Distinctive role of IgG1 and IgG3 isotypes in Fc gamma R-mediated functions

Polyclonal and monoclonal anti-Rh (D) antibodies of IgG1 and IgG3 subclass were evaluated for their capacity to sensitize erythrocytes and (i) to trigger monocyte and K-cell mediated antibody-dependent cellular cytotoxicity (ADCC); (ii) to mediate binding to monocyte and lymphocyte Fc gamma R; (iii) to stimulate phagocytosis by monocytes. All antibodies were equally effective in mediating monocyte or activated U937 cell ADCC but IgG1 was more active than IgG3 in K-cell mediated ADCC. IgG3-sensitized erythrocytes inhibited IgG1-induced lysis, suggesting that each subclass engages the same Fc gamma R receptor but that lysis requires a further 'signal' that the IgG3 molecule can not deliver. Two monoclonal IgG3 anti-D antibodies were shown to have higher binding (two times) and phagocytic (three times) indices than IgG1 antibody for monocytes; similar differences were observed for polyclonal IgG1 and IgG3 antibodies. The same pattern was observed in an EA rosette assay when a total lymphocyte population was used; however, this difference was not seen with a B-cell depleted (T+ null cell) lymphocyte population.