Functions of E2A-HEB Heterodimers in T-Cell Development Revealed by a Dominant Negative Mutation of HEB

Abstract

Lymphocyte development and differentiation are regulated by the basic helix-loop-helix (bHLH) transcription factors encoded by the E2A and HEB genes. These bHLH proteins bind to E-box enhancers in the form of homodimers or heterodimers and, consequently, activate transcription of the target genes. E2A homodimers are the predominant bHLH proteins present in B-lineage cells and are shown genetically to play critical roles in B-cell development. E2A-HEB heterodimers, the major bHLH dimers found in thymocyte extracts, are thought to play a similar role in T-cell development. However, disruption of either the E2A or HEB gene led to only partial blocks in T-cell development. The exact role of E2A-HEB heterodimers and possibly the E2A and HEB homodimers in T-cell development cannot be distinguished in simple disruption analysis due to a functional compensation from the residual bHLH homodimers. To further define the function of E2A-HEB heterodimers, we generated and analyzed a dominant negative allele of HEB, which produces a physiological amount of HEB proteins capable of forming nonfunctional heterodimers with E2A proteins. Mice carrying this mutation show a stronger and earlier block in T-cell development than HEB complete knockout mice. The developmental block is specific to the ␣/␤ T-cell lineage at a stage before the completion of V(D)J recombination at the TCR␤ gene locus. This defect is intrinsic to the T-cell lineage and cannot be rescued by expression of a functional T-cell receptor transgene. These results indicate that E2A-HEB heterodimers play obligatory roles both before and after TCR␤ gene rearrangement during the ␣/␤ lineage T-cell development.T lymphocytes are derived in the thymus following a stepwise developmental pathway. Each T cell acquires a unique T-cell receptor (TCR), composed of either an ␣/␤ or ␥/␦ heterodimer, on the cell surface after the V(D)J recombination at the corresponding TCR gene loci. The ␣/␤ cell lineage development in the thymus has been conveniently divided into several stages based on the expression of TCR and its coreceptor CD4 and CD8 surface molecules. The most immature population is negative for TCR, CD4, and CD8 expression (double negative, or DN). These cells progress and expand to the TCR low CD4 ϩ CD8 ϩ (double positive, or DP) stage, which makes up 70 to 80% of total cell mass of the thymus (11). DP cells are then subject to major histocompatibility complexmediated positive and negative selection before maturing into either TCR ϩ CD4 Ϫ CD8 ϩ cytotoxic T cells or TCR ϩ CD4 ϩ CD8 Ϫ helper T cells (single positive, or SP). These cytotoxic and helper T cells exit the thymus to peripheral lymph organs, where they provide and mediate antigen-specific immune responses, respectively.Lineage commitment and initiation of V(D)J recombination occur in the DN population, which is composed of less than 2% of total thymocytes in young adult mice. With additional markers such as CD44 and CD25, the DN cells can be further divided into precommitment (CD44 ϩ CD25 Ϫ , or DN1) and postcom...

The origin of hematopoietic cell type diversity

“…In contrast, T lineage differentiation is not affected by ectopic SCL expression in two transgenic models (Larson et al, 1996;Aplan et al, 1997;Herblot et al, 2002), indicating that transgene levels in these two models are not sufficient to inhibit E2A and HEB function in thymocytes, and requires collaboration with LMO proteins (Larson et al, 1996;Aplan et al, 1997). These observations are consistent with the finding that E2A interacts with HEB in driving thymocyte development (Sawada and Littman, 1993; Barndt et al, 2000). Another study, based on Notch1 signaling studies, indicates that a reduction in E2A function in the common lymphoid precursor is permissive for the T-cell lineage but not for the B-cell lineage, which requires an intact E2A activity (Busslinger et al, 2000).…”

Proviral activation of the tumor suppressor E2a contributes to T cell lymphomagenesis in EμMyc transgenic mice

“…The development of T lymphoid cells is partially arrested at the CD4/CD8 double negative (DN) stage before rearrangement of the TCRb genes (Bain et al, 1999). A similar block in T cell maturation is observed in mice expressing a dominant negative variant of HEB, a bHLH factor that is expressed in thymus and normally dimerizes with E2A (Barndt et al, 2000). In this context, dominant negative HEB dimerizes with E2A, but is unable to bind DNA.…”

Development of human plasmacytoid dendritic cells depends on the combined action of the basic helix‐loop‐helix factor E2‐2 and the Ets factor Spi‐B

“…In particular, E-proteins are crucial for development of lymphoid progenitors to the B-and T-cell lineages [26]. In the development of T cells, both E2A and HEB have been implicated [30][31][32]. E12 and E47 are essential for B-cell development by controlling either directly or indirectly the expression of Pax-5, a factor essential for B-cell lineage fate decision and securing of the B-cell identity until the mature stages of development [33][34][35][36][37].…”