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

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Abstract
The Vγ5 chain of Tcell receptor γδ is preferentially expressed by murine fetal thymocytes. The fetal Vγ5 gene is known to be homogeneous and lacks N nucleotide addition.We previously reported that Vγ5+ 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 Vγ5+ 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 Vγ5 is determined at the level of Tcell precursor generation. In the present report, we analyzed whether the junctional region of FL‐derived Vγ5 genes in the FL chimeras is of fetal type. Sequencing analysis showed that Vγ5 genes from FL chimeras contained extensive N nucleotide addition and were diverse both in nucleotide sequences and deduced amino acid sequences. Vγ5 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 Vγ5 gene is determined at the level of Tcell precursor generation but the homogeneous Vγ5 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).