Neonatal vaccination with bacille Calmette-Guérin promotes the dendritic development of hippocampal neurons.

Dendritic structure is sensitive to changes in the environment during brain development. Accumulating evidence has demonstrated that early immune activation can significantly affect neuronal development. Our study concentrated on the morphological study of neural dendrites and spines in the hippocampal CA1 area using Diolistic labeling with Sholl analysis and fractal analysis. The results revealed that Bacille Calmette-Guérin (BCG) vaccination enhanced dendritic complexity, as reflected by the increased number of intersections, number of branch points and fractal dimension, and promoted neurite outgrowth. In addition, BCG increased the density and promoted the maturation of dendritic spines. The alterations in dendritic structure and spine morphology were observed at 2 and 4 w, but the differences were more apparent at 4 w than at 2 w. However, no significant difference was observed at 8 w. Furthermore, we observed that BCG increased the expression of hippocampal brain derived neurotrophic factor (BDNF) and insulin-like growth factor 1 (IGF-1). Hippocampal BDNF/IGF-1 was positively correlated with apical dendritic length, fractal dimension, and spine density. Taken together, we show in this study that neonatal BCG vaccination promotes dendritic development in developing hippocampal CA1 neurons, most likely by increasing the expression of BDNF and IGF-1 in the hippocampus.