LETAL, A Tumor-Associated NKG2D Immunoreceptor Ligand, Induces Activation and Expansion of Effector Immune Cells

Serum immune mediators as novel predictors of response to anti-PD-1/PD-L1 therapy in non-small cell lung cancer patients with high tissue-PD-L1 expression

Non-small cell lung cancer (NSCLC) is the leading cause of cancer-related morbidity and mortality worldwide. Immune checkpoint inhibitors (ICIs) including anti-PD-1 and anti-PD-L1 antibodies, have significantly changed the treatment outcomes with better overall survival, but only 15-40% of the patients respond to ICIs therapy. The search for predictive biomarkers of responses is warranted for better clinical outcomes. We aim here to identify pre-treatment soluble immune molecules as surrogate biomarkers for tissue PD-L1 (TPD-L1) status and as predictors of response to anti-PD-1/PD-L1 therapy in NSCLC patients. Sera from 31 metastatic NSCLC patients, eligible for anti-PD-1/PD-L1 or combined chemoimmunotherapy, were collected prior to treatment. Analysis of soluble biomarkers with TPD-L1 status showed significant up/down regulation of the immune inhibitory checkpoint markers (sSiglec7, sSiglec9, sULBP4 and sPD-L2) in patients with higher TPD-L1 (TPD-L1 >50%) expression. Moreover, correlation analysis showed significant positive linear correlation of soluble PD-L1 (sPD-L1) with higher TPD-L1 expression. Interestingly, only responders in the TPD-L1 >50% group showed significant down regulation of the immune inhibitory markers (sPD-L2, sTIMD4, sNectin2 and CEA). When responders vs. non-responders were compared, significant down regulation of other immune inhibitory biomarkers (sCD80, sTIMD4 and CEA) was recorded only in responding patients. In this, the optimal cut-off values of CD80 <91.7 pg/ml and CEA <1614 pg/ml were found to be significantly associated with better progression free survival (PFS). Indeed, multivariate analysis identified the cutoff-value of CEA <1614 pg/ml as an independent predictor of response in our patients. We identified here novel immune inhibitory/stimulatory soluble mediators as potential surrogate/predictive biomarkers for TPD-L1 status, treatment response and PFS in NSCLC patients treated with anti-PD-1/PD-L1 therapy.

Regulation of NKG2D Stress Ligands and Its Relevance in Cancer Progression

Under cellular distress, multiple facets of normal homeostatic signaling are altered or disrupted. In the context of the immune landscape, external and internal stressors normally promote the expression of natural killer group 2 member D (NKG2D) ligands that allow for the targeted recognition and killing of cells by NKG2D receptor-bearing effector populations. The presence or absence of NKG2D ligands can heavily influence disease progression and impact the accessibility of immunotherapy options. In cancer, tumor cells are known to have distinct regulatory mechanisms for NKG2D ligands that are directly associated with tumor progression and maintenance. Therefore, understanding the regulation of NKG2D ligands in cancer will allow for targeted therapeutic endeavors aimed at exploiting the stress response pathway. In this review, we summarize the current understanding of regulatory mechanisms controlling the induction and repression of NKG2D ligands in cancer. Additionally, we highlight current therapeutic endeavors targeting NKG2D ligand expression and offer our perspective on considerations to further enhance the field of NKG2D ligand biology.

Molecular mechanisms of human herpes viruses inferring with host immune surveillance

Several human herpes viruses (HHVs) exert oncogenic potential leading to malignant transformation of infected cells and/or tissues. The molecular processes induced by viral-encoded molecules including microRNAs, peptides, and proteins contributing to immune evasion of the infected host cells are equal to the molecular processes of immune evasion mediated by tumor cells independently of viral infections. Such major immune evasion strategies include (1) the downregulation of proinflammatory cytokines/chemokines as well as the induction of anti-inflammatory cytokines/chemokines, (2) the downregulation of major histocompatibility complex (MHC) class Ia directly as well as indirectly by downregulation of the components involved in the antigen processing, and (3) the downregulation of stress-induced ligands for activating receptors on immune effector cells with NKG2D leading the way. Furthermore, (4) immune modulatory molecules like MHC class Ib molecules and programmed cell death1 ligand 1 can be upregulated on infections with certain herpes viruses. This review article focuses on the known molecular mechanisms of HHVs modulating the above-mentioned possibilities for immune surveillance and even postulates a temporal order linking regular tumor immunology with basic virology and offering putatively novel insights for targeting HHVs.

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.

Cutting an NKG2D Ligand Short: Cellular Processing of the Peculiar Human NKG2D Ligand ULBP4

Stress-induced cell surface expression of MHC class I-related glycoproteins of the MIC and ULBP families allows for immune recognition of dangerous "self cells" by human cytotoxic lymphocytes via the NKG2D receptor. With two MIC molecules (MICA and MICB) and six ULBP molecules (ULBP1-6), there are a total of eight human NKG2D ligands (NKG2DL). Since the discovery of the NKG2D-NKG2DL system, the cause for both redundancy and diversity of NKG2DL has been a major and ongoing matter of debate. NKG2DL diversity has been attributed, among others, to the selective pressure by viral immunoevasins, to diverse regulation of expression, to differential tissue expression as well as to variations in receptor interactions. Here, we critically review the current state of knowledge on the poorly studied human NKG2DL ULBP4. Summarizing available facts and previous studies, we picture ULBP4 as a peculiar ULBP family member distinct from other ULBP family members by various aspects. In addition, we provide novel experimental evidence suggesting that cellular processing gives rise to mature ULBP4 glycoproteins different to previous reports. Finally, we report on the proteolytic release of soluble ULBP4 and discuss these results in the light of known mechanisms for generation of soluble NKG2DL.

Genetics, genomics, and evolutionary biology of NKG2D ligands

Human and mouse NKG2D ligands (NKG2DLs) are absent or only poorly expressed by most normal cells but are upregulated by cell stress, hence, alerting the immune system in case of malignancy or infection. Although these ligands are numerous and highly variable (at genetic, genomic, structural, and biochemical levels), they all belong to the major histocompatibility complex class I gene superfamily and bind to a single, invariant, receptor: NKG2D. NKG2D (CD314) is an activating receptor expressed on NK cells and subsets of T cells that have a key role in the recognition and lysis of infected and tumor cells. Here, we review the molecular diversity of NKG2DLs, discuss the increasing appreciation of their roles in a variety of medical conditions, and propose several explanations for the evolutionary force(s) that seem to drive the multiplicity and diversity of NKG2DLs while maintaining their interaction with a single invariant receptor.

Cancer prevention and therapy through the modulation of the tumor microenvironment

Cancer arises in the context of an in vivo tumor microenvironment. This microenvironment is both a cause and consequence of tumorigenesis. Tumor and host cells co-evolve dynamically through indirect and direct cellular interactions, eliciting multiscale effects on many biological programs, including cellular proliferation, growth, and metabolism, as well as angiogenesis and hypoxia and innate and adaptive immunity. Here we highlight specific biological processes that could be exploited as targets for the prevention and therapy of cancer. Specifically, we describe how inhibition of targets such as cholesterol synthesis and metabolites, reactive oxygen species and hypoxia, macrophage activation and conversion, indoleamine 2,3-dioxygenase regulation of dendritic cells, vascular endothelial growth factor regulation of angiogenesis, fibrosis inhibition, endoglin, and Janus kinase signaling emerge as examples of important potential nexuses in the regulation of tumorigenesis and the tumor microenvironment that can be targeted. We have also identified therapeutic agents as approaches, in particular natural products such as berberine, resveratrol, onionin A, epigallocatechin gallate, genistein, curcumin, naringenin, desoxyrhapontigenin, piperine, and zerumbone, that may warrant further investigation to target the tumor microenvironment for the treatment and/or prevention of cancer.

Hypoxia-mediated immune evasion of pancreatic carcinoma cells

Hypoxia is one of the characteristics of human and animal tumors. To investigate the association between hypoxia and the immune evasion of cancer cells, the present study examined paraffin sections of pancreatic tissues from patients with pancreatic carcinoma, chronic pancreatitis and normal pancreatic tissue and established a series of PANC‑1 cell lines, which were cultured under various hypoxic and normoxic conditions. The results demonstrated that the expression of hypoxia‑inducible 1α (HIF‑1α) in pancreatic carcinoma was significantly higher compared with that in the chronic pancreatitis and normal pancreatic tissues, which revealed that a hypoxic microenvironment existed in pancreatic carcinoma. HIF‑1α was inversely correlated with major histocompatibility complex class I chain‑related (MIC) genes, which indicated that hypoxia was involved in tumor immune evasion. The cell experiments demonstrated that the mechanism involved shedding of the MIC from the membrane of the pancreatic carcinoma cells, which then formed soluble (s)MIC. The sMIC genes downregulated natural killer (NK) group 2, member D and the cytotoxic activity of NK cells. Depending on its activity, the nitric oxide‑cyclic guanosine monophosphate‑protein kinase G signaling pathway can either increase or inhibit immune evasion of pancreatic cancer cells.

The correlation between NK cell and liver function in patients with primary hepatocellular carcinoma

Background/Aims

This study aimed to detect the expression of natural killer (NK) cell receptor natural killer group 2D (NKG2D) in the peripheral blood of patients with primary hepatocellular carcinoma and to discuss the correlation between NK cell cytotoxicity and liver function.

Methods

The number of NK cells and the expression of NK cell receptor NKG2D in peripheral blood were determined by flow cytometry in patients with primary hepatocellular carcinoma, hepatitis B cirrhosis, chronic hepatitis B, and healthy controls.

Results

When compared with patients in the healthy and the chronic hepatitis B groups, the primary hepatocellular carcinoma group showed significant decreases in all parameters, including the cytotoxicity of NK cells on K562 cells, expression rate of NKG2D in NK cells, number of NKG2D⁺ NK cells, expression level of NKG2D, and number of NK cells (p<0.05). The activity of NK cells showed a positive correlation, whereas the Child-Pugh scores in the primary hepatocellular carcinoma and the hepatitis B cirrhosis groups showed a negative correlation with all parameters detected above.

Conclusions

The decrease of NK cell activity in patients with primary hepatocellular carcinoma is closely related to their lower expression of NKG2D. Liver function affects the expression of NKG2D and the activity of NK cells.

Nitric oxide signaling pathway activation inhibits the immune escape of pancreatic carcinoma cells

The aim of the present study was to investigate the effect of the nitric oxide signaling pathway on immune escape; thus, a tumorigenesis model was established using nude mice. The mice were inoculated with pancreatic carcinoma cells and divided into two groups, a glyceryl trinitrate (GTN) and a placebo group. When tumor volumes reached 150 mm³, the mice in the GTN group were treated with GTN transdermal patches (dose, 7.3 μg/h) while the mice in the placebo group were administered untreated patches. Following treatment, the tumor volume was recorded every 3-4 days and after 28 days, the tumors were analyzed. The results indicated that GTN treatment may reduce the levels of soluble major histocompatibility complex class I chain-related molecules, and natural killer group 2 member D, as well as inhibiting tumor growth.

The role of microRNAs in the control of innate immune response in cancer

Ligands for receptors of natural killer (NK) cells and CD8(+) cytotoxic T lymphocytes (CTL), such as the inhibitory nonclassical HLA-G, the activating stress-induced major histocompatibility complex class I-related antigens MICA and MICB, and/or the UL16-binding proteins (ULBPs), are often aberrantly expressed upon viral infection and neoplastic transformation, thereby preventing virus-infected or malignant-transformed cells from elimination by immune effector cells. Recently, it has been shown that ligands of both NK and CD8(+) T cells are regulated by a number of cellular and/or viral microRNAs (miRs). These miRs are involved in shaping the antiviral and/or antitumoral immune responses as well as neoplastic growth properties. This review summarizes the expression pattern and function of miRs directed against selected NK and T cell receptor ligands, their putative role in shaping immune surveillance and tumorigenicity, and their clinical relevance. In addition, the potential role of RNA-binding proteins in the post-transcriptional gene regulation of these ligands will be discussed.

Clinical Implications of Co-Inhibitory Molecule Expression in the Tumor Microenvironment for DC Vaccination: A Game of Stop and Go

The aim of therapeutic dendritic cell (DC) vaccines in cancer immunotherapy is to activate cytotoxic T cells to recognize and attack the tumor. T cell activation requires the interaction of the T cell receptor with a cognate major-histocompatibility complex-peptide complex. Although initiated by antigen engagement, it is the complex balance between co-stimulatory and co-inhibitory signals on DCs that results in T cell activation or tolerance. Even when already activated, tumor-specific T cells can be neutralized by the expression of co-inhibitory molecules on tumor cells. These and other immunosuppressive cues in the tumor microenvironment are major factors currently hampering the application of DC vaccination. In this review, we discuss recent data regarding the essential and complex role of co-inhibitory molecules in regulating the immune response within the tumor microenvironment. In particular, possible therapeutic intervention strategies aimed at reversing or neutralizing suppressive networks within the tumor microenvironment will be emphasized. Importantly, blocking co-inhibitory molecule signaling, often referred to as immune checkpoint blockade, does not necessarily lead to an effective activation of tumor-specific T cells. Therefore, combination of checkpoint blockade with other immune potentiating therapeutic strategies, such as DC vaccination, might serve as a synergistic combination, capable of reversing effector T cells immunosuppression while at the same time increasing the efficacy of T cell-mediated immunotherapies. This will ultimately result in long-term anti-tumor immunity.

Dendritic cells the tumor microenvironment and the challenges for an effective antitumor vaccination

Many clinical trials have been carried out or are in progress to assess the therapeutic potential of dendritic-cell- (DC-) based vaccines on cancer patients, and recently the first DC-based vaccine for human cancer was approved by the FDA. Herewith, we describe the general characteristics of DCs and different strategies to generate effective antitumor DC vaccines. In recent years, the relevance of the tumor microenvironment in the progression of cancer has been highlighted. It has been shown that the tumor microenvironment is capable of inactivating various components of the immune system responsible for tumor clearance. In particular, the effect of the tumor microenvironment on antigen-presenting cells, such as DCs, does not only render these immune cells unable to induce specific immune responses, but also turns them into promoters of tumor growth. We also describe strategies likely to increase the efficacy of DC vaccines by reprogramming the immunosuppressive nature of the tumor microenvironment.

Cervical cancer cell lines expressing NKG2D-ligands are able to down-modulate the NKG2D receptor on NKL cells with functional implications

BACKGROUND

Cervical cancer represents the third most commonly diagnosed cancer and the fourth leading cause of cancer-related deaths in women worldwide. Natural killer (NK) cells play an important role in the defense against viruses, intracellular bacteria and tumors. NKG2D, an activating receptor on NK cells, recognizes MHC class I chain-related molecules, such as MICA/B and members of the ULBP/RAET1 family. Tumor-derived soluble NKG2D-ligands have been shown to down-modulate the expression of NKG2D on NK cells. In addition to the down-modulation induced by soluble NKG2D-ligands, it has recently been described that persistent cell-cell contact can also down-modulate NKG2D expression. The goal of this study was to determine whether the NKG2D receptor is down-modulated by cell-cell contact with cervical cancer cells and whether this down-modulation might be associated with changes in NK cell activity.

RESULTS

We demonstrate that NKG2D expressed on NKL cells is down-modulated by direct cell contact with cervical cancer cell lines HeLa, SiHa, and C33A, but not with non-tumorigenic keratinocytes (HaCaT). Moreover, this down-modulation had functional implications. We found expression of NKG2D-ligands in all cervical cancer cell lines, but the patterns of ligand distribution were different in each cell line. Cervical cancer cell lines co-cultured with NKL cells or fresh NK cells induced a marked diminution of NKG2D expression on NKL cells. Additionally, the cytotoxic activity of NKL cells against K562 targets was compromised after co-culture with HeLa and SiHa cells, while co-culture with C33A increased the cytotoxic activity of the NKL cells.

CONCLUSIONS

Our results suggest that differential expression of NKG2D-ligands in cervical cancer cell lines might be associated with the down-modulation of NKG2D, as well as with changes in the cytotoxic activity of NKL cells after cell-cell contact with the tumor cells.

The immunomodulating roles of glycoproteins in epithelial ovarian cancer

The complexity of the immune system demands an intricate defense mechanism by tumors. Ovarian and other tumors employ specific glycoproteins and the associated glycan sequences to modulate immune responses. Glycoproteins enable tumor cells that express or secrete these molecules to evade immune cell attack and induce the immune system to promote tumor growth. This review focuses first on the immune environment in ovarian cancer, and the mechanisms of activation and inhibition that immune cells undergo in order to either attack or ignore a target cell. Next we illustrate the immunomodulatory roles of ovarian cancer-associated glycans and glycoproteins in 1. preventing immune synapse formation, 2. serving as ligands of immune cell receptors, 3. scavenging cytokines and chemokines, and 4. participating in the formation of autoantibodies against the tumor. The importance of these immunomodulating strategies from the view points of understanding the tumor immunology of ovarian tumors, potential origin of such mechanisms, and specific strategies to circumvent the glycoconjugate-mediated suppression of immune responses is discussed in this review.

Decreased NKG2D expression on NK cells correlates with impaired NK cell function in patients with gastric cancer

BACKGROUND

Although malignant diseases are known to be associated with immune suppression, the detailed mechanisms involved are still unknown. NKG2D is an activating cell surface receptor expressed by natural killer (NK) cells and CD8+ T cells, and the engagement of NKG2D is extremely important for NK cell activation. Although decreased NKG2D expression on NK cells is closely related to immune evasion by some cancers, the immunopathological importance of this phenomenon in gastric cancer patients remains unclear.

METHODS

NKG2D expression on NK cells was determined, using multicolor flow cytometry, to investigate the mechanisms responsible for immune evasion in gastric cancer patients.

RESULTS

NKG2D expression on NK cells from gastric cancer patients was significantly lower than that in healthy controls. Also, NKG2D expression in advanced gastric cancer was significantly lower than that in early gastric cancer. NK cells from patients with lymph node metastasis expressed significantly lower levels of NKG2D than the NK cells from those without lymph node metastasis, and NKG2D expression on NK cells in gastric cancer tissue was significantly lower than that of circulating NK cells. NKG2D expression on NK cells obtained from cancer patients was restored after 48 h in culture with RPMI containing 10% AB serum. Furthermore, NKG2D expression on NK cells obtained after surgery was significantly higher than that before surgery.

CONCLUSIONS

Decreased NKG2D expression on NK cells may be one of the key mechanisms responsible for NK cell dysfunction in gastric cancer.

Establishment, characterization, and successful adaptive therapy against human tumors of NKG cell, a new human NK cell line

Natural killer (NK) cells play important roles in adoptive cellular immunotherapy against certain human cancers. This study aims to establish a new human NK cell line and to study its role for adoptive cancer immunotherapy. Peripheral blood samples were collected from 54 patients to establish the NK cell line. A new human NK cell line, termed as NKG, was established from a Chinese male patient with rapidly progressive non-Hodgkin's lymphoma. NKG cells showed LGL morphology and were phenotypically identified as CD56(bright) NK cell with CD16(-), CD27(-), CD3(-), αβTCR(-), γδTCR(-), CD4(-), CD8(-), CD19(-), CD161(-), CD45(+), CXCR4(+), CCR7(+), CXCR1(-), and CX3CR1(-). NKG cells showed high expression of adhesive molecules (CD2, CD58, CD11a, CD54, CD11b, CD11c), an array of activating receptors (NKp30, NKp44, NKp46, NKG2D, NKG2C), and cytolysis-related receptors and molecules (TRAIL, FasL, granzyme B, perforin, IFN-γ). The cytotoxicity of NKG cells against tumor cells was higher than that of the established NK cell lines NK-92, NKL, and YT. NKG cell cytotoxicity depended on the presence of NKG2D and NKp30. When irradiated with 8 Gy, NKG cells were still with high cytotoxicity and activity in vitro and with safety in vivo, but without proliferation. Further, the irradiated NKG cells exhibited strong cytotoxicity against human primary ovarian cancer cells in vitro, and against human ovarian cancer in a mouse xenograft model. The adoptive transfer of NKG cells significantly inhibited the ovarian tumor growth, decreased the mortality rate and prolonged the survival, even in cases of advanced diseases. A number of NKG cells were detected in the ovarian tumor tissues during cell therapy. In use of the new human NK cell line, NKG would a promising cellular candidate for adoptive immunotherapy of human cancer.

Major histocompatibility complex class I-related chain A/B (MICA/B) expression in tumor tissue and serum of pancreatic cancer: role of uric acid accumulation in gemcitabine-induced MICA/B expression

Background

Major histocompatibility complex class I-related chain A and B (MICA/B) are two stress-inducible ligands that bind the immunoreceptor NKG2D and play an important role in mediating the cyotoxicity of NK and T cells. In this study, we sought to study MICA/B expression in pancreatic cancer and to determine whether and how genotoxic drugs such as gemcitabine can affect MICA/B expression and natural killer cytotoxity.

Methods

Seven pancreatic cancer cell lines were analyzed for MICA/B expression by flow cytometry and for their sensitivity to NK-92 cell killing by a ⁵¹Cr release assay. MICA/B expression in tumor tissues and sera of pancreatic cancer was analyzed by immunohistochemical staining (IHC) and ELISA, respectively.

Results

Two MICA/B-positive cell lines were sensitive to the cytotoxic activity of NK-92 cells. Other two MICA/B-positive cell lines and three MICA/B-negative cell lines were resistant to NK-92 cell killing. MICA/B expression was positive in 17 of 25 (68%) pancreatic ductal adenocarcinomas but not in normal pancreatic ductal epithelial cells. Serum MICA/B levels were significantly elevated in patients with pancreatic adenocarcinomas but did not correlate with the stage of pancreatic cancer and patient survival. Gemcitabine therapy led to increased serum MICA levels in 6 of 10 patients with detectable serum MICA. Allopurinol, an inhibitor of xanthine oxidoreductase that converts xanthine to uric acid, blocked uric acid production, MICA/B expression, and sensitivity to NK-92 cell killing toward a PANC-1 cancer cell line exposed to radiation and two genotoxic drugs, gemcitabine and 5-fluorouracil.

Conclusions

The levels of MICA/B expression in serum and tissue of pancreatic cancer are elevated. DNA damage-induced MICA/B expression is mediated through increased uric acid production.

NKG2D receptor regulates human effector T-cell cytokine production

Although innate immune signals shape the activation of naive T cells, it is unclear how innate signals influence effector T-cell function. This study determined the effects of stimulating the NKG2D receptor in conjunction with the TCR on human effector CD8(+) T cells. Stimulation of CD8(+) T cells through CD3 and NKG2D simultaneously or through a chimeric NKG2D receptor, which consists of NKG2D fused to the intracellular region of CD3ζ, activated β-catenin and increased expression of β-catenin-induced genes, whereas T cells stimulated through the TCR or a combination of the TCR and CD28 did not. Activation by TCR and NKG2D prevented expression and production of anti-inflammatory cytokines IL-10, IL-9, IL-13, and VEGF-α in a β-catenin- and PPARγ- dependent manner. NKG2D stimulation also modulated the cytokine secretion of T cells activated simultaneously through CD3 and CD28. These data indicate that activating CD8(+) T cells through the NKG2D receptor along with the TCR modulates signal transduction and the production of anti-inflammatory cytokines. Thus, human effector T cells alter their function depending on which innate receptors are engaged in conjunction with the TCR complex.

γδ T cell receptor ligands and modes of antigen recognition

T lymphocytes expressing the γδ-type of T cell receptors (TCRs) for antigens contribute to all aspects of immune responses, including defenses against viruses, bacteria, parasites and tumors, allergy and autoimmunity. Multiple subsets have been individualized in humans as well as in mice and they appear to recognize in a TCR-dependent manner antigens as diverse as small non-peptidic molecules, soluble or membrane-anchored polypeptides and molecules related to MHC antigens on cell surfaces, implying diverse modes of antigen recognition. We review here the γδ TCR ligands which have been identified along the years and their characteristics, with emphasis on a few systems which have been extensively studied such as human γδ T cells responding to phosphoantigens or murine γδ T cells activated by allogeneic MHC antigens. We discuss a speculative model of antigen recognition involving simultaneous TCR recognition of MHC-like and non-MHC ligands which could fit with most available data and shares many similarities with the classical model of MHC-restricted antigen recognition for peptides or lipids by T cells subsets with αβ-type TCRs.

Functional dichotomy between NKG2D and CD28-mediated co-stimulation in human CD8+ T cells

Both CD28 and NKG2D can function as co-stimulatory receptors in human CD8⁺ T cells. However, their independent functional contributions in distinct CD8⁺ T cell subsets are not well understood. In this study, CD8⁺ T cells in human peripheral blood- and lung-derived lymphocytes were analyzed for CD28 and NKG2D expression and function. We found a higher level of CD28 expression in PBMC-derived naïve (CD45RA⁺CD27⁺) and memory (CD45RA⁻CD27⁺) CD8⁺ T cells (CD28^Hi), while its expression was significantly lower in effector (CD45RA⁺CD27⁻) CD8⁺ T cells (CD28^Lo). Irrespective of the differences in the CD28 levels, NKG2D expression was comparable in all three CD8⁺ T cell subsets. CD28 and NKG2D expressions followed similar patterns in human lung-resident GILGFVFTL/HLA-A2-pentamer positive CD8⁺ T cells. Co-stimulation of CD28^Lo effector T cells via NKG2D significantly increased IFN-γ and TNF-α levels. On the contrary, irrespective of its comparable levels, NKG2D-mediated co-stimulation failed to augment IFN-γ and TNF-α production in CD28^Hi naïve/memory T cells. Additionally, CD28-mediated co-stimulation was obligatory for IL-2 generation and thereby its production was limited only to the CD28^Hi naïve/memory subsets. MICA, a ligand for NKG2D was abundantly expressed in the tracheal epithelial cells, validating the use of NKG2D as the major co-stimulatory receptor by tissue-resident CD8⁺ effector T cells. Based on these findings, we conclude that NKG2D may provide an expanded level of co-stimulation to tissue-residing effector CD8⁺ T cells. Thus, incorporation of co-stimulation via NKG2D in addition to CD28 is essential to activate tumor or tissue-infiltrating effector CD8⁺ T cells. However, boosting a recall immune response via memory CD8⁺ T cells or vaccination to stimulate naïve CD8⁺ T cells would require CD28-mediated co-stimulation.

The detection, treatment, and biology of epithelial ovarian cancer

Ovarian cancer is particularly insidious in nature. Its ability to go undetected until late stages coupled with its non-descript signs and symptoms make it the seventh leading cause of cancer related deaths in women. Additionally, the lack of sensitive diagnostic tools and resistance to widely accepted chemotherapy regimens make ovarian cancer devastating to patients and families and frustrating to medical practitioners and researchers. Here, we provide an in-depth review of the theories describing the origin of ovarian cancer, molecular factors that influence its growth and development, and standard methods for detection and treatment. Special emphasis is focused on interactions between ovarian tumors and the innate and adaptive immune system and attempts that are currently underway to devise novel immunotherapeutic approaches for the treatment of ovarian tumors.

The NKG2D ligand ULBP4 binds to TCRgamma9/delta2 and induces cytotoxicity to tumor cells through both TCRgammadelta and NKG2D

UL16-binding proteins (ULBPs) belong to a family of ligands for NKG2D activating receptor of human natural killer (NK) cells. We previously reported that RAET1E2, a soluble isoform of the RAET1E (ULBP4), inhibits NKG2D-mediated NK cytotoxicity. In this study, we examined whether ULBP4 could be recognized by gammadeltaT cells via TCRgammadelta. Here we show that immobilized soluble ULBP4 (rULBP4) induces the proliferation of human ovarian epithelial carcinoma- or colonic carcinoma-derived Vdelta2(+) T cells in vitro. These Vdelta2(+) T cells secrete Th1 cytokines and display a strong cytolytic activity toward ULBP4-transfected targets. We also show that ULBP4 binds to a soluble chimeric protein containing TCRgamma9/delta2 and activates TCR(-) Jurkat T cells transfected with TCRgamma9/delta2. Moreover, both TCRgammadelta and NKG2D are involved in ULBP4-induced activation and cytotoxicity of gammadeltaT cells. We found that ULBP4 is expressed not only on human tumor cells, but also on Epstein-Barr virus (EBV)-infected peripheral blood cells. Taken together, our data suggest that ULBP4 functions as a ligand for both TCRgammadelta and NKG2D and may play a key role in immune surveillance of tumor development and clearance of viral infection.

Four novel ULBP splice variants are ligands for human NKG2D

UL16-binding proteins [ULBPs, also termed as retinoic acid early transcripts (RAET1) molecules] are frequently expressed by malignant transformed cells and stimulate anti-tumor immune responses mediated by NKG2D-positive NK cells, CD8(+) alphabeta T cells and gammadelta T cells in vitro and in vivo. In this study, we identified four novel functional splice variants of ULBPs including ULBP4-I, ULBP4-II, ULBP4-III and RAET1G3 in HepG2 liver carcinoma cells, WISH human amnion cells, Hep-2 larynx carcinoma cells and K562 leukemia cells, respectively, by reverse transcription-PCR and T vector cloning strategy. Analysis of alignments of amino acid sequences of the splice variants illustrated that there were important modifications between splice variants and their individual parental ULBP. All ULBP4 splice variants (ULBP4-I, ULBP4-II and ULBP4-III) were type 1 membrane-spanning molecules and had the ability to bind with human NKG2D receptor in vitro. Ectopic expressions of ULBP4 and ULBP4 splice variants resulted in the enhanced cytotoxic sensitivity of target cells against NK cells, which could be blocked by anti-NKG2D mAb. Moreover, co-culture-free soluble forms of ULBP4 splice variants (their alpha1 + alpha2 ectodomains) and RAET1G3 (soluble splice variant of RAET1G2) with NK cells down-regulated the cell surface expression of NKG2D. Finally, immobilized in a plate-bound form of RAET1G3 stimulated NK cells to secrete IFN-gamma. Taken together, all the identified functional splice variants will help to advance our knowledge regarding the overall functions of ULBP gene family.

Whole tumor antigen vaccination using dendritic cells: comparison of RNA electroporation and pulsing with UV-irradiated tumor cells

Because of the lack of full characterization of tumor associated antigens for solid tumors, whole antigen use is a convenient approach to tumor vaccination. Tumor RNA and apoptotic tumor cells have been used as a source of whole tumor antigen to prepare dendritic cell (DC) based tumor vaccines, but their efficacy has not been directly compared. Here we compare directly RNA electroporation and pulsing of DCs with whole tumor cells killed by ultraviolet (UV) B radiation using a convenient tumor model expressing human papilloma virus (HPV) E6 and E7 oncogenes. Although both approaches led to DCs presenting tumor antigen, electroporation with tumor cell total RNA induced a significantly higher frequency of tumor-reactive IFN-gamma secreting T cells, and E7-specific CD8+ lymphocytes compared to pulsing with UV-irradiated tumor cells. DCs electroporated with tumor cell RNA induced a larger tumor infiltration by T cells and produced a significantly stronger delay in tumor growth compared to DCs pulsed with UV-irradiated tumor cells. We conclude that electroporation with whole tumor cell RNA and pulsing with UV-irradiated tumor cells are both effective in eliciting antitumor immune response, but RNA electroporation results in more potent tumor vaccination under the examined experimental conditions.

Regulation of ligands for the activating receptor NKG2D

The outcome of an encounter between a cytotoxic cell and a potential target cell depends on the balance of signals from inhibitory and activating receptors. Natural Killer group 2D (NKG2D) has recently emerged as a major activating receptor on T lymphocytes and natural killer cells. In both humans and mice, multiple different genes encode ligands for NKG2D, and these ligands are non-classical major histocompatibility complex class I molecules. The NKG2D-ligand interaction triggers an activating signal in the cell expressing NKG2D and this promotes cytotoxic lysis of the cell expressing the ligand. Most normal tissues do not express ligands for NKG2D, but ligand expression has been documented in tumour and virus-infected cells, leading to lysis of these cells. Tight regulation of ligand expression is important. If there is inappropriate expression in normal tissues, this will favour autoimmune processes, whilst failure to up-regulate the ligands in pathological conditions would favour cancer development or dissemination of intracellular infection.

Promiscuity and the single receptor: NKG2D

NKG2D (natural-killer group 2, member D) is a powerful activating receptor expressed by natural killer (NK) cells and T cells that regulates immune responses during infection, cancer and autoimmunity. NKG2D ligands comprise a diverse array of MHC-class-I-related proteins that are upregulated by cellular stress. Why is it beneficial for the host to have so many ligands for the same receptor? In this Opinion article, we propose that although competition with viruses is the most likely evolutionary drive for this diversity, there might be other explanations.

Chimeric NKG2D Receptor–Bearing T Cells as Immunotherapy for Ovarian Cancer

Despite advancements in the treatment of ovarian cancer, this disease continues to be a leading cause of cancer death in women. Adoptive transfer of tumor-reactive T cells is a promising antitumor therapy for many cancers. We designed a chimeric receptor linking NKG2D, a natural killer (NK) cell-activating receptor, to the CD3zeta chain of the T-cell receptor to target ovarian tumor cells. Engagement of chimeric NKG2D receptors (chNKG2D) with ligands for NKG2D, which are commonly expressed on tumor cells, leads to T-cell secretion of proinflammatory cytokines and tumor cytotoxicity. In this study, we show that >80% of primary human ovarian cancer samples expressed ligands for NKG2D on the cell surface. The tumor samples expressed MHC class I-related protein A, MICB, and UL-16 binding proteins 1 and 3. ChNKG2D-expressing T cells lysed ovarian cancer cell lines. We show that T cells from ovarian cancer patients that express chNKG2D secreted proinflammatory cytokines when cultured with autologous tumor cells. In addition, we show that chNKG2D T cells can be used therapeutically in a murine model of ovarian cancer. These data indicate that treatment with chNKG2D-expressing T cells is a potential immunotherapy for ovarian cancer.

The role of the NKG2D receptor for tumor immunity

Natural killer (NK) cells have originally been identified based on their capacity to kill transformed cells in a seemingly non-specific fashion. Over the last 15 years, knowledge on receptor ligand systems used by NK cells to specifically detect transformed cells has been accumulating rapidly. One of these receptor ligand systems, the NKG2D pathway, has received particular attention, and now serves as a paradigm for how the immune system is able to gather information about the health status of autologous host cells. In addition to its significance on NK cells, NKG2D, as well as other NK cell receptors, play significant roles on T cells. This review aims at summarizing recent insights into the regulation of NKG2D function, the control over NKG2D ligand expression and the role of NKG2D in tumor immunity. Finally, we will discuss first attempts to exploit NKG2D function to improve immunity to tumors.

NKG2D-mediated activation of cytotoxic lymphocytes: unique signaling pathways and distinct functional outcomes

NKG2D is an important immunosurveillance receptor that triggers a unique signal transduction pathway resulting in a variety of functional outcomes. NKG2D couples to the non-ITAM-containing DAP10 and initiates at least two signaling branches that are both required for cytotoxicity. Transformed, infected, or healthy cells can express NKG2D ligands, and NKG2D(+) lymphocytes can be unaffected, costimulated, or fully activated by the NKG2D-ligand interaction. The NKG2D-mediated response can be modulated by factors such as cytokine milieu and possibly the particular ligand expressed. Thus, this nontraditional NKG2D-DAP10 initiated signal pathway enables lymphocytes to differentially respond to ligands based on the cellular environment.

Innate Immune Recognition and Suppression of Tumors

In this chapter, we first summarized the strong evidence that now supports the existence of an effective cancer immune surveillance process that prevents cancer development in both mice and humans. We then focused the remainder of the chapter on methods of tumor recognition that contribute to natural host immune suppression of tumors. In particular, NKG2D is a type II transmembrane-anchored glycoprotein expressed as a disulfide-linked homodimer on the surface of all mouse and human natural killer cells (NK cells). Stimulation of NK cell through NKG2D triggers cell-mediated cytotoxicity and in some cases induces production of cytokines. NKG2D binds to family of ligands with structural homology to major histocompatibility complex (MHC) class I, however, NKG2D ligands often display upregulated surface expression on stressed cells and are frequently overexpressed by tumors unlike conventional MHC class I molecules. Evidence clearly implicate that NKG2D recognition plays an important role in tumor immune surveillance.

Cancer Immunosurveillance and Immunoediting: The Roles of Immunity in Suppressing Tumor Development and Shaping Tumor Immunogenicity

Cellular transformation and tumor development result from an accumulation of mutational and epigenetic changes that alter normal cell growth and survival pathways. For the last 100 years, there has been a vigorous debate as to whether the unmanipulated immune system can detect and eliminate such altered host derived cells despite the fact that cancer cells frequently express either abnormal proteins or abnormal levels of normal cellular proteins that function as tumor antigens. In this review, we discuss the current state of this argument and point out some of the recent key experiments demonstrating that immunity not only protects the host from cancer development (i.e., provides a cancer immunosurveillance function) but also can promote tumor growth, sometimes by generating more aggressive tumors. The terminology "cancer immunoediting" has been used to describe this dual host protective and tumor promoting action of immunity, and herein we summarize the ever-increasing experimental and clinical data that support the validity of this concept.

CD95L/FasL and TRAIL in tumour surveillance and cancer therapy

The membrane-bound death ligands CD95L/FasL and TRAIL, which activate the corresponding death receptors CD95/Fas, TRAILR1 and TRAILR2, induce apoptosis in many tumour cells, but can also elicit an inflammatory response. This chapter focuses on the relevance of CD95L/FasL and TRAIL for the tumour surveillance function of natural killer cells and cytotoxic T-cells and discuss current concepts of utilizing these ligands in tumour therapy.

NK Cell Receptors as Tools in Cancer Immunotherapy

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

Anti-lymphoma effect of gammadelta T cells

During the last few years, our knowledge about the activation and control of non-major histocompatibility complex (MHC)-restricted innate effector lymphocytes (such as natural killer (NK) cells, NK T cells and gammadelta T cells) has advanced enormously and immunotherapeutic strategies based on these cell types receive more and more attention. Apart from NK cells, several lines of evidence indicate that T cells, which express an alternative T cell receptor (TCR) composed of a CD3-associated gammadelta heterodimer, also contribute to the innate immune defense against tumors. Human gammadelta T cells represent a small subset of T cells (1-10% of peripheral blood T cells) and differ from conventional MHC-restricted ass T cells in recognition of a unique set of antigens ("phosphoantigens") and the lack of requirement of classical antigen-presenting molecules. Besides their role in the innate immune response against pathogens based on the recognition of distinctive microbial metabolic products (metabolites of the non-mevalonate pathway of isoprenoid synthesis), Vgamma9Vdelta2 T cells that constitute the dominant fraction of gammadelta T cells in humans exert potent cytotoxic activity, especially against lymphoid malignancies, mediated by as yet only partially determined pathway(s) of tumor recognition. This article will review available evidence from pre-clinical and early clinical studies regarding the contribution of gammadelta T cells in the defense against lymphoid malignancies and highlights some important issues that need to be addressed in the future.

Costimulation through NKG2D Enhances Murine CD8+ CTL Function: Similarities and Differences between NKG2D and CD28 Costimulation

Multiple studies have demonstrated that the NK cell activating receptor NKG2D can function as a costimulatory receptor for both mouse and human CD8+ T cells. However, it has recently been suggested that stimulation through NKG2D is insufficient for costimulation of CD8+ T cells. To aid in the delineation of NKG2D function in CTL responses, we investigated whether stimulation of NKG2D by the natural ligand RAE1epsilon was able to costimulate effector functions of a murine CTL line generated from DUC18 TCR transgenic mice. We found that NKG2D was able to costimulate DUC CTL responses and did so in a manner similar to CD28 costimulation. The T cells exhibited increased proliferation, IFN-gamma release, and cytotoxicity when presented antigenic peptide by P815 cells expressing RAE1epsilon or B7-1 compared with untransfected P815. In addition, both RAE1epsilon and B7-1 enhanced Ag-independent IFN-gamma secretion in response to IL-12 and IL-18 by DUC CTL. However, only costimulation through CD28 allowed for DUC CTL survival upon secondary stimulation, whereas ligation of NKG2D, but not CD28, induced DUC CTL to form an immune synapse with target cells in the absence of TCR stimulation. Understanding the outcomes of these differences may allow for a better understanding of T cell costimulation in general.

HSV oncolytic therapy upregulates interferon-inducible chemokines and recruits immune effector cells in ovarian cancer

Cooperation between oncolytic herpes simplex virus (HSV) and host effector immune mechanisms has been previously described. In the present study, we investigated the mechanism underlying such cooperation in a murine syngeneic model of ovarian carcinoma. Therapeutic administration of HSV-1716, a replication-restricted mutant, resulted in significant reduction of tumor growth and a significant survival advantage. Intratumoral injection of HSV-1716 induced expression of IFN-gamma, MIG, and IP-10 in the tumor. This was accompanied by a significant increase in the number of tumor-associated NK and CD8+ T cells expressing CXCR3 and CD25. Ascites from HSV-1716-treated animals efficiently induced in vitro migration of NK and CD8+ T cells, which was dependent on the presence of MIG and IP-10. Murine monocytes and dendritic cells (DCs) were responsible for the production of MIG and IP-10 upon HSV-1716 infection. In monocytes, this was partially abrogated by neutralizing antibodies against IFN-alpha and -beta, thus indicating a role for type-1 IFNs in the reported effect. Human ovarian carcinomas showed high numbers of monocytes and DCs. Upon HSV-1716 infection, human monocyte-derived DCs produced large amounts of IFN-gamma and upregulated MIG and IP-10 expression. These results indicate that HSV-1716 induces an inflammatory response that may facilitate antitumor immune response upon oncolytic therapy.

UL16 binding proteins

According to present concepts, innate immunity plays an important role in tumor surveillance and immune modulation. The state of NK cells depends on the balance between inhibitory and activating signals from corresponding receptors. As one of the activating receptors, NKG2D recognises some self ligands such as MICA/B in human and Rae1 in mice, which is dissimilar to those toll-like receptors that recognise some pathogen-derived ligands. NKG2D is expressed not only on NK cells, but on gammadelta T cells, CD8+ alphabeta T cells in normal individuals and CD4+ alphabeta T cells in rheumatoid arthritis patients and plays a different role on respective cells. Whereas NKG2D can only function as a costimulatory receptor on CD8+ alphabeta T cells under the domination of alphabeta TCR in spite of a deficiency of costimulatory molecule CD28, NKG2D can directly activate NK cells even in the presence of inhibitory signals from MHC-I and corresponding receptor complexes. Experiments in mice have identified that alternative splicing produces two distinct NKG2D polypeptides that associate differentially with the DAP10 and DAP12 signaling subunits and that differential expression of these isoforms and of signaling proteins determines whether NKG2D only functions as a costimulatory receptor in the adaptive immune system (CD8+ T cells) or as both a primary recognition unit and a costimulatory receptor in the innate immune system (natural killer cells and macrophages). This review summarizes the research achievements in a new ligand family (UL16 binding proteins) of NKG2D in human and shows the possible prospects of ULBP function and application.

The immunobiology of cancer immunosurveillance and immunoediting

The last fifteen years have seen a reemergence of interest in cancer immunosurveillance and a broadening of this concept into one termed cancer immunoediting. The latter, supported by strong experimental data derived from murine tumor models and provocative correlative data obtained by studying human cancer, holds that the immune system not only protects the host against development of primary nonviral cancers but also sculpts tumor immunogenicity. Cancer immunoediting is a process consisting of three phases: elimination (i.e., cancer immunosurveillance), equilibrium, and escape. Herein, we summarize the data supporting the existence of each of the three cancer immunoediting phases. The full understanding of the immunobiology of cancer immunosurveillance and immunoediting will hopefully stimulate development of more effective immunotherapeutic approaches to control and/or eliminate human cancers.

Prevalent expression of the immunostimulatory MHC class I chain-related molecule is counteracted by shedding in prostate cancer

The MHC class I chain-related molecules (MICs) have previously been shown to be induced on most epithelial tumor cells. Engagement of MIC by the activating immune receptor NKG2D triggers NK cells and augments antigen-specific CTL anti-tumor immunity. The MIC-NKG2D system was proposed to participate in epithelial tumor immune surveillance. Paradoxically, studies suggest that tumors may evade MIC-NKG2D-mediated immunity by MIC shedding-induced impairment of effector cell function. Here we demonstrate the first evidence to our knowledge of a significant correlation of MIC shedding and deficiency in NK cell function with the grade of disease in prostate cancer. MIC is widely expressed in prostate carcinoma. The presence of surface target MIC, however, is counteracted by shedding. A significant increase in serum levels of soluble MIC (sMIC) and deficiency in NK cell function was shown in patients with advanced cancer. Finally, the deficiency in NK cell function can be overcome by treatment with IL-2 or IL-15 in vitro. Our results suggest that (a) deficiency in MIC-NKG2D immune surveillance may contribute to prostate cancer progression, (b) sMIC may be a novel biomarker for prostate cancer, and (c) using cytokines to restore MIC-NKG2D-mediated immunity may have clinical significance for prostate cancer in cell-based adaptive immunotherapy.

Ovarian carcinoma expresses the NKG2D ligand Letal and promotes the survival and expansion of CD28- antitumor T cells

The role of the NKG2D immunoreceptor and its ligands in antitumor immune response is incompletely understood. Here, we report that effector immune cells infiltrating ovarian carcinoma are mostly CD8+ lymphocytes lacking CD28 but expressing the NKG2D costimulatory receptor. Human ovarian carcinoma expresses the novel NKG2D ligand lymphocyte effector cell toxicity-activating ligand (Letal). Letal was found to be an independent prognosticator of improved survival in advanced ovarian cancer. Higher levels of tumor-derived Letal were associated with stronger lymphocyte infiltration. Letal exerted marked costimulatory effects and induced type-1 polarization in CD8+CD28- tumor-infiltrating lymphocytes ex vivo. Letal engagement increased the expression of the glucose transporter Glut-1, enhanced glucose up-take, and protected CD8+ lymphocytes from cisplatin-induced killing. Letal also down-regulated the expression of Fas in CD8+ cells and rendered them resistant to Fas ligand-induced apoptosis. Our results indicate that Letal promotes tumor immune surveillance by promoting the survival and intratumoral expansion of antitumor cytotoxic lymphocytes. We propose that Letal could be used for the ex vivo expansion of apoptosis-resistant tumor-reactive cytotoxic lymphocytes for adoptive transfer.