Thymocyte development: an analysis of T cell receptor gene expression in 519 newborn thymocyte hybridomas

Abstract

We have examined the frequency of expression of individual α and β chain V gene families in a population of immature T cells that has not been selected or tolerized. To accomplish this, we generated 519 T cell hybridomas from freshly isolated thymocytes of newborn C57BL/10 mice and subjected RNA from these hybrids to hybridization analysis with 11 V_α, 16 V_β, C_γ and C_δ probes. Comparison of the expressed repertoire of V_β gene segments in this newborn thymocte population with similar data previously generated from adult peripheral T cells (Bill et al., Proc. Natl. Acad. Sci. USA 1988. 85: 9184.) revealed two V_β genes, V_β12 and V_β15, whose expression is decreased in the periphery possibly due to the effects of tolerance. An additional V_β gene segment (V_β10) and a V_β gene family (V_β5.1, V_β5.2) were expressed more frequently in the mature, peripheral population than in the newborn thymus. These findings may represent two instances of positive selection of T cells. Furthermore, unlike V_H gene segments, D_β-proximal V_β genes are not overrepresented on this collection of immature thymocyte hybridomas. A similar analysis of V_α gene family expression was hampered by the fact that V_α gene segments were expressed in only 15% of newborn thymocyte hybridomas (compared to 58% of adult hybridomas). An unexpectedly large fraction (57%) of those newborn hybrids expressing a V_α gene segment was also found to express C_δ mRNA and further examination revealed that several V_α gene probes were actually detecting δ chain mRNA. The most notable of these was the V_α7 gene family, which accounted for approximately one-third of the expressed V_α genes but was expressed exclusively as part of a γ chain mRNA. We found no examples of hybridomas co-expressing both full-length β and δ chain mRNA, despite significant numbers of hybridomas co-expressing full-length β and γ chain transcripts. This observation suggests that a large number of mature α/β cells may have proceeded through development without having gone through a stage where a δ chain message is expressed, thereby precluding their development into γ/δ cells.