Obesity is associated with an activated macrophage phenotype in multiple tissues that contributes to tissue inflammation and metabolic disease. To evaluate the mechanisms by which obesity potentiates myeloid activation, we evaluated the hypothesis that obesity activates myeloid cell production from bone marrow progenitors to potentiate inflammatory responses in metabolic tissues. High fat diet-induced obesity generated both quantitative increases in myeloid progenitors as well as a potentiation of inflammation in macrophages derived from these progenitors. In vivo, hematopoietic stem cells from obese mice demonstrated the sustained capacity to preferentially generate inflammatory CD11c+ adipose tissue macrophages after serial bone marrow transplantation. We identified that hematopoietic MyD88 was important for the accumulation of CD11c+ adipose tissue macrophage accumulation by regulating the generation of myeloid progenitors from HSCs. These findings demonstrate that obesity and metabolic signals potentiate leukocyte production and that dietary priming of hematopoietic progenitors contributes to adipose tissue inflammation.

Summary An adaptive immune response triggered by obesity is characterized by the activation of adipose tissue CD4+ T cells by unclear mechanisms. We have examined if interactions between adipose tissue macrophages (ATMs) and CD4+ T cells contribute to adipose tissue metainflammation. Intravital microscopy identifies dynamic antigen dependent interactions between ATMs and T cells in visceral fat. Mice deficient in major histocompatibility complex class II (MHCII) showed protection from diet-induced obesity. Deletion of MHCII expression in macrophages led to an adipose tissue specific decrease in the effector/memory CD4+ T cells, attenuation of CD11c+ ATM accumulation, and improvement in glucose intolerance by increasing adipose tissue insulin sensitivity. Ablation experiments demonstrated that the maintenance of proliferating conventional T cells is dependent on signals from CD11c+ ATMs in obese mice. These studies demonstrate the importance of MHC Class II restricted signals from ATMs that regulate adipose tissue T cell maturation and metainflammation.

Dynamic changes of adipose tissue leukocytes, including adipose tissue macrophage (ATM) and adipose tissue dendritic cells (ATDC) contribute to obesity-induced inflammation and metabolic disease. However, clear discrimination between ATDC and ATM in adipose tissue has limited progress in the field of immunometabolism. In this study, we utilize CD64 to distinguish ATM and ATDC and investigated the temporal and functional changes in these myeloid populations during obesity. Flow cytometry and immunostaining demonstrated that the definition of ATM as F4/80+CD11b+ cells overlaps with other leukocytes and that CD45+CD64+ is specific for ATM. The expression of core DC genes were enriched in CD11c+CD64− cells (ATDC), while core macrophage genes were enriched in CD45+CD64+ cells (ATM). CD11c+CD64− ATDC expressed MHCII and co-stimulatory receptors and had similar capacity to stimulate CD4+ T cell proliferation as ATM. ATDC were predominantly CD11b+ conventional DCs and made up the bulk of CD11c+ cells in adipose tissue with moderate high fat diet exposure. Mixed chimeric experiments with Ccr2−/− mice demonstrated that high-fat diet (HFD) induced ATM accumulation from monocytes was dependent on CCR2; while ATDC accumulation was less CCR2-dependent. ATDC accumulation during obesity was attenuated in Ccr7−/− mice and was associated with decreased adipose tissue inflammation and insulin resistance. CD45+CD64+ ATM and CD45+CD64−CD11c+ ATDC were identified in human obese adipose tissue and ATDC were increased in subcutaneous adipose tissue compared to omental. These results support a revised strategy for unambiguous delineation of ATM and ATDC and suggests that ATDC are independent contributors to adipose tissue inflammation during obesity.

Background: Diet-induced obesity leads to a chronic low grade inflammation with production of activated macrophages associated with systemic sexually dimorphic metabolic dysfunction. Results: Males have enhanced myelopoiesis and a proinflammatory response to obesity compared with females. Conclusion: Sex differences in myelopoiesis result in dimorphic responses to obesity-induced inflammation. Significance: Given differences in inflammatory responses, targeted treatment strategies are probably required for males and females.

The innate immune kinase TBK1 initiates inflammatory responses to combat infectious pathogens by driving production of type I interferons. TBK1 also controls metabolic processes and promotes oncogene-induced cell proliferation and survival. Here, we demonstrate that TBK1 activates mTOR complex 1 (mTORC1) directly. In cultured cells, TBK1 associates with and activates mTORC1 through site-specific mTOR phosphorylation (on S2159) in response to certain growth factor receptors (i.e., EGF-receptor but not insulin receptor) and pathogen recognition receptors (PRRs) (i.e., TLR3; TLR4), revealing a stimulus-selective role for TBK1 in mTORC1 regulation. By studying cultured macrophages and those isolated from genome edited mTOR S2159A knock-in mice, we show that mTOR S2159 phosphorylation promotes mTORC1 signaling, IRF3 nuclear translocation, and IFN-β production. These data demonstrate a direct mechanistic link between TBK1 and mTORC1 function as well as physiologic significance of the TBK1-mTORC1 axis in control of innate immune function. These data unveil TBK1 as a direct mTORC1 activator and suggest unanticipated roles for mTORC1 downstream of TBK1 in control of innate immunity, tumorigenesis, and disorders linked to chronic inflammation.

Within adipose tissue, multiple leukocyte interactions contribute to metabolic homeostasis in health as well as to the pathogenesis of insulin resistance with obesity. Adipose tissue macrophages (ATMs) are the predominant leukocyte population in fat and contribute to obesity-induced inflammation. Characterization of ATMs and other leukocytes in the stromal vascular fraction from fat has benefited from the use of flow cytometry and flow-assisted cell sorting techniques. These methods permit the immunophenotyping, quantification, and purification of these unique cell populations from multiple adipose tissue depots in rodents and humans. Proper isolation, quantification, and characterization of ATM phenotypes are critical for understanding their role in adipose tissue function and obesity-induced metabolic diseases. Here, we present the flow cytometry protocols for phenotyping ATMs in lean and obese mice employed by our laboratory.

Obesity causes dramatic proinflammatory changes in the adipose tissue immune environment, but relatively little is known regarding how this inflammation responds to weight loss (WL). To understand the mechanisms by which meta-inflammation resolves during WL, we examined adipose tissue leukocytes in mice after withdrawal of a high-fat diet. After 8 weeks of WL, mice achieved similar weights and glucose tolerance values as age-matched lean controls but showed abnormal insulin tolerance. Despite fat mass normalization, total and CD11c+ adipose tissue macrophage (ATM) content remained elevated in WL mice for up to 6 months and was associated with persistent fibrosis in adipose tissue. ATMs in formerly obese mice demonstrated a proinflammatory profile, including elevated expression of interferon-γ, tumor necrosis factor-α, and interleukin-1β. T-cell–deficient Rag1−/− mice showed a degree of ATM persistence similar to that in WT mice, but with reduced inflammatory gene expression. ATM proliferation was identified as the predominant mechanism by which ATMs are retained in adipose tissue with WL. Our study suggests that WL does not completely resolve obesity-induced ATM activation, which may contribute to the persistent adipose tissue damage and reduced insulin sensitivity observed in formerly obese mice.

The canonical IKKβ/NF-κB1 pathway has been well documented to promote insulin resistance; however, the noncanonical NIK/NF-κB2 pathway is poorly understood in obesity. Additionally, the contribution of counterregulatory hormones, particularly glucagon, to hyperglycemia in obesity remains unclear. Here we show that NIK promotes glucagon responses in obesity. Hepatic NIK was abnormally-activated in mice with dietary or genetic obesity. Systemic deletion of NIK decreased glucagon responses and hepatic glucose production (HGP). Obesity is associated with increased glucagon responses, and liver-specific inhibition of NIK decreased glucagon responses and HGP and protected against hyperglycemia and glucose intolerance. Conversely, hepatocyte-specific overexpression of NIK increased glucagon responses and HGP. In isolated livers and primary hepatocytes, NIK also promoted glucagon action and glucose production, at least in part by increasing CREB stability. Therefore, overactivation of liver NIK in obesity promotes hyperglycemia and glucose intolerance by increasing the hyperglycemic response to glucagon and other factors that activate CREB.