Adipose tissue lymphocytes: types and roles.

Besides adipocytes, specialized in lipid handling and involved in energy balance regulation, white adipose tissue (WAT) is mainly composed of other cell types among which lymphocytes represent a non-negligible proportion. Different types of lymphocytes (B, alphabetaT, gammadeltaT, NK and NKT) have been detected in WAT of rodents or humans, and vary in their relative proportion according to the fat pad anatomical location. The lymphocytes found in intra-abdominal, visceral fat pads seem representative of innate immunity, while those present in subcutaneous fat depots are part of adaptive immunity, at least in mice. Both the number and the activity of the different lymphocyte classes, except B lymphocytes, are modified in obesity. Several of these modifications in the relative proportions of the lymphocyte classes depend on the degree of obesity, or on leptin concentration, or even fat depot anatomical location. Recent studies suggest that alterations of lymphocyte number and composition precede the macrophage increase and the enhanced inflammatory state of WAT found in obesity. Lymphocytes express receptors to adipokines while several proinflammatory chemokines are produced in WAT, rendering intricate crosstalk between fat and immune cells. However, the evidences and controversies available so far are in favour of an involvement of lymphocytes in the control of the number of other cells in WAT, either adipocytes or immune cells and of their secretory and metabolic activities. Therefore, immunotherapy deserves to be considered as a promising approach to treat the endocrino-metabolic disorders associated to excessive fat mass development.