be called a “clinical experiment” to a routine and reliable practice, which has proven to be clinically relevant, life-saving and cost-effective when compared with non-transplantation management strategies of both chronic and acute end-stage organ failures. The kidney is the most frequently transplanted organ in the world (transplant-observatory¹). The two treatment options for end-stage renal disease (ESRD) are dialysis and/or transplantation. Compared with dialysis, transplantation is associated with significant improvements in quality of life and overall longevity. A strong relationship exists between allograft loss and human leukocyte antigens (HLA) antibodies (Abs). HLA Abs are not the only factor involved in graft loss, as multiple studies have shown that non-HLA antigens are also involved, even when a patient has a good HLA matche and receives standard immunosuppressive therapy. A deeper understanding of other biomarkers is therefore important, as it is likely to lead to better monitoring (and consequent success) of organ transplants. The objective is to fill the void left by extensive reviews that do not often dive this deep into the importance of MICA and NKG2D in allograft acceptance and their partnership in the immune response. There are few papers that explore the relationship between these two protagonists when it comes to kidney transplantation. This is especially true for the role of NKG2D in kidney transplantation. These reasons give a special importance to this review, which aims to be a helpful tool in the hands of researchers in this field.

Development and function of CD94-deficient natural killer cells

The CD94 transmembrane-anchored glycoprotein forms disulfide-bonded heterodimers with the NKG2A subunit to form an inhibitory receptor or with the NKG2C or NKG2E subunits to assemble a receptor complex with activating DAP12 signaling proteins. CD94 receptors expressed on human and mouse NK cells and T cells have been proposed to be important in NK cell tolerance to self, play an important role in NK cell development, and contribute to NK cell-mediated immunity to certain infections including human cytomegalovirus. We generated a gene-targeted CD94-deficient mouse to understand the role of CD94 receptors in NK cell biology. CD94-deficient NK cells develop normally and efficiently kill NK cell-susceptible targets. Lack of these CD94 receptors does not alter control of mouse cytomegalovirus, lymphocytic choriomeningitis virus, vaccinia virus, or Listeria monocytogenes. Thus, the expression of CD94 and its associated NKG2A, NKG2C, and NKG2E subunits is dispensable for NK cell development, education, and many NK cell functions.

Comparative analysis of NK-cell receptor expression and function across primate species: Perspective on antiviral defenses

Natural killer (NK) cells are lymphoid effectors that are involved in the innate immune surveillance against infected and/or tumor cells. Their function is under the fine-tuning control of cell surface receptors that display either inhibitory or activating function and in healthy condition, mediate self-tolerance. It is known that inhibitory receptors are characterized by clonal and stochastic distribution and are extremely sensible to any modification, downregulation or loss of MHC class I surface expression that are induced in autologous cells upon viral infection or cancer transformation. This alteration of the MHC class I expression weakens the strength of the inhibitory receptor-induced interaction, thus resulting in a prompt triggering of NK cell function, which ends up in the inhibition of tumor progression and proliferation of pathogen-infected cells. Thus, the inhibitory function of NK cells is only one face of the coin, since NK-cell activation is controlled by different arrays of activating receptors that finally are involved in the induction of cytolysis and/or cytokine release. Interestingly, the inhibitory NK-cell receptors that are involved in dampening NK cell-mediated responses evolved during speciation in different, often structurally unrelated surface-expressed molecules, all using a conserved signaling pathway. In detail, during evolution, the inhibitory receptors that assure the recognition of MHC class I molecules, originate in, at least, three different ways. This ended up in multigene families showing marked structural divergences that coevolved in a convergent way with the availability of appropriate MHC ligand molecules.

Human natural killer receptors, co-receptors, and their ligands

In the last 20 years, the study of human natural killer (NK) cells has moved from the first molecular characterizations of very few receptor molecules to the identification of a plethora of receptors displaying surprisingly divergent functions. Our laboratory has contributed to the description of inhibitory receptors and their signaling pathways, important in fine regulation in many cell types, but unknown until their discovery in the NK cells. Inhibitory function is central to regulating NK-mediated cytolysis, with different molecular structures evolving during speciation to assure its persistence. Only in the last ten years has it become possible to characterize the NK triggering receptors mediating natural cytotoxicity, leading to an appreciation of the existence of a cellular interaction network between effectors of both natural and adaptive immunity. This report reviews the contemporary history of molecular studies of receptors and ligands involved in NK cell function, characterizing the ligands of the triggering receptor and the mechanisms for finely regulating their expression in pathogen-infected or tumor cells.

Mistaken notions about natural killer cells

As natural killer (NK) cells were first described more than 30 years ago--a lifetime in recent immunological history--this is a good time to reflect on their transition from outcasts of mainstream immunology to prominent players in innate immunity. Whereas much of our existing knowledge about NK cells is taken as longstanding fact, it may be surprising to immunologists of a younger vintage, particularly those working on NK cells today, that some of this knowledge was, at least initially, unexpected. In this Perspective, I take an unconventional approach to discussing our progress in understanding NK cells.

CD94-NKG2A recognition of human leukocyte antigen (HLA)-E bound to an HLA class I leader sequence

The recognition of human leukocyte antigen (HLA)-E by the heterodimeric CD94-NKG2 natural killer (NK) receptor family is a central innate mechanism by which NK cells monitor the expression of other HLA molecules, yet the structural basis of this highly specific interaction is unclear. Here, we describe the crystal structure of CD94-NKG2A in complex with HLA-E bound to a peptide derived from the leader sequence of HLA-G. The CD94 subunit dominated the interaction with HLA-E, whereas the NKG2A subunit was more peripheral to the interface. Moreover, the invariant CD94 subunit dominated the peptide-mediated contacts, albeit with poor surface and chemical complementarity. This unusual binding mode was consistent with mutagenesis data at the CD94-NKG2A-HLA-E interface. There were few conformational changes in either CD94-NKG2A or HLA-E upon ligation, and such a "lock and key" interaction is typical of innate receptor-ligand interactions. Nevertheless, the structure also provided insight into how this interaction can be modulated by subtle changes in the peptide ligand or by the pairing of CD94 with other members of the NKG2 family. Differences in the docking strategies used by the NKG2D and CD94-NKG2A receptors provided a basis for understanding the promiscuous nature of ligand recognition by NKG2D compared with the fidelity of the CD94-NKG2 receptors.

Differential induction of CD94 and NKG2 in CD4 helper T cells. A consequence of influenza virus infection and interferon-gamma?

Influenza A virus causes worldwide epidemics and pandemics and the investigation of memory T helper (Th) cells that help maintain serological memory following infection is important for vaccine design. In this study we investigated CD94 and NKG2 gene expression in memory CD4 T-cell clones established from the spleens of C57BL/10 (H-2(b)) and BALB/c (H-2(d)) mice infected with influenza A virus (H3N2). CD94 and NKG2A/C/E proteins form heterodimeric membrane receptors that are involved in virus recognition. CD94 and NKG2 expression have been well characterized in natural killer (NK) and cytotoxic T cells. Despite CD94 being potentially an important marker for Th1 cells involved in virus infection, however, there has been little investigation of its expression or function in the CD4 T-cell lineage and no studies have looked at in-vivo-generated Th cells or memory cells. We show in this study that in-vivo-generated CD4 Th1 cells, but not Th2 cells, exhibited full-length CD94 and NKG2A gene expression following activation with viral peptide. For NKG2A, a novel 'short' (possibly redundant) truncated isoform was detectable in a Th2 cell clone. Another member of the NK receptor family, NKG2D, but not NKG2C or E, was also differentially expressed in Th1 cells. We show here that CD94 and NKG2A may exist as multiple isoforms with the potential to distinguish helper T-cell subsets.

Complexity in the cattle CD94/NKG2 gene families

Natural killer cell responses are controlled to a large extent by the interaction of an array of inhibitory and activating receptors with their ligands. The mostly nonpolymorphic CD94/NKG2 receptors in both humans and mice were shown to recognize a single nonclassical MHC class I molecule in each case. In this paper, we describe the CD94/NKG2 gene family in cattle. NKG2 and CD94 sequences were amplified from cDNA derived from four animals. Four CD94 sequences, ten NKG2A, and three NKG2C sequences were identified in total. In contrast to human, we show that cattle have multiple distinct NKG2A genes, some of which show minor allelic variation. All of the sequences designated NKG2A have two tyrosine-based inhibitory motifs in the cytoplasmic domain and one putative gene has, in addition, a charged residue in the transmembrane domain. NKG2C appears to be essentially monomorphic in cattle. All of the NKG2A sequences are similar apart from NKG2A-01, which, in contrast, shares the majority of its carbohydrate recognition domain with NKG2-C. Most of the genes appear to generate multiple alternatively spliced forms. These findings suggest that the CD94/NKG2A heterodimers in cattle, in contrast to other species, are binding several different ligands. Because NKG2C is not polymorphic, this raises questions as to the combined functional capacity of the CD94/NKG2 gene families in cattle.

The role of the NKG2D receptor for tumor immunity

Natural killer (NK) cells have originally been identified based on their capacity to kill transformed cells in a seemingly non-specific fashion. Over the last 15 years, knowledge on receptor ligand systems used by NK cells to specifically detect transformed cells has been accumulating rapidly. One of these receptor ligand systems, the NKG2D pathway, has received particular attention, and now serves as a paradigm for how the immune system is able to gather information about the health status of autologous host cells. In addition to its significance on NK cells, NKG2D, as well as other NK cell receptors, play significant roles on T cells. This review aims at summarizing recent insights into the regulation of NKG2D function, the control over NKG2D ligand expression and the role of NKG2D in tumor immunity. Finally, we will discuss first attempts to exploit NKG2D function to improve immunity to tumors.

Natural killer cell immune responses

Natural killer (NK) cells play a vital role in innate immune responses to infection; they express activation receptors that recognize virus-infected cells. Highly related to receptors recognizing tumor cells, the activation receptors trigger cytotoxicity and cytokine production. NK cells also express inhibitory receptors for major histocompatibility complex (MHC) class I molecules that block the action of the activation receptors. Although many ligands for NK cell receptors have MHC class I folds, recent studies also indicate ligands resembling the NK cell receptors themselves. A combination of immunologic, genetic, biophysical, and in vivo approaches is being employed to understand fully how these receptors contribute to NK cell activities in innate immunity to pathogens and tumors.

NKG2D-mediated activation of cytotoxic lymphocytes: unique signaling pathways and distinct functional outcomes

NKG2D is an important immunosurveillance receptor that triggers a unique signal transduction pathway resulting in a variety of functional outcomes. NKG2D couples to the non-ITAM-containing DAP10 and initiates at least two signaling branches that are both required for cytotoxicity. Transformed, infected, or healthy cells can express NKG2D ligands, and NKG2D(+) lymphocytes can be unaffected, costimulated, or fully activated by the NKG2D-ligand interaction. The NKG2D-mediated response can be modulated by factors such as cytokine milieu and possibly the particular ligand expressed. Thus, this nontraditional NKG2D-DAP10 initiated signal pathway enables lymphocytes to differentially respond to ligands based on the cellular environment.

Immature NK cells suppress dendritic cell functions during the development of leukemia in a mouse model

To analyze the mechanisms by which cancer cells escape from hosts' immune surveillance, we investigated the changes in immune status during the progression of leukemia induced by injecting mice with WEHI-3B cells. In the bone marrow (BM) of leukemic mice, only DX5(+)CD3(-) cells were continuously increased, despite the progression of leukemia. In addition, DX5(+)CD3(-) cells were rapidly increased in peripheral blood (PB) 20 days after inoculation. We also found that myeloid dendritic cells (DCs) expressing low levels of I-A(d) and having low allo-T cell stimulatory activity were markedly increased in PB and spleen. The increase in DX5(+) cells in BM was thought to be induced by soluble factors from leukemic cells. DX5(+) cells from leukemic mice were CD3(-), B220(-), Gr-1(-), CD14(-), CD94(-), Ly-49C/F(-), asialo GM1(+), CD25(+), CD122(+), Thy-1(bright), and c-kit(dim) and showed low killing activity against YAC-1 cells, suggesting that those DX5(+) cells were immature NK cells. NK cells from leukemic PB down-regulated the expression of I-A(d) on DCs, an effect mediated by TGF-beta. Moreover, these NK cells significantly suppressed the allo-T cell stimulatory activity of DCs, an effect requiring cell-to-cell contact between NK cells and DCs and thought to involve CD25. Importantly, NK cells from leukemic PB inhibited generation of autotumor-specific CTL induced by DCs in primary MLR or by DC immunization. In conclusion, we identified circulating immature NK cells with immunosuppressive activities. These cells may be important for understanding the involvement of the host immune system during the development of leukemia.

How TCRs bind MHCs, peptides, and coreceptors

Since the first crystal structure determinations of alphabeta T cell receptors (TCRs) bound to class I MHC-peptide (pMHC) antigens in 1996, a sizable database of 24 class I and class II TCR/pMHC complexes has been accumulated that now defines a substantial degree of structural variability in TCR/pMHC recognition. Recent determination of free and bound gammadelta TCR structures has enabled comparisons of the modes of antigen recognition by alphabeta and gammadelta T cells and antibodies. Crystal structures of TCR accessory (CD4, CD8) and coreceptor molecules (CD3epsilondelta, CD3epsilongamma) have further advanced our structural understanding of most of the components that constitute the TCR signaling complex. Despite all these efforts, the structural basis for MHC restriction and signaling remains elusive as no structural features that define a common binding mode or signaling mechanism have yet been gleaned from the current set of TCR/pMHC complexes. Notwithstanding, the impressive array of self, foreign (microbial), and autoimmune TCR complexes have uncovered the diverse ways in which antigens can be specifically recognized by TCRs.

NK cell recognition of mouse cytomegalovirus-infected cells

Abstract Natural killer (NK) cells and cytomegalovirus have been locked in an evolutionary arms race for millions of years in an attempt to overwhelm each other. Cytomegaloviruses deploy cunning disguises to avoid detection by NK cells. Studies of the mouse model of infection have shown that NK cells deploy multiple mechanisms to deal with mouse cytomegalovirus (MCMV) infection, which involve receptors of the C-lectin type superfamily. Remarkably, these receptors have two additional common features: They map to the same genetic region, known as the NK cell gene complex; and they recognize MHC class I-related structures. While reviewing these attack-counterattack measures, this chapter points to the central role that recognition of the MCMV-infected cells by NK cells plays in host resistance to infection.

The natural killer complex regulates severe malarial pathogenesis and influences acquired immune responses to Plasmodium berghei ANKA

The natural killer complex (NKC) is a genetic region of highly linked genes encoding several receptors involved in the control of NK cell function. The NKC is highly polymorphic, and allelic variability of various NKC loci has been demonstrated in inbred mice. Making use of BALB.B6-Cmv1r congenic mice, in which the NKC from disease-susceptible C57BL/6 mice has been introduced into the disease-resistant BALB/c background, we show here that during murine malaria infection, the NKC regulates a range of pathophysiological syndromes such as cerebral malaria, pulmonary edema, and severe anemia, which contribute to morbidity and mortality in human malaria. Parasitemia levels were not affected by the NKC genotype, indicating that control of malarial fatalities by the NKC cells does not operate through effects on parasite growth rate. Parasite-specific antibody responses and the proinflammatory gene transcription profile, as well as the TH1/TH2 balance, also appeared to be influenced by NKC genotype, providing evidence that this region, known to control innate immune responses via NK and/or NK T-cell activation, can also significantly regulate acquired immunity to infection. To date, NKC-encoded innate system receptors have been shown mainly to regulate viral infections. Our data provide evidence for critical NKC involvement in the broad immunological responses to a protozoan parasite.

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.

Structural and functional aspects of the Ly49 natural killer cell receptors

Natural killer cells are part of the first line of innate immune defence against virus-infected cells and cancer cells in the vertebrate immune system. They are called 'natural' killers because, unlike cytotoxic T cells, they do not require a previous challenge and preactivation to become active. The Ly49 NK receptors are type II transmembrane glycoproteins, structurally characterized as disulphide-linked homodimers. They share extensive homology with C-type lectins, and they are encoded by a multigene family that in mice maps on chromosome 6. A fine balance between inhibitory and activating signals regulates the function of NK cells. Inhibitory Ly49 molecules bind primarily MHC class I ligands, whereas the ligands for activating Ly49 molecules may include MHC class I, but also interestingly MHC class I-like molecules expressed by viruses, as is the case for Ly49H, which binds the m157 gene product of murine cytomegalovirus. In this study, we review the function and X-ray crystal structure of the Ly49 NK cell receptors hitherto determined (Ly49A, Ly49C and Ly49I), and the structural features of the Ly49/MHC class I interaction as revealed by the X-ray crystal structures of Ly49A/H-2Dd and the recently determined Ly49C/H-2Kb.

UL16 binding proteins

According to present concepts, innate immunity plays an important role in tumor surveillance and immune modulation. The state of NK cells depends on the balance between inhibitory and activating signals from corresponding receptors. As one of the activating receptors, NKG2D recognises some self ligands such as MICA/B in human and Rae1 in mice, which is dissimilar to those toll-like receptors that recognise some pathogen-derived ligands. NKG2D is expressed not only on NK cells, but on gammadelta T cells, CD8+ alphabeta T cells in normal individuals and CD4+ alphabeta T cells in rheumatoid arthritis patients and plays a different role on respective cells. Whereas NKG2D can only function as a costimulatory receptor on CD8+ alphabeta T cells under the domination of alphabeta TCR in spite of a deficiency of costimulatory molecule CD28, NKG2D can directly activate NK cells even in the presence of inhibitory signals from MHC-I and corresponding receptor complexes. Experiments in mice have identified that alternative splicing produces two distinct NKG2D polypeptides that associate differentially with the DAP10 and DAP12 signaling subunits and that differential expression of these isoforms and of signaling proteins determines whether NKG2D only functions as a costimulatory receptor in the adaptive immune system (CD8+ T cells) or as both a primary recognition unit and a costimulatory receptor in the innate immune system (natural killer cells and macrophages). This review summarizes the research achievements in a new ligand family (UL16 binding proteins) of NKG2D in human and shows the possible prospects of ULBP function and application.

Bone marrow allograft rejection mediated by a novel murine NK receptor, NKG2I

Natural killer (NK) cells mediate bone marrow allograft rejection. However, the molecular mechanisms underlying such a rejection remain elusive. In previous analyses, it has been shown that NK cells recognize allogeneic target cells through Ly-49s and CD94/NKG2 heterodimers. Here, we describe identification and characterization of a novel murine NK receptor, NKG2I, belonging to the NKG2 family. NKG2I, which was composed of 226 amino acids, showed ∼40% homology to the murine NKG2D and CD94 in the C-type lectin domain. Flow cytometric analysis with anti-NKG2I monoclonal antibody (mAb) revealed that expression of NKG2I was largely confined to NK and NKT cells, but was not seen in T cells. Furthermore, anti-NKG2I mAb inhibited NK cell–mediated cytotoxicity, whereas cross-linking of NKG2I enhanced interleukin 2– and interleukin 12–dependent interferon-γ production. Similarly, the injection of anti-NKG2I mAb before the allogeneic bone marrow transfer in vivo impinged on the function of NKG2I, resulting in the enhanced colony formation in the spleen. NKG2I is a novel activating receptor mediating recognition and rejection of allogeneic target cells.

Human CD94 gene expression: dual promoters differing in responsiveness to IL-2 or IL-15

CD94 is a C-type lectin required for the dimerization of the CD94/NKG2 family of receptors, which are expressed on NK cells and T cell subsets. Little is known about CD94 gene expression and the elements that regulate CD94 transcription. In this study, we report that CD94 gene expression is regulated by distal and proximal promoters that transcribe unique initial exons specific to each promoter. This results in two species of transcripts; the previously described CD94 mRNA and a novel CD94C mRNA. All NK cells and CD94(+), CD8(+) alphabeta T cells transcribe CD94 mRNA. Stimulation of NK and CD8(+) alphabeta T cells with IL-2 or IL-15 induced the transcription of CD94C mRNA. The distal and proximal promoters both contain elements with IFN-gamma-activated and Ets binding sites, known as GAS/EBS. Additionally, an unknown element, termed site A, was identified in the proximal promoter. EMSA analyses showed that constitutive factors could bind to oligonucleotide probes containing each element. After treatment of primary NK cells with IL-2 or IL-15, separate inducible complexes could be detected with oligonucleotide probes containing either the proximal or distal GAS/EBS elements. These elements are highly conserved between mice and humans, which suggests that both species regulate CD94 gene expression via mechanisms that predate their evolutionary divergence.

Proliferative and cytotoxic response of human natural killer cells exposed to transporter associated with antigen-processing-deficient cells

In transporter associated with antigen-processing (TAP)-deficient patients affected by a severe downmodulation of human leucocyte antigen class I (HLA-I) molecules, natural killer (NK) cells have an increased expression of the inhibitory receptor CD94/NKG2A. Focusing our attention on NK cells, we have investigated the phenotype, function and proliferative response of peripheral blood lymphocytes (PBLs) derived from healthy donors after coculturing with TAP (T2)- or HLA-I-deficient (721.221) cell lines and their related HLA-I-expressing transfectants (T3 and DT360, respectively). After 4 days, NK cells cocultured with T2 cells had a threefold increased CD94 expression compared to NK cells cocultured with T3. This increase was due to proliferation of the CD56brightCD94bright subset. In contrast, expression of other inhibitory receptors [killer cell immunoglobulin (Ig)-like receptors] was variable during time and was not related to HLA-I molecules expressed by stimulating cells. Similar results were obtained using HLA-I-deficient cells (721.221). The PBLs cocultured for 4 days with T2 cells displayed enhanced cytotoxic responses. The results suggest that CD56brightCD94bright NK cells are induced to proliferate and kill in response to a TAP-deficient environment. The changes seen in the NK-cell compartment were partially contributed by T lymphocytes present in the coculture. These data could explain the increased CD94 expression and autoimmune manifestations observed in TAP-deficient patients.

The IL-12 Signature: NK Cell Terminal CD56+high Stage and Effector Functions

We report that human peripheral NK cells expressing high CD56 levels (CD56(+high)) are terminally differentiated cells indistinguishable from mature NK cells recently activated in the presence of IL-12, and not a functionally distinct NK-cell subset or progenitors to mature CD56(+low) NK cells. CD56(+high) NK cells coexpress all differentiation Ags constitutive or inducible in mature (CD56(+)) NK cells, except CD16, present at lower level than on most mature NK cells. Also, activation markers, activating receptors and adhesion molecules, and most inducible receptors are expressed exclusively and constitutively and are inducible at higher levels on CD56(+high) than on CD56(+low) NK cells. Consistent with their activated phenotype, many CD56(+high) NK cells are cycling and mediate heightened effector functions (proliferation, IFN-gamma and IL-10 but not IL-13 production) in response to IL-12 and other NK cell-specific stimuli. Conversely, IL-12 induces on CD56(+low) NK cells all markers constitutively expressed on the CD56(+high) NK cells, concomitantly preventing the IL-2 (and IL-15)-inducible expression of NKp44 and CD16 re-expression after immune complex-induced down-modulation, and CD56(-/+low) NK cells acquire a CD56(+high) NK cell phenotype in short term in vitro culture with IL-12. The significance of these findings to the NK cell-mediated regulation of immune responses and NK cell development is discussed.

Ly49E expression points toward overlapping, but distinct, natural killer (NK) cell differentiation kinetics and potential of fetal versus adult lymphoid progenitors

Using a new antibody, we found previously that contrary to adult natural killer (NK) cells, fetal NK cells have a unique phenotype, as they exclusively express Ly49E. This can be explained by an intrinsic different NK differentiation potential of fetal versus adult lymphoid progenitors, by immaturity of fetal NK cells or by instability of Ly49E expression. Here, we show that adult progenitor cells were still capable of differentiating into Ly49E-expressing NK cells but at a much lower frequency. Surprisingly, Ly49E expression in vitro did not require stromal cells. Kinetic analysis in vivo showed that Ly49E was expressed early, together with CD94/NKG2 and Ly49G2, followed by Ly49C, and finally Ly49D. Transfer of sorted Ly49E-positive fetal NK cells showed stable Ly49E expression, and later, part of these cells up-regulated other Ly49 members. These data indicate that although there are intrinsic differences, there is no strict fetal and adult wave of NK cell differentiation.

Activated, but not resting, T cells can be recognized and killed by syngeneic NK cells

We demonstrate that IL-2-activated NK cells or lymphokine-activated killer cells recognize and kill syngeneic CD4(+) and CD8(+) T cells that have been activated by APCs. Induction with APC required TCR-specific Ag, and lysis was perforin mediated. Brefeldin A, which disrupts protein transport, inhibited the sensitivity induced by activation. In BALB/c, expression of NKG2D ligands correlated with lysis and could be inhibited by brefeldin A. As well, addition of anti-NKG2D mAb to a killing assay completely abrogated lysis. Transduction of mouse NKG2D into a human NK cell line, YTSeco, conferred upon it the ability to kill activated BALB/c T cells, indicating that NKG2D is necessary for recognition. Our data provide a basis for studying a role for NK cells in T cell regulation.

Selective associations with signaling proteins determine stimulatory versus costimulatory activity of NKG2D

Optimal lymphocyte activation requires the simultaneous engagement of stimulatory and costimulatory receptors. Stimulatory immunoreceptors are usually composed of a ligand-binding transmembrane protein and noncovalently associated signal-transducing subunits. Here, we report that alternative splicing leads to two distinct NKG2D polypeptides that associate differentially with the DAP10 and KARAP (also known as DAP12) signaling subunits. We found that differential expression of these isoforms and of signaling proteins determined whether NKG2D functioned as a costimulatory receptor in the adaptive immune system (CD8+ T cells) or as both a primary recognition structure and a costimulatory receptor in the innate immune system (natural killer cells and macrophages). This strategy suggests a rationale for the multisubunit structure of stimulatory immunoreceptors.

Variable receptors controlling activation and inhibition of NK cells

NK cells are important effector lymphocytes of innate immunity; they kill infected cells and produce cytokines that stimulate other immune effects. Once considered relatively homogeneous, NK cells are now seen to be highly diverse. Within an individual, expression of different combinations of inhibitory and stimulatory receptors creates a diverse NK cell repertoire, which exhibits specificity in the immune response. Rapid evolution of NK cell receptor gene families distinguishes members of a species and causes substantial species-specific differences in NK cell receptor systems. All known ligands for these diverse receptors are MHC class I molecules, or molecules of host or pathogen origin that are homologous to MHC class I.

The innate immune response to tumors and its role in the induction of T-cell immunity

Recent genetic studies have resurrected the concept that the adaptive and innate immune systems play roles in tumor surveillance. Natural killer (NK) cells recognize many tumor cells but not normal self cells, and they are thought to aid in the elimination of nascent tumors. Two main strategies are employed by NK cells to recognize tumor targets. Many tumor cells down-regulate class I major histocompatibility complex (MHC) molecules, thus releasing the NK cell from the inhibition provided by class I MHC-specific inhibitory receptors ('missing self recognition'). More recently, it has become clear that a stimulatory receptor expressed by NK cells, T cells and macrophages (NKG2D) recognizes ligands (MHC class I chain related [MIC], H6O, retinoic acid early inducible [Rae1] and UL16 binding proteins [ULBP]) that are up-regulated on tumor cells and virally infected cells but are not expressed well by normal cells. Ectopic expression of these ligands on tumor cells leads to the potent rejection of the tumors in vivo. Importantly, mice that previously rejected the ligand+ tumor cells develop T-cell immunity to the parental (ligand-) tumor cells. The recognition of induced-self ligands as a strategy to recognize abnormal self sets a precedent for a new immune recognition strategy of the innate immune system.

Viral and bacterial infections induce expression of multiple NK cell receptors in responding CD8(+) T cells

NK cells express several families of receptors that play central roles in target cell recognition. These NK cell receptors are also expressed by certain memory phenotype CD8(+) T cells, and in some cases are up-regulated in T cells responding to viral infection. To determine how the profile of NK receptor expression changes in murine CD8(+) T cells as they respond to intracellular pathogens, we used class I tetramer reagents to directly examine Ag-specific T cells during lymphocytic choriomeningitis virus and Listeria monocytogenes infections. We found that the majority of pathogen-specific CD8(+) T cells initiated expression of the inhibitory CD94/NKG2A heterodimer, the KLRG1 receptor, and a novel murine NK cell marker (10D7); conversely, very few Ag-specific T cells expressed Ly49 family members. The up-regulation of these receptors was independent of IL-15 and persisted long after clearance of the pathogen. The expression of CD94/NKG2A was rapidly initiated in naive CD8(+) T cells responding to peptide Ags in vitro and on many of the naive T cells that proliferate when transferred into lymphopenic (Rag-1(-/-)) hosts. Thus, CD94/NKG2A expression is a common consequence of CD8(+) T cell activation. Binding of the CD94/NKG2A receptor by its ligand (Qa-1(b)) did not significantly inhibit CD8(+) T cell effector functions. However, expression of CD94 and NKG2A transgenes partially inhibited early events of T cell activation. These subtle effects suggest that CD94/NKG2A-mediated inhibition of T cells may be limited to particular circumstances or may synergize with other receptors that are similarly up-regulated.

Genetic control of human NK cell repertoire

Through differential killer cell Ig-like receptor (KIR) and CD94:NKG2 gene expression, human NK cells generate diverse repertoires, each cell having an inhibitory receptor for autologous HLA class I. Using a new method for measuring repertoire difference that integrates multiple flow cytometry parameters, we found individual repertoire stability, but population variability. Correlating repertoire differences with KIR and HLA genotype for 85 sibling pairs reveals the dominant influence of KIR genotype; HLA genotype having a subtle, modulating effect on relative KIR expression frequencies. HLA and/or KIR genotype also influences CD94:NKG2A expression. After HLA-matched stem cell transplantation, KIR repertoires either recapitulated that of the donor or were generally depressed for KIR expression. Human NK cell repertoires are defined by combinations of variable KIR and HLA class I genes and conserved CD94:NKG2 genes.

Orderly and nonstochastic acquisition of CD94/NKG2 receptors by developing NK cells derived from embryonic stem cells in vitro

In mice there are two families of MHC class I-specific receptors, namely the Ly49 and CD94/NKG2 receptors. The latter receptors recognize the nonclassical MHC class I Qa-1(b) and are thought to be responsible for the recognition of missing-self and the maintenance of self-tolerance of fetal and neonatal NK cells that do not express Ly49. Currently, how NK cells acquire individual CD94/NKG2 receptors during their development is not known. In this study, we have established a multistep culture method to induce differentiation of embryonic stem (ES) cells into the NK cell lineage and examined the acquisition of CD94/NKG2 by NK cells as they differentiate from ES cells in vitro. ES-derived NK (ES-NK) cells express NK cell-associated proteins and they kill certain tumor cell lines as well as MHC class I-deficient lymphoblasts. They express CD94/NKG2 heterodimers, but not Ly49 molecules, and their cytotoxicity is inhibited by Qa-1(b) on target cells. Using RT-PCR analysis, we also report that the acquisition of these individual receptor gene expressions during different stages of differentiation from ES cells to NK cells follows a predetermined order, with their order of acquisition being first CD94; subsequently NKG2D, NKG2A, and NKG2E; and finally, NKG2C. Single-cell RT-PCR showed coexpression of CD94 and NKG2 genes in most ES-NK cells, and flow cytometric analysis also detected CD94/NKG2 on most ES-NK cells, suggesting that the acquisition of these receptors by ES-NK cells in vitro is nonstochastic, orderly, and cumulative.

Expression of inhibitory receptors Ly49E and CD94/NKG2 on fetal thymic and adult epidermal TCR V gamma 3 lymphocytes

Ly49 and CD94/NKG2 inhibitory receptors are predominantly expressed on murine NK cells, but they are also expressed on a subpopulation of peripheral CD8 memory TCR alphabeta lymphocytes. In this study we demonstrate that Ly49E and CD94/NKG2 receptors are expressed on mature TCR Vgamma3(+) cells in the fetal thymus. Expression correlated with a memory phenotype, such as expression of CD44, 2B4, and IL-2Rbeta (CD122), and absence of IL-2Ralpha (CD25) expression. No expression of Ly49A, C, D, G2, or I receptors was observed. This phenotype is similar to that of fetal thymic NK cells. Skin-located Vgamma3 T cells, the progeny of fetal thymic Vgamma3 cells, also expressed CD94/NKG2 and Ly49E but not the other members of the Ly49 family. The development and survival of Ly49E(+) or CD94/NKG2(+) Vgamma3 T lymphocytes was not dependent upon expression of MHC class I molecules. The cytotoxicity of TCR Vgamma3 cells was inhibited when Qdm, the ligand for CD94/NKG2, was presented by Qa1(b)-transfected target cells. Also, upon cross-linking of CD94/NKG2 with mAb 3S9, TCR Vgamma3 thymocytes were prevented from killing FcgammaR(+) P815 target cells. These effects were most pronounced in the CD94/NKG2(high) subpopulation as compared with the CD94/NKG2(low) subpopulation of Vgamma3 cells. Our data demonstrate that Vgamma3 T cells expressing inhibitory Ly49E and CD94/NKG2 receptors are mature and display a memory phenotype, and that CD94/NKG2 functions as an inhibitory receptor on these T lymphocytes.

The search for the missing 'missing-self' receptor on natural killer cells

In this article, I outline the studies that led to the discovery of Ly49A as the first 'missing-self ' receptor on natural killer cells. It is a more detailed and formal version of my acceptance speech at the Novartis Award ceremony at the International Congress of Immunology in Stockholm. Yet, it is impossible to mention my gratitude to all of my previous mentors and colleagues or to credit all the scientists and immunologists who preceded us by making contributions, large and small, to knowledge upon which our findings were based. It is therefore humbling to accept the 2001 Novartis Award for Basic Immunology along with Professors Klas Kärre and Lorenzo Moretta.

Implications of CD94 deficiency and monoallelic NKG2A expression for natural killer cell development and repertoire formation

Natural killer (NK) cells are believed to achieve self-tolerance through the expression of self-MHC-specific inhibitory receptors, such as members of the Ly49 and CD94/NKG2 families. Individual Ly49 genes are stochastically expressed by NK subsets and are expressed in a monoallelic fashion, but little is known about the mechanisms underlying CD94/NKG2A expression. We show here that, like Ly49 genes, mouse Nkg2a is stochastically and monoallelically expressed. Thus, a single general mechanism controls expression of all known MHC-specific receptors by mouse NK cells. In addition, we find that DBA/2J mice are naturally CD94-deficient and do not express cell-surface CD94/NKG2A receptors, even on neonatal NK cells. Thus, self-tolerance of neonatal NK cells cannot be attributed to CD94/NKG2A expression. Taken together, the results lead to a reconsideration of current models of NK cell development and self-tolerance.

A cluster of ten novel MHC class I related genes on human chromosome 6q24.2-q25.3

We have identified a novel family of human major histocompatibility complex (MHC) class I genes. This MHC class I related gene family is defined by 10 members, among which 6 encode potentially functional glycoproteins. The 180-kb cluster containing them has been generated by serial duplication and minimal diversification of an ancestral prototype. They are not located within the MHC on 6p21.3, but near the tip of its long arm at q24.2-q25.3, close to the human equivalent of the mouse H2-linked t-complex, a subchromosomal region syntenic to a segment of mouse chromosome 10 harboring the orthologous MHC class I related retinoic acid early transcript loci, Raet1a-d. Hence we have named the identified loci RAET1E-N. Human RAET1 products are all devoid of the membrane-proximal immunoglobulin-like alpha3 domain and most, but not all, are predicted to remain membrane-anchored via glycosylphosphatidylinositol linkage and are shown to display an atypical pattern of polymorphism. RAET1 transcripts are absent from hematopoietic tissues, but largely expressed in tumors. The involvement of orthologous mouse RAET1A-D/H60 in natural killer and T-cell activation through NKG2D engagement augurs a similar function for the human RAET1 proteins.

Conservation and variation in human and common chimpanzee CD94 and NKG2 genes

To assess polymorphism and variation in human and chimpanzee NK complex genes, we determined the coding-region sequences for CD94 and NKG2A, C, D, E, and F from several human (Homo sapiens) donors and common chimpanzees (Pan troglodytes). CD94 is highly conserved, while the NKG2 genes exhibit some polymorphism. For all the genes, alternative mRNA splicing variants were frequent among the clones obtained by RT-PCR. Alternative splicing acts similarly in human and chimpanzee to produce the CD94B variant from the CD94 gene and the NKG2B variant from the NKG2A gene. Whereas single chimpanzee orthologs for CD94, NKG2A, NKG2E, and NKG2F were identified, two chimpanzee paralogs of the human NKG2C gene were defined. The chimpanzee Pt-NKG2CI gene encodes a protein similar to human NKG2C, whereas in the chimpanzee Pt-NKG2CII gene the translation frame changes near the beginning of the carbohydrate recognition domain, causing premature termination. Analysis of a panel of chimpanzee NK cell clones showed that Pt-NKG2CI and Pt-NKG2CII are independently and clonally expressed. Pt-NKG2CI and Pt-NKG2CII are equally diverged from human NKG2C, indicating that they arose by gene duplication subsequent to the divergence of chimpanzee and human ancestors. Genomic DNA from 80 individuals representing six primate species were typed for the presence of CD94 and NKG2. Each species gave distinctive typing patterns, with NKG2A and CD94 being most conserved. Seven different NK complex genotypes within the panel of 48 common chimpanzees were due to differences in Pt-NKG2C and Pt-NKG2D genes.

Rae1 and H60 ligands of the NKG2D receptor stimulate tumour immunity

Natural killer (NK) cells attack many tumour cell lines, and are thought to have a critical role in anti-tumour immunity; however, the interaction between NK cells and tumour targets is poorly understood. The stimulatory lectin-like NKG2D receptor is expressed by NK cells, activated CD8+ T cells and by activated macrophages in mice. Several distinct cell-surface ligands that are related to class I major histocompatibility complex molecules have been identified, some of which are expressed at high levels by tumour cells but not by normal cells in adults. However, no direct evidence links the expression of these 'induced self' ligands with tumour cell rejection. Here we demonstrate that ectopic expression of the murine NKG2D ligands Rae1beta or H60 in several tumour cell lines results in potent rejection of the tumour cells by syngeneic mice. Rejection is mediated by NK cells and/or CD8+ T cells. The ligand-expressing tumour cells induce potent priming of cytotoxic T cells and sensitization of NK cells in vivo. Mice that are exposed to live or irradiated tumour cells expressing Rae1 or H60 are specifically immune to subsequent challenge with tumour cells that lack NKG2D ligands, suggesting application of the ligands in the design of tumour vaccines.

Natural killer cells and nitric oxide

Natural killer (NK) cells and nitric oxide (NO) are both important components of the natural or innate immune response. NK cells are large granular lymphocytes capable of destroying cells infected by virus or bacteria and susceptible tumor cells without prior sensitization and restriction by MHC antigens. They are abundant in blood, spleen, liver and lungs and are distinct from both T and B lymphocytes in their circulation patterns, profile of surface antigens, receptor repertoire and the way in which they discriminate between self and non-self. Uniquely, NK cells express receptors that can recognize and discriminate between normal and altered MHC class I determinants. NK cell cytotoxic activity is strongly induced by cytokines such as IL-2 and IL-12, and this activation is associated with synthesis of NO. Inhibitors of NO synthesis impair NK cell-mediated target cell killing, demonstrating a role for NO in NK cell function. Furthermore, NO itself can regulate NK cell activation. In this article, evidence that NO is a mediator of NK cell-mediated target cell killing, and that NO is a regulator of NK cell activation will be reviewed. Results of NO synthase gene deletion studies will be discussed, and rodent and human NK cells will be compared.

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.

The NK cell MHC class I receptor Ly49A detects mutations on H-2Dd inside and outside of the peptide binding groove

The NK cell inhibitory receptor Ly49A recognizes the mouse MHC class I molecule H-2D(d) and participates in the recognition of missing self. Previous studies indicated that the determinant recognized by Ly49A exists in alpha1/alpha2 domain of H-2D(d). Here we have substituted polymorphic as well as conserved residues of H-2D(d) alpha1/alpha2 domain (when compared with H-2K(d), which does not interact with Ly49A). We then tested the ability of the H-2D(d) mutants to interact with Ly49A by soluble Ly49A tetramer binding and NK cell cytotoxicity inhibition assays. Individual introduction of mutations converting the H-2D(d) residue into the corresponding H-2K(d) residue (N30D, D77S, or A99F) in H-2D(d) partially abrogated the interaction between Ly49A and H-2D(d). Introduction of the three mutations into H-2D(d) completely abolished Ly49A recognition. Individual introduction of D29N or R35A mutation into the residues of H-2D(d) that are conserved among murine MHC class I severely impaired the interaction. The crystal structure of H-2D(d) reveals that D77 and A99 are located in the peptide binding groove and that N30, D29, and R35 are in the interface of the three structural domains of MHC class I: alpha1/alpha2, alpha3, and beta(2)-microglobulin. These data suggest that Ly49A can monitor mutations in MHC class I inside and outside of the peptide binding groove and imply that inhibitory MHC class I-specific receptors are sensitive to mutations in MHC class I as well as global loss of MHC class I. Our results also provide insight into the molecular basis of Ly49A to distinguish MHC class I polymorphism.

Mouse CD94 participates in Qa-1-mediated self recognition by NK cells and delivers inhibitory signals independent of Ly-49

Inhibitory receptors expressed on NK cells recognize MHC class I molecules and transduce negative signals to prevent the lysis of healthy autologous cells. The lectin-like CD94/NKG2 heterodimer has been studied extensively as a human inhibitory receptor. In contrast, in mice, another lectin-like receptor, Ly-49, was the only known inhibitory receptor until the recent discovery of CD94/NKG2 homologues in mice. Here we describe the expression and function of mouse CD94 analyzed by a newly established mAb. CD94 was detected on essentially all NK and NK T cells as well as small fractions of T cells in all mouse strains tested. Two distinct populations were identified among NK and NK T cells, CD94(bright) and CD94(dull) cells, independent of Ly-49 expression. The anti-CD94 mAb completely abrogated the inhibition of target killing mediated by NK recognition of Qa-1/Qdm peptide on target cells. Importantly, CD94(bright) but not CD94(dull) cells were found to be functional in the Qa-1/Qdm-mediated inhibition. In the presence of the mAb, activated NK cells showed substantial cytotoxicity against autologous target cells as well as enhanced cytotoxicity against allogeneic and "missing self" target cells. These results suggest that mouse CD94 participates in the protection of self cells from NK cytotoxicity through the Qa-1 recognition, independent of inhibitory receptors for classical MHC class I such as Ly-49.

The non-classical MHC class I molecule Qa-1(b) inhibits classical MHC class I-restricted cytotoxicity of cytotoxic T lymphocytes

The CD94/NKG2A heterodimer is an inhibitory receptor expressed on a subset of mouse NK cells. CD94/NKG2A recognizes the non-classical MHC class I (class Ib) molecule Qa-1(b) and inhibits NK cytotoxicity. Qa-1(b) presents a peptide derived from the leader sequence of classical MHC class I molecules. Here, we examined the role of CD94/NKG2A in T cell-mediated cytotoxicity. Soluble tetrameric Qa-1(b) bound to almost all CD8(+), but not CD4(+), T cells. This binding seems to be mediated by CD8, because COS cells transfected with CD8 also bound Qa-1(b) tetramer. Therefore, the expression of CD94/NKG2 in T cells was further examined by single-cell RT-PCR. Most murine CD8(+) T cells constitutively expressed CD94 and NKG2A transcripts, whereas they were not detected in CD4(+) T cells. Co-expression of Qa-1(b) and D(k) on target cells significantly inhibited cytotoxicity of D(k)-specific cytotoxic T lymphocytes generated by mixed lymphocyte reaction, indicating that Qa-1(b) on antigen-presenting cells interacts with CD94/NKG2A on CD8 T cells and regulates classical MHC class I-restricted cytotoxic T cells. These results suggest a significant role of CD94/NKG2A as an inhibitory receptor on CD8(+) T cells.

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.

Ligands for the murine NKG2D receptor: expression by tumor cells and activation of NK cells and macrophages

Natural killer (NK) cells attack tumor and infected cells, but the receptors and ligands that stimulate them are poorly understood. Here we report the expression cloning of two murine ligands for the lectin-like receptor NKG2D. The two ligands, H-60 and Rae1 beta, are distant relatives of major histocompatibility complex class I molecules. NKG2D ligands are not expressed by most normal cells but are up-regulated on numerous tumor cells. We show that mouse NKG2D is expressed by NK cells, activated CD8+ T cells and activated macrophages. Expression of either NKG2D ligand by target cells triggers NK cell cytotoxicity and interferon-gamma secretion by NK cells, as well as nitric oxide release and tumor necrosis factor alpha transcription by macrophages. Thus, through their interaction with NKG2D, H-60 and Rae1 beta are newly identified potent stimulators of innate immunity.

Viral subversion of the immune system

This review describes the diverse array of pathways and molecular targets that are used by viruses to elude immune detection and destruction. These include targeting of pathways for major histocompatibility complex-restricted antigen presentation, apoptosis, cytokine-mediated signaling, and humoral immune responses. The continuous interactions between host and pathogens during their coevolution have shaped the immune system, but also the counter measures used by pathogens. Further study of their interactions should improve our ability to manipulate and exploit the various pathogens.