Regulation of Mast Cell Survival

Sammanfattning: Mast cells are long-lived effector cells of importance for both acute and chronic inflammations. Mast cells can be activated in many different ways, leading to the release of inflammatory mediators. In contrast to most other inflammatory cells, activated mast cells have the capacity to recover, regranulate and thereby be activated again. In this thesis I have investigated the mechanisms involved in regulating activation-induced mast cell survival. We have found that cross-linking of Fc?RI-bound IgE with an antigen (IgER-CL) induces a survival program in mast cells. Upon IgER-CL, mouse and human mast cells upregulate the pro-survival Bcl-2 family gene A1/Bfl-1. A1-/- mast cells degranulate upon Fc?RI activation but they cannot recover most likely due to the lack of A1. Sensitized and provoked A1-/- mice exhibit lower amounts of mast cells compared to littermate controls. In contrast to mast cells, no Bfl-1 expression or survival promotion can be detected in basophils after IgER-CL. Another mast cell secretagogue, an adenosine receptor agonist, neither promoted upregulation of A1 nor survival.Although it is well established that mast cell survival is dependent on stem cell factor (SCF), it has not been described how this process is regulated. We have found that SCF promotes survival through Akt-mediated inhibition of the forkhead transcription factor FOXO3a and its transcriptional target Bim, a BH3-only pro-apoptotic protein. SCF-treatment prevents upregulation of Bim protein expression and leads to an upregulation of Bim phosphorylation through PI3-kinase and MEK-dependent pathways. Overexpression of FOXO3a causes an upregulation of Bim and induces mast cell apoptosis, even in the presence of SCF. Taken together, the work in this thesis demonstrates that A1/Bfl-1 and Bim play key roles in mast cell survival. These findings might be of importance in understanding the mechanisms of mast cell longevity and hence for possible new therapeutics used for mast cell-associated inflammations.