Extended time in space has been shown to have impacts on the immune system; while studies vary on the extent of the impact it appears that extended time in space results in reduction in immune capacity. With plans to return to the Moon and as well as a manned mission to Mars, a better understanding of the effect of zero gravity on the development and function of the immune system is paramount to the health of astronauts spending extended time in space. While, true, zero gravity cannot be studied in Earth bound experiments, microgravity can be simulated through a rotating wall vessel (RWV) culture system. Phosphatidylinositol (PI) and its phosphorylated derivatives play a critical role in maintaining cellular membranes as well as regulating cell signaling complexes. We have determined that the regulation of PI-4 Kinase is critical for the normal regulation of leukocytes, particularly lymphocytes. To better understand the cellular and molecular impact of microgravity on immune function we will evaluate simulated gravity on the cellular structures and dynamics related to PI composition in leukocyte cell lines. This will be accomplished by he evaluation of organelle structure and function, PI composition, PI-4K and PI-5K activity, and stress responses (particularly ER stress) in leukocyte cell lines cultured in simulated microgravity.