The MHC class I-related receptor, FcRn, plays an essential role in the maternofetal transfer of γ-globulin in humans

Abstract

The transfer of maternal gamma-globulin (IgG) provides the neonate with humoral immunity during early life. In humans, maternal IgG is transported across the placenta during the third trimester of pregnancy. The expression of the MHC class I-related receptor, FcRn, in the human placenta suggests that this Fc receptor might be involved in the delivery of maternal IgG, but direct evidence to support this is lacking. In the current study an ex vivo placental model has been used to analyze the maternofetal transfer of a recombinant, humanized (IgG1) antibody in which His435 has been mutated to alanine (H435A). In vitro binding studies using surface plasmon resonance indicate that the mutation ablates binding of the antibody to recombinant mouse and human FcRn. Relative to the wild-type antibody, the H435A mutant is deficient in transfer across the placenta. Significantly, the mutation does not affect binding to Fc gamma RIII, an FcR that has been suggested in earlier studies to mediate the transfer of maternal IgG. The analyses demonstrate that binding of an IgG to FcRn is a prerequisite for transport across the perfused placenta. FcRn therefore plays a central role in the maternofetal delivery of IgG and this has implications for the use of protein engineering to improve the properties of therapeutic antibodies.

Cross-species Binding Analyses of Mouse and Human Neonatal Fc Receptor Show Dramatic Differences in Immunoglobulin G and Albumin Binding

“…Others have immobilized IgG and injected the receptor (45,46,50). In this situation, the actual affinities that were calculated were lower than those obtained here using the heterogeneous ligand binding model.…”

Generation and characterization of a high affinity anti-human FcRn antibody, rozanolixizumab, and the effects of different molecular formats on the reduction of plasma IgG concentration

“…The low binding affinity of rozanolixizumab for mouse FcRn was most likely due to Fc (and not Fab) binding, since the binding was comparable to that reported for human IgG Fc domain binding to mouse FcRn (60 – 270 × 10 3 pM). 35–37 The low rodent cross-reactivity was insufficient to support the study of rozanolixizumab in wild-type mice or rats; hence transgenic mice and cynomolgus monkeys were selected as the test species for in vivo studies.…”

Elucidation of intracellular recycling pathways leading to exocytosis of the Fc receptor, FcRn, by using multifocal plane microscopy

“…To express the MST-HN mutant [human IgG1-derived; (35)] as an antibody with a site-specific biotinylation peptide at the C terminus, codons encoding Gly-Ser-Leu-His-His-Ile-Leu-Asp-Ala-GlnLys-Met-Val-Trp-Asn-His-Arg were appended to the 3Ј end of the CH3 domain by using two rounds of PCR with overlapping oligonucleotides and an oligonucleotide close to the 5Ј end of the CH3 domain gene. The resulting product was digested with XmaI (by using an internal XmaI site overlapping codons 14-16 of the CH3 domain gene and a 3Ј primer encoded site) and used to replace the wild-type gene in a human Fc construct (47,48). The final heavy-chain expression construct was assembled as described (47) by using a vector (48) generously provided by J. Foote (Arrowsmith Technologies, Seattle, WA).…”

“…The resulting product was digested with XmaI (by using an internal XmaI site overlapping codons 14-16 of the CH3 domain gene and a 3Ј primer encoded site) and used to replace the wild-type gene in a human Fc construct (47,48). The final heavy-chain expression construct was assembled as described (47) by using a vector (48) generously provided by J. Foote (Arrowsmith Technologies, Seattle, WA). This construct was transfected into an NSO transfectant expressing an anti-hen egg lysozyme light-chain [(48) generously provided by J. Foote], and transfectants were selected as described (47,48).…”

“…The final heavy-chain expression construct was assembled as described (47) by using a vector (48) generously provided by J. Foote (Arrowsmith Technologies, Seattle, WA). This construct was transfected into an NSO transfectant expressing an anti-hen egg lysozyme light-chain [(48) generously provided by J. Foote], and transfectants were selected as described (47,48). Recombinant protein was purified by using lysozyme-Sepharose.…”