Innate immune cell interactions mediated by the SIRP receptor family

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In vertebrates, multiple systems collaborate to ensure recognition and elimination of cells that are aged, defective or infected, while preserving normal “self” tissues. Immune homeostasis requires a balance between activating receptors that promote responses and inhibitory receptors that maintain self-tolerance. The long-term goal of my program is to understand how these activating and inhibitory responses are integrated to maintain accurate self/non-self recognition. This interplay has been studied extensively in lymphocytes that mediate adaptive immunity, but is less well understood in innate immune cells such as macrophages (M?). We have shown that the innate immune receptor Signal Regulatory Protein-α (SIRPα) is an inhibitory receptor that recognizes a protein expressed on virtually all healthy cells. I propose to identify the signalling pathways and down-stream mediators by which SIRPα inhibits M? responses to normal self and is regulated to allow appropriate responses to non-self cells. ******SIRPα's ligand CD47 is a widely expressed cell surface protein “marker of self”. CD47 binds to the SIRPα producing a “don't-eat-me” that restrains M? phagocytosis and secretion of inflammatory mediators. Activating receptors have evolved to ensure recognition and removal of aged, cancerous or pathogen-infected cells. CD47 expression is reduced on aged or damaged cells diminishing SIRPα-mediated inhibitory signals, and allowing phagocytic removal of the target cell. The goal of the proposed program is to use SIRPα as a model system to determine how inhibitory signals are conveyed to control macrophage responses, and how these are integrated with activating receptors.******Activating receptors signal through well-defined phospho-kinase cascades. For example, M? orchestrate phagocytosis in response to activating signals through immunoglobulin Fc receptors. In contrast, inhibitory signalling by SIRPα is phosphatase-driven and not well understood. This knowledge gap is fuelled by a paucity of assays for phosphatase activity. We will use flow cytometry to define SIRPα-mediated signalling with antibodies that detect phospho-epitopes on critical kinases, phosphatases and adaptor proteins. In addition, we will use a genetic loss-of-function approach to identify the protein partners of SIRPα signalling in M? derived from mice with germ line mutations in phosphatases and adaptor proteins. Our long term goal is to define how innate immune cells balance activating and inhibitory signals to clear pathogens and also limit collateral damage to host tissues. The studies provide a paradigm for analysis of innate immune inhibitory signalling applicable to many systems, and will uncover new mechanisms of pathogen subversion of host mechanisms to ensure their persistence.**