Missing self-recognition of Ocil/Clr-b by inhibitory NKR-P1 natural killer cell receptors

Modulation of NK cell function by genetically coupled C-type lectin-like receptor/ligand pairs encoded in the human natural killer gene complex

Functional responses of natural killer (NK) cells including eradication of “harmful” cells and modulation of immune responses are regulated by a broad variety of activating and inhibitory NK receptors. Whereas the leukocyte receptor complex (LRC) encodes for NK receptors of the immunoglobulin superfamily, genes of C-type lectin-like NK receptors are clustered in the mammalian natural killer gene complex (NKC). Besides the thoroughly studied C-type lectin-like receptors NKG2D, CD94/NKG2x, and members of the murine Ly49 subfamily, the NKC also encodes for NK receptors of the less characterized NKRP1 subfamily. The prototypic mouse NKRP1 receptor is Nkrp1c (also known as NK1.1), while human members of the NKRP1 subfamily are NKRP1A, NKp80, and NKp65. The latter are not straight homologs of mouse NKRP1 receptors, but share distinct subfamily-specific traits classifying them as members of the NKRP1 subfamily. Ligands of the human NKPR1 receptors are likewise C-type lectin-like glycoproteins belonging to the CLEC2 subfamily (i.e., LLT1, AICL, and KACL), and are encoded in the NKC in tight genetic linkage to their respective receptors. Similarly, certain members of the mouse NKRP1 subfamily interact with genetically coupled CLEC2 glycoproteins, while the reasons for this intriguing tight genetic linkage remain unknown. Recent studies provided new and unique insights into the expression, interaction, and signaling of NKRP1 receptors and their ligands, thereby substantially advancing our understanding of their function and biology. Here, we review our current knowledge on NKRP1 receptors and their genetically linked CLEC2 ligands with an emphasis on the human receptor/ligand pairs NKRP1A-LLT1, NKp80-AICL, and NKp65-KACL.

Re-evaluation of the involvement of NK cells and C-type lectin-like NK receptors in modulation of immune responses by multivalent GlcNAc-terminated oligosaccharides

Recognition of glycosylation patterns is one of the basic features of innate immunity. Ability of C-type lectin-like receptors such as NKR-P1 to bind saccharide moieties has become recently a controversial issue. In the present study, binding assay with soluble fluorescently labeled recombinant rat NKR-P1A and mouse NKR-P1C proteins revealed apparently no affinity to the various neoglycoproteins. Lack of functional linkage between NKR-P1 and previously described saccharide binder was supported by the fact, that synthetic N-acetyl-D-glucosamine octabranched dendrimer on polyamidoamine scaffold (GN8P) did not change gene expression of NKR-P1 isoforms in C57BL/6 and BALB/c mice divergent in the NK gene complex (both in vitro and in vivo). Surprisingly, N-acetyl-D-glucosamine-coated tetrabranched polyamido-amine dendrimer specifically binds to NKT cells and macrophages but not to NK cells (consistently with changes in cytokine patterns). Despite the fact that GN8P has been tested as an immunomodulator in anti-cancer treatment animal models for many years, surprisingly no changes in cytokine profiles in serum relevant to anti-cancer responses using B16F10 and CT26 harboring mouse strains C57BL/6 and BALB/c are observed. Our results indicate possible indirect involvement of NK cells in GN8P mediated immune responses.

Structure of NKp65 bound to its keratinocyte ligand reveals basis for genetically linked recognition in natural killer gene complex

The natural killer (NK) gene complex (NKC) encodes numerous C-type lectin-like receptors that govern the activity of NK cells. Although some of these receptors (Ly49s, NKG2D, CD94/NKG2A) recognize MHC or MHC-like molecules, others (Nkrp1, NKRP1A, NKp80, NKp65) instead bind C-type lectin-like ligands to which they are genetically linked in the NKC. To understand the basis for this recognition, we determined the structure of human NKp65, an activating receptor implicated in the immunosurveillance of skin, bound to its NKC-encoded ligand keratinocyte-associated C-type lectin (KACL). Whereas KACL forms a homodimer resembling other C-type lectin-like dimers, NKp65 is monomeric. The binding mode in the NKp65-KACL complex, in which a monomeric receptor engages a dimeric ligand, is completely distinct from those used by Ly49s, NKG2D, or CD94/NKG2A. The structure explains the exceptionally high affinity of the NKp65-KACL interaction compared with other cell-cell interaction pairs (KD = 6.7 × 10(-10) M), which may compensate for the monomeric nature of NKp65 to achieve cell activation. This previously unreported structure of an NKC-encoded receptor-ligand complex, coupled with mutational analysis of the interface, establishes a docking template that is directly applicable to other genetically linked pairs in the NKC, including Nkrp1-Clr, NKRP1A-LLT1, and NKp80-AICL.

BACL is a novel brain-associated, non-NKC-encoded mammalian C-type lectin-like receptor of the CLEC2 family

Natural Killer Gene Complex (NKC)–encoded C-type lectin-like receptors (CTLRs) are expressed on various immune cells including T cells, NK cells and myeloid cells and thereby contribute to the orchestration of cellular immune responses. Some NKC-encoded CTLRs are grouped into the C-type lectin family 2 (CLEC2 family) and interact with genetically linked CTLRs of the NKRP1 family. While many CLEC2 family members are expressed by hematopoietic cells (e.g. CD69 (CLEC2C)), others such as the keratinocyte-associated KACL (CLEC2A) are specifically expressed by other tissues. Here we provide the first characterization of the orphan gene CLEC2L. In contrast to other CLEC2 family members, CLEC2L is conserved among mammals and located outside of the NKC. We show that CLEC2L-encoded CTLRs are expressed as non-glycosylated, disulfide-linked homodimers at the cell surface. CLEC2L expression is fairly tissue-restricted with a predominant expression in the brain. Thus CLEC2L-encoded CTLRs were designated BACL (brain-associated C-type lectin). Combining in situ hybridization and immunohistochemistry, we show that BACL is expressed by neurons in the CNS, with a pronounced expression by Purkinje cells. Notably, the CLEC2L locus is adjacent to another orphan CTLR gene (KLRG2), but reporter cell assays did neither indicate interaction of BACL with the KLRG2 ectodomain nor with human NK cell lines or lymphocytes. Along these lines, growth of BACL-expressing tumor cell lines in immunocompetent mice did not provide evidence for an immune-related function of BACL. Altogether, the CLEC2L gene encodes a homodimeric cell surface CTLR that stands out among CLEC2 family members by its conservation in mammals, its biochemical properties and the predominant expression in the brain. Future studies will have to reveal insights into the functional relevance of BACL in the context of its neuronal expression.

The evolutionary arms race between NK cells and viruses: who gets the short end of the stick?

NK cells are innate lymphocytes that play a key role in the control of various viral infections. Recent studies indicate that NK cells may acquire some features of adaptive immune cells, including the formation of long-lived memory cells. A large and growing body of data indicates that NK cells regulate the adaptive immune response as well. The function and the activation status of NK cells are tightly regulated by signals induced by a broad range of inhibitory and activating cell surface receptors and cytokines released by other immune cells. Here, we review the function of mouse NK-cell receptors involved in virus control and in the regulation of the adaptive immune response. In addition, we discuss viral strategies used to evade NK-cell-mediated control during infection. Finally, the role of several activating Ly49 receptors specific for mouse cytomegalovirus (MCMV), as well as some controversial issues in the field, will be discussed.

The hammerhead ribozyme: structure, catalysis, and gene regulation

The hammerhead ribozyme has long been considered a prototype for understanding RNA catalysis, but discrepancies between the earlier crystal structures of a minimal hammerhead self-cleaving motif and various biochemical investigations frustrated attempt to understand hammerhead ribozyme catalysis in terms of structure. With the discovery that a tertiary contact distal from the ribozyme's active site greatly enhances its catalytic prowess, and the emergence of new corresponding crystal structures of full-length hammerhead ribozymes, a unified understanding of catalysis in terms of the structure is now possible. A mechanism in which the invariant residue G12 functions as a general base, and the 2'-OH moiety of the invariant G8, itself forming a tertiary base pair with the invariant C3, is the general acid, appears consistent with both the crystal structure and biochemical experimental results. Originally discovered in the context of plant satellite RNA viruses, the hammerhead more recently has been found embedded in the 3'-untranslated region of mature mammalian mRNAs, suggesting additional biological roles in genetic regulation.

Mouse Nkrp1-Clr gene cluster sequence and expression analyses reveal conservation of tissue-specific MHC-independent immunosurveillance

The Nkrp1 (Klrb1)-Clr (Clec2) genes encode a receptor-ligand system utilized by NK cells as an MHC-independent immunosurveillance strategy for innate immune responses. The related Ly49 family of MHC-I receptors displays extreme allelic polymorphism and haplotype plasticity. In contrast, previous BAC-mapping and aCGH studies in the mouse suggest the neighboring and related Nkrp1-Clr cluster is evolutionarily stable. To definitively compare the relative evolutionary rate of Nkrp1-Clr vs. Ly49 gene clusters, the Nkrp1-Clr gene clusters from two Ly49 haplotype-disparate inbred mouse strains, BALB/c and 129S6, were sequenced. Both Nkrp1-Clr gene cluster sequences are highly similar to the C57BL/6 reference sequence, displaying the same gene numbers and order, complete pseudogenes, and gene fragments. The Nkrp1-Clr clusters contain a strikingly dissimilar proportion of repetitive elements compared to the Ly49 clusters, suggesting that certain elements may be partly responsible for the highly disparate Ly49 vs. Nkrp1 evolutionary rate. Focused allelic polymorphisms were found within the Nkrp1b/d (Klrb1b), Nkrp1c (Klrb1c), and Clr-c (Clec2f) genes, suggestive of possible immune selection. Cell-type specific transcription of Nkrp1-Clr genes in a large panel of tissues/organs was determined. Clr-b (Clec2d) and Clr-g (Clec2i) showed wide expression, while other Clr genes showed more tissue-specific expression patterns. In situ hybridization revealed specific expression of various members of the Clr family in leukocytes/hematopoietic cells of immune organs, various tissue-restricted epithelial cells (including intestinal, kidney tubular, lung, and corneal progenitor epithelial cells), as well as myocytes. In summary, the Nkrp1-Clr gene cluster appears to evolve more slowly relative to the related Ly49 cluster, and likely regulates innate immunosurveillance in a tissue-specific manner.

Mouse Clr-g, a ligand for NK cell activation receptor NKR-P1F: crystal structure and biophysical properties

Interactions between C-type lectin-like NK cell receptors and their protein ligands form one of the key recognition mechanisms of the innate immune system that is involved in the elimination of cells that have been malignantly transformed, virally infected, or stressed by chemotherapy or other factors. We determined an x-ray structure for the extracellular domain of mouse C-type lectin related (Clr) protein g, a ligand for the activation receptor NKR-P1F. Clr-g forms dimers in the crystal structure resembling those of human CD69. This newly reported structure, together with the previously determined structure of mouse receptor NKR-P1A, allowed the modeling and calculations of electrostatic profiles for other closely related receptors and ligands. Despite the high similarity among Clr-g, Clr-b, and human CD69, these molecules have fundamentally different electrostatics, with distinct polarization of Clr-g. The electrostatic profile of NKR-P1F is complementary to that of Clr-g, which suggests a plausible interaction mechanism based on contacts between surface sites of opposite potential.

MHC class I target recognition, immunophenotypes and proteomic profiles of natural killer cells within the spleens of day-14 chick embryos

Chicken natural killer (NK) cells are not well defined, so little is known about the molecular interactions controlling their activity. At day 14 of embryonic development, chick spleens are a rich source of T-cell-free CD8αα(+), CD3(-) cells with natural killing activity. Cell-mediated cytotoxicity assays revealed complex NK cell discrimination of MHC class I, suggesting the presence of multiple NK cell receptors. Immunophenotyping of freshly isolated and recombinant chicken interleukin-2-stimulated d14E CD8αα(+) CD3(-) splenocytes provided further evidence for population heterogeneity. Complex patterns of expression were found for CD8α, chB6 (Bu-1), CD1-1, CD56 (NCAM), KUL01, CD5, and CD44. Mass spectrometry-based proteomics revealed an array of NK cell proteins, including the NKR2B4 receptor. DAVID and KEGG analyses and additional immunophenotyping revealed NK cell activation pathways and evidence for monocytes within the splenocyte cultures. This study provides an underpinning for further investigation into the specificity and function of NK cells in birds.

Defining the human T helper 17 cell phenotype

T helper (Th) 17 cells represent a third effector arm of CD4 T cells and complement the function of the Th1 and Th2 cell lineages. Here, we provide an overview of the transcription factors, cytokines, chemokines, and cytokine and chemokine receptors that characterize the Th17 cell phenotype. Data relevant for human Th17 cells are emphasized, with a focus on the function of two markers that have recently been associated with human Th17 cells, CD161 and interleukin-4-induced gene 1 (IL4I1). Also considered is the basis of Th17 cell plasticity towards the Th1 lineage, and we suggest that this plasticity together with the limited expansion of Th17 cells in response to T cell receptor (TCR) triggering accounts for the rarity of human Th17 cells in inflamed tissues.

Channel catfish leukocyte immune-type receptor mediated inhibition of cellular cytotoxicity is facilitated by SHP-1-dependent and -independent mechanisms

Channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) are immunoregulatory proteins belonging to the immunoglobulin superfamily that likely play an important role in the regulation of teleost immune cell effector responses. IpLITRs are expressed by myeloid and lymphoid subsets and based on their structural features can be classified as either putative stimulatory or inhibitory forms. We have recently demonstrated at the biochemical and functional levels that stimulatory IpLITR-types induced intracellular signaling cascades resulting in immune cell activation. Alternatively, we have shown that putative inhibitory IpLITRs may abrogate immune cell responses by recruiting teleost Src homology 2 (SH2) domain-containing cytoplasmic phosphatases (SHP) to their tyrosine-containing cytoplasmic tails. In the present study, we used vaccinia virus to express recombinant chimeric proteins encoding the extracellular and transmembrane regions of human KIR2DL3 fused with the cytoplasmic tails of two putative inhibitory IpLITRs (i.e. IpLITR1.2a and IpLITR1.1b) in mouse spleen-derived cytotoxic lymphocytes. This approach allowed us to study the specific effects of IpLITR-induced signaling on lymphocyte killing of B cell targets (e.g. 721.221 cells) using a standard chromium release assay. Our results suggest that both IpLITR1.2a and IpLITR1.1b are potent inhibitors of lymphocyte-mediated cellular cytotoxicity. Furthermore, using a catalytically inactive SHP-1 mutant in combination with site-directed mutagenesis and co-immunoprecipitations, we also demonstrate that the IpLITR1.2a-mediated functional inhibitory response is SHP-1-dependent. Alternatively, IpLITR1.1b-mediated inhibition of cellular cytotoxicity is facilitated by both SHP-1-dependent and independent mechanisms, possibly involving the C-terminal Src kinase (Csk). The involvement of this inhibitory kinase requires binding to a tyrosine residue encoded in the unique membrane proximal cytoplasmic tail region of IpLITR1.1b. Overall, this represents the first functional information for inhibitory IpLITR-types and reveals that catfish LITRs engage SHP-dependent and -independent inhibitory signaling pathways to abrogate lymphocyte-mediated killing.

Natural killer cell subsets in man and rodents

NK cells are important contributors to the early immune defence against infected or transformed cells. They are rapidly activated in response to cytokines, whereby they exert their effector functions. NK cell responses are controlled by a multitude of receptors, which are expressed by subpopulations of NK cells with distinct phenotypes and functionalities. Direct comparisons between species are often difficult because of differences in the expression of NK cell receptors and other markers. In addition, NK cells change their phenotype and effector functions during differentiation, by tissue-specific factors, or upon activation, complicating interpretations. We will here review the similarities and differences between the major NK cell subsets in man and two well-characterized rodent models.

Examination of the stimulatory signaling potential of a channel catfish leukocyte immune-type receptor and associated adaptor

Expressed by various subsets of myeloid and lymphoid immune cells, channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) are predicted to play a key role in the initiation and termination of teleost cellular effector responses. These type I transmembrane proteins belong to the immunoglobulin superfamily and display features of immunoregulatory receptors with inhibitory and/or stimulatory signaling potential. Expanding on our previous work, which demonstrated that putative stimulatory IpLITR-types associated with the catfish adaptor proteins IpFcRγ and FcRγ-L, this study focuses on the functional significance of this immune receptor-adaptor signaling complex. Specifically, we generated an epitope-tagged chimeric receptor construct by fusing the extracellular domain of IpLITR 2.6b with the transmembrane region and cytoplasmic tail of IpFcRγ-L. This chimera was stably expressed in a rat basophilic leukemia (RBL) cell line, RBL-2H3, and following cross-linking of the surface receptor with an anti-hemagglutinin monoclonal antibody or opsonized microspheres, the chimeric teleost receptor induced cellular degranulation and phagocytic responses, respectively. Site-directed mutagenesis of the immunoreceptor tyrosine-based activation motif encoded within the cytoplasmic tail of the chimera confirmed that these functional responses were dependent on the phosphorylated tyrosines within this motif. Using a combination of phospho-specific antibodies and pharmacological inhibitors, we also demonstrate that the IpLITR/IpFcRγ-L-induced degranulation response requires the activity of Src homology 2 domain containing protein tyrosine phosphatases, phosphatidylinositol 3-kinase, protein kinase C, and mitogen-activated protein kinases but appears independent of the c-Jun N-terminal kinase and p38 MAP kinase pathways. In addition to this first look at stimulatory IpLITR-mediated signaling and its influence on cellular effector responses, the advantage of generating RBL-2H3 cells stably expressing a functional IpLITR-adaptor chimera will be discussed.

Natural killer cell activation enhances immune pathology and promotes chronic infection by limiting CD8+ T-cell immunity

Infections with HIV, hepatitis B virus, and hepatitis C virus can turn into chronic infections, which currently affect more than 500 million patients worldwide. It is generally thought that virus-mediated T-cell exhaustion limits T-cell function, thus promoting chronic disease. Here we demonstrate that natural killer (NK) cells have a negative impact on the development of T-cell immunity by using the murine lymphocytic choriomeningitis virus. NK cell-deficient (Nfil3(-/-), E4BP4(-/-)) mice exhibited a higher virus-specific T-cell response. In addition, NK cell depletion caused enhanced T-cell immunity in WT mice, which led to rapid virus control and prevented chronic infection in lymphocytic choriomeningitis virus clone 13- and reduced viral load in DOCILE-infected animals. Further experiments showed that NKG2D triggered regulatory NK cell functions, which were mediated by perforin, and limited T-cell responses. Therefore, we identified an important role of regulatory NK cells in limiting T-cell immunity during virus infection.

Structure of the H107R variant of the extracellular domain of mouse NKR-P1A at 2.3 Å resolution

The structure of the H107R variant of the extracellular domain of the mouse natural killer cell receptor NKR-P1A has been determined by X-ray diffraction at 2.3 Å resolution from a merohedrally twinned crystal. Unlike the structure of the wild-type receptor in space group I4(1)22 with a single chain per asymmetric unit, the crystals of the variant belonged to space group I4(1) with a dimer in the asymmetric unit. Different degrees of merohedral twinning were detected in five data sets collected from different crystals. The mutation does not have a significant impact on the overall structure, but led to the binding of an additional phosphate ion at the interface of the molecules.

Mechanisms of NK cell activation: CD4(+) T cells enter the scene

Natural killer (NK) cells are innate lymphocytes involved in immunosurveillance through their cytotoxic activity and their capacity to secrete inflammatory cytokines. NK cell activation is necessary to initiate effector functions and results from a complex series of molecular and cellular events. We review here the signals that trigger NK cells and discuss recent findings showing that, besides antigen-presenting cells, T cells can play a central role in the initiation of NK cell activation in lymph nodes.

Screening for ligands of C-type lectin-like receptors

In order to execute their immune functions, leukocytes interact with a broad range of cell types through cell surface receptors, such as those of the immunoglobulin and C-type lectin families, or indirectly through soluble factors. The characterization of activating and inhibitory counterparts of NK cell receptors on myeloid cells, as well as the identification of their physiological ligands, has provided new insights into the underlying mechanisms of immunity and homeostasis. Here, we describe methodology that can be employed to screen for endogenous ligands of type-II C-type lectin-like receptors using reporter cells and Fc fusion proteins.

Induction of lectin-like transcript 1 (LLT1) protein cell surface expression by pathogens and interferon-γ contributes to modulate immune responses

CD161 is a C-type lectin-like receptor expressed on human natural killer (NK) cells and subsets of T cells. CD161 has been described as an inhibitory receptor that regulates NK cell-mediated cytotoxicity and IFN-γ production. Its role on T cells has remained unclear. Studies have shown that triggering of CD161 enhances NK T cell proliferation and T cell-IFN-γ production while inhibiting TNF-α production by CD8(+) T cells. Lectin-like transcript 1 (LLT1), the ligand of CD161, was found to be expressed on Toll-like receptor (TLR)-activated plasmacytoid and monocyte-derived dendritic cells (DC) and on activated B cells. Using newly developed anti-LLT1 mAbs, we show that LLT1 is not expressed on the surface of circulating B and T lymphocytes, NK cells, monocytes, and dendritic cells but that LLT1 is up-regulated upon activation. Not only TLR-stimulated dendritic cells and B cells but also T cell receptor-activated T cells and activated NK cells up-regulate LLT1. Interestingly, IFN-γ increases LLT1 expression level on antigen-presenting cells. LLT1 is also induced on B cells upon viral infection such as Epstein-Barr virus or HIV infection and in inflamed tonsils. Finally, expression of LLT1 on B cells inhibits NK cell function but costimulates T cell proliferation or IFN-γ production, and coengagement of CD161 with CD3 increases IL-17 secretion. Altogether, our results point toward a role for LLT1/CD161 in modulating immune responses to pathogens.

CD161-expressing human T cells

Expression of the Natural Killer cell receptor CD161 has recently been identified on a subset of T cells, including both CD4+ T helper and CD8+ T cells. Expression of this molecule within the adult circulation is restricted to those T cells with a memory phenotype. However, the distinct properties of these T cell populations is yet to be fully determined, although expression of CD161 has been related to the secretion of interleukin-17, and therefore to a type 17 phenotype. Recent studies have aimed to determine both the origin of these cells and the significance of CD161 expression as either a marker of specific cell types or as an effector and regulator of lymphocyte function, and hence to characterize the role of these CD161+ cells within a variety of human diseases in which they have been implicated.

Analysis of the mouse 129-strain Nkrp1-Clr gene cluster reveals conservation of genomic organization and functional receptor-ligand interactions despite significant allelic polymorphism

The Nkrp1 (Klrb) family of NK cell receptors and their genetically linked Clr (Clec2) ligands are conserved between rodents and humans. Nonetheless, certain mouse and rat Nkrp1 genes exhibit significant allelic polymorphism between inbred strains. We previously demonstrated that the Nkrp1-Clr recognition system is genetically and functionally conserved between the B6 and BALB/c strains, with focused sequence divergence evident in certain genes (e.g., Nkrp1b,c). Here, we extend this finding by mapping the 129-strain Nkrp1-Clr gene cluster, which is structurally conserved yet displays significant sequence divergence relative to the B6 haplotype. In addition, we show that 129-strain NK cells possess comparable Nkrp1 and Clr transcript expression, and characterize several NKR-P1:Clr interactions that are functionally conserved between the B6 and 129 strains, including documented and novel receptor-ligand pairs. Thus, despite significant allelic polymorphism observed in the Nkrp1-Clr region, the overall genetic organization and functional repertoire appear to be conserved among mouse strains, in contrast to the striking variation observed in the corresponding Ly49 region. These data extend our knowledge of the complex genetically linked Nkrp1-Clr NK recognition system in mice.

Molecular basis for LLT1 protein recognition by human CD161 protein (NKRP1A/KLRB1)

Human Th17 cells express high levels of CD161, a member of the killer cell lectin-like receptor (KLR) family (also referred to as NK receptor-P1A (NKRP1A) or KLRB1), as a representative marker. CD161 is also expressed on natural killer (NK) cells and NKT cells. Lectin-like transcript 1 (LLT1), another KLR family member, was recently identified as a ligand for CD161. This interaction may play pivotal roles in the immunomodulatory functions of Th17 cells as well as those of NK and NKT cells. However, the molecular basis for the interaction is poorly understood. Here we show that the extracellular domain of CD161 bound directly to LLT1 with a K(d) of 48 μM and with the fast kinetics typical of cell-cell recognition receptors. Mutagenesis revealed that the similar membrane-distal β-sheet and loop regions of both CD161 and LLT1 were utilized for the binding, and notably, these regions correspond to the ligand-binding sites for major histocompatibility complex (MHC)-recognizing KLRs. Furthermore, we found a pair of detrimental mutations for both molecules that restored the binding. These results reveal a new template model for the recognition mode between the KLR family members and provide insights into the molecular mechanism underlying Th17/NK/NKT-mediated immune responses.

Molecular architecture of mouse activating NKR-P1 receptors

Receptors belonging to NKR-P1 family and their specific Clr ligands form an alternative missing self recognition system critical in immunity against tumors and viruses, elimination of tumor cells subjected to genotoxic stress, activation of T cell dependent immune response, and hypertension. The three-dimensional structure of the extracellular domain of the mouse natural killer (NK) cell receptor mNKR-P1Aex has been determined by X-ray diffraction. The core of the C-type lectin domain (CTLD) is homologous to the other CTLD receptors whereas one quarter of the domain forms an extended loop interacting tightly with a neighboring loop in the crystal. This domain swapping mechanism results in a compact interaction interface. A second dimerization interface resembles the known arrangement of other CTLD NK receptors. A functional dimeric form of the receptor is suggested, with the loop, evolutionarily conserved within this family, proposed to participate in interactions with ligands.

Phylogenetic and functional conservation of the NKR-P1F and NKR-P1G receptors in rat and mouse

Two clusters of rat Nkrp1 genes can be distinguished based on phylogenetic relationships and functional characteristics. The proximal (centromeric) cluster encodes the well-studied NKR-P1A and NKR-P1B receptors and the distal cluster, the largely uncharacterized, NKR-P1F and NKR-P1G receptors. The inhibitory NKR-P1G receptor is expressed only by the Ly49s3(+) NK cell subset as detected by RT-PCR, while the activating NKR-P1F receptor is detected in both Ly49s3(+) and NKR-P1B(+) NK cells. The mouse NKR-P1G ortholog is expressed by both NKR-P1D(-) and NKR-P1D(+) NK cells in C57BL/6 mice. The rat and mouse NKR-P1F and NKR-P1G receptors demonstrate a striking, cross-species conservation of specificity for Clr ligands. NKR-P1F and NKR-P1G reporter cells reacted with overlapping panels of tumour cell lines and with cells transiently transfected with rat Clr2, Clr3, Clr4, Clr6 and Clr7 and mouse Clrc, Clrf, Clrg and Clrd/x, but not with Clr11 or Clrb, which serve as ligands for NKR-P1 from the proximal cluster. These data suggest that the conserved NKR-P1F and NKR-P1G receptors function as promiscuous receptors for a rapidly evolving family of Clr ligands in rodent NK cells.

High-level expression of soluble form of mouse natural killer cell receptor NKR-P1C(B6) in Escherichia coli

Mouse NKR-P1C(B6) receptor corresponding to NK1.1 alloantigen is one of the most widespread surface markers of mouse NK and NKT cells in C57BL/6 mice detected by monoclonal antibody PK136. Although functional studies revealed the ability of this receptor to activate both natural killing and production of cytokines upon antibody crosslinking, the ligand for NKR-P1C(B6) remains unknown. In order to initiate ligand identification, structural studies, and epitope mapping experiments, we developed a simple and efficient expression and purification protocol allowing to produce large amounts of pure soluble monomeric mouse NKR-P1C(B6). Our protein encompassed approximately half of the stalk region and the entire C-terminal globular ligand binding domain. The identity of protein that was devoid of N-terminal initiation methionine and had all three expected disulfides closed was confirmed using high resolution ion cyclotron resonance mass spectrometry. Protein produced into inclusion bodies in Escherichia coli was efficiently refolded into a unique three dimensional structure as confirmed by NMR using (1)H-(15)N-HSQC spectra of uniformly labeled protein. The exceptional purity of the protein should allow its crystallization and detailed structural investigations, and is a prerequisite for its use as a probe in ligand identification and antibody epitope mapping experiments.

CELLISA: reporter cell-based immunization and screening of hybridomas specific for cell surface antigens

Monoclonal antibodies (mAbs) specific for cell surface antigens are an invaluable tool to study immune receptor expression and function. Here, we outline a generalized reporter cell-based approach to the generation and high-throughput screening of mAbs specific for cell surface antigens. Termed CELLISA, this technology hinges upon the capture of hybridoma supernatants in mAb arrays that facilitate ligation of an antigen of interest displayed on BWZ reporter cells in the form of a CD3ζ-fusion chimeric antigen receptor (zCAR); in turn, specific mAb-mediated cross-linking of zCAR on BWZ cells results in the production of β-galactosidase enzyme (β-gal), which can be assayed colorimetrically. Importantly, the BWZ reporter cells bearing the zCAR of interest may be used for immunization as well as screening. In addition, serial immunizations employing additional zCAR- or native antigen-bearing cell lines can be used to increase the frequency of the desired antigen-specific hybridomas. Finally, the use of a cohort of epitope-tagged zCAR (e.g., zCAR(FLAG)) variants allows visualization of the cell surface antigen prior to immunization, and coimmunization using these variants can be used to enhance the immunogenicity of the target antigen. Employing the CELLISA strategy, we herein describe the generation of mAb directed against an uncharacterized natural killer cell receptor protein.

Inhibitory C-type lectin receptors in myeloid cells

C-type lectin receptors encoded by the natural killer gene complex play critical roles in enabling NK cell discrimination between self and non-self. In recent years, additional genes at this locus have been identified with patterns of expression that extend to cells of the myeloid lineage where many of the encoded inhibitory receptors have equally important functions as regulators of immune homeostasis. In the present review we highlight the roles of some of these receptors including recent insights gained with regard to the identification of exogenous and endogenous ligands, mechanisms of cellular inhibition and activation, regulated expression within different cellular and immune contexts, as well as functions that include the regulation of bone homeostasis and involvement in autoimmunity.

Natural killer cell tolerance persists despite significant reduction of self MHC class I on normal target cells in mice

BACKGROUND

A major group of murine inhibitory receptors on Natural Killer (NK) cells belong to the Ly49 receptor family and recognize MHC class I molecules. Infected or transformed target cells frequently downmodulate MHC class I molecules and can thus avoid CD8(+) T cell attack, but may at the same time develop NK cell sensitivity, due to failure to express inhibitory ligands for Ly49 receptors. The extent of MHC class I downregulation needed on normal cells to trigger NK cell effector functions is not known.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we show that cells expressing MHC class I to levels well below half of the host level are tolerated in an in vivo assay in mice. Hemizygous expression (expression from only one allele) of MHC class I was sufficient to induce Ly49 receptor downmodulation on NK cells to a similar degree as homozygous expression, despite a strongly reduced cell surface level of MHC class I. Co-expression of weaker MHC class I ligands in the host did not have any further effect on the degree of Ly49 downmodulation. Furthermore, a single MHC class I allele could downmodulate up to three Ly49 receptors on individual NK cells. Only when NK cells simultaneously expressed several Ly49 receptors and hemizygous MHC class I levels, a putative threshold for Ly49 downmodulation was reached.

CONCLUSION

Collectively, our findings suggest that in interactions between NK cells and normal untransformed cells, MHC class I molecules are in most cases expressed in excess compared to what is functionally needed to ensure self tolerance and to induce maximal Ly49 downmodulation. We speculate that the reason for this is to maintain a safety margin for otherwise normal, autologous cells over a range of MHC class I expression levels, in order to ensure robustness in NK cell tolerance.

Structural variation of the mouse natural killer gene complex

The natural killer gene complex (NKC) on chromosome 6 contains clusters of genes that encode both activation and inhibitory receptors expressed on mouse natural killer (NK) cells. NKC genes, particularly belonging to the Nkrp1 and Ly49 gene families, display haplotype differences between different mouse strains and allelic polymorphisms of individual genes, as previously revealed by conventional analysis in a small number of inbred mouse strains. Herein we used array-based comparative genomic hybridization (aCGH) to efficiently compare the NKC in 21 mouse strains to the reference C57BL/6 strain. By using unsupervised clustering methods, we could sort these variations into the same groups as determined by previous RFLP analyses of Nkrp1 and Ly49 genes. Prospective analyses of aCGH and RFLP data validated these relationships. Moreover, aCGH data predicted monoclonal antibody reactivity with an allospecific determinant on molecules expressed by NK cells. Taken together, these data demonstrate the structural variation in the NKC between mouse strains as well as the usefulness of aCGH in analysis of complex, polymorphic gene clusters.

Characterization of alternatively spliced transcript variants of CLEC2D gene

Lectin-like transcript 1 (LLT1) encoded by CLEC2D gene is a C-type lectin-like molecule interacting with human CD161 (NKR-P1A) receptor expressed by natural killer cells and subsets of T cells. Using RT-PCR and sequencing, we identified several CLEC2D alternatively spliced transcript variants generated by exon skipping. In addition to the reported transcript variants 1 (LLT1) and 2, we identified a novel splice variant 4 and transcripts coding for putative soluble proteins. CLEC2D transcripts were detected primarily in hematopoietic cell lines and were found to be co-induced by the same activation signals. Although very low amounts of putative soluble CLEC2D protein isoforms could be produced by transfectants, CLEC2D isoforms 2 and 4 were efficiently expressed. By contrast to LLT1, which was detected on the cell surface, isoform 2 and 4 remained in the endoplasmic reticulum where they formed homodimers or heterodimers with LLT1. They failed to interact with CD161, leaving LLT1 as the sole ligand for this receptor. CLEC2D therefore uses gene splicing to generate protein isoforms that are structurally distinct and that have different biological activities.

Chemotherapy-induced genotoxic stress promotes sensitivity to natural killer cell cytotoxicity by enabling missing-self recognition

Natural killer (NK) cells can recognize and kill tumor cells lacking "self" markers, such as class I MHC, but the basis for this recognition is not completely understood. NKR-P1 receptors are members of the C-type lectin-related NK receptor superfamily that are conserved from rodents to humans. Identification of Clr ligands for the NKR-P1 receptors has facilitated functional analysis of MHC-independent target cell recognition by NK cells. One receptor-ligand pair, NKR-P1B:Clr-b, can mediate "missing-self" recognition of tumor and infected cells, but the role of this axis in sensing stressed cells remains unknown. Here, we show that Clr-b is rapidly downregulated in cells undergoing genotoxic and cellular stress at the level of both RNA and surface protein. Stress-mediated loss of Clr-b on leukemia cells enhanced cytotoxicity mediated by NKR-P1B(+) NK cells. Notably, Clr-b downregulation was coordinated functionally with stress-mediated upregulation of NKG2D ligands (but not class I MHC). Our findings highlight a unique role for the MHC-independent NKR-P1B:Clr-b missing-self axis in recognition of stressed cells, and provide evidence of two independent levels of Clr-b regulation in stressed cells.

Activating and inhibitory receptors of natural killer cells

Natural killer (NK) cells are potent immune effector cells that can respond to infection and cancer, as well as allowing maternal adaptation to pregnancy. In response to malignant transformation or pathogenic invasion, NK cells can secrete cytokine and may be directly cytolytic, as well as exerting effects indirectly through other cells of the immune system. To recognize and respond to inflamed or infected tissues, NK cells express a variety of activating and inhibitory receptors including NKG2D, Ly49 or KIR, CD94-NKG2 heterodimers and natural cytotoxicity receptors, as well as co-stimulatory receptors. These receptors recognize cellular stress ligands as well as major histocompatibility complex class I and related molecules, which can lead to NK cell responses. Importantly, NK cells must remain tolerant of healthy tissue, and some of these receptors can also prevent activation of NK cells. In this review, we describe the expression of prominent NK cell receptors, as well as expression of their ligands and their role in immune responses. In addition, we describe the main signaling pathways used by NK cell receptors. Although we now appreciate that NK cell biology is more complicated than first thought, there are still facets of their biology that remain unclear. These will be highlighted and discussed in this review.

Two complementary rat NK cell subsets, Ly49s3+ and NKR-P1B+, differ in phenotypic characteristics and responsiveness to cytokines

Two major subsets of rat NK cells can be distinguished based on their expression of the Ly49s3 or the NKR-P1B lectin-like receptor. Ly49s3(+) NK cells, but not NKR-P1B(+) NK cells, express a wide range of Ly49 receptors. Here, we have examined differences between these two subsets in their expression of certain NK cell-associated molecules as well as their responses to cytokines. A microarray analysis suggested several differentially expressed genes, including preferential expression of NKG2A/C receptors by NKR-P1B(+) NK cells. This was confirmed by staining with tetramers of RT.BM1, the putative ligand of CD94/NKG2, indicating that Ly49 and CD94/NKG2 receptors separate into distinct NK cell compartments. Further, expression of CD25 by Ly49s3(+) NK cells was associated with more rapid proliferation in response to IL-2 as compared with NKR-P1B(+) NK cells. Thus, certain inflammatory situations may preferentially expand the Ly49s3(+) NK cells. Moreover, freshly isolated Ly49s3(+) and NKR-P1B(+) NK cells produce similar amounts of cytokines, and a minor Ly49s3(-)NKR-P1B(-) double-negative NK subset appears to be hyporesponsive based on its significantly lower IFN-gamma production. Collectively, our data demonstrate divergent profiles of NKR-P1B(+) and Ly49s3(+) NK cells, indicating distinct tasks in vivo.

Interaction of C-type lectin-like receptors NKp65 and KACL facilitates dedicated immune recognition of human keratinocytes

Many well-known immune-related C-type lectin-like receptors (CTLRs) such as NKG2D, CD69, and the Ly49 receptors are encoded in the natural killer gene complex (NKC). Recently, we characterized the orphan NKC gene CLEC2A encoding for KACL, a further member of the human CLEC2 family of CTLRs. In contrast to the other CLEC2 family members AICL, CD69, and LLT1, KACL expression is mostly restricted to skin. Here we show that KACL is a non-disulfide-linked homodimeric surface receptor and stimulates cytotoxicity by human NK92MI cells. We identified the corresponding activating receptor on NK92MI cells that is encoded adjacently to the CLEC2A locus and binds KACL with high affinity. This CTLR, termed NKp65, stimulates NK cytotoxicity and release of proinflammatory cytokines upon engagement of cell-bound KACL. NKp65, a distant relative of the human activating NK receptor NKp80, possesses an amino-terminal hemITAM that is required for NKp65-mediated cytotoxicity. Finally, we show that KACL expression is mainly restricted to keratinocytes. Freshly isolated keratinocytes express KACL and are capable of stimulating NKp65-expressing cells in a KACL-dependent manner. Thus, we report a unique NKC-encoded receptor-ligand system that may fulfill a dedicated function in the immunobiology of human skin.

Structure of natural killer cell receptor KLRG1 bound to E-cadherin reveals basis for MHC-independent missing self recognition

The cytolytic activity of natural killer (NK) cells is regulated by inhibitory receptors that detect the absence of self molecules on target cells. Structural studies of missing self recognition have focused on NK receptors that bind MHC. However, NK cells also possess inhibitory receptors specific for non-MHC ligands, notably cadherins, which are downregulated in metastatic tumors. We determined the structure of killer cell lectin-like receptor G1 (KLRG1) in complex with E-cadherin. KLRG1 mediates missing self recognition by binding to a highly conserved site on classical cadherins, enabling it to monitor expression of several cadherins (E-, N-, and R-) on target cells. This site overlaps the site responsible for cell-cell adhesion but is distinct from the integrin alpha(E)beta(7) binding site. We propose that E-cadherin may coengage KLRG1 and alpha(E)beta(7) and that KLRG1 overcomes its exceptionally weak affinity for cadherins through multipoint attachment to target cells, resulting in inhibitory signaling.

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.

Intracellular bacterial infection and invariant NKT cells

The invariant (i) natural killer (NK)T cells represent a unique subset of T lymphocytes which express the V alpha 14 chain of the T cell receptor (TCR), that recognizes glycolipid antigens presented by the nonpolymorphic major histocompatibility complex (MHC) class I-like antigen presentation molecule CD1d, and they participate in protection against some microbial pathogens. Although iNKT cells have originally been regarded as T cells co-expressing NKR-P1B/C (NK1.1: CD 161), they do not seem to consistently express this marker, since NK1.1 surface expression on iNKT cells undergoes dramatic changes following facultative intracellular bacterial infection, which is correlated with functional changes of this cell population. Accumulating evidence suggests that NK1.1 allows recognition of "missing-self", thus controlling activation/inhibition of NK1.1-expressing cells. Therefore, it is tempting to suggest that iNKT cells participate in the regulation of host immune responses during facultative intracellular bacterial infection by controlling NK1.1 surface expression. These findings shed light not only on the unique role of iNKT cells in microbial infection, but also provide evidence for new aspects of the NK1.1 as a regulatory molecule on these cells.

Natural killer cell tolerance licensing and other mechanisms

Armed with potent cytotoxic and immunostimulatory effector functions, natural killer (NK) cells have the potential to cause significant damage to normal self cells unless controlled by self-tolerance mechanisms. NK cells identify and attack target cells based on integration of signals from activation and inhibitory receptors, whose ligands exhibit complex expression and/or binding patterns. Preservation of NK cell self-tolerance must therefore go beyond mere engagement of inhibitory receptors during effector functions. Herein, we review recent work that has uncovered a number of mechanisms to ensure self-tolerance of NK cells. For example, licensing of NK cells allows only NK cells that can engage self-MHC to become functionally competent, or licensed. The molecular mechanism of this phenomenon appears to require signaling by receptors that were originally identified in effector inhibition. However, the nature of the signaling event has not yet been defined, but new interpretations of several published experiments provide valuable clues. In addition, several other cell-intrinsic and -extrinsic mechanisms of NK cell tolerance are discussed, including activation receptor cooperation and synergy, cytokine stimulation, and the opposing roles of accessory and regulatory cells. Finally, NK cell tolerance is discussed as it relates to the clinic, such as KIR-HLA disease associations, tumor immunotherapy, and fetal tolerance.

NK cells and cancer immunosurveillance

Natural killer (NK) cells are lymphocytes of the innate immune system that monitor cell surfaces of autologous cells for an aberrant expression of MHC class I molecules and cell stress markers. Since their first description more than 30 years ago, NK cells have been implicated in the immune defence against tumours. Here, we review the broadly accumulating evidence for a crucial contribution of NK cells to the immunosurveillance of tumours and the molecular mechanisms that allow NK cells to distinguish malignant from healthy cells. Particular emphasis is placed on the activating NK receptor NKG2D, which recognizes a variety of MHC class I-related molecules believed to act as 'immuno-alerters' on malignant cells, and on tumour-mediated counterstrategies promoting escape from NKG2D-mediated recognition.

Regulation of NK cell responsiveness to achieve self-tolerance and maximal responses to diseased target cells

Inhibitory receptors specific for major histocompatibility complex (MHC) class I molecules govern the capacity of natural killer (NK) cells to attack class I-deficient cells ('missing-self recognition'). These receptors are expressed stochastically, such that the panel of expressed receptors varies between NK cells. This review addresses how the activity of NK cells is coordinated in the face of this variation to achieve a repertoire that is self-tolerant and optimally reactive with diseased cells. Recent studies show that NK cells arise in normal animals or humans that lack any known inhibitory receptors specific for self-MHC class I. These NK cells exhibit self-tolerance and exhibit functional hyporesponsiveness to stimulation through various activating receptors. Evidence suggests that hyporesponsiveness is induced because these NK cells cannot engage inhibitory MHC class I molecules and are therefore persistently over-stimulated by normal cells in the environment. Finally, we discuss evidence that hyporesponsiveness is a quantitative trait that varies depending on the balance of signals encountered by developing NK cells. Thus, a tuning process determines the functional set-point of NK cells, providing a basis for discriminating self from missing-self, and at the same time endowing each NK cell with the highest inherent responsiveness compatible with self-tolerance.

Negative signaling by inhibitory receptors: the NK cell paradigm

Receptors carrying immunoreceptor tyrosine-based inhibition motifs (ITIMs) in their cytoplasmic tail control a vast array of cellular responses, ranging from autoimmunity, allergy, phagocytosis of red blood cells, graft versus host disease, to even neuronal plasticity in the brain. The inhibitory function of many receptors has been deduced on the basis of cytoplasmic ITIM sequences. Tight regulation of natural killer (NK) cell cytotoxicity and cytokine production by inhibitory receptors specific for major histocompatibility complex class I molecules has served as a model system to study the negative signaling pathway triggered by an ITIM-containing receptor in the physiological context of NK-target cell interactions. Advances in our understanding of the molecular details of inhibitory signaling in NK cells have provided a conceptual framework to address how ITIM-mediated regulation controls cellular reactivity in diverse cell types.

Avian NK activities, cells and receptors

Natural killer (NK) activity has been examined in birds for over 30 years, but evidence that avian NK activity plays crucial roles in disease is only suggestive. In chickens, NK activity is mediated by TCR0 cells in the intestinal epithelium, but elsewhere subsets of alphabeta and gammadelta T cells (NKT cells) may be more important. There are few lectin-like NK receptor genes, located in the genomic region syntenic with the natural killer complex (NKC) as well as the major histocompatibility complex (MHC). In contrast, a huge number of Ig-like receptor genes are located in a region syntenic with the leukocyte receptor complex (LRC).