Abstract
Background: Platelets are critical for maintaining hemostasis, but inappropriate platelet activation can lead to pathogenic thrombosis. It has been demonstrated that the platelet integrin αIIbβ3 is essential for platelet aggregation and is also a major target antigen in immune thrombocytopenias (e.g. ITP). Current monoclonal antibodies (mAbs) against this protein complex have been generated using traditional methods involving cross-species immunization (e.g. mouse proteins into rat hosts). These approaches may generate a limited repertoire of anti-β3 mAbs since the antigenicity of the protein and the variety of epitopes targeted are based on amino acid sequence differences between the two species and integrin family members are highly conserved. Additionally, studies in murine models of ITP are hampered by the use of xenogeneic antibodies rather than syngeneic antibodies.
Methods: We developed a method to generate mouse anti-mouse β3 integrin mAbs utilising β3 gene deficient mice (β3−/−) immunized with wild-type platelets. To generate antibodies specific to the PSI domain (HPA-1 region) of β3 integrin, β3−/− mice were immunized with the recombinant murine PSI domain of β3 integrin. Platelet binding and specificity were determined by flow cytometry and western blot. In vitro effects on platelet function were measured using aggregometry. Different doses of mAbs (5, 10, and 15 μg/mouse) were injected intravenously to induce thrombocytopenia in vivo.
Results: A total of twelve mAbs were generated against native β3 integrin (JAN A1, B1, C1, D1 and DEC A1 and B1, 9D2, M1) or recombinant PSI domain (PSI A1, B1, C1, E1). The mAbs were specific for β3 integrin; no binding was observed using β3−/− platelets. Isotyping showed that DEC A1 and DEC B1 are IgG3, PSI E1 is IgG2b, and all other mAbs are IgG1. The anti-PSI domain mAbs recognized linear epitopes and the anti-native β3 mAbs recognized conformational epitopes. All mAbs, with the exception of JAN A1 and B1, cross-reacted with human platelets. JAN C1, JAN D1, DEC A1, 9D2, M1, and all anti-PSI antibodies inhibited mouse platelet aggregation. These antibodies, except DEC A1, 9D2 and M1, also inhibited human platelet aggregation. One anti-PSI domain antibody (PSI B1), however, directly induced human platelet aggregation in the absence of agonist in platelet rich plasma but not in PIPES buffer. This suggests that PSI B1 may initiate conformational changes in β3 integrin and promote fibrinogen binding. Six anti-β3 mAbs (JAN A1, B1, C1 and D1, 9D2 and M1) induced severe dose-dependent thrombocytopenia in mice, while the anti-PSI domain mAbs induced only a mild decrease in platelet count. Interestingly, the two IgG3 mAbs (DEC A1 and B1) did not induce thrombocytopenia.
Conclusion: This approach to generating mouse anti-mouse β3 integrin mAbs using β3−/− mice was successful. Different anti-β3 mAbs had different effects on platelet aggregation, and on the induction of thrombocytopenia. These mAbs may be useful reagents for research in thrombosis and immune thrombocytopenia and as novel anti-thrombotic therapeutics.
