Pro- versus anti-tumor activities of microglia/macrophages: Experimental and mathematical modeling approaches

Tumor infiltrating microglia/macrophages (TIMs) constitute the largest population of infiltrating cells in glioblastoma (GBM), the most aggressive brain tumor. Data from the clinic and from experimental work performed in murine and human models indicate that TIMs play a significant role in GBM biology as they support proliferation, migration and invasion of tumor cells. Evidence is amounting that tumor cells actually polarize TIMs towards this M2-like, tumor-supportive phenotype. We showed that toll-like receptor 3 ligand reverses this M2- into a M1-like phenotype. Pre-activated, M1-like polarized TIMs incubated with spheroids of GBM cells reduced migration, killed and phagocytosed tumor cells over a 15 day-period, indicating a sustained M1 activation of TIMs in absence of exogenously added stimuli. In order to analyse in more detail TIMs-GBM cells interactions, we have undertaken two approaches. We use spheroids of cells (tumor with/without TIMs) embedded in collagen matrix, in which TIMs can be implanted, as an experimental model. This three-dimensional in-vitro system is well suited for a qualitative and quantitative monitoring of cell proliferation, death, migration and invasion. Data experimentally generated are then implemented in a mathematical model that is, to our knowledge, the first one proposed to take into account tumor cells and TIMs in order to simulate GBM progressive behaviour. In a first step, the capacity of spheroids made of murine glioma cells to invade collagen matrices was evaluated in absence and presence of microglia. Invasion was monitored by photography of the spheroids and images were processed with Photoshop CS5 software. In order to achieve a precise determination of invasion, we chose to measure the diameter of the core plus the invasive rim as a read out of expansion rate. Untreated microglia promoted growth and invasion of tumor cells. Data generated through the proposed in-vitro system were comparable to and reproduced the insilico simulations obtained with the mathematical model, hence validating our mathematical and experimental approaches. We currently evaluate the rate of proliferation and death of tumor cells in various settings that modulate TIMs polarization, using flow cytometry and confocal (live) imaging. Data contributed by these two approaches should facilitate the delineation of a predictive model for tumor progression in a TIMs-enriched microenvironment with possible therapeutic fallouts.