Return to sender: Lymphocyte trafficking mechanisms as contributors to primary sclerosing cholangitis.

Primary sclerosing cholangitis (PSC) is an inflammatory disease of the biliary tree, characterised by stricturing bile duct disease and progression to liver fibrosis. The pathophysiology of PSC is still unknown. The concurrence with inflammatory bowel disease (IBD) in about 70% of cases has led to the hypothesis that gut-homing lymphocytes aberrantly traffic to the liver, contributing to disease pathogenesis in patients with both PSC and IBD (PSC-IBD). The discovery of mutual trafficking pathways of lymphocytes to target tissues, and expression of gut-specific adhesion molecules and chemokines in the liver has pointed in this direction. There is now increasing interest in using drugs that intervene with these trafficking pathways (e.g. vedolizumab, etrolizumab) for the treatment of PSC-IBD. In this review we discuss what is currently known about the immunological interactions between the gut and the liver in concomitant PSC and IBD, as well as potential therapeutic options for intervening in these mechanisms.