Interleukin (IL) 15 is a novel cytokine that activates human natural killer cells via components of the IL-2 receptor

Identification and cloning of a novel IL-15 binding protein that is structurally related to the alpha chain of the IL-2 receptor

Interleukin-15 (IL-15) is a novel cytokine of the four-helix bundle family which shares many biological activities with IL-2, probably due to its interaction with the IL-2 receptor beta and gamma (IL-2R beta and gamma c) chains. We report here the characterization and molecular cloning of a distinct murine IL-15R alpha chain. IL-15R alpha alone displays an affinity of binding for IL-15 equivalent to that of the heterotrimeric IL-2R for IL-2. A biologically functional heteromeric IL-15 receptor complex capable of mediating IL-15 responses was generated through reconstruction experiments in a murine myeloid cell line. IL-15R alpha is structurally similar to IL-2R alpha; together they define a new cytokine receptor family. The distribution of IL-15 and IL-15R alpha mRNA suggests that IL-15 may have biological activities distinct from IL-2.

The action of interleukin-2 receptor subunits defines a new type of signaling mechanism for hematopoietin receptors in hepatic cells and fibroblasts

The gene regulatory functions of the human IL-2 receptor (IL-2R) were reconstituted in transiently transfected hepatoma cells. The combination of IL-2R beta and -gamma mediated a strong stimulation via the cytokine response element of the alpha 1-acid glycoprotein gene and the hematopoietin receptor response element, but none via the IL-6 response element or the sis-inducible element. IL-2R alpha enhanced 10-fold the sensitivity of the IL-2R beta.gamma complex to respond to IL-2 or IL-15, but did not modify the specificity or the magnitude of maximal gene regulation. A homodimerizing chimeric receptor G-CSFR-IL-2R beta could mimic the IL-2R action. The IL-2R-mediated gene regulation was similar to that seen with receptors for IL-4 and IL-7, but differed from that for IL-6 type cytokines, thrombopoietin, erythropoietin, and growth hormone. The activation of STAT proteins by the IL-2R was assessed in transfected L-cells and COS-1 cells. Although IL-2R subunits were highly expressed in these cells, no STAT protein activation was detectable. Transient overexpression of JAK3 was unable to change the signaling specificity of the hematopoietin receptors in rat hepatoma, L-, and COS cells, but established a prominent activation of the IL-6 response elements by the IL-2R and IL-4R in HepG2 cells. The data support the model that the IL-2R and related hematopoietin receptors produce at least two separate signals which control gene expression.

Pathways from hematopoietic stem cells to thymocytes

A long-standing debate has been whether commitment to the T cell lineage occurs exclusively following thymus colonization, or whether prethymic T lineage restricted progenitors exist. Recently, the analysis of murine fetal blood for the presence of hematopoietic progenitor cells has led to the identification of a T lineage committed precursor population (designated prothymocytes). Fetal blood prothymocytes lack multipotent progenitor potential as shown by the fact that they fail to reconstitute B lymphocyte, myeloid and erythroid lineages. In addition to prothymocytes, fetal blood also contains a phenotypically distinct, pluripotent progenitor population which can reconstitute both T and B lymphocytes as well as myeloid and erythroid lineages. The identification of a circulating, T lineage restricted precursor population, which is also found in the blood of fetal athymic mice, provides strong evidence that T lineage commitment can precede thymus colonization. The thymus is not, however, exclusively colonized by prothymocytes. Under appropriate developmental conditions, multipotent precursor activity for non-T lineages such as B lymphocytes and thymic dendritic cells can be revealed within the intrathymic precursor pool. Moreover, evidence has been accumulated for a common progenitor for T cells and natural killer cells which may be distinct from multipotent intrathymic progenitors.