Activation of STING by cGAMP Regulates MDSCs to Suppress Tumor Metastasis via Reversing Epithelial-Mesenchymal Transition

Signaling by Type I Interferons in Immune Cells: Disease Consequences

This review addresses interferon (IFN) signaling in immune cells and the tumor microenvironment (TME) and examines how this affects cancer progression. The data reveal that IFNs exert dual roles in cancers, dependent on the TME, exhibiting both anti-tumor activity and promoting cancer progression. We discuss the abnormal IFN signaling induced by cancerous cells that alters immune responses to permit their survival and proliferation.

Co-delivery of doxorubicin and STING agonist cGAMP for enhanced antitumor immunity

Many chemotherapeutic agents can induce immunogenic cell death (ICD), which leads to the release of danger-associated molecular patterns (DAMPs) and tumor-associated antigens. This process promotes dendritic cells (DCs) maturation and cytotoxic T lymphocyte (CTL) infiltration. However, cancer cells can employ diverse mechanisms to evade the host immune system. Recent studies have shown that stimulator of interferon genes (STING) agonists, such as cGAMP, can amplify ICD-triggered immune responses and enhance the infiltration of immune cells into the tumor microenvironment (TME). Building upon these findings, we constructed a doxorubicin (DOX) and cGAMP co-delivery system (DOX/cGAMP@NPs) for melanoma and triple-negative breast cancer (TNBC) therapy. The results demonstrated that DOX could effectively destroy tumors and induce the release of DAMPs by ICD. Furthermore, in orthotopic 4T1 tumors mice model and subcutaneous B16 tumor mice model, cGAMP could promote the maturation of DCs and CD8+ T cell activation and infiltration by inducing the secretion of type I interferons and pro-inflammation cytokine, which amplified the antitumor immune response induced by DOX. This strategy also promoted the depletion of immunosuppressive cells, potentially alleviating the immunosuppressive TME. In conclusion, our study highlights the combination of DOX-induced ICD and the immune-enhancing properties of cGAMP holds significant implications for future research and clinical applications.

Exploring the roles and potential therapeutic strategies of inflammation and metabolism in the pathogenesis of vitiligo: a mendelian randomization and bioinformatics-based investigation

Introduction

Vitiligo, a common autoimmune acquired pigmentary skin disorder, poses challenges due to its unclear pathogenesis. Evidence suggests inflammation and metabolism's pivotal roles in its onset and progression. This study aims to elucidate the causal relationships between vitiligo and inflammatory proteins, immune cells, and metabolites, exploring bidirectional associations and potential drug targets.

Methods

Mendelian Randomization (MR) analysis encompassed 4,907 plasma proteins, 91 inflammatory proteins, 731 immune cell features, and 1400 metabolites. Bioinformatics analysis included Protein-Protein Interaction (PPI) network construction, Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. Subnetwork discovery and hub protein identification utilized the Molecular Complex Detection (MCODE) plugin. Colocalization analysis and drug target exploration, including molecular docking validation, were performed.

Results

MR analysis identified 49 proteins, 39 immune cell features, and 59 metabolites causally related to vitiligo. Bioinformatics analysis revealed significant involvement in PPI, GO enrichment, and KEGG pathways. Subnetwork analysis identified six central proteins, with Interferon Regulatory Factor 3 (IRF3) exhibiting strong colocalization evidence. Molecular docking validated Piceatannol's binding to IRF3, indicating a stable interaction.

Conclusion

This study comprehensively elucidates inflammation, immune response, and metabolism's intricate involvement in vitiligo pathogenesis. Identified proteins and pathways offer potential therapeutic targets, with IRF3 emerging as a promising candidate. These findings deepen our understanding of vitiligo's etiology, informing future research and drug development endeavors.

A homologous-targeting cGAS-STING agonist multimodally activates dendritic cells for enhanced cancer immunotherapy

Herein, we developed a doxorubicin (Dox)-loaded and 4T1 cancer cell membrane-modified hydrogenated manganese oxide nanoparticles (mHMnO-Dox) to elicit systemic antitumor immune responses. The results revealed that mHMnO-Dox actively recognized tumor cells and then effectively delivered Dox into the cells. Upon entering tumor cells, the mHMnO-Dox underwent rapid degradation and abundant release of Mn2+ and chemotherapeutic drugs. The released Mn2+ not only catalysed a Fenton-type reaction to produce excessive reactive oxygen species (ROS) but also activated the cGAS-STING pathway to boost dendritic cell (DC) maturation. This process increased cytotoxic T lymphocyte infiltration as well as natural killer cell recruitment into the tumor site. In addition, the released Dox could contribute to a chemotherapeutic effect, while activating DC cells and subsequently intensifying immune responses through immunogenic cell death (ICD) of tumor cells. Consequently, the mHMnO-Dox suppressed the primary and distal tumor growth and inhibited tumor relapse and metastasis, as well as prolonged the lifespan of tumor-bearing mice. Thus, the mHMnO-Dox multimodally activated DC cells to demonstrate synergistic antitumor activity, which was mediated via the activation of the cGAS-STING signalling pathway to regulate tumor microenvironment, ICD-mediated immunotherapy and ROS-mediated CDT. These findings suggest the therapeutic potential of mHMnO-Dox in cancer immunotherapy. STATEMENT OF SIGNIFICANCE: A cancer cell membrane-camouflaged hydrogenated mesoporous manganese oxide (mHMnO) has been developed as a cGAS-STING agonist and ICD inducer. The mHMnO effectively induced abundance of ROS production in cancer cells, which caused cancer cell death and then promoted DC maturation via tumour-associated antigen presentation. Meanwhile, the mHMnO significantly activated cGAS-STING pathway to facilitate DC maturation and cytotoxic T lymphocyte infiltration as well as natural killer cell recruitment, which further enhanced tumour immune response. In addition, the combination of the mHMnO and Dox could synergistically promote tumour ICD and then multimodally induce DC maturation, achieving an enhanced CIT. Overall, this study provides a potential strategy to design novel immunologic adjuvant for enhanced CIT.

Reciprocal relationship between cancer stem cells and myeloid-derived suppressor cells: implications for tumor progression and therapeutic strategies

Recently, there has been an increased focus on cancer stem cells (CSCs) due to their resilience, making them difficult to eradicate. This resilience often leads to tumor recurrence and metastasis. CSCs adeptly manipulate their surroundings to create an environment conducive to their survival. In this environment, myeloid-derived suppressor cells (MDSCs) play a crucial role in promoting epithelial-mesenchymal transition and bolstering CSCs' stemness. In response, CSCs attract MDSCs, enhancing their infiltration, expansion and immunosuppressive capabilities. This interaction between CSCs and MDSCs increases the difficulty of antitumor therapy. In this paper, we discuss the interplay between CSCs and MDSCs based on current research and highlight recent therapeutic strategies targeting either CSCs or MDSCs that show promise in achieving effective antitumor outcomes.

Discovery of VH domains that allosterically inhibit ENPP1

Ectodomain phosphatase/phosphodiesterase-1 (ENPP1) is overexpressed on cancer cells and functions as an innate immune checkpoint by hydrolyzing extracellular cyclic guanosine monophosphate adenosine monophosphate (cGAMP). Biologic inhibitors have not yet been reported and could have substantial therapeutic advantages over current small molecules because they can be recombinantly engineered into multifunctional formats and immunotherapies. Here we used phage and yeast display coupled with in cellulo evolution to generate variable heavy (VH) single-domain antibodies against ENPP1 and discovered a VH domain that allosterically inhibited the hydrolysis of cGAMP and adenosine triphosphate (ATP). We solved a 3.2 Å-resolution cryo-electron microscopy structure for the VH inhibitor complexed with ENPP1 that confirmed its new allosteric binding pose. Finally, we engineered the VH domain into multispecific formats and immunotherapies, including a bispecific fusion with an anti-PD-L1 checkpoint inhibitor that showed potent cellular activity.

Innate immunity: Looking beyond T-cells in radiation and immunotherapy combinations

Radiation therapy is an established and effective anti-cancer treatment modality. Extensive pre-clinical experimentation has demonstrated that the pro-inflammatory properties of irradiation may be synergistic with checkpoint immunotherapy. Radiation induces double-stranded DNA breaks (dsDNA). Sensing of the dsDNA activates the cGAS/STING pathway, producing Type 1 interferons essential to recruiting antigen-presenting cells (APCs). Radiation promotes cytotoxic CD8 T-cell recruitment by releasing tumour-associated antigens captured and cross-presented by surveying antigen-presenting cells. Radiation-induced vascular normalisation may further promote T-cell trafficking and drug delivery. Radiation is also immunosuppressive. Recruitment of regulatory T cells (Tregs) and innate cells such as myeloid-derived suppressive cells (m-MDSCs) all counteract the immunostimulatory properties of radiation. Many innate immune cell types operate at the interface of the adaptive immune response. Innate immune cells, such as m-MDSCs, can exert their immunosuppressive effects by expressing immune checkpoints such as PD-L1, further highlighting the potential of combined radiation and checkpoint immunotherapy. Several early-phase clinical studies investigating the combination of radiation and immunotherapy have been disappointing. A greater appreciation of radiotherapy's impact on the innate immune system is essential to optimise radioimmunotherapy combinations. This review will summarise the impact of radiotherapy on crucial cells of the innate immune system and vital immunosuppressive cytokines.

At the Crossroads of the cGAS-cGAMP-STING Pathway and the DNA Damage Response: Implications for Cancer Progression and Treatment

The evolutionary conserved DNA-sensing cGAS-STING innate immunity pathway represents one of the most important cytosolic DNA-sensing systems that is activated in response to viral invasion and/or damage to the integrity of the nuclear envelope. The key outcome of this pathway is the production of interferon, which subsequently stimulates the transcription of hundreds of genes. In oncology, the situation is complex because this pathway may serve either anti- or pro-oncogenic roles, depending on context. The prevailing understanding is that when the innate immune response is activated by sensing cytosolic DNA, such as DNA released from ruptured micronuclei, it results in the production of interferon, which attracts cytotoxic cells to destroy tumors. However, in tumor cells that have adjusted to significant chromosomal instability, particularly in relapsed, treatment-resistant cancers, the cGAS-STING pathway often supports cancer progression, fostering the epithelial-to-mesenchymal transition (EMT). Here, we review this intricate pathway in terms of its association with cancer progression, giving special attention to pancreatic ductal adenocarcinoma and gliomas. As the development of new cGAS-STING-modulating small molecules and immunotherapies such as oncolytic viruses involves serious challenges, we highlight several recent fundamental discoveries, such as the proton-channeling function of STING. These discoveries may serve as guiding lights for potential pharmacological advancements.

Anti-triple-negative breast cancer metastasis efficacy and molecular mechanism of the STING agonist for innate immune pathway

BACKGROUND

With high recurrence and metastatic rates, triple-negative breast cancer (TNBC) has few therapy choices. The innate immune stimulator of interferon genes protein (STING) pathway has emerged as a critical foundation for improving anticancer immunotherapy. Although 2',3'-cGAMP has been shown to have therapeutic potential as a STING agonist in subcutaneous solid tumour treatments in mice, the effect of cGAMP in metastatic malignancies has received less attention.

METHODS

Bioluminescence imaging technology was applied to monitor TNBC tumour cell metastasis in living mice. Serum biochemical test and blood routine examination of mice were used to demonstrate cGAMP administration had no toxicity. The activation of DCs and CD8+ T cells was demonstrated by flow cytometry. The pharmacological mechanism of cGAMP for suppressing breast tumour metastasis was also explored.

RESULTS

cGAMP treatment substantially suppressed tumour development and metastasis without adverse effects. cGAMP activated the cGAS-STING-IRF3 pathway, which modified the tumour immune milieu to reverse the Epithelial-Mesenchymal Transition (EMT) and PI3K/AKT pathways and prevent tumour metastasis. It was postulated and proven that cGAMP had a pharmacological mechanism for reducing breast tumour metastasis.

CONCLUSION

The findings suggest that cGAMP could be useful in the immunotherapy of immune-insensitive metastatic breast cancer.

Targeting STING in cancer: Challenges and emerging opportunities

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway is a key pathway through which the host regulates immune responses by recognizing cytoplasmic double-stranded DNA of abnormal origin, and it plays an important role in tumor growth as well as metastasis, with relevant molecular details constantly being explored and updated. The significant immunomodulatory effects make STING an attractive target for cancer immunotherapy, and STING agonists have been receiving great attention for their development and clinical translation. Despite exciting results in preclinical work, the application of STING agonists to cancer therapy remains challenging due to their poor pharmacokinetic and physicochemical properties, as well as toxic side effects they produce. Here, we summarize the dichotomous role of cGAS-STING in cancer and discuss the limitations of cancer immunotherapy based on STING activation as well as feasible strategies to overcome them to achieve tumor regression.

DNA sensing in cancer: Pro-tumour and anti-tumour functions of cGAS-STING signalling

The DNA sensor cGAS (cyclic GMP-AMP synthase) and its adaptor protein STING (Stimulator of Interferon Genes) detect the presence of cytosolic DNA as a sign of infection or damage. In cancer cells, this pathway can be activated through persistent DNA damage and chromosomal instability, which results in the formation of micronuclei and the exposure of DNA fragments to the cytosol. DNA damage from radio- or chemotherapy can further activate DNA sensing responses, which may occur in the cancer cells themselves or in stromal and immune cells in the tumour microenvironment (TME). cGAS-STING signalling results in the production of type I interferons, which have been linked to immune cell infiltration in 'hot' tumours that are susceptible to immunosurveillance and immunotherapy approaches. However, recent research has highlighted the complex nature of STING signalling, with tumours having developed mechanisms to evade and hijack this signalling pathway for their own benefit. In this mini-review we will explore how cGAS-STING signalling in different cells in the TME can promote both anti-tumour and pro-tumour responses. This includes the role of type I interferons and the second messenger cGAMP in the TME, and the influence of STING signalling on local immune cell populations. We examine how alternative signalling cascades downstream of STING can promote chronic interferon signalling, the activation of the transcription factor nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and the production of inflammatory cytokines, which can have pro-tumour functions. An in-depth understanding of DNA sensing in different cell contexts will be required to harness the anti-tumour functions of STING signalling.

Prognostic significance of STING expression in solid tumor: a systematic review and meta-analysis

Objective

Stimulator of interferon genes (STING) is a key regulator in initiating innate immune response from sensing cytosolic DNA. Recent studies have revealed that the cGAS-STING signaling pathway has a crucial role in tumor development and progression across cancer types. Herein, we conducted a meta-analysis to explore the relationship between the immunoexpression of STING and the survival outcome of patients in various solid tumors. Studies relevant to the subject were searched from PubMed, Embase, and Web of Science.

Results

Eleven studies including 2,345 patients were eligible for the analysis. STING expression in tumor cells was related to improved disease-free survival/recurrence-free survival (DFS/RFS) (HR = 0.656, 95% CI = 0.455-0.946, p = 0.024) but not with overall survival (OS) (HR = 0.779, 95% CI = 0.534-1.136, p = 0.194). STING expression in stromal cells, however, did not show significant correlation with DFS/RFS and OS (HR = 0.979, 95% CI = 0.565-1.697, p-value = 0.940 and HR = 1.295, 95% CI = 0.845-1.985, p = 0.235, respectively). In a subgroup analysis, STING expression in tumor cells was associated with better DFS (HR = 0.622, 95% CI = 0.428-0.903, p = 0.012). In tumor cells, favorable DFS/RFS were also related to studies from univariate analysis and the gastrointestinal system (HR = 0.667, 95% CI = 0.482-0.923, p = 0.015 and HR = 0.566, 95% CI = 0.330-0.971, p = 0.039).

Conclusions

STING expression in tumor cells is associated with favorable outcome in solid tumors.

Systematic review registration

https://www.crd.york.ac.uk/prospero/, registration number: CRD42023427027.

Targeting cGAS/STING signaling-mediated myeloid immune cell dysfunction in TIME

Myeloid immune cells (MICs) are potent innate immune cells serving as first responders to invading pathogens and internal changes to cellular homeostasis. Cancer is a stage of altered cellular homeostasis that can originate in response to different pathogens, chemical carcinogens, and internal genetic/epigenetic changes. MICs express several pattern recognition receptors (PRRs) on their membranes, cytosol, and organelles, recognizing systemic, tissue, and organ-specific altered homeostasis. cGAS/STING signaling is a cytosolic PRR system for identifying cytosolic double-stranded DNA (dsDNA) in a sequence-independent but size-dependent manner. The longer the cytosolic dsDNA size, the stronger the cGAS/STING signaling activation with increased type 1 interferon (IFN) and NF-κB-dependent cytokines and chemokines' generation. The present article discusses tumor-supportive changes occurring in the tumor microenvironment (TME) or tumor immune microenvironment (TIME) MICs, specifically emphasizing cGAS/STING signaling-dependent alteration. The article further discusses utilizing MIC-specific cGAS/STING signaling modulation as critical tumor immunotherapy to alter TIME.

Targeting myeloid-derived suppressor cells in tumor immunotherapy: Current, future and beyond

Myeloid-derived suppressor cells (MDSCs) are one of the major negative regulators in tumor microenvironment (TME) due to their potent immunosuppressive capacity. MDSCs are the products of myeloid progenitor abnormal differentiation in bone marrow, which inhibits the immune response mediated by T cells, natural killer cells and dendritic cells; promotes the generation of regulatory T cells and tumor-associated macrophages; drives the immune escape; and finally leads to tumor progression and metastasis. In this review, we highlight key features of MDSCs biology in TME that are being explored as potential targets for tumor immunotherapy. We discuss the therapies and approaches that aim to reprogram TME from immunosuppressive to immunostimulatory circumstance, which prevents MDSC immunosuppression activity; promotes MDSC differentiation; and impacts MDSC recruitment and abundance in tumor site. We also summarize current advances in the identification of rational combinatorial strategies to improve clinical efficacy and outcomes of cancer patients, via deeply understanding and pursuing the mechanisms and characterization of MDSCs generation and suppression in TME.

"Open Sesame" to the complexity of pattern recognition receptors of myeloid-derived suppressor cells in cancer

Pattern recognition receptors are primitive sensors that arouse a preconfigured immune response to broad stimuli, including nonself pathogen-associated and autologous damage-associated molecular pattern molecules. These receptors are mainly expressed by innate myeloid cells, including granulocytes, monocytes, macrophages, and dendritic cells. Recent investigations have revealed new insights into these receptors as key players not only in triggering inflammation processes against pathogen invasion but also in mediating immune suppression in specific pathological states, including cancer. Myeloid-derived suppressor cells are preferentially expanded in many pathological conditions. This heterogeneous cell population includes immunosuppressive myeloid cells that are thought to be associated with poor prognosis and impaired response to immune therapies in various cancers. Identification of pattern recognition receptors and their ligands increases the understanding of immune-activating and immune-suppressive myeloid cell functions and sheds light on myeloid-derived suppressor cell differences from cognate granulocytes and monocytes in healthy conditions. This review summarizes the different expression, ligand recognition, signaling pathways, and cancer relations and identifies Toll-like receptors as potential new targets on myeloid-derived suppressor cells in cancer, which might help us to decipher the instruction codes for reverting suppressive myeloid cells toward an antitumor phenotype.

Tailoring therapies to counter the divergent immune landscapes of breast cancer

Breast cancer remains a significant clinical concern affecting millions of women worldwide. Immunotherapy is a rapidly growing drug class that has revolutionized cancer treatment but remains marginally successful in breast cancer. The success of immunotherapy is dependent on the baseline immune responses as well as removing the brakes off pre-existing anti-tumor immunity. In this review, we summarize the different types of immune microenvironment observed in breast cancer as well as provide approaches to target these different immune subtypes. Such approaches have demonstrated pre-clinical success and are currently under clinical evaluation. The impact of combination of these approaches with already approved chemotherapies and immunotherapies may improve patient outcome and survival.

Bibliometric and Visualized Analysis of the Current Status on STING Signaling Pathway and Cancer

Cancer, as the second leading cause of death worldwide, has become an ongoing public health challenge and its treatment has received much attention, with immunotherapy becoming a hot research topic in recent years. The interferon gene stimulating factor (STING)-mediated signaling pathway has a “double-edged sword” role in cancer, which plays different roles in different types and stages of tumors. In this paper, we discuss the current research status, cooperation, and hotspots of STING signaling pathway in cancer from 2008–2022 using CiteSpace software based on the literature of cancer and STING signaling pathway. In addition, we predicted future research trends in this field by analysis, and the results showed that the STING signaling pathway is rapidly increasing in cancer research, and its role in tumor microenvironment and immunotherapy has become a new hot spot in current research and will continue to receive high attention.

AVA-NP-695 Selectively Inhibits ENPP1 to Activate STING Pathway and Abrogate Tumor Metastasis in 4T1 Breast Cancer Syngeneic Mouse Model

Cyclic GMP-AMP synthase (cGAS) is an endogenous DNA sensor that synthesizes cyclic guanosine monophosphate–adenosine monophosphate (2′3′-cGAMP) from ATP and GTP. 2′3′-cGAMP activates the stimulator of interferon genes (STING) pathway, resulting in the production of interferons and pro-inflammatory cytokines. Ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) is the phosphodiesterase that negatively regulates the STING pathway by hydrolyzing 2′3′-cGAMP. It has been established that the cGAS–STING pathway plays a major role in inhibiting tumor growth by upregulating T cell response. Herein, we demonstrate that AVA-NP-695, a selective and highly potent ENPP1 inhibitor, apart from the immunomodulatory effect also modulates cancer metastasis by negatively regulating epithelial–mesenchymal transition (EMT). We established that the combined addition of 2′3′-cGAMP and AVA-NP-695 significantly abrogated the transforming growth factor beta (TGF-ꞵ)-induced EMT in MDA-MB-231 cells. Finally, results from the in vivo study showed superior tumor growth inhibition and impact on tumor metastasis of AVA-NP-695 compared to Olaparib and PD-1 in a syngeneic 4T1 breast cancer mouse model. The translation of efficacy from in vitro to in vivo 4T1 tumor model provides a strong rationale for the therapeutic potential of AVA-NP-695 against triple-negative breast cancer (TNBC) as an immunomodulatory and anti-metastatic agent.

cGAS-STING pathway targeted therapies and their applications in the treatment of high-grade glioma

Median survival of patients with glioblastoma (GBM) treated with standard of care which consists of maximal safe resection of the contrast-enhancing portion of the tumor followed by radiation therapy with concomitant adjuvant temozolomide (TMZ) remains 15 months. The tumor microenvironment (TME) is known to contain immune suppressive myeloid cells with minimal effector T cell infiltration. Stimulator of interferon genes (STING) is an important activator of immune response and results in production of Type 1 interferon and antigen presentation by myeloid cells. This review will discuss important developments in STING agonists, potential biomarkers for STING response, and new combinatorial therapeutic approaches in gliomas.

Enhancing anti-tumour innate immunity by targeting the DNA damage response and pattern recognition receptors in combination with radiotherapy

Radiotherapy is one of the most effective and frequently used treatments for a wide range of cancers. In addition to its direct anti-cancer cytotoxic effects, ionising radiation can augment the anti-tumour immune response by triggering pro-inflammatory signals, DNA damage-induced immunogenic cell death and innate immune activation. Anti-tumour innate immunity can result from recruitment and stimulation of dendritic cells (DCs) which leads to tumour-specific adaptive T-cell priming and immunostimulatory cell infiltration. Conversely, radiotherapy can also induce immunosuppressive and anti-inflammatory mediators that can confer radioresistance. Targeting the DNA damage response (DDR) concomitantly with radiotherapy is an attractive strategy for overcoming radioresistance, both by enhancing the radiosensitivity of tumour relative to normal tissues, and tipping the scales in favour of an immunostimulatory tumour microenvironment. This two-pronged approach exploits genomic instability to circumvent immune evasion, targeting both hallmarks of cancer. In this review, we describe targetable DDR proteins (PARP (poly[ADP-ribose] polymerase); ATM/ATR (ataxia-telangiectasia mutated and Rad3-related), DNA-PKcs (DNA-dependent protein kinase, catalytic subunit) and Wee1 (Wee1-like protein kinase) and their potential intersections with druggable immunomodulatory signalling pathways, including nucleic acid-sensing mechanisms (Toll-like receptors (TLR); cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) and retinoic acid-inducible gene-I (RIG-I)-like receptors), and how these might be exploited to enhance radiation therapy. We summarise current preclinical advances, recent and ongoing clinical trials and the challenges of therapeutic combinations with existing treatments such as immune checkpoint inhibitors.

Radiotherapy combined with immunotherapy: the dawn of cancer treatment

Radiotherapy (RT) is delivered for purposes of local control, but can also exert systemic effect on remote and non-irradiated tumor deposits, which is called abscopal effect. The view of RT as a simple local treatment has dramatically changed in recent years, and it is now widely accepted that RT can provoke a systemic immune response which gives a strong rationale for the combination of RT and immunotherapy (iRT). Nevertheless, several points remain to be addressed such as the interaction of RT and immune system, the identification of the best schedules for combination with immunotherapy (IO), the expansion of abscopal effect and the mechanism to amplify iRT. To answer these crucial questions, we roundly summarize underlying rationale showing the whole immune landscape in RT and clinical trials to attempt to identify the best schedules of iRT. In consideration of the rarity of abscopal effect, we propose that the occurrence of abscopal effect induced by radiation can be promoted to 100% in view of molecular and genetic level. Furthermore, the “radscopal effect” which refers to using low-dose radiation to reprogram the tumor microenvironment may amplify the occurrence of abscopal effect and overcome the resistance of iRT. Taken together, RT could be regarded as a trigger of systemic antitumor immune response, and with the help of IO can be used as a radical and systemic treatment and be added into current standard regimen of patients with metastatic cancer.

Activation of STING in the pancreatic tumor microenvironment: A novel therapeutic opportunity

Pancreatic ductal adenocarcinoma (PDAC) is a cancer of poor prognosis that presents with a dense desmoplastic stroma that contributes to therapeutic failure. PDAC patients are mostly unresponsive to immunotherapy. However, hopes to elicit response to immunotherapy have emerged with novel strategies targeting the Stimulator of Interferon Genes (STING) protein, which is a major regulator of tumor-associated inflammation. Combination of STING agonists with conventional immunotherapy approaches has proven to potentiate therapeutic benefits in several cancers. However, recent data underscore that the output of STING activation varies depending on the cellular and tissue context. This suggests that tumor heterogeneity, and in particular the heterogeneity of the tumor microenvironment (TME), is a key factor determining whether STING activation would bear benefits for patients. In this review, we discuss the potential benefits of STING activation in PDAC. To this aim, we describe the major components of the PDAC TME, and the expected consequences of STING activation.

STING Agonist-Mediated Cytokine Secretion Is Accompanied by Monocyte Apoptosis

The cGAS-STING (cyclic GMP-AMP synthase-stimulator of interferon genes) pathway plays a crucial role in inducing an antiviral and antitumor immune response. We studied the effects of synthetic STING agonists on several immune populations and related cytokine production. In comparison with the toll-like receptor 7 (TLR7) agonist, STING agonists induced secretion of a broader proinflammatory cytokine spectrum. Unlike the TLR7 agonist, the structurally diverse STING agonists partially depleted B and NK cells and completely depleted CD14+ monocytes via induction of apoptosis. The TANK-binding kinase 1 inhibitor efficiently prevented interferon alpha (IFNα) secretion and cell depletion, suggesting their possible dependence on the cGAS-STING pathway activation. Finally, IFNα, tumor necrosis factor alpha, interleukin 6, and interleukin 1 beta secretion and CD14+ monocyte apoptosis were primary responses to STING agonists, whereas IFNγ was secreted secondarily. These findings bring new insights into the cGAS-STING pathway immunomodulation that is of future therapeutic importance.

The Function of cGAS-STING Pathway in Treatment of Pancreatic Cancer

The activation of stimulator of interferon genes (STING) signalling pathway has been suggested to promote the immune responses against malignancy. STING is activated in response to the detection of cytosolic DNA and can induce type I interferons and link innate immunity with the adaptive immune system. Due to accretive evidence demonstrating that the STING pathway regulates the immune cells of the tumor microenvironment (TME), STING as a cancer biotherapy has attracted considerable attention. Pancreatic cancer, with a highly immunosuppressive TME, remains fatal cancer. STING has been applied to the treatment of pancreatic cancer through distinct strategies. This review reveals the role of STING signalling on pancreatic tumors and other diseases related to the pancreas. We then discuss new advances of STING in either monotherapy or combination methods for pancreatic cancer immunotherapy.

Epithelial-mesenchymal transition: When tumor cells meet myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous myeloid cell population characterized by protumoral functions in the tumor immune network. An increasing number of studies have focused on the biological functions of MDSCs in tumor immunity. Epithelial-mesenchymal transition (EMT) is a cellular plasticity process accompanied by a loss of epithelial phenotypes and an acquisition of mesenchymal phenotypes. In general, tumor cells that undergo EMT are more likely to invade and metastasize. Recently, extensive evidence suggests that EMT is closely related to a highly immunosuppressive environment. This review will summarize the immunosuppressive capacities of MDSC subsets and their distinct role in tumor EMT and further discuss immunotherapy for tumor EMT by targeting MDSCs.

Radiotherapy and cGAS/STING signaling: Impact on MDSCs in the tumor microenvironment

Myeloid derived suppressor cells (MDSCs) are a highly heterogeneous population of immature immune cells with immunosuppressive functions that are recruited to the tumor microenvironment (TME). MDSCs promote tumor growth and progression by inhibiting immune effector cell proliferation and function. MDSCs are affected by both novel anti-cancer therapies targeting the immune system to promote anti-tumor immunity, as well as by conventional treatments such as radiotherapy. Following radiotherapy, cytoplasmic double stranded DNA stimulates the cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) pathway, resulting in type I interferon production. Effectiveness of radiotherapy and cGAS/STING signaling are closely intertwined: activation of cGAS and STING is key to generate systemic anti-tumor immunity after irradiation. This review focuses on how radiotherapy and cGAS/STING signaling in MDSCs and/or tumor cells impact MDSC recruitment, expansion and function. The influence of conventional and ablative radiotherapy treatment schedules, inflammatory response following radiotherapy, and hypoxia are discussed as MDSC modulators.

Interferons in cancer immunoediting: sculpting metastasis and immunotherapy response

Interferons (IFNs) are pleiotropic cytokines critical for regulation of epithelial cell functions and for immune system regulation. In cancer, IFNs contribute to tumor-intrinsic and -extrinsic mechanisms that determine the quality of antitumor immunity and response to immunotherapy. In this Review, we focus on the different types of tumor IFN sensitivity that determine dynamic tumor-immune interactions and their coevolution during cancer progression and metastasis. We extend the discussion to new evidence supporting immunotherapy-mediated immunoediting and the dual opposing roles of IFNs that lead to immune checkpoint blockade response or resistance. Understanding the intricate dynamic responses to IFN will lead to novel immunotherapeutic strategies to circumvent protumorigenic effects of IFN while exploiting IFN-mediated antitumor immunity.