Immune Cells Promote <scp>BDNF</scp> Expression by Infiltrated Macrophages via Interleukin 4 in the Cerebral Ischemia of Male Rats

We reported that infiltrated Ly6C⁺ macrophages express brain-derived neurotrophic factor (BDNF) only at the cerebral cortex infarct in a rat dMCAO model. However, the changein neuron-expressed BDNF, the niche components that induce the Ly6C⁺ cells to express BDNF, and the cellular sources of these components, remain unclear. In this study, immunofluorescence double staining was performed to label BDNF and Ly6C on brain sections at 3, 24, and 48 h following distal middle cerebral artery occlusion (dMCAO) of male rats, and to stain BDNF with Ly6C, IL-4R, and IL-10R. A neutralizing anti-IL-4 antibody was injected into the infarct, and the IL-4 and BDNF concentrations in the subareas of the infarct were determined using enzyme-linked immunosorbent assay. To find out the cellular sources of IL-4, the markers for microglia, T cells, and neurons were co-stained with IL-4 separately. In certain infarct subareas, the main BDNF-expressing cells shifted quickly from NeuN⁺ neurons to Ly6C⁺ cells during 24-48 h post-stroke, and the Ly6C⁺/BDNF⁺ cells mostly expressed IL-4 receptor. Following IL-4 neutralizing antibody injection, the BDNF, IL-4 protein levels, and BDNF⁺/Ly6C⁺ cells decreased significantly. The main IL-4-expressing cell type in this infarct subarea is not neuron either, but immune cells, including microglia, monocyte, macrophages, and T cells. The neurons, maintained BDNF and IL-4 expression in the peri-infarct area. In conclusion, in a specific cerebral subarea of the rat dMCAO model, IL-4 secreted by immune cells is one of the main inducers for Ly6C⁺ cells to express BDNF.