Uptake of vWF-anti-vWF complexes by platelets in suspension

Engineered human platelet-derived microparticles as natural vectors for targeted drug delivery

Drug targeting has opened a new paradigm in therapeutics with development of delivery vectors like liposomes and polymeric nanoparticles. Although their clinical application is crippled by limited biological adaptability. Off-target toxicity and biocompatibility still remains one of the critical problems in anticancer therapeutics that can be life-threatening. Here we report a quick, simple and facile method of engineering human platelets to generate drug loaded platelet-derived microparticles (PMPs) by top-down approach, which are biocompatible and naturally target leukemia cells. Drug loaded PMPs and cancer cell uptake were characterized by flow cytometry, confocal microscopy, Nanoparticle Tracking Analysis and fluorimetry. Effective drug delivery was tested in cancer cell lines as well as in clinical samples from leukemia patients. We explored that PMPs are capable of carrying multiple drug payloads, have long shelf life and can be harvested in large quantity in short period. Importantly, PMPs exhibited remarkably higher toxicity towards cancer cells than free drug and had lower escape into extravascular spaces. Transfer of drug to cancer cells of leukemia patients was significantly higher than free drug, when delivered through PMPs. Our experiments validated therapeutic application of PMPs as biocompatible drug delivery vector against cancer cells with minimal off-target delivery.

Platelets release pathogenic serotonin and return to circulation after immune complex-mediated sequestration

There is a growing appreciation for the contribution of platelets to immunity; however, our knowledge mostly relies on platelet functions associated with vascular injury and the prevention of bleeding. Circulating immune complexes (ICs) contribute to both chronic and acute inflammation in a multitude of clinical conditions. Herein, we scrutinized platelet responses to systemic ICs in the absence of tissue and endothelial wall injury. Platelet activation by circulating ICs through a mechanism requiring expression of platelet Fcγ receptor IIA resulted in the induction of systemic shock. IC-driven shock was dependent on release of serotonin from platelet-dense granules secondary to platelet outside-in signaling by αIIbβ3 and its ligand fibrinogen. While activated platelets sequestered in the lungs and leaky vasculature of the blood-brain barrier, platelets also sequestered in the absence of shock in mice lacking peripheral serotonin. Unexpectedly, platelets returned to the blood circulation with emptied granules and were thereby ineffective at promoting subsequent systemic shock, although they still underwent sequestration. We propose that in response to circulating ICs, platelets are a crucial mediator of the inflammatory response highly relevant to sepsis, viremia, and anaphylaxis. In addition, platelets recirculate after degranulation and sequestration, demonstrating that in adaptive immunity implicating antibody responses, activated platelets are longer lived than anticipated and may explain platelet count fluctuations in IC-driven diseases.

Platelets are covercytes, not phagocytes: Uptake of bacteria involves channels of the open canalicular system

Recent studies have suggested that uptake of bacteria by human platelets is more similar to ingestion of the same organisms by neutrophils than previously considered. Platelet pseudopods surround a bacterium by a process of circumferential adherence until the organism is fully enclosed in a vacuole consisting of internalized extracellular space surrounded by plasma membrane. The surface connected, open canalicular system (OCS) is not involved in the process of platelet phagocytosis. The present investigation has reviewed the literature on platelet-particle and platelet-bacterial interaction, and added a new study of the process of bacterial engulfment employing tannic acid as an electron dense tracer. The osmium black reaction product formed by tannic and osmic acids during fixation stained the glycocalyx on the platelet exterior surface and lining channels of the OCS. It also stained the vacuoles containing bacteria in platelets as well as channels of the OCS connected to the engulfment vacuole and exterior surface of the cell. Alpha granules releasing their contents into the OCS and into engulfment vacuoles were also stained by osmium black. The only way that tannic acid could reach and stain the interior surface of engulfment vacuoles during fixation was through patent channels of the OCS. In contrast, neutrophils do not contain an OCS, and phagocytic vacuoles containing organisms are never stained by tannic acid. Thus the platelet is a covercyte, not a phagocyte, and uptake of the bacteria does involve channels of the OCS.

On the interaction of fluorophore-encapsulating PEGylated lecithin liposomes with hamster and human platelets

Polyethylene glycol (PEG)-grafted phosphatidylcholine liposomes are used as drug carriers due to their low immunogenicity and prolonged circulation time. The interaction between sterically stabilized lecithin liposomes and platelets has not been investigated before, and deserves to be subjected to scrutiny inasmuch as the uptake of liposomes by platelets could be detrimental for drug delivery and primary hemostasis. Consequently, the interaction between resting and convulxin-activated hamster and human platelets and calcein- or 5,6-carboxyfluorescein-encapsulating PEGylated liposomes composed of distearoyl- and dipalmitoyl phosphatidylcholine and PEG-derivatized distearoyl phosphatidylethanolamine was investigated by flow cytometry, confocal microscopy, and a glass capillary thrombosis model. Fluorescently labeled liposomes of the same composition were subsequently assayed in vivo after 15 and 45 min of systemic circulation. Neither resting nor activated hamster and human platelets interacted with liposomes at 0.70 mM lipid concentration. An absence of any interaction was corroborated in the in vivo experiments. Alternatively, flow cytometry assays evinced that human platelets interact with liposomes at lipid concentrations of >or=1.35 mM. These interactions were more profound for activated platelets than resting platelets. We conclude that the use of PEGylated lecithin liposomes at lipid concentrations of <1.35 mM has no detrimental impact on liposomal drug delivery based on PEGylated lecithin liposomes, but that these drug carriers may be associated with a reduced targeting efficacy or compromised primary hemostatic system when used at concentrations of >or=1.35 mM. In contrast, these drug carriers may become valuable in thrombosis- and drug delivery-related research and applications at concentrations of >or=1.35 mM.