title: The role of mannose-binding lectin-associated serine protease-3 in activation of the alternative complement pathway.
abstract: Mannose-binding lectin (MBL)-associated serine proteases (MASPs) are responsible for activation of the lectin complement pathway. Three types of MASPs (MASP-1, MASP-2, and MASP-3) are complexed with MBL and ficolins in serum. Although MASP-1 and MASP-2 are known to contribute to complement activation, the function of MASP-3 remains unclear. In this study, we investigated the mechanism of MASP-3 activation and its substrate using the recombinant mouse MASP-3 (rMASP-3) and several different types of MASP-deficient mice. A proenzyme rMASP-3 was obtained that was not autoactivated during preparation. The recombinant enzyme was activated by incubation with Staphylococcus aureus in the presence of MBL-A, but not MBL-C. In vivo studies revealed the phagocytic activities of MASP-1/3-deficient mice and all MASPs (MASP-1/2/3)-deficient mice against S. aureus and bacterial clearance in these mice were lower than those in wild-type and MASP-2-deficient mice. Sera from all MASPs-deficient mice showed significantly lower C3 deposition activity on the bacteria compared with that of wild-type serum, and addition of rMASP-3 to the deficient serum restored C3 deposition. The low C3 deposition in sera from all MASPs-deficient mice was probably caused by the low level factor B activation that was ameliorated by the addition of rMASP-3. Furthermore, rMASP-3 directly activated factors B and D in vitro. These results suggested that MASP-3 complexed with MBL is converted to an active form by incubation with bacterial targets, and that activated MASP-3 triggered the initial activation step of the alternative complement pathway.
authors.affiliation: Department of Immunology, Fukushima Medical University, Fukushima 960-1295, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Iwaki
authors.forename: Kazuko
authors.initials: K
authors.lastname: Kanno
authors.forename: Minoru
authors.initials: M
authors.lastname: Takahashi
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Endo
authors.forename: Misao
authors.initials: M
authors.lastname: Matsushita
authors.forename: Teizo
authors.initials: T
authors.lastname: Fujita
mesh_terms: D000818:Animals; D015153:Blotting, Western; D002469:Cell Separation; D003170:Complement Pathway, Alternative / Q000276:immunology*; D004789:Enzyme Activation / Q000276:immunology*; D005434:Flow Cytometry; D006801:Humans; D017737:Macrophages, Peritoneal / Q000276:immunology / Q000378:metabolism; D037601:Mannose-Binding Lectin / Q000276:immunology / Q000378:metabolism; D050606:Mannose-Binding Protein-Associated Serine Proteases / Q000276:immunology* / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D010587:Phagocytosis / Q000276:immunology; D011994:Recombinant Proteins / Q000276:immunology / Q000378:metabolism
chemical_list: D037601:Mannose-Binding Lectin; D011994:Recombinant Proteins; C468572:MASP-3 protein, mouse; D050606:Mannose-Binding Protein-Associated Serine Proteases