Interaction of Ly-49D+ NK cells with H-2Dd target cells leads to Dap-12 phosphorylation and IFN-gamma secretion

The contribution of natural killer complex loci to the development of experimental cerebral malaria

Background

The Natural Killer Complex (NKC) is a genetic region of highly linked genes encoding several receptors involved in the control of NK cell function. The NKC is highly polymorphic and allelic variability of various NKC loci has been demonstrated in inbred mice, providing evidence for NKC haplotypes. Using BALB.B6-Cmv1^r congenic mice, in which NKC genes from C57BL/6 mice were introduced into the BALB/c background, we have previously shown that the NKC is a genetic determinant of malarial pathogenesis. C57BL/6 alleles are associated with increased disease-susceptibility as BALB.B6-Cmv1^r congenic mice had increased cerebral pathology and death rates during P. berghei ANKA infection than cerebral malaria-resistant BALB/c controls.

Methods

To investigate which regions of the NKC are involved in susceptibility to experimental cerebral malaria (ECM), intra-NKC congenic mice generated by backcrossing recombinant F2 progeny from a (BALB/c x BALB.B6-Cmv1^r) F1 intercross to BALB/c mice were infected with P. berghei ANKA.

Results

Our results revealed that C57BL/6 alleles at two locations in the NKC contribute to the development of ECM. The increased severity to severe disease in intra-NKC congenic mice was not associated with higher parasite burdens but correlated with a significantly enhanced systemic IFN-γ response to infection and an increased recruitment of CD8⁺ T cells to the brain of infected animals.

Conclusions

Polymorphisms within the NKC modulate malarial pathogenesis and acquired immune responses to infection.

Mouse Ly49G2+ NK cells dominate early responses during both immune reconstitution and activation independently of MHC

Natural killer (NK) cell subsets can be defined by the differential expression of inhibitory receptors for MHC class I molecules. Early after congenic HSCT, we found that Ly49G2(high) single-positive NK cells repopulated, displayed an activated phenotype, and were highly cytolytic. Over time, this subset was replaced with NK cells with a normal pattern of Ly49 expression. Treatment of mice with IL-2 also resulted in the rapid expansion of these Ly49G2(high) single-positive NK cells. Only the Ly49g (Klra7) Pro1 transcript was highly induced in both HSCT- and IL-2-treated recipients. MHC-independent expansion of the Ly49G2(+) subset was also observed after Listeria monocytogenes or mouse cytomegalovirus infection. Our data indicate that during reconstitution after HSCT and various activation stimuli, Ly49G2(+) NK cells represent the "first-responder" NK cells, which occur independently of NK-cell licensing via Ly49-MHC interactions. These data suggest that the inhibitory Ly49G2 receptor represents an activation marker on mouse NK cells under various conditions.

Activating and inhibitory receptors of natural killer cells

Natural killer (NK) cells are potent immune effector cells that can respond to infection and cancer, as well as allowing maternal adaptation to pregnancy. In response to malignant transformation or pathogenic invasion, NK cells can secrete cytokine and may be directly cytolytic, as well as exerting effects indirectly through other cells of the immune system. To recognize and respond to inflamed or infected tissues, NK cells express a variety of activating and inhibitory receptors including NKG2D, Ly49 or KIR, CD94-NKG2 heterodimers and natural cytotoxicity receptors, as well as co-stimulatory receptors. These receptors recognize cellular stress ligands as well as major histocompatibility complex class I and related molecules, which can lead to NK cell responses. Importantly, NK cells must remain tolerant of healthy tissue, and some of these receptors can also prevent activation of NK cells. In this review, we describe the expression of prominent NK cell receptors, as well as expression of their ligands and their role in immune responses. In addition, we describe the main signaling pathways used by NK cell receptors. Although we now appreciate that NK cell biology is more complicated than first thought, there are still facets of their biology that remain unclear. These will be highlighted and discussed in this review.

Cancer vaccines for established cancer: how to make them better?

If one envisions dendritic cells (DCs) as nature's adjuvant, then it is easy to predict that they would be advantageous for cancer immunotherapy. Advances in culture processes that generate large numbers of purified and functionally mature DCs raised the possibility that DCs might be promising clinical agents to generate effective immune responses against cancer. The use of mature DCs as cellular vaccines was proposed to be superior to conventional strategies aimed at treating cancer, yet a phase III clinical trial in patients with melanoma demonstrated no increased benefit of DCs over standard therapy. Despite this and other apparent failures, we propose that DC-based therapy should not be discarded but rather reassessed. The heterogeneity of DCs and their interaction with other innate cells and regulatory and effector pathways must be clearly understood before the full therapeutic benefit of DCs are recognized. Several aspects of DC vaccination require optimization including the following: effective delivery of vaccines to DCs in lymphoid tissues; incorporation of components that induce appropriate DC activation; and facilitation of innate and adaptive interactions and reduction of regulatory T-cell networks or suppressive microenvironments that hinder the function of immune effectors. Application of this knowledge is resulting in encouraging new data in pre-clinical settings, where multiple arms of the immune system are targeted for cancer therapy.

DAP10- and DAP12-associated receptors in innate immunity

The DAP10 and DAP12 signaling subunits are highly conserved in evolution and associate with a large family of receptors in hematopoietic cells, including dendritic cells, plasmacytoid dendritic cells, neutrophils, basophils, eosinophils, mast cells, monocytes, macrophages, natural killer cells, and some B and T cells. Some receptors are able to associate with either DAP10 or DAP12, which contribute unique intracellular signaling functions. Studies of humans and mice deficient in these signaling subunits have provided surprising insights into the physiological functions of DAP10 and DAP12, demonstrating that they can either activate or inhibit immune responses. DAP10- and DAP12-associated receptors have been shown to recognize both host-encoded ligands and ligands encoded by microbial pathogens, indicating that they play an important role in innate immune responses.

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.

Regulation of NK cell responsiveness to achieve self-tolerance and maximal responses to diseased target cells

Inhibitory receptors specific for major histocompatibility complex (MHC) class I molecules govern the capacity of natural killer (NK) cells to attack class I-deficient cells ('missing-self recognition'). These receptors are expressed stochastically, such that the panel of expressed receptors varies between NK cells. This review addresses how the activity of NK cells is coordinated in the face of this variation to achieve a repertoire that is self-tolerant and optimally reactive with diseased cells. Recent studies show that NK cells arise in normal animals or humans that lack any known inhibitory receptors specific for self-MHC class I. These NK cells exhibit self-tolerance and exhibit functional hyporesponsiveness to stimulation through various activating receptors. Evidence suggests that hyporesponsiveness is induced because these NK cells cannot engage inhibitory MHC class I molecules and are therefore persistently over-stimulated by normal cells in the environment. Finally, we discuss evidence that hyporesponsiveness is a quantitative trait that varies depending on the balance of signals encountered by developing NK cells. Thus, a tuning process determines the functional set-point of NK cells, providing a basis for discriminating self from missing-self, and at the same time endowing each NK cell with the highest inherent responsiveness compatible with self-tolerance.

NK gene complex dynamics and selection for NK cell receptors

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

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.

Stat4-dependent, T-bet-independent regulation of IL-10 in NK cells

Interleukin-10 (IL-10) is intensely studied, yet little is known about the mechanisms that control IL-10 expression. We identified striking similarities between IL-10 and interferon-gamma (IFN-gamma) regulation in mouse natural killer (NK) cells. Like IFN-gamma, IL-10 expression is induced by IL-2 and IL-12 and IL-2+IL-12 stimulation is synergistic. Unlike IFN-gamma, neither IL-18 nor Ly-49D cross-linking induced IL-10 expression however. Additionally, the IL-12 homologs IL-23 and IL-27 also do not regulate NK cell-specific IL-10. We determined that a small population of NK cells accounts for IL-10 production. The induction of IL-10 by IL-2+IL-12 treatment in NK cells appears to be biphasic, with an initial burst of expression which diminishes by 12 h but spikes again at 18 h. We determined that much like IFN-gamma, Stat4 is largely required for IL-12-induced IL-10. Conversely, we observed normal induction of IL-10 in T-bet-deficient NK cells. We identified a Stat4-binding element in the fourth intron of the Il10 gene, which is completely conserved between mouse and human. This intronic Stat4 motif is within a conserved noncoding sequence, which is also a target for cytokine-induced histone acetylation. These findings highlight tissue- and receptor-specific IL-10 regulatory mechanisms, which may be part of an early feedback loop.

NK cells stimulate recruitment of CXCR3+ T cells to the brain during Plasmodium berghei-mediated cerebral malaria

NK cells are cytotoxic lymphocytes that also secrete regulatory cytokines and can therefore influence adaptive immune responses. NK cell function is largely controlled by genes present in a genomic region named the NK complex. It has been shown that the NK complex is a genetic determinant of murine cerebral malaria pathogenesis mediated by Plasmodium berghei ANKA. In this study, we show that NK cells are required for cerebral malaria disease induction and the control of parasitemia. NK cells were found infiltrating brains of cerebral malaria-affected mice. NK cell depletion resulted in inhibition of T cell recruitment to the brain of P. berghei-infected animals. NK cell-depleted mice displayed down-regulation of CXCR3 expression and a significant reduction of T cells migrating in response to IFN-gamma-inducible protein 10, indicating that this chemokine pathway plays an essential role in leukocyte trafficking leading to cerebral disease and fatalities.

Non-T Cell Activation Linker (NTAL) Negatively Regulates TREM-1/DAP12-Induced Inflammatory Cytokine Production in Myeloid Cells

The engagement of triggering receptor expressed on myeloid cells 1 (TREM-1) on macrophages and neutrophils leads to TNF-alpha and IL-8 production and enhances inflammatory responses to microbial products. For signal transduction, TREM-1 couples to the ITAM-containing adapter DNAX activation protein of 12 kDa (DAP12). In general, ITAM-mediated signals lead to cell activation, although DAP12 was recently implicated in inhibitory signaling in mouse macrophages and dendritic cells. To date, signals downstream of the TREM-1 and DAP12 complex in myeloid cells are poorly defined. By analyzing receptor-induced tyrosine phosphorylation patterns, we discovered that the ligation of TREM-1 leads to tyrosine phosphorylation of the non-T cell activation linker (NTAL; also called linker of activation in B cells or LAB) in a myelomonocytic cell line and primary human granulocytes. Using RNA interference to decrease the expression levels of NTAL, we demonstrate that in NTAL knockdown cell lines the phosphorylation of ERK1/2 is enhanced. In addition, low levels of NTAL are correlated with decreased and delayed mobilization of Ca(2+) after TREM-1 triggering. Most importantly, we demonstrate that NTAL acts as a negative regulator of TNF-alpha and IL-8 production after stimulation via TREM-1. Our results show that activation signals delivered via DAP12 can be counterbalanced by the adaptor NTAL, identifying NTAL as gatekeeper of TREM-1/DAP12-induced signaling in myeloid cells.

Critical Residues at the Ly49 Natural Killer Receptor’s Homodimer Interface Determine Functional Recognition of m157, a Mouse Cytomegalovirus MHC Class I-Like Protein

NK cell function is regulated by Ly49 receptors in mice and killer cell Ig-like receptors in humans. Although inhibitory Ly49 and killer cell Ig-like receptors predominantly ligate classical MHC class I molecules, recent studies suggest that their activating counterparts recognize infection. The quintessential example is resistance to the mouse CMV in C57BL/6 mice, which depends on the functional recognition of m157, a mouse CMV-encoded MHC class I-like molecule, by Ly49H, an activating NK cell receptor. We have taken advantage of the natural variation in closely related members of the Ly49C-like receptors and the availability of Ly49 crystal structures to understand the molecular determinants of the Ly49H-m157 interaction and to identify amino acid residues discriminating between m157 binding and nonbinding receptors. Using a site-directed mutagenesis approach, we have targeted residues conserved in receptors binding to m157 (Ly49H and Ly49I(129)) but different from receptors lacking m157 recognition (Ly49C, Ly49I(B6), and Ly49U). Wild-type and mutant receptors were transfected into reporter cells, and physical binding as well as functional activation by m157 was studied. Our findings suggested that the Ly49 MHC class I contact "site 2," I226, may not be involved in m157 binding. In contrast, residue Y146 and G151, mapping at the receptor homodimer interface, are likely critical for functional recognition of the m157 glycoprotein. Our combined functional and three-dimensional modeling approach suggested that the architecture of the Ly49H dimer is crucial to accessing m157, but not MHC class I. These results link Ly49 homodimerization variability to the direct recognition of pathogen products.

Missing self recognition and self tolerance of natural killer (NK) cells

Natural killer cells express inhibitory receptors specific for polymorphic MHC molecules, which enables them to mediate "missing self recognition", the capacity to attack self cells that extinguish expression of MHC class I molecules. A key question is: how are NK cells rendered self-tolerant? It was proposed that all NK cells express at least one inhibitory receptor specific for self MHC, but we recently identified an NK cell subset that does not. Instead, these NK cells, like anergic B and T cells, are hyporesponsive to stimulation. These findings indicate that NK cell activity can be modulated independently of inhibitory receptors specific for MHC molecules, and that such modulation may contribute to self tolerance. This review summarizes current understanding of NK cell recognition and self tolerance.

Self-tolerance of natural killer cells

Natural killer (NK) cells, similar to other lymphocytes, acquire tolerance to self. This means that NK cells have the potential to attack normal self cells but that there are mechanisms to ensure that this does not usually occur. Self-tolerance is acquired by NK cells during their development, but the underlying molecular and cellular mechanisms remain poorly understood. Recent studies have produced important new information about NK-cell self-tolerance. Here, we review the evidence for and against possible mechanisms of NK-cell self-tolerance, with an emphasis on the role of MHC-specific receptors.

Regulation of Ly49D/DAP12 Signal Transduction by Src-Family Kinases and CD45

Activating, DAP12-coupled members of the Ly-49 family of NK cell receptors help control viral infections in mice. However, the kinases and/or phosphatases mediating tyrosine phosphorylation of Ly-49D-associated DAP12 have not been elucidated. In this study, we show for the first time that Src family tyrosine kinases are physically and functionally associated with Ly-49D/DAP12 signaling in murine NK cells. Specifically, we demonstrate the following: 1) inhibition of Src family kinases suppresses DAP12 phosphorylation and downstream DAP12 signals; 2) both Fyn and Lck are capable of phosphorylating DAP12; and 3) both kinases coimmunoprecipitate with the Ly-49D/DAP12 complex in NK cells. Although we detect enhanced phosphorylation of Fyn upon Ly-49D cross-linking in NK cells, Ly-49D-mediated events in both Fyn-/- and Fyn/Lck-/- mice appear normal, reinforcing the theme of redundancy in the ability of Src family kinases to initiate activation events. In contrast to disruption of specific Src family enzymes, Ly-49D/DAP12-mediated calcium mobilization and cytokine production by CD45 null NK cells are defective. Although others have ascribed the effects of CD45 mutation solely on the suppression of Src family activity, we demonstrate in this study that DAP12 is hyperphosphorylated in CD45 null NK cells, resulting in uncoordinated tyrosine-mediated signaling upon Ly-49D ligation. Therefore, although our data are consistent with a Src kinase activity proximally within DAP12 signaling, DAP12 also appears to be a substrate of CD45, suggesting a more complex role for this phosphatase than has been reported previously.

NK Cell Receptors as Tools in Cancer Immunotherapy

Natural killer (NK) cells were identified 30 years ago based on their ability to "spontaneously" kill tumor cells. The basis for NK cell recognition and activation is due to a variety of receptors that bind to specific ligands on tumor cells and normal cells. Some of these receptors have the ability to inhibit NK cell function, and other receptors activate NK cell function. Therapeutic strategies for cancer therapy are being developed based on preventing NK cell inhibition or using NK cell receptors to activate NK cells or T cells. There are intriguing clinical data from studies of bone marrow transplantation that support the idea that preventing NK cell inhibition by human leukocyte antigen (HLA) class I molecules can be a means to promote graft-versus-leukemia (GvL) effects and limit graft-versus-host disease (GvHD) in acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) patients. Experimental findings also support the blockade of NK cell inhibitory receptors as a way to protect against leukemia relapse. It may be possible to use our knowledge of NK cell activating receptors and their ligands to immunize patients with modified tumor cells to promote beneficial NK cell responses and development of host antitumor cytotoxic T lymphocytes (CTLs). Finally, new data support the idea of using modified NK cell receptors as a means to target patients' T cells against their own tumor cells and induce long-term immunity against them. Tumors are essentially tissues that have overcome normal regulation mechanisms, and therefore the ability to distinguish normal cells from abnormal cells is a key part of selectively attacking tumor cells. NK cells have various receptor systems designed to recognize infected and abnormal cells. Understanding NK cell receptors and their recognition mechanisms provides new tools for the development of immunotherapies against cancer.

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.

Impaired synaptic function in the microglial KARAP/DAP12-deficient mouse

Several proteins are expressed in both immune and nervous systems. However, their putative nonimmune functions in the brain remain poorly understood. KARAP/DAP12 is a transmembrane polypeptide associated with cell-surface receptors in hematopoeitic cells. Its mutation in humans induces Nasu-Hakola disease, characterized by presenile dementia and demyelinization. However, alteration of white matter occurs months after the onset of neuropsychiatric symptoms, suggesting that other neuronal alterations occur in the early phases of the disease. We hypothesized that KARAP/DAP12 may impact synaptic function. In mice deficient for KARAP/DAP12 function, long-term potentiation was enhanced and was partly NMDA receptor (NMDAR) independent. This effect was accompanied by changes in synaptic glutamate receptor content, as detected by the increased rectification of AMPA receptor EPSCs and increased sensitivity of NMDAR EPSCs to ifenprodil. Biochemical analysis of synaptic proteins confirmed these electrophysiological data. In mutants, the AMPA receptor GluR2 subunit expression was decreased only in the postsynaptic densities but not in the whole membrane fraction, demonstrating specific impairment of synaptic receptor accumulation. Alteration of the BNDF-tyrosine kinase receptor B (TrkB) signaling in the mutant was demonstrated by the dramatic decrease of synaptic TrkB with no change in other regulatory or scaffolding proteins. Finally, KARAP/DAP12 was detected only in microglia but not in neurons, astrocytes, or oligodendrocytes. KARAP/DAP12 may thus alter microglial physiology and subsequently synaptic function and plasticity through a novel microglia-neuron interaction.

A requirement for CD45 distinguishes Ly49D-mediated cytokine and chemokine production from killing in primary natural killer cells

Engagement of receptors on the surface of natural killer (NK) cells initiates a biochemical cascade ultimately triggering cytokine production and cytotoxicity, although the interrelationship between these two outcomes is currently unclear. In this study we investigate the role of the cell surface phosphatase CD45 in NK cell development and intracellular signaling from activating receptors. Stimulation via the major histocompatibility complex I-binding receptor, Ly49D on CD45(-/-) primary NK cells resulted in the activation of phosphoinositide-3-kinase and normal cytotoxicity but failed to elicit a range of cytokines and chemokines. This blockage is associated with impaired phosphorylation of Syk, Vav1, JNK, and p38, which mimics data obtained using inhibitors of the src-family kinases (SFK). These data, supported by analogous findings after CD16 and NKG2D stimulation of CD45(-/-) primary NK cells, place CD45 upstream of SFK in NK cells after stimulation via immunoreceptor tyrosine-based activation motif-containing receptors. Thus we identify CD45 as a pivotal enzyme in eliciting a precise subset of NK cell responses.

NK cell recognition

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

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

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

Reciprocal transfer of class I MHC allele specificity between activating Ly-49P and Ly-49W receptors by exchange of beta 4-beta 5 loop residues

Receptors of the Ly-49 multigene family regulate rodent NK cell functions. Ly-49Rs are highly polymorphic and exist in either activating or inhibitory forms. Examples of both Ly-49 receptor types have been shown to recognize class I MHC ligands. Ly-49Rs can distinguish between class I alleles, but the molecular basis of this discrimination is unknown. Two activating receptors, Ly-49P and Ly-49W, differ in class I recognition, recognizing H-2D(d), or H-2D(d) and D(k), respectively. In this report, we demonstrate that specificity for H-2D(k) can be transferred from Ly-49W to Ly-49P by substituting 3 aa predicted to reside in the beta4-beta5 loop of Ly-49W into Ly-49P. Replacement of these same residues of Ly-49W with corresponding residues in Ly-49P eliminates H-2D(k) recognition while still preserving H-2D(d) recognition. Further mutagenesis indicates that all 3 aa facilitate optimal class I specificity exchange. These results provide the first evidence for a specific site on Ly-49Rs, the beta4-beta5 loop, in determining class I MHC allele specificity.

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.

Analysis of cytokine regulators inducing interferon production by mouse uterine natural killer cells

In mice and women, terminal differentiation of uterine natural killer (uNK) cells commences during endometrial decidualization. Both proliferation and interferon (IFN)-gamma are induced. Uterine NK cell precursors appear to home from secondary lymphoid organs to decidualizing uteri and localize mesometrially to the central decidua basalis, the site of maternal arterial modification at Gestation Days (gd) 9.5-10. In mice, genetic absence of uNK cells results in absence of pregnancy-induced spiral artery modification. Administration of IFN-gamma to uNK-negative pregnant females induces arterial modifications without fetal loss. In this study, we investigated the roles of cytokines, known in other tissues to differentiate and activate NK cells, in induction of IFN-gamma production in normal mouse implantation sites. Fecundity evaluation, implantation site morphometry, and IFN-gamma quantification in interleukin (IL)-12p40(0/0), IL-18(0/0), dual IL-12p40(0/0)/IL-18(0/0) and congenic strains revealed the importance of both IL-12 and IL-18 in the induction of spiral artery modification and IFN-gamma synthesis. Immediately after implantation, IL-18 was localized transiently to decidual cells, but by gd8, IL-18 was produced solely by uNK cells, suggesting that early uNK cells are activated by stroma and lymphocyte-derived signals maintain later uNK cell activation. Mesometrial tissue of C57Bl/6J mice was examined by reverse transcription polymerase chain reaction assay in virgin, early postimplantation, and midgestation females for expression of the heterodimeric cytokines IL-23 (composed of IL-12p40 and a novel alpha chain), IL-27 (composed of two IL-12-related chains) and IL-27R. No expression was detected in virgin uteri. The four genes were induced by gd6, and uNK cells isolated from midgestation transcribed IL-23alpha and IL-27R. This study advances the understanding of uNK cell activation during normal pregnancy.

IL-4 synergistically enhances both IL-2- and IL-12-induced IFN-gamma expression in murine NK cells

Interleukin-4 (IL-4) is thought to influence T and natural killer (NK) cells by down-regulating T helper 1 (Th1)-type cytokines like interferon-gamma (IFN-gamma). While investigating IL-4 regulation of IFN-gamma expression, we found that IL-4 synergized with IL-2 or IL-12 to enhance IFN-gamma production and mRNA expression in spleen-derived, IL-2-cultured NK cells, as well as negatively sorted fresh DX5+/CD3- NK cells albeit at lower levels. The positive effect of IL-4 on IL-2-induced IFN-gamma production was dependent upon signal transducer and activator of transcription 6 (Stat6) because this response was virtually abrogated in Stat6-/- mice. Notably, though, IL-12 plus IL-4 synergy on IFN-gamma expression was intact in Stat6-/- mice. In exploring possible molecular mechanisms to account for the synergistic effects of IL-4 on murine NK cells, we found that IL-2 plus IL-4 stimulation resulted in a modest increase in tyrosine phosphorylation of Stat5, while IL-12 plus IL-4 treatment resulted in a more substantial increase in tyrosine-phosphorylated Stat4. Finally, to identify regions of the IFN-gamma promoter that may be involved, NK cells from human IFN-gamma promoter/luciferase transgenic mice were treated with cytokines. NK cells from proximal (-110 to +64) promoter region mice did not respond to cytokine stimulation; however, the intact -565 to +64 IFN-gamma promoter responded synergistically to IL-2 plus IL-4 and to IL-12 plus IL-4 in NK cells. These data demonstrate a role for IL-4 in enhancing IFN-gamma expression in murine NK cells that is partially dependent on Stat6 in IL-2 costimulation and completely independent of Stat6 in IL-12 costimulations.

Regulation of murine cerebral malaria pathogenesis by CD1d-restricted NKT cells and the natural killer complex

NKT cells are specialized cells coexpressing NK and T cell receptors. Upon activation they rapidly produce high levels of interferon-gamma (IFN-gamma) and interleukin-4 (IL-4) and are therefore postulated to influence T(H)1/T(H)2 immune responses. The precise role of the CD1/NKT cell pathway in immune response to infection remains unclear. We show here that CD1d-restricted NKT cells from distinct genetic backgrounds differentially influence T(H)1/T(H)2 polarization, proinflammatory cytokine levels, pathogenesis, and fatality in the P. berghei ANKA/rodent model of cerebral malaria. The functional properties of CD1d-restricted NKT cells vary according to expression of loci of the natural killer complex (NKC) located on mouse chromosome 6, which is shown here to be a significant genetic determinant of murine malarial fatalities.

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

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

Successful control of viruses by NK cells--a balance of opposing forces?

Natural killer (NK) cells play a crucial role in limiting the severity of diseases caused by a range of viruses. Recent data have shown that the effector functions of NK cells can be specifically stimulated when NK cell activation receptors engage cellular major histocompatibility complex (MHC) class I-like ligands induced after infection or by specific viral gene products. However, to counter this NK cell response viruses have evolved an array of strategies to subvert efficient NK cell activation. These data indicate that the balance of host NK cell responses and viral NK cell escape mechanisms can be strategically poised as each strives for survival.

Genetic control of innate immune responses against cytomegalovirus: MCMV meets its match

Cytomegalovirus (CMV) is a widespread pathogen that is responsible for severe disease in immunocompromised individuals and probably, associated with vascular disease in the general population. There is increasing evidence that cells of the innate immune system play a key role in controlling this important pathogen. This is particularly evident in the experimental murine CMV (MCMV) model of infection which has revealed an important role for natural killer (NK) cells in controlling early viral replication after infection with MCMV. In this model, different strains of inbred mice exhibit striking differences in their level of susceptibility to MCMV infection. Genetic studies, performed almost 10 years ago, revealed that this pattern of susceptibility/resistance can be attributed to a single genetic locus termed Cmv1 and recently several groups that have been working on the mapping and identification of Cmv1 have met with success. Interestingly, Cmv1 is allelic to a member of the Ly49 gene family, which encode activating or inhibitory transmembrane receptors present on the surface of NK cells. All Ly49 receptors characterized to date interact with MHC class I molecules on potential target cells, resulting in the accumulation of signals to the NK to either 'kill' or 'ignore' the cell based upon the repertoire of MHC class I molecules expressed. The identification of Cmv1 as Ly49H, a stimulatory member of the Ly49 family, adds an interesting twist to the Ly49 story. Although the ligand of Ly49H is not yet known, there is already compelling evidence that the ligand is upregulated on virally infected cells, resulting in specific activation of Ly49H-expressing NK cells. This review provides an historical perspective of the MCMV infection model from its inception to the discovery of the gene responsible for the phenotype and provides a basis for further experiments aimed at understanding the role of NK cells, in general, and Ly49H, in particular, in mediating resistance to cytomegalovirus.

A ligand for the murine NK activation receptor Ly-49D: activation of tolerized NK cells from beta 2-microglobulin-deficient mice

Mouse NK cells express inhibitory NK receptors that recognize target cell MHC class I molecules and activation receptors that are less well defined. The Ly-49D activation receptor on C57BL/6 NK cells recognizes Chinese hamster ovary cells and triggers natural killing. In this study, we demonstrate that a Chinese hamster classical MHC class I molecule is the ligand for Ly-49D in a reporter gene assay system as well as in NK cell killing assays. Ly-49D recognizes the Chinese hamster class I molecule better when it is expressed with Chinese hamster beta(2)-microglobulin (beta(2)m) than murine beta(2)m. However, it is still controversial that Ly-49D recognizes H-2D(d), as we were unable to demonstrate the specificity previously reported. Using this one ligand-one receptor recognition system, function of an NK activation receptor was, for the first time, investigated in NK cells that are tolerized in beta(2)m-deficient mice. Surprisingly, Ly-49D-killing activity against ligand-expressing targets was observed with beta(2)m-deficient mouse NK cells, albeit reduced, even though "tolerized" function of Ly-49D was expected. These results indicate that Ly-49D specifically recognizes the Chinese hamster MHC class I molecule associated with Chinese hamster beta(2)m, and indicate that the Ly-49D NK cell activation receptor is not tolerized in beta(2)m deficiency.

Regulation of nuclear gamma interferon gene expression by interleukin 12 (IL-12) and IL-2 represents a novel form of posttranscriptional control

Posttranscriptional control of gamma interferon (IFN-gamma) gene expression has not been extensively studied and is poorly understood. Our work describes a posttranscriptional mechanism that modulates IFN-gamma mRNA expression in stimulated natural killer (NK) cells through nuclear retention of the IFN-gamma mRNA. This is evidenced by the elevated and sustained nuclear accumulation of both precursor and processed IFN-gamma mRNAs in NK cells stimulated with interleukin-12 (IL-12). The elevated nuclear mRNA accumulation persists long after transcriptional activity has subsided and the rate of cytoplasmic IFN-gamma mRNA accumulation has dropped. The IL-12-induced nuclear retention of the IFN-gamma mRNA prevails until a secondary cytokine stimulus is received. The secondary stimulus, which is initiated by IL-2, mediates transcription-independent movement of the nuclear IFN-gamma mRNA. Concurrent with the nucleocytoplasmic movement of the IFN-gamma mRNA, we have observed increases in the amount of processed nuclear IFN-gamma mRNA that are greater than that seen for the unprocessed IFN-gamma mRNA. The increase in processed IFN-gamma mRNA appears to be due to increased mRNA stability which then promotes increased nucleocytoplasmic shuttling of the mature IFN-gamma mRNA. These data support a model whereby mobilization of nuclear IFN-gamma mRNA stores allows NK cells to rapidly and robustly respond to secondary cytokine activators in a transcription-independent manner, thus shortening the time for overall cellular response to inflammatory signals.

Cre recombinase-mediated inactivation of H-2Dd transgene expression: evidence for partial missing self-recognition by Ly49A NK cells

We have established H-2D(d)-transgenic (Tg) mice, in which H-2D(d) expression can be extinguished by Cre recombinase-mediated deletion of an essential portion of the transgene (Tg). NK cells adapted to the expression of the H-2D(d) Tg in H-2(b) mice and acquired reactivity to cells lacking H-2D(d), both in vivo and in vitro. H-2D(d)-Tg mice crossed to mice harboring an Mx-Cre Tg resulted in mosaic H-2D(d) expression. That abrogated NK cell reactivity to cells lacking D(d). In D(d) single Tg mice it is the Ly49A+ NK cell subset that reacts to cells lacking D(d), because the inhibitory Ly49A receptor is no longer engaged by its D(d) ligand. In contrast, Ly49A+ NK cells from D(d) x MxCre double Tg mice were unable to react to D(d)-negative cells. These Ly49A+ NK cells retained reactivity to target cells that were completely devoid of MHC class I molecules, suggesting that they were not anergic. Variegated D(d) expression thus impacts specifically missing D(d) but not globally missing class I reactivity by Ly49A+ NK cells. We propose that the absence of D(d) from some host cells results in the acquisition of only partial missing self-reactivity.

A DAP12-mediated pathway regulates expression of CC chemokine receptor 7 and maturation of human dendritic cells

Gene targeting of the adaptor molecule DAP12 in mice caused abnormal distribution and impaired antigen presentation capacity of dendritic cells (DCs). However, the DAP12-associated receptors expressed on DCs and their functions have not been identified yet. Here we show that the triggering receptor expressed on myeloid cells-2 (TREM-2) is a cell surface receptor on human monocyte-derived DCs, which is associated with DAP12. TREM-2/DAP12 promotes upregulation of CC chemokine receptor 7, partial DC maturation, and DC survival through activation of protein tyrosine kinases and extracellular signal-regulated kinase. In contrast to Toll-like receptor-mediated signaling, TREM2/DAP12 stimulation is independent of nuclear factor-kappaB and p38 stress-activated protein kinase. This novel DC activation pathway may regulate DC homeostasis and amplify DC responses to pathogens, explaining the phenotype observed in DAP12-deficient mice.

Activating Ly-49 NK receptors: central role in cytokine and chemokine production

In an attempt to understand potential novel functions of receptors in vivo, we evaluated gene expression after cross-linking the activating Ly-49D mouse NK receptor. Gene expression was evaluated using a mouse GEM 2 microarray chip (Incyte Genomics, St. Louis, MO). Each chip displays a total of 8734 elements. The strongly induced genes fell into two categories: 1) soluble factors and 2) apoptotic genes. The majority of the strongly induced mRNAs as analyzed by microarray hybridization were chemokine genes. RNase protection assays and chemokine protein production analysis validated the microarray results, as cross-linking the Ly-49D mouse NK receptor induced high levels of IFN-gamma, lymphotactin, macrophage-inflammatory protein (MIP)1alpha, and MIP1beta. This gene expression was specific because other chemokines were not induced by anti-Ly-49D receptors. In addition, a series of pharmacological inhibitors were used to identify the key signaling pathways involved in the cellular response. The primary Ly-49D signaling for IFN-gamma production is predominantly mediated through Src kinase pathways involving membrane proximal events, whereas MIP1alpha and MIP1beta gene induction is more complex and may involve multiple biochemical pathways. Thus, we conclude that a primary role for the activating NK receptors in vivo may be to trigger soluble factor production and regulation of the immune response. This would place NK cells and their activating Ly-49 receptors as important initiators of microbial immunity and key elements of the innate immune system.

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.

Molecular cloning and expression pattern of porcine myeloid DAP12-associating lectin-1

DAP12 is an ITAM-bearing membrane protein that is associated with activating receptors in natural killer cells, granulocytes, macrophages, and monocytes. Myeloid DAP12-associating lectin-1 (MDL-1) is a type II membrane protein that associates with DAP12. In this study, we report the molecular cloning of two isoforms of porcine MDL-1 cDNA from pulmonary alveolar macrophages. The porcine MDL-1 short form has 165 amino acids and 70% sequence identity with the mouse MDL-1 short form. The long form has 20 more amino acids in the stalk region and 71% sequence identity with human MDL-1 and 67% with the mouse MDL-1 long form. Porcine MDL-1 contains a conserved lysine in the transmembrane domain. There are six putative N-linked glycosylation sites in the MDL-1 long form. MDL-1 transcripts were detected exclusively in macrophages and monocytes by RT-PCR. When transfected into 293 cells, porcine MDL-1 is expressed on the cell surface associated with DAP12.

Ly-49W, an activating receptor of nonobese diabetic mice with close homology to the inhibitory receptor Ly-49G, recognizes H-2D(k) and H-2D(d)

The diversity and ligand specificity of activating Ly-49 receptors expressed by murine NK cells are largely unknown. We cloned a new Ly-49-activating receptor, expressed by NK cells of the nonobese diabetic mouse strain, which we have designated Ly-49W. Ly-49W is highly related to the known inhibitory receptor Ly-49G in its carbohydrate recognition domain, exhibiting 97.6% amino acid identity in this region. We demonstrate that the 4D11 and Cwy-3 Abs, thought to be Ly-49G specific, also recognize Ly-49W. Rat RNK-16 cells transfected with Ly-49W mediated reverse Ab-dependent cellular cytotoxicity of FcR-positive target cells, indicating that Ly-49W can activate NK-mediated lysis. We further show that Ly-49W is allo-MHC specific: Ly-49W transfectants of RNK-16 only lysed Con A blasts expressing H-2(k) or H-2(d) haplotypes, and Ab-blocking experiments indicated that H-2D(k) and D(d) are ligands for Ly-49W. Ly-49W is the first activating Ly-49 receptor demonstrated to recognize an H-2(k) class I product. Ly-49G and Ly-49W represent a new pair of NK receptors with very similar ligand-binding domains, but opposite signaling functions.

The ITAM-bearing transmembrane adaptor DAP12 in lymphoid and myeloid cell function

DAP12, an ITAM-bearing transmembrane adaptor protein, associates non-covalently with receptors in natural killer (NK) and myeloid cells, and provides signaling function via the Syk and ZAP-70 tyrosine kinase activation pathways. Humans and mice lacking DAP12 (DAP12(-/-)) show normal development of hematopoietic cells. However, DAP12(-/-) humans develop presenile dementia and bone cysts, and DAP12(-/-) mice show impaired immune responses.

The role of the DAP12 signal in mouse myeloid differentiation

DAP12 is a recently cloned, immunoreceptor tyrosine-based activation motif-bearing transmembrane adapter molecule that is associated with the NK-activating receptors. Previous reports showed that the DAP12 message could be detected not only in NK cells but also in granulocytes, monocytes, dendritic cells, and macrophages. In this study we found a significant level of DAP12 protein expression in macrophage-related cell lines and organs. Additionally, we observed increased expression of DAP12 after LPS-induced differentiation of M1 cells into macrophages. To examine the role of DAP12 in the myeloid cell lineage, we established M1 FLAG-DAP12 transfectants (FDAP-M1) and demonstrated the marked morphological changes in FDAP-M1 cells caused by signaling through DAP12. Cell surface phenotypic analysis showed up-regulation of macrophage markers CD11b, 2.4G2, and adhesion molecule B7-2. Additionally, after stimulation through DAP12, phosphorylated FLAG -DAP12 could be immunoprecipitated using anti-phosphotyrosine mAbs. Collectively, these findings indicate that direct DAP12 signaling has an important role in macrophage differentiation.

Ly-49P activates NK-mediated lysis by recognizing H-2Dd

Little is known regarding the ligand specificity of Ly-49 activating receptor subfamily members expressed by NK cells. A new Ly-49 activating receptor related to Ly-49A in its extracellular domain, designated Ly-49P, was recently cloned from 129 strain mice. We independently cloned an apparent allele of Ly-49P expressed by nonobese diabetic and nonobese diabetes-resistant mouse strain NK cells. We found it to be reactive with the A1 Ab thought to recognize a polymorphic epitope expressed only by the Ly-49A inhibitory receptor of the C57BL/6 strain. Rat RNK-16 cells transfected with Ly-49P mediated reverse Ab-dependent cellular cytotoxicity of FcR-positive target cells, indicating that Ly-49P can activate NK-mediated lysis. We determined that RNK-16 lysis of Con A blasts induced by Ly-49P was MHC dependent, resulting in efficient lysis of H-2Dd-bearing targets. We found that the Dd alpha1/alpha2 domain is required for Ly-49P-mediated RNK-16 activation, as determined by exon shuffling and transfection. Thus, Ly-49P is the second activating Ly-49 receptor demonstrated to induce NK cytotoxicity by recognizing a class I MHC molecule.