From thymus to periphery: Molecular basis of effector γδ-T cell differentiation

Abstract

The contributions of γδ T cells to immune (patho)physiology in many pre-clinical mouse models have been associated with their rapid and abundant provision of two critical cytokines, interferon-γ (IFN-γ) and interleukin-17A (IL-17). These are typically produced by distinct effector γδ T cell subsets that can be segregated on the basis of surface expression levels of receptors such as CD27, CD44 or CD45RB, among others. Unlike conventional T cells that egress the thymus as naïve lymphocytes awaiting further differentiation upon activation, a large fraction of murine γδ T cells commits to either IFN-γ or IL-17 expression during thymic development. However, extrathymic signals can both regulate pre-programmed γδ T cells; and induce peripheral differentiation of naïve γδ T cells into effectors. Here we review the key cellular events of "developmental pre-programming" in the mouse thymus; and the molecular basis for effector function maintenance vs plasticity in the periphery. We highlight some of our contributions towards elucidating the role of T cell receptor, co-receptors (like CD27 and CD28) and cytokine signals (such as IL-1β and IL-23) in these processes, and the various levels of gene regulation involved, from the chromatin landscape to microRNA-based post-transcriptional control of γδ T cell functional plasticity.

Keywords: Gamma-delta T cells; IL-17; Thymic T cell development; effector T cell differentiation.

FIGURE 1

Thymic development of γδ T cells throughout mouse ontogeny. Limited windows in time allow for the generation of specific γδ T cell subsets in the murine thymus. γδ T cell development starts during early fetal and continues throughout life. The combination of distinct progenitors and a changing thymic microenvironment, constrains γδ T cell development during ontogeny. γδ T cells undergo effector cell differentiation inside the thymus influenced by signals received through the TCR. Strong TCR signaling promotes the IFNγ effector program, while IL‐17‐producing γδ T cells develop upon no/weak TCR signals. The dynamic changes of γδ T cell development are further reflected in specific patterns of TCRγ chain rearrangement and expression

FIGURE 2

Expression status of cell surface receptors that segregate effector γδ T cell subsets. IL‐17⁺and IFNγ⁺γδ T cell subsets express different surface receptors enabling their identification and isolation based on these markers. The most commonly used markers are highlighted for IL‐17⁺γδ T (blue) and IFNγ⁺γδ T cells (red). Noted within brackets are surface markers expressed on particular subpopulations of the respective effector subsets

FIGURE 3

Epigenetically “active” gene loci associated with γδ T cell effector functions. CD27⁻and CD27⁺γδ T cells, corresponding to IL‐17⁺and IFNγ⁺γδ T cell subsets, respectively, display active dimethylated H3K4me2 marks in loci of genes associated with their effector functions (from the Chip‐Seq analysis in84). Loci of genes associated with IL‐17 production (blue) are only actively marked in CD27⁻T cells, whereas loci of genes related with IFNγ production (red) are actively marked in both subsets, highlighting the potential of IL‐17⁺γδ T cells to engage in both cytokine programs

FIGURE 4

Regulation of IL‐17 and IFNγ expression in γδ T cell subsets. Integration of different levels of gene expression in CD27⁻(IL‐17⁺) and CD27⁺(IFNγ⁺) γδ T cell subsets. CD27⁺IFNγ‐producing γδ T cells display active marks (H3K4me2) exclusively in IFNγ and related transcription factor loci, while the IL‐17 locus is repressed (as dictated by repressive H3K27me3 marks). By contrast, CD27⁻IL‐17‐producing γδ T cells display active chromatin marks (H3K4me2) in both IL‐17 and IFNγ gene loci, as well as their respective transcriptional regulators RORγt and T‐bet, which drive IL‐17 and IFNγ mRNA expression, respectively. However, co‐production of IL‐17 and IFNγ is only observed under limited and strongly inflammatory conditions. In the steady state, CD27⁻IL‐17‐producing γδ T cells express high levels of miR‐146a, which acts as a brake for IFNγ production by inhibiting the expression of Nod1, an inducer of IFNγ production in γδ T cells