Cytokines and Chemokines

Chemokines belong to a family of structurally related, low molecular weight (8 to 10 kDa), glycoproteins of approximately 70 to 80 amino acid residues. Most chemokines fall into two subfamilies (a) CXC chemokines, contain a single amino acid between the first and second cysteine residues; (b) CC chemokines that have adjacent cysteine residues. Most CXC chemokines are chemoattractants for neutrophils whereas CC chemokines generally attract monocytes, lymphocytes, basophils, and eosinophils. Within the CXC subfamily, the chemokines can be further divided into (a) CXC chemokines with characteristic ELR sequence (glutamic acid-leucine-arginine) immediately preceding the first cysteine residue near the amino terminus, and (b) CXC chemokines lacking the ELR domain.

The cytokine family of signaling molecules includes several interleukins, a variety of growth and colony-stimulating factors, ciliary neurotrophic factor, interferons, and several other molecules that exhibit pleiotropic effects on cell differentiation, tissue development and homeostasis. Cytokines share many properties with hormones and growth factors. Cytokines mediate communication among cells in the immune system through binding to specific receptors on target cells. Their biological actions vary widely depending upon the type of target tissue involved. They are endowed with anti-proliferative properties and regulate the synthesis of acute phase proteins following tissue injury, trauma, inflammation, and sepsis. The receptors for a large number of cytokines have been cloned and shown to be membrane-spanning glycoproteins with their amino termini in the extracellular space. Unlike receptors for growth factors, cytokine receptors generally lack identifiable catalytic activity. The major diagnostic feature of the ‘cytokine’ receptors is the presence, in the extracellular region of the receptor, of a domain containing multiple cysteine residues and a conserved amino acid motif, WSXWS (Trp-Ser-X-Trp-Ser) that functions in the recognition and binding of the ligand.