The lectin pathway of complement activation is an important component of innate immunity. Its activation has been thought to occur via recognition of pathogens by mannan-binding lectin (MBL) or ficolins in complex with MBL-associated serine protease (MASP)-2, followed by MASP-2 autoactivation and cleavage of C4 and C2 generating the C3 convertase. MASP-1 has been shown to aid MASP-2 convertase generation by auxiliary C2 cleavage.
Mannan binding lectin serine protease 1, also known as mannose-associated serine protease 1 (MASP-1), is a serine protease that functions as a component of the lectin pathway of complement activation. The complement pathway plays an essential role in the innate and adaptive immune response. MASP-1 is synthesized as a zymogen and is activated when it complexes with the pathogen recognition molecules of lectin pathway, the mannose-binding lectin and the ficolins. MASP-1 is also able to cleave fibrinogen and factor XIII and may may be involved in coagulation.
Mannan binding lectin serine protease 2, also known as mannose-binding protein-associated serine protease 2 (MASP-2), is a member of the peptidase S1 family of serine proteases. MASP-2 cleaves complement components C2 and C4 in order to generate C3 convertase in the lectin pathway of the complement system. MASP-2 also plays a role in the coagulation cascade through cleavage of prothrombin to form thrombin. MASP-2 is very similar to the C1s molecule, of the classical complement pathway, and they are thought to have a common evolutionary ancestor. When the carbohydrate-recognising heads of MBL bind to specifically arranged mannose residues on the surface of a pathogen, MASP-2 is activated to cleave complement components C4 and C2 into C4a, C4b, C2a, and C2b.
MASP (MBL/ficolin-associated serine protease) can activate the complement. Three distinct MASP, MASP-1, MASP-2 and MASP-3 have been identified in many species of vertebrates. Although the contribution of MASP-2 in activation of complement was well defined, the substrates for MASP-1 and MASP-3 were still obscure. Minoru et al. generated MASP-1- and MASP-3-deficient mice (Masp1/3-/-) to verify roles of MASP-1 and MASP-3 proteases in vivo. One major finding is that MASP-1, considered being a lectin pathway component—also acts as a pro-factor D (Df) convertase, the initiator of the alternative pathway. Their results emphasize a unique feature of MASP-1, participating two complement pathways.
Banda et al. demonstrated that Masp1/3−/− mice are highly resistant to collagen Ab-induced arthritis (CAIA) as evidenced by a significant decrease in the histological scores as compared with WT mice. Recent studies supported that the alternative pathway is both necessary and sufficient to induce disease in murine collagen Ab-induced arthritis (CAIA). This model mouse confirmed that Masp1/3-/- shows the abnormality of the alternative pathway.
The complement system is part of the innate immune system and contributes to the establishment of adaptive immune responses. The mannan-binding lectin pathway of the complement system is activated when mannan-binding lectin binds to carbohydrate structures on microorganisms. This happens through autoactivation of the mannan-binding lectin–associated serine protease 2 (MASP-2), which then cleaves complement factors C4 and C2, generating the C3 convertase C4bC2b. Activation of C3 initiates the alternative pathway and the formation of the membrane-attack complex. Complement fragments deposited on microorganisms facilitate phagocytosis and initiate inflammatory reactions. Like mannan-binding lectin, the recognition molecules L-ficolin and H-ficolin activate complement by a MASP-2–dependent mechanism. Three other proteins are associated with mannan-binding lectin and ficolins: mannan-binding lectin–associated proteases 1 and 3 (MASP-1 and MASP-3), serine proteases of unknown function, and MAp19, a fragment of MASP-2, with four additional amino acid residues.
Mannose-binding lectin-associated serine protease-2 (MASP-2) is an essential component of the lectin pathway of complement activation. MASP-2 deficiency is common because of genetic polymorphisms, but its impact on susceptibility to infection is largely unknown. A deficiency of mannan-binding lectin is associated with susceptibility to infections and with the development of immunologic disease. In addition, MASP-2 deficiency was associated with an increased risk of fever and neutropenia / FN in children treated with chemotherapy for cancer.
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2. Entrez Gene: mannan-binding lectin serine peptidase 1 (C4/C2 activating component of Ra-reactive factor)
3. Entrez Gene: mannan-binding lectin serine peptidase 2
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