Long-term characterization of activated microglia/macrophages facilitating the development of experimental brain metastasis through intravital microscopic imaging

Background Microglia/macrophages (M/Ms) with multiple functions derived from distinct activation states are key surveillants maintaining brain homeostasis. However, their activation status and role during the brain metastasis of malignant tumors have been poorly characterized. Methods Heterozygous CX3CR1-GFP transgenic mice were used to visualize the dynamic changes of M/Ms during the development of experimental brain metastasis through long-term intravital imaging equipped with redesigned bilateral cranial windows. The occurrence of experimental brain metastasis was evaluated after M/Ms were depleted with PLX3397, a CSF-1R inhibitor. The possible mediators of M/Ms in facilitating the brain metastasis were determined using reverse transcription-PCR, immunofluorescence, correlational analysis, and MMP inhibition. Results Here, we showed that M/Ms were persistently activated and facilitated the formation of melanoma brain metastasis in vivo. We observed that M/Ms gradually and massively accumulated in the metastasis, with a 2.89-fold increase. To precisely depict the dynamic changes in the activation state of M/Ms, we defined the branching parameter to quantify their morphological alterations. The quantitative data showed that the extent of activation of M/Ms in metastatic foci was enhanced, with a 2.27-fold increase from day 1 to day 21. Along with the activation, the M/Ms increased their moving velocity (4.15-fold) and established a rapid, confined, and discontinuous motility behavior. The occurrence of melanoma brain metastasis was significantly hindered under M/M elimination, indicating the key role of M/Ms in the experimental brain metastasis. Interestingly, we found that M/Ms highly expressed matrix metalloproteinase 3 (MMP3), which were strongly correlated with M/M activation and the decrease of tight junction protein zonula occludens-1 (ZO-1). An MMP inhibitor moderately decreased the occurrence of melanoma brain metastasis, suggesting that MMP3 secreted by M/Ms may facilitate melanoma cell growth. Conclusions Our results indicated that the activated M/Ms were essential in the development of melanoma brain metastasis, suggesting that M/Ms are a potential therapeutic target for tumor brain metastasis.