Cell-to-Cell Contact Is Critical for the Survival of Hematopoietic Progenitor Cells on Osteoblasts.

Osteoblasts constitute part of the stromal cell support system in marrow for hematopoiesis, however little is known as to how they interact with hematopoietic stem cells (HSCs). In vitro studies have demonstrated that the survival of HSCs in co-culture with osteoblasts requires intimate cell-to-cell contact. This suggests that the osteoblast-derived factor(s) that supports stem cell activities are either produced in very small quantities, are rapidly turned over, may be membrane-anchored and/or requires the engagement of cell-cell adhesion molecules yet to be determined. In the present report we found that survival of hematopoietic progenitor cells on osteoblasts is dependent upon the engagement of VLA-4 (α₄β₁) and VLA-5 (α₅ß₁) receptors using function blocking antibodies. Surprisingly, cell-to-cell contact is not absolutely required to support progenitor activity, but does not require receptor-ligand engagement of the VLA-4 and LFA-1 complexes, which can in part be replaced through the use of recombinant ligands (fibronectin, ICAM-1, VCAM-1). Moreover conditioned once these receptors were engaged, medium derived from HSCs grown on osteoblasts ligands supported significantly greater hematopoietic progenitors in vitro than did osteoblast-conditioned or HSC-conditioned medium alone. As an initial attempt to identify the activity we examined which genes are activated following the establishment of osteoblast-CD34⁺ cell co-cultures nine separate co-cultures were establsihed and the RNA was pooled and analyized on Affymetrix HG-U133A chips at 24 hours. Initially our analysis revealed that there were 259 genes that are up regulated at 24 hours, and 14 genes that are down regulated. Inspection revealed that 30 of these signals were repeated at least once suggesting that 206 genuine gene candidates were differentially expressed resulting from the co-culture. A significant proportion of the differentially expressed cDNAs represent intracellular signaling ligands 16.5% (n=34) and cell surface receptors 13.5% (n=28). Molecules associated with assembly of the extra cellular matrix or its degradation comprised 7.2% (n=15) of the differentially up regulated molecules. Molecules associated with intracellular signaling, novel sequences and intermediate metabolism comprised the majority of the remaining activities. Amoung the candidates of extra cellular signaling molecules, we noted that IL-6, LIF, MIP-1alpha and SDF-1 were identified in the microarray analysis. This observation was most gratifying as we had previously reported that IL-6, LIF and MIP-1α activities are critical components of an HSC-osteoblast microenvironment. Other notable cytokine messages for BMP-2, CCL7, FGF2b, GRO1α, GRO3, IGF1, IL1ß, IL-8, IL-11, LIF, PDGF-D and the receptors for CCL7 (CCR7). Elevations in mRNA for fibronectin, lysine hydroxylase-like proteins, laminin and Type I collagen suggest that the presence of hematopoietic cells also induces osteoblastic activities. While the identity of those molecules present in the co-cultured medium remain to be identified, the data suggests that hematopoietic cells cooperate with osteoblasts to assemble the various marrow microenvironments by directing the synthesis of osteoblast-derived cytokines to improve HSC survival.