Mouse Ly-49A interrupts early signaling events in natural killer cell cytotoxicity and functionally associates with the SHP-1 tyrosine phosphatase

Influence of Self-MHC Class I Recognition on the Dynamics of NK Cell Responses to Cytomegalovirus Infection

Although interactions between inhibitory Ly49 receptors and their self-MHC class I ligands in C57BL/6 mice are known to limit NK cell proliferation during mouse CMV (MCMV) infection, we created a 36-marker mass cytometry (CyTOF) panel to investigate how these inhibitory receptors impact the NK cell response to MCMV in other phenotypically measurable ways. More than two thirds of licensed NK cells (i.e., those expressing Ly49C, Ly49I, or both) in uninfected mice had already differentiated into NK cells with phenotypes indicative of Ag encounter (KLRG1+Ly6C-) or memory-like status (KLRG1+Ly6C+). These pre-existing KLRG1+Ly6C+ NK cells resembled known Ag-specific memory NK cell populations in being less responsive to IL-18 and IFN-α stimulation in vitro and by selecting for NK cell clones with elevated expression of a Ly49 receptor. During MCMV infection, the significant differences between licensed and unlicensed (Ly49C-Ly49I-) NK cells disappeared within both CMV-specific (Ly49H+) and nonspecific (Ly49H-) responses. This lack of heterogeneity carried into the memory phase, with only a difference in CD16 expression manifesting between licensed and unlicensed MCMV-specific memory NK cell populations. Our results suggest that restricting proliferation is the predominant effect licensing has on the NK cell population during MCMV infection, but the inhibitory Ly49-MHC interactions that take place ahead of infection contribute to their limited expansion by shrinking the pool of licensed NK cells capable of robustly responding to new challenges.

Genetic manipulation of primary human natural killer cells to investigate the functional and oncogenic roles of PRDM1

Extra-nodal natural killer (NK)/T-cell lymphoma, nasal type (ENKTCL) is a highly aggressive lymphoma, in which the tumor suppressor gene PRDM1 is frequently lost or inactivated. We employed two different CRISPR/Cas9 approaches to generate PRDM1-/- primary NK cells to study the role of this gene in NK-cell homeostasis. PRDM1-/- NK cells showed a marked increase in cloning efficiency, higher proliferation rate and less apoptosis compared with their wild-type counterparts. Gene expression profiling demonstrated a marked enrichment in pathways associated with proliferation, cell cycle, MYC, MYB and TCR/NK signaling in PRDM1-/- NK cells, but pathways associated with normal cellular functions including cytotoxic functions were downregulated, suggesting that the loss of PRDM1 shifted NK cells toward proliferation and survival rather than the performance of their normal functions. We were also able to further modify a PRDM1-deleted clone to introduce heterozygous deletions of common tumor suppressor genes in ENKTCL such as TP53, DDX3X, and PTPN6. We established an in vitro model to elucidate the major pathways through which PRDM1 mediates its homeostatic control of NK cells. This approach can be applied to the study of other relevant genetic lesions and oncogenic collaborations in lymphoma pathogenesis.

Licensing Natural Killers for Antiviral Immunity

Immunoreceptor tyrosine-based inhibitory motif (ITIM)-bearing receptors (IRs) enable discrimination between self- and non-self molecules on the surface of host target cells. In this regard, they have a vital role in self-tolerance through binding and activating intracellular tyrosine phosphatases which can inhibit cellular activation. Yet, self-MHC class I (MHC I)-specific IRs are versatile in that they can also positively impact lymphocyte functionality, as exemplified by their role in natural killer (NK) cell education, often referred to as ’licensing‘. Recent discoveries using defined mouse models of cytomegalovirus (CMV) infection have revealed that select self-MHC I IRs can increase NK cell antiviral defenses as well, whereas other licensing IRs cannot, or instead impede virus-specific NK responses for reasons that remain poorly understood. This review highlights a role for self-MHC I ‘licensing’ IRs in antiviral immunity, especially in the context of CMV infection, their impact on virus-specific NK cells during acute infection, and their potential to affect viral pathogenesis and disease.

Natural Killer Cells: Tumor Surveillance and Signaling

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

Regulation of MHC class I-independent NK cell education by SLAM family receptors

Seven members of signaling lymphocytic activation molecule (SLAM) family receptors (SFRs) are ubiquitously expressed on hematopoietic cells and they play critical roles in immune cell differentiation and activation. The engagement of these receptors transmits intracellular signaling mainly by recruiting SLAM-associated protein (SAP) and its related adaptors, EWS-FLI1-activated transcript-2 (EAT-2) and EAT-2-related transducer (ERT). The critical roles of SFRs and SAP-family adaptors are highlighted by the discovery that SAP is mutated in human X-linked lymphoproliferative (XLP1) disease in which the contact between T and B cells in germinal center and cytotoxic lymphocytes (NK cells and CD8⁺ T cells) function are severely compromised. These immune defects are closely associated with the defective antibody production and the high incidence of lymphoma in the patients with XLP1. In addition to these well-known functions, SLAM-SAP family is involved in NK cell education, a process describing NK cell functional competence. In this chapter, we will mainly discuss these unappreciated roles of SAP-dependent and SAP-independent SFR signaling in regulating MHC-I-independent NK cell education.

The Role of Adaptor Proteins in the Biology of Natural Killer T (NKT) Cells

Adaptor proteins contribute to the selection, differentiation and activation of natural killer T (NKT) cells, an innate(-like) lymphocyte population endowed with powerful immunomodulatory properties. Distinct from conventional T lymphocytes NKT cells preferentially home to the liver, undergo a thymic maturation and differentiation process and recognize glycolipid antigens presented by the MHC class I-like molecule CD1d on antigen presenting cells. NKT cells express a semi-invariant T cell receptor (TCR), which combines the Vα14-Jα18 chain with a Vβ2, Vβ7, or Vβ8 chain in mice and the Vα24 chain with the Vβ11 chain in humans. The avidity of interactions between their TCR, the presented glycolipid antigen and CD1d govern the selection and differentiation of NKT cells. Compared to TCR ligation on conventional T cells engagement of the NKT cell TCR delivers substantially stronger signals, which trigger the unique NKT cell developmental program. Furthermore, NKT cells express a panoply of primarily inhibitory NK cell receptors (NKRs) that control their self-reactivity and avoid autoimmune activation. Adaptor proteins influence NKT cell biology through the integration of TCR, NKR and/or SLAM (signaling lymphocyte-activation molecule) receptor signals or the variation of CD1d-restricted antigen presentation. TCR and NKR ligation engage the SH2 domain-containing leukocyte protein of 76kDa slp-76 whereas the SLAM associated protein SAP serves as adaptor for the SLAM receptor family. Indeed, the selection and differentiation of NKT cells selectively requires co-stimulation via SLAM receptors. Furthermore, SAP deficiency causes X-linked lymphoproliferative disease with multiple immune defects including a lack of circulating NKT cells. While a deletion of slp-76 leads to a complete loss of all peripheral T cell populations, mutations in the SH2 domain of slp-76 selectively affect NKT cell biology. Furthermore, adaptor proteins influence the expression and trafficking of CD1d in antigen presenting cells and subsequently selection and activation of NKT cells. Adaptor protein complex 3 (AP-3), for example, is required for the efficient presentation of glycolipid antigens which require internalization and processing. Thus, our review will focus on the complex contribution of adaptor proteins to the delivery of TCR, NKR and SLAM receptor signals in the unique biology of NKT cells and CD1d-restricted antigen presentation.

The Protein Phosphatase Shp1 Regulates Invariant NKT Cell Effector Differentiation Independently of TCR and Slam Signaling

Invariant NKT (iNKT) cells are innate lipid-reactive T cells that develop and differentiate in the thymus into iNKT1/2/17 subsets, akin to T_H1/2/17 conventional CD4 T cell subsets. The factors driving the central priming of iNKT cells remain obscure, although strong/prolonged TCR signals appear to favor iNKT2 cell development. The Src homology 2 domain-containing phosphatase 1 (Shp1) is a protein tyrosine phosphatase that has been identified as a negative regulator of TCR signaling. In this study, we found that mice with a T cell-specific deletion of Shp1 had normal iNKT cell numbers and peripheral distribution. However, iNKT cell differentiation was biased toward the iNKT2/17 subsets in the thymus but not in peripheral tissues. Shp1-deficient iNKT cells were also functionally biased toward the production of T_H2 cytokines, such as IL-4 and IL-13. Surprisingly, we found no evidence that Shp1 regulates the TCR and Slamf6 signaling cascades, which have been suggested to promote iNKT2 differentiation. Rather, Shp1 dampened iNKT cell proliferation in response to IL-2, IL-7, and IL-15 but not following TCR engagement. Our findings suggest that Shp1 controls iNKT cell effector differentiation independently of positive selection through the modulation of cytokine responsiveness.

Protein Tyrosine Phosphatases: Regulators of CD4 T Cells in Inflammatory Bowel Disease

Protein tyrosine phosphatases (PTPs) play a critical role in co-ordinating the signaling networks that maintain lymphocyte homeostasis and direct lymphocyte activation. By dephosphorylating tyrosine residues, PTPs have been shown to modulate enzyme activity and both mediate and disrupt protein-protein interactions. Through these molecular mechanisms, PTPs ultimately impact lymphocyte responses to environmental cues such as inflammatory cytokines and chemokines, as well as antigenic stimulation. Mouse models of acute and chronic intestinal inflammation have been shown to be exacerbated in the absence of PTPs such as PTPN2 and PTPN22. This increase in disease severity is due in part to hyper-activation of lymphocytes in the absence of PTP activity. In accordance, human PTPs have been linked to intestinal inflammation. Genome wide association studies (GWAS) identified several PTPs within risk loci for inflammatory bowel disease (IBD). Therapeutically targeting PTP substrates and their associated signaling pathways, such as those implicated in CD4⁺ T cell responses, has demonstrated clinical efficacy. The current review focuses on the role of PTPs in controlling CD4⁺ T cell activity in the intestinal mucosa and how disruption of PTP activity in CD4⁺ T cells can contribute to intestinal inflammation.

Immunoreceptor tyrosine-based inhibitory motif-dependent functions of an MHC class I-specific NK cell receptor

Natural killer (NK) cells express MHC class I (MHC-I)-specific receptors, such as Ly49A, that inhibit killing of cells expressing self-MHC-I. Self-MHC-I also "licenses" NK cells to become responsive to activating stimuli and regulates the surface level of NK-cell inhibitory receptors. However, the mechanisms of action resulting from these interactions of the Ly49s with their MHC-I ligands, particularly in vivo, have been controversial. Definitive studies could be derived from mice with targeted mutations in inhibitory Ly49s, but there are inherent challenges in specifically altering a single gene within a multigene family. Herein, we generated a knock-in mouse with a targeted mutation in the immunoreceptor tyrosine-based inhibitory motif (ITIM) of Ly49A that abolished the inhibitory function of Ly49A in cytotoxicity assays. This mutant Ly49A caused a licensing defect in NK cells, but the surface expression of Ly49A was unaltered. Moreover, NK cells that expressed this mutant Ly49A exhibited an altered inhibitory receptor repertoire. These results demonstrate that Ly49A ITIM signaling is critical for NK-cell effector inhibition, licensing, and receptor repertoire development.

Ly49 receptors: evolution, genetic diversity, and impact on immunity

Natural killer (NK) cells express cell surface receptors that recognize class I major histocompatibility complex (MHC-I) molecules to distinguish between healthy and unhealthy cells. The multigenic and polymorphic nature of the MHC-I genes has influenced the convergent evolution of similarly polymorphic and diversified NK cell receptor families: the C-type lectin-like Ly49 receptors in mice, and the killer cell immunoglobulin-like receptors (KIRs) in humans. Although structurally distinct, both receptor families have similar functions in terms of MHC-I recognition and downstream signal transduction, and they regulate multiple aspects of NK cell biology during development and after maturation as fully differentiated and functionally competent cells. The Ly49 gene locus has undergone rapid, lineage-specific expansions and contractions resulting in multiple distinct haplotypes of variable gene number, allelic diversity, and MHC-I ligand specificity. This in turn has influenced the type and degree of Ly49 receptor expression on NK cells, and their contribution to immunity in different mouse strains. In this review, we have attempted to describe the evolutionary processes that have shaped strain-specific Ly49 receptor repertoires, and their impact on NK cell functions during health and disease.

The early days of NK cells: an example of how a phenomenon led to detection of a novel immune receptor system - lessons from a rat model

In this review, I summarize some of the early research on NK cell biology and function that led to the discovery of a totally new receptor system for polymorphic MHC class I molecules. That NK cells both could recognize and kill tumor cells but also normal hematopoietic cells through expression of MHC class I molecules found a unifying explanation in the “missing self” hypothesis. This initiated a whole new area of leukocyte receptor research. The common underlying mechanism was that NK cells expressed receptors that were inhibited by recognition of unmodified “self” MHC-I molecules. This could explain both the killing of tumor cells with poor expression of MHC-I molecules and hybrid resistance, i.e., that F1 hybrid mice sometimes could reject parental bone marrow cells. However, a contrasting phenomenon termed allogeneic lymphocyte cytotoxicity in rats gave strong evidence that some of these receptors were activated rather than inhibited by recognition of polymorphic MHC-I. This was soon followed by molecular identification of both inhibitory and stimulatory Ly49 receptors in mice and rats and killer cell immunoglobulin-like receptors in humans that could be either inhibited or activated when recognizing their cognate MHC-I ligand. Since most of these receptors now have been molecularly characterized, their ligands and the intracellular pathways leading to activation or inhibition identified, we still lack a more complete understanding of how the repertoire of activating and inhibitory receptors is formed and how interactions between these receptors for MHC-I molecules on a single NK cell are integrated to generate a productive immune response. Although several NK receptor systems have been characterized that recognize MHC-I or MHC-like molecules, I here concentrate on the repertoires of NK receptors encoded by the natural killer cell gene complex and designed to recognize polymorphic MHC-I molecules in rodents, i.e., Ly49 (KLRA) receptors.

Ly49 receptors: innate and adaptive immune paradigms

The Ly49 receptors are type II C-type lectin-like membrane glycoproteins encoded by a family of highly polymorphic and polygenic genes within the mouse natural killer (NK) gene complex. This gene family is designated Klra, and includes genes that encode both inhibitory and activating Ly49 receptors in mice. Ly49 receptors recognize class I major histocompatibility complex-I (MHC-I) and MHC-I-like proteins on normal as well as altered cells. Their functional homologs in humans are the killer cell immunoglobulin-like receptors, which recognize HLA class I molecules as ligands. Classically, Ly49 receptors are described as being expressed on both the developing and mature NK cells. The inhibitory Ly49 receptors are involved in NK cell education, a process in which NK cells acquire function and tolerance toward cells that express “self-MHC-I.” On the other hand, the activating Ly49 receptors recognize altered cells expressing activating ligands. New evidence shows a broader Ly49 expression pattern on both innate and adaptive immune cells. Ly49 receptors have been described on multiple NK cell subsets, such as uterine NK and memory NK cells, as well as NKT cells, dendritic cells, plasmacytoid dendritic cells, macrophages, neutrophils, and cells of the adaptive immune system, such as activated T cells and regulatory CD8⁺ T cells. In this review, we discuss the expression pattern and proposed functions of Ly49 receptors on various immune cells and their contribution to immunity.

Mutagenesis of Ly49B reveals key structural elements required for promiscuous binding to MHC class I molecules and new insights into the molecular evolution of Ly49s

Ly49B is a potentially important immunoregulator expressed on mouse myeloid cells, and it is thus an unusual member of the wider Ly49 family whose members are ordinarily found on NK cells. Ly49B displays substantial sequence divergence from other Ly49s and in particular shares virtually no amino acid sequence identity with the residues that have been reported to bind to MHC class I (cI) ligands in other Ly49s. Despite this, we show in this study that the BALB/c, but not the C57, isoform of Ly49B displays promiscuous cI binding. Binding was not significantly affected by inactivation of any of the four predicted N-linked glycosylation sites of Ly49B, nor was it affected by removal of the unique 20-aa C-terminal extension found in Ly49B. However, transfer of these C-terminal 20 aa to Ly49A inhibited cI binding, as did the addition of a hemagglutinin tag to the C terminus of Ly49B, demonstrating unexpectedly that the C-terminal region of Ly49s can play a significant role in ligand binding. Systematic exchange of BALB/c and C57 residues revealed that Trp(166), Asn(167), and Cys(251) are of major importance for cI binding in Ly49B. These residues are highly conserved in the Ly49 family. Remarkably, however, Ly49B(BALB) variants that have C57 residues at positions 166 or 167, and are unable to bind cI multimers, regain substantial cI binding when amino acid changes are made at distal positions, providing an explanation of how highly divergent Ly49s that retain the ability to bind cI molecules might have evolved.

Interferon response factor 3 is crucial to poly-I:C induced NK cell activity and control of B16 melanoma growth

Interferon Response Factor 3 (IRF3) induces several NK-cell activating factors, is activated by poly-I:C, an experimental cancer therapeutic, but is suppressed during many viral infections. IRF3 Knockout (KO) mice exhibited enhanced B16 melanoma growth, impaired intratumoral NK cell infiltration, but not an impaired poly-I:C therapeutic effect due to direct suppression of B16 growth. IRF3 was responsible for poly-I:C decrease in TIM-3 expression by intratumoral dendritic cells, induction of NK-cell Granzyme B and IFN-γ, and induction of macrophage IL-12, IL-15, IL-6, and IRF3-dependent NK-activating molecule (INAM). Thus, IRF3 is a key factor controlling melanoma growth through NK-cell activities, especially during poly-I:C therapy.

Shp1 regulates T cell homeostasis by limiting IL-4 signals

Absence of the phosphatase Shp1 in T cells does not affect the TCR signaling threshold but results in IL-4 sensitivity and memory phenotype cells.

The protein-tyrosine phosphatase Shp1 is expressed ubiquitously in hematopoietic cells and is generally viewed as a negative regulatory molecule. Mutations in Ptpn6, which encodes Shp1, result in widespread inflammation and premature death, known as the motheaten (me) phenotype. Previous studies identified Shp1 as a negative regulator of TCR signaling, but the severe systemic inflammation in me mice may have confounded our understanding of Shp1 function in T cell biology. To define the T cell–intrinsic role of Shp1, we characterized mice with a T cell–specific Shp1 deletion (Shp1^fl/fl CD4-cre). Surprisingly, thymocyte selection and peripheral TCR sensitivity were unaltered in the absence of Shp1. Instead, Shp1^fl/fl CD4-cre mice had increased frequencies of memory phenotype T cells that expressed elevated levels of CD44. Activation of Shp1-deficient CD4⁺ T cells also resulted in skewing to the Th2 lineage and increased IL-4 production. After IL-4 stimulation of Shp1-deficient T cells, Stat 6 activation was sustained, leading to enhanced Th2 skewing. Accordingly, we observed elevated serum IgE in the steady state. Blocking or genetic deletion of IL-4 in the absence of Shp1 resulted in a marked reduction of the CD44^hi population. Therefore, Shp1 is an essential negative regulator of IL-4 signaling in T lymphocytes.

Building tolerance by dismantling synapses: inhibitory receptor signaling in natural killer cells

Cell surface receptors bearing immunotyrosine-based inhibitory motifs (ITIMs) maintain natural killer (NK) cell tolerance to normal host tissues. These receptors are difficult to analyze mechanistically because they block activating responses in a rapid and comprehensive manner. The advent of high-resolution single cell imaging techniques has enabled investigators to explore the cell biological basis of the inhibitory response. Recent studies using these approaches indicate that ITIM-containing receptors function at least in part by structurally undermining the immunological synapse between the NK cell and its target. In this review, we discuss these new advances and how they might relate to what is known about the biochemistry of inhibitory signaling in NK cells and other cell types.

Adiponectin deficiency suppresses lymphoma growth in mice by modulating NK cells, CD8 T cells, and myeloid-derived suppressor cells

Previously, we found that adiponectin (APN) suppresses IL-2-induced NK cell activation by downregulating the expression of the IFN-γ-inducible TNF-related apoptosis-inducing ligand and Fas ligand. Although the antitumor function of APN has been reported in several types of solid tumors, with few controversial results, no lymphoma studies have been conducted. In this study, we assessed the role of APN in immune cell function, including NK cells, CTLs, and myeloid-derived suppressor cells, in EL4 and B16F10 tumor-bearing APN knockout (KO) mice. We observed attenuated EL4 growth in the APNKO mice. Increased numbers of splenic NK cells and splenic CTLs were identified under naive conditions and EL4-challenged conditions, respectively. In APNKO mice, splenic NK cells showed enhanced cytotoxicity with and without IL-2 stimulation. Additionally, there were decreased levels of myeloid-derived suppressor cell accumulation in the EL4-bearing APNKO mice. Enforced MHC class I expression on B16F10 cells led to attenuated growth of these tumors in APNKO mice. Thus, our results suggest that EL4 regression in APNKO mice is not only due to an enhanced antitumor immune response but also to a high level of MHC class I expression.

Ly49Q positively regulates type I IFN production by plasmacytoid dendritic cells in an immunoreceptor tyrosine-based inhibitory motif-dependent manner

Plasmacytoid dendritic cells (pDC) are the major producers of type I IFN during the initial immune response to viral infection. Ly49Q, a C-type lectin-like receptor specific for MHC-I, possesses a cytoplasmic ITIM and is highly expressed on murine pDC. Using Ly49Q-deficient mice, we show that, regardless of strain background, this receptor is required for maximum IFN-α production by pDC. Furthermore, Ly49Q expression on pDC, but not myeloid dendritic cells, is necessary for optimal IL-12 secretion, MHC-II expression, activation of CD4(+) T cell proliferation, and nuclear translocation of the master IFN-α regulator IFN regulatory factor 7 in response to TLR9 agonists. In contrast, the absence of Ly49Q did not affect plasmacytoid dendritic cell-triggering receptor expressed on myeloid cells expression or pDC viability. Genetic complementation revealed that IFN-α production by pDC is dependent on an intact tyrosine residue in the Ly49Q cytoplasmic ITIM. However, pharmacological inhibitors and phosphatase-deficient mice indicate that Src homology 2 domain-containing phosphatase 1 (SHP)-1, SHP-2, and SHIP phosphatase activity is dispensable for this function. Finally, we observed that Ly49Q itself is downregulated on pDC in response to CpG exposure in an ITIM-independent manner. In conclusion, Ly49Q enhances TLR9-mediated signaling events, leading to IFN regulatory factor 7 nuclear translocation and expression of IFN-I genes in an ITIM-dependent manner that can proceed without the involvement of SHP-1, SHP-2, and SHIP.

SHP-1 phosphatase is a critical regulator in preventing natural killer cell self-killing

Balance of signals generated from the engaged activating and inhibitory surface receptors regulates mature NK cell activities. The inhibitory receptors signal through immunoreceptor tyrosine based inhibitory motifs (ITIM), and recruit phosphatases such as SHP-1 to inhibit NK cell activation. To directly examine the importance of SHP-1 in regulating activities and cell fate of mature NK cells, we used our established lentiviral-based engineering protocol to knock down the SHP-1 protein expression in primary C57BL/6NCrl cells. Gene silencing of the SHP-1 in primary NK cells abrogated the ability of ITIM-containing NK inhibitory receptors to suppress the activation signals induced by NK1.1 activating receptors. We followed the fates of stably transduced SHP-1 silenced primary NK cells over a longer period of time in IL-2 containing cultures. We observed an impaired IL-2 induced proliferation in the SHP-1 knockdown NK cells. More interestingly, these "de-regulated" SHP-1 knockdown NK cells mediated specific self-killing in a real-time live cell microscopic imaging system we developed to study NK cell cytotoxicity in vitro. Selective target recognition of the SHP-1 knockdown NK cells revealed also possible involvement of the SHP-1 phosphatase in regulating other NK functions in mature NK cells.

Effects of MHC class I alleles on licensing of Ly49A+ NK cells

NK cells are innate immune lymphocytes that can react to cells lacking self-MHC class I. However, NK cells that cannot engage self-MHC through an inhibitory receptor are resistant to stimulation through their activation receptors. To become licensed (i.e., functionally competent to be triggered through its activation receptors), an NK cell must engage host MHC class I via a MHC class I-specific inhibitory receptor, such as a member of the murine Ly49 family. To explore potential determinants of NK cell licensing on a single Ly49 receptor, we have investigated the relative licensing impacts of the b, d, k, q, r, and s H2 haplotypes on Ly49A(+) NK cells. The results indicate that licensing is essentially analog but is saturated by moderate-binding MHC class I ligands. Interestingly, licensing exhibited a strong inverse correlation with a measure of cis engagement of Ly49A. Finally, licensing of Ly49A(+) NK cells was found to be less sensitive to MHC class I engagement than Ly49A-mediated effector inhibition, suggesting that licensing establishes a margin of safety against NK cell autoreactivity.

'Unlicensed' natural killer cells dominate the response to cytomegalovirus infection

Natural killer (NK) cells expressing inhibitory receptors that bind to self-MHC class I are “licensed” or rendered functionally more responsive to stimulation, whereas “unlicensed” NK cells lacking receptors for self-MHC class I are hyporesponsive. Here we show that, contrary to the licensing hypothesis, unlicensed NK cells were the primary mediators of NK cell-mediated control of mouse cytomegalovirus infection in vivo. Depletion of unlicensed, but not licensed, NK cells impaired control of viral titers. Transfer of unlicensed NK cells was more protective than licensed NK cells. SHP-1 signaling limited proliferation of licensed, but not unlicensed NK cells during infection. Thus, “unlicensed” NK cells are critical for protection against viral infection.

Ly49Q, an ITIM-bearing NK receptor, positively regulates osteoclast differentiation

Osteoclasts, multinucleated cells that resorb bone, play a key role in bone remodeling. Although immunoreceptor tyrosine-based activation motif (ITAM)-mediated signaling is critical for osteoclast differentiation, the significance of immunoreceptor tyrosine-based inhibitory motif (ITIM) has not been well understood. Here we report the function of Ly49Q, an Ly49 family member possessing an ITIM motif, in osteoclastogenesis. Ly49Q is selectively induced by receptor activator of nuclear factor-kappaB (NF-kappaB) ligand (RANKL) stimulation in bone marrow-derived monocyte/macrophage precursor cells (BMMs) among the Ly49 family of NK receptors. The knockdown of Ly49Q resulted in a significant reduction in the RANKL-induced formation of tartrate-resistance acid phosphatase (TRAP)-positive multinucleated cells, accompanied by a decreased expression of osteoclast-specific genes such as Nfatc1, Tm7sf4, Oscar, Ctsk, and Acp5. Osteoclastogenesis was also significantly impaired in Ly49Q-deficient cells in vitro. The inhibitory effect of Ly49Q-deficiency may be explained by the finding that Ly49Q competed for the association of Src-homology domain-2 phosphatase-1 (SHP-1) with paired immunoglobulin-like receptor-B (PIR-B), an ITIM-bearing receptor which negatively regulates osteoclast differentiation. Unexpectedly, Ly49Q deficiency did not lead to impaired osteoclast formation in vivo, suggesting the existence of a compensatory mechanism. This study provides an example in which an ITIM-bearing receptor functions as a positive regulator of osteoclast differentiation.

Inhibitory Ly49 receptors on mouse natural killer cells

The Ly49 receptors, which are expressed in a stochastic manner on subsets of murine natural killer (NK) cells, T cells, and other cells, are encoded by the Klra gene family and include receptors with either inhibitory or activating function. All of the inhibitory Ly49 receptors are characterized by an immunoreceptor tyrosine-based inhibitory motif in their cytoplasmic domain, which upon phosphorylation recruits tyrosine or lipid phosphatases to dampen signals transmitted through other activating receptors. Most of the inhibitory Ly49 receptors recognize polymorphic epitopes on major histocompatibility complex (MHC) class I proteins as ligands. Here, we review the polymorphism, ligand specificity, and signaling capacity of the inhibitory Ly49 receptors and discuss how these molecules regulate NK cell development and function.

Distinct conformations of Ly49 natural killer cell receptors mediate MHC class I recognition in trans and cis

Certain cell-surface receptors engage ligands expressed on juxtaposed cells and ligands on the same cell. The structural basis for trans versus cis binding is not known. Here, we showed that Ly49 natural killer (NK) cell receptors bound two MHC class I (MHC-I) molecules in trans when the two ligand-binding domains were backfolded onto the long stalk region. In contrast, dissociation of the ligand-binding domains from the stalk and their reorientation relative to the NK cell membrane allowed monovalent binding of MHC-I in cis. The distinct conformations (backfolded and extended) define the structural basis for cis-trans binding by Ly49 receptors and explain the divergent functional consequences of cis versus trans interactions. Further analyses identified specific stalk segments that were not required for MHC-I binding in trans but were essential for inhibitory receptor function. These data identify multiple distinct roles of stalk regions for receptor function.

A Role for cis Interaction between the Inhibitory Ly49A receptor and MHC class I for natural killer cell education

Natural killer (NK) cells show enhanced functional competence when they express inhibitory receptors specific for inherited major histocompatibility complex class I (MHC-I) molecules. Current models imply that NK cell education requires an interaction of inhibitory receptors with MHC-I expressed on other cells. However, the inhibitory Ly49A receptor can also bind MHC-I ligand on the NK cell itself (in cis). Here we describe a Ly49A variant, which can engage MHC-I expressed on other cells but not in cis. Even though this variant inhibited NK cell effector function, it failed to educate NK cells. The association with MHC-I in cis sequestered wild-type Ly49A, and this was found to relieve NK cells from a suppressive effect of unengaged Ly49A. These data explain how inhibitory MHC-I receptors can facilitate NK cell activation. They dissociate classical inhibitory from educating functions of Ly49A and suggest that cis interaction of Ly49A is necessary for NK cell education.

Natural killer cell tolerance licensing and other mechanisms

Armed with potent cytotoxic and immunostimulatory effector functions, natural killer (NK) cells have the potential to cause significant damage to normal self cells unless controlled by self-tolerance mechanisms. NK cells identify and attack target cells based on integration of signals from activation and inhibitory receptors, whose ligands exhibit complex expression and/or binding patterns. Preservation of NK cell self-tolerance must therefore go beyond mere engagement of inhibitory receptors during effector functions. Herein, we review recent work that has uncovered a number of mechanisms to ensure self-tolerance of NK cells. For example, licensing of NK cells allows only NK cells that can engage self-MHC to become functionally competent, or licensed. The molecular mechanism of this phenomenon appears to require signaling by receptors that were originally identified in effector inhibition. However, the nature of the signaling event has not yet been defined, but new interpretations of several published experiments provide valuable clues. In addition, several other cell-intrinsic and -extrinsic mechanisms of NK cell tolerance are discussed, including activation receptor cooperation and synergy, cytokine stimulation, and the opposing roles of accessory and regulatory cells. Finally, NK cell tolerance is discussed as it relates to the clinic, such as KIR-HLA disease associations, tumor immunotherapy, and fetal tolerance.

Positive regulation of plasmacytoid dendritic cell function via Ly49Q recognition of class I MHC

Plasmacytoid dendritic cells (pDCs) are an important source of type I interferon (IFN) during initial immune responses to viral infections. In mice, pDCs are uniquely characterized by high-level expression of Ly49Q, a C-type lectin-like receptor specific for class I major histocompatibility complex (MHC) molecules. Despite having a cytoplasmic immunoreceptor tyrosine-based inhibitory motif, Ly49Q was found to enhance pDC function in vitro, as pDC cytokine production in response to the Toll-like receptor (TLR) 9 agonist CpG-oligonucleotide (ODN) could be blocked using soluble monoclonal antibody (mAb) to Ly49Q or H-2K^b. Conversely, CpG-ODN–dependent IFN-α production by pDCs was greatly augmented upon receptor cross-linking using immobilized anti-Ly49Q mAb or recombinant H-2K^b ligand. Accordingly, Ly49Q-deficient pDCs displayed a severely reduced capacity to produce cytokines in response to TLR7 and TLR9 stimulation both in vitro and in vivo. Finally, TLR9-dependent antiviral responses were compromised in Ly49Q-null mice infected with mouse cytomegalovirus. Thus, class I MHC recognition by Ly49Q on pDCs is necessary for optimal activation of innate immune responses in vivo.

NK cell receptors in rodents and cattle

Natural killer (NK) cells discriminate between normal syngeneic cells and infected, neoplastic or MHC-disparate allogeneic cells. The reactivity of NK cells appears to be regulated by a balance between activating receptors that recognize non-self or altered self, and inhibitory receptors recognizing normal, self-encoded MHC class I molecules. Subfamilies of NK receptors undergo rapid evolution, and appear to co-evolve with the MHC. We here review present views on the evolution and function of NK cell receptors, with an emphasis on knowledge gained in cattle and rodents.

Signal transduction during natural killer cell activation

Understanding of transmembrane signaling during NK-cell activation has greatly expanded during the past few years. The discovery and characterization of novel triggering and inhibitory receptors have revealed the complexity of these processes. This unit focuses on receptor-initiated signaling pathways that modulate NK functions. Establishing the roles of different signaling pathways in NK cells is a crucial step in the design of therapeutic approaches for selective enhancement or suppression of NK-cell activation.

Ly49 cluster sequence analysis in a mouse model of diabetes: an expanded repertoire of activating receptors in the NOD genome

The mouse Ly49 and human killer cell immunoglobulin-like receptors (KIR) gene clusters encode activating and inhibitory class I MHC receptors on natural killer (NK) cells. A direct correlation between the presence of multiple activating KIR and various human autoimmune diseases including diabetes has been shown. Previous studies have implicated NK cell receptors in the development of diabetes in the non-obese diabetic (NOD) inbred mouse strain. To assess the contribution of Ly49 to NOD disease acceleration the Ly49 gene cluster of these mice was sequenced. Remarkably, the NOD Ly49 haplotype encodes the largest haplotype and the most functional activating Ly49 of any known mouse strain. These activating Ly49 include three Ly49p-related and two Ly49h-related genes. The NOD cluster contains large regions highly homologous to both C57BL/6 and 129 haplotypes, suggesting unequal crossing over as a mechanism of Ly49 haplotype evolution. Interestingly, the 129-like region has duplicated in the NOD genome. Thus, the NOD Ly49 cluster is a unique mix of elements seen in previously characterized Ly49 haplotypes resulting in a disproportionately large number of functional activating Ly49 genes. Finally, the functionality of activating Ly49 in NOD mice was confirmed in cytotoxicity assays.

Signal transduction during natural killer cell activation

Natural killer (NK) cells are a subpopulation of lymphocytes that can mediate cytotoxicity of certain tumor cells, virus-infected cells, and normal cells. In addition to their cytotoxic potential, NK cells secrete a variety of cytokines and chemokines that can modulate the function, growth, and differentiation of other immune cells. These different responses are initiated by the interaction of specific NK surface receptors with defined soluble or cell-associated ligands. There are several different types of receptors on the NK cell surface including "triggering" receptors, adhesion molecules, cytokine receptors, and MHC-recognizing killer-cell inhibitory receptors. The functional response of an NK cell is the result of the integration of signals transduced by these different types of receptors. Some of these signaling pathways are similar to other lymphoid cells, but there are also unique features employed by NK cells. This overview focuses on receptor-initiated signaling pathways that modulate NK functions.

NK cells and their receptors

Despite early reports that natural killer (NK) cells are non-specific or have non-major histocompatibility complex (MHC)- restricted killing, it is now clear that NK cells express a panoply of receptors with defined specificity for ligands expressed on their cellular targets. The roles of these receptors in terms of physiological NK cell effector functions, such as cytotoxicity and cytokine production, are beginning to be unravelled. Inasmuch as NK cells accumulate in the uterus, an appreciation of NK cell receptor specificities and their physiological functions should provide valuable clues to the role of NK cells in the uterus and during pregnancy.

Inappropriate recruitment and activity by the Src homology region 2 domain-containing phosphatase 1 (SHP1) is responsible for receptor dominance in the SHIP-deficient NK cell

We have previously demonstrated that the NKR repertoire is profoundly disrupted by SHIP deficiency. This repertoire disruption is characterized by receptor dominance where inhibitory signals from 2B4 repress killing of complex targets expressing MHC class I and activating ligands. In this study, we examine the molecular basis of receptor dominance in SHIP-/- NK cells. In this study, we show that in SHIP-/- NK cells there is a pronounced bias toward the 2B4 long isoform. We have also characterized signaling molecules recruited to 2B4 in SHIP-/- NK cells. Interestingly, we find that approximately 10- to 16-fold more Src homology region 2 domain-containing phosphatase 1 (SHP1) is recruited to 2B4 in SHIP-/- NK cells when compared with wild type. Consistent with SHP1 overrecruitment, treatment with sodium orthovanadate or a novel inhibitor with micromolar activity against SHP1 restores the ability of SHIP-/- NK cells to kill Rae1+ RMA and M157+ targets. These findings define the molecular basis for hyporesponsiveness by SHIP-deficient NK cells.

Licensing of natural killer cells by self-major histocompatibility complex class I

Natural killer (NK) cells have potent capacities to immediately kill cellular targets and produce cytokines that may potentially damage normal self-tissues unless they are kept in check. Such tolerance mechanisms are incompletely understood. Here we discuss recent studies suggesting that NK cells undergo a host major histocompatibility complex (MHC) class I-dependent functional maturation process, termed 'licensing'. Ironically, licensing directly involves inhibitory receptors that recognize target cell MHC class I molecules and block activation of NK cells in effector responses. This process results in two types of tolerant NK cells: functionally competent (licensed) NK cells, whose effector responses are inhibited by self-MHC class I molecules through the same receptors that conferred licensing, and functionally incompetent (unlicensed) NK cells.

Ly49B Is Expressed on Multiple Subpopulations of Myeloid Cells

Using a novel mAb specific for mouse Ly49B, we report here that Ly49B, the last remaining member of the C57 Ly49 family to be characterized, is expressed at low levels on approximately 1.5% of spleen cells, none which are NK cells or T cells but which instead belong to several distinct subpopulations of myeloid cells defined by expression of CD11b and different levels of Gr1. Much larger proportions of bone marrow and peritoneal cells expressed Ly49B, all being CD11b+ and comprising multiple subpopulations defined by light scatter, F4/80, and Gr1 expression. Costaining for Ly49Q, also expressed on myeloid cells, revealed that Ly49B and Ly49Q were most strongly expressed on nonoverlapping subpopulations, Ly49Q(high) cells being mostly B220+CD4+ and/or CD8+, Ly49B+ cells lacking these markers. Myeloid populations that developed from bone marrow progenitors in vitro frequently coexpressed both Ly49B and Ly49Q, and Ly49B expression could be up-regulated by LPS, alpha-IFN, and gamma-IFN, often independently of Ly49Q. PCR analysis revealed that cultured NK cells and T cells contained Ly49B transcripts, and Ly49B expression could be detected on NK cells cultured in IL-12 plus IL-18, and on an immature NK cell line. Immunohistochemical studies showed that Ly49B expression in tissues overlapped with but was distinct from that of all other myeloid molecules examined, being particularly prominent in the lamina propria and dome of Peyer's patches, implicating an important role of Ly49B in gut immunobiology. In transfected cells, Ly49B was found to associate with SHP-1, SHP-2, and SHIP in a manner strongly regulated by intracellular phosphorylation events.

Evidence supporting a circadian control of natural killer cell function

Natural killer (NK) cells participate in the immune response against infection and cancer. An emerging body of epidemiological data supports that circadian homeostasis may constitute a factor risk for cancer development. Physiological rhythms under circadian control persist in the absence of light entrainment and ultimately rely on a molecular clock. We have previously shown that NK cell cytolytic activity follows a daily rhythm and that NK cells enriched from light-entrained rats present 24-h oscillations of clock genes, cytolytic factors, and cytokines. To investigate whether these oscillations are under a genuine circadian control, we assessed the daily expression of clock genes (Per1, Per2, Clock, and Bmal1), a clock-controlled gene (Dbp), cytolytic factors (granzyme B and perforin), and cytokines (IFN-gamma and TNF-alpha) in NK cells enriched from rats maintained in constant darkness (DD). In addition, we investigated whether the disruption of the NK cell clock by RNA interference (RNAi) affects the expression of cytolytic factors and cytokines. Persistent 24-h oscillations were found in the expression levels of clock genes, cytolytic factors, and cytokines in NK cells enriched from DD rats. In addition, RNAi-mediated Per2 knockdown caused a significant decrease of granzyme B and perforin levels in the rat derived NK cell line RNK16. Taken together, these results provide evidence supporting that NK cell function is under circadian regulation.

The balancing act: inhibitory Ly49 regulate NKG2D-mediated NK cell functions

NK cells use NKG2D receptor to recognize 'induced-self'. In apparent violation of the 'missing-self' hypothesis, NK cells stimulated through NKG2D can lyse target cells despite normal expression levels of MHC class I molecules. Although, 'overriding' of the inhibitory by the activating signals had been postulated the precise role of inhibitory Ly49 receptors on NKG2D-mediated activation has only started emerging. We propose that NKG2D-mediated activation is a function of 'altering the balance' in the signaling strength between the activating NKG2D and inhibiting Ly49 receptors. Balance in the signaling strength depends on the expression levels of activating ligands on the target cells. Qualitative and quantitative variations of MHC class I molecules expressed on the target cells also plays a major role in determining this 'altered-balance'. Consequently, the nature of Ly49 receptors expressed on specific NK subsets determines the level of NKG2D-mediated NK cell activation. These observations provide a firm basis of 'altered-balance' in NK signaling and describe an active interplay between inhibitory Ly49 and activating NKG2D receptors.

Anti-fibrotic activity of NK cells in experimental liver injury through killing of activated HSC

BACKGROUND/AIMS

We have investigated the role of natural killer (NK) cells in hepatic fibrogenesis. Mouse NK cells express both inhibitory/activating-killing-immunoglobulin-related-receptors (iKIR/aKIR) specific for Class-I-molecules.

METHODS

Hepatic fibrosis induced by carbon-tetrachloride (CCl4) was compared between wild-type (WT) male-BALBc; combined-immunodeficiency (SCID, lacking B/T-cells); and SCID-BEIGE-mice (lacking B/T/NK cells), and naive mice.

RESULTS

Hepatic fibrosis significantly increased in all CCl4-treated groups. SCID-BEIGE mice had more fibrosis than SCID-mice (P<0.0001) as assessed by morphometry of sirius-red stained tissue sections. Following fibrosis, hepatic NK cells significantly decreased, the aKIR:iKIR-ratio significantly increased while Class-I expression on HSC decreased (P<0.001). Both freshly isolated and in situ HSC displayed a significant increase in cellular apoptosis following fibrosis induction. Confocal microscopy demonstrated the direct adhesion of NK cells to HSC in mouse liver sections and in vitro human NK/HSC co-culture. In human HSC there was decreased Class-I expression and increased apoptosis as well, which was further increased following blocking of either HSC-related Class-I or NK-related killer inhibitory receptors. Apoptosis was inhibited by pre-incubation of NK cells with the granzyme inhibitor 3,4-dichloroisocoumarin.

CONCLUSIONS

During liver injury, NK cells have an anti-fibrotic activity at least in part through stimulation of HSC killing.

How do natural killer cells find self to achieve tolerance?

Natural killer (NK) cells provide innate defense against tumors and infections by virtue of potent capacities to immediately kill cellular targets and produce cytokines. These effector functions may potentially damage normal self-tissues unless they are kept in check by tolerance mechanisms that need clarification. Here, we discuss recent studies indicating that the NK cells acquire functional competence directly through engagement of their MHC-specific receptors by self-MHC. Ironically, these receptors were first identified in terms of recognizing target cell MHC class I molecules and inhibiting NK cells in effector responses. Other studies of NK cell tolerance are also discussed. Although these studies begin to clarify the means by which NK cell tolerance is achieved, much more investigation is needed because NK cell tolerance is relevant to clinical observations in patients with infections and cancer.

Killer cell Ig-like receptor-dependent signaling by Ig-like transcript 2 (ILT2/CD85j/LILRB1/LIR-1)

Inhibitory killer cell Ig-like receptors (KIR) signal by recruitment of the tyrosine phosphatase Src homology region 2 domain-containing phosphatase-1 to ITIM. In the present study, we show that, surprisingly, KIR lacking ITIM are able to signal and inhibit in the human NK cell line NK92, but not in mouse NK cells. Signaling by mutant KIR is weaker than the wild-type receptor, does not require the transmembrane or cytoplasmic tail of KIR, and is blocked by overexpression of a catalytically inactive Src homology region 2 domain-containing phosphatase-1 molecule. We also demonstrate that mutant KIR signaling is blocked by Abs, which disrupt the interaction between KIR and human leukocyte Ag-C or Abs, which block the interaction between Ig-like transcript 2 (ILT2) and the alpha3 domain of HLA class I molecules. Thus, although ILT2 expressed in NK92 is insufficient to signal in response to human leukocyte Ag-C alone, ILT2 can signal in a KIR-dependent manner revealing functional cooperation between receptors encoded by two distinct inhibitory receptor families.

Tight regulation of IFN-gamma transcription and secretion in immature and mature B cells by the inhibitory MHC class I receptor, Ly49G2

To complete their maturation and to participate in the humoral immune response, immature B cells that leave the bone marrow are targeted to specific areas in the spleen, where they differentiate into mature cells. Previously, we showed that immature B cells actively down-regulate their integrin-mediated migration to lymph nodes or sites of inflammation, enabling their targeting to the spleen to allow their final maturation. This inhibition is mediated by IFN-gamma, which is transcribed and secreted at low levels by these immature B cells and is down-regulated at the mature stage. The activating MHC class I receptor, Ly49D, which is expressed at high levels on immature B cells, stimulates this IFN-gamma secretion. In this study we show that B cells coexpress the inhibitory MHC class I receptor, Ly49G2. In addition, we demonstrate a tight regulation in the expression of the Ly49 family members on B cells that depends on their cell surface levels. High levels of Ly49G2 have a dominant inhibitory effect on Ly49D expressed at low levels on immature bone marrow and mature B cells, resulting in inhibition of IFN-gamma secretion. However, low levels of the inhibitory receptor, Ly49G2, coexpressed with high levels of the activating receptor, Ly49D, on the immigrating immature B cells enable the secretion of specific low levels of IFN-gamma. This expression pattern insures the inhibitory control of peripheral immature B cell to prevent premature encounter with an Ag while enabling entry to the lymph nodes during the mature stage.

Licensing of natural killer cells by host major histocompatibility complex class I molecules

Self versus non-self discrimination is a central theme in biology from plants to vertebrates, and is particularly relevant for lymphocytes that express receptors capable of recognizing self-tissues and foreign invaders. Comprising the third largest lymphocyte population, natural killer (NK) cells recognize and kill cellular targets and produce pro-inflammatory cytokines. These potentially self-destructive effector functions can be controlled by inhibitory receptors for the polymorphic major histocompatibility complex (MHC) class I molecules that are ubiquitously expressed on target cells. However, inhibitory receptors are not uniformly expressed on NK cells, and are germline-encoded by a set of polymorphic genes that segregate independently from MHC genes. Therefore, how NK-cell self-tolerance arises in vivo is poorly understood. Here we demonstrate that NK cells acquire functional competence through 'licensing' by self-MHC molecules. Licensing involves a positive role for MHC-specific inhibitory receptors and requires the cytoplasmic inhibitory motif originally identified in effector responses. This process results in two types of self-tolerant NK cells--licensed or unlicensed--and may provide new insights for exploiting NK cells in immunotherapy. This self-tolerance mechanism may be more broadly applicable within the vertebrate immune system because related germline-encoded inhibitory receptors are widely expressed on other immune cells.

Expression of regulator of G protein signalling proteins in natural killer cells, and their modulation by Ly49A and Ly49D

The small GTPase accelerators regulator of G protein signalling (RGS) proteins are important regulators of proximal signalling from G protein coupled receptors. Although natural killer (NK) cells express a number of G-protein coupled receptors, expression of RGS proteins has not been investigated. We analysed the expression of RGS proteins in rat NK cells, and detected mRNA for RGS1, RGS2, RGS5, RGS8, RGS16, and RGS18. Interestingly, when we included a panel of different leucocyte subsets, we found that RGS8 was selectively expressed by NK cells. NK cells are under control of both activating and inhibitory receptors and, utilizing a xenogeneic system where the mouse activating Ly49D or inhibitory Ly49A receptors were transfected into the rat RNK-16 cell line, the potential regulation of RGS proteins by single NK cell receptors was studied. We found that ligation of Ly49D led to a rapid and transient increase in message for RGS2, while Ly49A ligation up-regulated RGS2, RGS16, and RGS18 mRNA. Both receptors also induced a prolonged increase in RGS2 endogenous protein levels. These findings suggest that RGS proteins may be influenced by or involved in NK cell receptor events, suggesting a crosstalk between G-protein coupled receptors and NK cell receptors.

Inhibitory NK receptor Ly49Q is expressed on subsets of dendritic cells in a cellular maturation- and cytokine stimulation-dependent manner

Ly49Q is a member of the Ly49 family that is expressed on Gr-1+ cells but not on NK and NKT cells. Ly49Q appears to be involved in regulating cytoskeletal architectures through ITIM-mediated signaling. We provide evidence that dendritic cells (DCs) of certain maturational states expressed Ly49Q, and that IFN-alpha plays an important role in its regulation. Freshly prepared murine plasmacytoid pre-DCs as well as Flt3L-induced plasmacytoid pre-DCs expressed Ly49Q, whereas freshly prepared myeloid DCs did not. However, GM-CSF-induced myeloid DCs showed low levels of Ly49Q expression, and this was significantly enhanced by IFN-alpha. In contrast, other cytokines and ligands for TLRs such as TNF-alpha, IL-6, LPS, and CpG-ODN had little or no effect on Ly49Q expression. Plasmacytoid pre-DCs in all mouse strains examined expressed Ly49Q. Constitutive expression of Ly49Q on myeloid DCs was observed in three restricted mouse strains including 129, NZB, and NZW. As can be seen in other Ly49 family members, Ly49Q expression was affected by MHC class I expression. At the same time, Ly49Q possessed polymorphisms, including at least three alleles. The polymorphic residues lay within the stalk and carbohydrate recognition domain, and two of them, in loop 3 and loop 6 of the carbohydrate recognition domain, are located in the region implicated in the interaction of Ly49A with H-2D(d). Therefore, depending on IFN-alpha, our results imply that Ly49Q serves a role for the biological functions of certain DC subsets through recognition of MHC class I or related molecules.

Two structurally related rat Ly49 receptors with opposing functions (Ly49 stimulatory receptor 5 and Ly49 inhibitory receptor 5) recognize nonclassical MHC class Ib-encoded target ligands

The Ly49 family of lectin-like receptors in rodents includes both stimulatory and inhibitory members. Although NK alloreactivity in mice is regulated primarily by inhibitory Ly49 receptors, in rats activating Ly49 receptors are equally important. Previous studies have suggested that activating rat Ly49 receptors are triggered by polymorphic ligands encoded within the nonclassical class Ib region of the rat MHC, RT1-CE/N/M, while inhibitory Ly49 receptors bind to widely expressed classical class Ia molecules encoded from the RT1-A region. To further investigate rat Ly49-mediated regulation of NK alloreactivity, we report in this study the identification and characterization of two novel paired Ly49 receptors that we have termed Ly49 inhibitory receptor 5 (Ly49i5) and Ly49 stimulatory receptor 5 (Ly49s5). Using a new mAb (mAb Fly5), we showed that Ly49i5 is an inhibitory receptor that recognizes ligands encoded within the class Ib region of the u and l haplotypes, while the structurally related Ly49s5 is an activating receptor that recognizes class Ib ligands of the u haplotype. Ly49s5 is functionally expressed in the high NK-alloresponder PVG strain, but not in the low alloresponder BN strain, in which it is a pseudogene. Ly49s5 is hence not responsible for the striking anti-u NK alloresponse previously described in BN rats (haplotype n), which results from repeated alloimmunizations with u haplotype cells. The present studies support the notion of a complex regulation of rat NK alloreactivity by activating and inhibitory Ly49 members, which may be highly homologous in the extracellular region and bind similar class Ib-encoded target ligands.

Role of interaction between Ly49 inhibitory receptors and cognate MHC I molecules in IL2-induced development of NK cells in murine bone marrow cell cultures

Murine bone marrow (BM) cell preparations lack mature cytotoxic natural killer (NK) cells, but NK cells may be induced in these cell preparations by culturing with interleukin-2 (IL2). Present study was aimed at studying the role of interactions between Ly49 molecules and major histocompatibility complex (MHC) class I molecules during IL2-induced development of mature NK cells in BM cell cultures. Addition of monoclonal antibodies (mabs) specific to class I MHC molecules of H-2b haplotype, to block any interaction of MHC I molecules with their receptors, was found to inhibit NK cell development. Mouse NK cells express several types of Ly49 molecules including Ly49C, which is an inhibitory receptor specific to MHC I molecules of H-2b haplotype. Blocking Ly49-MHC I interaction by using anti-Ly49C mab inhibited the development of cytotoxic NK cells. Addition of anti-Ly49A (no specificity for H-2b MHC I molecules) or anti-Ly49D (activating receptor specific for MHC I molecules of many H-2 haplotypes including H-2b) mabs, however, had no effect on IL2-induced NK cell development in BM cells. Mabs specific to Ly49C molecule and MHC I molecules of H-2b haplotype inhibited the development of mature NK cells from highly purified NK precursor cell population. These results indicate that specific interaction between inhibitory self-reactive Ly49 molecules and MHC I molecules may be crucial for NK cell development. We propose a model in which Ly49-MHC I interaction may have a permissive role in allowing development of only such NK cell clones that expresses at least one self-reactive inhibitory Ly49 molecule so that lysis of autologous healthy cells by mature NK cells may be avoided.