BACKGROUND & AIMS: The hepatitis D virus (HDV) causes the most severe form of chronic hepatitis, often progressing to cirrhosis within 5–10 years. There is no curative treatment and the mechanisms underlying the accelerated liver disease progression are unknown. METHODS: Innate and adaptive immune responses were studied in blood and liver of 24 HDV-infected patients and 30 uninfected controls by multiparameter flow cytometry in correlation with disease severity and stage. RESULTS: The two main intrahepatic innate immune-cell populations, MAIT cells and NK cells, were reduced in the livers of HDV-infected patients compared to those of uninfected controls but were more frequently activated in the liver compared to the blood. Most intrahepatic CD8(+) T cells were memory cells or terminal effector memory cells, and the majority of activated and degranulating (CD107a(+)) HDV-specific and total CD8(+) T cells were liver-resident (CD69(+)CXCR6(+)). Unsupervised analysis of flow cytometry data identified an activated, memory-like, tissue-resident HDV-specific CD8(+) T-cell cluster with expression of innate-like NKp30 and NKG2D receptors. The size of this population correlated with liver enzyme activity (r=1.0). NKp30 and NKG2D expression extended beyond the HDV-specific to the total intrahepatic CD8(+) T-cell population suggesting global bystander activation. This was supported by the correlations between NKG2D expression and degranulation of intrahepatic CD8(+) T cells and between frequency of degranulating CD8(+) T cells and liver enzyme activity, and APRI score and by in vitro demonstration of cytokine-induced NKDG2D-dependent cytotoxicity. CONCLUSION: Antigen-nonspecific activation of liver-resident CD8(+) T cells may contribute to inflammation and disease stage in HDV infection.
