AXL Expression on Homeostatic Resident Liver Macrophages Is Reduced in Cirrhosis Following GAS6 Production by Hepatic Stellate Cells

BACKGROUND & AIMS: AXL and MERTK expression on circulating monocytes modulated immune responses in pa-tients with cirrhosis (CD14(+) HLA-DR+ AXL(+) ) and acute-on -chronic liver failure (CD14(+) MERTK+ ). AXL expression involved enhanced efferocytosis, sustained phagocytosis, but reduced tumor necrosis factor-alpha/interleukin-6 production and T-cell activation, suggesting a homeostatic function. Axl was expressed on murine airway in tissues contacting the external environment, but not interstitial lung-and tissue-resident synovial lining macrophages. Here, we assessed AXL expression on tissue macrophages in patients with cirrhosis. METHODS: Using multiplexed immunofluorescence we compared AXL expression in liver biopsies in cirrhosis (n = 22), chronic liver disease (n = 8), non-cirrhotic portal hypertension (n = 4), and healthy controls (n = 4). Phenotype and function of isolated primary human liver macrophages were character-ized by flow cytometry (cirrhosis, n = 11; control, n = 14) ex vivo. Also, AXL expression was assessed on peritoneal (n = 29) and gut macrophages (n = 16) from cirrhotic patients. Regulation of AXL expression was analyzed in vitro and ex vivo using primary hepatic stellate cells (HSCs), LX-2 cells, and GAS6 in co-culture experiments. RESULTS: AXL was expressed on resident (CD68(+)) but not tissue-infiltrating (MAC387(+)) liver macrophages, hepatocytes, HSCs, or sinusoidal endothelial cells. Prevalence of hepatic CD68(+)AXL(+) cells significantly decreased with cirrhosis pro-gression: (healthy, 90.2%; Child-Pugh A, 76.1%; Child-Pugh B, 64.5%; and Child-Pugh C, 18.7%; all P < .05) and negatively correlated with Model for End-Stage Liver Disease and C-reactive protein (all P < .05). AXL-expressing hepatic mac-rophages were CD68(high)HLA-DR(high)CD16(high)CD206(high). AXL expression also decreased on gut and peritoneal macrophages from cirrhotic patients but increased in regional lymph nodes. GAS6, enriched in the cirrhotic liver, appeared to be secreted by HSCs and down-regulate AXL in vitro. CONCLUSIONS: Decreased AXL expression on resident liver macrophages in advanced cirrhosis, potentially in response to activated HSC-secreted GAS6, suggests a role for AXL in the regulation of hepatic immune homeostasis.

Automated summary

This study found that AXL is expressed on tissue macrophages in patients with cirrhosis, but not on tissue-infiltrating liver macrophages, hepatocytes, and sinusoidal endothelial cells. The expression of AXL decreases significantly with cirrhosis progression and is negatively correlated with liver function and C-reactive protein. AXL-expressing liver macrophages show high expression of CD68, HLA-DR, CD16, and CD206. Additionally, AXL expression decreases on gut and peritoneal macrophages in cirrhotic patients but increases in regional lymph nodes. These findings suggest an important role for AXL in the regulation of hepatic immune homeostasis.