Human Fcγ RI triggering of the mononuclear phagocyte respiratory burst

Abstract

This study investigates the role played by Fcγ RI and Fcγ RII in triggering the respiratory burst induced in human monocyte-like U937 cells by monoclonal chimaeric anti-NIP antibodies expressing the human IgG subclass and mouse IgG2b heavy chains. Respiratory burst activity was measured as Superoxide generation. Four separate lines of evidence indicate a predominant role for Fcγ RI in triggering Superoxide generation induced by erythrocytes sensitized with up to the maximum of 100,000 IgG molecules per cell. Firstly, erythrocytes sensitized with mouse IgG2b anti-NIP antibodies which are not recognized by human Fcγ RI, did not induce a response but when residue Glu-235 was replaced by Leu to give the lower hinge sequence of mouse IgG2a which is recognized by Fcγ RI, the mutant bound to Fcγ RI and induced a response equal to 80% of that given by chimaeric human IgG3. Chimaeric human IgG3 antibodies with amino acid substitutions in the lower hinge showed reduced activity and the greatest reductions (< 32% of wild type antibody activity) were associated with changes at Leu-235 which is critical for recognition by Fcγ RI. Secondly, chimaeric human IgG4 antibodies which are not recognized by FcγRII, were able to induce Superoxide generation. The rank order of abilities of chimaeric human IgG subclass antibodies to induce responses was IgG3 > IgG1 > IgG4 ⪢ IgG2. Thirdly, responses induced by chimaeric human IgG were inhibited by concns of monomeric human IgG3 in the nM range. Finally, chimaeric human IgG3 induced responses were inhibited by anti-Fcγ RI, but not anti-Fcγ RII monoclonal antibodies. Consistent with a major role for FcγRI in triggering the responses of U937 cells, erythrocytes sensitized with chimaeric human IgG3 did not induce Superoxide generation by neutrophils which express FcγRII and FcγRIII, or eosinophils which express Fcγ RII, but neither of which expresses Fcγ RI.