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

We have examined the frequency of expression of individual alpha and beta 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 alpha, 16 V beta, C gamma and C delta probes. Comparison of the expressed repertoire of V beta gene segments in this newborn thymocyte 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 beta genes, V beta 12 and V beta 15, whose expression is decreased in the periphery possibly due to the effects of tolerance. An additional V beta gene segment (V beta 10) and a V beta gene family (V beta 5.1, V beta 5.2) 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 VH gene segments, D beta-proximal V beta genes are not overrepresented on this collection of immature thymocyte hybridomas. A similar analysis of V alpha gene family expression was hampered by the fact that V alpha 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 alpha gene segment was also found to express C delta mRNA and further examination revealed that several V alpha gene probes were actually detecting delta chain mRNA. The most notable of these was the V alpha 7 gene family, which accounted for approximately one-third of the expressed V alpha genes but was expressed exclusively as part of a delta chain mRNA. We found no examples of hybridomas co-expressing both full-length alpha and delta chain mRNA, despite significant numbers of hybridomas co-expressing full-length beta and gamma chain transcripts. This observation suggests that a large number of mature alpha/beta T cells may have proceeded through development without having gone through a stage where a delta chain message is expressed, thereby precluding their development into gamma/delta cells.