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 antig ens such as the A and B blood groups are predominantly IgG2. The conse quences of this isotype restriction for the immune destruction of red cells were investigated. Human IgG2 anti-D and IgG2 anti-A were isolat ed by affinity purification from an unusual anti-D serum (DEL) and ant i-A sera, respectively. These antibodies were compared with IgG1 and I gG3 monoclonal anti-D in in vitro functional assays of the interaction between IgG-coated red cells (EA-IgG) and cells bearing IgG Fc recept ors (Fc gamma R). Dimeric IgG2 anti-D bound efficiently to cell lines transfected with Fc gamma RIIa-H131, an allotypic form of Fc gamma RII a which readily interacts with IgG2, IgG1 and IgG3. Unexpectedly, howe ver, -D- phenotype red cells coated with IgG2 anti-D did not form rose ttes with these cells, whereas EA-IgG2 anti-A and EA-IgG1 and EA-IgG3 anti-D effectively formed rosettes with these transfectants at the sam e sensitization level (100000 molecules IgG/red cell). In antibody-dep endent cell-mediated cytotoxicity (ADCC) assays, lysis of EA-IgG2 anti -A was mediated via Fc gamma RIIa, whereas lysis of EA-IgG1 and EA-IgG 3 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 gamm a R interactions. The topography of the Rh D antigen, an integral memb rane protein, ensures that anti-D is bound near the lipid bilayer surr ounded by the glycocalyx. This may sterically hinder access of Fc gamm a RIIa-H131 to the Fc gamma R recognition site on the relatively infle xible 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 func tional interactions of red-cell-bound IgG2 anti-D and IgG2 anti-A with cells bearing Fc gamma receptors can occur.