Different Host Complement Systems and Their Interactions with Saliva from Lutzomyia longipalpis (Diptera,Psychodidae) and Leishmania infantum Promastigotes

Background

Lutzomyia longipalpis is the vector of Leishmania infantum in the New World, and its saliva inhibits classical and alternative human complement system pathways. This inhibition is important in protecting the insect´s midgut from damage by the complement. L. longipalpis is a promiscuous blood feeder and must be protected against its host’s complement. The objective of this study was to investigate the action of salivary complement inhibitors on the sera of different host species, such as dogs, guinea pigs, rats and chickens, at a pH of 7.4 (normal blood pH) and 8.15 (the midgut pH immediately after a blood meal). We also investigated the role of the chicken complement system in Leishmania clearance in the presence and absence of vector saliva.

Results

The saliva was capable of inhibiting classical pathways in dogs, guinea pigs and rats at both pHs. The alternative pathway was not inhibited except in dogs at a pH of 8.15. The chicken classical pathway was inhibited only by high concentrations of saliva and it was better inhibited by the midgut contents of sand flies. Neither the saliva nor the midgut contents had any effect on the avian alternative pathway. Fowl sera killed L. infantum promastigotes, even at a low concentration (2%), and the addition of L. longipalpis saliva did not protect the parasites. The high body temperature of chickens (40°C) had no effect on Leishmania viability during our assays.

Conclusion

Salivary inhibitors act in a species-specific manner. It is important to determine their effects in the natural hosts of Leishmania infantum because they act on canid and rodent complements but not on chickens (which do not harbour the parasite). Moreover, we concluded that the avian complement system is the probable mechanism through which chickens eliminate Leishmania and that their high body temperature does not influence this parasite.     

Influence of the intestinal anticoagulant in the feeding performance of triatomine bugs (Hemiptera; Reduviidae)

Triatomines are haematophagous insects in all post-embryonic life stages. They are vectors of Trypanosoma cruzi, the causative agent of Chagas disease. Their vectorial ability is influenced by their feeding performance, which varies greatly amongst species. Recent work showed that inhibition of the coagulation process in the anterior midgut (crop) environment considerably influences the blood meal size. In this work, we performed a comparative study of the level of anticoagulant activity in the saliva and crop contents of three triatomine species - Triatoma infestans, Triatoma brasiliensis and Rhodnius prolixus - and correlated this with their feeding performance on live hosts. Moreover, the feeding parameters on a large diameter vessel influenced by the crop anticoagulants were evaluated in detail. The anticoagulant activity was significantly higher in the crop contents than in salivary glands, varying from 1.6-fold higher for R. prolixus to 70-fold higher for T. brasiliensis. Amongst the species, T. brasiliensis had the lowest crop anticoagulant activity, the lowest concentration of thrombin inhibitor, and took the longest to feed. Triatoma brasiliensis nymphs that had their intestinal anticoagulant (brasiliensin) knocked down by RNA interference had the lowest capacity to maintain cibarial pump frequency at higher levels throughout the feeding process and consequently a lower ingestion rate (mg/min), even when fed under favourable conditions (large diameter vessel). However, the feeding difficulty for brasiliensin knockdown T. brasiliensis nymphs was reversed by treating the host mice with heparin (a potent systemic anticoagulant) before blood feeding. The results indicate that crop anticoagulant activity influences modulation of the blood-pumping frequency to the intestine and significantly affects the feeding efficiency of triatomine spp. on live hosts.     

The location of parasites within their hosts: The influence of host feeding and diet on the dispersion of adults of Nippostrongylus brasiliensis in the intestine of the rat

This paper is the first in a series to experimentally examine factors which determine the location of parasites within their hosts. Nippostrongylus brasiliensis is typically found one third of the way along the small intestine of the rat between days 7 and 12 of a primary infection. When rats were fasted for 2 days, N. brasiliensis became evenly distributed along the intestine and many individuals, mostly females, entered the caecum. Upon being fed again, the parasites were at their ‘normal site’ within 24 h. Experimental diets of α-cellulose, or simple dietary compounds caused the same re-distribution as did normal food. By sectioning the rat intestine when it was full of food and relating the dispersion of worms to the presence of intestinal contents, it was confirmed that N. brasiliensis is found amongst the intestinal contents of its host. The notion of a ‘site’ is critically discussed and the merit of ‘habitat’ is argued because the ‘site’ of N. brasiliensis would appear to be less a ‘place’ but rather a combination of environmental factors.     

Starvation at the larval stage increases the vector competence of Aedes aegypti females for Zika virus

Aedes aegypti is the primary vector of Zika virus (ZIKV), a flavivirus which typically presents itself as febrile-like symptoms in humans but can also cause neurological and pregnancy complications. The transmission cycle of mosquito-borne arboviruses such as ZIKV requires that various key tissues in the female mosquito get productively infected with the virus before the mosquito can transmit the virus to another vertebrate host. Following ingestion of a viremic blood-meal from a vertebrate, ZIKV initially infects the midgut epithelium before exiting the midgut after blood-meal digestion to disseminate to secondary tissues including the salivary glands. Here we investigated whether smaller Ae. aegypti females resulting from food deprivation as larvae exhibited an altered vector competence for blood-meal acquired ZIKV relative to larger mosquitoes. Midguts from small ‘Starve’ and large ‘Control’ Ae. aegypti were dissected to visualize by transmission electron microscopy (TEM) the midgut basal lamina (BL) as physical evidence for the midgut escape barrier showing Starve mosquitoes with a significantly thinner midgut BL than Control mosquitoes at two timepoints. ZIKV replication was inhibited in Starve mosquitoes following intrathoracic injection of virus, however, Starve mosquitoes exhibited a significantly higher midgut escape and population dissemination rate at 9 days post-infection (dpi) via blood-meal, with more virus present in saliva and head tissue than Control by 10 dpi and 14 dpi, respectively. These results indicate that Ae. aegypti developing under stressful conditions potentially exhibit higher midgut infection and dissemination rates for ZIKV as adults, Thus, variation in food intake as larvae is potentially a source for variable vector competence levels of the emerged adults for the virus.     

Trichinella spiralis Paramyosin Binds Human Complement C1q and Inhibits Classical Complement Activation

Background

Trichinella spiralis expresses paramyosin (Ts-Pmy) as a defense mechanism. Ts-Pmy is a functional protein with binding activity to human complement C8 and C9 and thus plays a role in evading the attack of the host’s immune system. In the present study, the binding activity of Ts-Pmy to human complement C1q and its ability to inhibit classical complement activation were investigated.

Methods and Findings

The binding of recombinant and natural Ts-Pmy to human C1q were determined by ELISA, Far Western blotting and immunoprecipitation, respectively. Binding of recombinant Ts-Pmy (rTs-Pmy) to C1q inhibited C1q binding to IgM and consequently inhibited C3 deposition. The lysis of antibody-sensitized erythrocytes (EAs) elicited by the classical complement pathway was also inhibited in the presence of rTs-Pmy. In addition to inhibiting classical complement activation, rTs-Pmy also suppressed C1q binding to THP-1-derived macrophages, thereby reducing C1q-induced macrophages migration.

Conclusion

Our results suggest that T. spiralis paramyosin plays an important role in immune evasion by interfering with complement activation through binding to C1q in addition to C8 and C9.     

Hyperglycemia-Induced Protein Kinase C Activation Inhibits Phagocytosis of C3b- and Immunoglobulin G–Opsonized Yeast Particles in Normal Human Neutrophils

The aim of this study was to investigate the effects of elevated glucose concentrations on complement receptor– and Fcγ receptor–mediated phagocytosis in normal human neutrophils. D-Glucose at 15 or 25 mM dose-dependently inhibited both complement receptor– and Fcγ receptor– mediated phagocytosis, as compared to that at a normal physiological glucose concentration. The protein kinase C (PKC) inhibitors GF109203X and Go6976 both dose dependently and completely reversed the inhibitory effect of 25 mM D-glucose on phagocytosis. Complement receptor– mediated phagocytosis was dose-dependently inhibited by the cell permeable diacylglycerol analogue 1,2-dioctanoylsn- glycerol (DAG), an effect that was abolished by PKC inhibitors. Furthermore, suboptimal inhibitory concentrations of DAG and glucose showed an additive inhibitory effect on complement receptor–mediated phagocytosis. The authors conclude that elevated glucose concentrations can inhibit complement receptor and Fcγ receptor–mediated phagocytosis in normal human neutrophils by activating PKCα and/or PKCβ, an effect possibly mediated by DAG.     

Anopheles Midgut Epithelium Evades Human Complement Activity by Capturing Factor H from the Blood Meal

Hematophagous vectors strictly require ingesting blood from their hosts to complete their life cycles. Exposure of the alimentary canal of these vectors to the host immune effectors necessitates efficient counteractive measures by hematophagous vectors. The Anopheles mosquito transmitting the malaria parasite is an example of hematophagous vectors that within seconds can ingest human blood double its weight. The innate immune defense mechanisms, like the complement system, in the human blood should thereby immediately react against foreign cells in the mosquito midgut. A prerequisite for complement activation is that the target cells lack complement regulators on their surfaces. In this work, we analyzed whether human complement is active in the mosquito midgut, and how the mosquito midgut cells protect themselves against complement attack. We found that complement remained active for a considerable time and was able to kill microbes within the mosquito midgut. However, the Anopheles mosquito midgut cells were not injured. These cells were found to protect themselves by capturing factor H, the main soluble inhibitor of the alternative complement pathway. Factor H inhibited complement on the midgut cells by promoting inactivation of C3b to iC3b and preventing the activity of the alternative pathway amplification C3 convertase enzyme. An interference of the FH regulatory activity by monoclonal antibodies, carried to the midgut via blood, resulted in increased mosquito mortality and reduced fecundity. By using a ligand blotting assay, a putative mosquito midgut FH receptor could be detected. Thereby, we have identified a novel mechanism whereby mosquitoes can tolerate human blood.     

Molecular basis of anticoagulant and anticomplement activity of the tick salivary protein Salp14 and its homologs

During feeding, a tick''s mouthpart penetrates the host''s skin and damages tissues and small blood vessels, triggering the extrinsic coagulation and lectin complement pathways. To elude these defense mechanisms, ticks secrete multiple anticoagulant proteins and complement system inhibitors in their saliva. Here, we characterized the inhibitory activities of the homologous tick salivary proteins tick salivary lectin pathway inhibitor, Salp14, and Salp9Pac from Ixodesscapularis in the coagulation cascade and the lectin complement pathway. All three proteins inhibited binding of mannan-binding lectin to the polysaccharide mannan, preventing the activation of the lectin complement pathway. In contrast, only Salp14 showed an appreciable effect on coagulation by prolonging the lag time of thrombin generation. We found that the anticoagulant properties of Salp14 are governed by its basic tail region, which resembles the C terminus of tissue factor pathway inhibitor alpha and blocks the assembly and/or activity of the prothrombinase complex in the same way. Moreover, the Salp14 protein tail contributes to the inhibition of the lectin complement pathway via interaction with mannan binding lectin–associated serine proteases. Furthermore, we identified BaSO₄-adsorbing protein 1 isolated from the tick Ornithodoros savignyi as a distant homolog of tick salivary lectin pathway inhibitor/Salp14 proteins and showed that it inhibits the lectin complement pathway but not coagulation. The structure of BaSO₄-adsorbing protein 1, solved here using NMR spectroscopy, indicated that this protein adopts a noncanonical epidermal growth factor domain–like structural fold, the first such report for tick salivary proteins. These data support a mechanism by which tick saliva proteins simultaneously inhibit both the host coagulation cascade and the lectin complement pathway.     

Borrelia valaisiana Resist Complement-Mediated Killing Independently of the Recruitment of Immune Regulators and Inactivation of Complement Components

Spirochetes belonging to the Borrelia (B.) burgdorferi sensu lato complex differ in their resistance to complement-mediated killing, particularly in regard to human serum. In the present study, we elucidate the serum and complement susceptibility of B. valaisiana, a genospecies with the potential to cause Lyme disease in Europe as well as in Asia. Among the investigated isolates, growth of ZWU3 Ny3 was not affected while growth of VS116 and Bv9 was strongly inhibited in the presence of 50% human serum. Analyzing complement activation, complement components C3, C4 and C6 were deposited on the surface of isolates VS116 and Bv9, and similarly the membrane attack complex was formed on their surface. In contrast, no surface-deposited components and no aberrations in cell morphology were detected for serum-resistant ZWU3 Ny3. While further investigating the protective role of bound complement regulators in mediating complement resistance, we discovered that none of the B. valaisiana isolates analyzed bound complement regulators Factor H, Factor H-like protein 1, C4b binding protein or C1 esterase inhibitor. In addition, B. valaisiana also lacked intrinsic proteolytic activity to degrade complement components C3, C3b, C4, C4b, and C5. Taken together, these findings suggest that certain B. valaisiana isolates differ in their capability to resist complement-mediating killing by human serum. The molecular mechanism utilized by B. valaisiana to inhibit bacteriolysis appears not to involve binding of the key host complement regulators of the alternative, classical, and lectin pathways as already known for serum-resistant Lyme disease or relapsing fever borreliae.     

A Scabies Mite Serpin Interferes with Complement-Mediated Neutrophil Functions and Promotes Staphylococcal Growth

Background

Scabies is a contagious skin disease caused by the parasitic mite Sarcoptes scabiei. The disease is highly prevalent worldwide and known to predispose to secondary bacterial infections, in particular by Streptococcus pyogenes and Staphylococcus aureus. Reports of scabies patients co-infected with methicillin resistant S. aureus (MRSA) pose a major concern for serious down-stream complications. We previously reported that a range of complement inhibitors secreted by the mites promoted the growth of S. pyogenes. Here, we show that a recently characterized mite serine protease inhibitor (SMSB4) inhibits the complement-mediated blood killing of S. aureus.

Methodology/Principal Findings

Blood killing of S. aureus was measured in whole blood bactericidal assays, counting viable bacteria recovered after treatment in fresh blood containing active complement and phagocytes, treated with recombinant SMSB4. SMSB4 inhibited the blood killing of various strains of S. aureus including methicillin-resistant and methicillin-sensitive isolates. Staphylococcal growth was promoted in a dose-dependent manner. We investigated the effect of SMSB4 on the complement-mediated neutrophil functions, namely phagocytosis, opsonization and anaphylatoxin release, by flow cytometry and in enzyme linked immuno sorbent assays (ELISA). SMSB4 reduced phagocytosis of S. aureus by neutrophils. It inhibited the deposition of C3b, C4b and properdin on the bacteria surface, but did not affect the depositions of C1q and MBL. SMSB4 also inhibited C5 cleavage as indicated by a reduced C5b-9 deposition.

Conclusions/Significance

We postulate that SMSB4 interferes with the activation of all three complement pathways by reducing the amount of C3 convertase formed. We conclude that SMSB4 interferes with the complement-dependent killing function of neutrophils, thereby reducing opsonization, phagocytosis and further recruitment of neutrophils to the site of infection. As a consequence secreted scabies mites complement inhibitors, such as SMSB4, provide favorable conditions for the onset of S. aureus co-infection in the scabies-infected microenvironment by suppressing the immediate host immune response.     

Post-feeding induction of trypsin in the midgut of Aedes aegypti L. (Diptera: Culicidae) is separable into two cellular phases

The induction of trypsin activity in the midgut of the mosquito, Aedes aegypti, was studied following meals of chicken blood, and several protein and peptide diets. Various concentrations of bovine serum albumin (BSA) in 0.15 M NaCl stimulated trypsin activity, in a similar fashion to the initial increase observed after a normal blood meal. Trypsin synthesis was also initiated when Ae. aegypti were fed on glutaraldehyde cross-linked BSA and on BSA fragments prepared by both pepsin and cyanogen bromide cleavage. Non-soluble proteins, in the form of glutaraldehyde-fixed erythrocyte ghosts, induced a delayed and reduced trypsin response, whilst small peptides from neutralized liver digests did not induce trypsin activity until 8–10 h after feeding. Metabolic inhibitors had varying effects on the post-feeding activity of trypsin stimulated by BSA feeding. Cycloheximide, a peptidyl transferase inhibitor prevented expression of all activity in vivo, whereas α-amanitin (RNA-polymerase inhibitor) did not affect trypsin activity in the first 10 h after feeding. At 20 μg/ml concentration in the diet, actinomycin D (RNA synthesis inhibitor) caused temporary superinduction followed by inhibition of trypsin activity, but at lower concentrations, the later phase of trypsin activity was inhibited. The results suggest that post-feeding induction of trypsin activity in Ae. aegypti is a two-phase process regulated at the midgut cellular level. The first phase of trypsin synthesis is stimulated by soluble proteins of variable molecular weights, and only involves translation of messenger RNA already available within the midgut cells. The second phase is stimulated by small peptides and requires complete synthesis of new mRNA from DNA.     

Selective inhibitors of digestive enzymes from Aedes aegypti larvae identified by phage display

Dengue is a serious disease transmitted by the mosquito Aedes aegypti during blood meal feeding. It is estimated that the dengue virus is transmitted to millions of individuals each year in tropical and subtropical areas. Dengue control strategies have been based on controlling the vector, Ae. aegypti, using insecticide, but the emergence of resistance poses new challenges. The aim of this study was the identification of specific protease inhibitors of the digestive enzymes from Ae. aegypti larvae, which may serve as a prospective alternative biocontrol method. High affinity protein inhibitors were selected by all of the digestive serine proteases of the 4th instar larval midgut, and the specificity of these inhibitors was characterized. These inhibitors were obtained from a phage library displaying variants of HiTI, a trypsin inhibitor from Haematobia irritans, that are mutated in the reactive loop (P1–P4′). Based on the selected amino acid sequence pattern, seven HiTI inhibitor variants were cloned, expressed and purified. The results indicate that the HiTI variants named T6 (RGGAV) and T128 (WNEGL) were selected by larval trypsin-like (IC₅₀ of 1.1 nM) and chymotrypsin-like enzymes (IC₅₀ of 11.6 nM), respectively. The variants T23 (LLGGL) and T149 (GGVWR) inhibited both larval chymotrypsin-like (IC₅₀ of 4.2 nM and 29.0 nM, respectively) and elastase-like enzymes (IC₅₀ of 1.2 nM for both). Specific inhibitors were successfully obtained for the digestive enzymes of Ae. aegypti larvae by phage display. Our data also strongly suggest the presence of elastase-like enzymes in Ae. aegypti larvae. The HiTI variants T6 and T23 are good candidates for the development as a larvicide to control the vector.     

Effects of Streptococcus pneumoniae strain background on complement resistance

Background

Immunity to infections caused by Streptococcus pneumoniae is dependent on complement. There are wide variations in sensitivity to complement between S. pneumoniae strains that could affect their ability to cause invasive infections. Although capsular serotype is one important factor causing differences in complement resistance between strains, there is also considerable other genetic variation between S. pneumoniae strains that may affect complement-mediated immunity. We have therefore investigated whether genetically distinct S. pneumoniae strains with the same capsular serotype vary in their sensitivity to complement mediated immunity.

Methodology and Principal Findings

C3b/iC3b deposition and neutrophil association were measured using flow cytometry assays for S. pneumoniae strains with different genetic backgrounds for each of eight capsular serotypes. For some capsular serotypes there was marked variation in C3b/iC3b deposition between different strains that was independent of capsule thickness and correlated closely to susceptibility to neutrophil association. C3b/iC3b deposition results also correlated weakly with the degree of IgG binding to each strain. However, the binding of C1q (the first component of the classical pathway) correlated more closely with C3b/iC3b deposition, and large differences remained in complement sensitivity between strains with the same capsular serotype in sera in which IgG had been cleaved with IdeS.

Conclusions

These data demonstrate that bacterial factors independent of the capsule and recognition by IgG have strong effects on the susceptibility of S. pneumoniae to complement, and could therefore potentially account for some of the differences in virulence between strains.     

Defense reaction of mosquitoes to filarial worms: role of the microfilarial sheath in the response of mosquitoes to inoculated Brugia pahangi microfilariae

The melanization response of Aedes trivittatus and A. aegypti (black-eyed Liverpool strain) against intrathoracically inoculated sheathed and chemically exsheathed Brugia pahangi microfilariae (mff) was assessed daily through 5 days postinoculation (PI). Response of A. aegypti against exsheathed mff was significantly reduced on all days compared with the response against sheathed mff, and a significantly greater percentage of exsheathed mff were alive through 4 days PI than were sheathed mff. The melanization response of A. trivittatus was nearly 100% effective against either sheathed or exsheathed mff by Day 2 PI. When mff were allowed to migrate through A. aegypti midguts in vitro before inoculation into intact A. aegypti, nearly 94% ($\text{120}\text{128}$) of the parasites recovered had avoided the response and were developing. Penetration of A. trivittatus midguts in vitro by mff before inoculation into intact A. trivittatus did not prevent a melanization response. Inoculation of mff into A. trivittatus following A. aegypti midgut penetration, however, resulted in almost 60% ($\text{98}\text{171}$) of the mff avoiding the response and developing as normal L₁ larvae after 5 days PI. The possibility of mff acquiring host antigens during midgut penetration and therefore avoiding recognition as nonself by mosquitoes, and (or) the possibility of the midgut environment modifying or stimulating mff to inhibit the response of mosquitoes are discussed.     

Inhibition of C3 Convertase Activity by Hepatitis C Virus as an Additional Lesion in the Regulation of Complement Components

We have previously reported that in vitro HCV infection of cells of hepatocyte origin attenuates complement system at multiple steps, and attenuation also occurs in chronically HCV infected liver, irrespective of the disease stage. However, none of these regulations alone completely impaired complement pathways. Modulation of the upstream proteins involved in proteolytic processing of the complement cascade prior to convertase formation is critical in promoting the function of the complement system in response to infection. Here, we examined the regulation of C2 complement expression in hepatoma cells infected in vitro with cell culture grown virus, and validated our observations using randomly selected chronically HCV infected patient liver biopsy specimens. C2 mRNA expression was significantly inhibited, and classical C3 convertase (C4b2a) decreased. In separate experiments for C3 convertase function, C3b deposition onto bacterial membrane was reduced using HCV infected patient sera as compared to uninfected control, suggesting impaired C3 convertase. Further, iC3b level, a proteolytically inactive form of C3b, was lower in HCV infected patient sera, reflecting impairment of both C3 convertase and Factor I activity. The expression level of Factor I was significantly reduced in HCV infected liver biopsy specimens, while Factor H level remained unchanged or enhanced. Together, these results suggested that inhibition of C3 convertase activity is an additional cumulative effect for attenuation of complement system adopted by HCV for weakening innate immune response.     

Tsetse GmmSRPN10 Has Anti-complement Activity and Is Important for Successful Establishment of Trypanosome Infections in the Fly Midgut

The complement cascade in mammalian blood can damage the alimentary tract of haematophagous arthropods. As such, these animals have evolved their own repertoire of complement-inactivating factors, which are inadvertently exploited by blood-borne pathogens to escape complement lysis. Unlike the bloodstream stages, the procyclic (insect) stage of Trypanosoma brucei is highly susceptible to complement killing, which is puzzling considering that a tsetse takes a bloodmeal every 2–4 days. In this study, we identified four tsetse (Glossina morsitans morsitans) serine protease inhibitors (serpins) from a midgut expressed sequence tag (EST) library (GmmSRPN3, GmmSRPN5, GmmSRPN9 and GmmSRPN10) and investigated their role in modulating the establishment of a T. brucei infection in the midgut. Although not having evolved in a common blood-feeding ancestor, all four serpins have an active site sharing remarkable homology with the human complement C1-inhibitor serpin, SerpinG1. RNAi knockdown of individual GmmSRPN9 and GmmSRPN10 genes resulted in a significant decreased rate of infection by procyclic form T. brucei. Furthermore, recombinant GmmSRPN10 was both able to inhibit the activity of human complement-cascade serine proteases, C1s and Factor D, and to protect the in vitro killing of procyclic trypanosomes when incubated with complement-activated human serum. Thus, the secretion of serpins, which may be part of a bloodmeal complement inactivation system in tsetse, is used by procyclic trypanosomes to evade an influx of fresh trypanolytic complement with each bloodmeal. This highlights another facet of the complicated relationship between T. brucei and its tsetse vector, where the parasite takes advantage of tsetse physiology to further its chances of propagation and transmission.     

The influence of Trypanosoma brucei infection on local immunoglobulin responses of rats to Nippostrongylus brasiliensis

Wedrychowicz H., Maclean J. M. and Holmes P. H., 1984. The influence of Trypanosoma brucei infection on local immunoglobulin responses of rats to Nippostrongylus brasiliensis. International Journalfor Parasitology14: 453–458. Serum, intestinal and lung immunoglobulin and antibody isotype responses to Nippostrongylus brasiliensis infection were studied in normal and trypanosome-infected Hooded Lister rats. Rats which received trypanosomes 7 days before N. brasiliensis infection had impaired responses of serum IgG and IgA. Bronchial and intestinal mucosal IgG was not reduced whilst IgA concentration in these sites was markedly diminished. Total immunoglobulin M levels in T. brucei parasitised rats were higher in both sera and mucosal sites. However, tests with radiolabelled adult nematode excretory-secretory antigens indicated that specific lung and intestinal IgM responses were reduced. Immunoglobulin A antibody responses were diminished most markedly in sera and lungs and also in the intestine while IgG antibodies were decreased in sera and intestine mucosae T. brucei infected rats had higher worm burdens than rats infected with N. brasiliensis alone but worm expulsion was not delayed. The results indicate that local as well as systemic antibody responses are reduced in trypanosome infected animals.     

Fasciola hepatica is refractory to complement killing by preventing attachment of mannose binding lectin (MBL) and inhibiting MBL-associated serine proteases (MASPs) with serpins

The complement system is a first-line innate host immune defence against invading pathogens. It is activated via three pathways, termed Classical, Lectin and Alternative, which are mediated by antibodies, carbohydrate arrays or microbial liposaccharides, respectively. The three complement pathways converge in the formation of C3-convertase followed by the assembly of a lethal pore-like structure, the membrane attack complex (MAC), on the pathogen surface. We found that the infectious stage of the helminth parasite Fasciola hepatica, the newly excysted juvenile (NEJ), is resistant to the damaging effects of complement. Despite being coated with mannosylated proteins, the main initiator of the Lectin pathway, the mannose binding lectin (MBL), does not bind to the surface of live NEJ. In addition, we found that recombinantly expressed serine protease inhibitors secreted by NEJ (rFhSrp1 and rFhSrp2) selectively prevent activation of the complement via the Lectin pathway. Our experiments demonstrate that rFhSrp1 and rFhSrp2 inhibit native and recombinant MBL-associated serine proteases (MASPs), impairing the primary step that mediates C3b and C4b deposition on the NEJ surface. Indeed, immunofluorescence studies show that MBL, C3b, C4b or MAC are not deposited on the surface of NEJ incubated in normal human serum. Taken together, our findings uncover new means by which a helminth parasite prevents the activation of the Lectin complement pathway to become refractory to killing via this host response, in spite of presenting an assortment of glycans on their surface.     

A Novel Peptide Can Mimic Extracellular Fibrinogen-Binding Protein to Block the Activation of Complement System

Extracellular fibrinogen-binding protein (Efb) of Staphylococcus aureus (S. aureus) is a bi-functional protein, which can specifically bind fibrinogen with its N terminus and inhibit deposition of C3b on the surface of S. aureus with its C terminus. Here, we screened the epitopes of Efb using phage display. Four peptides with consensus motif were screened. This consensus motif was identical to C terminus (161–164) of Efb. In the further investigation, it was found the synthesized peptide EC1 (154–165aa of Efb) could specifically bind C3/C3b and subsequently to block the activation of complement. Meanwhile, EC1 could inhibit the interaction between Efb and C3/C3b. Moreover, the interaction between the mutant protein of EmC1 (Efb without EC1) and C3 was decreased. And, the effect on the complement system of the mutant protein was dramatically declined compared with Efb. Our finding suggested that the peptide EC1 could mimic Efb to block complement system activation via binding C3.