7K; Supplementary Fig. PDAC metastatic tumors, which are refractory to immune checkpoint inhibitors, into tumors that respond to immune checkpoint inhibition therapies. Introduction The ability of the immune system to identify and destroy cancer cells is a primary defense mechanism against cancer. CD8+ cytotoxic T cells, also known as cytotoxic T cells (CTLs), are key effectors of the immune response against cancer (1) and their presence in tumors is associated with a good clinical outcome in many tumor types, including ovarian, colon, breast, and pancreatic cancer (2C4). The importance HBX 41108 of effector CD8+ T cell mediated anti-tumor immune response in oncogenesis is demonstrated by the clinical success HBX 41108 of immunotherapies (1,5). Particularly, the use of immune checkpoint inhibitors has recently been shown to be beneficial for many types of cancers, with anti-programmed cell death protein 1 (PD-1) inhibitors being one of the leading candidates (6). However, immune checkpoint inhibitors only work if CD8+ T cells are infiltrated in to tumors. Pancreatic tumors are particularly poorly infiltrated by CD8+ HBX 41108 T cells and thus, inhibition of immune checkpoint receptors alone did not show any benefit in pancreatic cancer patients (7,8). Pancreatic cancer is characterized by a rich and desmoplastic tumor stroma, also called tumor microenvironment (TME), identified by high numbers of activated fibroblasts, collagen deposition, and extensive myeloid cell infiltration, which all together critically impact the disease progression (9) and its response to therapy (10C12). The TME is also thought to be a major barrier to CD8+ T cell infiltration in pancreatic tumors (13,14) and it is necessary to overcome this immune/fibrotic-protective barrier for the successful use of immune checkpoint inhibitors (15,16). Macrophages represent a major component of tumor infiltrating immune cells and depending on the activation signals, macrophages can acquire a spectrum of phenotypic states. In respect to cancer, macrophages can be polarized into HBX 41108 M1-like inflammatory macrophages that activate a tumoricidal immune response (hereafter SLC22A3 also referred to as M1-like), or into anti-inflammatory, immunosuppressive macrophages (hereafter also referred to as M2-like) that potently suppress other anti-tumor immune effector cells and thereby promote tumor progression (15,17,18). High density of macrophages, especially those exhibiting an immunosuppressive M2-like phenotype, correlates with poor clinical outcome in most human cancers (17). Accordingly, inhibition of myeloid cell recruitment into tumors have resulted in increased CD8+ T cell infiltration and decreased tumor burden in mouse models and early-phase clinical trials (15,18C23). Yet, the mechanisms by which macrophages regulate T cell infiltration is only beginning to emerge. Pancreatic ductal adenocarcinoma (PDAC) is a very aggressive metastatic disease. Currently, surgical resection is the best treatment option for PDAC patients, but, unfortunately, by the time PDAC is diagnosed, the majority of patients (~ 80%) present with non-resectable metastatic cancer. Moreover, more than 60% of the patients whose tumors are removed, relapse with distant hepatic recurrence within the first 24 months after surgery (12,24). Thus, a better understanding of the mechanisms underlying the metastatic process in pancreatic cancer is critical to improve treatment and patient survival. We and others have recently identified that a desmoplastic TME also exists at the metastatic site in PDAC, which is mainly the liver, and that this fibro-inflammatory reaction is required for metastatic growth (25,26). However, whether and how the metastatic TME affects CD8+ T cell infiltration and function in the metastatic liver remains unexplored. Here, we found that macrophage-derived granulin is a key protein that supports CD8+ HBX 41108 T cell exclusion and resistance to anti-PD-1 (PD-1) treatment. In fact, we show that depletion of granulin converts PDAC metastatic tumors, which are refractory to anti-PD1 treatment, into tumors that respond to PD-1. Our findings provide pre-clinical evidence that support the rational for targeting granulin in combination with the immune checkpoint blocker PD-1 for the treatment of metastatic PDAC. Material and Methods Cells Murine pancreatic cancer cells KPC FC1199, from here on referred as KPC, were generated in the Tuveson lab (Cold Spring Harbor Laboratory, New.