Endo180, a member of the mannose receptor family, is constitutively recycled between clathrin-coated pits on the cell surface and intracellular endosomes. Its large extracellular domain contains an N-terminal cysteine-rich domain, a single fibronectin type II domain and eight C-type lectin-like domains. The second of these lectin-like domains has been shown to mediate Ca2+-dependent mannose binding. In addition, cross-linking studies have identified Endo180 as a urokinase plasminogen activator receptor-associated protein and this interaction can be blocked by collagen V. Here we demonstrate directly using in vitro assays, cell-based studies and tissue immunohistochemistry that Endo180 binds both to native and denatured collagens and provide evidence that this is mediated by the fibronectin type II domain. In cell culture systems, expression of Endo180 results in the rapid uptake of soluble collagens for delivery to lysosomal degradative compartments. Together with the observed restricted expression of Endo180 in both embryonic and adult tissue, we propose that Endo180 plays a physiological role in mediating collagen matrix remodelling during tissue development and homeostasis and that the observed receptor upregulation in pathological conditions may contribute to disease progression.

Fibroblasts are a diverse cell type and display clear topographic differentiation and positional memory. In a screen for fibroblast specific markers we have characterized four monoclonal antibodies to endosialin (TEM1/CD248). Previous studies have reported that endosialin is a tumour endothelium marker and is localized intracellularly. We demonstrate conclusively that endosialin is a cell surface glycoprotein and is predominantly expressed by fibroblasts and a subset of pericytes associated with tumour vessels but not by tumour endothelium. These novel antibodies will facilitate the isolation and classification of fibroblast and pericyte lineages as well as the further functional analysis of endosialin.

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Studies of stromal cell populations in lymphoid tissue (LT) have been hampered by a lack of selective markers. Here, we show that CD248 (Endosialin/TEM1) is a stromal marker that is differentially expressed on fibroblasts and pericytes in the thymus, lymph node and spleen. Expression is high during LT development but largely disappears in the adult. CD248 is re-expressed in a Salmonella-induced model of splenic enlargement; peak expression corresponding to the peak of splenic enlargement. These results suggest that CD248 expression helps define a subset of LT stromal cells which play a role in remodelling during tissue development, infection and repair.

CD248 (endosialin) is a transmembrane glycoprotein that is dynamically expressed on pericytes and fibroblasts during tissue development, tumour neovascularization and inflammation. Its role in tissue remodelling is associated with increased stromal cell proliferation and migration. We show that CD248 is also uniquely expressed by human, but not mouse (C57BL/6), CD8(+) naive T cells. CD248 is found only on CD8(+) CCR7(+) CD11a(low) naive T cells and on CD8 single-positive T cells in the thymus. Transfection of the CD248 negative T-cell line MOLT-4 with CD248 cDNA surprisingly reduced cell proliferation. Knock-down of CD248 on naive CD8 T cells increased cell proliferation. These data demonstrate opposing functions for CD248 on haematopoietic (CD8(+)) versus stromal cells and suggests that CD248 helps to maintain naive CD8(+) human T cells in a quiescent state.

Endo180, also known as the urokinase plasminogen activator receptor (uPAR)-associated protein (uPARAP), is one of the four members of the mannose receptor family, and is implicated in extracellular-matrix remodelling through its interactions with collagens, sugars and uPAR. The extracellular portion of Endo180 contains an amino-terminal cysteine-rich domain, a single fibronectin type II domain and eight C-type lectin-like domains. We have purified a soluble version of Endo180 and analysed it by single-particle electron microscopy to obtain a three-dimensional structure of the N-terminal part of the protein at a resolution of 17 Å and reveal, for the first time, the interactions between non-adjacent domains in the mannose receptor family. We show that for Endo180, the cysteine-rich domain contacts the second C-type lectin-like domain, thus providing structural insight into how modulation of its several ligand interactions may regulate Endo180 receptor function.

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