Recombinant human IgG molecules lacking Fcγ receptor I binding and monocyte triggering activities

Abstract

Subclasses of human IgG have a range of activity levels with different effector systems but each triggers at least one mechanism of cell destruction. We are aiming to engineer non‐destructive human IgG constant regions for therapeutic applications where depletion of cells bearing the target antigen is undesirable. The attributes required are a lack of killing via Fcγ receptors (R) and complement but retention of neonatal FcR binding to maintain placental transport and the prolonged half‐life of IgG. Eight variants of human IgG constant regions were made with anti‐RhD and CD52 specificities. The mutations, in one or two key regions of the CH2 domain, were restricted to incorporation of motifs from other subclasses to minimize potential immunogenicity. IgG2 residues at positions 233 – 236, substituted into IgG1 and IgG4, reduced binding to FcγRI by 10⁴‐fold and eliminated the human monocyte response to antibody‐sensitized red blood cells, resulting in antibodies which blocked the functions of active antibodies. If glycine 236, which is deleted in IgG2, was restored to the IgG1 and IgG4 mutants, low levels of activity were observed. Introduction of the IgG4 residues at positions 327, 330 and 331 of IgG1 and IgG2 had no effect on FcγRI binding but caused a small decrease in monocyte triggering.