The upper wells of 8-µm-pore-size Boyden chambers were coated with MDSC stained with Hoechst (1:1,000 dilution, Thermo Fisher Scientific) at a density of 103 cells/µL embedded in hydrogel composed by type 1 collagen (2 mg/mL, BD Bioscience) and Matrigel poor in growth factors (2 mg/mL, BD Bioscience) as previously described (58 (link)). HP was used at a concentration of 0.02 to 0.5 mg/mL based on the findings that murine serum levels are between 0.5 and 1.5 mg/mL in normal conditions. After 24 hours, cells were recovered from the lower chamber and were quantified by flow cytometry using a CytoFLEX (Beckman Coulter) and CyteExpert software. Experiments were repeated three times.