Extensive N nucleotide addition in junctional region of T cell receptor Vγ5 genes rearranged in fetal liver‐derived thymocytes in radiation chimera mice

The V_γ5 chain of Tcell receptor γ^δ is preferentially expressed by murine fetal thymocytes. The fetal V_γ5 gene is known to be homogeneous and lacks N nucleotide addition.We previously reported that V_γ5⁺ 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_γ5⁺ 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_γ5 is determined at the level of Tcell precursor generation. In the present report, we analyzed whether the junctional region of FL‐derived V_γ5 genes in the FL chimeras is of fetal type. Sequencing analysis showed that V_γ5 genes from FL chimeras contained extensive N nucleotide addition and were diverse both in nucleotide sequences and deduced amino acid sequences. V_γ5 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_γ5 gene is determined at the level of Tcell precursor generation but the homogeneous V_γ5 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 γ 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).