The Specificity and Isotype of Monoclonal Anti-D Antibodies Dictates Their Ability To Inhibit Opsonized Platelet Phagocytosis.

The use of polyclonal anti-D is the established treatment for prophylaxis against rhesus immunization in Rh(D) negative pregnant women and is effective in raising the platelet counts in patients with autoimmune thrombocytopenic purpura (AITP). Since the supply of plasma derived anti-D antibodies is limited and plasma products carry the risk of infectious complications, replacement with monoclonal anti-D antibodies is a promising alternative. This study analyzed the efficacy of polyclonal anti-D (WinRho®SDF, Cangene) and six human monoclonal anti-D antibodies with differeing isotypes and specificities (Table 1) for their ability to opsonized erythrocytes and modulate monocyte phagocytosis of opsonized platelets in a flow cytometric assay. Human platelets were labeled with CellTracker CMGreen, opsonized with various HLA or HPA1a specific antibodies, incubated with the monocytic leukemic cell line THP-1 for 2 h and intracellular fluorescence was compared at 4°C and 37°C. Various concentrations of anti-D opsonized erythrocytes were added to the assay and the changes in intracellular fluorescence was compared. The results demonstrate that the THP cells significantly phagocytosed opsonized platelets in an Fc-dependent manner e.g. the fold change in median channel fluorescence between 37°C and 4°C for intact IgG W6/32 opsonized platelets was 51.5 ± 5.2 (mean+SD, n=74) and only 2.1+2.1 if the platelets were labeled with F(ab’)₂ fragments. When erythrocytes were opsonized with the monoclonal anti-D antibodies that shared specificity but differed in IgG isotype (RhG1/RhG3 and BRAD3/BRAD5) and added to the assay, the IgG3 isotypes had a significantly greater ability to inhibit opsonized platelet phagocytosis compared with IgG1 isotypes and this inhibition was similar to that observed with polyclonal anti-D opsonized erythrocytes. However, in the one case where the monoclonal anti-D antibodies shared isotypes but differed in specificity (113/178), rRh9B8-113 monoclonal antibody significantly inhibited platelet phagocytosis whereas the rRh429-178 antibody did not. These results suggest that some human monoclonal antibodies can be as efficient as polyclonal anti-D in inhibiting opsonized platelet phagocytosis and the degree inhibition depends on the IgG isotype and specificity of the monoclonal antibodies. Thus, a defined cocktail of monoclonal anti-D antibodies could be prepared to mimic the effects and replace plasma-derived polyclonal anti-D.