The VEGF decoy receptor soluble Fms-like tyrosine kinase 1 binds to macrophages

Soluble Fms-like Tyrosine kinase-1 (sFLT1) is a native inhibitor of VEGF, best known for its antiangiogenic effects in preeclampsia. sFLT1 also reduces chronic inflammation and promotes tissue repair. In experimental diabetic nephropathy, we previously found that sFLT1 ameliorates kidney fibrosis and reduces the infiltration of macrophages. How sFLT1 regulates inflammation is still incompletely understood. Based on the direct association of sFLT1 with various cell types, we here studied whether sFLT1 interacts with macrophages to modulate inflammation.

Using various macrophage cell lines, sFLT1 cell surface binding was detected with flow cytometry. Enzyme studies, mass spectrometry and RNAseq were employed to identify potential sFLT1 cell surface interactors and effects of sFLT1 on macrophage signaling.

Soluble FLT1 binds to primary macrophages, THP-1 and RAW264.7 macrophages in vitro. Alternative activation with IL-4 increases sFLT1 binding in THP-1 macrophages, whereas proinflammatory activation with IFN-γ and LPS decreases binding. Binding of sFLT1 depends on heparan sulphates, and colocalizes with the membrane heparin sulfate proteoglycan neuropilin-1. Incubation with sFLT1 reduces the gene expression of chemokine receptors.

Our results show that sFLT1, while typically associated with angiogenesis, also directly interacts with macrophages. Alternative activation of macrophages by IL-4 strongly increases binding of sFLT1 to the cell surface membrane, possibly via the VEGF co-receptor neuropilin-1. Considering sFLT1's anti-inflammatory effects in animal studies, our findings indicate a novel function for sFLT1 to directly control anti-inflammatory macrophage function.