The role of visceral fat.

Until a decade ago, fat tissue had been exclusively considered as an endocrine organ. The emerging functional characterization of adipokines as well as adipocytes and preadipocytes suggested for the first time a close link between the endocrine and the immune system. This is emphasized by the changes of the expression pattern of adipokines when the fat tissue is adjacent to inflamed sites. In addition, adipokines are capable of regulating adaptive and acquired immune responses. Remarkably, adipocytes express functional pattern recognition receptors and can consequently respond to bacterial and viral antigens. This seems to be highly relevant for intestinal inflammation and here in particular transmural inflammation where bacteria or bacterial antigens translocalize into the mesenteric fat tissue. Besides phagocytosis of these antigens, adipocytes as well as preadipocytes can be activated resulting in a release of adipokines and chemokines mediating the infiltration of immune cells thus allowing for an immune response. Recent data suggest that the adipokine milieu of the fat tissue closely regulates the polarization of infiltrating immune cells. This is of increasing interest since the pattern of infiltrating cells allows for a characterization of the underlying disease. Thus, in obesity pro-inflammatory M1 macrophages dominate this site. Remarkably, in colorectal carcinoma the presence of M1 and M2 macrophages represents a prognostic marker for the disease course. In conclusion, the visceral fat tissue represents a complex organ with multifaceted function linking the endocrine and the immune system.