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Cell- Rather Than Antibody-Mediated Immunity Leads to the Development of Profound Thrombocytopenia during Experimental Plasmodium berghei Malaria¹

La Jolla Bioengineering Institute, La Jolla, CA 92037

Experimental malarial thrombocytopenia can reach life-threatening levels and is believed to be due to Abs targeting platelets for destruction by the reticuloendothelial system. However, we report that Abs account for at most 15% of platelet destruction as Plasmodium berghei-infected B cell-deficient mice exhibited profound thrombocytopenia (83%) as did C57BL/6 controls (98%). Further, no significant increase in Abs bound to intact platelets was observed during infection. P. berghei infection can enhance the activity of anti-platelet Abs as indicated by a significantly (p < 0.005) increased thrombocytopenia on day 4 of infection in mice that were administered a low dose anti-CD41 mAb compared with rat IgG1-injected controls. RAG1^–/– and CD4- plus CD8-deficient mice were markedly protected from thrombocytopenia (p < 0.005) and malarial pathogenesis. CD8- or TCR-deficient mice were not protected from thrombocytopenia and CD4-deficient mice were modestly protected. RAG1^–/– mice exhibited significantly (p < 0.05) lower levels of plasma TNF, IFN-, and IL-12 during infection. IFN^–/– and IL-12^–/– mice exhibited increased survival but similar thrombocytopenia to C57BL/6 controls. Collectively, these data indicate that thrombocytopenia is necessary but not sufficient for malarial pathogenesis and Abs are not the major contributors to malarial thrombocytopenia. Rather, we propose that both CD4⁺ and CD8⁺ T cell populations play key roles in malarial thrombocytopenia; a complex bidirectional interaction between cell-mediated immunity and platelets exists during experimental severe malaria that regulates both responses.