Recent advances in the structural analysis of the genes and proteins for immunoglobulin Fc domain receptors have provided a molecular characterization of this complex family. The wide cellular distribution of these receptors and their functional heterogeneity are reflected in the diversity of molecules which bind antibody and immune complexes. The detailed analysis of the IgG and IgE Fc receptors has indicated that these molecules have evolved from a common precursor through gene duplication. Similarities among these receptors, in both structure and function have emerged. Thus, the Fc receptors provide an example of a class of molecules in which conserved domains are combined with divergent sequences to yield a diversity of function.

Constant regions of antibodies influence toxin neutralization in a manner dependent on FcγR.

Replacement of aspartic acid by alanine at position 265 (D265A) in mouse IgG1 results in a complete loss of interaction between this isotype and low-affinity IgG Fc receptors (FcγRIIB and FcγRIII). However, it has not yet been defined whether the D265A substitution could exhibit similar effects on the interaction with two other FcγR (FcγRI and FcγRIV) and on the activation of complement. To address this question, 34-3C anti-RBC IgG2a and IgG2b switch variants bearing the D265A mutation were generated, and their effector functions and in vivo pathogenicity were compared with those of the respective wild-type Abs. The introduction of the D265A mutation almost completely abolished the binding of 34-3C IgG2a and IgG2b to all four classes of FcγR and the activation of complement. Consequently, these mutants were hardly pathogenic. Although oligosaccharide side chains of these mutants were found to contain higher levels of sialic acids than those of wild-type Abs, the analysis of enzymatically desialylated D265A variants ruled out the possibility that very poor Fc-associated effector functions of the D265A mutants were due to an increased level of the mutant Fc sialylation. Thus, our results demonstrate that aspartic acid at position 265 is a residue critically implicated in triggering the Fc-associated effector functions of IgG, probably by defining a crucial three-dimensional structure of the Fc region.

No abstract

Four murine IgG subclasses display markedly different Fc-associated effector functions because of their differential binding to three activating IgG Fc receptors (FcγRI, FcγRIII, and FcγRIV) and C1q. Previous analysis of IgG subclass switch variants of 34-3C anti-RBC monoclonal autoantibodies revealed that the IgG1 subclass, which binds only to FcγRIII and fails to activate complement, displayed the poorest pathogenic potential. This could be related to the presence of a three amino acid deletion at positions 233–235 in the CH2 domain uniquely found in this subclass. To address this question, IgG1 insertion and IgG2b deletion mutants at positions 233–235 of 34-3C anti-RBC Abs were generated, and their ability to initiate effector functions and their pathogenicity were compared with those of the respective wild-type Abs. The insertion of amino acid residues at positions 233–235 enabled the IgG1 subclass to bind FcγRIV but did not improve the binding to C1q. Accordingly, its pathogenicity was enhanced but still inferior to that of IgG2b. In contrast, the IgG2b deletion mutant lost its ability to bind to FcγRIV and activate complement. Consequently, its pathogenicity was markedly diminished to a level comparable to that of IgG1. Our results demonstrated that the initiation of FcγR- and complement-mediated effector functions of IgG2b was profoundly affected by the three amino acid deletion at positions 233–235, but that this natural three amino acid deletion could only partially explain the poor binding of IgG1 to FcγRIV and C1q. This indicates the lack in the IgG1 subclass of as yet unknown motifs promoting efficient interaction with FcγRIV and C1q.