Interactions of adipose tissue immune cells. In lean adipose tissue, IL-4 secreted by eosinophils and Th2 cells activates M2 type macrophages, which express arginase and anti-inflammatory cytokines such as IL-10. Regulatory T (Treg) cells also play an important role in anti-inflammatory responses via cell-cell contact or cytokine secretion involving IL-10. However, in obese adipose tissue the number of pro-inflammatory immune cells is increased and that of anti-inflammatory immune cells is decreased. Neutrophils, which are early responders to inflammatory responses, infiltrate the adipose tissue where they secrete elastase and also stimulate M1 type macrophage infiltration and pro-inflammatory cytokine secretion. In addition, levels of IFN-γ-secreting cell types, such as Th1 cells, CD8 T cells, and mast cells, are elevated in obese adipose tissue. B cells also play a pro-inflammatory role through secretion of obesity-induced IgG.|@|~(^,^)~|@|Model of adipocytes as antigen presenting cells. Adipocytes could act as antigen presenting cells via expression of key molecules for antigen presentation in obese adipose tissue. Adipocytes expressing MHC I could mediate CD8 T cell responses whereas those expressing MHC II molecules could regulate CD4 T cell responses. In addition, adipocytes could modulate the function and activation of iNKT cells via high expression of CD1d molecules in adipose tissue.

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