Real-time quantitative in vivo imaging of chemokine and chemokine receptors dynamics during intrathymic cell migration
Developmental Biology
Cell Biology
Final Report Abstract
The adaptive immune system consists of two T-cell sublineages, defined according to the expression of αβ or γδ T-cell receptors. Their development starts with the entry of common progenitors into the thymus and their migration into distinct thymic niches to interact with thymic epithelial cells (TECs), to develop either as αβ or γδ T-cells. Thus far, the lineage αβ/γδ lineage divergence is mostly understood to be regulated by intrinsic mechanisms. However, the proportion of αβ/γδ T-cells varies between vertebrates and how this process is regulated in species that tend to produce a high frequency of γδ T-cells (e.g., sheep, cattle, pigs, chickens, zebrafish, medaka and shark) remains unstudied. Such studies in other animals will provide the information needed to understand their immunological fitness. Using medaka as a model system, we unexpectedly found that αβ and γδ T-cells are spatially organized into two distinct thymic niches, in contrast to humans and mice. By combining genetic perturbation and live in toto imaging of transgenic reporter lines, we found that upon entry into the thymus, progenitors that will become αβ T-cells take a different migratory journey from those that will become γδ T-cells. We identify the chemokine receptor Ccr9b, which is first expressed in thymocytes before the commitment stage, as a part of the Notch1-Bcl11b gene regulatory network involved in αβ, but not γδ, T-cell development. Lack of Ccr9b affects thymocytes' correct positioning within the thymus and, surprisingly, shifts their fate choice towards the γδ lineage. On the other hand, thymic epithelial cells generate gradients of the chemokine Ccl25a (the ligand of Ccr9b) and the cytokine Interleukin-7 (Il-7) in opposing directions within the thymic niche. The overall lineage outcome shifted towards αβ or γδ lineage when the either Ccl25a or Il-7 levels were manipulated. Besides experimental data, we developed a cell-based computational model for T-cell development, which integrates spatial and quantitative parameter sets for cellular dynamics (cell motility, directionality, speed, and proliferation), and signaling molecules (Il-7/Il-7r, Delta4/Notch1, and Ccr9b). By simulating various scenarios in this model, we reveal how the interplay between extracellular signals, cellular behavior, and the onset of gene expression influences the lineage choice. With respect to the evolution of the adaptive immune system, this project might also explain the longstanding mystery about the ancient mechanism underlying T-cell lineage decisions in early vertebrates. We anticipate that our integrated approach cab be used to examine fundamental principles governing cell-fate choice and lineage commitment during T-cell development and leukemia formation.
Publications
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(2018) Making Thymus Visible: Understanding T-Cell Development from a New Perspective. Frontiers in Immunology. 9:375
Aghaallaei N, Bajoghli B
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(2019) Zebrafish and Medaka: Two Teleost Models of T-Cell and Thymic Development. Int J Mol Sci. Aug 26;20(17)
Bajoghli B, Dick AM, Claasen A, Doll L, Aghaallaei N
