CCL18

Chemokine (C-C motif) ligand 18 (CCL18) is a small cytokine belonging to the CC chemokine family. It was previously known as Pulmonary and activation-regulated chemokine (PARC), dendritic cell (DC)-chemokine 1 (DC-CK1), alternative macrophage activation-associated CC chemokine-1 (AMAC-1), and macrophage inflammatory protein-4 (MIP-4). The effector functions that CCL18 exerts on the is_associated_with::immune system is mainly on the is_associated_with::adaptive immune system, while being produced and secreted mainly by is_associated_with::innate immune system. Although the functions of CCL18 have been well studied is_associated_with::in vitro, physiological effects of CCL18 is_associated_with::in vivo have been difficult to characterize because there is no is_associated_with::murine analog of CCL18. Only recently has the receptor for CCL18 been identified, and further characterization of CCL18 interactions with its receptor will lead to better understanding of the physiological role of CCL18 in humans.

Gene and protein structure
The gene of CCL18 is most similar to is_associated_with::CCL3. CCL18 is located on Chromosome 17, along with many other is_associated_with::macrophage inflammatory proteins (MIP). The gene itself has 3 is_associated_with::exons and 2 is_associated_with::introns; but, unlike other is_associated_with::chemokines, CCL18 includes 2 pseudo-exons (exons that do not appear in the final peptide) in the first intron. Because of these pseudo-exons, it is believed that CCL18 arose as a result of a gene fusion event between CCL3-like protein encoding genes and gained a different function over time due to accumulating is_associated_with::mutations. CCL18 is a 89 is_associated_with::peptide-long protein, with a 20 peptide signalling sequence (to signal its secretion) at the N’ terminus which is cleaved in the is_associated_with::endoplasmic reticulum into a 69 peptide mature protein.



Chemotactic functions
is_associated_with::Chemokines are classed as a special type of is_associated_with::cytokine that is involved in immune cell trafficking. CCL18 in particular has some chemotactic functions for the innate immune system, but its functions are primarily involved with recruitment of the adaptive immune system. CCL18 is involved in attracting naïve T-cells, is_associated_with::T-regulatory cells, T-helper 2 cells, both immunosuppressive and immature Dendritic Cells,  basophils, and B-cells (naïve and effector). The is_associated_with::T-regulatory cells that CCL18 attracts are not classical T-regulatory cells; these cells do not express is_associated_with::FoxP3 as most T-regulatory cells do, and instead non-is_associated_with::antigen specifically exert their immunosuppressive functions by secreting IL-10. It is thought that these recruited cells maintain homeostasis under healthy conditions.

Receptor
The classical receptors for chemokines are is_associated_with::G-protein coupled receptors (GPCRs), which have 7 is_associated_with::transmembrane regions. Following this trend, it was thought that CCL18’s receptor is also probably a GPCR. However, for a long time, the physiological receptor has not been found until very recently. To date, are three receptors that have been proposed for CCL18: PITPNM3, is_associated_with::GPR30, and is_associated_with::CCR8. PITPNM3 is a CCL18 receptor, but PITPNM3 is only expressed on is_associated_with::breast cancer cells and not on T-cells nor B-cells, and PITPNM3-CCL8 binding induces is_associated_with::Pyk2 and Src mediated signaling, a cancer related signaling pathway, and subsequent metastasis of breast cancer. GPR30 is also reported to bind to CCL18, but binding of CCL18 does not induce chemotaxis; instead, binding of CCL18 to GPR30 blocks both activation of GPR30 its natural is_associated_with::ligands and reduces the ability of is_associated_with::CXCL12-dependant activation of is_associated_with::acute lymphocytic leukemia B cells. CCR8 is the most recently discovered receptor for CCL18, and the effects of CCR8-CCL18 interactions appear to be physiological, as CCL18 binding to CCR8 induces chemotaxis of Th2 cells. Furthermore, CCL18 binding is competitive with CCR8’s previously described ligand, is_associated_with::CCL1, further suggesting that CCL18 binds physiologically with CCR8. ) Further elucidation of the role of CCR8 in CCL18-mediated is_associated_with::pathologies would allow for better understanding of CCL18’s function in these diseases.

Effector functions
CCL18 has a plethora of functions that have been characterized in vitro and in vivo. Strangely, CCL18 seems to play a part in both activation of the immune system and the induction of tolerance and homeostasis at steady-state conditions.

Immune activation
The production of CCL18 is induced by T-helper 2 type cytokines, namely IL-4 and IL-13. Coupled with the fact that CCL18 is highly expressed in patients with allergic asthma and other hypersensitivity diseases, CCL18 seems to play an important role for generating and maintaining a T-helper 2 (Th2) type response. Furthermore, the addition of CCL18 as an is_associated_with::adjuvant for a malaria vaccine have shown efficacy, perhaps by recruiting immune cells to the site of vaccination. Finally, CCL18 is expressed by dendritic cells in the is_associated_with::germinal center of inflamed is_associated_with::lymph nodes, and recruits naïve B-cells for is_associated_with::antigen presentation. Perhaps aberrant CCL18 expression is involved in the generation of chronic Th2 response, leading to asthma or arthritis.

Immunosuppression
In addition to immune-activating effects, CCL18 also has strong immunosuppressive effects. CCL18 induces immature dendritic cells to differentiate into an immunosuppressive dendritic cell that is capable producing CCL18 which attract T-cells, suppressing effector T-cell function, and generating T-regulatory cells by secreting large amounts of IL-10. Furthermore, exposure to CCL18 by macrophages causes them to mature in the #M2 spectrum, which promotes immunosuppression and healing.

Involvement in disease
Aberrant CCL18 expression is observed in many diseases, and it is thought that these abnormal expression patterns play a key role in these diseases. This table shows a list of all the diseases that CCL18 is involved in.

Breast cancer
The most understood disease that CCL18 is involved in is in breast cancer, where CCL18 induces is_associated_with::metastasis of breast cancer cells by binding to PITPNM3. Perhaps CCL18, in breast cancers, is acting as an immunosuppressive cytokine by generating T-regulatory cells, generating immunosuppressive dendritic cells and macrophages, and recruiting effector T-cells to these dendritic cells and macrophages to abolish their anti-cancer functions and allowing the cancer to escape the immune system.

Autoimmunity and hypersensitivity
CCL18 is highly expressed in T-helper 2 mediated is_associated_with::hypersensitivity and is_associated_with::autoimmune diseases, such as is_associated_with::asthma and is_associated_with::arthritis. CCL18 is expressed at much higher levels in allergic patients compared to healthy patients and respond aggressively to innocuous antigens. Allergic patients also had higher amounts of activated T-cells in the lungs, suggesting that CCL18 recruitment of these cells is contributing to hypersensitivity. In addition to lung hypersensitivities, these patterns were also observed in is_associated_with::dermatitis patients. Furthermore, a similar pattern was also observed in arthritis patients, where CCL18 was expressed at much higher rates by dendritic cells in affected patients. However, in arthritis, perhaps the increased CCL18 is an attempt to suppress effector T-helper 1 cells that are self-reactive.