Figure 4

Both microglia and macrophages comprise the immune system in the cochlear sensory epithelium. (a) The Cd200-Cd200r1 system restricts microglial functions in the brain. Cd200 (green), the ligand of Cd200r1, was mainly expressed in HCs, but also weakly in SCs and GERCs. Cd200r1 (green) was expressed in SCs and GERCs. These findings indicate that Cd200-Cd200r1-specific intercellular signalling restricts SC/GERC functions to macrophages. During virus infection, Cd200r1 mRNA was significantly downregulated (Mock 9 h: n = 3, TMEV 9 h: n = 4, Mock 16 h: n = 4, TMEV 16 h: n = 3), despite little change in Cd200 expression (Mock 9 h: n = 3, TMEV 9 h: n = 4, Mock 16 h: n = 3, TMEV 16 h: n = 3) (*P < 0.05, **P < 0.01, t-test). These findings indicate that virus infection removes the Cd200-Cd200r1 restriction on SCs and GERCs, which induces their differentiation into macrophages. (b) In uninfected cochleae, Iba1-positive cells (green) were concentrically arranged beneath OPCs and DC1s, which might be cochlear resident microglia (arrowhead; outer PC: OPC, inner PC: IPC, DC1: first row of DCs, DC2: second row of DCs, DC3: third row of DCs). (c) Cochlear resident microglial cells were not observed in their original location during the virus infection. Scale bars, 20 µm. Error bars, standard errors.