PURPOSE: Tissue factor (TF) initiates coagulation in vivo by converting prothrombin to thrombin. Although TF is known to be expressed by monocytes and endothelial cells, TF expression by anucleate platelets is debated. We investigated the in-vivo expression of mature, spliced TF mRNA by platelets isolated from septic patients.
METHODS: 36 patients meeting criteria for sepsis and providing consent were prospectively enrolled within 24 hours of admission to the ICU. Demographics were collected and whole blood was drawn from an existing central line. Patients with platelet transfusions were excluded. Control patients were healthy volunteers, aged 18-50, who were not on any medications. Leukocyte-free platelets were immediately isolated from samples using standardized protocols. Platelets from controls and cases were assayed in parallel to eliminate differences due to inter-assay variability. TF mRNA was detected using primers that targeted sequences in exons one and six of the RNA encoding for the TF protein. Platelet-derived TF activity was calculated using an Actichrome® TF assay. A paired t-test analyzed differences in TF expression in cases and controls (significance set at p<0.05).
RESULTS: Of the septic patients, the average age was 56±16 years, 56% were male, and mean APACHE II score was 25. The mean platelet count was 234. Overall, 61% of samples from patients with sepsis expressed mature, spliced TF mRNA. In comparison, platelets from healthy volunteers exclusively expressed immature TF pre-mRNA. Side-by-side comparisons of TF activity in platelets from healthy controls demonstrated that TF activity was significantly increased in cells from patients with sepsis. Patients with bacteremia were more likely to express mature TF than patients without bacteremia (64% versus 47%, respectively). Patients who expressed mature TF had significantly higher APACHE II scores than those who did not express mature TF (21±10 versus 27±22, p<0.05).
CONCLUSION: These novel results demonstrate that platelets from septic patients express mature, spliced TF mRNA.
CLINICAL IMPLICATIONS: Expression of mature TF by platelets may contribute to the dysregulated hemostasis present during sepsis, and may be a target for therapeutic intervention.
DISCLOSURE: Matthew Rondina, None.