PROJECT SUMMARY Obesity-induced inflammation is a well-established mechanistic link between obesity and diabetes. Adipose tissue macrophages (ATMs) lie at the center of the adipose tissue immune network. Many knowledge gaps exist regarding ATM biology and its relationship to human metabolic disease. The mechanisms of ATM accumulation, how their activation state relates to type 2 diabetes mellitus (DM), and how they communicate with preadipocytes and adipocytes to regulate nutrient storage are incompletely understood. The goal of this proposal is to address these knowledge gaps using complementary studies in human adipose tissue samples and mouse models. The scientific premise for the hypotheses in this project is rooted in the observation from our groups that visceral adipose tissue from obese (DM) subjects have higher CD206+ ATMs, fewer preadipocytes, larger adipocytes, and manifest adipocyte metabolic dysfunction compared to obese non-DM subjects. We propose a model whereby the expansion of CD206+ ATMs by in situ proliferation blocks preadipocyte proliferation and differentiation to generate a dysfunctional adipose tissue environment. We will evaluate CSF1 as a putative activator of CD206+ ATMs in humans and evaluate the function of CCL18 as a CD206+ ATM secreted chemokine that mediates ATM-preadipocyte communication. If completed our proposal will significantly advance our understanding of how human metabolic inflammation develops independent of obesity and lead to substantial revisions in the current models of ATM function. To evaluate our model, we propose to complete three specific aims: 1) To define mechanisms of CD206+ hATM proliferation and its relationship to adipocyte hypertrophy and DM status. 2) To identify mechanisms underlying CD206+ ATM-preadipocyte crosstalk and resultant preadipocyte and adipocyte metabolic dysfunction. 3) To evaluate the role of ATM-derived CCL18 in the regulation of adipose tissue inflammation and metabolism. The experimental approach for all aims utilize tissue samples from a large bariatric surgery cohort with diversity in age and sex, and assays of inflammatory and metabolic function. This study will accomplish its goals using a team science approach between surgeons and basic scientists to close the gap between our understanding of metainflammation in human and murine models. If completed our study can impact health by identifying new biomarkers for DM risk independent of obesity and new pathways for therapeutic interventions.