A New Murine Model of Immune Thrombocytopenia: Evidence of Both Antibody- and CD8+ T Cell-Mediated Platelet Destruction.

Immune thrombocytopenic purpura (ITP) is an autoimmune-mediated bleeding disorder in which platelets are opsonized by autoantibodies and prematurely destroyed by Fc receptor (R)-mediated phagocytosis in the reticuloendothelial system (RES). Although the pathogenesis of the disorder has been considered to be primarily antibody mediated, recent reports have indicated that cell-mediated (e.g. CD8+ T cell) mechanisms can also lead to platelet destruction at least in those patients with ITP who have no detectible antibodies. To study these mechanisms, we developed a murine model of immune thrombocytopenia by transferring spleen cells (SC) from GPIIIa-knockout (KO) mice immunized against wild type (WT; GPIIIa-positive) platelets into syngeneic severe combined immunodeficient (SCID) mice. Results show that compared with naive SC, transfer of as few as 5x10⁴ immune GPIIIa-KO SC caused significant thrombocytopenia and a bleeding diathesis and death in 60 percent of recipient SCID mice within 10 days post-transfer. Bleeding occurred primarily in the gut, lungs, subcutaneous tissues and brain. When the immune GPIIIa-KO SC were first depleted of CD4+T cells and transferred, thrombocytopenia or bleeding mortality occurred. In contrast, CD8+ T cell depletion of the GPIIIa-KO SC before transfer did not affect their ability to cause thombocytopenia nor bleeding. Interestingly, depletion of CD19+ B cells before transfer did not affect the ability of the SC to induce thrombocytopenia but prevented all bleeding. These results suggest that both antibody-mediated and CD8+ T cell-mediated platelet destruction occur in this animal model of immune thrombocytopenia, however, it appears that only antibody-mediated thrombocytopenia is associated with bleeding mortality. Because the immune thrombocytopenia is against a platelet-specific antigen e.g. GPIIIa (like in ITP), it will not only allow for the study of disease pathogenesis but may also be important for testing new immunospecific therapeutics to increase platelet counts in patients with ITP.