The tumour microenvironment in pancreatic cancer - clinical challenges and opportunities.

Mechanistic Insights into Self-Reinforcing Processes Driving Abnormal Histogenesis During the Development of Pancreatic Cancer

We have collectively been spoiled by the astounding clinical benefit of antimicrobials. Much like the discovery and use of penicillin to eradicate once deadly infections, we continue to desperately search for the next “magic bullet” to kill cancer while sparing the non‐transformed cells. Greater appreciation for the molecular intricacies of malignancy has resulted in dedicated pursuit of cancer genomics and large‐scale informatics to identify “drugable” targets within the cancer cell itself. However, studies at the bench elucidating a dynamic relationship between tumor and microenvironment have become more common and demonstrate promise for novel therapeutic intervention.

Patients with advanced-stage pancreatic ductal adenocarcinoma (PDAC) predominantly receive chemotherapy, and despite initial responses in some patients, most will have disease progression and often dismal outcomes. This lack of clinical effectiveness partly reflects not only cancer cell-intrinsic factors but also the presence of a tumour microenvironment (TME) that precludes access of both systemic therapies and circulating immune cells to the primary tumour, as well as supporting the growth of PDAC cells. Combined with improved preclinical models of PDAC, advances in single-cell spatial multi-omics and machinelearning-based models have provided novel methods of untangling the complexities of the TME. In this Review, we focus on the desmoplastic stroma and both the intratumoural and intertumoural heterogeneity of PDAC, with an emphasis on cancer-associated fibroblasts and their surrounding immune cell niches. We describe new approaches in converting the immunologically 'cold' PDAC TME into a 'hot' TME by priming T cell activation, overcoming T cell exhaustion and unravelling myeloid cell-mediated immunosuppression. Furthermore, we explore integrated targets involving the TME, such as points of convergence among tumour, stromal and immune cell metabolism as well as oncogenic KRAS signalling. Finally, building on our experience with failed clinical trials in the past, we consider how this evolving comprehensive understanding of the TME will ensure future success in developing more effective therapies for patients with PDAC.

KRAS mutations have been observed in nearly 95% of pancreatic ductal adenocarcinoma (PDAC), however targeting KRAS remains a major therapeutic challenge. Previous studies from our group have discovered a novel interaction between KRAS and Argonaute 2 (AGO2) and have uncovered a significant role of this interaction in regulating KRAS signaling. Knockout of AGO2 in genetically engineered mouse models of KRAS driven pancreatic and lung cancer dramatically impacted tumor progression. Intriguingly, in pancreatic cancer loss of AGO2 expression resulted in early pancreatic intraepithelial lesions (PanINs) that failed to progress to PDAC. We observed increased senescence in these AGO2 knockout lesions that abrogated PDAC progression. Further, there was pronounced infiltration of immune cells in pancreata lacking AGO2 in comparison to wild type. We particularly observed a 20-fold increase in natural killer (NK) cells population in pancreata lacking AGO2. This instigated us to evaluate potential role of AGO2 in regulating immune microenvironment in pancreatic cancer. Gene set enrichment analysis of AGO2 knockout SW1990 PDAC cells revealed significant upregulation of inflammatory and interferon response pathways. We observed increased expression of several signaling proteins implicated in immune activation pathways. Further characterization of these knockout cells indicated upregulation of MHC proteins that consequently added to mechanistic insights. Additionally, we are also exploring syngeneic models of pancreatic cancer and we will present findings from our ongoing studies to evaluate the impact of AGO2 knockout on immune axis. Overall, our findings suggest potential involvement of AGO2 in regulating immune microenvironment in pancreatic cancer. Citation Format: Jennifer Hon, Carson Kenum, Pushpinder S. Bawa, Vijaya L. Dommeti, Anastasia A. Sahu, Miriam Gandham, Chi-Chiang Li, Zainab I. Taher, Sylvia Zelenka-Wang, Jean C. Tien, Sunita Shankar, Seema Chugh, Arul M. Chinnaiyan. Role of Argonaute 2 in regulation of immune microenvironment in pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2538.

Pancreatic ductal adenocarcinoma is one of the most aggressive and lethal cancers. Surgical resection is the only curable treatment option, but it is available for only a small fraction of patients at the time of diagnosis. With current therapeutic regimens, the average 5-year survival rate is less than 10% in pancreatic cancer patients. Immunotherapy has emerged as one of the most promising treatment options for multiple solid tumors of advanced stage. However, its clinical efficacy is suboptimal in most clinical trials on pancreatic cancer. Current studies have suggested that the tumor microenvironment is likely the underlying barrier affecting immunotherapy drug efficacy in pancreatic cancer. In this review, we discuss the role of the tumor microenvironment in pancreatic cancer and the latest advances in immunotherapy on pancreatic cancer.

Background: Despite of recent advances in chemotherapies, PDAC remains aggressive malignancies and the leading cause of cancer-related death in the world. TGF-β is strongly involved in the tumor microenvironment in PDAC, and dysregulation of TGF-β signaling is a frequent molecular disturbance in progression and metastasis. Vactosertib is an orally bioavailable TGF-β signaling inhibitor that targets the TGF-β type I receptor kinase. In in vivo studies, vactosertib in combination with FOLFOX improves pancreatic cancer survival by suppressing cell migration, invasion, and EMT, highlighting a potential clinical application of this approach for PDAC patients. Based on this preclinical study, we develop a phase 1b study to determine the RP2D and to evaluate the safety of vactosertib in combination with FOLFOX in patients with metastatic PDAC who have failed first-line gemcitabine with nab-paclitaxel. Methods: Eligible patients have histologically confirmed PDAC who have failed first-line gemcitabine/nab-paclitaxel with adequate organ function and performance status. This study is composed of two parts; dose escalation and dose expansion. In the dose escalation part, different dose levels of vactosertib (50 mg bid, 100 mg bid, and 200 mg bid) for escalation were tested, starting with dose level 0 (DL 0, 100 mg bid) with 3 to 6 subjects recruited in each cohort. DL -1 was only planned to test when DL 0 was unacceptable. In the dose expansion part, an additional backfill cohort was planned to open for determination of the final RP2D. Patients in each cohort were planned to receive vactosertib 50-200 mg orally twice per day 1-5 & day 8-12 with oxaliplatin 85 mg/m2 on day 1, LV 200mg/m2 IV on day 1, 5FU 200mg/m2 bolus on day 1 and continuous 5-FU 2400mg/m2 infusion over 48 hours every 2 weeks. The primary endpoints were to determine RP2D and to evaluate safety of this combination. The key secondary endpoints were progression free survival (PFS), overall response rate (ORR), disease control rate (DCR) based on RECIST 1.1 and overall survival. Results: A total of 16 patients were enrolled, 3 in DL 0, 4 in DL 1 and 9 in DL 1 backfill cohort. No dose limiting toxicities (DLTs) were observed and the RP2D was established as vactosertib 200 mg orally twice per day 1-5 & day 8-12 with FOLFOX. The vactosertib related adverse events (AEs) of any grade included fatigue, nausea, vomiting and anorexia. Three of 13 patients (23.1%) had partial response and 5 (38.5%) stable disease as best response, with clinical benefit rate of 61.5 % in DL 1, while there were no PR or SD in DL 0. Median PFS was 4.2 months [95% confidence interval (CI), 1.3–7.1] and median OS was 9.3 months [95% confidence interval (CI), 4.86–13.7]. Conclusion: In this phase Ib study, we demonstrated the feasibility and safety of adding vactosertib to FOLFOX in 2nd line setting, which needs to be further investigated. Updated results, including PK analyses, safety profiles, and clinical outcomes will be presented. This study was prospectively registered on ClinicalTrials.gov, NCT03666832. Citation Format: Joon Oh Park, Seung Tae Kim, Eunji Hong, Jung Yong Hong, Young Suk Park, Seong-Jin Kim. Phase 1b Study of 2nd line vactosertib plus oxaliplatin with 5FU/LV (FOLFOX) in patients with metastatic pancreatic ductal adenocarcinoma (PDAC) who have failed first-line gemcitabine with nab-paclitaxel [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr A017.

Introduction: Tumor budding is considered to be the morphologic correlate of epithelial-mesenchymal-transition (EMT) and is an independent prognostic factor in pancreatic ductal adenocarcinoma (PDAC). Here we explore the interactions between tumor budding cells, stromal cells and immune cells in the microenvironment of the tumor buds in PDAC. Methods: Multi-punch tissue-microarrays (TMAs) containing punches from the tumor center and the tumor front, including tumor buds, from a well characterized cohort of 120 PDAC patients, were stained immunohistochemically and by mRNA-in-situ-Hybridization (mRNA-ISH)for the EMT markers E-cadherin, β-catenin, Snail, ZEB1, ZEB2, N-cadherin and Twist and for the immune cell markers CD8, CD4, Foxp3, M1- and M2-macrophages. Expression in budding- and stromal cells and immune-cell counts in the microenvironment of the tumor buds were compared with findings in the main tumor. Results: Tumor buds showed loss of E-cadherin and b-catenin and overexpressed ZEB1 and ZEB2 compared with the neoplastic cells of the main tumor both at protein and mRNA level (p<0.0001 respectively). Stromal cells surrounding tumor buds showed increased protein and mRNA levels of the transcription factors Snail, ZEB1 and ZEB2. The immune microenvironment of the tumor buds composed of numerous Foxp3+cells and M1-macrophages while M2-macrophages were significantly reduced and CD8+cells were almost absent, in contrast to the main tumor. Conclusions: Tumor budding cells show a shift towards EMT-promoting profiles at protein and mRNA level, compared with the neoplastic cells of the main tumor. Stromal cells surrounding tumor buds express high levels of Snail, ZEB1 and ZEB2 suggesting that some stromal cells may represent complete mesenchymally transformed tumor cells or alternatively that there is a special phenotype of cancer associated fibroblasts supporting EMT-Type tumor-budding through cellular crosstalk in the tumor microenvironment of PDAC. Moreover, there is a tumor favoring immune cell composition in the immediate microenvironment of the tumor buds. Our findings suggest a close interaction of the stromal and immune response with the EMT process in PDAC. The combined assessment of host-associated factors such as stromal and immune response and tumor-associated factors such as EMT-type tumor-budding could help us to achieve superior prognostication and patient stratification than either factor alone. Citation Format: Eva Karamitopoulou, Martin Wartenberg, José A. Galván, Inti Zlobec, Alessandro Lugli, Aurel Perren. Tumor microenvironment in pancreatic cancer (PDAC): interplay between tumor cells, stromal cells and immune cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 447. doi:10.1158/1538-7445.AM2015-447

BackgroundPancreatic ductal adenocarcinoma (PDAC) is a highly lethal malignancy, characterized by a desmoplastic stromal reaction and an immunosuppressive tumor microenvironment (TME)1. The metabolic stress within the PDAC TME promotes autophagy,...

Despite the use of multi-agent conventional chemotherapy regimens, metastatic pancreatic ductal adenocarcinoma (PDAC) remains one of the most lethal solid tumors with a 5-year survival of 9%. PDAC is characterized by a complex tumor microenvironment with multilayered interplay between neoplastic and stromal cells and the natural extracellular matrix (ECM). To date, models that recapitulate the complexity of these features are only partially reproduced ex vivo and lack the capacity to recreate PDAC tumor microenvironment physiology, fluid flow and the ECM architecture. To address these issues, we developed Curiochips MPS platform for metastatic PDAC on-a-chip. To replicate an accurate PDAC complex tumor microenvironment, we investigated human pancreatic cancer cell lines with/without SMAD4 mutation as a test model. SMAD4 is among the most frequently altered genes in PDAC and its mutation accelerates pancreatic cancer cell migration. To tailor cell-ECM interactions and recreate natural biochemical environment of tumors, we examined various ECM conditions. ECM substitutes such as type I collagen and fibrin gel offer mechanical properties to closely mimic the tumor ECM. We accessed the effect of ECM condition on the pancreatic stellate cells (PSC) activation and pancreatic cancer cell migration using PANC-1 (SMAD4-WT) and BxP3 (SMAD4-null) cell lines. Notably, type I collagen induced more PSC activation than fibrin gel. Interestingly BxP3 spheroids migrated toward to the endothelial compartment compared to PANC-1. Angiogenesis of endothelial cells was more prominent in co-culture with BxP3 than PANC-1. Furthermore, we are building the metastatic model using PDAC patient derived organoids and co-culture with primary PSC in different types of ECM. Preliminary analysis showed functional responses such as 3D organoid migration and aggregation, PSC activation and migration controlled by specific properties of EMC. Our data highlight the importance of the ECM composition on PDAC migration, PSC activation and angiogenesis in the context of genetic heterogeneity and presence/absence oncogenic mutation. The 3D co-culture platform, Curiochips offers suitable and tailored solution to investigate tumor-stroma interaction and potentially can be used as a platform for drug discovery and precision oncology to target PDAC desmoplastic tumor microenvironment. Citation Format: Sumin Kim, Yeongmin Choi, Jihye Baek, Bharti Garg, Jan Pencik, Andrew M. Lowy, Herve Tiriac, Sanghee Yoo. Building a 3D co-culture model for metastatic pancreatic ductal cancer using Curiochips microphysiological system [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1328.

αvβ5 integrin is highly expressed in pancreatic ductal adenocarcinoma (PDAC) and predicts poor prognosis in PDAC patients. We previously showed that transforming growth factor-β (TGF-β) produced by cancer-associated fibroblasts and cancer epithelial cells induces the expression of αvβ5 integrin in PDAC, contributing to the αvβ5-rich tumor microenvironment (TME). Here, we show that the αvβ5 integrin is a critical activator of TGF-β in PDAC and that the iRGD tumor-penetrating peptide effectively inhibits the activation cascade causing a dynamic modification of the PDAC TME. TGF-β is secreted as a latent complex (LAP-TGF-β) and activated by various stimuli including receptor-dependent mechanisms. αv integrins potently activate TGF-β by binding to the RGD peptide motif within LAP-TGF-β to mechanically release active TGF-β from the complex. Our results show that the αvβ5 integrin is a major activator of TGF-β in PDAC cells because deleting the integrin significantly reduced TGF-β signaling and target gene expression despite the presence of other αv integrins on the cells. The iRGD peptide, which binds to αv integrins (and neuropilin-1) and delivers drugs deep into the tumor tissue, inhibited the integrin-mediated activation of TGF-β in the PDAC cells in an RGD-dependent manner. iRGD effectively inhibited TGF-β signaling in vivo evidenced by a dramatic suppression of phosphorylated Smad-2 in orthotopic tumors in syngeneic PDAC mice. Various changes in the TME accompanied the effect, such as reduced regulatory T cells, altered distribution of CD8+ T cells, normalization of blood vessels, and reduced stromal fibers. Similar results were obtained in humanized orthotopic PDAC mice that allow proper characterization of human immune response to tumor antigens. Adding an immune checkpoint blockade (ICB) to the iRGD monotherapy led to enhanced anti-tumor effects in both mouse models. Of note, iRGD, even as monotherapy, caused a striking inhibition of liver metastasis formation in humanized PDAC mice. Our findings suggest that PDAC has assembled a mechanism to maintain high levels of active TGF-β and αvβ5 integrin expression that likely contributes to its desmoplastic and drug resistant nature. iRGD interferes with the mechanism to cause dynamic TME modification and sensitization to ICB. The results justify the concept of the ongoing iLSTA clinical trial that studies the effect of iRGD-based immunotherapy in patients with locally advanced PDAC (ACTRN12623000223639). Citation Format: Yukihito Kuroda, Kodai Suzuki, Yuki Kunisada, Hotatka Kawai, Henri Havia, Tero A. Järvinen, Moriya Tsuji, Kazuki N. Sugahara. The iRGD tumor-penetrating peptide inhibits TGF-β activation mediated by an αvβ5 integrin-rich tumor microenvironment in pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2684.

Interleukin 1α Sustains the Expression of Inflammatory Factors in Human Pancreatic Cancer Microenvironment by Targeting Cancer-Associated Fibroblasts

Rationale: Pancreatic ductal adenocarcinoma (PDAC) tumors are notorious for their extensive non-malignant stroma in which the most abundant cells are cancer-associated fibroblasts (CAFs). CAFs are comprised of distinct subtypes with seemingly diverse roles within the PDAC tumor microenvironment (TME) and have been shown to influence disease progression and therapy response. However, most insights into PDAC CAF heterogeneity have been based on KPC (KrasLSL-G12D/+; Trp53LSL-R172H/+; Pdx1-Cre) mouse models, which recapitulate the pathophysiology of human PDAC but not its genetic complexity, representing only ~5% of patient tumor mutation profiles. We hypothesized that further understanding of CAF heterogeneity in PDAC tumors with commonly occurring mutations is essential for improving patient stratification and treatment strategies, and consequently, survival of this disease. To start exploring this, we developed and analyzed new PDAC models with deletion of Smad4, which is inactivated in ~30% of PDAC cases, in the context of Kras and Trp53 mutations. Methods: Smad4 deletion was performed using CRISPR-Cas9 technology in pancreatic cancer organoids derived from KPC tumors to generate matched KPC Smad4 knockout organoids. To assess whether this mutation profile modulates the PDAC TME, we cultured pancreatic stellate cells (PSCs), a precursor of CAFs, in media conditioned from KPC and KPC Smad4 knockout organoids and assessed the activation of inflammatory CAF (iCAF) and myofibroblastic CAF (myCAF) markers by RT-qPCR. We also established co-cultures of PSCs and organoids and performed bulk RNA-sequencing on flow-sorted CAFs and cancer cells. Finally, orthotopically-grafted organoid (OGO) transplantation mouse models were generated and pancreatic tumors were evaluated by ultrasound-based imaging, single-cell RNA-sequencing, immunohistochemistry, and flow cytometry. Results: We found that Smad4 deletion in PDAC cells, in the context of Kras and Trp53 mutations, promotes PDAC progression, as expected, and alters the PDAC TME. In particular, Smad4 deletion leads to a decrease in matrix-producing myCAFs, CD105-positive CAFs and antigen-presenting apCAFs, while increasing potentially immunosuppressive iCAFs. Additionally, pancreatic tumors with Smad4 deletion are depleted for macrophages, while enriched for neutrophils. Conclusions and Significance: This study demonstrates how distinct cancer genetics differentially shape the PDAC TME and establishes a workflow for ongoing investigation of additional relevant mutation profiles. This approach could lead to better PDAC patient stratification based on the profile of both cancer and stromal cells for more effective and targeted therapies. Citation Format: Eloise G. Lloyd, Gianluca Mucciolo, Muntadher Jihad, Judhell Sandoval Manansala, Sara Pinto Teles, Giulia Biffi. Smad4 deletion alters the composition of the tumor microenvironment in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr C063.

In pancreatic ductal adenocarcinoma (PDAC) the tumor microenvironment (TME) plays a crucial role in therapy resistance, metastasis, and recurrence. ProAgio is a novel therapeutic cytotoxin rationally designed to bind outside the RGD ligand pocket of the alphaV-beta3 integrin pair and then kill cells expressing this target. Pre-clinical studies demonstrated the safety and anti-tumor efficacy of ProAgio in animal models, and ProAgio is currently being tested in a Phase I clinical trial. The effect of ProAgio on TME cellular components has not been closely examined. We hypothesized that ProAgio remodels the PDAC TME to decrease stromal density and local immunosuppression by killing stromal cells thought to express the integrin pair, such as cancer associated fibroblasts (CAFs), proliferating endothelial cells, and tumor associated macrophages (TAMs). To identify potential cellular targets of ProAgio in the PDAC TME, we re-analyzed publicly available single cell RNA-seq datasets of human and mouse PDAC samples. We assessed integrin co-expression using flow cytometry, performed immunophenotyping and histopathological analysis to identify TME cellular and structural alterations induced by ProAgio. Integrins αV and β3 were both expressed at the RNA level in TAMs, myeloid cells, lymphocytes (T, B and NK), endothelial cells and CAFs. In the autochthonous KPC mouse model, we confirmed that granulocytes, macrophages, monocytes, CD4+ and CD8+T cells co-expressed integrin αVβ3 at the protein level. Amongst CAFs, inflammatory CAFs (iCAFs) had the highest percentage of cells co-expressing αV and β3 integrins. No anti-tumor effect was observed after 14 days of ProAgio treatment in a syngeneic KPC-derived PDAC orthotopic model, however, in the KPC autochthonous model the same treatment retarded tumor growth within 1 week. ProAgio treated autochthonous tumors displayed no changes in immune components or CAF subsets, but increased collagen area was observed. In addition, ProAgio treatment resulted in higher blood vessel area while integrin β3 protein levels remained stable. In conclusion, short-course ProAgio remodels the TME and delays tumor growth in an autochthonous PDAC model with mature stromal components, but does not deplete CAFs, TAMs or collagen area. Future studies will seek to refine our understanding of the cell subtypes affected by more prolonged ProAgio treatment and to determine the cellular and inflammatory mediators responsible for these changes. Citation Format: Mayrel Palestino Dominguez, Philip Homan, Xianyu Zhang, Sandra Navas Reyes, Theresa Guerin, Laura Bassel, Liu Zhi-ren, Serguei Kozlov, Christine Alewine. Targeting integrin alpha V beta 3 remodels the tumor microenvironment in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Pancreatic Cancer; 2023 Sep 27-30; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84(2 Suppl):Abstract nr B044.

<div>Abstract<p>The ability of disseminated cancer cells to evade the immune response is a critical step for efficient metastatic progression. Protection against an immune attack is often provided by the tumor microenvironment that suppresses and excludes cytotoxic CD8<sup>+</sup> T cells. Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive metastatic disease with unmet needs, yet the immunoprotective role of the metastatic tumor microenvironment in pancreatic cancer is not completely understood. In this study, we find that macrophage-derived granulin contributes to cytotoxic CD8<sup>+</sup> T-cell exclusion in metastatic livers. Granulin expression by macrophages was induced in response to colony-stimulating factor 1. Genetic depletion of granulin reduced the formation of a fibrotic stroma, thereby allowing T-cell entry at the metastatic site. Although metastatic PDAC tumors are largely resistant to anti–PD-1 therapy, blockade of PD-1 in granulin-depleted tumors restored the antitumor immune defense and dramatically decreased metastatic tumor burden. These findings suggest that targeting granulin may serve as a potential therapeutic strategy to restore CD8<sup>+</sup> T-cell infiltration in metastatic PDAC, thereby converting PDAC metastatic tumors, which are refractory to immune checkpoint inhibitors, into tumors that respond to immune checkpoint inhibition therapies.</p><p><b>Significance:</b> These findings uncover a mechanism by which metastatic PDAC tumors evade the immune response and provide the rationale for targeting granulin in combination with immune checkpoint inhibitors for the treatment of metastatic PDAC.</p><p><b>Graphical Abstract:</b> <a href="http://cancerres.aacrjournals.org/content/canres/78/15/4253/F1.large.jpg" target="_blank">http://cancerres.aacrjournals.org/content/canres/78/15/4253/F1.large.jpg</a>. <i>Cancer Res; 78(15); 4253–69. ©2018 AACR</i>.</p></div>

<div>Abstract<p>The ability of disseminated cancer cells to evade the immune response is a critical step for efficient metastatic progression. Protection against an immune attack is often provided by the tumor microenvironment that suppresses and excludes cytotoxic CD8<sup>+</sup> T cells. Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive metastatic disease with unmet needs, yet the immunoprotective role of the metastatic tumor microenvironment in pancreatic cancer is not completely understood. In this study, we find that macrophage-derived granulin contributes to cytotoxic CD8<sup>+</sup> T-cell exclusion in metastatic livers. Granulin expression by macrophages was induced in response to colony-stimulating factor 1. Genetic depletion of granulin reduced the formation of a fibrotic stroma, thereby allowing T-cell entry at the metastatic site. Although metastatic PDAC tumors are largely resistant to anti–PD-1 therapy, blockade of PD-1 in granulin-depleted tumors restored the antitumor immune defense and dramatically decreased metastatic tumor burden. These findings suggest that targeting granulin may serve as a potential therapeutic strategy to restore CD8<sup>+</sup> T-cell infiltration in metastatic PDAC, thereby converting PDAC metastatic tumors, which are refractory to immune checkpoint inhibitors, into tumors that respond to immune checkpoint inhibition therapies.</p><p><b>Significance:</b> These findings uncover a mechanism by which metastatic PDAC tumors evade the immune response and provide the rationale for targeting granulin in combination with immune checkpoint inhibitors for the treatment of metastatic PDAC.</p><p><b>Graphical Abstract:</b> <a href="http://cancerres.aacrjournals.org/content/canres/78/15/4253/F1.large.jpg" target="_blank">http://cancerres.aacrjournals.org/content/canres/78/15/4253/F1.large.jpg</a>. <i>Cancer Res; 78(15); 4253–69. ©2018 AACR</i>.</p></div>