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NK cells have been implicated to affect the outcome of numerous liver diseases. In particular, members of the killer‐cell Ig‐like receptor (KIR) family, predominantly expressed by NK cells, have been associated with the outcome of hepatitis C virus infection and clearance of hepatocellular carcinoma. Inhibitory KIRs tune NK cell function through interaction with HLA class I, a process termed education. Nevertheless, the impact of the hepatic environment on NK cell education is incompletely understood. Therefore, we investigated the composition and function of hepatic KIR‐expressing NK cells. Matched PBMC and hepatic lymphocytes were isolated from 20 individuals undergoing liver surgery and subsequently phenotypically analyzed for expression of KIRs and markers for tissue residency using flow cytometry. NK cell function was determined by co‐culturing NK cells with the target cell line 721.221 and subsequent assessment of CD107a, IFN‐γ, and TNF‐α expression. Liver‐resident CXCR6+/CD56Bright NK cells lacked KIRs and were predominantly educated through NKG2A, while CXCR6−/CD16+ NK cells expressed KIRs and resembled peripheral blood NK cells. Hepatic NK cells showed lower response rates compared to peripheral blood NK cells; in particular, CXCR6+ NK cells were hyporesponsive to stimulation with target cells. The high proportion of educated NK cells in both subsets indicates the importance of self‐inhibitory receptors for the balance between maintenance of self‐tolerance and functional readiness. However, the reduced functionality of hepatic NK cells may reflect the impact of the tolerogenic hepatic environment on NK cells irrespective of NK cell education.
The transcription factor promyelocytic leukemia zinc finger protein (PLZF) is involved in the development of natural killer (NK) cells and innate lymphoid cells, including liver‐resident NK cells in mice. In human NK cells, the role of PLZF in liver residency is still unknown. Expression of PLZF in matched human peripheral blood‐ and liver‐derived NK cells and the association of PLZF expression with surface molecules and transcription factors relevant for tissue residency were investigated using multiparameter flow cytometry and assessing single‐cell messenger RNA (mRNA) levels. Intrahepatic cluster of differentiation (CD)56bright NK cells expressed significantly higher levels of PLZF than peripheral blood CD56bright NK cells, which were predominantly PLZFlo. Expression of PLZF was highest within C‐X‐C motif chemokine receptor 6 (CXCR6)+CD69+ liver‐resident NK cells among intrahepatic CD56bright NK cell populations. Association of PLZF with liver‐residency markers was also reflected at mRNA levels. A small PLZFhiCD56bright NK cell population was identified in peripheral blood that also expressed the liver‐residency markers CXCR6 and CD69 and shared functional characteristics with liver‐resident NK cells. Conclusion: PLZF is implicated as part of a transcriptional network that promotes liver residency of human NK cells. Expression of liver‐homing markers on peripheral blood PLZFhiCD56bright NK cells identifies an intermediate population potentially contributing to the maintenance of liver‐resident NK cells.
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