A Novel B7-H6-Targeted IgG-Like T Cell-Engaging Antibody for the Treatment of Gastrointestinal Tumors

PURPOSE

Advanced-stage gastrointestinal cancers represent a high unmet need requiring new effective therapies. We investigated the antitumor activity of a novel T cell-engaging antibody (B7-H6/CD3 ITE) targeting B7-H6, a tumor-associated antigen that is expressed in gastrointestinal tumors.

EXPERIMENTAL DESIGN

Membrane proteomics and IHC analysis identified B7-H6 as a tumor-associated antigen in gastrointestinal tumor tissues with no to very little expression in normal tissues. The antitumor activity and mode of action of B7-H6/CD3 ITE was evaluated in in vitro coculture assays, in humanized mouse tumor models, and in colorectal cancer precision cut tumor slice cultures.

RESULTS

B7-H6 expression was detected in 98% of colorectal cancer, 77% of gastric cancer, and 63% of pancreatic cancer tissue samples. B7-H6/CD3 ITE-mediated redirection of T cells toward B7-H6-positive tumor cells resulted in B7-H6-dependent lysis of tumor cells, activation and proliferation of T cells, and cytokine secretion in in vitro coculture assays, and infiltration of T cells into tumor tissues associated with tumor regression in in vivo colorectal cancer models. In primary patient-derived colorectal cancer precision-cut tumor slice cultures, treatment with B7-H6/CD3 ITE elicited cytokine secretion by endogenous tumor-infiltrating immune cells. Combination with anti-PD-1 further enhanced the activity of the B7-H6/CD3 ITE.

CONCLUSION

These data highlight the potential of the B7-H6/CD3 ITE to induce T cell-redirected lysis of tumor cells and recruitment of T cells into noninflamed tumor tissues, leading to antitumor activity in in vitro, in vivo, and human tumor slice cultures, which supports further evaluation in a clinical study.

LY6G6D is a selectively expressed colorectal cancer antigen that can be used for targeting a therapeutic T-cell response by a T-cell engager

Colorectal cancer (CRC) is one of the most common cancers worldwide and demands more effective treatments. We sought to identify tumor selective CRC antigens and their therapeutic potential for cytotoxic T-cell targeting by transcriptomic and immunohistochemical analysis. LY6G6D was identified as a tumor selectively expressed CRC antigen, mainly in the microsatellite stable (MSS) subtype. A specific anti LY6G6D/CD3 T cell engager (TcE) was generated and demonstrated potent tumor cell killing and T cell activation in vitro. Ex vivo treatment of primary patient-derived CRC tumor slice cultures with the LY6G6D/CD3 TcE led to IFNγ secretion in LY6G6D positive tumor samples. In vivo, LY6G6D/CD3 TcE monotherapy demonstrated tumor regressions in pre-clinical mouse models of engrafted human CRC tumor cells and PBMCs. Lastly, 2D and 3D cocultures of LY6G6D positive and negative cells were used to explore the bystander killing of LY6G6D negative cells after specific activation of T cells by LY6G6D positive cells. LY6G6D/CD3 TcE treatment was shown to lyse target negative cells in the vicinity of target positive cells through a combined effect of IFNγ, TNFα and Fas/FasL. In summary, LY6G6D was identified as a selectively expressed CRC antigen that can be utilized to potently re-direct and activate cytotoxic T-cells to lyse LY6G6D expressing CRC using a TcE. This effect can be spread to target negative neighboring tumor cells, potentially leading to improved therapeutic efficacy.

Human papillomavirus infection and lung adenocarcinoma: special benefit is observed in patients treated with immune checkpoint inhibitors

BACKGROUND

Human papilloma virus (HPV) has been associated with the development and modulation of response in a series of neoplasms. In the case of lung adenocarcinoma, its role in etiology and pathogenesis is still controversial. Considering that this infection brings foreign epitopes, it could be of prognostic significance in patients with lung adenocarcinoma treated with immunotherapy.

METHODS

In a retrospective cohort study we evaluated the presence of HPV genomic material in lung adenocarcinoma primary lesions with the INNO-LiPA platform. Viral replication was also evaluated by detecting the presence of oncoprotein E6/E7 messenger RNA (mRNA) by quantitative RT-PCR. To confirm possible hypotheses regarding viral oncogenesis, vascular endothelial growth factor (VEGF) and hypoxia-inducible factor 1 (HIF1) were evaluated with stromal fibrosis and immunoscore.

RESULTS

A total of 133 patients were included in the analysis, of whom 34 tested positive for HPV, reaching an estimated prevalence of 25.6% [95% confidence interval (CI) 18.2% to 32.9%]. E6/7 mRNA was identified in 28 out of the 34 previously positive cases (82.3%). In immune checkpoint inhibitor (ICI)-treated patients, the median overall survival reached 22.3 months [95% CI 19.4 months- not reached (NR)] for HPV-negative and was not reached in HPV-positive (HPV+) ones (95% CI 27.7-NR; P = 0.008). With regard to progression-free survival, HPV- patients reached a median of 9.2 months (95% CI 7.9-11.2 months) compared to 14.3 months (95% CI 13.8-16.4 months) when HPV was positive (P = 0.001). The overall response rate for HPV+ patients yielded 82.4% compared to 47.1% in negative ones. No differences regarding programmed death-ligand 1, VEGF, HIF1, stromal fibrosis, or immunoscore were identified.

CONCLUSIONS

In patients with HPV+ lung adenocarcinoma, a significant benefit in overall response and survival outcomes is observed.

Abstract PR09: ADU-S100 (MIW815) synergizes with checkpoint blockade to elicit an antitumor CD8+ T-cell response to control distal tumors

Stimulator of interferon genes (STING) is a critical component of an innate immune pathway that activates robust antiviral and antitumor responses in mouse models. Activation of the STING pathway by intratumoral (IT) injection of synthetic cyclic dinucleotides (CDNs) is being explored as a cancer therapy and has shown potent antitumor activity in preclinical models. Here we assessed the benefit of combining immune checkpoint blockade with ADU-S100 (MIW815), a CDN under clinical evaluation, in different syngeneic mouse tumor models. In mice bearing dual flank 4T1 mammary carcinoma tumors resistant to anti-PD-1 treatment, adding a single dose of ADU-S100 with anti-PD-1 induced eradication of both injected and noninjected tumors, leading to near-complete responses, demonstrating that ADU-S100 potentiates the activity of checkpoint blockade. Tumor control was CD8+ T cell-dependent and correlated with an enhanced CD8+ T-cell effector profile in both the periphery and in noninjected tumors. Combining a single injection of ADU-S100 with anti-PD-1 also elicited enhanced tumor control in the dual flank MC-38 colon carcinoma model compared to ADU-S100 or anti-PD-1 treatment alone. Those mice cured by combination treatment were also protected from tumor rechallenge. Moreover, in the poorly immunogenic B16.F10 model, adding ADU-S100 to the ineffective combination therapy of anti-PD-1 and anti-CTLA-4 induced tumor-specific CD8+ T-cell responses and tumor control, leading to multiple complete responses and durable immunity in surviving animals. Together, these results highlight the immune correlates of STING-mediated antitumor efficacy and illustrate the potential of combining ADU-S100 with checkpoint inhibitors for the treatment of human cancer. Clinical trials of ADU-S100 in combination with anti-PD-1 or with anti-CTLA-4 are ongoing and could further elucidate the immunologic mechanism of action and therapeutic effect in humans.

This abstract is also being presented as Poster B47.

Citation Format: Weiwen Deng, Anthony L. Desbien, Kelsey Sivick Gauthier, Gabrielle Reiner, Leticia Corrales, Tamara Schroeder, Laura Hix Glickman, Natalie H. Surh, Brian Francica, Justin J. Leong, Ken Metchette, Lianxing Zheng, Charles Cho, Yan Feng, Jeffery M. McKenna, Steven L. Bender, Chudi Ndubaku, Meredith L. Leong, Andrea van Elsas, Sarah M. McWhirter. ADU-S100 (MIW815) synergizes with checkpoint blockade to elicit an antitumor CD8+ T-cell response to control distal tumors [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr PR09.

Blockade of the Complement C5a/C5aR1 Axis Impairs Lung Cancer Bone Metastasis by CXCL16-mediated Effects

RATIONALE

C5aR1 (CD88), a receptor for complement anaphylatoxin C5a, is a potent immune mediator. Its impact on malignant growth and dissemination of non-small cell lung cancer cells is poorly understood.

OBJECTIVES

To investigate the contribution of the C5a/C5aR1 axis to the malignant phenotype of non-small cell lung cancer cells, particularly in skeletal colonization, a preferential lung metastasis site.

METHODS

Association between C5aR1 expression and clinical outcome was assessed in silico and validated by immunohistochemistry. Functional significance was evaluated by lentiviral gene silencing and ligand l-aptamer inhibition in in vivo models of lung cancer bone metastasis. In vitro functional assays for signaling, migration, invasion, metalloprotease activity, and osteoclastogenesis were also performed.

MEASUREMENTS AND MAIN RESULTS

High levels of C5aR1 in human lung tumors were significantly associated with shorter recurrence-free survival, overall survival, and bone metastasis. Silencing of C5aR1 in lung cancer cells led to a substantial reduction in skeletal metastatic burden and osteolysis in in vivo models. Furthermore, metalloproteolytic, migratory, and invasive tumor cell activities were modulated in vitro by C5aR1 stimulation or gene silencing. l-Aptamer blockade or C5aR1 silencing significantly reduced the osseous metastatic activity of lung cancer cells in vivo. This effect was associated with decreased osteoclastogenic activity in vitro and was rescued by the exogenous addition of the chemokine CXCL16.

CONCLUSIONS

Disruption of C5aR1 signaling in lung cancer cells abrogates their tumor-associated osteoclastogenic activity, impairing osseous colonization. This study unveils the role played by the C5a/C5aR1 axis in lung cancer dissemination and supports its potential use as a novel therapeutic target.

Abstract 631: Intratumoral activation of STING with a synthetic cyclic dinucleotide elicits antitumor CD8 T-cell immunity that effectively combines with checkpoint inhibitors

Activation of the STING pathway by intratumoral (IT) injection of synthetic cyclic dinucleotides (CDNs) induces stable tumor regression in preclinical models, yet the underlying immune correlates are not fully understood. ADU-S100, a CDN under clinical evaluation, was administered IT with an optimized dosing regimen to explore the immune requirements for antitumor efficacy in mouse syngeneic tumor models. We show that CD8+ T cells are necessary and sufficient for durable antitumor immunity elicited by ADU-S100 and that activation of STING in hematopoietic cells mediates CD8+ T cell induction. Both type I IFN and TNFα, which are induced by STING pathway activation, influence the antitumor immune response. The combination of ADU-S100 and anti-PD1 treatment enhances CD8+ T cell-dependent, noninjected tumor control that correlates with an enhanced effector profile of CD8+ T cells in the tumor. Combination of ADU-S100 with checkpoint inhibition also enhances durable immunity in a poorly immunogenic tumor model. Together, these results elucidate the immune correlates to STING-mediated antitumor efficacy and highlight the potential of combining STING agonists with checkpoint inhibition in the clinic.

Citation Format: Anthony L. Desbien, Kelsey Sivick Gauthier, Leticia Corrales, Gabrielle Reiner, Laura Hix Glickman, George Katibah, Thomas E. Hudson, Uyen Vu, Natalie H. Surh, Brian Francica, Weiwen Deng, David B. Kanne, Justin J. Leong, Chudi Ndubaku, Ken Metchette, Jeffery M. McKenna, Steven L. Bender, Meredith L. Leong, Thomas W. Dubensky Jr., Andrea van Elsas, Sarah M. McWhirter. Intratumoral activation of STING with a synthetic cyclic dinucleotide elicits antitumor CD8 T-cell immunity that effectively combines with checkpoint inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 631.

Cancer Immunotherapy Targets Based on Understanding the T Cell-Inflamed Versus Non-T Cell-Inflamed Tumor Microenvironment

Most cancers express tumor antigens that can be recognized by T cells of the host. The fact that cancers become clinically evident nonetheless implies that immune escape must occur. Two major subsets of human melanoma metastases have been identified based on gene expression profiling. One subgroup has a T cell-inflamed phenotype that includes expression of chemokines, T cell markers, and a type I IFN signature. In contrast, the other major subset lacks this phenotype and has been designated as non-T cell-inflamed. The mechanisms of immune escape are likely distinct in these two phenotypes, and therefore the optimal immunotherapeutic interventions necessary to promote clinical responses may be different. The T cell-inflamed tumor microenvironment subset shows the highest expression of negative regulatory factors, including PD-L1, IDO, FoxP3⁺ Tregs, and evidence for T cell-intrinsic anergy. Therapeutic strategies to overcome these inhibitory mechanisms are being pursued, and anti-PD-1 mAbs have been FDA approved. The presence of multiple inhibitory mechanisms in the same tumor microenvironment argues that combination therapies may be advantageous, several of which are in clinical testing. A new paradigm may be needed to promote de novo inflammation in cases of the non-T cell-infiltrated tumor microenvironment. Natural innate immune sensing of tumors appears to occur via the host STING pathway, type I IFN production, and cross-priming of T cells via CD8α⁺ DCs. New strategies are being developed to engage this pathway therapeutically, such as through STING agonists. The molecular mechanisms that mediate the presence or absence of the T cell-inflamed tumor microenvironment are being elucidated using parallel genomics platforms. The first oncogene pathway identified that mediates immune exclusion is the Wnt/β-catenin pathway, suggesting that new pharmacologic strategies to target this pathway should be developed to restore immune access to the tumor microenvironment.

Innate immune mediators in cancer: between defense and resistance

Chronic inflammation in the tumor microenvironment and evasion of the antitumor effector immune response are two of the emerging hallmarks required for oncogenesis and cancer progression. The innate immune system not only plays a critical role in perpetuating these tumor-promoting hallmarks but also in developing antitumor adaptive immune responses. Thus, understanding the dual role of the innate system in cancer immunology is required for the design of combined immunotherapy strategies able to tackle established tumors. Here, we review recent advances in the understanding of the role of cell populations and soluble components of the innate immune system in cancer, with a focus on complement, the adapter molecule Stimulator of Interferon Genes, natural killer cells, myeloid cells, and B cells.

Innate immune signaling and regulation in cancer immunotherapy

A pre-existing T cell-inflamed tumor microenvironment has prognostic utility and also can be predictive for response to contemporary cancer immunotherapies. The generation of a spontaneous T cell response against tumor-associated antigens depends on innate immune activation, which drives type I interferon (IFN) production. Recent work has revealed a major role for the STING pathway of cytosolic DNA sensing in this process. This cascade of events contributes to the activation of Batf3-lineage dendritic cells (DCs), which appear to be central to anti-tumor immunity. Non-T cell-inflamed tumors lack chemokines for Batf3 DC recruitment, have few Batf3 DCs, and lack a type I IFN gene signature, suggesting that failed innate immune activation may be the ultimate cause for lack of spontaneous T cell activation and accumulation. With this information in hand, new strategies for triggering innate immune activation and Batf3 DC recruitment are being developed, including novel STING agonists for de novo immune priming. Ultimately, the successful development of effective innate immune activators should expand the fraction of patients that can respond to immunotherapies, such as with checkpoint blockade antibodies.

Abstract B020: STING activation in the tumor microenvironment using a synthetic human STING-activating cyclic dinucleotide induces potent antitumor immunity

Stimulator of Interferon Genes (STING) is a critical component of the cytosolic DNA sensing pathway of the innate immune system. STING is activated by cyclic dinucleotides (CDNs), a product of the intracellular enzyme, cyclic GMP-AMP synthase (cGAS), in response to presence of cytosolic DNA, including tumor-derived DNA. Production of type I interferon within the tumor microenvironment (TME), mediated by the STING pathway, leads to the priming and activation of systemic tumor antigen-specific CD8+ T-cell immunity and tumor regression. Therapeutic activation of STING through intratumoral (IT) administration of CDNs results in anti-tumor efficacy and long-lived survival in several mouse syngeneic tumor models. Rational design of synthetic CDN derivatives has shown that certain modifications alter STING binding, increase cellular potency, enhance maturation of human dendritic cells to promote in vitro T cell expansion, and are able to broadly activate all human STING haplotypes. Mechanistic studies in mouse tumor models demonstrate that CDNs mediate anti-tumor immunity by inducing an acute innate immune response, leading to collapse of the injected tumor, and promoting a tumor-specific CD8+ T cell response that protects against tumor re-challenge. Anti-tumor efficacy is enhanced by combination with immune checkpoint inhibitors, informing future clinical development. The ability to elicit innate and adaptive anti-tumor immunity via activation of STING in the TME demonstrates that CDNs have high translational potential for the treatment of patients with advanced/metastatic solid tumors. The design of an ongoing Phase 1 first-in-human clinical study to evaluate the safety, tolerability and possible antitumor activity of ADU-S100 in subjects with cutaneously accessible tumors and lymphomas will also be presented.

Citation Format: Sarah M. McWhirter, Laura Hix Glickman, Tony Desbien, Kelsey Sivick Gauthier, David Kanne, Shailaja Kasibhatla, Jie Li, AnneMarie Culazzo Pferdekamper, George Katibah, Ed Lemmens, Leticia Corrales, Meredith Leong, Chudi Ndubaku, Justin Leong, Leonard Sung, Lianxing Zheng, Charles Cho, Yan Feng, Jeffery M. McKenna, John A. Tallarico, Steven L. Bender, Thomas W. Dubensky, Jr.. STING activation in the tumor microenvironment using a synthetic human STING-activating cyclic dinucleotide induces potent antitumor immunity [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B020.

Abstract SY39-02: Direct activation of STING in the tumor microenvironment leads to potent and systemic tumor regression and immunity

Recent reports have provided the mechanistic insight of how innate immune activation promotes priming of anti-tumor immunity and inform the development of clinical approaches to facilitate this process. Spontaneous T cell infiltration of melanoma lesions in humans is correlated with a type I interferon (IFN) transcriptional profile in the tumor microenvironment (TME) and infiltration of lymphocytes, indicative of ongoing innate immune recognition within the tumor. Substantial evidence indicates that tumor infiltrating lymphocytes (TILs), including activated CD8+ T cells, is predictive of a positive clinical outcome in response to several immunotherapy strategies. Similarly, in mice bearing melanoma, there is a correlation between expression of IFN-β by tumor-resident dendritic cells (DCs), and spontaneous priming of tumor-specific immunity. Induction of IFN-β expression and co-regulated IFN-responsive genes and pro-inflammatory chemokines is dependent upon activation of the STING (Stimulator of Interferon Genes) pathway, mediated through sensing of tumor dsDNA in TME-resident CD8α+ DCs by cyclic GMP-AMP (cGAMP) synthase (cGAS), which in turn synthesizes cGAMP. The cyclic dinucleotide (CDN) cGAMP produced by cGAS is the natural STING agonist ligand. Thus, the cGAS-STING signaling axis has emerged as a central node for sensing damage in the host. We hypothesized that direct activation of the STING pathway in the TME by intratumoral (IT) injection of specific CDNs would be an effective therapeutic strategy to promote broad tumor-initiated T cell priming against an individual's tumor antigen repertoire. There are five variant human STING alleles that exist at varying frequencies. While the natural STING ligand cGAMP activates signaling in all variants, other natural CDNs, including those produced by bacteria, have structural differences and are unable to activate particular STING variants, such as the REF (R232H) allele, informing the development of compounds that activate all human STING alleles. We sought to develop synthetic CDN compounds with increased activity in human cells as well as the ability to engage all known polymorphic human STING molecules. Using human 293T cell lines engineered to express the various STING proteins, we screened a large panel of CDN derivatives that varied in purine nucleotide base, structure of the phosphate bridge linkage, and substitution of the non-bridging oxygen atoms at the phosphate bridge with sulfur atoms. ADU-S100 is composed of two adenosine monophosphate (AMP) analogues cyclized via a 2’-5’ (non-canonical) and a 3’-5’ (canonical) phosphodiester bond, and was selected for clinical translation based on properties of enhanced cellular uptake, human STING activation, stability and anti-tumor efficacy, as compared to bacterial and mammalian derived CDNs. Induced cytokine expression from a panel of donor human peripheral blood mononuclear cells (PBMCs) expressing a variety of STING haplotypes, including donors with a homozygous haplotype for the refractory human REF allele, indicates that ADU-S100 activates STING across a diverse human population. Direct engagement of STING through IT administration of ADU-S100 results in effective anti-tumor therapy and long-term survival in various mouse syngeneic tumor models. IT injection of ADU-S100 also generates substantial systemic immune responses capable of rejecting distant metastases and provided long-lived immunologic memory.

A bell-shaped ADU-S100 dose response curve (which varied based on tumor model) delineated regression of injected tumor, induction of tumor-specific CD8+ T cell immunity, and regression of distal non-injected tumors (abscopal effect), and/or protection against autologous tumor challenge. At low dose levels, regression of the treated tumor was suboptimal. At optimal doses, regression of the treated and distal untreated tumors, or protection against tumor re-challenge was observed, and correlated with induction of a robust tumor Ag-specific CD8+ T cell response. At higher dose levels there was a loss of protection against tumor re-challenge which correlated with increases in acute systemic cytokines and reduction in CD8+ T cell responses even though growth of the treated tumor was inhibited. These results suggest that the mechanism of ADU-S100-induced tumor regression is due to both an acute pro-inflammatory cytokine response and also tumor-specific CD8+ T cell immunity. The local anti-tumor effect without systemic immunity is consistent with well-established data in which excessive innate immune stimulation and induction of pro-inflammatory cytokines such as TNF-α are known to inhibit both priming of CD8+ T cell immunity and establishment of a stable and self-renewing memory CD8+ T cell population. In addition, anti-tumor efficacy was enhanced by combination with immune checkpoint inhibitors, for example α-PD1, informing future clinical development. By virtue of the ability to elicit innate and T cell-mediated anti-tumor immunity in the TME, these results demonstrate that CDNs have high translational potential for the treatment of patients with advanced/metastatic solid tumors. A Phase 1 clinical study to evaluate the safety and tolerability and possible anti-tumor effects in subjects with cutaneously accessible non UV-induced and UV-induced malignancies or lymphomas given repeated IT doses of ADU-S100 is planned.

Citation Format: Laura Hix Glickman, Leticia Corrales, David B. Kanne, Shailaja Kasibhatla, Jie Li, Anne Marie Culazzo Pferdekamper, Kelsey Sivick Gauthier, George E. Katibah, Justin J. Leong, Leonard Sung, Ken Metchette, Weiwen Deng, Anthony L. Desbien, Chudi Ndubaku, Lianxing Zheng, Charles Cho, Yan Feng, Jeffery M. McKenna, John A. Tallarico, Steven L. Bender, Sarah M. McWhirter, Thomas F. Gajewski, Thomas W. Dubensky. Direct activation of STING in the tumor microenvironment leads to potent and systemic tumor regression and immunity. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr SY39-02.

The host STING pathway at the interface of cancer and immunity

A major subset of human cancers shows evidence for spontaneous adaptive immunity, which is reflected by the presence of infiltrating CD8+ T cells specific for tumor antigens within the tumor microenvironment. This observation has raised the question of which innate immune sensing pathway might detect the presence of cancer and lead to a natural adaptive antitumor immune response in the absence of exogenous infectious pathogens. Evidence for a critical functional role for type I IFNs led to interrogation of candidate innate immune sensing pathways that might be triggered by tumor presence and induce type I IFN production. Such analyses have revealed a major role for the stimulator of IFN genes pathway (STING pathway), which senses cytosolic tumor-derived DNA within the cytosol of tumor-infiltrating DCs. Activation of this pathway is correlated with IFN-β production and induction of antitumor T cells. Based on the biology of this natural immune response, pharmacologic agonists of the STING pathway are being developed to augment and optimize STING activation as a cancer therapy. Intratumoral administration of STING agonists results in remarkable therapeutic activity in mouse models, and STING agonists are being carried forward into phase I clinical testing.

STING Pathway Activation Stimulates Potent Immunity against Acute Myeloid Leukemia

Type I interferon (IFN), essential for spontaneous T cell priming against solid tumors, is generated through recognition of tumor DNA by STING. Interestingly, we observe that type I IFN is not elicited in animals with disseminated acute myeloid leukemia (AML). Further, survival of leukemia-bearing animals is not diminished in the absence of type I IFN signaling, suggesting that STING may not be triggered by AML. However, the STING agonist, DMXAA, induces expression of IFN-β and other inflammatory cytokines, promotes dendritic cell (DC) maturation, and results in the striking expansion of leukemia-specific T cells. Systemic DMXAA administration significantly extends survival in two AML models. The therapeutic effect of DMXAA is only partially dependent on host type I IFN signaling, suggesting that other cytokines are important. A synthetic cyclic dinucleotide that also activates human STING provided a similar anti-leukemic effect. These data demonstrate that STING is a promising immunotherapeutic target in AML.

Antagonism of the STING Pathway via Activation of the AIM2 Inflammasome by Intracellular DNA

Recent evidence has indicated that innate immune sensing of cytosolic DNA in dendritic cells via the host STING pathway is a major mechanism leading to spontaneous T cell responses against tumors. However, the impact of the other major pathway triggered by intracellular DNA, the absent in melanoma 2 (AIM2) inflammasome, on the functional output from the stimulator of IFN genes (STING) pathway is poorly understood. We found that dendritic cells and macrophages deficient in AIM2, apoptosis-associated specklike protein, or caspase-1 produced markedly higher IFN-β in response to DNA. Biochemical analyses showed enhanced generation of cyclic GMP-AMP, STING aggregation, and TANK-binding kinase 1 and IFN regulatory factor 3 phosphorylation in inflammasome-deficient cells. Induction of pyroptosis by the AIM2 inflammasome was a major component of this effect, and inhibition of caspase-1 reduced cell death, augmenting phosphorylation of TANK-binding kinase 1/IFN regulatory factor 3 and production of IFN-β. Our data suggest that in vitro activation of the AIM2 inflammasome in murine macrophages and dendritic cells leads to reduced activation of the STING pathway, in part through promoting caspase-1-dependent cell death.

Tumor and Host Factors Controlling Antitumor Immunity and Efficacy of Cancer Immunotherapy

Despite recent clinical advances in immunotherapy, a fraction of cancer patients fails to respond to these interventions. Evidence from preclinical mouse models as well as clinical samples has provided evidence that the extent of activated T cell infiltration within the tumor microenvironment is associated with clinical response to immunotherapies including checkpoint blockade. Therefore, understanding the molecular mechanisms mediating the lack of T cell infiltration into the tumor microenvironment will be instrumental for the development of new therapeutic strategies to render those patients immunotherapy responsive. Recent data have suggested that major sources of intersubject heterogeneity include differences in somatic mutations in specific oncogene pathways between cancers of individual subjects and also environmental factors including commensal microbial composition. Successful identification of such causal factors should lead to new therapeutic approaches that may facilitate T cell entry into noninflamed tumors and expand the fraction of patients capable of responding to novel immunotherapies.

Abstract A173: Inhibition of the STING pathway by the AIM2 inflammasome

Recognition of tumor-derived DNA by the cGAS/STING pathway in the cytosol of antigen-presenting cells leads to production of type I IFN, facilitating adaptive immunity against tumors. But cytosolic DNA is also recognized by AIM2, leading to the assembly of the AIM2 inflammasome and the production of mature IL-1 and IL-18. As stimulation of the STING pathway by endogenous tumor-derived DNA or exogenous therapeutic agonists have shown to trigger an immune response against tumors, a key question was whether the AIM2 inflammasome would have a positive or negative regulatory role, given the sharing of the same ligand by both pathways. In vivo, sorted CD45+ cells from implanted B16 tumors in AIM2-/- animals showed a higher IFN-β expression than the same cells sorted from WT mice, suggesting that AIM2 plays a negative role in the induction of type I IFN in vivo. Experiments in vitro confirmed that macrophages and dendritic cells from AIM2-, ASC-, or caspase-1-deficient mice produced substantially higher IFN-β in response to tumor-derived DNA. Biochemical analysis revealed that all steps in the STING pathway were enhanced in the inflammasome deficient cells: generation of cGAMP, aggregation of STING, and phosphorylation of TBK1 and IRF3. Studies investigating the biological mechanism by which activation of the AIM2 inflammasome inhibits the STING pathway revealed that pyroptosis induced by activation of the AIM2 inflammasome was critically involved. Pre-incubation with a caspase-1 inhibitor abolished cell death, and recovered a significant proportion of phosphorylation of TBK1/IRF3 and production of IFN-β. Our data suggest that the activation of AIM2 inflammasome inhibits the STING pathway mainly by cell death derived from the activation of the inflammasome. This crosstalk may have potential consequences for anti-tumor immunity which should be explored in vivo.

Citation Format: Leticia Corrales, Seng-Ryong Woo, Thomas F. Gajewski. Inhibition of the STING pathway by the AIM2 inflammasome. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A173.

Commensal Bifidobacterium promotes antitumor immunity and facilitates anti-PD-L1 efficacy

T cell infiltration of solid tumors is associated with favorable patient outcomes, yet the mechanisms underlying variable immune responses between individuals are not well understood. One possible modulator could be the intestinal microbiota. We compared melanoma growth in mice harboring distinct commensal microbiota and observed differences in spontaneous antitumor immunity, which were eliminated upon cohousing or after fecal transfer. Sequencing of the 16S ribosomal RNA identified Bifidobacterium as associated with the antitumor effects. Oral administration of Bifidobacterium alone improved tumor control to the same degree as programmed cell death protein 1 ligand 1 (PD-L1)-specific antibody therapy (checkpoint blockade), and combination treatment nearly abolished tumor outgrowth. Augmented dendritic cell function leading to enhanced CD8(+) T cell priming and accumulation in the tumor microenvironment mediated the effect. Our data suggest that manipulating the microbiota may modulate cancer immunotherapy.

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(2',5')pA(3',5')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 individual's 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.

Molecular Pathways: Targeting the Stimulator of Interferon Genes (STING) in the Immunotherapy of Cancer

Novel immunotherapy approaches are transforming the treatment of cancer, yet many patients remain refractory to these agents. One hypothesis is that immunotherapy fails because of a tumor microenvironment that fails to support recruitment of immune cells, including CD8(+) T cells. Therefore, new approaches designed to initiate a de novo antitumor immune response from within the tumor microenvironment are being pursued. Recent evidence has indicated that spontaneous activation of the Stimulator of Interferon Genes (STING) pathway within tumor-resident dendritic cells leads to type I IFN production and adaptive immune responses against tumors. This pathway is activated in the presence of cytosolic DNA that is detected by the sensor cyclic GMP-AMP synthase (cGAS) and generates cyclic GMP-AMP (cGAMP), which binds and activates STING. As a therapeutic approach, intratumoral injection of STING agonists has demonstrated profound therapeutic effects in multiple mouse tumor models, including melanoma, colon, breast, prostate, and fibrosarcoma. Better characterization of the STING pathway in human tumor recognition, and the development of new pharmacologic approaches to engage this pathway within the tumor microenvironment in patients, are important areas for clinical translation.

Abstract A10: Activation of the STING pathway enhances immunity and improves survival in a murine myeloid leukemia model

Type I interferon (IFN) production by innate immune cells is critical to prime spontaneous T cell responses against solid tumors. Until recently, the tumor-derived signals which stimulate host type I IFN production have remained elusive. However, emerging data suggest that tumor-derived DNA is sufficient to induce IFN-β production in responding cells through activation of a cytosolic DNA sensing receptor called STING (Stimulator of Interferon Genes). STING-mediated IFN-β production, in turn, leads to functional priming of antigen-specific T cells which mediate tumor rejection in pre-clinical solid tumor models. However, because of their disseminated growth pattern, hematological cancers, such as acute leukemia, may not release sufficient quantities of DNA upon apoptosis to activate the STING pathway in innate immune cells, such as dendritic cells (DCs). The failure of acute leukemias to induce a type I IFN response may contribute to their ability to generate T cell tolerance in the host. Thus, we hypothesized that activating the STING pathway would promote a type I IFN response sufficient to facilitate DC-mediated priming of leukemia-specific T cells and to prolong survival of mice with acute myeloid leukemia (AML). Murine C1498 AML cells engineered to express a model peptide antigen called SIY (C1498.SIY) were inoculated intravenously into syngeneic C57BL/6 mice which were then treated with a selective murine STING agonist, DMXAA (5,6-dimethylxanthenone-4-acetic acid), or vehicle control. Preliminary experiments confirmed that DMXAA administration resulted in robust IFN-β production by C57BL/6 spleen cells (90-fold induction over vehicle control-treated spleen cells). STING pathway activation via DMXAA treatment of C1498.SIY-bearing animals resulted in a massive expansion (10-fold over vehicle control-treated animals) of endogenous SIY antigen-specific T cells as determined by SIY/Kb pentamer analysis. Intracellular cytokine staining of SIY-peptide restimulated spleen cells from DMXAA-treated, leukemia-bearing animals, revealed that the expanded SIY-specific CD8+ T cells were functionally active, and produced high-levels of IFN-γ. To determine if this enhanced immune response translated to a benefit in survival, C1498.SIY AML cell-challenged mice were treated with a single dose of DMXAA or vehicle control five days after tumor inoculation. Among mice treated with DMXAA, 80% of mice rejected the leukemia and survived long-term, while control-treated mice succumbed to AML within approximately 3 weeks, as expected. DMXAA-treated mice surviving a primary C1498.SIY cell challenge also rejected a secondary challenge with parental C1498 cells (SIY-negative), suggesting the DMXAA treatment led to potent immunological memory against native C1498-expressed antigens. DMXAA treatment of mice following a systemic challenge with parental C1498 cells also led to enhanced survival compared to control-treated animals. This effect was T and or B cell-dependent, as DMXAA treatment of RAG-/- mice had no effect on survival. Collectively, these data suggest that DMXAA-mediated activation of the STING pathway upstream of type I IFN potently enhances adaptive immunity to AML which culminates in prolonged survival or even disease cure in treated animals. It will be important to determine whether the STING pathway may be successfully manipulated in other murine leukemia models. If so, then STING may be a promising therapeutic target to enhance endogenous immunity in AML patients.

Citation Format: Emily K. Curran, Xiufen Chen, Leticia Corrales, Justin Kline. Activation of the STING pathway enhances immunity and improves survival in a murine myeloid leukemia model. [abstract]. In: Proceedings of the AACR Special Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(17 Suppl):Abstract nr A10.

Direct Activation of STING in the Tumor Microenvironment Leads to Potent and Systemic Tumor Regression and Immunity

Spontaneous tumor-initiated T cell priming is dependent on IFN-β production by tumor-resident dendritic cells. On the basis of recent observations indicating that IFN-β expression was dependent upon activation of the host STING pathway, we hypothesized that direct engagement of STING through intratumoral (IT) administration of specific agonists would result in effective anti-tumor therapy. After proof-of-principle studies using the mouse STING agonist DMXAA showed a potent therapeutic effect, we generated synthetic cyclic dinucleotide (CDN) derivatives that activated all human STING alleles as well as murine STING. IT injection of STING agonists induced profound regression of established tumors in mice and generated substantial systemic immune responses capable of rejecting distant metastases and providing long-lived immunologic memory. Synthetic CDNs have high translational potential as a cancer therapeutic.