The Wiskott-Aldrich syndrome (WAS) is a severe immune deficiency, caused by mutations of WASP, that belongs to a family of proteins that control de novo actin nucleation. It is unclear whether defects in humoral immunity observed in patients with WAS and in WASP-/- mice reflect a B-cell intrinsic role of WASP for B cell differentiation, and function, and whether N-WASP may play a compensatory role in these processes. We will test the hypothesis that lack of expression of WASP and/or N-WASP affects B lymphocyte maturation, homing and function in a cell-intrinsic fashion. To this purpose, we will study in vivo competition models between WASP+ and WASP- cells in humans and mice. We will also develop conditional knock-out models in which expression of WASP and/or N-WASP is ablated in B lymphocytes. Specifically, we will: 1) analyze the role of WASP in B cell development and maturation, through the analysis of in vivo competition models both in mice and in humans. The proportion of memory and naive B cells will be analyzed among WASP+ and WASP- cells in carriers of XLT. We will also analyze the role of WASP in germinal center reaction and somatic hypermutation following immunization in WASP+/- mice and in WASP+/- mice in which expression of N-WASP is deleted in B cells. 2) test the hypothesis that the B-cell specific lack of WASP and/or N-WASP affects B cell maturation, homing and function in vivo. To this purpose, we will develop a conditional model of WASP deficiency in B cells. We will test the peripheral distribution and homing of B cells, and response to immunization in mice with B-cell specific lack of WASP and/or N-WASP. We will analyze susceptibility of mice with B-cell-specific deficiency of WASP to invasive S. pneumoniae infection, and explore possible defects in the number of IgM memory B cells in patients with WASP gene mutations. 3) test the hypothesis that the B-cell specific lack of WASP and/or N-WASP affects B cell function in vitro. To this purpose, chemotaxis, activation and class-switch recombination will be studied in vitro in B cells from mice with B-cell specific lack of WASP and/or N-WASP. We anticipate that the results of .this project will allow a better understanding of the biology of WAS, and will be important for development of novel forms of treatment of WAS, including gene therapy.