Development of a B cell repertoire that reacts with a diverse range of foreign antigens and yet remains self- tolerant is critical for effective immune responses against pathogens and the prevention of autoimmunity. Disruptions in this process can lead to immunodeficiency, autoimmune diseases such as systemic lupus erythematosus (SLE), and B cell malignancy. An understanding of the signals that mediate proper B cell development and function will likely lead to the development of novel therapeutic agents for these disorders. One important component of several B cell signaling pathways, including but not limited to the B cell antigen receptor (BCR) pathway, is PI3 kinase (PI3K). The product of PI3K, PIP3, binds to the PH domains of a number of critical B cell signaling molecules including Akt and Btk, recruiting them to the plasma membrane where they can form signaling complexes, become activated, gain access to substrates, and transduce downstream signals. PI3K signaling is critical for many aspects of B cell development and activation. The controlled fluctuation of PI3K signaling throughout B cell development in the bone marrow promotes a repertoire enriched in functional but non-autoreactive BCRs by regulating the stages at which Rag proteins are, or are not, expressed. In the periphery, PI3K promotes B cell activation and proliferation, controls the choice between plasma cell differentiation and class switching, and regulates B cell subset distribution. Consistent with its critical role throughout the life of a B cell, PI3K signaling is tightly regulated. The vast majority of research on the negative regulation of PI3K signaling in B cells has focused on two inositol phosphatases, PTEN and SHIP, that act downstream of PI3K to dephosphorylate its product. Much less attention has been paid to upstream regulators of PI3K that modulate its catalytic activity. PIK3IP1, a transmembrane protein that interacts with the p110 catalytic subunit of PI3K, has recently been shown to downregulate the activity of PI3K in several cell types. While its role in B cells is unknown, its expression pattern during B cell development strongly suggests that PIK3IP1 controls at least some PI3K mediated functions in the B lineage, a hypothesis which we will test as follows. In Aim 1, we will overexpress and knockdown PI3KIP1 in B cell lines, primary splenic B cells, and primary pre-B cells in IL-7 cultures and measure PI3K mediated events such as BCR signaling, the choice between plasma cell differentiation and class switching, and Rag expression. In Aim 2, we will generate mice with B cell specific deletion of PI3KIP1 and perform similar assays as in Aim 1. We will also measure effects of PI3KIP1 deficiency on B cell development and subset distribution, humoral immune responses, the development of autoantibodies and receptor editing. These results of these studies will provide important information on a novel regulator of PI3K activity and may shed light on new therapeutic approaches for immunodeficiency, autoimmunity, or B cell malignancy.