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Review
. 2014 Nov;262(1):134-52.
doi: 10.1111/imr.12216.

A decade of progress in adipose tissue macrophage biology

Andrea A Hill  1 , W Reid Bolus, Alyssa H Hasty
Affiliations
Review

A decade of progress in adipose tissue macrophage biology

Andrea A Hill et al. Immunol Rev. 2014 Nov.

Abstract

One decade has passed since seminal publications described macrophage infiltration into adipose tissue (AT) as a key contributor to inflammation and obesity-related insulin resistance. Currently, a PubMed search for 'adipose tissue inflammation' reveals over 3500 entries since these original reports. We now know that resident macrophages in lean AT are alternatively activated, M2-like, and play a role in AT homeostasis. In contrast, the macrophages in obese AT are dramatically increased in number and are predominantly classically activated, M1-like, and promote inflammation and insulin resistance. Mediators of AT macrophage (ATM) phenotype include adipokines and fatty acids secreted from adipocytes as well as cytokines secreted from other immune cells in AT. There are several mechanisms that could explain the large increase in ATMs in obesity. These include recruitment-dependent mechanisms such as adipocyte death, chemokine release, and lipolysis of fatty acids. Newer evidence also points to recruitment-independent mechanisms such as impaired apoptosis, increased proliferation, and decreased egress. Although less is known about the homeostatic function of M2-like resident ATMs, recent evidence suggests roles in AT expansion, thermoregulation, antigen presentation, and iron homeostasis. The field of immunometabolism has come a long way in the past decade, and many exciting new discoveries are bound to be made in the coming years that will expand our understanding of how AT stands at the junction of immune and metabolic co-regulation.

Keywords: adipose tissue; inflammation; insulin resistance; macrophages.

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Figures

Fig. 1
Fig. 1. Macrophages in lean and obese adipose tissue
As adipose tissue (AT) transitions from a lean to an obese state, the adipocytes undergo hypertrophy and macrophages numbers increase. The resident macrophages in lean AT are M2-like, interstitially spaced, and likely play key roles in AT homeostasis. The macrophages in obese AT are M1-like, are localized to crown-like structures, and secrete pro-inflammatory cytokines. This leads to local and systemic insulin resistance.
Fig. 2
Fig. 2. Multiple mechanisms exist for macrophage recruitment to adipose tissue
Many publications have focused on recruitment-independent mechanisms that can account for macrophage recruitment to adipose tissue. In addition, there is recent evidence for recruitment-independent mechanism such as apoptosis, proliferation, and egress to also contribute to total adipose tissue macrophage numbers.
Fig. 3
Fig. 3. Roles for resident macrophages in adipose tissue homeostasis
Potential roles for resident M2-like macrophages in lean adipose tissue (AT) include AT development and expansion, iron metabolism, antigen presentation, and catecholamine synthesis.
See this image and copyright information in PMC

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