Progression of NAFLD and NASH is mediated by proinflammatory tissue-resident T cells

Background:

The occurrence of non-alcoholic fatty liver disease (NAFLD) is closely associated with obesity and metabolic syndrome and is the leading cause of chronic liver disease in developed countries. Most often, hepatic steatosis leads to inflammation and metabolic dysfunction, which are the hallmarks of non-alcoholic steatohepatitis (NASH). Pathological immune responses are thought to be the main drivers of NAFLD progression but the mechanisms of pathogenesis remain incompletely understood. Therefore we aim to characterize the immunologic parameters that are involved in the progression of NAFLD.

Methods:

We collected PBMCs liver and mesenteric fat tissue samples from obese patients with and without NAFLD and/or NASH after informed consent. Biopsies were taken during bariatric surgery and patients were subsequently monitored for 12 months. 58 patients were included in the study (obese without NAFLD [n = 12], patients with NAFLD without inflammation [n = 28] and patients with NASH [n = 15]). Mediators involved in local immune responses were characterized by single cell RNA sequencing (scRNAseq) of CD3 T cells in liver and mesenterial fat tissue of two patients and subsequently validated by flow cytometry.

Results:

Thirteen disparate T cell subtypes were identified by scRNAseq. Effector CD8 T cells, MAIT cells and activated γδ T cells seemed to be enriched within the diseased tissue. CD8 T cells were accumulated within hepatic tissue whereas CD4 T cells were predominantly found in the fat tissue. Subsequent flow cytometric analysis revealed similar results. Furthermore, natural killer T (NKT) cells and NK cells were highly abundant in the hepatic tissue. We further elucidated the distribution of cytotoxic granules within tissue cells and circulation more deeply. Cytotoxic granules containing perforin, granulysin and granzymes were predominantly found within the CD8 T cell population. The majority of cytotoxic granules were detected within subpopulations of innate immune cells (especially NKT cells, NK cells and γδ T cells). Moreover, peripheral blood NKT cells and γδ T cells contained more cytotoxic granules than their counterparts in the liver.

Discussion:

Collectively, our preliminary data suggest that CD8 T cells and innate immune cell subsets are likely to mediate liver damage and fibrosis progression in NASH patients. Especially the role of the innate T cell subsets warrants further exploration towards their specific role in progression from NAFLD to NASH.