Selective induction of Arg1 expression in a subset of microglia. Brains after 16 h intracerebroventricular treatment with vehicle (A,C) and IL4 (B,D) were formalin fixed and embedded in paraffin and analyzed by immunohistochemistry to visualize Iba1 (A,B) or Arg1 (C,D) in the frontal cortex. Arg1 expression is observed selectively in microglia-like cells after IL4 treatment and is restricted to a subset of cells (D). Scale bar 100 μm. The results obtained by counting the number of Arg1-positive microglia cells and the total number of Iba1-positive microglia cells demonstrated that an ≈ 35% subset of microglia cells is Arg1 responder microglia (E). Veh., vehicle.