THE NATURAL HUMAN-IGG ANTI-F(AB')(2) ANTIBODY RECOGNIZES A CONFORMATIONAL IGG1 HINGE EPITOPE

Natural IgG anti-F(ab')(2) Abs are part of the physiologic immune repe rtoire and have important immunoregulatory functions. Although previou s work suggested that some of these Abs recognize epitopes located in the constant region of the F(ab')(2) molecule, an exact epitope mappin g has not been performed. We found that the anti-F(ab')(2) Ab binds st rongly to F(ab')(2) but only weakly to Fab fragments. Fab fragments ar e lacking the core and lower hinge region. In our experiments, we show that the IgG anti-F(ab')(2) Ab binds strongly to a synthetic double c hain peptide (225-237/225'-237') comprising the core and lower hinge r egion of the human IgG1 molecule. In contrast, it binds only weakly to the same peptide in monomeric form (225-237) or to a short double cha in hinge peptide (225-232/225'-232'). The double chain peptides compri se a cyclic region between the two cystine bridges and an exocyclic re gion. Previous nuclear magnetic resonance analyses showed that the cyc lic portion of the short double chain hinge peptide adopts the same co nformation as that found in the intact IgG1 molecule. The dichroic pro perties of the short and long double chain hinge peptides indicate tha t they have identical conformations in their cyclic regions, but have different conformations in their exocyclic regions. The conformational differences in the exocyclic regions explain the binding of the Ab to the long double chain hinge peptide and the lack of binding to the sh ort one. The circular dichroism spectrum of the monomeric hinge peptid e, which is not recognized by the Ab, is consistent with the absence o f an ordered peptide structure. These findings lead us to conclude tha t the IgG anti-F(ab')(2) Ab recognizes a conformational IgG1 hinge epi tope.