Local proliferation of macrophages in adipose tissue during obesity-induced inflammation

Obesity is frequently associated with low-grade inflammation of adipose tissue (AT), and the increase in adipose tissue macrophages (ATMs) is linked to an increased risk of type 2 diabetes. Macrophages have been regarded as post-mitotic, but recent observations have challenged this view. In this study, we tested the hypothesis that macrophages proliferate within AT in diet-induced obesity in mice and humans. We studied the expression of proliferation markers by immunofluorescence, PCR and flow cytometry in three different models of mouse obesity as well as in humans (n = 239). The cell fate of dividing macrophages was assessed by live imaging of AT explants. We show that ATMs undergo mitosis within AT, predominantly within crown-like structures (CLS). We found a time-dependent increase in ATM proliferation when mice were fed a high-fat diet. Upregulation of CD206 and CD301 in proliferating ATMs indicated preferential M2 polarisation. Live imaging within AT explants from mice revealed that macrophages emigrate out of the CLS to become resident in the interstitium. In humans, we confirmed the increased expression of proliferation markers of CD68(+) macrophages in CLS and demonstrated a higher mRNA expression of the proliferation marker Ki67 in AT from obese patients. Local proliferation contributes to the increase in M2 macrophages in AT. Our data confirm CLS as the primary site of proliferation and a new source of ATMs and support a model of different recruitment mechanisms for classically activated (M1) and alternatively activated (M2) macrophages in obesity.