PROJECT SUMMARY/ABSTRACT: A critical need exists to identify factors which place sensitized individuals at risk for anaphylaxis in order to devise targeted therapies to limit reaction severity. In the absence of the proposed work, allergic individuals – frequently children – will be at continuously increasing risk for anaphylaxis with potentially fatal consequences. Recently the Principle Investigator (PI) identified increased TPSAB1 copy number as the common cause of elevated basal serum tryptase (BST), a well-established risk factor for severe anaphylaxis. The long-term goal of this project is to develop therapies targeting the symptoms associated with increased TPSAB1 expression, including anaphylaxis, in humans. The principle objective of this proposal is to determine how increased TPSAB1 expression in myeloid cells alters their reciprocal relationship with neighboring cells to promote severe allergic reactions. Based upon clinical observations and published reports, the central hypothesis for this proposal is that elevated BST due to increased TPSAB1-derived tryptase expression promotes myeloproliferation and anaphylaxis in humans. Our rationale is that through identification of the mechanism(s) leading to TPSAB1 over-expression and by defining the activities of these tryptases in vivo, new therapeutic strategies will be developed to prevent anaphylaxis and target myeloid dyscrasias. To test our central hypothesis, we propose three specific aims to: (1) elucidate mechanisms by which specific tryptase expression patterns promote severe allergic reactions; (2) identify pathways governing TPSAB1 gene expression and associated myeloid proliferation; (3) define additional pathways essential to severe allergic reactions by identifying novel genetic mutations leading to elevated BST and anaphylaxis. This project is significant because it will establish mechanisms by which elevated BST and associated myeloproliferation can promote severe allergic reactions and anaphylaxis, and identify targets for therapeutic intervention. This project is based upon our identification of the role that TPSAB1 copy number variation and gene expression play in BST levels. This major conceptual advance was enabled by state-of-the-art genotyping and gene expression assays developed by the PI. In this project, these tools will be applied in an innovate manner: 1) to devise a non-invasive strategy to identify patients in whom clonal myeloid disease is highly probable; and 2) to dissect the relationship(s) between tryptase isoform expression, elevated BST, anaphylaxis, and clonal myeloid disease. Furthermore, the central hypothesis will be tested in an innovative system, employing a humanized / bone marrow xenotransplantation mouse model that optimizes myeloid engraftment, not previously used to study anaphylaxis. The proposed studies will provide important insights into the pathways governing severe allergic reactions in humans, and bridge the disparate fields of allergy a...