Non-alcoholic steatohepatitis (NASH) is a form of non-alcoholic fatty liver disease (NAFLD) with more severe inflammation-induced liver damage. Microbial products, such as endotoxin, may contribute to the pathogenesis of NASH. In this study, we investigated the effect of serum endotoxin on CD4 T cell inflammation. Age and sex-matched non-obese healthy subjects, subjects with non-alcoholic fatty liver (NAFL) but not steatohepatitis, and NASH patients were recruited for this study. The latter two groups were additionally matched in BMI and diabetes status. We first showed that compared to healthy subjects and NAFL patients, NASH patients presented significantly higher levels of serum endotoxin. Concurrently, NASH patients presented a Th17 bias that was associated with high endotoxin levels. To examine whether endotoxin could directly mediate IL-17 expression from CD4 T cells, naive CD4 T cells were stimulated with varying levels of endotoxin. In healthy subjects and NAFL patients, endotoxin did not act directly on naive CD4 T cells but required the presence of antigen-presenting cells to upregulate IL-17. Inhibition of TLR4 in macrophages, but not in CD4 T cells, could impair endotoxin-mediated IL-17 upregulation. In NASH patients, however, endotoxin at high levels directly, but minimally, increased IL-17 production. We further found that naive CD4 T cells from NASH patients presented significantly higher TLR4 than naive CD4 T cells from healthy subjects and NAFL patients, and CD3/CD28 stimulation could significantly elevate TLR4 expression by naive CD4 T cells. Overall, these data demonstrate that endotoxin promote Th17 bias in NASH patients.Copyright © 2020 Elsevier Ltd. All rights reserved.