A probable role for trail-induced apoptosis in the pathogenesis of marrow failure. Implications from an in vitro model and from marrow of aplastic anemia patients.

Aplastic anemia (AA) is a syndrome of hematopoietic failure involving increased apoptosis of stem cells. In order to investigate the molecular mechanisms participated in the process of marrow failure, we created an in vitro model of hematopoietic cell suppression, by continuous addition of TNF-alpha and IFN-gamma in an vitro long-term bone marrow culture system. An up-regulation of Fas expression was observed in CD34+ cells in cytokine treated cultures, compared to controls. This was accompanied by significant TRAIL and decreased caspase 3 mRNA expression, whereas the expression of Bcl-2 family members was low (Bcl-xl) or absent (Bcl-2, Bax). The expression of these apoptotic genes was also investigated in aplastic anemia patients. Apart from Fas mRNA expression in total marrow and/or CD34+ cells, TRAIL mRNA expression was found only in CD34+ cells in active disease while in total marrow cell compartment this remains a constant finding even in patients in remission. The above data are in agreement with previous studies proposing a major role for the extrinsic apoptosis pathway in the pathogenesis of aplastic anemia and additionally introduce TRAIL as a probable important molecule in the process.

Markers of endothelial and in vivo platelet activation in patients with essential thrombocythemia and polycythemia vera.

We investigated endothelial and in vivo platelet activation in a cohort of 52 patients with essential thrombocythemia (ET) and polycythemia vera (PV) before and after cytoreductive treatment, 22 healthy controls, and 17 patients with acute cerebrovascular ischemia (ACVI) and normal platelet counts. We measured platelet expression of CD62P and CD63 antigens and levels of soluble vascular cell adhesion molecule 1 (sVCAM-1). We found increased in vivo platelet activation in all patients with ET and PV, both before and after cytoreductive treatment, compared with controls. In patients with arterial thrombosis, platelet expression of CD62P, and in patients with erythromelalgia, expression of both markers was higher compared with expression in patients without thrombotic complications. In patients with ET and PV before and after treatment, sVCAM-1 expression was increased compared with expression in controls but also compared with expression in patients with ACVI and normal platelet counts. In patients with arterial thrombosis and erythromelalgia, in vivo platelet activation correlated with the level of sVCAM-1. Our findings indicated that in vivo platelet activation reflects intrinsic platelet defects in patients with ET and PV, persists after cytoreductive treatment, and results in endothelial damage, probably through release of angiogenic factors and/or activation of white blood cells.

Clinical relevance of balance between type 1 and type 2 immune responses of lymphocyte subpopulations in aplastic anaemia patients.

Immune dysfunction, which leads to the suppression of haemopoiesis by cytokines that are secreted by activated T lymphocytes, is considered to play a key role in the pathogenesis of acquired aplastic anaemia (AAA). We investigated the intracytoplasmic expression of type-1 [interferon gamma (IFN-gamma), interleukin (IL)-2] and type-2 (IL-4, IL-10) cytokines in CD4+ and CD8+ T cells before and after in vitro activation in 16 patients with AAA and 17 normal controls. Untreated or refractory patients had a significantly higher proportion of unstimulated CD4+ and CD8+ T cells that produced IFN-gamma and IL-2 whereas the IL-4 and IL-10 producing T cells did not differ from that of controls, resulting in a shift of IFN-gamma/IL-4 ratio towards a type-1 response. Patients in remission had also increased proportion of IFN-gamma-producing unstimulated CD4+ and CD8+ cells, with a parallel rise of IL-4- and IL-10-producing cells and normal IFN-gamma/IL-4 ratio. These data indicate that, in newly diagnosed and refractory patients with AAA, CD4+ cells are polarized towards a type-1 response that in turn leads to activation of cytotoxic CD8+ cells and finally to haemopoietic stem cell destruction. The type-1 response persists in patients in remission although this effect is compensated by the increase of IL-4 and IL-10 production.