Thymocyte-dendritic cell interactions near sources of CCR7 ligands in the thymic cortex

Thymocyte maturation requires the migration through distinct regions in the thymus and interation with diverse cell types. In characterizing structures of the thymus, we describe Dendritic Cells (DCs) that are tightly associated with a cortical capillary network. We find that thymocytes expressing a transgenic, positive selecting TCR (P14) interact more extensively with DCs compared to thymocytes bearing polyclonal TCRs. In addition, overexpression of the chemokine receptor CCR7 in thymocytes results in a significant increase in DC‐thymocyte interactions in the cortex. Intriguingly, the cortical source of CCR7 ligand, CCL21, is not DCs but rather the blood vessels of the cortex which are in close contact with the cortical DCs. These observations suggest that CCR7 ligands produced by vasculature in the thymic cortex attract thymocytes that are undergoing positive selection, thus increasing the potential for cell‐cell interactions involved in T cell repertoire selection.