Hematopoietic humanized mice (hu-mice) have been developed to study the human immune system in an experimental in vivo model. These mice bear a human immune system that can be manipulated with methodologies similar to those utilized in mice. Hu-mice, therefore, can be used as an effective translational system to investigate whether observations made in the mouse immune system hold true in the human, and to explore the natural variation of outbred human immune responses. Despite recent advances in the establishment of this animal model, however, hu-mice do not yet provide an optimal environment to the human immune system. With regard to B cells, the current hu-mouse model does not support efficient and prolonged generation and/or maintenance of mature B cells limiting the validity of this model in studies that require mature B cell functions. BAFF isan important homeostatic factor for B cells both in mice and in humans. In addition to sustaining the survival of mature B cells, BAFF also promotes Ig class switch and, therefore humoral responses. Recent studies have suggested that the inefficient survival of mature B cell in hu-mice might be in part due to the absence of human BAFF. In support of these observations, we have found that increased production of human BAFF correlates with higher numbers of human mature B cells in hu-mice, but that expression of this human cytokine becomes detectable only in some of these animal chimeras and at late time points. Here we propose to genetically engineer a novel mouse strain that expresses human BAFF in place of mouse BAFF. Our hypothesis is that hu-mice expressing endogenous human BAFF will support faster and increased generation of mature human B cells and promote antibody responses of better quality and magnitude. To achieve these goals, we will develop the following two aims: 1) to generate human BAFF knock- in mice; and 2) to characterize the development and function of mature B cells in human BAFF knock-in hu- mice. These studies aim at increasing the relevance of hu-mice as experimental animal models for the investigation of the human immune system and, more specifically, of B cell and humoral responses. The innovation of this work lies in the generation of the human BAFF knock-in mouse strain and in the characterization of the human B cell population developing in hu-mice that express human BAFF. Our hypothesis is that human BAFF knock-in hu-mice will display a larger human mature B cell population and heightened antigen-specific antibody responses, improving this model for studies of human antibody responses and B cell biology.