Abstract A60: Expression of NFκB p50 in pancreatic tumor stroma drives M2 differentiation and limits the control of tumors by radiation therapy.

Abstract

Radiation therapy aims to kill cancer cells with a minimum of normal tissue toxicity. Dying cancer cells have been proposed to be a source of tumor antigens and may release endogenous immune adjuvants into the tumor environment; for these reasons, radiation therapy may be an effective modality to initiate new anti-tumor adaptive immune responses that can target residual disease and distant metastases. However, pancreatic tumors engender an environment dominated by M2 differentiated tumor macrophages that support tumor invasion, metastases and escape from immune control. Finally, primary tumor analysis is feasible in only the small fraction of patients who have resectable pancreatic cancer. In this study, we assess the impact of radiation therapy on macrophage phenotype in a mouse model, and we employ in vitro assays and a genetically modified mouse model to understand the mechanism by which the phenotype switch may occur. To investigate changes in the phenotype of tumor macrophages following radiation therapy in wild type mice, we FACS sort tumor macrophages from Panc02 tumors. Finally, we quantify using flow cytometry (FACS) populations of immune cells in peripheral whole blood samples from 85 patients with pancreatic masses undergoing resection, and analyze human intratumoral macrophages by FACS sort and immunohistochemistry. We demonstrate that established Panc02 tumors in immune competent mice display a transient local response to radiation therapy followed by local recurrence and progressive growth of metastases that is independent of treatment. We next demonstrate that following radiation therapy of pancreatic tumors, there is a local influx of tumor macrophages that polarize towards immune suppression. We have previously shown that we can distinguish mature tumor macrophages from immature myeloid and MDSC populations by expression of Gr1 and IA (MHC class II). To isolate these sub-populations, we first gate CD11b+ cells in the untreated or irradiated tumors, then FACS sort out the macrophage population. RNA was purified from CD11b+IA+ macrophages from untreated or irradiated tumors, and gene expression microarray analysis confirmed the isolated macrophage phenotype; there was abundant expression of CD14, F4/80, and low or absent expression of B cell, T cell and endothelial markers. We demonstrate using in vitro models that this macrophage polarization is mediated by transcriptional regulation by NFκB p50, and that in mice lacking NFκB p50, radiation therapy is both curative and results in long-term protection against rechallenge. Finally, early follow up of the human data demonstrates abnormal myeloid expansions in peripheral whole blood of some patients with advanced stages of pancreatic cancer, but not in patients with benign disease. Further time will be necessary to correlate these myeloid expansions with survival and recurrence. In conclusion, we propose that despite the opportunity for increased antigen- specific adaptive immune responses, the intrinsic processes of macrophage-driven repair following radiation therapy in pancreatic cancer may limit the ability to control residual disease if the tumor environment is not targeted. Citation Format: Pippa Newell, Marka Crittenden, Talicia Savage, Benjamin Cottam, Paul Hansen, Michael Gough. Expression of NFκB p50 in pancreatic tumor stroma drives M2 differentiation and limits the control of tumors by radiation therapy. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Progress and Challenges; Jun 18-21, 2012; Lake Tahoe, NV. Philadelphia (PA): AACR; Cancer Res 2012;72(12 Suppl):Abstract nr A60.