Infused platelet-like particles infiltrate the bone marrow. (A) cMpl^−/− mice were infused with wild-type (MPL⁺), CMPTX-labeled platelets (2 donors/recipient; 4 × 10⁸ total platelets) before an injection of LPS (5 µg/g) or saline control. Mice were euthanized after 20 hours. (B) Transfusion efficiency was measured immediately after platelet infusion (baseline) and 4 hours post–platelet infusion and LPS administration. Shown is a representative scatter plot and gating strategy for selection of CD41⁺, CMPTX⁺ events. Flow cytometry of whole blood revealed donor platelets constituted a mean of 68% of circulating platelets at baseline and 39% after 4 hours. Twenty hours postplatelet infusion, MPL expression in bone marrow was measured by immunohistochemistry on whole bone marrow (femurs). (C) Positive control: wild-type C57BL/6 mouse showing positive MPL staining of MKs. Insert shows 2 EVs. (D) Negative control: cMpl^−/− mouse showing no MPL staining. (E) cMpl^−/− mouse infused with MPL⁺ platelets: note an observable increase in brown punctate staining corresponding to MPL⁺ platelet-like particles. Insert shows platelet-like particles that appear to be bound to a BM cell. (F) cMpl^−/− mouse infused with MPL⁺ platelets and LPS: insert shows platelet-like particles that appear to be extravasating through a sinusoid. Scale bars, 50 µm. (G) Flow cytometry of flushed bone marrow was then used to quantify CMPTX⁺ events in the BM after platelet infusion (±LPS) compared with unstained bone marrow control. Data are mean ± standard error of the mean (SEM) of 3 independent experiments, *P < .05 (Student t test).