Abstract 1343: Competition for dendritic cells limits engineered TCR-T cell activation in tumor-draining lymph nodes and impairs synergy with PD-L1 blockade.

Abstract Adoptive cell therapy with engineered T cells (i.e. CAR-T and TCR-T) has transformed outcomes for patients with hematologic malignancies, but their efficacy in solid tumors remains limited. To improve responses, these therapies are increasingly combined with PD-1/PD-L1 blockade, though results remain variable. For endogenous T cells, PD-1/PD-L1 blockade expands PD-1+Tcf1+ progenitor exhausted T cells (Tpex) in tumor-draining lymph nodes (TdLNs), where dendritic cells (DCs) maintain them through antigen presentation. However, it is unclear whether engineered T cells form an analogous Tpex reservoir in TdLNs that can replenish effector T cells in tumors and respond to PD-1/PD-L1 blockade. To test this, we used the Kras/p53 model of lung cancer expressing ovalbumin (KPOva) to compare endogenous and engineered TCR-T cells targeting the same neoantigen. OT-I transgenic CD8 T cells were expanded in vitro to generate Ova-specific engineered TCR-T cells, and Ova-specific endogenous T cells (Endog-T) were identified with a pMHC tetramer. Consistent with prior reports, Endog-T in untreated KPOva mice accumulated in TdLNs and adopted a PD-1+Tcf1+ Tpex phenotype. In contrast, in mice treated with cyclophosphamide (Cy) lymphodepletion and infusion of 1x106 TCR-T cells, TCR-T also accumulated in TdLNs, but they displayed significantly reduced PD-1 expression compared to Endog-T. Despite lower PD-1 levels, TCR-T cells in TdLNs produced less IL-2 and exhibited impaired tumor control relative to Endog-T, indicating they adopted a hypofunctional state. Further, while PD-L1 blockade improved anti-tumor immunity in untreated mice, it drove terminal differentiation of TCR-T in tumors and failed to enhance TCR-T-mediated tumor control. Mechanistically, lymphodepletion reduced TdLN DC numbers by ∼70%, increasing the TCR-T:DC ratio from ∼1:1 to ∼200:1. Imaging revealed large swarms of TCR-T cells competing for contact with limited numbers of DCs in TdLNs. However, doubling DC numbers in TdLNs with anti-CD40 and Flt3L vaccination was insufficient to overcome this imbalance and failed to increase TCR-T activation. In contrast, infusing 100-fold fewer TCR-T (1x104) significantly reduced the TCR-T:DC ratio to ∼9:1, which restored TCR-T PD-1 expression, IL-2 production, and expansion capacity. Importantly, this reduced competition in TdLNs enabled TCR-T cells to respond to PD-L1 blockade, resulting in improved tumor control. Our data suggest that Cy lymphodepletion and high-dose TCR-T induces competition for DCs in TdLNs, which limits their activation, function, and synergy with PD-L1 blockade. Thus, strategies that reduce competition here may promote Tpex formation and sensitize TCR-T therapy to PD-L1 blockade—and potentially vaccination—to improve control of solid tumors. Citation Format: William Samuel Nutt, Emma Bingham, Jessica Huang, Andrew Snyder, Mitchell Kluesner, Victor Zepeda, Dani Miller, Sebastian La Rosa, Megha Sarvothama, Sarah Garrison, Michael Gerner, Shivani Srivastava. Competition for dendritic cells limits engineered TCR-T cell activation in tumor-draining lymph nodes and impairs synergy with PD-L1 blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1343.