γδTCR-independent origin of neonatal γδ T cells prewired for IL-17 production

Abstract

A classical view of T cell lineages consists of two major clades of T cells expressing either the αβ or γδ T cell receptor (TCR). However, genome-wide assessments indicate molecular clusters segregating T cell subsets that are preprogrammed for effector function (innate) from those that mediate conventional adaptive response, regardless of the TCR types. Within this paradigm, γδ T cells remain the prototypic innate-like lymphocytes, many subsets of which are programmed during intrathymic development for committed peripheral tissue localization and effector responses. Emerging evidence for innate γδ T cell lineage choice dictated by developmental gene programs rather than the sensory TCR is discussed in this review.

Figure 1.. Ontogeny and programmed tissue tropism of innate T and B cells.

Development of innate T and B cells begins with the emergence of hematopoietic progenitors from the aorta-gonad- mesonephros or yolk sac (AGM/YS) regions and the fetal liver (FL). Natural antibody producing B-1 B cells that predominantly home to the peritoneal cavity have been demonstrated to develop from both sites, starting ~E9.5. The first T cell subset observed in mice, Vγ3TCR⁺ DETC, originates from FL HSCs. Subsequently, two subsets of Tγδ17 cells emerge, expressing either Vγ4TCR and Vγ2TCR, the latter of which are primarily derived from SOX13⁺ DN1d cells termed Soxpro. Tγδ17 cells are distributed widely in non-lymphoid tissues, but only the primary tissue sites are depicted here. In newborns these cells are required for organogenesis, tissue homeostasis and body thermogenesis, depending on their tissue localization. All innate γδ T cell subsets function and tissue homing property are specified during thymic differentiation. FL HSCs migrate to the fetal bone marrow (BM) to establish long-term residence and sustain adult hematopoietic output. The next phase of innate T cell development also switches to the BM origin, although the initial tissue homing T cells at birth are likely from FL HSCs. MAIT and NKT cells with restricted TCR repertoire (αβTCR or γδTCR) begin to be produced predominantly after birth, some from fetal progenitors with biased innate lymphoid gene programming. Gut homing CD8αα⁺ intestinal intraepithelial T cells expressing either αβTCR or γδTCR are generated from the neonatal thymus and originate from BM HSCs. Lastly, additional subsets of non-mucosal homing Vγ1.1⁺ and Vγ2⁺ cells develop skewed toward IFNγ production as well as “naïve” γδ T cells that acquire effector specificity in peripheral tissues. This phase of lymphatic and blood borne γδ T cell development coincides with the developmental window when conventional naïve γδ T cells begin to be exported from the thymus to populate the secondary lymphoid tissues. Dashed lines indicate developmental pathways awaiting definitive experimental verification. Garman nomenclature for Tcrg genes is used in this figure and throughout this manuscript; the alternative Tonegawa nomenclature is provided for ease of reference. Note that tissues of residence presented here are not exhaustive; for example, Vγ4⁺ Tγδ17 cells also home to the gut, while Vγ2⁺ Tγδ17 cells mount responses in the eye and skin- draining LNs. FRT, female reproductive tract; LN, lymph node.

Figure 2.. A model of neonatal Vγ2+ Tγδ17 cell differentiation from dedicated progenitors.

The nature of the cell types and factors involved in directing Tγδ17 cell programming in progenitors and fostering a supportive niche for Tγδ17 development remains incompletely resolved. We propose that the initial specification of Tγδ17 progenitor called Soxpro occurs independent of TCR signaling and takes place during development transitions of YS progenitors as they migrate to the fetal thymus. Whether this transition involves trafficking to FL is currently unknown, but conventional FL lymphoid progenitors (LMPPs and CLPs) do not appear to be a major source of neonatal Tγδ17 cells. In the thymus Soxpro is contained within the DN1d subset (cKit⁻CD24⁺ DN1 precursors) and some may transit to the DN1e stage (cKit-CD24- DN1, which has dramatically diminished Sox13 transcript amounts) before expressing γδTCR on the cell surface. Sox13 expression is dynamic (as depicted by the light bulb), as it is turned on (or maintained) in immature (CD24⁺) γδTCR+ thymocytes and turned down in mature (CD24⁻) counterparts. TCR signaling likely act as a permissive developmental checkpoint, at the transition from DN1 subsets, which include Soxpro, to immature CD24⁺Vγ2TCR⁺ thymocytes. In perinatal mice with genetically attenuated TCR signaling, alterations in the Tγδ17 gene signature are noted prior to the expression CD73, a marker induced by TCR signaling. Using Il17a reporter mice, we also observed that effector function is acquired prior to the expression of CD73. These results suggest a two-step model of TCR signaling requirement for Tγδ17 cell maturation, before CD73 expression and in the TCR-dependent acquisition of CD73, which is associated with the loss of CD24 and final maturation. Roles for cell- extrinsic thymic Notch, WNT and TGFβ in Tγδ17 cell development have been reported in genetic models, while IL-1β, IL-21, IL-23 have also been proposed to be involved based on data from in vitro assays (not depicted in the Figure). While the precise identity and supportive thymic niche for Tγδ17 cell differentiation is unknown, mouse models with compromised thymic architecture have implicated a role for cortical epithelial cells (cTEC), although involvements of medullary epithelial cells (mTEC) have also been suggested. One emerging model of unique cTEC-Soxpro-mTEC triad to maintain and facilitate Tγδ17 differentiation is currently under investigation. These studies also emphasize that caution is necessary in interpreting γδ T cell developmental assays employing non- thymic stromal cells.