Ficolins are pattern recognition receptors that bind to acetyl groups present in the carbohydrates of bacterial surfaces and mediate activation of the lectin pathway of the complement cascade. [1]
Structure
Ficolins (Fi+Col+Lin) are a group of oligomeric lectins with N-terminal collagen-like domain and a C-terminal fibrinogen-like domain. The primary ficolin structure contains 288 amino acids. The combination of collagen-like and fibrinogen-like domain allows the protein to form a basic subunit containing a triple helical tail and a trio of globular heads. [2]
Ficolins are produced in the liver by hepatocytes and in the lung by alveolar cells type II, neutrophils and monocytes. [3]
Role in innate immunity
We now know that innate immune recognition mechanisms are sophisticated. Exocrine secretions provide a variety of soluble factors that are able to protect the body from potential pathogens. [4]
Together with pentraxins, collectins and C1q molecules, ficolins constitute the soluble pattern-recognition molecules (PRMs) which play an important role in humoral innate immunity. [4] Ficolins recognise carbohydrate structures on pathogens' surfaces as their pathogen-associated molecular pattern (PAMP) and activate the lectin pathway of the complement cascade. [3] [5] Specifically, ficolins bind to acetyl groups present in certain bacterial molecules, such as N-acetylglucosamine, a component of peptidoglycan in the bacterial cell wall. [1] [6] When ficolins bind to their PAMP ligands by their C-terminal fibrinogen-like domain, [1] they initiate the proteolytic complement cascade, facilitated by the mannose-binding protein-associated serine proteases (MASPs) that ficolins are associated to and co-circulate with. [1] [6] Serine proteases then cleave a number of soluble complement proteins leading to complement activation, opsonisation, generation of proinflammatory mediators, and cell lysis. [7]
Collectins and ficolins are also called collagenous lectins. The collectin family constitutes calcium-dependent proteins. In contrast, the ficolin family does not bind to PAMPs in a calcium-dependent way. [3]
Types of ficolin
Three ficolins have been identified in humans:
- M-ficolin (FCN1), monocyte ficolin
- L-ficolin (FCN2), liver ficolin
- H-ficolin (FCN3), hakata antigen. [2] [3]
Ficolin-1 and ficolin-2 are encoded be a gene localised on chromosome 9 (9q34) and they share approximately 80% identity in amino sequence. Whereas, ficolin-3 is encoded by chromosome 1 and therefore it has only about 50% identity with the other two ficolins. [2] A cross-reactivity of the ficolins in human serum has been observed. [7]
Clinical references
The concentration of ficolins in healthy serum is between 3 and 5 μg/mL. [2]
As Ficolin-2 and 3 are expressed by hepatocytes, their levels decrease in advanced liver diseases like cirrhosis. Low ficolin levels contribute to cirrhosis-associated immune dysfunction. [8]
Immunologist Jeak L. Ding and her team found that natural IgG (nIgG; a non-specific immunoglobulin of adaptive immunity) is not quiescent, but plays a crucial role in immediate immune defense by collaborating with ficolin (an innate immune protein). [9]
References
- ^ a b c d Merle, Nicolas S.; Church, Sarah Elizabeth; Fremeaux-Bacchi, Veronique; Roumenina, Lubka T. (2015). "Complement System Part I - Molecular Mechanisms of Activation and Regulation". Frontiers in Immunology. 6: 262. doi: 10.3389/fimmu.2015.00262. PMC 4451739. PMID 26082779. 262.
- ^ a b c d Kilpatrick, David C.; Chalmers, James D. (2012). "Human L-Ficolin (Ficolin-2) and Its Clinical Significance". Journal of Biomedicine and Biotechnology. 2012: 138797. doi: 10.1155/2012/138797. ISSN 1110-7243. PMC 3303570. PMID 22500076.
- ^ a b c d Matsushita, Misao (2018). "Chapter 5 - Ficolins". In Barnum, Scott R.; Schein, Theresa N. (eds.). The Complement FactsBook (Second ed.). Elsevier. pp. 45–56. doi: 10.1016/B978-0-12-810420-0.00005-5. ISBN 978-0-12-810420-0.
- ^ a b Hajishengallis, George; Russell, Michael W. (2015). "Chapter 15 - Innate Humoral Defense Factors". In Mestecky, Jiri; Strober, Warren; Russell, Michael W.; Kelsall, Brian L.; Cheroutre, Hilde; Lambrecht, Bart N. (eds.). Mucosal Immunology (Fourth ed.). Elsevier. pp. 251–270. doi: 10.1016/b978-0-12-415847-4.00015-x. ISBN 978-0-12-415847-4.
- ^ Endo, Yuichi; Matsushita, Misao; Fujita, Teizo (June 2007). "Role of ficolin in innate immunity and its molecular basis". Immunobiology. 212 (4–5): 371–379. doi: 10.1016/j.imbio.2006.11.014. ISSN 0171-2985. PMID 17544822.
- ^ a b Krarup, Anders; Thiel, Steffen; Hansen, Annette; Fujita, Teizo; Jensenius, Jens C. (2004). "L-ficolin Is a Pattern Recognition Molecule Specific for Acetyl Groups". Journal of Biological Chemistry. 279 (46): 47513–47519. doi: 10.1074/jbc.M407161200. PMID 15331601.
- ^ a b Jarlhelt, Ida; Pilely, Katrine; Clausen, Jytte Bryde; Skjoedt, Mikkel-Ole; Bayarri-Olmos, Rafael; Garred, Peter (2020-02-24). "Circulating Ficolin-2 and Ficolin-3 Form Heterocomplexes". The Journal of Immunology. 204 (7): 1919–1928. doi: 10.4049/jimmunol.1900694. ISSN 0022-1767. PMID 32094208. S2CID 211477247.
- ^ Foldi, Ildiko; Tornai, Tamas; Tornai, David; Sipeki, Nora; Vitalis, Zsuzsanna; Tornai, Istvan; Dinya, Tamas; Antal-Szalmas, Peter; Papp, Maria (2017). "Lectin-complement pathway molecules are decreased in patients with cirrhosis and constitute the risk of bacterial infections". Liver International. 37 (7): 1023–1031. doi: 10.1111/liv.13368. hdl: 2437/234045. ISSN 1478-3231. PMID 28109038. S2CID 4724419.
- ^ Panda, Saswati; Ding, Jeak L. (2015). "Natural Antibodies Bridge Innate and Adaptive Immunity". The Journal of Immunology. 194 (1): 13–20. doi: 10.4049/jimmunol.1400844. PMID 25527792.