Immunometabolism of Tissue-Resident Macrophages - An Appraisal of the Current Knowledge and Cutting-Edge Methods and Technologies

Abstract

Tissue-resident macrophages exist in unique environments, or niches, that inform their identity and function. There is an emerging body of literature suggesting that the qualities of this environment, such as the types of cells and debris they eat, the intercellular interactions they form, and the length of time spent in residence, collectively what we call habitare, directly inform their metabolic state. In turn, a tissue-resident macrophage's metabolic state can inform their function, including whether they resolve inflammation and protect the host from excessive perturbations of homeostasis. In this review, we summarize recent work that seeks to understand the metabolic requirements for tissue-resident macrophage identity and maintenance, for how they respond to inflammatory challenges, and for how they perform homeostatic functions or resolve inflammatory insults. We end with a discussion of the emerging technologies that are enabling, or will enable, in situ study of tissue-resident macrophage metabolism.

Keywords: development; homeostasis; inflammation; metabolism; tissue-resident macrophages.

Figure 1

Overview of metabolic pathways induced in pro-inflammatory macrophages. Shown is the canonical pro-inflammatory macrophage metabolic program (Canonical Pathways) as well as some of the more predominantly defined intermediates that accumulate in pro-inflammatory macrophages (Intermediates). Also included are some tissue-resident macrophage (TRM)-specific metabolism observations, including that TRMs in some tissues upregulate OXPHOS and glutaminolysis, that lipids (such as oxidized phosphoplipids and high/low-density lipoproteins) can drive a hyper-metabolic state, and in some contexts, glucose is shunted into glycogenesis.

Figure 2

Overview of metabolic pathways induced in pro-resolving macrophages. Shown is the canonical anti-inflammatory/pro-resolving macrophage metabolic program (Canonical Pathways) as well as some of the more predominantly defined intermediates that accumulate in pro-resolving macrophages (Intermediates). Also included are some tissue-resident macrophage (TRM)-specific metabolism observations, including that apoptotic cells and/or cytokine (e.g., IL-4/IL-13) induce upregulation of glycolysis, that succinate accumulates and signals via SUCNR1 to drive inflammation resolution, and that novel pathways such as desmosterol synthesis are induced in specific tissues, such as the CNS.

Figure 3

Schematic of spatial metabolomic analysis using MALDI Mass Spectrometry imaging. (A) 1. Isolated whole or sectioned tissue can be analyzed using a combination of untargeted approaches involving various MALDI matrix substances, such as 9-AA, allowing for profiling of a broad range of metabolite sizes and classes. 2. Quantitative analysis of metabolites of interest can be performed using two interconnected approaches: On-tissue spotting and metabolite concentration curves. On-tissue spotting involves taking a slurry of known metabolites (at known concentrations) and dabbing them onto tissues of interest. This allows for an internal reference standard to control for matrix deposition and laser excitation. Metabolite concentration curves allow for quantification of the concentration of a specific metabolite or finite number of metabolites of interest. 2. Metabolic flux analysis can be performed by adopting conventional in vivo infusion of ¹³C-labeled metabolites, such as uniformly-labeled ¹³C-Glucose into mice. (B) 1. Samples are cut entirely into serial sections. Interleaved sections are used for MALDI or H&E/immunofluorescence staining. 2. Slides used for MALDI are sprayed with matrix of interest using a standard MALDI matrix sprayer. Up to two slides can be sprayed and run at a given time, and slides with applied matrix should be imaged immediately. 3. Slides are then analyzed using a MALDI MSI (such as the SolariX XR MALDI-FT-ICR instrument, pictured). Slides are analyzed at a desired pixel resolution within the limits of the machine (e.g., 20-micron), meaning that the MALDI laser is rastered over the whole tissue surface, exciting 20-micron regions at a time. 4) Mass spectrometry peaks are subsequently analyzed for identity (see method) and mapped back onto H&E or IF stained slides.