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

Different NK cell surface phenotypes defined by the DX9 antibody are due to KIR3DL1 gene polymorphism

KIR3DL1 and KIR3DL2 are NK cell receptors for polymorphic HLA-B and -A determinants. The proportion of NK cells that bind anti-KIR3DL1-specific Ab DX9 and their level of binding vary between individuals. To determine whether these differences are due to KIR polymorphism, we assessed KIR3D gene diversity in unrelated individuals and families. Both KIR3DL1 and KIR3DL2 are highly polymorphic genes, with KIR3DS1 segregating like an allele of KIR3DL1. A KIR haplotype lacking KIR3DL1 and KIR3DS1 was defined. The two KIR3DL1 alleles of a heterozygous donor were expressed by different, but overlapping, subsets of NK cell clones. Sequence variation in KIR3DL1 and KIR3DL2 appear distinct; recombination is more evident in KIR3DL1, and point mutation is more evident in KIR3DL2. The KIR3DL1 genotype correlates well with levels of DX9 binding by NK cells, but not with the frequency of DX9-binding cells. Different KIR3DL1 alleles determine high, low, and no binding of DX9 Ab. Consequently, heterozygotes for high and low binding KIR3DL1 alleles have distinct subpopulations of NK cells that bind DX9 at high and low levels, giving characteristic bimodal distributions in flow cytometry. The Z27 Ab gave binding patterns similar to those of DX9. Four KIR3DL1 alleles producing high DX9 binding phenotypes were distinguished from four alleles producing low or no binding phenotypes by substitution at one or more of four positions in the encoded protein: 182 and 283 in the extracellular Ig-like domains, 320 in the transmembrane region, and 373 in the cytoplasmic tail.

Cloning and characterization of a novel ITIM containing lectin-like immunoreceptor LLIR and its two transmembrane region deletion variants

A novel full-length cDNA was cloned from human dendritic cells (DC) by subtractive cloning and RACE. The deduced protein is a type II lectin-like membrane protein that contains an ITIM proximal to N terminal and is designated as lectin-like immunoreceptor (LLIR). The gene of LLIR is located in a region of chromosomal 12p13 and shows highest homologous with ASGPR. Two alternatively spliced transmembraneless variants of LLIR were identified by RT-PCR and named as LLIRv1 and LLIRv2. RT-PCR and immunoblotting analysis revealed that LLIR was expressed with much higher level in immature DC than in mature DC. The ITIM in LLIR was demonstrated to bind SHP-1 in HL-60 cell after the tyrosine had been phosphorylated. In addition, the mRNA expression level of LLIRv2 was raised when leukemia cells were induced to differentiate by PMA.

Arrangement of the ILT gene cluster: a common null allele of the ILT6 gene results from a 6.7-kbp deletion

The leukocyte receptor cluster (LRC) is a highly polymorphic region of human chromosome 19q13.4 that encompasses at least 24 members of the immunoglobulin superfamily (Ig-SF). The centromeric end of the LRC contains eight Ig-SF loci, namely LAIR1 and seven ILT genes. All ILT genes conform to prototypic ILT gene structures. ILT6 is the only member of the ILT family that lacks a transmembrane and cytoplasmic domain. Close examination of the ILT6 genomic sequence reveals high similarity of this locus with the organization of activating ILT genes. However, the ILT6 transcript runs through the putative splice site of exon 8 that encodes for an extracellular stalk region, leading to a premature in-frame stop codon. Downstream of exon 8 are three pseudo exons that are not included in any of the known ILT6 transcripts, but share high homology to the equivalent region in activating ILT loci, suggesting that these genes have evolved from a common ancestral sequence. Comparison of two haplotypes over this region revealed a remarkable polymorphism with respect to the ILT6 gene which lacks exons 1-7 in one allele, reminiscent of the presence/absence variation displayed by the closely related and genetically linked KIR loci. Detailed sequence analysis of the two LAIR/ILT clusters suggests that the two complexes may have evolved from an inverted duplication.

Identification and characterization of the gene for a novel C-type lectin (CLECSF7) that maps near the natural killer gene complex on human chromosome 12

We report the identification and characterization of a novel C-type lectin gene, named HECL (HGMW-approved symbol CLECSF7), that maps close to the natural killer gene complex on human chromosome 12p13. Sequence analysis revealed a complete open reading frame of 549 bp comprising several putative glycosylation and phosphorylation sites as well as a C-terminal C-type carbohydrate-recognition domain. Homology analysis revealed that HECL exhibits a significant degree of divergence from the natural killer cell receptors that comprise the natural killer gene complex. These natural killer cell receptors all belong to group V of the C-type lectin superfamily. HECL, however, is most closely related to the sole group II C-type lectins reported to map near this region of the genome, the murine Nkcl and Mpcl genes. Like Nkcl, HECL is expressed in a variety of hematopoietic cell types and has a complete Ca(2+)-binding site 2. Despite the presence of critical amino acids for sugar binding in Ca(2+)-binding site 2, HECL does not seem to bind carbohydrate. Moreover, HECL is the first non-receptor-like C-type lectin to map near the natural killer gene complex.

Participation of innate and acquired immunity in atherosclerosis

Coronary artery disease, the major manifestation of atherosclerosis, is the leading cause of death in the Western world. However, the pathogenesis of atherosclerosis is still poorly understood. Controversy exists regarding the participation of innate immunity involving macrophages and natural killer (NK) cells vs antigen-specific acquired immunity involving lymphocytes. Macrophages predominate in atherosclerotic lesions. NK cells, although smaller in number, are present as well. Furthermore, T lymphocytes that participate in acquired immunity are frequently observed in lesions and can modulate lesion progression. By using mouse models of hyperlipidemia, our laboratory is addressing in vivo the participation of both innate inflammatory responses and acquired immune responses in atherosclerosis.

CD8+ T cells rapidly acquire NK1.1 and NK cell-associated molecules upon stimulation in vitro and in vivo

NKT cells express both NK cell-associated markers and TCR. Classically, these NK1.1+TCRalphabeta+ cells have been described as being either CD4+CD8- or CD4-CD8-. Most NKT cells interact with the nonclassical MHC class I molecule CD1 through a largely invariant Valpha14-Jalpha281 TCR chain in conjunction with either a Vbeta2, -7, or -8 TCR chain. In the present study, we describe the presence of significant numbers of NK1.1+TCRalphabeta+ cells within lymphokine-activated killer cell cultures from wild-type C57BL/6, CD1d1-/-, and Jalpha281-/- mice that lack classical NKT cells. Unlike classical NKT cells, 50-60% of these NK1.1+TCRalphabeta+ cells express CD8 and have a diverse TCR Vbeta repertoire. Purified NK1.1-CD8alpha+ T cells from the spleens of B6 mice, upon stimulation with IL-2, IL-4, or IL-15 in vitro, rapidly acquire surface expression of NK1.1. Many NK1.1+CD8+ T cells had also acquired expression of Ly-49 receptors and other NK cell-associated molecules. The acquisition of NK1.1 expression on CD8+ T cells was a particular property of the IL-2Rbeta+ subpopulation of the CD8+ T cells. Efficient NK1.1 expression on CD8+ T cells required Lck but not Fyn. The induction of NK1.1 on CD8+ T cells was not just an in vitro phenomenon as we observed a 5-fold increase of NK1.1+CD8+ T cells in the lungs of influenza virus-infected mice. These data suggest that CD8+ T cells can acquire NK1.1 and other NK cell-associated molecules upon appropriate stimulation in vitro and in vivo.

Stress renders T cell blasts sensitive to killing by activated syngeneic NK cells

Exposure of primary T cell blasts to stress in the forms of heat, hydrogen peroxide, or high-density growth conditions resulted in a state of enhanced susceptibility to killing by syngeneic IL-2-activated NK cells or lymphokine-activated killer cells, but not to killing by CTL. Cytotoxicity was perforin mediated and was not due to decreased target expression of total MHC class I. The levels of stress used had little effect on cell viability. For thermal stress, sensitization increased with temperature, required a minimum exposure time, and disappeared when cells were given a long enough recovery time. Our data support a model that predicts that activated NK cells play a role in the immunosurveillance of nontransformed stressed cells in normal animals.

Sequence-ready BAC contig, physical, and transcriptional map of a 2-Mb region overlapping the mouse chromosome 6 host-resistance locus Cmv1

The host-resistance locus Cmv1 controls viral replication of mouse cytomegalovirus (MCMV) in the spleen of infected mice. Cmv1 maps on distal chromosome 6, very tightly linked to the Ly49 gene family within a 0.35-cM interval defined proximally by Cd94/Nkg2d and distally by D6Mit13/D6Mit111/D6Mit219/Prp/Kap. To facilitate the cloning of the gene, we have created a high-resolution physical map of the Cmv1 genetic interval that is based on long-range restriction mapping by pulsed-field gel electrophoresis, fluorescence in situ hybridization analysis of interphase nuclei, and the assembly of a cloned contig. A contig of BAC and YAC clones was assembled using probes derived from the minimal genetic interval. Individual clones from the region were validated by (1) restriction digest fingerprinting, (2) STS content mapping, (3) Southern hybridizations, and (4) sequencing and mapping of clone ends. This contig contains 25 YACs anchored by 71 STSs and 73 BACs anchored by 40 STSs. We also report the cloning of 31 new STSs and 18 new polymorphic markers. A minimum tiling path was defined that consists of either 4 YACs or 13 BACs covering 1.82 Mb between D6Ott8, the closest proximal marker, and D6Ott115, the closest distal marker. Gene distribution in the region includes 14 Ly49 genes as well as 3 new additional transcripts. This high-resolution, sequence-ready BAC contig provides a backbone for the identification of Cmv1 and its relationship with genes involved in innate immunity.

The genomic organization of the mouse CD94 C-type lectin gene

The mouse natural killer (NK) gene complex is located on chromosome 6 and contains a number of genes encoding C-type lectin receptors which have been found to regulate NK cell function. Among these are CD94 and four NKG2 genes. Like its human counterpart, the mouse CD94 protein associates with different NKG2 isoforms and recognizes the atypical MHC class I molecule Qa-1b. Here, the genomic organization of the mouse CD94 gene was determined by analysing a BAC clone containing the CD94 gene. The mouse CD94 gene contains six exons separated by five introns. Exons I and II encode the 5' untranslated region (UTR) and the transmembrane domain. Exon III encodes the stalk region and exons IV-VI encode the carbohydrate recognition domain (CRD). Furthermore, we cloned and sequenced the CD94 promoter region, and putative regulatory DNA elements were identified. Further studies on the CD94 promoter region may help to elucidate the restricted expression pattern of CD94 in NK cells and a subpopulation of T cells.

The mouse tumor cell lines EL4 and RMA display mosaic expression of NK-related and certain other surface molecules and appear to have a common origin

As a potential means for facilitating studies of NK cell-related molecules, we examined the expression of these molecules on a range of mouse tumor cell lines. Of the lines we initially examined, only EL4 and RMA expressed such molecules, both lines expressing several members of the Ly49 and NKRP1 families. Unexpectedly, several of the NK-related molecules, together with certain other molecules including CD2, CD3, CD4, CD32, and CD44, were often expressed in a mosaic manner, even on freshly derived clones, indicating frequent switching in expression. In each case examined, switching was controlled at the mRNA level, with expression of CD3zeta determining expression of the entire CD3-TCR complex. Each of the variable molecules was expressed independently, with the exception that CD3 was restricted to cells that also expressed CD2. Treatment with drugs that affect DNA methylation and histone acetylation could augment the expression of at least some of the variable molecules. The striking phenotypic similarity between EL4 and RMA led us to examine the state of their TCRbeta genes. Both lines had identical rearrangements on both chromosomes, indicating that RMA is in fact a subline of EL4. Overall, these findings suggest that EL4 is an NK-T cell tumor that may have retained a genetic mechanism that permits the variable expression of a restricted group of molecules involved in recognition and signaling.

Plasticity in the organization and sequences of human KIR/ILT gene families

The approximately 1-Mb leukocyte receptor complex at 19q13.4 is a key polymorphic immunoregion containing all of the natural killer-receptor KIR and related ILT genes. When the organization of the leukocyte receptor complex was compared from two haplotypes, the gene content in the KIR region varied dramatically, with framework loci flanking regions of widely variable gene content. The ILT genes were more stable in number except for ILT6, which was present only in one haplotype. Analysis of Alu repeats and comparison of KIR gene sequences, which are over 90% identical, are consistent with a recent origin. KIR genesis was followed by extensive duplication/deletion as well as intergenic sequence exchange, reminiscent of MHC class I genes, which provide KIR ligands.

Cloning of a second dendritic cell-associated C-type lectin (dectin-2) and its alternatively spliced isoforms

Using a subtractive cDNA cloning strategy, we isolated previously five novel genes that were expressed abundantly by the murine dendritic cell (DC) line XS52, but not by the J774 macrophage line. One of these genes encoded a unique, DC-associated C-type lectin, termed "dectin-1." Here we report the characterization of a second novel gene that was also expressed in a DC-specific manner. Clone 1B12 encoded a type II membrane-integrated polypeptide of 209 amino acids containing a single carbohydrate recognition domain motif in the COOH terminus. The expression pattern of this molecule, termed "dectin-2," was almost indistinguishable from that for dectin-1; that is, both were expressed abundantly at mRNA and protein levels by the XS52 DC line, but not by non-DC lines, and both were detected in spleen and thymus, as well as in skin resident DC (i.e. Langerhans cells). Interestingly, reverse transcriptase-polymerase chain reaction and immunoblotting revealed multiple bands of dectin-2 transcripts and proteins suggesting molecular heterogeneity. In fact, we isolated additional cDNA clones encoding two distinct, truncated dectin-2 isoforms. Genomic analyses indicated that a full-length dectin-2 (alpha isoform) is encoded by 6 exons, whereas truncated isoforms (beta and gamma) are produced by alternative splicing. We propose that dectin-2 and its isoforms, together with dectin-1, represent a unique subfamily of DC-associated C-type lectins.

A sequence-ready physical map of the region containing the human natural killer gene complex on chromosome 12p12.3-p13.2

We developed a sequence-ready physical map of a part of human chromosome 12p12.3-p13.2 where the natural killer gene complex (NKC) is located. The NKC includes a cluster of genes with structure similar to that of the Ca(2+)-dependent lectin superfamily of glycoproteins that are expressed on the surface of most natural killer (NK) cells and a subset of T cells. These killer cell lectin-like receptors (KLR) are involved in NK target cell recognition, leading to activation or inhibition of NK cell function. We used a number of sequence-tagged site (STS) markers from this region to screen two large insert bacterial artificial chromosome (BAC) libraries and a bacteriophage P1-derived (PAC) chromosome library. The clones were assembled into contiguous sets by STS content analysis. The 72-BAC and 11-PAC contig covers nearly 2 Mb of DNA and provides an average marker resolution of 26 kb. We have precisely localized 17 genes, 5 expressed sequence tags, and 49 STSs within this contig. Of this total number of STS, 30 are newly developed by clone-end sequencing. We established the order of the genes as tel-M6PR-MAFAL (HGMW-approved symbol KLRG1)-A2M-PZP-A2MP-NKRP1A (HGMW-approved symbol KLRB1)-CD69-AICL (HGMW-approved symbol CLECSF2)-KLRF1-OLR1-CD94 (HGMW-approved symbol KLRD1)-NKG2D (HGMW-approved symbol D12S2489E)-PGFL-NKG2F (HGMW-approved symbol KLRC4)-NKG2E (HGMW-approved symbol KLRC3)-NKG2A (HGMW-approved symbol KLRC1)-LY49L (HGMW-approved symbol KLRA1)-cen. This map would facilitate the cloning of new KLR genes and the complete sequencing of this region.

Cytomegalovirus MHC class I homologues and natural killer cells: an overview

Viruses that establish a persistent infection with their host have evolved numerous strategies to evade the immune system. Consequently, they are useful tools to dissect the complex cellular processes that comprise the immune response. Rapid progress has been made in recent years in defining the role of cellular MHC class I molecules in regulating the response of natural killer (NK) cells. Concomitantly, the roles of the MHC class I homologues encoded by human and mouse cytomegaloviruses in evading or subverting NK cell responses has received considerable interest. This review discusses the results from a number of studies that have pursued the biological function of the viral MHC class I homologues. Based on the evidence from these studies, hypotheses for the possible role of these intriguing molecules are presented.

Coevolution of host and virus: the pathogenesis of virulent and attenuated strains of myxoma virus in resistant and susceptible European rabbits

Myxoma virus was introduced into the European rabbit population of Australia in 1950. Although the virus was initially highly lethal in rabbits, there was rapid selection for less virulent strains of virus and innately resistant rabbits. To investigate the basis of resistance to myxoma virus, we have compared the pathogensis of the virulent strain of myxoma virus originally released into Australia and an attenuated, naturally derived field strain of myxoma virus. This was done in laboratory rabbits, which have not been selected for resistance, and in wild rabbits that have developed significant resistance. Wild rabbits were able to recover from infection with virus that was always lethal in laboratory rabbits. Laboratory rabbits were able to control and recover from infection with attenuated virus. This virus caused a trivial disease in wild rabbits. There was little difference between laboratory and wild rabbits in titers of either virulent or attenuated virus in the skin at the inoculation site. However, resistant wild rabbits had a 10- to 100-fold lower titer of virulent virus within the lymph node draining the inoculation site and controlled virus replication in tissues distal to the draining lymph node. Replication of virus in lymphocytes or fibroblasts cultured from wild and laboratory rabbits demonstrated that resistance was not due to altered cellular permissivity for replication. Neutralizing antibodies were present in both susceptible and resistant rabbits, suggesting that these have no significant role in resistance. We hypothesise that resistance is due to an enhanced innate immune response that allows the rabbit to mount an effective cellular immune response.

Treatment of allograft recipients with donor-specific transfusion and anti-CD154 antibody leads to deletion of alloreactive CD8+ T cells and prolonged graft survival in a CTLA4-dependent manner

A two-element protocol consisting of one donor-specific transfusion (DST) plus a brief course of anti-CD154 mAb greatly prolongs the survival of murine islet, skin, and cardiac allografts. To study the mechanism of allograft survival, we determined the fate of tracer populations of alloreactive transgenic CD8+ T cells in a normal microenvironment. We observed that DST plus anti-CD154 mAb prolonged allograft survival and deleted alloreactive transgenic CD8+ T cells. Neither component alone did so. Skin allograft survival was also prolonged in normal recipients treated with anti-CD154 mAb plus a depleting anti-CD8 mAb and in C57BL/6-CD8 knockout mice treated with anti-CD154 mAb monotherapy. We conclude that, in the presence of anti-CD154 mAb, DST leads to an allotolerant state, in part by deleting alloreactive CD8+ T cells. Consistent with this conclusion, blockade of CTLA4, which is known to abrogate the effects of DST and anti-CD154 mAb, prevented the deletion of alloreactive transgenic CD8+ T cells. These results document for the first time that peripheral deletion of alloantigen-specific CD8+ T cells is an important mechanism through which allograft survival can be prolonged by costimulatory blockade. We propose a unifying mechanism to explain allograft prolongation by DST and blockade of costimulation.

Expression of HLA class I-specific inhibitory receptors in human cytolytic T lymphocytes: a regulated mechanism that controls T-cell activation and function

Different families of major histocompatibility complex (MHC)-specific inhibitory receptors (NKRs) play a major role in natural killer (NK) cell function, allowing discrimination between normal cells and cells that do not express adequate amounts of MHC class I antigens. This occurs in most instances as a consequence of viral infection or tumor transformation. In T lymphocytes, expression of NKR is mostly confined to activated CD8+ cytolytic T lymphocytes (CTLs). While NKR expression by CTLs may be viewed as a mechanism preventing damages to normal cells by those CTLs that have acquired NK-like activity, it may also down regulate TCR-mediated T cell activation, thus, impairing CTL functions. The finding that certain cytokines can modulate killer inhibitory receptor expression in CTLs is of major interest and might be instrumental in novel therapeutic approaches aimed at the down regulation ofT-cell function in transplantation or autoimmunity.

Cutting Edge: Cross-Talk Between Cells of the Innate Immune System: NKT Cells Rapidly Activate NK Cells

α-Galactosylceramide (α-GalCer) is a glycolipid with potent antitumor properties that binds to CD1d molecules and activates mouse Vα14 and human Vα24 NKT cells. Surprisingly, we found that, as early as 90 min after α-GalCer injection in vivo, NK cells also displayed considerable signs of activation, including IFN-γ production and CD69 induction. NK activation was not observed in RAG- or CD1-deficient mice, and it was decreased by pretreatment with anti-IFN-γ Abs, suggesting that, despite its rapid induction, it was a secondary event that depended on IFN-γ release by NKT cells. At later time points, B cells and CD8 T cells also began to express CD69. These findings identify a high-speed communication network between the innate and adaptive immune systems in vivo that is initiated upon NKT cell activation. They also suggest that the antitumor effects of α-GalCer result from the sequential recruitment of distinct innate and adaptive effector lymphocytes.

Natural Killing of Xenogeneic Cells Mediated by the Mouse Ly-49D Receptor

NK lymphocytes lyse certain xenogeneic cells without prior sensitization. The receptors by which NK cells recognize xenogeneic targets are largely uncharacterized but have been postulated to possess broad specificity against ubiquitous target ligands. However, previous studies suggest that mouse NK cells recognize xenogeneic targets in a strain-specific manner, implicating finely tuned, complex receptor systems in NK xenorecognition. We speculated that mouse Ly-49D, an activating NK receptor for the MHC I ligand, H2-Dd, might display public specificities for xenogeneic target structures. To test this hypothesis, we examined the lysis of xenogeneic targets by mouse Ly-49D transfectants of the rat NK cell line RNK-16 (RNK.Ly-49D). Of the xenogeneic tumor targets tested, RNK.Ly-49D, but not untransfected RNK-16, preferentially lysed tumor cells derived from Chinese hamsters and lymphoblast targets from rats. Ly-49D-dependent recognition of Chinese hamster cells was independent of target N-linked glycosylation. Mouse Ly-49D also specifically stimulated the natural killing of lymphoblast targets derived from wild-type and MHC-congenic rats of the RT1lv1 and RT1l haplotypes, but not of the RT1c, RT1u, RT1av1, or RT1n haplotypes. These studies demonstrate that Ly-49D can specifically mediate cytotoxicity against xenogeneic cells, and they suggest that Ly-49D may recognize xenogeneic MHC-encoded ligands.

Mono-allelic Ly49 NK cell receptor expression

Each cell is equipped with two copies (alleles) of each autosomal gene. While the vast majority use both alleles, occasional genes are expressed from a single allele. The reason for mono-allelic expression is not always evident and can serve distinct purposes. First, it may facilitate the tight control over the dosage of certain gene products such as some growth factors and their receptors or X-linked genes. Second, the differential usage of the two parental alleles may reflect the mechanisms that ensure mono-specificity, e.g. olfactory receptors, T and B cell receptors. The context of allele-specific expression of the murine Ly49 natural killer (NK) cell receptor genes suggests that their allele-specific expression reflects a process that generates clonal variability.

Clonal acquisition of the Ly49A NK cell receptor is dependent on the trans-acting factor TCF-1

Families of clonally expressed major histocompatibility complex (MHC) class I-specific receptors provide specificity to and regulate the function of natural killer (NK) cells. One of these receptors, mouse Ly49A, is expressed by 20% of NK cells and inhibits the killing of H-2D(d) but not D(b)-expressing target cells. Here, we show that the trans-acting factor TCF-1 binds to two sites in the Ly49A promoter and regulates its activity. Moreover, we find that TCF-1 determines the size of the Ly49A NK cell subset in vivo in a dosage-dependent manner. We propose that clonal Ly49A acquisition during NK cell development is regulated by TCF-1.

Chronic allograft rejection. Do the Th2 cells preferentially induced by indirect alloantigen recognition play a dominant role?

Chronic rejection has been the major obstacle to the long-term allograft survival in the clinic. Although the etiology of this rejection reaction is multifactorial, alloantigen-specific immune activation plays the most critical role. We herein hypothesize that CD4+ Th2 cells that are preferentially induced by the indirect recognition of allogeneic histocompatibility antigens late in transplantation may play the most critical role in the initiation and/or maintenance of chronic allograft rejection. Immunosuppression used to prevent acute rejection and the nature of antigen-presenting cells and alloligands in the graft may all contribute to immune deviation to the Th2 response. This response may be further perpetuated by type 2 cytokines conceivably produced by activated macrophages, NK cells, and CD8+ T cells in the graft. Cytokines and growth factors induced by this type 2 response, in turn, allow for activation of B, endothelial, and smooth muscle cells that collectively contribute to the pathogenesis of chronic allograft rejection by producing alloantibodies and growth hormones required for interstitial fibrosis, extracellular matrix deposition, and vascular neointimal hyperplasia.

Stochastic Acquisition of Qa1 Receptors During the Development of Fetal NK Cells In Vitro Accounts in Part But Not in Whole for the Ability of These Cells to Distinguish Between Class I-Sufficient and Class I-Deficient Targets

Fetal mouse NK cells are grossly deficient in the expression of Ly49 molecules yet show a limited ability to distinguish between wild-type and MHC class I-deficient target cells. In this paper we report that during their development in vitro from immature thymic progenitors, a proportion of C57BL/6 fetal NK cells acquires receptors for a soluble form of the nonclassical class I molecule Qa1b associated with the Qdm peptide, but not for soluble forms of the classical class I molecules Kb and Db. The acquisition of these Qa1 receptors occurs in a stochastic manner that is strictly controlled by cytokines, and in particular is strongly inhibited by IL-4. All fetal NK clones tested, including those that lack detectable Qa1 receptors, express mRNA for CD94 and for both inhibitory and noninhibitory members of the NKG2 family. Fetal NK cells lacking receptors for Qa1 (and also for classical class I molecules) cannot distinguish between wild-type and class I-deficient blasts but, surprisingly, distinguish efficiently between certain wild-type and class I-deficient tumor cells. A variant line that lacks several members of the NKG2 family kills both types of tumor cell equally well, suggesting the existence of NKG2-containing inhibitory receptors that recognize as yet undefined nonclassical class I molecules of restricted distribution.

Localization on a Physical Map of the NKC-Linked Cmv1 Locus Between Ly49b and the Prp Gene Cluster on Mouse Chromosome 6

The Cmv1 locus controls NK cell-mediated resistance to infection with murine CMV. Our recent genetic analysis of backcross mice demonstrated that the NK gene complex (NKC)-linked Cmv1 locus should reside between the Ly49 and Prp gene clusters on distal mouse chromosome 6. We have aligned yeast artificial chromosome (YAC) inserts in a contig spanning the interval between the Ly49 and Prp gene clusters. This YAC contig includes 13 overlapping YAC inserts that span more than 2 megabases (Mb) in C57BL/6 (B6) mice. Since we have identified genomic clones that span the Ly49-Prp gene region, we hypothesize that at least one should contain the Cmv1 locus. To narrow the Cmv1 critical region, we developed novel NKC genetic markers and used these to genotype informative backcross and intra-NKC recombinant congenic mouse DNA samples. These data suggest that Cmv1 resides on a single YAC insert within an interval that corresponds to a physical distance of ∼390 kb. This high resolution, integrated physical and genetic NKC map will facilitate identification of Cmv1 and other NKC-linked loci that regulate NK cell-mediated immunity.

APCs Express DCIR, a Novel C-Type Lectin Surface Receptor Containing an Immunoreceptor Tyrosine-Based Inhibitory Motif

We have identified a novel member of the calcium-dependent (C-type) lectin family. This molecule, designated DCIR (for dendritic cell (DC) immunoreceptor), is a type II membrane glycoprotein of 237 aa with a single carbohydrate recognition domain (CRD), closest in homology to those of the macrophage lectin and hepatic asialoglycoprotein receptors. The intracellular domain of DCIR contains a consensus immunoreceptor tyrosine-based inhibitory motif. A mouse cDNA, encoding a homologous protein has been identified. Northern blot analysis showed DCIR mRNA to be predominantly transcribed in hematopoietic tissues. The gene encoding human DCIR was localized to chromosome 12p13, in a region close to the NK gene complex. Unlike members of this complex, DCIR displays a typical lectin CRD rather than an NK cell type extracellular domain, and was expressed on DC, monocytes, macrophages, B lymphocytes, and granulocytes, but not detected on NK and T cells. DCIR was strongly expressed by DC derived from blood monocytes cultured with GM-CSF and IL-4. DCIR was mostly expressed by monocyte-related rather than Langerhans cell related DC obtained from CD34+ progenitor cells. Finally, DCIR expression was down-regulated by signals inducing DC maturation such as CD40 ligand, LPS, or TNF-α. Thus, DCIR is differentially expressed on DC depending on their origin and stage of maturation/activation. DCIR represents a novel surface molecule expressed by Ag presenting cells, and of potential importance in regulation of DC function.

A new monoclonal antibody reactive with several Ly49 NK cell receptors mediates redirected lysis of target cells

We produced a novel hamster monoclonal antibody (MAb), 14B11, that recognizes the majority of mouse natural-killer (NK) cells. Transfection studies demonstrated that 14B11 MAb binds a subset of Ly49 receptors, including three putative inhibitory receptors, Ly49F, I, and C. No binding to Ly49A, B, D, or G was detected. In addition, 14B11 was shown to bind the putative activating receptor Ly49H, which required co-transfection of the signaling molecule DAP12 for detectable cell surface expression. Thus, 14B11 is the first reported MAb to bind Ly49H and F. At the functional level, 14B11 MAb enhanced the lysis by IL-2 activated NK cells of an FcR+ target cell line (Daudi), but not an FcR- target cell (EL-4). Because F(ab')2 fragments of 14B11 failed to enhance lytic activity, the enhancement of lysis by intact antibody is apparently due to "redirected lysis," in which stimulatory receptors on the NK cell are bridged by antibody to Fc receptors on the target cell. Cell separation experiments demonstrated that the 14B11-dependent redirected lysis was markedly increased using NK cell populations that had been depleted of Ly49F,+ I,+ or C+ NK cells. Because such depletions are expected to enrich for Ly49H+ NK cells, these results suggest that the enhancement of lysis mediated by 14B11 MAb may be due to stimulation of the activating Ly49H receptor. In conjunction with other anti-Ly49 MAbs, the 14B11 MAb will be useful in further studies of Ly49 receptor function and specificity.

N-linked oligosaccharides can protect target cells from the lysis mediated by NK cells but not by cytotoxic T lymphocytes: role of NKG2-A

We have previously shown that glycophorin A (GPA), inserted by electropulsation into the membrane of K562 cells, protected them from natural killer (NK) cell-mediated cytotoxicity and the unique N-linked oligosaccharide of GPA was essential for resistance to occur. The present study demonstrates that the protection level conferred by GPA is similar to the resistance induced by HLA-Cw3 expressed by transfected K562 cells. A monoclonal antibody against NKG2-A, an NK inhibitory receptor interacting with HLA class I antigens and belonging to the C-type lectin receptor, was able to restore the ability of NK cells to lyse K562 cells expressing HLA-Cw3 at the cell membrane but not electroinserted-GPA, suggesting that the N-linked oligosaccharide of GPA cannot be a ligand for NKG2-A. GPA was then electroinserted into the membrane of two lymphoblastoid B-cell lines: one was sensitive to NK cell-mediated lysis, the other was susceptible to cytotoxic CD8+ T-lymphocyte (CTL)-mediated cytotoxicity. The electroinserted GPA protected the target cells from NK-mediated cytotoxicity, whereas it did not modify the cell susceptibility to lysis by CTL. Endoglycosidase F treatment abolished the resistance towards NK cell-mediated lysis, suggesting that N-linked glycans could inhibit mechanisms used by NK cells to exert their cytotoxic function in agreement with our previous results.

Mouse NKR-P1B, a Novel NK1.1 Antigen with Inhibitory Function

The mouse NK1.1 Ag originally defined as NK cell receptor (NKR)-P1C (CD161) mediates NK cell activation. Here, we show that another member of the mouse CD161 family, NKR-P1B, represents a novel NK1.1 Ag. In contrast to NKR-P1C, which functions as an activating receptor, NKR-P1B inhibits NK cell activation. Association of NKR-P1B with Src homology 2-containing protein tyrosine phosphatase-1 provides a molecular mechanism for this inhibition. The existence of these two NK1.1 Ags with opposite functions suggests a potential role for NKR-P1 molecules, such as those of the Ly-49 gene family, in regulating NK cell function.

The chromosomal duplication model of the major histocompatibility complex

The major histocompatibility complex (MHC) is a genetic region that has been extensively studied by immunologists, molecular biologists, and evolutionary biologists. Nevertheless, our knowledge of how the MHC acquired its present-day organization is quite limited. The recent discovery that the mammalian genome contains regions paralogous to the MHC has led us to the proposal that the MHC region of jawed vertebrates arose as a result of ancient chromosomal duplications. Here, I review the current status of this proposal.

The Role of LY49 NK Cell Subsets in the Regulation of Murine Cytomegalovirus Infections

The distributions and functions of NK cell subsets, as defined by the expression of Ly49 NK cell receptors, were examined in murine CMV (MCMV)-infected mice. MCMV induced a reduction in NK1.1+ cell number in the spleen and an increase in the peritoneal exudate cells. Within the splenic NK1.1+ population, proportional increases in Ly49A+ and Ly49G2+ cells but decreases in Ly49C+ and Ly49D+ cells were observed 3 days post-MCMV infection, but within the peritoneal NK1.1+ cell populations there were proportional decreases in Ly49A+ cells and increases in Ly49C+, Ly49D+, and Ly49G2+ cells. Lymphocytic choriomeningitis virus did not elicit a comparable NK cell subset distribution. Lymphokine-activated killer cells were sorted into different Ly49 NK cell subsets and adoptively transferred into C57BL/6 suckling mice. Regulation of MCMV synthesis in these suckling mice was shown to be an IFN-γ-dependent, perforin- and Cmv-1-independent process, and each NK cell subset mediated anti-viral activity. In adult C57BL/6 mice, the control of MCMV in the spleen is mediated by a perforin-dependent mechanism, regulated in part by the Cmv-1 gene, which maps closely to the Ly49 family. In vivo depletions of either one or two of the Ly49 subsets in adult mice did not affect the ability of the residual NK cells to regulate MCMV synthesis. These data provide evidence of NK cell subset distribution and function in MCMV infection, but no individual subset was required for the Cmv-1-like regulation of MCMV synthesis.

The human gene encoding the lectin-type oxidized LDL receptor (OLR1) is a novel member of the natural killer gene complex with a unique expression profile

LOX-1 is an endothelial receptor for oxidized low-density lipoprotein that plays essential roles in atherogenesis. LOX-1 has the highest homology with C-type lectin receptors expressed on natural killer cells. In the present study, we cloned and characterized the human LOX-1 gene (HGMW-approved symbol OLR1). The gene structure of LOX-1 resembles that of the natural killer cell receptors. Fluorescence in situ hybridization and analyses of a yeast artificial chromosome contig revealed that the human LOX-1 gene is located in the natural killer gene complex on chromosome 12p12-p13, where the genes of the natural killer cell receptors cluster. In contrast, the expression pattern of LOX-1 is different from that of the natural killer cell receptors; LOX-1 is expressed in vascular-rich organs, but not in lymphocytes. A 1753-bp fragment of the 5' flanking region of the LOX-1 gene had a functional promoter activity. This region contains binding sites for several transcription factors, including the STAT family and NF-IL6, and the expression of LOX-1 was upregulated by several cytokines. These results demonstrate that the human LOX-1 gene is a new member of the natural killer gene complex with a unique expression profile.

External and Internal Calibration of the MHC Class I-Specific Receptor Ly49A on Murine Natural Killer Cells

Expression of the H-2Dd-specific inhibitory receptor Ly49A on murine NK cells is subject to MHC class I-dependent modulation in vivo. As a result, NK cells in H-2Dd-transgenic mice express low cell surface levels of Ly49A, whereas NK cells from nontransgenic C57BL/6 (B6) mice express high levels. The purpose of this study was to assess the role of MHC class I molecules on the NK cell itself vs those on surrounding cells in this calibration and to test whether the Ly49A levels are subject to regulation in mature NK cells also. Analysis of transgenic mice with mosaic expression of an H-2Dd/Ld transgene showed that MHC class I molecules on surrounding cells (external ligands) and on the NK cell itself (internal ligands) played distinct roles in the determination of Ly49A levels. External ligands were involved in down-regulation of Ly49A levels in vivo, whereas internal ligands kept the down-regulated levels of Ly49A low upon NK cell activation in vitro. Furthermore, in an experimental system based on adoptive transfer of spleen cells, receptor down-regulation of Ly49A occurred as a rapid adaptation process in mature NK cells after interaction with the H-2Dd ligand in vivo. This suggests that Ly49 levels are not fixed but can be changed in mature NK cells when they are exposed to a changed MHC class I environment.

SLP-76 is a direct substrate of SHP-1 recruited to killer cell inhibitory receptors

Activation of immune system cells via antigen-, Fc-, or natural killer cell-triggering-receptor stimulation is aborted by co-engagement of inhibitory receptors. Negative signaling by killer cell inhibitory receptors and related receptors depends on the Src homology 2 (SH2)-containing protein tyrosine phosphatase SHP-1. Using a combination of direct binding and functional assays, we demonstrated that the SH2 domain-containing leukocyte protein 76 (SLP-76) is a specific target for dephosphorylation by SHP-1 in T cells and natural killer cells. Furthermore, we showed that tyrosine-phosphorylated SLP-76 is required for optimal activation of cytotoxic lymphocytes, suggesting that the targeted dephosphorylation of SLP-76 by SHP-1 is an important mechanism for the negative regulation of immune cell activation by inhibitory receptors.

Inhibitory and Stimulatory Functions of Paired Ig-Like Receptor (PIR) Family in RBL-2H3 Cells

In this study, we demonstrate potent regulatory function of the murine killer cell inhibitory receptor-like molecules, paired Ig-like receptors (PIRs) or p91, using chimeric receptors expressed on the rat basophilic leukemia cell line RBL-2H3. One of the chimeras, which has the transmembrane and cytoplasmic domain of PIR-B fused to the extracellular portion of type IIB receptor for IgG, was able to inhibit the type I receptor for IgE-mediated degranulation response upon coaggregation. This chimera also suppressed cytoplasmic Ca2+ mobilization in the presence and absence of calcium ion in the extracellular medium. Tyrosine to phenylalanine point mutations at the third and fourth immunoreceptor tyrosine-based inhibitory motif-like sequences of PIR-B attenuated the inhibitory effects on degranulation and on cytoplasmic Ca2+ mobilization, indicating the important role of these tyrosines for the delivery of negative signal. In contrast, the cross-linking of another chimeric receptor composed of the type IIB receptor for IgG extracellular portion and the transmembrane and short cytoplasmic sequence of PIR-A elicited Ca2+ mobilization and degranulation. These results indicate that PIR molecules may regulate cellular functions both positively and negatively.

Natural killer cell receptors

In killing of cellular targets, natural killer cells employ receptors that activate them and receptors specific for MHC class I that inhibit their activation. Progress in understanding the inhibitory receptors has been rapid, and indications are that they fall into two distinct structural types that appear to utilize the same inhibitory signaling cascade; meanwhile, components of the activation cascade are being elucidated, permitting us to integrate the pathways involved.

The C-type lectin superfamily in the immune system

Protein-carbohydrate interactions serve multiple functions in the immune system. Many animal lectins (sugar-binding proteins) mediate both pathogen recognition and cell-cell interactions using structurally related Ca(2+)-dependent carbohydrate-recognition domains (C-type CRDs). Pathogen recognition by soluble collections such as serum mannose-binding protein and pulmonary surfactant proteins, and also the macrophage cell-surface mannose receptor, is effected by binding of terminal monosaccharide residues characteristic of bacterial and fungal cell surfaces. The broad selectivity of the monosaccharide-binding site and the geometrical arrangement of multiple CRDs in the intact lectins explains the ability of the proteins to mediate discrimination between self and non-self. In contrast, the much narrower binding specificity of selectin cell adhesion molecules results from an extended binding site within a single CRD. Other proteins, particularly receptors on the surface of natural killer cells, contain C-type lectin-like domains (CTLDs) that are evolutionarily divergent from the C-type lectins and which would be predicted to function through different mechanisms.

Murine Nkg2d and Cd94 are clustered within the natural killer complex and are expressed independently in natural killer cells

Natural killer (NK) cells express C-type lectin-like receptors, encoded in the NK gene complex, that interact with major histocompatibility complex class I and either inhibit or activate functional activity. Human NK cells express heterodimers consisting of CD94 and NKG2 family molecules, whereas murine NK cells express homodimers belonging to the Ly-49 family. The corresponding orthologues for other species, however, have not been described. In this report, we used probes derived from the expressed sequence tag database to clone C57BL/6-derived cDNAs homologous to human NKG2-D and CD94. Among normal tissues, murine NKG2-D and CD94 transcripts are highly expressed only in activated NK cells, including both Ly-49A+ and Ly-49A- subpopulations. Additionally, mNKG2-D is expressed in murine NK cell clones KY-1 and KY-2, whereas mCD94 expression is observed only in KY-1 cells but not KY-2. Last, we have finely mapped the physical location of the Cd94 (centromeric) and Nkg2d (telomeric) genes between Cd69 and the Ly49 cluster in the NK complex. Thus, these data indicate the expanding complexity of the NK complex and the corresponding repertoire of C-type lectin-like receptors on murine NK cells.

NK cell receptors

NK cells are regulated by opposing signals from receptors that activate and inhibit effector function. While positive stimulation may be initiated by an array of costimulatory receptors, specificity is provided by inhibitory signals transduced by receptors for MHC class I. Three distinct receptor families, Ly49, CD94/NKG2, and KIR, are involved in NK cell recognition of polymorphic MHC class I molecules. A common pathway of inhibitory signaling is provided by ITIM sequences in the cytoplasmic domains of these otherwise structurally diverse receptors. Upon ligand binding and activation, the inhibitory NK cell receptors become tyrosine phosphorylated and recruit tyrosine phosphatases, SHP-1 and possibly SHP-2, resulting in inhibition of NK cell-mediated cytotoxicity and cytokine expression. Recent studies suggest these inhibitory NK cell receptors are members of a larger superfamily containing ITIM sequences, the inhibitory receptor superfamily (IRS).

Modulation of natural killer cell cytotoxicity in human cytomegalovirus infection: the role of endogenous class I major histocompatibility complex and a viral class I homolog

Natural killer (NK) cells have been implicated in early immune responses against certain viruses, including cytomegalovirus (CMV). CMV causes downregulation of class I major histocompatibility complex (MHC) expression in infected cells; however, it has been proposed that a class I MHC homolog encoded by CMV, UL18, may act as a surrogate ligand to prevent NK cell lysis of CMV-infected cells. In this study, we examined the role of UL18 in NK cell recognition and lysis using fibroblasts infected with either wild-type or UL18 knockout CMV virus, and by using cell lines transfected with the UL18 gene. In both systems, the expression of UL18 resulted in the enhanced killing of target cells. We also show that the enhanced killing is due to both UL18-dependent and -independent mechanisms, and that the killer cell inhibitory receptors (KIRs) and CD94/NKG2A inhibitory receptors for MHC class I do not play a role in affecting susceptibility of CMV-infected fibroblasts to NK cell-mediated cytotoxicity.

Sequence analysis of a 62-kb region overlapping the human KLRC cluster of genes

The NKG2 family of genes (HGMW-approved symbol KLRC) contains at least four members (NKG2-A, -C, -E, and -F) which are localized to human chromosome 12p12.3-p13.2. This region, called the natural killer (NK) complex, encodes for lectin-like genes preferentially expressed on NK cells. One of them, the human CD94 gene (HGMW-approved symbol KLRD1), encodes for a protein that has been shown to be covalently associated with the NKG2-A molecule. In this report, we showed that the NKG2 and CD94 genes are localized in a small region (< 350 kb) and we mapped them in the following order: (NKG2-C/NKG2-A)/NKG2-E/NKG2-F/NKG2-D/CD 94. Sequence analysis of 62 kb spanning the NKG2-A, -E, -F, and -D loci allowed the identification of two LINE elements that could have been involved in the duplication of the NKG2 genes. Presence of one MIR and one L1ME2 element at homologous positions in the NKG2-A and NKG2-F genes is consistent with the existence of rodent NKG2 gene(s). Finally, we mapped the 5'-ends of the NKG2-A transcripts into two separate regions showing the existence of two separate transcriptional control regions upstream of the NKG2-A locus and defining putative promoter elements for these genes.

LAG-3 does not define a specific mode of natural killing in human

LAG-3 is a gene localized on the band p13 of human chromosome 12, close to the NK gene complex (NKC), expressed on activated NK cells and encoding a receptor for MHC class II molecules. Recently, LAG-3 has been proposed to define a specific mode of natural killing in mice. The putative role of LAG-3 on human natural killer cytotoxicity has been examined with specific monoclonal antibodies and a recombinant soluble form of LAG-3. Neither the antibodies, which are able to block the interaction with the ligand, nor the recombinant product, which has retained its binding capacity for MHC class II, had an effect on the natural killing of various target cells. Furthermore, in redirected killing assays, none of these antibodies were able to modulate either positively or negatively the cytotoxicity. Taken together, these data show that LAG-3 has no transducing activity involved in NK cytotoxicity, ruling out the existence of a specific mode of natural killing defined by this molecule in humans.

Regulation of KIR expression in human T cells: a safety mechanism that may impair protective T-cell responses

Killer-cell inhibitory Receptors (KIRs) are a new family of major histocompatibility complex (MHC) class I-specific receptors. KIRs allow natural killer cells to identify and lyse self cells that do not express sufficient amounts of MHC class I molecules. Here, Maria Cristina Mingari and colleagues view the expression of KIRs by cytolytic T lymphocytes and their regulation by certain cytokines as a double-edged sword.

Fetal Mouse NK Cell Clones Are Deficient in Ly49 Expression, Share a Common Broad Lytic Specificity, and Undergo Continuous and Extensive Diversification In Vitro

NK cells obtained by exposing mouse fetal thymocytes to appropriate combinations of IL-4, IL-2, and PMA are phenotypically indistinguishable from cultured adult splenic NK cells with the exception that they generally lack measurable expression of all of the inhibitory Ly49 molecules that can currently be detected with Abs (Ly49A, -C, -G, and -I) and of the activating molecule Ly49D. Despite this deficiency, fetal NK cells have a similar specificity to Ly49-expressing adult splenic NK cells. Individual fetal NK cell clones display an essentially invariant and broad specificity similar to that of polyclonal populations of fetal or adult NK cells, although significant differences in the fine specificity of clones can occasionally be detected. Most remarkably, cloned fetal NK cell lines display heterogeneous expression of a restricted set of surface molecules that includes 10A7, Ly6C, 3C2, CD8, certain isoforms of CD45, and also, occasionally, Ly49 molecules. This heterogeneity is not related to the cell cycle or activation status of the cells, and micromanipulation recloning demonstrates unambiguously that it is not due to a lack of a single cell origin. Diversity is generated rapidly and the capacity for diversification appears to persist indefinitely in vitro. The expression of individual variable Ags is independent and stochastic, resulting in fetal NK “clones” being potentially composed of hundreds of phenotypically distinct cells. We hypothesize that fetal NK cells behave as progenitor cells that are undergoing a process of rapid, extensive, and continuous diversification and that are individually capable of generating and regenerating a complex NK cell repertoire.