Neurofibromatosis-1 Heterozygosity Increases Microglia in a Spatially- and Temporally-Restricted Pattern Relevant to Mouse Optic Glioma Formation and Growth

While carcinogenesis requires the acquisition of driver mutations in progenitor cells, tumor growth and progression is heavily influenced by the local microenvironment. Previous studies from our laboratory have employed Neurofibromatosis-1 (NF1) genetically engineered mice to characterize the role of stromal cells and signals to optic glioma formation and growth. Previously, we have shown that Nf1+/- microglia in the tumor microenvironment are critical cellular determinants of optic glioma proliferation. To define the role of microglia in tumor formation and maintenance further, we employed CD11b-TK mice, in which resident brain microglia (CD11b+, CD68+, Iba1+, CD45low cells) can be ablated at specific times following ganciclovir (GCV) administration. GCV-mediated microglia reduction reduced Nf1 optic glioma proliferation during both tumor maintenance and tumor development. We identified the developmental window during which microglia are increased in the Nf1+/- optic nerve and demonstrated that this accumulation reflected delayed microglia dispersion. The increase in microglia in the Nf1+/- optic nerve was associated with reduced expression of the chemokine receptor, CX3CR1, such that reduced Cx3cr1 expression in Cx3cr1-GFP heterozygous knockout mice led to a similar increase in optic nerve microglia. These results establish a critical role for microglia in the development and maintenance of Nf1 optic glioma.