Effect of NKG2D ligand expression on host immune responses

Effect of bevacizumab plus XELOX (CapeOX) chemotherapy on liver natural killer cell activity in colorectal cancer with resectable liver metastasis

AIM

We investigated the chemotherapy effect of resectable colorectal cancer with liver metastasis (CRLM) on the function of intrahepatic immune cells.

METHODS

We classified patients into adjuvant chemotherapy (bevacizumab+CapeOX) after hepatectomy group (group A) and neoadjuvant chemotherapy followed by hepatectomy group (group B), and collected peripheral blood mononuclear cells (PBMC) and liver mononuclear cells (LMNC) to ascertain phenotypic and functional differences.

RESULTS

There were no significant differences in lymphocyte fractions of either PBMC or LMNC between groups, except for the significantly lower percentage of natural killer (NK) cells in LMNC in group B than in group A. Significantly higher percentage of natural-killer group 2, member D (NKG2D)- positive NK cells in PBMC and percentage of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-, NKp30-, and signal regulatory protein β (SIRPβ)-positive NK cells in LMNC were found in group B. Furthermore, significantly higher expressions of NKG2D and SIRPβ in peripheral blood NK cells and of NKp46 and CD122 in liver NK cells were found in group B. When LMNC were incubated with interleukin (IL)-2 in vitro, no difference was observed in the expression of these molecules in NK cells between groups. Consistently, there was no difference in the cytotoxic activity of those LMNC against a colon adenocarcinoma cell line between groups.

CONCLUSION

Colorectal cancer with liver metastasis patients treated with neoadjuvant chemotherapy showed enhanced expression of activation markers on peripheral blood and liver NK cells in comparison with patients who did not receive therapy; however, the difference in those function remains unclear. These results suggest that neoadjuvant chemotherapy does not have a negative impact on intrahepatic immune cells in resectable CRLM patients.

NKG2D gene variation and susceptibility to viral bronchiolitis in childhood

BACKGROUND

Genetic factors associated with bronchiolitis are inadequately characterized. We therefore inspected a selected subpopulation of our previous genome-wide association study (GWAS) of bronchiolitis for overlap with known quantitative trait loci (QTLs) to identify susceptibility loci that potentially affect mRNA and protein levels.

METHODS

GWAS included a Finnish-Swedish case-control population (n = 187), matched for age and site. We integrated GWAS variants (p < 10^-4) with QTL data. We subsequently verified allele-specific expression of identified QTLs by flow cytometry. Association of the resulting candidate loci with bronchiolitis was tested in three additional cohorts from Finland and Denmark (n = 1201).

RESULTS

Bronchiolitis-susceptibility variant rs10772271 resided within QTLs previously associated with NKG2D (NK group 2, member D) mRNA and protein levels. Flow cytometric analysis confirmed the association with protein level in NK cells. The GWAS susceptibility allele (A) of rs10772271 (odds ratio [OR] = 2.34) corresponded with decreased NKG2D expression. The allele was nominally associated with bronchiolitis in one Finnish replicate (OR = 1.50), and the other showed directional consistency (OR = 1.43). No association was detected in Danish population CONCLUSIONS: The bronchiolitis GWAS susceptibility allele was linked to decreased NKG2D expression in the QTL data and in our expression analysis. We propose that reduced NKG2D expression predisposes infants to severe bronchiolitis.

New Insights Into the Regulation of Natural-Killer Group 2 Member D (NKG2D) and NKG2D-Ligands: Endoplasmic Reticulum Stress and CEA-Related Cell Adhesion Molecule 1

Natural-killer group 2 member D (NKG2D) is a well-characterized activating receptor expressed by natural killer (NK) cells, NKT cells, activated CD8⁺ T cells, subsets of γδ⁺ T cells, and innate-like T cells. NKG2D recognizes multiple ligands (NKG2D-ligands) to mount an innate immune response against stressed, transformed, or infected cells. NKG2D-ligand surface expression is tightly restricted on healthy cells through transcriptional and post-transcriptional mechanisms, while transformed or infected cells express the ligands as a danger signal. Recent studies have revealed that unfolded protein response pathways during endoplasmic reticulum (ER) stress result in upregulation of ULBP-related protein via the protein kinase RNA-like ER kinase-activating factor 4-C/EBP homologous protein (PERK-ATF4-CHOP) pathway, which can be linked to the pathogenesis of autoimmune diseases. Transformed cells, however, possess mechanisms to escape NKG2D-mediated immune surveillance, such as upregulation of carcinoembryonic antigen (CEA)-related cell adhesion molecule 1 (CEACAM1), a negative regulator of NKG2D-ligands. In this review, we discuss mechanisms of NKG2D-ligand regulation, with a focus on newly discovered mechanisms that promote NKG2D-ligand expression on epithelial cells, including ER stress, and mechanisms that suppress NKG2D-ligand-mediated killing of cancer cells, namely by co-expression of CEACAM1.

Critical Contribution of NK Group 2 Member D Expressed on Invariant Natural Killer T Cells in Concanavalin A-Induced Liver Hepatitis in Mice

Natural killer group 2D (NKG2D) is a well-characterized activating receptor expressed on many immune cells, including invariant natural killer T (iNKT) cells. These cells were shown to be responsible of liver injury in the model of concanavalin A (Con A)-induced hepatitis, considered to be an experimental model of human autoimmune hepatitis. In this study, we investigated whether NKG2D plays a role in the hepatitis induced by iNKT cell-mediated immune response to Con A. By using killer cell lectin-like receptor subfamily K, member 1 deficient (Klrk1^−/−) mice, we found that the absence of NKG2D reduced the hepatic injury upon Con A administration. This was not due to an intrinsic functional defect of NKG2D-deficient iNKT cells as mice missing NKG2D have normal distribution and function of iNKT cells. Furthermore, increased resistance to Con A-induced hepatitis was confirmed using neutralizing anti-NKG2D antibodies. The reduced pathogenic effect of Con A in the absence of NKG2D correlates with a reduction in pathogenic cytokine production and FAS-Ligand (FAS-L) expression by iNKT cells. We also found that Con A administration led to an increase in the retinoic acid early inducible (RAE-1) surface expression on wild-type hepatocytes. Finally, we found that Con A has no direct action on FAS-L expression or cytokine production by iNKT cells and thus propose that NKG2D-L expression on stressed hepatocytes promote cytotoxic activity of iNKT cells via its interaction with NKG2D contributing to hepatic injury. In conclusion, our results highlight NKG2D as an essential receptor required for the activation of iNKT cells in Con A-induced hepatitis and indicate that it represents a potential drug target for prevention of autoimmune hepatitis.

Murine CMV Expressing the High Affinity NKG2D Ligand MULT-1: A Model for the Development of Cytomegalovirus-Based Vaccines

The development of a vaccine against human cytomegalovirus (CMV) has been a subject of long-term medical interest. The research during recent years identified CMV as an attractive vaccine vector against infectious diseases and tumors. The immune response to CMV persists over a lifetime and its unique feature is the inflationary T cell response to certain viral epitopes. CMV encodes numerous genes involved in immunoevasion, which are non-essential for virus growth in vitro. The deletion of those genes results in virus attenuation in vivo, which enables us to dramatically manipulate its virulence and the immune response. We have previously shown that the murine CMV (MCMV) expressing RAE-1γ, one of the cellular ligands for the NKG2D receptor, is highly attenuated in vivo but retains the ability to induce a strong CD8⁺ T cell response. Here, we demonstrate that recombinant MCMV expressing high affinity NKG2D ligand murine UL16 binding protein-like transcript (MULT-1) (MULT-1MCMV) inserted in the place of its viral inhibitor is dramatically attenuated in vivo in a NK cell-dependent manner, both in immunocompetent adult mice and in immunologically immature newborns. MULT-1MCMV was more attenuated than the recombinant virus expressing RAE-1γ. Despite the drastic sensitivity to innate immune control, MULT-1MCMV induced an efficient CD8⁺ T cell response to viral and vectored antigens. By using in vitro assay, we showed that similar to RAE-1γMCMV, MULT-1 expressing virus provided strong priming of CD8⁺ T cells. Moreover, MULT-1MCMV was able to induce anti-viral antibodies, which after passing the transplacental barrier protect offspring of immunized mothers from challenge infection. Altogether, this study further supports the concept that CMV expressing NKG2D ligand possesses excellent characteristics to serve as a vaccine or vaccine vector.

Elevated lactate dehydrogenase (LDH) can be a marker of immune suppression in cancer: Interplay between hematologic and solid neoplastic clones and their microenvironments

Metabolism of neoplastic cells is shifted toward high glucose uptake and enhanced lactate production. Lactate dehydrogenase (LDH), which is comprised of two major subunits, LDH-A and LDH-B, reversibly catalyzes the conversion of pyruvate to lactate or lactate to pyruvate. LDH-A has a higher affinity for pyruvate and is a key enzyme in the glycolytic pathway. Elevated LDH is a negative prognostic biomarker not only because it is a key enzyme involved in cancer metabolism, but also because it allows neoplastic cells to suppress and evade the immune system by altering the tumor microenvironment. LDH-A alters the tumor microenvironment via increased production of lactate. This leads to enhancement of immune-suppressive cells, such as myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs), and dendritic cells (DCs); and inhibition of cytolytic cells, such as natural killer (NK) cells and cytotoxic T-lymphocytes (CTLs). By promoting immune-suppression in the tumor microenvironment, LDH-A is able to promote resistance to chemo/radio/targeted therapy. Here we discuss the evidence that LDH is both a metabolic and an immune surveillance prognostic biomarker and its elevation is harbinger of negative outcome in both solid and hematologic neoplasms.

The combination of ionizing radiation and proteasomal inhibition by bortezomib enhances the expression of NKG2D ligands in multiple myeloma cells

Bortezomib, which is a potent proteasome inhibitor, has been used as a first-line drugs to treat multiple myeloma for a few decades, and radiotherapy has frequently been applied to manage acute bone lesions in the patients. Therefore, it was necessary to investigate what the benefits might be if the two therapies were applied simultaneously in the treatment of multiple myeloma. Since it was known that radiotherapy and proteasome inhibitors could increase the expression of NKG2D ligands through induction of protein synthesis and suppression of protein degradation of NKG2D ligands, respectively, we supposed that the combined treatment might further enhance the expression of NKG2D ligands. In this study, we analyzed the expression level of NKG2D ligands using multiplex PCR and flow cytometry after treatment of IM-9 and RPMI-8226 myeloma cells with bortezomib and ionizing radiation; we then assayed the susceptibility to NK-92 cells. Although the expression of only some kinds of NKG2D ligands were increased by treatment with bortezomib alone, five kinds of NKG2D ligands that we assayed were further induced at the surface protein level after combined treatment with ionizing radiation and bortezomib. Furthermore, combined treatment made myeloma cells more susceptible to NK-92 cells, compared with treatment with bortezomib alone. In conclusion, the combination therapy of ionizing radiation plus the proteasome inhibitor bortezomib is a promising therapeutical strategy for enhancing NK cell-mediated anticancer immune responses.

Decreased expression of the NKG2D ligand ULBP4 may be an indicator of poor prognosis in patients with nasopharyngeal carcinoma

U16-binding protein 4 (ULBP4), a human ligand for natural killer group 2, member D (NKG2D) receptor on NK cells and subsets of T cells, is thought to activate anticancer immune responses. However, the expression pattern and prognostic effect of ULBP4 in nasopharyngeal carcinoma (NPC) has not been investigated. We first compared ULBP4 expression between archival 15 NPC tissues and 8 normal nasopharynx (NP) tissues using qPCR. Then ULBP4 expression among 111 NPC specimens was validated on immunohistochemical examination. In addition, the association of ULBP4 expression with clinical characteristics and survival outcomes was analyzed. Furthermore, the impact of ULBP4 expression in NPC cells on the cytotoxic activity of NK cells was investigated. Both mRNA and protein ULBP4 expressions of NPC tissues were significantly lower than those in normal NP tissues. However, no association of ULBP4 expression with clinical characteristics was observed. Patients with NPC having decreased expression of UBLP4 had significantly poorer overall survival (OS), progression-free survival (PFS), and distant metastasis-free survival (DMFS) than those with preserved levels of ULBP4. On multivariate analyses, low expression of ULBP4 was of borderline significance for OS, PFS, and DMFS (P = 0.060, 0.053, and 0.076, respectively). Further, LDH analysis demonstrated that the cytotoxic activitity of NK cells against C666-1 or 5-8F NPC cells with lenti-ULBP4 was considerably increased as compared to those with lenti-vector at various E/T ratios. Hence, restoration of ULBP4 expression may be a novel therapeutic strategy for treatment of NPC. However, further study is required to confirm these findings.

NKG2D and its ligands in cancer

NKG2D is an activating immune receptor expressed by NK and effector T cells. Induced expression of NKG2D ligand on tumor cell surface during oncogenic insults renders cancer cells susceptible to immune destruction. In advanced human cancers, tumor cells shed NKG2D ligand to produce an immune soluble form as a means of immune evasion. Soluble NKG2D ligands have been associated with poor clinical prognosis in cancer patients. Harnessing NKG2D pathway is considered a viable avenue in cancer immunotherapy over recent years. In this review, we will discuss the progress and perspectives.

NKG2D and Its Ligands: "One for All, All for One"

The activating receptor NKG2D is peculiar in its capability to bind to numerous and highly diversified MHC class I-like self-molecules. These ligands are poorly expressed on normal cells but can be induced on damaged, transformed or infected cells, with the final NKG2D ligand expression resulting from multiple levels of regulation. Although redundant molecular mechanisms can converge in the regulation of all NKG2D ligands, different stimuli can induce specific cellular responses, leading to the expression of one or few ligands. A large body of evidence demonstrates that NK cell activation can be triggered by different NKG2D ligands, often expressed on the same cell, suggesting a functional redundancy of these molecules. However, since a number of evasion mechanisms can reduce membrane expression of these molecules both on virus-infected and tumor cells, the co-expression of different ligands and/or the presence of allelic forms of the same ligand guarantee NKG2D activation in various stressful conditions and cell contexts. Noteworthy, NKG2D ligands can differ in their ability to down-modulate NKG2D membrane expression in human NK cells supporting the idea that NKG2D transduces different signals upon binding various ligands. Moreover, whether proteolytically shed and exosome-associated soluble NKG2D ligands share with their membrane-bound counterparts the same ability to induce NKG2D-mediated signaling is still a matter of debate. Here, we will review recent studies on the NKG2D/NKG2D ligand biology to summarize and discuss the redundancy and/or diversity in ligand expression, regulation, and receptor specificity.

Fas ligand and lytic granule differentially control cytotoxic dynamics of natural killer cell against cancer target

Interaction dynamics between Natural Killer (NK) cells and cancer targets have been the topic of many previous investigations, but the underlying rate-limiting kinetics and heterogeneity remain poorly understood. In this study, using quantitative single cell microscopy assay, we elucidate the differential dynamic control of NK-cancer cell interaction by multiple cytotoxic pathways. We found primary human NK cell, unlike NK cell line, killed adherent cancer target mainly by lytic granule-independent mechanism, in particular through Fas ligand (FasL). And the distinct kinetics of FasL and lytic granule pathway resulted in significant cell-to-cell variability. Killing by FasL occurred slowly, requiring transient, often multiple NK-cancer cell conjugations that gradually activated caspase-8, while lytic granule triggered rapid cytotoxicity by a switch-like induction of granzyme-B upon a single, prolonged conjugation. Moreover, interleukin 2 was observed to enhance both cytotoxic mechanisms by promoting target recognition by NK cell and increasing NK-cancer cell interaction frequency. Our results not only identify the key points of variation in the rate-limiting kinetics of NK-cancer cell cytotoxic interaction but also point to the importance of non-lytic granule mechanism for developing NK cell therapy.

More than Decoration: Roles for Natural Killer Group 2 Member D Ligand Expression by Immune Cells

The activating immune receptor natural killer group 2 member D (NKG2D), which is expressed by natural killer cells and T cell subsets, recognizes a number of ligands expressed by "stressed" or damaged cells. NKG2D has been extensively studied for its role in tumor immunosurveillance and antiviral immunity. To date, the majority of studies have focused on NKG2D-mediated killing of target cells expressing NKG2D ligands. However, with a number of reports describing expression of NKG2D ligands by cells that are not generally considered stressed, it is becoming clear that some healthy cells also express NKG2D ligands. Expression of these ligands by cells within the skin, intestinal epithelium, and the immune system suggests other immune functions for NKG2D ligand expression in addition to its canonical role as a "kill me" signal. How NKG2D ligands function in this capacity is just now starting to be unraveled. In this review, we examine the expression of NKG2D ligands by immune cells and discuss current literature describing the effects of this expression on immunity and immune regulation.

A possible mechanism of impaired NK cytotoxicity in cancer patients: Down-regulation of DAP10 by TGF-β1

Aims and background

Elevated TGF-β1 secretion and down-modulation of NKG2D underlies impaired NK cytotoxicity in cancer patients. However, the molecular mechanism of immunosuppression by TGF-β1 is not yet clarified.

Methods

IL-2-activated human NK cells were cultured with TGF-β1. Protein levels of NKG2D and DAP10 were examined by FACS or immunoblot analyses. Real-time RT-PCR was performed to quantify the transcription levels. MAPK inhibitors were used to investigate intracellular signaling.

Results

TGF-β1 down-regulated total and surface NKG2D, which was partially dependent on transcriptional regulation. TGF-β1 treatment of human NK cells resulted in significant changes in both transcriptional and translational levels of DAP10. Moreover, treatment with bafilomycin A1 or folimycin restored total NKG2D levels in TGF-β1-treated NK cells. The impaired NKG2D down-modulation by TGF-β1 was not associated with activation of the MAPK signaling pathway.

Conclusions

TGF-β1 down-modulates surface NKG2D expression by controlling the transcriptional and translational levels of DAP10.

Influence of NKG2D Genetic Variants on Response to Anti-TNF Agents in Patients with Rheumatoid Arthritis

A natural killer group 2 member D (NKG2D) acts as a powerful activating and co-stimulatory receptor on immune effector cells including NK and T cells. Disruptions within the NKG2D signalling pathway may trigger an exacerbated immune response and promote autoimmune reactions. The objective of the study was to evaluate a plausible role of polymorphisms within the NKG2D gene as a predictor of how effective anti-tumor necrosis factor (TNF) therapy is in rheumatoid arthritis (RA) patients. A total of 280 RA patients receiving anti-TNF therapy were genotyped for NKG2D rs2255336 (A > G), rs1049174 (C > G), and rs1154831 (C > A). Clinical response was evaluated according to the European League against Rheumatism (EULAR) criteria at the 12th and 24th week. Both the NKG2D rs225336 and rs1049174 polymorphisms were significantly associated with efficacy of TNF inhibitors. Inefficient therapy was more frequently observed in patients with rs2255336 GG or rs1049174 CC genotype as compared to other genotypes (p-value = 0.003 and p-value = 0.004, respectively). The presence of the rs2255336 G or the rs1049174 C allele correlated with a worse EULAR response (p-value = 0.002, p-value = 0.031, respectively). Moreover, patients carrying the rs2255336 or rs1049174 heterozygous genotype achieved better EULAR responses than patients with homozygous genotypes (p-value = 0.010 and p-value = 0.002, respectively). Data from the present study provides evidence that NKG2D polymorphisms may affect response to anti-TNF inhibitors in RA patients.

NKG2D ligands in glioma stem-like cells: expression in situ and in vitro

Glioblastoma multiforme (GBM) is a highly malignant brain tumor. Tumor stem cells have a major influence on tumor malignancy, and immunological escape mechanisms, involving the Natural Killer Group 2, member D (NKG2D) receptor-ligand-system, are key elements in tumor immuno-surveillance. We analyzed the expression profile and localization of NKG2D ligands (NKG2DL) and embryonic and neural stem cell markers in solid human GBM and stem-like cells isolated from glioma cell lines by qRT-PCR and immunohistochemistry, including quantitative analysis. We also evaluated the effect of Temozolomide (TMZ), the standard chemotherapeutic agent used in GBM therapy, on NKG2DL expression. NKG2DL-positive cells were mostly found scattered and isolated, were detectable in glial fibrillary acidic protein (GFAP)-positive tumor regions and partly in the penumbra of tumor vessels. NKG2DL were found in a distinct tumor stem-like cell subpopulation and were broadly costained with each other. Quantitative analysis revealed, that dependent on the individual NKG2DL investigated, cell portions costained with different stem cell markers varied between small (Musashi-1) and high (KLf-4) amounts. However, a costaining of NKG2DL with CD3γ, typically found in T cells, was also observable, whereas CD11b as a marker for tumor micoglia cells was only rarely costained with NKG2DL. Stem-like cells derived from the glioma cell lines T98G and U251MG showed a distinct expression pattern of NKG2DL and stem cell markers, which seemed to be balanced in a cell line-specific way. With differentiation, T98G displayed less NKG2DL, whereas in U251MG, only expression of most stem cell markers decreased. In addition, stimulation with TMZ led to a significant upregulation of NKG2DL in stem-like cells of both lines. As stem-like glioma cells tend to show a higher expression of NKG2DL than more differentiated tumor cells and TMZ treatment supports upregulation of NKG2DL, the NKG2D system might play an important role in tumor stem cell survival and in GBM therapy.

Intestinal Inflammation-Mediated Clearance of Amebic Parasites Is Dependent on IFN-γ

Intestinal amebiasis is a major cause of diarrhea. However, research on host-amebae interactions has been hampered owing to a lack of appropriate animal models. Recently, a mouse model of intestinal amebiasis was established, and using it, we reported that Entamoeba moshkovskii colonized the intestine in a manner similar to that of the pathogenic Entamoeba histolytica In this study, we evaluated the protective mechanisms present against amebae using this model. CBA/J mice infected with E. histolytica had a persistent infection without apparent symptoms. In contrast, E. moshkovskii-infected mice rapidly expelled the ameba, which was associated with weight loss, diarrhea, and intestinal damage characterized by apoptosis of intestinal epithelial cells (IECs). Expression of NKG2D on intestinal intraepithelial lymphocytes (IELs) and IFN-γ-producing cells in Peyer's patches were significantly induced after infection with E. moshkovskii but not with E. histolytica IFN-γ-deficient mice infected with E. moshkovskii showed no obvious symptoms. Notably, none of these mice expelled E. moshkovskii, indicating that IFN-γ is responsible not only for intestinal symptoms but also for the expulsion of amebae. Furthermore, apoptosis of IECs and expression of NKG2D on IELs observed in E. moshkovskii-infected mice did not occur in the absence of IFN-γ. In vivo blocking of NKG2D in mice infected with E. moshkovskii enabled ameba to survive longer and remarkably reduced apoptotic IECs. Our results clearly demonstrate a novel protective mechanism exerted by IFN-γ against intestinal amebae, including induction of cytotoxicity of IELs toward IECs.

Downregulation of the stress-induced ligand ULBP1 following SV40 infection confers viral evasion from NK cell cytotoxicity

Polyomaviruses are a diverse family of viruses which are prevalent in the human population. However, the interactions of these viruses with the immune system are not well characterized. We have previously shown that two human polyomaviruses, JC and BK, use an identical microRNA to evade immune attack by Natural Killer (NK) cells. We showed that this viral microRNA suppresses ULBP3 expression, a stress induced ligand for the killer receptor NKG2D. Here we show that Simian Virus 40 (SV40) also evades NK cell attack through the down regulation of another stress-induced ligand of NKG2D, ULBP1. These findings indicate that NK cells play an essential role in fighting polyomavirus infections and further emphasize the importance of various members of the ULBP family in controlling polyomavirus infection.

Naïve T cells are activated by autologous HCMV-infected endothelial cells through NKG2D and can control HCMV transmission in vitro

Apart from classical antigen-presenting cells (APCs) like dendritic cells and macrophages, there are semiprofessional APCs such as endothelial cells (ECs) and Langerhans' cells. Human cytomegalovirus (HCMV) infects a wide range of cell types including the ECs which are involved in the trafficking and homing of T cells. By investigating the interaction of naïve T cells obtained from HCMV-seronegative umbilical cord blood with autologous HCMV-infected human umbilical vein ECs (HUVECs), we could show that the activation of naïve T cells occurred after 1 day of peripheral blood mononuclear cell (PBMC) exposure to HCMV-infected HUVECs. The percentage of activated T cells increased over time and the activation of naïve T cells was not induced by either autologous uninfected HUVECs or by autologous HCMV-infected fibroblasts. The activation of T cells occurred also when purified T cells were co-cultured with HCMV-infected HUVECs. In addition, in most of the donors only CD8⁺ T cells were activated, when the purified T cells were exposed to HCMV-infected HUVECs. The activation of naïve T cells was inhibited when the NKG2D receptor was blocked on the surface of T cells and among the different NKG2D ligands, we identified two ligands (ULBP4 and MICA) on HCMV-infected HUVECs which might be the interaction partners of the NKG2D receptor. Using a functional cell culture assay, we could show that these activated naïve T cells specifically inhibited HCMV transmission. Altogether, we identified a novel specific activation mechanism of naïve T cells from the umbilical cord by HCMV-infected autologous HUVECs through interaction with NKG2D.

How Mucosal Epithelia Deal with Stress: Role of NKG2D/NKG2D Ligands during Inflammation

Mucosal epithelia encounter both physicochemical and biological stress during their life and have evolved several mechanisms to deal with them, including regulation of immune cell functions. Stressed and damaged cells need to be cleared to control local inflammation and trigger tissue healing. Engagement of the activating NKG2D receptor is one of the most direct mechanisms involved in the recognition of stressed cells by the immune system. Indeed, injured cells promptly express NKG2D ligands that in turn mediate the activation of lymphocytes of both innate and adaptive arms of the immune system. This review focuses on different conditions that are able to modulate NKG2D ligand expression on the epithelia. Special attention is given to the mechanisms of immunosurveillance mediated by natural killer cells, which are finely tuned by NKG2D. Different types of stress, including viral and bacterial infections, chronic inflammation, and cigarette smoke exposure, are discussed as paradigmatic conditions for NKG2D ligand modulation, and the implications for tissue homeostasis are discussed.

A Protective Role for NKG2D-H60a Interaction via Homotypic T Cell Contact in Nonobese Diabetic Autoimmune Diabetes Pathogenesis

The NK group 2 member D (NKG2D) immune receptor is implicated in both human and mouse autoimmune diabetes. However, the significance of NKG2D in diabetes pathogenesis has been unclear due to conflicting reports as to the importance of this receptor in the NOD mouse model. In this study we demonstrate that NKG2D expression affects NOD diabetes development by at least two previously undescribed, and opposing, mechanisms. First, we demonstrate that the NKG2D ligand H60a is induced on activated NOD T cells, and that NKG2D-H60a interaction during CD8⁺ T cell differentiation into CTLs generally decreases the subsequent CTL effector cytokine response. This corresponds to an increase in diabetes development in NKG2D-deficient compared with wild-type NOD mice under microbiota-depleted conditions. Second, we demonstrate that NKG2D promotes NOD diabetes development through interaction with the microbiota. Together these findings reveal a previously undescribed role for NKG2D ligand expression by activated T cells in CTL development. Further, they demonstrate that NKG2D has both diabetogenic and antidiabetogenic roles in NOD diabetes development.

Anti-NKG2D monoclonal antibody (NNC0142-0002) in active Crohn's disease: a randomised controlled trial

OBJECTIVE

Anti-NKG2D (NNC0142-0002) is an antagonising human immunoglobulin G4 monoclonal antibody that binds to natural killer group 2 member D (NKG2D) receptors, which are expressed by T cells and innate lymphoid cells, and may be linked to mucosal damage in Crohn's disease (CD).

DESIGN

Seventy-eight patients (aged ≥18 and ≤75 years) with CD for ≥3 months, Crohn's Disease Activity Index (CDAI) ≥220 and ≤450 and either C-reactive protein ≥10 mg/L or endoscopic evidence of inflammation, were randomised 1:1 to a single subcutaneous (SC) dose of 2 mg/kg anti-NKG2D or placebo. Primary endpoint was change in CDAI (ΔCDAI) from baseline to week 4. Prespecified significance level was 10% for CDAI endpoints. A futility analysis was instituted due to slow recruitment.

RESULTS

Primary endpoint was not significantly different between anti-NKG2D and placebo (week 4 ΔCDAI=-16); however, there was a significant difference by week 12 (ΔCDAI=-55; p≤0.10). Significant improvements were noted in the non-failure to biologics subgroup (treated with anti-NKG2D (n=28)) from week 1 onward. Greater effects of anti-NKG2D were also observed in patients with baseline CDAI ≥330. Frequencies of adverse events (AEs) were comparable between anti-NKG2D and placebo. Most AEs were mild (49%) or moderate (43%). No antidrug antibodies were observed.

CONCLUSIONS

A single SC dose of 2 mg/kg anti-NKG2D did not reduce disease activity at week 4 versus placebo, but the difference was significant at week 12, and effects were evident in key subgroups. These data support further development of anti-NKG2D in IBD.

TRIAL REGISTRATION NUMBER

NCT01203631.

CD20-Specific Immunoligands Engaging NKG2D Enhance γδ T Cell-Mediated Lysis of Lymphoma Cells

Human γδ T cells are innate-like T cells which are able to kill a broad range of tumour cells and thus may have potential for cancer immunotherapy. The activating receptor natural killer group 2 member D (NKG2D) plays a key role in regulating immune responses driven by γδ T cells. Here, we explored whether recombinant immunoligands consisting of a CD20 single-chain fragment variable (scFv) linked to a NKG2D ligand, either MHC class I chain-related protein A (MICA) or UL16 binding protein 2 (ULBP2), could be employed to engage γδ T cells for tumour cell killing. The two immunoligands, designated MICA:7D8 and ULBP2:7D8, respectively, enhanced cytotoxicity of ex vivo-expanded γδ T cells against CD20-positive lymphoma cells. Both Vδ1 and Vδ2 γδ T cells were triggered by MICA:7D8 or ULBP2:7D8. Killing of CD20-negative tumour cells was not induced by the immunoligands, indicating their antigen specificity. MICA:7D8 and ULBP2:7D8 acted in a dose-dependent manner and induced cytotoxicity at nanomolar concentrations. Importantly, chronic lymphocytic leukaemia (CLL) cells isolated from patients were sensitized by the two immunoligands for γδ T cell cytotoxicity. In a combination approach, the immunoligands were combined with bromohydrin pyrophosphate (BrHPP), an agonist for Vδ2 γδ T cells, which further enhanced the efficacy in target cell killing. Thus, employing tumour-directed recombinant immunoligands which engage NKG2D may represent an attractive strategy to enhance antitumour cytotoxicity of γδ T cells.

Anti-NKG2D mAb: A New Treatment for Crohn's Disease?

Crohn’s disease (CD) and ulcerative colitis (UC) are immunologically-mediated, debilitating conditions resulting from destructive inflammation of the gastrointestinal tract. The pathogenesis of IBD is incompletely understood, but is considered to be the result of an abnormal immune response with a wide range of cell types and proteins involved. Natural Killer Group 2D (NKG2D) is an activating receptor constitutively expressed on human Natural Killer (NK), γδ T, mucosal-associated invariant T (MAIT), CD56⁺ T, and CD8⁺ T cells. Activation of NKG2D triggers cellular proliferation, cytokine production, and target cell killing. Research into the NKG2D mechanism of action has primarily been focused on cancer and viral infections where cytotoxicity evasion is a concern. In human inflammatory bowel disease (IBD) this system is less characterized, but the ligands have been shown to be highly expressed during intestinal inflammation and the following receptor activation may contribute to tissue degeneration. A recent phase II clinical trial showed that an antibody against NKG2D induced clinical remission of CD in some patients, suggesting NKG2D and its ligands to be of importance in the pathogenesis of CD. This review will describe the receptor and its ligands in intestinal tissues and the clinical potential of blocking NKG2D in Crohn’s disease.

The Molecular Mechanism of Natural Killer Cells Function and Its Importance in Cancer Immunotherapy

Natural killer (NK) cells are innate immune cells that show strong cytolytic function against physiologically stressed cells such as tumor cells and virus-infected cells. NK cells show a broad array of tissue distribution and phenotypic variability. NK cells express several activating and inhibitory receptors that recognize the altered expression of proteins on target cells and control the cytolytic function. NK cells have been used in several clinical trials to control tumor growth. However, the results are encouraging only in hematological malignancies but not very promising in solid tumors. Increasing evidence suggests that tumor microenvironment regulate the phenotype and function of NK cells. In this review, we discussed the NK cell phenotypes and its effector function and impact of the tumor microenvironment on effector and cytolytic function of NK cells. We also summarized various NK cell-based immunotherapeutic strategies used in the past and the possibilities to improve the function of NK cell for the better clinical outcome.

Cytotoxicity of Natural Killer Cells Activated Through NKG2D Contributes to the Development of Bronchiolitis Obliterans in a Murine Heterotopic Tracheal Transplant Model

Bronchiolitis obliterans after lung transplantation is a major cause of postoperative mortality in which T cell-mediated immunity is known to play an important role. However, the exact contribution of natural killer (NK) cells, which have functions similar to CD8⁺ T cells, has not been defined. Here, we assessed the role of NK cells in murine bronchiolitis obliterans through heterotopic tracheal transplantations and found a greater percentage of NK cells in allografts than in isografts. Depletion of NK cells using an anti-NK1.1 antibody attenuated bronchiolitis obliterans in transplant recipients compared with controls. In terms of NK cell effector functions, an improvement in bronchiolitis obliterans was observed in perforin-KO recipient mice compared to wild type (WT). Furthermore, we found upregulation of NKG2D-ligand in allografts and demonstrated the significance of this using grafts expressing Rae-1, a murine NKG2D-ligand, which induced severe bronchiolitis obliterans in WT and Rag-1 KO recipients. This effect was ameliorated by injection of anti-NKG2D blocking antibody. Together, these results suggest that cytotoxicity resulting from activation of NK cells through NKG2D leads to the development of murine bronchiolitis obliterans.

Dual Face of Vγ9Vδ2-T Cells in Tumor Immunology: Anti- versus Pro-Tumoral Activities

Vγ9Vδ2-T cells are considered as potent effector cells for tumor immunotherapy through directly killing tumor cells and indirectly regulating other innate and adaptive immune cells to establish antitumoral immunity. The antitumoral activity of Vγ9Vδ2-T cells is governed by a complicated set of activating and inhibitory cell receptors. In addition, cytokine milieu in tumor microenvironment can also induce the pro-tumoral activities and functional plasticity of Vγ9Vδ2-T cells. Here, we review the anti- versus pro-tumoral activities of Vγ9Vδ2-T cells and discuss the mechanisms underlying the recognition, activation, differentiation and regulation of Vγ9Vδ2-T cells in tumor immunosurveillance. The comprehensive understanding of the dual face of Vγ9Vδ2-T cells in tumor immunology may improve the therapeutic efficacy and clinical outcomes of Vγ9Vδ2-T cell-based tumor immunotherapy.

Heterogeneity of colon cancer: from bench to bedside

The large bowel shows biomolecular, anatomical and bacterial changes that proceed from the proximal to the distal tract. These changes account for the different behaviour of colon cancers arising from the diverse sides of the colon-rectum as well as for the sensitivity to the therapy, including immunotherapy. The gut microbiota plays an important role in the modulation of the immune response and differs between the right colon cancer and the left colorectal cancer. The qualitative and quantitative difference of the commensal bacteria between the right side and the left side induces epigenetic changes in the intestinal epithelial cells as well as in the resident immune population. The second player in the pathological homeostasis of colorectal cancer is the differences of the genetic features of cancer cells and the different effects that microsatellite instability, chromosomal instability and the CpG island methylator phenotype induce on the immunological organisation of the tumour microenvironment. The third player is the immunological composition of the tumour microenvironment, which changes under the influence of both genetic structures and gut microbiota. All these three players influence each other. This review describes these three aspects, highlights their interactions and discusses data from reported clinical trials.

In Vitro Culture with Interleukin-15 Leads to Expression of Activating Receptors and Recovery of Natural Killer Cell Function in Acute Myeloid Leukemia Patients

Despite recent progress in the therapeutic approach of malignant hemopathies, their prognoses remain frequently poor. Immunotherapy could open a new window of great interest in this setting. Natural killer (NK) cells constitute an important area of research for hematologic malignancies, because this subpopulation is able to kill target cells spontaneously without previous sensitization, representing a novel tool in the treatment of them. Abnormal NK cytolytic function is observed in several hematological malignancies, including acute myeloid leukemia (AML) and myelodysplastic syndromes. Several mechanisms are involved in this abnormal function, such as decreased expression of activating receptors, increased expression of inhibitory receptors or defective expression of NK cell ligands on target cells. New immunotherapies are focused in identifying factors that could increase the expression of these activating receptors, to counteract inhibitory receptors expression, and therefore, to improve the NK cell cytotoxic capacities against tumor cells. In this work, we analyze the effect of interleukin (IL)-15 on the expression of NK cell-activating receptors that play a crucial role in the lysis of blasts from AML patients. Our results showed that IL-15 increased the surface expression of NKp30 on NK cells from healthy donors and AML patients with the consequent improvement of NK cell cytotoxicity. Besides, the upregulation of NKp30 induced by IL-15 is associated with an improvement of NK-mediated myeloid dendritic cells (DCs) maturation. NK cells cultured with IL-15 showed an upregulation of NKp30, which is associated with an increase anti-tumor activity and with an improved maturation of immature DCs. In our in vitro model, IL-15 exerted a great activating stimulus that could be used as novel immunotherapy in AML patients.

Cutting Edge: IL-2-Induced Expression of the Amino Acid Transporters SLC1A5 and CD98 Is a Prerequisite for NKG2D-Mediated Activation of Human NK Cells

Priming of human NK cells with IL-2 is necessary to render them functionally competent upon NKG2D engagement. We examined the underlying mechanisms that control NKG2D responsiveness in NK cells and found that IL-2 upregulates expression of the amino acid transporters SLC1A5 and CD98. Using specific inhibitors to block SLC1A5 and CD98 function, we found that production of IFN-γ and degranulation by CD56^bright and CD56^dim NK cells following NKG2D stimulation were dependent on both transporters. IL-2 priming increased the activity of mTORC1, and inhibition of mTORC1 abrogated the ability of the IL-2-primed NK cells to produce IFN-γ in response to NKG2D-mediated stimulation. This study identifies a series of IL-2-induced cellular changes that regulates the NKG2D responsiveness in human NK cells.

Natural Killer Cells - Their Role in Tumour Immunosurveillance

An important component of the innate immune system, the natural killer cells that originate from the lymphoid cell lineage, hold tremendous potential as an effective therapeutic tool to combat a variety of cancers. Their vast capability to kill altered cells such as opsonized cells (antibody coated), tumour cells, genotoxically changed cells without affecting the healthy cells of the body, make them an effective therapeutic agent for various types of cancers. Besides, through interplay and molecular crosstalk via several cytokines, they also augment the adaptive immune response by, promoting the differentiation, activation and recruitment of component cells of the system. With the current advance knowledge of Natural Killer (NK) cells, their receptor-ligand interactions involved in functional regulation, various mechanistic approaches involving the role of cytokines led to desired modulation of NK cell activity in a tailor-made manner, for triggering clinically relevant responces. Several strategies have been adopted by researchers, to augment the efficacy of NK cells. Still many challenges exist for increasing the therapeutic relevance of these cells.

Cutting Edge: NKG2D Signaling Enhances NK Cell Responses but Alone Is Insufficient To Drive Expansion during Mouse Cytomegalovirus Infection

NK cells play a critical role in host defense against viruses. In this study, we investigated the role of NKG2D in the expansion of NK cells after mouse CMV (MCMV) infection. Wild-type and NKG2D-deficient (Klrk1^-/- ) Ly49H⁺ NK cells proliferated robustly when infected with MCMV strains engineered to allow expression of NKG2D ligands, which enhanced the response of wild-type NK cells. Naive NK cells exclusively express NKG2D-L, which pairs only with DAP10, whereas NKG2D-S expressed by activated NK cells pairs with DAP10 and DAP12, similar to Ly49H. However, NKG2D alone was unable to drive robust expansion of Ly49H^- NK cells when mice were infected with these MCMV strains, likely because NKG2D-S was only transiently expressed postinfection. These findings demonstrate that NKG2D augments Ly49H-dependent proliferation of NK cells; however, NKG2D signaling alone is inadequate for expansion of NK cells, likely due to only transient expression of the NKG2D-DAP12 complex.

Intestinal epithelial cell endoplasmic reticulum stress promotes MULT1 up-regulation and NKG2D-mediated inflammation

Hosomi et al. show that intestinal epithelial cell–specific deletion of X-box–binding protein 1, an unfolded protein response–related transcription factor, results in CHOP-dependent increased expression of specific natural killer group 2 member D (NKG2D) ligands. This activates NKG2D-expressing intraepithelial group 1 ILCs and promotes small intestinal inflammation.

Endoplasmic reticulum (ER) stress is commonly observed in intestinal epithelial cells (IECs) and can, if excessive, cause spontaneous intestinal inflammation as shown by mice with IEC-specific deletion of X-box–binding protein 1 (Xbp1), an unfolded protein response–related transcription factor. In this study, Xbp1 deletion in the epithelium (Xbp1^ΔIEC) is shown to cause increased expression of natural killer group 2 member D (NKG2D) ligand (NKG2DL) mouse UL16-binding protein (ULBP)–like transcript 1 and its human orthologue cytomegalovirus ULBP via ER stress–related transcription factor C/EBP homology protein. Increased NKG2DL expression on mouse IECs is associated with increased numbers of intraepithelial NKG2D-expressing group 1 innate lymphoid cells (ILCs; NK cells or ILC1). Blockade of NKG2D suppresses cytolysis against ER-stressed epithelial cells in vitro and spontaneous enteritis in vivo. Pharmacological depletion of NK1.1⁺ cells also significantly improved enteritis, whereas enteritis was not ameliorated in Recombinase activating gene 1^−/−;Xbp1^ΔIEC mice. These experiments reveal innate immune sensing of ER stress in IECs as an important mechanism of intestinal inflammation.

Adoptively transferred Vγ9Vδ2 T cells show potent antitumor effects in a preclinical B cell lymphomagenesis model

A central issue for adoptive cellular immunotherapy is overcoming immunosuppressive signals to achieve tumor clearance. While γδ T cells are known to be potent cytolytic effectors that can kill a variety of cancers, it is not clear whether they are inhibited by suppressive ligands expressed in tumor microenvironments. Here, we have used a powerful preclinical model where EBV infection drives the de novo generation of human B cell lymphomas in vivo, and autologous T lymphocytes are held in check by PD-1/CTLA-4-mediated inhibition. We show that a single dose of adoptively transferred Vδ2+ T cells has potent antitumor effects, even in the absence of checkpoint blockade or activating compounds. Vδ2+ T cell immunotherapy given within the first 5 days of EBV infection almost completely prevented the outgrowth of tumors. Vδ2+ T cell immunotherapy given more than 3 weeks after infection (after neoplastic transformation is evident) resulted in a dramatic reduction in tumor burden. The immunotherapeutic Vδ2+ T cells maintained low cell surface expression of PD-1 in vivo, and their recruitment to tumors was followed by a decrease in B cells expressing PD-L1 and PD-L2 inhibitory ligands. These results suggest that adoptively transferred PD-1lo Vδ2+ T cells circumvent the tumor checkpoint environment in vivo.

NKG2D ligand expression in Crohn's disease and NKG2D-dependent stimulation of CD8+ T cell migration

Interaction between the activating NKG2D receptor on lymphocytes and its ligands MICA, MICB, and ULBP1-6 modulate T and NK cell activity and may contribute to the pathogenesis of Crohn's disease (CD). NKG2D ligands are generally not expressed on the cell surface of normal, non-stressed cells, but expression of MICA and MICB in CD intestine has been reported. In this exploratory study, we further characterize the expression of NKG2D and its ligands, including the less well-described ULBP4-6, in CD, and test if NKG2D ligand interactions are involved in the migration of activated T cells into the affected mucosal compartments. Intestinal tissue from CD patients and healthy controls were analyzed by flow cytometry, mass cytometry, and immunohistochemistry for expression of NKG2D and ligands, and for cytokine release. Furthermore, NKG2D-dependent chemotaxis of activated CD8⁺ T cells across a monolayer of ligand-expressing human intestinal endothelial cells was examined. Activated lymphocytes down-regulated NKG2D expression upon accumulation in inflamed CD intestine. NKG2D expression on CD56⁺ T and γδ T cells from inflamed tissue seemed inversely correlated with CRP levels and cytokine release. B cells, monocytes, mucosal epithelium, and vascular endothelium expressed NKG2D ligands in inflamed CD intestine. The expression of NKG2D ligands was correlated with cytokine release, but was highly variable between patients. Stimulation of vascular intestinal endothelial cells in vitro induced expression of NKG2D ligands, including MICA/B and ULBP2/6. Blockade of NKG2D on CD8⁺ T cells inhibited the migration over ligand-expressing endothelial cells. Intestinal induction of NKG2D ligands and ligand-induced down-regulation of NKG2D in CD suggest that the NKG2D-ligand interaction may be involved in both the activation and recruitment of NKG2D⁺ lymphocytes into the inflamed CD intestine.

The frog skin host-defense peptide frenatin 2.1S enhances recruitment, activation and tumoricidal capacity of NK cells

Frog skin is a source of peptides with various biological properties. Frenatin 2.1S, derived from norepinephrine-stimulated skin secretions of the Orinoco lime tree frog Sphaenorhynchus lacteus, exhibits immunostimulatory effects as demonstrated by the promotion of proinflammatory phenotypes of mononuclear cells in mouse peritoneal cavity and spleen. The aim of this study was to identify the populations of host cells sensitive to the action of frenatin 2.1S in vivo and to study its effects on their functional antitumor capacity. A single injection of frenatin 2.1S (100μg) in BALB/c mice increased the presence of peritoneal CD11c⁺ dendritic cells and CD3⁺ T cells 24h after administration and there was a significant increase in the number of IL-17 and CXCR3 expressing inflammatory T cells. Frenatin 2.1S treatment also increased the number of TNF-α expressing F4/80⁺ proinflammatory M1 macrophages. The most striking finding of the study is the marked increase of the number of peritoneal natural killer (NK) cells following frenatin 2.1S injection. Further, frenatin 2.1S administration led to activation of NK cells as evaluated by increased expression of NKG2D, FasL, CD69 and CD107a. The increased ratio of interferon-γ vs. IL-10 producing NK cells is further indication of the proinflammatory action of frenatin 2.1S. Peptide treatment enhanced the tumoricidal action of peritoneal NK cells on 4T1 mouse mammary carcinoma cells as revealed by the real-time automated monitoring of cell status. Our data demonstrate that frenatin 2.1S promotes activation and cytotoxic capacity of NK cells and should be regarded as a candidate for antitumor immunotherapy.

NKG2D+CD4+ T Cells Kill Regulatory T Cells in a NKG2D-NKG2D Ligand- Dependent Manner in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) features a decreased pool of CD4⁺CD25⁺Foxp3⁺ T regulatory (Treg) cells. We had previously observed NKG2D⁺CD4⁺ T cell expansion in contrast to a decreased pool of Treg cells in SLE patients, but whether NKG2D⁺CD4⁺ T cells contribute to the decreased Treg cells remains unclear. In the present study, we found that the NKG2D⁺CD4⁺ T cells efficiently killed NKG2D ligand (NKG2DL)⁺ Treg cells in vitro, whereby the surviving Treg cells in SLE patients showed no detectable expression of NKG2DLs. It was further found that MRL/lpr lupus mice have significantly increased percentage of NKG2D⁺CD4⁺ T cells and obvious decreased percentage of Treg cells, as compared with wild-type mice. Adoptively transferred NKG2DL⁺ Treg cells were found to be efficiently killed in MRL/lpr lupus mice, with NKG2D neutralization remarkably attenuating this killing. Anti-NKG2D or anti-interferon-alpha receptor (IFNAR) antibodies treatment in MRL/lpr mice restored Treg cells numbers and markedly ameliorated the lupus disease. These results suggest that NKG2D⁺CD4⁺ T cells are involved in the pathogenesis of SLE by killing Treg cells in a NKG2D-NKG2DL-dependent manner. Targeting the NKG2D-NKG2DL interaction might be a potential therapeutic strategy by which Treg cells can be protected from cytolysis in SLE patients.

TGF-β1 improving abnormal pregnancy outcomes induced by Toxoplasma gondii infection: Regulating NKG2D/DAP10 and killer subset of decidual NK cells

Our current aim was to investigate whether injection of TGF-β1 played an important role in improving abnormal pregnancy outcomes with T. gondii infection and how the TGF-β1 regulated. Results showed that TGF-β1 exhibited improved pregnancy outcomes induced by T. gondii infection. dNK cytotoxicity was increased with T. gondii infection while decreased with TGF-β1 treatment. dNK cytotoxicity related NKG2D/DAP10 expression, perforin, granzyme, IFN-γ and killer subsets were all increased with T. gondii infection while decreased after TGF-β1 treatment. In addition, anti-TGF-β1 antibodies could aggregate the cytotoxicity of dNK cells and the levels of molecules above. These results indicated that TGF-β1 treatment could improve the abnormal pregnancy outcomes with T. gondii infection by decreasing the cytotoxicity of dNK cells mediated by NKG2D/DAP10 pathway and killer subset. These results suggested that TGF-β1 might be a potential immunoprotective method for the treatment of abnormal pregnancy outcomes following T. gondii infection.

NKG2D: A versatile player in the immune system

NKG2D is known as a potent activating receptor of the immune system. It is expressed on a multitude of immune cells, including NK cells and different subsets of T cells. NKG2D recognizes various MHC I-like ligands that are induced on target cells exposed to stressors such as viral infection, DNA damage and oncological transformation. NKG2D drives or facilitates cytotoxic and cytokine responses towards cells expressing its ligands to eliminate the threat. Therefore, NKG2D is usually classified as a sensor that translates cellular stress into activation signals for immune cells. However, more recently it has become evident that NKG2D plays a role beyond direct killing of target cells. Lack of NKG2D affects development of NK cells in the bone marrow, resulting in hyperreactive NK cells. NKG2D deficiency on CD8 T cells affects the ability of effector cells to produce cytokines in response to T cell receptor engagement and reduces their capacity to establish immunological memory. Although NKG2D is not expressed on B cells subsets, lack of this receptor in hematopoietic precursors affects B cell development. Homing of mature B2 cells is altered in NKG2D-deficient mice and they have a strong reduction in peripheral B1a cell numbers, resulting in increased susceptibility to bacterial infections. The exact molecular mechanisms via which NKG2D mediates these versatile functions is still being explored, but appears to depend on the control of activation thresholds, either in hematopoietic precursors or mature immune cell subsets. In this review, we will elaborate on the underappreciated developmental and regulatory roles of NKG2D.

Natural Killer Group 2D Ligand Depletion Reconstitutes Natural Killer Cell Immunosurveillance of Head and Neck Squamous Cell Carcinoma

Head and neck squamous cell carcinoma (HNSCC) is a highly heterogeneous and aggressive tumor originating from the epithelial lining of the upper aero-digestive tract accounting for 300,000 annual deaths worldwide due to failure of current therapies. The natural killer group 2D (NKG2D) receptors on natural killer (NK) cells and several T cell subsets play an important role for immunosurveillance of HNSCC and are thus targeted by tumor immune evasion strategies in particular by shedding of various NKG2D ligands (NKG2DLs). Based on plasma and tumor samples of 44 HNSCC patients, we found that despite compositional heterogeneity the total plasma level of NKG2DLs correlates with NK cell inhibition and disease progression. Strikingly, based on tumor spheroids and primary tumors of HNSCC patients, we found that NK cells failed to infiltrate HNSCC tumors in the presence of high levels of NKG2DLs, demonstrating a novel mechanism of NKG2DL-dependent tumor immune escape. Therefore, the diagnostic acquisition of the plasma level of all NKG2DLs might be instrumental for prognosis and to decipher a patient cohort, which could benefit from restoration of NKG2D-dependent tumor immunosurveillance. Along these lines, we could show that removal of shed NKG2DLs (sNKG2DLs) from HNSCC patients’ plasma restored NK cell function in vitro and in individual patients following surgical removal of the primary tumor. In order to translate these findings into a therapeutic setting, we performed a proof-of-concept study to test the efficacy of adsorption apheresis of sNKG2DLs from plasma after infusion of human MICA in rhesus monkeys. Complete removal of MICA was achieved after three plasma volume exchanges. Therefore, we propose adsorption apheresis of sNKG2DLs as a future preconditioning strategy to improve the efficacy of autologous and adoptively transferred immune cells in cellular cancer immunotherapy.

Targeting neoantigens to augment antitumour immunity

The past decade of cancer research has been marked by a growing appreciation of the role of immunity in cancer. Mutations in the tumour genome can cause tumours to express mutant proteins that are tumour specific and not expressed on normal cells (neoantigens). These neoantigens are an attractive immune target because their selective expression on tumours may minimize immune tolerance as well as the risk of autoimmunity. In this Review we discuss the emerging evidence that neoantigens are recognized by the immune system and can be targeted to increase antitumour immunity. We also provide a framework for personalized cancer immunotherapy through the identification and selective targeting of individual tumour neoantigens, and present the potential benefits and obstacles to this approach of targeted immunotherapy.

Vigilin Regulates the Expression of the Stress-Induced Ligand MICB by Interacting with Its 5' Untranslated Region

NK cells are part of the innate immune system, and are able to identify and kill hazardous cells. The discrimination between normal and hazardous cells is possible due to an array of inhibitory and activating receptors. NKG2D is one of the prominent activating receptors expressed by all human NK cells. This receptor binds stress-induced ligands, including human MICA, MICB, and UL16-binding proteins 1-6. The interaction between NKG2D and its ligands facilitates the elimination of cells under cellular stress, such as tumor transformation. However, the mechanisms regulating the expression of these ligands are still not well understood. Under normal conditions, the NKG2D ligands were shown to be posttranscriptionally regulated by cellular microRNAs and RNA-binding proteins (RBPs). Thus far, only the 3' untranslated regions (UTRs) of MICA, MICB, and UL16-binding protein 2 were shown to be regulated by RBPs and microRNAs, usually resulting in their downregulation. In this study we investigated whether MICB expression is controlled by RBPs through its 5'UTR. We used an RNA pull-down assay followed by mass spectrometry and identified vigilin, a ubiquitously expressed multifunctional RNA-binding protein. We demonstrated that vigilin binds and negatively regulates MICB expression through its 5'UTR. Additionally, vigilin downregulation in target cells led to a significant increase in NK cell activation against said target cells. Taken together, we have discovered a novel mode of MICB regulation.

Natural Killer Group 2, Member D/NKG2D Ligands in Hematopoietic Cell Transplantation

Natural killer group 2, member D (NKG2D) is an invariant activatory receptor present on subsets of natural killer and T lymphocytes. It stimulates the cytolytic effector response upon engagement of its various stress-induced ligands NKG2D ligands (NKG2DL). Malignant transformation and conditioning treatment prior to hematopoietic cell transplantation (HCT) are stress factors leading to the activation of the NKG2D/NKG2DL signaling in clinical settings. In the context of HCT, NKG2D-bearing cells can kill both tumor and healthy cells expressing NKG2DL. The NKG2D/NKG2DL engagement has therefore a key role in the regulation of one of the most salient issues in allogeneic HCT, i.e., maintaining a balance between graft-vs.-leukemia effect and graft-vs.-host disease. The present review summarizes the current state of our knowledge pertaining to the role of the NKG2D and NKG2DL in HCT.

Role of NKG2D in cytokine-induced killer cells against lung cancer

It has been previously demonstrated that cytokine-induced killer (CIK) cells possess potent cytotoxicity against various cancer cells, including lung cancer cells. However, the mechanism by which CIK cells recognize lung cancer cells has not been understood. The interaction between killer cell lectin like receptor K1 (NKG2D) receptor and NKG2D ligands was demonstrated to serve an important role in target cell killing by natural killer cells. The present study investigated whether NKG2D receptor and NKG2D ligand interactions are involved in the CIK-directed killing of lung cancer cells. The expression of MICA and ULBP2 was detected in tumor and healthy tissue samples. The expression of MICA and ULBP2 in tumor tissue samples was higher compared with that in the healthy control tissue. The expression of NKG2D ligands was analyzed in A549 and Q56 cells through reverse transcription-quantitative polymerase chain reaction and flow cytometry. The results demonstrated that the lung cancer cell lines markedly expressed the NKG2D ligands. Furthermore, NKG2D ligand-expressing lung cancer cells were targeted by CIK cells, which was partially blocked by treating CIK cells with an antibody against NKG2D. The data of the current study has demonstrated that the NKG2D-NKG2D ligand interaction serves an essential role in mediating lung cancer cell killing by CIK cells.

MYC: Master Regulator of Immune Privilege

Cancers are often initiated by genetic events that activate proto-oncogenes or inactivate tumor-suppressor genes. These events are also crucial for sustained tumor cell proliferation and survival, a phenomenon described as oncogene addiction. In addition to this cell-intrinsic role, recent evidence indicates that oncogenes also directly regulate immune responses, leading to immunosuppression. Expression of many oncogenes or loss of tumor suppressors induces the expression of immune checkpoints that regulate the immune response, such as PD-L1. We discuss here how oncogenes, and in particular MYC, suppress immune surveillance, and how oncogene-targeted therapies may restore the immune response against tumors.

The immunoreceptor NKG2D promotes tumour growth in a model of hepatocellular carcinoma

Inflammation is recognized as one of the drivers of cancer. Yet, the individual immune components that possess pro- and anti-tumorigenic functions in individual cancers remain largely unknown. NKG2D is a potent activating immunoreceptor that has emerged as an important player in inflammatory disorders besides its well-established function as tumour suppressor. Here, we provide genetic evidence of an unexpected tumour-promoting effect of NKG2D in a model of inflammation-driven liver cancer. Compared to NKG2D-deficient mice, NKG2D-sufficient mice display accelerated tumour growth associated with, an increased recruitment of memory CD8⁺T cells to the liver and exacerbated pro-inflammatory milieu. In addition, we show that NKG2D contributes to liver damage and consequent hepatocyte proliferation known to favour tumorigenesis. Thus, the NKG2D/NKG2D-ligand pathway provides an additional mechanism linking chronic inflammation to tumour development in hepatocellular carcinoma. Our findings expose the need to selectively target the types of cancer that could benefit from NKG2D-based immunotherapy.

Expression of NKG2D immunoreceptor ligands on tumour cells is believed to inhibit tumour growth through engaging NKG2D-expressing immune cells. Here, the authors show that in a model of liver cancer the NKG2D/NKG2D-ligand pathway can also promote tumour formation by sustaining an inflammatory environment.

NKG2D Promotes B1a Cell Development and Protection against Bacterial Infection

NKG2D is a potent activating receptor that is expressed on cytotoxic immune cells such as CD8 T and NK cells, where it promotes cytotoxicity after binding stress ligands on infected or transformed cells. On NK cell precursors NKG2D modulates proliferation and maturation. Previously, we observed that NKG2D deficiency affects peripheral B cell numbers. In this study, we show that NKG2D regulates B1a cell development and function. We find that mice deficient for NKG2D have a strong reduction of B1a cell numbers. As a result, NKG2D-deficient mice produce significantly less Ag-specific IgM Abs upon immunization with T cell-independent Ags, and they are more susceptible to Gram-negative sepsis. Klrk1^-/- B1a cells are also functionally impaired and they fail to provide protection against Francisella novicida upon adoptive transfer. Using mixed bone marrow chimeric mice, we show that the impact of NKG2D deficiency on B1a cell development is cell intrinsic. No changes in homeostatic turnover and homing of B cells were detectable, limiting the effects of NKG2D to modulation of the hematopoietic development of B1a cells. Using conditional ablation, we demonstrate that the effect of NKG2D on B1a cell development occurs at a developmental stage that precedes the common lymphoid progenitor. Our findings reveal an unexpected new role for NKG2D in the regulation of B1a cell development. The protective effects of this activating receptor therefore reach beyond that of cytotoxic cells, stimulating the immune system to fight bacterial infections by promoting development of innate-like B cells.