Age-related adiposity has been linked to chronic inflammatory diseases in late-life. To date, the studies on adipose tissue leukocytes and aging have not taken into account the heterogeneity of adipose tissue macrophages (ATMs), nor have they examined how age impacts other leukocytes such as T cell in fat. Therefore, we have performed a detailed examination of ATM subtypes in young and old mice using state of the art techniques. Our results demonstrate qualitative changes in ATMs with aging that generate a decrease in resident Type 2 (M2) ATMs. The profile of ATMs in old fat shifts towards a pro-inflammatory environment with increased numbers of CD206-CD11c- (double negative) ATMs. The mechanism of this aging-induced shift in the phenotypic profile of ATMs was found to be related to a decrease in PPARγ expression in ATMs and alterations in chemokine/chemokine receptor expression profiles. Furthermore, we have revealed a profound and unexpected expansion of adipose tissue T (ATT) cells in visceral fat with aging that includes a significant induction of regulatory T cells (Tregs) in fat. Our findings demonstrate a unique inflammatory cell signature in the physiologic context of aging adipose tissue that differs from those induced in setting of diet-induced obesity.

The proinflammatory activation of leukocytes in adipose tissue contributes to metabolic disease. How crosstalk between immune cells initiates and sustains adipose tissue inflammation remains an unresolved question. We have examined the hypothesis that adipose tissue macrophages (ATMs) interact with and regulate the function of T cells. Dietary obesity was shown to activate the proliferation of effector memory CD4+ T cells in adipose tissue. Our studies further demonstrate that ATMs are functional antigen-presenting cells that promote the proliferation of interferon-γ–producing CD4+ T cells in adipose tissue. ATMs from lean and obese visceral fat process and present major histocompatibility complex (MHC) class II–restricted antigens. ATMs were sufficient to promote proliferation and interferon-γ production from antigen-specific CD4+ T cells in vitro and in vivo. Diet-induced obesity increased the expression of MHC II and T-cell costimulatory molecules on ATMs in visceral fat, which correlated with an induction of T-cell proliferation in that depot. Collectively, these data indicate that ATMs provide a functional link between the innate and adaptive immune systems within visceral fat in mice.

Neuropeptide Y (NPY) is induced in peripheral tissues such as adipose tissue with obesity. The mechanism and function of NPY induction in fat are unclear. Given the evidence that NPY can modulate inflammation, we examined the hypothesis that NPY regulates the function of adipose tissue macrophages (ATMs) in response to dietary obesity in mice. NPY was induced by dietary obesity in the stromal vascular cells of visceral fat depots from mice. Surprisingly, the induction of Npy was limited to purified ATMs from obese mice. Significant basal production of NPY was observed in cultured bone marrow derived macrophage and dendritic cells (DCs) and was increased with LPS stimulation. In vitro, addition of NPY to myeloid cells had minimal effects on their activation profiles. NPY receptor inhibition promoted DC maturation and the production of IL-6 and TNFα suggesting an anti-inflammatory function for NPY signaling in DCs. Consistent with this, NPY injection into lean mice decreased the quantity of M1-like CD11c+ ATMs and suppressed Ly6chi monocytes. BM chimeras generated from Npy−/− donors demonstrated that hematopoietic NPY contributes to the obesity-induced induction of Npy in fat. In addition, loss of Npy expression from hematopoietic cells led to an increase in CD11c+ ATMs in visceral fat with high fat diet feeding. Overall, our studies suggest that NPY is produced by a range of myeloid cells and that obesity activates the production of NPY in adipose tissue macrophages with autocrine and paracrine effects.

Adipose tissue macrophages (ATMs) accumulate in fat during obesity and resemble foam cells in atherosclerotic lesions, suggesting that common mechanisms underlie both inflammatory conditions. CX3CR1 and its ligand fractalkine/CX3CL1 contribute to macrophage recruitment and inflammation in atherosclerosis, but their role in obesity-induced adipose tissue inflammation is unknown. Therefore, we tested the hypothesis that CX3CR1 regulates ATM trafficking to epididymal fat and contributes to the development of adipose tissue inflammation during diet-induced obesity. Cx3cl1 and Cx3cr1 expression was induced specifically in epididymal fat from mice fed a high-fat diet (HFD). CX3CR1 was detected on multiple myeloid cells within epididymal fat from obese mice. To test the requirement of CX3CR1 for ATM trafficking and obesity-induced inflammation, Cx3cr1+/GFP and Cx3cr1GFP/GFP mice were fed a HFD. Ly-6cLow monocytes were reduced in lean Cx3cr1GFP/GFP mice; however, HFD-induced monocytosis was comparable between strains. Total ATM content, the ratio of type 1 (CD11c+) to type 2 (CD206+) ATMs, expression of inflammatory markers, and T-cell content were similar in epididymal fat from obese Cx3cr1+/GFP and Cx3cr1GFP/GFP mice. Cx3cr1 deficiency did not prevent the development of obesity-induced insulin resistance or hepatic steatosis. In summary, our data indicate that CX3CR1 is not required for the recruitment or retention of ATMs in epididymal adipose tissue of mice with HFD-induced obesity even though CX3CR1 promotes foam cell formation. This highlights an important point of divergence between the mechanisms regulating monocyte trafficking to fat with obesity and those that contribute to foam cell formation in atherogenesis.