Abstract 1639: Human immune system mouse model of ER+ metastatic breast cancer
Authors: Olukoya Ayodeji , Bahnassy Shaymaa , Spoelstra Nicole S. , Jin Lu , Ranjit Suman , Elias Anthony , Richer Jennifer K. , Riggins Rebecca B. 
Published: June 2022

Abstract
In a recent clinical trial in heavily pretreated estrogen receptor alpha-positive (ER+) metastatic breast cancer (MBC), ~23% of patients reached the primary endpoint of clinical benefit of &gt;24 weeks on Fulvestrant plus the anti-androgen Enzalutamide. ESR1 mutant biopsies had more T regulatory cells (t test p&lt;0.021) and macrophages (p&lt;0.003) than ESR1 wild type (WT), which had more TP53 mutations and higher levels of cytotoxic T effector cells (p&lt;0.01). In addition, PD-L1 increased in post- vs. pre-treatment biopsies (paired t test p&lt;0.002). This increase in PD-L1 post-treatment warrants testing if the addition of a checkpoint inhibitor to the Fulvestrant-Enzalutamide combination will enhance therapy efficacy in ER+ MBC resistant to standard endocrine therapy. However, the lack of immune competent ER+ breast cancer pre-clinical models is a major barrier to these studies. Human immune system (HIS) mouse models were developed as a platform in which to test immune checkpoint inhibitors in other human cancers, and one HIS model was recently shown to support the growth and immune infiltration of ER+ patient-derived xenograft (PDX) primary tumors.
We tested whether HIS mice are a robust model in which to define ER+ tumor-immune microenvironment interactions at metastatic sites. We implanted NCG mice (NOD-Prkdcem26Cd52Il2rgem26Cd22/NjuCrl) humanized by engraftment of human umbilical cord blood-derived CD34+ (huCD34+) stem cells with the estrogen-independent HCI-013EI (013EI) PDX, derived from an ER+, invasive lobular MBC which harbors the Y537S ESR1 mutation. Mice were monitored for tumor latency and growth rate, and primary tumors and organs were collected for immunohistochemistry, single-cell RNA sequencing (scRNAseq), and phasor-fluorescence lifetime imaging microscopy (FLIM).
The ER mutated PDX 013EI had a longer tumor latency in HIS (median 77 days) vs. age-matched immune deficient control mice (27.5 days, log-rank p=0.043). Once formed, 013EI tumors in HIS mice grew slower than those in controls (mixed-effects analysis, p=0.035). HIS mice had visible lung, liver, and/or gastrointestinal tract metastases despite small primary tumors. Lung metastases from HIS mice retain ER expression and are infiltrated by human CD4+, CD8+, and CD68+ immune cells. Initial scRNAseq analysis also suggests infiltration by diverse immune cell populations in primary tumors from HIS mice. Initial phasor-FLIM for NADH autofluorescence suggests that 013EI tumors are highly glycolytic in HIS mice.
In conclusion, our results show that huCD34+ HIS mice effectively support growth, metastasis, and immune infiltration of an ESR1 mutant MBC PDX. Ongoing studies will further define the tumor-immune microenvironment in this and ESR1 WT MBC PDX, focusing on immune cell infiltration, PD-L1 expression, steroid receptor status, metabolic signaling and changes in these parameters following novel combinations of endocrine, immunotherapy and metabolic agents.
Citation Format: Ayodeji Olukoya, Shaymaa Bahnassy, Nicole S. Spoelstra, Lu Jin, Suman Ranjit, Anthony Elias, Jennifer K. Richer, Rebecca B. Riggins. Human immune system mouse model of ER+ metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1639.