The natural killer gene complex: a genetic basis for understanding natural killer cell function and innate immunity

Genome-wide identification and expression analysis of C-type lectins in discus fish (Symphysodon aequifasciatus) during parental care

Discus fish (Symphysodon aequifasciatus) exhibit a unique parental care behavior: adult discus produces secretion through their skin, on which the larvae live after birth. The immune components in the skin mucus of parental discus would change during different parental care. C-type lectins (CTLs) could identify and eliminate pathogenic microorganisms and play important roles in innate immunity. Studies on CTLs of discus fish especially during parental care, however, are scarce. Here, we identified 186 CTL genes that distributed in 27 linkage groups based on discus genome. Phylogenetic analysis showed that S. aequifasciatus CTL (SaCTL) members were grouped into 14 subfamilies. A total of 80 gene replication events occurred, of which 15 pairs were subjected to segmental duplication and 65 pairs underwent tandem duplication. Ka/Ks ranged from 0.11 (SaCTL25/SaCTL158) to 0.68 (SaCTL36/SaCTL69), all undergoing purifying selection. RNA-seq analysis revealed that SaCTL members, including duplicated genes, in the skin of parental discus show distinct expression patterns in different care stages and between male and female parents. The SaCTL11 was differentially expressed in most care stages and reached the maximum after eggs spawned, but the expression of its paired SaCTL14 was low in each stage. The SaCTL39 increased first and then decreased, reaching a peak in eggs spawned, while paired SaCTL48 first decreased and then increased, reaching a peak in hatched eggs. The SaCTL50 was differentially expressed only in female fish during care, but not in male fish. These results provide new insights into the evolution and potential functional differentiation of CTLs in discus fish during parental care.

Natural variation in oxytocin receptor signaling causes widespread changes in brain transcription: a link to the natural killer gene complex

Oxytocin (OXT) is a highly conserved neuropeptide that modulates social cognition, and variation in its receptor gene (Oxtr) is associated with divergent social phenotypes. The cellular mechanisms connecting Oxtr genotype to social phenotype remain obscure. We exploit an association between Oxtr polymorphisms and striatal-specific OXTR density in prairie voles to investigate how OXTR signaling influences the brain transcriptome. We discover widespread, OXTR signaling-dependent transcriptomic changes. Interestingly, OXTR signaling robustly modulates gene expression of C-type lectin-like receptors (CTLRs) in the natural killer gene complex, a genomic region associated with immune function. CTLRs are positioned to control microglial synaptic pruning; a process important for shaping neural circuits. Similar relationships between OXTR RNA and CTLR gene expression were found in human striatum. These data suggest a potential molecular mechanism by which variation in OXTR signaling due to genetic background and/or life-long social experiences, including nurturing/neglect, may affect circuit connectivity and social behavior.

Polysaccharide from Codium fragile Induces Anti-Cancer Immunity by Activating Natural Killer Cells

Natural polysaccharides exhibit beneficial immune modulatory effects, including immune stimulatory and anti-cancer activities. In this study, we examined the effect of Codium fragile polysaccharide (CFP) on natural killer (NK) cell activation, and its effect on tumor-bearing mice. Intravenous CFP treatment of C57BL/6 mice resulted in the upregulation of CD69, which is a marker associated with NK cell activation. In addition, intracellular levels of interferon (IFN)-γ and the cytotoxic mediators perforin and granzyme B were markedly increased in response to the CFP treatment of splenic NK cells. IFN-γ production by NK cells was directly induced by CFP, whereas the upregulation of CD69 and cytotoxic mediators required IL-12. Finally, intraperitoneal treatment with CFP prevented CT-26 (murine carcinoma) tumor cell infiltration in the lungs, without significantly reducing the body weight. In addition, treatment with CFP prevented B16 melanoma cell infiltration in the lung of C57BL/6 mice. Moreover, the anti-tumor effect was diminished by the depletion of NK cells. Therefore, these data suggest that CFP may be used as an NK cell stimulator to produce a phenomenon that contributes to anti-cancer immunity.

Decrease of CD69 levels on TCR Vα7.2+CD4+ innate-like lymphocytes is associated with impaired cytotoxic functions in chronic hepatitis B virus-infected patients

Hepatitis B virus (HBV) infection is a major cause of chronic liver disease that may progress to liver cirrhosis and hepatocellular carcinoma. Host immune responses represent the key determinants of HBV clearance or persistence. Here, we investigated the role of the early activation marker, CD69 and effector cytokines, granzyme B (GrB) and IFN-γ in the exhaustion of innate-like TCR Vα7.2⁺CD4⁺T cells, in 15 individuals with chronic HBV (CHB) infection where six were HBV DNA⁺ and nine were HBV DNA^-. The percentage of cytokine-producing T cells and MAIT cells were significantly perturbed in HBV patients relative to healthy controls (HCs). The intracellular expression of GrB and IFN-γ was significantly reduced in MAIT cells derived from HBV-infected patients as compared to HCs, and the levels correlated with the percentage and levels [mean fluorescence intensity (MFI)] of CD69 expression. The total expression of CD69 (iMFI) was lower in CHB patients as compared to HCs. The frequency of CD69⁺ cells correlated with the levels of cytokine expression (MFI), particularly in CHB patients as compared to HCs. In summary, the polyfunctionality of peripheral T cells was significantly reduced among CHB patients, especially in the TCR Vα7.2⁺CD4⁺T cells, and the levels of cytokine expression correlated with functional cytokine levels.

NK cells and virus-related cancers

Natural killer (NK) cells become activated during viral infections and can play roles in such infections by attacking virus-infected cells or by regulating adaptive immune responses. Experimental models suggest that NK cells may also have the capacity to restrain virus-induced cancers. Here, we discuss the seven viruses linked to human cancers and the evidence of NK cell involvement in these systems.

Cytomegalovirus following stem cell transplantation: from pharmacologic to immunologic therapy

Human cytomegalovirus is a large DNA virus that is well-equipped to evade both innate and adaptive host immune responses and to establish lifelong latency. It is a major opportunistic pathogen in immunocompromised hosts. Following allogeneic transplantation, immune responses are often inadequate to inhibit viral reactivation, resulting in progressive tissue damage, manifesting as overt human cytomegalovirus disease that usually presents as pneumonitis, colitis or hepatitis. Currently available antiviral pharmacotherapies are limited by toxicities if used prophylactically, and by a lack of efficacy in established human cytomegalovirus disease. Efforts have therefore focused on molecular diagnostic surveillance protocols that allow earlier intervention and the development of adoptive immunotherapeutic strategies to hasten host immune reconstitution.

NK cells controlling virus-specific T cells: Rheostats for acute vs. persistent infections

Viral infections characteristically induce a cytokine-driven activated natural killer (NK) cell response that precedes an antigen-driven T cell response. These NK cells can restrain some but not all viral infections by attacking virus-infected cells and can thereby provide time for an effective T cell response to mobilize. Recent studies have revealed an additional immunoregulatory role for the NK cells, where they inhibit the size and functionality of the T cell response, regardless of whether the viruses are themselves sensitive to NK cells. This subsequent change in T cell dynamics can alter patterns of immunopathology and persistence and implicates NK cells as rheostat-like regulators of persistent infections.

Natural killer T cell deficiency in active adult-onset Still's Disease: correlation of deficiency of natural killer T cells with dysfunction of natural killer cells

OBJECTIVE

To examine the levels and functions of natural killer (NK) and natural killer T (NKT) cells, investigate relationships between NK and NKT cells, and determine the clinical relevance of NKT cell levels in patients with adult-onset Still's disease (AOSD).

METHODS

Patients with active untreated AOSD (n = 20) and age- and sex-matched healthy controls (n = 20) were studied. NK and NKT cell levels were measured by flow cytometry. Peripheral blood mononuclear cells were cultured in vitro with α-galactosylceramide (αGalCer). NK cytotoxicity against K562 cells and proliferation indices of NKT cells were estimated by flow cytometry.

RESULTS

Percentages and absolute numbers of NKT cells were significantly lower in the peripheral blood of AOSD patients than in that of healthy controls. Proliferative responses of NKT cells to αGalCer were also lower in patients, and this was found to be due to proinflammatory cytokines and NKT cell apoptosis. In addition, NK cytotoxicity was found to be significantly lower in patients than in healthy controls, but NK cell levels were comparable in the 2 groups. Notably, this NKT cell deficiency was found to be correlated with NK cell dysfunction and to reflect active disease status. Furthermore, αGalCer-mediated NK cytotoxicity, showing the interaction between NK and NKT cells, was significantly lower in AOSD patients than in healthy controls.

CONCLUSION

These findings demonstrate that NK and NKT cell functions are defective in AOSD patients and suggest that these abnormalities contribute to innate immune dysfunction in AOSD.

All is fair in virus-host interactions: NK cells and cytomegalovirus

The infection of mice with mouse cytomegalovirus (MCMV) as a model of human cytomegalovirus (HCMV) infection has been particularly informative in elucidating the role of innate and adaptive immune response mechanisms during infection. Millions of years of co-evolution between cytomegaloviruses (CMV) and their hosts has resulted in numerous attempts to overwhelm each other. CMVs devote many genes to modulating the host natural killer (NK) cell response and NK cells employ many strategies to cope with CMV infection. While focusing on these attack-counterattack measures, this review will discuss several novel mechanisms of immune evasion by MCMV, the role of Ly49 receptors in mediating resistance to MCMV, and the impact of the initial NK cell response on the shaping of adaptive immunity.

Differential activation of NK cells by influenza A pseudotype H5N1 and 1918 and 2009 pandemic H1N1 viruses

Natural killer (NK) cells are the effectors of innate immunity and are recruited into the lung 48 h after influenza virus infection. Functional NK cell activation can be triggered by the interaction between viral hemagglutinin (HA) and natural cytotoxicity receptors NKp46 and NKp44 on the cell surface. Recently, novel subtypes of influenza viruses, such as H5N1 and 2009 pandemic H1N1, transmitted directly to the human population, with unusual mortality and morbidity rates. Here, the human NK cell responses to these viruses were studied. Differential activation of heterogeneous NK cells (upregulation of CD69 and CD107a and gamma interferon [IFN-gamma] production as well as downregulation of NKp46) was observed following interactions with H5N1, 1918 H1N1, and 2009 H1N1 pseudotyped particles (pps), respectively, and the responses of the CD56(dim) subset predominated. Much stronger NK activation was triggered by H5N1 and 1918 H1N1 pps than by 2009 H1N1 pps. The interaction of pps with NK cells and subsequent internalization were mediated by NKp46 partially. The NK cell activation by pps showed a dosage-dependent manner, while an increasing viral HA titer attenuated NK activation phenotypes, cytotoxicity, and IFN-gamma production. The various host innate immune responses to different influenza virus subtypes or HA titers may be associated with disease severity.

Decrease in intrahepatic CD56+ lymphocytes in gastric and colorectal cancer patients with liver metastases

The aim of the study was to examine the main intrahepatic lymphocyte subpopulations, namely CD3(+) lymphocytes, natural killer (NK)-like T lymphocytes (NKT) expressing the CD3(+) CD56(+) phenotype, CD56(+) NK cells, CD4(+), and CD8(+) T cells in livers of patients with gastric and colorectal cancer with and without hepatic metastases. The proportion of each lymphocyte subset was determined in 34 patients with gastric or colorectal cancer (18 with and 16 without liver metastasis) by two-color flow cytometry after extraction of hepatic mononuclear cell fraction. The distribution of lymphocyte subpopulations in selected areas of liver metastases and adjacent liver tissue was evaluated using immunohistochemistry for CD4, CD8, and CD56. Flow cytometry analysis revealed a significant decrease in the proportion of CD3(+) CD56(+) cells in metastatic livers, but not in nonmetastatic livers (11.9 +/- 10.3 vs 24.2 +/- 13.6%, p = 0.02). The percentage of intrahepatic CD3(-)CD56(+) cells was also decreased in patients with metastases compared to those without (10.1 +/- 11.6 vs 16.6 +/- 8.9%, p = 0.039). Immunohistochemically, three types of lymphocytes (CD4(+), CD8(+), and CD56(+)) were present in the metastatic tissue, although the number of CD56(+) cells was almost twice lower. We found a low prevalence of tumor-infiltrating CD4(+), CD8(+), and CD56(+) cells in livers with multiple metastases, whereas in cases with solitary metastasis a higher degree of lymphocyte infiltration was observed. The number of CD3(-)CD56(+) and CD3(+) CD56(+) cells was decreased in metastatic livers compared to those unaffected by metastases. Therefore the prevalence of tumor-infiltrating lymphocytes seems to be related to the progression of metastatic liver disease. Depletion of hepatic innate lymphocytes may reveal susceptibility to metastatic liver disease and could be a reason for the escape of metastatic cells from the mechanisms of liver immune control.

NK gene complex dynamics and selection for NK cell receptors

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

Analysis of individual natural killer cell responses

Typical assays for natural killer (NK) cell function assess the responses of entire NK cell populations. It is now possible to determine the responses of individual NK cells. Herein, two representative assays are described along with examples of how they have helped clarify current understanding of NK cell biology.

Regulation of class I major histocompatibility complex receptor expression in natural killer cells: one promoter is not enough!

The class I major histocompatibility complex (MHC) receptors expressed by natural killer (NK) cells play an important role in regulating their function. The number and type of inhibitory receptors expressed by NK cells must be tightly controlled in order to avoid the generation of dominantly inhibited NK cells. The selective stochastic expression of the class I MHC receptors generates a variegated NK cell population capable of discriminating subtle changes in MHC expression on potential target cells. The molecular mechanisms controlling the cell-specific and probabilistic expression of these receptors are without doubt very complex. The traditional approach of considering a core promoter modulated by upstream enhancer elements is likely too simplistic a paradigm to adequately explain the regulation of these genes, as well as other gene clusters that are not expressed in an 'all or none' fashion. Our studies on the regulation of the mouse Ly49 and human killer immunoglobulin-like receptor (KIR) clusters of class I MHC receptor genes have revealed the presence of multiple transcripts in both sense and antisense orientations. In both systems, an antisense promoter overlaps a promoter that produces sense transcripts, creating a bidirectional element. In the Ly49 genes, the competing promoters behave as probabilistic switches, and it is likely that the human bidirectional promoters will have a similar property. The antisense transcripts generated in the Ly49 genes are far removed from the promoter responsible for Ly49 expression in mature NK cells, whereas the antisense KIR transcripts detected are within the adult promoter region. This finding suggests that the mechanism of promoter regulation in the KIR genes may be quite different from that of the Ly49 genes. This review summarizes the current state of knowledge regarding class I MHC receptor gene regulation. The models proposed for the control of the probabilistic expression of the Ly49 and KIR genes are discussed in the context of current knowledge regarding the complex control of other well-studied gene clusters such as the beta-globin and cytokine clusters.

Mapping of quantitative trait loci determining NK cell-mediated resistance to MHC class I-deficient bone marrow grafts in perforin-deficient mice

NK cells reject allogeneic and MHC class I-deficient bone marrow (BM) grafts in vivo. The mechanisms used by NK cells to mediate this rejection are not yet thoroughly characterized. Although perforin plays a major role, perforin-independent mechanisms are involved as well. C57BL/6 mice deficient in perforin (B6 perforin knockout (PKO)) reject class I-deficient TAP-1 KO BM cells as efficiently as normal B6 mice. In contrast, perforin-deficient 129S6/SvEvTac mice (129 PKO) cannot mediate this rejection while normal 129 mice efficiently reject. This suggests that in 129, but not in B6, mice, perforin is crucial for NK cell-mediated rejection of MHC class I-deficient BM grafts. To identify loci linked to BM rejection in perforin-deficient mice, we generated backcross 1 progeny by crossing (129 x B6)F(1) PKO mice to 129 PKO mice. In transplantation experiments, >350 backcross 1 progeny were analyzed and displayed a great variation in ability to reject TAP-1 KO BM grafts. PCR-based microsatellite mapping identified four quantitative trait loci (QTL) on chromosomes 2, 4, and 8, with the QTL on chromosome 8 showing the highest significance, as well as a fifth epistatic QTL on chromosome 3. This study describes the first important step toward identifying BM graft resistance gene(s).

Differential resistance/susceptibility patterns to pneumovirus infection among inbred mouse strains

Respiratory syncytial virus (RSV) is a prominent cause of airway morbidity in children under 1 yr of age. It is assumed that host factors influence the severity of the disease presentation and thus the need for hospitalization. As a first step toward the identification of the underlying genes involved, this study was undertaken to establish whether inbred mouse strains differ in susceptibility to pneumonia virus of mice (PVM), the murine counterpart of RSV, which has been shown to accurately mimic the RSV disease of children. With this purpose in mind, double-chamber plethysmography and carbon monoxide uptake data were collected daily for 7 days after inoculation of PVM in six inbred strains of mice. In parallel, histological examinations and lung viral titration were carried out from day 5 to day 7 after inoculation. Pulmonary structure/function values reflected the success of viral replication in the lungs and revealed a pattern of continuous variation, with resistant, intermediate, and susceptible strains. The results suggest that SJL (resistant) and 129/Sv (susceptible) strains should be used in crossing experiments aimed at identifying genes controlling pneumovirus replication by the positional cloning approach. Similarly, crossing experiments using BALB/c or C57BL/6 (resistant) and DBA/2 or 129/Sv (susceptible) will allow the identification of the genes involved in the control of pulmonary inflammation during pneumovirus infection.

The Novel Inhibitory NKR-P1C Receptor and Ly49s3 Identify Two Complementary, Functionally Distinct NK Cell Subsets in Rats

The proximal region of the NK gene complex encodes the NKR-P1 family of killer cell lectin-like receptors which in mice bind members of the genetically linked C-type lectin-related family, while the distal region encodes Ly49 receptors for polymorphic MHC class I molecules. Although certain members of the NKR-P1 family are expressed by all NK cells, we have identified a novel inhibitory rat NKR-P1 molecule termed NKR-P1C that is selectively expressed by a Ly49-negative NK subset with unique functional characteristics. NKR-P1C(+) NK cells efficiently lyse certain tumor target cells, secrete cytokines upon stimulation, and functionally recognize a nonpolymorphic ligand on Con A-activated lymphoblasts. However, they specifically fail to kill MHC-mismatched lymphoblast target cells. The NKR-P1C(+) NK cell subset also appears earlier during development and shows a tissue distribution distinct from its complementary Ly49s3(+) subset, which expresses a wide range of Ly49 receptors. These data suggest the existence of two major, functionally distinct populations of rat NK cells possessing very different killer cell lectin-like receptor repertoires.

Heterogeneous Human NK Cell Responses toPlasmodium falciparum-Infected Erythrocytes

Human NK cells can respond rapidly to Plasmodium falciparum-infected RBC (iRBC) to produce IFN-gamma. In this study, we have examined the heterogeneity of this response among malaria-naive blood donors. Cells from all donors become partially activated (up-regulating CD69, perforin, and granzyme) upon exposure to iRBC but cells from only a subset of donors become fully activated (additionally up-regulating CD25, IFN-gamma, and surface expression of lysosomal-associated membrane protein 1 (LAMP-1)). Although both CD56dim and CD56bright NK cell populations can express IFN-gamma in response to iRBC, CD25 and LAMP-1 are up-regulated only by CD56dim NK cells and CD69 is up-regulated to a greater extent in this subset; by contrast, perforin and granzyme A are preferentially up-regulated by CD56bright NK cells. NK cells expressing IFN-gamma in response to iRBC always coexpress CD69 and CD25 but rarely LAMP-1, suggesting that individual NK cells respond to iRBC either by IFN-gamma production or cytotoxicity. Furthermore, physical contact with iRBC can, in a proportion of donors, lead to NK cell cytoskeletal reorganization suggestive of functional interactions between the cells. These observations imply that individuals may vary in their ability to mount an innate immune response to malaria infection with obvious implications for disease resistance or susceptibility.

Sendai virus-induced alterations in lung structure/function correlate with viral loads and reveal a wide resistance/susceptibility spectrum among mouse strains

The Paramyxoviridae family includes some of the most important and ubiquitous disease-causing viruses of infants and children, most of which cause significant infections of the respiratory tract. Evidence is accumulating in humans that genetic factors are involved in the severity of clinical presentation. As a first step toward the identification of the genes involved, this study was undertaken to establish whether laboratory mouse strains differ in susceptibility to Sendai virus, the murine counterpart of human type-1 parainfluenza virus which, historically, has been used extensively in studies that have defined the basic biological properties of paramyxoviruses in general. With this purpose in mind, double-chamber plethysmography data were collected daily for 7 days after inoculation of Sendai virus in six inbred strains of mice. In parallel, histological examinations and lung viral titration were carried out from day 5 to day 7 after inoculation. Pulmonary structure/function values closely reflected the success of viral replication in the lungs and revealed a pattern of continuous variation with resistant, intermediate, and susceptible strains. The results unambiguously suggest that BALB/c (resistant) and 129Sv (susceptible) strains should be used in crossing experiments aimed at identifying the genes involved in resistance to Paramyxoviridae by the positional cloning approach.

A focused microarray approach to functional glycomics: transcriptional regulation of the glycome

Glycosylation is the most common posttranslational modification of proteins, yet genes relevant to the synthesis of glycan structures and function are incompletely represented and poorly annotated on the commercially available arrays. To fill the need for expression analysis of such genes, we employed the Affymetrix technology to develop a focused and highly annotated glycogene-chip representing human and murine glycogenes, including glycosyltransferases, nucleotide sugar transporters, glycosidases, proteoglycans, and glycan-binding proteins. In this report, the array has been used to generate glycogene-expression profiles of nine murine tissues. Global analysis with a hierarchical clustering algorithm reveals that expression profiles in immune tissues (thymus [THY], spleen [SPL], lymph node, and bone marrow [BM]) are more closely related, relative to those of nonimmune tissues (kidney [KID], liver [LIV], brain [BRN], and testes [TES]). Of the biosynthetic enzymes, those responsible for synthesis of the core regions of N- and O-linked oligosaccharides are ubiquitously expressed, whereas glycosyltransferases that elaborate terminal structures are expressed in a highly tissue-specific manner, accounting for tissue and ultimately cell-type-specific glycosylation. Comparison of gene expression profiles with matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) profiling of N-linked oligosaccharides suggested that the alpha1-3 fucosyltransferase 9, Fut9, is the enzyme responsible for terminal fucosylation in KID and BRN, a finding validated by analysis of Fut9 knockout mice. Two families of glycan-binding proteins, C-type lectins and Siglecs, are predominately expressed in the immune tissues, consistent with their emerging functions in both innate and acquired immunity. The glycogene chip reported in this study is available to the scientific community through the Consortium for Functional Glycomics (CFG) (http://www.functionalglycomics.org).

Epistasis between mouse Klra and major histocompatibility complex class I loci is associated with a new mechanism of natural killer cell-mediated innate resistance to cytomegalovirus infection

Experimental infection with mouse cytomegalovirus (MCMV) has been used to elucidate the intricate host-pathogen mechanisms that determine innate resistance to infection. Linkage analyses in F(2) progeny from MCMV-resistant MA/My (H2 (k)) and MCMV-susceptible BALB/c (H2 (d)) and BALB.K (H2 (k)) mouse strains indicated that only the combination of alleles encoded by a gene in the Klra (also called Ly49) cluster on chromosome 6, and one in the major histocompatibility complex (H2) on chromosome 17, is associated with virus resistance. We found that natural killer cell-activating receptor Ly49P specifically recognized MCMV-infected cells, dependent on the presence of the H2 (k) haplotype. This binding was blocked using antibodies to H-2D(k) but not antibodies to H-2K(k). These results are suggestive of a new natural killer cell mechanism implicated in MCMV resistance, which depends on the functional interaction of the Ly49P receptor and the major histocompatibility complex class I molecule H-2D(k) on MCMV-infected cells.

Expression of Killer Immunoglobulin-like Receptors on Peripheral Blood NK Cell Subsets of Women with Recurrent Spontaneous Abortions or Implantation Failures

PROBLEM

Decidual natural killer (NK) cells express inhibitory receptors (killer immunoglobulin-like receptors, KIRs), which bind to ligands on trophoblast cells (human leucocyte antigen, HLA-C). This interaction appears to block NK cytotoxicity against trophoblast cells. In this study, we investigated the expression of inhibitory and activating receptors in peripheral blood NK cells of women with recurrent spontaneous abortion (RSA) or implantation failures.

METHOD OF STUDY

CD56(dim)/CD16(+), CD56(bright)/CD16(-) NK cells and CD56(+)/CD3(+) NKT cells of women with RSA or in vitro fertilization (IVF) failures and normal controls were analyzed for the expression of CD158a, CD158b inhibitory KIRs or CD161-activating receptors, by flow cytometric analysis.

RESULTS

CD158a and CD158b inhibitory receptor expression by CD56(dim)/CD16(+) and CD56(bright)/CD16(-) NK cells were significantly decreased, and CD161-activating receptor expression by CD56(+)/CD3(+) NKT cells was significantly increased in women with implantation failures when compared with normal controls.

CONCLUSIONS

An imbalance between inhibitory and activating receptor expression was found in NK cells of women with implantation failures. This imbalance may explain the adverse reproductive outcome.

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

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

Killer immunoglobulin-like receptors

Killer Ig-like receptors (KIRs) are surface inhibitory receptors specific for allelic forms of human leukocyte antigen (HLA) class I molecules, which are expressed by natural killer (NK) cells and a subset of T lymphocytes. Upon engagement with HLA class I molecules, KIRs block NK cell activation and function. Cells lacking HLA class I molecules are promptly killed by NK cells because of the predominant effect of several activating NK receptors. The NK-mediated killing of these cells might represent an important defence mechanism, antagonizing spreading of pathogens and tumours. Evidence has been accumulated that KIR-encoding genes have evolved and diversified rapidly in primates and in humans. Similar to HLA loci, KIR sequences are highly polymorphic and, moreover, KIR haplotypes greatly vary in the number of the type of genes they contain. KIR gene expression is regulated by mechanisms of DNA methylation. As recently shown, the HLA class I regulated control of NK cell function can be exploited in an allogeneic bone marrow transplantation setting to eradicate acute myeloid leukaemias.

Thomsen-Friedenreich (T) antigen expression increases sensitivity of natural killer cell lysis of cancer cells

In this study, we demonstrate a correlation between T antigen expression on a panel of human carcinoma cell lines and their sensitivity to porcine NK cell lysis. Specifically, the more T antigen is expressed, the more sensitive the cancer cells are to porcine NK cell lysis. Furthermore, this correlation also exists for these cells and their ability to induce tumors in vivo. In this porcine animal model, the less T antigen is expressed, the more prolific the tumor growth in vivo and vice versa. Using the human colorectal adenocarcinoma cell line SW-48, we used limiting dilution to clone 2 populations of cells, one expressing high and the other low levels of T antigen, clones 143 and 111, respectively. In these cloned cells, the clone that expressed more T antigen was more NK-sensitive in vitro and weakly induced tumor growth in vivo. Inversely, the clone that expressed less T antigen clone was more NK-resistant in vitro and grew more prolific tumors in vivo. Using soluble T antigen in a competitive inhibition assay, there was a decrease in porcine NK cell killing of the T antigen+ human cell line Colo 320HSR. Taken together, these findings suggest a novel role for T antigen in the NK cell recognition of cancer cells, specifically as markers for NK sensitivity in carcinoma cell lines. The significance of T antigens as targets for NK cell-mediated lysis is novel and identifies NK cell-T antigen interactions as potentially significant in the immunotherapy of cancer and its associated metastases.

Clustering of C-Type Lectin Natural Killer Receptor-Like Loci in the Bony Fish Oreochromis niloticus

The genome of the cichlid (teleost) fish Oreochromis niloticus contains a set of genes which encode group V C-type lectin proteins homologous to the mammalian NKG2/CD94 family of natural killer (NK) cell receptors. To determine the genomic organization of these killer cell-like receptor (KLR) genes, an O. niloticus BAC library was screened with a cDNA probe derived previously from an expressed sequence tag of the related cichlid species Paralabidochromis chilotes. Four distinct KLR-bearing BAC clones were analysed, three of which could be assembled into a contig. One of the clones was sequenced in its entirety, whereas the others were partially sequenced to identify the KLR loci borne by them. Altogether, 28 distinct KLR loci were identified, of which at least 26 occupy a single chromosomal region, the KLR complex. One half of the loci appear to be occupied by pseudogenes. Compared to the human NK cell receptor complex, the Oreochromis KLR complex is more compact and, apart from transposons, appears to contain only KLR loci. The gene density of the complex is one KLR locus per 18 kb of sequence. All the KLR loci constituting the complex are derived from a single most recent common ancestor, which is estimated to have existed 7.7 million years ago. The 180 kb of the determined sequence is a mosaic of blocks of similar segments reflecting a complex history of duplications, deletions and rearrangements. The transposons found in the sequenced part belong to the TC1, Xena, CR1 and TX1 families.

Isolation of a human homolog of osteoclast inhibitory lectin that inhibits the formation and function of osteoclasts

Osteoclast inhibitory lectin (OCIL) is a newly recognized inhibitor of osteoclast formation. We identified a human homolog of OCIL and its gene, determined its regulation in human osteoblast cell lines, and established that it can inhibit murine and human osteoclast formation and resorption. OCIL shows promise as a new antiresorptive.

INTRODUCTION

Murine and rat osteoclast inhibitory lectins (mOCIL and rOCIL, respectively) are type II membrane C-type lectins expressed by osteoblasts and other extraskeletal tissues, with the extracellular domain of each, expressed as a recombinant protein, able to inhibit in vitro osteoclast formation.

MATERIALS AND METHODS

We isolated the human homolog of OCIL (hOCIL) from a human fetal cDNA library that predicts a 191 amino acid type II membrane protein, with the 112 amino acid C-type lectin region in the extracellular domain having 53% identity with the C-type lectin sequences of rOCIL and mOCIL. The extracellular domain of hOCIL was expressed as a soluble recombinant protein in E. coli, and its biological effects were determined.

RESULTS AND CONCLUSIONS

The hOCIL gene is 25 kb in length, comprised of five exons, and is a member of a superfamily of natural killer (NK) cell receptors encoded by the NK gene complex located on chromosome 12. Human OCIL mRNA expression is upregulated by interleukin (IL)-1alpha and prostaglandin E2 (PGE2) in a time-dependent manner in human osteogenic sarcoma MG63 cells, but not by dexamethasone or 1,25 dihydroxyvitamin D3. Soluble recombinant hOCIL had biological effects comparable with recombinant mOCIL on human and murine osteoclastogenesis. In addition to its capacity to limit osteoclast formation, OCIL was also able to inhibit bone resorption by mature, giant-cell tumor-derived osteoclasts. Thus, a human homolog of OCIL exists that is highly conserved with mOCIL in its primary amino acid sequence (C-lectin domain), genomic structure, and activity to inhibit osteoclastogenesis.

Identification of upstream cis-acting regulatory elements controlling lineage-specific expression of the mouse NK cell activation receptor, NKR-P1C

Mouse NKR-P1C (NK1.1) is a homodimeric type II transmembrane protein expressed on natural killer (NK) cells, NKT cells, and on CD117+ progenitor thymocytes capable of giving rise to cells of the T and NK lineages. Although its physiological ligands remain unknown, NKR-P1C engagement with a monoclonal antibody (mAb) leads to interferon-gamma (IFN-gamma) production and the directed release of cytotoxic granules from NK cells. We have cloned and sequenced a approximately 10-kb genomic fragment corresponding to the 5'-flanking region of the C57Bl/6 mouse NKR-P1C gene. A transcriptional initiation site has been mapped in NK cells and an NK1.1+ T cell line by primer extension and rapid amplification of 5'-cDNA ends (5'-RACE) techniques. Although the 5'-flanking region of NKR-P1C is TATA-less, we have identified an initiator region and a downstream promoter element, which together constitute the principal minimal functional promoter. Computational analysis of the 10-kb 5'-flanking region revealed potential regulatory factor binding sites. DNaseI hypersensitivity assays identified a single hypersensitive site (HS) about a 9-kb upstream of the transcriptional initiation site. This site, termed HS1, was able to act as a transcriptional enhancer element in an NK cell line, while minimally affecting transcription in non-NK cell lines. Moreover, the HS1 element was shown to function as a promoter, with a transcript detected only in fetal NK1.1+ cells. An additional promoter and two non-coding exons were also characterized. These results identify the minimal upstream cis-acting elements, and point to a complex regulatory mechanism involved in the lineage-specific control of NKR-P1C expression in NK lymphocytes.

Regulation of human short-term repopulating cell (STRC) engraftment in NOD/SCID mice by host CD122+ cells

OBJECTIVE

NOD/SCID and NOD/SCID B2m(null) mice are used for the in vivo study of human hematopoietic stem cells (HSC). A previously unrecognized HSC in cord blood, termed short-term repopulating cell (STRC), has been identified using NOD/SCID B2m(null) mice. However, only low levels of STRC engraft in NOD/SCID mice, presumably due to their higher levels of NK cell activity. The objective of these studies was to deplete NK cells both by genetic manipulation of the hosts and by antibody depletion of cell populations that may regulate engraftment with human STRC.

METHODS

C57BL/6-SCID mice and immunodeficient NOD mice genetically deleted in NK cell activity were injected intravenously with human cord blood cells to quantify STRC engraftment. Cohorts of these mice were also treated with anti-NK1.1 or anti-CD122 (IL-2r beta-chain) antibodies.

RESULTS

Human STRC fail to engraft in C57BL/6-SCID mice treated with anti-NK1.1 or with anti-CD122 antibody that targets mouse NK and myeloid cells. NOD/SCID mice, NOD-Rag1(null) mice, and NOD-Rag1(null)Pfp(null) mice that are genetically deleted in NK cell cytotoxic activity support only low levels of STRC engraftment. In contrast, STRC engraft at high levels in all three strains of immunodeficient NOD mice treated with anti-CD122 antibody.

CONCLUSION

Injection of anti-CD122 antibody leads to high levels of STRC engraftment in immunodeficient NOD mice, but not in C57BL/6-SCID mice. These data document that depletion of NK cells is required, and that additional murine host innate immune factors, presumably myeloid cells, are important in regulating human STRC engraftment.

Mechanism of activation of human peripheral blood NK cells at the single cell level by Echinacea water soluble extracts: recruitment of lymphocyte-target conjugates and killer cells and activation of programming for lysis

Echinacea purpurea, a plant originally used by native Americans to treat respiratory infections, has also been shown to exert immunomodulatory activities both in vivo and in vitro. However, the mechanism underlying Echinacea-induced immunomodulation remains largely unknown. This study examined in vitro the effects of soluble extracts of E. purpurea on natural killer (NK) cells present in human peripheral blood mononuclear cells (PBMC). Flow cytometric methods were used to examine activation, cytotoxicity, NK-target binding, and killer cell frequency. Treatment of PBMC with Echinacea overnight resulted in the activation of CD69 expression and increase in mean fluorescence intensity in both the CD16+ and CD16+CD56+ NK subsets. However, the frequency of CD16+ cells was decreased as well as the mean fluorescence intensity was down-regulated. NK cytotoxicity was augmented 100% at the concentration of 0.1 microg/ml of Echinacea in a short time (4-h) assay. Examination at the single cell level revealed augmentation of the frequency of CD56+ NK-target conjugates and a plateau was reached after 30-60 min of incubation. Likewise, the frequency of CD56+ killer cells in the conjugates was also significantly increased by Echinacea. There was recruitment of non-conjugated CD56+ cells into CD16+ NK-target conjugates and activation of the NK-target non-killer conjugates into killer cells. These findings demonstrate that Echinacea extracts are potent activators of NK cytotoxicity. Echinacea augments the frequency of NK target conjugates and activates the programming for lysis of NK cells.

Ly49 genes in non-rodent mammals

The Ly49 family of natural killer (NK) receptors is encoded by a highly polymorphic multigene family in the mouse and is also present in multiple copies in the rat. However, this gene exists as a single copy in primates and is mutated to non-function in humans. We recently showed that the cow also likely has only one Ly49 gene, but it is unclear what the Ly49 gene content is for other mammals. We have now isolated Ly49 cDNAs from the domestic cat, dog and pig and show that the corresponding gene appears to be single copy in these three species. The open reading frame is intact in all the genes and the putative proteins contain an immune tyrosine-based inhibition motif (ITIM), suggesting a role as an inhibitory receptor. In contrast to the other mammals, several Ly49-like genes appear to exist in the horse, indicating that amplification of this locus has occurred in a non-rodent lineage. Finally, phylogenetic analysis suggests that the rodent Ly49 genes have evolved more rapidly than their counterparts in mammals where the gene has remained as a single copy.

Identification of a new murine lectin-like gene in close proximity to CD94

The cytolytic activities of natural killer (NK) cells are controlled by immune receptors expressed on the NK cell surface that interact with MHC I molecules on the target cell. In the murine system, the receptors that perform this function are members of the c-type lectin superfamily and are found in a 4 Mbp cluster termed the natural killer gene complex (NKC) on chromosome 6. To date, over 30 receptor genes, which largely exhibit lymphoid-restricted expression, have been cloned from this region. With the completion of genomic sequencing in this region, computer prediction of genes based in part on expressed sequence tag (EST) evidence has allowed the identification of potential new genes in the cluster. In this study, we investigated one of these predicted genes and showed that it is a bona fide gene, which is transcribed in both the mouse and rat but is not present in humans. A phylogenetic comparison also showed that the novel gene, Klre-1, is most closely related to CD94. Strikingly, the Klre-1 gene is located within very close proximity (2.3 kb) to the CD94 gene. Consequently, we examined the expression patterns of Klre-1 by quantitative real-time polymerase chain reaction and found a correlation between expression of CD94 and Klre-1 in primary cells as well as cell lines. This discovery presents further possibilities for characterising transcriptional control of genes in the NKC.

Transgenic expression of the activating natural killer receptor Ly49H confers resistance to cytomegalovirus in genetically susceptible mice

Natural resistance to infection with mouse cytomegalovirus (MCMV) is controlled by a dominant locus, Cmv1. Cmv1 is linked to the Ly49 family of natural killer receptors on distal chromosome 6. While some studies localized Cmv1 as distal to the Ly49 gene cluster, genetic and functional analysis identified Ly49h as a pivotal factor in resistance to MCMV. The role of these two independent genomic domains in MCMV resistance was evaluated by functional complementation using transgenesis of bacterial artificial chromosomes (BAC) in genetically susceptible mice. Phenotypic and genetic characterization of the transgenic animals traced the resistance gene to a single region spanning the Ly49h gene. The appearance of the Ly49H protein in NK cells of transgenic mice coincided with the emergence of MCMV resistance, and there was a threshold Ly49H protein level associated with full recovery. Finally, transgenic expression of Ly49H in the context of either of the two independent susceptibility alleles, Cmv1(sBALB) or Cmv1(sFVB), conferred resistance to MCMV infection. These results demonstrate that Ly49h is necessary and sufficient to confer MCMV resistance, and formally demonstrate allelism between Cmv1 and Ly49h. This panel of transgenic animals provides a unique resource to study possible pleiotropic effect of Cmv1.

Natural killer cell activation receptors in innate immunity to infection

Natural killer (NK) cells are best known for their capacity to kill tumors but they are also critical in early innate responses to infection, especially herpesviruses. Recent studies indicate that NK cell receptors involved in tumor target specificity are also involved in responses to viral infections.

Fc receptor homologs: newest members of a remarkably diverse Fc receptor gene family

Newfound relatives of the classical Fc receptors (FcR) have been provisionally named the Fc receptor homologs (FcRH). The recent identification of eight human and six mouse FcRH genes substantially increases the size and functional potential of the FcR family. The extended family of FcR and FcRH genes spans approximately 15 Mb of the human chromosome 1q21-23 region, whereas in mice this family is split between chromosomes 1 and 3. The FcRH genes encode molecules with variable combinations of five subtypes of immunoglobulin (Ig) domains. The presence of a conserved sequence motif in one Ig domain subtype implies Ig Fc binding capability for many FcRH family members that are preferentially expressed by B lineage cells. In addition, most FcRH family members have consensus tyrosine-based activating and inhibitory motifs in their cytoplasmic domains, while the others lack features typical of transmembrane receptors. The FcRH family members, like the classical FcRs, come in multiple isoforms and allelic variations. The unique individual and polymorphic properties of the FcR/FcRH members indicate a remarkably diverse Fc receptor gene family with immunoregulatory function.

Innate immunity to cytomegalovirus: the Cmv1 locus and its role in natural killer cell function

The identification and characterization of genetic loci that contribute to patterns of susceptibility/resistance to infection provide important insights into the mechanisms of innate and adaptive immunity. Genetic heterogeneity across the population makes the characterization of such traits in humans technically difficult; however, inbred animal models represent an ideal tool for such analyses. This review illustrates the power of mouse genetics as utilized for the identification and characterization of the locus conferring early resistance to murine cytomegalovirus infection, Cmv1. This locus encodes an activating C-type lectin receptor of the Ly49 family that promotes natural killer (NK) cell cytolysis of infected cells. Although NK cells are usually able to detect and destroy virally infected cells via recognition of the downregulation of MHC class I molecules, the Cmv1 locus provides the first example of an NK receptor that is able to mediate clearance of viral infection via direct recognition of a virally encoded protein.

Src-dependent Syk activation controls CD69-mediated signaling and function on human NK cells

CD69 C-type lectin receptor represents a functional triggering molecule on activated NK cells, capable of directing their natural killing function. The receptor-proximal signaling pathways activated by CD69 cross-linking and involved in CD69-mediated cytotoxic activity are still poorly understood. Here we show that CD69 engagement leads to the rapid and selective activation of the tyrosine kinase Syk, but not of the closely related member of the same family, ZAP70, in IL-2-activated human NK cells. Our results indicate the requirement for Src family kinases in the CD69-triggered activation of Syk and suggest a role for Lck in this event. We also demonstrate that Syk and Src family tyrosine kinases control the CD69-triggered tyrosine phosphorylation and activation of phospholipase Cgamma2 and the Rho family-specific exchange factor Vav1 and are responsible for CD69-triggered cytotoxicity of activated NK cells. The same CD69-activated signaling pathways are also observed in an RBL transfectant clone, constitutively expressing the receptor. These data demonstrate for the first time that the CD69 receptor functionally couples to the activation of Src family tyrosine kinases, which, by inducing Syk activation, initiate downstream signaling pathways and regulate CD69-triggered functions on human NK cells.

MCMV glycoprotein gp40 confers virus resistance to CD8+ T cells and NK cells in vivo

The susceptibility of certain inbred mouse strains to murine cytomegalovirus (MCMV) is related to their inability to generate a strong natural killer (NK) cell response. We addressed here whether the MCMV susceptibility of the BALB/c strain is due to viral functions that control NK cell activation in a strain-specific manner. MCMV expresses two proteins, gp48 and gp40, that are encoded by the genes m06 and m152, respectively; they down-regulate major histocompatibility complex (MHC) class I expression at the plasma membrane. Using MCMV deletion mutants and revertants, we found that gp40 but not gp48 controls NK cell activation. Absence of gp40 improved antiviral NK cell control in BALB/c, but not C57BL/6, mice. Down-regulation of H-60, the high-affinity ligand for the NKG2D receptor, was the mechanism by which gp40 modulates NK cell activation. Thus, a single herpesvirus protein has a dual function in inhibiting both the adaptive as well as the innate immune response.

The role of an exposed loop in the alpha2 domain in the mouse MHC class I H-2Dd molecule for recognition by the monoclonal antibody 34-5-8S and the NK-cell receptor Ly49A

Natural killer (NK) cells express major histocompatibility complex (MHC) class I-specific inhibitory receptors. The region mediating the protective effect of the MHC class I molecule H-2Dd (Dd), recognized by the inhibitory receptor Ly49A, has been mapped to the alpha1/alpha2 domains. Here we have focused on an exposed loop in the N-terminal part of the alpha2 domain, which constitutes a major structural motif that differs between Dd (strong binding to Ly49A) and Db (weak binding to Ly49A at best). We mutated the residues 103, 104 and 107 in Dd to the corresponding amino acids in Db. The Dd mutant molecule retained the ability to be stabilized by a Dd-binding peptide. However, the mutation totally abolished the recognition by the conformational dependent monoclonal antibody (MoAb) 34-5-8S, known to inhibit the interaction between Dd and Ly49A. In addition, there was a marked impairment of the binding to Ly49A as evaluated by the ability of tetramers of the Dd mutant molecule to bind to Ly49A-transfected reporter cells and spleen cells. These results demonstrate that the introduced changes at positions 103, 104 and 107 directly or indirectly affect the epitopes for the MoAb 34-5-8S and the Ly49A receptor.

Autoreactivity by design: innate B and T lymphocytes

Innate B and T lymphocytes are a subset of lymphocytes that express a restricted set of semi-invariant, germ-line-encoded, autoreactive antigen receptors. Although they have long been set apart from mainstream immunological thought, they now seem to represent a distinct immune-recognition strategy that targets conserved stress-induced self-structures, rather than variable foreign antigens. Innate lymphocytes regulate a range of infectious, tumour and autoimmune conditions. New studies have shed light on the principles and mechanisms that drive their unique development and function, and show their resemblance to another subset of innate lymphocytes, the natural killer cells.

Differentially induced expression of C-type lectins in activated lymphocytes

The human NK gene complex encodes for the leucocyte C-type lectins, CD69, AICL (activation-induced C-type lectin), LLT1 (lectin-like transcript), CD161/NKR-P1A, CD94, and for NKG-2 molecules. These gene products have been implicated in the regulation of the function of natural killer (NK) cells and other lymphocytes. In this study the expression of C-type lectins during the early activation of PMA-stimulated peripheral blood lymphocytes was examined. To investigate the influence of de novo protein synthesis on activation-dependent expression of C-type lectins, cells were cultured in presence of cycloheximide (CHX) and mRNA levels were analyzed by semi-quantitative reverse transcription-polymerase chain reaction. Upregulated levels of CD69, AICL, and LLT1, but less pronounced changes of CD161/NKR-P1A and CD94 mRNA were found at early time points of cellular activation. CD69 was superinduced by CHX at the nuclear precursor transcript and the mRNA level suggesting that regulation of transcriptional activity and mRNA stability contribute to extent of CD69 mRNA accumulation. CHX treatment resulted also in an overexpression of AICL, LLT1, and CD161/NKR-P1A mRNAs. Conversely, CHX blocked CD94 mRNA expression in PMA-stimulated cells, demonstrating that this process is dependent on new protein synthesis. Expression kinetics in context with susceptibility to CHX indicate that the mechanisms responsible for upregulated CD69, AICL, and LLT1 expression are distinct from those which control CD161/NKR-P1A or CD94 expression. J. Cell. Biochem. Suppl. 36: 201-208, 2001.

Molecular and genomic characterization of human DLEC, a novel member of the C-type lectin receptor gene family preferentially expressed on monocyte-derived dendritic cells

We have identified a novel gene encoding a protein designated DLEC (dendritic cell lectin), which is a type II membrane glycoprotein of 213 amino acids and belongs to the human calcium-dependent (C-type) lectin family. The cytoplasmic tail of DLEC lacks consensus signaling motifs and its extracellular region shows a single carbohydrate recognition domain (CRD), closest in homology to the dendritic cell immunoreceptor (DCIR) CRD. The DLEC gene has been localized linked to DCIR on the telomeric region of the NK gene complex. RT-PCR and Northern blot analyses show that DLEC mRNA is preferentially expressed in monocyte-derived dendritic cells.

Identification of a human homologue of the dendritic cell-associated C-type lectin-1, dectin-1

Previously we identified the novel type II lectin receptor, dectin-1, that is expressed preferentially by murine antigen presenting dendritic cells (DC) and is involved in co-stimulation of T cells by DC. To identify the human homologue (DECTIN-1), we employed degenerative PCR amplification of mRNA isolated from DC and subsequent cDNA cloning. DECTIN-1 is a type II lectin receptor with high homology to type II lectin receptors expressed by natural killer (NK) cells. It contains an immunoreceptor tyrosine-based activation motif within the cytoplasmic domain. Human DECTIN-1 mRNA is expressed predominantly by peripheral blood leukocytes and preferentially by DC. The mRNA likely encodes a 33 kDa glycoprotein. In human epidermis, the protein is expressed selectively by Langerhans cells, which are an epidermal subset of DC. A truncated form of DECTIN-1 RNA (termed T beta) encodes for a polypeptide lacking almost the entire neck domain, which is required for accessibility of the carbohydrate recognition domain to ligands. Genome analysis showed the deleted amino acid sequence in T beta to be encoded by an exon, indicating that T beta RNA is produced by alternative splicing. DECTIN-1 gene maps to chromosome 12, between p13.2 and p12.3, close to the NK gene complex (12p13.1 to p13.2) which contains genes for NK lectin receptors. Our results indicate that human DECTIN-1 shares many features with mouse dectin-1, including the generation of neck domain-lacking isoforms, which may down-regulate the co-stimulatory function of dectin-1.

Identification of committed NK cell progenitors in adult murine bone marrow

Natural killer (NK) cells play important roles in innate immunity by lysing tumor and virally infected cells and by producing cytokines including interferon-gamma. While NK cell progenitors have been described in the fetal thymus, NK cell generation from hematopoietic stem cells (HSC) in the bone marrow (BM) occurs throughout life, and in athymic mice and humans. Interleukin (IL)-15 promotes NK development in vitro and is essential for the generation of normal numbers of NK cells in vivo. By characterizing BM cells expressing IL-15 receptor components, we found marked heterogeneity within the IL-2 receptor beta chain(+) (CD122(+)) subset, which included cells uniquely committed to the NK lineage. These CD122(+) NK cell precursors (NKP) are negative for markers used to identify mature NK cells, including NK1.1, DX5 and members of Ly-49 family, and fail to demonstrate natural cytotoxicity against susceptible target cells. In vitro culture of NKP generates mature lytic NK1.1(+) cells at high frequencies, while they do not give rise to T, B, myeloid or erythroid cells under appropriate conditions. NKP lack transcripts associated with early B and T cell differentiation (pTalpha, lambda5 and CD3epsilon), but express a group of genes (IL-15Ralpha, Id2, GATA-3 and Ets-1) and the 2B4 marker, which may define NK cell commitment. We propose that NKP represent the earliest adult BM precursor uniquely restricted to the NK cell lineage.

Vital involvement of a natural killer cell activation receptor in resistance to viral infection

Natural killer (NK) cells are lymphocytes that can be distinguished from T and B cells through their involvement in innate immunity and their lack of rearranged antigen receptors. Although NK cells and their receptors were initially characterized in terms of tumor killing in vitro, we have determined that the NK cell activation receptor, Ly-49H, is critically involved in resistance to murine cytomegalovirus in vivo. Ly-49H requires an immunoreceptor tyrosine-based activation motif (ITAM)-containing transmembrane molecule for expression and signal transduction. Thus, NK cells use receptors functionally resembling ITAM-coupled T and B cell antigen receptors to provide vital innate host defense.

Differential requirement for the transcription factor PU.1 in the generation of natural killer cells versus B and T cells

PU.1 is a member of the Ets family of transcription factors required for the development of various lymphoid and myeloid cell lineages, but its role in natural killer (NK) cell development is not known. The study shows that PU.1 is expressed in NK cells and that, on cell transfer into alymphoid Rag2/gammac(-/-) mice, hematopoietic progenitors of PU.1(-/-) fetal liver cells could generate functional NK cells but not B or T cells. Nevertheless, the numbers of bone marrow NK cell precursors and splenic mature NK cells were reduced compared to controls. Moreover, PU.1(-/-) NK cells displayed reduced expression of the receptors for stem cell factor and interleukin (IL)-7, suggesting a nonredundant role for PU.1 in regulating the expression of these cytokine receptor genes during NK cell development. PU.1(-/-) NK cells also showed defective expression of inhibitory and activating members of the Ly49 family and failed to proliferate in response to IL-2 and IL-12. Thus, despite the less stringent requirement for PU.1 in NK cell development compared to B and T cells, PU.1 regulates NK cell differentiation and homeostasis.

Susceptibility to mouse cytomegalovirus is associated with deletion of an activating natural killer cell receptor of the C-type lectin superfamily

Cytomegalovirus is the leading cause of congenital viral disease and the most important opportunistic infection in immunocompromised patients. We have used a mouse experimental infection model (MCMV) to study the genetic parameters of host/virus interaction. Susceptibility to infection with MCMV is controlled by Cmv1, a chromosome 6 locus that regulates natural killer (NK) cell activity against virally infected targets. Here, we use a positional cloning strategy to isolate the gene mutated at the Cmv1 locus. Cmv1 maps within a 0.35-cM interval defined by markers D6Ott8 and D6Ott115, which corresponds to a physical distance of 1.6 Mb (refs. 6-8). A transcript map of the region identified 19 genes, including members of the killer cell lectin-like receptor family a (Klra, formerly Ly49; refs. 9-12), which encode inhibitory or activating NK cell receptors that interact with MHC class I molecules. Klra genes have different copy numbers and genomic organization, and are highly polymorphic among inbred strains, making it difficult to distinguish between normal allelic variants and distinct Klra genes, or possible mutations associated with Cmv1. The recombinant inbred strain BXD-8/Ty (BXD-8; ref. 18), derived from Cmv1r C57BL/6 (B6, resistant) and Cmv1s DBA/2 (susceptible), is of particular interest because it is highly susceptible to MCMV infection despite having a B6 haplotype at Cmv1. We determined that MCMV susceptibility in BXD-8 is associated with the deletion of Klra8 (formerly Ly49h).

Crystal structure of the C-type lectin-like domain from the human hematopoietic cell receptor CD69

CD69, one of the earliest specific antigens acquired during lymphoid activation, acts as a signal-transducing receptor involved in cellular activation events, including proliferation and induction of specific genes. CD69 belongs to a family of receptors that modulate the immune response and whose genes are clustered in the natural killer (NK) gene complex. The extracellular portion of these receptors represent a subfamily of C-type lectin-like domains (CTLDs), which are divergent from true C-type lectins and are referred to as NK-cell domains (NKDs). We have determined the three-dimensional structure of human CD69 NKD in two different crystal forms. CD69 NKD adopts the canonical CTLD fold but lacks the features involved in Ca(2+) and carbohydrate binding by C-type lectins. CD69 NKD dimerizes noncovalently, both in solution and in crystalline state. The dimer interface consists of a hydrophobic, loosely packed core, surrounded by polar interactions, including an interdomain beta sheet. The intersubunit core shows certain structural plasticity that may facilitate conformational rearrangements for binding to ligands. The surface equivalent to the binding site of other members of the CTLD superfamily reveals a hydrophobic patch surrounded by conserved charged residues that probably constitutes the CD69 ligand-binding site.