Bone Marrow Contains High Levels of Microparticles

Background: Microparticles (MP) are shed in humans from a broad variety of cells and play an important role in activation of coagulation, cell to cell interaction and transport of membrane components. Alternations in plasma levels have been found in a variety of diseases, mostly associated with thrombosis and inflammation. However, little is known about tissue levels of MP. The aim of this pilot study was to investigate the bone marrow compartment as a possible source of circulating MP.

Material and Methods: MP were analysed in bone marrow blood and peripheral blood in 12 healthy stem cell donors. MPs were isolated and measured by flow-cytometric double-staining (FACScalibur, BD) with Anexin V and cell-specific antibodies for hematopoietic cells, platelets, white blood cells, red blood cells and endothelial cells. Statistical analysis was performed with SPSS for Windows 20.0.

Results: Total MP levels differed between bone marrow and peripheral blood: 14.8 x10⁹/l [8.5–19.3] vs. 9.2 x10⁹/l [3.8–14.8] (median value [25-75 percentile]), p=0.060. Pattern of MP´s origin varied likewise: in bone marrow blood (bmb), main cell source of MP were CD235a positive red blood cells/erythropoetic cells (6.4x10⁹/l [3.7–13.7], 43.2% of total bmb MP), CD61 positive platelets/megacaryocytes (4.1x10⁹/l [1.7–10.1], 27.7% of total bmb MP), CD45 positive leucocytes/myeloic and lymphatic progenitor cells (3.8x10⁹/l [2.7-4.9], 25.7% of total bmb MP) and CD62e positive endothelial cells (0.3x10⁹/l [0.1-1.0], 2.0% of total bmb MP). In contrast, MP in peripheral blood (pb) mainly derived from CD61 positive platelets/megacaryocytes (8.5x10⁹/l [2.4–14.4], 92.0% of total pb MP), CD45 positive leucocytes/myeloic and lymphatic progenitor cells (0.5x10⁹/l [0.2–1.1], 4.5% of total pb MP), CD235a positive red blood cells/erythropoetic cells (0.2x10⁹/l [0.1–0.6], 1.8% of total pb MP), and CD62e positive endothelial cells (0.1x10⁹/l [0.0–0.1], 0.9% of total pb MP). Mean levels of tissue factor bearing MP were low in bone marrow blood as well as in peripheral blood (0.19 x10⁹/l [0.16 – 0.29] and 0.08 x10⁹/l [0.05 – 0.12], respectively, p=0.004).

Conclusion: Within our study we were able to detect MP in bone marrow blood. When comparing MP from bone marrow and peripheral blood, major differences with regard to cell source and concentration was present. Our data suggest that the blood – bone marrow barrier is not only existentant for cells, but also for circulating MP.