IgG receptors and their role in antibody trafficking

Abstract

Immunoglobulins (Ig) are the results of an adaptive immune response and link humoral and cellular immunity in host defence with IgG being the most abundant of the Ig classes. IgG can bind via its fragment crystallizable (Fc) to a variety of IgG Fc receptor systems, among others Fc gamma receptors (FcγRs), C1q of the complement cascade, and the neonatal Fc receptor (FcRn). While FcγRs and the complement system mediate cellular and soluble immune effector functions upon binding to complexes IgG on e.g. pathogens, FcRn’s main task is to transport monomeric IgG across epithelial barriers like the placenta as well as to mediate the relatively long serum half-life of IgG. Recently, a role of FcRn in regulating adaptive immune responses to IgG immune complexes has been reported. Systemic lowering of IgG by FcRn antagonism is clinically investigated as a strategy to treat IgG-mediated autoimmunity. One of such approaches is to antagonize FcRn with a modified human IgG1 Fc fragment bearing five amino acid substitutions (M252Y, S254T, T256E, H433K, N434F, called MST-HN), which exhibits stronger binding to FcRn than WT IgG. Tailoring IgG binding to Fc receptors has gained increasing importance for the rational design of monoclonal antibodies for therapy. During this PhD study, we investigated interactions of IgG with FcγRs, C1q and FcRn. We discovered how several permutations in the lower hinge region of IgG affect binding of the IgG Fc to FcγRs and C1q and found that a single amino acid deletion of a glycine at position 236 from IgG1 differentiates complement from FcγR binding. We also uncovered how the structurally and functionally distinct fragments antigen binding (Fab) influence binding of the IgG Fc to FcRn. We describe how the Fab regions sterically hinder IgG binding to FcRn and how they affect cellular trafficking of IgG with consequences for in vivo half-life and FcRn blocking efficacy of an IgG with MST-HN substitutions. Additionally, we found that FcγRs and FcRn cooperate in the cellular trafficking of monomeric IgG in myeloid cells, that are indispensable for efficient recycling of IgG. We describe how FcγRs mediate the internalization of IgG in such cells as well as how specific engagement of FcγRs can be exploited to increase IgG delivery to FcRn. We apply our findings in the context of therapeutic targeting of FcRn by an Fc-MST-HN. We hypothesize that FcRn is necessary to rescue IgG after efficient internalization by FcγRs and that expression of FcγRs and FcRn have evolutionarily co-evolved in myeloid cells. Altogether, the findings have broad implications for rational design of therapeutic antibodies.