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Pathologic interaction between megakaryocytes and polymorphonuclear leukocytes in myelofibrosis
A Schmitt1, H Jouault, J Guichard
1INSERM U.474 and Laboratoire d'Hématologie, Hôpital Henri Mondor, Créteil, France.
Abstract:
Idiopathic myelofibrosis (MF) is a myeloproliferative syndrome characterized by an increase in bone marrow collagen. Megakaryocytes (Mks), which store growth factors in their alpha granules, are known to be involved in the pathogenesis of MF. Previously, mice given bone marrow grafts infected with a retrovirus carrying murine thrombopoietin (TPO) complementary DNA developed a disease resembling human idiopathic MF. In this study, we used this murine model (TPO mice) to determine whether release of alpha granules is responsible for fibroblast activation and development of fibrosis. The intracellular trafficking of several alpha-granule proteins (von Willebrand factor, fibrinogen, and transforming growth factor beta (TGF beta), which are stored in the granule matrix; and alpha(IIb)beta(3) integrin and P-selectin (CD62p), which are located in the alpha-granule membrane) was studied with immune electron microscopy in bone marrow Mks from TPO mice. P-selectin immunolabeling increased consistently and was occasionally found lining the demarcation membrane system. Evidence of extensive emperipolesis was also found in TPO mouse Mks, involving almost exclusively neutrophil and eosinophil polymorphonuclear (PMN) cells with altered morphologic features. In parallel, the host Mks had myeloperoxidase-positive granules scattered in their cytoplasm, associated with marked ultrastructural cytoplasmic alterations and ruptured alpha-granule membranes. Similar observations were made in bone marrow biopsy specimens from 12 patients with idiopathic MF; indeed, there was an increased rate of emperipolesis involving mostly PMN cells, abnormal P-selectin expression, and mutual subcellular PMN and Mk alterations. This study indicates that in idiopathic MF, abnormal P-selectin distribution in Mks induces selective sequestration of PMN cells. This results in a release of alpha-granular proteins and growth factors, which in turn induces fibroblast activation and fibrosis deposition. (Blood. 2000;96:1342-1347)
Insights
Idiopathic myelofibrosis involves abnormal megakaryocytes releasing growth factors. This study shows abnormal P-selectin in megakaryocytes attracts neutrophils, causing fibrosis.
Area of Science:
- Hematology
- Oncology
- Cell Biology
Background:
- Idiopathic myelofibrosis (MF) is a myeloproliferative neoplasm characterized by bone marrow fibrosis.
- Megakaryocytes (Mks) are implicated in MF pathogenesis due to their growth factor content.
- A murine model using thrombopoietin (TPO) mimics human idiopathic MF.
Purpose of the Study:
- To investigate the role of alpha-granule release from Mks in fibroblast activation and fibrosis in MF.
- To examine intracellular trafficking of alpha-granule proteins in a TPO-induced murine MF model.
Main Methods:
- Immune electron microscopy was used to study alpha-granule protein trafficking in Mks from TPO mice.
- Analysis included intracellular proteins (von Willebrand factor, fibrinogen, TGF-beta) and membrane proteins (alpha(IIb)beta(3) integrin, P-selectin).
- Bone marrow biopsy specimens from idiopathic MF patients were compared with the murine model.
Main Results:
- TPO mice Mks showed increased P-selectin, sometimes at the demarcation membrane system.
- Extensive emperipolesis involving neutrophils and eosinophils was observed in TPO mouse Mks.
- Similar findings, including abnormal P-selectin and emperipolesis, were seen in human idiopathic MF bone marrow.
Conclusions:
- Abnormal P-selectin distribution in Mks contributes to neutrophil sequestration in idiopathic MF.
- This process leads to the release of alpha-granular proteins and growth factors.
- These factors activate fibroblasts, promoting fibrosis deposition in the bone marrow.