Interleukin 6(IL-6) is a multifunctional protein produced by lymphoid and non-lymphoid cells and by normal and transformed cells, including T cells, monocyte/macrophages, fibroblasts, hepatocytes, vascular endothelial cells, cardiac myxomas, bladder cell carcinomas, myelomas, astrogliomas and glioblastomas. The production of IL-6 in these cells is regulated, either positively or negatively, by a variety of signals including mitogens, antigenic stimulation, lipopolysaccharides, IL-1, TNF, PDGF and viruses. For reviews on IL-6, see references 1-5.

The human IL-6 cDNA sequence predicts a protein of 212 amino acid (aa) residues in length with two potential N-glycosylation sites. The hydrophobic N-terminal 28 aa residue signal peptide is cleaved to produce a mature protein of 184 amino acids with four cysteine residues and a predicted molecular mass of 21 kDa (6-9). The mouse IL-6 cDNA sequence shows a homology of 42% at the aa level when compared with the human sequence (10). On the basis of sequence similarity and gene structural motif similarity, IL-6 can be grouped in a family of cytokines that also includes OSM, G-CSF, LIF, and CNTF. All of these cytokines are predicted to have a four helix bundle structure similar to that found for growth hormone, suggesting that they all evolved from a common ancestral gene (11-13).

The effects of IL-6 on different cells are numerous and varied. The effect on B cells is stimulation of differentiation and antibody secretion (6, 14 - 17). IL-6 also affects T cells, acting as a co-stimulant with sub-optimal concentrations of PHA or Con A to stimulate IL-2 production and IL-2 receptor expression.

IL-6 exhibits growth factor activity for mature thymic or peripheral T-cells and reportedly enhances the differentiation of cytotoxic T-cells in the presence of IL-2 or IFN-γ (18-20). IL-6 stimulates production of acute phase proteins by hepatocytes (21) and has colony-stimulating activity on hematopoietic stem cells. IL-6 has growth factor activities and will stimulate the growth of myeloma/hybridoma/ plasmacytoma cells, EBV-transformed B cells, keratinocytes and mesangial cells (4, 5).