Profibrogenic role of IL-15 through IL-15 receptor alpha-mediated trans-presentation in the carbon tetrachloride-induced liver fibrosis model

Abstract

Background: Inflammatory cytokines play key pathogenic roles in liver fibrosis. IL-15 is a proinflammatory cytokine produced by myeloid cells. IL-15 promotes pathogenesis of several chronic inflammatory diseases. However, increased liver fibrosis has been reported in mice lacking IL-15 receptor alpha chain (IL-15Rα), suggesting an anti-fibrogenic role for IL-15. As myeloid cells are key players in liver fibrosis and IL-15 signaling can occur independently of IL-15Rα, we investigated the requirement of IL-15 and IL-15Rα in liver fibrosis.

Methods: We induced liver fibrosis in Il15^-/- , Il15ra^-/- and wildtype C57BL/6 mice by the administration of carbon tetrachloride (CCl₄). Liver fibrosis was evaluated by Sirius red and Mason's trichrome staining and α-smooth muscle acting immunostaining of myofibroblasts. Gene expression of collagens, matrix modifying enzymes, cytokines and chemokines was quantified by RT-qPCR. The phenotype and the numbers of intrahepatic lymphoid and myeloid cell subsets were evaluated by flow cytometry.

Results: Both Il15^-/- and Il15ra^-/- mice developed markedly reduced liver fibrosis compared to wildtype control mice, as revealed by reduced collagen deposition and myofibroblast content. Il15ra^-/- mice showed further reduction in collagen deposition compared to Il15^-/- mice. However, Col1a1 and Col1a3 genes were similarly induced in the fibrotic livers of wildtype, Il15^-/- and Il15ra^-/- mice, although notable variations were observed in the expression of matrix remodeling enzymes and chemokines. As expected, Il15^-/- and Il15ra^-/- mice showed markedly reduced numbers of NK cells compared to wildtype mice. They also showed markedly less staining of CD45⁺ immune cells and CD68⁺ macrophages, and significantly reduced inflammatory cell infiltration into the liver, with fewer pro-inflammatory and anti-inflammatory monocyte subsets compared to wildtype mice.

Conclusion: Our findings indicate that IL-15 exerts its profibrogenic role in the liver by promoting macrophage activation and that this requires trans-presentation of IL-15 by IL-15Rα.

Keywords: IL-15; IL-15Rα; carbon tetra chloride (CCl) 4; liver fibrosis; pre-clinical study.