Xenochimeric animals produced by infusing human hematopoietic cells into immunodeficient mice have provided a valuable surrogate small animal model to study human lymphocyte development, maintenance, and function in vivo. Attempts to effect complete human B cell reconstitution, however, in non-obese diabetic (NOD) mice with scid or rag mutations have met with varying success. Functional long-lived B cells fail to develop when immature cord blood cells are used to reconstitute NOD/scids. Likewise, B cells engraft poorly if at all when mature human peripheral blood lymphocytes are transferred. Peripheral B cell maturation and survival are completely dependent on the newly described B cell stimulator (BLyS), also known as BAFF, TALL and THANK. Because peripheral B cell maturation ceased in the chimeras at a BlyS dependent checkpoint, we hypothesized murine BLyS in the NOD recipients is limiting, because of quantity, location or species restrictions for utilization. We present preliminary experiments to show robust B cell engraftment in NOD-rag-/-Pfp-/- recipients of human PBL supplemented daily with recombinant human BLyS. No human B cells develop in mice without BLyS supplement. From these data, we formulated the hypothesis that the failure of human B cells to develop and survive in xenochimeras is due to BlyS deficiency. We propose the following experimental aims to develop a stable transfer environment for human B cells, and to examine in vivo the function of BLyS on human peripheral B cell homeostasis. Aim 1. To develop a NOD-rag-/-Pfp-/- transgenic mouse that can produce recombinant human BlyS in a regulated fashion. Aim 2. To determine if human BLyS from a transgene will support the peripheral maturation of long lived naive B cells from human cord blood cells; and to determine if human BLyS will support the survival of mature peripheral B cells. Aim 3. To determine if the B cells in the NOD-rag-/-Pfp-/- BLyS transgenics can be induced to mount recall or de novo antibody responses to challenge with vaccine. The new transgenic mice we propose to produce will provide both new insights and a new small animal model system for determining human B cell development and function in vivo. Furthermore, These transgenic mice will form the basis of a new test system to study the efficacy of adjuvants and vaccines currently being developed against agents of bioterrorism.