Role of Regulatory T cells in B cell Development and Tolerance

According to the National Institute of Health, autoimmune diseases are estimated to affect approximately 5-8% of the US population and the prevalence is rising. Autoimmune diseases consist of roughly 80 distinct clinical conditions many of which present with autoantibody production. Autoantibody mediated autoimmune diseases result from B cells that display specificity towards self-antigens and inappropriately secrete autoantibodies against these self-antigens. The generation of self-specific B cells accounts for ~50-75% of all newly generated B cells. In healthy individuals autoimmunity and autoantibody production is prevented by immune tolerance mechanisms.;One tolerance mechanism, known as B cell anergy, is a mechanism by which selfspecific B cells are held in a functionally impaired, quiescent state unable to secrete antibodies. The process of B cell anergy is reversible and anergic B cells therefore represent a pathogenic cell population that if dysregulated can contribute to autoimmunity. In fact, dysregulation of anergic B cell populations has been reported in certain autoimmune disorders. Although the process of B cell anergy is a crucial tolerance mechanism in the prevention of autoimmunity the regulatory mechanisms that govern B cell anergy are poorly understood.;Regulatory T cells (Tregs) are a subset of CD4+ T cells which express the master gene transcription factor Foxp3 and participate in the maintenance of immune tolerance and the prevention of autoimmunity. A crucial role for Tregs as a regulator of immune tolerance is evident in patients with immune dysregulation, polyendocrinopathy, X-linked disorder (IPEX). Many IPEX patients possess a mutation in the foxp3 gene resulting in a premature stop codon and consequently lack functional Tregs. IPEX patients present with a multitude of immune disorders including extreme autoantibody production and autoimmunity. The high serum titers of autoantibodies in these patients suggest that Tregs play a central role in the maintenance of B cell tolerance. However, the mechanisms by which Tregs maintain B cell tolerance remains to be resolved.;Our studies to elucidate the mechanism by which Tregs modulates B cell tolerance were conducted using a mouse model (Foxp3- mouse), which possesses the same point mutation found in human IPEX patients. Using this model we discovered and characterized defects in B cell development and tolerance in both the bone marrow and periphery of Treg deficient mice. In subsequent experiments we demonstrated that the loss of B cell tolerance in the absence of Tregs is due to a loss of B cell anergy.;To delineate the mechanism by which B cell anergy is altered in the absence of Tregs we employed an inducible model of Treg deficiency to carefully analyze the kinetic breakdown of B cell tolerance in adult mice. Using this model, we determined that the onset of autoantibody production coincides with a significant expansion of helper T cells and that helper T cells were necessary and sufficient to drive the loss of B cell anergy, in the absence of Tregs. Further analysis revealed that the expanded T cell subsets, in the absence of Tregs, consisted of three distinct populations including a CXCR5+GL-7 - T follicular helper cell (TFH) population, a CXCR5 +GL-7+ germinal center T follicular helper cell (GC TFH) population, and a novel previously undescribed CXCR5 -GL-7+ T cell population. In subsequent experiments it was determined that the GC TFH and CXCR5-GL-7 + populations from Treg deficient mice were sufficient to initiate the breakdown of B cell anergy.