Human markers for IgG2 and IgG4 appear to be on the same molecule in the chimpanzee

It has been reported that all four immunoglobulin G (IgG) subclasses present in human serum are also present in chimpanzee serum, as detected with antibodies specific for the human IgG subclasses. We used monoclonal antibodies (mAb) specific for human IgG subclasses to measure concentrations of the four subclasses in the chimpanzee sera. Initial ELISA studies indicated that epitopes for all four human subclasses are present in chimpanzee sera. The concentrations of IgG1, IgG2 and IgG3 were similar in human and chimpanzee sera, but the registered concentrations of IgG4 were different. Absorption of IgG2-reactive material from chimpanzee serum with IgG2 mAb resulted in removal of IgG4-reactive material as well. Conversely, absorption of IgG4-reactive material removed IgG2-reactive material. IgG2-reactive material, isolated from chimpanzee serum using solid-phase anti-IgG2 mAb, reacted with anti-IgG4 mAb, and isolated IgG4-reactive material reacted with anti-IgG2 mAb. Three anti-IgG2 mAb and five anti-IgG4 mAb, each of which react with separate epitopes on their respective human isotype, were used in these studies. We conclude that chimpanzee serum contains only three IgG isotypes related to those of humans, one of which contains determinants related to both human IgG2 and IgG4.