Early-life stromal niches orchestrate B lymphopoiesis at the brain's borders.

The dura mater serves as a critical immunological niche for the central nervous system, yet the mechanisms governing the emergence of this niche in early life remain understudied. Here, we chart the trajectory of dural immune development, uncovering a distinctive function for the murine dura as a transient niche for B lymphopoiesis in the early-postnatal window. Shared embryonic progenitors initiate dural B cell development in concert with a multi-organ wave of extramedullary lymphopoiesis that contributes distinctively to the peripheral B cell pool. In the dura, B cells develop locally in discrete sinus-proximal foci, occupying an anatomically defined and developmentally restricted fibroblast niche. Sinus-proximal fibroblasts express the pro-hematopoietic chemokine CXCL12, and local deletion of this crucial factor severely impairs dural B lymphopoiesis. These data reveal a critical function for dural fibroblasts in shaping the early-life B cell compartment and provide a model for how extramedullary niches may support early-life leukocyte production.