Abstract
This thesis focuses on human FcγRI, a high affinity receptor for antibodies of the IgG isotype. IgG is the most abundant antibody type in blood and all currently FDA approved therapeutic antibodies are of the IgG isotype. FcγRI, a member of the activating Fcγ receptors, exists as a complex of
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a ligand binding α-chain and the ITAM containing FcR γ-chain. Like other leukocyte FcR, most downstream functions are mediated via the ITAM signaling motif. However, several biological functions of FcγRI, including antigen presentation and endocytosis, appear ITAM independent, supporting a role in signaling for the α-chain. Furthermore, high affinity IgG binding by FcγRI is unique within the FcγR family. This property may hamper binding of immune complexes due to IgG saturation under serum conditions, thus making the biological role of this receptor in immunity unclear. In addition, insight in the functioning of FcγRI may be relevant for antibody therapeutic strategies. In this thesis functional implications of proteins interacting with the ‘head’ (Chapter 2) and the ‘tail’ of FcγRI (Chapter 3-5) were studied. Chapter 2 documents the effects of cytokine stimulation on monomeric and multivalent IgG binding to the class I IgG receptor. Allthough it was previously thought that, due to its high affinity, FcγRI is consistently saturated with serum IgG, We observed cytokine stimulation to enhance binding of multivalent IgG. This cytokine enhanced binding allowed association of immune complexes to IgG saturated FcγRI. In chapter 3, we address the functional implications of the association between FcγRI and the actin binding protein, fi lamin A. Filamin A interaction was found to stabilize FcγRI surface expression, thereby preventing default routing of the receptor to lysosomal compartments. The interaction between protein 4.1G and FcγRI is characterized in Chapter 4. Alanine scans and alignments with other 4.1G binding proteins suggested a membrane proximal 4.1G bindingsite on the FcγRI α-chain. In Chapter 5, the functional consequences of 4.1G and FcγRI interaction are addressed. Confocal laser scanning microscopy studies revealed a potential role for 4.1G during ingestion of large particles by macrophages. A summarizing discussion is provided in Chapter 6.
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