Sphingosine kinase-1 regulates neutrophil recruitment during allergic inflammation

Abstract

Current health costs for allergy are more than US$300 billion annually worldwide. Although anti-histamines and steroids are the mainstay treatment for allergic inflammation, their effectiveness is varied. Rapid recruitment of neutrophils to sites of inflammation is associated with allergic diseases, such as dermatitis and anaphylaxis, yet they have been largely ignored as a treatment target. Thus, a better understanding of neutrophils in allergic inflammation is required to develop new therapeutic options. Leukocyte infiltration to a site of inflammation is an important process for the development of allergic diseases and the roles of eosinophils and mast cells have been well described in allergic inflammation. By contrast, neutrophil infiltration has not been fully examined even though they are one of the first responders to be recruited to the inflammatory site(s) and become a dominant producer of histamine over time to enhance the recruitment of other inflammatory cells. Herein, this thesis focuses on the better understanding of neutrophil infiltration during allergic inflammation. The classical paradigm of leukocyte recruitment suggests that this localization of cells, including neutrophils, from the circulation to a site of inflammation occurs via specific interactions of adhesion molecules. During allergic inflammation, the adhesion molecule P-selectin, which is pre-formed and pre-stored in Weibel Palade bodies of vascular endothelial cells (ECs), is rapidly exocytosed to the EC surface to recruit neutrophils. The mechanisms underpinning rapid P-selectin exocytosis by ECs are not fully understood. This study investigated the hypothesis that sphingosine kinase (SK) is a regulator for P-selectin-induced neutrophil recruitment during allergic inflammation. SK is a highly conserved lipid kinase, which is ubiquitously expressed at varying levels in different cell types, and catalyses the phosphorylation of sphingosine to form the bioactive molecule sphingosine-1-phosphate (S1P). Fingolimod (Gilenya) is an FDA approved oral drug currently used to treat multiple sclerosis via its effects on SK and S1P axis. Herein, I reveal that Fingolimod can inhibit histamine-induced neutrophil recruitment in vitro and in vivo. In addition, I show that topical application of Fingolimod can attenuate the production of inflammatory chemoattractants and ear swelling in two mouse models of allergic inflammation. Finally, I demonstrate that Fingolimod blocks calcium influx in histamine-treated ECs. Taken together, I have begun to unravel previously unknown processes underpinning neutrophil recruitment during allergic inflammation. Overall, this study provided evidence that SK can be a therapeutic target to prevent and treat allergic inflammation via regulation of excessive neutrophil recruitment. Our findings also reveal a new indication for Fingolimod wherein it can be applied topically to treat allergic-associated diseases. By doing so we may well provide a new treatment option for acute allergic inflammation and possibly fatal anaphylaxis.