Systemic immune reprogramming by extracranial melanoma reshapes the melanoma brain metastasis microenvironment

Introduction: Melanoma brain metastases (MBM) remain a critical unmet need, with median survival of only ~6 weeks after immune checkpoint blockade (ICB) failure. This lethality is driven by an immunosuppressive tumor immune microenvironment enriched for tumor-associated macrophages (TAMs) and dysfunctional dendritic cells (DCs). Emerging evidence suggests bidirectional crosstalk between extracranial and intracranial tumors may reshape systemic immunity and influence therapeutic outcomes, yet the mechanisms remain unclear. We therefore investigated how an extracranial melanoma alters the cranial tumor-immune microenvironment and modulates ICB efficacy. Methodology: A multi-site murine MBM model was established using YUMM1.7 melanoma cells. Subcutaneous (extracranial) tumors were implanted 48 h prior to stereotactic intracranial injection. Tumor progression was monitored by bioluminescence imaging (BLI). At day 21, blood and major tissues (skin, brain, and organs) were excised and analyzed using flow cytometry (tumor and TAMs, DC, T-cell profiling) and Lunaphore COMET™ multiplex immunofluorescence (mIF) imaging for PD-1/PD-L1, and LRP-1 expression on TAMs/DCs/T-cells, to better understand the tumor-immune microenvironment modulation in both intracranial and extracranial site. Kaplan-Meier survival was plotted to determine the tumor growth dynamics. Results: The multi-site MBM model was successfully generated and validated by BLI and histologic immune cell infiltration at both tumor sites. Mice bearing both intracranial and extracranial tumors showed significantly delayed intracranial tumor growth compared with mice bearing intracranial tumors alone (p<0.05), indicating systemic immune reprogramming driven by the extracranial tumor. mIF further revealed differential expression of PD-1/PD-L1 interactions and LRP-1 on TAMs, DCs, and T cells in multi-tumor mice relative to intracranial-only controls (p<0.05), highlighting dynamic modulation of the intracranial TiME. Conclusion: Extracranial melanoma reshapes the cranial immune landscape and attenuates intracranial tumor progression through systemic tumor-immune microenvironment reprogramming. PD-1/PD-L1 and LRP-1 emerge as key regulators of immunosuppression in MBM, supporting combinatorial ICB strategies targeting these axes. Ongoing studies leveraging LRP-1 knockout models aim to refine mechanistic understanding and improve immunotherapeutic efficacy in MBM, addressing a major gap in neuro-oncology.