New Lives Given by Cell Death: Macrophage Differentiation Following Their Encounter with Apoptotic Leukocytes during the Resolution of Inflammation

Obesity and insulin resistance, the cardinal features of metabolic syndrome, are closely associated with a state of low-grade inflammation. In adipose tissue chronic overnutrition leads to macrophage infiltration, resulting in local inflammation that potentiates insulin resistance. For instance, transgenic expression of Mcp1 (also known as chemokine ligand 2, Ccl2) in adipose tissue increases macrophage infiltration, inflammation and insulin resistance. Conversely, disruption of Mcp1 or its receptor Ccr2 impairs migration of macrophages into adipose tissue, thereby lowering adipose tissue inflammation and improving insulin sensitivity. These findings together suggest a correlation between macrophage content in adipose tissue and insulin resistance. However, resident macrophages in tissues display tremendous heterogeneity in their activities and functions, primarily reflecting their local metabolic and immune microenvironment. While Mcp1 directs recruitment of pro-inflammatory classically activated macrophages to sites of tissue damage, resident macrophages, such as those present in the adipose tissue of lean mice, display the alternatively activated phenotype. Despite their higher capacity to repair tissue, the precise role of alternatively activated macrophages in obesity-induced insulin resistance remains unknown. Using mice with macrophage-specific deletion of the peroxisome proliferator activated receptor-gamma (PPARgamma), we show here that PPARgamma is required for maturation of alternatively activated macrophages. Disruption of PPARgamma in myeloid cells impairs alternative macrophage activation, and predisposes these animals to development of diet-induced obesity, insulin resistance, and glucose intolerance. Furthermore, gene expression profiling revealed that downregulation of oxidative phosphorylation gene expression in skeletal muscle and liver leads to decreased insulin sensitivity in these tissues. Together, our findings suggest that resident alternatively activated macrophages have a beneficial role in regulating nutrient homeostasis and suggest that macrophage polarization towards the alternative state might be a useful strategy for treating type 2 diabetes.

Macrophages are found in close proximity with collagen-producing myofibroblasts and indisputably play a key role in fibrosis. They produce profibrotic mediators that directly activate fibroblasts, including transforming growth factor-beta1 and platelet-derived growth factor, and control extracellular matrix turnover by regulating the balance of various matrix metalloproteinases and tissue inhibitors of matrix metalloproteinases. Macrophages also regulate fibrogenesis by secreting chemokines that recruit fibroblasts and other inflammatory cells. With their potential to act in both a pro- and antifibrotic capacity, as well as their ability to regulate the activation of resident and recruited myofibroblasts, macrophages and the factors they express are integrated into all stages of the fibrotic process. These various, and sometimes opposing, functions may be performed by distinct macrophage subpopulations, the identification of which is a growing focus of fibrosis research. Although collagen-secreting myofibroblasts once were thought of as the master "producers" of fibrosis, this review will illustrate how macrophages function as the master "regulators" of fibrosis. Copyright Thieme Medical Publishers.

Interleukin (IL)-13 is a major inducer of fibrosis in many chronic infectious and autoimmune diseases. In studies of the mechanisms underlying such induction, we found that IL-13 induces transforming growth factor (TGF)-beta(1) in macrophages through a two-stage process involving, first, the induction of a receptor formerly considered to function only as a decoy receptor, IL-13Ralpha(2). Such induction requires IL-13 (or IL-4) and tumor necrosis factor (TNF)-alpha. Second, it involves IL-13 signaling through IL-13Ralpha(2) to activate an AP-1 variant containing c-jun and Fra-2, which then activates the TGFB1 promoter. In vivo, we found that prevention of IL-13Ralpha(2) expression reduced production of TGF-beta(1) in oxazolone-induced colitis and that prevention of IL-13Ralpha(2) expression, Il13ra2 gene silencing or blockade of IL-13Ralpha(2) signaling led to marked downregulation of TGF-beta(1) production and collagen deposition in bleomycin-induced lung fibrosis. These data suggest that IL-13Ralpha(2) signaling during prolonged inflammation is an important therapeutic target for the prevention of TGF-beta(1)-mediated fibrosis.