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Background & Aims: The hepatitis D virus (HDV) causes the most severe form of chronic hepatitis, often progressing to cirrhosis within 5 to 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 patients infected with HDV and 30 uninfected controls by multiparameter flow cytometry in correlation with disease severity and stage., Results: The 2 main intrahepatic innate immune-cell populations, mucosal-associated invariant T cells and natural killer (NK) cells, were reduced in the livers of patients infected with HDV compared with those of uninfected controls but were more frequently activated in the liver compared with the blood. Most intrahepatic cluster of differentiation (CD) 8-positive (CD8 + ) T cells were memory cells or terminal effector memory cells, and most of the activated and degranulating (CD107a + ) HDV-specific and total CD8 + T cells were liver-resident (CD69 + C-X-C motif chemokine receptor 6 + ). Unsupervised analysis of flow cytometry data identified an activated, memory-like, tissue-resident HDV-specific CD8 + T-cell cluster with expression of innate-like NK protein 30 (NKp30) and NK group 2D (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 (i) NKG2D expression with degranulation of intrahepatic CD8 + T cells, (ii) frequency of degranulating CD8 + T cells with liver enzyme activity and the aspartate aminotransferase-to-platelet ratio index score, and by the in vitro demonstration of cytokine-induced NKG2D-dependent cytotoxicity., Conclusion: Antigen-nonspecific activation of liver-resident CD8 + T cells may contribute to inflammation and disease stage in HDV infection., (Published by Elsevier Inc.)