Retinoic acid early inducible genes define a ligand family for the activating NKG2D receptor in mice

Proliferating cell nuclear antigen is a novel inhibitory ligand for the natural cytotoxicity receptor NKp44

NK cells play an important role in the early immune response to cancer. The NKp44 activating receptor is the only natural cytotoxicity receptor that is expressed exclusively by primate NK cells, yet its cellular ligands remain largely unknown. Proliferating cell nuclear Ag (PCNA) is overexpressed in cancer cells. In this study, we show that the NKp44 receptor recognizes PCNA. Their interaction inhibits NK cell function through NKp44/ITIM. The physical interaction of NKp44 and PCNA is enabled by recruitment of target cell PCNA to the NK immunological synapse. We demonstrate that PCNA promotes cancer survival by immune evasion through inhibition of NKp44-mediated NK cell attack.

Mechanisms of antigen presentation to T cells in murine graft-versus-host disease: cross-presentation and the appearance of cross-presentation

Recipient antigen-presenting cells (APCs) initiate GVHD by directly presenting host minor histocompatibility antigens (miHAs) to donor CD8 cells. However, later after transplantation, host APCs are replaced by donor APCs, and if pathogenic CD8 cells continue to require APC stimulation, then donor APCs must cross-present host miHAs. Consistent with this, CD8-mediated GVHD is reduced when donor APCs are MHC class I(-). To study cross-presentation, we used hosts that express defined MHC class I K(b)-restricted miHAs, crossed to K(b)-deficient backgrounds, such that these antigens cannot be directly presented. Cross-priming was surprisingly efficient, whether antigen was restricted to the hematopoietic or nonhematopoietic compartments. Cross-primed CD8 cells were cytolytic and produced IFN-γ. CD8 cells were exclusively primed by donor CD11c(+) cells, and optimal cross-priming required that they are stimulated by both type I IFNs and CD40L. In studying which donor APCs acquire host miHAs, we made the surprising discovery that there was a large-scale transfer of transmembrane proteins from irradiated hosts, including MHC class I-peptide complexes, to donor cells, including dendritic cells. Donor dendritic cells that acquired host MHC class I-peptide complexes were potent stimulators of peptide-specific T cells. These studies identify new therapeutic targets for GVHD treatment and a novel mechanism whereby donor APCs prime host-reactive T cells.

Expression of the RAE-1 family of stimulatory NK-cell ligands requires activation of the PI3K pathway during viral infection and transformation

Natural killer (NK) cells are lymphocytes that play a major role in the elimination of virally-infected cells and tumor cells. NK cells recognize and target abnormal cells through activation of stimulatory receptors such as NKG2D. NKG2D ligands are self-proteins, which are absent or expressed at low levels on healthy cells but are induced upon cellular stress, transformation, or viral infection. The exact molecular mechanisms driving expression of these ligands remain poorly understood. Here we show that murine cytomegalovirus (MCMV) infection activates the phosphatidylinositol-3-kinase (PI3K) pathway and that this activation is required for the induction of the RAE-1 family of mouse NKG2D ligands. Among the multiple PI3K catalytic subunits, inhibition of the p110α catalytic subunit blocks this induction. Similarly, inhibition of p110α PI3K reduces cell surface expression of RAE-1 on transformed cells. Many viruses manipulate the PI3K pathway, and tumors frequently mutate the p110α oncogene. Thus, our findings suggest that dysregulation of the PI3K pathway is an important signal to induce expression of RAE-1, and this may represent a commonality among various types of cellular stresses that result in the induction of NKG2D ligands.

Human and mouse cytomegaloviruses (HCMV and MCMV) are members of the Herpesvirus family. Both viruses cause disease in individuals with a compromised immune system, such as transplant patients and AIDS patients. Natural killer (NK) cells are essential players in the immune response against these viruses. NK cells recognize self-proteins, such as NKG2D ligands, that are poorly expressed on healthy cells but are upregulated on cells that are undergoing stress, such as infection and tumor development. The biological processes associated with NKG2D ligand expression in infected cells are unknown. The PI3K pathway, which controls many cellular processes, is activated by a variety of viruses to prime cells for efficient viral replication. We observed that MCMV activates the PI3K pathway and that this activation is required for NKG2D ligand expression. We also found that the expression of NKG2D ligands on cancer cell lines is dependent on this pathway. Our data suggest that NKG2D ligand expression, and thus recognition of infected and cancer cells by NK cells, is associated with a dysregulation in the PI3K pathway.

Engagement of NK receptor NKG2D, but not 2B4, results in self-reactive CD8+ T cells and autoimmune vitiligo

In this study, we demonstrate that engagement of two different natural killer receptors (NKRs) can lead to contrasting effects in the development of self-reactive CD8+T cells and autoimmune vitiligo. Specifically, using a mouse model, we show that CD8+T-cell targeting of a melanocyte antigen, tyrosinase-related protein-1 (TRP-1) in combination with delivery of the NKG2D ligands (Rae-1ϵ or H60), results in strong CD8+T-cell responses against TRP-1 and in the development of autoimmune vitiligo. In contrast, targeting of TRP-1 in combination with delivery of CD48, the natural ligand for the NKR 2B4, leads to reduced formation of TRP-1-reactive CD8+T-cell responses and decreased development of vitiligo. These data indicate that autoimmune vitiligo is limited by insufficient signals, despite plentiful self-reactive T cells in the peripheral immune system. To our knowledge, this is the first experimental evidence supporting the role of NKRs in modulating CD8+T-cell autoimmune vitiligo. This study supports the utilization of NKR signaling as a therapeutic avenue toward prevention of vitiligo and other autoimmune diseases.

Regulation of immune cell function and differentiation by the NKG2D receptor

NKG2D is one of the most intensively studied immune receptors of the past decade. Its unique binding and signaling properties, expression pattern, and functions have been attracting much interest within the field due to its potent antiviral and anti-tumor properties. As an activating receptor, NKG2D is expressed on cells of the innate and adaptive immune system. It recognizes stress-induced MHC class I-like ligands and acts as a molecular sensor for cells jeopardized by viral infections or DNA damage. Although the activating functions of NKG2D have been well documented, recent analysis of NKG2D-deficient mice suggests that this receptor may have a regulatory role during NK cell development. In this review, we will revisit known aspects of NKG2D functions and present new insights in the proposed influence of this molecule on hematopoietic differentiation.

Cancer immunoediting of the NK group 2D ligand H60a

Cancer immunoediting describes the process whereby highly immunogenic tumor cells are removed, or edited, from the primary tumor repertoire by the immune system. In immunodeficient mice, the editing process is hampered, and "unedited" tumor cells can be recovered and studied. In this study, we compared unedited and edited tumors for their expression of NK group 2D (NKG2D) ligands, a family of surface proteins expressed on tumor cells that can activate NK cell cytotoxic activity. We found that the expression of the NKG2D ligand H60a was more heterogeneous in groups of unedited 3'-methylcholanthrene sarcoma cell lines compared with that in edited 3'-methylcholanthrene sarcoma cell lines (i.e., some unedited cell lines expressed very high levels of H60a, whereas other unedited and edited cell lines expressed very low levels). We also found that some highly immunogenic cell lines displayed a bimodal distribution consisting of H60a-hi and H60a-lo cells. In one of these cell lines, the H60a-hi cells could be removed by passaging the cells through RAG2(-/-) mice, resulting in edited cell lines that were poor targets for NK cells and that displayed progressive tumor growth. This editing of H60a-hi cells required NK cells and NKG2D. Our studies show that the expression of H60a on tumors cells can be actively modulated by the immune system, thereby implicating this NKG2D ligand in tumor immunosurveillance.

Thy1+ NK [corrected] cells from vaccinia virus-primed mice confer protection against vaccinia virus challenge in the absence of adaptive lymphocytes

While immunological memory has long been considered the province of T- and B- lymphocytes, it has recently been reported that innate cell populations are capable of mediating memory responses. We now show that an innate memory immune response is generated in mice following infection with vaccinia virus, a poxvirus for which no cognate germline-encoded receptor has been identified. This immune response results in viral clearance in the absence of classical adaptive T and B lymphocyte populations, and is mediated by a Thy1⁺ subset of natural killer (NK) cells. We demonstrate that immune protection against infection from a lethal dose of virus can be adoptively transferred with memory hepatic Thy1⁺ NK cells that were primed with live virus. Our results also indicate that, like classical immunological memory, stronger innate memory responses form in response to priming with live virus than a highly attenuated vector. These results demonstrate that a defined innate memory cell population alone can provide host protection against a lethal systemic infection through viral clearance.

Immunological memory is a hallmark of adaptive immunity and provides the basis for our ability to become ‘immune’ to pathogens to which we have previously been exposed, and provides the basis for vaccination. For decades, the paradigm held that only the classical adaptive lymphocytes were capable of forming and maintaining protective immunological memory. Recently, several papers have shown the capacity of an innate cell population, a subset of natural killer (NK) cells, to exhibit certain aspects of immunological memory. Here we show that innate memory forms in response to infection with vaccinia virus and resides in a discrete subset of NK cells. We further demonstrate that this innate memory provides significant host protection against a subsequent systemic infection with a lethal dose of vaccinia virus, in some cases resulting in the complete clearance of detectable virus. We also demonstrate that priming with live, replicating virus stimulates innate memory more robustly than a highly attenuated vector. These findings shed new light on this emergent area of immunology, and hold significant implications for harnessing innate memory as part of creating novel vaccination strategies.

Transforming growth factor-beta-activated kinase 1 regulates natural killer cell-mediated cytotoxicity and cytokine production

Carma1, a caspase recruitment domain-containing membrane-associated guanylate kinase, initiates a unique signaling cascade via Bcl10 and Malt1 in NK cells. Carma1 deficiency results in reduced phosphorylation of JNK1/2 and activation of NF-κB that lead to impaired NK cell-mediated cytotoxicity and cytokine production. However, the precise identities of the downstream signaling molecules that link Carma1 to these effector functions were not defined. Here we show that transforming growth factor-β (TGF-β)-activated kinase 1 (TAK1) is abundantly present in NK cells, and activation via NKG2D results in its phosphorylation. Lack of Carma1 considerably reduced TAK1 phosphorylation, demonstrating the dependence of TAK1 on Carma1 in NKG2D-mediated NK cell activations. Pharmacological inhibitor to TAK1 significantly reduced NK-mediated cytotoxicity and its potential to generate IFN-γ, GM-CSF, MIP-1α, MIP-1β, and RANTES. Conditional in vivo knockdown of TAK1 in NK cells from Mx1Cre(+)TAK1(fx/fx) mice resulted in impaired NKG2D-mediated cytotoxicity and cytokine/chemokine production. Inhibition or conditional knockdown of TAK1 severely impaired the NKG2D-mediated phosphorylation of ERK1/2 and JNK1/2 and activation of NF-κB and AP1. Our results show that TAK1 links Carma1 to NK cell-mediated effector functions.

Differential survival of γδT cells, αβT cells and NK cells upon engagement of NKG2D by NKG2DL-expressing leukemic cells

Herein, we show that γδT, CD8(+) αβT lymphocytes and natural killer (NK) cells display a different sensitivity to survival signals delivered via NKG2D surface receptor. All the three effector cell populations activate Akt1/PKBalpha through the engagement of this molecule. Upon binding to leukemic cells expressing NKG2D ligands (NKG2DL), including chronic lymphocytic leukemias treated with transretinoic acid, most γδT (>60%) and half CD8(+) αβT cells (about 50%) received a survival signal, at variance with the majority of NK cells (>80%) that underwent apoptosis by day 5. Interestingly, oligomerization of NKG2D in γδT or CD8(+) αβT cells, led to a significant rise in nuclear/cytoplasmic ratio of both NF-kBp52 and RelB, the two NF-kB subunits mainly involved in the transcription of antiapoptotic proteins of the Bcl family. Indeed, the ratio between the antiapoptotic protein Bcl-2 or Bcl-x(L) and the proapoptotic protein Bax raised in γδT or CD8(+) αβT cells following NKG2D engagement by specific monoclonal antibodies or by NKG2DL expressing leukemic cells. Conversely, nuclear translocation of NF-kBp52 or RelB did not increase, nor the Bcl-2/Bax or the Bcl-x(L) /Bax ratios changed significantly, in NK cells upon oligomerizaton of NKG2D. Of note, transcripts for α5 importin, responsible for nuclear translocation of NF-kBp52/Rel B heterodimer, are significantly higher in γδT and CD8(+) αβT cells than in NK cells. These biochemical data may explain, at least in part, why γδT and CD8(+) αβT cells are cytolytic effector cells more resistant to target-induced apoptosis than NK cells.

How the virus outsmarts the host: function and structure of cytomegalovirus MHC-I-like molecules in the evasion of natural killer cell surveillance

Natural killer (NK) cells provide an initial host immune response to infection by many viral pathogens. Consequently, the viruses have evolved mechanisms to attenuate the host response, leading to improved viral fitness. One mechanism employed by members of the β-herpesvirus family, which includes the cytomegaloviruses, is to modulate the expression of cell surface ligands recognized by NK cell activation molecules. A novel set of cytomegalovirus (CMV) genes, exemplified by the mouse m145 family, encode molecules that have structural and functional features similar to those of host major histocompatibility-encoded (MHC) class I molecules, some of which are known to contribute to immune evasion. In this review, we explore the function, structure, and evolution of MHC-I-like molecules of the CMVs and speculate on the dynamic development of novel immunoevasive functions based on the MHC-I protein fold.

Natural killer cells mediate severe liver injury in a murine model of halothane hepatitis

Severe halothane (HAL)-induced hepatotoxicity occurs in one in 6000-30,000 patients by an unknown mechanism. Female sex is a risk factor in humans and rodents. We tested the hypothesis that a sex difference in natural killer (NK) cell activity contributes to HAL-induced liver injury. HAL (15 mmol/kg, ip) treatment resulted in severe liver injury by 12 h in female, wild-type BALB/cJ mice, and the magnitude of liver injury varied with stage of the estrous cycle. Ovariectomized (OVX) mice developed only mild liver injury. Plasma interferon-gamma (IFN-γ) was elevated 10-fold in HAL-treated females compared with similarly treated male mice or with OVX female mice. IFN-γ knockout mice were resistant to severe HAL-induced liver injury. The deactivation of NK cells with anti-asialo GM1 treatment attenuated liver injury and the increase in plasma IFN-γ compared with immunoglobulin G-treated control mice. Mice with a mutated form of perforin, a protein involved in granule-mediated cytotoxicity, were protected from severe liver injury. Furthermore, HAL increased the activity of NK cells in vivo, as indicated by increased surface expression of CD69, an early activation marker. In response to HAL, NK cell receptor ligands on the surface of hepatocytes were expressed in a manner that can activate NK cells. These results confirm the sexual dimorphic hepatotoxic response to HAL in mice and suggest that IFN-γ and NK cells have essential roles in the development of severe HAL-induced hepatotoxicity.

Studies on the antigenicity of the NKG2D ligand H60a in tumour cells

H60a is a minor histocompatibility antigen expressed in BALB and 129/Sv but not C57BL/6 mouse strains. The majority of CD8+ T cells in C57BL/6 mice responding to BALB.B splenocytes are specific for H60a. Interestingly, H60a is expressed constitutively on tumour cells, but its nature as a tumour rejection antigen, as a parallel to its function as a transplant rejection antigen, has not been studied. In this report, we show that tumour cells that constitutively express H60a at the cell surface can be recognized by H60a-specific T cells. Furthermore, when H60a-expressing sarcoma cell lines are transplanted into C57BL/6 mice, H60a-specific T cells can be found at high percentages among the tumour-infiltrating CD8+ T cells. These findings were seen in C57BL/6 but not F1 (C57BL/6×129) mice (which express H60a), suggesting that endogenous tolerance mechanisms suppress the antigenic properties of H60a. Our findings have implications for the generation of tumour vaccines against human natural killer group 2D ligands, such as MHC class I chain-like gene A, that are also transplantation antigens.

Essential role for retinoic acid in the promotion of CD4(+) T cell effector responses via retinoic acid receptor alpha

Vitamin A and its metabolite, retinoic acid (RA) are implicated in the regulation of immune homeostasis via the peripheral induction of regulatory T cells. Here we showed RA was also required to elicit proinflammatory CD4(+) helper T cell responses to infection and mucosal vaccination. Retinoic acid receptor alpha (RARα) was the critical mediator of these effects. Antagonism of RAR signaling and deficiency in RARα (Rara(-/-)) resulted in a cell-autonomous CD4(+) T cell activation defect, which impaired intermediate signaling events, including calcium mobilization. Altogether, these findings reveal a fundamental role for the RA-RARα axis in the development of both regulatory and inflammatory arms of adaptive immunity and establish nutritional status as a broad regulator of adaptive T cell responses.

The role of natural killer cells in the defense against Listeria monocytogenes lessons from a rat model

Ly49 receptors in rodents, like killer cell immunoglobulin-like receptors in humans, regulate natural killer (NK) cell activity. Although inhibitory Ly49 receptors clearly recognize classical major histocompatibility complex class I (MHC-I) molecules, the role for the activating Ly49 receptors has been less well understood. Here, we discuss recent data from a rat model for listeriosis. Rats depleted of NK cells, or more specifically the Ly49 receptor-bearing cells, showed increased bacterial loads in their spleen. Athymic nude rats with no functional T cells but increased numbers of Ly49-expressing NK cells were more resistant to infection, indicating a central role of NK cells in early immune defense against Listeria in this species. Listeria infection of macrophages or enteric epithelial cells led to upregulation of MHC-I, including nonclassical (Ib) molecules not regularly recognized by T cells. We have shown that activating Ly49 receptors are more efficiently stimulated when binding to upregulated class Ib antigens on infected cells. From this we postulate that activating Ly49 receptors may have a sentinel function in the early immune response against Listeria in detecting diseased cells 'flagged' by increased MHC-Ib expression.

Construction of a plasmid for co-expression of mouse membrane-bound form of IL-15 and RAE-1ε and its biological activity

Interleukin 15 (IL-15) is a pivotal cytokine for the proliferation and activation of a specific group of immune cells such as natural killer (NK), IFN-producing killer dendritic cells (IKDC) and CD8 T cells. RAE-1ε, the ligand for the activating NKG2D receptor, which also play an important role in the proliferation and activation of NK cells and IKDCs. In this study, a membrane-bound form of IL-15 (termed mb15) encoding sequence and RAE-1ε gene were obtained by SOE-PCR or PCR amplification. The amplified mb15 and RAE-1ε gene were then digested and inserted into the multiple cloning site1 (MCS1) and MCS2 of pVITRO2-mcs vector, respectively. A recombinant eukaryotic expression vector for co-expression of mb15 and RAE-1ε was successfully constructed. After it was transfected to BaF3 cells, the expression of IL-15 and RAE-1ε in recombinant BaF3/mb15/RAE-1ε cells were verified by RT-PCR, western blot and FCM analysis. Furthermore, BaF3/mb15/RAE-1ε cells had the ability of promoting NK cells proliferation and IFN-γ secretion. In conclusion, BaF3/mb15/RAE-1ε cells were successfully constructed, which is very useful for further studies, especially for the expansion and activation of certain subsets of immune cells such as NK cells and IKDCs.

Activation receptor-induced tolerance of mature NK cells in vivo requires signaling through the receptor and is reversible

NK cell responses are determined by signals received through activating and inhibitory cell surface receptors. Ly49H is an NK cell-specific activating receptor that accounts for the genetic resistance to murine CMV (MCMV). The Ly49H receptor has been shown to interact with two adaptor proteins (DAP12 and DAP10). In the context of MCMV infection, interaction of m157 (the MCMV-encoded ligand for Ly49H) with Ly49H results in activation of Ly49H-expressing NK cells. Chronic exposure of Ly49H with m157, however, induces tolerance in these same cells. The mechanism of this tolerance remains poorly understood. Using a transgenic mouse model, we demonstrate that induction of tolerance in Ly49H(+) NK cells by chronic exposure to m157, in vivo, requires signaling through the Ly49H adaptor protein DAP12, but not the DAP10 adaptor protein. Furthermore, mature Ly49H-expressing NK cells from wild-type mice can acquire a tolerant phenotype by 24 h posttransfer into a transgenic C57BL/6 mouse that expresses m157. The tolerant phenotype can be reversed, in vivo, if tolerant NK cells are transferred to mice that do not express the m157 protein. Thus, continuous activating receptor engagement can induce a transient tolerance in mature NK cells in vivo. These observations provide new insight into how activating receptor engagement shapes NK cell function and has important implications in how NK cells respond to tumors and during chronic viral infection.

Delivery of NKG2D ligand using an anti-HER2 antibody-NKG2D ligand fusion protein results in an enhanced innate and adaptive antitumor response

NKG2D ligands link the innate and adapative immune response by activating the receptors expressed on effector cells of both the innate (NK) and adaptive immune systems (CD8(+) T cells). In this study, we explored the potential therapeutic utility of this intersection by fusing the murine NKG2D ligand Rae-1β to the 3' end of an anti-HER2 IgG3 antibody containing an intact Fc domain (anti-HER2 IgG3-Rae-1β), thereby targeting an NK cell activation signal to HER2+ breast tumor cells. The antitumor efficacy of this anti-HER2-Rae-1β fusion protein was examined in a mouse mammary tumor model engineered to express HER2 (EMT6-HER2 cells). We observed an enhanced cytotoxic response of NK effectors against EMT-HER2 cells in vitro. Mice implanted on one flank with EMT6-HER2 cells and contralaterally with control EMT6 cells exhibited rapid regression of EMT6-HER2 tumors but delayed regression of contralateral EMT6 tumors. IFNγ was implicated, given a lack of antitumor efficacy in IFNγ(-/-) mice. Depletion of either NK cells or CD8(+) T cells abrogated tumor growth inhibition, suggesting essential roles for each in the observed antitumor activity. Mice rejecting EMT6-HER2 tumors after anti-HER2-Rae-1β treatment showed markedly decreased tumor growth when rechallenged with EMT6-HER2 or EMT6 cells, whereas both EMT6 and EMT6-HER2 cells grew in control mice, indicating the development of an adaptive memory response. Our findings demonstrate that administration of an antibody-NKG2D ligand fusion protein can enhance innate and adaptive immune antitumor responses, also evoking additional nontargeted antigens to enhance the potential clinical utility of this approach.

Construction, expression, and purification of chimeric protein reagents based on immunoglobulin fc regions

Recombinant fusion proteins incorporating experimental protein domains fused to immunoglobulin Fc regions have become widely utilized in studies of protein-ligand interactions. The advantages of these systems include an inherent increase in avidity provided by the multimerization of Fc regions, combined with robust detection methods based on numerous commercially available secondary reagents directed against the Fc tag. We describe a set of methods for subcloning, expression, and purification of chimeric protein reagents containing a protein domain (or domains) of interest fused to a C-terminal moiety derived from the Fc region of either IgG or IgM.

Studies of the H60a locus in C57BL/6 and 129/Sv mouse strains identify the H60a 3'UTR as a regulator of H60a expression

The minor histocompatibility antigen 60 (H60a) is expressed in BALB/C and 129/Sv but not in C57BL/6 strains of mice. We recently found that IFNγ down-regulates H60a, but the mechanism of regulation is not known. To better understand the regulation of H60a, we examined the genomic locus of H60a in 129/Sv and C57BL/6 strains. We found that the upstream regulatory region of H60a was present and functional in both strains. Interestingly, IFNγ can down-regulate H60a transcripts in cell lines from 129/Sv but not C57BL/6 strains of mice, suggesting that IFNγ-dependent regulation of H60a proceeds through cis elements other than the conserved promoter region. We determined that the regulation of H60a by IFNγ proceeds through the 3'UTR of H60a, which is present in 129/Sv, but not C57BL/6 cells. We also found that the H60a 3'UTR and microRNAs can contribute to the level of constitutive expression of H60a in tumor cell lines. We conclude that in 129/Sv strain mice, H60a can be regulated by its 3'UTR through IFNγ and unknown microRNAs. Since H60a mediates NK cell target recognition, our studies identify a cis element that can regulate virus and tumor surveillance.

Linkage disequilibrium of polymorphic RAET1 genes in Thais

Retinoic acid early transcripts-1 (RAET1) or unique long 16 (UL-16) binding proteins (ULBPs) is a gene cluster encoding for molecules acting as ligands to natural killer group 2 D (NKG2D), a receptor expressed on immune cells. Binding of these ligands to the receptor activates immune cells leading to killing of tumor cells and also viral-infected cells. The information on polymorphism of RAET1 is limited. In this report, we analyze the linkages between four polymorphic RAET1 genes: RAET1E, RAET1G, RAET1H and RAET1L, in 318 unrelated Thais. The strongest linkage disequilibrium was found between RAET1E and RAET1G, with P-value, D' and r(2) of <5.0 x 10(-5), 0.707 and 0.840, respectively. RAET1E(*)001 was found to be in linkage disequilibrium with RAET1G(*)002, and RAET1E(*)002 with RAET1G(*)001. Evidently, there were possible RAET1 haplotypes with haplotype frequencies of more than 10% consisting of RAET1E(*)001; RAET1G(*)002; RAET1H(*)001; RAET1L(*)001 and RAET1E(*)002; RAET1G(*)001; RAET1H(*)002; RAET1L(*)003. This study provides basic information on polymorphisms of RAET1 and possible RAET1 haplotypes in Thais.

Identification of a polyI:C-inducible membrane protein that participates in dendritic cell-mediated natural killer cell activation

The novel polyI:C-inducible membrane protein INAM triggers dendritic cell–mediated natural killer cell activation.

In myeloid dendritic cells (mDCs), TLR3 is expressed in the endosomal membrane and interacts with the adaptor toll/interleukin 1 receptor homology domain–containing adaptor molecule 1 (TICAM-1; TRIF). TICAM-1 signals culminate in interferon (IFN) regulatory factor (IRF) 3 activation. Co-culture of mDC pretreated with the TLR3 ligand polyI:C and natural killer (NK) cells resulted in NK cell activation. This activation was triggered by cell-to-cell contact but not cytokines. Using expression profiling and gain/loss-of-function analyses of mDC genes, we tried to identify a TICAM-1–inducing membrane protein that participates in mDC-mediated NK activation. Of the nine candidates screened, one contained a tetraspanin-like sequence and satisfied the screening criteria. The protein, referred to as IRF-3–dependent NK-activating molecule (INAM), functioned in both the mDC and NK cell to facilitate NK activation. In the mDC, TICAM-1, IFN promoter stimulator 1, and IRF-3, but not IRF-7, were required for mDC-mediated NK activation. INAM was minimally expressed on NK cells, was up-regulated in response to polyI:C, and contributed to mDC–NK reciprocal activation via its cytoplasmic tail, which was crucial for the activation signal in NK cells. Adoptive transfer of INAM-expressing mDCs into mice implanted with NK-sensitive tumors caused NK-mediated tumor regression. We identify a new pathway for mDC–NK contact-mediated NK activation that is governed by a TLR signal-derived membrane molecule.

The NKG2D ligands RAE-1δ and RAE-1ε differ with respect to their receptor affinity, expression profiles and transcriptional regulation

BACKGROUND

RAE-1 is a ligand of the activating receptor NKG2D expressed by NK cells, NKT, γδT and some CD8(+)T lymphocytes. RAE-1 is overexpressed in tumor cell lines and its expression is induced after viral infection and genotoxic stress. We have recently demonstrated that RAE-1 is expressed in the adult subventricular zone (SVZ) from C57BL/6 mice. RAE-1 is also expressed in vitro by neural stem/progenitor cells (NSPCs) and plays a non-immune role in cell proliferation. The C57BL/6 mouse genome contains two rae-1 genes, rae-1δ and rae-1ε encoding two different proteins. The goals of this study are first to characterize the in vivo and in vitro expression of each gene and secondly to elucidate the mechanisms underlying their respective expression, which are far from known.

PRINCIPAL FINDINGS

We observed that Rae-1δ and Rae-1ε transcripts are differentially expressed according to tissues, pathological conditions and cell lines. Embryonic tissue and the adult SVZ mainly expressed Rae-1δ transcripts. The NSPCs derived from the SVZ also mainly expressed RAE-1δ. The interest of this result is especially related to the observation that RAE-1δ is a weak NKG2D ligand compared to RAE-1ε. On the contrary, cell lines expressed either similar levels of RAE-1δ and RAE-1ε proteins or only RAE-1ε. Since the protein expression correlated with the level of transcripts for each rae-1 gene, we postulated that transcriptional regulation is one of the main processes explaining the difference between RAE-1δ and RAE-1ε expression. We indeed identified two different promoter regions for each gene: one mainly involved in the control of rae-1δ gene expression and the other in the control of rae-1ε expression.

CONCLUSIONS/SIGNIFICANCE

RAE-1δ and RAE-1ε differ with respect to their function and the control of their expression. Immune function would be mainly exerted by RAE-1ε and non-immune function by RAE-1δ.

Sialylation of 3-methylcholanthrene-induced fibrosarcoma determines antitumor immune responses during immunoediting

Sialylation of tumor cells is involved in various aspects of their malignancy (proliferation, motility, invasion, and metastasis); however, its effect on the process of immunoediting that affects tumor cell immunogenicity has not been studied. We have shown that in mice with impaired immunoediting, such as in IL-1α(-/-) and IFNγ(-/-) mice, 3-methylcholanthrene-induced fibrosarcoma cells are immunogenic and concomitantly bear low levels of surface sialylation, whereas tumor cells derived from wild type mice are nonimmunogenic and bear higher levels of surface sialylation. To study immune mechanisms whose interaction with tumor cells involves surface sialic acid residues, we used highly sialylated 3-methylcholanthrene-induced nonimmunogenic fibrosarcoma cell lines from wild type mice, which were treated with sialidase to mimic immunogenic tumor cell variants. In vivo and in vitro experiments revealed that desialylation of tumor cells reduced their growth and induced cytotoxicity by NK cells. Moreover, sialidase-treated tumor cells better activated NK cells for IFN-γ secretion. The NKG2D-activating receptor on NK cells was shown to be involved in interactions with desialylated ligands on tumor cells, the nature of which is still not known. Thus, the degree of sialylation on tumor cells, which is selected during the process of immunoediting, has possibly evolved as an important mechanism of tumor cells with low intrinsic immunogenicity or select for tumor cells that can evade the immune system or subvert its function. When immunoediting is impaired, such as in IFN-γ(-/-) and IL-1α(-/-) mice, the overt tumor consists of desialylayed tumor cells that interact better with immunosurveillance cells.

Intravital imaging reveals distinct dynamics for natural killer and CD8(+) T cells during tumor regression

Recognition of NKG2D ligands by natural killer (NK) cells plays an important role during antitumoral responses. To address how NKG2D engagement affects intratumoral NK cell dynamics, we performed intravital microscopy in a Rae-1β-expressing solid tumor. This NKG2D ligand drove NK cell accumulation, activation, and motility within the tumor. NK cells established mainly dynamic contacts with their targets during tumor regression. In sharp contrast, cytotoxic T lymphocytes (CTLs) formed stable contacts in tumors expressing their cognate antigen. Similar behaviors were observed during effector functions in lymph nodes. In vitro, contacts between NK cells and their targets were cytotoxic but did not elicit sustained calcium influx nor adhesion, whereas CTL contact stability was critically dependent on extracellular calcium entry. Altogether, our results offer mechanistic insight into how NK cells and CTLs can exert cytotoxic activity with remarkably different contact dynamics.

Natural killer cell tolerance persists despite significant reduction of self MHC class I on normal target cells in mice

BACKGROUND

A major group of murine inhibitory receptors on Natural Killer (NK) cells belong to the Ly49 receptor family and recognize MHC class I molecules. Infected or transformed target cells frequently downmodulate MHC class I molecules and can thus avoid CD8(+) T cell attack, but may at the same time develop NK cell sensitivity, due to failure to express inhibitory ligands for Ly49 receptors. The extent of MHC class I downregulation needed on normal cells to trigger NK cell effector functions is not known.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we show that cells expressing MHC class I to levels well below half of the host level are tolerated in an in vivo assay in mice. Hemizygous expression (expression from only one allele) of MHC class I was sufficient to induce Ly49 receptor downmodulation on NK cells to a similar degree as homozygous expression, despite a strongly reduced cell surface level of MHC class I. Co-expression of weaker MHC class I ligands in the host did not have any further effect on the degree of Ly49 downmodulation. Furthermore, a single MHC class I allele could downmodulate up to three Ly49 receptors on individual NK cells. Only when NK cells simultaneously expressed several Ly49 receptors and hemizygous MHC class I levels, a putative threshold for Ly49 downmodulation was reached.

CONCLUSION

Collectively, our findings suggest that in interactions between NK cells and normal untransformed cells, MHC class I molecules are in most cases expressed in excess compared to what is functionally needed to ensure self tolerance and to induce maximal Ly49 downmodulation. We speculate that the reason for this is to maintain a safety margin for otherwise normal, autologous cells over a range of MHC class I expression levels, in order to ensure robustness in NK cell tolerance.

Chemotherapy-induced genotoxic stress promotes sensitivity to natural killer cell cytotoxicity by enabling missing-self recognition

Natural killer (NK) cells can recognize and kill tumor cells lacking "self" markers, such as class I MHC, but the basis for this recognition is not completely understood. NKR-P1 receptors are members of the C-type lectin-related NK receptor superfamily that are conserved from rodents to humans. Identification of Clr ligands for the NKR-P1 receptors has facilitated functional analysis of MHC-independent target cell recognition by NK cells. One receptor-ligand pair, NKR-P1B:Clr-b, can mediate "missing-self" recognition of tumor and infected cells, but the role of this axis in sensing stressed cells remains unknown. Here, we show that Clr-b is rapidly downregulated in cells undergoing genotoxic and cellular stress at the level of both RNA and surface protein. Stress-mediated loss of Clr-b on leukemia cells enhanced cytotoxicity mediated by NKR-P1B(+) NK cells. Notably, Clr-b downregulation was coordinated functionally with stress-mediated upregulation of NKG2D ligands (but not class I MHC). Our findings highlight a unique role for the MHC-independent NKR-P1B:Clr-b missing-self axis in recognition of stressed cells, and provide evidence of two independent levels of Clr-b regulation in stressed cells.

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.

Junb regulates arterial contraction capacity, cellular contractility, and motility via its target Myl9 in mice

Cellular contractility and, thus, the ability to alter cell shape are prerequisites for a number of important biological processes such as cytokinesis, movement, differentiation, and substrate adherence. The contractile capacity of vascular smooth muscle cells (VSMCs) is pivotal for the regulation of vascular tone and thus blood pressure and flow. Here, we report that conditional ablation of the transcriptional regulator Junb results in impaired arterial contractility in vivo and in vitro. This was exemplified by resistance of Junb-deficient mice to DOCA-salt-induced volume-dependent hypertension as well as by a decreased contractile capacity of isolated arteries. Detailed analyses of Junb-deficient VSMCs, mouse embryonic fibroblasts, and endothelial cells revealed a general failure in stress fiber formation and impaired cellular motility. Concomitantly, we identified myosin regulatory light chain 9 (Myl9), which is critically involved in actomyosin contractility and stress fiber assembly, as a Junb target. Consistent with these findings, reexpression of either Junb or Myl9 in Junb-deficient cells restored stress fiber formation, cellular motility, and contractile capacity. Our data establish a molecular link between the activator protein-1 transcription factor subunit Junb and actomyosin-based cellular motility as well as cellular and vascular contractility by governing Myl9 transcription.

Intact NKG2D-independent function of NK cells chronically stimulated with the NKG2D ligand Rae-1

Human tumors frequently express membrane-bound or soluble NK group 2, member D (NKG2D) ligands. This results in chronic engagement of NKG2D on the surfaces of NK and CD8(+) T cells and rapid internalization of the receptor. Although it is well appreciated that this phenomenon impairs NKG2D-dependent function, careful analysis of NKG2D-independent functions in cells chronically stimulated through NKG2D is lacking. Using a mouse model of chronic NKG2D ligand expression, we show that constant exposure to NKG2D ligands does not functionally impair NK cells and CD8(+) T cells in the context of viral infection.

NK cells and gammadelta T cells mediate resistance to polyomavirus-induced tumors

NK and γδ T cells can eliminate tumor cells in many experimental models, but their effect on the development of tumors caused by virus infections in vivo is not known. Polyomavirus (PyV) induces tumors in neonatally infected mice of susceptible strains and in adult mice with certain immune deficiencies, and CD8+ αβ T cells are regarded as the main effectors in anti-tumor immunity. Here we report that adult TCRβ knockout (KO) mice that lack αβ but have γδ T cells remain tumor-free after PyV infection, whereas TCRβ×δ KO mice that lack all T cells develop tumors. In addition, E26 mice, which lack NK and T cells, develop the tumors earlier than TCRβ×δ KO mice. These observations implicate γδ T and NK cells in the resistance to PyV-induced tumors. Cell lines established from PyV-induced tumors activate NK and γδ T cells both in culture and in vivo and express Rae-1, an NKG2D ligand. Moreover, these PyV tumor cells are killed by NK cells in vitro, and this cytotoxicity is prevented by treatment with NKG2D-blocking antibodies. Our findings demonstrate a protective role for NK and γδ T cells against naturally occurring virus-induced tumors and suggest the involvement of NKG2D-mediated mechanisms.

Virus-induced tumors account for a large fraction of malignancies in both humans and mice. These tumors express viral antigens and have been thought to be controlled mostly by αβ TCR+ CD8 T lymphocytes that are specific for viral peptides. We found that mice lacking αβ T cells are protected from the formation of tumors induced by the small DNA virus polyoma (PyV) if they have γδ T and NK cells. Moreover, cell lines we established from the virus-induced tumors induced NK and γδ T cell activation, and expressed Rae-1, a cellular stress molecule which serves as ligand for NKG2D, an activating receptor on NK and γδ T cells. NK and γδ T cells seemed to mount antitumor but not antiviral responses, as their presence did not change the amount of persisting virus significantly. Our studies suggest that mice have a multipronged host defense against PyV-induced tumors that includes γδ T and NK cells in addition to αβ T cell responses. Merkel cell virus, a tumor causing polyomavirus in humans, is closely related to PyV with a similar biology, making it very important to understand mechanisms involved in host control of tumor development in the course of these life-long persistent infections.

Effect of NKG2D ligand expression on host immune responses

Natural killer group 2, member D (NKG2D) is an activating receptor present on the surface of natural killer (NK) cells, some NKT cells, CD8(+) cytotoxic T cells, gammadelta T cells, and under certain conditions CD4(+) T cells. Present in both humans and mice, this highly conserved receptor binds to a surprisingly diverse family of ligands that are distant relatives of major histocompatibility complex class I molecules. There is increasing evidence that ligand expression can result in both immune activation (tumor clearance, viral immunity, autoimmunity, and transplantation) and immune silencing (tumor evasion). In this review, we describe this family of NKG2D ligands and the various mechanisms that control their expression in stressed and normal cells. We also discuss the host response to both membrane-bound and secreted NKG2D ligands and summarize the models proposed to explain the consequences of this differential expression.

In vivo tumor cell rejection induced by NK cell inhibitory receptor blockade: maintained tolerance to normal cells even in the presence of IL-2

Missing-self-reactivity can be mimicked by blocking self-specific inhibitory receptors on NK cells, leading to increased rejection of syngeneic tumor cells. Using a mouse model, we investigated whether Ab-mediated blocking of inhibitory receptors, to a degree where NK cells rejected syngeneic tumor cells, would still allow self-tolerance toward normal syngeneic cells. Ly49C/I inhibitory receptors on C57BL/6 (H-2(b)) NK cells were blocked with F(ab')(2) fragments of the mAb 5E6. Inhibitory receptor blockade in vivo caused rejection of i.v. inoculated fluorescence-labeled syngeneic lymphoma line cells but not of syngeneic spleen cells, BM cells or lymphoblasts. The selective rejection of tumor cells was NK cell-dependent and specifically induced by Ly49C/I blockade. Moreover, selective tumor rejection was maintained after treatment with 5E6 F(ab')(2) for 9 wk, arguing against the induction of NK cell anergy or autoreactivity during this time. Combination therapy using 5E6 F(ab')(2) together with high dose IL-2 treatment further increased lymphoma cell rejection. In addition, combination therapy reduced growth of melanoma cell line tumors established by s.c. inoculation 3 days before start of treatment. Our results demonstrate that inhibitory receptor blockade does not result in attack on normal cells, despite potent reactivity against MHC class I-expressing tumors.

Comparative genomic analysis of mammalian NKG2D ligand family genes provides insights into their origin and evolution

NKG2D is a major activating receptor of natural killer cells. Its ligands are major histocompatibility complex (MHC) class I-like molecules whose expression is induced by cellular stresses such as infections and tumorigenesis. Humans have two families of NKG2D ligands (NKG2DL): MHC class I-related chains (MIC) encoded in the MHC and UL16-binding proteins (ULBP) encoded outside the MHC. By contrast, mice have only the latter family of ligands; instead, they have non-MHC-encoded MILL molecules that are closely related to MIC, but do not function as NKG2DL. To gain insights into the origin and evolution of MIC, ULBP, and MILL gene families, we conducted comparative genomic analysis of NKG2DL family genes in five mammalian species. In the opossum MHC, we identified a ULBP-like gene adjacent to a previously described MIC-like gene, suggesting that ULBP genes were originally encoded in the MHC. The opossum genome also contained a transcribed MILL-like gene in a region syntenic to the rodent regions encoding MILL molecules. These observations indicate that MIC-, ULBP-, and MILL-like genes emerged before the divergence of placental and marsupial mammals. Comparison of the human, cattle, rat, mouse, and opossum genomes indicates that after emigration from the MHC, ULBP genes underwent extensive duplications in each species. In mice, some of the ULBP genes appear to have been translocated telomerically on the same chromosome, forming a major cluster of existent NKG2DL genes.

Direct interaction of the mouse cytomegalovirus m152/gp40 immunoevasin with RAE-1 isoforms

Cytomegaloviruses (CMVs) are ubiquitous species-specific viruses that establish acute, persistent, and latent infections. Both human and mouse CMVs encode proteins that inhibit the activation of natural killer (NK) cells by downregulating cellular ligands for the NK cell activating receptor, NKG2D. The MCMV glycoprotein m152/gp40 downregulates the surface expression of RAE-1 to prevent NK cell control in vivo. So far, it is unclear if there is a direct interaction between m152 and RAE-1 and, if so, if m152 interacts differentially with the five identified RAE-1 isoforms, which are expressed as two groups in MCMV-susceptible or -resistant mouse strains. To address these questions, we expressed and purified the extracellular domains of RAE-1 and m152 and performed size exclusion chromatography binding assays as well as analytical ultracentrifugation and isothermal titration calorimetry to characterize these interactions quantitatively. We further evaluated the role of full-length and naturally glycosylated m152 and RAE-1 in cotransfected HEK293T cells. Our results confirmed that m152 binds RAE-1 directly, relatively tightly (K(d) < 5 microM), and with 1:1 stoichiometry. The binding is quantitatively different depending on particular RAE-1 isoforms, corresponding to the susceptibility to downregulation by m152. A PLWY motif found in RAE-1beta, although contributing to its affinity for m152, does not influence the affinity of RAE-1gamma or RAE-1delta, suggesting that other differences contribute to the RAE-1-m152 interaction. Molecular modeling of the different RAE-1 isoforms suggests a potential site for the m152 interaction.

NK cells mediate reduction of GVHD by inhibiting activated, alloreactive T cells while retaining GVT effects

Natural killer (NK) cells suppress graft-versus-host disease (GVHD) without causing GVHD themselves. Our previous studies demonstrated that allogeneic T cells and NK cells traffic similarly after allogeneic bone marrow transplantation (BMT). We therefore investigated the impact of donor NK cells on donor alloreactive T cells in GVHD induction. Animals receiving donor NK and T cells showed improved survival and decreased GVHD score compared with controls receiving donor T cells alone. Donor T cells exhibited less proliferation, lower CD25 expression, and decreased interferon-gamma (IFN-gamma) production in the presence of NK cells. In vivo, we observed perforin- and Fas ligand (FasL)-mediated reduction of donor T cell proliferation and increased T cell apoptosis in the presence of NK cells. Further, activated NK cells mediated direct lysis of reisolated GVHD-inducing T cells in vitro. The graft-versus-tumor (GVT) effect was retained in the presence of donor NK cells. We demonstrate a novel mechanism of NK cell-mediated GVHD reduction whereby donor NK cells inhibit and lyse autologous donor T cells activated during the initiation of GVHD.

Comparative analysis of NK-cell receptor expression and function across primate species: Perspective on antiviral defenses

Natural killer (NK) cells are lymphoid effectors that are involved in the innate immune surveillance against infected and/or tumor cells. Their function is under the fine-tuning control of cell surface receptors that display either inhibitory or activating function and in healthy condition, mediate self-tolerance. It is known that inhibitory receptors are characterized by clonal and stochastic distribution and are extremely sensible to any modification, downregulation or loss of MHC class I surface expression that are induced in autologous cells upon viral infection or cancer transformation. This alteration of the MHC class I expression weakens the strength of the inhibitory receptor-induced interaction, thus resulting in a prompt triggering of NK cell function, which ends up in the inhibition of tumor progression and proliferation of pathogen-infected cells. Thus, the inhibitory function of NK cells is only one face of the coin, since NK-cell activation is controlled by different arrays of activating receptors that finally are involved in the induction of cytolysis and/or cytokine release. Interestingly, the inhibitory NK-cell receptors that are involved in dampening NK cell-mediated responses evolved during speciation in different, often structurally unrelated surface-expressed molecules, all using a conserved signaling pathway. In detail, during evolution, the inhibitory receptors that assure the recognition of MHC class I molecules, originate in, at least, three different ways. This ended up in multigene families showing marked structural divergences that coevolved in a convergent way with the availability of appropriate MHC ligand molecules.

Signaling and ligand interaction of ILT7: receptor-mediated regulatory mechanisms for plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) are specialized dendritic cells (DCs) that produce large amounts of type I interferon (IFN) after Toll-like receptor (TLR) activation. Human pDCs preferentially express immunoglobulin-like transcript 7 (ILT7; LILRA4), which couples with a signaling adapter to activate a prominent immunoreceptor tyrosine-based activation motif (ITAM)-mediated signaling pathway. ILT7 protein directly binds to and can be activated by bone marrow stromal cell antigen 2 (BST2; CD317) protein, the expression of which is found on cells pre-exposed to IFN or on the surface of human cancer cells. The interaction between ILT7 and BST2 functions to assure an appropriate TLR response by pDCs during viral infection and likely participates in pDC-tumor crosstalk. Two opposing modes of receptor-mediated regulatory mechanisms work jointly to fine tune the innate immunity of pDCs.

Downregulation of uridine-cytidine kinase like-1 decreases proliferation and enhances tumor susceptibility to lysis by apoptotic agents and natural killer cells

Natural killer (NK) cells target and kill tumor cells by direct anti-tumor cytotoxicity. NK lytic-associated molecule (NKLAM) is a protein involved in this cytolytic function. Acting as an E3 ubiquitin ligase, NKLAM binds to and ubiquitinates a novel protein, uridine-cytidine kinase like-1 (UCKL-1), targeting it for degradation. However, UCKL-1's function in tumor cell survival and NK cell cytotoxicity is unknown. UCKL-1's homology to uridine kinases and over expression in tumor cells suggests a role for UCKL-1 in tumor growth and/or survival. We propose that NKLAM and UCKL-1 interact in the tumor cell, where degradation of UCKL-1 leads to increased tumor cell apoptosis. Here we use RNA interference to downregulate UCKL-1 expression in K562 erythroleukemia cells. It was seen that downregulation of UCKL-1 initiated apoptosis and slowed the cell cycle, resulting in lower growth in the small interfering UCKL-1 RNA treated K562 cell culture. In addition, the chemotherapeutic agent staurosporine was seen to be more effective in inducing cell death by apoptosis in UCKL-1 depleted K562 cells compared with controls. We also found that UCKL-1 depleted K562 cells were more susceptible to NK mediated cytolysis than controls. These results indicate a role for UCKL-1 in tumor cell survival and suggest possible therapeutic potential of UCKL-1 inhibitors in cancer treatment.

Nature killer cells in the central nervous system

Natural killer (NK) cells, a prominent component of the innate immune system, are large granular lymphocytes that respond rapidly to a variety of insults via cytokine secretion and cytolytic activity. Recently, there has been growing insight into the biological functions of NK cells, in particular into their roles in infection, tumorurveillance and autoimmunity. Under these pathophysiological circumstances, NK cells readily home to the central nervous system (CNS) tissues to combat infection and presumably to curb progression of tumor. Bystander neuronal and/or glial cell damage can occur in this setting. Paradoxically, NK cells appear to have an inhibitory role for autoimmune responses within the CNS. As in the periphery, NK cells act in concert with T cells and other lymphocytes responsible for CNS pathology and immune regulation. Insights into the molecular signals and pathways governing the diverse biological effects of NK cells are keys for designing NK cell-based therapy for CNS infections, tumor and autoimmune diseases.NK cells readily accumulate in homing to CNS tissues under the pathophysiological situations. This process is tightly controlled by a number of chemokines and chemokine receptors. There is ample of evidence that NK cells within the CNS contribute to the control of infections and might limit progression of certain tumor. Bystander neuronal and/or glial cell damage can occur. In certain autoimmune conditions of the CNS, NK cells appear to have an inhibitory role. Disassociation of disease-inhibiting versus disease-promoting effects of NK cells is a key to harnessing NK cells for therapeutic purposes. To achieve this goal, a generation of genetic models with selective NK cell deficiency, and development of reagents (antibodies) for visualizing subsets of NK cells in situ will be necessary.

An integrated view of the regulation of NKG2D ligands

NKG2D is one of the best characterized activating receptors and is expressed on natural killer cells and on various T-cell subsets. This receptor recognizes several different ligands that are induced by cellular stresses. In this review, we described the mechanisms controlling the expression of NKG2D ligands, with the emphasis on post-transcriptional and post-translational regulation.

ULBP6/RAET1L is an additional human NKG2D ligand

To date five ULBP/RAET (UL16-binding protein, also known as retinoic acid early transcript) genes, encoded on human chromosome 6q24.2-q25.3, have been shown to encode ligands of the activating immunoreceptor NKG2D. Here, we show that a sixth gene, ULBP6/RAET1L, is a polymorphic locus that expresses a functional transcript. ULBP6 had a more restricted expression profile in cell lines and primary human tissues than other NKG2D ligands, but expression was detected in several human papillomavirus-positive cervical carcinoma cell lines and was inducible on infection with human CMV. ULBP6 bound to recombinant NKG2D as well as the human CMV immune evasion molecule UL16. By confocal microscopy we show that UL16 retains ULBP6 inside the cell, preventing it from reaching the cell surface. Expression of ULBP6 on target cells induced a significant increase in NK-cell killing. Comparison of ULBP6 with ULBP4 and ULBP5 indicated that differences in recombinant NKG2D binding correlated with differences in NK-cell activation.

The functional impairment of natural killer cells during influenza virus infection

Natural killer (NK) cells have a critical role in clearing influenza virus, which primarily infects the lung epithelial cells. However, the ability of influenza virus to infect and manipulate NK cells has not been studied. In this context, we hypothesized that influenza virus can target NK cells leading to a functional impairment in their ability to mediate cytotoxicity and cytokine/chemokine generations. Here, we show influenza virus, PR8, can enter and infect NK cells. This infection did not alter the expression levels of activating, inhibitory or developmental receptors of NK cells. However, infection of NK cells by PR8 reduced the cytotoxicity to tumor cells that represent 'induced-self' and 'missing-self'. PR8-infection also significantly downregulated the NCR1, NKG2D, Nkpr1c, Ly49D and CD244 receptors-mediated generation of pro-inflammatory cytokines and chemokines. Mutations in the non-structural protein 1 (NS1) of influenza virus further augmented the functional impairment of NK cells. Our observations show the presence of a new, but yet to be explored, mechanism by which the influenza virus can evade immune detection.

NKG2D ligand expression in human colorectal cancer reveals associations with prognosis and evidence for immunoediting

PURPOSE

NKG2D (natural killer group 2, member D) binds to cellular ligands of the MIC and ULBP/RAET family. These ligands have restricted expression in normal tissue, but are frequently expressed on primary tumors. The role of NKG2D ligands is thought to be important in carcinogenesis but its prognostic effect has not been investigated in such a large cohort.

EXPERIMENTAL DESIGN

In our study, 462 primary colorectal tumors were screened for the expression of all MIC/ULBP/RAET proteins and NK cell infiltration. Tumor microarray technology was used for the purpose of this investigation.

RESULTS

NKG2D ligands were expressed by the majority of colorectal tumors; however, the level of expression varied considerably. High expression of MIC (68 versus 56 months) or RAET1G (74 versus 62 months) showed improved patient survival. Tumors expressing high levels of MIC and RAET1G showed improved survival of 77 months over tumors that expressed high levels of one ligand or low levels of both. High-level expression of all ligands was frequent in tumor-node-metastasis stage I tumors, but became progressively less frequent in stages II, III, and IV tumors. Expression of MIC was correlated with NK cellular infiltration.

CONCLUSION

The observations presented are consistent with an immunoediting mechanism that selects tumor cells that have lost or reduced their expression of NKG2D ligands. The combination of MIC and tumor-node-metastasis stage was found to be the strongest predictor of survival, splitting patients into eight groups and suggesting prognostic value in clinical assessment. Of particular interest were stage I patients with low expression of MIC who had a similar survival to stage III patients, and may be candidates for adjuvant therapy.

HOXB4-transduced embryonic stem cell-derived Lin-c-kit+ and Lin-Sca-1+ hematopoietic progenitors express H60 and are targeted by NK cells

Embryonic stem (ES) cells are a novel source of cells, especially hematopoietic progenitor cells that can be used to treat degenerative diseases in humans. However, there is a need to determine how ES cell-derived progenitors are regulated by both the adaptive and innate immune systems post transplantation. In this study, we demonstrate that hematopoietic progenitor cells (HPCs) derived from mouse ES cells ectopically expressing HOXB4 fail to engraft long-term in the presence of NK cells. In particular, the H60-expressing Lin(-)c-kit(+) and Lin(-)Sca-1(+) subpopulations were preferentially deleted in Rag2(-/-), but not in Rag2(-/-)gamma(c)(-/-) mice. Up-regulation of class I expression on HPCs prevented their lysis by NK cells, and Ab-mediated depletion of NK cells restored long-term HPC engraftment. In contrast to the notion that ES-derived cells are immune-privileged, we show in this study that NK cells form a formidable barrier to the long-term engraftment of ES cell-derived hematopoietic progenitors.

Adoptively transferred ex vivo expanded gammadelta-T cells mediate in vivo antitumor activity in preclinical mouse models of breast cancer

In contrast to antigen-specific alphabeta-T cells (adaptive immune system), gammadelta-T cells can recognize and lyse malignantly transformed cells almost immediately upon encounter in a manner that does not require the recognition of tumor-specific antigens (innate immune system). Given the well-documented capacity of gammadelta-T cells to innately kill a variety of malignant cells, efforts are now actively underway to exploit the antitumor properties of gammadelta-T cells for clinical purposes. Here, we present for the first time preclinical in vivo mouse models of gammadelta-T cell-based immunotherapy directed against breast cancer. These studies were explicitly designed to approximate clinical situations in which adoptively transferred gammadelta-T cells would be employed therapeutically against breast cancer. Using radioisotope-labeled gammadelta-T cells, we first show that adoptively transferred gammadelta-T cells localize to breast tumors in a mouse model (4T1 mammary adenocarcinoma) of human breast cancer. Moreover, by using an antibody directed against the gammadelta-T cell receptor (TCR), we determined that localization of adoptively transferred gammadelta-T cells to tumor is a TCR-dependant process. Additionally, biodistribution studies revealed that adoptively transferred gammadelta-T cells traffic differently in tumor-bearing mice compared to healthy mice with fewer gammadelta-T cells localizing into the spleens of tumor-bearing mice. Finally, in both syngeneic (4T1) and xenogeneic (2Lmp) models of breast cancer, we demonstrate that adoptively transferred gammadelta-T cells are both effective against breast cancer and are otherwise well-tolerated by treated animals. These findings provide a strong preclinical rationale for using ex vivo expanded adoptively transferred gammadelta-T cells as a form of cell-based immunotherapy for the treatment of breast cancer. Additionally, these studies establish that clinically applicable methods for radiolabeling gammadelta-T cells allows for the tracking of adoptively transferred gammadelta-T cells in tumor-bearing hosts.