The thymus road to a T cell: migration, selection, and atrophy

Abstract

The thymus plays a pivotal role in generating a highly-diverse repertoire of T lymphocytes while preventing autoimmunity. Thymus seeding progenitors (TSPs) are a heterogeneous group of multipotent progenitors that migrate to the thymus via CCR7 and CCR9 receptors. While NOTCH guides thymus progenitors toward T cell fate, the absence or disruption of NOTCH signaling renders the thymus microenvironment permissive to other cell fates. Following T cell commitment, developing T cells undergo multiple selection checkpoints by engaging with the extracellular matrix, and interacting with thymic epithelial cells (TECs) and other immune subsets across the different compartments of the thymus. The different selection checkpoints assess the T cell receptor (TCR) performance, with failure resulting in either repurposing (agonist selection), or cell death. Additionally, environmental cues such as inflammation and endocrine signaling induce acute thymus atrophy, contributing to the demise of most developing T cells during thymic selection. We discuss the occurrence of acute thymus atrophy in response to systemic inflammation. The thymus demonstrates high plasticity, shaping inflammation by abrogating T cell development and undergoing profound structural changes, and facilitating regeneration and restoration of T cell development once inflammation is resolved. Despite the challenges, thymic selection ensures a highly diverse T cell repertoire capable of discerning between self and non-self antigens, ultimately egressing to secondary lymphoid organs where they complete their maturation and exert their functions.

Keywords: T cell development; acute thymus atrophy; thymocyte cell death; thymus colonization; thymus morphology; thymus organogenesis.

Figure 1

Overview of the structural layers and cellular composition of the thymus lobe. The figure illustrates a zoom view of the four different morphological layers from the outer to the inner: capsula, cortex, corticomedullary junction (CMJ) and medulla and the diversity of cell types contained within those layers with their reported anatomical localization within the thymus. The schematic does not represent their relative numbers within the thymus tissue.

Figure 2

Current model of thymus colonization by MPPs during fetal development and thorough adulthood. During fetal development, CXCR4 plays a role in aiding MMPs being mobilized from either AGM, yolk sac, or fetal liver even prior complete formation of the blood circulation system. MPPs, primarily LMPPs and CLPs migrate to the thymus in a CCR9-, CCR7-dependent manner. Shortly after birth, the bone marrow becomes the primary site of MPPs residency and export. Analogous to during fetal development, progenitors are mobilized into circulation via CXCR4 and c-Kit and later guide towards the thymus by CCL21 and CCL25 (CCR7 and CCR9 ligands) by chemotaxis. Lastly PSGL-1 plays a role in the transmigration of the TSPs for entering the thymus.

Figure 3

Spatiotemporal model of T cell development in the thymus. 1) TSPs enter the thymus through blood vessels typically irrigating the thymus in the medulla and CMJ in a process mediated by PGSL-1. Then, the thymus microenvironment instructs them to become ETPs or DN1 entering the T cell differentiation path. 2) DN1 move towards the cortex where they become DN2 which proliferate robustly. This migration through the cortex is mediated by chemokine gradients promoting the migration of the developing thymocytes via CCR7 and CXCR4. 3) In the outer cortex, DN3 undergo TCR rearrangement, thereby irreversibly committing towards T cell fate. 4) Next, DN thymocytes encounter αβ- vs γδ- lineage choice based on the MHC-TCR interaction strength, and NOTCH signaling among other factors. In this step, their TCR is tested in a process known as β- or γδ- selection, where cells must interact with MHC to receive a survival signal. Failure to engage MHC results in programmed cell death at the DN3 stage. 5) Later on, DN3 move forward in the development and become DN4 and DP, staying quiescent for several days. At this point, the polarization of the migration reverses. This inward migration of DP is regulated by S1P, CXCL12 and CD69. 6) DP interact with cTECs and undergo positive selection based on the TCR-MHC strength and affinity interactions. 7) Cells displaying low-moderate TCR-MHC interaction are positively selected. 8) Cells with high TCR-MHC affinity mostly undergo cell death, 9) while a small fraction that recognize self-antigens are rescue to become regulatory T cells (Tregs) or other types of unconventional T-cells (95). 10) Positively selected cells undergo another selection round by interacting with APCs, either DCs or mTECs. 11) Successful interactions lead to their maturation into SP CD4⁺ or CD8⁺ thymocytes and naïve T cells that are ready to egress to the periphery and complete their development in secondary lymphoid organs.

Figure 4

Cell death selection checkpoints during thymic T-cell development and molecules that control life vs death decisions. Early thymus progenitors (ETPs) are uncommited progenitors that enter the T cell differentiation path instructed by the thymus stroma. The earlier stage is known as DN1 (defined as Lin^- CD44⁺ CD25^- c-Kit^hi), later becoming DN2 (defined as Lin^- CD44⁺ CD25⁺ c-Kit⁺). The survival of these progenitors is controlled mainly by IL-7 availability, SCF - c-KIT interactions, NOTCH signaling and BCL-2 and MCL-1. Later, when the machinery of TCR recombination these cells are irreversibly committed to the T cell fate and become DN3a (defined as Lin- CD44^- CD25⁺ c-Kit^-/lo CD27⁺ CD28^-) and later DN3b (defined as Lin- CD44^- CD25⁺ c-Kit^- CD27⁺ CD28⁺). During this stage the TCR chain is tested and BCL2A1 and CXCR4 were reported to control the survival of DN3 while FASL and glucocorticoids promote cell death when the TCR is defective. Then, DN3 cells move forward in the development acquiring a DN4 phenotype (characterized as Lin- CD44^- CD25^- CD27⁺ CD28⁺) and ultimately a DP phenotype (characterized as CD4⁺ CD8⁺). These cells remain quiescent for some days and later undergo additional selection rounds where the TCR:MHC affinity is tested. TCR:MHC interaction, BCL-XL, IL-7, RIPK3 and NF-κB are some of the signals that dictates their survival, while BH3-only proteins BIM and PUMA and their mitochondrial pore forming targets BAX and BAK are implicated in the cell death execution when the TCR:MHC interaction is not successful. Lastly, DP thymocytes will adopt a SP CD4⁺ or CD8⁺ phenotype and will egress from the thymus to secondary lymphoid organs where they complete their maturation into different effector and helper subsets. A small fraction of SP CD4+ cells that show high TCR:MHC interaction can be rescued from cell death to become regulatory T cells (T_regs, defined as CD4⁺ FOXP3⁺ CD25⁺) or unconventional T cells (iNKT, MAIT, etc.).

Figure 5

Anatomical, morphological, cellular and molecular differences between the healthy thymus, inflammation induced-acute thymus atrophy, and age-related thymus involution. (A) Morphological and cellular characteristics of a young-healthy thymus vs (B) aged-individual thymus. (C) Common morphological, cellular and molecular alterations produced in inflammation-induced acute thymus atrophy in response to different triggers such as infections, steroid hormones, malnutrition, irradiation, and pregnancy.