In vitro cultures of P knowlesi are sensitive to platelets and PF4. The growth of (A) P knowlesi (n = 4) and (B) P falciparum (n = 3) treated with different platelet concentrations or Tyrodes buffer for 48 hours. (C) The growth of P knowlesi treated with platelet lysate, with and without anti-PF4 antibodies or immunoglobulin G isotype control (n = 2). (D) P knowlesi PF4 dose-response curve (n = 2). (E) The growth of P knowlesi treated with platelets (60 million/mL), platelet lysate or PF4 (0.5 µM), and cocultured in standard wells or Transwells (n = 2). (F) Micrographs showing platelets bound to uninfected and P knowlesi-infected cells. (G) Percentage platelet binding to uninfected, P knowlesi (n = 4) or P falciparum iRBCs (n = 3), determined by flow cytometry. (H) Percentage TUNEL-labeled (TUNEL⁺) P knowlesi parasites costained for PF4 (PF4⁺) or not PF4-stained (PF4⁻; n = 3). (I) Micrographs showing a PF4⁺TUNEL⁺P knowlesi infected cell after platelet treatment. Scale bars, 5 µm. Images were taken at 630× magnification on an Axio Observer inverted fluorescence microscope coupled to an Axiocam 503 monochrome camera with ZEN 2 software (Carl Zeiss, Germany). Bars indicate means of replicate data points. Kruskal-Wallis test or 1-way ANOVA used for statistical comparisons, *P < .05 and **P < .01. DIC, differential interference contrast.