Brain metastasis is the cause of death of more than 40% of all cancer patients and is five times more prevalent than primary brain tumors. Melanoma, lung, and breast cancers are the three most common cancers metastasizing to the brain. Currently, radiation and chemotherapy are the gold standard for the treatment of brain metastasis. However, despite the efficacy of current T cell-based immunotherapies in primary cancers, recent clinical trials have demonstrated little to no benefit in brain metastasis patients. Thus, we predict that tertiary lymphoid structures (TLS) could provide the cellular niches for increased T cell influx and activity. TLS are ectopic lymphoid structures consisting of clusters of B and CD4+ T cells with the presence of high endothelial venules (HEVs) and follicular dendritic cells (FDCs) as hallmarks of TLS formation. B cells and TLS correlate with superior response to immunotherapies and greater overall survival in solid tumors. Despite the positive prognostics of TLS and B cells in solid primary tumors, they are critically understudied in brain metastasis. However, the expression of CXCL13, a key initiating factor of TLS, has been shown to correlate with greater overall survival in melanoma brain metastasis patients. In this study, we hypothesize that B cell infiltration and TLS signatures in brain metastases would correlate with improved anti-tumor immunity and better overall survival. Utilizing multispectral imaging, we demonstrate that despite lacking TLS with canonical hallmarks (HEVs and FDCs), brain metastasis patients have specialized lymphoid structures consisting of proliferating B and T cells, potentially early TLS. We observed that the presence of these lymphoid structures correlates with increased CD8+ T cell infiltration. More specifically, CD8+ T cells are more likely to localize intratumorally in melanoma brain metastasis and in non-tumor regions in LBM patients. Additionally, we identified more active lymphoid aggregates by Ki67 staining in melanoma-brain metastasis patients relative to lung-brain metastasis, suggesting that the primary tumor could influence the type of lymphoid structures in the brain. We also found a correlation between TLS at the primary tumor site and increased B cell infiltration and early TLS formation at the metastatic site in lung cancer patients. Utilizing spatial proteomics and transcriptomics, we aim to carry out an in-depth analysis of the immune activation of the lymphoid structures and key molecular pathways in the tumors of brain metastasis. In conclusion, uncovering differences in B cells and lymphoid aggregates will set the ground truth for structure formation in brain metastases, which will help elucidate key immune targets related to TLS function in these immunologically unique tumors.

Citation Format: Noor Nader, Elaine Byrnes, Sheryl Kunning, Thomas Pearce, Gabriel Sica, Timothy Burns, Laura Stabile, Xiaoran Zhang, Tullia Bruno. Lymphoid aggregates dictate immune activity in melanoma and lung brain metastases [abstract]. In: Proceedings of the AACR Special Conference on Brain Cancer; 2023 Oct 19-22; Minneapolis, Minnesota. Philadelphia (PA): AACR; Cancer Res 2024;84(5 Suppl_1):Abstract nr B005.