RIG-I Resists Hypoxia-Induced Immunosuppression and Dedifferentiation

Revitalizing antitumor immunity: Leveraging nucleic acid sensors as therapeutic targets

Nucleic acid sensors play a critical role in recognizing and responding to pathogenic nucleic acids as danger signals. Upon activation, these sensors initiate downstream signaling cascades that lead to the production and release of pro-inflammatory cytokines, chemokines, and type I interferons. These immune mediators orchestrate diverse effector responses, including the activation of immune cells and the modulation of the tumor microenvironment. However, careful consideration must be given to balancing the activation of nucleic acid sensors to avoid unwanted autoimmune or inflammatory responses. In this review, we provide an overview of nucleic acid sensors and their role in combating cancer through the perception of various aberrant nucleic acids and activation of the immune system. We discuss the connections between different programmed cell death modes and nucleic acid sensors. Finally, we outline the development of nucleic acid sensor agonists, highlighting how their potential as therapeutic targets opens up new avenues for cancer immunotherapy.

Efficacy and safety of natural killer cells injection combined with XELOX chemotherapy in postoperative patients with stage III colorectal cancer in China: a prospective randomised controlled clinical trial study protocol

BACKGROUND

Colorectal cancer (CRC) is the second most frequently diagnosed cancer and the fifth leading cause of cancer-related death in China. However, resistance to multiple chemotherapeutics after surgery leads to failure of the main therapy to CRC. Natural killer (NK) cells are innate cytotoxic lymphocytes that exhibit strong cytotoxic activity against tumour cells. NK cell-based therapy, either alone or in combination with chemotherapy, has achieved favourable results and holds promise for addressing recurrence and metastasis in CRC patients after surgery.

METHODS AND ANALYSIS

This is a prospective, randomised controlled clinical trial to evaluate efficacy and safety of interleukin 2 activated NK cells injection combined with XELOX (capecitabine plus oxaliplatin)-based chemotherapy for postoperative CRC patients. Participants will be randomly divided into treatment group and control group, and every group includes 40 patients. The treatment group will also receive NK cells (5×109) with+XELOX-based chemotherapy, while the control group will receive only XELOX-based chemotherapy. This treatment will be repeated for eight cycles (6 months). The follow-up period lasts about 3 years, during which CEA, CA19-9, CA125, enhancement CT and colonoscopy will be conducted. The primary endpoints of this study are progression-free survival and overall survival, while the secondary endpoint is safety (number and severity of adverse events). Additionally, we aim to identify cancer stem cells in peripheral blood and predictive biomarkers (cytokines secreted by NK cells and activated markers of NK cells) that indicate patients who achieve an effective response.

ETHICS AND DISSEMINATION

The study has been approved by the Clinical Research Ethics Committee of our hospital (approval number 2023LLSC006) and the Chinese Clinical Trials. It will be conducted in accordance with the Declaration of Helsinki. Written informed consent will be obtained from all participants. The study findings will be submitted to peer-reviewed journals for publication.

TRIAL REGISTRATION NUMBER

Chinese Clinical Trials Registry (ChiCTR2300075861).

Harnessing innate immune pathways for therapeutic advancement in cancer

The innate immune pathway is receiving increasing attention in cancer therapy. This pathway is ubiquitous across various cell types, not only in innate immune cells but also in adaptive immune cells, tumor cells, and stromal cells. Agonists targeting the innate immune pathway have shown profound changes in the tumor microenvironment (TME) and improved tumor prognosis in preclinical studies. However, to date, the clinical success of drugs targeting the innate immune pathway remains limited. Interestingly, recent studies have shown that activation of the innate immune pathway can paradoxically promote tumor progression. The uncertainty surrounding the therapeutic effectiveness of targeted drugs for the innate immune pathway is a critical issue that needs immediate investigation. In this review, we observe that the role of the innate immune pathway demonstrates heterogeneity, linked to the tumor development stage, pathway status, and specific cell types. We propose that within the TME, the innate immune pathway exhibits multidimensional diversity. This diversity is fundamentally rooted in cellular heterogeneity and is manifested as a variety of signaling networks. The pro-tumor effect of innate immune pathway activation essentially reflects the suppression of classical pathways and the activation of potential pro-tumor alternative pathways. Refining our understanding of the tumor's innate immune pathway network and employing appropriate targeting strategies can enhance our ability to harness the anti-tumor potential of the innate immune pathway and ultimately bridge the gap from preclinical to clinical application.

Harnessing the innate immune system by revolutionizing macrophage-mediated cancer immunotherapy

Immunotherapy is a promising and safer alternative to conventional cancer therapies. It involves adaptive T-cell therapy, cancer vaccines, monoclonal antibodies, immune checkpoint blockade (ICB), and chimeric antigen receptor (CAR) based therapies. However, most of these modalities encounter restrictions in solid tumours owing to a dense, highly hypoxic and immune-suppressive microenvironment as well as the heterogeneity of tumour antigens. The elevated intra-tumoural pressure and mutational rates within fastgrowing solid tumours present challenges in efficient drug targeting and delivery. The tumour microenvironment is a dynamic niche infiltrated by a variety of immune cells, most of which are macrophages. Since they form a part of the innate immune system, targeting macrophages has become a plausible immunotherapeutic approach. In this review, we discuss several versatile approaches (both at pre-clinical and clinical stages) such as the direct killing of tumour-associated macrophages, reprogramming pro-tumour macrophages to anti-tumour phenotypes, inhibition of macrophage recruitment into the tumour microenvironment, novel CAR macrophages, and genetically engineered macrophages that have been devised thus far. These strategies comprise a strong and adaptable macrophage-toolkit in the ongoing fight against cancer and by understanding their significance, we may unlock the full potential of these immune cells in cancer therapy.

RIG-I immunotherapy overcomes radioresistance in p53-positive malignant melanoma

Radiotherapy induces DNA damage, resulting in cell-cycle arrest and activation of cell-intrinsic death pathways. However, the radioresistance of some tumour entities such as malignant melanoma limits its clinical application. The innate immune sensing receptor retinoic acid-inducible gene I (RIG-I) is ubiquitously expressed and upon activation triggers an immunogenic form of cell death in a variety of tumour cell types including melanoma. To date, the potential of RIG-I ligands to overcome radioresistance of tumour cells has not been investigated. Here, we demonstrate that RIG-I activation enhanced the extent and immunogenicity of irradiation-induced tumour cell death in human and murine melanoma cells in vitro and improved survival in the murine B16 melanoma model in vivo. Transcriptome analysis pointed to a central role for p53, which was confirmed using p53-/- B16 cells. In vivo, the additional effect of RIG-I in combination with irradiation on tumour growth was absent in mice carrying p53-/- B16 tumours, while the antitumoural response to RIG-I stimulation alone was maintained. Our results identify p53 as a pivotal checkpoint that is triggered by RIG-I resulting in enhanced irradiation-induced tumour cell death. Thus, the combined administration of RIG-I ligands and radiotherapy is a promising approach to treating radioresistant tumours with a functional p53 pathway, such as melanoma.

Hypoxia alters the immune response in mouse peritoneal macrophages infected with influenza a virus with truncated NS1 protein

Macrophages are the most abundant cells in infected tissue and are involved in the clearing infection, and immunomodulation of the innate and adaptive immune response. NS80 virus of influenza A virus, which encodes only the first 80 aa of the NS1 protein, suppresses the immune host response and is associated with enhanced pathogenicity. Hypoxia promotes infiltration of peritoneal macrophages into the adipose tissue and production of cytokines. To understand the role of hypoxia in the regulation of immune response, macrophages were infected with A/WSN/33 (WSN) and NS80 virus, and transcriptional profiles of the RIG-I-like receptor signalling pathway and expression of cytokines were evaluated in normoxia and hypoxia. Hypoxia inhibited the proliferation of IC-21 cells, downregulated the RIG-I-like receptor signalling pathway, and inhibited transcriptional activity of IFN-α, IFN-β, IFN-ε, and IFN-λ mRNA in infected macrophages. While transcription of IL-1β and Casp-1 mRNAs were increased in infected macrophages in normoxia, hypoxia resulted in decreased transcription activity of IL-1β and Casp-1 mRNAs. Hypoxia significantly affected expression of the translation factors IRF4, IFN-γ, and CXCL10 involved in regulation of immune response and polarization of the macrophages. The expression of pro-inflammatory cytokines such as sICAM-1, IL-1α, TNF-α, CCL2, CCL3, CXCL12, and M-CSF was to a large extent affected in uninfected and infected macrophages cultivated in hypoxia. The NS80 virus increased the expression of M-CSF, IL-16, CCL2, CCL3, and CXCL12, especially under hypoxia. The results show that hypoxia may play an important role in peritoneal macrophage activation, regulates the innate and adaptive immune response, changes production of pro-inflammatory cytokines, promotes macrophage polarization, and could affect the function of other immune cells.

Bone Marrow Macrophages Induce Inflammation by Efferocytosis of Apoptotic Prostate Cancer Cells via HIF-1α Stabilization

The clearance of apoptotic cancer cells by macrophages, known as efferocytosis, fuels the bone-metastatic growth of prostate cancer cells via pro-inflammatory and immunosuppressive processes. However, the exact molecular mechanisms remain unclear. In this study, single-cell transcriptomics of bone marrow (BM) macrophages undergoing efferocytosis of apoptotic prostate cancer cells revealed a significant enrichment in their cellular response to hypoxia. Here, we show that BM macrophage efferocytosis increased hypoxia inducible factor-1alpha (HIF-1α) and STAT3 phosphorylation (p-STAT3 at Tyr705) under normoxic conditions, while inhibitors of p-STAT3 reduced HIF-1α. Efferocytosis promoted HIF-1α stabilization, reduced its ubiquitination, and induced HIF-1α and p-STAT3 nuclear translocation. HIF-1α stabilization in efferocytic BM macrophages resulted in enhanced expression of pro-inflammatory cytokine MIF, whereas BM macrophages with inactive HIF-1α reduced MIF expression upon efferocytosis. Stabilization of HIF-1α using the HIF-prolyl-hydroxylase inhibitor, Roxadustat, enhanced MIF expression in BM macrophages. Furthermore, BM macrophages treated with recombinant MIF protein activated NF-κB (p65) signaling and increased the expression of pro-inflammatory cytokines. Altogether, these findings suggest that the clearance of apoptotic cancer cells by BM macrophages triggers p-STAT3/HIF-1α/MIF signaling to promote further inflammation in the bone tumor microenvironment where a significant number of apoptotic cancer cells are present.

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.

Emerging role of RNA sensors in tumor microenvironment and immunotherapy

RNA sensors detect foreign and endogenous RNAs to protect the host by initiating innate and adaptive immune response. In tumor microenvironment (TME), activation of RNA sensors induces tumor-inhibitory cytotoxic T lymphocyte responses and inhibits the activity of immunosuppressive cells though stimulating type I IFN signaling pathway. These characteristics allow RNA sensors to be prospective targets in tumor immunotherapy. Therefore, a comprehensive understanding of the roles of RNA sensors in TME could provide new insight into the antitumor immunotherapy. Moreover, RNA sensors could be prominent triggering targets to synergize with immunotherapies. In this review, we highlight the diverse mechanisms of RNA sensors in cancer immunity and their emerging contributions in cancer immunotherapy, including monotherapy with RNA sensor agonists, as well as combination with chemotherapy, radiotherapy, immune checkpoint blockade or cancer vaccine.

RIG-I-induced innate antiviral immunity protects mice from lethal SARS-CoV-2 infection

The SARS-CoV-2 pandemic has underscored the need for rapidly usable prophylactic and antiviral treatments against emerging viruses. The targeted stimulation of antiviral innate immune receptors can trigger a broad antiviral response that also acts against new, unknown viruses. Here, we used the K18-hACE2 mouse model of COVID-19 to examine whether activation of the antiviral RNA receptor RIG-I protects mice from lethal SARS-CoV-2 infection and reduces disease severity. We found that prophylactic, systemic treatment of mice with the specific RIG-I ligand 3pRNA, but not type I interferon, 1–7 days before viral challenge, improved survival of mice by up to 50%. Survival was also improved with therapeutic 3pRNA treatment starting 1 day after viral challenge. This improved outcome was associated with lower viral load in oropharyngeal swabs and in the lungs and brains of 3pRNA-treated mice. Moreover, 3pRNA-treated mice exhibited reduced lung inflammation and developed a SARS-CoV-2-specific neutralizing antibody response. These results demonstrate that systemic RIG-I activation by therapeutic RNA oligonucleotide agonists is a promising strategy to convey effective, short-term antiviral protection against SARS-CoV-2 infection, and it has great potential as a broad-spectrum approach to constrain the spread of newly emerging viruses until virus-specific therapies and vaccines become available.

Prognostic value of PD-L1 expression combined with hypoxia-associated immunosuppression in hepatocellular carcinoma

Background: Hypoxia and immunosuppression are two properties of cancer. This study intends to establish the potential relationship between these two hallmarks in hepatocellular carcinoma (HCC). Materials & methods: A bioinformatics analysis of data obtained from the Cancer Genome Atlas and a retrospective single-center analysis based on a tissue microarray were utilized in this study. Results: We identified a hypoxia-high subtype of patients with immunosuppressive HCC which represented a poor prognosis in the Cancer Genome Atlas cohort. Immunohistochemical analysis of the tissue microarray showed that tumor PD-L1 expression was positively linked to HIF-1α expression, pro-tumor immunocyte infiltration and poor survival in HCC patients. Conclusion: This study provides evidence supporting the correlation between hypoxic signals and immunosuppression in HCC; the combined use of them might improve survival prediction and act as a potential predictor for PD-1/PD-L1 therapy.

Role of Hypoxia in the Interferon Response

In solid tumors, as the tumor grows and the disease progresses, hypoxic regions are often generated, but in contrast to most normal cells which cannot survive under these conditions, tumour cells adapt to hypoxia by HIF-driven mechanisms. Hypoxia can further promote cancer development by generating an immunosuppressive environment within the tumour mass, which allows tumour cells to escape the immune system recognition. This is achieved by recruiting immunosuppressive cells and by upregulating molecules which block immune cell activation. Hypoxia can also confer resistance to antitumor therapies by inducing the expression of membrane proteins that increase drug efflux or by inhibiting the apoptosis of treated cells. In addition, tumor cells require an active interferon (IFN) signalling pathway for the success of many anticancer therapies, such as radiotherapy or chemotherapy. Therefore, hypoxic effects on this pathway needs to be addressed for a successful treatment.

Intracellular virus sensor MDA5 mutation develops autoimmune myocarditis and nephritis

Mutations in IFIH1 gene encoding viral RNA sensor MDA5 have been reported responsible for many interferonopathies, including Aicardi-Goutières syndrome (AGS) and monogenic lupus, however, the pathological link between IFIH1 mutations and various autoimmune symptoms remains unclear. Here, we generated transgenic mice expressing human MDA5 R779H mutant (R779H Tg), reported in AGS and monogenic lupus patient. Mice spontaneously developed myocarditis and nephritis with upregulation of type I IFNs in the major organs. R779H Tg Mavs-/- and R779H Tg Ifnar-/- showed no phenotypes, indicating direct MDA5-signaling pathway involvement. Rag-2 deficiency and bone marrow cells transfer from wild type to adult mice did not prevent myocarditis development, while mice with cardiomyocyte-specific expression of hMDA5 R779H showed cardiomegaly and high expression of inflammatory cytokines. Taken together, our study clarifies that type I IFNs production and chemokines from cardiomyocytes starts in neonatal period and is critical for the development of myocarditis. Activated lymphocytes and auto-antibodies exacerbate the pathogenesis but are dispensable for the onset.

Directing hypoxic tumor microenvironment and HIF to illuminate cancer immunotherapy's existing prospects and challenges in drug targets

Cancer is now also reflected as a disease of the tumor microenvironment, primarily supposed to be a decontrolled genetic and cellular expression disease. Over the past two decades, significant and rapid progress has been made in recognizing the dynamics of the tumor's microenvironment and its contribution to influencing the response to various anti-cancer therapies and drugs. Modulations in the tumor microenvironment and immune checkpoint blockade are interesting in cancer immunotherapy and drug targets. Simultaneously, the immunotherapeutic strategy can be done by modulating the immune regulatory pathway; however, the tumor microenvironment plays an essential role in suppressing the antitumor's immunity by its substantial heterogeneity. Hypoxia inducible factor (HIF) is a significant contributor to solid tumor heterogeneity and a key stressor in the tumor microenvironment to drive adaptations to prevent immune surveillance. Checkpoint inhibitors here halt the ability of cancer cells to stop the immune system from activating, and in turn, amplify your body's immune system to help destroy cancer cells. Common checkpoints that these inhibitors affect are the PD-1/PD-L1 and CTLA-4 pathways and important drugs involved are Ipilimumab and Nivolumab, mainly along with other drugs in this group. Targeting the hypoxic tumor microenvironment may provide a novel immunotherapy strategy, break down traditional cancer therapy resistance, and build the framework for personalized precision medicine and cancer drug targets. We hope that this knowledge can provide insight into the therapeutic potential of targeting Hypoxia and help to develop novel combination approaches of cancer drugs to increase the effectiveness of existing cancer therapies, including immunotherapy.

Hypoxia Promotes Pancreatic Cancer Cell Dedifferentiation to Stem-Like Cell Phenotypes With High Tumorigenic Potential by the HIF-1α/Notch Signaling Pathway

OBJECTIVES

This study aimed to investigate the effect and mechanism of hypoxia on pancreatic cancer (PC) cell dedifferentiation and tumorigenic potential.

METHODS

Inhibition of hypoxia-inducible factor 1α (HIF-1α) and overexpression of Notch1 in PC HS766T cell lines were by lentiviral transfection. The expression of stem cell-specific markers C-X-C motif chemokine receptor 4, CD44, and Nestin was detected by immunofluorescence and Western blot assays. Cell invasion capacity was examined by Transwell assay. Tumorigenic potential was measured in an in situ tumor transplantation experiment. The expression of HIF-1α, Notch signals, and apoptosis signals was examined by Western blot assay.

RESULTS

Hypoxia promoted PC cells to dedifferentiate into stem-like cells by upregulating HIF-1α and activating Notch signals. Silencing of HIF-1α significantly repressed cell dedifferentiation and invasion, whereas overexpression of Notch1 reversed the effect of HIF-1α repression. In situ tumor transplantation experiment further confirmed that hypoxia promoted tumorigenic ability through upregulating HIF-1α. Moreover, the expression of HIF-1α and Notch1 was significantly increased in human PC tissues, and high expression of HIF-1α was correlated with poor survival rate.

CONCLUSIONS

Hypoxia promoted PC cell dedifferentiation to stem-like cell phenotypes with high tumorigenic potential by activating HIF-1α/Notch signaling pathway, indicating a novel role in regulating PC progression.

Hypoxia and Extracellular Acidification as Drivers of Melanoma Progression and Drug Resistance

Hypoxia and elevated extracellular acidification are prevalent features of solid tumors and they are often shown to facilitate cancer progression and drug resistance. In this review, we have compiled recent and most relevant research pertaining to the role of hypoxia and acidification in melanoma growth, invasiveness, and response to therapy. Melanoma represents a highly aggressive and heterogeneous type of skin cancer. Currently employed treatments, including BRAF V600E inhibitors and immune therapy, often are not effective due to a rapidly developing drug resistance. A variety of intracellular mechanisms impeding the treatment were discovered. However, the tumor microenvironment encompassing stromal and immune cells, extracellular matrix, and physicochemical conditions such as oxygen level or acidity, may also influence the therapy effectiveness. Hypoxia and acidification are able to reprogram the metabolism of melanoma cells, enhance their survival and invasiveness, as well as promote the immunosuppressive environment. For this reason, these physicochemical features of the melanoma niche and signaling pathways related to them emerge as potential therapeutic targets.

The multifaceted functions of RNA helicases in the adaptive cellular response to hypoxia: From mechanisms to therapeutics

Hypoxia is a hallmark of cancer. Hypoxia-inducible factor (HIF), a master player for sensing and adapting to hypoxia, profoundly influences genome instability, tumor progression and metastasis, metabolic reprogramming, and resistance to chemotherapies and radiotherapies. High levels and activity of HIF result in poor clinical outcomes in cancer patients. Thus, HIFs provide ideal therapeutic targets for cancers. However, HIF biology is sophisticated, and currently available HIF inhibitors have limited clinical utility owing to their low efficacy or side effects. RNA helicases, which are master players in cellular RNA metabolism, are usually highly expressed in tumors to meet the increased oncoprotein biosynthesis demand. Intriguingly, recent findings provide convincing evidence that RNA helicases are crucial for the adaptive cellular response to hypoxia via a mutual regulation with HIFs. More importantly, some RNA helicase inhibitors may suppress HIF signaling by blocking the translation of HIF-responsive genes. Therefore, RNA helicase inhibitors may work synergistically with HIF inhibitors in cancer to improve treatment efficacy. In this review, we discuss current knowledge of how cells sense and adapt to hypoxia through HIFs. However, our primary focus is on the multiple functions of RNA helicases in the adaptive response to hypoxia. We also highlight how these hypoxia-related RNA helicases can be exploited for anti-cancer therapeutics.

Hypoxia Induces Transcriptional and Translational Downregulation of the Type I IFN Pathway in Multiple Cancer Cell Types

Hypoxia is a common phenomenon in solid tumors and is strongly linked to hallmarks of cancer. Recent evidence has shown that hypoxia promotes local immune suppression. Type I IFN supports cytotoxic T lymphocytes by stimulating the maturation of dendritic cells and enhancing their capacity to process and present antigens. However, little is known about the relationship between hypoxia and the type I IFN pathway, which comprises the sensing of double-stranded RNA and DNA (dsRNA/dsDNA) followed by IFNα/β secretion and transcriptional activation of IFN-stimulated genes (ISG). In this study, we determined the effects of hypoxia on the type I IFN pathway in breast cancer and the mechanisms involved. In cancer cell lines and xenograft models, mRNA and protein expressions of the type I IFN pathway were downregulated under hypoxic conditions. This pathway was suppressed at each level of signaling, from the dsRNA sensors RIG-I and MDA5, the adaptor MAVS, transcription factors IRF3, IRF7, and STAT1, and several ISG including RIG-I, IRF7, STAT1, and ADAR-p150. Importantly, IFN secretion was reduced under hypoxic conditions. HIF1α- and HIF2α-mediated regulation of gene expression did not explain most of the effects. However, ATAC-seq data revealed in hypoxia that peaks with STAT1 and IRF3 motifs had decreased accessibility. Collectively, these results indicate that hypoxia leads to an overall downregulation of the type I IFN pathway due to repressed transcription and lower chromatin accessibility in an HIF1/2α-independent manner, which could contribute to immunosuppression in hypoxic tumors. SIGNIFICANCE: These findings characterize a new mechanism of immunosuppression by hypoxia via downregulation of the type I IFN pathway and its autocrine/paracrine effects on tumor growth.

The Innate Immune Signalling Pathways: Turning RIG-I Sensor Activation Against Cancer

Over the last 15 years, the ability to harness a patient's own immune system has led to significant progress in cancer therapy. For instance, immunotherapeutic strategies, including checkpoint inhibitors or adoptive cell therapy using chimeric antigen receptor T-cell (CAR-T), are specifically aimed at enhancing adaptive anti-tumour immunity. Several research groups demonstrated that adaptive anti-tumour immunity is highly sustained by innate immune responses. Host innate immunity provides the first line of defence and mediates recognition of danger signals through pattern recognition receptors (PRRs), such as cytosolic sensors of pathogen-associated molecular patterns (PAMPs) and damage-associated molecular pattern (DAMP) signals. The retinoic acid-inducible gene I (RIG-I) is a cytosolic RNA helicase, which detects viral double-strand RNA and, once activated, triggers signalling pathways, converging on the production of type I interferons, proinflammatory cytokines, and programmed cell death. Approaches aimed at activating RIG-I within cancers are being explored as novel therapeutic treatments to generate an inflammatory tumour microenvironment and to facilitate cytotoxic T-cell cross-priming and infiltration. Here, we provide an overview of studies regarding the role of RIG-I signalling in the tumour microenvironment, and the most recent preclinical studies that employ RIG-I agonists. Lastly, we present a selection of clinical trials designed to prove the antitumour role of RIG I and that may result in improved therapeutic outcomes for cancer patients.

Natural killer cells in cancer biology and therapy

The tumor microenvironment is highly complex, and immune escape is currently considered an important hallmark of cancer, largely contributing to tumor progression and metastasis. Named for their capability of killing target cells autonomously, natural killer (NK) cells serve as the main effector cells toward cancer in innate immunity and are highly heterogeneous in the microenvironment. Most current treatment options harnessing the tumor microenvironment focus on T cell-immunity, either by promoting activating signals or suppressing inhibitory ones. The limited success achieved by T cell immunotherapy highlights the importance of developing new-generation immunotherapeutics, for example utilizing previously ignored NK cells. Although tumors also evolve to resist NK cell-induced cytotoxicity, cytokine supplement, blockade of suppressive molecules and genetic engineering of NK cells may overcome such resistance with great promise in both solid and hematological malignancies. In this review, we summarized the fundamental characteristics and recent advances of NK cells within tumor immunometabolic microenvironment, and discussed potential application and limitations of emerging NK cell-based therapeutic strategies in the era of presicion medicine.

Targeting the innate immunoreceptor RIG-I overcomes melanoma-intrinsic resistance to T cell immunotherapy

Understanding tumor resistance to T cell immunotherapies is critical to improve patient outcomes. Our study revealed a role for transcriptional suppression of the tumor-intrinsic HLA class I (HLA-I) antigen processing and presentation machinery (APM) in therapy resistance. Low HLA-I APM mRNA levels in melanoma metastases before immune checkpoint blockade (ICB) correlated with nonresponsiveness to therapy and poor clinical outcome. Patient-derived melanoma cells with silenced HLA-I APM escaped recognition by autologous CD8+ T cells. However, targeted activation of the innate immunoreceptor RIG-I initiated de novo HLA-I APM transcription, thereby overcoming T cell resistance. Antigen presentation was restored in interferon-sensitive (IFN-sensitive) but also immunoedited IFN-resistant melanoma models through RIG-I-dependent stimulation of an IFN-independent salvage pathway involving IRF1 and IRF3. Likewise, enhanced HLA-I APM expression was detected in RIG-Ihi (DDX58hi) melanoma biopsies, correlating with improved patient survival. Induction of HLA-I APM by RIG-I synergized with antibodies blocking PD-1 and TIGIT inhibitory checkpoints in boosting the antitumor T cell activity of ICB nonresponders. Overall, the herein-identified IFN-independent effect of RIG-I on tumor antigen presentation and T cell recognition proposes innate immunoreceptor targeting as a strategy to overcome intrinsic T cell resistance of IFN-sensitive and IFN-resistant melanomas and improve clinical outcomes in immunotherapy.

Hypoxia-Driven Immune Escape in the Tumor Microenvironment

The tumor microenvironment is a complex ecosystem comprised of many different cell types, abnormal vasculature and immunosuppressive cytokines. The irregular growth kinetics with which tumors grow leads to increased oxygen consumption and, in turn, hypoxic conditions. Hypoxia has been associated with poor clinical outcome, increased tumor heterogeneity, emergence of resistant clones and evasion of immune detection. Additionally, hypoxia-driven cell death pathways have traditionally been thought of as tolerogenic processes. However, as researchers working in the field of immunotherapy continue to investigate and unveil new types of immunogenic cell death (ICD), it has become clear that, in some instances, hypoxia may actually induce ICD within a tumor. In this review, we will discuss hypoxia-driven immune escape that drives poor prognostic outcomes, the ability of hypoxia to induce ICD and potential therapeutic targets amongst hypoxia pathways.

High RIG-I expression in ovarian cancer associates with an immune-escape signature and poor clinical outcome

Ovarian cancer (OC) is the most lethal gynecological malignancy, with platinum-based chemotherapy remaining the mainstay for adjuvant treatment after surgery. The lack of indication for immunotherapy may at least in part result from the lack of suitable biomarkers allowing stratification of potentially responding patients. In this monocentric study of 141 cases with OC, we used real-time quantitative PCR to assess the expression of retinoic acid-inducible gene-I (RIG-I) in primary tumor and healthy ovarian control tissues. RIG-I expression was correlated to various clinicopathological characteristics as well as to a set of molecular and immunological markers. The prognostic significance of RIG-I expression was queried in univariate and multivariate analyses and validated in an independent cohort. RIG-I was overexpressed in the cancerous ovary and correlated with a higher tumor grade. The more aggressive Type-II cancers and cancers with inactivating p53 mutations exhibited higher RIG-I expression. RIG-I levels were also elevated in cancers that recurred after remission or were platinum-refractory. Survival analyses disclosed RIG-I as an independent marker of poor outcome in OC. Continuative analyses revealed the molecular and immunological correlates of RIG-I expression in the tumor microenvironment, including interferon production and a distinct immune-regulatory signature involving checkpoint molecules (PD-L1/PD-1), the RNA-editing enzyme ADAR1 and the regulatory T cell-specific transcription factor FoxP3. We conclude that high RIG-I expression associates with poor outcome in OC, which is explainable by local immunosuppression in the tumor bed. RIG-I expression may inform checkpoint blockade and/or RIG-I agonistic targeting in a subset of high-risk OC patients.

Intratumoral delivery of RIG-I agonist SLR14 induces robust antitumor responses

Cytosolic nucleic acid-sensing pathways can be triggered to enhance immune response to cancer. In this study, we tested the antitumor activity of a unique RIG-I agonist, stem loop RNA (SLR) 14. In the immunogenic tumor models, we observed significant tumor growth delay and an extended survival in SLR14-treated mice. SLR14 also greatly improved antitumor efficacy of anti-PD1 antibody over single-agent treatment. SLR14 was mainly taken up by CD11b+ myeloid cells in the tumor microenvironment, and many genes associated with immune defense were significantly up-regulated after treatment, accompanied by increase in the number of CD8+ T lymphocytes, NK cells, and CD11b+ cells in SLR14-treated tumors. Strikingly, SLR14 dramatically inhibited nonimmunogenic B16 tumor growth, and the cured mice developed an immune memory. Furthermore, a systemic antitumor response was observed in both bilateral and tumor metastasis models. Collectively, our results demonstrate that SLR14 is a promising therapeutic RIG-I agonist for cancer treatment, either alone or in combination with existing immunotherapies.

Interferons α and β in cancer: therapeutic opportunities from new insights

Over the past decade, preclinical and clinical research have confirmed the essential role of interferons for effective host immunological responses to malignant cells. Type I interferons (IFNα and IFNβ) directly regulate transcription of >100 downstream genes, which results in a myriad of direct (on cancer cells) and indirect (through immune effector cells and vasculature) effects on the tumour. New insights into endogenous and exogenous activation of type I interferons in the tumour and its microenvironment have given impetus to drug discovery and patient evaluation of interferon-directed strategies. When combined with prior observations or with other effective modalities for cancer treatment, modulation of the interferon system could contribute to further reductions in cancer morbidity and mortality. This Review discusses new interferon-directed therapeutic opportunities, ranging from cyclic dinucleotides to genome methylation inhibitors, angiogenesis inhibitors, chemoradiation, complexes with neoantigen-targeted monoclonal antibodies, combinations with other emerging therapeutic interventions and associations of interferon-stimulated gene expression with patient prognosis - all of which are strategies that have or will soon enter translational clinical evaluation.