Developmental progression of fetal HEB-/- precursors to the pre-T-cell stage is restored by HEBAlt

Gene knockout studies have shown that the E-protein transcription factor HEB is required for normal thymocyte development. We have identified a unique form of HEB, called HEBAlt, which is expressed only during the early stages of T-cell development, whereas HEBCan is expressed throughout T-cell development. Here, we show that HEB-/- precursors are inhibited at the beta-selection checkpoint of T-cell development due to impaired expression of pT alpha and function of CD3 epsilon, both of which are necessary for pre-TCR signaling. Transgenic expression of HEBAlt in HEB-/- precursors, however, upregulated pT alpha and allowed development to CD4(+)CD8(+) stage in fetal thymocytes. Moreover, HEBAlt did overcome the CD3 epsilon signaling defect in HEB-/- Rag-1(-/-) thymocytes. The HEBAlt transgene did not permit Rag-1(-/-) precursors to bypass beta-selection, indicating that it was not acting as a dominant negative inhibitor of other E-proteins. Therefore, our results provide the first mechanistic evidence that HEBAlt plays a critical role in early T-cell development and show that it can collaborate with fetal thymic stromal elements to create a regulatory environment that supports T-cell development past the beta-selection checkpoint.