The long-term goal of our research is to determine the mechanism responsible for beneficial alteration of the physiology and cellular function of adipose tissue, which improves insulin signaling function, suppresses systemic inflammation, and promotes healthy aging. Several studies with mice, rats, and humans have indicated that obesity causes insulin resistance and has negative effects on longevity. However, our studies with long-living Ames dwarf mice indicate that these growth hormone deficient animals have altered functions of adipose tissue that promote healthy phenotypes regardless of a predisposition to obesity during aging. Following our previous studies, we are determined to discover the mechanism responsible for high insulin sensitivity, low inflammation, and healthy aging. We propose the general hypothesis that the quality (cellular composition), and not quantity, of white adipose tissue (WAT) regulates high insulin sensitivity, protects against systemic inflammation, and promotes healthy aging. In our proposed studies we will (i) study whether WAT residing Treg cells help maintain a healthy immune balance and low inflammation by promoting mesenchymal stem cells (MSCs) self-renewal, (ii) investigate the consequences of Treg cells depletion on metabolism and inflammation, (iii) elucidate the mechanism responsible for self-renewal of adipose derived MSCs in df/df mice, and (iv) determine the regulatory role of Treg cells and MSCs on the endocrine function of adipocytes. We propose the following specific aims: Aim 1. To determine the role of WAT residing Treg cells and MSCs on insulin sensitivity and inflammatory status in df/df and N/df mice during aging: (A) investigate if expansion of Treg cells will promote self-renewal of WAT residing MSCs and healthy insulin sensitivity, and (B) study whether depletion of Treg cells will have a negative effect on insulin sensitivity, inflammation, and self-renewal of WAT residing MSCs populations. Aim 2. To determine the molecular mechanism by which fat-resident and infiltrating Treg cells and MSCs regulate the endocrine activity of adipocytes.