Von Willebrand Factor (VWF) cleaving protease ADAMTS13 is responsible for proteolysis of ultra large VWF multimers in the circulation. ADAMTS13 is synthesized by hepatic stellate cells in the liver. Also endothelial cells have been suggested to contribute to the synthesis of ADAMTS13. In patients with acquired thrombotic thrombocytopenic purpura auto-reactive antibodies are formed primarily against the spacer domain of ADAMTS13. Previously we have shown that monocyte-derived dendritic cells were able to endocytose ADAMTS13 via the macrophage mannose receptor and subsequently process it into peptides and present it on MHC class II. However, it is currently unclear which receptor contributes to the clearance of ADAMTS13 from the circulation. The half-life of ADAMTS13 was measured following plasma infusion in patients with congenital TTP and was found to vary between 2.1 and 3.3 days.
Internalization of ADAMTS13 by tissue-resident macrophages may contribute to its clearance from the circulation. Here we investigated endocytic mechanisms contributing to the uptake of ADAMTS13 by macrophages. Human monocyte-derived macrophages (MDMs) were used to monitor the uptake of fluorescently labelled recombinant ADAMTS13 by flow cytometry. Internalization of ADAMTS13 was blocked upon addition of the cell-permeable dynamin-inhibitor dynasore. Partial blockage of ADAMTS13 internalization was observed employing mannan; uptake however was not affected by a blocking antibody directed towards the macrophage mannose receptor CD206. A pull-down with ADAMTS13 and subsequent mass spectrometric analysis identified the hemoglobin scavenger receptor CD163 as a candidate receptor for ADAMTS13. CD163 is primarily expressed by the monocyte-macrophage lineage and is highly expressed on type-2 macrophages present in the bone marrow, the red pulp of the spleen and in the liver on Kupffer cells. Blocking experiments with a monoclonal anti-CD163 antibody EDHu-1 resulted in a decreased ADAMTS13 internalization by macrophages. Pronounced inhibition of ADAMTS13 uptake by EDHu-1 was observed in macrophages in which the expression of CD163 was boosted upon incubation with IL-10. In agreement with these findings CD163-expressing CHO cells but not CHO CD163-/- cells were capable of rapidly internalizing ADAMTS13. Surface plasmon resonance revealed high affinity binding of ADAMTS13 to soluble CD163 containing scavenger receptor cysteine-rich (SRCR) domain 1-9. Our results position CD163 as a novel binding partner for ADAMTS13 and suggest that binding of ADAMTS13 to CD163 promotes its internalization by macrophages.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.