Lectin pathway

Last updated June 09, 2024

The lectin pathway or MBL pathway is a type of cascade reaction in the complement system, similar in structure to the classical complement pathway,^[1] in that, after activation, it proceeds through the action of C4 and C2 to produce activated complement proteins further down the cascade. In contrast to the classical complement pathway, the lectin pathway does not recognize an antibody bound to its target. The lectin pathway starts with mannose-binding lectin (MBL) or ficolin binding to certain sugars.

In this pathway, mannose-binding lectin binds to mannose, glucose, or other sugars with 3- and 4-OH groups placed in the equatorial plane, in terminal positions on carbohydrate or glycoprotein components of microorganisms including bacteria such as Salmonella , Listeria , and Neisseria strains. Fungal pathogens such as Candida albicans and Cryptococcus neoformans as well as some viruses such as HIV-1 and Respiratory syncytial virus (RSV) are bound by MBL.

Mannan-binding lectin, also called mannose-binding protein, is a protein belonging to the collectin family that is produced by the liver and can initiate the complement cascade by binding to pathogen surfaces.

MBL

MBL forms oligomers of subunits, which are trimers (such that 6- and 18-subunit oligomers correspond to a dimer and a hexamer, respectively). Multimers of MBL form a complex with MASP1 (Mannose-binding lectin-Associated Serine Protease), MASP2 and MASP3, that are protease zymogens. The MASPs are very similar to C1r and C1s molecules of the classical complement pathway, respectively. When the carbohydrate-recognising heads of MBL bind to specifically arranged mannose residues on the surface of a pathogen, MASP-1 and MASP-2 are activated to cleave complement components C4 and C2 into C4a, C4b, C2a, and C2b. In f, two smaller MBL-associated proteins (MAps) are found in complex with MBL. MBL-associated protein of 19 kDa (MAp19) and MBL-associated protein of 44 kDa (Map44). MASP-1, MASP-3 and MAp44 are alternative splice products of the MASP1 gene, while MASP-2 and MAp19 are alternative splice products of the MASP-2 gene. MAp44 has been suggested to act as a competitive inhibitor of lectin pathway activation, by displacing MASP-2 from MBL, hence preventing cleavage of C4 and C2 ^[2]

C3 convertase

C4b tends to bind to bacterial cell membranes. If it is not then inactivated, it will combine with C2a to form the classical C3 convertase (C4bC2a) on the surface of the pathogen, as opposed to the alternative C3 convertase (C3bBb) involved in the alternative pathway. C4a and C2b act as potent cytokines, with C4a causing degranulation of mast cells and basophils and C2b acting to increase vascular permeability.^[3] Historically, the larger fragment of C2 was called C2a but some publications now refer to it as C2b in keeping with the convention of assigning 'b' to the larger fragment.^[4]

Clinical significance

Mannose-binding Lectin deficiency - These individuals are prone to recurrent infections, including infections of the upper respiratory tract and other body systems. People with this condition may also contract more serious infections such as pneumonia and meningitis. Depending on the type of infection, the symptoms caused by the infections vary in frequency and severity.^[5] Although the clinical significance of MBL-Deficiency is debated.^[6]

Infants and young children with mannose-binding lectin deficiency seem to be more susceptible to infections, but adults can also develop recurrent infections. In addition, affected individuals undergoing chemotherapy or taking drugs that suppress the immune system are especially prone to infections.^[5]

Related Research Articles

The complement system, also known as complement cascade, is a part of the humoral, innate immune system and enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen's cell membrane. Despite being part of the innate immune system, the complement system can be recruited and brought into action by antibodies generated by the adaptive immune system.

The classical complement pathway is one of three pathways which activate the complement system, which is part of the immune system. The classical complement pathway is initiated by antigen-antibody complexes with the antibody isotypes IgG and IgM.

<span class="mw-page-title-main">C3-convertase</span>

C3 convertase belongs to family of serine proteases and is necessary in innate immunity as a part of the complement system which eventuate in opsonisation of particles, release of inflammatory peptides, C5 convertase formation and cell lysis.

Pattern recognition receptors (PRRs) play a crucial role in the proper function of the innate immune system. PRRs are germline-encoded host sensors, which detect molecules typical for the pathogens. They are proteins expressed mainly by cells of the innate immune system, such as dendritic cells, macrophages, monocytes, neutrophils, as well as by epithelial cells, to identify two classes of molecules: pathogen-associated molecular patterns (PAMPs), which are associated with microbial pathogens, and damage-associated molecular patterns (DAMPs), which are associated with components of host's cells that are released during cell damage or death. They are also called primitive pattern recognition receptors because they evolved before other parts of the immune system, particularly before adaptive immunity. PRRs also mediate the initiation of antigen-specific adaptive immune response and release of inflammatory cytokines.

Complement component 3, often simply called C3, is a protein of the immune system that is found primarily in the blood. It plays a central role in the complement system of vertebrate animals and contributes to innate immunity. In humans it is encoded on chromosome 19 by a gene called C3.

<span class="mw-page-title-main">C5-convertase</span> Serine protease that plays key role in innate immunity.

C5 convertase is an enzyme belonging to a family of serine proteases that play key role in the innate immunity. It participates in the complement system ending with cell death.

Complement C2 is a protein that in humans is encoded by the C2 gene. The protein encoded by this gene is part of the classical pathway of the complement system, acting as a multi-domain serine protease. Deficiency of C2 has been associated with certain autoimmune diseases.

Mannan-binding lectin serine protease 1 also known as mannose-associated serine protease 1 (MASP-1) is an enzyme that in humans is encoded by the MASP1 gene.

Mannan-binding lectin serine protease 2 also known as mannose-binding protein-associated serine protease 2 (MASP-2) is an enzyme that in humans is encoded by the MASP2 gene.

C4b-binding protein (C4BP) is a protein complex involved in the complement system where it acts as inhibitor. C4BP has an octopus-like structure with a central stalk and seven branching alpha-chains. The main form of C4BP in human blood is composed of 7 identical alpha-chains and one unique beta-chain, which in turn binds anticoagulant, vitamin K-dependent protein S.

Complement decay-accelerating factor, also known as CD55 or DAF, is a protein that, in humans, is encoded by the CD55 gene.

Collectins (collagen-containing C-type lectins) are a part of the innate immune system. They form a family of collagenous Ca²⁺-dependent defense lectins, which are found in animals. Collectins are soluble pattern recognition receptors (PRRs). Their function is to bind to oligosaccharide structure or lipids that are on the surface of microorganisms. Like other PRRs they bind pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) of oligosaccharide origin. Binding of collectins to microorganisms may trigger elimination of microorganisms by aggregation, complement activation, opsonization, activation of phagocytosis, or inhibition of microbial growth. Other functions of collectins are modulation of inflammatory, allergic responses, adaptive immune system and clearance of apoptotic cells.

Mannose-binding lectin (MBL), also called mannan-binding lectin or mannan-binding protein (MBP), is a lectin that is instrumental in innate immunity as an opsonin and via the lectin pathway.

Complement component 4 (C4), in humans, is a protein involved in the intricate complement system, originating from the human leukocyte antigen (HLA) system. It serves a number of critical functions in immunity, tolerance, and autoimmunity with the other numerous components. Furthermore, it is a crucial factor in connecting the recognition pathways of the overall system instigated by antibody-antigen (Ab-Ag) complexes to the other effector proteins of the innate immune response. For example, the severity of a dysfunctional complement system can lead to fatal diseases and infections. Complex variations of it can also lead to schizophrenia. The C4 protein was thought to derive from a simple two-locus allelic model, which however has been replaced by a much more sophisticated multimodular RCCX gene complex model which contain long and short forms of the C4A or C4B genes usually in tandem RCCX cassettes with copy number variation, that somewhat parallels variation in the levels of their respective proteins within a population along with CYP21 in some cases depending on the number of cassettes and whether it contains the functional gene instead of pseudogenes or fragments. Originally defined in the context of the Chido/Rodgers blood group system, the C4A-C4B genetic model is under investigation for its possible role in schizophrenia risk and development.

C3b is the larger of two elements formed by the cleavage of complement component 3, and is considered an important part of the innate immune system. C3b is potent in opsonization: tagging pathogens, immune complexes (antigen-antibody), and apoptotic cells for phagocytosis. Additionally, C3b plays a role in forming a C3 convertase when bound to Factor B, or a C5 convertase when bound to C4b and C2b or when an additional C3b molecule binds to the C3bBb complex.

C3a is one of the proteins formed by the cleavage of complement component 3; the other is C3b. C3a is a 77 residue anaphylatoxin that binds to the C3a receptor (C3aR), a class A G protein-coupled receptor. It plays a large role in the immune response.

MBL deficiency or mannose-binding lectin deficiency is an illness that has an impact on immunity. Low levels of mannose-binding lectin, an immune system protein, are present in the blood of those who have this illness. It's unclear if this deficiency increases the risk of recurrent infections in those who are affected.

Mannose-binding lectin-associated protein of 44 kDa (MAp44) is a protein arising from the human MASP1 gene. MASP-1, MASP-3 and MAp44 are alternative splice products of the MASP1 gene. MAp44 has been suggested to act as a competitive inhibitor of lectin pathway activation, by displacing MASP-2 from MBL, hence preventing cleavage of C4 and C2

Mannan-binding lectin-associated serine protease-2 is an enzyme. This enzyme catalyses the following chemical reaction

The C1 complex is a protein complex involved in the complement system. It is the first component of the classical complement pathway and is composed of the subcomponents C1q, C1r and C1s.

References

↑ Wallis R, Mitchell DA, Schmid R, Schwaeble WJ, Keeble AH (2010). "Paths reunited: Initiation of the classical and lectin pathways of complement activation". Immunobiology. 215 (1): 1–11. doi:10.1016/j.imbio.2009.08.006. PMC 2824237 . PMID 19783065.
↑ Degn, Søren; Annette G. Hansen; Rudi Steffensen; Christian Jacobsen; Jens C. Jensenius; Steffen Thiel (November 2009). "MAp44, a Human Protein Associated with Pattern Recognition Molecules of the Complement System and Regulating the Lectin Pathway of Complement Activation". Journal of Immunology. 183 (11): 7371–7378. doi: 10.4049/jimmunol.0902388 . PMID 19917686.
↑ Stanley, Jacqueline (1 January 2002). Essentials of Immunology & Serology. Cengage Learning. p. 103. ISBN 978-0766810648.
↑ First Aid for the USMLE Step 1 2015
1 2 "Mannose-binding lectin deficiency". Genetics Home Reference. US National Library of Medicine. Retrieved 23 October 2016. This article incorporates text from this source, which is in the public domain .
↑ Bradley, D. T.; Bourke, T. W.; Fairley, D. J.; Borrow, R.; Shields, M. D.; Young, I. S.; Zipfel, P. F.; Hughes, A. E. (August 2012). "Genetic susceptibility to invasive meningococcal disease: MBL2 structural polymorphisms revisited in a large case-control study and a systematic review". International Journal of Immunogenetics. 39 (4): 328–337. doi:10.1111/j.1744-313X.2012.01095.x. PMID 22296677. S2CID 205900750.

External links

https://ghr.nlm.nih.gov/condition/mannose-binding-lectin-deficiency#diagnosis
Ali, Youssif M.; Lynch, Nicholas J.; Haleem, Kashif S.; Fujita, Teizo; Endo, Yuichi; Hansen, Soren; Holmskov, Uffe; Takahashi, Kazue; Stahl, Gregory L.; Dudler, Thomas; Girija, Umakhanth V.; Wallis, Russell; Kadioglu, Aras; Stover, Cordula M.; Andrew, Peter W.; Schwaeble, Wilhelm J. (5 July 2012). "The Lectin Pathway of Complement Activation Is a Critical Component of the Innate Immune Response to Pneumococcal Infection". PLOS Pathogens. 8 (7): e1002793. doi: 10.1371/journal.ppat.1002793 . PMC 3390405 . PMID 22792067.

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[pmid19783065-1] Wallis R, Mitchell DA, Schmid R, Schwaeble WJ, Keeble AH (2010). "Paths reunited: Initiation of the classical and lectin pathways of complement activation". Immunobiology. 215 (1): 1–11. doi:10.1016/j.imbio.2009.08.006. PMC 2824237 . PMID 19783065.

[2] Degn, Søren; Annette G. Hansen; Rudi Steffensen; Christian Jacobsen; Jens C. Jensenius; Steffen Thiel (November 2009). "MAp44, a Human Protein Associated with Pattern Recognition Molecules of the Complement System and Regulating the Lectin Pathway of Complement Activation". Journal of Immunology. 183 (11): 7371–7378. doi: 10.4049/jimmunol.0902388 . PMID 19917686.

[3] Stanley, Jacqueline (1 January 2002). Essentials of Immunology & Serology. Cengage Learning. p. 103. ISBN 978-0766810648.

[4] First Aid for the USMLE Step 1 2015

[NIH-5] 1 2 "Mannose-binding lectin deficiency". Genetics Home Reference. US National Library of Medicine. Retrieved 23 October 2016. This article incorporates text from this source, which is in the public domain .

[6] Bradley, D. T.; Bourke, T. W.; Fairley, D. J.; Borrow, R.; Shields, M. D.; Young, I. S.; Zipfel, P. F.; Hughes, A. E. (August 2012). "Genetic susceptibility to invasive meningococcal disease: MBL2 structural polymorphisms revisited in a large case-control study and a systematic review". International Journal of Immunogenetics. 39 (4): 328–337. doi:10.1111/j.1744-313X.2012.01095.x. PMID 22296677. S2CID 205900750.

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