Nutritional status is critically important for normal immune function in individuals. While malnutrition reduces immune function and increases markedly the risks and mortality from severe infections, obesity heightens immune reactivity and predisposes to systemic inflammation that fosters the development of inflammatory disorders including autoimmunity, type 2 diabetes, coronary artery and renovascular diseases. Studies during the last decade implicate a novel mechanism by which malnutrition causes immune dysfunction: a deficit of the adipocyte hormone leptin, resulting from generalized wasting and depletion of stores of adipose tissue. Leptin has now been shown to be a critical regulator of immune cell number and function. Leptin deficiency results in altered immune responses, affecting both innate and adaptive immunity. Leptin is particularly important for T cells, as leptin-deficient individuals have decreased T cell number, decreased CD4+ T helper cells, and abnormal T cell function, resulting in increased susceptibility to intracellular infections. And while the pathogenesis of obesity-related inflammation is incompletely understood, there is evidence that heightened production of leptin and other inflammatory adipocytokines play a central role. There is, therefore, a critical need to clarify the mechanisms by which leptin and other adipocytokines communicate nutritional status to influence immune cell metabolism and function.
As key players in the adaptive immune response, lymphocyte (T and B cell) survival and function must be tightly controlled to maintain proper immunity while preventing inappropriate inflammatory immune responses or immune-deficiency. We have found that T cell glucose uptake and metabolism are highly regulated and can exert strong effects on T cell differentiation and function, and that leptin is required for normal T cell glucose uptake and metabolism in activated cells.
Our long-term goal is to investigate the interactions between adipocytes and immune cells, specifically lymphocytes, in order to better understand the causes of immune dysregulation in both obesity and malnutrition. Our current studies include the following:
1. Identify the signals required to allow peripheral T cells to become sensitive to leptin.
2. Test the hypothesis that leptin has a cell intrinsic role in peripheral T cell metabolism and function in vivo.
3. Explore the molecular mechanisms by which leptin alters T cell metabolism, function and immunity.
4. Understand the role of AMP kinase (AMPK) activation as an important mediator of glucose metabolism following leptin stimulation in T cells.
5. Understand how nutritional changes alter T cell function and immunity and disrupt T cell homeostasis.
6. Understand how leptin levels influence T cell function both during and after acute malnutrition and affect morbidity and mortality from viral infection in malnourished animals.