Abstract 5188: Interrogating the role of the immune microenvironment in the response of brain metastases to immunotherapy using new preclinical melanoma models

Abstract Brain metastases (BrM) remain an intractable, deadly complication for advanced melanoma patients and efficient therapeutic strategies are desperately needed. The tumor microenvironment (TME) plays an important role in response to therapy. However, studies addressing the contribution of the TME to therapy efficacy for BrM are lacking, mostly due to limited access to human samples and scarcity of appropriate preclinical models. Here, we describe two novel isogenic immunocompetent BrM models generated by intracardiac injection of UV-induced mouse melanoma cell lines, representative of mutant-RAS human melanoma subtypes. We used these models to test immune checkpoint blockade (ICB) therapy and to interrogate the role of the TME in therapeutic efficacy. To evaluate response, we developed and applied a new machine-learning method to quantify metastatic burden. We investigated the TME by high-parametric flow cytometry and single-cell RNA sequencing (scRNA-seq). We showed that the models have distinct metastatic behaviors, with BR1 being mostly brain tropic and BR3 displaying widespread metastases. Notably, BR1 BrM were sensitive to ICB with a better response to anti-PD-L1/anti-CTLA-4 combination therapy as compared to monotherapies. In contrast, BR3 BrM were resistant to both mono- and combination therapies. Interestingly, we found that ICB efficacy on extracranial BR3 metastases is organ-dependent. Characterization of the BrM immune microenvironment before and after treatment revealed dramatic differences between the models. Untreated BR1 BrM showed significant recruitment of T cells, dendritic cells, and natural killer cells, while neutrophils were enriched in untreated ICB-resistant BR3 BrM. Moreover, we uncovered phenotypically distinct microglia populations exclusively present in ICB-sensitive BR1 BrM that positively correlated with T cell infiltration. Consistent with this finding, scRNA-seq showed upregulation of genes encoding for T cell-attracting chemokines and antigen presentation uniquely in the BR1-associated microglia. Post-treatment analysis of the brain TME highlighted beneficial changes induced by ICB in the responsive BR1 model, including increased recruitment of CD8 T cells with an activated phenotype, while a mild recruitment of exhausted T cells was observed in the resistant BR3 model. Altogether, our data emphasize the importance of interrogating the BrM TME to understand therapeutic response. Our unique BrM models, mirroring the diversity of ICB response observed in patients, provide a robust platform for the much-needed mechanistic studies to optimize BrM therapy. Deciphering the contribution of the newly identified BR1 BrM-associated microglia to ICB efficacy will be crucial to the identification of novel therapeutic targets. Citation Format: Amélie Lopès, Jessica Rappaport, Eva Pérez Guijarro, Quanyi Chen, Emily Wu, Isabella Church, April Huang, Jessica Bridge, Sung Chin, Cari Smith, Charli Gruen, Khiem C. Lam, Romina E. Araya, Antonella Sassano, Chi-Ping Day, Glenn Merlino, Romina S. Goldszmid. Interrogating the role of the immune microenvironment in the response of brain metastases to immunotherapy using new preclinical melanoma models. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5188.