Microglia/Macrophages in the Brain Metastatic Cascade

Brain metastases are a frequent and devastating complication occurring in up to 40% of patients with metastatic cancer. Due to the limited treatment opportunities of established brain metastases, the survival of patients is very limited ranging only from few weeks to months. The prevention of brain metastases with drugs targeting specific events in the development is a promising new clinical approach. Understanding the key factors regulating the development of brain metastases is the basis for finding new drug targets. Microglia cells are the main effector cells of the brain immune system In brain tumors microglia cells have various functions including cytotoxicity via nitric oxide release, antigen presentation and phagocytic activity. However, microglia cells might also be recruited by the brain tumor to provide tumor promoting factors. So far, little is known on the interaction of microglia cells and BM cancer cells and whether the interaction is tumor suppressing or promoting. Importantly, the impact of the interaction might change during the single steps of BM formation, namely extravasation, growth and dormancy in the perivascular niche, tumor cell invasion, induction of angiogenesis, and macrometastatic outgrowth. Within the present project, we hypothesize that the interaction of microglia cells and brain metastases cancer cells is pivotal for the successful outgrowth of macrometastases and that the inhibition of microglia cells could facilitate a preventive treatment approach. Using repetitive in vivo multiphoton laser microscopy, the steps of the brain metastatic cascade and the involved cell-cell interactions are investigated in real time over many weeks in a brain metastases mouse model. The used transgenetic mouse model facilitates to follow single brain metastases initiating cells in real time over the steps of brain metastatic cascade including arrest in the brain vascular system, extravasation as well as induction of neoangiogenesis and proliferation. Further, the interaction with microglia cells as well as circulating macrophages and the vascular system can be visualized, facilitating a deep insight on the involved interaction and their time depended importance. Based on the results a microglia or macrophages inhibiting compound is selected and investigated for brain metastases treatment and brain metastases preventive strategies. In addition, key proteins of microglia function are investigated in a unique large series of over 600 human brain metastases specimens and correlated with clinical characteristics including survival times. In conclusion, the results of the present project will provide a deeper in sight on the pivotal steps of brain metastases formation and provide the scientific background for new drugable targets for brain metastases treatment and prevention. This will help to improve the treatment and life of patients suffering from brain metastases.

Brain metastases are a frequent and devastating complication of metastatic cancer. Recently, even more patients were reported to suffer from this quality of life and life time limiting complication than previously believed. Treatment possibilities are currently limited and brain metastases cannot be cured. Therefore, given the limited treatment options prevention of brain metastases before their development as symptomatic disease is a reasonable new approach. However, the exact hallmarks of the brain metastatic cascade and more importantly treatable targets are still widely unknown. Therefore, the aim of this project was to understand the specific hallmarks of the brain metastatic cascade in order to formulate promising targets for preventive treatments in the future. In order to study the brain metastatic process in detail, we developed a unique mouse model that allows us to follow a single brain metastatic initiating cell in real time. This model therefore allows us to investigate important cell interactions and also to apply specific drugs and investigate their preventive potential. We focused especially on the interaction of the brain metastasis initialing cell and the microglia cells, which resemble the innate immune system of the brain. Microglia cells can be explained as the bodyguards of the brain as they prevent overwhelming infections in this delicate organ very effectively. However, tumor cells are known to utilize cells in their local microenvironment for their proliferation and therefore it has been a long standing question whether microglia cells are actually friends or foes of the brain metastasis initiating cells. Here, we discovered that an early interaction with microglia cell is crucial for the brain metastasis initiating cells to survive. Therefore, microglia cell support the early steps of the brain metastatic cascade and are actually friends during this step. In contrary, a dense interaction with microglia cells in the stadium of micrometastases (not yet symptomatic disease) resulted in a growth inhibition. Thereby the interaction changed from an initial protective relationship with the cancer cells to becoming a foe that inhibits the tumor growth. The relationship of brain metastasis cells and microglia cells is therefore with an initially supporting character changing to a tumor suppressing during the further growth of the brain metastasis. The exact, targetable molecular mechanisms of this interaction are currently mainly unknown, however we identified one targetable drug that has been in clinical trails. In our study, this specific inhibitor was not able to prevent the process of brain metastasis establishment. Therefore we need to investigate this interaction in more detail to discover further treatment options. In conclusion, we were able to establish a unique model to study the process of brain metastasis development what is the foundation to further understanding this process and investigate new preventive strategies in the future.