The activation of the adaptor protein STING depends on its interactions with the phospholipid PI4P

Activation of the endoplasmic reticulum (ER)-resident adaptor protein STING, a component of a cytosolic DNA-sensing pathway, induces the transcription of genes encoding type I interferons (IFNs) and other proinflammatory factors. Because STING is activated at the Golgi apparatus, control of the localization and activation of STING is important in stimulating antiviral and antitumor immune responses. Through a genome-wide CRISPR interference screen, we found that STING activation required the Golgi-resident protein ACBD3, which promotes the generation of phosphatidylinositol 4-phosphate (PI4P) at the trans-Golgi network, as well as other PI4P-associated proteins. Appropriate localization and activation of STING at the Golgi apparatus required ACBD3 and the PI4P-generating kinase PI4KB. In contrast, STING activation was enhanced when the lipid-shuttling protein OSBP, which removes PI4P from the Golgi apparatus, was inhibited by the US Food and Drug Administration-approved antifungal itraconazole. The increase in the abundance of STING-activating phospholipids at the trans-Golgi network resulted in the increased production of IFN-β and other cytokines in THP-1 cells. Furthermore, a mutant STING that could not bind to PI4P failed to traffic from the ER to the Golgi apparatus in response to a STING agonist, whereas forced relocalization of STING to PI4P-enriched areas elicited STING activation in the absence of stimulation with a STING agonist. Thus, PI4P is critical for STING activation, and manipulating PI4P abundance may therapeutically modulate STING-dependent immune responses.

Induced CD45 Proximity Potentiates Natural Killer Cell Receptor Antagonism

Natural killer (NK) cells are a major subset of innate immune cells that are essential for host defense against pathogens and cancer. Two main classes of inhibitory NK receptors (NKR), KIR and CD94/NKG2A, play a key role in suppressing NK activity upon engagement with tumor cells or virus-infected cells, limiting their antitumor and antiviral activities. Here, we find that single-chain NKR antagonists linked to a VHH that binds the cell surface phosphatase CD45 potentiate NK and T activities to a greater extent than NKR blocking antibodies alone in vitro. We also uncovered crosstalk between NKG2A and Ly49 that collectively inhibit NK cell activation, such that CD45-NKG2A and CD45-Ly49 bispecific molecules show synergistic effects in their ability to enhance NK cell activation. The basis of the activity enhancement by CD45 ligation may reflect greater antagonism of inhibitory signaling from engagement of MHC I on target cells, combined with other mechanisms, including avidity effects, tonic signaling, antagonism of weak inhibition from engagement of MHC I on non-target cells, and possible CD45 segregation within the NK cell-target cell synapse. These results uncover a strategy for enhancing the activity of NK and T cells that may improve cancer immunotherapies.

Synergy of a STING agonist and an IL-2 superkine in cancer immunotherapy against MHC I-deficient and MHC I+ tumors

Cyclic dinucleotides (CDN) and Toll-like receptor (TLR) ligands mobilize antitumor responses by natural killer (NK) cells and T cells, potentially serving as complementary therapies to immune checkpoint therapy. In the clinic thus far, however, CDN therapy targeting stimulator of interferon genes (STING) protein has yielded mixed results, perhaps because it initiates responses potently but does not provide signals to sustain activation and proliferation of activated cytotoxic lymphocytes. To improve efficacy, we combined CDN with a half life-extended interleukin-2 (IL-2) superkine, H9-MSA (mouse serum albumin). CDN/H9-MSA therapy induced dramatic long-term remissions of the most difficult to treat major histocompatibility complex class I (MHC I)–deficient and MHC I+ tumor transplant models. H9-MSA combined with CpG oligonucleotide also induced potent responses. Mechanistically, tumor elimination required CD8 T cells and not NK cells in the case of MHC I+ tumors and NK cells but not CD8 T cells in the case of MHC-deficient tumors. Furthermore, combination therapy resulted in more prolonged and more intense NK cell activation, cytotoxicity, and expression of cytotoxic effector molecules in comparison with monotherapy. Remarkably, in a primary autochthonous sarcoma model that is refractory to PD-1 checkpoint therapy, the combination of CDN/H9-MSA with checkpoint therapy yielded long-term remissions in the majority of the animals, mediated by T cells and NK cells. This combination therapy has the potential to activate responses in tumors resistant to current therapies and prevent MHC I loss accompanying acquired resistance of tumors to checkpoint therapy.

Binary outcomes of enhancer activity underlie stable random monoallelic expression

Mitotically stable random monoallelic gene expression (RME) is documented for a small percentage of autosomal genes. We developed an in vivo genetic model to study the role of enhancers in RME using high-resolution single-cell analysis of natural killer (NK) cell receptor gene expression and enhancer deletions in the mouse germline. Enhancers of the RME NK receptor genes were accessible and enriched in H3K27ac on silent and active alleles alike in cells sorted according to allelic expression status, suggesting enhancer activation and gene expression status can be decoupled. In genes with multiple enhancers, enhancer deletion reduced gene expression frequency, in one instance converting the universally expressed gene encoding NKG2D into an RME gene, recapitulating all aspects of natural RME including mitotic stability of both the active and silent states. The results support the binary model of enhancer action, and suggest that RME is a consequence of general properties of gene regulation by enhancers rather than an RME-specific epigenetic program. Therefore, many and perhaps all genes may be subject to some degree of RME. Surprisingly, this was borne out by analysis of several genes that define different major hematopoietic lineages, that were previously thought to be universally expressed within those lineages: the genes encoding NKG2D, CD45, CD8α, and Thy-1. We propose that intrinsically probabilistic gene allele regulation is a general property of enhancer-controlled gene expression, with previously documented RME representing an extreme on a broad continuum.

Tumor-induced disruption of the blood-brain barrier promotes host death

Cancer patients often die from symptoms that manifest at a distance from any tumor. Mechanisms underlying these systemic physiological perturbations, called paraneoplastic syndromes, may benefit from investigation in non-mammalian systems. Using a non-metastatic Drosophila adult model, we find that malignant-tumor-produced cytokines drive widespread host activation of JAK-STAT signaling and cause premature lethality. STAT activity is particularly high in cells of the blood-brain barrier (BBB), where it induces aberrant BBB permeability. Remarkably, inhibiting STAT in the BBB not only rescues barrier function but also extends the lifespan of tumor-bearing hosts. We identify BBB damage in other pathological conditions that cause elevated inflammatory signaling, including obesity and infection, where BBB permeability also regulates host survival. IL-6-dependent BBB dysfunction is further seen in a mouse tumor model, and it again promotes host morbidity. Therefore, BBB alterations constitute a conserved lethal tumor-host interaction that also underlies other physiological morbidities.

Innate immune cells in the tumor microenvironment

The tumor immune microenvironment (TIME) is a complex ecosystem that contains adaptive and innate immune cells that have tumor-promoting and anti-tumor effects. There is still much to learn about the diversity, plasticity, and functions of innate immune cells in the TIME and their roles in determining the response to immunotherapies. Experts discuss recent advances in our understanding of their biology in cancer as well as outstanding questions and potential therapeutic avenues.

Abstract IA05: Innate immune defenses against cancer: Potential for mobilizing NK cells for cancer immunotherapy

Immunotherapies that mobilize CD8 T cells have revolutionized cancer therapy in recent years. A likely limitation of CD8 T cells in immunotherapy is that many tumors display few neoantigens and/or lack MHC I expression. Natural killer cells have emerged as exciting targets for new generations of immunotherapeutics. We investigated innate cues necessary for mobilizing spontaneous anti-tumor responses against tumors in mice and identified the cGAS-STING pathway as a critical pathway that is activated by cancer cells and mobilizes both NK cell and T cell mediated tumor rejection. Cyclic dinucleotides (CDNs) are STING agonists that, when injected into tumors, superactivate cellular immunity. We investigated the efficacy of CDNs in mobilizing anti-tumor activity in 6 independent gene edited models of MHC I-deficient tumors. Due to the absence of MHC I, CD8 T cells played no role in these responses. When transplanted syngeneic tumors of all 6 models were established and then treated intratumorally with CDNs (a single treatment, or in some cases 3 early treatments), tumor rejection rapidly ensued, which was NK-dependent and independent of CD8 T cells. In 5 of 6 MHC I-deficient tumor models tested, CDN therapy resulted in the indefinite survival of between 30% and 100% of the animals, with no evidence of residual tumors, suggesting the animals were cured. In one model (MHC I-deficient B16 melanoma cells) early rejection of the tumors was followed by relapse in all of the animals. Tumor rejection induced by CDNs was IFN-I-dependent, and conditional knockout experiments indicated that IFN-I acted directly on NK cells but also indirectly on DCs. After CDN therapy, DCs in the draining lymph nodes upregulated IL-15/IL-15R complexes on the membrane, and tumor rejection was blocked by neutralizing IL-15/IL15R. These data suggested that CDNs induce trans-presentation of IL15 by DCs to NK cells to support the tumor rejection response. CDN-induced tumor rejection was markedly enhanced by combining CDNs with IL-2 superkine (“super2”, in collaboration with C. Garcia), resulting in long-term tumor free survival in nearly all of the mice in the MHC I-deficient B16 and MC-38 models, indicating striking synergy between CDNs and super2. Our results suggest great potential for combining agonists of innate immune responses with superkines for treating cancers that are invisible to CD8 T cells, including MHC I-deficient cancers and potentially cancers with low neoantigen expression.

Citation Format: David H. Raulet. Innate immune defenses against cancer: Potential for mobilizing NK cells for cancer immunotherapy [abstract]. In: Abstracts: AACR Virtual Special Conference: Tumor Immunology and Immunotherapy; 2020 Oct 19-20. Philadelphia (PA): AACR; Cancer Immunol Res 2021;9(2 Suppl):Abstract nr IA05.

Millikelvin-resolved ambient thermography

Temperature sensitivity of thermography is boosted by over 15 times to achieve millikelvin-resolution near ambient temperature.

Combination therapy to enhance NK cell anti-tumor responses

Current cancer immunotherapies are targeted at mobilizing CD8 T cells, although numerous tumors have few or no antigens for CD8 T cells, or have lost MHC I, thereby evading CD8 T cells. There is therefore a need for improved therapies that mobilize other immune cells, such as NK cells, to the tumor microenvironment. A recent therapy candidate, cyclic dinucleotide (CDN), activates the cGAS-STING pathway of the innate immune system. Intratumoral CDN injections induce type I IFNs and other mediators that amplify the CD8 T cell response and induce regressions of primary tumors. MHC I-deficient tumors are not susceptible to CD8 T cell-mediated rejection, but data from our lab demonstrates that STING activation still leads to long-term tumor regressions, mediated by NK cells and in some cases CD4 T cells, in 30–100% of animals in 5/6 transplant models tested. In order to improve upon CDN monotherapy, we are testing combinations of CDN therapy with other therapies to target and activate NK cells in the tumor microenvironment and prevent or delay the onset of desensitization. Our results show that engineered IL-2 family cytokines synergize with CDN therapy to mobilize anti-tumor responses by NK cells and in some cases CD4 T cells. These anti-tumor responses are due to increased recruitment of NK cells to the tumor and sustained activation of these recruited NK cells. Our combination therapy regimen shows enhanced systemic effects, both in vivo in abscopal tumor models, and ex vivo by cytolysis of target cells mediated by splenocytes. Overall, our work demonstrates the impact of a novel combination therapy in mobilizing powerful NK cell-mediated anti-tumor activity, providing evidence for the potential of NK cell-targeted immunotherapeutics for cancer.

NK cells mediate clearance of CD8+ T cell-resistant tumors in response to STING agonists

Several immunotherapy approaches that mobilize CD8⁺ T cell responses stimulate tumor rejection, and some, such as checkpoint blockade, have been approved for several cancer indications and show impressive increases in patient survival. However, tumors may evade CD8⁺ T cell recognition via loss of MHC molecules or because they contain few or no neoantigens. Therefore, approaches are needed to combat CD8⁺ T cell-resistant cancers. STING-activating cyclic dinucleotides (CDNs) are a new class of immune-stimulating agents that elicit impressive CD8⁺ T cell-mediated tumor rejection in preclinical tumor models and are now being tested in clinical trials. Here, we demonstrate powerful CDN-induced, natural killer (NK) cell-mediated tumor rejection in numerous tumor models, independent of CD8⁺ T cells. CDNs enhanced NK cell activation, cytotoxicity, and antitumor effects in part by inducing type I interferon (IFN). IFN acted in part directly on NK cells in vivo and in part indirectly via the induction of IL-15 and IL-15 receptors, which were important for CDN-induced NK activation and tumor control. After in vivo administration of CDNs, dendritic cells (DCs) up-regulated IL-15Rα in an IFN-dependent manner. Mice lacking the type I IFN receptor specifically on DCs had reduced NK cell activation and tumor control. Therapeutics that activate NK cells, such as CDNs, checkpoint inhibitors, NK cell engagers, and cytokines, may represent next-generation approaches to cancer immunotherapy.

Author Correction: SLC19A1 transports immunoreactive cyclic dinucleotides

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

SLC19A1 transports immunoreactive cyclic dinucleotides

The accumulation of DNA in the cytosol serves as a key immunostimulatory signal associated with infections, cancer and genomic damage^1,2. Cytosolic DNA triggers immune responses by activating the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway³. The binding of DNA to cGAS activates its enzymatic activity, leading to the synthesis of a second messenger, cyclic guanosine monophosphate-adenosine monophosphate (2'3'-cGAMP)^4-7. This cyclic dinucleotide (CDN) activates STING⁸, which in turn activates the transcription factors interferon regulatory factor 3 (IRF3) and nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB), promoting the transcription of genes encoding type I interferons and other cytokines and mediators that stimulate a broader immune response. Exogenous 2'3'-cGAMP produced by malignant cells⁹ and other CDNs, including those produced by bacteria^10-12 and synthetic CDNs used in cancer immunotherapy^13,14, must traverse the cell membrane to activate STING in target cells. How these charged CDNs pass through the lipid bilayer is unknown. Here we used a genome-wide CRISPR-interference screen to identify the reduced folate carrier SLC19A1, a folate-organic phosphate antiporter, as the major transporter of CDNs. Depleting SLC19A1 in human cells inhibits CDN uptake and functional responses, and overexpressing SLC19A1 increases both uptake and functional responses. In human cell lines and primary cells ex vivo, CDN uptake is inhibited by folates as well as two medications approved for treatment of inflammatory diseases, sulfasalazine and the antifolate methotrexate. The identification of SLC19A1 as the major transporter of CDNs into cells has implications for the immunotherapeutic treatment of cancer¹³, host responsiveness to CDN-producing pathogenic microorganisms¹¹ and-potentially-for some inflammatory diseases.

Targetable mechanisms driving immunoevasion of persistent senescent cells link chemotherapy-resistant cancer to aging

Cellular senescence is a tumor suppressive mechanism that can paradoxically contribute to aging pathologies. Despite evidence of immune clearance in mouse models, it is not known how senescent cells (SnCs) persist and accumulate with age or in tumors in individuals. Here, we identify cooperative mechanisms that orchestrate the immunoevasion and persistence of normal and cancer human SnCs through extracellular targeting of natural killer receptor signaling. Damaged SnCs avoid immune recognition through MMPs-dependent shedding of NKG2D-ligands reinforced via paracrine suppression of NKG2D receptor-mediated immunosurveillance. These coordinated immunoediting processes are evident in residual, drug-resistant tumors from cohorts of >700 prostate and breast cancer patients treated with senescence-inducing genotoxic chemotherapies. Unlike in mice, these reversible senescence-subversion mechanisms are independent of p53/p16 and exacerbated in oncogenic RAS-induced senescence. Critically, the p16INK4A tumor suppressor can disengage the senescence growth arrest from the damage-associated immune senescence program, which is manifest in benign nevi lesions where indolent SnCs accumulate over time and preserve a non-pro-inflammatory tissue microenvironment maintaining NKG2D-mediated immunosurveillance. Our study shows how subpopulations of SnCs elude immunosurveillance, and reveals secretome-targeted therapeutic strategies to selectively eliminate -and restore the clearance of- the detrimental SnCs that actively persist after chemotherapy and accumulate at sites of aging pathologies.

Contribution of NK cells to immunotherapy mediated by PD-1/PD-L1 blockade

Checkpoint blockade immunotherapy targeting the PD-1/PD-L1 inhibitory axis has produced remarkable results in the treatment of several types of cancer. Whereas cytotoxic T cells are known to provide important antitumor effects during checkpoint blockade, certain cancers with low MHC expression are responsive to therapy, suggesting that other immune cell types may also play a role. Here, we employed several mouse models of cancer to investigate the effect of PD-1/PD-L1 blockade on NK cells, a population of cytotoxic innate lymphocytes that also mediate antitumor immunity. We discovered that PD-1 and PD-L1 blockade elicited a strong NK cell response that was indispensable for the full therapeutic effect of immunotherapy. PD-1 was expressed on NK cells within transplantable, spontaneous, and genetically induced mouse tumor models, and PD-L1 expression in cancer cells resulted in reduced NK cell responses and generation of more aggressive tumors in vivo. PD-1 expression was more abundant on NK cells with an activated and more responsive phenotype and did not mark NK cells with an exhausted phenotype. These results demonstrate the importance of the PD-1/PD-L1 axis in inhibiting NK cell responses in vivo and reveal that NK cells, in addition to T cells, mediate the effect of PD-1/PD-L1 blockade immunotherapy.

Dysregulated cellular functions and cell stress pathways provide critical cues for activating and targeting natural killer cells to transformed and infected cells

Natural killer (NK) cells recognize and kill cancer cells and infected cells by engaging cell surface ligands that are induced preferentially or exclusively on these cells. These ligands are recognized by activating receptors on NK cells, such as NKG2D. In addition to activation by cell surface ligands, the acquisition of optimal effector activity by NK cells is driven in vivo by cytokines and other signals. This review addresses a developing theme in NK cell biology: that NK-activating ligands on cells, and the provision of cytokines and other signals that drive high effector function in NK cells, are driven by abnormalities that arise from transformation or the infected state. The pathways include genomic damage, which causes self DNA to be exposed in the cytosol of affected cells, where it activates the DNA sensor cGAS. The resulting signaling induces NKG2D ligands and also mobilizes NK cell activation. Other key pathways that regulate NKG2D ligands include PI-3 kinase activation, histone acetylation, and the integrated stress response. This review summarizes the roles of these pathways and their relevance in both viral infections and cancer.

Tumor-derived CSF-1 induces the NKG2D ligand RAE-1δ on tumor-infiltrating macrophages

NKG2D is an important immunoreceptor expressed on the surface of NK cells and some T cells. NKG2D recognizes a set of ligands typically expressed on infected or transformed cells, but recent studies have also documented NKG2D ligands on subsets of host non-tumor cells in tumor-bearing animals and humans. Here we show that in transplanted tumors and genetically engineered mouse cancer models, tumor-associated macrophages are induced to express the NKG2D ligand RAE-1δ. We find that a soluble factor produced by tumor cells is responsible for macrophage RAE-1δ induction, and we identify tumor-derived colony-stimulating factor-1 (CSF-1) as necessary and sufficient for macrophage RAE-1δ induction in vitro and in vivo. Furthermore, we show that induction of RAE-1δ on macrophages by CSF-1 requires PI3K p110α kinase signaling. Thus, production of CSF-1 by tumor cells leading to activation of PI3K p110α represents a novel cellular and molecular pathway mediating NKG2D ligand expression on tumor-associated macrophages.

Endothelial cells express NKG2D ligands and desensitize antitumor NK responses

Natural Killer (NK) cells confer protection from tumors and infections by releasing cytotoxic granules and pro-inflammatory cytokines upon recognition of diseased cells. The responsiveness of NK cells to acute stimulation is dynamically tuned by steady-state receptor-ligand interactions of an NK cell with its cellular environment. Here, we demonstrate that in healthy WT mice the NK activating receptor NKG2D is engaged in vivo by one of its ligands, RAE-1ε, which is expressed constitutively by lymph node endothelial cells and highly induced on tumor-associated endothelium. This interaction causes internalization of NKG2D from the NK cell surface and transmits an NK-intrinsic signal that desensitizes NK cell responses globally to acute stimulation, resulting in impaired NK antitumor responses in vivo.

White blood cells called “natural killer cells” are part of the first line of immune defense. Often called NK cells for short, one job of these cells is to help prevent cancer by killing tumor cells. If an NK cell spots a tumor cell, it must become energized so that it can deliver the killing blow, which comes in the form of a packet of cell-killing “cytotoxic” granules. Yet tumor cells look very similar to healthy cells, and NK cells must be able to tell the difference to be effective.

Molecules on the outer surface of the NK cell control how the cell recognizes tumors, and deliver the signals the cell needs to become energized. One of these surface molecules is called NKG2D. It interacts with “partner” molecules found on the surface of cancer cells and tells the NK cell to attack. These partner molecules are not usually found on healthy cells, helping the immune system to tell the difference.

After NKG2D interacts with its partner molecules, it moves inside the NK cell. This makes the cell less able to become energized. If the NK cells do not encounter any partner molecules in healthy mice, blocking the interactions should have no effect on NKG2D levels. But now, Thompson et al. find that blocking one of these interactions increased the levels of NKG2D on the surface of NK cells in healthy mice. Further experiments revealed that NK cells in mice constantly encounter an NKG2D partner molecule called RAE-1ε.

A search for the source of RAE-1ε in healthy mice pointed to blood vessels inside the lymph nodes. NK cells pass through theses organs as part of their normal path around the body. Thompson et al. also saw that NK cells from healthy mice were less responsive than NK cells from mutant mice that lacked RAE-1ε. As a result of their encounters with RAE-1ε in healthy mice, the NK cells were less able to kill tumor cells. Blocking the interaction between NKG2D and RAE-1ε in mice re-energized their NK cells. More cells were able to enter tumors in these mice and the cells became better at killing tumors.

Together these findings increase the current understanding of the biological processes that control NK cells. Further research may lead to new treatments for diseases like cancer. But first, scientists need to find out whether NK cells behave in the same way in humans as they do in mice. If so, developing ways to block the interaction could re-energize human NK cells to better kill cancer cells.

Listening to each other: Infectious disease and cancer immunology

The immune system provides defense against tumors and pathogens. Here, we propose that by elucidating the shared principles of immunity that underlie cancer and infectious disease, oncologists and microbiologists can learn from each other and achieve the deeper mechanistic understanding critical the development of therapeutic approaches.

An RNA-Based Fluorescent Biosensor for High-Throughput Analysis of the cGAS-cGAMP-STING Pathway

In mammalian cells, the second messenger (2'-5',3'-5') cyclic guanosine monophosphate-adenosine monophosphate (2',3'-cGAMP), is produced by the cytosolic DNA sensor cGAMP synthase (cGAS), and subsequently bound by the stimulator of interferon genes (STING) to trigger interferon response. Thus, the cGAS-cGAMP-STING pathway plays a critical role in pathogen detection, as well as pathophysiological conditions including cancer and autoimmune disorders. However, studying and targeting this immune signaling pathway has been challenging due to the absence of tools for high-throughput analysis. We have engineered an RNA-based fluorescent biosensor that responds to 2',3'-cGAMP. The resulting "mix-and-go" cGAS activity assay shows excellent statistical reliability as a high-throughput screening (HTS) assay and distinguishes between direct and indirect cGAS inhibitors. Furthermore, the biosensor enables quantitation of 2',3'-cGAMP in mammalian cell lysates. We envision this biosensor-based assay as a resource to study the cGAS-cGAMP-STING pathway in the context of infectious diseases, cancer immunotherapy, and autoimmune diseases.

A Herpesviral induction of RAE-1 NKG2D ligand expression occurs through release of HDAC mediated repression

Natural Killer (NK) cells are essential for control of viral infection and cancer. NK cells express NKG2D, an activating receptor that directly recognizes NKG2D ligands. These are expressed at low level on healthy cells, but are induced by stresses like infection and transformation. The physiological events that drive NKG2D ligand expression during infection are still poorly understood. We observed that the mouse cytomegalovirus encoded protein m18 is necessary and sufficient to drive expression of the RAE-1 family of NKG2D ligands. We demonstrate that RAE-1 is transcriptionally repressed by histone deacetylase inhibitor 3 (HDAC3) in healthy cells, and m18 relieves this repression by directly interacting with Casein Kinase II and preventing it from activating HDAC3. Accordingly, we found that HDAC inhibiting proteins from human herpesviruses induce human NKG2D ligand ULBP-1. Thus our findings indicate that virally mediated HDAC inhibition can act as a signal for the host to activate NK-cell recognition.

Differential Role of Hematopoietic and Nonhematopoietic Cell Types in the Regulation of NK Cell Tolerance and Responsiveness

Many NK cells express inhibitory receptors that bind self-MHC class I (MHC I) molecules and prevent killing of self-cells, while enabling killing of MHC I-deficient cells. But tolerance also occurs for NK cells that lack inhibitory receptors for self-MHC I, and for all NK cells in MHC I-deficient animals. In both cases, NK cells are unresponsive to MHC I-deficient cells and hyporesponsive when stimulated through activating receptors, suggesting that hyporesponsiveness is responsible for self-tolerance. We generated irradiation chimeras, or carried out adoptive transfers, with wild-type (WT) and/or MHC I-deficient hematopoietic cells in WT or MHC I-deficient C57BL/6 host mice. Unexpectedly, in WT hosts, donor MHC I-deficient hematopoietic cells failed to induce hyporesponsiveness to activating receptor stimulation, but did induce tolerance to MHC I-deficient grafts. Therefore, these two properties of NK cells are separable. Both tolerance and hyporesponsiveness occurred when the host was MHC I deficient. Interestingly, infections of mice or exposure to inflammatory cytokines reversed the tolerance of NK cells that was induced by MHC I-deficient hematopoietic cells, but not the tolerance induced by MHC I-deficient nonhematopoietic cells. These data have implications for successful bone marrow transplantation, and suggest that tolerance induced by hematopoietic cells versus nonhematopoietic cells may be imposed by distinct mechanisms.

Bacterial Manipulation of NK Cell Regulatory Activity Increases Susceptibility to Listeria monocytogenes Infection

Natural killer (NK) cells produce interferon (IFN)-γ and thus have been suggested to promote type I immunity during bacterial infections. Yet, Listeria monocytogenes (Lm) and some other pathogens encode proteins that cause increased NK cell activation. Here, we show that stimulation of NK cell activation increases susceptibility during Lm infection despite and independent from robust NK cell production of IFNγ. The increased susceptibility correlated with IL-10 production by responding NK cells. NK cells produced IL-10 as their IFNγ production waned and the Lm virulence protein p60 promoted induction of IL-10 production by mouse and human NK cells. NK cells consequently exerted regulatory effects to suppress accumulation and activation of inflammatory myeloid cells. Our results reveal new dimensions of the role played by NK cells during Lm infection and demonstrate the ability of this bacterial pathogen to exploit the induction of regulatory NK cell activity to increase host susceptibility.

Natural killer (NK) cells are an innate immune cell population known to promote antiviral immunity through cytolysis and production of cytokines. Yet, some pathogens encode proteins that cause increased NK cell activation. Here, using a model of systemic infection by the bacterial pathogen Listeria monocytogenes (Lm), we show that NK cell activation increases host susceptibility. Activated NK cells increased bacterial burdens in infected tissues despite their early production of the pro-inflammatory cytokine IFNγ. We found that the ability of NK cells to exacerbate infection was independent from their production of IFNγ and instead due to subsequent production of the anti-inflammatory cytokine IL-10. A single bacterial protein, p60, was sufficient to elicit NK cell production of both early IFNγ and delayed IL-10. IL-10-production by NK cells has been shown to occur in other systems, but our studies are first to show how this “regulatory” response impacts the course of a bacterial infection. We found that IL-10 producing NK cells suppress accumulation and activation of inflammatory myeloid cells. Our studies suggest that the exploitation of NK cell regulatory activity provides selective pressure for the evolution of pathogen proteins that promote NK cell activation.

Neutrophils Suppress Intraluminal NK Cell-Mediated Tumor Cell Clearance and Enhance Extravasation of Disseminated Carcinoma Cells

Immune cells promote the initial metastatic dissemination of carcinoma cells from primary tumors. In contrast to their well-studied functions in the initial stages of metastasis, the specific roles of immunocytes in facilitating progression through the critical later steps of the invasion-metastasis cascade remain poorly understood. Here, we define novel functions of neutrophils in promoting intraluminal survival and extravasation at sites of metastatic dissemination. We show that CD11b(+)/Ly6G(+) neutrophils enhance metastasis formation via two distinct mechanisms. First, neutrophils inhibit natural killer cell function, which leads to a significant increase in the intraluminal survival time of tumor cells. Thereafter, neutrophils operate to facilitate extravasation of tumor cells through the secretion of IL1β and matrix metalloproteinases. These results identify neutrophils as key regulators of intraluminal survival and extravasation through their cross-talk with host cells and disseminating carcinoma cells.

SIGNIFICANCE

This study provides important insights into the systemic contributions of neutrophils to cancer metastasis by identifying how neutrophils facilitate intermediate steps of the invasion-metastasis cascade. We demonstrate that neutrophils suppress natural killer cell activity and increase extravasation of tumor cells. Cancer Discov; 6(6); 630-49. ©2016 AACR.This article is highlighted in the In This Issue feature, p. 561.

A forward genetic screen reveals novel independent regulators of ULBP1, an activating ligand for natural killer cells

Recognition and elimination of tumor cells by the immune system is crucial for limiting tumor growth. Natural killer (NK) cells become activated when the receptor NKG2D is engaged by ligands that are frequently upregulated in primary tumors and on cancer cell lines. However, the molecular mechanisms driving NKG2D ligand expression on tumor cells are not well defined. Using a forward genetic screen in a tumor-derived human cell line, we identified several novel factors supporting expression of the NKG2D ligand ULBP1. Our results show stepwise contributions of independent pathways working at multiple stages of ULBP1 biogenesis. Deeper investigation of selected hits from the screen showed that the transcription factor ATF4 drives ULBP1 gene expression in cancer cell lines, while the RNA-binding protein RBM4 supports ULBP1 expression by suppressing a novel alternatively spliced isoform of ULBP1 mRNA. These findings offer insight into the stress pathways that alert the immune system to danger.

DOI:http://dx.doi.org/10.7554/eLife.08474.001

Cancer is caused by a series of mutations that result in uncontrolled cell growth and division. Yet, the body's immune system can often detect and destroy abnormal cells before they cause tumors and disease. Natural killer cells are part of the immune system and have receptors on their surface that allow them to tell the difference between healthy host cells and host cells that are stressed or abnormal. Some of these receptors activate the natural killer cells when they bind to their target molecules. Other receptors have the opposite effect and inhibit the natural killer cells. Activation occurs when the signaling from the activating receptors is stronger than the signals from the inhibitory receptors.

One of the well-studied activating receptors recognizes a number of proteins and molecules that are produced by abnormal or tumor cells, including a protein called ULBP1. This protein is absent from the surface of healthy cells but is found in abundance on tumor cells. However, it is still not clear what drives tumor cells to produce ULBP1 (or other molecules) that are recognized by natural killer cell receptors.

Now, Gowen et al. report on a genetic screen that has revealed numerous genes that regulate the levels of ULBP1 in human cells. Many of these genes had independent effects that when added together accounted for most of the ULBP1 present on the cell surface.

Gowen et al. then explored some of the ‘regulators’ encoded by these genes in more detail. One called ATF4, which had previously been linked to stress responses, was shown to increase the expression of the gene for ULBP1 in cancer cells. Another regulator called RBM4 instead acted in a different way and at a later stage in ULBP1 production.

All together, these findings offer insight into the stress pathways that alert the immune system to abnormal cells. The next challenge will be investigating how these pathways might be exploited for cancer immunotherapy.

DOI:http://dx.doi.org/10.7554/eLife.08474.002

Cytokine therapy restores antitumor responses of NK cells rendered anergic in MHC I-deficient tumors

Recent extraordinary advances in cancer immunotherapy rely primarily on marshaling T cell responses. Here we discuss how NK cell responses can be amplified. We find that MHC I-deficient tumors induce anergy of NK cells but that cytokine therapy restores NK cell activity and increases the survival of mice bearing MHC I-deficient tumors.

Antitumor immunity. A shed NKG2D ligand that promotes natural killer cell activation and tumor rejection

Immune cells, including natural killer (NK) cells, recognize transformed cells and eliminate them in a process termed immunosurveillance. It is thought that tumor cells evade immunosurveillance by shedding membrane ligands that bind to the NKG2D-activating receptor on NK cells and/or T cells, and desensitize these cells. In contrast, we show that in mice, a shed form of MULT1, a high-affinity NKG2D ligand, causes NK cell activation and tumor rejection. Recombinant soluble MULT1 stimulated tumor rejection in mice. Soluble MULT1 functions, at least in part, by competitively reversing a global desensitization of NK cells imposed by engagement of membrane NKG2D ligands on tumor-associated cells, such as myeloid cells. The results overturn conventional wisdom that soluble ligands are always inhibitory and suggest a new approach for cancer immunotherapy.

Cytokine therapy reverses NK cell anergy in MHC-deficient tumors

Various cytokines have been evaluated as potential anticancer drugs; however, most cytokine trials have shown relatively low efficacy. Here, we found that treatments with IL-12 and IL-18 or with a mutant form of IL-2 (the "superkine" called H9) provided substantial therapeutic benefit for mice specifically bearing MHC class I-deficient tumors, but these treatments were ineffective for mice with matched MHC class I+ tumors. Cytokine efficacy was linked to the reversal of the anergic state of NK cells that specifically occurred in MHC class I-deficient tumors, but not MHC class I+ tumors. NK cell anergy was accompanied by impaired early signal transduction and was locally imparted by the presence of MHC class I-deficient tumor cells, even when such cells were a minor population in a tumor mixture. These results demonstrate that MHC class I-deficient tumor cells can escape from the immune response by functionally inactivating NK cells, and suggest cytokine-based immunotherapy as a potential strategy for MHC class I-deficient tumors. These results suggest that such cytokine therapies would be optimized by stratification of patients. Moreover, our results suggest that such treatments may be highly beneficial in the context of therapies to enhance NK cell functions in cancer patients.

Evidence for natural killer cell memory

Natural killer (NK) cells are generally considered to be part of the innate immune system. Over the past few years, however, evidence has accumulated suggesting that NK cells have certain features that are characteristic of the adaptive immune system. NK cells reportedly respond in an antigen-specific manner to a variety of small molecules and certain viruses, and mediate enhanced responses to these antigens upon secondary exposure. In infections with mouse cytomegalovirus (MCMV), MCMV-specific NK cells undergo clonal expansion, and display increased effector function after the resolution of the infection. In addition, inflammatory conditions resulting from exposure to certain cytokines seem to promote prolonged effector function in NK cells in an antigen-non-specific fashion. Taken together, these studies reveal new aspects of NK biology, and suggest that NK cells, like T and B cells, may carry out memory responses and may also exhibit greater capacity to distinguish antigens than was previously recognized.

NK cell self tolerance, responsiveness and missing self recognition

Natural killer (NK) cells represent a first line of defense against pathogens and tumor cells. The activation of NK cells is regulated by the integration of signals deriving from activating and inhibitory receptors expressed on their surface. However, different NK cells respond differently to the same stimulus, be it target cells or agents that crosslink activating receptors. The processes that determine the level of NK cell responsiveness have been referred to collectively as NK cell education. NK cell education plays an important role in steady state conditions, where potentially auto-reactive NK cells are rendered tolerant to the surrounding environment. According to the "tuning" concept, the responsiveness of each NK cell is quantitatively adjusted to ensure self tolerance while at the same time ensuring useful reactivity against potential threats. MHC-specific inhibitory receptors displayed by NK cells play a major role in tuning NK cell responsiveness, but recent studies indicate that signaling from activating receptors is also important, suggesting that the critical determinant is an integrated signal from both types of receptors. An important and still unresolved question is whether NK cell education involves interactions with a specific cell population in the environment. Whether hematopoietic and/or non-hematopoietic cells play a role is still under debate. Recent results demonstrated that NK cell tuning exhibits plasticity in steady state conditions, meaning that it can be re-set if the MHC environment changes. Other evidence suggests, however, that inflammatory conditions accompanying infections may favor high responsiveness, indicating that inflammatory agents can over-ride the natural tendency of NK cells to adjust to the steady state environment. These findings raise many questions such as whether viruses and tumor cells manipulate NK cell responsiveness to evade immune-recognition. As knowledge of the underlying processes grows, the possibility of modulating NK cell responsiveness for therapeutic purposes is becoming increasingly attractive, and is now under serious investigation in clinical studies.

RAE1 ligands for the NKG2D receptor are regulated by STING-dependent DNA sensor pathways in lymphoma

The immunoreceptor NKG2D originally identified in natural killer (NK) cells recognizes ligands that are upregulated on tumor cells. Expression of NKG2D ligands (NKG2DL) is induced by the DNA damage response (DDR), which is often activated constitutively in cancer cells, revealing them to NK cells as a mechanism of immunosurveillance. Here, we report that the induction of retinoic acid early transcript 1 (RAE1) ligands for NKG2D by the DDR relies on a STING-dependent DNA sensor pathway involving the effector molecules TBK1 and IRF3. Cytosolic DNA was detected in lymphoma cell lines that express RAE1 and its occurrence required activation of the DDR. Transfection of DNA into ligand-negative cells was sufficient to induce RAE1 expression. Irf3(+/-);Eμ-Myc mice expressed lower levels of RAE1 on tumor cells and showed a reduced survival rate compared with Irf3(+/+);Eμ-Myc mice. Taken together, our results suggest that genomic damage in tumor cells leads to activation of STING-dependent DNA sensor pathways, thereby activating RAE1 and enabling tumor immunosurveillance.

Immunosurveillance of senescent cancer cells by natural killer cells

We recently dissected how senescent tumors can trigger complementing signaling pathways that mobilize natural killer (NK) cells to eliminate malignant cells. In addition to cell-intrinsic effects on proliferation, senescence induces the production of chemokine (C-C motif) ligand 2 (CCL2), which recruits NK cells to mediate direct tumoricidal effects. Hence, senescence activates a cancer cell-extrinsic oncosuppression program.

Recognition of tumors by the innate immune system and natural killer cells

In recent years, roles of the immune system in immune surveillance of cancer have been explored using a variety of approaches. The roles of the adaptive immune system have been a major emphasis, but increasing evidence supports a role for innate immune effector cells such as natural killer (NK) cells in tumor surveillance. Here, we discuss some of the evidence for roles in tumor surveillance of innate immune cells. In particular, we focus on NK cells and other immune cells that express germline-encoded receptors, often labeled NK receptors. The impact of these receptors and the cells that express them on tumor suppression is summarized. We discuss in detail some of the pathways and events in tumor cells that induce or upregulate cell-surface expression of the ligands for these receptors, and the logic of how those pathways serve to identify malignant, or potentially malignant cells. How tumors often evade tumor suppression mediated by innate killer cells is another major subject of the review. We end with a discussion on some of the implications of the various findings with respect to possible therapeutic approaches.

Immune surveillance of unhealthy cells by natural killer cells

Pathogenic and oncogenic insults result in the induction of intrinsic defense mechanisms such as cell-death pathways and senescence, and extrinsic pathways that mobilize immune responses to destroy unhealthy cells. Both protective mechanisms presumably evolved to limit the damage these insults could inflict on the host. After viral infection or malignant transformation, unhealthy cells can be directly sensed by natural killer (NK) and some T cells via the activating receptor NKG2D. All NK cells and subsets of T cells express NKG2D. The NKG2D/ligand system represents a major recognition mechanism for detection and elimination of unhealthy cells. Here we discuss different pathways, including stress pathways, that are responsible for cell-surface display of ligands for NKG2D, which are self-proteins that are minimally expressed by normal cells. We also discuss new results indicating that efficient elimination of tumor cells that display NKG2D ligands depends on the recruitment of NK cells and other immune cells to the tumor, which can be regulated by distinct mechanisms, including the p53-dependent production of chemokines by senescent tumors. The cooperative effect of pathways that induce the display of NKG2D ligands and distinct pathways that mobilize immune cells provides a higher degree of specificity to the NK cell response.

p53-dependent chemokine production by senescent tumor cells supports NKG2D-dependent tumor elimination by natural killer cells

p53 induction regulates NK cell recruitment via CCL2, leading to NKG2D-dependent elimination of senescent tumors.

The induction of cellular senescence is an important mechanism by which p53 suppresses tumorigenesis. Using a mouse model of liver carcinoma, where cellular senescence is triggered in vivo by inducible p53 expression, we demonstrated that NK cells participate in the elimination of senescent tumors. The elimination of senescent tumor cells is dependent on NKG2D. Interestingly, p53 restoration neither increases ligand expression nor increases the sensitivity to lysis by NK cells. Instead, p53 restoration caused tumor cells to secrete various chemokines with the potential to recruit NK cells. Antibody-mediated neutralization of CCL2, but not CCL3, CCL4 or CCL5, prevented NK cell recruitment to the senescent tumors and reduced their elimination. Our findings suggest that elimination of senescent tumors by NK cells occurs as a result of the cooperation of signals associated with p53 expression or senescence, which regulate NK cell recruitment, and other signals that induce NKG2D ligand expression on tumor cells.

A selective role of NKG2D in inflammatory and autoimmune diseases

The NKG2D activating receptor has been implicated in numerous autoimmune diseases. We tested the role of NKG2D in models of autoimmunity and inflammation using NKG2D knockout mice and antibody blockade experiments. The severity of experimental autoimmune encephalitis (EAE) was decreased in NKG2D-deficient mice when the disease was induced with a limiting antigen dose, but unchanged with an optimal antigen dose. Surprisingly, however, NKG2D deficiency had no detectable effect in several other models, including two models of type 1 diabetes, and a model of intestinal inflammation induced by poly(I:C). NKG2D antibody blockade in normal mice also failed to inhibit disease in the NOD diabetes model or the intestinal inflammation model. Published evidence using NKG2D knockout mice demonstrated a role for NKG2D in mouse models of atherosclerosis and liver inflammation, as well as in chronic obstructive pulmonary disease. Therefore, our results suggest that NKG2D plays selective roles in inflammatory diseases.

HLA reduces killer cell Ig-like receptor expression level and frequency in a humanized mouse model

NK cells use NK cell receptors to be able to recognize and eliminate infected, transformed, and allogeneic cells. Human NK cells are prevented from killing autologous healthy cells by virtue of inhibitory NKRs, primarily killer cell Ig-like receptors (KIR) that bind "self" HLA class I molecules. Individual NK cells stably express a selected set of KIR, but it is currently disputed whether the fraction of NK cells expressing a particular inhibitory KIR is influenced by the presence of the corresponding HLA ligand. The extreme polymorphism of the KIR and HLA loci, with wide-ranging affinities for individual KIR and HLA allele combinations, has made this issue particularly hard to tackle. In this study, we used a transgenic mouse model to investigate the effect of HLA on KIR repertoire and function in the absence of genetic variation inside and outside the KIR locus. These H-2K(b-/-) and H-2D(b-/-) mice lacked ligands for inhibitory Ly49 receptors and were transgenic for HLA-Cw3 and a KIR B haplotype. In this reductionist system, the presence of HLA-Cw3 reduced the frequency of KIR2DL2(+) cells, as well as the surface expression levels of KIR2DL2. In addition, in the presence of HLA-Cw3, the frequency of NKG2A(+) cells and the surface expression levels of NKG2A were reduced. In line with these findings, both transgene-encoded KIR and endogenous NKG2A contributed to the rejection of cells lacking HLA-Cw3. These findings support the idea that HLA influences the human KIR repertoire.

Regulation of ligands for the NKG2D activating receptor

NKG2D is an activating receptor expressed by all NK cells and subsets of T cells. It serves as a major recognition receptor for detection and elimination of transformed and infected cells and participates in the genesis of several inflammatory diseases. The ligands for NKG2D are self-proteins that are induced by pathways that are active in certain pathophysiological states. NKG2D ligands are regulated transcriptionally, at the level of mRNA and protein stability, and by cleavage from the cell surface. In some cases, ligand induction can be attributed to pathways that are activated specifically in cancer cells or infected cells. We review the numerous pathways that have been implicated in the regulation of NKG2D ligands, discuss the pathologic states in which those pathways are likely to act, and attempt to synthesize the findings into general schemes of NKG2D ligand regulation in NK cell responses to cancer and infection.

RAE-1 ligands for the NKG2D receptor are regulated by E2F transcription factors, which control cell cycle entry

E2F transcription factors regulate expression of RAE-1 family NKG2D ligands in cancer cells and normal proliferating cells to promote wound healing and immune recognition.

The NKG2D stimulatory receptor expressed by natural killer cells and T cell subsets recognizes cell surface ligands that are induced on transformed and infected cells and facilitate immune rejection of tumor cells. We demonstrate that expression of retinoic acid early inducible gene 1 (RAE-1) family NKG2D ligands in cancer cell lines and proliferating normal cells is coupled directly to cell cycle regulation. Raet1 genes are directly transcriptionally activated by E2F family transcription factors, which play a central role in regulating cell cycle entry. Induction of RAE-1 occurred in primary cell cultures, embryonic brain cells in vivo, and cells in healing skin wounds and, accordingly, wound healing was delayed in mice lacking NKG2D. Transcriptional activation by E2Fs is likely coordinated with posttranscriptional regulation by other stress responses. These findings suggest that cellular proliferation, as occurs in cancer cells but also other pathological conditions, is a key signal tied to immune reactions mediated by NKG2D-bearing lymphocytes.

Infection-induced regulation of natural killer cells by macrophages and collagen at the lymph node subcapsular sinus

Infection leads to heightened activation of natural killer (NK) cells, a process that likely involves direct cell-to-cell contact, but how this occurs in vivo is poorly understood. We have used two-photon laser-scanning microscopy in conjunction with Toxoplasma gondii mouse infection models to address this question. We found that after infection, NK cells accumulated in the subcapsular region of the lymph node, where they formed low-motility contacts with collagen fibers and CD169(+) macrophages. We provide evidence that interactions with collagen regulate NK cell migration, whereas CD169(+) macrophages increase the activation state of NK cells. Interestingly, a subset of CD169(+) macrophages that coexpress the inflammatory monocyte marker Ly6C had the most potent ability to activate NK cells. Our data reveal pathways through which NK cell migration and function are regulated after infection and identify an important accessory cell population for activation of NK cell responses in lymph nodes.

NKG2D mediates NK cell hyperresponsiveness and influenza-induced pathologies in a mouse model of chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is characterized by peribronchial and perivascular inflammation and largely irreversible airflow obstruction. Acute disease exacerbations, due frequently to viral infections, lead to enhanced disease symptoms and contribute to long-term progression of COPD pathology. Previously, we demonstrated that NK cells from cigarette smoke (CS)-exposed mice exhibit enhanced effector functions in response to stimulating cytokines or TLR ligands. In this article, we show that the activating receptor NKG2D is a key mediator for CS-stimulated NK cell hyperresponsiveness, because CS-exposed NKG2D-deficient mice (Klrk1(-/-)) did not exhibit enhanced effector functions as assessed by cytokine responsiveness. NK cell cytotoxicity against MHC class I-deficient targets was not affected in a COPD model. However, NK cells from CS-exposed mice exhibit greater cytotoxic activity toward cells that express the NKG2D ligand RAET1ε. We also demonstrate that NKG2D-deficient mice exhibit diminished airway damage and reduced inflammation in a model of viral COPD exacerbation, which do not affect viral clearance. Furthermore, adoptive transfer of NKG2D(+) NK cells into CS-exposed, influenza-infected NKG2D-deficient mice recapitulated the phenotypes observed in CS-exposed, influenza-infected wild-type mice. Our findings indicate that NKG2D stimulation during long-term CS exposure is a central pathway in the development of NK cell hyperresponsiveness and influenza-mediated exacerbations of COPD.

Immune activation resulting from NKG2D/ligand interaction promotes atherosclerosis

BACKGROUND

The interplay between the immune system and abnormal metabolic conditions sustains and propagates a vicious feedback cycle of chronic inflammation and metabolic dysfunction that is critical for atherosclerotic progression. It is well established that abnormal metabolic conditions, such as dyslipidemia and hyperglycemia, cause various cellular stress responses that induce tissue inflammation and immune cell activation, which in turn exacerbate the metabolic dysfunction. However, molecular events linking these processes are not well understood.

METHODS AND RESULTS

Tissues and organs of humans and mice with hyperglycemia and hyperlipidemia were examined for expression of ligands for NKG2D, a potent immune-activating receptor expressed by several types of immune cells, and the role of NKG2D in atherosclerosis and metabolic diseases was probed with the use of mice lacking NKG2D or by blocking NKG2D with monoclonal antibodies. NKG2D ligands were upregulated in multiple organs, particularly atherosclerotic aortas and inflamed livers. Ligand upregulation was induced in vitro by abnormal metabolites associated with metabolic dysfunctions. Using apolipoprotein E-deficient mouse models, we demonstrated that preventing NKG2D functions resulted in a dramatic reduction in plaque formation, suppressed systemic and organ inflammation mediated by multiple immune cell types, and alleviated abnormal metabolic conditions.

CONCLUSIONS

The NKG2D/ligand interaction is a critical molecular link in the vicious cycle of chronic inflammation and metabolic dysfunction that promotes atherosclerosis and might be a useful target for therapeutic intervention in the disease.

Abstract CN08-03: Oncogenic stress regulates sensitivity of tumor cells to elimination by natural killer cells and T cells

Natural killer (NK) cell receptors regulate the capacity of NK cells and in some cases T cells to attack tumor cells and infected cells. Cancer cells become susceptible to NK cells by up-regulating stimulatory ligands, such as the Raet1/Mult1/H60/ULBP/MICA families of proteins recognized by the NKG2D receptor. This presentation will discuss the role of activating receptors such as NKG2D in tumor surveillance in vivo and the molecular mechanisms and signaling pathways responsible for induction of NKG2D ligands in cancer cells and their relationship to major pathways regulating tumorigenesis. Evidence will be presented that tumor associated stress pathways induce NKG2D ligands by acting at distinct levels of biogenesis (transcription, mRNA stabilization, protein stabilization). The role of NKG2D recognition in mobilizing lymphocytes against tumors in vivo will be discussed.

Supported by grants from NCI, NIAID and Prostate Cancer Foundation.

Citation Information: Cancer Prev Res 2011;4(10 Suppl):CN08-03.

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.

Innate or adaptive immunity? The example of natural killer cells

Natural killer (NK) cells were originally defined as effector lymphocytes of innate immunity endowed with constitutive cytolytic functions. More recently, a more nuanced view of NK cells has emerged. NK cells are now recognized to express a repertoire of activating and inhibitory receptors that is calibrated to ensure self-tolerance while allowing efficacy against assaults such as viral infection and tumor development. Moreover, NK cells do not react in an invariant manner but rather adapt to their environment. Finally, recent studies have unveiled that NK cells can also mount a form of antigen-specific immunologic memory. NK cells thus exert sophisticated biological functions that are attributes of both innate and adaptive immunity, blurring the functional borders between these two arms of the immune response.

Stress-regulated targeting of the NKG2D ligand Mult1 by a membrane-associated RING-CH family E3 ligase

NKG2D is a stimulatory receptor expressed by NK cells and some T cell subsets. Expression of the self-encoded ligands for NKG2D is presumably tightly regulated to prevent autoimmune disorders while allowing detection of infected cells and developing tumors. The NKG2D ligand Mult1 is regulated at multiple levels, with a final layer of regulation controlling protein stability. In this article, we report that Mult1 cell-surface expression was prevented by two closely related E3 ubiquitin ligases membrane-associated RING-CH (MARCH)4 and MARCH9, members of an E3 family that regulates other immunologically active proteins. Lysines within the cytoplasmic domain of Mult1 were essential for this repression by MARCH4 or MARCH9. Downregulation of Mult1 by MARCH9 was reversed by heat-shock treatment, which resulted in the dissociation of the two proteins and increased the amount of Mult1 at the cell surface. These results identify Mult1 as a target for the MARCH family of E3 ligases and show that induction of Mult1 in response to heat shock is due to regulated association with its E3 ligases.