A study of the mechanism by which beta2-adrenergic receptor stimulation on a B cell regulates IgE production.

摘要

The goal of this dissertation was to determine if the beta2AR on the B cell is a direct target for norepinephrine (NE) in vivo and to determine the mechanism by which beta 2AR stimulation regulates the level of IgE. Our laboratory and others reported previously that NE release in vivo and beta 2AR stimulation in vitro increase the amount of IgE produced by a B cell. Also, our laboratory reported that beta2AR stimulation increases the amount of either IgG1 or IgE produced per B cell, without affecting class switch recombination (CSR). The mechanism responsible for the increase in IgG1 involves a beta2AR-dependent activation of the cAMP/PKA/CREB pathway that increases OCA-B expression and binding with Oct-2 on the 3'-IgH enhancer to increase the rate of IgG 1 production. Early data in this dissertation using B cells isolated from CREB-dominant negative mice revealed that CREB was necessary for the beta 2AR-induced increase in IgG1 but not the beta2AR-induced increase in IgE. These data were the first to suggest that the beta 2AR may activate different signaling pathways to regulate the levels of IgG1 or IgE produced. Thus, the hypothesis tested in this dissertation is that NE stimulates the beta2AR on a B cell to activate a unique signaling pathway that increases the level of IgE, independently of the pathway activated to increase the level of IgG1. The present data using an animal model of allergic asthma are the first to show that NE stimulates the beta2AR on the B cell directly. Molecular techniques were used to show that the beta2AR activates a unique signaling pathway to increase the amount of IgE produced per cell, without affecting CSR or the level of IgG1. Using gene-deficient mice, pharmacological agents, and shRNA, we showed that the mechanism involves a beta 2AR-dependent activation of the cAMP/PKA pathway that leads to phosphorylation and inactivation of HePTP, which is bound to inactive p38 MAPK. Phosphorylation of HePTP releases p38 MAPK into the cytoplasm of the cell where it is phosphorylated by upstream MAPK kinase molecules that were activated by CD40 receptor stimulation. The increase in phosphorylated p38 MAPK augments the production of soluble CD23 (sCD23), which interacts with unidentified receptors on the surface of the B cell resulting in an increase the level of IgE, possibly via stimulation of the CD21/CD19 complex. Thus, our findings suggest that beta2AR stimulation on a B cell modulates the level of IgE in vivo and in vitro, independently of CSR and the level of IgG 1. Furthermore, the production of IgE and IgG1 is dependent on IL-4 production by the T helper 2 (Th2) cell, which does not express the beta 2AR. Data presented in this dissertation provide evidence that repression of the beta2AR in a Th2 cell is mediated by histone and DNA methylation. The overall significance of this dissertation is that it is the first study to identify the beta2AR on the B cell as a target for NE-induced modulation of an immune response in vivo and the first to identify the mechanism by which beta2AR stimulation increases the level of IgE. Our findings also identify mechanism by which beta 2AR is repressed in a Th2 cell. The knowledge gained from this work contributes to the understanding of how the level of IgE is regulated in vivo, as well as how current allergic asthma therapies may affect the production of IgE, which might contribute the severity of disease. In addition, these findings identify novel molecular targets for therapeutic interventions to selectively regulate the beta2AR-induced effects on IgE.