Although it is well established that early expression of TCRb transgenes in the thymus leads to efficient inhibition of both endogenous TCRb and TCRc rearrangement (also known as allelic and "isotypic" exclusion, respectively) the role of pTa in these processes remains controversial. Here, we have systematically re-evaluated this issue using three independent strains of TCRb-transgenic mice that differ widely in transgene expression levels, and a sensitive intracellular staining assay that detects endogenous TCRVb expression in individual immature thymocytes. In the absence of pTa, both allelic and isotypic exclusion were reversed in all three TCRb-transgenic strains, clearly demonstrating a general requirement for pre-TCR signaling in the inhibition of endogenous TCRb and TCRc rearrangement. Both allelic and isotypic exclusion were pTa dose dependent when transgenic TCRb levels were subphysiological. Moreover, pTa-dependent allelic and isotypic exclusion occurred in both ab and cd T cell lineages, indicating that pre-TCR signaling can potentially be functional in cd precursors. Finally, levels of endogenous RAG1 and RAG2 were not down-regulated in TCRb-transgenic immature thymocytes undergoing allelic or isotypic exclusion. Collectively, our data reveal a critical but lineage-nonspecific role for pTa in mediating both allelic and isotypic exclusion in TCRb-transgenic mice.
IntroductionT cells can be divided into ab and cd lineages based on their expression of TCRab or TCRcd. During development in the thymus, ab and cd lineages separate at an early CD4 -CD8 -double-negative (DN) stage, but the precise mechanism of lineage divergence remains controversial.One molecule that has been postulated to play a key role in ab/cd lineage commitment is the pTa chain. pTa is a critical component of the pre-TCR which consists, in addition, of a functionally rearranged TCRb chain and CD3 signaling components. Signaling through the pre-TCR has been reported to be responsible for many important functions during the development of ab lineage cells, including TCRb allelic exclusion [1-3], TCRc silencing [4], and extensive proliferation during the transition from DN to CD4 + CD8 + double-positive (DP) stages [5]. Since pTa -/-mice have an increased number of cd cells that harbor an increased frequency of in-frame TCRb rearrangements [6], it has also been proposed that the pre-TCR favors ab lineage commitment at the expense of cd cells [6]. Since functions of pTa are often difficult to study in normal mice, several groups have crossed pTa-deficient mice with TCRb-transgenic (Tg) mice to assess various putative roles for the pre-TCR during thymus development [7,8]. However, the results of these studies are in many cases controversial and it is still not known whether transgenic "artifacts" caused by overexpression or premature expression of the transgenic TCRb chain may obscure the contribution of pTa to the observed phenotypes.In order to re-evaluate this issue in a systematic manner, we have utilized three independent TCRb Tg mouse...