Epitope specificity and isotype of monoclonal anti-D antibodies dictate their ability to inhibit phagocytosis of opsonized platelets

Abstract

Rh immune globulin (WinRho SDF; Cangene, Mississauga, ON, Canada) is an effective treatment for autoimmune thrombocytopenic purpura; however, maintaining a sustained supply for its use in autoimmune thrombocytopenic purpura and its primary indication, hemolytic disease of the newborn, makes the development of alternative reagents desirable. We compared Rh immune globulin and 6 human monoclonal anti-D antibodies (MoAnti-D) with differing isotypes and specificities for their ability to opsonize erythrocytes and inhibit platelet phagocytosis in an in vitro assay. Results demonstrated that opsonization of erythrocytes with Rh immune globulin significantly (P < .001) reduced phagocytosis of fluorescently labeled opsonized platelets in an Fcdependent manner. Of the MoAnti-D that shared specificity but differed in isotype, only IgG3 antibodies could significantly (P < .001) inhibit platelet phagocytosis. In contrast, 2 MoAnti-D shared isotypes and differed in specificity; however, only one could significantly (P < .001) inhibit platelet phagocytosis. The results suggest that MoAnti-D epitope specificity and isotypes are critical requirements for optimal inhibition of opsonized platelet phagocytosis.

Molecular genetics and clinical applications for RH

“…identical epitope specificity, and Fc portions that do not elicit red cell sequestration, in effect to producing potential ‘drugs’ to prevention immune destruction of red cells in utero by the fetus 89. Potentially, these or similar anti-D can be produced with recombinant technology and substitute for human blood-derived RhIg preparation 90. It would be worthwhile to use such drugs in a clinical setting in the not-too-distant future.…”

Intravenous immunoglobulin products: an update on their mechanisms of action

“…Subsequently, few studies attempted to understand why monoclonal anti‐D preparations are not effective in ITP. Recently, however, it was shown that the ability of monoclonal anti‐D to inhibit opsonized platelet phagocytosis in vitro was dictated by both the antibodies epitope specificity and isotype [59]. These results have reopened the debate as to whether monoclonal anti‐D preparations can be produced to mimic polyclonal anti‐D and perhaps be therapeutically effective in ITP.…”

“…This may suggest that anti‐D does possess significant amounts of anti‐idiotypes that can neutralize the ability of autoantibodies to bind to platelets. On the other hand, the murine models of ITP have found that both IVIG and monoclonal anti‐RBC antibodies can raise platelet counts without the requirement of anti‐idiotypes [59]. The controversies lend credence to the notion that more research is required.…”

Back to the future: recombinant polyclonal antibody therapeutics

“…A biclonal mixture of the Brad5 and Brad3 antibodies, binding the same loop 6/7 epitope and using IgG1 and IgG3 isotypes respectively, showed efficacy similar to RhIG but only at a three-fold higher dose [36 ]. A careful analysis of protection conferred by combinations of six antibodies, suggested that the specific epitope recognized and antibody isotype are key variables [39].…”