MFe(hi) adipose tissue macrophages compensate for tissue iron perturbations in mice.

Resident adipose tissue macrophages (ATMs) play multiple roles to maintain tissue homeostasis, such as removing excess free fatty acids and regulation of the extracellular matrix. The phagocytic nature and oxidative resiliency of macrophages not only allows them to function as innate immune cells but also to respond to specific tissue needs, such as iron homeostasis. MFe(hi) ATMs are a subtype of resident ATMs that we recently identified to have twice the intracellular iron content as other ATMs and elevated expression of iron-handling genes. Although studies have demonstrated that iron homeostasis is important for adipocyte health, little is known about how MFe(hi) ATMs may respond to and influence adipose tissue iron availability. Two methodologies were used to address this question: dietary iron supplementation and intraperitoneal iron injection. Upon exposure to high dietary iron, MFe(hi) ATMs accumulated excess iron, whereas the iron content of MFe(lo) ATMs and adipocytes remained unchanged. In this model of chronic iron excess, MFe(hi) ATMs exhibited increased expression of genes involved in iron storage. In the injection model, MFe(hi) ATMs incorporated high levels of iron, and adipocytes were spared iron overload. This acute model of iron overload was associated with increased numbers of MFe(hi) ATMs; 17% could be attributed to monocyte recruitment and 83% to MFe(lo) ATM incorporation into the MFe(hi) pool. The MFe(hi) ATM population maintained its low inflammatory profile and iron-cycling expression profile. These studies expand the field's understanding of ATMs and confirm that they can respond as a tissue iron sink in models of iron overload.