The vascular endothelial growth factor (VEGF) receptor FLT-1 has been shown to be involved in vasculogenesis and angiogenesis. The receptor is characterized by seven Ig-like loops within the extracellular domain. Upon VEGF binding FLT-1 becomes phosphorylated, which has been thought to be preceded by receptor dimerization. To further investigate high affinity binding of VEGF to FLT-1 and ligand-induced receptor dimerization, we expressed in Sf9 cells the entire extracellular domain comprising all seven Ig-like loops: sFLT-1(7) and several truncated mutants consisting of loop one, one and two, one to three, one to four, and one to five. The corresponding proteins, named sFLT-1(1), (2), (3), (4), and (5) were purified. Only mutants sFLT-1(3) to (7) were able to bind 125 I-VEGF with high affinity. No binding of VEGF was observed with sFLT-1(1) and sFLT-1(2), indicating that the first three Ig-like loops are involved in high affinity binding of VEGF. The binding of VEGF to sFLT-1(3) could be competed with placenta growth factor (PlGF), a VEGF-related ligand, suggesting that high affinity binding of VEGF and PlGF is mediated by the same or closely related contact sites on sFLT-1. Deglycosylation of the sFLT-1(3), (4), (5), and (7) did not abolish VEGF binding. Furthermore, unglycosylated sFLT-1(3), expressed in Escherichia coli, was able to bind VEGF with similar affinity as sFLT-1(3) or sFLT-1(7), both expressed in Sf9 cells. This indicates that receptor glycosylation is not essential for high affinity binding. Dimerization of the extracellular domains of FLT-1 upon addition of VEGF was detected with all mutants containing the Ig-like loop four. Although sFLT-1(3) was able to bind VEGF, dimerization of this mutant was inefficient, indicating that sites on Ig-like loop four are essential to stabilize receptor dimers.

Globally accessible preventive and therapeutic molecules against SARS-CoV-2 are urgently needed. DARPin molecules are an emerging class of novel therapeutics based on naturally occurring repeat proteins (∼15 kDa in size) and can be rapidly produced in bacteria in large quantities. Here, we report the identification of 380 DARPin molecules specifically targeting the SARS-CoV-2 spike protein selected from a naïve library of 1012 DARPin molecules. Using extensive biophysical and biochemical characterization, (pseudo)virus neutralization assays and cryo-EM analysis, 11 mono-DARPin molecules targeting either the receptor binding domain (RBD), the S1 N-terminal-domain (NTD) or the S2 domain of the SARS-CoV-2 spike protein were chosen. Based on these 11 mono-DARPin molecules, 31 anti-SARS-CoV-2 multi-DARPin molecules were constructed which can broadly be grouped into 2 types; multi-paratopic RBD-neutralizing DARPin molecules and multi-mode DARPin molecules targeting simultaneously RBD, NTD and the S2 domain. Each of these multi-DARPin molecules acts by binding with 3 DARPin modules to the SARS-CoV-2 spike protein, leading to potent inhibition of SARS-CoV-2 infection down to 1 ng/ml (12 pM) and potentially providing protection against viral escape mutations. Additionally, 2 DARPin modules binding serum albumin, conferring an expected half-life of about 3 weeks in humans, were included in the multi-DARPin molecules. The protective efficacy of one multi-DARPin molecule was studied in a Golden Syrian hamster SARS-CoV-2 infection model, resulting in a significant reduction in viral load and pathogenesis. In conclusion, the multi-DARPin molecules reported here display very high antiviral potency, high-production yield, and a long systemic half-life, and thereby have the potential for single-dose use for prevention and treatment of COVID-19.

It was shown before that the soluble form of VEGFR-1 (sVEGFR-1) is present in serum of pregnant women. The aim of the present study was to investigate the presence of this endogenous vascular endothelial growth factor-A (VEGF-A) antagonist in human serum in more detail. sVEGFR-1 was detected in human serum and plasma from normal healthy male and female donors by ELISA. sVEGFR-1 levels ranged from non-detectable up to 440 pg/ml, with no significant difference between male and female donors. In addition, vein endothelial cells (ECs) from an intact vascular bed, the umbilical cord, were shown to secrete sVEGFR-1. Furthermore, human peripheral blood monocytes, a non-EC type expressing VEGFR-1, were shown to contribute to the sVEGFR-1 detectable in human serum and plasma for the first time. EC- and monocyte-derived sVEGFR-1 proved capable of inhibiting the VEGF-induced proliferation and migration of ECs in vitro. Finally, secretion of sVEGFR-1 was increased by the angiogenic factor basic fibroblast growth factor (bFGF) in human ECs and was also enhanced in lipopolysaccharide-activated human monocytes. In human umbilical vein endothelial cells, both the membrane-bound and the sVEGFR-1 seem to be equally regulated on the mRNA as well as the protein level. The presence of an sVEGFR-1 in human serum and plasma of normal male and female donors strongly suggests that it plays an important role as a naturally occurring VEGF antagonist in the regulation and availability of VEGF-mediated biological activities in vivo.

We assessed effects of reactive oxygen metabolites on renin release ofjuxtaglomerular cells (JG-cells) prepared in primary culture from mouse kidneys. Renin activity was measured in culture supernatants and cells. Basal renin release was increased by incubation of JG-cells with xanthine/xanthine oxidase from 26 + 1% up to 58 + 3% of total activity. This increase was slightly inhibited by superoxide dismutase, and was eliminated by addition of catalase, implicating H202 as an intermediate product in the stimulatory cascade. H202 applied exogeneously dosedependently stimulated renin release up to 55 + 2%; this effect was also prevented by catalase. We propose that reactive oxygen metabolites stimulate renin release in isolated JG-cells. This could have important implications in inflammatory kidney diseases.

PD-1/PD-L1 blockade has revolutionized the field of immunooncology. Despite the relative success, the response rate to anti-PD-1 therapy requires further improvements. Our aim was to explore the enhancement of T-cell function by using novel PD-1-blocking proteins and compare with clinically approved monoclonal antibodies (mAbs). We isolated T-cells from the ascites and tumor of 17 patients with advanced epithelial ovarian cancer (EOC) and analyzed the effects using the mAbs nivolumab and pembrolizumab and two novel engineered ankyrin repeat proteins (DARPin® proteins). PD-1 blockade with either mAb or DARPin® molecule significantly increased the release of IFN-γ, granzyme B, IL-2, and TNF-α, demonstrating successful reinvigoration. The monovalent DARPin® protein was less effective compared to its bivalent equivalent, demonstrating that bivalency brings an additional benefit to PD-1 blockade. Overall, we found a higher fold increase of lymphokine secretion in response to the PD-1 blockade by tumor-derived T-cells; however, the absolute amounts were significantly lower compared to the release from ascites-derived T-cells. Our results demonstrate that PD-1 blockade can only partially reinvigorate functionally suppressed T-cells from EOC patients. This warrants further investigation preferably in combination with other therapeutics. The study provides an early pilot proof-of-concept for the potential use of DARPin® proteins as eligible alternative scaffold proteins to block PD-1.