Abstract C174: MerTK as a putative biomarker for treatment response in triple negative breast cancer
Authors: Mehall Bridget E , Iida Mari , Kostecki Kourtney L , Adams Jillian M , Wheeler Deric L 
Published: December 2023

Abstract
Triple Negative Breast Cancer (TNBC) is characterized by the lack of estrogen, progesterone, and HER2 receptors. TNBC is a highly invasive subtype of breast cancer, with limited targeted therapies. Current therapeutic options for TNBC patients include surgery, chemotherapy, and radiation but disease recurrence is common. Anti-PD-1 and anti-PD-L1 therapeutic antibodies have been approved for the treatment of TNBC in combination with chemotherapy, but biomarkers for response and new therapeutic targets are still needed. The receptor tyrosine kinase MerTK has been associated with many types of cancer and is highly expressed in TNBC. To investigate the role of MerTK in TNBC, we created a syngeneic cell line (4T1) overexpressing MerTK to evaluate tumor growth, the tumor immune microenvironment (TIME), and response to immune checkpoint inhibitors (ICIs) in vivo. First, we tested the effect of MerTK on tumor growth in vivo by inoculating the 4T1 cell line, which has very low MerTK expression, and two 4T1 MerTK-overexpressing clones into eight Balb/c mice per group subcutaneously on the flank. We measured tumor growth over 27 days and found that MerTK-overexpressing tumors exhibited a statistically significant delay in tumor growth when compared to the parental 4T1 tumors. This indicates that MerTK may be inducing a hotter TIME in MerTK overexpressing tumors, as this slowed growth was not observed in athymic nude mice. To test this hypothesis, we evaluated the TIME of these tumors by flow cytometry, which revealed robust expansion of macrophage, natural killer cell and T cell compartments, and reduction in tumor-infiltrating myeloid derived suppressor cells. These results suggest that MerTK is playing a yet unidentified role in promoting infiltration of anti-tumor immune cells into the TIME and excluding pro-tumor immune cells. Further, we saw robust changes in genes associated with cytokine signaling pathways including CD38, CCL2 and CCL5 by Nanostring nCounter analysis in MerTK-overexpressing 4T1 cells compared to parental 4T1 cells. Finally, there is evidence that hotter tumors respond well to ICI therapy, and we hypothesized that MerTK-overexpressing tumors would exhibit a significant response to ICI when compared to parental 4T1 tumors. Indeed, our data showed a robust and durable response marked by a significant reduction in tumor growth in the MerTK-overexpressing tumors compared to parental 4T1 tumors in Balb/c mice when treated with either anti-PD-L1 or anti-CTLA-4 antibodies. Together, these results suggest that MerTK overexpression could serve as a predictive biomarker for response to immune therapy in patients with TNBC.
Citation Format: Bridget E Mehall, Mari Iida, Kourtney L Kostecki, Jillian M Adams, Deric L Wheeler. MerTK as a putative biomarker for treatment response in triple negative breast cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr C174.