Platelets neutralize viral infection in vivo depending on their serostatus. (A) Transfusion setup: iDTR^PLT mice expressing an inducible diphtheria toxin receptor (iDTR) in the megakaryocytic lineage were rendered thrombocytopenic by repeated injections of diphtheria toxin (DT). Thrombocytopenic iDTR^PLT mice were transfused with donor platelets (PLT), infected with IAV, and euthanized 7 days after infection. (B) Platelet count in iDTR^PLT mice with and without transfusion of seronegative (PLT neg.) or seropositive (PLT pos.) donor platelets (n = 4-12). (C-H) Thrombocytopenic iDTR^PLT mice were transfused with seronegative or seropositive donor platelets in buffer, infected with IAV, and evaluated for disease severity 7 days later (n = 5). (C) Pulmonary viral load was quantified by measuring relative messenger RNA expression of IAV HA. Representative images (D) and quantification (E) of lung sections stained with hematoxylin and eosin and scored for edema formation (arrows) and alveolar hemorrhage (*). The boxes in panel D (left panels) are shown at higher magnification in (the right panels. (F-G) Lung sections were stained for CD41 (yellow), CD144 (red), and nuclei (blue), and platelet infiltration was determined by calculating the CD41-positive area per nucleus. Representative images (F) and quantification (G). The boxes in panel F are shown at higher magnification in the corresponding panel to the immediate right. (H) Interferon response in lung tissue was quantified by measuring relative mRNA expression of interferon response factor 5 (IRF5). (I-J) Thrombocytopenic iDTR^PLT mice were transfused with seronegative platelets in diluted seronegative plasma (Platelets: neg.; light blue), seronegative platelets in diluted seropositive plasma (Plasma: pos.; red), or seropositive platelets in diluted seronegative plasma (Platelets: pos.; dark blue) and infected with IAV (n = 5-6). Pulmonary viral load (I) and interferon response (J) were assessed as in panels C and H, respectively. *P < .05.