Abstract B058: Rewiring CD8+ T cell responses to PD-1 immune checkpoint blockade in PDAC via the inhibitory receptor PSGL-1

Abstract Pancreatic ductal adenocarcinoma (PDAC) is an aggressive, poorly immunogenic cancer and non-responsive to all currently available treatments, including immune checkpoint blockade (ICB) therapies. As survival rates remain low (13%), there is an urgent need for new therapeutic options. A hallmark of PDAC is the limited infiltration of anti-tumor cytotoxic CD8+ T cells. Analysis of single-cell RNA-sequencing data of PDAC tumors from treatment naïve patients reveals high expression of P-selectin glycoprotein ligand 1 (PSGL-1) on tumor-infiltrating CD8+ T cells. PSGL-1 is an intrinsic negative regulator of CD8+ T cells that promotes the development of functional exhaustion. We therefore hypothesized that PSGL-1-mediated immune suppression is a critical barrier to effective anti-tumor immunity in PDAC. To test this, we used preclinical models of orthotopic injection PDAC tumor cells into pancreata, recapitulating patient responses to PDAC with limited T cell infiltrates and uncontrolled tumor growth in C57BL/6 (wildtype) control mice and compared anti-tumor immune responses in PSGL-1KO (C57BL/6 background) mice. At 28 days post-injection of KPC. 4662 PDAC tumor cells, tumor burden in PSGL-1KO mice was reduced by 50% compared to controls, with 45% of PSGL-1KO animals exhibiting 70% reduction. Total immune infiltration (CD45+) was 2-fold greater in tumors from PSGL-1KO mice, with significant increases in infiltrating CD4+ and CD8+ T cells. Detailed analyses revealed that infiltrating CD8+ T cells from PSGL-1KO mice were less terminally differentiated towards exhaustion. PSGL-1KO PD-1+ CD8+ T cells expressed less TOX expression and fewer cells co-expressed CD38 and CD101 while more expressed CX3CR1 compared to controls. Intratumoral PSGL-1KO CD8+ T cells also retained greater functionality as well as indicated by inflammatory cytokine production (IFNg, TNFa) and Granzyme B expression. Moreover, PSGL-1KO mice showed protection against metastatic disease to the lungs. We find that protection by PSGL-1KO mice is T cell dependent as transient depletion of CD4+ and CD8+ T cells prior to tumor cell implantation results in the loss of PDAC tumor growth inhibition by PSGL-1KO mice. Less-differentiated, proliferation-competent precursors of exhausted CD8+ T cells (TPEX) are essential to PD-1 ICB responsiveness in patients and preclinical models. Given that PSGL-1-deficiency limits terminal differentiation of CD8+ T cells in PDAC, we hypothesized that PSGL-1-deficiency would promote responsiveness to PD-1 ICB, which is ineffective as a monotherapy in PDAC patients and mouse models. While therapeutic treatment with anti-PD-1 had no effect on the tumor growth in the control animals, PD-1 ICB resulted in tumor ablation in 85% of PSGL-1KO animals, demonstrating a profound synergistic effect of PD-1 inhibition in the absence of PSGL-1. From these studies, we conclude that PSGL- 1 is a critical inhibitor anti-tumor T cell immunity in PDAC and represents a novel ICB target with high translational potential for promoting immune responses to PDAC tumors. Citation Format: Jennifer L Hope, Yijuan Zhang, Hannah A. F. Hetrick, Evelyn S. Sanchez Hernandez, Gabriele Romano, Sreeja Roy, Michelle Lin, Ashley B Palete, Swetha Maganti, Dennis C Otero, Katelyn T Byrne, Cosimo Commisso, Linda M Bradley. Rewiring CD8T cell responses to PD-1 immune checkpoint blockade in PDAC via the inhibitory receptor PSGL-1 abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pancreatic Cancer Research; 2024 Sep 15-18; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2024;84 (17 Suppl₂): Abstract nr B058.