Thymic precursors of TCRαβ(+)CD8αα(+) intraepithelial lymphocytes are negative for CD103

Bcl-2 Is Necessary to Counteract Bim and Promote Survival of TCRαβ+CD8αα+ Intraepithelial Lymphocyte Precursors in the Thymus

The precursors of TCRαβ+CD8αα+ intraepithelial lymphocytes (IEL) arise in the thymus through a complex process of agonist selection. We and others have shown that the proapoptotic protein, Bim, is critical to limit the number of thymic IEL precursors (IELp), as loss of Bim at the CD4+CD8+ double-positive stage of development drastically increases IELp. The factors determining this cell death versus survival decision remain largely unknown. In this study, we used CD4CreBcl2f/f mice to define the role of the antiapoptotic protein Bcl-2 and CD4CreBcl2f/fBimf/f mice to determine the role of Bcl-2 in opposing Bim to promote survival of IELp. First, in wild-type mice, we defined distinct subpopulations within PD-1+CD122+ IELp, based on their expression of Runx3 and α4β7. Coexpression of α4β7 and Runx3 marked IELp that were most dependent upon Bcl-2 for survival. Importantly, the additional loss of Bim restored Runx3+α4β7+ IELp, showing that Bcl-2 antagonizes Bim to enable IELp survival. Further, the loss of thymic IELp in CD4CreBcl2f/f mice also led to a dramatic loss of IEL in the gut, and the additional loss of Bim restored gut IEL. The loss of gut IEL was due to both reduced seeding by IELp from the thymus as well as a requirement for Bcl-2 for peripheral IEL survival. Together, these findings highlight subset-specific and temporal roles for Bcl-2 in driving the survival of TCRαβ+CD8αα+ IEL and thymic IELp.

TCR-α/β CD4- CD8- double negative T cells arise from CD8+ T cells

The cellular origin of CD4^- CD8^- (double negative, DNT) TCR-α/β⁺ T cells remains unknown. Available evidence indicates that they may derive from CD8⁺ T cells, but most published data have been obtained using cells that bear an invariant transgenic T cell receptor that recognizes an Ag that is not present in normal mice. Here, we have used complementary fate mapping and adoptive transfer experiments to identify the cellular lineage of origin of DNT cells in wild-type mice with a polyclonal T cell repertoire. We show that TCR-α/β⁺ DNT cells can be traced back to CD8⁺ and CD4⁺ CD8⁺ double positive cells in the thymus. We also demonstrate that polyclonal DNT cells generated in secondary lymphoid organs proliferate upon adoptive transfer and can regain CD8 expression in lymphopenic environment. These results demonstrate the cellular origin of DNT cells and provide a conceptual framework to understand their presence in pathological circumstances.

A committed postselection precursor to natural TCRαβ+ intraepithelial lymphocytes

The intestine is a major immune organ with several specialized lymphoid structures and immune cells. Among these are thymus-derived natural intraepithelial lymphocytes (IELs) that lack expression of the classical co-receptors CD4 or CD8αβ (double negative (DN)). Natural IELs are both αβ⁺ and γδ⁺ T cells that play important roles in the maintenance of the epithelial barrier at steady state and during inflammation. The transcription factor T-bet is essential for the peripheral development of natural IELs, but its role during thymic development has remained less clear. Here we show that a T-bet gradient in DN TCRαβ⁺NK1.1^- thymocytes (IEL precursors (IELPs)) determines IEL fate in natural TCRαβ⁺ IELs. Employing T-bet ZsGreen reporter mice in in vitro cultures and in vivo transfer experiments, we demonstrate that with increasing expression of T-bet, DN TCRαβ⁺NK1.1^- thymocytes are gradually restricted to a DN IEL fate. Furthermore, we show that the natural TCRαβ⁺ IELs seed the intestine within the first month of life. This in turn is preceded by the appearance of T-bet^- and T-bet⁺ IELPs that egress from the thymus in a sphingosine-1-phosphate (S1P)-dependent manner. In summary, the use of T-bet reporter mice has enabled us to identify and refine an immediate and clearly committed postselection precursor of natural TCRαβ⁺ IELs.

Thymic progenitors of TCRαβ+ CD8αα intestinal intraepithelial lymphocytes require RasGRP1 for development

Golec et al. show that RasGRP1, a critical Ras activator in thymocytes, is required for TCRαβ⁺CD8αα IEL development by regulating the survival of a heterogeneous population of thymic progenitors that receive a strong TCR signal. Therefore, RasGRP1 is necessary for thymic selection events stemming from strong or weak TCR signals.

Strong T cell receptor (TCR) signaling largely induces cell death during thymocyte development, whereas weak TCR signals induce positive selection. However, some T cell lineages require strong TCR signals for differentiation through a process termed agonist selection. The signaling relationships that underlie these three fates are unknown. RasGRP1 is a Ras activator required to transmit weak TCR signals leading to positive selection. Here, we report that, despite being dispensable for thymocyte clonal deletion, RasGRP1 is critical for agonist selection of TCRαβ⁺CD8αα intraepithelial lymphocyte (IEL) progenitors (IELps), even though both outcomes require strong TCR signaling. Bim deficiency rescued IELp development in RasGRP1^−/− mice, suggesting that RasGRP1 functions to promote survival during IELp generation. Additionally, expression of CD122 and the adhesion molecules α₄β₇ and CD103 define distinct IELp subsets with differing abilities to generate TCRαβ⁺CD8αα IEL in vivo. These findings demonstrate that RasGRP1-dependent signaling underpins thymic selection processes induced by both weak and strong TCR signals and is differentially required for fate decisions derived from a strong TCR stimulus.

CD8αα intraepithelial lymphocytes arise from two main thymic precursors

TCRαβ⁺CD4^-CD8α⁺CD8β^- intestinal intraepithelial lymphocytes (CD8αα IELs) are an abundant population of thymus-derived T cells that protect the gut barrier surface. We sought to better define the thymic IEL precursor (IELp) through analysis of its maturation, localization and emigration. We defined two precursor populations among TCRβ⁺CD4^-CD8^- thymocytes by dependence on the kinase TAK1 and rigorous lineage-exclusion criteria. Those IELp populations included a nascent PD-1⁺ population and a T-bet⁺ population that accumulated with age. Both gave rise to intestinal CD8αα IELs after adoptive transfer. The PD-1⁺ IELp population included more strongly self-reactive clones and was largely restricted by classical major histocompatibility complex (MHC) molecules. Those cells localized to the cortex and efficiently emigrated in a manner dependent on the receptor S1PR1. The T-bet⁺ IELp population localized to the medulla, included cells restricted by non-classical MHC molecules and expressed the receptor NK1.1, the integrin CD103 and the chemokine receptor CXCR3. The two IELp populations further differed in their use of the T cell antigen receptor (TCR) α-chain variable region (V_α) and β-chain variable region (V_β). These data provide a foundation for understanding the biology of CD8αα IELs.