Cutting edge: KIR antisense transcripts are processed into a 28-base PIWI-like RNA in human NK cells

Killer Ig-like receptors (KIRs) are expressed in a variegated, clonally restricted fashion on NK cells and are important determinants of NK cell function. Although silencing of individual KIR genes is strongly correlated with the presence of CpG dinucleotide methylation within the promoter, the mechanism responsible for silencing has not been identified. Our results show that antisense transcripts mediate KIR transcriptional silencing through a novel PIWI-like 28-base small RNA. Although PIWI RNA-mediated silencing of transposable elements within germ cells have been described, this is the first report that identifies a PIWI-like RNA in an immune somatic cell lineage and identifies a mechanism that may be broadly used in orchestrating immune development.

The transcription factor c-Myc enhances KIR gene transcription through direct binding to an upstream distal promoter element

The killer cell immunoglobulin-like receptor (KIR) repertoire of natural killer (NK) cells determines their ability to detect infected or transformed target cells. Although epigenetic mechanisms play a role in KIR gene expression, work in the mouse suggests that other regulatory elements may be involved at specific stages of NK-cell development. Here we report the effects of the transcription factor c-Myc on KIR expression. c-Myc directly binds to, and promotes transcription from, a distal element identified upstream of most KIR genes. Binding of endogenous c-Myc to the distal promoter element is significantly enhanced upon interleukin-15 (IL-15) stimulation in peripheral blood NK cells and correlates with an increase in KIR transcription. In addition, the overexpression of c-Myc during NK-cell development promotes transcription from the distal promoter element and contributes to the overall transcription of multiple KIR genes. Our data demonstrate the significance of the 5' promoter element upstream of the conventional KIR promoter region and support a model whereby IL-15 stimulates c-Myc binding at the distal KIR promoter during NK-cell development to promote KIR transcription. This finding provides a direct link between NK-cell activation signals and KIR expression required for acquisition of effector function during NK-cell education.

Antisense transcripts negatively regulate transcription of multiple variegated killer immunoglobulin-like receptor (KIR) genes (136.38)

The objective of this study is to understand the epigenetic mechanisms that regulate Killer Immunoglobulin-Like Receptor (KIR) gene regulation in human natural killer (NK) cells. We have previously identified both bidirectional transcription within the previously characterized KIR promoter and a novel promoter element upstream of the previously characterized promoter. Based on these findings, we propose that antisense transcripts homologous to the KIR promoter region participate in the establishment of the epigenetic modifications that silence individual KIR genes during human NK cell development. To test this prediction, we overexpressed KIR3DL1 antisense transcripts in developing NK cells and found that mature cells from these cultures exhibited a four-fold decrease in KIR expression relative to GFP-expressing controls. We have also identified 324 base pair and 28 base pair double-stranded RNA molecules homologous to the KIR3DL1 promoter region, suggesting that a siRNA mechanism may be responsible for establishment of the promoter methylation patterns within the KIR locus. Understanding the mechanisms of KIR expression, a requisite for NK cell education/licensing, may allow us to understand the acquisition of effector function and manipulate it for therapeutic benefit.

R01 HL 55417

Genetic control of variegated KIR gene expression: polymorphisms of the bi-directional KIR3DL1 promoter are associated with distinct frequencies of gene expression

Natural killer (NK) cells play an important role in the detection and elimination of tumors and virus-infected cells by the innate immune system. Human NK cells use cell surface receptors (KIR) for class I MHC to sense alterations of class I on potential target cells. Individual NK cells only express a subset of the available KIR genes, generating specialized NK cells that can specifically detect alteration of a particular class I molecule or group of molecules. The probabilistic behavior of human KIR bi-directional promoters is proposed to control the frequency of expression of these variegated genes. Analysis of a panel of donors has revealed the presence of several functionally relevant promoter polymorphisms clustered mainly in the inhibitory KIR family members, especially the KIR3DL1 alleles. We demonstrate for the first time that promoter polymorphisms affecting the strength of competing sense and antisense promoters largely explain the differential frequency of expression of KIR3DL1 allotypes on NK cells. KIR3DL1/S1 subtypes have distinct biological activity and coding region variants of the KIR3DL1/S1 gene strongly influence pathogenesis of HIV/AIDS and other human diseases. We propose that the polymorphisms shown in this study to regulate the frequency of KIR3DL1/S1 subtype expression on NK cells contribute substantially to the phenotypic variation across allotypes with respect to disease resistance.

Natural killer (NK) cells represent a specialized blood cell that plays an important role in the detection of virus-infected or cancer cells. NK cells recognize and kill diseased cells using receptors for self antigens (HLA) that are frequently altered on aberrant cells. The HLA receptors are known as Killer cell Immunoglobulin-like Receptors, or KIR. Humans possess from four to 14 KIR receptor genes in their genome, and individual NK cells express a subset of the available KIR genes, generating specialized NK cells that detect alterations in specific HLA proteins. The mechanism of this unusual selective gene activation was recently shown by our group to be controlled by a probabilistic bi-directional promoter switch that turns on a given gene at a pre-determined frequency in the NK cell population. The current study shows that the properties of the switches in terms of the relative activity of forward (on) versus reverse (off) promoter activity is directly correlated with the frequency at which a given gene is expressed within the NK cell population. These results have important implications for our understanding of the role of NK cells in viral resistance and bone marrow transplants.

Novel KIR3DL1 alleles and their expression levels on NK cells: convergent evolution of KIR3DL1 phenotype variation?

KIR3DL1 shows extensive polymorphism, and its variation has functional significance in terms of cell-surface expression levels and inhibitory capacity. We characterized nine KIR3DL1 alleles (*022, *028, *029, *033, *035, *051, *052, *053, and *054), four of which were identified for the first time in this study, and compared them to known alleles in phylogenetic analysis. Blood was available from eight individuals with these alleles, and cell-surface expression on NK cells could be determined for six of them using the KIR3DL1-specific Ab DX9. Four of the alleles were expressed at clearly detectable levels, and two others showed exceptionally low levels of expression. Site-directed mutagenesis demonstrated that single amino acid changes can result in either diminished or enhanced DX9 staining compared with the respective related KIR3DL1 allotypes. These results raise the possibility that KIR3DL1 evolution maintains variation in KIR3DL1 cell-surface expression levels, potentially due to the effect of such variation on functional capacity.

NF-kappa B p50/p65 affects the frequency of Ly49 gene expression by NK cells

In mice, acquisition of Ly49 receptors characterizes one of the developmental stages of NK cells. We previously described a novel Ly49 promoter, Pro1, involved in Ly49 gene regulation in immature NK cells. Pro1 transcriptional activity requires a NF-kappaB binding site; however, only NF-kappaB/p50 binding to this element was observed. Cotransfection of NF-kappaB/p65 with Ly49g Pro1 in LNK cells induced a decrease in the transcriptional activity of the core promoter. Moreover, decreasing NF-kappaB/p65 protein expression by RNA interference increases Pro1 transcriptional activity. A high rate of NF-kappaB/p65 degradation in LNK cells correlates with Pro1 activity, since treatment with the proteasome inhibitor MG132 increased levels of NF-kappaB/p65 protein and decreased Pro1 activity. In addition, analysis of the Ly49 repertoire in NF-kappaB/p50 null mice reveals a decrease in the proportion of NK cells expressing a given Ly49 molecule. The defect in Ly49 expression is observed in the bone marrow and the spleen with a similar altered pattern of developmental stages in each tissue. The frequency of Ly49 expression in NF-kappaB/p52 null mice is slightly increased, indicating the specific role of NF-kappaB/p50 in Ly49 gene activation. These results suggest that NF-kappaB p50/p65 plays a major role in the initiation of Ly49 gene expression in NK cells.

A mutation in KIR3DS1 that results in truncation and lack of cell surface expression

The KIR gene cluster exhibits a high degree of polymorphism in terms of gene content as well as allelic polymorphism, and data suggest that it is evolving rapidly. The KIR3DL1 locus is one of the most polymorphic loci within this cluster and is unique in that it encodes an activating receptor KIR3DS1, as well as multiple inhibitory KIR3DL1 allotypes. Because KIR3DS1 has been implicated in a number of diseases, we tested for the presence of KIR3DS1 variants that might affect its expression and activating capacity. Preliminary FACS analysis indicated that indeed some individuals with the KIR3DS1 allele showed no cell surface expression of the molecule. Sequencing analysis identified a variant with a complex deletion/substitution mutation in exon 4 (which encodes the D1 extracellular domain), resulting in a premature stop codon. We subsequently genotyped 3,960 unrelated individuals and determined the frequencies of this allele across geographically distinct world populations. The data indicate that the null KIR3DS1 allele is uncommon, arose on a single haplotype, and spread across geographically distinct populations.

Detection of KIR3DS1 on the Cell Surface of Peripheral Blood NK Cells Facilitates Identification of a Novel Null Allele and Assessment of KIR3DS1 Expression during HIV-1 Infection

KIR3DL1 is a highly polymorphic killer cell Ig-like receptor gene with at least 23 alleles described, including its activating counterpart, KIR3DS1. Recently, the KIR3DS1 allele has been shown to slow progression to AIDS in individuals expressing HLA-Bw4 with isoleucine at position 80. However, due to the lack of a specific Ab, KIR3DS1 expression and function is not well characterized. In this study, we demonstrate KIR3DS1 expression on a substantial subset of peripheral natural killer cells through its recognition by the mAb Z27. The fidelity of this detection method was confirmed by analysis of KIR3DS1 transfectants and the identification of a novel KIR3DS1 null allele. Interestingly, KIR3DS1 is also expressed by a small proportion of CD56(+) T cells. We show that ligation of KIR3DS1 by Z27 leads to NK cell IFN-gamma production and degranulation as assessed by expression of CD107a. Furthermore, we document the persistence of KIR3DS1(+) NK cells in HIV-1 viremic patients. The high frequency of KIR3DS1 expression, along with its ability to activate NK cells, and its maintenance during HIV-1 viremia are consistent with the epidemiological data suggesting a critical role for this receptor in controlling HIV-1 pathogenesis.

Autoimmunity, spontaneous tumourigenesis, and IL-15 insufficiency in mice with a targeted disruption of the tumour suppressor gene Fus1

The Fus1 gene resides in the critical 3p21.3 human chromosomal region deleted in lung and breast cancers. Recently, the tumour suppressor properties of Fus1 were confirmed experimentally by intra-tumoural administration of Fus1 that suppressed experimental lung metastasis in mice. We generated Fus1-deficient mice that were viable, fertile, and demonstrated a complex immunological phenotype. Animals with a disrupted Fus1 gene developed signs of autoimmune disease, such as vasculitis, glomerulonephritis, anaemia, circulating autoantibodies, and showed an increased frequency of spontaneous vascular tumours. Preliminary analysis of immune cell populations revealed a consistent defect in NK cell maturation in Fus1 null mice that correlated with changes in the expression of IL-15. Injection of IL-15 into Fus1 knockout mice completely rescued the NK cell maturation defect. Based on these results, we propose the hypothesis that Fus1 deficiency affects NK cell maturation through the reduction of IL-15 production but does not directly alter their developmental capacity. Since acquired immunity was not affected in Fus1-deficient animals, we suggest a relationship between the Fus1 protein and the regulation of innate immunity via IL-15 production. The increased frequency of spontaneous cancers and the development of an autoimmune syndrome in Fus1 null mice imply that these mice could serve as a model for studying molecular mechanisms of anti-tumour immunity and autoimmunity.

Functional evidence supporting the predicted role of KIR3DS1 in the host response to HIV: KIR3DS1 is an activating receptor expressed by human Natural Killer cells (52.5)

KIR3DL1 is a highly polymorphic inhibitory Killer cell Immunoglobulin-like (KIR) Receptor gene with at least 23 alleles described, including its activating counterpart KIR3DS1. Recently, the KIR3DS1 allele has been shown to slow progression to AIDS in HLABw4*80I individuals. However, due to the lack of specific anti-KIR3DS1 antibody, expression and function have never been reported. In a study of 64 donors, we demonstrate KIR3DS1 expression on a substantial subset of peripheral Natural Killer (NK) cells through its recognition by the monoclonal antibody Z27. The fidelity of this detection method was confirmed by analysis of KIR3DS1 transfectants and the identification of a novel KIR3DS1 null allele. Furthermore, we show that ligation of KIR3DS1 by Z27 leads to NK cell degranulation and interferon gamma secretion. Finally, we document the persistence of KIR3DS1+ NK cells in HIV1 viremic patients. In conclusion, the high frequency of KIR3DS1 expression, together with its ability to activate NK cells, and its maintenance during HIV-1 viremia support the epidemiological data suggesting a critical role for this receptor in controlling HIV-1 pathogenesis.

This project has been funded in part with Federal funds from the NCI and the NIH, under DHHS #N01-C0-12400.

Genetic control of variegated KIR gene expression: polymorphisms of the bi-directional KIR3DL1 promoter are associated with distinct frequencies of gene expression (35.37)

The recent discovery of the bi-directional nature of the KIR gene promoters suggests that the relative strength of competing sense and antisense promoter elements might affect the probability of gene expression. Analysis of a panel of donors has revealed the presence of several functionally relevant promoter polymorphisms, clustered mainly in the inhibitory KIR family members, especially the KIR3DL1 alleles. The percentage of peripheral blood NK cells expressing either KIR3DL1 or KIR3DS1 was evaluated in individuals heterozygous at this locus. The KIR3DL1*001 and KIR3DS1 alleles were found to be expressed on a significantly higher percentage of NK cells than all other KIR3DL1 alleles. High frequency of expression was associated with a specific promoter polymorphism at a Sp1 transcription factor-binding site, and this change increased the relative promoter activity in the sense orientation. The KIR2DS4 promoter region is nearly identical to the KIR3DL1 promoter, and several polymorphisms are present, however none of the KIR2DS4 polymorphisms affect promoter activity in vitro and no association was observed between promoter polymorphisms and frequency of expression.

This research was supported in part by the Intramural Research Program of the NIH, National Cancer Institute, Center for Cancer Research. Funded by NCI Contract N01-CO-12400

Identification of bidirectional promoters in the human KIR genes

Although the class I MHC receptors expressed by human and mouse natural killer (NK) cells have distinct molecular origins, they are functional analogues that are expressed in a variegated pattern. The murine Ly49 class I receptors contain bidirectional promoters that have been proposed to control the probabilistic expression of these genes. Whether similar elements are present in the human killer Ig-like receptor (KIR) genes is a fundamental question. A detailed analysis of the 2 kb intergenic region separating the KIR2DL4 gene and the adjacent KIR3DL1 gene revealed that additional promoter elements exist in the human KIR genes. Remarkably, the previously characterized KIR3DL1 proximal promoter possesses bidirectional promoter activity that maps to an 88 bp DNA fragment containing CREB, AML, Sp1 and Ets transcription factor binding sites. Individual KIR genes and alleles possess bidirectional promoters with distinct properties. Analysis of KIR(+)and KIR(-) NK cells and NK precursors indicates that reverse transcripts from the bidirectional promoter are found in cells that lack KIR protein expression, but are not present in mature KIR-expressing NK cells, suggesting that reverse transcription from the proximal promoter blocks gene activation in immature NK and precursor cells.

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.

Identification of distal KIR promoters and transcripts

A more complete understanding of the transcriptional control of the human and murine class I MHC receptors will help to shed light on the mechanism of selective, stochastic, gene activation that operates in these gene families. Studies of the murine Ly49 class I MHC receptor genes have revealed an important role for distal transcripts originating upstream of the proximal promoter. To date, there have been no reports of distal promoters within the functionally analogous human KIR family of class I MHC receptors. In the current study, reverse transcriptase-polymerase chain reaction (RT-PCR) and RNase protection assays were used to reveal the presence of distal KIR transcripts initiating upstream of the previously characterized proximal KIR promoter. The intergenic promoter elements detected were associated with repetitive elements of the Alu and L1 families. Unlike the proximal KIR promoter, the distal promoter regions were not NK cell-specific. KIR genes expressed in a variegated manner produced a low level of distal transcripts containing a large 5' untranslated region. In contrast, the highly expressed KIR2DL4 gene possessed a higher level of spliced distal transcripts that were capable of producing KIR2DL4 protein. The identification of distal KIR promoter elements suggests that intergenic transcripts may influence the expression of KIR genes.

Transcriptional Regulation of NK Cell Receptors

The stochastic expression of individual members of NK cell receptor gene families on subsets of NK cells has attracted considerable interest in the transcriptional regulation of these genes. Each receptor gene can contain up to three separate promoters with distinct properties. The recent discovery that an upstream promoter can function as a probabilistic switch element in the Ly49 gene family has revealed a novel mechanism of variegated gene expression. An important question to be answered is whether or not the other NK cell receptor gene families contain probabilistic switches. The promoter elements currently identified in the Ly49, NKR-P1, CD94, NKG2A, and KIR gene families are described.

A single-gene biomarker identifies breast cancers associated with immature cell type and short duration of prior breastfeeding

The pathogenesis of breast cancers that do not express estrogen receptors or Her-2/neu receptors (ER-/HER2- phenotype) is incompletely understood. We had observed markedly elevated gene expression of gamma-aminobutyric acid type A (GABA(A)) receptor subunit pi (GABApi, GABRP) in some breast cancers with ER-/HER2- phenotype. In this study, transcriptional profiles (TxPs) were obtained from 82 primary invasive breast cancers by oligonucleotide microarrays. Real-time reverse transcription-polymerase chain reaction (RT-PCR) was used to measure GABApi gene expression in a separate cohort of 121 invasive breast cancers. GABApi gene expression values from TxP and RT-PCR were standardized and compared with clinicopathologic characteristics in the 203 patients. GABApi gene expression was increased in 16% of breast cancers (13/82 TxP, 20/ 121 RT-PCR), particularly in breast cancers with ER-/HER2- phenotype (60%), and breast cancers with basal-like genomic profile (60%). The profile of genes coexpressed with GABApi in these tumors was consistent with an immature cell type. In multivariate linear regression analysis, the level of GABApi gene expression was associated with ER-/HER2- phenotype (P < 0.0001), younger age at diagnosis (P = 0.0003), and shorter lifetime duration of breastfeeding (< or = 6 months) in all women (P = 0.017) and specifically in parous women (P = 0.013). GABApi gene expression was also associated with combinations of high grade with ER-/HER2- phenotype (P = 0.002), and with Hispanic ethnicity (P = 0.036). GABApi gene expression is increased in breast cancers of immature (undifferentiated) cell type and is significantly associated with shorter lifetime history of breastfeeding and with high-grade breast cancer in Hispanic women.

Complete elucidation of a minimal class I MHC natural killer cell receptor haplotype

The BALB/c inbred mouse is widely used in models of infectious disease, transplantation, and cancer. The differences in the immune responses of BALB/c compared to C57BL/6 mice are especially valuable for the identification of immune regulation genes. One striking immune variance between these mice is in the function of natural killer (NK) cells, and there is strong evidence implicating differential expression of Ly49 genes. In this study, the complete BALB/c Ly49 gene cluster has been sequenced and found to contain six functional genes and two pseudogenes. Compared to C57BL/6 mice, there is a 200 kb region absent in the BALB/c cluster including a complete lack of Ly49h-related genes, which explains the increased susceptibility of BALB/c to cytomegalovirus infection. In addition, there is no BALB/c Ly49d allele, explaining the inability of BALB/c NK cells to kill certain tumor cells. The Ly49 region has now been sequenced in three different inbred mouse strains, and comparisons indicate that the evolution of each haplotype is not straightforward and has involved large-scale deletions/insertions, gene recombination, and unequal crossing over between divergent haplotypes. This study confirms that relatively small murine class I MHC receptor haplotypes exist, analogous to observations made of human killer cell Ig-like receptor gene haplotypes.

Direct sequence comparison of two divergent class I MHC natural killer cell receptor haplotypes

The murine Ly49 gene family encoding natural killer cell receptors for class I MHC is an example of a rapidly evolving cluster of immune response genes. Determining the genomic sequence of the 129S6/SvEvTac (129S6) Ly49 cluster and comparing it to the known sequence of the C57BL/6 (B6) region provided insight into the mechanisms of Ly49 gene evolution. 129S6 contains 20 Ly49, many of which are pseudogenes and 40% of the genes have no counterpart in the B6 genome. The difference in gene content between these two strains is primarily the result of distinct patterns of gene duplication. Phylogenetic analyses of individual exons showed that Ly49 genes form distinct sub-families and an ancestral haplotype can be surmised. Dotplot analysis supports limited allelism in the two haplotypes; however, large regions of variation punctuate these islands of co-linearity. These variable regions contain a high concentration of repetitive elements that are predicted to contribute to the dynamic evolution of this cluster. The extreme variation in Ly49 haplotype content between mouse strains provides a genetic explanation for the documented differences in natural killer cell phenotype, and also indicates that differences in natural killer cell function observed between B6 and 129-derived gene-targeted mice should be interpreted with caution.

Identification of probabilistic transcriptional switches in the Ly49 gene cluster: a eukaryotic mechanism for selective gene activation

Murine natural killer cells selectively express members of the Ly49 family of class I MHC receptors; however, the molecular mechanism controlling probabilistic expression of Ly49 proteins has not been defined. A pair of overlapping, divergent promoters discovered in the Ly49g gene functions as a molecular switch that can produce a forward transcript containing the coding region of the gene (on position) or a noncoding transcript in the opposite direction (off position), and this element maintains transcription in the chosen direction. Competition of C/EBP and TBP transcription factors for overlapping binding sites determines the relative strength of the competing promoters and the probability of transcription in a given direction. Similar elements precede all Ly49 family members, and the relative strength of the forward promoter in each inhibitory Ly49 gene correlates with the percentage of natural killer cells that express a given receptor, supporting a promoter competition model of selective gene activation.

Independent Control ofLy49gAlleles: Implications for NK Cell Repertoire Selection and Tumor Cell Killing

A novel murine NK cell-reactive mAb, AT8, was generated. AT8 recognizes Ly49G from 129/J, BALB/c, and related mouse strains, but does not bind to Ly49G(B6). Costaining with AT8 and a Ly49G(B6)-restricted Ab (Cwy-3) provides the first direct evidence that Ly49G protein is expressed from both alleles on a significant proportion of NK cells from four different types of F(1) hybrid mice. The observed level of biallelic Ly49G expression reproducibly followed the product rule in both freshly isolated and cultured NK cells. Surprisingly, the percentage of NK cells expressing both Ly49G alleles could be dramatically increased in vitro and in vivo through IL-2R- and IFN receptor-dependent signaling pathways, respectively. Unexpectedly, Ly49G(B6+) NK cells in an H-2(d), but not H-2(b), background were more likely to lyse D(d+) and Chinese hamster ovary tumor cells than Ly49G(BALB/129+) NK cells. Furthermore, Ly49G(B6+) NK cells also proliferated to a higher degree in response to poly(I:C) than NK cells expressing a non-Ly49G(B6) allele in an H-2(d), but not H-2(b), background. These results suggest that Ly49G(B6) has a lower affinity for H-2D(d) than Ly49G(BALB/129), and the genetic background calibrates the responsiveness of NK cells bearing self-specific Ly49. Other H-2D(d) receptors on the different Ly49G(+) NK cell subsets were unequally coexpressed, possibly explaining the disparate responses of Ly49G(B6+) NK cells in different hybrid mice. These data indicate that the stochastic mono- and biallelic expression of divergent Ly49G alleles increases the range of MHC affinities and the functional potential in the total NK cell population of heterozygous mice.

Receptor glycosylation regulates Ly-49 binding to MHC class I

Murine NK cells express the Ly-49 family of class I MHC-binding receptors that control their ability to lyse tumor or virally infected host target cells. X-ray crystallography studies have identified two predominant contact sites (sites 1 and 2) that are involved in the binding of the inhibitory receptor, Ly-49A, to H-2D(d). Ly-49G2 (inhibitory) and Ly-49D (activating) are highly homologous to Ly-49A and also recognize H-2D(d). However, the binding of Ly-49D and G(2) to H-2D(d) is of lower affinity than Ly-49A. All Ly-49s contain N-glycosylation motifs; however, the importance of receptor glycosylation in Ly-49-class I interactions has not been determined. Ly-49D and G(2) contain a glycosylation motif (NTT (221-223)), absent in Ly-49A, adjacent to one of the proposed binding sites for H-2D(d) (site 2). The presence of a complex carbohydrate group at this critical site could interfere with class I binding. In this study, we are able to demonstrate for the first time that Ly-49D binds H-2D(d) in the presence of mouse beta(2)-microglobulin. We also demonstrate that glycosylation of the NTT (221-23) motif of Ly-49D inteferes with recognition of H-2D(d). Alteration of the Ly-49D-NTT (221-23) motif to abolish glycosylation at this site resulted in enhanced H-2D(d) binding and receptor activation. Furthermore, glycosylation of Ly-49G2 at NTT (221-23) also reduces receptor binding to H-2D(d) tetramers. Therefore, the addition of complex carbohydrates to the Ly-49 family of receptors may represent a mechanism by which NK cells regulate affinity for host class I ligands.

Regulation of natural killer cell function

Individuals lacking natural killer (NK) cells have persistent viral infections and as a consequence die prematurely. In addition, mice with decreased NK cell function are more susceptible to carcinogen-induced cancers. Current evidence strongly suggests that downregulation of MHC by certain tumors and virally-infected cells results in NK cell attack due to the inability to trigger inhibitory Ly49, KIR, and NKG2A/CD94 class Ia and Ib MHC receptors. Extreme haplotype diversity is present in both mouse and human chromosomal segments coding for NK cell class Ia MHC receptors resulting in different numbers and types of receptors being expressed in individuals and different inbred mouse strains. Whether the absence or presence of a particular NK cell receptor gene is advantageous or deleterious for an individual with respect to immunity to pathogens and cancer is a question of paramount importance. Recent advances in our understanding of NK cell function are due to the identification of activating NK cell receptors, such as Ly49H and NKG2D, for specific viral and tumor ligands (m157 and Rae1, respectively). In a clinical setting, such MHC class I receptor diversity is advantageous with respect to preventing leukemic relapse in individuals treated for leukemia and receiving bone marrow transplants. Further delineation of NK cell receptors and tumor ligands will help researchers to exploit the innate immune system to better treat such diseases.

Aberrant DAP12 signaling in the 129 strain of mice: implications for the analysis of gene-targeted mice

NK cells are implicated in antiviral responses, bone marrow transplantation and tumor immunosurveillance. Their function is controlled, in part, through the Ly49 family of class I binding receptors. Inhibitory Ly49s suppress signaling, while activating Ly49s (i.e., Ly49D) activate NK cells via the DAP12 signaling chain. Activating Ly49 signaling has been studied primarily in C57BL/6 mice, however, 129 substrains are commonly used in gene-targeting experiments. In this study, we show that in contrast to C57BL/6 NK cells, cross-linking of DAP12-coupled receptors in 129/J mice induces phosphorylation of DAP12 but not calcium mobilization or cytokine production. Consistent with poor-activating Ly49 function, 129/J mice reject bone marrow less efficiently than C57BL/6 mice. Sequence analysis of receptors and DAP12 suggests no structural basis for inactivity, and both the 129/J and C57BL/6 receptors demonstrate normal function in a reconstituted receptor system. Most importantly, reconstitution of Ly49D in 129/J NK cells demonstrated that the signaling deficit is within the NK cells themselves. These unexpected findings bring into question any NK analysis of 129/J, 129Sv, or gene-targeted mice derived from these strains before complete backcrossing, and provide a possible explanation for the differences observed in the immune response of 129 mice in a variety of models.

Identification of a novel Ly49 promoter that is active in bone marrow and fetal thymus

The analysis of several Ly49 genes has identified a tissue-specific promoter adjacent to the previously defined first exon. The current study reveals the presence of an additional Ly49 promoter (Pro-1) and two noncoding exons upstream of the previously defined promoter (Pro-2). DNA sequences homologous to Pro-1 are present 4-10 kb upstream of Pro-2 in all Ly49 genes examined, and Pro-1 transcripts were detected from the Ly49a, e, g, o, and v genes. Pro-1 activity can be detected in bone marrow, embryonic thymus, freshly isolated liver NK cells, and the murine LNK cell line, but it does not function in adult thymus, sorted NK-T cells, spleen NK cells, or the EL-4 T cell line, even though these cells express Ly49 proteins. Luciferase reporter assays identified a Pro-1 core promoter region that functions in the LNK cell line but not EL-4 cells. The novel promoter is not active in mature NK cells, suggesting that Pro-1 represents an early Ly49 promoter.

A BAC contig map of the Ly49 gene cluster in 129 mice reveals extensive differences in gene content relative to C57BL/6 mice

The murine Ly49 gene family is functionally analogous to the human killer cell Ig-like receptor (KIR) family of class I major histocompatibility complex (MHC) receptors. The number of KIR genes varies dramatically between individuals; however, the organization of the Ly49 genes has only been determined for the C57BL/6 (B6) mouse. The organization of the 129 Ly49 loci was determined from a BAC contig map by PCR and Southern blot analysis. In addition to the 10 Ly49 genes known from previous studies of the 129/J strain, 8 new genes were localized to the 129 Ly49 cluster. A gene order of Ly49q(1), e, (v, q(2)), e/c(2), l/r, s, t, e/c(1), r, u, u/i, i(1), g, p/d, (i(2), p), and o was determined. The 129 Ly49 gene cluster is predicted to span approximately 600 kb. These results indicate that Ly49 gene numbers can be significantly different between inbred mouse strains, analogous to the haplotype differences observed in the human KIR genes.

The murine Ly49 family: form and function

The activity of natural killer (NK) cells is regulated by surface receptors that recognize class I MHC. Murine NK cells express a large family of lectin-related receptors (Ly49s) to perform this function, while human NK cells utilize a separate group of proteins containing Ig-related domains (KIRs). Although these receptor families are not structurally related, the Ly49 family appears to be the functional equivalent of human KIRs, since it uses similar signal transduction pathways for either activation or inhibition of NK cell function. Therefore, lessons learned from the study of the murine MHC class I receptor system may be relevant to human NK function. This review summarizes the current state of knowledge of the Ly49 family.

The ever-expanding Ly49 gene family: repertoire and signaling

The mouse lectin-related Ly49 family and the human killer cell Ig-like receptor (KIR) family represent structurally distinct, yet functionally analogous, class I MHC receptors that are expressed on natural killer cells and some T cells. The functional similarity of these two families has been borne out by the demonstration of identical signal transduction pathways associated with each receptor family. The Ly49 family therefore provides a useful model system to study the role of this dass of receptors in the regulation of the immune system. Recent data relating to the Ly49 repertoire in several mouse strains has revealed an additional evolutionary parallel between KIR and Ly49 receptor families. There is now an appreciation of the variation in the number and type of Ly49s expressed in different mouse strains, similar to the previously demonstrated differences in the number of KIR genes found in humans. This review summarizes the current members of the Ly49 gene family, their MHC class I recognition and associated signal transduction pathways.

Class I MHC-binding characteristics of the 129/J Ly49 repertoire

The Ly49 family of NK cell receptors and its MHC-binding characteristics have only been well characterized in C57BL/6 (B6) mice. Previous studies have shown that 129/J mice express unique Ly49 genes that are not found in the B6 strain. Screening of a 129/J cDNA library led to the discovery of 10 distinct full-length Ly49-related coding sequences (Ly49e, g, i, o, p, r, s, t, u, and v). Although 129/J mice share identical class I MHC (K(b) and D(b)) transcripts with B6 mice, only one Ly49 is identical in the two strains (Ly49E). In addition to the previously characterized Ly49P, two new activating Ly49 proteins were discovered, Ly49R and U. The MHC specificity of the total 129/J Ly49 repertoire was evaluated with soluble class I MHC tetramers and found to be distinct compared with the B6 Ly49 repertoire. Ly49V bound to many types of class I MHC, suggesting that Ly49V(+) NK cells may monitor host cells for a global down-regulation in MHC levels. An activating receptor, Ly49R, was shown to bind soluble class I molecules to a moderate degree, a result not previously observed for other activating Ly49 proteins. Furthermore, tetramer-binding results were confirmed functionally with cytotoxicity assays using sorted 129/J NK cells. This study shows that the Ly49 repertoire and its MHC-binding characteristics can be very different among inbred mouse strains. Ly49 divergence should be considered when using 129-derived embryonic stem cells for the production of gene-targeted mice, especially when an immune or NK-derived phenotype is under scrutiny.

Unusual expression of human lymphocyte antigen class II in normal renal microvascular endothelium

BACKGROUND

The human lymphocyte antigen (HLA) class II proteins (DR, DQ, and DP) and DM, a protein involved in loading antigenic peptide onto the class II molecules, have a coordinate regulation that facilitates antigen presentation to CD4+ T cells. CIITA is a specific transcription factor responsible for the coordinate regulation of these genes. DR expression in the kidney was described to be constitutive on renal microvascular endothelium in the early 1980s, but expression of other genes involved in class II antigen presentation (DQ, DP, DM, and CIITA) has not been characterized.

METHODS

Expression of the HLA class II proteins, DM, and CIITA in normal human kidney cortex was evaluated by immunofluorescence microscopy, Northern blots, and reverse transcriptase-polymerase chain reaction (RT-PCR).

RESULTS

The endothelium of glomerular and peritubular capillaries constitutively express DR, as indicated by colocalization of DR and CD31 antibodies. However, the endothelium of larger renal blood vessels is devoid of class II proteins. Capillaries that express DR do not have detectable DQ, DP, or DM by immunofluorescence. Northern blots identified DR, DP, and DM mRNAs but not DQ mRNA. CIITA was amplified by RT-PCR at a level that could account for the class II expressed by the microvascular endothelium.

CONCLUSION

The renal microvascular endothelium constitutively expresses DR without the other class II proteins or DM. This discoordinate expression of HLA class II genes is unusual and may contribute to the kidney's ability to control CD4+ T-cell responses.

Identification of the Ly49L protein: evidence for activating counterparts to inhibitory Ly49 proteins

Previous studies have indicated that NK cells from different strains of inbred mice may express distinct Ly49 repertoires. Screening of NK cells from the CBA/J mouse for inhibitory and activating Ly49s revealed a novel DAP12-associated receptor that was immunoprecipitated with the Ly49G-specific mAb 4D11. Degenerate primers were designed to amplify and clone Ly49 cDNAs from CBA/J NK cells. A novel activating Ly49 cDNA was identified, which bears strong homology to the partially sequenced Ly49l gene found in C57BL/6 mice. Transfection of Ly49l into a DAP12+ cell line and subsequent immunoprecipitation experiments showed that Ly49L is likely the activating Ly49 detected by the 4DD11 antibody in CBA/J NK cells. Antibody-mediated cross-linking of Ly49L induced DAP12 phosphorylation, providing evidence that Ly49L is a functional activating receptor. Comparison of the extracellular domains of Ly49 family members indicates that all known activating members have an inhibitory counterpart with a highly related extracellular region.

Ly49 gene expression in different inbred mouse strains

Mouse natural killer cells express receptors for class I MHC in the form of the Ly49 family of proteins. The Ly49 family contains at least 13 expressed members (A, B, C, D, E, F, G, H, I, J, L, O, and P) and is further subdivided into activating and inhibitory subfamilies based on intracellular and transmembrane characteristics. The level of sequence identity between different members varies dramatically. However, comparison of the extracellular domain has revealed that several of the Ly49 molecules also form "pairs," where one member is activating and the other is inhibitory. Until recently, most Ly49 molecules described have come from the C57B1/6 strain of inbred mice. Using molecular cloning and immunochemical analysis we have found that different mouse strains express novel Ly49 molecules. Comparison of the allelic forms of some Ly49 molecules has shown that the dividing line between different genes and different alleles is blurred.

Thymic vasculature: organizer of the medullary epithelial compartment?

The epithelial component of the thymic environment is organized into discrete cortical and medullary compartments that mediate different aspects of thymocyte differentiation. The processes controlling the growth and organization of these epithelial compartments are poorly defined. In this study we have used a novel approach to define the three-dimensional organization of thymic epithelial (TE) compartments to demonstrate that the organization of the medullary TE compartment is very complex. A spatial relationship of medullary thymic epithelium with vascular elements of the thymus was demonstrated by simultaneous immunohistochemical labeling of vascular elements and medullary TE. Medullary TE was often arranged as perivascular cuffs surrounding intermediate-sized vessels, but was not associated with either the capillary network or large centrally located vessels. Similar analyses of RAG-2(-/-) thymi revealed a striking physical association of medullary TE with vascular elements. Ultrastructural analysis of the RAG-2(-/-) thymus indicated a preferential association of focal accumulations of medullary TE with post-capillary venules. These data suggest that discrete segments of the thymic vasculature provide cues that act in concert with thymocyte-derived stimuli to effect normal development of the thymic environment.

Characterization of the Ly49I promoter

Fourteen potential Ly49 genes have been identified in the C57B1/6 mouse strain, and cDNAs containing a complete coding region have been isolated for 10 members of this gene family. Ly49 proteins are primarily expressed in natural killer (NK) cells. Although the sequence of the Ly49a promoter region has been published, no study of the cell-specific activity of the promoter has been reported. A 12-kb genomic fragment of the Ly49I gene was isolated and characterized by DNA sequencing. Approximately 5 kb of DNA sequence upstream of the first Ly49I exon was determined and this region was used to perform promoter analysis using luciferase reporter plasmid constructs. A core promoter was identified that was preferentially transcribed in a Ly49-expressing cell line, EL-4. Electrophoretic mobility shift assays using oligonucleotide probes from the core Ly49i promoter and comparable regions from the Ly49a promoter demonstrated the importance of TATA-related elements in generating EL-4 and NK cell-specific DNA/protein complexes.

Structure/function relationship of activating Ly-49D and inhibitory Ly-49G2 NK receptors

Murine NK cells express Ly-49 family receptors capable of either inhibiting or activating lytic function. The overlapping patterns of expression of the various receptors have complicated their precise biochemical characterization. Here we describe the use of the Jurkat T cell line as the model for the study of Ly-49s. We demonstrate that Ly-49D is capable of delivering activation signals to Jurkat T cells even in the absence of the recently described Ly-49D-associated chain, DAP-12. Ly-49D signaling in Jurkat leads to tyrosine phosphorylation of TCRzeta and requires Syk/Zap70 family kinases and arginine 54 of Ly-49D, suggesting that Ly-49D signals via association with TCRzeta. Coexpression studies in 293-T cells confirmed the ability of Ly-49D to associate with TCRzeta. In addition, we have used this model to study the functional interactions between an inhibitory Ly-49 (Ly-49G2) and an activating Ly-49 (Ly-49D). Ly-49G2 blocks activation mediated by Ly-49D in an immunoreceptor tyrosine-based inhibitory motif (ITIM)-dependent manner. In contrast, Ly-49G2 was incapable of inhibiting activation by the TCR even though human killer cell inhibitory receptor (KIR) (KIR3DL2(GL183)) effectively inhibits TCR. Both the ability of Ly-49G2 to block Ly-49D activation and the failure of Ly-49G2 to inhibit TCR signaling were confirmed in primary murine NK cells and NK/T cells, respectively. These data demonstrate the dominant effects of the inhibitory receptors over those that activate and suggest an inability of the Ly-49 type II inhibitory receptors to efficiently inhibit type I transmembrane receptor signaling in T cells and NK cells.

Structure/Function Relationship of Activating Ly-49D and Inhibitory Ly-49G2 NK Receptors

Murine NK cells express Ly-49 family receptors capable of either inhibiting or activating lytic function. The overlapping patterns of expression of the various receptors have complicated their precise biochemical characterization. Here we describe the use of the Jurkat T cell line as the model for the study of Ly-49s. We demonstrate that Ly-49D is capable of delivering activation signals to Jurkat T cells even in the absence of the recently described Ly-49D-associated chain, DAP-12. Ly-49D signaling in Jurkat leads to tyrosine phosphorylation of TCRζ and requires Syk/Zap70 family kinases and arginine 54 of Ly-49D, suggesting that Ly-49D signals via association with TCRζ. Coexpression studies in 293-T cells confirmed the ability of Ly-49D to associate with TCRζ. In addition, we have used this model to study the functional interactions between an inhibitory Ly-49 (Ly-49G2) and an activating Ly-49 (Ly-49D). Ly-49G2 blocks activation mediated by Ly-49D in an immunoreceptor tyrosine-based inhibitory motif (ITIM)-dependent manner. In contrast, Ly-49G2 was incapable of inhibiting activation by the TCR even though human killer cell inhibitory receptor (KIR) (KIR3DL2(GL183)) effectively inhibits TCR. Both the ability of Ly-49G2 to block Ly-49D activation and the failure of Ly-49G2 to inhibit TCR signaling were confirmed in primary murine NK cells and NK/T cells, respectively. These data demonstrate the dominant effects of the inhibitory receptors over those that activate and suggest an inability of the Ly-49 type II inhibitory receptors to efficiently inhibit type I transmembrane receptor signaling in T cells and NK cells.

Cloning and characterization of a novel activating Ly49 closely related to Ly49A

The majority of the known Ly49 family members have been isolated from either C57BL/6 (B6) or BALB/c mice. Interestingly, the anti-Ly49 Ab reactivities observed in 129/J mice are different from those of B6 mice. Furthermore, immunoprecipitation of 129/J NK cell lysates with YE1/32 and YE1/48, Abs specific for the inhibitory Ly49A in B6, resulted in detection of the activation-associated DAP12 molecule. These results indicated a need for a more detailed study of this strain. Therefore, a cloning strategy was devised to isolate Ly49 cDNAs from 129/J mice. An immunoreceptor tyrosine-based inhibitory motif-containing, Ly49D-related clone was discovered that we have named Ly49O, and one immunoreceptor tyrosine-based inhibitory motif-lacking, Ly49A-related clone was discovered that we have named Ly49P. No anti-Ly49 mAb reacted with Ly49O, whereas the molecule encoded by the Ly49P cDNA was found to react with YE1/32 and YE1/48. Ly49P was found to associate with mouse DAP12, and Ab-mediated cross-linking of Ly49P resulted in mouse DAP12 phosphorylation and Ca2+ mobilization, indicating that Ly49P is a competent activation receptor. Ly49P, therefore, represents a novel member of the Ly49 activating receptor subfamily.

Cloning and Characterization of a Novel Activating Ly49 Closely Related to Ly49A

The majority of the known Ly49 family members have been isolated from either C57BL/6 (B6) or BALB/c mice. Interestingly, the anti-Ly49 Ab reactivities observed in 129/J mice are different from those of B6 mice. Furthermore, immunoprecipitation of 129/J NK cell lysates with YE1/32 and YE1/48, Abs specific for the inhibitory Ly49A in B6, resulted in detection of the activation-associated DAP12 molecule. These results indicated a need for a more detailed study of this strain. Therefore, a cloning strategy was devised to isolate Ly49 cDNAs from 129/J mice. An immunoreceptor tyrosine-based inhibitory motif-containing, Ly49D-related clone was discovered that we have named Ly49O, and one immunoreceptor tyrosine-based inhibitory motif-lacking, Ly49A-related clone was discovered that we have named Ly49P. No anti-Ly49 mAb reacted with Ly49O, whereas the molecule encoded by the Ly49P cDNA was found to react with YE1/32 and YE1/48. Ly49P was found to associate with mouse DAP12, and Ab-mediated cross-linking of Ly49P resulted in mouse DAP12 phosphorylation and Ca2+ mobilization, indicating that Ly49P is a competent activation receptor. Ly49P, therefore, represents a novel member of the Ly49 activating receptor subfamily.

Comparison of capillary zone and immunosubtraction with agarose gel and immunofixation electrophoresis for detecting and identifying monoclonal gammopathies

Capillary zone electrophoresis (CZE) and immuno-subtraction electrophoresis (ISE) were evaluated for ability to detect and immunotype monoclonal proteins, compared with agarose gel electrophoresis (AGE) and immunofixation electrophoresis (IFE), respectively. Six hundred seventeen serum samples were analyzed with CZE and AGE to determine sensitivity and specificity in detecting IFE-confirmed monoclonal gammopathies. Both techniques detected all monoclonal spikes due to IgM (n = 8), IgG (n = 38), and free light chains (n = 3). Agarose gel electrophoresis, however, detected only 11 of 14 (79%) IgA monoclonal spikes detected with CZE. In a second study, 78 serum samples, 48 of which had a monoclonal gammopathy confirmed with IFE, were evaluated with ISE. Only 60% to 75% of the monoclonal gammopathies were correctly immunotyped with ISE by 4 readers blinded to the IFE immunotype. Thus CZE was more sensitive than AGE in detecting low concentrations of monoclonal proteins, but ISE is less accurate than IFE in determining the immunotype of the monoclonal gammopathy.

Induction of DAP12 phosphorylation, calcium mobilization, and cytokine secretion by Ly49H

The ability of several Ly49 family members to inhibit natural killer (NK) cell functions through recruitment of SHP-1 phosphatase has been reported. In contrast, the mechanisms underlying the activating signal generated by Ly49D are poorly understood. A homodimeric phosphoprotein (pp16) that physically and functionally associates with Ly49D has been described. In this study, a rabbit anti-mouse pp16 antiserum was generated and used to demonstrate that pp16 corresponds to the recently described DAP12 molecule. In addition, we show that a second Ly49 family member that lacks an immunoreceptor tyrosine-based inhibitory motif and contains a charged residue in the transmembrane domain, Ly49H, also associates with DAP12. Furthermore, we show that engagement of the Ly49H/DAP12 complex results in phosphorylation of DAP12, intracellular calcium mobilization, and tumor necrosis factor secretion in transfected cells. These results thus provide evidence that Ly49H is an activating receptor that associates with DAP12, previously described as a pp16 component of the Ly49D receptor complex.

Role of a STAT binding site in the regulation of the human perforin promoter

The pore-forming protein perforin is preferentially expressed in NK and cytotoxic T cells. To investigate the molecular regulation of human perforin gene transcription, the activity of the human perforin promoter was analyzed in human NK and T cell lines using various promoter fragments linked to a luciferase reporter gene. A core promoter was identified within 55 bp upstream of the transcription start site. This promoter region contains a guanine/cytosine box and has basal activity in YT, Kit225-k6, and Jurkat cells. A strong enhancer activity was identified between positions -1136 and -1076, a region that includes a STAT-like element. This enhancer region was active in YT cells, which have constitutive perforin expression and activated STAT3 protein, but not in Kit225-k6 or Jurkat cells, which do not have constitutive perforin expression. Mutation of the STAT binding site resulted in a dramatic down-regulation of promoter activity. Electrophoretic mobility shift assays, using a probe containing the STAT element of the perforin promoter, indicated that this element can bind STAT3 from YT cells. Moreover, the STAT element was shown to bind STAT5a/b induced by IL-2 as well as STAT1alpha induced by IL-6 in human NK cells. Together, these results suggest that STAT proteins play a key role in perforin gene transcription and provide a model by which cytokines can regulate perforin gene expression.

Role of a STAT Binding Site in the Regulation of the Human Perforin Promoter

The pore-forming protein perforin is preferentially expressed in NK and cytotoxic T cells. To investigate the molecular regulation of human perforin gene transcription, the activity of the human perforin promoter was analyzed in human NK and T cell lines using various promoter fragments linked to a luciferase reporter gene. A core promoter was identified within 55 bp upstream of the transcription start site. This promoter region contains a guanine/cytosine box and has basal activity in YT, Kit225-k6, and Jurkat cells. A strong enhancer activity was identified between positions −1136 and −1076, a region that includes a STAT-like element. This enhancer region was active in YT cells, which have constitutive perforin expression and activated STAT3 protein, but not in Kit225-k6 or Jurkat cells, which do not have constitutive perforin expression. Mutation of the STAT binding site resulted in a dramatic down-regulation of promoter activity. Electrophoretic mobility shift assays, using a probe containing the STAT element of the perforin promoter, indicated that this element can bind STAT3 from YT cells. Moreover, the STAT element was shown to bind STAT5a/b induced by IL-2 as well as STAT1α induced by IL-6 in human NK cells. Together, these results suggest that STAT proteins play a key role in perforin gene transcription and provide a model by which cytokines can regulate perforin gene expression.

DAP12-mediated signal transduction in natural killer cells. A dominant role for the Syk protein-tyrosine kinase

The murine Ly49 family contains nine genes in two subgroups: the inhibitory receptors (Ly49A, B, C, E, F, G2, and I) and the noninhibitory receptors (Ly49D and H). Unlike their inhibitory counterparts, Ly49D and H do not contain immunoreceptor tyrosine-based inhibitory motifs but associate with a recently described co-receptor, DAP12, to transmit positive signals to natural killer (NK) cells. DAP12 is also expressed in myeloid cells, but the receptors coupled to it there are unknown. Here we document the signaling pathways of the Ly49D/DAP12 complex in NK cells. We show that ligation of Ly49D results in 1) tyrosine phosphorylation of several substrates, including phospholipase Cgamma1, Cbl, and p44/p42 mitogen-activated protein kinase, and 2) calcium mobilization. Moreover, we demonstrate that although human DAP12 reportedly binds the SH2 domains of both Syk and Zap-70, ligation of Ly49D leads to activation of Syk but not Zap-70. Consistent with this observation, Ly49D/DAP12-mediated calcium mobilization is blocked by dominant negative Syk but not by catalytically inactive Zap-70. These data demonstrate the dependence of DAP12-coupled receptors on Syk and suggest that the outcome of Ly49D/DAP12 engagement will be regulated by Cbl and culminate in the activation of transcription factors.

Phenotypic and functional characterization of a panel of cytotoxic murine NK cell clones that are heterogeneous in their enhancement of Ig secretion in vitro

NK cells not only function as cytotoxic effector cells, but also have immunoregulatory roles including the enhancement of Ig secretion. To have a stable and uniform population of NK cells to study their role in Ig secretion, we generated murine NK clones. Thus, culture of splenocytes from mice that were homozygous for a mutation in the p53 tumor suppressor gene (p53-KO) with IL-2 and poly(IC) resulted in a long-term NK line, from which four stable clones were derived. This approach also yielded a long-term NK line from splenocytes of normal C57BL/6 mice. Identification of the clones as members of the NK lineage was based on large granular morphology, expression of NK-TR and absence of TCR gene rearrangement. Flow cytometry revealed that all clones expressed IL-2R alpha and beta, chains and B220, but no CD3, NK1.1, DX5 or Ly-49. RT-PCR analysis showed heterogeneity in NK1.1 gene expression, and demonstrated expression of perforin and several granzymes in all clones. Three out of four clones lysed YAC-1, but not P815 target cells, corresponding to a pattern of NK specificity. All NK clones enhanced Ig secretion in an in vitro model for T cell-independent type 2 antigens, albeit to varying degrees. We found no correlation between the degree of helper activity of the NK clones and the level of their cytotoxic activity on YAC-1 targets. Thus, we established murine NK clones, and show that they mediate both cytotoxicity and enhancement of Ig secretion.

Characterization of an associated 16-kDa tyrosine phosphoprotein required for Ly-49D signal transduction

Ly-49D is an activating receptor on NK cells that does not become tyrosine phosphorylated upon activation. This report demonstrates that immunoprecipitation of Ly-49D, following pervanadate treatment or specific Ab cross-linking, coprecipitates a 16-kDa tyrosine-phosphorylated protein (pp16). Immunoblotting experiments and data from TCR-zeta/Fc epsilonRIgamma double knockout mice confirm that pp16 is not TCR-zeta, TCR-eta, or Fc epsilonRIgamma. Association of pp16 with Ly-49D involves a transmembrane arginine since mutation to leucine (Ly-49D[R54L]) abolishes association with pp16 in transfected P815 cells. In addition, Ly-49D(R54L) transfectants fail to mediate Ca2+ mobilization following Ab cross-linking. Therefore, signaling through Ly49D on NK cells depends on association with a distinct tyrosine phosphoprotein (pp16) in a manner analogous to that of TCR and FcR. Expression of this novel signaling peptide in both the NK and myeloid lineages indicates that pp16 is likely involved in the signal transduction cascade of additional receptor families.