The medulla controls effector primed γδT-cell development in the adult mouse thymus

γδT cells are produced in the thymus throughout life and provide immunity at epithelial-rich sites. Unlike conventional αβT cells, γδT-cell development involves intrathymic acquisition of effector function, with priming for either IL17 or IFN-γ production occurring during embryonic or adult life, respectively. How the thymus controls effector-primed γδT-cell generation in adulthood is poorly understood. Here, we distinguished de novo γδT cells from those undergoing thymus recirculation and/or retention using Rag2GFP mice alongside markers of maturation/effector priming including CD24, CD25, CD73, and IFN-γ, the latter by crossing with IFN-γ^YFP GREAT mice. We categorize newly developing γδT-cells into an ordered sequence where CD25⁺ CD73^- IFN-γ^YFP- precursors are followed sequentially by CD25^- CD73⁺ IFN-γ^YFP- intermediates and CD25^- CD73⁺ IFN-γ^YFP+ effectors. To determine intrathymic requirements controlling this sequence, we examined γδT-cell development in Relb^-/- thymus grafts that lack medullary microenvironments. Interestingly, medulla deficiency did not alter CD25⁺ γδT-cell precursor generation, but significantly impaired development of effector primed stages. This impact on γδT-cell priming was mirrored in plt/plt mice lacking the medullary chemoattractants CCL19 and CCL21, and also Ccl21a^-/- but not Ccl19^-/- mice. Collectively, we identify the medulla as an important site for effector priming during adult γδT-cell development and demonstrate a specific role for the medullary epithelial product CCL21 in this process.