sTREM2 protein increases the expression of proinflammatory cytokines and induces morphological changes of microglia in mouse brain. (A) Fc and sTREM2-Fc proteins were analyzed by Western blotting after injection into the hippocampi of WT or Trem2-KO mice. (B) The hippocampi were dissected from Fc- or sTREM2-Fc–injected WT or Trem2-KO mice. After RNA extraction, the relative mRNA levels of IL-1β, IL-6, and TNF in the hippocampi shown as bar graph were determined by quantitative real-time PCR. β-Actin was used as an internal control (n = 4 per group, unpaired Student’s t test). The mRNA level of Fc-injected WT mice served as a control. (C) Coronal sections from Fc- or sTREM2-Fc–injected WT or Trem2-KO mice were stained with DAPI (blue) for nuclei, TREM2 (green) for sTREM2-Fc protein, and Iba1 (red) for microglia. Representative z stack images of CA3 region are shown. Bar, 50 µm. Iba1-labeled images on the right represent enlarged microglia; bar, 10 µm. (D–I) Analyses of morphological parameters include the area of cell body and total length of processes (D and G for CA3 region, E and H for CA2 region, F and I for CA1 region). At least 30 cells each from three mice were selected to quantify the morphological parameters (unpaired Student’s t test). Data represent mean ± SEM. *, P < 0.05; **, P < 0.01; ***, P < 0.001.