Acidic pH increases the avidity of FcγR for immune complexes

Abstract

The interaction of immunoglobulin G (IgG) antibodies with FcγR constitutes a critical mechanism through which IgG antibody effector functions are mediated. In the current work we have examined whether human neutrophil FcγR exhibit pH dependence in their association with IgG. Binding assays were performed in culture medium adjusted to different pH values. It was found that the binding of either heat-aggregated human IgG (AIgG), soluble immune complexes (sIC) or IgG-coated erythrocytes (IgG-E) was markedly higher at pH 6.5 than at pH 7.3. This effect was not observed when saturation of FcγR was achieved, suggesting that acidic pH increases the avidity of FcγR for IC without modifying the total binding capacity. Similar results were observed for the binding of AIgG to either monocytes, natural killer (NK) or K562 cells, suggesting that acidic pH increases the avidity of both, FcγRII and FcγRIII. Additional experiments were performed to analyse whether the binding of IgG to FcγRI also showed pH dependence. To this aim, we employed interferon-γ-treated human neutrophils and mouse inflammatory macrophages, previously incubated with blocking antibodies directed to FcγRII and FcγRIII. Acidic pH did not enhance the binding of AIgG nor monomeric IgG under these experimental conditions. Further studies are required to determine whether the enhancement of FcγR avidity for IC could be attributed to titration of histidine(s) residues on the Fc fragment of IgG.