Discovering Macrophage Functions Using In Vivo Optical Imaging Techniques

Abstract

Macrophages are an important component of host defense and inflammation and play a pivotal role in immune regulation, tissue remodeling, and metabolic regulation. Since macrophages are ubiquitous in human bodies and have versatile physiological functions, they are involved in virtually every disease, including cancer, diabetes, multiple sclerosis, and atherosclerosis. Molecular biological and histological methods have provided critical information on macrophage biology. However, many in vivo dynamic behaviors of macrophages are poorly understood and yet to be discovered. A better understanding of macrophage functions and dynamics in pathogenesis will open new opportunities for better diagnosis, prognostic assessment, and therapeutic intervention. In this article, we will review the advances in macrophage tracking and analysis with in vivo optical imaging in the context of different diseases. Moreover, this review will cover the challenges and solutions for optical imaging techniques during macrophage intravital imaging.

Keywords: bioluminescence imaging; cell tracking; function; intravital microscopy; macrophage; optical imaging.

Figure 1

Optical imaging techniques for macrophage tracking in vivo. (A) Bioluminescence imaging (BLI) and intravital microscopy are the two of most commonly used non-invasive optical techniques for macrophage imaging in animal models. (B) Spectra of fluorescent probes for macrophage labeling: intrinsic components (NADH, FAD, and collagen), genetic probes (fluorescent proteins), and commercial chemical probes (fluorescent dye-dextran conjugates).

Figure 2

Imaging macrophages in the context of cancer. (A) Live imaging on mouse tumor showed that resident CD11c-EYFP⁺ microglia (R) were gradually replaced by EGFP⁺/EYFP⁺ monocyte-derived macrophages (M) and dendritic cells with glioma development (reproduced under the terms of the CC BY 4.0 license, Copyright© 2016 Springer Nature.) (68). (B) Live imaging in mouse cornea showed that one enhanced green fluorescent protein (EGFP)-labeled macrophage (M)-guided mCherry-labeled endothelial branch elongation by promoting endothelial tip cell sprouting [reproduced under the terms of the CC BY 4.0 license, Copyright© 2015 Hsu et al. (73)]. (C) Texas red dextran-labeled macrophages (M) co-migrated with EGFP-expressing tumor cells (T) as part of a migratory stream according to live imaging in vivo [reproduced with permission from Ref. (78), Copyright© 2011 the Company of Biologists Ltd.].

Figure 3

Imaging macrophages in the context of wound healing. (A) Intravital imaging showed that mCherry-labeled macrophages engulfed green fluorescent protein-expressing necrotic/apoptotic neutrophils (N) at the wound site in transgenic Xenopus larva [reproduced under the terms of the CC BY 4.0 license, Copyright© 2015 Paredes et al. (83)]. (B) Time-lapse imaging showed that ECFP⁺ monocytes and PE-labeled neutrophils concomitantly infiltrated the wound bed through microhemorrhages in the skin wound of mice [reproduced under the terms of the CC BY 4.0 license, Copyright© 2014 Rodero et al.] (84).

Figure 4

Imaging macrophages in the context of obesity. (A) Long-term imaging showed that enhanced green fluorescent protein (EGFP)-expressing macrophages (M) migrated toward and accumulated around dying adipocytes (A) stained with BODIPY to form crown-like structure (CLS) in ex vivo cultured adipose tissue explants from a mouse (reproduced under the terms of the CC BY 4.0 license, Copyright© 2015 the American Physiological Society) (92). (B) EGFP-expressing macrophages were involved in degradation of dead adipocytes in adipose tissue explants from a mouse (reproduced under the terms of the CC BY 4.0 license, Copyright© 2015 the American Physiological Society) (92).