Phagocytosis of co-developing neutrophil progenitors by dendritic cells in a culture of human CD34(+) cells with granulocyte colony-stimulating factor and tumor necrosis factor-alpha

IL-6 Generated from Human Hematopoietic Stem and Progenitor Cells through TLR4 Signaling Promotes Emergency Granulopoiesis by Regulating Transcription Factor Expression

Emergency granulopoiesis, also known as demand-adapted granulopoiesis, is defined as the response of an organism to systemic bacterial infections, and it results in neutrophil mobilization from reservoir pools and increased myelopoiesis in the bone marrow. Indirect and direct initiating mechanisms of emergency granulopoiesis have been hypothesized. However, the detailed mechanism of hyperactive myelopoiesis in the bone marrow, which leads to granulocyte left shift, remains unknown. In this study, we report that TLR4 is expressed on granulo-monocytic progenitors, as well as mobilized human peripheral blood CD34⁺ cells, which account for 0.2% of monocytes in peripheral blood, and ∼ 10% in bone marrow. LPS, a component of Gram-negative bacteria that results in a systemic bacterial infection, induces the differentiation of peripheral blood CD34⁺ cells into myelocytes and monocytes in vitro via the TLR4 signaling pathway. Moreover, CD34⁺ cells directly responded to LPS stimulation by activating the MAPK and NF-κB signaling pathways, and they produced IL-6 that promotes emergency granulopoiesis by phosphorylating C/EBPα and C/EBPβ, and this effect was suppressed by the action of an IL-6 receptor inhibitor. This work supports the finding that TLR is expressed on human hematopoietic stem and progenitor cells, and it provides evidence that human hematopoietic stem and progenitor cells can directly sense pathogens and produce cytokines exerting autocrine and/or paracrine effects, thereby promoting differentiation.

CpG-ODN 2006 and human parvovirus B19 genome consensus sequences selectively inhibit growth and development of erythroid progenitor cells

Recent studies have shown that anemia is commonly observed after exposure to pathogens or pathogen-derived products, which are recognized via Toll-like receptor 9 (TLR9). In the current study, we demonstrate that CpG oligodeoxynucleotide-2006, a TLR9 ligand with phosphodiester (PO; 2006-PO) but not with the phosphorothioate backbone, selectively inhibits the erythroid growth derived from human CD34(+) cells. The 2006-PO was internalized by the erythroid progenitors within 30 minutes; however, expression of TLR9 mRNA was not detected in these cells. The 2006-PO directly inhibited burst-forming unit-erythroid growth, resulted in the accumulation of cells in S and G(2)/M phases, and increased cell size and frequency of apoptotic cells. These features were similar to those observed in erythroid progenitors infected with human parvovirus B19 that causes pure red cell aplasia. The consensus sequence of 2006-PO was defined as 5'-GTTTTGT-3', which was located in the P6-promoter region of B19 and inhibited erythroid growth in a sequence-specific manner and down-regulated expression of erythropoietin receptor (EPOR) mRNA and EPOR. B19 genome extracted from serum also inhibited erythroid growth and down-regulated expression of EPOR on glycophorin A(+) cells. These results provide a possible insight into our understanding of the mechanisms of human parvovirus B19-mediated inhibition of erythropoiesis.

Delayed addition of tumor necrosis factor (TNF) antagonists inhibits the generation of CD11c+ dendritic cells derived from CD34+ cells exposed to TNF-alpha

We have developed a method that cells exhibiting typical dendritic cell (DC) characteristics are generated from human CD34(+) cells and phagocytose cogenerating erythroid progenitor cells in the presence of tumor necrosis factor-alpha (TNF-alpha), interleukin-3, stem cell factor and erythropoietin. Using this system, we titrated the effects of TNF antagonists, etanercept and infliximab, on TNF-alpha activity. We found that 1 microg/ml etanercept dramatically inhibited the generation of CD11c(+) cells accompanying with a complete recovery of the generation of erythroid progenitors. Infliximab at 200 microg/ml exhibited a similar effect to that observed for etanercept. The delayed addition of etanercept to this culture system at day five resulted in significant inhibitory effects on the generation of CD11c(+), CD4(+) and CD86(+) cells. These results indicate that TNF antagonists administered at a concentration that is achievable in vivo, neutralize the biologic effects of TNF-alpha in generating CD11c(+) cells and that a delay in the administration of these antagonists for as long as 5 days partially inhibits the biologic activity of TNF-alpha. These findings may contribute to a great understanding of anti-TNF therapy in patients with an overproduction of cytokines such as hemophagocytic syndromes.