Structural and functional aspects of the Ly49 natural killer cell receptors

Natural Killer Cells: Tumor Surveillance and Signaling

Natural killer (NK) cells play a pivotal role in cancer immunotherapy due to their innate ability to detect and kill tumorigenic cells. The decision to kill is determined by the expression of a myriad of activating and inhibitory receptors on the NK cell surface. Cell-to-cell engagement results in either self-tolerance or a cytotoxic response, governed by a fine balance between the signaling cascades downstream of the activating and inhibitory receptors. To evade a cytotoxic immune response, tumor cells can modulate the surface expression of receptor ligands and additionally, alter the conditions in the tumor microenvironment (TME), tilting the scales toward a suppressed cytotoxic NK response. To fully harness the killing power of NK cells for clinical benefit, we need to understand what defines the threshold for activation and what is required to break tolerance. This review will focus on the intracellular signaling pathways activated or suppressed in NK cells and the roles signaling intermediates play during an NK cytotoxic response.

Natural Killer Cell Receptor Genes in Camels: Another Mammalian Model

Due to production of special homodimeric heavy chain antibodies, somatic hypermutation of their T-cell receptor genes and unusually low diversity of their major histocompatibility complex genes, camels represent an important model for immunogenetic studies. Here, we analyzed genes encoding selected natural killer cell receptors with a special focus on genes encoding receptors for major histocompatibility complex (MHC) class I ligands in the two domestic camel species, Camelus dromedarius and Camelus bactrianus. Based on the dromedary genome assembly CamDro2, we characterized the genetic contents, organization, and variability of two complex genomic regions, the leukocyte receptor complex and the natural killer complex, along with the natural cytotoxicity receptor genes NCR1, NCR2, and NCR3. The genomic organization of the natural killer complex region of camels differs from cattle, the phylogenetically most closely related species. With its minimal set of KLR genes, it resembles this complex in the domestic pig. Similarly, the leukocyte receptor complex of camels is strikingly different from its cattle counterpart. With KIR pseudogenes and few LILR genes, it seems to be simpler than in the pig. The syntenies and protein sequences of the NCR1, NCR2, and NCR3 genes in the dromedary suggest that they could be human orthologues. However, only NCR1 and NCR2 have a structure of functional genes, while NCR3 appears to be a pseudogene. High sequence similarities between the two camel species as well as with the alpaca Vicugna pacos were observed. The polymorphism in all genes analyzed seems to be generally low, similar to the rest of the camel genomes. This first report on natural killer cell receptor genes in camelids adds new data to our understanding of specificities of the camel immune system and its functions, extends our genetic knowledge of the innate immune variation in dromedaries and Bactrian camels, and contributes to studies of natural killer cell receptors evolution in mammals.

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.

Kbtbd2 inhibits the cytotoxic activity of immortalized NK cells through down-regulating mTOR signaling in a mouse hepatocellular carcinoma model

Natural killer cell (NK cell)-based immunotherapy is a promising therapeutic strategy for hepatocellular carcinoma (HCC). However, the molecular mechanisms underlying the regulation of NK cell function in the tumor sites are not completely elucidated. In this study, we identified the enhanced expression of kelch repeat and BTB (POZ) domain containing 2 (Kbtbd2) in intratumoral NK cells in a mouse HCC implantation model as a negative regulator of NK cells. To investigate this interaction, we used a Tet-on inducible expression system to control Kbtbd2 expression in an immortalized mouse NK cell line KIL C.2. With this approach, we found that overexpression of Kbtbd2 reduced KIL C.2 cell proliferation, decreased expression certain of Ly49 receptor family members, and substantially impaired cytotoxic activity of KIL C.2 cells in vitro. Moreover, phosphorylation of mTOR and its target 4E-binding protein 1 was reduced in Kbtbd2-expressing KIL C.2 cells, along with down-regulated phosphorylation of Erk1/2. Adoptively transferred Kbtbd2-expressing KIL C.2 cells exhibited weaker tumoricidal effect on hepatocellular carcinoma cells in the HCC implantation model, in comparison with transferred control KIL C.2 cells. Taken together, our investigation indicates that Kbtbd2 is an inhibitory molecule for the tumoricidal activity of KIL C.2 cells and perhaps intratumoral NK cells.

Recognition of the Major Histocompatibility Complex (MHC) Class Ib Molecule H2-Q10 by the Natural Killer Cell Receptor Ly49C

Murine natural killer (NK) cells are regulated by the interaction of Ly49 receptors with major histocompatibility complex class I molecules (MHC-I). Although the ligands for inhibitory Ly49 were considered to be restricted to classical MHC (MHC-Ia), we have shown that the non-classical MHC molecule (MHC-Ib) H2-M3 was a ligand for the inhibitory Ly49A. Here we establish that another MHC-Ib, H2-Q10, is a bona fide ligand for the inhibitory Ly49C receptor. H2-Q10 bound to Ly49C with a marginally lower affinity (∼5 μm) than that observed between Ly49C and MHC-Ia (H-2K(b)/H-2D(d), both ∼1 μm), and this recognition could be prevented by cis interactions with H-2K in situ To understand the molecular details underpinning Ly49·MHC-Ib recognition, we determined the crystal structures of H2-Q10 and Ly49C bound H2-Q10. Unliganded H2-Q10 adopted a classical MHC-I fold and possessed a peptide-binding groove that exhibited features similar to those found in MHC-Ia, explaining the diverse peptide binding repertoire of H2-Q10. Ly49C bound to H2-Q10 underneath the peptide binding platform to a region that encompassed residues from the α1, α2, and α3 domains, as well as the associated β2-microglobulin subunit. This docking mode was conserved with that previously observed for Ly49C·H-2K(b) Indeed, structure-guided mutation of Ly49C indicated that Ly49C·H2-Q10 and Ly49C·H-2K(b) possess similar energetic footprints focused around residues located within the Ly49C β4-stand and L5 loop, which contact the underside of the peptide-binding platform floor. Our data provide a structural basis for Ly49·MHC-Ib recognition and demonstrate that MHC-Ib represent an extended family of ligands for Ly49 molecules.

Nonobese diabetic natural killer cells: a barrier to allogeneic chimerism that can be reduced by rapamycin

BACKGROUND

Induction of allogeneic hematopoietic chimerism is a promising strategy to induce tolerance to donor islets for treating type 1 diabetes. Successful induction of chimerism requires overcoming host alloimmunity. In diabetes-prone nonobese diabetic (NOD) mice, this is challenging due to their general tolerance resistance. Although the adaptive alloimmunity of NOD mice is a known barrier to allogeneic chimerism, whether NOD natural killer (NK) cells are an additional barrier has not been examined. Because NOD NK cells exhibit functional defects, they may not inhibit chimerism generation.

METHODS

Antibody depletion of NK cells in vivo, or transplantation of F1 hybrid donor cells to eliminate the "missing-self" trigger of NK cells, was preformed to test the NK-mediated rejection of donor bone marrow cells. We also studied the capacity of rapamycin to block the NK cell response against allogeneic cells in vivo.

RESULTS

Depleting NK cells or rendering them unresponsive to the donor greatly improved the level of chimerism obtained in NOD mice. Rapamycin significantly reduced the resistance to allogeneic chimerism mounted by NOD NK cells; however, it was much less effective than NK cell depletion by antibodies.

CONCLUSIONS

Contrary to the view that NOD NK cells are defective, we found these cells to be a substantial barrier to allogeneic chimerism in the presence or absence of adaptive immunity. Moreover, rapamycin will need to be combined with other approaches to fully overcome the NK cell barrier.

Ly49P recognition of cytomegalovirus-infected cells expressing H2-Dk and CMV-encoded m04 correlates with the NK cell antiviral response

Natural killer (NK) cells are crucial in resistance to certain viral infections, but the mechanisms used to recognize infected cells remain largely unknown. Here, we show that the activating Ly49P receptor recognizes cells infected with mouse cytomegalovirus (MCMV) by a process that requires the presence of H2-D^k and the MCMV m04 protein. Using H2 chimeras between H2-D^b and -D^k, we demonstrate that the H2-D^k peptide-binding platform is required for Ly49P recognition. We identified m04 as a viral component necessary for recognition using a panel of MCMV-deletion mutant viruses and complementation of m04-deletion mutant (Δm04) virus infection. MA/My mice, which express Ly49P and H2-D^k, are resistant to MCMV; however, infection with Δm04 MCMV abrogates resistance. Depletion of NK cells in MA/My mice abrogates their resistance to wild-type MCMV infection, but does not significantly affect viral titers in mice infected with Δm04 virus, implicating NK cells in host protection through m04-dependent recognition. These findings reveal a novel mechanism of major histocompatability complex class I–restricted recognition of virally infected cells by an activating NK cell receptor.

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.

Macrophage migration inhibitory factor has a MHC class I-like motif and function

Macrophage migration inhibitory factor (MIF) is found in immune-privileged sites and inhibits cytotoxicity mediated by CD3-ve lymphokine-activated killer cells (LAK). The mechanism by which MIF attenuates LAK cytotoxicity is unknown. We provide evidence that MIF has a major histocompatibility complex (MHC) class I-like motif. A monoclonal antibody (OX18) that binds a conserved region of rat MHC class I proteins binds native MIF. Anti-MIF polyclonal antibodies bind MHC class I. Epitope mapping suggests OX18 binds a loop of MHC class I bound by several receptors for MHC class I. A sequence (PRPEG) within the proposed OX18-binding site on MHC class I exists with a short insertion in MIF. OX18 does not bind MIF that is denatured by SDS-PAGE. This suggests the OX18 epitope is dependent on higher order structure in MIF. Interestingly, MIF inhibits binding of tetramers of MHC class I (H2D(b)) to LAK cells, suggesting it may bind to receptors for MHC class I. MIF may be an example where small regions of MHC class I are used by endogenous and viral proteins to control cytotoxicity mediated by immune cells.

Comparative modeling of marsupial MHC class I molecules identifies structural polymorphisms affecting functional motifs

Major histocompatibility complex (MHC) class I molecules are transmembrane glycoproteins that present antigenic peptides to CD8+ T cells and are subsequently important for the initiation of an immune response. In this study novel MHC class I sequences from the tammar wallaby (Macropus eugenii) have been characterized. Analysis and comparative modeling of these and existing marsupial molecules reveals potential functional polymorphisms within peptide-binding grooves, MHC assembly motifs and the T cell receptor recognition interface. In addition, we show that a previously identified marsupial-specific insertion is within a region, which is known as a putative NK cell receptor (Ly49A) binding site in the mouse, suggesting that this site may be functionally active in marsupials. Further, the analysis highlighted differences in structural and sequence based grouping of marsupial MHC class I molecules.

The leukocyte receptor complex in chicken is characterized by massive expansion and diversification of immunoglobulin-like Loci

The innate and adaptive immune systems of vertebrates possess complementary, but intertwined functions within immune responses. Receptors of the mammalian innate immune system play an essential role in the detection of infected or transformed cells and are vital for the initiation and regulation of a full adaptive immune response. The genes for several of these receptors are clustered within the leukocyte receptor complex (LRC). The purpose of this study was to carry out a detailed analysis of the chicken (Gallus gallus domesticus) LRC. Bacterial artificial chromosomes containing genes related to mammalian leukocyte immunoglobulin-like receptors were identified in a chicken genomic library and shown to map to a single microchromosome. Sequencing revealed 103 chicken immunoglobulin-like receptor (CHIR) loci (22 inhibitory, 25 activating, 15 bifunctional, and 41 pseudogenes). A very complex splicing pattern was found using transcript analyses and seven hypervariable regions were detected in the external CHIR domains. Phylogenetic and genomic analysis showed that CHIR genes evolved mainly by block duplications from an ancestral inhibitory receptor locus, with transformation into activating receptors occurring more than once. Evolutionary selection pressure has led not only to an exceptional expansion of the CHIR cluster but also to a dramatic diversification of CHIR loci and haplotypes. This indicates that CHIRs have the potential to complement the adaptive immune system in fighting pathogens.

The immune system developed to cope with a diverse array of pathogens, including infectious organisms. The detection of these pathogens by cells of the immune system is mediated by a large set of specific receptor proteins. Here the authors seek to understand how a particular subset of cell surface receptors of the domestic chicken, the chicken Ig-like receptors (CHIR), has evolved. They demonstrate that at least 103 such receptor loci are clustered on a single microchromosome and provide the first detailed analysis of this region. The sequences of the CHIR genes suggest the presence of inhibitory, activating, and bifunctional receptors, as well as numerous incomplete loci (pseudogenes) that appear to have evolved by duplications of an ancestral inhibitory receptor gene. Multiple regions of very high sequence variability were also identified within CHIR loci which, together with considerable expansion of the number of these genes, suggest that CHIR polypeptides are involved in critical functions in the immune system of the chicken.

Immune-related, lectin-like receptors are differentially expressed in the myeloid and lymphoid lineages of zebrafish

The identification of C-type lectin (Group V) natural killer (NK) cell receptors in bony fish has remained elusive. Analyses of the Fugu rubripes genome database failed to identify Group V C-type lectin domains (Zelensky and Gready, BMC Genomics 5:51, 2004) suggesting that bony fish, in general, may lack such receptors. Numerous Group II C-type lectin receptors, which are structurally similar to Group V (NK) receptors, have been characterized in bony fish. By searching the zebrafish genome database we have identified a multi-gene family of Group II immune-related, lectin-like receptors (illrs) whose members possess inhibiting and/or activating signaling motifs typical of Group V NK receptors. Illr genes are differentially expressed in the myeloid and lymphoid lineages, suggesting that they may play important roles in the immune functions of multiple hematopoietic cell lineages.

Cytokines as critical co-stimulatory molecules in modulating the immune response of natural killer cells

Cytokines are involved in directing the activation of natural killer (NK) cells. NK cells are involved in the recognition of cells that have been altered; thus they do not recognize specific insults to the host, but when activated, are capable of destroying infected cells directly, as well as promoting the recruitment and response of the other components of the immune system by the release of cytokines and chemokines. It is these properties that have made NK cells a critical part of innate immunity and adaptive immunity, and they play a principal role linking innate and adaptive immunity by the recruitment of an adaptive immune response to an innate immune reaction.