Folic acid-mediated fibrosis is driven by C5a receptor 1-mediated activation of kidney myeloid cells

We have previously reported that increased expression and activation of kidney cell complement components play an important role in the pathogenesis of renal scarring. Here, we used floxed green fluorescent protein (GFP)-C5a receptor 1 (C5aR1) knockin mice (GFP-C5ar1^fl/^fl) and the model of folic acid (FA)-induced kidney injury to define the cell types and potential mechanisms by which increased C5aR1 activation leads to fibrosis. Using flow cytometry and confocal microscopy, we identified macrophages as the major interstitial cell type showing increased expression of C5aR1 in FA-treated mice. C5ar1^fl/^fl.Lyz2Cre^þ/- mice, in which C5aR1 has been specifically deleted in lysozyme M-expressing myeloid cells, experienced reduced fibrosis compared with control C5ar1^fl/^fl mice. Examination of C5aR1-expressing macrophage transcriptomes by gene set enrichment analysis demonstrated that these cells were enriched in pathways corresponding to the complement cascade, collagen formation, and the NABA matrisome, strongly pointing to their critical roles in tissue repair/scarring. Since C5aR1 was also detected in a small population of platelet-derived growth factor receptor-b^þ GFP^þ cells, we developed C5ar1^fl/^fl.Foxd1Cre^þ/- mice, in which C5aR1 is deleted specifically in pericytes, and found reduced FA-induced fibrosis. Primary cell cultures of platelet-derived growth factor receptor-b^þ pericytes isolated from FA-treated C5ar1^fl/^fl.Foxd1Cre^þ/- mice showed reduced secretion of several cytokines, including IL-6 and macrophage inflammatory protein-2, compared with pericytes isolated from FA-treated control GFP-C5ar1^fl/^fl mice. Collectively, these data imply that C5a/C5aR1 axis activation primarily in interstitial cells contributes to the development of renal fibrosis. NEW & NOTEWORTHY This study used novel green fluorescent protein C5a receptor 1 floxed mice and the model of folic acid-mediated kidney fibrosis to demonstrate the pathogenic role of increased expression of this complement receptor on macrophages.