Activation of p53 in Immature Myeloid Precursor Cells Controls Differentiation into Ly6c+CD103+ Monocytic Antigen-Presenting Cells in Tumors

CD103⁺ dendritic cells are critical for cross-presentation of tumor antigens. Here we have shown that during immunotherapy, large numbers of cells expressing CD103 arose in murine tumors via direct differentiation of Ly6c⁺ monocytic precursors. These Ly6c⁺CD103⁺ cells could derive from bone-marrow monocytic progenitors (cMoPs) or from peripheral cells present within the myeloid-derived suppressor cell (MDSC) population. Differentiation was controlled by inflammation-induced activation of the transcription factor p53, which drove upregulation of Batf3 and acquisition of the Ly6c⁺CD103⁺ phenotype. Mice with a targeted deletion of p53 in myeloid cells selectively lost the Ly6c⁺CD103⁺ population and became unable to respond to multiple forms of immunotherapy and immunogenic chemotherapy. Conversely, increasing p53 expression using a p53-agonist drug caused a sustained increase in Ly6c⁺CD103⁺ cells in tumors during immunotherapy, which markedly enhanced the efficacy and duration of response. Thus, p53-driven differentiation of Ly6c⁺CD103⁺ monocytic cells represents a potent and previously unrecognized target for immunotherapy.

Abstract B32: Inhibiting DNA methylation causes an interferon response in cancer via dsRNA including endogenous retroviruses

DNA methyltransferase inhibitors (DNMTis) upregulate immune attraction, including the interferon response, in solid tumors. We now define viral defense signaling as one mechanism for this. In epithelial ovarian cancer cells DNMTis upregulate viral defense by cytosolic sensing of double-stranded RNA (dsRNA), triggering a Type I Interferon response, upregulation of downstream interferon response genes, and increased apoptosis. Knockdown of the dsRNA sensors TLR3 and MAVS and inhibition of the interferon alpha/beta receptor blunt the DNMTi induced dsRNA response. DNMTis cause apoptosis of cancer cells, which is partially rescued by inhibiting the interferon alpha/beta receptor. We observe upregulation and demethylation of hypermethylated endogenous retroviruses (ERVs) and overexpression of individual ERVs whose sense and anti-sense transcripts may be key candidates for triggering the above signaling. Overexpression of ERVs alone is sufficient to trigger an interferon response in the absence of DNMTis. Basal levels of ERV and viral defense gene expression significantly correlate in primary OC and basal expression of the viral defense signature separates primary TCGA samples for multiple tumor types into low versus high expression groups. In melanoma patients treated with an immune checkpoint therapy, high viral defense signature expression in tumors significantly associates with durable clinical response and DNMTi treatment sensitizes to anti-CTLA4 therapy in a pre-clinical melanoma model. We thus define a major mechanism for how DNMTis may induce cancer cells to increase immune attraction and possibly sensitize patients to immunotherapy. Experiments determining which Aza-upregulated molecules on tumor cells are necessary for attraction and activation of host immune cells are ongoing.

Citation Format: Katherine B. Chiappinelli, Pamela L. Strissel, Alexis Desrichard, Huili Li, Christine Henke, Benjamin Akman, Alexander Hein, Neal S. Rote, Leslie M. Cope, Alexandra Snyder, Vladimir Makarov, Sadna Budhu, Jedd Wolchok, Cynthia A. Zahnow, Taha Mergoub, Timothy A. Chan, Reiner Strick, Stephen B. Baylin. Inhibiting DNA methylation causes an interferon response in cancer via dsRNA including endogenous retroviruses. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Sep 24-27, 2015; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2016;76(2 Suppl):Abstract nr B32.

Inhibiting DNA Methylation Causes an Interferon Response in Cancer via dsRNA Including Endogenous Retroviruses

We show that DNA methyltransferase inhibitors (DNMTis) upregulate immune signaling in cancer through the viral defense pathway. In ovarian cancer (OC), DNMTis trigger cytosolic sensing of double-stranded RNA (dsRNA) causing a type I interferon response and apoptosis. Knocking down dsRNA sensors TLR3 and MAVS reduces this response 2-fold and blocking interferon beta or its receptor abrogates it. Upregulation of hypermethylated endogenous retrovirus (ERV) genes accompanies the response and ERV overexpression activates the response. Basal levels of ERV and viral defense gene expression significantly correlate in primary OC and the latter signature separates primary samples for multiple tumor types from The Cancer Genome Atlas into low versus high expression groups. In melanoma patients treated with an immune checkpoint therapy, high viral defense signature expression in tumors significantly associates with durable clinical response and DNMTi treatment sensitizes to anti-CTLA4 therapy in a pre-clinical melanoma model.