Expression of DII4 and CCL25 in Foxn1-negative epithelial cells in the post-natal thymus

Foxn1 transcription factor is known to be essential for development of the thymic organ. We analyzed whether Foxn1 expression in thymic epithelial cells is necessary for the expression of functional molecules such as Delta-like 4 (Dll4) and CCL25, and whether maintenance of these molecular expressions depends on the Foxn1 transcription factor. We show that almost all thymic epithelial cells in the early thymus anlagen express Foxn1, and Dll4 and CCL25 are limitedly expressed in Foxn1-positive epithelial cells. The results are consistent with previous reports suggesting the indispensability of Foxn1 for epithelial cell differentiation which enables these cells to induce the expressions of CCL25 (Bleul, C. C. and Boehm, T. 2000. Chemokines define distinct microenvironments in the developing thymus. Eur. J. Immunol. 30:3371), Dll1 and Dll4 (Tsukamoto, N., Itoi, M., Nishikawa, M. and Amagai, T. 2005. Lack of Delta like 1 and 4 expressions in nude thymus anlages. Cell. Immunol. 234:77). On the other hand, the expression of Foxn1 was not detectable in a large number of post-natal thymic epithelial cells. Both Foxn1-positive and -negative epithelial cells seem to express Dll4 and CCL25. Therefore, the expressions of Dll4 and CCL25 are independent of Foxn1 transcription factor in the post-natal thymus. These results indicate that in the post-natal thymus, epithelial cells may maintain the expressions of those functional molecules without the aid of Foxn1 transcription factor.