*4.1. Renal Biopsy Study*

Renal biopsies were selected from patients that had a biopsy proven acute renal allograft rejection, as previously described [20]. Patient and transplantation characteristics are summarized in Supplementary Table S1. Frozen biopsy tissue sections (4 μm) were fixed in acetone, endogenous peroxidase was blocked with H2O2, and slides were blocked with 1% bovine serum albumin and 5% normal human serum in PBS. Sections were then incubated with specific antibodies directed against CD34 (BD Biosciences, Breda, The Netherlands) and CD73 (BD Biosciences, Breda, The Netherlands) followed by appropriate secondary antibodies that were HRP-conjugated (Jackson Immunoresearch, Westgrove, PA, USA). Stainings were visualized using Nova RED (Vector Labs, Peterborough, UK). Quantification of immunohistological staining results was performed using image J software.
