PROJECT SUMMARY The objective of this research is to understand the role and mechanism by which chemoattractant and integrin signaling plays in the bone marrow selection of B cells and in shaping the peripheral primary B cell repertoire and its degree of self-reactivity. The fact that 30% of the millions of B cells produced daily express a B cell receptor with high-avidity for self-antigen suggests we may not be healthy and live a full life in the absence of central B cell tolerance. Intriguingly, individuals affected by common autoimmune diseases display defects of central B cell tolerance manifesting with much higher numbers of bone marrow autoreactive B cells entering the peripheral tissue. However, these individuals also have defects in peripheral tolerance. A better understanding of how newly generated high-avidity autoreactive B cells are controlled by or escape tolerance can have profound implications for the development of therapies that may reduce the self-reactive burden of the circulating B cell population in individuals predisposed to autoimmunity. We have recently documented that the chemokine receptor CXCR4 is expressed at higher levels by autoreactive B cells undergoing central tolerance than non-self-reactive B cells licensed to leave the bone marrow. Moreover, in vivo treatment of mice with a CXCR4 antagonist, releases high-avidity autoreactive B cells from the bone marrow into the circulation from where they relocate into the spleen. The mechanisms by which CXCR4 signaling retains self-reactive B cells within the bone marrow is not clear. It is known that CXCR4 signaling can modulate the function of integrins, molecules that regulate cell adhesion to tissues. Chemokine receptors and integrins can also be regulated by PI3K signaling or can themselves promote the activity of PI3K, which is important for the positive selection of immature B cells out of the bone marrow. The goal of this grant is to define the contribution of CXCR4 and integrin receptors to central B cell tolerance and the impact of central B cell tolerance on peripheral tolerance and autoimmune responses. Experiments in Aim 1 will establish whether CXCR4 signaling controls the bone marrow retention of autoreactive B cells in human beings. Aim 2 will resolve the contribution of integrins in bone marrow B cell selection and the crosstalk between CXCR4, integrins and PI3K. Experiments in Aim 3 will test the hypothesis that the CXCR4-mediated central B cell tolerance process is necessary to reduce the chances of systemic autoantibody and autoimmune responses. Overall, these studies are significant because they generate a deeper mechanistic understanding of how B cell tolerance can be breached during the development of autoreactive B cells to raise the autoreactive capacity of the primary circulating B cell repertoire and the general risk for autoimmunity.