Abstract A124: Early bystander effect of vaccine boosting of CAR T cells reinvigorates endogenous anti-tumor T cell immunity to regress pancreatic cancer

Abstract Pancreatic ductal adenocarcinoma (PDAC) is one of the most difficult-to-treat cancers. This therapeutic challenge is primarily driven by a stroma-dense immunosuppressive tumor microenvironment that limits therapeutic T-cell infiltration and impairs anti-tumor T-cell activities. For example, mesothelin-targeted chimeric antigen receptor (MSLN-CAR) T cells demonstrated minimal efficacy in PDAC patients in early-phase clinical trials, with stable disease being the best observed outcome. We recently showed that a tailored synthetic vaccine can specifically stimulate adoptively transferred tumor-targeting chimeric antigen receptor (CAR) T cells in vivo in the lymph node microenvironment, markedly enhancing CAR T cell fitness, tumor infiltration, and anti-tumor activity (Ma et al, Science, 2019; Cell, 2023; Grzywa et al, Nat BME, 2025). Recently, we investigated vaccine boosting of MSLN-CAR T cells to enhance their infiltration and cytotoxicity against immunologically cold and therapy-resistant PDACs and observed superior tumor control. Unexpectedly, we also observed a pronounced immediate PDAC tumor control even in the control group where mice received tumor-ignorant (FITC-targeting) CAR T cells and a single dose of synthetic FITC vaccine boost, with >70% reduction of tumor volume in the first 24 hours that lasts >7 days. Notably, this early tumor control is independent of the target antigen specificity of CAR T cells but dependent on the CAR-specific vaccine boost, suggesting a previously unappreciated early anti-tumor bystander effect associated with vaccine boosting of CAR T cells. This marked CAR antigen-independent early tumor control is completely lost in Rag1-knockout (no endogenous T cells) and BatF3-knockout (deficient in antigen cross-presentation) recipient mice, highlighting the determining role of pre-existing PDAC-reactive endogenous T cells. Comprehensive serological profiling demonstrated that synthetic FITC vaccine boosting through the FITC-CAR induced a rapid but non-toxic cytokine and chemokine release, including IFNγ, TNFα, IL-6, and CCL2. Notably, IFNγ blockade completely abolished this early tumor control, at least partially through blocking the induction of the class I major histocompatibility complexes, which present PDAC antigens to endogenous T cells. These findings collectively suggest that the potent bystander tumor control mediated by vaccine-boosting of CAR T cells is critically dependent on IFNγ, which crosstalks with both the PDAC tumor and the pre-existing endogenous anti-tumor T cells. Therefore, synthetic vaccine boosting of tumor-agnostic CAR T cells likely offers a novel therapeutic strategy that can be used independently or in combination with other therapeutic modalities for treating PDAC tumors. Citation Format: Tomasz Grzywa, Ryan Tannir, Gregory Beatty, Leyuan Ma. Early bystander effect of vaccine boosting of CAR T cells reinvigorates endogenous anti-tumor T cell immunity to regress pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pancreatic Cancer Research—Emerging Science Driving Transformative Solutions; Boston, MA; 2025 Sep 28-Oct 1; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl_3):Abstract nr A124.