The development and establishment of the B cell repertoire is the net result of both genetic environmental forces. The primary event at the genetic level is Ig gene rearrangement resulting in numerous possible combinations of genes which can be further modified by somatic events such as N segment addition and somatic mutations. Environmental forces in the form of self and exogenous Ags also shape the repertoire by positively or negatively selecting B cells according to the specificity of their Ig receptors. These are dynamic processes beginning with the earliest expression of immunoglobulins in fetal life and continuing throughout life. These studies will analyze the early repertoires in mice with the goal of determining genetic and selective mechanisms responsible for differences in the early immune system compared to that of the adult. Mechanisms responsible for the self-reactivity, polyreactivity and connectivity which characterize the fetal and neonatal repertoire will be determined in B cells. Immunoglobulin transgenic mouse models will be used to define the antigens involved in the selection process, to determine the phenotypes of B cells at different stages of differentiation and selection, and seek out the fetal and anatomical sites where positive and negative selection of B cells occurs. These studies will include a global analysis of the early repertoire with respect to the idiotype/anti- idiotype interactions that occur early in development. The impact of Tdt expression on the diversity of immunoglobulin CDR3 regions and the subsequent effects on fetal perinatal and adult B cells will be determined. This question will be addressed by the use of transgenic mice in which N region additions will be introduced during stages of B cell development when such additions are normally absent or minimal. The molecular and cellular differences between B cell subsets will be compared by study of precursor/progeny relationships using newly developed monoclonal antibodies as cellular markers and the use of transgenic mice in which B cells constitutively express CD5. The overall goal of these studies is to understand the differentiative events during early development that mold the B cell repertoire. It will be determined whether a restricted perinatal B cell repertoire is more important to the development of a healthy immune system than a highly diverse adult B cell repertoire. The experiments outlined in this proposal will aid in our understanding of the role that these fetal and neonatal B cells play in establishment and maintenance of the normal immune system and will provide insight into their roles in autoimmune diseases, B cell neoplasia and immunodeficiency diseases.