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The multiple effects of interleukin-1 (IL-1) on hematopoietic cells are mainly documented in disturbed hematopoiesis, but its production and participation during constitutive hematopoiesis are still unproven, To assess the involvement of IL-1 in the regulation of steady-state hematopoiesis in vivo, we have investigated the consequences of IL-1 receptor blockade by recombinant human IL-1 receptor antagonist (rhIL-1Ra) in normal CBA/H mice treated with two i.p. injections of rhIL-1Ra (2 x 50 mu g/mouse) seventeen and two hours before sacrifice. The cellularity, the number of granulocyte-macrophage (CFU-GM), the number of erythroid (BFU-E) progenitor cells and the percentage of these cells in S phase of the cell cycle, as well as the morphologically recognizable cells in bone marrow were estimated, In peripheral blood, hematocrit, the number and differential count of nucleated cells, the number of erythrocytes and the percentage of reticulocytes were determined, IL-1Ra treatment significantly reduced the number of femoral CFU-GM and BFU-E, while all the other analyzed parameters were not different from the level obtained in control, non-treated animals. These findings show that a number of bone marrow IL-l-responsive cells were affected by the IL-1 receptor blockade, indicating that the expression of IL-1 receptors and endogenous IL-1 secretion occur as part of constitutive hematopoiesis.

Co-culture of haematopoietic cells with a stromal cell layer does not mimic the physiological, micro-environmental niche, whose major feature is a low oxygen (O-2) concentration. Thus, in order to study the effects of IL-17 in a context which better approximates the physiological state, we investigated its effects on cell expansion, colony-forming ability, and the phenotypical profile of normal, human blood CD34(+) cells co-cultured for five days with MSC layers at various O-2 concentrations (20%, 12.5% and 3% O-2). We demonstrated that IL-17 enhances CD34(+) and total CFC production during the five days of MSC/CD34(+) co-culture. This effect depends upon the O-2 concentration, reaching its maximum at 3% O-2, and is more pronounced on erythroid progenitors (BFU-E). In addition, the stimulation of IL-6 production by IL-17 in MSC cultures and co-cultures is enhanced by low O-2 concentration. The expression of some differentiation markers (CD34, CD13 and CD41) on haematopoietic cells in co-cultures also depends upon the oxygen concentration. Our results strengthen the concept that physiological levels of O-2 (mistakenly called hypoxia), should be considered as an important environmental factor that significantly influences cytokine activity.

Recent studies have shown that the T cell-derived cytokine, interleukin-17 (IL-17), stimulates hematopoiesis, specifically granulopoiesis inducing expansion of committed and immature progenitors in bone marrow. Our previous results pointed to its role in erythropoiesis too, demonstrating significant stimulation of BFU-E and suppression of CFU-E growth in the bone marrow from normal mice. As different sensitivities of erythroid and myeloid progenitor cells to nitric oxide (NO) were found, we considered the possibility that the observed effects of IL-17 were mediated by NO. The effects of recombinant mouse IL-17, NO donor (sodium nitroprusside - SNP) and two NO synthases inhibitors (L-NAME and aminoguanidine) on erythroid progenitor cells growth, as well as the ability of IL-17 to induce nitric oxide production in murine bone marrow cells, were examined. In addition, we tested whether the inhibition of CFU-E colony formation by IL-17 could be corrected by erythropoietin (Epo), the principal regulator of erythropoiesis. We demonstrated that IL-17 can stimulate low level production of NO in murine bone marrow cells. Exogenously added NO inhibited CFU-E colony formation, whereas both L-NAME and aminoguanidine reversed the CFU-E suppression by IL-17 in a dose-dependent manner. The inhibition of CFU-E by IL-17 was also corrected by exposure to higher levels of Epo. The data obtained demonstrated that at least some of the IL-17 effects in bone marrow related to the inhibition of CFU-E, were mediated by NO generation. The fact that Epo also overcomes the inhibitory effect of IL-17 on CFU-E suggests the need for further research on their mutual relationship and co-signalling.