T cell lineage determination precedes the initiation of TCRβ gene rearrangement

Loss of dendritic cell potential is one of the major events in intrathymic T cell development, during which the progenitors become determined to the T cell lineage. However, it remains unclear whether this event occurs in synchrony with another important event, TCRβ chain gene rearrangement, which has been considered the definitive sign of irreversible T cell lineage commitment. To address this issue, we used transgenic mice in which GFP expression is controlled by the lck proximal promoter. We found that the double-negative (DN) 2 stage can be subdivided into GFP^- and GFP⁺ populations, representing functionally different developmental stages in that the GFP ^-DN2, but not GFP⁺DN2, cells retain dendritic cell potential. The GFP⁺DN2 cells were found to undergo several rounds of proliferation before the initiation of TCRβ rearrangement as evidenced by the diversity of D-Jβ rearrangements seen in T cells derived from a single GFP⁺DN2 progenitor. These results indicated that the determination step of progenitors to the T cell lineage is a separable event from TCRβ rearrangement.