Cell-Autonomous Defects in Thymic Epithelial Cells Disrupt Endothelial-Perivascular Cell Interactions in the Mouse Thymus

Jerrod L Bryson,Nancy R Manley,Yousuke Takahama,Fumi Saito,Bernard Hughes Iii,Ellen R Richie,Ann V Griffith

doi:10.1371/journal.pone.0065196

Abstract

The thymus is composed of multiple stromal elements comprising specialized stromal microenvironments responsible for the development of self-tolerant and self-restricted T cells. Here, we investigated the ontogeny and maturation of the thymic vasculature. We show that endothelial cells initially enter the thymus at E13.5, with PDGFR-β+ mesenchymal cells following at E14.5. Using an allelic series of the thymic epithelial cell (TEC) specific transcription factor Foxn1, we showed that these events are delayed by 1–2 days in Foxn1 Δ/Δ mice, and this phenotype was exacerbated with reduced Foxn1 dosage. At subsequent stages there were fewer capillaries, leaky blood vessels, disrupted endothelium - perivascular cell interactions, endothelial cell vacuolization, and an overall failure of vascular organization. The expression of both VEGF-A and PDGF-B, which are both primarily expressed in vasculature-associated mesenchyme or endothelium in the thymus, were reduced at E13.5 and E15.5 in Foxn1 Δ/Δ mice compared with controls. These data suggest that Foxn1 is required in TECs both to recruit endothelial cells and for endothelial cells to communicate with thymic mesenchyme, and for the differentiation of vascular-associated mesenchymal cells. These data show that Foxn1 function in TECs is required for normal thymus size and to generate the cellular and molecular environment needed for normal thymic vascularization. These data further demonstrate a novel TEC-mesenchyme-endothelial interaction required for proper fetal thymus organogenesis.

Highlights

Organ vascularization is essential for the delivery of oxygen and nutrients to developing tissues and is required for normal tissue growth and homeostasis
By E13.5, a network of nascent vascular endothelial structures were present within the thymus and associated with the centrally localized Keratin 5+ (K5) subset of thymic epithelial cells (Figure 2F, I–J and 3A)
These data are consistent with a model in which PDGFR-b+ mesenchyme and endothelial cells first encapsulate the E12.5 thymus, and that vascular branches subsequently penetrate the thymus capsule, followed by PDGFR-b+ mesenchymal cells migrating along the endothelium into the thymic rudiment

Summary

Introduction

Organ vascularization is essential for the delivery of oxygen and nutrients to developing tissues and is required for normal tissue growth and homeostasis. Vasculature is primarily comprised of endothelial and perivascular support cells [1]. In addition to their essential role in providing oxygen and nutrients to tissues, recent reports have highlighted the versatility of the developing endothelia in organs [2]. The vascular network plays a critical role in organ function, and has been implicated in organ development. The thymus provides a specialized microenvironment that supports the development of self-MHC restricted and self-tolerant T cells. The postnatal thymus is comprised predominantly of hematopoietic-derived cells (mainly thymocytes, and dendritic cells and macrophages), in close association with a complex network of non-hematopoietic-derived stromal cells (epithelial, mesenchymal, and endothelial cells) [8]

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Results

Conclusion