In hematopoietic malignancies the bone marrow (BM) stroma has been reported to be in dynamic contact with neoplastic cells in a way that malignant cells are supported by various mechanisms including blocking of apoptosis, induction of resistance to therapeutic drugs, and suppression of potential anti-tumorigenic immunity. Here, we studied the immunomodulatory properties of cellular factors derived from BM stroma cells and their influence on maturation of dendritic cells (DC). To circumvent the inter-individual heterogeneity often observed with primary stroma, novel BM stroma cell lines derived from BM stroma of two AML patients (before and after chemotherapy) and one healthy donor were established. The surface marker profile of the cell lines is in accordance with an MSC pattern. Furthermore, differential expression patterns were observed for molecules such as HLA-G, MICA, MICB, ULBP2, ILT-4 and intracellular IL-10 known to be implicated in the regulatory network of the immune system. To monitor DC maturation in presence and absence of BM stroma cell fragments, obtained by ultra-sonication, surface markers including CD40, CD80, CD83, CD86, and CD274 were investigated before maturation on monocytes, three days after BM stroma fragment addition and on terminal differentiated DCs. In addition, the expression of classical (HLA class I, HLA-DR) and non-classical immunoregulatory molecules (HLA-G, HLA-E) were determined. For terminal differentiation of DCs on day seven, the medium was exchanged and 1000 U/ml IL-4, 1000 U/ml GM-CSF, 2.0 U/ml IL-1β and 0.2 U/ml TNFα were added to complete the maturation process by giving a “danger” signal. A markedly decreased expression of differentiation (up to −36%) and antigen presentation molecules (up to −68%) of DCs cultured in presence of AML BM stroma cell fragments was observed compared to DC maturation without AML BM stroma cell fragments. In particular, the decreased HLA-DR expression could not be restored by the addition of the pro-inflammatory cytokines at day seven. In comparison, the addition of BM stroma cell fragments derived from AML stroma had a higher impact on the decrease of HLA-DR expression than BM stroma cell fragments of the healthy donor cells. This indicates that molecules derived from the AML stroma cells are implicated in mechanisms impairing DC maturation and function, which may promote immune escape during leukemia development.
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Asterisk with author names denotes non-ASH members.