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

Abstract

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 À/À precursors are inhibited at the b-selection checkpoint of T-cell development due to impaired expression of pTa and function of CD3e, both of which are necessary for pre-TCR signaling. . Transcription factors that have been implicated in T-cell specification and commitment include Notch1/RBP-J, TCF-1 (Tcf7), GATA-3 (GATA-3), E2A (TCF3) and HEB (TCF12) [2]. Signaling from Notch receptor, IL-7R and pre-TCR are integrated into this regulatory background to form a network of gene interactions that drive T-cell differentiation and commitment. Surprisingly, however, the gene networks that operate during T-cell development are among the least understood within hematopoiesis.The earliest T-cell precursors in the thymus lack the expression of CD4 and CD8 and are referred to as double ) T cells. Signaling through the pre-TCR is an absolute requirement for progression from the DN to the DP stage of development [5]. The pre-TCR complex is composed of a rearranged TCRb chain, a pTa chain, and CD3 chains, which are necessary for the initiation of the pre-TCR signal. Gene targeting studies have shown that CD3e is required for the development beyond the DN3 stage [6], whereas the loss of either CD3 g or CD3 d results in a mild block [7]. Successful pre-TCR signaling leads to cellular proliferation, differentiation, survival, and allelic exclusion of the TCRb gene, events collectively referred to as b-selection [8,9]. Id3 (ID3) upregulation occurs rapidly following pre-TCR signaling [10] and thus serves as a useful marker for evaluating passage through the b-selection checkpoint.Gene knockout studies have shown that the E-protein transcription factors HEB and E2A play critical and non-redundant roles in normal T-cell development [11]. E2A À/À precursors can become DP cells even in the Rag-deficient background, which is indicative of a defective ''gatekeeping'' function that prevents differentiation of cells lacking TCRb rearrangements [12]. HEB À/À mice, by contrast, display an accumulation of ISP cellsand an overall thymic hypocellularity [13]. Moreover, HEB directly upregulates pTa [14,15]. However, the exact nature of the block in HEB À/À T-cell development has not been well established, and the mechanisms by which HEB uniquely regulates early T-cell development have not been defined. We have previously shown that the HEB gene locus encodes two transcription factors: a long form, HEBCan, that is closely related to E2A and a shorter form, HEBAlt. Although E2A and HEBCan are expressed throughout T-cell development, the expression of HEBAlt is restricted to the DN stages [16]. Therefore, we wanted to investigate whether HEBAlt was involved in regulating the genes required for pre-TCR signaling. To ad...

Transcriptional Regulation of Innate and Adaptive Lymphocyte Lineages

“…It is thought that HEB collaborates with E47 to enforce cell cycle arrest preceding the β-selection checkpoint. HEBAlt appears to function redundantly with HEB, given that ectopic expression of HEBAlt in Heb-deficient cells restored the generation of DP progenitors (241). HEB proteins may also play a role in inhibiting myeloid and NK cell fates in T cell progenitors (242).…”

An Overview of the Intrathymic Intricacies of T Cell Development

“…Separate transcriptional start sites in HEB give rise to canonical and shorter alternate (HEBAlt) forms (26). A developmental block in HEB 2/2 cells observed at the b-selection checkpoint can be partially restored with transgenic expression of HEBAlt, bypassing b-selection, to the DP stage, thus implicating HEBAlt as a critical regulator of early T-lineage genes (45). In addition, HEBAlt limits myeloid cell outcomes by collaborating with intracellular Notch (46).…”

HEB in the Spotlight: Transcriptional Regulation of T-Cell Specification, Commitment, and Developmental Plasticity

“…GATA3 is essential for CD4 gene expression, whereas Runx3 is a direct repressor of CD4 [108, 109]. We found that although HEB deficiency at the DN3 stage did not affect the expression of GATA3, transgenic reconstitution of HEB −/− cells with HEBAlt resulted in the upregulation of CD4 to generate DPs [110]. Therefore, another possibility is that HEB factors, and HEBAlt in particular, function in repressing Runx3 protein expression or activity.…”

“…Furthermore, we identified pT α and CD3 signaling components as specific targets of HEBAlt during β -selection [110]. In addition, HEB −/− mice also had a defect in T-cell commitment, with compromised Notch1 function and a tendency to become DN1-like cells [106].…”