The effect of TGF-β on PD-L1 expression on PDAC TAMs.

Abstract

764 Background: Pancreatic Ductal Adenocarcinoma (PDAC) has less than a 10% five year survival and will become the second leading cause of US cancer mortality in the next decade. Immunotherapy, such as checkpoint inhibition against anti-Programmed death-ligand 1 (PD-L1) has not been successful in the treatment of PDAC patients. Both tumor associated macrophages (TAMs) and the TGF-β protein are ubiquitous in PDAC tumors. We hypothesize that TGF-β increases the overall number of TAMs and degree of PD-L1 expression of TAMs in PDAC. Methods: Our lab has a mouse pancreatic cancer cell line derived from a genetically engineered mouse model (KPC mice that spontaneously form PDAC tumors that are similar to human PDAC). We orthotopically implanted this cell line into the pancreas of immunocompetent C57BL/6 (B6) mice. In groups of 5 each, mice were treated with saline (control) or TGF-β. We investigated tumor burden, the number of TAMs (CD45+, CD11b+F4/80hi, Ly6C−, Ly6G−/lo) in the tumors with flow cytometry and the percentage of TAMs expressing PD-L1 in the pancreas and metastatic lesions in the liver. Results: As a percent of leukocytes in the tumor, PDAC liver metastases had more TAMs compared to tumors in the pancreas (33 ± 5% vs 10 ± 4%, P = 0.001). Compared to controls, TGF-β treatment significantly increased the percent of PD-L1 expressing TAMs (32 ± 6 % vs 12 ± 5%, P = 0.013, see Figure) in the pancreas but no effect was evident on TAM density. In liver metastases, treatment with TGF-β decreased the overall TAM density (P = 0.039) but did not affect the number of PD-L1 positive TAMs. Conclusions: TGF-β plays pivotal role in the progression of PDAC and demonstrates context dependent activity. Our results suggest that an immunosuppressive effect mediated by PD-L1 expression on TAMs may be initiated by TGF-β. Future investigations will focus on understanding the role of the PDAC – TAM interaction to develop effective immune therapies for PDAC patients.