Natural killer cell maturation markers in the human liver and expansion of an NKG2C+KIR+ population

Background: selected populations of murine natural killer (NK) cells possess memory features to haptens, cytokines, and viruses. Liver-specific adhesion molecules CXCR6 and CD49a have been identified as surface markers in mice. In people, expansion of long-lived terminally differentiated NKG2C+ populations occur in the blood after viral infection. We aimed to compare intrahepatic and blood NK cell receptor expression to determine the existence of CD49a+ and CXCR6+ NK cells in human liver and define the maturation status of NKG2C+ NK cells at this site.Methods: tissue samples were taken from the liver margin of 39 patients with hepatic metastases and flushed with chelating buffer followed by collagenase or mechanical digestion. Paired peripheral blood samples were taken from 15 patients, the remainder being unpaired. Mononuclear cells were isolated by ficoll separation and cell surface staining performed for CD3, CD56, CD16, CD57, CD117, CD161, CD158a, CD158b, CD49a, CD49b, CXCR6, NKG2C, and NKp46. Statistical analysis to compare intrahepatic and blood NK cell receptor expression included the median, IQR, and Mann-Whitney U testFindings: frequencies of NK cell precursors were similar in the liver and the blood (0·91% [0·62–3·26] vs 0·87 [0·41–1·52]); however, expression of all later markers of maturity were reduced including CD16 (47% [40·4–61·4] vs 88·7 [82·2–93·2], p<0·0001), CD57 (30·7% [25·0–53·9] vs 73·4 [70·4–87·6], p=0·0003), and KIR (11·2% [7·5–14·5] vs 26·7 [17·3–30·8], p<0·0001). Expanded hepatic CD16– NK cells were particularly immature with reduced CD57 and increased CD161 compared with the blood. NKG2C+ NK cells were found in similar frequencies in liver and blood. The hepatic NKG2C+ population was terminally differentiated, as in the circulation, but demonstrated a three-fold increase in KIR expression compared with NKG2C– counterparts, which was not seen in the blood. As in previously published research in mice, CD49a+ and CXCR6+ NK cells were liver resident (6·5% [3·9–14·6] liver vs 2·1 [1·3–4·3] blood, p<0·0001, and 65·3 [48·1–75·2] vs 4·5 [1·43–12·12], p=0·0039, respectively). Both populations were immature, with reduced KIR expression.Interpretation: we have shown that the liver contains an expanded population of immature CD16– NK cells. These cells might traffic from the blood and then differentiate into hepatic-specific CD49a+ and CXCR6+ NK cells. The function of these subsets is unknown but their immaturity hints against memory. Terminally differentiated NKG2C+ cells show KIR expansion in the human liver and probably represent an antigen-experienced population, raising the question of whether the liver is a site of NK cell memory acquisitio