Antibody therapy is coming of age, with 15 monoclonal antibodies approved for therapeutic use in the United States and many others currently undergoing clinical trials (1). The advent of antibody engineering over the past two decades has contributed to the recent clinical success of therapeutic antibodies. The development of chimeric (2) and humanized (3) antibodies not only reduced the potent immunogenicity of rodent antibodies in humans but also improved the serum halflives and efficacy of such therapeutics compared with rodent antibodies. Phage display (4) and other display technologies have led to the ability to increase the affinity of antibodies for their target antigens. More recently, antibody engineering has been used to modify the effector functions of antibodies by altering their binding to C1q (5) and various Fc␥ receptors (6).The neonatal Fc receptor (FcRn) 1 is a heterodimer that comprises a transmembrane ␣ chain with structural homology to the extracellular domains of the ␣ chain of major histocompatibility complex class I molecules, and a soluble light chain consisting of ␤2-microglubulin (␤2m) (7). FcRn mediates both transcytosis of maternal IgG to the fetus or neonate and IgG homeostasis in adults (8). Evidence for the latter role initially came from studies indicating an unusually short serum halflife for IgG antibodies in ␤2m-deficient mice (9 -11). This observation led to the generation of mutant mouse hinge-Fc fragments with enhanced binding to FcRn and increased serum persistence in mice (12). Recently, several studies have identified human IgG 1 mutants with enhanced FcRn binding (6, 13), although no improvement in the serum half-lives of these mutants was observed in mice (13) or reported in primates.The binding of IgG to FcRn is sharply pH-dependent; IgG binds to FcRn under mildly acidic conditions and is released under slightly basic conditions (14). It has been hypothesized that pinocytosed IgG antibodies are captured by FcRn in acidified endosomes, rescued from degradation in lysosomes, recycled back to the cell surface, and returned to the circulation (8). Mutagenesis studies have identified both the mouse (15, 16) and human (17) Fc residues believed to be important in mediating pH-dependent binding. The results of the mutagenesis studies are consistent with the interpretation of a crystallographic study of the Fc⅐FcRn interaction (18). In the current study, molecular modeling was used to identify residues in the human IgG Fc near the FcRn binding site that, when mutated, might alter binding to FcRn without affecting the pH dependence of this interaction. Following exhaustive mutagenesis at these positions, several IgG 2 mutants were identified with improved binding to FcRn at pH 6.0 that retained the property of pH-dependent release. A pharmacokinetics study in rhesus monkeys showed that two mutant IgG 2 antibodies with increased FcRn binding affinity had considerably longer serum half-lives than the wild-type antibody. EXPERIMENTAL PROCEDURESMolecular Modeling-Molecular models o...

The serum half-life of IgG Abs is regulated by the neonatal Fc receptor (FcRn). By binding to FcRn in endosomes, IgG Abs are salvaged from lysosomal degradation and recycled to the circulation. Several studies have demonstrated a correlation between the binding affinity of IgG Abs to FcRn and their serum half-lives in mice, including engineered Ab fragments with longer serum half-lives. Our recent study extended this correlation to human IgG2 Ab variants in primates. In the current study, several human IgG1 mutants with increased binding affinity to human FcRn at pH 6.0 were generated that retained pH-dependent release. A pharmacokinetics study in rhesus monkeys of one of the IgG1 variants indicated that its serum half-life was ∼2.5-fold longer than the wild-type Ab. Ag binding was unaffected by the Fc mutations, while several effector functions appeared to be minimally altered. These properties suggest that engineered Abs with longer serum half-lives may prove to be effective therapeutics in humans.

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The human G1m1 allotype comprises two amino acids, D12 and L14, in the CH3 domain of IGHG1. Although the G1m1 allotype is prevalent in human populations, ∼40% of Caucasiods are homozygous for the nG1m1 allotype corresponding to E12 and M14. Peptides derived from the G1m1 region were tested for their ability to induce CD4+ T-cell proliferative responses in vitro. A peptide immediately downstream from the G1m1 sequence was recognized by CD4+ T cells in a large percentage of donors (peptide CH315−29). CD4+ T-cell proliferative responses to CH315−29 were found at an increased frequency in nG1m1 homozygous donors. Homozygous nG1m1 donors possessing the HLA-DRB1*07 allele displayed the highest magnitudes of proliferation. CD4+ T cells from donors homozygous for nG1m1 proliferated to G1m1-carrying Fc-fragment proteins, whereas CD4+ T cells from G1m1 homozygous donors did not. The G1m1 sequence creates an enzymatic cleavage site for asparaginyl endopeptidase in vitro. Proteolytic activity at D12 may allow the presentation of the CH315−29 peptide, which in turn may result in the establishment of tolerance to this peptide in G1m1-positive donors. Homozygous nG1m1 patients may be more likely to develop CD4+ T-cell-mediated immune responses to therapeutic antibodies carrying the G1m1 allotype.