Abstract 661: LAG-3 blockade enhances the anti-tumor efficacy of PD-1 blockade combined with radiotherapy in a syngeneic murine triple-negative breast cancer model

The anti-tumor efficacy of PD-1 blockade and local radiotherapy (RT) is often limited by diverse mechanisms that suppress CD8+ T cell responses. Lymphocyte-activation gene 3 (LAG-3) is an inhibitory receptor expressed on exhausted CD8+ T cells and regulatory T cells (Tregs), and its blockade has been shown to enhance the anti-tumor effects of PD-1 blockade. Therefore, we hypothesized that combining LAG-3 blockade with PD-1 blockade and local RT would overcome treatment resistance in triple-negative breast cancer (TNBC). Using the 4T1 syngeneic mouse TNBC model, we evaluated the anti-tumor effects and immune cell phenotypes following treatments with local RT, PD-1 blockade, and LAG-3 blockade. We found that the triple combination of local RT, PD-1 blockade, and LAG-3 blockade resulted in the greatest tumor growth delay in both irradiated (primary) and unirradiated (secondary) tumors, with a significant reduction in metastatic lung nodules. The proportion of tumor-infiltrating CD8+ T cells increased with triple combination therapy while the proportion of Tregs decreased compared to the PD-1 + RT combination in both primary and secondary tumors. Consequently, the CD8+ T cell to Treg ratio was highest in the triple combination group. The proportion of tumor-specific CD8+ T cells increased following triple combination therapy not only in tumor microenvironment but also among splenic CD8+ T cells. Interestingly, the proportions of CD8+ T cells, as well as CD4+ T cells, with an effector memory phenotype were most significantly elevated by the triple combination therapy. In tumor-draining lymph nodes, the proportion of conventional type 1 dendritic cells capable of antigen cross-presentation to CD8+ T cells was increased by the triple combination therapy. These findings suggest that LAG-3 blockade enhances the therapeutic efficacy of local RT and PD-1 blockade by promoting T cell-mediated anti-tumor immune responses. This study highlights the potential of combining LAG-3 blockade with local RT and immune checkpoint inhibitors for the treatment of TNBC. (Work supported by grant from the National Research Foundation of Korea #2023R1A2C3003782) Citation Format: Yoomin Kim, Seung Hyuck Jeon, Nawon Park, Seongmin Kim, In Ah Kim. LAG-3 blockade enhances the anti-tumor efficacy of PD-1 blockade combined with radiotherapy in a syngeneic murine triple-negative breast cancer model [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 661.