title: The neonatal Fc receptor (FcRn) binds independently to both sites of the IgG homodimer with identical affinity.
abstract: The neonatal Fc receptor (FcRn) is expressed by cells of epithelial, endothelial and myeloid lineages and performs multiple roles in adaptive immunity. Characterizing the FcRn/IgG interaction is fundamental to designing therapeutic antibodies because IgGs with moderately increased binding affinities for FcRn exhibit superior serum half-lives and efficacy. It has been hypothesized that 2 FcRn molecules bind an IgG homodimer with disparate affinities, yet their affinity constants are inconsistent across the literature. Using surface plasmon resonance biosensor assays that eliminated confounding experimental artifacts, we present data supporting an alternate hypothesis: 2 FcRn molecules saturate an IgG homodimer with identical affinities at independent sites, consistent with the symmetrical arrangement of the FcRn/Fc complex observed in the crystal structure published by Burmeister et al. in 1994. We find that human FcRn binds human IgG1 with an equilibrium dissociation constant (KD) of 760 ± 60 nM (N = 14) at 25°C and pH 5.8, and shows less than 25% variation across the other human subtypes. Human IgG1 binds cynomolgus monkey FcRn with a 2-fold higher affinity than human FcRn, and binds both mouse and rat FcRn with a 10-fold higher affinity than human FcRn. FcRn/IgG interactions from multiple species show less than a 2-fold weaker affinity at 37°C than at 25°C and appear independent of an IgG's variable region. Our in vivo data in mouse and rat models demonstrate that both affinity and avidity influence an IgG's serum half-life, which should be considered when choosing animals, especially transgenic systems, as surrogates.
authors: a Protein Engineering; Rinat-Pfizer Inc. ; South San Francisco , CA USA.
authors: Yasmina Noubia
authors: YN
authors: Abdiche
authors: Yik Andy
authors: YA
authors: Yeung
authors: Javier
authors: J
authors: Chaparro-Riggers
authors: Ishita
authors: I
authors: Barman
authors: Pavel
authors: P
authors: Strop
authors: Sherman Michael
authors: SM
authors: Chin
authors: Amber
authors: A
authors: Pham
authors: Gary
authors: G
authors: Bolton
authors: Dan
authors: D
authors: McDonough
authors: Kevin
authors: K
authors: Lindquist
authors: Jaume
authors: J
authors: Pons
authors: Arvind
authors: A
authors: Rajpal
mesh_terms: D000818:Animals; D015395:Histocompatibility Antigens Class I / Q000737:chemistry* / Q000276:immunology; D006801:Humans; D007141:Immunoglobulin Fc Fragments / Q000737:chemistry* / Q000276:immunology; D007074:Immunoglobulin G / Q000737:chemistry* / Q000276:immunology; D008252:Macaca fascicularis; D051379:Mice; D051381:Rats; D011961:Receptors, Fc / Q000737:chemistry* / Q000276:immunology; D013045:Species Specificity; D020349:Surface Plasmon Resonance
chemical_list: D015395:Histocompatibility Antigens Class I; D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G; D011961:Receptors, Fc; C100688:Fc receptor, neonatal
keywords: CFCA, calibration-free concentration analysis; FcRn; FcRn, neonatal Fc receptor; IgG; RU, response units; Rmax, maximum binding response; SPR; SPR, surface plasmon resonance; WT, wild-type; anti-Id, anti-idiotypic; cyFcRn, cynomolgus monkey FcRn; cyIgG, cynomolgus monkey IgG; hErbB2, human ErbB2; hFcRn, human FcRn; hIgG, human IgG; label-free biosensor; mAb, monoclonal antibody; mFcRn, mouse FcRn; neonatal Fc receptor; pI, isoelectric point; rFcRn, rat FcRn; rIgG, rat IgG