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 β-selection checkpoint of T-cell development due to impaired expression of pTα and function of CD3ε, both of which are necessary for pre-TCR signaling. Transgenic expression of HEBAlt in HEB(-/-) precursors, however, upregulated pTα and allowed development to CD4(+) CD8(+) stage in fetal thymocytes. Moreover, HEBAlt did overcome the CD3ε signaling defect in HEB(-/-) Rag-1(-/-) thymocytes. The HEBAlt transgene did not permit Rag-1(-/-) precursors to bypass β-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 β-selection checkpoint.