Figure 3.

Potential “monocyte-to-microglia” transition after neonatal LPS/HI injury. (A-D) At 4 d post-LPS/HI, the ipsilateral hippocampus (mainly in the CA, but not DG subfield) was populated by GFP⁺, RFP⁺, and RFP⁺/GFP⁺ double-positive amoeboid cells, without obvious tissue damage in R/G mice. (E-L) GFP⁺, RFP⁺, and RFP⁺/ GFP⁺ amoeboid cells were still detectable in the ipsilateral CA3 subfield of few NeuN⁺ neurons in R/G mice at 14 d post-LPS/HI (F-H). In contrast, the contralateral hippocampus only showed ramified GFP⁺ microglia at 14 after LPS/HI (E). The majority of GFP⁺ amoeboid cells in ipsilateral CA3 subfield were Iba1⁺ (I-J) and TNFα⁺ (K-L) at 14 d post-LPS/HI. (M-T'') Intravenous grafting of CCR2^RFP+; actin-GFP⁺ monocytes to wildtype P10 mice at 1 h after LPS/HI injury to assess their potential metamorphosis (M). At 1 d post-transfer, many round RFP⁺/ GFP⁺ double-positive cells were detected in the contralateral CPx (N) and, to a lesser degree, in ipsilateral CPx (O) and brain parenchyma (P, P'). At 3 d post-transfer, there were more GFP⁺-only than RFP⁺/ GFP⁺ double-positive amoeboid cells in the ipsilateral brain parenchyma (Q). At 7 d post-transfer, GFP⁺-only cells with a ramified microglia-like morphology were detected in the ipsilateral hemisphere (R). These GFP⁺-only microglia-like cells were Iba1⁺ (S) and often contained the EdU that was injected after adoptive transfer (T-T''). DG: dentate gyrus; CA: cornu ammonis. N = 3 mice for each time point in A-L, and n = 6 in N-P, each time point. Scale bar: 50 μm.