THE STOICHIOMETRY AND AFFINITY OF THE INTERACTION OF MURINE FC FRAGMENTS WITH THE MHC CLASS I-RELATED RECEPTOR, FCRN

The binding of recombinant wild type and mutant Fc-hinge fragments to soluble, FcRn expressed in insect cells has been analysed. The mutant Fc-hinge fragments are derived from murine IgG1 with mutation of resid ues located at the CH2-CH3 domain interface (Ile253, His310, Gln311, H is433 and Asn434; EU numbering). These mutant Fc-hinge fragments have previously been shown to be deficient in neonatal transcytosis in suck ling mice and also have abnormally short serum half lives. The mutated residues are highly conserved in human and rodent gammaglobulins (IgG s) and are also involved in binding to staphylococcal protein A. This study demonstrates that the Fc mutants have lower binding affinities f or recombinant FcRn and mutations in the CH2 domain have a greater eff ect than those in the CH3 domain. There is an excellent correlation be tween affinity and transcytosis or the control of catabolism, and this provides further evidence in support of the close overlap of the site s of IgG/Fc involved in these processes. The stoichiometry of the FcRn :Fc interaction has also been investigated and has been found to be 1: 1, indicating that binding of FcRn to one CH2-CH3 domain interface sit e precludes an FcRn:Fc interaction at the second site. Copyright (C) 1 996 Elsevier Science Ltd.