Effects of human cytomegalovirus infection on ligands for the activating NKG2D receptor of NK cells: up-regulation of UL16-binding protein (ULBP)1 and ULBP2 is counteracted by the viral UL16 protein

Human cytomegalovirus-encoded UL16 discriminates MIC molecules by their alpha2 domains

Human CMV infection results in MHC class I down-regulation and induction of NKG2D ligand expression favoring NK recognition of infected cells. However, human CMV-encoded UL16 counteracts surface expression of several NKG2D ligands by intracellular retention. Interestingly, UL16 interacts with MICB, but not with the closely related MICA, and with UL16-binding proteins (ULBP) ULBP1 and ULBP2, which are only distantly related to MICB, but not with ULPB3 or ULBP4, although all constitute ligands for NKG2D. Here, we dissected the molecular basis of MICA-MICB discrimination by UL16 to elucidate its puzzling binding behavior. We report that the UL16-MICB interaction is independent of glycosylation and demonstrate that selective MICB recognition by UL16 is governed by helical structures of the MICB alpha2 domain. Transplantation of the MICB alpha2 domain confers UL16 binding capacity to MICA, and thus, diversification of the MICA alpha2 domain may have been driven by the selective pressure exerted by UL16.

Intracellular trafficking of the HCMV immunoevasin UL16 depends on elements present in both its cytoplasmic and transmembrane domains

Expression of the UL16 glycoprotein leads to down-regulation of NKG2D-ligands from the surface of the human cytomegalovirus (HCMV)-infected cell. The molecular elements responsible for UL16 trafficking and intracellular localization were investigated by preparing various chimeric proteins and mutants, using CD8 as a reporter molecule. A YQRL motif, present in UL16's cytoplasmic tail was functional for internalization, but the presence of the transmembrane domain modified the fate of the molecule after internalization. Various elements of the transmembrane domain that affected the trafficking of the protein were identified; however, their influence was modified in turn by the presence of the cytoplasmic tail of UL16. Strikingly, the extremely slow maturation rate of the native viral protein was only reproduced by the chimera that contained both transmembrane and cytoplasmic regions of UL16. These findings add data to a topic of increasing interest and importance: the role of the transmembrane domain of a protein in controlling its intracellular trafficking. In addition, they provide a new insight into the mechanism of action of the viral immunoevasin UL16.

NKG2D ligands are expressed on stressed human airway epithelial cells

Immune surveillance of the airways is critical to maintain the integrity and health of the lung. We have identified a family of ligands expressed on the surface of stressed airway epithelial cells whose function is to bind the NKG2D-activating receptor found on several pulmonary lymphocytes, including natural killer cells, γδ+ T cells, and CD8+ T cells. We employed real-time PCR and flow cytometry in normal and transformed airway epithelial cell to demonstrate that major histocompatibility complex class I chain-related (MIC) B and the UL-16 binding protein (ULBP) ligands (ULBP1–4) are ubiquitously expressed at the mRNA level in all cell lines. MICA/B surface expression was present on 70% of transformed cell lines but was undetectable on primary cells. We demonstrate that MICA/B and ULBP 1, 2, 3, and 4 expression is rare or absent on the cell surface of unstimulated normal human bronchial epithelial cells although transcripts and intracellular proteins are present. Normal human bronchial epithelial cells exposed to 0.3 mM hydrogen peroxide exhibit an induction of all ligands examined on the cell surface. Surface expression is independent of changes in transcript level or total cellular protein and is mediated by the ERK family of mitogen-activated protein kinases. The induction of NKG2D ligands on stressed airway epithelial cells represents a potentially important mechanism of immune cell activation in regulation of pulmonary health and disease.

The human cytomegalovirus glycoprotein UL16 traffics through the plasma membrane and the nuclear envelope

The human cytomegalovirus (HCMV) UL16 gene encodes a glycoprotein that interferes with the immune response to the virus-infected cell. In vitro, UL16 interacts with MICB and ULBPs that are ligands for the stimulatory receptor NKG2D, expressed on NK cells and CD8(+)T cells. UL16 expression has been shown to promote intracellular accumulation of MICB, ULBP1 and 2 and thus, interfere with the immune response to HCMV-infected cells. The mechanism that has been suggested for UL16-mediated MICB downmodulation is retention in the ER. Here, we studied the intracellular localization and maturation of UL16 and MICB in HCMV-infected cells and transfectant systems. UL16 trafficked through the ER, TGN and progressed to the plasma membrane, after which the protein was internalized. Strikingly, UL16 was also observed in the inner nuclear membrane. MICB was also localized in the TGN in HCMV-infected cells. These data suggest that MICB trafficking might be affected after its transit through the ER.

NKG2D and cytotoxic effector function in tumor immune surveillance

NKG2D is a type II transmembrane-anchored glycoprotein expressed as a disulfide-linked homodimer on the surface of all mouse and human natural killer cells (NK cells). Stimulation of NK cells through NKG2D triggers cell-mediated cytotoxicity and in some cases induces the production of cytokines. NKG2D binds to family of ligands with structural homology to MHC class I, however, unlike conventional MHC class I molecules, NKG2D ligands often display up-regulated surface expression on stressed cells and are frequently over expressed by tumors. Recent evidence clearly implicates that NKG2D recognition plays an important role in tumor immune surveillance and that NKG2D primarily acts to trigger perforin-mediated apoptosis. The data begin to place the NKG2D pathway into the context of other recognition-effector systems used by NK cells.

Down-regulation of the NKG2D ligand MICA by the human cytomegalovirus glycoprotein UL142

Human cytomegalovirus (HCMV) employs a variety of strategies to modify or evade the host immune response, and natural killer (NK) cells play a crucial role in controlling cytomegalovirus infections in mice and humans. Activation of NK cells through the receptor NKG2D/DAP10 leads to killing of NKG2D ligand-expressing cells. We have previously shown that HCMV is able to down-regulate the surface expression of some NKG2D ligands, ULBP1, ULBP2, and MICB via the viral glycoprotein UL16. Here, we show that the viral gene product UL142 is able to down-regulate another NKG2D ligand, MICA, leading to protection from NK cytotoxicity. UL142 is not able to affect surface expression of all MICA alleles, however, which may reflect selective pressure on the host to thwart viral immune evasion, further supporting an important role for the MICA-NKG2D interaction in immune surveillance.

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.

Regulation of NKG2D ligand gene expression

The activating immunoreceptor NKG2D has seven known host ligands encoded by the MHC class I chain-related MIC and ULBP/RAET genes. Why there is such diversity of NKG2D ligands is not known but one hypothesis is that they are differentially expressed in different tissues in response to different stresses. To explore this, we compared expression patterns and promoters of NKG2D ligand genes. ULBP/RAET genes were transcribed independent of each other in a panel of cell lines. ULBP/RAET gene expression was upregulated on infection with human cytomegalovirus; however, a clinical strain, Toledo, induced expression more slowly than did a laboratory strain, AD169. ULBP4/RAET1E was not induced by infection with either strain. To investigate the mechanisms behind the similarities and differences in NKG2D ligand gene expression a comparative sequence analysis of NKG2D ligand gene putative promoter regions was conducted. Sequence alignments demonstrated that there was significant sequence diversity; however, one region of high similarity between most of the genes is evident. This region contains a number of potential transcription factor binding sites, including those involved in shock responses and sites for retinoic acid-induced factors. Promoters of some NKG2D ligand genes are polymorphic and several sequence alterations in these alleles abolished putative transcription factor binding.

A transmembrane-anchored rat RAE-1-like transcript as a ligand for NKR-P2, the rat ortholog of human and mouse NKG2D

NKG2D is a potent activating immunoreceptor that has been extensively characterized in NK cells and shown to lend costimulatory functions in CD8(+) T cells under certain conditions. The list is growing of ligands for NKG2D, which are distantly related to MHC class I molecule and are often up-regulated during cellular distress. Here, we describe the cloning of a novel (and incidentally the first) ligand for NKR-P2, the rat ortholog of human and mouse NKG2D, termed as rat RAE-1-like transcript (RRLT). The newly identified ligand is homologous to mouse RAE-1 family of proteins, but differs from them in being transmembrane anchored. The protein localizes to cell surface, and is expressed in a variety of tumor cell lines and tissues. Ectopic expression of the ligand induces NK cell activation, and renders the target cells susceptible to NK cell killing. In addition, the role of RRLT-NKR-P2 interaction in tumor killing has been demonstrated.

Immunoselection by natural killer cells of PIGA mutant cells missing stress-inducible ULBP

The mechanism by which paroxysmal nocturnal hemoglobinuria (PNH) clones expand is unknown. PNH clones harbor PIGA mutations and do not synthesize glycosylphosphatidylinositol (GPI), resulting in deficiency of GPI-linked membrane proteins. GPI-deficient blood cells often expand in patients with aplastic anemia who sustain immune-mediated marrow injury putatively induced by cytotoxic cells, hence suggesting that the injury allows PNH clones to expand selectively. We previously reported that leukemic K562 cells preferentially survived natural killer (NK) cell-mediated cytotoxicity in vitro when they acquired PIGA mutations. We herein show that the survival is ascribable to the deficiency of stress-inducible GPI-linked membrane proteins ULBP1 and ULBP2, which activate NK and T cells. The ULBPs were detected on GPI-expressing but not on GPI-deficient K562 cells. In the presence of antibodies to either the ULBPs or their receptor NKG2D on NK cells, GPI-expressing cells were as less NK sensitive as GPI-deficient cells. NK cells therefore spared ULBP-deficient cells in vitro. The ULBPs were identified only on GPI-expressing blood cells of a proportion of patients with PNH but none of healthy individuals. Granulocytes of the patients partly underwent killing by autologous cytotoxic cells, implying ULBP-associated blood cell injury. In this setting, the lack of ULBPs may allow immunoselection of PNH clones.

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.

NK cell receptors involved in the response to human cytomegalovirus infection

Human cytomegalovirus (HCMV) infection is a paradigm of the complexity reached by host-pathogen interactions. To avoid recognition by cytotoxic T lymphocytes (CTL) HCMV inhibits the expression of HLA class I molecules. As a consequence, engagement of inhibitory killer immunoglobulin-like receptors (KIR), CD94/NKG2A, and CD85j (ILT2 or LIR-1) natural killer cell receptors (NKR) specific for HLA class I molecules is impaired, and infected cells become vulnerable to an NK cell response driven by activating receptors. In addition to the well-defined role of the NKG2D lectin-like molecule, the involvement of other triggering receptors (i.e., activating KIR, CD94/NKG2C, NKp46, NKp44, and NKp30) in the response to HCMV is being explored. To escape from NK cell-mediated surveillance, HCMV interferes with the expression of NKG2D ligands in infected cells. In addition, the virus may keep NK inhibitory receptors engaged preserving HLA class I molecules with a limited role in antigen presentation (i.e., HLA-E) or, alternatively, displaying class I surrogates. Despite considerable progress in the field, a number of issues regarding the involvement of NKR in the innate immune response to HCMV remain uncertain.

Role of NK cell-activating receptors and their ligands in the lysis of mononuclear phagocytes infected with an intracellular bacterium

We studied the role of NK cell-activating receptors and their ligands in the lysis of mononuclear phagocytes infected with the intracellular pathogen Mycobacterium tuberculosis. Expression of the activating receptors NKp30, NKp46, and NKG2D were enhanced on NK cells by exposure to M. tuberculosis-infected monocytes, whereas expression of DNAX accessory molecule-1 and 2B4 was not. Anti-NKG2D and anti-NKp46 inhibited NK cell lysis of M. tuberculosis-infected monocytes, but Abs to NKp30, DNAX accessory molecule-1, and 2B4 had no effect. Infection of monocytes up-regulated expression of the NKG2D ligand, UL-16 binding protein (ULBP)1, but not expression of ULBP2, ULBP3, or MHC class I-related chain A or chain B. Up-regulation of ULBP1 on infected monocytes was dependent on TLR2, and anti-ULBP1 abrogated NK cell lysis of infected monocytes. The dominant roles of NKp46, NKG2D, and ULBP1 were confirmed for NK cell lysis of M. tuberculosis-infected alveolar macrophages. We conclude that NKp46 and NKG2D are the principal receptors involved in lysis of M. tuberculosis-infected mononuclear phagocytes, and that ULBP1 on infected cells is the major ligand for NKG2D. Furthermore, TLR2 contributes to up-regulation of ULBP1 expression.

Human NK Cells Inhibit Cytomegalovirus Replication through a Noncytolytic Mechanism Involving Lymphotoxin-Dependent Induction of IFN-β

NK cells play a key role in host defense against the beta-herpesvirus CMV through perforin-dependent cytolysis. In this study, we show that human NK cells can also control human CMV (HCMV) infection by a noncytolytic mechanism involving induction of IFN-beta in the virus-infected cell. Both IL-2-activated primary NK cells and an IL-2-dependent NK cell line (NK-92) exhibited potent, noncytolytic anti-HCMV activity at very low E:T cell ratios (<0.1:1). Activated NK cells expressed lymphotoxin (LT)alphabeta on their cell surface, and secreted LTalpha and TNF, all of which contributed to the NF-kappaB-dependent release of IFN-beta from infected fibroblasts. IFN-beta produced by fibroblasts and NK cell-produced IFN-gamma combined to inhibit HCMV replication after immediate early gene expression. These results highlight an efficient mechanism used by NK cells to activate IFN-beta expression in the infected target cell that contributes to the arrest of virion production and virus spread without cellular elimination.

NKG2D in NK and T Cell-Mediated Immunity

One of the best characterized NK cell receptors is NKG2D, a highly conserved C-type lectin-like membrane glycoprotein expressed on essentially all NK cells, as well as on gammadelta-TcR+ T cells and alphabeta-TcR+ CD8+ T cells, in humans and mice. Here we review recent studies implicating NKG2D in T cell and NK cell-mediated immunity to viruses and tumors, and its potential role in autoimmune diseases and allogeneic bone marrow transplantation.

NKG2D function protects the host from tumor initiation

The activation NKG2D receptor has been shown to play an important role in the control of experimental tumor growth and metastases expressing ligands for NKG2D; however, a function for this recognition pathway in host protection from de novo tumorigenesis has never been demonstrated. We show that neutralization of NKG2D enhances the sensitivity of wild-type (WT) C57BL/6 and BALB/c mice to methylcholanthrene (MCA)-induced fibrosarcoma. The importance of the NKG2D pathway was additionally illustrated in mice deficient for either IFN-γ or tumor necrosis factor–related apoptosis-inducing ligand, whereas mice depleted of natural killer cells, T cells, or deficient for perforin did not display any detectable NKG2D phenotype. Furthermore, IL-12 therapy preventing MCA-induced sarcoma formation was also largely dependent on the NKG2D pathway. Although NKG2D ligand expression was variable or absent on sarcomas emerging in WT mice, sarcomas derived from perforin-deficient mice were Rae-1⁺ and immunogenic when transferred into WT syngeneic mice. These findings suggest an important early role for the NKG2D in controlling and shaping tumor formation.

Functional dissection of HCMV US11 in mediating the degradation of MHC class I molecules

The human cytomegalovirus (HCMV) gene product US11 dislocates MHC I heavy chains from the endoplasmic reticulum (ER) and targets them for proteasomal degradation in the cytosol. To identify the structural and functional domains of US11 that mediate MHC class I molecule degradation, we constructed truncated mutants and chimeric proteins, and analyzed these to determine their intracellular localization and their ability to degrade MHC class I molecules. We found that only the luminal domain of US11 was essential to confer ER localization to the protein but that the ability to degrade MHC class I molecules required both the transmembrane domain and the luminal domain of US11. By analyzing a series of point mutants of the transmembrane domain, we were also able to identify Gln(192) and Gly(196) as being crucial for the functioning of US11, suggesting that these residues may play a critical role in interacting with the components of the protein degradation machinery.

NK cell recognition

The integrated processing of signals transduced by activating and inhibitory cell surface receptors regulates NK cell effector functions. Here, I review the structure, function, and ligand specificity of the receptors responsible for NK cell recognition.

Role of tumor cell immune escape mechanisms in cytomegalovirus-mediated oncomodulation

It has been known for a long time that cytomegalovirus (CMV) has evolved mechanisms that allow the escape from the host immune surveillance. In the past, many efforts have been done to elucidate the molecular mechanisms underlying this virus-mediated immune escape and thus virus persistence. However, it is unknown, whether CMV may also impair immune responses directed against tumor cells. This might have severe consequences on tumor progression and may explain the growing evidence for CMV-mediated oncomodulation. This review summarizes recent work on CMV-mediated immune escape mechanisms of tumor cells and oncomodulation and proposes novel aspects that may be important for understanding the CMV-associated tumor progression.

NKG2D is a costimulatory receptor for human naive CD8+ T cells

In humans, all alpha beta CD8+ T cells express NKG2D, but in mouse, it is only expressed by activated and memory CD8+ T cells. We purified human naive CD8+ T cells to show that NKG2D serves as a costimulatory receptor for TCR induced Ca2+ mobilization and proliferation. The resulting effector cells are skewed toward a type 1 phenotype and produce high levels of IFN-gamma and TNF-alpha. NKG2D ligands, MHC class I chain-related (MIC)A, MICB, and UL16-binding proteins are expressed on the proliferating cells and NKG2D is down-regulated. The addition of the homeostatic cytokines IL-7 and IL-15 to the culture medium not only enhances proliferation but also counteracts the down-regulation of NKG2D, more so than the addition of IL-2. These results indicate that NKG2D can regulate the priming of human naive CD8+ T cells, which may provide an alternative mechanism for potentiating and channeling the immune response.

The activating NKG2D receptor assembles in the membrane with two signaling dimers into a hexameric structure

The activating NKG2D receptor plays a critical role in innate and adaptive immune responses by natural killer cells and subpopulations of T cells. The human receptor assembles with the DAP10 signaling dimer, and it is thought that one NKG2D homodimer pairs with a single DAP10 dimer by formation of two salt bridges between charged transmembrane (TM) residues. However, direct stoichiometry measurements demonstrated that one NKG2D homodimer assembles with four DAP10 chains. Selective mutation of one of the basic TM residues of NKG2D resulted in loss of two DAP10 chains, indicating that each TM arginine serves as an interaction site for a DAP10 dimer. Assembly of the hexameric structure was cooperative because this mutation also significantly reduced NKG2D dimerization. A monomeric NKG2D TM peptide was sufficient for assembly with a DAP10 dimer, indicating that the interaction between these proteins occurs in the membrane environment. Formation of a three-helix interface among the TM domains involved ionizable residues from all three chains, the TM arginine of NKG2D and both TM aspartic acids of the DAP10 dimer. The organization of the TM domains thus shows similarities to the T cell antigen receptor-CD3 complex, in particular to the six-chain assembly intermediate between T cell antigen receptor and the CD3delta epsilon and CD3gamma epsilon dimers. Binding of a single ligand can thus result in phosphorylation of four DAP10 chains, which may be relevant for the sensitivity of NKG2D receptor signaling, in particular in situations of low ligand density.

Effect of human cytomegalovirus on expression of MHC class I-related chains A

The MHC-encoded MHC class I-related chains A (MICA) glycoproteins are known to enhance the functions of NK and T cells by ligating the stimulating receptor NKG2D and appear to play an important role in host defense. Human CMV (HCMV) evades the immune response in many different ways, but has not previously been found to down-regulate MICA. We have found that a common form of MICA, which has a nucleotide insertion in exon 5 corresponding to the transmembrane region and no cytoplasmic tail, was increased on the surface of fibroblasts HFS-13 compared with the mock-infected sample of the same cells that had been cultured to confluence. However, an astrocytoma cell line, U373, which has a full-length variant of MICA, showed that the expression of MICA was decreased after HCMV infection. Retroviral transduction of different MICA alleles into fibroblasts HFF-D, which express no MICA of their own, established that full-length MICA was down-regulated by HCMV, and the truncated form was not. Fibroblasts with decreased MICA due to HCMV infection were found to be protected from NK cell killing, whereas in the presence of the truncated form of MICA, the virus-infected cells were destroyed. Thus, the truncated form of MICA, which is the most common, has a mutation that allows it to persist on the surface and hinder efforts of the virus to evade the immune response.

Clinical and immunologic aspects of cytomegalovirus infection in solid organ transplant recipients

Primary cytomegalovirus (CMV) infection is a major cause of morbidity and mortality in recipients after solid organ transplantation (SOT). Widespread and prolonged use of antiviral drugs has changed the natural course of CMV disease by delaying its onset and causing drug resistance. CMV induces a strong cellular immune response, even in immunosuppressed patients, and has developed strategies to evade this immune surveillance. This review summarizes challenges in managing CMV infection in transplant recipients and highlights current insights in the cellular immune response against CMV.

Viral modulation of NK cell immunity

Natural killer (NK) cells have been implicated in innate immune responses against viruses such as herpesviruses, which cause persistent infections in the host. In response to the selective pressure that is exerted by NK cells, many viruses have evolved strategies either to evade detection by NK cells or to modulate the activity of NK cells. Here, we review the unique relationship that exists between NK cells and viruses, with a focus on herpesviruses.

NK cells in innate immunity

NK cells have an important role in innate immune responses, particularly in anti-viral immunity. Recent studies have revealed a molecular basis for NK cell recognition of virus-infected cells, implicating the activating KIR and Ly49 receptors and NKG2D in this process. Additionally, mutual cooperation between NK cells and dendritic cells suggests that these innate cells can shape the nature of an adaptive immune response. These findings, as well as advances in understanding NK cell development and homeostasis, indicate that NK cell biology is more sophisticated than previously appreciated.

Mutual interference of HIV and natural killer cell-mediated immune response

Natural killer (NK) cells represent important early effector cells in innate immune defense as they exert their functions without prior sensitization. They participate in regulation of innate and adaptive immune responses and hematopoiesis by producing various cytokines and chemokines. In addition, NK cells lyse virally infected and malignant cells raising them to multifunctional members of the first line of defense. Unlike other lymphocytes they lack specific antigen receptors. They rather bind cells using ubiquitous molecules and communicate via a pattern of receptors specific for MHC-I molecules with their counterparts. In general, successful binding of the receptors delivers an inhibitory signal to NK cells thus sparing the target cell from lysis. In contrast, down-regulated or altered MHC-I expression as frequently observed during virus infection or on malignant cells prevents ligation of inhibitory receptors and MHC-I paralyzing inhibition and thus inducing lysis of the target cell. In human immunodeficiency virus (HIV) infection NK cells are of central importance since they can combat viral infection itself and opportunistic pathogens like fungi and protozoa that usually spread during the course of HIV infection. However, various studies have reported alterations in HIV patients affecting NK cell numbers and functions that might negatively influence course and severity of the disease. This review will focus on the mutual interference of NK cells and the HI virus.

Imprint of human cytomegalovirus infection on the NK cell receptor repertoire

Expression of the activating CD94/NKG2C killer lectin-like receptor (KLR) specific for HLA-E was analyzed in peripheral blood lymphocytes (PBLs) from healthy adult blood donors; the expression of other natural killer (NK) cell receptors (ie, CD94/NKG2A, KIR, CD85j, CD161, NKp46, NKp30, and NKG2D) was also studied. Human cytomegalovirus (HCMV) infection as well as the HLA-E and killer immunoglobulin-like receptor (KIR) genotypes were considered as potentially relevant variables associated with CD94/NKG2C expression. The proportion of NKG2C(+) lymphocytes varied within a wide range (<0.1% to 22.1%), and a significant correlation (r = 0.83; P < .001) between NKG2C(+) NK and T cells was noticed. The HLA-E genotype and the number of activating KIR genes of the donors were not significantly related to the percentage of NKG2C(+) lymphocytes. By contrast, a positive serology for HCMV, but not for other herpesviruses (ie, Epstein-Barr and herpes simplex), turned out to be strongly associated (P < .001) with increased proportions of NKG2C(+) NK and T cells. Remarkably, the CD94/NKG2C(+) population expressed lower levels of natural cytotoxicity receptors (NCRs) (ie, NKp30, NKp46) and included higher proportions of KIR(+) and CD85j(+) cells than CD94/NKG2A(+) cells. Altogether, these data support that HCMV infection selectively shapes the natural killer cell receptor (NKR) repertoire of NK and T cells from healthy carrier individuals.

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.

A Structural basis for the association of DAP12 with mouse, but not human, NKG2D

Prior studies have revealed that alternative mRNA splicing of the mouse NKG2D gene generates receptors that associate with either the DAP10 or DAP12 transmembrane adapter signaling proteins. We report that NKG2D function is normal in human patients lacking functional DAP12, indicating that DAP10 is sufficient for human NKG2D signal transduction. Further, we show that human NKG2D is incapable of associating with DAP12 and provide evidence that structural differences in the transmembrane of mouse and human NKG2D account for the species-specific difference for this immune receptor.

Human MHC class I chain related (MIC) genes: their biological function and relevance to disease and transplantation

Major histocompatibility complex (MHC) class I chain related (MIC) molecules show homology with classical human leukocyte antigen (HLA) molecules, but they do not combine with beta2 microglobulin, do not bind peptide and are not expressed on normal circulating lymphocytes. In response to stress, MIC proteins are expressed on the cell surface of freshly isolated gastric epithelium, endothelial cells and fibroblasts and engage the activating natural killer cell receptor NKG2D, which is found on many cells within the immune system. Despite the highly polymorphic nature of MIC genes, only one polymorphic position has been identified that appears to affect the binding of NKG2D. Alleles with a methionine at codon 129 have a 10-50-fold greater capacity to complex NKG2D than alleles with a valine at this position. Renal and pancreatic grafts with evidence of both acute and chronic rejection have been shown to express MIC proteins, and anti-MIC antibodies have been identified in the serum of these patients. Some MIC molecules which are expressed by tumours appear to shed and solubilize in plasma. This soluble form of MIC engages cells expressing NKG2D, rendering them inactive, and impairs tumour cytolysis. Similarly, a protein encoded by human cytomegalovirus (CMV) prevents MICB surface expression and subsequent NKG2D interaction. Whereas the benefit of solid organ transplantation may be hindered by the expression of MIC molecules on grafts, tumours and viruses may take advantage of the expression of MIC molecules on transformed and virus-infected cells in order to evade this recognition pathway.

Specific recognition of virus-infected cells by paired NK receptors

NK cells show cytotoxicity and cytokine production upon recognition of virus-infected cells and play an important role in conferring protective immunity against virus infection. Patients who lack functional NK cells are susceptible to severe infectious diseases such as herpesvirus infection. In addition, when NK cells are removed from cytomegalovirus-resistant mice, the virus titre after infection is markedly increased and the mice are likely to die due to infection. However, the exact mechanism of how NK cells recognise virus-infected cells has remained unclear for a long time. Recent findings of the presence of virus-specific NK cell receptors and their ligands on virus-infected cells have provided a new vision of a protective role of NK cells in virus infection. Furthermore, the recognition of virus-infected cells by paired receptors consisting of activating and inhibitory receptors was found to correlate with the degree of host susceptibility to virus infection. Current topics on the role of NK cells in anti-virus immunity are reviewed.

Consequences of human cytomegalovirus mimicry

The HCMV genome has evolved with its host by incorporating a series of genes that are homologous to, or functionally mimic, cellular genes. Some are designed to counteract the stress of infection on the host cell, notably the viral antiapoptotic proteins (vICA, vMIA). Others potentially help the infected cell maintain a low immunologic profile. These include virus-encoded chemokine receptors (UL33, UL78, US27, US28), FcRs (gp TRL11/IRL11, gp UL119-118), and proteins that directly or indirectly thwart natural killer cell activity (UL16, gpUL40). In addition, some viral proteins may play a role in immunopathology because of fortuitous cross-reactivity with host cell proteins. This overview discusses how these proteins affect the life of the host cell and its immediate neighbors.

Natural killer cell receptors for major histocompatibility complex class I and related molecules in cytomegalovirus infection

Downmodulation of major histocompatibility complex (MHC) class I molecules by cytomegalovirus (CMV) impairs the engagement of specific leucocyte-inhibitory receptors, rendering infected cells vulnerable to natural killer (NK) cells. Members of the murine Ly49 and human KIR families, CD85j (ILT2 or leucocyte Ig-like receptor-1), as well as the CD94/NKG2A-inhibitory killer lectin-like receptor (KLR) fulfil this surveillance role. On the other hand, NK-activating receptors specific to ligands expressed on virus-infected cells may overcome the control by inhibitory receptors. In this regard, NKG2D and Ly49H lectin-like molecules trigger NK-cell functions recognizing, respectively class I-related stress-inducible molecules and the m157 murine CMV glycoprotein. Among a variety of immune evasion strategies, CMV promotes the synthesis of class I surrogates and selectively preserves the expression of some class I molecules in infected cells; moreover, CMV interferes with the expression of ligands for NKG2D. We herein review these aspects of the host-pathogen interaction, discussing a number of open issues.