CCR7 and Autoimmunity: Of the CCR7 ligands, it has recently been shown that CCL21a is the major regulator of CCR7‐mediated SP thymocyte migration, and in its absence there is a failure in thymic tolerance that leads to autoimmunity.43 Importantly, the thymus medulla is also rich in a heterogeneous mixture of DCs, which play a key role in both negative selection and Foxp3+ T‐cell development.