MKs deliver MHC class I complexes containing OVA antigen to proplatelets in vitro. MKs were pulsed with OVA (green) and proplatelet formation (A) was monitored for 24 hours as OVA antigens were gradually packaged into proplatelets during their formation. This observation was confirmed at 24 hours postpulse (B). OVA signal was found in mature proplatelets, presenting the typical β1 tubulin peripheral coil (B). Scale bar represents 10 μM. MHCI-OVA complexes were also transferred to proplatelet during thrombopoiesis (C). Scale bar represents 2 μM. The processing of OVA antigens did not affect MK proplatelet formation (D).