Expression and Factor-Dependent Modulation of the Interleukin-3 Receptor Subunits on Human Hematopoietic Cells

Interleukin-3 (IL-3) regulates growth and differentiation of multipotential as well as lineage-committed progenitor cells. The human IL-3 receptor (IL-3R) consists of the α and common β (βc) subunits. The α subunit (IL-3Rα) is specific for IL-3 and binds IL-3 with low affinity. In contrast, the βC subunit does not bind any cytokine by itself, but forms a high-affinity receptor with IL-3Rα. As the same βC subunit also forms high-affinity receptors for IL-5 and granulocyte-macrophage colony-stimulating factor (GM-CSF) with the respective cytokine-specific α subunit, the expression of the α subunits is responsible for specificity of cytokines. To examine the expression of IL-3Rα, we have developed a monoclonal antibody (MoAb), N3A. N3A specifically bound to cells expressing IL-3Rα and immunoprecipitated a 75 Kd glycoprotein, which became 43 Kd on N-glycosidase digestion. N3A and an anti-βc antibody, CRS1, were used in double color fluorescence-activated cell sorter (FACS) staining with several lineage markers to see the IL-3R expression pattern in peripheral blood (PB), cord blood (CB), and bone marrow (BM) cells. Both IL-3R sub-units were expressed on myeloid cell lineages (CD13+, CD14+, CD15Lo, or CD33+). To further study the IL-3R expression on hematopoietic progenitor cells, the CD34+ populations were isolated from both BM and CB cells. Those populations showed positive staining profiles with the N3A MoAb and were weakly stained with the CRS1 MoAb. Furthermore, anti c-kit antibody staining of the CD34+ fraction from CB, but not from BM, showed two intensities and the IL-3Rα expression seemed to be higher in a fraction of low c-kit expression. Because IL-1, IL-6, G-CSF, stem cell factor (SCF), interferon (IFN)-γ, and tumor necrosis factor (TNF)-α are known to enhance IL-3- dependent colony formation, we have examined whether this enhancement could be correlated with upregulation of the IL-3R expression. Incubation of CD34+ cells with TNF-α for 2 days significantly increased the level of βC and G-CSF increased the number of cells with high level expression of α, while other factors did not affect the IL-3R expression. Thus, different cytokines appear to have different mechanisms for enhancement of IL-3-dependent proliferation.