Here, we demonstrated that mast cells are distributed differentially in abdominal fat depots and lymph nodes in leptin-deficient obese mice. With respect to abdominal fat depots, leptin deficiency-induced obesity was accompanied by a substantial increase (20-fold) in the density of mast cells in epididymal fat, while a remarkable decrease (11-fold) in the density of mast cells in inguinal (subcutaneous) fat was observed. This divergent alteration in the density of mast cells was confirmed by CD117/c-kit protein expression analysis. Furthermore, the proportion of mast cells immunoreactive for TNF-α was significantly greater in epididymal than in inguinal fat. Leptin deficiency-induced obesity was associated with increased mast cells in abdominal lymph nodes. We found no significant difference in the density of mast cells in skeletal muscle, liver, spleen, and thymus between leptin deficient and control mice.
The structural and functional differences between fat depots in various anatomical locations have been the subject of much interest [12–14]. Since the discovery of adipose tissue inflammation in obesity and its impact on systemic insulin sensitivity, numerous studies have examined immune responses in fat depots in obese rodents and humans [11, 17, 18]. Our group recently reported that a high-fat diet (60% calories from fat) fed for 20 weeks led to an increase in mast cells in epididymal (90-fold), perinephric (24-fold), and mesenteric (7-fold) fat depots of 6-month-old C57BL/6 mice . However, we found no significant difference in the density of mast cells in inguinal fat between diet-induced obese and control mice . We believe that the differences observed between these two studies can be explained by at least three factors: 1) the duration of obesity; 2) the experimental diet; and 3) the proinflammatory properties of leptin, as discussed below.
Similar to other inflammatory diseases, adipose tissue inflammation in obesity is a dynamic pathophysiological process . Neutrophils infiltrate visceral fat of mice within a week after high-fat feeding . This is followed by macrophage accumulation and formation of crown-like structures [3, 4]. Over time, adipose tissue inflammation in obesity is associated with collagen deposition and tissue fibrosis . Thus, the type and severity of inflammatory changes in a specific fat depot is in part a function of the duration of obesity. Moreover, since fat depots have different propensity for obesity-associated inflammation, at any given time-point the severity of inflammation is different from one fat depot to another. In addition, the development of obesity and associated adipose tissue inflammation also depends on the type of diet consumed. We believe that a high-fat diet leads to more severe adipocyte injury and associated adipose tissue inflammation than the intake of larger quantities of a standard chow, as it is the case with leptin-deficient mice in the present study.
Pivotal in the regulation of energy homeostasis, metabolism, and neuroendocrine functions, leptin also plays an important role in innate and adaptive immune responses. Leptin can induce TNF-α and IL-6 production by monocytes . Leptin can also augment the ability of macrophages to phagocytize pathogens . For example, macrophages harvested from leptin-deficient mice showed reduced phagocytosis of bacteria . Since leptin is required for lymphopoiesis, leptin receptor-deficient mice have fewer circulating B- and CD4+ T-lymphocytes and are unable to correct irradiation-induced lymphopenia . By stimulating IL-2 and IFN-γ and suppressing IL-4 production, Leptin may favor proinflammatory T-lymphocyte responses as well . Leptin is also found to be important in the development and activation of natural killer cells . Thus, a lower degree of adipose tissue inflammation in ob/ob mice can also be accounted for by diminished innate and adaptive immune responses due to leptin deficiency.
A proinflammatory cytokine with diverse biological effects, TNF-α plays a critical role in the pathogenesis of obesity-linked insulin resistance . Mast cells may contain preformed TNF-α . Moreover, upon proper stimulation, TNF-α protein and gene expression can be upregulated in mast cells . We have previously shown that mast cells in the epididymal fat of diet-induced obese mice store and secrete TNF-α . Here we showed that the majority of mast cells in epididymal and subcutaneous adipose tissue of both leptin-deficient obese and control mice are immunoreactive for TNF-α. However, we found that the proportion of mast cells immunoreactive for TNF-α was significantly higher in epididymal than in subcutaneous adipose tissue. Although other inflammatory and non-inflammatory cells in adipose tissue are capable of expressing TNF-α, a lower proportion of mast cells immunoreactive for TNF-α might be an important mechanism for the resilience of subcutaneous adipose tissue to metabolic challenges and consequent adipose tissue inflammation in obesity.
Lymph nodes are strategically located lymphoid tissues where innate immune responses can result in the induction of adaptive immunity . Macrophages, B- and T-lymphocytes form the bulk of a lymph node. Although lymph nodes are involved in most immune responses, little is known about immunological reactions occurring in regional lymph nodes draining fat depots in obesity. A recent report indicated that increased T-lymphocyte activation and apoptosis was associated with decreased numbers of CD4+ and CD8+ T-lymphocytes in mesenteric lymph nodes of diet-induced obese mice . Moreover, decreased proportions of CD8+ T-lymphocytes and higher proportions of helper T-cell subsets in mesenteric lymph nodes of genetically obese rats compared to lean controls .
Mast cells are scattered in the cortical and medullary sinuses of murine lymph nodes . While much is known about immune responses that take place in a lymph node, the immunological functions of mast cells in a lymph node remain largely elusive. It has been shown that mast cells residing in a lymph node can facilitate recruitment of T-lymphocytes by secreting chemokines such as macrophage inflammatory protein-1B . Furthermore, mast cells in inflamed tissues can cause enlargement and activation of a draining lymph node by releasing TNF-α that is transported to draining lymph nodes . In addition, it has been shown that dermal mast cells can migrate to draining lymph nodes and induce adaptive immune responses [30, 32]. The present study is the first to show an increase in the density of mast cells in regional lymph nodes draining abdominal fat depots in obesity. The increased numbers of mast cells in the abdominal lymph nodes could be due to 1) increased mitotic activity of resident mast cells, 2) enhanced recruitment of precursor cells, 3) immigration from inflamed adipose tissue, and/or 4) decreased emigration. Although the contributions of mast cells to the activation of the immune system are widely recognized, under certain conditions mast cells can suppress immune responses . Thus, further studies are required to delineate the role of mast cells in the lymph nodes draining inflamed fat depots in obesity.
The present study describes the effects of leptin deficiency-induced obesity on the distribution of mast cells in subcutaneous and visceral fat depots and regional lymph nodes. Important differences were noted between subcutaneous and epididymal fat depots. Increased density of mast cells in abdominal lymph nodes in obesity represents an anatomical link between adipose tissue inflammation and adaptive immune responses in lymph nodes. This work will stimulate design and implementation of mechanistic studies addressing immunological functions of mast cells in adipose and secondary lymphoid tissues in obesity.