Approximately 15% of the total platelet count is sufficient to prevent lethality and peripheral organ hemorrhage during LCMV Armstrong infection. (A) Circulating platelet counts 24 hours after intraperitoneal injection of PBS, 40 μg, or 80 μg of anti-GPIIb monoclonal antibody. Groups of 5 animals per treatment, of at least 2 independent experiments. Error bars are SEM. (B) Survival of LCMV Armstrong infection after platelet depletion treatment. Mice were injected intraperitoneally with PBS, 40 μg, or 80 μg of anti-GPIIb monoclonal antibody and infected 12 hours later with 2 × 10⁶ PFU of LCMV Armstrong intravenously. Depletion treatment was repeated 2 and 4 days after infection and survival was monitored daily for 16 days. The plot shows at least 8 animals per group from the combination of 2 experiments. (C) Signs of hemorrhage observed in infected animals treated with 80 μg of anti-GPIIb antibody. Animals treated as in panel B were killed at day 8 after infection. Skin, brains, and guts were dissected and photographed and lungs were histologically processed, H&E-stained, and micrographs were taken (100× and 40× insets). Arrows indicate hemorrhagic spots. (D) Kinetics of the thrombocytopenia induced by the anti-GPIIb treatment and the LCMV Armstrong infection (40 μg of anti-GPIIb, 12 hours before and, 2 and 4 days after LCMV Armstrong infection or PBS injection). Platelet counts were determined at the indicated time points for nontreated and noninfected mice (black bars), treated and noninfected mice (open bars), nontreated and infected mice (gray bars), and treated and infected mice (checkered bars). Error bars represent SEM (*P < .05). Data of 2 separate experiments, with 5 animals per group per experiment.