Role of gp91^phox in hepatic macrophage programming and alcoholic liver disease

Hepatic macrophages (MΦs) are important in the development and progression of alcoholic liver disease (ALD). This study investigates the role of gp91^phox (nicotinamide adenine dinucleotide phosphate oxidase 2) in the severity of ALD and specifically in regulating hepatic MΦ efferocytic capability and the subsequent reprogramming associated with resolution of inflammation. After 4 weeks of ethanol feeding, more severe ALD developed in gp91^phox-/- mice than in wild-type (WT) C57Bl/6J mice, evidenced by increased liver injury and inflammation. This phenomenon was not sex dependent, and thus the majority of experiments were performed with female mice. While total hepatic MΦ numbers did not differ between genotypes, hepatic infiltrating MΦs (IMs) were slightly more numerous in gp91^phox-/- mice, and both IMs and resident Kupffer cells displayed enhanced proinflammatory and reduced tissue-restorative programming compared with these cells from WT mice. The ratio of proinflammatory IMs with higher expression of Ly6C (Ly6C^hi) to anti-inflammatory IMs with lower expression of Ly6C (Ly6C^low) was significantly higher in gp91^phox-/- mice compared to WT mice. Greater numbers of apoptotic cells accumulated in the liver of gp91^phox-/- mice compared to WT mice, and receptors for binding and engulfing apoptotic cells were expressed at much lower levels on both Kupffer cells and IMs of gp91^phox-/- mice. Interactions with apoptotic cells (binding and engulfment) in vitro were significantly fewer for gp91^phox-/- MΦs than for WT MΦs, resulting in diminished expression of tissue restorative mediators by hepatic MΦs of gp91^phox-/- mice. Conclusion: gp91^phox plays a critical role in the differentiation of proinflammatory hepatic MΦs to a tissue-restorative phenotype, likely through programming for efferocytosis, and thereby lessens the severity of ALD. These findings enhance our understanding of the tissue environmental cues that regulate MΦ phenotypes. This knowledge could help in designing MΦ-targeting strategies to prevent and treat ALD. (Hepatology Communications 2017;1:765-779).