β-Selection: Abundance of TCRβ(–)/γδ(–) CD44(–)CD25(–) (DN4) cells in the foetal thymus

Expression of TCRβ and pre-TCR signalling are essential for differentiation of CD4(–)CD8(–) double negative (DN) thymocytes to the CD4(+)CD8(+) double-positive (DP) stage. Thymocyte development in adult Rag1, Rag2 or TCRβδ-deficient mice is arrested at the DN3 stage leading to the assumption that pre-TCR signalling and β-selection occur at, and are obligatory for, the transition from DN3 to DN4. We show that the majority of DN3 and DN4 cells that differentiate during early embryogenesis in wild-type mice do not express intracellular (ic) TCRβ/γδ. These foetal icTCRβ(−)/γδ(−) DN4 cells were T lineage as determined by expression of Thy1 and icCD3 and TCRβ DJ rearrangement. In addition, in the foetal Rag1(–/–) thymus, a normal percentage of DN4 cells were present. In wild-type mice after hydrocortisone-induced synchronisation of differentiation, the majority of DN4 cells that first emerged did not express icTCRβ/γδ, showing that adult thymocytes can also differentiate to the DN4 stage independently of pre-TCR signalling. Pre-TCR signalling induced expansion in the DN4 population, but lack of TCRβ/γδ expression did not immediately induce apoptosis. Our data demonstrate in vivo differentiation from DN3 to DN4 cell in the absence of TCRβ/γδ expression in the foetal thymus, and after hydrocortisone treatment of adult mice.