Abstract B069: The role of tumor-bound MerTK in promoting sensitivity to immune checkpoint therapy in triple-negative breast cancer

Abstract Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer characterized by the lack of estrogen, progesterone, and HER2 receptor expression. Treatment options for TNBC patients are limited, and high rates of disease recurrence are a critical problem. The anti-PD-1 antibody pembrolizumab is approved to treat PD-L1-positive, advanced-stage TNBC, and all early-stage TNBC cases. Clinical trial data indicate that PD-L1 expression may be an unreliable biomarker for predicting response to pembrolizumab, underscoring the need to identify alternative biomarkers with greater predictive accuracy. MerTK is a member of the TAM family of receptor tyrosine kinases and is expressed in many solid tumors, including TNBC. We investigated the role of tumor-bound MerTK in TNBC and its contribution to immune checkpoint inhibitor (ICI) response. First, we analyzed 119 human TNBC clinical samples using immunohistochemistry (IHC) and immunofluorescence, revealing that 30-40% of TNBC cases exhibit high MerTK expression. Further analysis of this cohort demonstrated a correlation between elevated MerTK expression and increased infiltration of CD8+ T cells, B cells, macrophages, and NK cells. Next, we developed a series of syngeneic mouse lines (4T1 and EMT6) overexpressing MerTK and determined its impact on tumor growth and the tumor immune microenvironment (TIME). In immunodeficient models (NCG and athymic nude), MerTK-overexpressing tumors showed no significant difference in growth relative to vector controls. Strikingly, MerTK-overexpressing clones exhibited a significant reduction in tumor growth in syngeneic BALB/c mice. This attenuation of tumor growth was associated with infiltration of anti-tumor immune cells, including M1 macrophages, CD4+ T cells, CD8+ T cells, NKT cells, and NK cells, and exclusion of pro-tumor cell populations including Tregs and PMN-MDSCs. These results suggest that overexpression of MerTK in TNBC tumors leads to an antitumor TIME. Next, we hypothesized that TNBC tumors expressing MerTK may exhibit enhanced sensitivity to ICI. To test this hypothesis, we inoculated BALB/c mice with 4T1 vector and MerTK-overexpressing clones, followed by treatment with either anti-PD-L1 or anti-CTLA-4 antibodies. MerTK-overexpressing tumors exhibited a robust and durable response (33% complete regression without recurrence) to ICI compared to vector controls. ICI treatment increased CD8+ T cell and NK cell activity, as demonstrated by flow cytometry, NanoString nCounter, and IHC analysis. Additionally, utilizing a human microphysiological system and human TNBC cells expressing MerTK, we showed that PBMCs migrate toward MerTK-overexpressing tumor spheroids relative to vector control. Finally, we demonstrate that tumor-bound MerTK and PD-L1 expression are independent in our 4T1 mouse model and human TNBC clinical samples. Collectively, our data suggest that expression of MerTK in TNBC leads to a proinflammatory TIME and increased sensitivity to ICI, suggesting that MerTK may serve as a reliable predictive biomarker for ICI response in TNBC. Citation Format: Bridget E Crossman, Mari Iida, Regan L Harmon, Candie Y Lin, Sheena C Kerr, Karla Esbona, Irene M Kang, Ravi Salgia, Deric L Wheeler. The role of tumor-bound MerTK in promoting sensitivity to immune checkpoint therapy in triple-negative breast cancer [abstract]. In: Proceedings of the AACR IO Conference: Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2025 Feb 23-26; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2025;13(2 Suppl):Abstract nr B069.