Role of the interleukin-15/macrophage axis in the immune response of liver fibrosis

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Inflammation has been recognized as a crucial aspect of liver fibrosis, however therapeutic approaches targeting cytokines for this condition have been so far unsuccessful. This failure is in part due to our incomplete understanding of the complex process of inflammation in liver fibrosis. Interleukin-15 (IL-15) is a potent cytokine with key functions in NK and NK-T cell physiology. Recently, it was reported that IL-15 receptor alpha (IL-15RA) KO mice showed enhanced susceptibility to liver fibrosis. However, our preliminary studies reveal an opposite response in IL-15 KO mice, showing reduced liver fibrosis compared to wild type mice. This controversy could be caused by the congenital defects in both IL-15 and IL-15RA constitutive KO mice. However, additional preliminary results support a pro-fibrotic role of IL-15. First, we observed a strong induction of IL-15 in primary hepatocytes stimulated with TGFb and detected several pro-fibrotic cytokines strongly induced by IL-15 in primary macrophages. Second, we observed induction of IL-15 in mouse and human fibrotic and cirrhotic liver. Therefore, our preliminary results strongly suggest IL-15 promotes fibrosis by inducing a pro-fibrotic phenotype in macrophages. Elucidating the complex role of the IL-15/macrophage axis in liver fibrosis requires a more refined knowledge of inflammation during fibrosis, including identification of cells expressing IL-15, Omics-based analysis of liver leukocytes, hepatocytes and non-parenchymal cells, and experimental in vivo conditions whereby deletion of IL-15 or IL-15RA is achieved, without altering the homeostatic milieu of liver leukocytes. In this application, we will approach this by first identifying the main cell source of IL-15 using IL-15-CFP reporter mice, and establishing correlations between IL-15 expression and the inflammatory microenvironment in liver fibrosis in mouse and human. Second, by analyzing the severity of liver fibrosis after tamoxifen-controlled, Cre-dependent deletion of IL-15 or IL-15RA in adult mice. Third, by transcriptomics analyses of hepatocytes, stellate cells, macrophages, NK and NK-T cells from fibrotic mice of control and IL-15 and IL-15RA-deleted. Finally, we will perform functional in vivo analyses of macrophages, NK and NK-T cells, using multiphoton intravital microscopy (i.e. motility, cytotoxicity towards myofibroblasts) by adoptive cell transfer of fluorescently-labeled cells into fibrotic mice (control and IL-15 or IL-15RA-deleted). In addition, we will use in vitro co-culture systems of macrophages, NK/NK-T cells and stellate cells to identify the key factors (i.e. secreted cytokines, surface molecules) mediating the pro-fibrotic effects of IL-15. We expect that our project will unveil the contribution and the mechanisms by which IL-15 influences immune responses in liver fibrosis, setting the basis for diagnostic and therapeutic applications focused on this novel molecular player in liver disease.