Abstract 1559: Inhibition of integrin αvβ8 enhances immune checkpoint induced anti-tumor immunity by acting across immunologic synapse in syngeneic models of breast cancer
Authors: Reszka-Blanco Natalia J. , Yadav Vinod , Krumpoch Megan , Cappellucci Laura , Cui Dan , Dowling James E. , Gwara Elizabeth , Harrison Bryce , Lee Dooyoung , Lin Fu-Yang , Luus Lia , Monroy Meghan , Moy Terence I. , Nebelitsky Eugene , Qiao Qi , Sullivan Andrew , Troast Dawn , Lippa Blaise , Rogers Bruce , Ray Adrian S. 
Published: July 2021

Abstract
Introduction We explored whether integrin αvβ8 inhibition potentiates immune checkpoint blockade (ICB) in syngeneic orthotopic models of breast cancer. Integrin αvβ8 mediates cell type specific and tissue localized activation of TGFβ1/3 to regulate the immune system. For example, αvβ8 expressed on dendritic cells (DC) in the intestine has been shown to be a key mediator of tolerance, maintaining gut immunologic homeostasis.
Methods Efficacy was evaluated in combination with anti-PD-1 in EMT6 and PyMT breast cancer syngeneic mouse models. A potent αvβ8 inhibitor was orally administered at 60mg/kg BID for 21 days. Anti-αvβ8 or non-isoform specific anti-TGFβ mAbs were dosed TIWx3 at 7 and 10mg/kg, respectively. Tumor volumes are presented as mean±SEM. Statistics were performed by t test, one-way ANOVA, or log-rank test. Flow cytometry and transcriptome analysis on bulk and single-cell levels were used to assess the mechanism of action in EMT6.
Results A similar αvβ8 expression pattern on DC, macrophages and regulatory T cells (Treg) was observed in mouse models and human tumors. Combination of oral αvβ8 inhibitor with anti-PD-1 was efficacious in the primary ICB resistant EMT6 model and resulted in superior tumor regression during treatment (p=0.0003) and improved survival with 5/12 complete responders relative to 0/12 in anti-PD-1 alone. Across studies the αvβ8 inhibitor phenocopied the results obtained with αvβ8 and TGFβ mAbs (n=3 independent studies). Complete responders re-challenged 89 days after treatment with EMT6 or 4T1 tumors showed no EMT6 tumor growth, suggesting the combination induced long-term immunologic memory. Analysis of tumors by flow cytometry showed combination resulted in increased CD8 T cell infiltrates (p=0.0006), T cell activation (CD8+CD69+, p=0.0194) and IFN-γ expression (CD8+IFN-γ+, p=0.0021). Single cell transcriptomic analysis of lymph nodes showed that αvβ8 inhibition potentiated DC co-stimulation (Cd40, Cd83/6) and migration (Ccr7, Cxcl16, Ccl22). Moreover, combination treatment led to tumor infiltrated Treg dysfunction including downregulation of Ctla4, Il10 and Tigit, and upregulation of Ifn-γ. The observation of these anti-tolerance and pro-inflammatory signatures in DC and Treg has not been described previously. Anti-tumor efficacy was driven by immune-mediated mechanisms as confirmed by a CD8 depletion study. Efficacy was confirmed in PyMT breast cancer model.
Conclusions An αvβ8 inhibitor in combination with anti-PD-1 showed efficacy in syngeneic mouse models, supported by increased T cell infiltrates and evidence of reduced tumor tolerance. These results show that an orally administered αvβ8 targeted inhibitor is a potent modulator of anti-tumor immune response acting across the immunologic synapse, and is a promising therapeutic approach to ICB refractory tumors.
Citation Format: Natalia J. Reszka-Blanco, Vinod Yadav, Megan Krumpoch, Laura Cappellucci, Dan Cui, James E. Dowling, Elizabeth Gwara, Bryce Harrison, Dooyoung Lee, Fu-Yang Lin, Lia Luus, Meghan Monroy, Terence I. Moy, Eugene Nebelitsky, Qi Qiao, Andrew Sullivan, Dawn Troast, Blaise Lippa, Bruce Rogers, Adrian S. Ray. Inhibition of integrin αvβ8 enhances immune checkpoint induced anti-tumor immunity by acting across immunologic synapse in syngeneic models of breast cancer [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 1559.