Human platelet-TLR7 mediates aggregation of platelets with granulocytes through endosomal signaling. Blood from human donors was treated with Loxo immediately after draw and stained with CD41 (platelets) and CD14 (leukocytes) antibodies. Aggregates between platelets and granulocytes were measured by flow cytometry. (A) Dot plots gated around the granulocyte population. FSC-H, forward scatter; SSC-H, side scatter. (B) Quantitation of CD41, CD14-positive aggregates from A. The following conditions were applied: R837 (2 μg/mL; n = 8); Loxo (1 mM; n = 5); Pam₃CSK₄ (10 μg/mL; males n = 10). (C) CD41, CD14 aggregates in human blood (n = 3) posttreatment with specific antagonist (IRS661, 2.8 μM) or the endosomal inhibitor chloroquine (Cl, 10 μM). (D-E) Platelets (denoted Plt) and neutrophils (denoted PMN) were isolated from human blood and labeled with CD41 and CD14, respectively. (D) Quantitation of CD41, CD14-positive aggregates in each fraction stimulated with either TLR7 agonist (Loxo) or pretreated with IRS661 or Cl (for 30 minutes) and then stimulated with Loxo for 15 minutes. The fractions were mixed together for 15 minutes, and aggregates were measured (n = 4) in each experiment. (E) SEM images of platelets and WBC stimulated together with different agonists. (F) SEM image of a WBC and a platelet with viral particles at the end of its pseudopodia (see circles). Bar in E and F represents 4 μm, except in the magnified image, where it is 2 μm. Data are average ± SD and were analyzed by Student t test (3B) or 1-way ANOVA (C, P < .05; E, P < .0001) followed by Bonferroni test (P values are noted on the graph).