Pcdna6.0 expression vector

pDONR223-MERTK (RRID:Addgene_23900) was a gift from William Hahn and David Root, and MerTK was subcloned into the BamHI/NotI restriction sites of the pcDNA6.0 expression vector (Life Technologies, Carlsbad, CA, USA). Transfection was performed using Lipofectamine3000 and Opti-MEM (Life Technologies) according to the manufacturer’s instructions. Antibiotic selection was started 48 h after transfection via addition of blasticidin (1 µg/mL) to the growth media. For CRISPR editing, SUM102-MerTKC2 cells (3 × 10⁴ cells) were transfected with two predesigned TrueGuide sgRNA targeting endoglin (CRISPR768842 and CRISPR1060035, Invitrogen, Waltham, MA, USA) or TrueGuide sgRNA Negative Control (#A35526) using Invitrogen TrueCut Cas9 v2 and Lipofectamine CRISPRMAX (Invitrogen) according to the manufacturer’s recommendations. Two days after transfection, cells were harvested and stained with MerTK (APC anti-human mouse MerTK #367612, BioLegend, San Diego, CA, USA) as well as endoglin (PE anti-endoglin, #800504, BioLegend) antibodies. Then one cell per well was sorted in a 96-well plate by BD FACSAria Cell Sorters (BD, Franklin Lakes, NJ, USA) for clonal isolation and expansion.