Mertk on tumor macrophages is a therapeutic target to prevent tumor recurrence following radiation therapy

Hallmarks of pancreatic cancer: spotlight on TAM receptors

Pancreatic ductal adenocarcinoma (PDAC) represents the most prevalent type of pancreatic cancer and ranks among the most aggressive tumours, with a 5-year survival rate of less than 11%. Projections indicate that by 2030, it will become the second leading cause of cancer-related deaths. PDAC presents distinctive hallmarks contributing to its dismal prognosis: (i) late diagnosis, (ii) heterogenous and complex mutational landscape, (iii) high metastatic potential, (iv) dense fibrotic stroma, (v) immunosuppressive microenvironment, and (vi) high resistance to therapy. Mounting evidence has shown a role for TAM (Tyro3, AXL, MerTK) family of tyrosine kinase receptors in PDAC initiation and progression. This review aims to describe the impact of TAM receptors on the defining hallmarks of PDAC and discuss potential future directions using these proteins as novel biomarkers for early diagnosis and targets for precision therapy in PDAC, an urgent unmet clinical need.

In the Eyes of the Beholder-New Mertk Knockout Mouse and Re-Evaluation of Phagocytosis versus Anti-Inflammatory Functions of MERTK

Greg Lemke's laboratory was one of the pioneers of research into the TAM family of receptor tyrosine kinases (RTKs). Not only was Tyro3 cloned in his laboratory, but his group also extensively studied mice knocked out for individual or various combinations of the TAM RTKs Tyro3, Axl, and Mertk. Here we primarily focus on one of the paralogs-MERTK. We provide a historical perspective on rodent models of loss of Mertk function and their association with retinal degeneration and blindness. We describe later studies employing mouse genetics and the generation of newer knockout models that point out incongruencies with the inference that loss of MERTK-dependent phagocytosis is sufficient for severe, early-onset photoreceptor degeneration in mice. This discussion is meant to raise awareness with regards to the limitations of the original Mertk knockout mouse model generated using 129 derived embryonic stem cells and carrying 129 derived alleles and the role of these alleles in modifying Mertk knockout phenotypes or even displaying Mertk-independent phenotypes. We also suggest molecular approaches that can further Greg Lemke's scintillating legacy of dissecting the molecular functions of MERTK-a protein that has been described to function in phagocytosis as well as in the negative regulation of inflammation.

Inhibition of MER proto-oncogene tyrosine kinase by an antisense oligonucleotide enhances treatment efficacy of immunoradiotherapy

BACKGROUND

The combination of radiotherapy and immunotherapy (immunoradiotherapy) has been increasingly used for treating a wide range of cancers. However, some tumors are resistant to immunoradiotherapy. We have previously shown that MER proto-oncogene tyrosine kinase (MerTK) expressed on macrophages mediates resistance to immunoradiotherapy. We therefore sought to develop therapeutics that can mitigate the negative impact of MerTK. We designed and developed a MerTK specific antisense oligonucleotide (ASO) and characterized its effects on eliciting an anti-tumor immune response in mice.

METHODS

344SQR cells were injected into the right legs on day 0 and the left legs on day 4 of 8-12 weeks old female 129sv/ev mice to establish primary and secondary tumors, respectively. Radiation at a dose of 12 Gy was given to the primary tumors on days 8, 9, and 10. Mice received either anti-PD-1, anti-CTLA-4 or/and MerTK ASO starting from day 1 post tumor implantation. The composition of the tumor microenvironment and the level of MerTK on macrophages in the tumor were evaluted by flow cytometry. The expression of immune-related genes was investigated with NanoString. Lastly, the impact of MerTK ASO on the structure of the eye was histologically evaluated.

RESULTS

Remarkably, the addition of MerTK ASO to XRT+anti-PD1 and XRT+anti-CTLA4 profoundly slowed the growth of both primary and secondary tumors and significantly extended survival. The ASO significantly reduced the expression of MerTK in tumor-associated macrophages (TAMs), reprograming their phenotype from M2 to M1. In addition, MerTK ASO increased the percentage of Granzyme B+ CD8+ T cells in the secondary tumors when combined with XRT+anti-CTLA4. NanoString results demonstrated that the MerTK ASO favorably modulated immune-related genes for promoting antitumor immune response in secondary tumors. Importantly, histological analysis of eye tissues demonstrated that unlike small molecules, the MerTK ASO did not produce any detectable pathology in the eyes.

CONCLUSIONS

The MerTK ASO can significantly downregulate the expression of MerTK on TAMs, thereby promoting antitumor immune response. The combination of MerTK ASO with immunoradiotherapy can safely and significantly slow tumor growth and improve survival.

Potential roles for efferocytosis in glioblastoma immune evasion

Glioblastoma is an aggressive and incurable brain cancer. This cancer establishes both local and systemic immunosuppression that creates a major obstacle to effective immunotherapies. Many studies point to tumor-resident myeloid cells (primarily microglia and macrophages) as key mediators of this immunosuppression. Myeloid cells exhibit a high level of plasticity with respect to their phenotype and are capable of both stimulating and repressing immune responses. How glioblastomas recruit myeloid cells and exploit them to avoid the immune system is an active area of research. Macrophages can acquire an immunosuppressive phenotype as a consequence of exposure to cytokines such as TGFB1 or IL4; in addition, macrophages can acquire an immunosuppressive phenotype as a consequence of the engulfment of apoptotic cells, a process referred to as efferocytosis. There is substantial evidence that glioblastoma cells are able to secrete cytokines and other factors that induce an immunosuppressive phenotype in macrophages and microglia. However, less is known about the contribution of efferocytosis to immunosuppression in glioblastoma. Here I review the literature in this area and discuss the potential of efferocytosis inhibition to improve glioblastoma response to immunotherapy.

Efferocytosis: An accomplice of cancer immune escape

The clearance of apoptotic cells by efferocytes such as macrophages and dendritic cells is termed as "efferocytosis", it plays critical roles in maintaining tissue homeostasis in multicellular organisms. Currently, available studies indicate that efferocytosis-related molecules and pathways are tightly associated with cancer development, metastasis and treatment resistance, efferocytosis also induces an immunosuppressive tumor microenvironment and assists cancer cells escape from immune surveillance. In this study, we reviewed the underlying mechanisms of efferocytosis in mediating the occurrence of cancer immune escape, and then emphatically summarized the strategies of using efferocytosis as therapeutic target to enhance the anti-tumor efficacies of immune checkpoint inhibitors, hoping to provide powerful evidences for more effective therapeutic regimens of malignant tumors.

The immunogenic radiation and new players in immunotherapy and targeted therapy for head and neck cancer

Although treatment modalities for head and neck cancer have evolved considerably over the past decades, survival rates have plateaued. The treatment options remained limited to definitive surgery, surgery followed by fractionated radiotherapy with optional chemotherapy, and a definitive combination of fractionated radiotherapy and chemotherapy. Lately, immunotherapy has been introduced as the fourth modality of treatment, mainly administered as a single checkpoint inhibitor for recurrent or metastatic disease. While other regimens and combinations of immunotherapy and targeted therapy are being tested in clinical trials, adapting the appropriate regimens to patients and predicting their outcomes have yet to reach the clinical setting. Radiotherapy is mainly regarded as a means to target cancer cells while minimizing the unwanted peripheral effect. Radiotherapy regimens and fractionation are designed to serve this purpose, while the systemic effect of radiation on the immune response is rarely considered a factor while designing treatment. To bridge this gap, this review will highlight the effect of radiotherapy on the tumor microenvironment locally, and the immune response systemically. We will review the methodology to identify potential targets for therapy in the tumor microenvironment and the scientific basis for combining targeted therapy and radiotherapy. We will describe a current experience in preclinical models to test these combinations and propose how challenges in this realm may be faced. We will review new players in targeted therapy and their utilization to drive immunogenic response against head and neck cancer. We will outline the factors contributing to head and neck cancer heterogeneity and their effect on the response to radiotherapy. We will review in-silico methods to decipher intertumoral and intratumoral heterogeneity and how these algorithms can predict treatment outcomes. We propose that (a) the sequence of surgery, radiotherapy, chemotherapy, and targeted therapy should be designed not only to annul cancer directly, but to prime the immune response. (b) Fractionation of radiotherapy and the extent of the irradiated field should facilitate systemic immunity to develop. (c) New players in targeted therapy should be evaluated in translational studies toward clinical trials. (d) Head and neck cancer treatment should be personalized according to patients and tumor-specific factors.

Advances in Radiotherapy Immune Modulation: From Bench-to-Bedside and Back Again

Pre-clinical and clinical data clearly demonstrate the immune modulatory effects of radiotherapy (RT) but clinical trials testing RT + immunotherapy have been equivocal. An improved understanding of the immune modulatory effects of RT and how practical parameters of RT delivery (site and number of lesions, dose, fractionation, timing) influence these effects are needed to optimally combine RT with immunotherapy. Additionally, increased exploration of immunotherapy combinations with RT, beyond immune checkpoint inhibitors, are needed. A "bench-to-bedside and back again" approach will improve our understanding of RT immune modulation and allow for the implementation of more effective RT + immunotherapy strategies.

Myeloid MyD88 restricts CD8+ T cell response to radiation therapy in pancreatic cancer

Radiation therapy induces immunogenic cell death in cancer cells, whereby released endogenous adjuvants are sensed by immune cells to direct adaptive immune responses. TLRs expressed on several immune subtypes recognize innate adjuvants to direct downstream inflammatory responses in part via the adapter protein MyD88. We generated Myd88 conditional knockout mice to interrogate its contribution to the immune response to radiation therapy in distinct immune populations in pancreatic cancer. Surprisingly, Myd88 deletion in Itgax (CD11c)-expressing dendritic cells had little discernable effects on response to RT in pancreatic cancer and elicited normal T cell responses using a prime/boost vaccination strategy. Myd88 deletion in Lck-expressing T cells resulted in similar or worsened responses to radiation therapy compared to wild-type mice and lacked antigen-specific CD8+ T cell responses from vaccination, similar to observations in Myd88-/- mice. Lyz2-specific loss of Myd88 in myeloid populations rendered tumors more susceptible to radiation therapy and elicited normal CD8+ T cell responses to vaccination. scRNAseq in Lyz2-Cre/Myd88fl/fl mice revealed gene signatures in macrophages and monocytes indicative of enhanced type I and II interferon responses, and improved responses to RT were dependent on CD8+ T cells and IFNAR1. Together, these data implicate MyD88 signaling in myeloid cells as a critical source of immunosuppression that hinders adaptive immune tumor control following radiation therapy.

Radiation-induced tumor immune microenvironments and potential targets for combination therapy

As one of the four major means of cancer treatment including surgery, radiotherapy (RT), chemotherapy, immunotherapy, RT can be applied to various cancers as both a radical cancer treatment and an adjuvant treatment before or after surgery. Although RT is an important modality for cancer treatment, the consequential changes caused by RT in the tumor microenvironment (TME) have not yet been fully elucidated. RT-induced damage to cancer cells leads to different outcomes, such as survival, senescence, or death. During RT, alterations in signaling pathways result in changes in the local immune microenvironment. However, some immune cells are immunosuppressive or transform into immunosuppressive phenotypes under specific conditions, leading to the development of radioresistance. Patients who are radioresistant respond poorly to RT and may experience cancer progression. Given that the emergence of radioresistance is inevitable, new radiosensitization treatments are urgently needed. In this review, we discuss the changes in irradiated cancer cells and immune cells in the TME under different RT regimens and describe existing and potential molecules that could be targeted to improve the therapeutic effects of RT. Overall, this review highlights the possibilities of synergistic therapy by building on existing research.

Relationship between the tumor microenvironment and the efficacy of the combination of radiotherapy and immunotherapy

Activating and recruiting the immune system is critical for successful cancer treatment. Since the discovery of immune checkpoint inhibitors, immunotherapy has become the standard of care for many types of cancers. However, many patients fail to respond to immunotherapy. Further research is needed to understand the mechanisms of resistance and adjuvant therapies that can help sensitize patients to immunotherapies. Here, we will discuss how radiotherapy can change the tumor microenvironment and work synergistically with immunotherapy. We will examine different pre-clinical models focusing on their limitations and their unique advantages in studying the efficacy of treatments and the tumor microenvironment. We will also describe emerging findings from clinical trials testing the combination of immunotherapy and radiotherapy.

The role of dendritic cells in radiation-induced immune responses

Dendritic cells perform critical functions in bridging innate and adaptive immunity. Their ability to sense adjuvant signals in their environment, migrate on maturation, and cross-present cell-associated antigens enables these cells to carry antigen from tissue sites to lymph nodes, and thereby prime naïve T cells that cannot enter tissues. Despite being an infrequent cell type in tumors, we discuss how dendritic cells impact the immune environment of tumors and their response to cancer therapies. We review how radiation therapy of tumors can impact dendritic cells, through transfer of cell associated antigens to dendritic cells and the release of endogenous adjuvants, resulting in increased antigen presentation in the tumor-draining lymph nodes. We explore how tumor specific factors can result in negative regulation of dendritic cell function in the tumor, and the impact of direct radiation exposure to dendritic cells in the treatment field. These data suggest an important role for dendritic cell subpopulations in activating new T cell responses and boosting existing T cell responses to tumor associated antigens in tumor draining lymph nodes following radiation therapy. It further justifies a focus on the needs of the lymph node T cells to improve systemic anti-immunity following radiation therapy.

Inhibiting efferocytosis reverses macrophage-mediated immunosuppression in the leukemia microenvironment

Background

Previous studies show that the spleen and bone marrow can serve as leukemia microenvironments in which macrophages play a significant role in immune evasion and chemoresistance. We hypothesized that the macrophage driven tolerogenic process of efferocytosis is a major contributor to the immunosuppressive leukemia microenvironment and that this was driven by aberrant phosphatidylserine expression from cell turnover and cell membrane dysregulation.

Methods

Since MerTK is the prototypic efferocytosis receptor, we assessed whether the MerTK inhibitor MRX2843, which is currently in clinical trials, would reverse immune evasion and enhance immune-mediated clearance of leukemia cells.

Results

We found that inhibition of MerTK decreased leukemia-associated macrophage expression of M2 markers PD-L1, PD-L2, Tim-3, CD163 and Arginase-1 compared to vehicle-treated controls. Additionally, MerTK inhibition led to M1 macrophage repolarization including elevated CD86 and HLA-DR expression, and increased production of T cell activating cytokines, including IFN-β, IL-18, and IL-1β through activation of NF-κB. Collectively, this macrophage repolarization had downstream effects on T cells within the leukemia microenvironment, including decreased PD-1+Tim-3+ and LAG3+ checkpoint expression, and increased CD69+CD107a+ expression.

Discussion

These results demonstrate that MerTK inhibition using MRX2843 altered the leukemia microenvironment from tumor-permissive toward immune responsiveness to leukemia and culminated in improved immune-mediated clearance of AML.

Control of tumor-associated macrophage responses by nutrient acquisition and metabolism

Metazoan tissue specification is associated with integration of macrophage lineage cells in sub-tissular niches to promote tissue development and homeostasis. Oncogenic transformation, most prevalently of epithelial cell lineages, results in maladaptation of resident tissue macrophage differentiation pathways to generate parenchymal and interstitial tumor-associated macrophages that largely foster cancer progression. In addition to growth factors, nutrients that can be consumed, stored, recycled, or converted to signaling molecules have emerged as crucial regulators of macrophage responses in tumor. Here, we review how nutrient acquisition through plasma membrane transporters and engulfment pathways control tumor-associated macrophage differentiation and function. We also discuss how nutrient metabolism regulates tumor-associated macrophages and how these processes may be targeted for cancer therapy.

The role of macrophage TAM receptor family in the acute-to-chronic progression of liver disease: From friend to foe?

Hepatic macrophages, the key cellular components of the liver, emerge as essential players in liver inflammation, tissue repair and subsequent fibrosis, as well as tumorigenesis. Recently, the TAM receptor tyrosine kinase family, consisting of Tyro3, Axl and MerTK, was found to be a pivotal modulator of macrophages. Activation of macrophage TAM receptor signalling promotes the efferocytosis of apoptotic cells and skews the polarization of macrophages. After briefly reviewing the mechanisms of TAM receptor signalling in macrophage polarization, we focus on their role in liver diseases from acute injury to chronic inflammation, fibrosis and then to tumorigenesis. Notably, macrophage TAM receptor signalling seems to be a two-edged sword for liver diseases. On one hand, the activation of TAM receptor signalling inhibits inflammation and facilitates tissue repair during acute liver injury. On the other hand, continuous activation of the signalling contributes to the process of chronic inflammation into fibrosis and tumorigenesis by evoking hepatic stellate cells and inhibiting anti-tumour immunity. Therefore, targeting macrophage TAM receptors and clarifying its downstream pathways will be exciting prospects for the precaution and treatment of liver diseases, particularly at different stages or statuses.

Tissue-specific modifier alleles determine Mertk loss-of-function traits

Knockout (KO) mouse models play critical roles in elucidating biological processes behind disease-associated or disease-resistant traits. As a presumed consequence of gene KO, mice display certain phenotypes. Based on insight into the molecular role of said gene in a biological process, it is inferred that the particular biological process causally underlies the trait. This approach has been crucial towards understanding the basis of pathological and/or advantageous traits associated with Mertk KO mice. Mertk KO mice suffer from severe, early-onset retinal degeneration. MERTK, expressed in retinal pigment epithelia, is a receptor tyrosine kinase with a critical role in phagocytosis of apoptotic cells or cellular debris. Therefore, early-onset, severe retinal degeneration was described to be a direct consequence of failed MERTK-mediated phagocytosis of photoreceptor outer segments by retinal pigment epithelia. Here, we report that the loss of Mertk alone is not sufficient for retinal degeneration. The widely used Mertk KO mouse carries multiple coincidental changes in its genome that affect the expression of a number of genes, including the Mertk paralog Tyro3. Retinal degeneration manifests only when the function of Tyro3 is concomitantly lost. Furthermore, Mertk KO mice display improved anti-tumor immunity. MERTK is expressed in macrophages. Therefore, enhanced anti-tumor immunity was inferred to result from the failure of macrophages to dispose of cancer cell corpses, resulting in a pro-inflammatory tumor microenvironment. The resistance against two syngeneic mouse tumor models observed in Mertk KO mice is not, however, phenocopied by the loss of Mertk alone. Neither Tyro3 nor macrophage phagocytosis by alternate genetic redundancy accounts for the absence of anti-tumor immunity. Collectively, our results indicate that context-dependent epistasis of independent modifier alleles determines Mertk KO traits.

N-glycosylation stabilizes MerTK and promotes hepatocellular carcinoma tumor growth

Despite the evidences of elevated expression of Mer tyrosine kinase (MerTK) in multiple human cancers, mechanisms underlying the oncogenic roles of MerTK in hepatocellular carcinoma (HCC) remains undefined. We explored the functional effects of MerTK and N-Glycosylated MerTK on HCC cell survival and tumor growth. Here, we show that MerTK ablation increases reactive oxygen species (ROS) production and promotes the switching from glycolytic metabolism to oxidative phosphorylation in HCC cells, thus suppressing HCC cell proliferation and tumor growth. MerTK is N-glycosylated in HCC cells at asparagine 294 and 454 that stabilizes MerTK to promote oncogenic transformation. Moreover, we observed that nuclear located non-glycosylated MerTK is indispensable for survival of HCC cells under stress. Pathologically, tissue microarray (TMA) data indicate that MerTK is a pivotal prognostic factor for HCC. Our data strongly support the roles of MerTK N-glycosylation in HCC tumorigenesis and suggesting N-glycosylation inhibition as a potential HCC therapeutic strategy.

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MerTK promotes the switching from oxidative phosphorylation to glycolytic metabolism in HCC cells.

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MerTK is N-glycosylated in HCC cells at asparagine 294 and 454 that stabilizes MerTK to promote HCC tumor growth.

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The nuclear located non-glycosylated MerTK is indispensable for survival of HCC cells under stress.

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MerTK is a pivotal prognostic factor for HCC and its N-glycosylation inhibition is a potential HCC therapeutic strategy.

The paradox of radiation and T cells in tumors

In this review we consider what appears to be a paradox in immunotherapies based around radiation therapy. The paradox is based on three main points. 1. That T cells are needed for radiation's efficacy; 2. That tumor-specific T cells are enriched in the field of treatment; and 3. That radiation kills T cells in the treatment field. We discuss evidence of the effect of radiation on T cells in the field given their ongoing movement in and out of tissues and the tumor, and how the movement of T cells impacts the treated primary tumor and untreated distant metastases. Given this evidence, we revisit the paradox to understand how the extraordinary efficacy of radiation and immunity in preclinical models is dependent on this radiation sensitive cell.

Mertk: An emerging target in cancer biology and immuno-oncology

Mertk, a type I Receptor Tyrosine Kinase (RTK) and member of the TAM (Tyro3, Axl, and Mertk) family of homologous tyrosine kinases, has important roles in signal transduction both homeostatically on normal cells as well as patho-physiologically on both tumor-associated macrophages and malignant cells by its overexpression in a wide array of cancers. The main ligands of Mertk are Vitamin K-modified endogenous proteins Gas6 and Protein S (ProS1), heterobifunctional modular proteins that bind Mertk via two carboxyl-terminal laminin-like globular (LG) domains, and an N-terminal Gla domain that binds anionic phospholipids, whereby externalized phosphatidylserine (PS) on stressed viable and caspase-activated apoptotic cells is most emblematic. Recent studies indicate that Vitamin K-dependent γ-carboxylation on the N-terminal Gla domain of Gas6 and Protein S is necessary for PS binding and Mertk activation, implying that Mertk is preferentially active in tissues where there is high externalized PS, such as the tumor microenvironment (TME) and acute virally infected tissues. Once stimulated, activated Mertk can provide a survival advantage for cancer cells as well as drive compensatory proliferation. On monocytes and tumor-associated macrophages, Mertk promotes efferocytosis and acts as an inhibitory receptor that impairs host anti-tumor immunity, functioning akin to a myeloid checkpoint inhibitor. In recent years, inhibition of Mertk has been implicated in a dual role to enhance the sensitivity of cancer cells to cytotoxic agents along with improving host anti-tumor immunity with anti-PD-1/PD-L1 immunotherapy. Here, we examine the rationale of Mertk-targeted immunotherapies, the current and potential therapeutic strategies, the clinical status of Mertk-specific therapies, and potential challenges and obstacles for Mertk-focused therapies.

Targeting MERTK and AXL in EGFR Mutant Non-Small Cell Lung Cancer

MERTK and AXL are members of the TAM family of receptor tyrosine kinases and are abnormally expressed in 69% and 93% of non-small cell lung cancers (NSCLCs), respectively. Expression of MERTK and/or AXL provides a survival advantage for NSCLC cells and correlates with lymph node metastasis, drug resistance, and disease progression in patients with NSCLC. The TAM receptors on host tumor infiltrating cells also play important roles in the immunosuppressive tumor microenvironment. Thus, MERTK and AXL are attractive biologic targets for NSCLC treatment. Here, we will review physiologic and oncologic roles for MERTK and AXL with an emphasis on the potential to target these kinases in NSCLCs with activating EGFR mutations.

Effects of Radiation on the Tumor Microenvironment

A malignant tumor consists of malignant cells as well as a wide array of normal host tissues including stroma, vasculature, and immune infiltrate. The interaction between cancer and these host tissues is critical as these host tissues play a variety of roles in supporting or resisting disease progression. Radiotherapy (RT) has direct effects on malignant cells, but, also, critically important effects on these other components of the tumor microenvironment (TME). Given the growing role of immune checkpoint inhibitors and other immunotherapy strategies, understanding how RT affects the TME, particularly the immune compartment, is essential to advance RT in this new era of cancer therapy. The interactions between RT and the TME are complex, affecting the innate and adaptive arms of the immune system. RT can induce both proinflammatory effects and immune suppressive effects that can either promote or impede antitumor immunity. It is likely that the initial proinflammatory effects of RT eventually lead to rebound immune-suppression as chronic inflammation sets in. The exact kinetics and nature of how RT changes the TME likely depends on timing, dose, fractionation, site irradiated, and tumor type. With increased understanding of the effects of RT on the TME, in the future it is likely that we will be able to personalize RT by varying the dose, site, and timing of intervention to generate the desired response to partner with immunotherapy strategies.

The complex roles of efferocytosis in cancer development, metastasis, and treatment

When tumor cells are killed by targeted therapy, radiotherapy, or chemotherapy, they trigger their primary tumor by releasing pro-inflammatory cytokines. Microenvironmental interactions can also promote tumor heterogeneity and development. In this line, several immune cells within the tumor microenvironment, including macrophages, dendritic cells, regulatory T-cells, and CD8⁺ and CD4⁺ T cells, are involved in the clearance of apoptotic tumor cells through a process called efferocytosis. Although the efficiency of apoptotic tumor cell efferocytosis is positive under physiological conditions, there are controversies regarding its usefulness in treatment-induced apoptotic tumor cells (ATCs). Efferocytosis can show the limitation of cytotoxic treatments, such as chemotherapy and radiotherapy. Since cytotoxic treatments lead to extensive cell mortality, efferocytosis, and macrophage polarization toward an M2 phenotype, the immune response may get involved in tumor recurrence and metastasis. Tumor cells can use the anti-inflammatory effect of apoptotic tumor cell efferocytosis to induce an immunosuppressive condition that is tumor-tolerant. Since M2 polarization and efferocytosis are tumor-promoting processes, the receptors on macrophages act as potential targets for cancer therapy. Moreover, researchers have shown that efferocytosis-related molecules/pathways are potential targets for cancer therapy. These include phosphatidylserine and calreticulin, Tyro3, Axl, and Mer tyrosine kinase (MerTK), receptors of tyrosine kinase, indoleamine-2,3-dioxygenase 1, annexin V, CD47, TGF-β, IL-10, and macrophage phenotype switch are combined with conventional therapy, which can be more effective in cancer treatment. Thus, we set out to investigate the advantages and disadvantages of efferocytosis in treatment-induced apoptotic tumor cells.

Tyro3, Axl, Mertk receptor-mediated efferocytosis and immune regulation in the tumor environment

Three structurally related tyrosine receptor cell surface kinases, Tyro3, Axl, and Mertk (TAM) have been recognized to modulate immune function, tissue homeostasis, cardiovasculature, and cancer. The TAM receptor family appears to operate in adult mammals across multiple cell types, suggesting both widespread and specific regulation of cell functions and immune niches. TAM family members regulate tissue homeostasis by monitoring the presence of phosphatidylserine expressed on stressed or apoptotic cells. The detection of phosphatidylserine on apoptotic cells requires intermediary molecules that opsonize the dying cells and tether them to TAM receptors on phagocytes. This complex promotes the engulfment of apoptotic cells, also known as efferocytosis, that leads to the resolution of inflammation and tissue healing. The immune mechanisms dictating these processes appear to fall upon specific family members or may involve a complex of different receptors acting cooperatively to resolve and repair damaged tissues. Here, we focus on the role of TAM receptors in triggering efferocytosis and its consequences in the regulation of immune responses in the context of inflammation and cancer.

Structure-activity relationship of 7-aryl-2-anilino-pyrrolopyrimidines as Mer and Axl tyrosine kinase inhibitors

The TAM (Axl, Mer, and Tyro3) family is implicated in the survival and chemoresistance of tumours and has emerged as a potential therapeutic target. A novel series of 7-aryl-2-anilino-pyrrolopyrimidines were identified as potent Axl/Mer tyrosine kinase inhibitors without significant inhibition of Tyro3. A representative compound 27 exhibited IC₅₀ values of 2 nM and 16 nM for Mer and Axl, respectively, and considerable inhibition for Mer phosphorylation in cells. Docking studies suggested that the formation of a salt bridge between the nitrogen of the aniline moiety with ASP678 of the Mer kinase domain as well as an interaction with the hinge region that most kinase inhibitors have in common would be essential to retain activity. These results could provide useful information for finding promising inhibitors of Axl/Mer for the treatment of cancer.

Defining Immunogenic and Radioimmunogenic Tumors

In the cancer literature tumors are inconsistently labeled as ‘immunogenic’, and experimental results are occasionally dismissed since they are only tested in known ‘responsive’ tumor models. The definition of immunogenicity has moved from its classical definition based on the rejection of secondary tumors to a more nebulous definition based on immune infiltrates and response to immunotherapy interventions. This review discusses the basis behind tumor immunogenicity and the variation between tumor models, then moves to discuss how these principles apply to the response to radiation therapy. In this way we can identify radioimmunogenic tumor models that are particularly responsive to immunotherapy only when combined with radiation, and identify the interventions that can convert unresponsive tumors so that they can also respond to these treatments.

MERTK Inhibition: Potential as a Treatment Strategy in EGFR Tyrosine Kinase Inhibitor-Resistant Non-Small Cell Lung Cancer

Epidermal growth factor tyrosine kinase inhibitors (EGFR-TKIs) are currently the most effective treatment for non-small cell lung cancer (NSCLC) patients, who carry primary EGFR mutations. However, the patients eventually develop drug resistance to EGFR-TKIs after approximately one year. In addition to the acquisition of the EGFR T790M mutation, the activation of alternative receptor-mediated signaling pathways is a common mechanism for conferring the insensitivity of EGFR-TKI in NSCLC. Upregulation of the Mer receptor tyrosine kinase (MERTK), which is a member of the Tyro3-Axl-MERTK (TAM) family, is associated with a poor prognosis of many cancers. The binding of specific ligands, such as Gas6 and PROS1, to MERTK activates phosphoinositide 3-kinase (PI3K)/Akt and mitogen-activated protein kinase (MAPK) cascades, which are the signaling pathways shared by EGFR. Therefore, the inhibition of MERTK can be considered a new therapeutic strategy for overcoming the resistance of NSCLC to EGFR-targeted agents. Although several small molecules and monoclonal antibodies targeting the TAM family are being developed and have been described to enhance the chemosensitivity and converse the resistance of EGFR-TKI, few have specifically been developed as MERTK inhibitors. The further development and investigation of biomarkers which can accurately predict MERTK activity and the response to MERTK inhibitors and MERTK-specific drugs are vitally important for obtaining appropriate patient stratification and increased benefits in clinical applications.

Small molecular drugs reshape tumor microenvironment to synergize with immunotherapy

Recently, immune checkpoint blockade (ICB), especially anti-programmed death 1 (anti-PD-1) and anti-programmed death-ligand 1 (anti-PD-L1) therapy, has become an increasingly appealing therapeutic strategy for cancer patients. However, only a small portion of patients responds to anti-PD treatment. Therefore, treatment strategies are urgently needed to reverse the ICB-resistant tumor microenvironment (TME). It has become clear that the TME has diminished innate sensing that is critical to activate adaptive immunity. In addition, tumor cells upregulate various immunosuppressive factors to diminish the immune response and resist immunotherapy. In this review, we briefly update the current small molecular drugs that could synergize with immunotherapy, especially anti-PD therapy. We will discuss the modes of action by those drugs including inducing innate sensing and limiting immunosuppressive factors in the TME.

Axl and Mertk Receptors Cooperate to Promote Breast Cancer Progression by Combined Oncogenic Signaling and Evasion of Host Antitumor Immunity

Despite the promising clinical benefit of targeted and immune checkpoint blocking therapeutics, current strategies have limited success in breast cancer, indicating that additional inhibitory pathways are required to complement existing therapeutics. TAM receptors (Tyro-3, Axl, and Mertk) are often correlated with poor prognosis because of their capacities to sustain an immunosuppressive environment. Here, we ablate Axl on tumor cells using CRISPR/Cas9 gene editing, and by targeting Mertk in the tumor microenvironment (TME), we observed distinct functions of TAM as oncogenic kinases, as well as inhibitory immune receptors. Depletion of Axl suppressed cell intrinsic oncogenic properties, decreased tumor growth, reduced the incidence of lung metastasis and increased overall survival of mice when injected into mammary fat pad of syngeneic mice, and demonstrated synergy when combined with anti-PD-1 therapy. Blockade of Mertk function on macrophages decreased efferocytosis, altered the cytokine milieu, and resulted in suppressed macrophage gene expression patterns. Mertk-knockout mice or treatment with anti-Mertk-neutralizing mAb also altered the cellular immune profile, resulting in a more inflamed tumor environment with enhanced T-cell infiltration into tumors and T-cell-mediated cytotoxicity. The antitumor activity from Mertk inhibition was abrogated by depletion of cytotoxic CD8α T cells by using anti-CD8α mAb or by transplantation of tumor cells into B6.CB17-Prkdc SCID mice. Our data indicate that targeting Axl expressed on tumor cells and Mertk in the TME is predicted to have a combinatorial benefit to enhance current immunotherapies and that Axl and Mertk have distinct functional activities that impair host antitumor response. SIGNIFICANCE: This study demonstrates how TAM receptors act both as oncogenic tyrosine kinases and as receptors that mediate immune evasion in cancer progression.

Next Generation Imaging Techniques to Define Immune Topographies in Solid Tumors

In recent years, cancer immunotherapy experienced remarkable developments and it is nowadays considered a promising therapeutic frontier against many types of cancer, especially hematological malignancies. However, in most types of solid tumors, immunotherapy efficacy is modest, partly because of the limited accessibility of lymphocytes to the tumor core. This immune exclusion is mediated by a variety of physical, functional and dynamic barriers, which play a role in shaping the immune infiltrate in the tumor microenvironment. At present there is no unified and integrated understanding about the role played by different postulated models of immune exclusion in human solid tumors. Systematically mapping immune landscapes or "topographies" in cancers of different histology is of pivotal importance to characterize spatial and temporal distribution of lymphocytes in the tumor microenvironment, providing insights into mechanisms of immune exclusion. Spatially mapping immune cells also provides quantitative information, which could be informative in clinical settings, for example for the discovery of new biomarkers that could guide the design of patient-specific immunotherapies. In this review, we aim to summarize current standard and next generation approaches to define Cancer Immune Topographies based on published studies and propose future perspectives.

Macrophage-Mediated Tumor Cell Phagocytosis: Opportunity for Nanomedicine Intervention

Macrophages are one of the most abundant non-malignant cells in the tumor microenvironment, playing critical roles in mediating tumor immunity. As important innate immune cells, macrophages possess the potential to engulf tumor cells and present tumor-specific antigens for adaptive antitumor immunity induction, leading to growing interest in targeting macrophage phagocytosis for cancer immunotherapy. Nevertheless, live tumor cells have evolved to evade phagocytosis by macrophages via the extensive expression of anti-phagocytic molecules, such as CD47. In addition, macrophages also rapidly recognize and engulf apoptotic cells (efferocytosis) in the tumor microenvironment, which inhibits inflammatory responses and facilitates immune escape of tumor cells. Thus, intervention of macrophage phagocytosis by blocking anti-phagocytic signals on live tumor cells or inhibiting tumor efferocytosis presents a promising strategy for the development of cancer immunotherapies. Here, the regulation of macrophage-mediated tumor cell phagocytosis is first summarized, followed by an overview of strategies targeting macrophage phagocytosis for the development of antitumor therapies. Given the potential off-target effects associated with the administration of traditional therapeutics (for example, monoclonal antibodies, small molecule inhibitors), we highlight the opportunity for nanomedicine in macrophage phagocytosis intervention.

Single cell analysis reveals distinct immune landscapes in transplant and primary sarcomas that determine response or resistance to immunotherapy

Immunotherapy fails to cure most cancer patients. Preclinical studies indicate that radiotherapy synergizes with immunotherapy, promoting radiation-induced antitumor immunity. Most preclinical immunotherapy studies utilize transplant tumor models, which overestimate patient responses. Here, we show that transplant sarcomas are cured by PD-1 blockade and radiotherapy, but identical treatment fails in autochthonous sarcomas, which demonstrate immunoediting, decreased neoantigen expression, and tumor-specific immune tolerance. We characterize tumor-infiltrating immune cells from transplant and primary tumors, revealing striking differences in their immune landscapes. Although radiotherapy remodels myeloid cells in both models, only transplant tumors are enriched for activated CD8+ T cells. The immune microenvironment of primary murine sarcomas resembles most human sarcomas, while transplant sarcomas resemble the most inflamed human sarcomas. These results identify distinct microenvironments in murine sarcomas that coevolve with the immune system and suggest that patients with a sarcoma immune phenotype similar to transplant tumors may benefit most from PD-1 blockade and radiotherapy.

Targeting Tumor-Associated Macrophages in the Pediatric Sarcoma Tumor Microenvironment

For many pediatric sarcoma patients, multi-modal therapy including chemotherapy, radiation, and surgery is sufficient to cure their disease. However, event-free and overall survival rates for patients with more advanced disease are grim, necessitating the development of novel therapeutic approaches. Within many pediatric sarcomas, the normal immune response, including recognition and destruction of cancer cells, is lost due to the highly immune suppressive tumor microenvironment (TME). In this setting, tumor cells evade immune detection and capitalize on the immune suppressed microenvironment, leading to unchecked proliferation and metastasis. Recent preclinical and clinical approaches are aimed at understanding this immune suppressive microenvironment and employing cancer immunotherapy in an attempt to overcome this, by renewing the ability of the immune system to recognize and destroy cancer cells. While there are several factors that drive the attenuation of immune responses in the sarcoma TME, one of the most remarkable are tumor associated macrophage (TAMs). TAMs suppress immune cytolytic function, promote tumor growth and metastases, and are generally associated with a poor prognosis in most pediatric sarcoma subtypes. In this review, we summarize the mechanisms underlying TAM-facilitated immune evasion and tumorigenesis and discuss the potential therapeutic application of TAM-focused drugs in the treatment of pediatric sarcomas.

Reprogramming the tumour microenvironment by radiotherapy: implications for radiotherapy and immunotherapy combinations

Radiotherapy (RT) is a highly effective anti-cancer therapy delivered to around 50-60% of patients. It is part of therapy for around 40% of cancer patients who are cured of their disease. Until recently, the focus of this anti-tumour efficacy has been on the direct tumour cytotoxicity and RT-induced DNA damage. Recently, the immunomodulatory effects of RT on the tumour microenvironment have increasingly been recognized. There is now intense interest in potentially using RT to induce an anti-tumour immune response, which has led to rethinking into how the efficacy of RT could be further enhanced. Following the breakthrough of immune check point inhibitors (ICIs), a new era of immuno-oncology (IO) agents has emerged and established immunotherapy as a routine part of cancer treatment. Despite ICI improving outcomes in many cancer types, overall durable responses occur in only a minority of patients. The immunostimulatory effects of RT make combinations with ICI attractive to potentially amplify anti-tumour immunity resulting in increased tumour responses and improved outcomes. In contrast, tumours with profoundly immunosuppressive tumour microenvironments, dominated by myeloid-derived cell populations, remain a greater clinical challenge and RT may potentially further enhance the immunosuppression. To harness the full potential of RT and IO agent combinations, further insights are required to enhance our understanding of the role these immunosuppressive myeloid populations play, how RT influences these populations and how they may be therapeutically manipulated in combination with RT to improve outcomes further. These are exciting times with increasing numbers of IO targets being discovered and IO agents undergoing clinical evaluation. Multidisciplinary research collaborations will be required to establish the optimal parameters for delivering RT (target volume, dose and fractionation) in combination with IO agents, including scheduling to achieve maximal therapeutic efficacy.

Dual targeting of TAM receptors Tyro3, Axl, and MerTK: Role in tumors and the tumor immune microenvironment

In both normal and tumor tissues, receptor tyrosine kinases (RTKs) may be pleiotropically expressed. The RTKs not only regulate ordinary cellular processes, including proliferation, survival, adhesion, and migration, but also have a critical role in the development of many types of cancer. The Tyro3, Axl, and MerTK (TAM) family of RTKs (Tyro3, Axl, and MerTK) plays a pleiotropic role in phagocytosis, inflammation, and normal cellular processes. In this article, we highlight the cellular activities of TAM receptors and discuss their roles in cancer and immune cells. We also discuss cancer therapies that target TAM receptors. Further research is needed to elucidate the function of TAM receptors in immune cells toward the development of new targeted immunotherapies for cancer.

Cell Death in the Tumor Microenvironment: Implications for Cancer Immunotherapy

The physiological fate of cells that die by apoptosis is their prompt and efficient removal by efferocytosis. During these processes, apoptotic cells release intracellular constituents that include purine nucleotides, lysophosphatidylcholine (LPC), and Sphingosine-1-phosphate (S1P) that induce migration and chemo-attraction of phagocytes as well as mitogens and extracellular membrane-bound vesicles that contribute to apoptosis-induced compensatory proliferation and alteration of the extracellular matrix and the vascular network. Additionally, during efferocytosis, phagocytic cells produce a number of anti-inflammatory and resolving factors, and, together with apoptotic cells, efferocytic events have a homeostatic function that regulates tissue repair. These homeostatic functions are dysregulated in cancers, where, aforementioned events, if not properly controlled, can lead to cancer progression and immune escape. Here, we summarize evidence that apoptosis and efferocytosis are exploited in cancer, as well as discuss current translation and clinical efforts to harness signals from dying cells into therapeutic strategies.

MERTK in cancer therapy: Targeting the receptor tyrosine kinase in tumor cells and the immune system

The receptor tyrosine kinase MERTK is aberrantly expressed in numerous human malignancies, and is a novel target in cancer therapeutics. Physiologic roles of MERTK include regulation of tissue homeostasis and repair, innate immune control, and platelet aggregation. However, aberrant expression in a wide range of liquid and solid malignancies promotes neoplasia via growth factor independence, cell cycle progression, proliferation and tumor growth, resistance to apoptosis, and promotion of tumor metastases. Additionally, MERTK signaling contributes to an immunosuppressive tumor microenvironment via induction of an anti-inflammatory cytokine profile and regulation of the PD-1 axis, as well as regulation of macrophage, myeloid-derived suppressor cell, natural killer cell and T cell functions. Various MERTK-directed therapies are in preclinical development, and clinical trials are underway. In this review we discuss MERTK inhibition as an emerging strategy for cancer therapy, focusing on MERTK expression and function in neoplasia and its role in mediating resistance to cytotoxic and targeted therapies as well as in suppressing anti-tumor immunity. Additionally, we review preclinical and clinical pharmacological strategies to target MERTK.

MERTK in cancer therapy: Targeting the receptor tyrosine kinase in tumor cells and the immune system

The receptor tyrosine kinase MERTK is aberrantly expressed in numerous human malignancies, and is a novel target in cancer therapeutics. Physiologic roles of MERTK include regulation of tissue homeostasis and repair, innate immune control, and platelet aggregation. However, aberrant expression in a wide range of liquid and solid malignancies promotes neoplasia via growth factor independence, cell cycle progression, proliferation and tumor growth, resistance to apoptosis, and promotion of tumor metastases. Additionally, MERTK signaling contributes to an immunosuppressive tumor microenvironment via induction of an anti-inflammatory cytokine profile and regulation of the PD-1 axis, as well as regulation of macrophage, myeloid-derived suppressor cell, natural killer cell and T cell functions. Various MERTK-directed therapies are in preclinical development, and clinical trials are underway. In this review we discuss MERTK inhibition as an emerging strategy for cancer therapy, focusing on MERTK expression and function in neoplasia and its role in mediating resistance to cytotoxic and targeted therapies as well as in suppressing anti-tumor immunity. Additionally, we review preclinical and clinical pharmacological strategies to target MERTK.

LC3-Associated Phagocytosis (LAP): A Potentially Influential Mediator of Efferocytosis-Related Tumor Progression and Aggressiveness

One aim of cancer therapies is to induce apoptosis of tumor cells. Efficient removal of the apoptotic cells requires coordinated efforts between the processes of efferocytosis and LC3-associated phagocytosis (LAP). However, this activity has also been shown to produce anti-inflammatory and immunosuppressive signals that can be utilized by live tumor cells to evade immune defense mechanisms, resulting in tumor progression and aggressiveness. In the absence of LAP, mice exhibit suppressed tumor growth during efferocytosis, while LAP-sufficient mice show enhanced tumor progression. Little is known about how LAP or its regulators directly affect efferocytosis, tumor growth and treatment responses, and identifying the mechanisms involved has the potential to lead to the discovery of novel approaches to target cancer cells. Also incompletely understood is the direct effect of apoptotic cancer cells on LAP. This is particularly important as induction of apoptosis by current cytotoxic cancer therapies can potentially stimulate LAP following efferocytosis. Herein, we highlight the current understanding of the role of LAP and its relationship with efferocytosis in the tumor microenvironment with a view to presenting novel therapeutic strategies.

Spatial Density and Distribution of Tumor-Associated Macrophages Predict Survival in Non-Small Cell Lung Carcinoma

The respective antitumoral and protumoral roles of M1 and M2 tumor-associated macrophages (TAM) typify the complexity of macrophage function in cancer. In lung cancer, density and topology of distinct TAM phenotypes at the tumor center (TC) versus the invasive margin (IM) are largely unknown. Here, we investigated TAM subtype density and distribution between TC and IM in human lung cancer and TAM associations with overall survival. Macrophages isolated from adjacent nontumor tissue (NM), the TC (TC-TAM), and the IM (IM-TAM) were analyzed with RNA-sequencing (RNA-seq). Lung tumor tissue microarrays from 104 patient samples were constructed. M1 and M2 TAMs were identified using multiplex immunofluorescence staining and a tumor cell-TAM proximity analysis was performed. RNA-seq identified marked differences among NM, TC-TAM, and IM-TAM. On the basis of a panel of five selected markers (CD68, IL12, CCR7, CD163, and ALOX15), M2 predominance over M1 and M2 proximity to tumor cells was observed, especially at IM. Tumor cell proximity to TAM was linked with tumor cell survival and hypoxia was associated with accumulation of M2 TAM. Notably, lower density of M1 TC-TAM and higher proximity of tumor cells to M2 IM-TAM or lower proximity to M1 IM-TAM were linked with poor survival. In addition, three novel molecules (UBXN4, MFSD12, and ACTR6) from RNA-seq served as potential prognostic markers for lung cancer, and M2 predominance and juxtaposition of M2 TAM near tumor cells were associated with poor survival. Together, our results reveal the marked heterogeneity of TAM populations in different tumor regions, with M2 TAM predominance, particularly at IM. SIGNIFICANCE: This study underlines the significance of the density, spatial distribution, and gene expression of TAM phenotypes as prognostic factors for overall survival in lung cancer. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/20/4414/F1.large.jpg.

MerTK inhibition decreases immune suppressive glioblastoma-associated macrophages and neoangiogenesis in glioblastoma microenvironment

Background

Glioblastoma-associated macrophages and microglia (GAMs) are the predominant immune cells in the tumor microenvironment. Activation of MerTK, a receptor tyrosine kinase, polarizes GAMs to an immunosuppressive phenotype, promoting tumor growth. Here, the role of MerTK inhibition in the glioblastoma microenvironment is investigated in vitro and in vivo.

Methods

Effects of MRX-2843 in glioblastoma microenvironment regulation were determined in vitro by cell viability, cytokine array, in vitro tube formation, Western blotting, and wound healing assays. A syngeneic GL261 orthotopic glioblastoma mouse model was used to evaluate the survival benefit of MRX-2843 treatment. Multiplex fluorescent immunohistochemistry was used to evaluate the expression of CD206, an anti-inflammatory marker on GAMs, and angiogenesis in murine brain tumor tissues.

Results

MRX-2843 inhibited cell growth and induced apoptosis in human glioblastoma cells and decreased protein expression of phosphorylated MerTK, AKT, and ERK, which are essential for cell survival signaling. Interleukin-8 and C-C motif chemokine ligand 2, the pro-glioma and pro-angiogenic cytokines, were decreased by MRX-2843. Decreased vascular formation and numbers of immunosuppressive (CD206+) GAMs were observed following MRX-2843 treatment in vivo, suggesting that in addition to alleviating immunosuppression, MRX-2843 also inhibits neoangiogenesis in the glioma microenvironment. These results were supported by a prolonged survival in the syngeneic mouse orthotopic GL261 glioblastoma model following MRX-2843 treatment.

Conclusion

Our findings suggest that MRX-2843 has a therapeutic benefit via promoting GAM polarization away from immunosuppressive condition, inhibiting neoangiogenesis in the glioblastoma microenvironment and inducing tumor cell death.

Engineering nanoparticles to reprogram radiotherapy and immunotherapy: recent advances and future challenges

Nanoparticles (NPs) have been increasingly studied for radiosensitization. The principle of NPs radio-enhancement is to use high-atomic number NPs (e.g. gold, hafnium, bismuth and gadolinium) or deliver radiosensitizing substances, such as cisplatin and selenium. Nowadays, cancer immunotherapy is emerged as a promising treatment and immune checkpoint regulation has a potential property to improve clinical outcomes in cancer immunotherapy. Furthermore, NPs have been served as an ideal platform for immunomodulator system delivery. Owing to enhanced permeability and retention (EPR) effect, modified-NPs increase the targeting and retention of antibodies in target cells. The purpose of this review is to highlight the latest progress of nanotechnology in radiotherapy (RT) and immunotherapy, as well as combining these three strategies in cancer treatment. Overall, nanomedicine as an effective strategy for RT can significantly enhance the outcome of immunotherapy response and might be beneficial for clinical transformation.

Dendritic Cell Maturation Defines Immunological Responsiveness of Tumors to Radiation Therapy

Radiation therapy is capable of directing adaptive immune responses against tumors by stimulating the release of endogenous adjuvants and tumor-associated Ags. Within the tumor, conventional type 1 dendritic cells (cDC1s) are uniquely positioned to respond to these signals, uptake exogenous tumor Ags, and migrate to the tumor draining lymph node to initiate cross-priming of tumor-reactive cytotoxic CD8⁺ T cells. In this study, we report that radiation therapy promotes the activation of intratumoral cDC1s in radioimmunogenic murine tumors, and this process fails to occur in poorly radioimmunogenic murine tumors. In poorly radioimmunogenic tumors, the adjuvant polyinosinic-polycytidylic acid overcomes this failure following radiation and successfully drives intratumoral cDC1 maturation, ultimately resulting in durable tumor cures. Depletion studies revealed that both cDC1 and CD8⁺ T cells are required for tumor regression following combination therapy. We further demonstrate that treatment with radiation and polyinosinic-polycytidylic acid significantly expands the proportion of proliferating CD8⁺ T cells in the tumor with enhanced cytolytic potential and requires T cell migration from lymph nodes for therapeutic efficacy. Thus, we conclude that lack of endogenous adjuvant release or active suppression following radiation therapy may limit its efficacy in poorly radioimmunogenic tumors, and coadministration of exogenous adjuvants that promote cDC1 maturation and migration can overcome this limitation to improve tumor control following radiation therapy.

Targeting MerTK Enhances Adaptive Immune Responses After Radiation Therapy

PURPOSE

The role of MerTK, a member of the Tyro3-Axl-MerTK family of receptor tyrosine kinase, in the immune response to radiation therapy (RT) is unclear. We investigated immune-mediated tumor control after RT in murine models of colorectal and pancreatic adenocarcinoma using MerTK wild-type and knock-out hosts and whether inhibition of MerTK signaling with warfarin could replicate MerTK knock-out phenotypes.

METHODS AND MATERIALS

Wild-type and MerTK^-/- BALB/c mice were grafted in the flanks with CT26 tumors and treated with computed tomography guided RT. The role of macrophages and CD8 T cells in the response to radiation were demonstrated with cell depletion studies. The role of MerTK in priming immune responses after RT alone and with agonist antibodies to the T cell costimulatory molecule OX40 was evaluated in a Panc02-SIY model antigen system. The effect of warfarin therapy on the in-field and abscopal response to RT was demonstrated in murine models of colorectal adenocarcinoma. The association between warfarin and progression-free survival for patients treated with SABR for early-stage non-small cell lung cancer was evaluated in a multi-institutional retrospective study.

RESULTS

MerTK^-/- hosts had better tumor control after RT compared with wild-type mice in a macrophage and CD8 T cell-dependent manner. MerTK^-/- mice showed increased counts of tumor antigen-specific CD8 T cells in the peripheral blood after tumor-directed RT alone and in combination with agonist anti-OX40. Warfarin therapy phenocopied MerTK^-/- for single-flank tumors treated with RT and improved abscopal responses for RT combined with anti-CTLA4. Patients on warfarin therapy when treated with SABR for non-small cell lung cancer had higher progression-free survival rates compared with non-warfarin users.

CONCLUSIONS

MerTK inhibits adaptive immune responses after SABR. Because warfarin inhibits MerTK signaling and phenocopies genetic deletion of MerTK in mice, warfarin therapy may have beneficial effects in combination with SABR and immune therapy in patients with cancer.

Using Preclinical Data to Design Combination Clinical Trials of Radiation Therapy and Immunotherapy

Immunotherapies are rapidly entering the clinic as approved treatments for diverse cancer pathologies. Radiation therapy is an integral partner in cancer therapy, commonly as part of complicated multimodality approaches that optimize patient outcomes. Preclinical studies have demonstrated that the success of radiation therapy in tumor control is due in part to immune mechanisms, and that outcomes following radiation therapy can be improved through combination with a range of immunotherapies. However, preclinical models of cancer are very different from patient tumors, and the way these preclinical tumors are treated is often very different from standard of care treatment of patients. This review examines the preclinical and clinical data for the role of the immune system in radiation therapy outcomes, and how to integrate preclinical findings into clinical trials, using ongoing studies as examples.

The Immune Microenvironment and Cancer Metastasis

The dynamic interplay between neoplastic cells and the immune microenvironment regulates every step of the metastatic process. Immune cells contribute to invasion by secreting a cornucopia of inflammatory factors that promote epithelial-to-mesenchymal transition and remodeling of the stroma. Cancer cells then intravasate to the circulatory system assisted by macrophages and use several pathways to avoid recognition by cytotoxtic lymphocytes and phagocytes. Circulating tumor cells that manage to adhere to the vasculature and encounter premetastic niches are able to use the associated myeloid cells to extravasate into ectopic organs and establish a dormant microscopic colony. If successful at avoiding repetitive immune attack, dormant cells can subsequently grow into overt, clinically detectable metastatic lesions, which ultimately account to most cancer-related deaths. Understanding how disseminated tumor cells evade and corrupt the immune system during the final stages of metastasis will be pivotal in developing new therapeutic modalities that combat metastasis.

The Biology of Immune-Active Cancers and Their Regulatory Mechanisms

The development of cancer results from the evolutionary balance between the proliferating aptitude of cancer cells and the response of the host's tissues. Some cancers are characterized by genetic instability dependent upon impaired DNA repair mechanisms that lead to the chaotic disruption of multiple cellular functions often in excess of the cancer survival needs and may exert broad effects on surrounding tissues, some beneficial and some detrimental to cancer growth. Among them, inflammatory processes that accompany wound healing may initiate a reaction of the host against the neo-formation. This is possibly triggered by the release by dying cancer cells of molecules known as Damage-Associated Molecular Patterns (DAMPs) following a process termed Immunogenic Cell Death (ICD) that initiates an immune response through innate and adaptive mechanisms. Indeed, analysis of large cancer data sets has shown that ICD is strictly associated with the activation of other immune effector or immune-regulatory pathways. Here, we will describe how immune activation and compensatory immune-regulatory mechanisms balance anti-cancer immune surveillance and the roles that innate and adaptive immunity play including the weight that neo-epitopes may exert as initiators and sculptors of high-affinity memory and effector immune responses against cancer. We will discuss the evolutionary basis for the existence of immune checkpoints and how several theories raised to explain cancer resistance to immunotherapy represent a facet of a similar evolutionary phenomenon that we described in the Theory of Everything. We will show how the biology of immunogenicity and counterbalancing immune regulation is widespread across cancers independent of their ontogenesis while subtle idiosyncratic differences are discernible among them. Finally, we will suggest that overcoming immune resistance implies distinct approaches relevant to the immune context of individual cancers.

Blockade of the Phagocytic Receptor MerTK on Tumor-Associated Macrophages Enhances P2X7R-Dependent STING Activation by Tumor-Derived cGAMP

Clearance of apoptotic cells by macrophages prevents excessive inflammation and supports immune tolerance. Here, we examined the effect of blocking apoptotic cell clearance on anti-tumor immune response. We generated an antibody that selectively inhibited efferocytosis by phagocytic receptor MerTK. Blockade of MerTK resulted in accumulation of apoptotic cells within tumors and triggered a type I interferon response. Treatment of tumor-bearing mice with anti-MerTK antibody stimulated T cell activation and synergized with anti-PD-1 or anti-PD-L1 therapy. The anti-tumor effect induced by anti-MerTK treatment was lost in Sting^gt/gt mice, but not in Cgas^-/- mice. Abolishing cGAMP production in Cgas^-/- tumor cells, depletion of extracellular ATP, or inactivation of the ATP-gated P2X7R channel also compromised the effects of MerTK blockade. Mechanistically, extracellular ATP acted via P2X7R to enhance the transport of extracellular cGAMP into macrophages and subsequent STING activation. Thus, MerTK blockade increases tumor immunogenicity and potentiates anti-tumor immunity, which has implications for cancer immunotherapy.

Status of Immune Oncology: Challenges and Opportunities

This volume is intended to review the methods used to identify biomarkers predictive of cancer responsiveness to immunotherapy. The successful development of clinically actionable biomarkers depends upon three features: (a) their biological role with respect to malignant transformation and tumor progression; (b) the ability to detect them with robust, reliable, and clinically applicable assays; and (c) their prognostic or predictive value, as validated in clinical trials.Identifying biomarkers that have predictive value for patient selection based on the likelihood of benefiting from anticancer immunotherapy is a lengthy and complex process. To date, few predictive biomarkers for anticancer immunotherapy have been robustly analytically and clinically validated (i.e., PD-L1 expression as measured by IHC assays and microsatellite instability (MSI)/dMMR as measured by PCR or IHC, respectively).This introductory chapter to this book focuses on scientific and technical aspects relevant to the identification and validation of predictive biomarkers for immunotherapy. We emphasize that methods should address both the biology of the tumor and the tumor microenvironment. Moreover, the identification of biomarkers requires highly sensitive, multiplexed, comprehensive techniques, especially for application in clinical care. Thus, in this chapter, we will define the outstanding questions related to the immune biology of cancer as a base for development of the biomarkers and assays using diverse methodologies. These biomarkers will likely be identified through research that integrates conventional immunological approaches along with high-throughput genomic and proteomic screening and the host immune response of individual patients that relates to individual tumor biology and immune drugs' mechanism of action.Checkpoint inhibitor therapy (CIT) is by now an accepted modality of cancer treatment. However, immune resistance is common, and most patients do not benefit from the treatment. The reasons for resistance are diverse, and approaches to circumvent it need to consider genetic, biologic, and environmental factors that affect anticancer immune response. Here, we propose to systematically address fundamental concepts based on the premise that malignant cells orchestrate their surroundings by interacting with innate and adaptive immune sensors. This principle applies to most cancers and governs their evolution in the immune-competent host. Understanding the basic requirement(s) for this evolutionary process will guide biomarker discovery and validation and ultimately guide to effective therapeutic choices. This volume will also discuss novel biomarker approaches aimed at informing an effective assay development from a mechanistic point of view, as well as the clinical implementation (i.e., patient enrichment) for immune therapies.

The Paradox of Cancer Immune Exclusion: Immune Oncology Next Frontier

Checkpoint inhibitor therapy (CIT) has revolutionized cancer treatment but it has also reached a standstill when an absent dialog between cancer and immune cells makes it irrelevant. This occurs with high prevalence in the context of "immune silent" and, even perhaps, "immune-excluded" tumors. The latter are characterized by T cells restricted to the periphery of cancer nests. Since in either case T cells do not come in direct contact with most cancer cells, CIT rests immaterial. Adoptive cell therapy (ACT), may also be affected by limited access to antigen-bearing cancer cells. While lack of immunogenicity intuitively explains the immune silent phenotype, immune exclusion is perplexing. The presence of T cells at the periphery suggests that chemo-attraction recruits them and an immunogenic stimulus promotes their persistence. However, what stops the T cells from infiltrating the tumors' nests and reaching the germinal center (GC)? Possibly, a concentric gradient of increased chemo-repulsion or decreased chemo-attraction demarcates an abrupt "do not trespass" warning. Various hypotheses suggest physical or functional barriers but no definitive consensus exists over the weight that each plays in human cancers. On one hand, it could be hypothesized that the intrinsic biology of cancer cells may degenerate from a "cancer stem cell" (CSC)-like phenotype in the GC toward a progressively more immunogenic phenotype prone to immunogenic cell death (ICD) at the periphery. On the other hand, the intrinsic biology of the cancer cells may not change but it is the disorderly architecture of the tumor microenvironment (TME) that alters in a centripetal direction cancer cell metabolism, both directly and indirectly, the function of surrounding stromal cells. In this chapter, we examine whether the paradoxical exclusion of T cells from tumors may serve as a model to understand the requirements for tumor immune infiltration and, correspondingly, we put forth strategies to restore the dialog between immune cells and cancer to enhance the effectiveness of immune oncology (IO) approaches.