Abstract
Age-associated thymic involution is characterized by decreased thymopoiesis, adipocyte deposition and changes in the expression of various thymic microenvironmental factors. In this work, we characterized the distribution of fat-storing cells within the aging thymus. We found an increase of unilocular adipocytes, ERTR7(+) and CCR5(+ )fat-storing multilocular cells in the thymic septa and parenchymal regions, thus suggesting that mesenchymal cells could be immigrating and differentiating in the aging thymus. We verified that the expression of CCR5 and its ligands, CCL3, CCL4 and CCL5, were increased in the thymus with age. Hypothesizing that the increased expression of chemokines and the CCR5 receptor may play a role in adipocyte recruitment and/or differentiation within the aging thymus, we examined the potential role for CCR5 signaling on adipocyte physiology using 3T3-L1 pre-adipocyte cell line. Increased expression of the adipocyte differentiation markers, PPARgamma2 and aP2 in 3T3-L1 cells was observed under treatment with CCR5 ligands. Moreover, 3T3-L1 cells demonstrated an ability to migrate in vitro in response to CCR5 ligands. We believe that the increased presence of fat-storing cells expressing CCR5 within the aging thymus strongly suggests that these cells may be an active component of the thymic stromal cell compartment in the physiology of thymic aging. Moreover, we found that adipocyte differentiation appear to be influenced by the proinflammatory chemokines, CCL3, CCL4 and CCL5.