Staining for Iba1, CD11c, and iNOS/Arg1 in P9 corpus callosum of APCfl/fl control (a) and Olig2-cre:APCfl/fl (b) mice. There is marked upregulation of CD11c expression (which marks both activated microglia and infiltrated macrophages) and iNOS expression in P9 Olig2-cre:APCfl/fl brain around OPC perivascular clusters (stained with dapi) compared to APCfl/fl brain. (c-d) Use of CX3CR1-GFP: CCR2-RFP mice (which label microglia green, and macrophages red) crossed into Olig2cre:APC fl/fl identifies these cells as predominantly activated microglia around OPC clusters. (e) Quantification of CCR2-RFP+ cells in P9 corpus callosum of APCfl/fl control and around OPC clusters in P9 CC of Olig2-cre:APCfl/fl (n=6 animals; APCfl/fl:CCR2-RFP vs.Olig2-cre:APCfl/fl:CCR2-RFP ***p = 0.0002). Data were analyzed by unpaired two-sided Student’s t test. Scale bars, 40 μm (a, b), 20 μm (c, d). *** P < 0.001. Values are mean ± s.d. For all staining results, experiments were repeated at least three times independently with similar results.