TF has been reported in association with many types of cancers, including adenocarcinomas of the pancreas, stomach, colon, ovary, lung, and breast [
7]. With the exception of leukemias, the mechanism by which TF from cancer cells gains access to the blood has not been clearly elucidated. In one patient with Trousseau’s syndrome, Callander and Rapaport observed tumor cells that stained intensely for TF invading a blood vessel and in direct contact with the blood [
14]. This observation supported the long-standing conjecture that cancer cells within blood were responsible for the hypercoagulable state of cancer [
15,
16]. Our studies indicate that an additional mechanism for exposure of blood to TF is through membrane microvesicles. This mechanism was suggested a quarter of a century ago by Dvorak
et al., [
17] who demonstrated in 1981 that several mouse carcinoma cell lines shed procoagulant membrane microvesicles both in culture and into ascites when implanted
in vivo. Nevertheless, only a few studies have reported the presence of microvesicles in the blood of cancer patients, and none, to our knowledge, have specifically investigated the role of TF-bearing microvesicles in Trousseau’s syndrome. We describe a patient who developed a severe form of Trousseau’s syndrome with an underlying giant-cell lung carcinoma and who had extraordinarily high concentrations of TF in his blood, mostly associated with microvesicles. As in previously reported cases [
14], the thrombotic diathesis in this patient began long before there was any evidence of malignancy. However, certain aspects of this patient’s course suggest that the high TF level may not fully explain his clinical syndrome. For example, both animals that receive TF [
18] and patients with sepsis [
19] – a condition characterized by a high level of TF in blood – develop DIC rather than overt macrovascular thrombosis. It is thus somewhat surprising that the patient we report did not have full-blown DIC. A possible explanation for this is that most of the TF in the patient’s blood was cryptic [
12], incapable of initiating coagulation. The TF-VIIa activity in his plasma was only 5-fold greater than in controls, despite the TF antigen level being forty-onefold higher. However, the activity measurement was complicated by the fact that the patient’s plasma contained heparin; the assay therefore likely grossly underestimated the TF activity in his plasma during those times when he was not receiving heparins.
A clue as to how TF-VIIa activity may be regulated comes from the study of monocyte-derived TF-bearing microvesicles. The TF-VIIa activity in these microvesicles increases when they interact with platelets [
8,
20]. Could a similar mechanism operate in the case we describe? Monocyte-derived TF-bearing microvesicles bind and fuse with activated platelets through a mechanism dependent on P-selectin on platelets and P-selectin glycoprotein ligand-1 (PSGL-1) on the monocyte microvesicles [
8]. PSGL-1 is a membrane mucin [
21] and, as with carcinoma mucins, its interaction with P-selectin is blocked by heparin [
22]. Could mucins on the surface of cancer-derived TF-bearing microvesicles bind P-selectin on activated platelets and initiate thrombosis? Another intriguing possibility is that TF may play a role in Trousseau’s syndrome independent of its coagulant function. TF has been shown to also possess adhesive functions [
23,
24]. Could TF-bearing microvesicles cross-link other cells by virtue of their dense coating of TF, producing cell aggregates capable of occluding a vessel? Such a scenario would explain the relative inefficacy of oral anticoagulants in the treatment of Trousseau’s syndrome. Therapies targeted to the mechanisms of Trousseau’s syndrome will probably lead to more effective treatments of this frequently devastating condition. Anti-TF agents, including recombinant TFPI [
25] and anti-TF monoclonal antibodies [
26], are currently being tested in various clinical settings (e.g. sepsis and coronary heart disease). We believe that anti-TF therapies may prove to be particularly useful in cases of Trousseau’s syndrome with elevated blood TF levels.