Comment on "Mannan-binding lectin-associated serine protease (MASP)-1 is crucial for lectin pathway activation in human serum, whereas neither MASP-1 nor MASP-3 is required for alternative pathway function"

Key Components of the Complement Lectin Pathway Are Not Only Required for the Development of Inflammatory Arthritis but Also Regulate the Transcription of Factor D

The complement system plays an important role in the pathogenesis of rheumatoid arthritis (RA). Besides driving lectin pathway (LP) activation, the mannan-binding lectin (MBL)-associated serine proteases (MASPs) also play a key role in regulating the alternative pathway (AP). We evaluated the effects of N-acetylgalactosamine (GalNAc)-conjugated MASP-1 and MASP-2 duplexes in vitro and in mice with and without arthritis to examine whether knockdown of MASP-1 and MASP-2 expression affects the development of arthritis. GalNAc-siRNAs for MASP-1 and MASP-2 demonstrated robust silencing of MASP-1 or MASP-2 at pM concentrations in vitro. To evaluate the impact of silencing in arthritic mice, we used the collagen antibody-induced arthritis (CAIA) mouse model of RA. Mice were injected a 10 mg/kg dose of GalNAc-siRNAs 3x s.q. prior to the induction of CAIA. Liver gene expression was examined using qRT-PCR, and protein levels were confirmed in the circulation by sandwich immunoassays and Western blot. At day 10, CAIA mice separately treated with MASP-1 and MASP-2 duplexes had a specific reduction in expression of liver MASP-1 (70–95%, p < 0.05) and MASP-2 (90%, p < 0.05) mRNA, respectively. MASP-1-siRNA treatment resulted in a 95% reduction in levels of MASP-1 protein in circulation with no effect on MASP-2 levels and clinical disease activity (CDA). In mice injected with MASP-2 duplex, there was a significant (p < 0.05) 90% decrease in ex vivo C4b deposition on mannan, with nearly complete elimination of MASP-2 in the circulation. MASP-2 silencing initially significantly decreased CDA by 60% but subsequently changed to a 40% decrease vs. control. Unexpectedly, GalNAc-siRNA-mediated knockdown of MASP-1 and MASP-2 revealed a marked effect of these proteins on the transcription of FD under normal physiological conditions, whereas LPS-induced inflammatory conditions reversed this effect on FD levels. LPS is recognized by Toll-like receptor 4 (TLR4), we found MBL not only binds to TLR4 an interaction with a K_d of 907 nM but also upregulated FD expression in differentiated adipocytes. We show that MASP-2 knockdown impairs the development of RA and that the interrelationship between proteins of the LP and the AP may extend to the transcriptional modulation of the FD gene.

Complement in the Initiation and Evolution of Rheumatoid Arthritis

The complement system is a major component of the immune system and plays a central role in many protective immune processes, including circulating immune complex processing and clearance, recognition of foreign antigens, modulation of humoral and cellular immunity, removal of apoptotic and dead cells, and engagement of injury resolving and tissue regeneration processes. In stark contrast to these beneficial roles, however, inadequately controlled complement activation underlies the pathogenesis of human inflammatory and autoimmune diseases, including rheumatoid arthritis (RA) where the cartilage, bone, and synovium are targeted. Recent studies of this disease have demonstrated that the autoimmune response evolves over time in an asymptomatic preclinical phase that is associated with mucosal inflammation. Notably, experimental models of this disease have demonstrated that each of the three major complement activation pathways plays an important role in recognition of injured joint tissue, although the lectin and amplification pathways exhibit particularly impactful roles in the initiation and amplification of damage. Herein, we review the complement system and focus on its multi-factorial role in human patients with RA and experimental murine models. This understanding will be important to the successful integration of the emerging complement therapeutics pipeline into clinical care for patients with RA.

Mannan-binding lectin-associated serine protease-1 (MASP-1) mediates immune responses against Aeromonas hydrophila in vitro and in vivo in grass carp

The mannan-binding lectin-associated serine protease-1 (MASP-1) gene is a crucial component of the lectin pathway in the complement and coagulation cascade. Although MASP-1 has been found in the immune system of teleosts, its immune functions in response to bacterial infection are unclear. In this study, we identified a MASP-1 homolog (gcMASP-1) in the grass carp (Ctenopharyngodon idella). The full-length 3308-bp gcMASP-1 cDNA includes a 2160-bp open reading frame encoding a protein composed of 719 amino acids with epidermal growth factor-like, complement control protein, and trypsin-like domains. gcMASP-1 shares a high similarity with MASP-1 counterparts in other species, and it is most closely related to Cyprinus carpio MASP-1 and Sinocyclocheilus anshuiensis MASP-1. Transcription of gcMASP-1 was widely distributed in different tissues and induced by Aeromonas hydrophila in vivo and in vitro. Expression of gcMASP-1 was also affected by lipopolysaccharide and flagellin stimulation in vitro. In cells over-expressing gcMASP-1, transcript levels of almost all components, except gcMBL and gcC5, were significantly enhanced, and gcIL1β, gcTNF-α, gcIFN, gcCD59, gcC5aR1, and gcITGβ-2 were significantly upregulated after exposure to A. hydrophila; gcMASP-1 interference downregulated the transcript levels after A. hydrophila challenge. In addition, gcMASP-1 activated NF-κB signaling. These findings indicate the vital role of gcMASP-1 in innate immunity in C. idella.

Lessons learned from mice deficient in lectin complement pathway molecules

The lectin pathway of the complement system is initiated when the pattern-recognition molecules, mannose-binding lectin (MBL), ficolins or collectin-11, bind to invading pathogens or damaged host cells. This leads to activation of MBL/ficolin/collectin-11 associated serine proteases (MASPs), which in turn activate downstream complement components, ultimately leading to elimination of the pathogen. Mice deficient in the key molecules of lectin pathway of complement have been generated in order to build knowledge of the molecular mechanisms of the lectin pathway in health and disease. Despite differences in the genetic arrangements of murine and human orthologues of lectin pathway molecules, the knockout mice have proven to be valuable models to explore the effect of deficiency states in humans. In addition, new insight and unexpected findings on the diverse roles of lectin pathway molecules in complement activation, pathogen infection, coagulation, host tissue injury and developmental biology have been revealed by in vivo investigations. This review provides an overview of the mice deficient in lectin pathway molecules and highlights some of the most important findings that have resulted from studies of these.

Essential role for the lectin pathway in collagen antibody-induced arthritis revealed through use of adenovirus programming complement inhibitor MAp44 expression

Previous studies using mannose-binding lectin (MBL) and complement C4-deficient mice have suggested that the lectin pathway (LP) is not required for the development of inflammatory arthritis in the collagen Ab-induced arthritis (CAIA) model. MBL, ficolins and collectin-11 are key LP pattern recognition molecules that associate with three serine proteases-MASP-1, MASP-2, and MASP-3-and with two MBL-associated proteins designated sMAP and MBL-associated protein of 44kDA (MAp44). Recent studies have shown that MAp44, an alternatively spliced product of the MASP-1/3 gene, is a competitive inhibitor of the binding of the recognition molecules to all three MASPs. In these studies, we examined the effect of treatment of mice with adenovirus (Ad) programmed to express human MAp44 (AdhMAp44) on the development of CAIA. AdhMAp44 and Ad programming GFP (AdGFP) expression were injected i.p. in C57BL/6 wild type mice prior to the induction of CAIA. AdhMAp44 significantly reduced the clinical disease activity (CDA) score by 81% compared with mice injected with AdGFP. Similarly, histopathologic injury scores for inflammation, pannus, cartilage and bone damage, as well as C3 deposition in the cartilage and synovium, were significantly reduced by AdhMAp44 pretreatment. Mice treated with AdmMAp44, programming expression of mouse MAp44, also showed significantly decreased CDA score and histopathologic injury scores. In addition, administration of AdhMAp44 significantly diminished the severity of Ross River virus-induced arthritis, an LP-dependent model. Our study provides conclusive evidence that an intact complement LP is essential to initiate CAIA, and that MAp44 may be an appropriate treatment for inflammatory arthritis.

C3 dysregulation due to factor H deficiency is mannan-binding lectin-associated serine proteases (MASP)-1 and MASP-3 independent in vivo

Uncontrolled activation of the complement alternative pathway is associated with complement-mediated renal disease. Factor B and factor D are essential components of this pathway, while factor H (FH) is its major regulator. In complete FH deficiency, uncontrolled C3 activation through the alternative pathway results in plasma C3 depletion and complement-mediated renal disease. These are dependent on factor B. Mannan-binding lectin-associated serine proteases 1 and 3 (MASP-1, MASP-3) have been shown recently to contribute to alternative pathway activation by cleaving pro-factor D to its active form, factor D. We studied the contribution of MASP-1 and MASP-3 to uncontrolled alternative pathway activation in experimental complete FH deficiency. Co-deficiency of FH and MASP-1/MASP-3 did not ameliorate either the plasma C3 activation or glomerular C3 accumulation in FH-deficient mice. Our data indicate that MASP-1 and MASP-3 are not essential for alternative pathway activation in complete FH deficiency.

Roles of adipocytes and fibroblasts in activation of the alternative pathway of complement in inflammatory arthritis in mice

The complement system is involved in mediation of joint damage in rheumatoid arthritis, with evidence suggesting activation of both the classical and alternative pathway (AP). The AP is both necessary and sufficient to mediate collagen Ab-induced arthritis, an experimental animal model of immune complex-induced joint disease. The AP in mice is dependent on MASP-1/3 cleavage of pro-factor D (pro-FD) into mature factor D (FD). The objectives of the current study were to determine the cells synthesizing MASP-1/3 and pro-FD in synovial tissue. Collagen Ab-induced arthritis was studied in wild-type C57BL/6 mice, and the localization of mRNA and protein for FD and MASP-1/3 in synovial adipose tissue (SAT) and fibroblast-like synoviocytes (FLS) was determined using various techniques, including laser capture microdissection. SAT was the sole source of mRNA for pro-FD. Cultured differentiated 3T3 adipocytes, a surrogate for SAT, produced pro-FD but no mature FD. FLS were the main source of MASP-1/3 mRNA and protein. Using cartilage microparticles (CMPs) coated with anti-collagen mAb and serum from MASP-1/3(-/-) mice as a source of factor B, pro-FD in 3T3 supernatants was cleaved into mature FD by MASP-1/3 in FLS supernatants. The mature FD was eluted from the CMP, and was not present in the supernatants from the incubation with CMP, indicating that cleavage of pro-FD into mature FD by MASP-1 occurred on the CMP. These results demonstrate that pathogenic activation of the AP can occur in the joint through immune complexes adherent to cartilage and the local production of necessary AP proteins by adipocytes and FLS.