BACKGROUND CD4+ T cells are involved in the pathogenesis of immunoglobulin A nephropathy (IgAN); T helper (Th) 1, Th17 and Th22 cells promote the occurrence and amplification of inflammatory reactions, while regulatory T (Treg) cells produce the opposite effects. However, whether Th9 cells, a subset of CD4+ T cells, participate in IgAN development is still unknown. METHODS Human peripheral blood mononuclear cells (PBMCs) were isolated from IgAN patients for Th9 cells detection by flow cytometry. Wild-type (WT) mouse was used to establish an IgAN mouse model while C3aR and C5aR inhibitor treated IgAN mouse. Kidney disease and function was assessed by histology and albumin-to-creatinine ratio. C3aR and C5aR expression was examined by immunohistochemical (IHC) assay. Th9 cell proportions in the blood of IgAN mouse was detected. C3a, C5a and interleukin (IL)-9 levels were tested by ELISA. Moreover, co-culture system between human mesangial cells (HMCs) and CD4+ T cells were constructed with or without C3a, C5a and anti-CCL20 mAb stimulation for transwell assay to examine Th9 cell chemotaxis. RESULTS We observed the numbers of Th9 cell and the levels of IL-9 were increased in IgAN patients and IgAN mice. Furthermore, C3a and C5a level in serum and kidney, C3aR and C5aR expression was increased in IgAN mice compared to WT mice. Most interestingly, C3aR and C5aR inhibitor could reduce kidney damage, Th9 cell numbers and IL-9 levels. We also observed that C3a and C5a enhanced CCL20 production in HMCs. Notably, C3a and C5a also increased the recruitment of Th9 cells and IL-9 levels by HMCs through enhancing the CCL20-CCR6 pathway. CONCLUSIONS Our results support that C3a and C5a increase the production of CCL20 by HMCs and consequently augment Th9 cell recruitment and IL-9 levels, resulting in IgAN exacerbation.