Summary The goal of our research is to understand how defects in calcium influx in cells of the immune system cause immunodeficiency in patients with inherited mutations in genes regulating calcium influx. Since the same patients also suffer from anemia caused by autoantibodies against red blood cells, a second goal of our research is to understand how calcium influx maintains immunological self-tolerance and prevents autoimmunity. Our central hypothesis is that CRAC channels, which mediate calcium influx across the cell membrane, are required for the function of different cells in the innate and adaptive arms of the immune system that provide immunity to infection and prevent autoimmunity. CRAC channels are the main source of calcium influx in most immune cells. They are formed by the channel protein ORAI1 and are activated by STIM1 and STIM2. We have identified the first patients with mutations in ORAI1 and STIM1 genes who suffer from a disease we called CRAC channelopathy that is characterized by immunodeficiency, autoimmunity, and several non-immunological defects. Because CRAC channelopathy is a rare disease and patient samples are limited, we generated mice that lack CRAC channels in T cells. By investigating immune responses in these mice and by validating key results in samples of patients with CRAC channelopathy, we have been able to define a critical role of CRAC channels in T cell-mediated immune responses, in particular for cellular and antibody-mediated immunity to infection and for limiting immune responses during chronic infection that would otherwise cause harmful inflammation. However, we are still far from having a complete picture of how CRAC channels regulate immunity to infection. Beyond T cells, CRAC channels may also regulate innate immune responses mediated by dendritic cells and neutrophils. Studies so far have yielded conflicting data whether CRAC channels are required for innate immune cell function, and their role for innate immunity to infection has not been studied. Besides immunity to infection, CRAC channels are essential for immunological self-tolerance by controlling the development of regulatory T cells, which suppress the function of other immune cells and thereby prevent autoimmunity. CRAC channel-deficient patients have fewer Treg cells, potentially explaining their autoimmunity. How CRAC channels control the function of Treg cells and prevent autoimmunity is not understood. To address these questions, we propose the following three specific aims: (1) We will analyze inherited defects in ORAI1 and STIM1 genes of patients to understand the role of CRAC channels for immune function and the molecular regulation of CRAC channels. (2) We will determine if CRAC channels are required for innate immune responses by dendritic cells and neutrophils to fight bacterial and fungal infections. (3) We will determine how CRAC channels control regulatory T cell function and prevent autoimmunity, in particular autoimmune hemolytic anemia, by studying CRAC channel-deficient mice and patients. Our studies will provide fundamental insights how CRAC channels regulate immunity to infection and prevent autoimmunity.