The role of the capsular polysaccharide in the activation of the alternative pathway by the pneumococcus.

Previous studies have shown that when pneumococci are incubated in normal, nonimmune serum, they activate the alternative pathway and opsonically active C3b is fixed to the surface of the organism. Other studies have demonstrated that C3-dependent opsonization via the alternative pathway plays a significant role in the nonimmune host's defense against the pneumococcus. The present studies concern the role of the capsular polysaccharide in initiating the activation of the alternative pathway by the pneumococcus. Some pneumococcal capsular polysaccharide types, but not all, are able to activate the alternative pathway. Soluble purified capsular polysaccharide types 1, 4 and 25 activate the alternative pathway, whereas types 2, 3, 14, and 19 do not. Since the capsular polysaccharides exist in their native form attached to the pneumococcal surface, selected capsular polysaccharides were also tested for their ability to activate the alternative pathway when attached to a particulate carrier, sheep erythrocytes. Capsular polysaccharide types 2 and 3 failed to activate the alternative pathway when attached to sheep erythrocytes, paralleling the results obtained when these capsular polysaccharides were in solution. In contrast, the type 25 capsular polysaccharide not only activated the alternative pathway when attached to sheep erythrocytes, as it had when in solution, but it also initiated alternative pathway-mediated lysis of the erythrocytes. The capsular polysaccharide is not required for the activation of the alternative pathway by the pneumococcus. Although all types of encapsulated pneumococci are able to activate the alternative pathway, not all the purified capsular polysaccharide types are able to do so. In addition, a nonencapsulated pneumococcus, derived originally from a type 2 organism, activates the alternative pathway as well as a fully encapsulated type 2 pneumococcus.     

Schistosoma mansoni: Activation of the alternative pathway of human complement by schistosomula

Schistosomula of Schistosoma mansoni newly transformed from cercariae by either the mechanical or skin penetration procedures, as well as 5-day-old schistosomula recovered from the lungs of mice, were tested for their ability to activate the human alternative complement pathway. Newly transformed larvae prepared by both methods, although less active than cercariae, were found to activate the pathway to a comparable degree as judged by the consumption of fluid phase C3 and factor B and the conversion of native C3 into a component with a more anodal electrophoretic mobility. The alternative pathway activating capacity could not be blocked or enhanced by pretreating the larvae with purified IgG or F(ab′)₂ fragments prepared from human sera containing antibodies directed against schistosomula. In contrast to newly transformed parasites, 5-day-old schistosomula recovered from mouse lungs failed to activate the alternative pathway as judged by either the C3 or B consumption assays or the C3 conversion assay. This developmental change could not be reversed by treating lung stage larvae with neuraminidase and heparinase, enzymes which are known to alter the activating capacity of other particulate substances or with chondroitinase ABC or trypsin.     

Metabolic activity is necessary for activation of T suppressor cells by B cells

Ag-primed B cells must express cell-surface IgM, but not IgD or Ia Ag, and must remain metabolically active, in order to activate suppressor T cells (Ts) specific for type III pneumococcal polysaccharide. Ag-primed B cells that were gamma-irradiated with 1000r, or less, retained the ability to activate Ts; however, Ag-primed B cells exposed to UV light were not able to do so. gamma-Irradiated and UV-treated Ag-primed B cells both expressed comparable levels of cell-surface IgM, and both localized to the spleen after in vivo transfer; neither could proliferate in vitro in response to mitogens. By contrast, gamma-irradiated primed B cells were still able to synthesize proteins, whereas UV-treated primed B cells could not. These findings suggest that in order for Ag-primed B cells to activate Ts, they must a) express cell-associated IgM (sIgM) antibody bearing the idiotypic determinants of antibody specific for type III pneumococcal polysaccharide, and b) be able to synthesize protein for either the continued expression of sIgM after cell transfer, or for the elaboration of another protein molecule that is also required for the activation of Ts; this molecule does not appear to be Ia Ag.     

Role of the property of C-reactive protein to activate the classical pathway of complement in protecting mice from pneumococcal infection

C-reactive protein (CRP) is not an acute-phase protein in mice, and therefore, mice are widely used to investigate the functions of human CRP. It has been shown that CRP protects mice from pneumococcal infection, and an active complement system is required for full protection. In this study, we assessed the contribution of CRP's ability of activating the classical pathway of complement in the protection of mice from lethal infection with virulent Streptococcus pneumoniae type 3. We used two CRP mutants, Y175A and K114A. The Y175A CRP does not bind C1q and does not activate complement in human serum. The K114A CRP binds C1q and activates complement more efficiently than wild-type CRP. Passively administered, both CRP mutants and the wild-type CRP protected mice from infection equally. Infected mice injected with wild-type or mutant CRP had reduced bacteremia, resulting in lower mortality and increased longevity compared with mice that did not receive CRP. Thus, the protection of mice was independent of CRP-mediated activation of the classical pathway of complement. To confirm that human CRP does not differentiate between human and mouse complement, we analyzed the binding of human CRP to mouse C1q. Surprisingly, CRP did not react with mouse C1q, although both mutant and wild-type CRP activated mouse C3, indicating species specificity of CRP-C1q interaction. We conclude that the mouse is an unfit animal for exploring CRP-mediated activation of the classical complement pathway, and that the characteristic of CRP to activate the classical complement pathway has no role in protecting mice from infection.     

Activation of the human complement cascade by bacterial cell walls, peptidoglycans, water-soluble peptidoglycan components, and synthetic muramylpeptides--studies on active components and structural requirements

Cell walls isolated from 29 strains of 24 gram-positive bacterial species, whose peptidoglycans belong to the group A type of Schleifer and Kandler's classification, with one exception (Arthrobacter sp.), were shown to activate the complement cascade in pooled fresh human serum mainly through the alternative pathway and partly through the classical one. The complement-activating effect of cell walls (5 species) possessing group B type peptidoglycan, except those of Corynebacterium insidiosum, was weaker than that of the walls with group A type peptidoglycan. Preparations of peptidoglycan isolated from cell walls of Staphylococcus aureus, Streptococcus pyogenes, and Lactobacillus plantarum also activated the alternative pathway of the complement cascade, but less effectively than the respective parent cell walls. A water-soluble "polymer" of peptidoglycan subunits (SEPS), which was prepared from Staphylococcus epidermidis peptidoglycans by treatment with a cross-bridge degrading endopeptidase, retained most of the complement-activating ability of the parent cell walls. A peptidoglycan "monomer," SEPS-M, which was obtained by hydrolysis of the glycan chain of SEPS with endo-N-acetylmuramidase to disaccharide units did not activate complement. In conformity with this finding, neither synthetic N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) nor MDP-L-Lys-D-Ala activated the complement cascade. Among several lipophilic derivatives of MDP, 6-O-(3-hydroxy-3-docosylhexacosanoyl)-MDP-L-Lys-D-Ala (BH48-MDP-L-Lys-D-Ala) and 6-O-(2-tetradecylhexadecanoyl)-MDP (B30-MDP) were shown to activate complement through the alternative as well as the classical pathway and exclusively through the classical pathway, respectively. The finding that a D-isoasparagine analog of B30-MDP caused the same effect as the parent molecule strongly suggests that the activation of complement by B30-MDP is different from that caused by cell wall peptidoglycans and a water-soluble "polymer" of peptidoglycan subunits.     

Structure and function of pneumolysin, the multifunctional, thiol-activated toxin of Streptococcus pneumoniae

Pneumolysin is a thiol-activated, membrane-damaging, multifunctional toxin and a known virulence factor of Streptococcus pneumoniae. The toxin can interfere with the functioning of both cellular and soluble components of the human immune system which protects against pneumococcal infection. Different amino acids within the toxin which are important in promoting oligomerization of the toxin in membranes and for the generation of functional lesions have been identified by site-directed mutagenesis. Pneumolysin can also activate the classical pathway of complement, and this appears to involve antibody binding (via Fc) by a region of the toxin homologous to C-reactive protein, a human acute-phase protein also capable of classical pathway activation and implicated in host defence against pneumococcal infection.     

Purification and polar localization of pneumococcal LytB,a putative endo-beta-N-acetylglucosaminidase: the chain-dispersing murein hydrolase

The DNA region encoding the mature form of a pneumococcal murein hydrolase (LytB) was cloned and expressed in Escherichia coli. LytB was purified by affinity chromatography, and its activity was suggested to be the first identified endo-beta-N-acetylglucosaminidase of Streptococcus pneumoniae. LytB can remove a maximum of only 25% of the radioactivity from [(3)H]choline-labeled pneumococcal cell walls in in vitro assays. Inactivation of the lytB gene of wild-type strain R6 (R6B mutant) led to the formation of long chains but did not affect either total cell wall hydrolytic activity at the stationary phase of growth or development of genetic competence. Longer chains were formed when the lytB mutation was introduced into the M31 strain (M31B mutant), which harbors a complete deletion of lytA, which codes for the major autolysin. Furthermore, the use of this mutant revealed that LytB is the first nonautolytic murein hydrolase of pneumococcus. Purified LytB added to pneumococcal cultures of R6B or M31B was capable of dispersing, in a dose-dependent manner, the long chains characteristic of these mutants into diplococci or short chains, the typical morphology of R6 and M31 strains, respectively. In vitro acetylation of purified pneumococcal cell walls did not affect the activity of LytB, whereas that of the LytA amidase was drastically reduced. On the other hand, the use of a translational fusion between the gene (gfp) coding for the green fluorescent protein (GFP) and lytB supports the notion that LytB accumulates in the cell poles of either the wild-type R6, lytB mutants, or ethanolamine-containing cells (EA cells). The GFP-LytB fusion protein was also able to unchain the lytB mutants but not the EA cells. In contrast, translational fusion protein GFP-LytA preferentially bound to the equatorial regions of choline-containing cells but did not affect their average chain length. These observations suggest the existence of specific receptors for LytB that are positioned at the polar region on the pneumococcal surface, allowing localized peptidoglycan hydrolysis and separation of the daughter cells.     

Effect of membrane phospholipids on activation of the alternative complement pathway

Although some of the membrane glycoproteins that serve as activators or regulators of C activation have been identified, the influence of membrane lipids has not been studied extensively. A model of alternative C pathway activation was established using liposomes composed of cholesterol and synthetic phospholipids. Liposomes containing phosphatidylcholine (PC) as the sole phospholipid did not activate C as measured by C3 binding after incubation in normal human serum containing 2.5 mM MgCl2 and 10 mM EGTA. When phosphatidylethanolamine (PE) was included as 20% or more of the phospholipid, C3 binding was observed. C3 binding to liposomes was inhibited by salicylhydroxamic acid indicating binding through the C3 thioester bond. The phospholipid composition did not influence C3 binding to liposomes in an unregulated system of C3, B, D, and P indicating equivalent C3b binding sites on activating and nonactivating liposomes. When the regulatory proteins H and I were added to the other components, liposomes containing PE bound three times more C3 than PC liposomes suggesting that the phospholipid affects C3 regulation. This was tested directly in a radiolabeled H binding assay. In the presence of equal amounts of C3b, PC liposomes showed a greater number of high affinity H binding sites than PE liposomes. Using different PE derivatives, C activation could be directly related to the phospholipid polar head group. Liposomes containing PE, trinitrophenyl-PE or monomethyl-PE did activate the alternative C pathway, whereas those containing dimethyl-PE, PC, or phosphatidylserine did not. These studies provide evidence that primary and secondary amino groups on lipid membranes can decrease the interaction between H and C3b and provide sites for alternative pathway activation.     

Interchange of functional domains switches enzyme specificity: construction of a chimeric pneumococcal-clostridial cell wall lytic enzyme

Bacterial autolysins are endogenous enzymes that specifically cleave covalent bonds in the cell wall. These enzymes show both substrate and bond specificities. The former is related to their interaction with the insoluble substrate whereas the latter determine their site of action. The bond specificity allows their classification as muramidases (lysozymes), glucosaminldases, amidases, and endopeptidases. To demonstrate that the autolysin (LYC muramidase) of Clostridium acetobutylicum ATCC824 presents a domainal organization, a chimeric gene (clc) containing the regions coding for the catalytic domain of the LYC muramidase and the choline-binding domain of the pneumococcal phage CPL1 muramidase has been constructed by in vitro recombination of the corresponding gene fragments. This chimeric construction codes for a choline-binding protein (CLC) that has been purified using affinity chromatography on DEAE-cellulose. Several biochemical tests demonstrate that this rearrangement of domains has generated an enzyme with a choline-dependent muramidase activity on pneumococcal cell walls. Since the parental LYC muramidase was cholineindependent and unable to degrade pneumococcal cell walls, the formation of this active chimeric enzyme by exchanging protein domains between two enzymes that specifically hydrolyse cell walls of bacteria belonging to different genera shows that a switch on substrate specificity has been achieved. The chimeric CLC muramidase behaved as an autolytic enzyme when it was adsorbed onto a live autolysin-defective mutant of Streptococcus pneumoniae. The construction described here provides experimental support for the theory of modular evolution which assumes that novel proteins have evolved by the assembly of preexisting polypeptide units.     

Activation by some T-independent antigens and B cell mitogens of the alternative pathway of the complement system.

A number of T-independent antigens and B cell mitogens were examined for their ability to activate C3 via the alternative pathway of the complement system. Loss of hemolytically active C3, generation of anaphylatoxin activity, and immunoelectrophoretic conversion of C3 and factor B, were checked in normal and C4-deficient guinea pig serum, and, in some cases, in normal human serum. As judged by their activity in these assays, 10 lipopolysaccharides of different origin and constitution, pneumococcus type III polysaccharide, levan, dinitrophenylated aminoethyl-dextran, dinitrophenylated (D-glutamic acid, D-lysin) copolymer, polymerized flagellin, and pokeweed mitogen were all capable of initiating the alternative pathway, but differed with respect to their potency, their relative activity in the presence or absence of C4, and their ability to inhibit C3-turnover at high concentrations. Polyvinylpyrrolidone of intermediate molecular weight (4 x 10(4) daltons) was only active if the most sensitive assay was used (anaphylatoxin generation). Other species of polyvinylpyrrolidone, depolymerized pneumococcal polysaccharide, aminoethyl-dextran, [D-glutamic acid, D-lysin] copolymer, phytohemagglutinin and concanavalin A failed to activate C3. C3-consumption by concanavalin A was due to nonspecific binding.     

Serum opsonic deficiency produced by Streptococcus pneumoniae and by capsular polysaccharide antigens.

The opsonic requirements for phagocytosis of S. pneumoniae types 6, 7, 18, and 23 were determined in normal and C2 deficient serum, and in normal serum chelated with magnesium ethyleneglycoltetraacetic acid. All four strains were effectively opsonized via the alternative complement pathway, a finding suggesting that the capsular polysaccharides of these strains activated complement via the alternative pathway. Since bacteremic pneumococcal disease is often associated with circulating capsular polysaccharide, it was considered that this cellular component may activate complement in vivo and impair host defenses by producing an opsonic defect for pneumococci. To examine this hypothesis, serum was incubated with suspensions of whole S. pneumoniae types 6, 7, 18, or 23 or with purified capsular polysaccharide from each of these types, and residual complement activity and opsonic capacity were measured. Hemolytic C 3--9 complement activity and opsonic capacity for 3H-thymidine labeled Salmonella typhimurium, a species effectively opsonized via the alternative pathway, were reduced in serum following incubation. Polysaccharide concentrations as low as 1 microgram/ml inhibited serum opsonic capacity for salmonella. Whole pneumococci and pneumococcal capsular polysaccharide also inhibited the opsonic activity of human C2 deficient serum for salmonella, further evidence for activation of complement via the alternative pathway. Pneumococcal capsular polysaccharide markedly inhibited the opsonic capacity of normal serum for the homologous pneumoccal type. Thus, amounts of pneumococcal capsular polysaccharide, similar to those found in the serum of patients with pneumococcal disease, bring about decomplementation of serum via activation of the alternative pathway and inhibit pneumococcal opsonization.     

The antimicrobial activity of C-reactive protein

Recent studies in transgenic mice confirmed that C-reactive protein is protective against microbial pathogens. This is consistent with its ability in vitro to bind microbes, activate the complement classical pathway, and engage FcgammaRI and FcgammaRII. However, in transgenic mice protection also requires the alternative pathway of complement, and FcgammaRI is dispensable.     

C-reactive protein increases cytokine responses to Streptococcus pneumoniae through interactions with Fc gamma receptors

Streptococcus pneumoniae is the most common organism responsible for community acquired pneumonia and meningitis. In pneumococcal pneumonia, a strong local inflammatory cytokine response reduces the frequency of bacteremia and increases survival. The initiation of this cytokine response by innate recognition of bacterial cell wall components through TLR has been described, but the role of soluble innate mediators has received limited attention. C-reactive protein (CRP) is an acute phase protein that binds phosphocholine residues on S. pneumoniae cell walls. CRP interacts with phagocytic cells through FcgammaRI and FcgammaRII and activates the classical complement pathway. CRP is protective in mouse pneumococcal bacteremia by increasing complement-dependent clearance and killing of bacteria. We studied the cytokine response of PBMC stimulated with CRP-opsonized S. pneumoniae to determine the effect of CRP interaction with FcgammaR. CRP dramatically increased the production of TNF-alpha and IL-1beta in response to S. pneumoniae. These increases were blocked by phosphocholine, which inhibits CRP binding to S. pneumoniae, by inhibitors of FcgammaR signaling, and by mAb to FcgammaRI and FcgammaRII. A mutated rCRP with decreased FcgammaR binding had a decreased ability to stimulate TNF-alpha release, compared with wild-type CRP. Individuals who were homozygous for the R-131 allele of FcgammaRIIA, which has a higher affinity for CRP, showed higher responses to CRP-opsonized bacteria than did individuals homozygous for the H-131 allele, further implicating this receptor. The results indicate that CRP recognition of S. pneumoniae and binding to FcgammaR may enhance the early protective cytokine response to infection.     

The molecular characterization of the first autolytic lysozyme of Streptococcus pneumoniae reveals evolutionary mobile domains

A biochemical approach to identify proteins with high affinity for choline-containing pneumococcal cell walls has allowed the localization, cloning and sequencing of a gene (lytC ) coding for a protein that degrades the cell walls of Streptococcus pneumoniae. The lytC gene is 1506 bp long and encodes a protein (LytC) of 501 amino acid residues with a predicted M r of 58 682. LytC has a cleavable signal peptide, as demonstrated when the mature protein (about 55 kDa) was purified from S. pneumoniae. Biochemical analyses of the pure, mature protein proved that LytC is a lysozyme. Combined cell fractionation and Western blot analysis showed that the unprocessed, primary product of the lytC gene is located in the pneumococcal cytoplasm whereas the processed, active form of LytC is tightly bound to the cell envelope. In vivo experiments demonstrated that this lysozyme behaves as a pneumococcal autolytic enzyme at 30 degrees C. The DNA region encoding the 253 C-terminal amino acid residues of LytC has been cloned and expressed in Escherichia coli. The truncated protein exhibits a low, but significant, choline-independent lysozyme activity, which suggests that this polypeptide adopts an active conformation. Self-alignment of the N-terminal part of the deduced amino acid sequence of LytC revealed the presence of 11 repeated motifs. These results strongly suggest that the lysozyme reported here has changed the general building plan characteristic of the choline-binding proteins of S. pneumoniae and its bacteriophages, i.e. the choline-binding domain and the catalytic domain are located, respectively, at the N-terminal and the C-terminal moieties of LytC. This work illustrates the natural versatility exhibited by the pneumococcal genes coding for choline-binding proteins to fuse separated catalytic and substrate-binding domains and create new and functional mature proteins.     

A proatherogenic role for C-reactive protein in vivo

PURPOSE OF REVIEW: We have selectively reviewed some of the latest papers on the mechanistic role of C-reactive protein in atherosclerotic cardiovascular disease. RECENT DEVELOPMENTS: C-reactive protein is known to activate the classic pathway of the complement system. One paper examined the role of C-reactive protein in complement activation by enzymatically remodeled LDL proteins. Enzymatically remodeled LDL was found to induce complement activation with or without C-reactive protein, but in the presence of C-reactive protein the activation of complement halted before its terminal sequence. Complement activation by C-reactive protein in atherogenesis remains controversial. Different laboratories have reported the multi-organ origin of C-reactive protein. The atherosclerotic lesion itself is another place where C-reactive protein could be produced. Numerous studies have continued to dissect the potential diverse proatherogenic actions of C-reactive protein on cultured vascular cells. Caution must be exercised in inadequately controlled studies that have unwittingly used commercial C-reactive protein preparations contaminated by other bioactive components. In contrast to in-vitro experiments, in-vivo studies that support a proatherogenic role of C-reactive protein are less likely to be subject to misinterpretation. SUMMARY: Evidence suggests that C-reactive protein is a proatherogenic molecule that plays an active role. The amount of C-reactive protein in lesions is determined by its plasma levels and its local production. The biological effect of C-reactive protein on atherosclerosis development seems to encompass a complex network of interactions with other players in immunity and inflammation, such as the complement system, as well as a direct effect of C-reactive protein on the cells involved in lesion growth and development.     

The Sho1 adaptor protein links oxidative stress to morphogenesis and cell wall biosynthesis in the fungal pathogen Candida albicans

The Sho1 adaptor protein is an important element of one of the two upstream branches of the high-osmolarity glycerol (HOG) mitogen-activated protein (MAP) kinase pathway in Saccharomyces cerevisiae, a signal transduction cascade involved in adaptation to stress. In the present work, we describe its role in the pathogenic yeast Candida albicans by the construction of mutants altered in this gene. We report here that sho1 mutants are sensitive to oxidative stress but that Sho1 has a minor role in the transmission of the phosphorylation signal to the Hog1 MAP kinase in response to oxidative stress, which mainly occurs through a putative Sln1-Ssk1 branch of the HOG pathway. Genetic analysis revealed that double ssk1 sho1 mutants were still able to grow on high-osmolarity media and activate Hog1 in response to this stress, indicating the existence of alternative inputs of the pathway. We also demonstrate that the Cek1 MAP kinase is constitutively active in hog1 and ssk1 mutants, a phenotypic trait that correlates with their resistance to the cell wall inhibitor Congo red, and that Sho1 is essential for the activation of the Cek1 MAP kinase under different conditions that require active cell growth and/or cell wall remodeling, such as the resumption of growth upon exit from the stationary phase. sho1 mutants are also sensitive to certain cell wall interfering compounds (Congo red, calcofluor white), presenting an altered cell wall structure (as shown by the ability to aggregate), and are defective in morphogenesis on different media, such as SLAD and Spider, that stimulate hyphal growth. These results reveal a role for the Sho1 protein in linking oxidative stress, cell wall biogenesis, and morphogenesis in this important human fungal pathogen.     

Identification of a lytic enzyme of Clostridium acetobutylicum that degrades choline-containing pneumococcal cell walls

Abstract A protein that degrades pneumococcal walls containing choline, but not ethanolamine, in the teichoic acids has been isolated and purified from supernatants obtained from cultures of Clostridium acetobutylicum . The analyses of the degradation products of [³H]choline-labeled cell walls treated with this enzyme indicated that the purified protein, showing an apparent M _r of 115 000, is an N-acetylmuramyl- l -alanine amidase. Our results also suggest that C. acetobutylicum contains choline in its cell wall.     

Two components of cysteine oxidase in rat liver

Purified cysteine oxidase in rat liver is composed of two distinct proteins. These proteins are able to be fractionated by DEAE-cellulose column chromatography. It appears that one of them is a catalytic protein named protein-B having tightly bound iron as a prosthetic group, while the other is either a modifier or activating protein named protein-A. Protein-B is found to exist in both an active and an inactive form. Inactive protein-B is activated by incubation with substrate cysteine under anaerobic condition. Activated protein-B alone exhibited an extremely low catalytic activity but in the presence of protein-A remarkable increase in activity was observed.     

The role of ficolins in the lectin pathway of innate immunity

Ficolins are a family of oligomeric proteins consisting of an N-terminal collagen-like domain and a C-terminal globular fibrinogen-like domain. They are novel lectins that employ the fibrinogen-like domain as a functional domain. Ficolins specifically recognize N-acetyl compounds such as N-acetylglucosamine, components of bacterial and fungal cell walls, and certain bacteria. Like mannose-binding lectin (MBL), ficolins circulate in complexes with MBL-associated serine proteases (MASPs). MASP complexes form with ficolins and MBL, thereby activating the complement through the lectin pathway. Upon binding of ficolins and MBL to carbohydrates on pathogens, MASPs convert to active forms, and subsequently activate the complement. The activated complements lead to pathogen phagocytosis, aggregation and lysis. In humans, three ficolins (L-, M- and H-ficolins) have been identified, which exhibit differences in tissue expression, protein location site, ligand-binding and bacteria-recognition, suggesting a specific role of each ficolin. In addition, these ficolins form complexes with three MASPs (MASP-1, MASP-2 and MASP-3) and two nonenzymatic proteins (sMAP and MAP-1), suggesting a highly sophisticated organization and regulated activation of the ficolin-dependent lectin pathway. This review provides an overview of our current knowledge of ficolins, especially human ficolins and their mouse homologues. We also discuss their possible physiological roles in innate immunity, especially their defensive role against bacterial infection.