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Platelets are involved in host defense via clearance of bacteria from the circulation, interaction with virus particles, and uptake of various size particulates. There is a growing interest in micro- and nanoparticles for drug delivery and there is evidence that the properties of these particles critically influence their interaction and uptake by various tissues and cells including platelets. Virus mediated gene therapy applications are still challenged by the resultant thrombocytopenia and the mechanism(s) of platelet-foreign particles interaction remains unclear. We studied the specifics of platelet interaction with an active biological agent (adenovirus) and inert latex microspheres (MS) and investigated the role of platelet proteins in this interaction. We show that activated and not resting platelets internalize MS, without influencing platelet aggregation. In contrast, adenovirus induces and potentiates ADP-induced platelet aggregation and results in rapid expression of P-selectin. Platelets then internalize adenovirus and viral particles appear inside the open canalicular system. Inhibition of platelet αIIbβ3, GPIbα, and P-selectin decreases both platelet aggregation and internalization of MS. Inhibition of αIIbβ3 and αVβ3 does not abolish adenovirus platelet internalization and adenovirus-induced platelet activation is maintained. Our study demonstrates that platelets react differentially with foreign particles and that αIIbβ3 is a key player in platelet engulfing of foreign particles but not in mediating adenovirus internalization. Other platelet candidate molecules remain to be investigated as potential targets for management of adenovirus-induced thrombocytopenia.

Adenovirus has a wide tissue tropism. The virus attaches to the surface of cells via the fiber protein knob binding to the Coxsackie and Adenovirus receptor known as CAR. Virus entry inside cells is facilitated by integrins αVβ3 and αVβ5. Mice platelets are shown to be the predominant Ad binding blood cell type and the virus is documented inside platelets. CAR was identified on human platelets in one study yet contradicted in another. The presence of CAR appears to be the most reasonable initial step for virus entry into platelets and is a key to the understanding of platelet adenovirus interaction. This study aimed to re investigate the presence of CAR on human platelets. Platelets were tested by indirect immune-fluorescence using rabbit H-300 polyclonal anti-CAR antibody and goat anti-rabbit IgG F(ab')2 Texas Red antibodies, alongside with CAR positive and negative controls. Platelets were found to express CAR on their surface and in contrast to the previous study only 3.5 ± 1.9% of the tested platelets did express CAR. In addition, CAR was seen within intracellular aggregates localized at the sites of cell-cell contacts indicating that CAR expression might be upregulated in response to platelet stimulation. We confirm the presence of CAR on human platelets, we provide explanation to some of the discrepancies in this regards and we add that this receptor is localized at the sites of intercellular interaction.