Deletion of Myeloid Interferon Regulatory Factor 4 (Irf4) in Mouse Model Protects against Kidney Fibrosis after Ischemic Injury by Decreased Macrophage Recruitment and Activation

Significance Statement Macrophage proliferation and polarization to the M2 phenotype play a key role in AKI recovery. However, M2 macrophages also can produce profibrotic factors and their persistence may contribute to interstitial fibrosis. The authors demonstrated that mice with macrophage-specific deletion of the gene encoding IFN regulatory factor 4 (Irf4), a mediator of myeloid polarization, exhibited decreased renal fibrosis after severe AKI, in association with less-activated macrophages. Bone marrow–derived monocytes from such mice had diminished chemotactic responses, with decreased activation of the PI3 kinase/AKT pathway. Renal macrophage infiltration in response to AKI was markedly decreased in these mice and in wild-type mice with inhibition of AKT activity. These studies provide novel insights into the role of IRF4 by demonstrating its important function to mediate monocyte recruitment to injured tissue. Background AKI is characterized by abrupt and reversible kidney dysfunction, and incomplete recovery leads to chronic kidney injury. Previous studies by us and others have indicated that macrophage infiltration and polarization play key roles in recovery from AKI. The role in AKI recovery played by IFN regulatory factor 4 (IRF4), a mediator of polarization of macrophages to the M2 phenotype, is unclear. Methods We used mice with myeloid or macrophage cell–specific deletion of Irf4 (MΦ Irf4 −/− ) to evaluate Irf4’s role in renal macrophage polarization and development of fibrosis after severe AKI. Results Surprisingly, although macrophage Irf4 deletion had a minimal effect on early renal functional recovery from AKI, it resulted in decreased renal fibrosis 4 weeks after severe AKI, in association with less-activated macrophages. Macrophage Irf4 deletion also protected against renal fibrosis in unilateral ureteral obstruction. Bone marrow–derived monocytes (BMDMs) from MΦ Irf4 −/− mice had diminished chemotactic responses to macrophage chemoattractants, with decreased activation of AKT and PI3 kinase and increased PTEN expression. PI3K and AKT inhibitors markedly decreased chemotaxis in wild-type BMDMs, and in a cultured macrophage cell line. There was significant inhibition of homing of labeled Irf4 −/− BMDMs to postischemic kidneys. Renal macrophage infiltration in response to AKI was markedly decreased in MΦ Irf4 −/− mice or in wild-type mice with inhibition of AKT activity. Conclusions Deletion of Irf4 from myeloid cells protected against development of tubulointerstitial fibrosis after severe ischemic renal injury in mice, due primarily to inhibition of AKT-mediated monocyte recruitment to the injured kidney and reduced activation and subsequent polarization into a profibrotic M2 phenotype.