Chemokine receptors are G protein-coupled receptors containing 7 transmembrane domains that are found on the surface of leukocytes. Chemokines function by activating specific G protein–coupled receptors, which results in, among other functions, the migration of inflammatory and noninflammatory cells to the appropriate tissues or compartments within tissues. Approximately 19 different chemokine receptors have been characterized to date, which are divided into four families depending on the type of chemokine they bind; CXCR that bind CXC chemokines, CCR that bind CC chemokines, CX3CR1 that binds the sole CX3C chemokine (CX3CL1), and XCR1 that binds the two XC chemokines (XCL1 and XCL2).
This illustration explains the ligand-binding patterns of the seven-transmembrane domain G-protein-coupled human chemokine receptors. Receptors CXCR1–CXCR3, CCR1–CCR5, CCR7, CCR8, CCR10 and XCR1 all bind several chemokines. By contrast, CCR6, CCR9, CX3CR1 and CXCR4–CXCR6 bind only one ligand each. Duffy and D6 are considered to be 'deceptors', as they bind ligands but do not signal, thereby acting as a negative feedback for chemokine responses.
Chemokine receptors share many structural features; they are similar in size (with about 350 amino acids), have a short, acidic N-terminal end, seven helical transmembrane domains with three intracellular and three extracellular hydrophilic loops, and an intracellular C-terminus containing serine and threonine residues important for receptor regulation. The first two extracellular loops of chemokine receptors each has a conserved cysteine residue that allow formation of a disulfide bridge between these loops. G proteins are coupled to the C-terminal end of the chemokine receptor to allow intracellular signaling after receptor activation, while the N-terminal domain of the chemokine receptor determines ligand binding specificity.
Chemokine receptors have a key role in the pathogenesis of autoimmune diseases, inflammation and viral infection. However, with the exception of selective CCR5 antagonists for HIV, the promise of obtaining new therapeutics related to chemokine receptors has not yet been realized. Some of the failures have occurred in recent years in the clinical trials of chemokine receptor antagonists. Reasons include the lack of predictability of animal models and redundancy of the target. A potential solution could be to develop drugs that target more than one receptor--known as polypharmacology--which could be a novel way to generate effective therapeutics.
Chemokine receptors that have been shown to play an import pathophysiological role in the liver are CCR1, CCR2, CCR5, D6, CXCR2 and CXCR3. Interestingly, most of these receptors have already been targeted by specific antagonists in early human trials and a CCR5 antagonist is already licensed for use in HIV infection. Most of these trials have been performed in autoimmune and infectious human diseases, but no controlled clinical trials have yet been performed in patients with liver diseases. Nevertheless, in light of growing evidence that chemokines are important mediators of liver damage, these trials seem to be on the clinical horizon.