Role of innate and adaptive immunity in obesity-associated metabolic disease (original) (raw)

Macrophages are key mediators of inflammation in AT and are the most abundant immune cells that infiltrate obese AT (37, 38). They comprise 40% to 60% of AT immune cells in obese mice compared to 10% to 15% of AT immune cells in lean mice, and secrete the majority of inflammatory cytokines in response to obesity (12, 17). Furthermore, macrophages are mainly found in interstitial spaces between adipocytes in the AT of lean individuals, but are primarily found in CLSs around large dying adipocytes during a state of obesity (39). Macrophages are a heterogeneous population characterized by varied surface markers and cytokine secretory patterns. Based partly on their surface phenotype, they are often classified as M1 or M2 cells, although many macrophages do not fall unequivocally into either classification. M1, or “classically activated,” macrophages are induced by proinflammatory mediators such as LPS and IFN-γ and secrete proinflammatory cytokines including IL-6, IL-1β, inducible NOS (iNOS), and TNF-α. M2, or “alternatively activated,” macrophages play a role in tissue repair, angiogenesis, and resolution of inflammation. These macrophages are induced by the antiinflammatory cytokines IL-4 and IL-13 and secrete high levels of antiinflammatory IL-10, IL-1 decoy receptor, and arginase, thereby blocking IL-1β and iNOS activity. Surface markers of M2 differ from those of M1; the classical differentiating marker is CD11c (negative in M2 and positive in M1), but other markers of inflammation also segregate with M1 (CD163, CD172, CD44) and M2 (arginase 1, CD206, CD301) phenotypes. Lean mice demonstrate a dominant M2 phenotype, whereas obese mice demonstrate a dominant M1 phenotype (40). In human obesity, classical M1 markers may not typify proinflammatory ATMs even in the face of functional and cytokine phenotypes typical of classical inflammation. Several studies have described mixed M1/M2 phenotypes in ATMs from obese mice and humans, suggesting that the ATMs adopt more complex states in vivo (41, 42). Indeed, one comparison of surface markers expressed in obese human ATMs versus bronchial macrophages from cystic fibrosis patients with chronic bacterial infection demonstrated absence of CD11c positivity and other classic M1 surface markers. A plasma membrane proteomics analysis in the latter study suggested that a unique metabolically activated macrophage phenotype was present, characterized by surface markers distinct from classic M1 markers (43). Macrophage phenotype also exhibits plasticity, changing rapidly in response to external stimuli. For example, while M2 macrophages dominate in AT of lean mice, a “phenotypic switch” occurs in DIO mice, characterized by a shift in polarization toward M1 over days to weeks (40, 44). An M2 phenotypic shift can also be induced by PPARγ activation, which protects against M1 activation and IR (45), possibly by promoting lipid storage in adipocytes and preventing lipotoxicity and adipocyte death (46).

The signals that regulate recruitment of macrophages to AT are not fully elucidated, but are almost certainly the product of cross-talk with adipocytes or with other immune cells. In general, these signals attract bone marrow–-derived monocytes to AT, where they become activated macrophages of varying phenotypes. The two leading mechanisms of macrophage recruitment to AT include secretion of chemokines and activation of pattern-recognition receptors (PRRs) on macrophages, including the NLR and TLR families, which recognize danger signals from dying and stressed cells, LPS from gut pathogens that reach the circulation, and lipid signals. MCP-1, a well-characterized macrophage chemoattractant, secreted by adipocytes as well as macrophages, is more abundant in obese versus lean mice and humans, and is elevated in VAT versus SAT (47). It is induced by IL-1β, TNF-α, IL-8, IL-4, and IL-6 and, when under- or overexpressed, alters monocyte recruitment to AT. Another chemokine that may contribute to macrophage recruitment is leukotriene B4 (LTB4), which is secreted by adipocytes and also stimulates chemotaxis of neutrophils; blockade of LTB4 prevents induction of diet-induced IR in mice (48). Fractaline (CX3CL1) and its receptor CX3CR1 participate in recruitment and adhesion of monocytes and T cells in atherosclerosis and have been implicated in macrophage recruitment to AT. CX3CL1 is expressed in adipocytes and upregulated in obese human AT, and contributes to adhesion of monocytes to adipocytes and glucose intolerance (49). In addition to the classical chemokines MCP-1 and LTB4, an axon-guiding molecule, semaphorin 3E (SEMA3E), also exhibits chemoattractant properties for macrophages. Expression of SEMA3E and its receptor, plexinD1, is increased in the AT of DIO mice. Blockade of the interaction between the SEMA3E and plexinD1 results in a decreased number of infiltrated macrophages in AT and improved IR (50). Finally, studies in mice implicate macrophage migration inhibitory factor, a proinflammatory chemokine secreted by numerous cell types, including adipocytes, in ATM accumulation, proinflammatory cytokine secretion, activation of JNK and IKKB pathways, and impaired insulin signaling in AT and liver (51).

Obesity not only induces the production of chemoattractants that direct the recruitment of macrophages into AT, but also results in the release of signals that promote macrophage retention. A recent study suggested that a neuronal guidance molecule, netrin-1, promotes macrophage retention in AT. Netrin-1 is increased in obese mice and humans and was reported to block macrophage emigration from AT via binding to its receptor, UNC5B, on macrophages (52). The overall importance of this process to macrophage accumulation and inflammation in AT remains to be determined.

PRRs, including TLRs, are present on macrophages and other innate immune cells, and recognize a variety of danger signals as well as metabolic alterations. PRRs play a critical role in the innate immune system by activating proinflammatory signaling pathways in response to microbial pathogens (53). For example, TLR4 binds to LPS on gram-negative bacterial cell walls and activates both IKKB and JNK, thereby triggering transcription of proinflammatory genes that encode cytokines, chemokines, and other effectors of the innate immune response (54). TLR4 responds not only to LPS, but also to free fatty acids (FFAs), which trigger inflammation and IR via TLR4 activation; this has been demonstrated in TLR4-knockout models, which exhibited decreases in NF-κB activation, proinflammatory gene expression, and IR in response to lipid infusion (55, 56). The NLR family is another group of PRRs that may participate in macrophage recruitment and activation. In macrophages these receptors recognize non-microbe signals, activating the NLRP3 inflammasome, which stimulates IL-1β and IL-18 production via caspase-1 activation. NLRP3-knockout animals are protected from IR and have more metabolically active adipose cells compared with WT animals (57). NLRP3 activation, with associated IL-1β and IL-18 secretion, has been observed in response to stimulation by lipotoxicity-related increases in intracellular ceramide (58), palmitate (59), islet amyloid polypeptide (60), serum amyloid A (61), cholesterol crystals (62), and oxidized LDL cholesterol (63). Other purported but not yet proven stimuli include AT hypoxia (64) and adipocyte death (65), both of which occur during obesity and have been shown to trigger inflammation through as yet unclear molecular pathways.

To date, few studies have evaluated macrophage activation and/or phenotype in obese humans. In one human study, M1 ATMs were localized to CLS surrounding adipocytes, expressed higher levels of proinflammatory genes including IL1B, IL6, IL8, TNFA, and CCL3, and were enriched for transcripts encoding mitochondrial, proteasomal, and lysosomal proteins, as well as fatty acid metabolism enzymes and T cell chemoattractants (66). These findings not only suggest that M1 macrophages in humans are proinflammatory and cluster around single adipocytes, as demonstrated in mice (13), but also indicate that they may play a role in lipid scavenging, as in mice (46), and antigen presentation to T cells, thus promoting an adaptive immune response. This study also demonstrated a direct association between the M1/M2 ratio and IR. Another study in non-obese humans with and without T2D found no association between M1/M2 ratio and T2D or insulin sensitivity, but did find a significant direct association with adipose cell size (67). A third study in non-obese humans showed that CD14+ macrophages in SAT, quantified by flow cytometry, correlated with central obesity, liver fat, and IR. Additionally, the expression of the macrophage activation marker CD11b correlated with expression of MCP-1 in SAT (68). Interestingly, weight gain of 2.7 kg over 28 days did not induce inflammation in AT despite an 11% worsening of IR. On the other hand, weight loss following bariatric surgery in morbidly obese subjects was associated with decreases in markers of activated macrophages and proinflammatory T cells in SAT (69) and peripheral blood (70–72). Independent of body weight, limited studies have shown a relationship between inflammation in human AT and IR, with greater frequency of mononuclear cells and expression of genes related to macrophage recruitment and activation including CD68, MCP1, IL6, and IL8 (73). Of note, the frequency of CLSs in human obesity is lower than in obese mice, which may reflect the fact that most human studies are cross-sectional (73), whereas obese mice are typically studied after weight perturbation.