Immune complexes in chronic Chagas disease patients are formed by exovesicles from Trypanosoma cruzi carrying the conserved MASP N-terminal region.

The exovesicles (EVs) are involved in pathologic host-parasite immune associations and have been recently used as biomarkers for diagnosis of infectious diseases. The release of EVs by Trypanosoma cruzi, the causative agent of Chagas disease, has recently been described, with different protein cargoes including the MASP multigene family of proteins MASPs are specific to this parasite and characterized by a conserved C-terminal (C-term) region and an N-terminal codifying for a signal peptide (SP). In this investigation, we identified immature MASP proteins containing the MASP SP in EVs secreted by the infective forms of the parasite. Those EVs are responsible for the formation of immune complexes (ICs) containing anti-MASP SP IgGs in patients with different (cardiac, digestive and asymptomatic) chronic Chagas disease manifestations. Moreover, purified EVs as well as the MASP SP inhibit the action of the complement system and also show a significant association with the humoral response in patients with digestive pathologies. These findings reveal a new route for the secretion of MASP proteins in T. cruzi, which uses EVs as vehicles for immature and misfolded proteins, forming circulating immune complexes. Such complexes could be used in the prognosis of digestive pathologies of clinical forms of Chagas disease.

Recombinant expression of the autocatalytic complement protease MASP-1 is crucially dependent on co-expression with its inhibitor, C1 inhibitor.

MASP-1 is a protease of the lectin pathway of complement. It is homologous with MASP-2, previously thought both necessary and sufficient for lectin pathway activation. Recently MASP-1 has taken centre stage with the observation that it is crucial to the activation of MASP-2 and thus central to complement activation. Numerous additional functions have been suggested for MASP-1 and its importance is obvious. Yet, thorough analyses of proteolytic activities and physiological roles in the human scenario have been hampered by difficulties in purifying or producing full-length human MASP-1. We present the successful expression of full-length recombinant human MASP-1 entirely in the zymogen form in a mammalian expression system. We found that the catalytic activity of MASP-1 suppresses its expression through rapid auto-activation and auto-degradation. This auto-degradation was not inhibited by the addition of inhibitors to the culture medium, and it was subsequently found to occur intracellularly. Numerous mutations aimed at attenuating auto-activation or preventing auto-degradation failed to rescue expression, as did also attempts at stabilizing the protease by co-expression with MBL or ficolins or expression in hepatocyte cell lines, representing the natural site of synthesis. The active protease was finally produced through co-expression with the serine protease inhibitor C1 inhibitor. We demonstrate that the expressed protease is capable of binding MBL and auto-activating, and is catalytically active. We have generalized the concept to the expression also of MASP-2 entirely in its zymogen form and with improved yields. We suggest a general advantage of expressing aggressive, autocatalytic proteases with their cognate inhibitors.

Studies of the pattern recognition molecule H-ficolin: specificity and purification.

Ficolins are pattern recognition molecules of the innate immune system. H-ficolin is found in plasma associated with mannan-binding lectin-associated serine proteases (MASPs). When H-ficolin binds to microorganisms the MASPs are activated, which in turn activate the complement system. H-ficolin is the most abundant ficolin in humans, yet its ligand binding characteristics and biological role remain obscure. We examined the binding of H-ficolin to Aerococcus viridans as well as to a more defined artificial target, i.e. acetylated bovine serum albumin. A strict dependence for calcium ions and inhibition at high NaCl concentration was found. The binding to acetylated bovine serum albumin was inhibited by acetylsalicylic acid and sodium acetate as well as by N-acetylated glucosamine and galactosamine (GlcNAc and GalNAc) and glycine (GlyNAc). The binding to A. viridans was sensitive to the same compounds, but, importantly, higher concentrations were needed for inhibition. N-Acetylated cysteine was also inhibitory, but this inhibition was parallel with reduction in the oligomerization of H-ficolin and thus represents structural changes of the molecule. Based on our findings, we developed a procedure for the purification of H-ficolin from serum, involving PEG precipitation, affinity chromatography on Sepharose derivatized with acetylated serum albumin, ion exchange chromatography, and gel permeation chromatography. The purified H-ficolin was observed to elute at 700 kDa, similar to what we find for H-ficolin in whole serum. MASP-2 was co-purified with H-ficolin, and the purified H-ficolin·MASP-2 complex could activate complement as measured by cleavage of complement factor C4. This study extends our knowledge of the specificity of this pattern recognition molecule, and the purified product will enable further studies.

Purification, crystallization and preliminary X-ray analysis of human mannose-binding lectin-associated serine protease-1 (MASP-1) catalytic region.

MASP-1, a multidomain serine protease, is a component of the lectin pathway of complement. Its precise function is unknown, although it seems to enhance the complement-activating capacity of MASP-2, a related enzyme. MASP-1 has also been implicated as playing a role in blood coagulation. It is mostly found associated with mannose-binding lectin (MBL) and ficolins. Early attempts to crystallize MASP-1 failed because of the inhomogeneity of the purified material. MASP-1 was shown by acidic nondenaturing PAGE to be composed of differently charged species, which are most likely to be the products of deamidation occurring during the refolding procedure. Sequential cation-exchange and anion-exchange chromatography resulted in a homogeneous material, which was successfully crystallized. The best crystal diffracted to 2.55 A resolution and belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 68.4, b = 70.4, c = 121.4 A. The crystal structure of MASP-1 may help in understanding the function of this mysterious serine protease.

[Prokaryotic expression of N terminal fragment of mannan-binding lectin associated serine protease-1].

AIM: To express the N-terminal fragment of human mannan-binding lectin (MBL) associated serine proteases-1 (MASP1-N) in E.coli. METHODS: The target sequence in pGEM-MASP1 plasmid that contains human MBL-MASP1 cDNA was amplified by PCR, inserted into prokaryotic expression vector pGEX4T-1 and identified by restriction mapping and sequencing. The recombinant expression vector was transformed into E.coli BL21 (DE3) cells. The expressed product was purified by GSTrap Immobilized Metal Affinity Chromatography(IMAC) and identified by SDS-PAGE and Western blot assay, its binding-activity with the collagen-like region of human MBL(MBL-CLR) and with recombinant human MBL was analyzed by an indirect enzyme-linked immunosorbent assayèELISAé. RESULTS: The DNA fragment of 860 bp, which encode the N-terminal region of human MASP1, was amplified from pGEM-MASP1 plasmid and the recombinant expression vector, pGEX4T-MASP1-N, was constructed, whose restriction maps and sequence were consistent with those expected. The component of M(r) 60 000 in the purified recombinant product was found by SDS-PAGE and could be recognized by anti-GST antibody in Western blot assay. The purified recombinant product could react with human MBL-CLR and human MBL in the indirect ELISA. CONCLUSION: The prokaryotic cell strain that expresses efficiently recombinant human MASP1-N(rhMASP1-N) protein and the purified rhMASP1-N protein were successfully obtained, which provides the basis for further research of MASP1 molecule.

L-ficolin binding and lectin pathway activation by acetylated low-density lipoprotein.

L-ficolin, like mannan-binding lectin (MBL), is a lectin pathway activator present in normal human plasma. Upon binding ligand, l-ficolin similarly initiates C4 cleavage via the serine protease MBL-associated serine protease-2 (MASP-2). We sought further insight into l-ficolin binding reactions and MASP-2 activation by passing plasma through GlcNAc-derivatized Sepharose. l-Ficolin bound in 1.0 M NaCl-ethylenediamine tetraacetic acid (EDTA), and remained bound in NaCl-free EDTA, while MASP-2 eluted in proenzyme form ( approximately 20% yield, > 40 000-fold purification). L-Ficolin was eluted with GlcNAc in 1.0 M NaCl ( approximately 10% yield, > 3000-fold purification), with trace amounts of C3, alpha(2)-macroglobulin and both native and activated MASP-2. These preparations were utilized to investigate l-ficolin reactivities with acetylated low-density lipoprotein (A-LDL) as a model ligand in albumin-free systems. L-Ficolin bound strongly to A-LDL in the absence as well as presence of calcium, including saline-EDTA, and was optimal in 1.0 M NaCl-EDTA, but binding failed to occur in EDTA in the absence of NaCl. The addition of l-ficolin to immobilized A-LDL resulted in activation of MASP-2 in unmodified but not ficolin-depleted plasma unless l-ficolin was restored. We conclude that A-LDL is a useful ligand for investigation of l-ficolin function; both binding and activation are optimally examined in systems free of albumin; and ligand binding in 1.0 M NaCl in EDTA can be useful in the isolation of l-ficolin and native MASP-2.

Localization and characterization of the mannose-binding lectin (MBL)-associated-serine protease-2 binding site in rat ficolin-A: equivalent binding sites within the collagenous domains of MBLs and ficolins.

Ficolins and mannose-binding lectins (MBLs) are the first components of the lectin branch of the complement system. They comprise N-terminal collagen-like domains and C-terminal pathogen-recognition domains (fibrinogen-like domains in ficolins and C-type carbohydrate-recognition domains in MBLs), which target surface-exposed N-acetyl groups or mannose-like sugars on microbial cell walls. Binding leads to activation of MBL-associated serine protease-2 (MASP-2) to initiate complement activation and pathogen neutralization. Recent studies have shown that MASP-2 binds to a short segment of the collagen-like domain of MBL. However, the interaction between ficolins and MASP-2 is relatively poorly understood. In this study, we show that the MASP-2 binding site on rat ficolin-A is also located within the collagen-like domain and encompasses a conserved motif that is present in both MBLs and ficolins. Characterization of this motif using site-directed mutagenesis reveals that a lysine residue in the X position of the Gly-X-Y collagen repeat, Lys(56) in ficolin-A, which is present in all ficolins and MBLs known to activate complement, is essential for MASP-2 binding. Adjacent residues also make important contributions to binding as well as to MASP activation probably by stabilizing the local collagen helix. Equivalent binding sites and comparable activation kinetics of MASP-2 suggest that complement activation by ficolins and MBLs proceeds by analogous mechanisms.

A true autoactivating enzyme. Structural insight into mannose-binding lectin-associated serine protease-2 activations.

Few reports have described in detail a true autoactivation process, where no extrinsic cleavage factors are required to initiate the autoactivation of a zymogen. Herein, we provide structural and mechanistic insight into the autoactivation of a multidomain serine protease: mannose-binding lectin-associated serine protease-2 (MASP-2), the first enzymatic component in the lectin pathway of complement activation. We characterized the proenzyme form of a MASP-2 catalytic fragment encompassing its C-terminal three domains and solved its crystal structure at 2.4 A resolution. Surprisingly, zymogen MASP-2 is capable of cleaving its natural substrate C4, with an efficiency about 10% that of active MASP-2. Comparison of the zymogen and active structures of MASP-2 reveals that, in addition to the activation domain, other loops of the serine protease domain undergo significant conformational changes. This additional flexibility could play a key role in the transition of zymogen MASP-2 into a proteolytically active form. Based on the three-dimensional structures of proenzyme and active MASP-2 catalytic fragments, we present model for the active zymogen MASP-2 complex and propose a mechanism for the autoactivation process.

[Purification and separation of mannan-binding lectin (MBL) and MBL-associated serine proteases complex from human plasma].

OBJECTIVE: To separate mannan-binding lectin (MBL) and MBL-associated serine proteases (MASPs) from human plasma. METHODS: A two-step affinity chromatography on underivatized sepharose 4B was employed for purification of MBL-MASP complex, followed by gel filtration on a Sephacryl S-300 column for separation of MBL and MASPs from the complex. The purification procedures were performed at 4 degrees Celsius with the addition of two proteolytic inhibitors, phenyl methylsulfonyl fluoride and 1,10-phenanthroline during affinity chromatography but not in the gel filtration buffer. RESULTS: Preparations of highly purified MBL and proenzyme MASPs were obtained. The purified MBL was shown by SDS-PAGE and Western blotting to be a functional multimer composed of 28,000 and 32,000 peptide chains, with high bioactivity as demonstrated by ligand-binding assay and yeast agglutination experiment. CONCLUSION: A simple and convenient procedure is established successfully for the purification of MBL and the proenzyme MASPs.