78: Insights into the function of an anti-leukaemia antibody: structural studies of CSL362 bound to soluble CD123

The sc family of cytokines (GM-CSF, IL-3 and IL-5) are largely products of activated T cells and regulate the survival, proliferation, differentiation and functional activation of hematopoietic cells following infection or bleeding. They are variously able to target myeloid hematopoietic stem and progenitor cells as well as mature neutrophils, eosinophils, macrophages and mast cells. Recent evidence suggests that this family of cytokines plays a role in other biological systems and importantly also in cancer, through the development and metastasis of solid tumours. Intriguingly, CD123, the IL-3 receptor alpha subunit (IL3R α ) is overexpressed by stem cells and primary blast cells of patients with acute myelogenous leukaemia (AML) and this correlates with poor prognosis leading to the development of multiple strategies that target CD123. We have now solved the structure of soluble IL3R α , in complex with a Fab fragment of CSL362, a humanised monoclonal antibody that binds IL3R α , blocks IL-3 function and is optimised for NK cell-mediated killing of leukaemic cells [1] . CSL362 is currently in a Phase 1 clinical trial for the treatment of patients with AML (Clinical Trials Gov. Identifier: NCT01632852). The three domain IL3R α structure unexpectedly revealed two alternative conformations, an open and a closed form based on the orientation of the N-terminal domain (NTD). The open conformation has not previously been reported for other Type I cytokine receptors with a similar domain structure, IL5R α , IL13R α 1 and IL13R α 2. The IL3R α structure will be presented together with data that supports a mechanism of IL-3 recognition and receptor signalling that may be applicable to other members of the Type I cytokine receptor superfamily. An unexpected dual mechanism of IL-3 antagonism utilised by CSL362 that involves direct antagonism of IL-3 binding as well as blockade of IL-3 receptor assembly, will also be presented.