Characterization of murine bone marrow and spleen-derived stromal cells: analysis of leukocyte marker and growth factor mRNA transcript levels.

Stromal cells are believed to regulate lympho-hematopoiesis through direct cell-cell interactions and the release of growth factors. Many questions remain, however, about their lineage derivation and functional heterogeneity. We previously prepared a panel of stromal cell lines from murine spleen and bone marrow and characterized them based on their ability to support lymphocyte growth in long-term cultures. These cells are now compared with respect to their expression of various immunoglobulin superfamily and cytokine genes by Northern blot analysis. These results indicate that although stromal cells appear to be mesodermal in origin, they are not closely related developmentally to the hematopoietic progenitor cells they support. The potential production of at least six cytokines was demonstrated. All clones constitutively expressed mRNA for macrophage colony stimulating factor, interleukin-6, transforming growth factor beta and neuroleukin. The most potent lymphocyte supporting clones also made interleukin 7 constitutively. Previous findings had suggested that these clones responded to exogenous stimuli and this has now been demonstrated in terms of induced expression of IL-6 and G/M-CSF mRNA. Interleukin 6 mRNA levels were markedly upregulated by exposure of cells to LPS, TNF, IL-1, IL-6, IL-7, and EGF. G/M-CSF mRNA levels were "superinduced" by the combination of LPS and cycloheximide, a protein synthesis inhibitor. These responses are similar to ones documented by investigators working with endothelial cells and fibroblasts. Together, these data suggest that stromal cells are a multifunctional component of the lymphopoietic microenvironment and may be active participants in a complex, cytokine-mediated regulatory network.