The purpose of this work is to further the understanding on how the activation of inflammatory cells is regulated. As dysregulation of inflammatory cell activation is known to cause disease, the knowledge gained is likely to advance the development of new therapies in many diseases such as autoimmune disorders, allergies, and asthma. Results: The objectives of the past year were met in the following manner. Our studies on the role of sphingosine kinases demonstrated that two sphingosine kinases are activated upon IgE Fc receptor engagement, and that this requires the activity of both Lyn and Fyn kinases. The product of sphingosine kinase activity, sphingosine-1-phosphate, is important for normal mast cell degranulation and chemotaxis. This work revealed the interdependence of Src and sphinogosine kinase family members in IgE-dependent mast cell activation and suggests that this is a common mechanism for many immune cells. Additional studies also provide new insights on the importance of two known isoforms of sphingosine kinases (1 and 2) in the activation of mast cells. These studies demonstrate a key role for sphingosine kinase 2 in regulating the activation of mast cells through its ability to cause calcium influx. However, in vivo studies demonstrated that both sphingosine kinase 1 and sphingosine kinase 2 play a role in how mast cells respond to an allergenic challenge. Strikingly, the role of sphingosine kinase 1 was dominant as it regulated the amount of sphingosine-1-phosphate in the blood. Elevated levels of sphingosine-1-phosphate in the blood were found to cause increased mast cell responsiveness, whereas low levels led to a resistance of mast cells to an allergenic challenge. In addition, we have succeeded in developing animal models for the study of the relationship between atopic dermatitis and asthma, as well as models to determine how mast cells traffic in vivo. Conclusions and Significance: In summary, our studies have shown that a complementary family of receptors (those for sphingosine-1-phosphate), whose transactivation occurs following IgE Fc receptor stimulation, are important for full mast cell responses and chemotaxis. The studies demonstrated that generation of sphingosine-1-phosphate is key for normal mast cell function and that this is regulated through activation-dependent coupling of sphingosine kinases to the IgE receptor by Fyn and Lyn. In addition, we identified sphingosine-1-phosphate as an important modulator of mast cell responsiveness in vivo. These findings may have important implications in anaphylactic shock, since it is unclear why two individuals with similar allergies react differently to an allergenic challenge. This possible connection that high blood levels of sphingosine-1-phosphate are permissive for anaphylaxis is now being explored in studies measuring the levels of this lipid in allergic individuals.