Key Points

  • RBC antibody-induced anemia is not a sufficient condition to predict a successful antibody for the treatment of a murine model of ITP

  • RBC-specific antibodies which successfully block platelet phagocytosis in vitro predicted a successful outcome in a murine model of ITP

Polyclonal anti-D is a first line therapy for immune thrombocytopenia (ITP). Monoclonal antibodies are desirable alternatives, but none have yet proven successful despite their ability to opsonize erythrocytes and cause anemia. Here we examined twelve murine erythrocyte-specific antibodies of different specificity and subtypes and found eight of these antibodies could induce anemia in antigen positive mice. Of these eight antibodies, only five ameliorated ITP. All antibodies were examined for their in vitro ability to support macrophage-mediated phagocytosis of erythrocytes. Antibodies which supported erythrocyte phagocytosis in vitro successfully ameliorated ITP in vivo. To examine the ability of each antibody to inhibit phagocytosis of platelets, the antibodies were used to sensitize erythrocytes in vitro and these were added to a platelet phagocytosis assay. Antibodies which inhibited platelet phagocytosis in vitro also all ameliorated ITP in vivo. We conclude that inducing anemia is not a sufficient condition for amelioration of ITP but that the antibody's ability to prevent platelet phagocytosis in vitro predicted its ability to ameliorate ITP. We suggest that inhibition of in vitro platelet phagocytosis may prove to be a valuable tool for determining which erythrocyte antibodies would likely be candidates for clinical use in ITP.

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