A Pro-Metastatic Subpopulation of the Microenvironment that Controls Local Immunity in Brain Metastasis

Introduction

Brain metastasis is the most frequent brain tumor and remains an unmet clinical need. Targeted therapy and immunotherapy have a limited impact on the progression of the disease in most patients. Since local therapies (neurosurgery and radiation) are palliative, innovative therapeutic approaches are of great interest.

The growth of metastatic cells in the brain modify the local environment. We hypothesized that identification of metastases-associated molecular alterations in the microenvironment might uncover important aspects of the biology and lead to novel therapeutic opportunities to benefit a higher number of patients.

Material and method

Experimental and human brain metastasis were evaluated for a specific signalling pathway detected in the tumor microenvironment but absent in the normal brain. A genetically engineered mouse model that target this signalling pathway, specifically activated in a brain resident cell type when associated with metastases, was combined with brain metastatic cancer cells from lung cancer and melanoma. Ex vivo (organotypic brain cultures) and in vivo experiments were used in combination with genetic and pharmacologic strategies to probe the pro-metastatic behaviour of this component of the microenvironment. One of the inhibitors was validated in lung adenocarcinoma patients with brain metastasis. The subpopulation of the microenvironment was characterized using RNAseq and proteomics and mediators of its influence on metastatic cells validated through functional experiments.

Results and discussion

Phosphorylation of STAT3 was found to specifically label a subpopulation of reactive astrocytes (pSTAT3+ RA) in seven experimental brain metastasis models and 89% of human brain metastases from different primary tumors. Targeting STAT3 genetically or pharmacologically impairs the viability of brain metastasis even at advanced stages of the colonization (macrometastases). pSTAT3+ RA are functionally different from their counterparts since they acquire stem-cell like properties and are able to negatively influence adaptive immunity and promote pro-tumor macrophages/microglia within the metastatic lesion. The use of a blood-brain barrier permeable, non-toxic and orally bioavailable STAT3 inhibitor impaired brain metastases in experimental models and lung adenocarcinoma patients.

Conclusion

We provide the proof-of-concept for the development of a therapy targeting a resident cell type reprogrammed by the sustained presence of cancer cells in the brain. The identified molecular alteration labels a subpopulation of reactive astrocytes that behave as a local hub regulating immunosuppression. This finding applies to experimental and human brain metastases from breast cancer, lung cancer and melanoma suggesting that brain-specific therapies could be combined with others to improve the poor outcome of these patients.