Jason R. Baird

Radiotherapy Combined with Novel STING-Targeting Oligonucleotides Results in Regression of Established Tumors

Cytotoxic therapies prime adaptive immune responses to cancer by stimulating the release of tumor-associated antigens. However, the tumor microenvironment into which these antigens are released is typically immunosuppressed, blunting the ability to initiate immune responses. Recently, activation of the DNA sensor molecule STING by cyclic dinucleotides was shown to stimulate infection-related inflammatory pathways in tumors. In this study, we report that the inflammatory pathways activated by STING ligands generate a powerful adjuvant activity for enhancing adaptive immune responses to tumor antigens released by radiotherapy. In a murine model of pancreatic cancer, we showed that combining CT-guided radiotherapy with a novel ligand of murine and human STING could synergize to control local and distant tumors. Mechanistic investigations revealed T-cell-independent and TNFα-dependent hemorrhagic necrosis at early times, followed by later CD8 T-cell-dependent control of residual disease. Clinically, STING was found to be expressed extensively in human pancreatic tumor and stromal cells. Our findings suggest that this novel STING ligand could offer a potent adjuvant for leveraging radiotherapeutic management of pancreatic cancer.

Optimizing Timing of Immunotherapy Improves Control of Tumors by Hypofractionated Radiation Therapy

The anecdotal reports of promising results seen with immunotherapy and radiation in advanced malignancies have prompted several trials combining immunotherapy and radiation. However, the ideal timing of immunotherapy with radiation has not been clarified. Tumor bearing mice were treated with 20Gy radiation delivered only to the tumor combined with either anti-CTLA4 antibody or anti-OX40 agonist antibody. Immunotherapy was delivered at a single timepoint around radiation. Surprisingly, the optimal timing of these therapies varied. Anti-CTLA4 was most effective when given prior to radiation therapy, in part due to regulatory T cell depletion. Administration of anti-OX40 agonist antibody was optimal when delivered one day following radiation during the post-radiation window of increased antigen presentation. Combination treatment of anti-CTLA4, radiation, and anti-OX40 using the ideal timing in a transplanted spontaneous mammary tumor model demonstrated tumor cures. These data demonstrate that the combination of immunotherapy and radiation results in improved therapeutic efficacy, and that the ideal timing of administration with radiation is dependent on the mechanism of action of the immunotherapy utilized.

Expression of Arginase I in Myeloid Cells Limits Control of Residual Disease after Radiation Therapy of Tumors in Mice

An accumulating body of evidence demonstrates that radiation therapy can generate adaptive immune responses that contribute to tumor control. However, in the absence of additional immune therapy, the adaptive immune response is insufficient to prevent tumor recurrence or affect distant disease. It has been shown in multiple models that tumor-infiltrating myeloid cells exhibit alternative activation phenotypes and are able to suppress adaptive immune responses, and recent data suggests that the myeloid response in tumors treated with cytotoxic therapy limits tumor control. We hypothesized that tumor myeloid cells inhibit the adaptive immune response after radiation therapy through expression of the enzyme arginase I. Using a myeloid cell-specific deletion of arginase I in mice, we demonstrate an improved tumor control after radiation therapy. However, tumors still recurred despite the conditional knockdown of arginase I. Since multiple alternative factors may combine to inhibit adaptive immunity, we propose that targeting macrophage differentiation may be a more effective strategy than targeting individual suppressive pathways.

TGFβ inhibition prior to hypofractionated radiation enhances efficacy in preclinical models

Young and colleagues demonstrate in syngeneic mouse models of colorectal and pancreatic cancers that TGFβ inhibition with the oral, small-molecule inhibitor SM16 enhanced adaptive immunity in the tumor microenvironments and significantly improved the efficacy of subsequent radiotherapy. The immune infiltrate in colorectal cancer has been correlated with outcome, such that individuals with higher infiltrations of T cells have increased survival independent of the disease stage. For patients with lower immune infiltrates, overall survival is limited. Because the patients with colorectal cancer studied have received conventional cancer therapies, these data may indicate that the pretreatment tumor environment increases the efficacy of treatments such as chemotherapy, surgery, and radiotherapy. This study was designed to test the hypothesis that an improved immune environment in the tumor at the time of treatment will increase the efficacy of radiotherapy. We demonstrate that inhibition of TGFβ using the orally available small-molecule inhibitor SM16 improved the immune environment of tumors in mice and significantly improved the efficacy of subsequent radiotherapy. This effect was not due to changes in radiosensitivity, epithelial–mesenchymal transition, or changes in vascular function in the tumor; rather, this effect was dependent on adaptive immunity and resulted in long-term protective immunity in cured mice. These data demonstrate that immunotherapy is an option to improve the immune status of patients with poor tumor infiltrates and that pretreatment improves the efficacy of radiotherapy. Cancer Immunol Res; 2(10); 1011–22. ©2014 AACR.

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

Radiation therapy provides a means to kill large numbers of cancer cells in a controlled location resulting in the release of tumor-specific antigens and endogenous adjuvants. However, by activating pathways involved in apoptotic cell recognition and phagocytosis, irradiated cancer cells engender suppressive phenotypes in macrophages. We demonstrate that the macrophage-specific phagocytic receptor, Mertk is upregulated in macrophages in the tumor following radiation therapy. Ligation of Mertk on macrophages results in anti-inflammatory cytokine responses via NF-kB p50 upregulation, which in turn limits tumor control following radiation therapy. We demonstrate that in immunogenic tumors, loss of Mertk is sufficient to permit tumor cure following radiation therapy. However, in poorly immunogenic tumors, TGFb inhibition is also required to result in tumor cure following radiation therapy. These data demonstrate that Mertk is a highly specific target whose absence permits tumor control in combination with radiation therapy.

A Novel Agonist of the TRIF Pathway Induces a Cellular State Refractory to Replication of Zika, Chikungunya, and Dengue Viruses

ABSTRACT The ongoing concurrent outbreaks of Zika, Chikungunya, and dengue viruses in Latin America and the Caribbean highlight the need for development of broad-spectrum antiviral treatments. The type I interferon (IFN) system has evolved in vertebrates to generate tissue responses that actively block replication of multiple known and potentially zoonotic viruses. As such, its control and activation through pharmacological agents may represent a novel therapeutic strategy for simultaneously impairing growth of multiple virus types and rendering host populations resistant to virus spread. In light of this strategy’s potential, we undertook a screen to identify novel interferon-activating small molecules. Here, we describe 1-(2-fluorophenyl)-2-(5-isopropyl-1,3,4-thiadiazol-2-yl)-1,2-dihydrochromeno[2,3-c]pyrrole-3,9-dione, which we termed AV-C. Treatment of human cells with AV-C activates innate and interferon-associated responses that strongly inhibit replication of Zika, Chikungunya, and dengue viruses. By utilizing genome editing, we investigated the host proteins essential to AV-C-induced cellular states. This showed that the compound requires a TRIF-dependent signaling cascade that culminates in IFN regulatory factor 3 (IRF3)-dependent expression and secretion of type I interferon to elicit antiviral responses. The other canonical IRF3-terminal adaptor proteins STING and IPS-1/MAVS were dispensable for AV-C-induced phenotypes. However, our work revealed an important inhibitory role for IPS-1/MAVS, but not TRIF, in flavivirus replication, implying that TRIF-directed viral evasion may not occur. Additionally, we show that in response to AV-C, primary human peripheral blood mononuclear cells secrete proinflammatory cytokines that are linked with establishment of adaptive immunity to viral pathogens. Ultimately, synthetic innate immune activators such as AV-C may serve multiple therapeutic purposes, including direct antimicrobial responses and facilitation of pathogen-directed adaptive immunity. IMPORTANCE The type I interferon system is part of the innate immune response that has evolved in vertebrates as a first line of broad-spectrum immunological defense against an unknowable diversity of microbial, especially viral, pathogens. Here, we characterize a novel small molecule that artificially activates this response and in so doing generates a cellular state antagonistic to growth of currently emerging viruses: Zika virus, Chikungunya virus, and dengue virus. We also show that this molecule is capable of eliciting cellular responses that are predictive of establishment of adaptive immunity. As such, this agent may represent a powerful and multipronged therapeutic tool to combat emerging and other viral diseases. IMPORTANCE The type I interferon system is part of the innate immune response that has evolved in vertebrates as a first line of broad-spectrum immunological defense against an unknowable diversity of microbial, especially viral, pathogens. Here, we characterize a novel small molecule that artificially activates this response and in so doing generates a cellular state antagonistic to growth of currently emerging viruses: Zika virus, Chikungunya virus, and dengue virus. We also show that this molecule is capable of eliciting cellular responses that are predictive of establishment of adaptive immunity. As such, this agent may represent a powerful and multipronged therapeutic tool to combat emerging and other viral diseases.

Tumor cure by radiation therapy and checkpoint inhibitors depends on pre-existing immunity

Radiation therapy is a source of tumor antigen release that has the potential to serve as an endogenous tumor vaccination event. In preclinical models radiation therapy synergizes with checkpoint inhibitors to cure tumors via CD8 T cell responses. To evaluate the immune response initiated by radiation therapy, we used a range of approaches to block the pre-existing immune response artifact initiated by tumor implantation. We demonstrate that blocking immune responses at tumor implantation blocks development of a tumor-resident antigen specific T cell population and prevents tumor cure by radiation therapy combined with checkpoint immunotherapy. These data demonstrate that this treatment combination relies on a pre-existing immune response to cure tumors, and may not be a solution for patients without pre-existing immunity.

Amplifying IFN-γ Signaling in Dendritic Cells by CD11c-Specific Loss of SOCS1 Increases Innate Immunity to Infection while Decreasing Adaptive Immunity

Although prophylactic vaccines provide protective humoral immunity against infectious agents, vaccines that elicit potent CD8 T cell responses are valuable tools to shape and drive cellular immunity against cancer and intracellular infection. In particular, IFN-γ–polarized cytotoxic CD8 T cell immunity is considered optimal for protective immunity against intracellular Ags. Suppressor of cytokine signaling (SOCS)1 is a cross-functional negative regulator of TLR and cytokine receptor signaling via degradation of the receptor–signaling complex. We hypothesized that loss of SOCS1 in dendritic cells (DCs) would improve T cell responses by accentuating IFN-γ–directed immune responses. We tested this hypothesis using a recombinant Listeria monocytogenes vaccine platform that targets CD11c+ DCs in mice in which SOCS1 is selectively deleted in all CD11c+ cells. Unexpectedly, in mice lacking SOCS1 expression in CD11c+ cells, we observed a decrease in CD8+ T cell response to the L. monocytogenes vaccine. NK cell responses were also decreased in mice lacking SOCS1 expression in CD11c+ cells but did not explain the defect in CD8+ T cell immunity. We found that DCs lacking SOCS1 expression were functional in driving Ag-specific CD8+ T cell expansion in vitro but that this process was defective following infection in vivo. Instead, monocyte-derived innate TNF-α and inducible NO synthase–producing DCs dominated the antibacterial response. Thus, loss of SOCS1 in CD11c+ cells skewed the balance of immune response to infection by increasing innate responses while decreasing Ag-specific adaptive responses to infectious Ags.

Preparative immunotherapy with anti-OX40 and anti-CTLA4 improves the response to chemotherapy

Meeting abstracts Recent studies have reported that decreased T cell infiltrate alone, or co-ordinate with increased macrophage infiltrate, correlate with decreased survival in a range of cancers, including patients with pancreatic cancer. Importantly, in mouse models of pancreatic cancer,

Abstract IA10: Effective immunotherapy regimens incorporating highly active human STING-activating cyclic dinucleotide derivatives

Innate immune sensing in the tumor microenvironment is a critical step in promoting tumor infiltrating lymphocytes (TILs) and spontaneous anti-tumor T cell priming. Transcriptional profiling analysis of melanoma patients has revealed that tumors with a T cell-inflamed immunophenotype are characterized by a type I IFN (IFN) transcriptional signature. Studies in mice support the notion that IFN produced by tumor-resident dendritic cells (DCs) plays a critical role in spontaneous T cell priming against tumor antigens, which is dependent upon the host Stimulator of Interferon Genes (STING) pathway. STING mediates host innate defense by responding to cytosolic nucleic acids, either through direct binding of cyclic dinucleotides (CDNs) produced by bacteria, or through binding of a structurally distinct CDN produced by host cyclic GMP-AMP synthetase in response to cytosolic double-stranded DNA. While CDN adjuvants have been explored previously in mice, we sought to develop compounds that activate human STING. We therefore synthesized a panel of cyclic dinucleotides (CDNs) that varied by purine nucleotide base, internucleotide phosphate bridge linkage, or by substitution of non-bridging oxygen atoms in the phosphate bridge with sulfur. We screened and selected from among these compounds based on their capacity to activate all known human STING alleles expressed in stably transformed reporter cell lines, stimulate the activation of human PBMCs, and impact significant antitumor efficacy in several mouse tumor models, without significant local or systemic toxicity. Strikingly, direct IT injection of particular CDN derivative molecules into two-week established flank B16 melanoma, CT26 colon, or 4T1 breast tumors profoundly inhibited tumor growth that was durable and correlated with induction of lasting systemic antigen-specific CD8+ T cell immunity that conferred complete protection against tumor re-challenge, or significantly inhibited the growth of distal untreated tumors. Induction of cytokines, tumor antigen-specific immunity, and antitumor efficacy was entirely STING-dependent. We selected dithio-[Rp,Rp]-c[A(29,59)pA(39,59)p], a synthetic CDN molecule that has significantly higher activity than natural STING ligands, as the lead molecule for continued development. We next tested whether direct activation of STING within the tumor microenvironment would enhance the absocopal effect resulting from irradiating the CDN-treated tumor. Treatment of one tumor in mice bearing established bilateral Panc02 flank tumors, with a suboptimal dose of 10 Gy of radiotherapy (RT) in combination with IT CDN injection resulted in rapid local and systemic induction of inflammatory mediators, and vascular damage that spread through the injected tumor without causing detectable damage to normal tissues. Compared to RT alone, CDN injection resulted in significantly enhanced adaptive-immune mediated control of the contralateral tumor. The synthetic CDN molecule described here was significantly more potent than IT TLR ligands, indicating its high translational potential as an approach to elicit effective unbiased T cell priming against an individual9s unique tumor antigen repertoire. Citation Format: Laura Hix Glickman, Leticia Corrales, Sarah M. McWhirter, David B. Kanne, Kelsey E. Sivick, Jason R. Baird, Edward Lemmens, Justin J. Leong, Ken Metchette, Mark Crittenden, Michael Gough, Thomas F. Gajewski, Thomas W. Dubensky, Jr.. Effective immunotherapy regimens incorporating highly active human STING-activating cyclic dinucleotide derivatives. [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy: A New Chapter; December 1-4, 2014; Orlando, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2015;3(10 Suppl):Abstract nr IA10.