Megakaryocyte-stromal cell interactions: Effect on megakaryocyte proliferation, proplatelet production, and survival.

Bone marrow stromal cells provide a proper environment for the development of hematologic lineages. The incorporation of different stromal cells into in vitro culture systems would be an attractive model to study megakaryopoiesis and thrombopoiesis. Our objective was to evaluate the participation of different types of stromal cells in in vitro megakaryopoiesis, thrombopoiesis, and megakaryocyte (MK) survival. CD34-positive progenitors from umbilical cord blood were differentiated into MK precursors and then cocultured with umbilical vein endothelial cells (HUVECs), bone marrow mesenchymal stem cells (MSCs), skin fibroblasts (SFs) (all human), or the mouse fibroblast cell line L929. The number of MKs (CD61-positive cells) was increased in the presence of HUVECs and SFs, whereas L929 cells decreased total and mature MK counts. With respect to thrombopoiesis, HUVECs increased proplatelet (PP)-producing MKs, while MSCs, L929 cells, and SFs had the opposite effect (immunofluorescence staining and microscopic analysis). MK survival was enhanced in MSC and SF co-cultures, as assessed by evaluation of pyknotic nuclei. However, HUVECs and L929 did not prevent apoptosis of MKs. Reciprocally, the cloning efficiency of MSCs was decreased in the presence of MKs, while the ability of stromal cells (either MSCs or SFs) to produce the extracellular matrix proteins type III collagen, fibronectin, dermatan sulfate, heparan sulfate, and prolyl 4-hydroxylase subunit ß was preserved. These data indicate that each stromal cell type performs distinctive functions that differentially modulate MK growth and platelet production and, at the same time, that MKs also modify stromal cell behavior. Overall, our results highlight the relevance of considering the influence of stromal cells in MK research as well as the close interplay of different cell types within the bone marrow milieu.

Regulatory and effector T-cells are differentially modulated by Dexamethasone.

It is assumed that the ratio between effector T cells (Teff) and regulatory T cells (Tregs) controls the immune reactivity within the T-cell compartment. The purpose of this study was to investigate if Dexamethasone (Dex) affects Teff and Tregs subsets. Dex induced on Tregs a dose and time-dependent apoptosis which resulted in a relative increase of Teff. After TCR activation, Dex induced a strong proliferative inhibition of Teff, but a weaker proliferative inhibition on Tregs. These effects were modulated by IL-2, which not only restored the proliferative response, but also prevented Dex-induced apoptosis. The highest dose of IL-2 prevented apoptosis on all FOXP3+CD4+ T cells. Meanwhile, the lowest dose only rescued activated Tregs (aTregs), probably related to their CD25 higher expression. Because Dex did not affect the suppressor capacity of aTregs either, our results support the notion that under Dex treatment, the regulatory T-cell compartment maintains its homeostasis.