Abstract 1590: An experimental tumor model resistant to anti-PD-1 is sensitive to local mRNA immunotherapy

Abstract

Abstract Immune checkpoint blockade elicits durable anti-cancer responses in the clinic, however a large proportion of patients do not benefit from treatment. In an effort to better understand acquired resistance to checkpoint blockade, we generated a mouse tumor model exhibiting in vivo resistance to anti-PD-1 antibody treatment. MC38 tumors acquired resistance to PD-1 blockade following serial in vivo passaging. Lack of sensitivity to PD-1 blockade was not attributed to dysregulation of PD-L1 or β2M expression, as both were expressed at similar levels in parental and resistant cells. Similarly, IFNγ signaling and antigen processing and presentation pathways were functional in both parental and resistant cell lines. RNA-sequencing revealed substantial differences in global gene expression, with tumors resistant to anti-PD-1 displaying a marked reduction in expression of immune-related genes relative to parental MC38 tumors. Indeed, resistant tumors exhibited reduced immune infiltration across multiple cell types, including T and NK cells. Pathway analysis revealed activation of TGFβ and Notch signaling in anti-PD-1 resistant tumors, and activation of these pathways was associated with poorer survival in human cancer patients. While pharmacological inhibition of TGFβ and Notch in combination with PD-1 blockade decelerated tumor growth, local administration of a mixture of mRNAs encoding immune-stimulatory cytokines (IL12sc, GM-CSF, IL15s and IFNα) potently induced regression of resistant tumors and resulted in complete tumor remission. Overall, this study describes a novel anti-PD-1 resistant mouse tumor model and underscores the role of two well-defined signaling pathways in response to immune checkpoint blockade. Furthermore, our data highlights the potential of intratumoral mRNA therapy in overcoming acquired resistance to PD-1 blockade. Citation Format: Marie Bernardo, Tatiana Tolstykh, Yu-an Zhang, Dinesh S. Bangari, Hui Cao, Kerstin A. Heyl, Joon Sang Lee, Natalia Malkova, Jack Pollard, Hui Qu, Fangxian Sun, Dmitri Wiederschain, Timothy R. Wagenaar. An experimental tumor model resistant to anti-PD-1 is sensitive to local mRNA immunotherapy [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 1590.