Medullary stromal cells synergize their production and capture of CCL21 for T-cell emigration from neonatal mouse thymus

The release of newly selected αβT cells from the thymus is key in establishing a functional adaptive immune system. Emigration of the first cohorts of αβT cells produced during the neonatal period is of particular importance, because it initiates formation of the peripheral αβT-cell pool and provides immune protection early in life. Despite this, the cellular and molecular mechanisms of thymus emigration are poorly understood. We examined the involvement of diverse stromal subsets and individual chemokine ligands in this process. First, we demonstrated functional dichotomy in the requirement for CCR7 ligands and identified CCL21, but not CCL19, as an important regulator of neonatal thymus emigration. To explain this ligand-specific requirement, we examined sites of CCL21 production and action and found Ccl21 gene expression and CCL21 protein distribution occurred within anatomically distinct thymic areas. Although Ccl21 transcription was limited to subsets of medullary epithelium, CCL21 protein was captured by mesenchymal stroma consisting of integrin α7⁺ pericytes and CD34⁺ adventitial cells at sites of thymic exit. This chemokine compartmentalization involved the heparan sulfate–dependent presentation of CCL21 via its C-terminal extension, explaining the absence of a requirement for CCL19, which lacks this domain and failed to be captured by thymic stroma. Collectively, we identified an important role for CCL21 in neonatal thymus emigration, revealing the importance of this chemokine in initial formation of the peripheral immune system. Moreover, we identified an intrathymic mechanism involving cell-specific production and presentation of CCL21, which demonstrated a functional synergy between thymic epithelial and mesenchymal cells for αβT-cell emigration.