Interleukin 6 (IL-6) is a pleiotropic growth factor with a wide range of biological activities in immune regulation, hematopoiesis, and oncogenesis. IL-6 is produced by a variety of cell types including T cells, B cells, monocytes and macrophages, fibroblasts, hepatocytes, vascular endothelial cells, and various tumor cell lines. On its own or in combination with other factors such as IL-2 and interferon-γ, IL-6 stimulates the proliferation of B cells, T cells, and hybridoma cells (Nordan et al.; Van Snick et al.; Gauldie et al.; Mihara et al.; Tanaka et al). In combination with cytokines such as IL-3, GM-CSF, and SCF, IL-6 has been shown to promote hematopoietic progenitor cell proliferation and differentiation in vitro. IL-6 signals through a cell surface type I cytokine receptor complex consisting of the ligand-binding IL-6α (CD126) and the signal-transducing gp130 subunits. The binding of IL-6 to its receptor system includes activation of the JAK/STAT signaling pathway (Mihara et al.; Peters et al; Tanaka et al.).