Reduced red blood cell destruction by antibody fragments.

Alloantibodies and autoantibodies to blood group antigens can cause immune RBC destruction, resulting in transfusion reactions and autoimmune hemolytic diseases.1 Antibodies to human glycophorin A (GPA)–associated antigens can cause hemolytic transfusion reactions and HDN.2 Ter-119, a rat IgG2b monoclonal antibody, is a mouse erythroid-specific antibody that recognizes GPA3 and causes autoimmune hemolytic anemia after injection into mice.4 This indicates that the Ter-119 antibody pathogenicity is analogous to that of human GPA antibodies. Because the Fc portion of the antibody molecule mediates its effector functions, including binding to Fc receptors and complement components,5 we hypothesized that blood group–specific antibodies would be rendered nonpathogenic by removal of their Fc domains. We chose to use the F(ab′)2 fragment as compared with the monovalent Fab fragment because of its higher binding affinity (avidity) for the target antigens. Although the use of antibody fragments directed against a variety of antigens has been described, there have been only two published studies that addressed their in vivo application for antibodies directed against RBC antigens. Specifically, in two previous reports, the in vivo effects of F(ab′)2 prepared from anti-D were compared to whole IgG but conflicting results had been obtained.6,7 In one report, the antibody fragment preparation resulted in complete clearance of D+ RBCs,7 whereas in the other report, only about one-third of the RBCs were destroyed.6 The explanation given for these unexpected results was that the antibody fragment preparations used for the studies had different degrees of contamination with intact IgG molecules, which were responsible for RBC destruction. In this study, we compared the in vivo effects of intact Ter-119 antibody with those of a highly purified preparation of its F(ab′)2 fragment. We found that direct injection of Ter-119 antibody into mice resulted in a more severe anemia compared with that observed in mice injected with the F(ab′)2 fragment. In addition, mouse RBCs sensitized in vitro with the Ter-119 F(ab′)2 fragment exhibited prolonged survival in vivo as compared with RBCs coated with the whole IgG molecule.