Characterization of tryptic fragments of human complement factor C3

Auxiliary activation of the complement system and its importance for the pathophysiology of clinical conditions

Activation and regulation of the cascade systems of the blood (the complement system, the coagulation/contact activation/kallikrein system, and the fibrinolytic system) occurs via activation of zymogen molecules to specific active proteolytic enzymes. Despite the fact that the generated proteases are all present together in the blood, under physiological conditions, the activity of the generated proteases is controlled by endogenous protease inhibitors. Consequently, there is remarkable little crosstalk between the different systems in the fluid phase. This concept review article aims at identifying and describing conditions where the strict system-related control is circumvented. These include clinical settings where massive amounts of proteolytic enzymes are released from tissues, e.g., during pancreatitis or post-traumatic tissue damage, resulting in consumption of the natural substrates of the specific proteases and the available protease inhibitor. Another example of cascade system dysregulation is disseminated intravascular coagulation, with canonical activation of all cascade systems of the blood, also leading to specific substrate and protease inhibitor elimination. The present review explains basic concepts in protease biochemistry of importance to understand clinical conditions with extensive protease activation.

Proteinases, Their Extracellular Targets, and Inflammatory Signaling

Given that over 2% of the human genome codes for proteolytic enzymes and their inhibitors, it is not surprising that proteinases serve many physiologic-pathophysiological roles. In this context, we provide an overview of proteolytic mechanisms regulating inflammation, with a focus on cell signaling stimulated by the generation of inflammatory peptides; activation of the proteinase-activated receptor (PAR) family of G protein-coupled receptors (GPCR), with a mechanism in common with adhesion-triggered GPCRs (ADGRs); and by proteolytic ion channel regulation. These mechanisms are considered in the much wider context that proteolytic mechanisms serve, including the processing of growth factors and their receptors, the regulation of matrix-integrin signaling, and the generation and release of membrane-tethered receptor ligands. These signaling mechanisms are relevant for inflammatory, neurodegenerative, and cardiovascular diseases as well as for cancer. We propose that the inflammation-triggering proteinases and their proteolytically generated substrates represent attractive therapeutic targets and we discuss appropriate targeting strategies.

Induction of complement C3a receptor responses by kallikrein-related peptidase 14

Activation of the complement system is primarily initiated by pathogen- and damage-associated molecular patterns on cellular surfaces. However, there is increasing evidence for direct activation of individual complement components by extrinsic proteinases as part of an intricate crosstalk between physiological effector systems. We hypothesized that kallikrein-related peptidases (KLKs), previously known to regulate inflammation via proteinase-activated receptors, can also play a substantial role in innate immune responses via complement. Indeed, KLKs exemplified by KLK14 were efficiently able to cleave C3, the point of convergence of the complement cascade, indicating a potential modulation of C3-mediated functions. By using in vitro fragmentation assays, mass spectrometric analysis, and cell signaling measurements, we pinpointed the generation of the C3a fragment of C3 as a product with potential biological activity released by the proteolytic action of KLK14. Using mice with various complement deficiencies, we demonstrated that the intraplantar administration of KLK14 results in C3-associated paw edema. The edema response was dependent on the presence of the receptor for C3a but was not associated with the receptor for the downstream complement effector C5a. Our findings point to C3 as one of the potential substrates of KLKs during inflammation. Given the wide distribution of the KLKs in tissues and biological fluids where complement components may also be expressed, we suggest that via C3 processing, tissue-localized KLKs can play an extrinsic complement-related role during activation of the innate immune response.

Complement C3b/C3d and cell surface polyanions are recognized by overlapping binding sites on the most carboxyl-terminal domain of complement factor H

Factor H (FH) is a potent suppressor of the alternative pathway of C in plasma and when bound to sialic acid- or glycosaminoglycan-rich surfaces. Of the three interaction sites on FH for C3b, one interacts with the C3d part of C3b. In this study, we generated recombinant constructs of FH and FH-related proteins (FHR) to define the sites required for binding to C3d. In FH, the C3d-binding site was localized by surface plasmon resonance analysis to the most C-terminal short consensus repeat domain (SCR) 20. To identify amino acids of FH involved in binding to C3d and heparin, we compared the sequences of FH and FHRs and constructed a homology-based molecular model of SCR19-20 of FH. Subsequently, we created an SCR15-20 mutant with substitutions in five amino acids that were predicted to be involved in the binding interactions. These mutations reduced binding of the SCR15-20 construct to both C3b/C3d and heparin. Binding of the wild-type SCR15-20, but not the residual binding of the mutated SCR15-20, to C3d was inhibited by heparin. This indicates that the heparin- and C3d-binding sites are overlapping. Our results suggest that a region in the most C-terminal domain of FH is involved in target recognition by binding to C3b and surface polyanions. Mutations in this region, as recently reported in patients with familial hemolytic uremic syndrome, may lead to indiscriminatory C attack against self cells.

Each of the three binding sites on complement factor H interacts with a distinct site on C3b

Factor H (fH) restricts activation of the alternative pathway of complement at the level of C3, both in the fluid phase and on self-structures, but allows the activation to proceed on foreign structures. To study the interactions between fH and C3b we used surface plasmon resonance analysis (Biacore(R)) and eight recombinantly expressed fH constructs containing fragments of the 20 short consensus repeat domains (SCRs) of fH. We analyzed the binding of these constructs to C3b and its cleavage products C3c and C3d. Three binding sites for C3b were found on fH. Site 1 was localized to the five amino-terminal SCRs (SCR1-5), and its reciprocal binding site on C3b was found to be lost upon the cleavage of C3b to C3c and C3d. Site 2 on fH was localized by exclusion probably within or near SCRs 12-14 (fragment SCR8-20 bound to C3b, C3c, and C3d; SCR8-11 did not bind to C3b at all; and SCR15-20 bound only to the C3d part of C3b). Site 3 on fH for C3b was localized to the carboxyl-terminal SCRs 19-20, and its reciprocal binding site was mapped to the C3d part of C3b. In conclusion, we confirmed and mapped three binding sites on fH for C3b and demonstrated that the three binding sites on fH interact with distinct sites on C3b. Multiple reciprocal interactions between C3b and fH can provide a basis for the different reactivity of the alternative pathway with different target structures.

Functional properties of complement factor H-related proteins FHR-3 and FHR-4: binding to the C3d region of C3b and differential regulation by heparin

The human factor H-related proteins FHR-3 and FHR4 are members of a family of proteins related to the complement factor H. Here, we report that the two proteins bind to the C3d region of complement C3b. The apparent K(A) values for the interactions of FHR-3 and FHR-4 with C3b are 7.5 x 10(6) M(-1) and 2.9 x 10(6) M(-1), respectively. Binding studies performed with C3b-coated pneumococci confirmed the results obtained with the biosensor system. A C-terminal construct of factor H showed similar binding characteristics. The interaction of FHR-3, but not of FHR4, with opsonised pneumococci was inhibited by heparin.

Phosphorylation of complement component C3 after synthesis in U937 cells by a putative protein kinase, casein kinase 2, which is regulated by CD11b: evidence that membrane-bound proteases preferentially cleave phosphorylated C3

It was our aim in this study to investigate the possibility that the third component of complement (C3) is phosphorylated during synthesis and secretion in U937 cells. Labelling of U937 cells with [32P]Pi, followed by immunoprecipitation of C3 from cell lysates and culture supernatants at different time points, showed that C3 was phosphorylated intracellularly immediately before release into the medium, which initiated cleavage of the protein into an iC3b-like fragment. Stimulation of CD11b/CD18 increased phosphorylation 7-fold, from a basal level of 2%. The phosphorylation sites in C3 did not resemble those described previously for casein kinase (CK) 1, cAMP-dependent protein kinase A or calcium- and phospholipid-dependent protein kinase C. Instead, protein kinase CK2 was suggested inasmuch as: (1) CK2 was detected both on the cell surface and on shed microparticles; (2) phosphorylation of purified C3 by microparticles was abolished by a monoclonal antibody, anti-CK2; (3) the [32P]Pi tag of both phosphorylated C3 (secreted from U937 cells) and of microparticle-phosphorylated C3, which was cleaved either by membrane proteases or by leucocyte elastase, was found in a 40 and a 70 kDa polypeptide; (4) both secreted C3 and C3 phosphorylated in vitro were much more susceptible to cleavage by proteases. Generation of C3 fragments provides a means by which U937 cells can stimulate nearby cells which are expressing complement receptors. The present study demonstrates that the cleavage of C3 is controlled by an intracellular phosphorylation event regulated by CD11b/CD18.

Isolation of the porcine complement activation fragments C3c and C3d and their use in the preparation of monospecific antibodies

The hemolytic test to date has been the sole analytic technique applied to study the complement reaction in the swine. To improve the analytical possibilities for this species we have developed polyclonal antibody reagents with specificities for the C3c and C3d activation fragments of swine C3. Access to these reagents, by which activation products can be analysed in tissues and biological fluids, will offer new possibilities for a more precise analysis of the complement reaction.

Conformational epitopes of C3 reflecting its mode of binding to an artificial polymer surface

The aim of the study was to investigate the incompletely understood mechanisms of complement (C) activation and binding on artificial biomaterials. Polystyrene in the form of microtitre plates was used as target for C binding, detectable by ELISA using monoclonal anti-C3 antibodies specific for conformational epitopes expressed by bound C3 and C3 fragments. C3 binding in whole blood/plasma/serum is maximal at low dilutions and occurs predominantly by C activation. At higher dilutions, C3 binding occurs at approximately 1/3 of maximal levels and is solely an effect of adsorption. C3 adsorption in the lower serum dilution range, occurs at low but clearly detectable levels. Comparative epitope analysis between C3 fragments, actively bound to polystyrene in the presence of serum, and of iC3b bound to sheep erythrocytes, clearly indicates that C3 binding/activation on polystyrene takes place as a C3 convertase-mediated reaction, which in serum/plasma is followed by a secondary factor I-dependent degradation of the bound C3b into iC3b. The neo-epitope analysis of serum-contacting polystyrene revealed that the adsorbed C3, throughout the entire serum dilution range tested, deposits in a state closely similar to that observed for purified C3 at a high packing density. Polystyrene surfaces with adsorbed purified C3 expressing this epitope profile were found to mediate APW dependent deposition of C3b in pig serum, presumably by forming a hybrid convertase with porcine Bb. These data therefore suggest that adsorbed C3 on serum-contacting polystyrene surfaces may initiate complement activation via the APW.

Purification and characterization of porcine C3. Studies of the biologically active protein and its split products

Separation techniques for obtaining pure and biologically active swine C3 have been improved in this study. Using these procedures and through the further characterization of porcine C3, the possibilities for developing more specific techniques for the analysis of the complement system in swine have been improved. Plasma was initially treated with protease inhibitors, polyethylene glycol (PEG)-fractionation, plasminogen-depletion and a rapid chromatographic desalting step. The essential fractionation was carried out by DEAE-Sephacel chromatography. Contaminants were removed by size-exclusion (Sepharose CL-6B)- and hydroxylapatite-chromatography. The final recovery reached 56% with 73% retaining specific hemolytic activity. The amino acid composition (98.33%), the functional compatibility and the secondary structure of fragments and intact protein indicate a high degree of homology with human C3. In contrast with the findings of earlier studies was the considerable immunologic cross-reactivity observed with human C3, and the size difference between the human and the swine C3-beta subunit, which was found to be 10 kDa lighter than the human analogue. The finding that the swine C3b/iC3b/C3c fragments do not separate from C3 by agarose electrophoresis, unlike the human analogues, demonstrated that this commonly used simple parameter for the detection of complement activation cannot be used in the porcine model.

Conformational differences between surface-bound and fluid-phase complement-component-C3 fragments. Epitope mapping by cDNA expression

In previous studies a subset of complement-component-C3 (C3) epitopes, C3(D), expressed in denatured and surface-bound C3 and C3 fragments, has been described. These epitopes were detected by antibodies raised against denatured C3. In the present study we used a cDNA expression strategy to localize epitopes recognized by monoclonal and polyclonal anti-C3(D) antibodies. First, DNAse I digestion of C3 cDNA was used to generate 200-300 bp fragments. These cDNA fragments were expressed as beta-galactosidase-C3 fusion proteins using the lambda gt11 vector. The fusion proteins were tested by Western-blot analysis for reactivity with monoclonal and polyclonal anti-C3 antibodies, and the location of the epitopes were determined by sequencing the cDNA fragments. Affinity-purified polyclonal anti-C3(D) antibodies specific for denatured C3 reacted strongly with the C3 fusion fragments corresponding to segments of the 40 kDa subunit of C3c (residues 1477-1510) and the C3d fragment (residues 1117-1155 and 1234-1294) of C3. Adsorption of the polyclonal antibodies with a mixture of EAC3b and EAC3bi (degradation fragments of C3 bound to sheep erythrocytes) abolished binding to fusion proteins spanning the C3d region, but not the 40 kDa fragment of C3c. No effect was seen with the corresponding soluble C3 fragments. The monoclonal anti-C3(D) antibodies (mAbs) 7D326.1 and 7D331.1, specific for EAC3b and EAC3bi, bound to a fusion protein corresponding to amino acid residues 1312-1404, whereas mAb 7D9.2, specific for EAC3d, reacted with a fusion protein spanning amino acid residues 1082-1118. mAbs 4SD11.1 and 4SD18.1, which did not bind to any physiological C3 fragment, detected a fusion protein covering residues 1477-1510. In summary, the segments of C3 represented by amino acid residues 1082-1118, 1117-1155, 1234-1294 and 1312-1404 accommodate C3(D) epitopes that are expressed by erythrocyte-bound C3 fragments, but not by the corresponding fluid-phase fragment, whereas the segments spanning residues 973-1026 and 1477-1510 contain C3(D) epitopes that are exposed exclusively in denatured C3 and therefore hidden in physiological fragments of the protein.

C3d-mediated negative and positive signals on the proliferation of human B cells separated from blood

Soluble C3d applied to human blood-derived B lymphocytes inhibited anti-mu, T cell-produced growth factor, and EBV-induced DNA synthesis in serum-free culture. C3d added to the B cell cultures 1 and 2 days after the stimulus, still exerted inhibition, though with gradually diminishing efficiency. C3d, fixed on zymosan or attached to the culture wells, induced [3H]thymidine incorporation of the B cells in serum-free medium. The concentration of C3d used to coat the wells was critical, with optimal stimulatory effect of 8.3 micrograms/ml. These C3d molecules were shown to be denatured. Our results are in line with earlier data on B cells derived from mouse spleen and human tonsils showing that depending on the way of presentation and its amounts, the natural ligand of CR2 can exert negative or positive signals. Moreover, we demonstrate that C3d can inhibit even the proliferative stimulus of EBV.

Neoantigens in complement component C3 as detected by monoclonal antibodies. Mapping of the recognized epitopes by synthetic peptides

The different fragments of the third complement component, C3, generated upon complement activation/inactivation have the ability to bind to several other complement components and receptors as well as to proteins of foreign origin. These multiple reactivities of C3 fragments are associated with a series of conformational changes occurring in the C3 molecule during its degradation. The conformations acquired by the different C3 fragments are also associated with the exposure of neoantigenic epitopes that are specific for (a) particular fragment(s). In order to study these epitopes and thus the conformational changes occurring in C3, monoclonal antibodies (mAbs) recognizing such epitopes were produced in Balb/c mice after immunization with denatured human C3. Two of the three antibodies (7D84.1 and 7D264.6) presented in this study recognized predominantly surface-bound iC3b, and one mAb (7D323.1) recognized both surface-bound and fluid-phase iC3b. Although none of the mAbs recognized any other fluid-phase C3 fragment, all three antibodies detected micro-titre-plate-fixed C3b and iC3b, but not C3c or C3d. In addition to the reaction with human C3, mAb 7D323.1 also bound to micro-titre-plate-fixed rabbit C3. The epitopes recognized by the three mAbs were further localized by using synthetic peptides that were designed on the basis of the differential binding of the mAbs to the C3 fragments. All three antibodies reacted with C3-(924-965)-peptide, which represents the region of C3 between the kallikrein-cleavage site (923-924) and the elastase-cleavage site (965-966). On the basis of the binding of the mAbs to five different overlapping peptides spanning the region between residues 924 and 965 of the human C3 sequence, and the sequence similarity between human C3 and rabbit C3 within this area, the epitopes recognized by these antibodies are mapped. The contribution of the individual amino acid residues in the formation of the epitopes is discussed.

Isolation and characterization of the third complement component of axolotl (Ambystoma mexicanum)

1. Using a monoclonal anti-human C3 antibody and a polyclonal anti-cobra venom factor antibody as probes, a protein homologous to the mammalian third complement component (C3) was purified from axolotl plasma and found to be axolotl C3. 2. Axolotl C3 consists of two polypeptide chains (Mr = 110,000 and 73,000) linked by disulfide bonds. An internal thiolester bond in the alpha chain was identified by the incorporation of [14C]methylamine and NH2-terminal sequence from the C3d fragment of C3. 3. Digestion of C3 by trypsin resulted in the cleavage of both the alpha and beta chains, generating fragments with a cleavage pattern similar to that of human C3. 4. The amino acid composition of axolotl C3 and the amino acid sequences of the thiolester site (and the surrounding amino acids), the cleavage site for the C3-convertase, and one of the factor I cleavage sites are similar to C3 from other vertebrates. 5. In contrast to human C3, which has concanavalin A binding carbohydrates on both the alpha and beta chains, only the beta chain of axolotl C3 contains such carbohydrates.

Ligands of CR2 do not interfere with C3 fragment fixation or enhanced NK sensitivity of Raji cells treated with human serum

Raji cells activate the alternative complement pathway (ACP) and fix C3 fragments when incubated in human serum (HS). Earlier experiments have shown that CR2 molecules are involved in this phenomenon and the opsonized cells have elevated sensitivity to the lytic effect of CR3-bearing NK cells. We show here that Raji cells treated with CR2 site-specific ligands, (C3d, OKB-7 and HB-5 mAbs, and a synthetic peptide which binds to CR2) generated and bound C3 fragments after exposure to HS. The elevated lytic sensitivity of HS-treated cells was not altered by the presence of the various CR2 ligands. Thus, the membrane-bound C3 fragments are not fixed at the C3dg receptor binding site.

Distinctive expression of neoantigenic C3(D) epitopes on bound C3 following activation and binding to different target surfaces in normal and pathological human sera

Binding of C3 to sheep erythrocytes in a serum-free milieu (EAC14oxy2, EAC142) has previously been shown to mimic the antigenic change that occurs upon denaturation of C3 in sodium dodecyl sulphate (SDS), whereby neoantigenic C3(D) epitopes are exposed. The present paper deals with C3 bound to various target surfaces which are known to modulate the functional properties of C3 in different ways. Bound C3 fragments on serum-treated human aggregated gammaglobulin, zymosan, rabbit and sheep erythrocytes, and on circulating immune complexes isolated from sera of patients with rheumatoid arthritis and systemic lupus erythematosus, were shown to be mainly in the iC3b form. By RIAs, employing polyclonal antibodies, the range of C3(D) antigenic epitopes of 125I-labelled SDS denatured C3 expressed by the particle-bound iC3b was monitored. The physiologically bound iC3b on all tested particles expressed wide range of C3(D) epitopes and each type of particle-bound C3 exposed its individual range. By competition ELISA specific C3(D) alpha epitopes were monitored, employing monoclonal antibodies. A distinct difference in the expression of these epitopes was observed in iC3b bound to various test particles in the presence of normal serum and in iC3b present on circulating immune complexes from pathological sera. Considering that the neoantigenic C3(D) epitopes have been shown to be associated with different functions of C3, the distinctive antigenic expression of each type of serum-treated particle might reflect different functional forms of the protein.

Elevated NK-mediated lysis of Raji and Daudi cells carrying fixed iC3b fragments

Raji and Daudi cells were opsonized with C3b, iC3b, and C3d fragments by using purified complement components. The sensitivity of C3-opsonized cells to lysis mediated by low density blood lymphocytes was studied. Raji and Daudi cells carrying C3b or C3d fragments were lysed with similar efficiencies as the nonopsonized cells. The presence of iC3b on the target surface imposed elevated NK sensitivity. The iC3b-mediated enhancement of NK lysis was inhibited when iC3b fragments or rabbit anti-human C3 antibodies were included into the lytic assays. These results indicate that the iC3b fragments fixed on the targets bind to the CR3 on the lymphocytes. Results obtained in immobilized conjugate-lytic assays showed that iC3b-opsonized targets interact more readily with the lymphocytes. This was reflected by the elevated proportion of lymphocytes that were bound to the iC3b-carrying targets. The proportions of conjugates in which target damage occurred were similar with the control and with the iC3b-carrying cells. It seems therefore that opsonization of targets with iC3b leads to recruitment of effector lymphocytes due to contact with their CR3. However, once the effector-target contact is established, the triggering of lytic function does not seem to be influenced by the iC3b/CR3 bridge.

Binding and activation of C4 and C3 on the red cell surface by non-complement enzymes

We investigated the binding of C4 and C3 to red cell surfaces by non-complement enzymes. Cell bound C components were quantitated by a radioimmunoassay, the chain structure of bound components was analyzed by Western blotting and the hemolytic activity of bound components was determined. Trypsin, chymotrypsin, plasmin, elastase, thrombin, kallikrein and enzymes from Bacillus subtilis, Staphylococcus aureus and Streptomyces griseus all were found capable of binding C4b and C3b to sheep red cells. C4b bound by any of these enzymes was hemolytically active; both classical and alternate pathway activity of C3 could be demonstrated for most enzymes except plasmin and thrombin. In addition, trypsin and the bacterial enzymes were also able to generate the classical pathway C3-convertase from C4b + C2. The hemolytic efficiency of enzyme bound C4b and C3b was about the same as for these molecules bound by complement enzymes. In contrast, the process of binding by the non-complement enzymes was several hundred-fold less efficient than by cell bound complement enzymes. The results demonstrate that several enzymes can replace the C1 and C42 enzymes in the classical pathway and are able to initiate the alternative pathway by activating C3 and binding C3b to the cell surface.

Alternative pathway of complement activation by stimulated T lymphocytes. II. Elevation of cytotoxic potential against complement receptor-carrying cell lines

Exposure of lectin-stimulated (concanavalin A, phytohemagglutinin and pokeweed mitogen) blood lymphocytes to human serum or to purified C3 increased their cytotoxic capacity towards complement receptor positive targets such as Raji and Daudi cells. The lysis of complement receptor-negative lymphoblastoid cell lines was not influenced. The lytic capacity of lymphocytes exposed to 12-O-tetradecanoylphorbol 13-acetate was not elevated by human serum. Lectin-stimulated lymphocytes were previously shown to activate and bind C3. The results using lymphocytes activated in different ways and targets with or without complement receptor expression suggest that the C3b deposited on lymphocytes binds to the complement receptor on the targets. This contact elevates the avidity between the two cells as indicated also by the increased frequency of the lymphocyte-target conjugates. On the basis of immune adherence the C3 fragment bound on the lymphocytes was identified as C3b. The increase of the conjugate formation and cytotoxicity was abrogated when the target cells, Raji, were pre-exposed to purified C3d which occupy the CR2 receptor. The majority of lymphocytes responsible for the cytotoxicity were CD8+.

Trypanosoma lewisi: restriction of alternative complement pathway C3/C5 convertase activity

The rat parasite Trypanosoma lewisi was incubated in vitro with rat or human serum, washed, and extracted in detergent. Extracts were fractionated by electrophoresis in denaturing gels, transferred to nitrocellulose, allowed to renature, then immunoblotted with polyclonal antibodies to rat complement component C3 and human complement components C3, C5, and factor B. Molecules that reacted with these antibodies were detected in the extracts. Fragments of rat C3 were detected in extracts of parasites that had not been exposed to serum in vitro. Additional complement deposition occurred during in vitro incubations; human complement components deposited in vitro could be distinguished from rat components deposited in vivo. Complement deposition in vitro required magnesium ions and did not occur when heat inactivated serum was used. Components reacting with antibodies to human C3 included a group of bands with molecular weights higher than C3 alpha or beta chains. Blotting with affinity purified, chain specific antibodies demonstrated that a 68 kDa component on parasites is C3 beta and that a 44 kDa molecule is derived from C3 alpha. A 73 kDa component that was difficult to resolve from C3 beta is probably also a C3 alpha fragment. This suggests that an inactive iC3b-like molecule is present on parasites. Kinetic studies showed that cleavage of C3 alpha is rapid and that the amount of C3 alpha fragments and C3 beta on intact parasites reached a steady state after 15 min. When parasites were trypsinized prior to incubation in C5 or C6 deficient serum, the rate and extent of C3 and C5 deposition increased. Unprocessed C3 alpha' and C5 alpha' chains were detected. Trypsinized parasites were lysed by the alternative complement pathway in normal serum. Intact parasites could be lysed by complement in the presence of antibody. The data support our previous suggestion that trypsin sensitive surface proteins on intact T. lewisi limit alternative pathway activity by restricting C3/C5 convertase activity.