Heparan sulfate is essential for thymus growth

Abstract

Thymus organogenesis and T cell development are coordinated by various soluble and cell-bound molecules. Heparan sulfate (HS) proteoglycans can interact with and immobilize many soluble mediators, creating fields or gradients of secreted ligands. While the role of HS in the development of many organs has been studied extensively, little is known about its function in the thymus. Here, we examined the distribution of HS in the thymus and the effect of its absence on thymus organogenesis and T cell development. We found that HS was expressed most abundantly on the thymic fibroblasts and at lower levels on endothelial, epithelial, and hematopoietic cells. To study the function of HS in the thymus, we eliminated most of HS in this organ by genetically disrupting the glycosyltransferase Ext1 that is essential for its synthesis. The absence of HS greatly reduced the size of the thymus in fetal thymic organ cultures and in vivo , in mice, and decreased the production of T cells. However, no specific blocks in T cell development were observed. Wild-type thymic fibroblasts were able to physically bind the homeostatic chemokines CCL19, CCL21, and CXCL12 ex vivo . However, this binding was abolished upon HS degradation, disrupting the CCL19/CCL21 chemokine gradients and causing impaired migration of dendritic cells in thymic slices. Thus, our results show that HS plays an essential role in the development and growth of the thymus and in regulating interstitial cell migration.

The fibroblast: An emerging key player in thymic T cell selection

“… 137 , 138 Cell‐surface binding of CCL21 is mediated by the heparan sulfate strongly expressed by these cells, and consistent with this, the EXT family genes, Ext1 and Ext2 , that encode the glycosyl transferases for heparan sulfate biosynthesis are highly expressed in mFbs and pericytes. 138 James et al reported that CCL21 captured by the blood‐vessel surrounding fibroblasts and pericytes promotes T cell emigration from the neonatal mouse thymus, suggesting a synergy between TEC‐dependent production and mesenchymal cell‐dependent immobilization of chemokines for controlling T cell migration. 137 A study by Hsu et al demonstrated that elimination of heparan sulfate in the thymus by genetic deletion of Ext1 resulted in a marked decrease in the number of thymic fibroblasts and TECs as well as thymocytes.…”

“…Two very recent reports demonstrated that CCL21 is displayed on the surface of mFbs and pericytes around blood vessels 137,138 . Cell‐surface binding of CCL21 is mediated by the heparan sulfate strongly expressed by these cells, and consistent with this, the EXT family genes, Ext1 and Ext2 , that encode the glycosyl transferases for heparan sulfate biosynthesis are highly expressed in mFbs and pericytes 138 . James et al reported that CCL21 captured by the blood‐vessel surrounding fibroblasts and pericytes promotes T cell emigration from the neonatal mouse thymus, suggesting a synergy between TEC‐dependent production and mesenchymal cell‐dependent immobilization of chemokines for controlling T cell migration 137 .…”

“…Two very recent reports demonstrated that CCL21 is displayed on the surface of mFbs and pericytes around blood vessels 137,138 . Cell‐surface binding of CCL21 is mediated by the heparan sulfate strongly expressed by these cells, and consistent with this, the EXT family genes, Ext1 and Ext2 , that encode the glycosyl transferases for heparan sulfate biosynthesis are highly expressed in mFbs and pericytes 138 .…”

“…James et al reported that CCL21 captured by the blood‐vessel surrounding fibroblasts and pericytes promotes T cell emigration from the neonatal mouse thymus, suggesting a synergy between TEC‐dependent production and mesenchymal cell‐dependent immobilization of chemokines for controlling T cell migration 137 . A study by Hsu et al demonstrated that elimination of heparan sulfate in the thymus by genetic deletion of Ext1 resulted in a marked decrease in the number of thymic fibroblasts and TECs as well as thymocytes 138 . Thus, heparan sulfate produced by mesenchymal cells may also be important for thymic stromal cell homeostasis via the immobilization of secreted proteins including chemokines, although the mechanism has not yet been elucidated.…”

“… 137 A study by Hsu et al demonstrated that elimination of heparan sulfate in the thymus by genetic deletion of Ext1 resulted in a marked decrease in the number of thymic fibroblasts and TECs as well as thymocytes. 138 Thus, heparan sulfate produced by mesenchymal cells may also be important for thymic stromal cell homeostasis via the immobilization of secreted proteins including chemokines, although the mechanism has not yet been elucidated.…”