Hematopoiesis occurs in close association with a complex network of cells loosely termed the hematopoietic microenvironment. Analysis of the mechanisms of microenvironmental regulation of hematopoiesis has been hindered by the complexity of the microenvironment as well as the heterogeneity of hematopoietic stem cells and early progenitor cells. We have established immortalized primate bone marrow-derived stromal cell lines to facilitate analysis of the interactions of hematopoietic cells with the microenvironment in a large animal species. One such line, PU-34, was found to produce a variety of growth factors, including an activity that stimulates the proliferation of an interleukin 6-dependent murine plasmacytoma cell line. A cDNA encoding the plasmacytoma stimulatory activity was isolated through functional expression cloning in mammalian cells. The nucleotide sequence contained a single long reading frame of 597 nucleotides encoding a predicted 199-amino acid polypeptide. The amino acid sequence of this cytokine, designated interleukin 11 (IL-11), did not display significant similarity with any other sequence in the GenBank data base. Preliminary biological characterization indicates that in addition to stimulating plasmacytoma proliferation, IL-11 stimulates the T-cell-dependent development of immunoglobulin-producing B cells and synergizes with IL-3 in supporting murine megakaryocyte colony formation. These properties implicate IL-11 as an additional multifunctional regulator in the hematopoietic microenvironment.

We used functional expression cloning in mammalian cells to identify a cDNA clone encoding a hematopoietic growth factor that is mitogenic for the factor-dependent human megakaryoblastic leukemic cell line, MO7E. Analysis of the sequence of this cDNA revealed striking similarity to that of a recently reported novel murine growth factor for helper T- cell clones designated T-cell growth factor P40. The mRNA for the human P40 protein is expressed by several different human T-cell lines and by mitogen-stimulated peripheral blood lymphocytes. The recombinant protein displays substantial size heterogeneity typical of other glycoprotein cytokines. These properties plus the observation that this cytokine may well act within both the lymphoid and myeloid lineages warrant the designation of P40 as interleukin-9.

The authors studied the role that interleukin (IL)-11 plays during the early stages of megakaryocyte (MK) development by investigating its in vitro effects on cell subpopulations enriched for bone marrow primitive progenitor cells and early and late committed progenitor cells. Progenitor subpopulations were isolated from bone marrow of normal or 5-fluorouracil (5FU)-treated mice and separated by sorting based on the surface antigens Sca-1, c-kit, and CD34. Functional analysis of the cell subpopulations, 5FU Lin−Sca-1+c-kit+ or normal bone marrow (NBM) Lin−Sca-1+c-kit+CD34−cells, indicated that exposure of these cells to recombinant human (rh)IL-11 in combination with steel factor (SF) stimulates the formation of colonies in methylcellulose and their proliferation in single cell-containing liquid cultures. Kinetic studies of MK progenitor generation, in response to SF and rhIL-11, demonstrated that a significant number of the progenitors produced are committed to the MK lineage. RhIL-11 also synergized with both SF and IL-3 to stimulate MK colony growth from NBM Lin−Sca-1+c-kit+ cells (early progenitors) and NBM Lin−Sca-1−c-kit+ cells (committed late progenitors). In the presence of IL-3, NBM, Lin−Sca-1−c-kit+ cells responded more strongly to rhIL-11 than SF. Consistent with these results is the observation that IL-11 receptor  chain mRNA is present in all the progenitor cells from which the MKs are derived. This cell culture and RNA analysis suggest that murine bone marrow primitive progenitor cells and early and late progenitor cells are direct targets of rhIL-11 and that rhIL-11 has the potential to promote megakaryocyte development at several very early stages. (Blood, 2000;95:503-509)