Abstract
A hallmark of immune cell trafficking is directional guidance via gradients of soluble or surface bound chemokines. Vascular endothelial cells produce, transport and deposit either their own chemokines or chemokines produced by the underlying stroma. Endothelial heparan sulfate (HS) was suggested to be a critical scaffold for these chemokine pools, but it is unclear how steep chemokine gradients are sustained between the lumenal and ablumenal aspects of blood vessels. Addressing this question by semi-quantitative immunostaining of HS moieties around blood vessels with a pan anti-HS IgM mAb, we found a striking HS enrichment in the basal lamina of resting and inflamed post capillary skin venules, as well as in high endothelial venules (HEVs) of lymph nodes. Staining of skin vessels with a glycocalyx probe further suggested that their lumenal glycocalyx contains much lower HS density than their basolateral extracellular matrix (ECM). This polarized HS pattern was observed also in isolated resting and inflamed microvascular dermal cells. Notably, progressive skin inflammation resulted in massive ECM deposition and in further HS enrichment around skin post capillary venules and their associated pericytes. Inflammation-dependent HS enrichment was not compromised in mice deficient in the main HS degrading enzyme, heparanase. Our results suggest that the blood vasculature patterns steep gradients of HS scaffolds between their lumenal and basolateral endothelial aspects, and that inflammatory processes can further enrich the HS content nearby inflamed vessels. We propose that chemokine gradients between the lumenal and ablumenal sides of vessels could be favored by these sharp HS scaffold gradients.
Highlights
Chemokines are structurally related chemotactic cytokines which guide the adhesion and motility of many cell types including immune cells [1,2]
Endothelial cells (ECs) can be readily depicted by VE-cadherin immunostaining, and post capillary venules were distinguished from other blood vessels by a-SMA immunostaining, since this protein is highly elevated on pericytes associated with post capillary venules (Figure S1)
It is believed that net chemotactic gradients between the apical and basolateral aspects of the post capillary venules are essential for the highly directional nature of leukocyte diapedesis across the vessel wall, but the evidence for such net gradients is still weak
Summary
Chemokines are structurally related chemotactic cytokines which guide the adhesion and motility of many cell types including immune cells [1,2]. Unlike lipid chemoattractants, which operate in their membrane bound lipophilic forms [5,6,7], most chemokines share high affinity binding sites for HS glycosaminoglycans (GAGs) [8]. These sites do not overlap or compete with chemokine binding to their G-protein coupled receptors (GPCRs) [8,9], and chemokine presentation by HS scaffolds retains, but in many cases augments, GPCR signaling on adherent or motile leukocytes [10,11]. Loss of lymphatic endothelial HS in Ext1-deficient lymphatic vessels impaired both CCL21 presentation and DC migration towards and across lymphatic endothelial barriers [13]
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