The role of platelets in lung disease is unclear, although increased platelet activation has been demonstrated in adult respiratory distress syndrome and chronic obstructive pulmonary disease. While the role of platelets in IPF is unknown, platelet trapping in the lungs of mice following intravenous bleomycin administration strongly correlated with subsequent collagen deposition, suggesting a role in fibrogenesis in this animal model. In the present study we measured markers of platelet activation in IPF patients under basal conditions and following stimulation with platelet agonists and investigated the effect of plasma from IPF patients on platelet activation. Our data demonstrate that IPF is associated with platelet hyperactivity that may be caused by the plasma environment. We have demonstrated platelet hyperactivity in patients with IPF using two mechanistically different agonists and three different markers of platelet activation, namely platelet-monocyte aggregate formation, platelet P-selectin expression, and platelet Dimesna fibrinogen binding. Increased platelet-monocyte aggregate formation is a sensitive marker of platelet activation and demonstrates a functional consequence of the increased platelet reactivity. These data provide robust evidence of increased platelet reactivity in IPF. To explore whether the hyperactivity phenotype in IPF was the result of an alteration of the platelets or their environment we used the plasma swap approach, which has been used successfully in previous studies to examine the role of plasma. Incubation of washed control platelets in IPF plasma increased platelet activation as assessed by P-selectin expression both under basal conditions and after stimulation with ADP. Platelet hyperactivity was not observed when platelets were incubated with control plasma. This indicates that the plasma environment in IPF is responsible for the observed increased platelet reactivity. Further investigation is required to identify the plasma factor or factors responsible for this effect. The two patient 
groups in this study were well matched in terms of age and gender. At the time of undertaking the platelet assays there was no evidence-based disease modifying treatment for IPF and therefore it was local practice not to treat IPF patients with immunosuppressant drugs. This is reflected in our cohort with only 1 IPF patient receiving long term low dose prednisolone. One patient in the control group was also taking long term low dose prednisolone as treatment for COPD. Although not statistically significant, there was a higher prevalence of cerebrovascular disease in the IPF group. There is a recognised association between cerebrovascular disease and platelet activation, but repeat analysis of our data following exclusion of patients with a history of cerebrovascular disease did not alter our conclusions. The platelet assays used in this study are not affected by aspirin and reanalysis of the data following exclusion of patients using antiplatelet therapies did not alter our conclusions. It has recently been demonstrated that patients with COPD have increased levels of Diperodon circulating platelet-monocyte aggregates compared to age and smoking status matched controls. The high prevalence of COPD in our control group may therefore result in the differences between the groups being underestimated, adding further support to the significance of the observed increased platelet reactivity in patients with IPF. The current smokers in this study were evenly distributed between IPF and control groups, indicating that the documented increased platelet activation and surface expression of P-selectin in smokers will not have influenced our conclusions. It is recognised that the choice of ex-vivo anticoagulant can impact platelet-monocyte association.