TCR repertoire in early fetal mouse thymus

We investigated the rearrangement and expression of TCR genes in mouse fetal thymus organ culture, a system that avoids subsequent entry of hematopoietic precursor cells. The first observable rearranged TCR gene was homogeneous V gamma 2-J gamma 2, detectable as early as fetal day 11 (d11) in the thymic primordia. The productive TCR was homogeneous V gamma 5-J gamma 1, first detectable in d13 thymocytes, followed by adult-type TCR gamma (V gamma 4 and V gamma 7). Sequence analysis of TCR revealed five types of V-J junctional sequences. In the very early stage, a homogeneous V-J junction is generated via a short homology sequence in the coding region (Type I), while a short homology sequence in the P-nucleotide rather than the coding region is used in the following stage (Type II). In the later embryonic stages, diverse V-J junctions are generated by well-known mechanisms, such as P-nucleotide (Type III), N-region insertion (Type IV) or trimming of the coding ends (Type V). These findings suggest that the generation of homogeneous TCR gamma (V gamma 2 and V gamma 5) in the early fetal stages is due to the intrinsic rearrangement mechanisms and is in stage specific manner.