Immune-mediated Thrombocytopenia is More Severe in Human FcγRIIa Transgenic Mice than in Wild-type Mice • 782

Abstract

Human FcγRIIa is the platelet receptor for IgG. FcγRIIa is also expressed in man along with the FcγRI/γ and FcγRIIa/γ complexes on spleen macrophages. Mice lack an equivalent of FcγRIIa although it is likely to be a key receptor in immune-mediated thrombocytopenias. We established transgenic mouse lines that express human platelet FcγRIIa at levels equivalent to human platelets in order to determine if immune-mediated thrombocytopenia would be more severe in these transgenic mice which more fully recapitulate the human Fc receptor endowment. We showed that the human receptor is functional on mouse platelets using platelet aggregation stimulated by antibody in vitro. To determine whether this receptor was functional in vivo, we injected mice from this transgenic line with 70 μg of 4A5 antibody, a previously characterized rat anti-mouse platelet antibody found to cause moderate thrombocytopenia in wild type mice. We compared the transgenic mice to wild type mice. We followed the time course of platelet counts in mice injected with saline or isotype control (n=6 each) versus wild type mice (n=6) and FcγRIIa transgenic mice (n=9) receiving a single intraperitoneal dose of antibody on day 0. Intravenous injection gave comparable nadir platelet counts, with more rapid kinetics as expected. The average platelet count in wild type mice decreased to a nadir of 0.60 × 106 / μL on day 6 from a pre-treatment count of 1.13 × 106/ μL following injection of the antibody, in keeping with literature results on this antibody. In FcγRIIa transgenic mice there was a greater response to 4A5. The average count reached a nadir on day 6 after injection of 0.12 × 106 / μL (n = 9; statistically significantly different from wild-type, p < 0.05), while the pre-treatment and recovery counts of the transgenic mice were similar to the controls, with platelet counts of 0.96 × 106 / μL. In FcR γ-chain knockout mice which express no FcγRI or RIII, no 4A5-induced thrombocytopenia was seen (n = 6). Human FcγRIIa transgenic mice have more severe antibody-mediated thrombocytopenia in vivo, suggesting that the human FcγRIIa receptor is functional in vivo in these transgenic mice and contributes to the increased clearance of platelets. Our human FcγRIIa transgenic mice will be a valuable model of the pathophysiology and therapy of human immune-mediated thrombocytopenia disorders, including neonatal alloimmune thrombocytopenia.