Background & aims: Natural killer (NK) cells are known to exert strong antiviral activity. Killer cell lectin-like receptor subfamily G member 1 (KLRG1) is expressed by terminally differentiated NK cells and KLRG1-expressing lymphocytes are known to expand following chronic viral infections. We aimed to elucidate the previously unknown role of KLRG1 in the pathogenesis of chronic hepatitis B (CHB).

Methods: KLRG1+ NK cells were taken from the blood and liver of healthy individuals and patients with CHB. The phenotype and function of these cells was assessed using flow cytometry and in vitro stimulation.

Results: Patients with CHB had a higher frequency of KLRG1+ NK cells compared to healthy controls (blood 13.4 vs. 2.3%, p <0.0001 and liver 23.4 vs. 2.6%, p <0.01). KLRG1+ NK cells were less responsive to K562 and cytokine stimulation, but demonstrated enhanced cytotoxicity (9.0 vs. 4.8%, p <0.05) and IFN-γ release (8.0 vs. 1.5%, p <0.05) via antibody dependent cellular cytotoxicity compared to their KLRG1- counterparts. KLRG1+ NK cells possessed a mature phenotype, demonstrating stronger cytolytic activity and IFN-γ secretion against hepatic stellate cells (HSCs) than KLRG1- NK cells. Moreover, KLRG1+ NK cells more effectively induced primary HSC apoptosis in a TRAIL-dependent manner. Increased KLRG1+ NK cell frequency in the liver and blood was associated with lower fibrosis stage (F0/F1) in patients with CHB. Finally, the expression of CD44, degranulation and IFN-γ production were all increased in KLRG1+ NK cells following stimulation with osteopontin, the CD44 ligand, suggesting that HSC-derived osteopontin may cause KLRG1+ NK cell activation.

Conclusions: KLRG1+ NK cells likely play an antifibrotic role during the natural course of CHB infection. Harnessing this antifibrotic function may provide a novel therapeutic approach to treat liver fibrosis in patients with CHB.

Lay summary: Individuals that are chronically infected with hepatitis B virus (HBV) possess an increased number of immune cells, called natural killer (NK) cells expressing the surface marker KLRG1 in the blood and liver. Here, we demonstrate that these specific NK cells are able to kill activated stellate cells in the liver. Because activated stellate cells contribute to liver scarring, i.e. fibrosis, and subsequent liver dysfunction in individuals with chronic HBV infection, KLRG1+ NK cells are a novel immune cell type that can limit liver scarring.