Chemokines, a subgroup of cytokines, are small (size between 8 and 10 kDa), chemotactically active proteins (signal proteins). They are common in all vertebrates, in some virus species and bacteria. In humans, about 50 chemokines are currently known. A highly conserved structural feature of all chemokines is a fixed group of cysteine residues stabilized by 1 or 2 disulfide bridges. This key structural position in the molecule is responsible for their fixed 3-dimensional structure (see Chemokines below).
In CC chemokines the cysteines follow each other directly, in CXC chemokines (see figure) they are separated by 1, in CXXXC chemokines by 3 other amino acids. Chemokines are produced and secreted by a variety of immune cells. They mediate their signals by means of specific chemokine receptors via G-proteins. Some chemokines have proinflammatory effects, while others have regulatory effects in tissue formation and homeostasis.
Increased chemokine expression is found in chronic inflammatory diseases such as HIV infection, atopic eczema, bronchial asthma, allergic rhinitis and psoriasis vulgaris. Chemokines play a positive role, for example, in wound healing, haematopoiesis (blood formation) or the defence against infections. The fact that chemokine receptors are not only present on inflammatory cells but also on tumor cells and endothelial cells suggests that they are also involved in the migration of tumor cells or the metastatic behavior of the various tumors.