Studies of innate and adaptive lymphocytes in human liver diseases and viral infections

Abstract: The immune system, including innate and adaptive lymphocytes, is involved in determining the outcome of many human diseases. Two lymphocyte subsets, natural killer (NK) cells and T cells, which are present in both peripheral blood and in tissues, will be further discussed in the context of acute and chronic viral infections and inflammatory liver diseases in this thesis. NK cells are important players in the early defense against many viral infections. To improve our understanding of human NK cell responses in acute viral infections, we here comprehensively characterized peripheral blood NK cells in patients with acute dengue virus infection, causing dengue fever, from early after symptom debut (paper I). In particular, less mature NK cell subsets were robustly activated, and our data further suggested an IL-18- dependent mechanism for driving the observed response. Responding NK cells exhibited a distinct chemokine receptor imprint indicative of skin homing and we could identify a corresponding NK cell subset in the skin from patients with acute infection. In chronic viral infections, such as chronic hepatitis B (CHB), NK cells and T cells are generally dysfunctional. This dysfunction may contribute to the hosts inability to clear the infection. Nucleos(t)ide analogue (NA) therapy suppresses hepatitis B virus (HBV) replication, but rarely cures CHB. Stopping long-term NA therapy leads to viral relapse and liver inflammation but eventually to functional cure in a fraction of patients. Here, we found that structured NA treatment discontinuation in CHB patients augmented peripheral blood NK cell natural cytotoxicity responses 12 weeks following treatment cessation. This enhanced functionality was associated with liver inflammation, particularly in patients with subsequent functional cure (paper II). Furthermore, T cells from the CHB patients achieving a functional cure displayed a more activated phenotype. In vitro stimulation with HBVspecific peptides further revealed enhanced peripheral blood T cell functionality that could be boosted with PD-L1 blockade (paper III). In addition to the analyses of lymphocytes in peripheral blood, we investigated the role of an unconventional T cell subset, mucosal invariant T (MAIT) cells, in peripheral blood and bile ducts of patients suffering from primary sclerosing cholangitis. The immunological mechanisms in this rare chronic progressive inflammatory disease of the biliary tract are largely unknown. While MAIT cells were enriched in the bile ducts, numbers and function of circulating MAIT cells were strongly reduced (paper IV). Further analyses of the biliary tract immunological landscape revealed that TcRab CD8ab effector memory T cells represented the dominant intraepithelial immune cell population. These biliary-resident T cells co-expressed gut- and liver-homing receptors and displayed a Th1/Th17 functional profile (paper V). In summary, we could show a significant contribution of NK cells and T cells to the immune response in acute and chronic viral infections, whereas the characterization of biliary-resident T cells has just begun and their function in immunopathogenesis remains to be explored. Altogether, the investigation of immunological mechanisms underlying a variety of human diseases adds to our understanding of human immune cell functionality as well as presents strategies for future treatment development.