This study reports the cloning and characterization of human interleukin-17 (hIL-17), a cytokine produced by activated memory CD4+ T cells. hIL-17 is a glycoprotein of 155 amino acids secreted as a homodimer. It induces stromal cells to produce proinflammatory and hematopoietic cytokines such as IL-6, IL-8, and granulocyte-colony-stimulating factor (G-CSF), as well as prostaglandin E2. When cultured in the presence of hIL-17, fibroblasts can sustain the proliferation of CD34+ hematopoietic progenitors and their preferential maturation into neutrophils. These findings suggest that hIL-17 may be an early initiator of T cell-dependent inflammatory reactions and a component of the cytokine network that connects the immune system to hematopoiesis. T lymphocytes produce cytokines that are involved in cell growth, inflammation, immunity, differentiation, and repair. These cytokines are not produced constitutively by T cells but are induced after T cell activation. The human counterpart of murine IL-17, hIL-17, is a cytokine produced only by activated memory T cells that induces stromal cells to secrete cytokines involved in inflammatory and hematopoietic processes. hIL-17 is secreted by activated CD4+ T cells as a mixture of glycosylated and nonglycosylated homodimers. It induces the secretion of IL-6, IL-8, PGE2, and G-CSF from synovial fibroblasts. The effects of hIL-17 on cytokine production are specific and can be blocked by an anti-hIL-17 mAb. hIL-17 also induces the secretion of IL-6 and GM-CSF in combination with TNF-α or IFN-γ. hIL-17 is expressed mainly in activated CD4+ memory T cells and is not expressed in resting T cells or other cell types. hIL-17 induces the growth and differentiation of CD34+ hematopoietic progenitors when cultured with synoviocytes. The results suggest that hIL-17 plays a role in hematopoiesis by inducing the secretion of cytokines such as IL-6 and G-CSF, which promote the production of neutrophils. The unique cellular source of hIL-17 and its ability to induce the secretion of IL-6, IL-8, and G-CSF by stromal cells may be tailored to contribute to the inducible hematopoiesis observed after antigenic stimulation. The study also highlights the potential role of hIL-17 in the immune response and hematopoiesis, and suggests that its absence may lead to reduced enThis study reports the cloning and characterization of human interleukin-17 (hIL-17), a cytokine produced by activated memory CD4+ T cells. hIL-17 is a glycoprotein of 155 amino acids secreted as a homodimer. It induces stromal cells to produce proinflammatory and hematopoietic cytokines such as IL-6, IL-8, and granulocyte-colony-stimulating factor (G-CSF), as well as prostaglandin E2. When cultured in the presence of hIL-17, fibroblasts can sustain the proliferation of CD34+ hematopoietic progenitors and their preferential maturation into neutrophils. These findings suggest that hIL-17 may be an early initiator of T cell-dependent inflammatory reactions and a component of the cytokine network that connects the immune system to hematopoiesis. T lymphocytes produce cytokines that are involved in cell growth, inflammation, immunity, differentiation, and repair. These cytokines are not produced constitutively by T cells but are induced after T cell activation. The human counterpart of murine IL-17, hIL-17, is a cytokine produced only by activated memory T cells that induces stromal cells to secrete cytokines involved in inflammatory and hematopoietic processes. hIL-17 is secreted by activated CD4+ T cells as a mixture of glycosylated and nonglycosylated homodimers. It induces the secretion of IL-6, IL-8, PGE2, and G-CSF from synovial fibroblasts. The effects of hIL-17 on cytokine production are specific and can be blocked by an anti-hIL-17 mAb. hIL-17 also induces the secretion of IL-6 and GM-CSF in combination with TNF-α or IFN-γ. hIL-17 is expressed mainly in activated CD4+ memory T cells and is not expressed in resting T cells or other cell types. hIL-17 induces the growth and differentiation of CD34+ hematopoietic progenitors when cultured with synoviocytes. The results suggest that hIL-17 plays a role in hematopoiesis by inducing the secretion of cytokines such as IL-6 and G-CSF, which promote the production of neutrophils. The unique cellular source of hIL-17 and its ability to induce the secretion of IL-6, IL-8, and G-CSF by stromal cells may be tailored to contribute to the inducible hematopoiesis observed after antigenic stimulation. The study also highlights the potential role of hIL-17 in the immune response and hematopoiesis, and suggests that its absence may lead to reduced en