Platelet functional abnormalities in pediatric patients with kaposiform hemangioendothelioma/Kasabach-Merritt phenomenon.

Kaposiform hemangioendothelioma (KHE) is a rare vascular tumor of infancy which is commonly associated with a life-threatening thrombocytopenic condition, Kasabach-Merritt phenomenon (KMP). Platelet CLEC-2 - tumor podoplanin interaction is considered as the key mechanism of platelet clearance in these patients. Here we aimed to assess platelet functionality in such patients. Three groups by 6-9 children were enrolled: a) with KHE/KMP without hematologic response to therapy (HR), b) with KHE/KMP with HR, and c) healthy children. Platelet functionality was assessed by continuous and endpoint flow cytometry, low-angle light scattering aggregometry (LaSca), fluorescent microscopy of blood smears and ex vivo thrombi formation. Platelet integrin activation in response to a combination of CRP (GPVI agonist) and TRAP-6 (PAR1 agonist), as well as calcium mobilization and integrin activation in response to CRP or rhodocytin (CLEC-2 agonist) alone, were significantly diminished in groups (a) and (b). At the same time platelet responses to ADP with or without TRAP-6 were unaltered. Thrombi formation from collagen in parallel plate flow chambers was also noticeably decreased in groups (a) and (b). In silico analysis of these results predicted diminished amounts of CLEC-2 on platelet surface of patients, which was further confirmed by immunofluorescence microscopy and flow cytometry. Additionally, we also noted a decrease in GPVI levels on platelets from group (a). In KHE/KMP platelet responses induced by CLEC-2 or GPVI activation are impaired due to diminished amount of receptors on platelet surface. This impairment correlates with the severity of the disease and resolves as patient recovers.