PECAM1 and renal fibrosis: Presumably, these macrophages were initially CD45+ or CD68+ and then acquired the expression of nonspecific markers, such as the endothelial/platelet marker CD31 or the mesenchymal marker α-SMA, similar to tumour-associated macrophages [51] and alveolar macrophages of human lung allografts in patients with acute rejection [52] which gain CD31 expression, or to the macrophage-to-myofibroblast transition observed in kidney biopsies obtained from patients with renal fibrosis [53] or the recipients of kidney transplants [54].