B cell differentiation factor, BCDF, BSF-2, HPGF, HSF, MGI-2, B-cell stimulatory factor 2,Hybridoma growth factor, CTL differentiation factor, CDF, IL-6, HGF.

SDS

IL-6 is produced by many different cell types. The main sources in vivo are stimulated monocytes, fibroblasts, and endothelial cells. Macrophages, T-cells and B-lymphocytes, granulocytes, smooth muscle cells, eosinophils, chondrocytes, osteoblasts, mast cells, glial cells, and keratinocytes also produce IL6 after stimulation. Glioblastoma cells constitutively produce IL6 and the factor can be detected also in the cerebrospinal fluid. Human milk also contains IL6 (see: MGF, milk growth factor).

Cardiac myxomas and cervical and bladder carcinomas secrete large amounts of IL6 into the conditioned medium. Very high levels of IL6 are also detected in the synovial fluid of patients with rheumatoid arthritis but not with osteoarthritis.

Physiological stimuli for the synthesis of IL6 are IL1, bacterial endotoxins, TNF, PDGF, and Oncostatin M. In fibroblasts the synthesis of IL6 is stimulated by IFN-beta, TNF-alpha, PDGF, and viral infections. In thymic stromal cells the synthesis of IL6 can be induced by NGF. IL6 can also stimulate or inhibits its own synthesis, depending upon the cell type. For the constitutive production of IL6 see also: IAP (intracisternal A particles). In epithelial, endothelial, and fibroblastic cells secretion of IL6 is induced by IL17.

IL6 is produced also in the anterior lobe of the pituitary and can be induced by bacterial endotoxins and all substances elevating intracellular levels of cAMP (prostaglandin E2, forskolin, cholera toxin). VIP (vasoactive intestinal peptide) also stimulates the synthesis of IL6 in the anterior lobe of the pituitary while GHreleasing hormone is inactive. Macrolide antibiotics spiramycin stimulate the synthesis of IL6 in human monocytes following cell activation by bacterial lipopolysaccharides but not the synthesis of IL1 or TNF-alpha.

Glucocorticoids inhibit the synthesis of IL6 and IL4 or TGF-beta reduce its synthesis. The synthesis of IL6 by human alveolar macrophages is inhibited by IL4.

In serum IL6 is complexed with Alpha-2-Macroglobulin (Alpha2M) which protects IL6 from cleavage by proteases and possibly functions as a transport protein.

The release of IL6 from isolated rat macrophages in culture is stimulated by Histogranin.

IL6 is a protein of 185 amino acids glycosylated at positions 73 and 172. It is synthesized as a precursor protein of 212 amino acids. Monocytes express at least five different molecular forms of IL6 with molecular masses of 21.5-28 kDa. They mainly differ by post-translational alterations such as glycosylation and phosphorylation.

IL6 isolated from various cell types shows some microheterogeneity in its N-terminus. A 42-45 kDa form has been observed in plasma that is probably complexed with a carrier protein Alpha-2-Macroglobulin (Alpha2M) Murine and human IL6 show 65 % sequence homology at the DNA level and 42 % homology at the protein level. For a truncated form of IL6 with antagonist activities see: tIL6. IL6 is a member of a family of cytokines, which also includes LIF, CNTF, Oncostatin M, IL11, and CT-1. All known members of the IL6 cytokine family induce hepatic expression of acute phase proteins.

IL6-Delta-4 is an isoform constituting a splice variant of IL6 that lacks exon 4. The protein lacks two amino acids necessary for IL6 homodimerization and two of six amino acids required for interaction with IL6 receptor beta. This isoform forms a stable complex with IL6 receptor alpha. IL6-Delta-4 may compete with native IL6 for binding to the IL6 receptor alpha and regulate IL6 bioactivities because of its inability to transmit IL6 receptor beta-mediated signaling (Bihl et al, 2002).