IL‐16 is differentially expressed in the developing human fetal brain by microglial cells in zones of neuropoesis

Microglial cells are regulators of tissue homeostasis in the adult central nervous system and readily participate in pathological processes, orchestrating tissue remodeling. Cytokines produced by microglial cells are markers of cell activation and contribute to reactive processes. In this paper, we have studied the expression of IL‐16 (leukocyte chemoattractant factor), a natural soluble ligand to the CD4 molecule, in human fetal brains from the 11th to the 20th. week of gestation by immunohistochemistry. Interleukin (IL)‐16+ cells were detected already at the 11th gestational week, accumulating with aging in cortical layers (P80%) revealed morphological hallmarks of activated microglia. We observed that IL‐16 cells coexpress LCA (>80%) and MRP‐8, an activation‐associated Ca2+ binding S‐100 family member (>80%). In contrast, only few IL‐16+ cells proliferated (PCNA+, 20–40%) or co‐expressed the HLA‐DR, ‐DP, or ‐DQ antigen (