Tmic-02. pre-surgical immune checkpoint blockade reshapes the immune landscape and spatial architecture of metastatic brain tumors

Abstract Brain tumors reside in a modified microenvironment where inflammatory processes are heavily regulated. Nevertheless, our recent study showed a significant immune infiltration in recurrent glioblastoma (rGBM) treated with neoadjuvant PD-1 checkpoint blockade therapy. In comparison to GBM, the intracranial response of brain metastases (BrM) to immunotherapy has been less well studied. To test this question, we investigated the effect of pre-surgical immune checkpoint blockade (ICB) on the immune compartments of BrM, using multiplex immunofluorescence, single-cell RNA sequencing and spatial transcriptomics on a total of 17 patients. We found that ICB naïve BrM featured an immune exclusion phenotype with accumulation of immune cells in the fibrovascular stroma, necrotic and hemorrhagic peritumoral areas. Following ICB treatment, T cells and monocyte-derived macrophages were then observed within the tumor parenchyma. Computational receptor-ligand analysis suggested that the tumor-infiltrating T cells and macrophages were highly engaged by checkpoint receptor-ligand pairing, which may drive the T cells into an advanced exhausted state. To further explore the organization and interactions between the immune, stroma and tumor cells in BrM, we utilized the sequencing-based spatial transcriptomics technology. Our spatial transcriptomics data further highlighted the distinct spatial architectures between ICB naïve and –treated BrM tumors. It also pinpointed a unique macrophage subset that are preferentially co-localized with the brain vasculature cells and confined the immune infiltrates to the perivascular space. In summary, our study revealed ICB-induced marked changes in the immune composition and spatial organization in BrM and offers insights into new combination therapies that can help improve the clinical efficacy of immunotherapy for BrM patients.