Competition and collaboration: GATA-3, PU.1, and Notch signaling in early T-cell fate determination

Abstract

T-cell precursors remain developmentally plastic for multiple cell generations after entering the thymus, preserving access to developmental alternatives of macrophage, dendritic-cell, and even mast-cell fates. The underlying regulatory basis of this plasticity is that early T-cell differentiation depends on transcription factors which can also promote alternative developmental programs. Interfactor competition, together with environmental signals, keep these diversions under control. Here the pathways leading to several lineage alternatives for early pro-T-cells are reviewed, with close focus on the mechanisms of action of three vital factors, GATA-3, PU.1, and Notch-Delta signals, whose counterbalance appears to be essential for T-cell specification.

Figure 1

T cell regulatory requirements in relation to other hematopoietic pathways. Schematic summary of the transcription factor requirements for T-lineage specification from hematopoietic stem cells (thick arrows), indicating alternative hematopoietic pathways that are naturally promoted by the same factors (thin arrows). For reviews and updated references, see [,,–162]. Note that although GATA-3 is T-lineage specific under normal conditions shown here, it is also capable of driving a non-T differentiation program even in early T-lineage cells if expressed at an elevated level.

Figure 2

T cell developmental stages. Stages of early T-cell development in the young postnatal mouse are shown in the context of their ordered migration through different compartments of the thymus (rev. in [28,153]). Cells in the ETP and DN2 stages undergo extensive proliferation (>10 cell cycles in all), and then the only other major phase of proliferation is immediately following β-selection. To the left are shown the developmental fates that cells at each stage can display if they are removed from the thymic microenvironment at the indicated stages.

Figure 3

Stepwise respecification of ETP and DN2 cells by GATA-3 overexpression. Early, intermediate, and late stages in the reprogramming of ETP or DN2 cells by forced expression of GATA-3. The stages where Notch signaling exerts inhibition are shown. Adapted from Supplementary Figure 6, ref. [36]. Note that susceptibility of cells to these interactions is limited to the stages before DN3, when an unidentified commitment function blocks access to the mast-cell pathway.

Figure 4

Myeloid diversion pathways for advanced pro-T cells by re-introduction of C/EBPα or PU.1. Comparison of the sequences of gene expression changes induced by transduction of pro-T cells with PU.1 or with C/EBPα, from first effects (day 1–2) through lineage conversion (4–9 days). Effects summarized in the figure were measured on purified DN3 pro-T cells from adult mouse thymus, transduced with C/EBP factors [54] or on DN2/3 stage fetal thymocytes, transduced with PU.1 [58]. “Pro-T genes” include Rag1, Lck, LAT, and ZAP70, the products of which are needed for TCR function. Continued progress along the myeloid pathway depends on the lack of Notch-Delta signals in PU.1 overexpressing cells. The pathway branch indicated by “+DL” shows how reintroduction of Notch-Delta signaling can stall or reverse the reprogramming of fetal pro-T cells by PU.1. Although high-level PU.1 is inhibitory for proliferation and progression of T-lineage cells beyond the DN3 stage, Notch signaling and neutral viability support (a Bcl2 transgene) can restore substantial T-lineage differentiation even beyond β-selection. Major gene targets that are differentially affected by PU.1 in the presence or absence of Notch-Delta signaling are shown (data from ref. [58]).

Figure 5

Schematic of counterbalancing and dose-dependent regulatory forces that act on T-cell precursors through the ETP and DN2 stages. Regulators named in red denote forces diverting cells from the T-cell pathway. Of these, only EBF and Pax5 are not normally active in the thymus during these stages. Notch signaling acts as a constraint on all of these alternatives. This dynamic equilibrium is normally resolved at the DN3 stage, in part by the downregulation of PU.1, but almost certainly through other regulatory changes as well [163].