Hepatic Macrophage Abundance and Phenotype in Aging and Liver Iron Accumulation

Abstract

Liver macrophages serve important roles in iron homeostasis through phagocytosis of effete erythrocytes and the export of iron into the circulation. Conversely, intracellular iron can alter macrophage phenotype. Aging increases hepatic macrophage number and nonparenchymal iron, yet it is unknown whether age-related iron accumulation alters macrophage number or phenotype. To evaluate macrophages in a physiological model of iron loading that mimicked biological aging, young (6 mo) Fischer 344 rats were given one injection of iron dextran (15 mg/kg), and macrophage number and phenotype were evaluated via immunohistochemistry. A separate group of old (24 mo) rats was treated with 200 mg/kg deferoxamine every 12 h for 4 days. Iron administration to young rats resulted in iron concentrations that matched the values and pattern of tissue iron deposition observed in aged animals; however, iron did not alter macrophage number or phenotype. Aging resulted in significantly greater numbers of M1 (CD68+) and M2 (CD163+) macrophages in the liver, but neither macrophage number nor phenotype were affected by deferoxamine. Double-staining experiments demonstrated that both M1 (iNOS+) and M2 (CD163+) macrophages contained hemosiderin, suggesting that macrophages of both phenotypes stored iron. These results also suggest that age-related conditions other than iron excess are responsible for the accumulation of hepatic macrophages with aging.

Monocyte and Macrophage Function Diversity

“…Iron was suggested to shift macrophage phenotype to the M2 subtype, but its effect on macrophage proliferation was unknown. Bloomer [ 11 ] studied how macrophage phenotype, number, and accumulation of iron in the liver change with age. He showed in the rat model that aging increases the number of M1 (CD68+) and M2 (CD163+) macrophages in the liver and that both macrophage subtypes stored iron.…”