Foundations of Immunometabolism and Implications for Metabolic Health and Disease

Abstract

Highly ordered interactions between immune and metabolic responses are evolutionarily conserved and paramount for tissue and organismal health. Disruption of these interactions underlies the emergence of many pathologies, particularly chronic non-communicable diseases such as obesity and diabetes. Here, we examine decades of research identifying the complex immunometabolic signaling networks and the cellular and molecular events that occur in the setting of altered nutrient and energy exposures and offer a historical perspective. Furthermore, we describe recent advances such as the discovery that a broad complement of immune cells play a role in immunometabolism and the emerging evidence that nutrients and metabolites modulate inflammatory pathways. Lastly, we discuss how this work may eventually lead to tangible therapeutic advancements to promote health.

Figure 1. The evolution of immunometabolism

Over the course of evolution, the Drosophila fat body, where liver, adipose tissue and the principle immune organ is situated in a single structure, has given rise to the distinct metabolic and immune organs observed in modern mammals. However, despite this seeming delegation of tasks, highly regulated interactions and crosstalk that are required to maintain immune and metabolic homeostasis remain as part of this evolutionary history. In the setting of obesity, this gives rise to activation and infiltration of immune cells into metabolic tissues and chronic activation of inflammatory pathways in both stromal and immune components, triggering stress kinase activation which impinges on the signaling of metabolic hormones such as insulin, and leading to impaired glucose and lipid homeostasis. The fundamental principles of this transition from an "immunometabolic" adaptive to maladaptive state could be depicted in a simple framework wherein the pathogen sensing, immune signaling and metabolic responses are signaled through Toll-like, TNF (tumor necrosis factor), and, insulin receptors. Each one of the highly conserved signaling components that construct this principle framework of metaflammation could be enriched and expanded with many more molecules and signaling networks in higher organisms and humans. Wengen (Drosophila TNF receptor), dlInR (Drosophila insulin receptor), Toll (Toll receptor), TNFR (TNF receptor), INSR (Insulin receptor), TLR (Toll like receptor).

Figure 2. Timeline of immunometabolism

Summary of the major findings that have given rise to the field of immunometabolism. The line graph reflects the total number of publications (261,764) describing links between immunity and metabolism over the last several decades, found with the search terms “metabolism and inflammation” in PubMed. The major findings that laid the ground work for this field are highlighted. At the bottom of the figure processes that marked each decade are illustrated. The arrows point to the critical discoveries that have initiated and stimulated the expansion of the field of immune-metabolism. Publications in this field have proliferated rapidly in the last decades, as the field has recognized the impact of innate and adaptive immunity in metabolism, identified signaling networks and molecular mediators as well as immunomodulatory effects of lipid species and progressed towards new translational avenues with exciting proof of principle studies. These issues are covered in more detail in the manuscript.

Figure 3. Adipose tissue-immune cell interactions

Interactions with immune effectors and stromal components are critical for tissue maintenance and health. Immune cells and their interactions with adipocytes, for example, are critical for adipose tissue homeostasis and response to acute inflammatory signals. The expression of multiple anti-inflammatory cytokines, and the presence of alternatively activated macrophages, immunomodulatory Tregs, and other cell types in the adipose tissue in the lean state has been well documented and may have adaptive roles in tissue health and maintenance. In the setting of obesity, chronic overnutrition and metabolic stresses trigger an inflamed state in the adipose tissue, characterized by enhanced immune infiltration, skewing of macrophage polarization towards the inflammatory phenotype, altered B cell antigen production, and expression of pro-inflammatory cytokines. In this case, the maladaptive chronic, non-resolving metabolic inflammation (metaflammation) contributes to tissue dysfunction, disease, and premature death, as best exemplified in obesity and associated metabolic complications. Although adipose tissue is depicted in this scheme as the first discovered and most studied site, similar interactions and transitions are observed in other key metabolic organs, including liver, pancreas, and brain.