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.

Intravenous immunoglobulin (IVIG) are purified IgG preparations made from the pooled plasma from thousands of healthy donors and are being tested in preclinical mouse models. Inherent challenges, however, are the pluripotency of IVIG and its xenogeneicity in animals. IVIG can alter the viability of human neutrophils via agonistic antibodies to Fas and Siglec-9. In this study, we compared the effects of IVIG on human and mouse neutrophils using different death assays. Different commercial IVIG preparations similarly induced cytokine-dependent death in human neutrophils, whereas they had no effects on the survival of either peripheral blood or bone marrow neutrophils from C57BL/6 or BALB/c mice. F(ab’)2 but not Fc fragments of IVIG induced death of human neutrophils, whereas neither of these IVIG fragments, nor agonistic monoclonal antibodies to human Fas or Siglec-9 affected the viability of mouse neutrophils. Pooled mouse IgG, which exhibited a different immunoprofile compared to IVIG, also had no effect on mouse cells. Together, these observations demonstrate that effects of IVIG on neutrophil survival are not adequately reflected in current mouse models, despite the key role of these cells in human inflammatory and autoimmune diseases.

Natural polyreactive IgG antibodies are found in the sera of all healthy individuals. The in vitro exposure of pooled human IgG to protein-destabilizing chemical or physical factors has been previously shown to result in the exposure of their "hidden" polyspecificity. We hypothesize that such an enhancement of their pre-existing immunoreactivity may occur in vivo in the aggressive microenvironment of inflammation sites. An increase in the antigen binding intensity as well as of the number of recognized antigens was observed in the sera of IgG-infused immunodeficient SCID mice with induced local inflammation. The expansion of the IgG pathogen-binding repertoire may have important biological consequences.