Cellular origin and procoagulant activity of tissue factor-exposing microparticles in cancer patients

Background: In patients with cancer, tissue factor-exposing microparticles (TF-exposing MP) have been associated with disease progression and thrombosis. The cellular origin and coagulant activity of TF-exposing MP, however, remain disputed. Therefore, we investigated the cellular origin of the TF-exposing MP and the procoagulant activity in cancer patients. Methods: The cellular origin of TF-exposing MP was investigated by flow cytometry in a cohort of 209 cancer patients (59 pancreatic, 97 gastrointestinal, 23 breast, 15 lung, 5 prostate cancer and 10 other types), and 22 healthy controls. We first determined the numbers of TF-exposing MP in plasma from all patients and measured TF-exposing MP coagulant activity in a fibrin generation test. Based on previous results, a prolongation of the clotting time in the presence of an inhibitory antibody against factor VIIa above 13% was considered abnormal. Second, we selected those patients with numbers of TF-exposing MP above the 95th percentile, and determined the cellular origin of TF-exposing MP in these patients. Results: The numbers of TF-exposing MP were increased in the cancer patients compared to the healthy subjects (median: 2.0 vs 0.40×105/mL; p=0.01). 30% of the cancer patients had an abnormal FGT test, indicating TF-exposing MP coagulant activity. There was no correlation between the number and coagulant activity of TF-exposing MP (r=0.029, p=0.685). 13 patients had TF-exposing MP above the 95th percentile. Of these TF-exposing MP, 5.9% (median; interquartile range (IQR) 0.69-54) double stained with CD227 (MUC-1) and 19% (0.62-41) with CD24, both markers of cancer cells (Fig. 1). Furthermore, 28% (11-63) of the TF-exposing MP stained for CD61, platelet glycoprotein IIIa, 3.9% (0.39-55) for the monocyte LPS-receptor (CD14), while 3.0% (1.4-3.5) of TF-MP stained for the erythrocyte marker CD235, and 0.97% (0.53-4.5) for the granulocyte marker CD66b. (Figure presented) Discussion: Levels of TF-exposing MP in cancer patients are higher than in healthy subjects. Strikingly, but not unexpectedly, we could not demonstrate a relationship between levels of TF-exposing MP and coagulant activity. Therefore, at least part of the TF exposed on circulating MP may be involved in other TF-dependent functions, such as angiogenesis or signal transduction. Most TF-exposing MP seem to originate from cancer cells, as they stained double positive with typical tumour cell markers such as CD227 and CD24. However, most TF-exposing MP also labelled positive for typical blood cell CD antigens, therefore these microparticles might be of mixed cellular origin, being shed after a tumour cell has incorporated blood cell characteristics. This phenomenon is specific rather than an artefact, since no double-labelling was observed with some markers such as CD235.