Abstract B94: Fibroblast activation protein alpha-expressing stromal cells mediate immune suppression in the KPC mouse model of pancreatic ductal adenocarcinoma.

Abstract

Local immune suppression may account for the paradox that cancers survive in the face of spontaneous or vaccine-induced immune responses. We have previously demonstrated that a stromal cell marked by fibroblast activation protein alpha (FAP) mediates immune suppression in established transplanted tumor models (Kraman et al. Science 2010). This stromal cell is present in 90% of human adenocarcinomas, including pancreatic ductal adenocarcinoma (PDAC). Transplanted models do not always reflect human cancer, so we investigated FAP+ cells in a spontaneous model of PDAC. The KPC mouse uses the Cre recombinase system to target expression of mutations common in human pancreatic cancer (KrasG12D and p53R172H) to pancreatic ductal cells. The mouse develops a spectrum of lesions, from pancreatic intraepithelial neoplasia (PanIN) to PDAC. FAP+ cells are present in PanIN and PDAC. We have previously generated a BAC transgenic mouse that expresses the primate diphtheria toxin receptor under the regulatory elements of the fap gene (FAP-DTR mouse). This allows depletion of FAP+ cells in vivo by administration of diphtheria toxin (DTX). The KPC mouse was crossed with the FAP-DTR mouse to study the effects of FAP+ cell depletion. Tumor growth is arrested during one week of FAP+ cell depletion (mean growth 27.7% treated vs. 85.1% vehicle, p=0.0001). After 72 hours depletion, the percentage of neutrophils in the tumor doubles as measured by flow cytometry (27.8% treated vs. 14.3% vehicle, p=0.0367). This influx is also evident by immunofluorescence microscopy (p=0.0056). There is also a threefold increase in tumor necrosis factor-α mRNA (p=0.0446). These features are suggestive of tissue necrosis. CD8α+ T cells purified from the spleens of tumor-bearing or wild-type mice were incubated with tumor cells in interferon-γ secretion ELISpot assays. T cells purified from tumor-bearing, but not wild-type mice, reacted with the tumor cells indicating a spontaneous antitumor response. Additionally, T cells prepared from one tumor bearing mouse also react with the tumor of another mouse, suggesting that tumor antigens may be shared between KPC tumors. To investigate whether control following FAP+ cell depletion is dependent on this immune response, we administered depleting monoclonal antibodies to CD4 and CD8α concurrently with DTX. Depletion of FAP+ cells has no effect following T cell depletion (mean growth 72.4% treated vs. 80.8% vehicle, p=0.60), indicating that tumor control is immune dependent. In summary, depletion of FAP+ stromal cells results in immune-dependent control of murine PDAC. This control is rapid, suggesting that the preexisting immune response detected by ELISpot assay is effective: T cell exhaustion or selection of nonimmunogenic clones (“cancer immunoediting”) do not account for escape of this tumor from immunosurveillance. Rather, the immune suppressive microenvironment, dependent on the FAP+ cell, masks the tumor from the immune system. If this were the case in humans, a cancer vaccine might not be needed. A therapy that neutralizes immune suppression mediated by FAP+ cells might allow an existing or vaccine induced antitumor response to control tumor growth. Citation Format: James O. Jones, Christine Feig, Edward W. Roberts, Richard J.B. Wells, David A. Tuveson, Douglas T. Fearon. Fibroblast activation protein alpha-expressing stromal cells mediate immune suppression in the KPC mouse model of pancreatic ductal adenocarcinoma. [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 B94.