Antigen topography is critical for interaction of IgG2 anti-red-cell antibodies with Fc gamma receptors.

IgG antibodies to the Rh D polypeptide on red cells are normally IgG1 or IgG3, whereas antibodies produced in response to carbohydrate antigens such as the A and B blood groups are predominantly IgG2. The consequences of this isotype restriction for the immune destruction of red cells were investigated. Human IgG2 anti-D and IgG2 anti-A were isolated by affinity purification from an unusual anti-D serum (DEL) and anti-A sera, respectively. These antibodies were compared with IgG1 and IgG3 monoclonal anti-D in in vitro functional assays of the interaction between IgG-coated red cells (EA-IgG) and cells bearing IgG Fc receptors (Fc gamma R). Dimeric IgG2 anti-D bound efficiently to cell lines transfected with Fc gamma RIIa-H131, an allotypic form of Fc gamma RIIa which readily interacts with IgG2, IgG1 and IgG3. Unexpectedly, however, -D- phenotype red cells coated with IgG2 anti-D did not form rosettes with these cells, whereas EA-IgG2 anti-A and EA-IgG1 and EA-IgG3 anti-D effectively formed rosettes with these transfectants at the same sensitization level (100,000 molecules IgG/red cell). In antibody-dependent cell-mediated cytotoxicity (ADCC) assays, lysis of EA-IgG2 anti-A was mediated via Fc gamma RIIa, whereas lysis of EA-IgG1 and EA-IgG3 anti-D was mediated via Fc gamma RI or Fc gamma RIII; EA-IgG2 anti-D was inactive in all functional assays. These experiments suggest that both IgG subclass and antigen structure affect functional IgG-Fc gamma R interactions. The topography of the Rh D antigen, an integral membrane protein, ensures that anti-D is bound near the lipid bilayer surrounded by the glycocalyx. This may sterically hinder access of Fc gamma RIIa-H131 to the Fc gamma R recognition site on the relatively inflexible IgG2 anti-D, but not to that of IgG1 or IgG3 anti-D. In contrast, IgG2 bound to the A antigen on glycoproteins is not so constrained. The topography of the D and A antigens may thus determine whether functional interactions of red-cell-bound IgG2 anti-D and IgG2 anti-A with cells bearing Fc gamma receptors can occur.