Infectious complications in patients with cirrhosis frequently initiate episodes of decompensation and substantially contribute to the high mortality. Mechanisms of the underlying immuneparesis remain underexplored. TAM receptors (TYRO3/AXL/MERTK) are important inhibitors of innate immune responses. To understand the pathophysiology of immuneparesis in cirrhosis, we detailed TAM receptor expression in relation to monocyte function and disease severity prior to the onset of acute decompensation. TNF-α/IL-6 responses to lipopolysaccharide were attenuated in monocytes from patients with cirrhosis (n = 96) compared with controls (n = 27) and decreased in parallel with disease severity. Concurrently, an AXL-expressing (AXL+) monocyte population expanded. AXL+ cells (CD14+CD16highHLA-DRhigh) were characterised by attenuated TNF-α/IL-6 responses and T cell activation but enhanced efferocytosis and preserved phagocytosis of Escherichia coli Their expansion correlated with disease severity, complications, infection, and 1-yr mortality. AXL+ monocytes were generated in response to microbial products and efferocytosis in vitro. AXL kinase inhibition and down-regulation reversed attenuated monocyte inflammatory responses in cirrhosis ex vivo. AXL may thus serve as prognostic marker and deserves evaluation as immunotherapeutic target in cirrhosis.

Cells require oxygen for aerobic metabolism, which may also result in the production of reactive oxygen species (ROS) as a by-product. Under low oxygen conditions, ROS formation has been reported to either increase or decrease. We addressed this physiological response for the first time in zebrafish embryonic fibroblasts (Z3) and used a hydrogen peroxide (H2O2)-specific fluorescent protein (roGFP2-Orp1) either targeted to the mitochondria or expressed in the cytosol. Microfluidic live-cell imaging measurements showed that oxygen deprivation in Z3 cells results in decreased or stable H2O2 levels within the mitochondria or the cytosol, respectively, and that the reductive shift recorded in the mitochondrial matrix is directly dependent on oxygen concentration. The response was accompanied by a transient increase in extracellular acidification rate (ECAR) and a lower cellular reducing potential as assessed by the viability stain alamarBlue. Complex I and III inhibition with Rotenone and Antimycin A led to H2O2 production under normoxia but these inhibitors were not able to avert the reductive shift under hypoxia. Only by system-wide inhibition of flavin-containing oxidases with Diphenyleneiodonium (DPI) were we able to decrease the reductive shift, while selective inhibition of NADPH oxidases with the inhibitor Apocynin had no effect on the hypoxia response. Since DPI also led to a strong increase in ECAR we found that, in order to keep the cytosolic H2O2 levels stable, glycolytic metabolism was of fundamental importance. According to our experiments with the glucose-6-phosphate dehydrogenase inhibitor 6-Aminonicotinamide, this was attributable to the pentose phosphate pathway producing reducing equivalents required for ROS degradation.

AXL and MERTK expression on circulating monocytes modulated immune responses in patients 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-α/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.