Context-Dependent Immunomodulatory Effects of MEK Inhibition are Enhanced with T-cell Agonist Therapy

MEK inhibition (MEKi) is proposed to enhance antitumor immunity but has demonstrated mixed results as an immunomodulatory strategy in human clinical trials. MEKi exerts direct immunomodulatory effects on tumor cells and tumor-infiltrating lymphocytes, but these effects have not been independently investigated. Here we modeled tumor-specific MEKi through CRISPR/Cas-mediated genome editing of tumor cells (MEK1 KO) and pharmacologic MEKi with cobimetinib in a RAS-driven model of colorectal cancer. This approach allowed us to distinguish tumor-mediated and tumor-independent mechanisms of MEKi immunomodulation. MEK1 KO tumors demonstrated upregulation of JAK/STAT signaling; enhanced MHCI expression, CD8+ T-cell infiltration and T-cell activation; and impaired tumor growth that is immune-dependent. Pharmacologic MEKi recapitulated tumor-intrinsic effects but simultaneously impaired T-cell activation in the tumor microenvironment. We confirmed a reduction in human peripheral lymphocyte activation from a clinical trial of anti-PD-L1 (atezolizumab) with or without cobimetinib in biliary tract cancers. Impaired activation of tumor-infiltrating lymphocytes treated with pharmacologic MEKi was reversible and was rescued with the addition of a 41BB agonist. Collectively, these data underscore the ability of MEKi to induce context-dependent immunomodulatory effects and suggest that T cell-agonist therapy maximizes the beneficial effects of MEKi on the antitumor immune response.

Abstract 2730: Breast pulmonary metastases and associated myeloid derived suppressor cells are resistant to the effects of entinostat with checkpoint inhibitor in a murine tumor model

Introduction: Immune checkpoint inhibitors (ICIs) are an effective strategy to engage the adaptive arm of the immune system in several solid cancers. While approved for metastatic triple negative breast cancer and breast cancer with microsatellite instability or mismatch repair deficiencies, ICIs are still under investigation in HER2+ breast cancer. In preclinical studies, the effects of ICIs are dampened by immunosuppressive cells, such as myeloid derived suppressor cells (MDSC), in the breast tumor microenvironment (TME). Previous studies in mice showed that addition of entinostat, a histone deacetylase inhibitor, to ICIs improved survival and reduced immunosuppression in an early HER2+ breast tumor model. Here, we evaluated the addition of entinostat to ICIs in a mouse model of metastatic HER2+ breast cancer. We hypothesize that the different immune cell composition within the TMEs of breast and lung metastases will affect mechanisms of response to this treatment combination.

Methods: mmTV-NeuN transgenic mice (NeuN) were challenged with syngeneic NT2.5-LM cells that spontaneously metastasize to the lungs. Mice were treated with different combinations of entinostat, anti-CTLA-4, anti-PD-1, and anti-HER2 for 3 weeks for analysis of survival and metastatic tumor burden. For analyses of tumor-infiltrating immune cells in pulmonary metastases, flow cytometry of dissociated lungs was done 6 weeks after tumor injection.

Results: Unlike the previously published model of early-stage HER2+ breast cancer, combinations of entinostat and ICIs did not improve survival in NT2.5-LM bearing mice. Anti-HER2 therapy was the only agent to improve survival, and its effect was hindered by the addition of entinostat and ICIs. The numbers of pulmonary metatastases among treatment groups were not significantly different. Although the combination of entinostat and ICIs increased the percentage of cytotoxic CD8 T cells in the lungs, it also increased the percentage of the more suppressive monocytic-MDSCs and decreased the percentage of less suppressive granulocytic-MDSCs. Whereas entinostat decreased suppressive activity of MDSCs from primary tumors, entinostat did not significantly change functional markers of MDSCs in the lung.

Conclusion: Entinostat and ICIs did not reduce breast metastases in the lung, nor did their combination improve survival. Previous studies showed that entinostat and ICIs reduced MDSC suppressive function within the TME. Reduced suppression was not observed in lung metastases. Investigations are underway to define mechanisms responsible for the differential effect of entinostat on MDSC phenotype in primary tumors but not in the metastatic niche.

Citation Format: Julie K. Jang, Christine Rafie, Sofi Castanon, Brian J. Christmas, Kayla A. Cruz, Skylar Woolman, Evanthia T. Roussos Torres. Breast pulmonary metastases and associated myeloid derived suppressor cells are resistant to the effects of entinostat with checkpoint inhibitor in a murine tumor model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2730.

Multi-omic profiling of lung and liver tumor microenvironments of metastatic pancreatic cancer reveals site-specific immune regulatory pathways

BACKGROUND

The majority of pancreatic ductal adenocarcinomas (PDAC) are diagnosed at the metastatic stage, and standard therapies have limited activity with a dismal 5-year survival rate of only 8%. The liver and lung are the most common sites of PDAC metastasis, and each have been differentially associated with prognoses and responses to systemic therapies. A deeper understanding of the molecular and cellular landscape within the tumor microenvironment (TME) metastasis at these different sites is critical to informing future therapeutic strategies against metastatic PDAC.

RESULTS

By leveraging combined mass cytometry, immunohistochemistry, and RNA sequencing, we identify key regulatory pathways that distinguish the liver and lung TMEs in a preclinical mouse model of metastatic PDAC. We demonstrate that the lung TME generally exhibits higher levels of immune infiltration, immune activation, and pro-immune signaling pathways, whereas multiple immune-suppressive pathways are emphasized in the liver TME. We then perform further validation of these preclinical findings in paired human lung and liver metastatic samples using immunohistochemistry from PDAC rapid autopsy specimens. Finally, in silico validation with transfer learning between our mouse model and TCGA datasets further demonstrates that many of the site-associated features are detectable even in the context of different primary tumors.

CONCLUSIONS

Determining the distinctive immune-suppressive features in multiple liver and lung TME datasets provides further insight into the tissue specificity of molecular and cellular pathways, suggesting a potential mechanism underlying the discordant clinical responses that are often observed in metastatic diseases.

Effects of B cell-activating factor on tumor immunity

Immunotherapies that modulate T cell function have been firmly established as a pillar of cancer therapy, whereas the potential for B cells in the antitumor immune response is less established. B cell-activating factor (BAFF) is a B cell-activating cytokine belonging to the TNF ligand family that has been associated with autoimmunity, but little is known about its effects on cancer immunity. We find that BAFF upregulates multiple B cell costimulatory molecules; augments IL-12a expression, consistent with Be-1 lineage commitment; and enhances B cell antigen-presentation to CD4+ Th cells in vitro. In a syngeneic mouse model of melanoma, BAFF upregulates B cell CD40 and PD-L1 expression; it also modulates T cell function through increased T cell activation and TH1 polarization, enhanced expression of the proinflammatory leukocyte trafficking chemokine CCR6, and promotion of a memory phenotype, leading to enhanced antitumor immunity. Similarly, adjuvant BAFF promotes a memory phenotype of T cells in vaccine-draining lymph nodes and augments the antitumor efficacy of whole cell vaccines. BAFF also has distinct immunoregulatory functions, promoting the expansion of CD4+Foxp3+ Tregs in the spleen and tumor microenvironment (TME). Human melanoma data from The Cancer Genome Atlas (TCGA) demonstrate that BAFF expression is positively associated with overall survival and a TH1/IFN-γ gene signature. These data support a potential role for BAFF signaling as a cancer immunotherapy.

Abstract 2352: Epigenetic modulation— unlocking the potential of checkpoint inhibition in advanced breast cancer

Immune checkpoint inhibition (ICI) has revolutionized treatment in immunogenic cancers by enabling infiltration of T cells into the tumor microenvironment (TME) and promoting cytotoxic signaling pathways. Tumors with complex immunosuppressive TME’s such as breast cancer present unique therapeutic obstacles as response rates to ICI remain low. Such tumors often recruit myeloid-derived suppressor cells (MDSCs) whose functioning prohibit both T cell activation and infiltration. Our current work aims to uncover the efficacy of ICI in advanced HER2 positive (HER2+) disease and to enhance response rates to these promising therapies by altering the metastatic TME epigenetically. Using a HER-2/neu transgenic mouse model, we syngeneically tumor challenge the NT2.5LM metastatic cell line to evaluate survival outcomes and metastatic burden upon treatment with combinations of the HDAC inhibitor entinostat (ENT), and the checkpoint inhibitors anti-PD-1 and anti-CTLA-4. We show that in the HER2+ mouse model of advanced disease, combining ENT + ICIs improves survival, and ENT + anti-CTLA-4 most significantly improves survival and decreases metastatic burden among responders. By investigating immune changes in sites of metastases, we show that treatment with ENT + ICIs significantly increases infiltration and proliferation of CD8+ T cells, increasing effector T cell infiltration, cytokine production, and markers of activation in cytotoxic T cells in the lung. Flow cytometry, ex vivo co-culture assays, western blots, and other functional assays performed on MDSCs and TIL elucidate further mechanisms behind response. We have found that the metastatic sites of animals treated with ENT + ICIs have significantly decreased infiltration of granulocytic-MDSCs and increased infiltration of monocytic-MDSCs, leading to the apparent cytotoxic anti-tumor response. In mouse models of early stage disease, ENT + ICI therapy alters MDSC infiltration and function in primary tumors, allowing for a more robust adaptive immune response. A significant anti-tumor effect is also seen in the metastatic state, though the function of MDSCs is not consistently altered, suggesting a key mechanism of ENT synergy with ICI to incite immune response and survival benefit that remains to be elucidated. In summary, addition of ICIs to ENT is beneficial in models of advanced disease by altering the recruitment of suppressive cells into the metastatic microenvironment, changing the dynamic interaction of T cells and tumor cells causing a robust anti-tumor response. These novel findings provide insight into how these combination therapies may function in patients with advanced stages of HER2+ breast cancer and suggest that responses are linked to stage of disease and likely follow different mechanisms of action within the different tumor microenvironments.

Citation Format: Christine Rafie, Kayla Cruz, Skylar Woolman, Todd Armstrong, Elizabeth Jaffee, Evanthia Roussos Torres. Epigenetic modulation— unlocking the potential of checkpoint inhibition in advanced breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2352.

A CD40 Agonist and PD-1 Antagonist Antibody Reprogram the Microenvironment of Nonimmunogenic Tumors to Allow T-cell-Mediated Anticancer Activity

In cancers with tumor-infiltrating lymphocytes (TILs), monoclonal antibodies (mAbs) that block immune checkpoints such as CTLA-4 and PD-1/PD-L1 promote antitumor T-cell immunity. Unfortunately, most cancers fail to respond to single-agent immunotherapies. T regulatory cells, myeloid derived suppressor cells (MDSCs), and extensive stromal networks within the tumor microenvironment (TME) dampen antitumor immune responses by preventing T-cell infiltration and/or activation. Few studies have explored combinations of immune-checkpoint antibodies that target multiple suppressive cell populations within the TME, and fewer have studied the combinations of both agonist and antagonist mAbs on changes within the TME. Here, we test the hypothesis that combining a T-cell-inducing vaccine with both a PD-1 antagonist and CD40 agonist mAbs (triple therapy) will induce T-cell priming and TIL activation in mouse models of nonimmunogenic solid malignancies. In an orthotopic breast cancer model and both subcutaneous and metastatic pancreatic cancer mouse models, only triple therapy was able to eradicate most tumors. The survival benefit was accompanied by significant tumor infiltration of IFNγ-, Granzyme B-, and TNFα-secreting effector T cells. Further characterization of immune populations was carried out by high-dimensional flow-cytometric clustering analysis and visualized by t-distributed stochastic neighbor embedding (t-SNE). Triple therapy also resulted in increased infiltration of dendritic cells, maturation of antigen-presenting cells, and a significant decrease in granulocytic MDSCs. These studies reveal that combination CD40 agonist and PD-1 antagonist mAbs reprogram immune resistant tumors in favor of antitumor immunity.

Entinostat Converts Immune-Resistant Breast and Pancreatic Cancers into Checkpoint-Responsive Tumors by Reprogramming Tumor-Infiltrating MDSCs

Immune-checkpoint inhibition (ICI) has revolutionized treatment in cancers that are naturally immunogenic by enabling infiltration of T cells into the tumor microenvironment (TME) and promoting cytotoxic signaling pathways. Tumors possessing complex immunosuppressive TMEs such as breast and pancreatic cancers present unique therapeutic obstacles as response rates to ICI remain low. Such tumors often recruit myeloid-derived suppressor cells (MDSCs), whose functioning prohibits both T-cell activation and infiltration. We attempted to sensitize these tumors to ICI using epigenetic modulation to target MDSC trafficking and function to foster a less immunosuppressive TME. We showed that combining a histone deacetylase inhibitor, entinostat (ENT), with anti-PD-1, anti-CTLA-4, or both significantly improved tumor-free survival in both the HER2/neu transgenic breast cancer and the Panc02 metastatic pancreatic cancer mouse models. Using flow cytometry, gene-expression profiling, and ex vivo functional assays, we characterized populations of tumor-infiltrating lymphocytes (TILs) and MDSCs, as well as their functional capabilities. We showed that addition of ENT to checkpoint inhibition led to significantly decreased suppression by granulocytic MDSCs in the TME of both tumor types. We also demonstrated an increase in activated granzyme-B-producing CD8+ T effector cells in mice treated with combination therapy. Gene-expression profiling of both MDSCs and TILs identified significant changes in immune-related pathways. In summary, addition of ENT to ICI significantly altered infiltration and function of innate immune cells, allowing for a more robust adaptive immune response. These findings provide a rationale for combination therapy in patients with immune-resistant tumors, including breast and pancreatic cancers.

Combining STING-based neoantigen-targeted vaccine with checkpoint modulators enhances antitumor immunity in murine pancreatic cancer

Tumor neoantigens arising from somatic mutations in the cancer genome are less likely to be subject to central immune tolerance and are therefore attractive targets for vaccine immunotherapy. We utilized whole-exome sequencing, RNA sequencing (RNASeq), and an in silico immunogenicity prediction algorithm, NetMHC, to generate a neoantigen-targeted vaccine, PancVAX, which was administered together with the STING adjuvant ADU-V16 to mice bearing pancreatic adenocarcinoma (Panc02) cells. PancVAX activated a neoepitope-specific T cell repertoire within the tumor and caused transient tumor regression. When given in combination with two checkpoint modulators, namely anti-PD-1 and agonist OX40 antibodies, PancVAX resulted in enhanced and more durable tumor regression and a survival benefit. The addition of OX40 to vaccine reduced the coexpression of T cell exhaustion markers, Lag3 and PD-1, and resulted in rejection of tumors upon contralateral rechallenge, suggesting the induction of T cell memory. Together, these data provide the framework for testing personalized neoantigen-based combinatorial vaccine strategies in patients with pancreatic and other nonimmunogenic cancers.

Abstract 4936: Combination CD40 agonist and PD-1 antagonist antibody therapy enhances vaccine induced T cell responses in non-immunogenic cancers

A hallmark of many non-immunogenic cancers is the lack of tumor infiltrating lymphocytes (TIL) and/or failure to mount a robust anti-tumor T cell response via multiple mechanisms. The presence of T regulatory cells and myeloid derived suppressor cells (MDSCs) serve to dampen the immune response, and furthermore, tumor antigen-specific T cell tolerance limits the efficacy of therapeutic cancer vaccines. CD40 signaling is critical to the decision of whether cytotoxic T lymphocytes become primed or tolerized. Administration of monoclonal CD40 agonistic antibody (Ab) has been shown to promote CD8 activation in vivo, and likely alters the myeloid component of the tumor microenvironment. Our study asks the question of whether combining a T cell inducing vaccine and PD1 inhibition with CD40 agonistic Ab can induce T cell priming and TIL activation in non-immunogenic solid malignancies. We utilized mouse models of pancreatic ductal adenocarcinoma (PDAC) and breast cancer to assess the effects of drug combinations on intratumoral immune responses. Tumor-bearing mice were treated with a GM-CSF secreting vaccine (GVAX) + anti-PD1 Ab alone or in combination with CD40 Ab, or isotype control Ab, and monitored for survival. A separate cohort of mice were analyzed by immunohistochemistry and multi-color flow cytometry to assess T cell infiltration/activation and myeloid maturation. In a hemisplenectomy model of PDAC in which tumor cells were surgically implanted into wild-type recipients, mice treated with isotype control Abs succumbed to disease with extensive liver and peritoneal metastases at 35-70 days. GVAX + anti-PD1 Ab treatment displayed some efficacy, although 70% of mice eventually developed fatal liver metastases. In contrast, CD40 Ab was highly active, with 90% long-term survival afforded by a single administration of Ab, and mice treated with GVAX + anti-PD1 Ab + CD40 Ab had 100% survival. Similar trends in treatment efficacy were observed following subcutaneous tumor implantation of PDAC tumor cells in the lower limb. In an orthotopic model in which HER2/neu-expressing breast tumor cells were implanted into the mammary fat pad of syngeneic neu-N mice, we demonstrated delayed tumor progression and increased median survival in mice treated with GVAX + anti-PD1 Ab + CD40 Ab relative to either therapy alone. Further characterization of immune populations was carried out by high dimensional flow cytometric analysis utilizing PhenoGraph clustering and visualized by t-SNE. Changes were observed in monocytic and dendritic cell infiltration and maturation in the tumors of combination-treated mice. A significant decrease in granulocytic MDSCs was associated with response, as well as an increase in mature antigen presenting cells. In conclusion, GVAX, anti-PD1 and CD40 agonist Ab have potential synergy in modulating anti-tumor immunity in non-immunogenic cancers.

Citation Format: Hayley S. Ma, Evanthia Roussos Torres, Bibhav Poudel, Tara Robinson, Brian Christmas, Kayla Cruz, Skylar Woolman, Christine Rafie, Blake Scott, Valerie Wall, Todd Armstrong, Elizabeth Jaffee. Combination CD40 agonist and PD-1 antagonist antibody therapy enhances vaccine induced T cell responses in non-immunogenic cancers [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 4936.