TOLL-LIKE RECEPTOR 9 AND INTFERON-GAMMA RECEPTOR SIGNALING SUPPRESS THE B CELL FATE OF UNCOMMITTED PROGENITORS IN MICE Sheena Baratono Edward M. Behrens Hyperinflammatory syndromes come in a variety of flavors with different immunogens and cytokines leading to variations in organ pathology. This inflammation can be driven by viral, autoimmune, or neoplastic conditions leading to profound and widespread organ pathology including hepatitis, fever, splenomegaly, cytokinemia and pan-cytopenias, including T and B lymphopenias. The developing immune cells in the bone marrow are known to express a variety of cytokine and inflammatory receptors yet the effects of signaling through these receptors on hematopoiesis is incomplete. Since many infectious and autoimmune syndromes result in sustained TLR9 stimulation, understanding the effects of TLR9 driven inflammation on hematopoiesis is important for both questions of pathogenesis as well as possible therapeutic interventions that might help to restore homeostasis. Utilizing in vitro and in vivo techniques, we demonstrate that B lymphopoiesis is inhibited during TLR9 driven inflammation from the Ly-6D+ CLP stage onwards with different effects inhibiting development at multiple stages across B cell development. We show that TLR9 signaling directly decreases in vitro B cell yields from CLPs, Pro and Pre B cells yet increases T cell yields. While TLR9 can inhibit B cell growth in vitro, using mixed bone marrow chimeras, we show in vivo that this TLR9 intrinsic effect is masked, likely due to other TLR9 induced inflammatory factors that also mediate decreases in B lymphopoiesis. This led us to demonstrate that IFN-γ also directly inhibits B cell differentiation in vitro as well as when induced by TLR9 in vivo. Microarray and RT-PCR analysis of Ly-6D- CLPs points to HOXa9 as an early B-cell directing transcription factor that is altered by TLR9 induced inflammation, as it and many of its known targets are decreased in Ly-6D- CLPs. Additionally EBF-1, another factor essential for initiation of the B cell program was transcriptionally decreased in Ly-6D- and Ly-6D+ CLPs. Our work demonstrates both cellular and molecular targets that lead to altered B-lymphopoiesis in sustained TLR9 driven inflammation that may be relevant in a number of infectious and autoimmune/inflammatory settings.