Differential STAT3, STAT5, and NF-κB activation in human hematopoietic progenitors by endogenous interleukin-15: implications in the expression of functional molecules

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Abstract
Different forms of interleukin-15 (IL-15) have been identified and shown to elicit different transduction pathways whose impact on hematopoiesis is poorly understood. We demonstrated herein that hematopoietic CD34+ cells constitutively produced endogenous secreted IL-15 (ES-IL-15) that activated different transcription factors and controlled the expression of several functional proteins, depending on the progenitor source. Thus, nuclear factor-κ B (NF-κ B) was activated in bone marrow (BM) and cord blood (CB) progenitors, whereas signal transducer and activator of transcription 3 (STAT3) and STAT5 activation was restricted to peripheral granulocyte—colony-stimulating factor (G-CSF)—mobilized and BM progenitors, respectively. ES-IL-15 acts through autocrine/paracrine loops controlled by high-affinity receptors involving IL-15 receptor α (IL-15R α). Furthermore, ES-IL-15 was found to differentially control the expression of several functional molecules important for hematopoietic differentiation. Indeed, in BM precursors, neutralizing anti—IL-15 monoclonal antibody (mAb) inhibits the expression of the γ c chain and of the chemokine stromal derived factor-1 (SDF-1) but had no effect on vascular cell adhesion molecule 1 (VCAM-1) and β 1 integrin adhesion molecule expression. Conversely, in CB progenitors, anti—IL-15 mAb inhibited VCAM-1 and β 1 integrin expression without affecting γ c chain expression and, most important, up-regulated SDF-1 expression. In conclusion, unprimed human hematopoietic CD34+ cells secrete cell-unbound IL-15, which activates through autocrine/paracrine loop distinct signaling pathways, depending on the progenitor source, thereby influencing the expression of several molecules important in the control of hematopoiesis. (Blood. 2003;102:109-117)

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