Targeting FcRn to Generate Antibody-Based Therapeutics

Abstract

The MHC class I-related receptor FcRn serves multiple roles ranging from the regulation of levels of IgG isotype antibodies and albumin throughout the body to the delivery of antigen into antigen loading compartments in specialized antigen-presenting cells. In parallel with studies directed towards understanding FcRn at the molecular and cellular levels, there has been an enormous expansion in the development of engineering strategies involving FcRn to modulate the dynamic behavior of antibodies, antigens, and albumin. In this review article, we focus on a discussion of FcRn-targeted approaches that have resulted in the production of novel antibody-based platforms with considerable potential for use in the clinic.

Keywords: FcRn; FcRn-targeted therapeutics; antibody engineering; pharmacokinetics.

Figure 1.

The molecular nature of the IgG-FcRn interaction. A. Schematic representation of the IgG-FcRn interaction. The constant regions and variable domains (Fv) of the antibody IgG are colored blue and red, respectively. FcRn is a heterodimer of an α-chain (green) and β2-microglobulin (β2m, black) and is shown in its transmembrane form. B. The location of the residues (Ile253, His310, His435 and Tyr436) at the CH2-CH3 domain interface of the Fc region of human IgG1. These residues play a central role in binding to FcRn. The figure was drawn using the x-ray crystallographic structure of human IgG1-derived Fc fragment [100] and Pymol (PyMOL Molecular Graphics System, Version 2.0 Schrödinger, LLC).

Figure 2.

Model for the activity of FcRn as an IgG homeostat. IgG enters cells by fluid-phase pinocytosis in small tubulovesicular transport carriers that fuse with larger, FcRn-positive early (sorting), acidic endosomes in which binding to FcRn can occur. Bound IgG molecules are recycled and released by exocytosis involving fusion of recycling compartments with the plasma membrane. By contrast, IgG that does not bind to FcRn in sorting endosomes enters lysosomes and is degraded.

Figure 3.

Enhancement of antigen-clearance by acid-switching. A. pH-independent binding. Antibodies that bind to antigen with similar affinity in the pH range 6.0–7.4 enter cells by fluid-phase pinocytosis in small tubulovesicular TCs that fuse with early (sorting) endosomes in which binding to FcRn can occur, followed by recycling of the IgG:antigen complex and exocytic release at the plasma membrane. This results in the buffering effect that prolongs the half-life of the targeted antigen. B. Acid-switched binding. By contrast, pH-dependent or acid-switched antibodies have substantially higher affinity for antigen at near neutral pH relative to acidic, endosomal pH. Following internalization into cells by fluid-phase pinocytosis in small tubulovesicular TCs, the complexes enter acidic sorting endosomes in which antigen dissociates from antibody and enters the lysosomal pathway. The FcRn-bound antibody is recycled and exocytosed. This results in increased clearance of antigen combined with recycling of antibody for re-use.

Figure 4.

Antibody engineering strategies to reduce endogenous IgG levels. A. In the presence of an inhibitor of FcRn such as an Abdeg that binds to FcRn with increased affinity at both near neutral and acidic pH, the inhibitor is internalized by receptor (FcRn)-mediated processes and competes with endogenous IgGs for FcRn binding in acidic endosomes. Consequently, endogenous IgG molecules are driven into lysosomes and are degraded. The inhibitor shown binds through its Fc region to FcRn, whereas other classes of inhibitors can be peptide-based inhibitors or antibodies that bind through their variable domains [,–89,92]. B. A specific class of engineered proteins comprising antibody Fc regions fused to an antigen (one antigen molecule per Fc fragment) can be used to deplete antigen-specific antibodies. This fusion protein, named a Seldeg (for selective degradation of antigen-specific antibodies), is engineered to bind to FcRn via its Fc region with increased affinity at both near neutral and acidic pH. Seldeg-antibody complexes are internalized into FcRn-expressing cells by receptor-mediated uptake, and subsequently follow the constitutive degradation pathway of FcRn into lysosomes [83], leading to the breakdown of the targeted antibody. Due to their different mechanism of action, Seldegs can be used at lower doses than Abdegs and do not affect the levels of antibodies that are not specific for antigen.