Towards a molecular understanding of the differential signals regulating alphabeta/gammadelta T lineage choice

Abstract

While insights into the molecular processes that specify adoption of the alphabeta and gammadelta fates are beginning to emerge, the basis for control of specification remains highly controversial. This review highlights the current models attempting to explain T lineage commitment. Recent observations support the hypothesis that the T cell receptor (TCR) provides instructive cues through differences in TCR signaling intensity and/or longevity. Accordingly, we review evidence addressing the importance of differences in signal strength/longevity, how signals differing in intensity/longevity may be generated, and finally how such signals modulate the activity of downstream effectors to promote the opposing developmental fates.

Figure 1. αβ/γδ lineage commitment during thymocyte development

TCR γ, δ, and β gene rearrangement begins in DN2 thymocytes. αβ/γδ lineage commitment is thought to occur between the onset of gene rearrangement and arrival at the DN3 stage. Precursors that have committed to the γδ lineage express the γδ TCR complex, but usually not CD4 or CD8. In contrast, commitment to the αβ lineage usually occurs in response to pre-TCR signaling and is characterized by development of thymocytes to the DP stage. While commitment to the αβ and γδ lineage is most often directed by signals from the pre-TCR and γδ TCR complexes, respectively, these decisions are not irrevocably tied to the receptor isotype. Rather, they are determined by the nature of the TCR signal, with weaker signals favoring adoption of the αβ fate and stronger signals promoting adoption of the γδ fate.

Figure 2. Model by which strong TCR signals render γδ lineage cells Notch independent

Development beyond the β-selection checkpoint requires suppression of E protein function. We hypothesize that T lineage fate and developmental characteristics are determined by the extent to which E protein activity is repressed in a model encompassing graded suppression of E protein function by TCR signals of differing strength. Pre-TCR signals are too weak by themselves to suppress E proteins beyond the threshold required for the αβ lineage differentiation program. They require assistance from Notch to do so, providing an explanation for the Notch-dependence of αβ lineage differentiation to the DP stage. γδ lineage commitment, in contrast, is dictated by strong TCR signals capable of suppressing E protein function beyond the threshold required for γδ lineage commitment, and do so without assistance from Notch. Notch is able to contribute to E protein suppression both through Id3 induction and by ERK-dependent degradation of E proteins; however, our data suggest Notch represses E proteins primarily by inducing their degradation. Lightning bolt size denotes signals of differing strength.

Figure 3. Signal duration and lineage commitment

A. ERK signals can differ in amplitude or duration. Transient ERK activation decays prior to synthesis of the protein encoded by immediate early genes (IEG), resulting in their rapid degradation and failure to accumulate. In contrast, during prolonged signals ERK signals persist until IEG protein products are expressed, enabling them to physically dock with ERK. Physical interaction enables ERK to phosphorylate and stabilize the IEG protein so that it accumulates, leading to protein levels disproportionate to the encoding mRNA. B. γδ commitment is associated with disproportionate increases in the IEG protein Egr1. The mRNA and protein levels of Egr1 were measured in thymocytes committing to the γδ lineage (KN6⁺Lig⁺) and the αβ lineage (KN6⁺Lig⁻) and normalized to control (KN6⁻). Egr1 protein levels were disproportionately high in cells committing to the γδ lineage, consistent with the notion that γδ lineage commitment involves signals of increased duration. C. γδ lineage commitment is accompanied by ERK signals lasting longer than those occurring in cells committing to the αβ lineage.