Protocol to generate murine organotypic brain cultures for drug screening and evaluation of anti-metastatic efficacy

Organotypic brain cultures are short-term assays that phenotypically and functionally recapitulate brain metastatic cells in vivo. Here, we present a protocol to generate murine organotypic brain cultures for drug screening. We describe steps for sectioning of murine brains and plating of organotypic cultures. We then detail evaluation of the anti-metastatic effect of chemical compounds through bioluminescence imaging before and after drug treatment. Combined with downstream applications, this protocol allows comprehensive characterizations of both cancer cells and the tumor-associated microenvironment. For complete details on the use and execution of this protocol, please refer to Zhu et al. (2022).¹.

Abstract 5105: TIMP1 mediates astrocyte-dependent local immunosuppression in brain metastasis

Brain metastasis is an unmet clinical need, affecting between 10-30% of cancer patients with 200000 to 400000 newly diagnosis per annum in the US. Recently, several clinical trials have reported benefits using immunotherapy to treat brain metastasis. However, variability of the responses is broad and high benefit is found mainly in asymptomatic brain metastasis, while the benefit is dramatically reduced in the clinically relevant stage. 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 in these advanced stages of the disease. Our current study further exploited the heterogeneity within the metastasis-associated microenvironment as a resource to identify novel therapeutic vulnerabilities to improve the benefits of immunotherapies based on immune checkpoint blocking antibodies (ICB) in symptomatic brain metastasis.

Our results demonstrate that reactive astrocytes are strong immunomodulatory cells in brain tumors. We have identified the molecular profile of disease-associated glial cells and defined its connection to modulatory activities on specific lymphocyte populations in experimental brain metastasis as well as human-derived samples. scRNASeq and high content multiplex immunofluorescence allowed us to report a novel local immunomodulatory axis dependent on TIMP1 (astrocytes)/CD63 (CD8 T cells), which is present in brain metastasis patients with high immunoscore and would imply an additional immunosuppressive signal for potential ICB responders in brain metastasis.

Genetic and pharmacologic approaches targeting this STAT3-dependent local immunomodulatory axis have allowed us to define the rationale to combine immune checkpoint blockade with a STAT3 inhibitor, which we previously used in patients. We proved that such combined immunotherapy boost the systemic activation of T cells while also preventing the local blockade. Additionally, our comprehensive strategy includes the possibility to stratify patients that are best qualified to benefit from this therapy by measuring TIMP1 in liquid biopsies from CSF. Even more, our data using Patient Derived Organotypic Cultures (PDOC) from fresh brain metastasis neurosurgeries confirms that our therapeutic strategy might benefit brain metastases generated from any primary source.

In conclusion, we describe an immunosuppressive mechanism in the brain microenvironment that could explain the lack of response to ICB in patients with advanced brain metastasis. Our finding provides the rationale to implement complementary approaches targeting local immunosuppression to increase the benefit of immunotherapy in symptomatic brain metastasis.

Citation Format: Neibla Priego, Pedro García-Gómez, Ana de Pablos-Aragoneses, María Perea-García, Laura Álvaro-Espinosa, Carolina Hernández-Oliver, Elena Martínez-Saez, Ángel Pérez-Núñez, Aurelio Hernández-Laín, Rebeca Sanz-Pamplona, Marc Schmitz, Stephen J. Crocker, Diego Serrano, Asís Palazón, RENACER Red Nacional de Metástasis Cerebral, Manuel Valiente. TIMP1 mediates astrocyte-dependent local immunosuppression in brain metastasis. [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 5105.