In vitro studies have defined an essential role for stromal cells in supporting B-cell development, including production of lymphopoietic cytokines. It has been suggested that stromal cells are equivalent to adventitial reticular cells in the marrow; however, evidence of reticular cells producing cytokines has been difficult to obtain. Staining of bone marrow (BM) sections with antibodies to interleukin-7 (IL-7) showed a reticular pattern, mimicking that obtained using antibodies to vascular cell adhesion molecule 1 (VCAM-1), a molecule present on both stromal cells in vitro and reticular cells. To more closely examine cytokine production within normal marrow, an immunomagnetic separation scheme was devised to directly enrich VCAM-1+ stromal cells. Twenty to thirty percent of cells isolated in the VCAM-1+ fraction shared characteristics with stromal cells from long term BM cultures, including cellular morphology and expression of alkaline phosphatase and alpha actin. These were termed “reticular stromal” cells. Immunohistochemical staining showed that virtually all of the latter cells possessed cytoplasmic IL-7 protein, and about half expressed stem cell factor. In contrast with cultured stromal cells, very few had detectable macrophage-colony-stimulating factor. These data constitute the first report of cytokine expression by marrow reticular cells in vivo. The implications of this data with respect to the existence of stromal cell subsets and their regulation of lymphopoiesis is discussed.
Adherent murine stromal cells support long-term in vitro lymphopoiesis or myelopoiesis dependent on the culture conditions used. A cell line, TC-1, isolated from long-term liquid murine marrow cultures under conditions approaching those permissive for lymphoid growth, has been found to produce an activity that acts synergistically with interleukin- 3 (IL-3) or colony-stimulating factor-1 (CSF-1) to stimulate in vitro myeloid colonies, but which has no intrinsic colony-stimulating activity. We report here the presence of multiple growth factors in conditioned medium (CM) from the TC-1 line, including granulocyte- macrophage colony-stimulating factor (GM-CSF) (bioassay with antibody blocking and messenger RNA [mRNA] analysis), granulocyte CSF (G-CSF) and IL-4 (factor-dependent cell line bioassay), and CSF-1 (radioimmunoassay, mRNA) along with a pre-B cell inducing activity, which appears separate from these CSFs and segregates with the myeloid synergizing activity through anion exchange, sizing, and Conconavalin A chromatography. Because these activities are not yet purified to homogeneity, their identity or lack of identity remains an open question. Assays of TC-1 CM or cellular mRNA analysis have given negative results for IL-1, IL-2, IL-3, IL-6, and IL-7, and IL-6 does not stimulate pre-B cells in this assay. However, IL-4 and G-CSF do stimulate in vitro induction of pre-B cells from pre-B and B-cell- depleted Balb/C marrow and are present in CM by selective cell line assay. A monoclonal antibody to IL-4 that inhibited its pre-B inducing activity did not inhibit pre-B inducing activity of TC-1 CM. These data suggest the existence of a unique synergizing and pre-B inducing factor(s) in TC-1 CM. Given the known capacity of subliminal levels of growth factors to act synergistically, an alternate possibility is that these biologic phenomena represent the actions of low concentrations of growth factors acting synergistically and possibly associated with some core protein.
In suspensions of murine bone marrow, many stromal cells are tightly entwined with hematopoietic cells. These cellular aggregations appear to exist normally within the marrow. Previous studies showed that lymphocytes and stem cells adhered to stromal cells via vascular cell adhesion molecule 1 (VCAM1). Injection of anti-VCAM1 antibody into mice disrupts the aggregates, showing the importance of VCAM1 in the adhesion between stromal cells and hematopoietic cells in vivo. Early hematopoietic stem cells were shown to be enriched in aggregates by using a limiting-dilution culture assay. Myeloid progenitors responsive to WEHI-3CM in combination with stem cell factor (c-kit ligand) and B220- B-cell progenitors responsive to insulin-like growth factor-1 in combination with interleukin-7 are not enriched. We propose a scheme of stromal cell-hematopoietic cell interactions based on the cell types selectively retained within the aggregates. The existence of these aggregates as native elements of bone marrow organization presents a novel means to study in vivo stem cell-stromal cell interaction.
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