A DNA-based cancer vaccine enhances lymphocyte cross talk by engaging the NKG2D receptor

Global trends in tumor microenvironment-related research on tumor vaccine: a review and bibliometric analysis

Background

Tumor vaccines have become crucial in cancer immunotherapy, but, only a limited number of phase III clinical trials have demonstrated clinical efficacy. The crux of this issue is the inability of tumor vaccines to effectively harmonize the tumor microenvironment with its intricate interplay. One factor that can hinder the effectiveness of vaccines is the natural immunosuppressive element present in the tumor microenvironment. This element can lead to low rates of T-cell response specific to antigens and the development of acquired resistance. Conversely, anticancer vaccines alter the tumor microenvironment in conflicting manners, inducing both immune activation and immunological evasion. Hence, comprehending the correlation between tumor vaccines and the tumor microenvironment would establish a foundation for forthcoming tumor treatment.

Objective

Our review explores the realm of research pertaining to tumor vaccinations and the tumor microenvironment. Our objective is to investigate the correlation between tumor vaccines and the tumor microenvironment within this domain. We then focus our review on the dominant international paradigms in this research field and visually illustrates the historical progression and emergent patterns observed in the past.

Methods

From January 1, 1999 to February 7, 2023, 1420 articles on the interplay between tumor vaccines and the tumor microenvironment were published, according to The Clarivate Web of Science (WOS) database used in our review. A bibliometric review was designed for this collection and consisted of an evaluation. The evaluation encompassed various discernible attributes, including the year of publication, the journals in which the articles were published, the authors involved, the affiliated institutions, the geographical locations of the institutions, the references cited, and the keywords employed.

Results

Between the years 1999 and 2022, publications saw a significant increase, from 3 to 265 annually. With 72 papers published, Frontiers in Immunology had the most manuscripts published. The Cancer Research publication garnered the highest number of citations, amounting to 2874 citations. The United States exerts significant dominance in the subject, with the National Cancer Institute being recognized as a prominent institution in terms of both productivity and influence. Furthermore, Elizabeth M. Jaffee was recognized as the field's most prolific and influential author with 24 publications and 1,756 citations. The co-occurrence cluster analysis was conducted on the top 197 keywords, resulting in the identification of five distinct clusters. The most recent high-frequency keywords, namely immune therapy, dendritic cell, tumor microenvironment, cancer, and vaccine, signify the emerging frontiers in the interaction between tumor vaccines and the tumor microenvironment.

Conclusion

Our review uncovers insights into contemporary trends, global patterns of collaboration, fundamental knowledge, research areas of high interest, and emerging frontiers in the field of TME-targeted vaccines.

Bidirectional crosstalk between therapeutic cancer vaccines and the tumor microenvironment: Beyond tumor antigens

Immunotherapy has rejuvenated cancer therapy, especially after anti-PD-(L)1 came onto the scene. Among the many therapeutic options, therapeutic cancer vaccines are one of the most essential players. Although great progress has been made in research on tumor antigen vaccines, few phase III trials have shown clinical benefits. One of the reasons lies in obstruction from the tumor microenvironment (TME). Meanwhile, the therapeutic cancer vaccine reshapes the TME in an ambivalent way, leading to immune stimulation or immune escape. In this review, we summarize recent progress on the interaction between therapeutic cancer vaccines and the TME. With respect to vaccine resistance, innate immunosuppressive TME components and acquired resistance caused by vaccination are both involved. Understanding the underlying mechanism of this crosstalk provides insight into the treatment of cancer by directly targeting the TME or synergizing with other therapeutics.

Application of neoantigens in malignant tumor treatment and prognosis evaluation

In the last decade, there have been an endless number of cancer therapy strategies, and the study of tumor neoantigens provides a new direction for cancer immune therapy. With the development of deep sequencing, especially whole-exome sequencing, great opportunities have been provided to precise immune therapy of malignant tumors. This article systematically summarizes the research on tumor neoantigens in the past ten years and the challenges most likely to be encountered, describes the role of neoantigens in the treatment of malignant tumors, and discusses their possible clinical applications.

Vaccine-elicited CD8+ T cells cure mesothelioma by overcoming tumor-induced immunosuppressive environment

Eradicating malignant tumors by vaccine-elicited host immunity remains a major medical challenge. To date, correlates of immune protection remain unknown for malignant mesothelioma. In this study, we demonstrated that antigen-specific CD8(+) T-cell immune response correlates with the elimination of malignant mesothelioma by a model PD-1-based DNA vaccine. Unlike the nonprotective tumor antigen WT1-based DNA vaccines, the model vaccine showed complete and long-lasting protection against lethal mesothelioma challenge in immunocompetent BALB/c mice. Furthermore, it remained highly immunogenic in tumor-bearing animals and led to therapeutic cure of preexisting mesothelioma. T-cell depletion and adoptive transfer experiments revealed that vaccine-elicited CD8(+) T cells conferred to the protective efficacy in a dose-dependent way. Also, these CD8(+) T cells functioned by releasing inflammatory IFNγ and TNFα in the vicinity of target cells as well as by initiating TRAIL-directed tumor cell apoptosis. Importantly, repeated DNA vaccinations, a major advantage over live-vectored vaccines with issues of preexisting immunity, achieve an active functional state, not only preventing the rise of exhausted PD-1(+) and Tim-3(+) CD8(+) T cells but also suppressing tumor-induced myeloid-derived suppressive cells and Treg cells, with the frequency of antigen-specific CD8(+) T cells inversely correlating with tumor mass. Our results provide new insights into quantitative and qualitative requirements of vaccine-elicited functional CD8(+) T cells in cancer prevention and immunotherapy.

Use of attenuated bacteria as delivery vectors for DNA vaccines

Live, attenuated bacterial vaccines (LBV) are promising candidates for the induction of a broad-based immune response directed at recombinant heterologous antigens and the corresponding pathogen. LBVs allow vaccination through the mucosal surfaces and specific targeting of professional antigen-presenting cells located at the inductive sites of the immune system. A novel approach exploits attenuated intracellular bacteria as delivery vectors for eukaryotic antigen-expression plasmids (so-called DNA vaccines). Candidate carrier bacteria include attenuated strains of Gram-positive and Gram-negative bacteria. These bacteria have been shown to deliver DNA vaccines to human cells in vitro and have also proven their in vivo efficacy in several experimental animal models of infectious diseases and different cancers. The clinical assessment of the safety, immunogenicity and efficacy of these candidate strains will be the next challenging step towards live bacterial DNA vaccines.

Enhancement of DNA vaccine efficacy by targeting the xenogeneic human chorionic gonadotropin, survivin and vascular endothelial growth factor receptor 2 combined tumor antigen to the major histocompatibility complex class II pathway

BACKGROUND

A number of strategies have been used to improve the efficacy of the DNA vaccine for the treatment of tumors. These strategies, ranging from activating CD4+ T cell, manipulating antigen presentation and/or processing to anti-angiogenesis, focus on one certain aspect in the functioning of the vaccine. Therefore, their combination is necessary for rational DNA vaccines design by synergizing different regimens and overcoming the limitations of each strategy.

METHODS

A DNA fragment (HSV) encoding the C terminal 37 amino acids of human chorionic gonadotropin β chain (hCGβ), 5 different HLA-restricted cytotoxic T lymphocyte epitopes from human survivin and the third and fourth extracellular domains of vascular endothelial growth factor receptor 2 (VEGFR2) was inserted into the sequence between the luminal and transmembrane domain of human lysosome-associated membrane protein-1 cDNA for the construction of a novel DNA vaccine.

RESULTS

This novel vaccine, named p-L/HSV, has a potent antitumor effect on the LL/2 lung carcinoma model in syngeneic C57BL/6 mice. The immunologic mechanism involved in the antitumor effect referred to the activation of both cellular and humoral immune response. In addition, the tumor vasculature was abrogated as observed by immunohistochemistry in p-L/HSV immunized mice. Furthermore, the immunized mice received an additional boost with p-L/HSV 6 months later and showed a strong immune recall response.

CONCLUSIONS

The present study indicates that the strategies of combining antitumor with antiangiogenesis and targeting the tumor antigen to the major histocompatibility complex class II pathway cooperate well. Such a study may shed new light on designing vaccine for cancer in the future.

Enhancement of cancer vaccine therapy by systemic delivery of a tumor-targeting Salmonella-based STAT3 shRNA suppresses the growth of established melanoma tumors

Cancer vaccine therapies have only achieved limited success when focusing on effector immunity with the goal of eliciting robust tumor-specific T-cell responses. More recently, there is an emerging understanding that effective immunity can only be achieved by coordinate disruption of tumor-derived immunosuppression. Toward that goal, we have developed a potent Salmonella-based vaccine expressing codon-optimized survivin (CO-SVN), referred to as 3342Max. When used alone as a therapeutic vaccine, 3342Max can attenuate growth of aggressive murine melanomas overexpressing SVN. However, under more immunosuppressive conditions, such as those associated with larger tumor volumes, we found that the vaccine was ineffective. Vaccine efficacy could be rescued if tumor-bearing mice were treated initially with Salmonella encoding a short hairpin RNA (shRNA) targeting the tolerogenic molecule STAT3 (YS1646-shSTAT3). In vaccinated mice, silencing STAT3 increased the proliferation and granzyme B levels of intratumoral CD4(+) and CD8(+) T cells. The combined strategy also increased apoptosis in tumors of treated mice, enhancing tumor-specific killing of tumor targets. Interestingly, mice treated with YS1646-shSTAT3 or 3342Max alone were similarly unsuccessful in rejecting established tumors, whereas the combined regimen was highly potent. Our findings establish that a combined strategy of silencing immunosuppressive molecules followed by vaccination can act synergistically to attenuate tumor growth, and they offer a novel translational direction to improve tumor immunotherapy.

NKT ligand-loaded, antigen-expressing B cells function as long-lasting antigen presenting cells in vivo

We had previously shown that activated NKT cells licensed B cells to be immunogenic antigen-presenting cells and helped to elicit a wide spectrum of cancer targeted immune responses. In the current study, we sought to verify the safety of αGalCer-loaded, and adenovirus-transduced B cell-based vaccines, together with mechanism of action. Intravenously injected αGalCer-loaded, antigen-expressing B cells rapidly localized in the spleen and directly primed CD8(+) T cells in an antigen-specific manner. The transferred antigen was sustained for at least 30 days. While some injected B cells produced nonspecific IgG, the antigen-specific IgG response was completely dependent on endogenous B cells. The liver was one of the main tissues where injected B cells were retained; however, we could not find the signs of liver toxicity. Our results demonstrate that αGalCer-loaded, antigen-expressing B cells behave as "antigen-presenting" cells that stimulate endogenous antigen-specific T cells and B cells in vivo without significant toxicity.

Changes to the natural killer cell repertoire after therapeutic hepatitis B DNA vaccination

BACKGROUND

Improvements to the outcome of adaptive immune responses could be achieved by inducing specific natural killer (NK) cell subsets which can cooperate with dendritic cells to select efficient T cell responses. We previously reported the induction or reactivation of T cell responses in chronic hepatitis B patients vaccinated with a DNA encoding hepatitis B envelope proteins during a phase I clinical trial.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we examined changes in the peripheral NK cell populations occurring during this vaccine trial using flow cytometry analysis. Despite a constant number of NK cells in the periphery, a significant increase in the CD56(bright) population was observed after each vaccination and during the follow up. Among the 13 different NK cell markers studied by flow cytometry analysis, the expression of CD244 and NKG2D increased significantly in the CD56(bright) NK population. The ex vivo CD107a expression by CD56(bright) NK cells progressively increased in the vaccinated patients to reach levels that were significantly higher compared to chronically HBV-infected controls. Furthermore, modifications to the percentage of the CD56(bright) NK cell population were correlated with HBV-specific T cell responses detected by the ELISPOT assay.

CONCLUSIONS/SIGNIFICANCE

These changes in the CD56(bright) population may suggest a NK helper effect on T cell adaptive responses. Activation of the innate and adaptive arms of the immune system by DNA immunization may be of particular importance to the efficacy of therapeutic interventions in a context of chronic infections.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00988767.

Immunostimulatory RNA oligonucleotides induce an effective antitumoral NK cell response through the TLR7

RNA oligonucleotides containing immune-activating sequences promote the development of cytotoxic T cell and B cell responses to Ag. In this study, we show for the first time that immunostimulatory RNA oligonucleotides induce a NK cell response that prevents growth of NK-sensitive tumors. Treatment of mice with immunostimulatory RNA oligonucleotides activates NK cells in a sequence-dependent manner, leading to enhanced IFN-gamma production and increased cytotoxicity. Use of gene-deficient mice showed that NK activation is entirely TLR7-dependent. We further demonstrate that NK activation is indirectly induced through IL-12 and type I IFN production by dendritic cells. Reconstitution of TLR7-deficient mice with wild-type dendritic cells restores NK activation upon treatment with immunostimulatory RNA oligonucleotides. Thus, by activating both NK cells and CTLs, RNA oligonucleotides stimulate two major cellular effectors of antitumor immunity. This dual activation may enhance the efficacy of immunotherapeutic strategies against cancer by preventing the development of tumor immune escape variants.

Mutual activation of CD4+ T cells and monocytes mediated by NKG2D-MIC interaction requires IFN-gamma production in systemic lupus erythematosus

The activating receptor NKG2D is mainly expressed by human CD8(+) T cells and NK cells but normally absent on CD4(+) T cells. However, a subset of autoreactive NKG2D(+)CD4(+) T cells has been found to exist in some autoimmune disease such as rheumatoid arthritis (RA) and to participate in the imbalance of immune response and inflammation. Up to date this observation has been extended to some autoimmune diseases such as RA and Crohn's disease and the mechanism underlying the presence of this type of NKG2D(+)CD4(+) T cells has not been delineated yet. In this study, we found that a substantial proportion of CD4(+) T cells expressed NKG2D in the PBMC of SLE patients. We also found that monocytes in SLE aberrantly expressed the NKG2D ligand of MHC class I chain-related (MIC) molecules and membrane-bound IL-15 (mIL-15) at the cell surface. When cultured with the sera from SLE patients, the monocytes from healthy volunteers could be induced to express MIC and mIL-15. However, this induced expression of MIC and mIL-15 could be blocked with anti-IFN-gamma receptor (anti-IFN-gammaR) antibody. We further demonstrated that NKG2D could be induced on normal CD4(+) T cells either cocultured with monocytes from patients with SLE, or monocytes from healthy volunteers but pretreated with IFN-gamma. Moreover, Th1 cytokines were found to be produced by NKG2D(+)CD4(+) T cells in the coculture system. By transwell assay, we found that both NKG2D expression and Th1 cytokines production depended on the cell-cell contact. These results indicate that the elevated sera IFN-gamma may be responsible for MIC and mIL-15 induction on monocytes in SLE; mIL-15 on monocytes contribute to NKG2D receptor induction on a subset of CD4(+) T cells. Moreover, CD14(+) monocytes promote NKG2D(+)CD4(+) T cells activation through the NKG2D-MIC engagement in the pathogenesis of SLE.

NKG2D ligands in tumor immunity

The activating receptor NKG2D (natural-killer group 2, member D) and its ligands play an important role in the NK, gammadelta(+) and CD8(+) T-cell-mediated immune response to tumors. Ligands for NKG2D are rarely detectable on the surface of healthy cells and tissues, but are frequently expressed by tumor cell lines and in tumor tissues. It is evident that the expression levels of these ligands on target cells have to be tightly regulated to allow immune cell activation against tumors, but at the same time avoid destruction of healthy tissues. Importantly, it was recently discovered that another safeguard mechanism controlling activation via the receptor NKG2D exists. It was shown that NKG2D signaling is coupled to the IL-15 receptor pathway in a cell-specific manner suggesting that priming of NKG2D-mediated activation depends on the cellular microenvironment and the distinct cellular context. This review will provide a broad overview of our up-to-date knowledge of the NKG2D receptor and its ligands in the context of tumor immunology. Strategies to amplify NKG2D-mediated antitumor responses and counteract tumor immune escape mechanisms will be discussed.

alpha-Galactosylceramide-loaded, antigen-expressing B cells prime a wide spectrum of antitumor immunity

Most of the current tumor vaccines successfully elicit strong protection against tumor but offer little therapeutic effect against existing tumors, highlighting the need for a more effective vaccine strategy. Vaccination with tumor antigen-presenting cells can induce antitumor immune responses. We have previously shown that NKT-licensed B cells prime cytotoxic T lymphocytes (CTLs) with epitope peptide and generate prophylactic/therapeutic antitumor effects. To extend our B cell vaccine approach to the whole antigen, and to overcome the MHC restriction, we used a nonreplicating adenovirus to transduce B cells with antigenic gene. Primary B cells transduced with an adenovirus-encoding truncated Her-2/neu (AdHM) efficiently expressed Her-2/neu. Compared with the moderate antitumor activity induced by vaccination with adenovirus-transduced B cells (B/AdHM), vaccination with alpha-galactosylceramide-loaded B/AdHM (B/AdHM/alpha GalCer) induced significantly stronger antitumor immunity, especially in the tumor-bearing mice. The depletion study showed that CD4(+), CD8(+) and NK cells were all necessary for the therapeutic immunity. Confirming the results of the depletion study, B/AdHM/alpha GalCer vaccination induced cytotoxic NK cell responses but B/AdHM did not. Vaccination with B/AdHM/alpha GalCer generated Her-2/neu-specific antibodies more efficiently than B/AdHM immunization. More importantly, B/AdHM/alpha GalCer could prime Her-2/neu-specific cytotoxic T cells more efficiently and durably than B/AdHM. CD4(+) cells appeared to be necessary for the induction of antibody and CTL responses. Our results demonstrate that, with the help of NKT cells, antigen-transduced B cells efficiently induce innate immunity as well as a wide range of adaptive immunity against the tumor, suggesting that they could be used to develop a novel cellular vaccine.

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.

NKG2D-deficient mice are defective in tumor surveillance in models of spontaneous malignancy

Ligands for the NKG2D stimulatory receptor are frequently upregulated on tumor lines, rendering them sensitive to natural killer (NK) cells, but the role of NKG2D in tumor surveillance has not been addressed in spontaneous cancer models. Here, we provided the first characterization of NKG2D-deficient mice, including evidence that NKG2D was not necessary for NK cell development but was critical for immunosurveillance of epithelial and lymphoid malignancies in two transgenic models of de novo tumorigenesis. In both models, we detected NKG2D ligands on the tumor cell surface ex vivo, providing needed evidence for ligand expression by primary tumors. In a prostate cancer model, aggressive tumors arising in NKG2D-deficient mice expressed higher amounts of NKG2D ligands than did similar tumors in wild-type mice, suggesting an NKG2D-dependent immunoediting of tumors in this model. These findings provide important genetic evidence for surveillance of primary tumors by an NK receptor.

The NKG2D receptor: sensing stressed cells

The activating killer cell lectin-like receptor NKG2D plays a key role in the natural killer (NK) cell-mediated lysis of tumours and infected cells. Unlike other receptors, the ligands recognised by NKG2D are 'induced-self' ligands on stressed cells. This system requires precise regulation because inappropriate expression of NKG2D ligands might compromise NK cell activation. For therapeutic purposes it is essential to understand the mechanisms that regulate the expression and function of the NKG2D system. This review focuses on the importance of the signalling pathways involved in the regulation of the NKG2D receptor and its ligand expression in arming the immune response against infected or tumour cells and for the identification of new molecular targets and therapeutic strategies.

MHC class I chain-related molecules induced on monocytes by IFN-gamma promote NK cell activation

NKG2D receptor-ligand interaction triggers NK cell-mediated cytolysis and IFN-gamma secretion. IFN-gamma produced by NK cells has been found to promote the interaction between NK cells and monocytes; however, the underlying mechanism remains elusive. We demonstrate here that IFN-gamma exclusively induced or upregulated the expression of MHC class I chain-related (MIC) molecules, which are ligands of the NKG2D receptor, on the surface of human monocytes of the PBMC population. The IFN-gamma-induced MIC molecules on monocytes played an essential role in triggering the activation of NK cells because mAb-mediated masking of the MIC molecules and the inhibition of cell-to-cell contact using transwell inserts significantly abolished NK cell activation. Meanwhile, membrane-bound IL-15 (mIL-15) was concomitantly induced with MIC molecules on IFN-gamma-treated monocytes and played an essential role in protecting NK cells cocultured with monocytes from MIC-induced NKG2D down-modulation. Therefore, we conclude that the IFN-gamma-induced MIC molecules participated in monocyte/NK cell interaction and that this interaction also involved mIL-15.

RAET1E2, a soluble isoform of the UL16-binding protein RAET1E produced by tumor cells, inhibits NKG2D-mediated NK cytotoxicity

UL16-binding proteins (ULBPs, also termed as retinoic acid early transcripts, encoded by RAET1 genes), a family of ligands for NKG2D in humans, are frequently expressed by tumor cells and mediate cytotoxicities of natural killer (NK) cells and CD8(+) alphabeta T cells to tumor cells. ULBP1, ULBP2, ULBP3, and RAET1L link to membrane through glycosylphosphatidylinositol, whereas RAET1E and RAET1G contain transmembrane and cytoplasmic domains. Proteolytic cleavage of ULBP2 produces truncated and soluble forms that may counteract NKG2D-mediated tumor immune surveillance. In this study, we report that RAET1E can produce a soluble, 35-kDa protein (termed as RAET1E2) lacking the transmembrane region by selective splicing in tumor cells. The expressions of both RAET1E2 transcripts and protein can be found in different tumor cells and tissues. Preincubation of NK-92 cells, a human NK cell line, with culture supernatants from tumor cell lines expressing RAET1E2 or RAET1E2 gene-transfected COS-7 cells resulted in decreased expression of NKG2D on NK-92 cells. Furthermore, incubation of NK-92 cells with recombinant RAET1E2 protein also decreased the surface expression of NKG2D and resulted in marked reduction in cytotoxicities to MGC-803, HepG2, or K562 tumor cells. Taken together, our data provide strong evidence for an immune escape mechanism of tumors via alternative splicing of ULBP RNA to generate a free soluble ULBP protein, RAET1E2, that may impair NKG2D-mediated NK cell cytotoxicity to tumors.

DNA damage response and development of targeted cancer treatments

Some of the most effective anticancer treatments in clinical use induce DNA damage. The majority of treatments cause severe side effects because they do not specifically target cancer cells but also affect other proliferating cells. Detection of genomic lesions activates the DNA damage response, which determines cell fate according to the extent of damage. If the damage is manageable, the DNA damage response arrests cell cycle progression and induces DNA repair to prevent replication of damaged DNA. If the damage is beyond repair, cells undergo apoptosis. Recently we have shown that the DNA damage response also alerts the innate immune system by inducing the expression of ligands for the activating immune receptor NKG2D. The potential of cancer drugs that target components of the DNA damage response and therapeutic hypotheses to improve current cancer therapies are discussed.

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.