BSBM-03 A NOVEL COMBINED IMMUNOTHERAPY TO IMPROVE THERAPEUTIC RESPONSES IN SYMPTOMATIC BRAIN METASTASIS BASED ON TARGETING A SUBPOPULATION OF IMMUNOSUPPRESSIVE ASTROCYTES

Abstract The diagnosis of brain metastasis involves high morbidity and mortality. Recently, immune checkpoint blockade antibodies (ICB) have shown clinical benefits mainly when applied to asymptomatic brain metastasis patients. However, variability is broad and responses to this therapy drop considerably when treating clinically relevant disease. Although potentially involved in the lack of response to immunotherapy, corticoids do not seem to fully explain this situation. Thus, it is currently unknown how to effectively target symptomatic brain metastases with immunotherapy. We previously reported a clinically relevant protumoral program driven by STAT3 activation in a subpopulation of reactive astrocytes during advanced stages of the disease. By further exploiting astrocyte heterogeneity, we have found a potential strategy to improve the number of responders to immunotherapy in symptomatic brain metastasis. Specifically, we have developed a comprehensive strategy including genetic and pharmacologic approaches to define a novel immunosuppressive axis involving astrocyte-secreted TIMP1 signaling on CD63+ CD8+ T cells. Based on these findings, we developed a combined immunotherapy to boost the systemic activation of T cells with ICB while blocking local TIMP1-dependent immunosuppression in various preclinical models. Furthermore, the detection of TIMP1 in the CSF provides a biomarker to select patients who would benefit the most from the combined immunotherapy. Even more, our data using Patient Derived Organotypic Cultures from fresh brain metastasis neurosurgeries confirmed that our therapeutic strategy is valid for symptomatic brain metastases from any primary source. In conclusion, our study has identified a novel combined immunotherapy that could be especially relevant for symptomatic brain metastases. Our findings have emerged from a strong scientific rationale whereby a specific subpopulation of astrocytes is shown to activate the local immunosuppressive environment for brain-infiltrating CD8+ T cells.