Abstract 6382: Targeting mast cells restores T cell infiltration and sensitizes sarcomas to PD-L1 inhibition
Authors: Panagi Myrofora , Mpekris Fotios , Voutouri Chrysovalantis , Michael Christina , Constantinidou Anastasia , Martin John D. , Stylianopoulos Triantafyllos 
Published: June 2022

Abstract
Despite recent advances in cancer research, developments in sarcoma treatment have been slow. The efficacy of chemotherapy in sarcoma is limited by the pathophysiology of surrounding tumor stroma. Abnormalities in the tumor microenvironment (TME) result in stiffening of the tumor and accumulation of mechanical stresses which in turn cause the extreme deformation and collapse of intratumoral vessels. Strategies to induce TME reprogramming with the aim to restore mechanical properties of the tissue as well as tumor blood vessel functionality involve the use of “mechanotherapeutics”. Herein, we exploited the potential of the antihistamine drug and mast cell stabilizer, ketotifen, to induce mechanotherapeutic effects in murine models of soft tissue and bone sarcoma. Mast cells are the first population of immune cells drawn to the tumorigenic nidus where they interact with multiple components of the TME. Yet, their role in tumor stroma remains enigmatic as they either promote or inhibit tumor development depending on the conditions. In agreement with other mechanotherapeutics, we found that ketotifen induces vascular normalization in a dose dependent manner as indicated by the increase in perfusion, higher open lumen fraction and pericyte coverage of the vessels. Moreover, in vivo ultrasound shear wave elastography confirmed the mechanotherapeutic properties of ketotifen in reducing tissue stiffness, while further ex vivo validation indicated that TME reprogramming is mediated by a significant reduction in cancer associated fibroblast and inhibition of collagen synthesis as well as of that of other extracellular matrix components. Furthermore, we found that ketotifen modulates the immune microenvironment to promote immunostimulation. Immune stimulation was mediated by both intratumoral T cell accumulation and higher cytotoxic to regulatory T cell ratio concomitantly with a pronounced reduction in tissue hypoxia and upregulation of leukocyte adhesion molecules on endothelium. As a result of ketotifen-induced TME reprogramming, the addition of ketotifen to doxorubicin-aPDL1 combination treatment exhibited therapeutic superiority as opposed to doxorubicin-aPDL1, offering a durable tumor remission and immunological memory following tumor rechallenge experiments. Taken together, our findings demonstrate that ketotifen has a dual role in shaping the TME: i) reducing intratumoral physical forces to improve perfusion and ii) establishing favorable immunogenic conditions. Therefore, incorporating such agents in clinical practice holds great promise in the treatment of refractory, advanced sarcomas.
Citation Format: Myrofora Panagi, Fotios Mpekris, Chrysovalantis Voutouri, Christina Michael, Anastasia Constantinidou, John D. Martin, Triantafyllos Stylianopoulos. Targeting mast cells restores T cell infiltration and sensitizes sarcomas to PD-L1 inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6382.