INCREASING THE SERUM PERSISTENCE OF AN IGG FRAGMENT BY RANDOM MUTAGENESIS

The major histocompatibility complex (MHC) class I-related receptor Fc Rn is involved in regulating serum gammaglobulin (IgG) levels in mice. With the aim of increasing the serum half-life of a recombinant murin e Fc gamma 1 fragment, the affinity far binding to FcRn at pH 6.0 has been increased by random mutagenesis of Thr252, Thr254, and Thr256 fol lowed by selection using bacteriophage display. These residues were ch osen as they are in proximity to the FcRn-IgG (Fc) interaction site. T wo mutants with higher affinity (due to lower off-rates) than the wild -type Fc have been isolated and analyzed in pharmacokinetic studies in mice. The mutant with the highest affinity has a significantly longer serum half-life than the wild type fragment, despite its lower off-ra te from FcRn at pH 7.4. The results provide support for the involvemen t of FcRn in the homeostasis of serum IgGs in mice. The indications th at a homologous FcRn regulates IgG levels in humans suggest that this approach has implications for increasing the serum persistence of ther apeutic antibodies.