The proper functioning of the pathways that are involved in the sensing and management of nutrients is central to metabolic homeostasis and is therefore among the most fundamental requirements for survival. Metabolic systems are integrated with pathogen-sensing and immune responses, and these pathways are evolutionarily conserved. This close functional and molecular integration of the immune and metabolic systems is emerging as a crucial homeostatic mechanism, the dysfunction of which underlies many chronic metabolic diseases, including type 2 diabetes and atherosclerosis. In this Review we provide an overview of several important networks that sense and manage nutrients and discuss how they integrate with immune and inflammatory pathways to influence the physiological and pathological metabolic states in the body.The integration of metabolism and immunity (or of nutrient-and pathogen-sensing pathways) can be traced back to an evolutionary need for survival, which resulted in the co-development of the organ systems and signalling pathways that mediate these two processes 1 . The pressure to survive would have favoured energy efficiency and storage to prepare for times of food deprivation and for mounting a potent immune response to defend the host against infectious agents. However, the initiation and maintenance of immunity is a metabolically costly endeavour and cannot operate efficiently under conditions of energy deficit 2,3 . For example, fever is associated with a 7-13% increase in caloric energy consumption per 1°C increase in body temperature, the energy expenditure of which is estimated to equate to 9.4×10 6 joules; this is approximately the energy cost of a 70 kg person walking 45 km 4,5 . Sepsis can increase the human metabolic rate by 30-60% 6 . Furthermore, the production and maintenance of phagocytes during infection is thought to result in an energy consumption of approximately 7.9×10 5 joules 4 .It is also clear that starvation and malnutrition can impair immune function; a total reduction in body fat has been shown to result in a decrease in the energy that is available for immune responses in rodents 7 . In addition, conditions that trigger an immune response during starvation can severely reduce the survival of insects 8 . Therefore, immune defence is subject to a tradeoff between other energy-demanding processes, such as reproduction, thermoregulation and lactation. Interestingly, energy surplus (which is typical of individuals who are obese or suffer from metabolic syndrome) can also impair immune responses and induce chronic inflammation (see later). Therefore, a balanced energy flux and maintainance of favourable metabolic homeostasis are required for the proper functioning of the immune system. These processes may have been optimized through the close coordination and co-evolution of metabolic and immune responses, and of the organs that are involved in these processes.Correspondence to G.S.H. ghotamis@hsph.harvard.edu. Competing interests statementThe author(s) declare(s) competi...

Macrophages exhibit endoplasmic reticulum (ER) stress when exposed to lipotoxic signals associated with atherosclerosis, although the pathophysiological significance and the underlying mechanisms remain unknown. Here, we demonstrate that mitigation of ER stress with a chemical chaperone results in marked protection against lipotoxic death in macrophages and prevents macrophage fatty acid binding protein-4 (aP2) expression. Utilizing genetic and chemical models, we show that aP2 is the predominant regulator of lipid-induced macrophage ER stress. Lipid chaperone effects are mediated by the production of phospholipids rich in monounsaturated fatty acids and bioactive lipids that render macrophages resistant to lipid-induced ER stress. Furthermore, aP2’s impact on macrophage lipid metabolism and ER stress response is mediated by upregulation of key lipogenic enzymes by the liver X receptor. Our results demonstrate the central role for lipid chaperones in regulating ER homeostasis in macrophages in atherosclerosis and that ER responses can be modified, genetically or chemically, to protect the organism against the deleterious effects of hyperlipidemia.

After activation, cells of the myeloid lineage undergo robust metabolic transitions, as well as discrete epigenetic changes, that can dictate both ongoing and future inflammatory responses. In atherosclerosis, in which macrophages play central roles in the initiation, growth, and ultimately rupture of arterial plaques, altered metabolism is a key feature that dictates macrophage function and subsequent disease progression. This Review explores how factors central to the plaque microenvironment (for example, altered cholesterol metabolism, oxidative stress, hypoxia, apoptotic and necrotic cells, and hyperglycemia) shape the metabolic rewiring of macrophages in atherosclerosis as well as how these metabolic shifts in turn alter macrophage immune-effector and tissue-reparative functions. Finally, this overview offers insight into the challenges and opportunities of harnessing metabolism to modulate aberrant macrophage responses in disease.

Obesity and the associated pathologies including dyslipidemia, insulin resistance, type 2 diabetes, and cardiovascular disease constitute a major threat to global human health. Yet, the genetic factors that differentially predispose individuals to this cluster of pathologies are unclear. The fatty acid-binding protein aP2 is a cytoplasmic lipid chaperon expressed in adipocytes and macrophages. Mice with aP2 deficiency are partially resistant to obesityinduced insulin resistance and type 2 diabetes, have lower circulating triglycerides, and exhibit marked protection against atherosclerosis. Here, we demonstrate a functionally significant genetic variation at the aP2 locus in humans that results in decreased adipose tissue aP2 expression due to alteration of the CAAT box͞enhancer-binding protein binding and reduced transcriptional activity of the aP2 promoter. In population genetic studies with 7,899 participants, individuals that carry this T-87C polymorphism had lower serum triglyceride levels and significantly reduced risk for coronary heart disease and type 2 diabetes compared with subjects homozygous for the WT allele. Taken together, our results indicate that reduction in aP2 activity in humans generate a metabolically favorable phenotype that is similar to aP2 deficiency in experimental models.adipocyte ͉ FABP4 ͉ triglyceride ͉ macrophage ͉ metabolic syndrome