ABSTRACT
Interleukin-7 represents an exceptional and multi-faceted cytokine. It is an exceptional cytokine, since it mediates effective lymphopoiesis in mice in a nonredundant fashion. Targeted gene deletion of other cytokines, such as IL-2, IL-4, or IL-10 revealed that these molecules are not required for the initial development and function of immune cells in mice [1-3]. In contrast, IL-7 gene deletion leads to severe lymphopenia in mice [4]. IL-7 also has a somewhat greater complexity since it is secreted, in contrast to IL-2, or IL-4, by both immune and non-immune cells including stromal cell lines [5], B-cells [6], monocytes/macrophages [7], follicular dendritic cells [8], keratinocytes [9-11], and gut epithelial cells [12-14]. Which cells are capable of secreting significant amounts of IL-7 in vivo now needs to be reexamined. Loco-regional IL-7 secretion may contribute to the paracrine IL-7 mediated effects and account for the defects observed in IL-7 knock-out mice.
