Extensive N nucleotide addition in junctional region of T cell receptor V gamma 5 genes rearranged in fetal liver-derived thymocytes in radiation chimera mice.

The V gamma 5 chain of T cell receptor gamma delta is preferentially expressed by murine fetal thymocytes. The fetal V gamma 5 gene is known to be homogeneous and lacks N nucleotide addition. We previously reported that V gamma 5+ cells were detected among donor-derived thymocytes from irradiated C3H/He mice soon after reconstitution with AKR/J fetal liver (FL) cells, but that only a few V gamma 5+ were detected among donor-derived thymocytes from irradiated C3H/He mice reconstituted with adult bone marrow (ABM) cells. The results suggest that preferential use of V gamma 5 is determined at the level of T cell precursor generation. In the present report, we analyzed whether the junctional region of FL-derived V gamma 5 genes in the FL chimeras is of fetal type. Sequencing analysis showed that V gamma 5 genes from FL chimeras contained extensive N nucleotide addition and were diverse both in nucleotide sequences and deduced amino acid sequences. V gamma 5 genes from donor-derived thymocytes of ABM chimeras, which by polymerase chain reaction were revealed to be less frequent than those of FL chimeras, also showed extensive N addition. Furthermore, the terminal deoxynucleotidyl transferase (TdT) gene, which encodes an enzyme that adds N nucleotides into the junctional region, was expressed at high level in the donor-derived thymocytes of FL chimeras and normal 8-week-old AKR/J thymocytes but at low level in day 17 fetal AKR/J thymocytes. Our results suggest that the preferential rearrangement of the V gamma 5 gene is determined at the level of T cell precursor generation but the homogeneous V gamma 5 sequence of fetal type is generated only in the fetal thymic microenvironment where TdT gene expression is low or absent. The nomenclature of murine TcR gamma chains is according to Reilly et al. (Nature 1986. 321: 878). The relationship between the different nomenclature systems is summarized in Takagaki et al. (J. Immunol. 1989. 141: 2112).