We have previously reported that complement activation precedes the development of kidney
fibrosis; however, little is known about the cellular mechanisms involved in this transition. We hypothesized that increased expression of C1 complex
protease C1r, the initiator of complement activation, contributes to tubulointerstitial
fibrosis and tested this idea in mice with global deletion of C1r. Although expression of C1r in untreated wild-type (WT) mice was higher in the liver compared with kidney tissue, administration of
folic acid (FA) led to upregulation of C1r
mRNA and
protein levels only in kidney tissue. Immunohistochemistry and in situ hybridization experiments localized increased expression of C1r and C1s
proteases to renal tubular epithelial cells. C1r-null mice had reduced acute tubular injury and
inflammation measured 2 days after FA administration compared with WT mice. C1r deletion reduced expression of C1s, C3 fragment formation, and organ
fibrosis measured 14 days after FA administration. Differential gene expression performed in kidney tissue demonstrated that C1r-null mice had reduced expression of genes associated with the
acute phase response,
complement, proliferation of connective tissue cells (e.g.,
platelet-derived growth factor receptor-β), and reduced expression of genes associated with
inflammation compared with FA-treated WT mice. In vitro experiments in renal epithelial cells demonstrated that C1s expression is dependent on increased C1r expression and that
interferon-γ induces the expression of these two
proteases. We conclude that increased expression of C1 complex
proteases is associated with increased tissue
inflammation and
complement C3 formation and represents an important pathogenic mechanism leading to FA-mediated tubulointerstitial
fibrosis.