Recruitment of regulatory T cells with rCCL17 promotes M2 microglia/macrophage polarization through TGFβ/TGFβR/Smad2/3 pathway in a mouse model of intracerebral hemorrhage.

Aims: Intracerebral hemorrhage (ICH) is a severe neurological condition with high mortality and morbidity. Microglia activation and peripheral inflammatory cells infiltration play an important role in ICH prognosis. Previous studies demonstrated that regulatory T cells (Tregs) ameliorated neuroinflammation following experimental ICH. However, the molecular mechanism underlying such effects of Tregs remains unclear. The objective was to examine how Tregs recruitment induced by recombinant CC chemokine ligand 17 (rCCL17) influences microglia/macrophage polarization in an intrastriatal autologous blood injection ICH animal model, and to determine if TGFβ/TGFβ-R/Smad2/3 pathway was involved.

Methods: 380 adult CD1 mice (male, eight weeks old) were subjected to sham surgery or autologous blood injection induced ICH. A CD25-specific mouse antibody or isotype control mAb was injected intraventricular (i.c.v) 48 h prior to ICH induction to deplete Tregs. rCCL17, a CC chemokine receptor 4 (CCR4) ligand, was delivered intranasally at 1 h post-ICH. SB431542, a specific inhibitor of TGF-β was administered intraperitoneally 1 h before ICH induction. Following the ICH, neurobehavioral testing, brain edema, hematoma volume, hemoglobin content, western blotting, double immunofluorescence labeling, and immunohistochemistry were performed.

Results: Endogenous expressions of CCL17, Tregs marker Foxp3, and the number of Tregs in perihematomal region increased following ICH. Tregs depletion with a CD25 antibody aggravated neurological deficits and brain edema, increased inflammatory cytokines, neutrophil infiltration, oxidative stress, and reduced the rate of hematoma resolution in ICH mice. rCCL17 treatment increased the number of Tregs in the brain, ameliorated neurological deficits and brain edema after ICH, and promoted microglia/macrophage polarization toward M2 phenotype which was reversed with CD25 antibody. Moreover, rCCL17 increased the expressions of brain TGF-β/phosphorylated-Smad2/3 which was abrogated with the selective TGFβ inhibitor SB431542.

Conclusions: rCCL17-mediated Tregs recruitment may be a potential therapeutic strategy to promote M2 microglia/macrophages polarization and alleviate early brain injury following ICH.