Evaluation of the human platelet response in vivo using a NOD-scid mouse model

Lisa-Marie Holbrook1, Christopher Moore2, Michael Emerson1. 1Imperial College London, NHLI, SW72AZ, London, United Kingdom, 2UCL, Neuroscience, Physiology and Pharmacology, WC1E6BT, London, United Kingdom.

The response of human platelets to activatory stimuli under in vitro conditions has been well characterised, however human platelet responses in vivo have been less widely studied. Within the circulation, platelet function is influenced by a range of regulatory substances such as nitric oxide and prostacyclin which are derived from the vascular endothelium and are absent in conventional in vitro aggregometry assays. The aim of this study was therefore to assess the functional responses of human platelets in vivo in the presence of an intact vascular endothelium. This was performed using the immunodeficient NOD-scid strain of mice and responses determined using a mouse model of platelet thromboembolism developed by our group (Tymvios et al., 2008). Human and mouse platelets labelled with indium oxine111 were infused intravenously into the femoral veins of untreated and platelet-depleted mice. Radiolabelled platelet thromboembolism following collagen injection (50-100µg/kg for mouse platelets or 100-200µg/kg for human platelets) was recorded in the lungs by means of a radiation detecting probe and spectrometer. Experiments using mouse platelets, with collagen as an agonist yielded similar responses to those observed previously in other mouse strains and no difference in response was observed between the untreated and platelet-depleted mice. We were also able to record human platelet aggregation responses in real-time in NOD-scid mice. Human platelets were, however, found to produce a greater thromboembolic response within the platelet depleted NOD-SCID mice than their untreated counterparts (maximum response was observed to be an 8.9% mean increase above baseline levels compared with 1.8% increase observed in the untreated mice in response to 100µg/kg collagen, measured by one-way ANOVA, ** p<0.01 , n = 4) and that the human platelets require greater doses of collagen (100-200µg/kg) than mouse platelets (50-100µg/kg) to achieve measurable responses. Further investigations are underway to determine the differences between mouse and human platelet in vivo responsiveness. In this study, we demonstrate that human platelet responses can be induced and measured within a living, physiological model with an intact vascular endothelium. Our model could be used as a pharmacological tool for assessing the effects of compounds upon human platelet function in vivo. Also, this model has wider implications as the use of human platelets contributes to the reduction of animal numbers required for thromboembolism experiments.