Objective@#To investigate the effect and mechanism of renal fibrosis after macrophage depletion in C3-deficient unilateral ureteral obstruction mice.@*Methods@#Renal interstitial fibrosis model was established by unilateral ureteral obstruction (UUO) in male C3-deficient mice and age-matched C57BL/6 WT mice (8-12 weeks of age). Mice were randomly divided into 4 groups, including sham operation in wild type group (WT/sham) (n=18), UUO operation in wild type group (WT/UUO) (n=18), sham operation in C3-deficient group (C3KO/sham) (n=18), and UUO operation in C3-deficient group (C3KO/UUO) (n=18). The expression of complement C3 was detected by immunohistochemical staining and renal interstitial macrophages were assessed by immunofluorescence staining. Tubulointerstitial fibrosis was observed by both HE staining and Masson staining after 14 days of UUO. Collagen accumulation and score of tubulointerstitial injury were obtained. Wild type and C3-deficient UUO mice were treated by liposome clodronate in early or late stage respectively and then interstitially infiltrated macrophages and renal fibrosis were analysed. Mice were sacrificed randomly at 3,7,14 days after UUO and obstructed kidneys were collected. Macrophage phenotype was detected by double-labeling immunofluorescence with F4/80 and iNOS for the M1, F4/80 and CD206 for the M2 macrophage subpopulation. iNOS, Arg-1 and CD206 were also detected by western blot.@*Results@#C3 deficient mice exhibited attenuated renal fibrosis, reduced collagen accumulation and tubulointerstitial injury score compared with WT mice (P<0.01). Meanwhile, macrophage depletion in early or late stage of UUO reduced renal fibrosis in WT mice, but had no effect on C3-deficient UUO mice. Decreased accumulation of M1 macrophages and expression of iNOS, increased accumulation of M2 macrophages and expression of Arg-1, CD206 were found in C3 deficient mice compared with WT mice in early stage of UUO (P<0.01).@*Conclusion@#Renal fibrosis is not reduced after depletion of macrophages in C3 deficient UUO mice due to the altered macrophage polarization.