Neutrophils alleviate fibrosis in the CCl4‐induced mouse chronic liver injury model

Tribbles pseudokinase 1 (Trib1) is a negative regulator of CCAAT/enhancer binding protein α (C/EBPα) and is known to induce granulopoiesis while suppressing monocyte differentiation. Loss of Trib1 was previously shown to increase the neutrophil population in the spleen but lead to M2‐like macrophage reduction. Because M2 macrophages are anti‐inflammatory and promote tissue repair by producing fibrogenic factors, we investigated liver fibrosis in Trib1‐deficient mice. Interestingly, loss of Trib1 suppressed fibrosis in the CCl4‐induced chronic liver injury model. Trib1 knockout increased neutrophils but had a minimal effect on the macrophage population in the liver. Hepatic expressions of neutrophil matrix metalloproteinases (Mmp)8 and Mmp9 were increased, but the production of fibrogenic factors, including transforming growth factor β1, was not affected by loss of Trib1. These results suggest that neutrophils are responsible for the suppression of fibrosis in Trib1‐deficient liver. Consistently, transplantation of Trib1‐deficient bone marrow cells into wild‐type mice alleviated CCl4‐induced fibrosis. Furthermore, expression of chemokine (C‐X‐C motif) ligand 1 (Cxcl1) by adeno‐associated viral vector in the normal liver recruited neutrophils and suppressed CCl4‐induced fibrosis; infusion of wild‐type neutrophils in CCl4‐treated mice also ameliorated fibrosis. Using recombinant adeno‐associated virus‐mediated expression of Mmp8 and Mmp9 alleviated liver fibrosis. Finally, neutrophil depletion by infusion of Ly6G antibody significantly enhanced CCl4‐induced fibrosis. Conclusion: While neutrophils are well known to exacerbate acute liver injury, our results demonstrate a beneficial role of neutrophils in chronic liver injury by promoting fibrolysis.