Abstract 1359: Rewiring CD8+T cell responses to PD-1 immune checkpoint blockade in PDAC by preventing terminal exhaustion via the inhibitory receptor PSGL-1

Abstract Pancreatic ductal adenocarcinoma (PDAC) is an aggressive, poorly immunogenic cancer with increasing incidence and a poor 5-year survival rate of 12%. PDAC tumors are refractory to all currently available treatments, including immune checkpoint blockade (ICB) therapies, and demonstrate limited infiltration of cytotoxic CD8+ T cells that kill tumor cells. There is therefore an urgent, unmet need for novel therapeutic options. Analysis of single-cell RNA-sequencing data of patient PDAC revealed high expression of the novel immune checkpoint, PSGL-1, on infiltrating CD8+ T cells. We hypothesize that PSGL-1-mediated immune suppression is a critical barrier to effective anti-tumor immunity in PDAC. To test this, we utilized a preclinical model of PDAC where orthotopic injection of the KPC. 4662 tumor cell line into pancreata recapitulates patient tumor responses with limited T cell infiltrates and uncontrolled tumor growth in C57BL/6 (wildtype) control mice and compared to PSGL-1KO (C57BL/6 background) mice. At endpoint, 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 a significant increase in infiltrating CD3+ T cells. Detailed analyses revealed that infiltrating CD8+ T cells from PSGL-1KO mice were not only more abundant, but also less differentiated towards terminal exhaustion with sustained TCT-1 expression compared to controls. Moreover, PSGL-1KO mice showed protection against metastatic disease. Since our data support a critical role for T cell driven anti-tumor immunity to PDAC, we transiently depleted CD4+ T cells, CD8+ T cells, or both just prior to tumor cell implantation. While depletion of CD8+ T cells did not impede tumor control by PSGL-1KO mice, depletion of all T cells or only CD4+ T cells resulted in the loss of PDAC tumor growth inhibition, demonstrating a key role of CD4+ cells in the response. Given our previous studies demonstrating an intrinsic role for PSGL-1 in development of terminal T cell exhaustion in melanoma, including loss of TCF-1+ CD8+ T cells, 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. To test this prediction, PD-1 ICB was therapeutically administered to control and PSGL-1KO mice. While treatment with anti-PD-1 had no effect on the tumor growth in the control animals, PD-1 ICB resulted in complete 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 of T cell anti-tumor responses in PDAC. PSGL-1 therefore 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. Hetrick, Gabriele Romano, Sreeja Roy, Michelle Lin, Ashley B. Palete, Swetha Maganti, Dennis C. Otero, Cosimo Commisso, Linda M. Bradley. Rewiring CD8+T cell responses to PD-1 immune checkpoint blockade in PDAC by preventing terminal exhaustion via the inhibitory receptor PSGL-1 abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 1359.