In the thymus, CCRL1 is abundant in cTECs but not mTECs or thymocytes [20]. In fetal mice, CCRL1 regulates the migration of thymocyte precursors before vascularization [19]. It has been reported that CCRL1 deficiency results in thymus enlargement in adult mice, in association with altered thymocyte development and autoimmunity [21]. Thus, CCRL1 is important for optimal thymus homeostasis and normal thymocyte development. To analyze the expression of CCRL1 in TECs during embryogenesis,

Ribeiro et al. [18] use CCRL1-EGFP-knockin mice, in which EGFP is expressed under the control of CCRL1 gene expression [20]. By crossing CCRL1-EGFP-knockin mice with IL-7-YFP-transgenic EPZ-6438 research buy mice, and by flow cytometry analysis of embryonic TECs, the authors show that CCRL1 expression progressively increases during fetal cTEC development. The emergence of CCRL1-EGFPhigh cells, which are class II MHChigh CD40high cTECs, is diminished in RAG2/IL2Rγ double-deficient mice, in which thymocyte development is arrested at an early stage. From these results, the authors conclude that CCRL1high cTECs represent late-appearing mature cTECs, and that the development of those mature cTECs is regulated by

the signals provided by developing this website thymocytes. These results agree with previous reports showing that thymocyte-derived signals are necessary for the late maturation of cTECs [4-6]. Ribeiro et al. [18] also show that CCRL1+UEA1–CD80– cTECs isolated from E15.5 fetal thymus give rise to UEA1+CD80+ mTECs, when cultured in the presence of RANK and CD40 stimulation in RTOCs, suggesting that CCRL1+ fetal cTECs contain mTEC progenitor activity. These results agree with the recent reports discussed above showing that pTECs progress through a stage in which they express cTEC-associated molecules before diversifying into mTECs [11, 14-16] (Fig. 1). Perhaps very more interestingly, Ribeiro et al. [18] go beyond the confirmation of other studies to report that CCRL1-EGFPlow cells in the thymus are not restricted to CD205+ Ly51+ cTECs but also contain UEA1+ mTECs, despite the fact that CCRL1-EGFPhigh cells are

limited to cTECs but not mTECs. The CCRL1-EGFPlow CD80+ UEA1+ mTECs were detectable only after birth. Gene expression analysis showed that this late-appearing subpopulation of mTECs, which was identified by the CCRL1-EGFPlow CD80+ phenotype, contained large amounts of Aire and RANK mRNAs but a nondetectable amount of CCL21 mRNA. Ribeiro et al. [18] further demonstrate that the combination of RANK and CD40 signals, the ligands of which are produced by positively selected thymocytes [8, 10], is important for the development of CCRL1-EGFPlow mTECs. Thus, the analysis of CCRL1-EGFP reporter mice suggests a novel heterogeneity in postnatal mTECs. It has been shown that mTECs are heterogeneous in terms of the expression of various molecules, including class II MHC, CD80, Aire, and CCL21 [22-26]. White et al.