Antagonism of CCL5 chemokine receptors during regression of liver fibrosis is associated with an intrahepatic shift towards pro-inflammatory macrophages

Abstract

Aims: Chemokines mediate the progression but also the regression of liver fibrosis. We could recently show that antagonism of the chemokine CCL5 with Met-CCL5 accelerates the regression of liver fibrosis (Berres et al. JCI 2010). Here we aimed to investigate the underlying mechanisms for the faster resolution of liver scarring in this model. Methods: C57BL/6 mice were treated with CCl4 for eight weeks to induce severe fibrosis. At peak of fibrosis (3 days after the last CCl4 injection), mice received either Met-CCL5 (10µg/day) or vehicle for the next 7 days. Mice were sacrificed 6 and 10 days after the last CCl4 injection and analyzed for their fibrosis and immune phenotype. Results: Both groups of animals (Met-CCL5 and vehicle treated) displayed a regression of liver fibrosis. However, the regression was accelerated in Met-CCL5 treated mice as evidenced by a lower Sirius red positive area and reduced hydroxyproline content at day 10 (both P < 0.05). This faster resolution of liver scarring was associated with a higher number of intrahepatic Gr1+ (Ly6C+) monocytes at all investigated time points (P < 0.05). Furthermore, the frequency of iNOS and TNF-alpha positive macrophages was increased in Met-CCL5 treated mice compared to vehicle treated mice. The shift to a more pro-inflammatory intrahepatic macrophage subset was confirmed by RT-PCRs of total liver tissue. Compared to vehicle treated mice, animals injected with Met-CCL5 displayed higher mRNA levels of iNOS (P < 0.05), but lower mRNA levels of IL–10 (P < 0.01) and arginase (P=0.2). Conclusions: The faster resolution of liver scarring in mice treated with an antagonist of CCL5 receptors is strongly associated with a shift of intrahepatic macrophage subsets to a pro-inflammatory phenotype. The results confirm earlier findings that monocyte/macrophage subsets differentially contribute to progression and resolution of liver fibrosis and that this balance can be successfully modulated by chemokine antagonists.