Exposure of IgG to an acidic environment results in molecular modifications and in enhanced protective activity in sepsis

IgG molecules are exposed on a regular basis to acidic conditions during immunoaffinity purification procedures, as well as during the production of some therapeutic immunoglobulin preparations. This exposure is known to induce in them an antigen‐binding polyreactivity. The molecular mechanisms and the possible biological significance of this phenomenon remain, however, poorly understood. In addition to the previously reported ability of these modified IgG antibodies to interact with a large panel of self‐antigens, enhanced binding to non‐self‐antigens (bacterial), an increased ability to engage in F(ab′)2/F(ab′)2 (idiotype/anti‐idiotype) interactions and an increased functional antigen‐binding affinity are reported here. The newly acquired ‘induced polyreactivity’ of low‐pH buffer‐exposed IgG is related to structural changes in the immunoglobulin molecules, and is at least partly attributable to the enhanced role of the hydrophobic effect in their interactions with antigen. Our results suggest that data from many previous studies on monoclonal and polyclonal IgG antibodies purified by low‐pH buffer elution from protein A or protein G immunoaffinity columns should be reconsidered, as the procedure itself may have dramatically affected their antigen‐binding behavior and biological activity. Low‐pH buffer‐treated pooled therapeutic immunoglobulins acquire novel beneficial properties, as passive immunotherapy with the pH 4.0 buffer‐exposed, but not with the native therapeutic intravenous immunoglobulin preparation, improves the survival of mice with bacterial lipopolysaccharide‐induced septic shock.