Abstract 5609: Macrophages in secondary lymphoid organs regulate t cell tolerance and immunosurveillance in pancreatic cancer
Authors: Bengsch Fee , Liu Anni , Graham Kathleen , Beatty Gregory L. 
Published: July 2017

Abstract
Pancreatic ductal adenocarcinoma (PDAC) is a deadly disease that shows striking resistance to standard therapies. Immunotherapy is an exciting new treatment strategy for PDAC, but has yet to reproducibly demonstrate clinical efficacy. This finding may reflect poor effector T cell infiltration into PDAC or mechanisms of peripheral T cell tolerance. We hypothesize that successful immunotherapy in PDAC will require strategies that reverse elements of innate immunity that regulate T cell tolerance. This hypothesis is based on our prior studies in the KrasG12D/+;Trp53R172H/+;Pdx-1-Cre (KPC) mouse model of PDAC where macrophages residing outside of the tumor microenvironment were found to regulate the efficacy of a vaccine strategy involving the combination of chemotherapy and a CD40 agonist (FGK45). Specifically, we found that depletion of peripheral phagocytes using clodronate-encapsulated liposomes (CEL) was necessary for gemcitabine chemotherapy and a CD40 agonist to induce productive anti-tumor T cell immunity against spontaneously arising PDAC tumors in the KPC model.
We now investigate the mechanism by which CEL-depleted phagocytes regulate T cell tolerance in PDAC using the KPC mouse model and an orthotopic implantation model. We found that CEL depletes macrophages in the spleen and abdominal lymph nodes, thereby reversing the pathologic accumulation of macrophages in these secondary lymphoid organs that is induced by PDAC tumors. CEL targeted two macrophage subsets in the lymph node, CD11b+CD169+F4/80neg subcapsular sinus macrophages (SSM) and CD11b+CD169+F4/80+ medullary sinus macrophages (MSM). These macrophage populations may be exposed to tumor-derived antigens and apoptotic cell vesicles entering the lymph node, and have been previously identified as central orchestrators of tolerance in autoimmune models. We found that both fluorescently-labeled liposomes and OVA-FITC peptide injected into the peritoneal cavity were specifically targeted to SSM and MSM with little uptake by dendritic cells (DC). In contrast, depletion of these first-line phagocytes by CEL shifted the cellular uptake of these model antigens increasing uptake by CD11b+CD11c+ DC which are more efficient in antigen presentation and show increased expression of co-stimulatory molecules after FGK45 treatment. Using an orthotopic model of PDAC, we then determined that CEL can enhance the T cell stimulatory capacity of gemcitabine chemotherapy in combination with FGK45 as seen by a significant increase in CD38, CD44, CD69 and PD-1 expression on CD4 and CD8 T cells in secondary lymphoid organs.
Together, our findings suggest that tumor-derived antigens may be sequestered by tolerogenic macrophages in peritumoral lymph nodes that act to impair optimal induction of T cell responses. Thus, strategies aimed at disrupting the tolerogenic functions of these macrophages may offer a novel approach for improving immunotherapy in PDAC.
Citation Format: Fee Bengsch, Anni Liu, Kathleen Graham, Gregory L. Beatty. Macrophages in secondary lymphoid organs regulate t cell tolerance and immunosurveillance in pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5609. doi:10.1158/1538-7445.AM2017-5609