Remodeling the Th1 polarized systemic environment contributes to neurogenesis and cognitive function via the Wnt7a pathway in neonatal mice

•BCG-serum induces a Th1 polarized systemic environment in neonatal mice.•Remodeling Th1 polarized systemic environment improves hippocampal neurogenesis and cognitive function.•Remodeling Th1 polarized systemic environment activates canonical Wnt7a/β-catenin signaling pathway in hippocampus.•Remodeling Th1 polarized systemic environment elevates hippocampal BDNF levels implicated in hippocampal neurogenesis.•The effect induced by remodeling Th1 polarized systemic environment can be prevented by DKK1 in BCG-serum mice.

Neonatal Bacillus Calmette-Guérin (BCG) vaccination results in a positive effect on hippocampal neurogenesis and cognition. Serum cytokines are considered to be the chief culprit. In this study, serum from BCG-treated mice was identified as Th1 polarized serum. The serum showed an increased ratio of IFN-γ to IL-4 and decreased levels of TNF-α and IL-6. After Th1 polarized serum was injected intraperitoneally into postnatal mice, the levels of cytokines and ratio of IFN-γ to IL-4 in the serum and hippocampus of postnatal mice showed a similar alteration as those in Th1 polarized serum. This result indicated that the immune homeostatic milieu in postnatal mice was broken and the Th1 polarized systemic environment in the BCG-serum group was remodeled. The BCG-serum group displayed more BrdU+/DCX+ cells, BrdU+/NeuN+ cells, Nestin+ cells and better cognitive abilities. In neural stem cells, the Wnt7a/β-catenin signaling pathway was activated and exposure to the Wnt7a antagonist Dickkopf-1 inhibited BCG-serum-induced Wnt7a/β-catenin signaling, neurogenesis and cognitive function. Additionally, BCG-serum was associated with elevations in hippocampal brain-derived neurotrophic factor (BDNF) levels, and BDNF expression in the BCG-serum group was offset by Dickkopf-1 treatment. By rebalancing the Th1 polarized systemic environment in neonatal mice, it is possible that treatment with BCG-serum promotes hippocampal neurogenesis and improves cognitive functions, which are associated with Wnt7a/β-catenin-BDNF signaling.