Elevated plasma thrombopoietic activity in patients with metastatic cancer-related thrombocytosis

High platelet counts are occasionally seen in patients suffering from progressive malignant disorders. While granulocyte colony-stimulating factor (G-CSF) has been implicated in paraneoplastic leukemoid reactions, the stimulus for thrombocytosis is unknown. Our purpose in this study was to determine if plasma from cancer patients with thrombocytosis contains a factor or factors with thrombopoietic activity.

We tested the effects of plasma obtained from 5 individuals with advanced tumors and high platelet counts and from 4 patients with advanced cancer and normal platelet counts on megakaryocytic differentiation of two megakaryoblastic cell lines (Dami and HEL). Differentiation was evaluated by assessing the expression of the platelet-specific cell-surface antigens CD41 (HUPL-mI) and glycoprotein IIb-IIIa using an immunocytochemical staining score. In addition, plasma samples from 7 of the 9 patients and from 5 additional cancer patients with thrombocytosis were assayed for the levels of interleukin (IL)-3, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), G-CSF, and IL-1 beta protein using an enzyme-linked immunosorbent assay (ELISA).

Expression of platelet-specific cell-surface antigen was increased in HEL cells after exposure to plasma from all 5 of the cancer patients with thrombocytosis, and in Dami cells after exposure to plasma from 4 of the 5. Similar, but less significant, results were found when these cells were incubated with control combinations of recombinant GM-CSF plus IL-6 or of IL-3 plus IL-6. Platelet-specific cell-surface-antigen expression was not increased in HEL or Dami cells after exposure to the plasma from the 4 cancer patients with normal platelet counts or to normal control plasma. ELISA revealed elevated levels of IL-6 in the plasma from 4 patients with thrombocytosis (38, 40, 63, and 99 pg/mL). In addition, GM-CSF concentration was high in 3 of these 4 patients (33, 47, and 127 pg/mL), and the G-CSF level was elevated in 1 (543 pg/mL). IL-1 beta and IL-3 levels were undetectable.

Our data suggest that the thrombocytosis observed in individuals with advanced malignant disease is mediated by a humoral mechanism. Levels of IL-6, GM-CSF, and G-CSF are elevated in some of these patients, but the plasma concentrations are generally lower than those required for in vitro induction of megakaryocytic differentiation. Plasma from patients with paraneoplastic thrombocytosis may therefore contain thrombopoietins that have not yet been identified, and which might have clinical usefulness.