Breast Regression Protein–39/Chitinase 3–Like 1 Promotes Renal Fibrosis after Kidney Injury via Activation of Myofibroblasts

The normal response to kidney injury includes a robust inflammatory infiltrate of PMNs and macrophages. We previously showed that the small secreted protein breast regression protein–39 (BRP-39), also known as chitinase 3–like 1 (CHI3L1) and encoded by the Chi3l1 gene, is expressed at high levels by macrophages during the early stages of kidney repair and promotes tubular cell survival via IL-13 receptor α 2 (IL13R α 2)–mediated signaling. Here, we investigated the role of BRP-39 in profibrotic responses after AKI. In wild-type mice, failure to resolve tubular injury after unilateral ischemia-reperfusion injury (U-IRI) led to sustained low-level Chi3l1 mRNA expression by renal cells and promoted macrophage persistence and severe interstitial fibrosis. Analysis of macrophages isolated from wild-type kidneys 14 days after U-IRI revealed high-level expression of the profibrotic BRP-39 receptor Ptgdr2 / Crth2 and expression of the profibrotic markers Lgals3 , Pdgfb , Egf , and Tgfb . In comparison, injured kidneys from mice lacking BRP-39 had significantly fewer macrophages, reduced expression of profibrotic growth factors, and decreased accumulation of extracellular matrix. BRP-39 depletion did not affect myofibroblast accumulation but did attenuate myofibroblast expression of Col1a1 , Col3a1 , and Fn1 . Together, these results identify BRP-39 as an important activator of macrophage-myofibroblast crosstalk and profibrotic signaling in the setting of maladaptive kidney repair.