DENV infection results in thrombocytopenia because of increased platelet activation and uptake. (A) Platelet counts of C57Bl/6 mice infected with DENV2 (1 × 10⁶ PFU via the intraperitoneal route; n = 5-9 per group). Blood and spleens of DENV2-infected mice were isolated, and cells were stained for platelet marker CD41 and activation marker CD62P and analyzed by flow cytometry (n = 5 per group). (B) There was increased activation of platelets in the blood at days 1 and 3 postinfection, with resolution at day 6. (C) Increased platelet activation in the spleen was observed 72 hours postinfection. Flow cytometry gating strategy is presented in supplemental Figure 2A. (D) Representative eosin and methylene blue staining of peripheral blood smears 24 hours postinfection. Some individual platelets are indicated by arrows. Platelet aggregates are surrounded by a dashed line. Scale bar, 25 μm. (E) Platelets were labeled in vivo to track their turnover by injecting mice with an antibody against platelet GP1b before DENV infection. Blood was taken at indicated points, stained for CD41, and analyzed by flow cytometry (n = 5 per group). Significantly reduced labeled platelets were observed in DENV-infected mice compared with uninfected control mice at 72 hours postinfection, indicating platelet destruction. (F) Representative flow cytometry plots of labeled circulating platelets 3 days postinfection. (G-I) Spleens were harvested from DENV2-infected mice 3 days postinfection; stained for CD41, CD11b, and F4/80; and analyzed by flow cytometry (n = 5 per group). (G) A significant reduction in total splenic labeled platelets was observed during DENV infection. (H) Representative flow diagram for macrophages containing platelets (CD11b⁺F4/80⁺GP1b⁺). Flow cytometry gating strategy is presented in supplemental Figure 2B. (I) Significantly increased numbers of macrophages had phagocytosed platelets (CD11b⁺F4/80⁺GP1b⁺) in spleens of DENV-infected mice compared with control, uninfected mice (n = 4-5 per group). Error bars represent the standard error of the mean. P values were determined by 1-way ANOVA for panel A, or by Student’s unpaired t test for all other panels, and error bars represent the standard error of the mean. ns, not significant; *P < .05; **P < .01; ***P < .001; ****P < .0001.