Contribution of C5-mediated mechanisms to host defence against Echinococcus granulosus hydatid infection

An Excretory Protein of Echinococcus multilocularis Inhibits Complement Classical Pathway Activation

Introduction

Alveolar echinococcosis is a lethal zoonosis caused by Echinococcus multilocularis (E.m) larvae. The mechanism by which E.m evades host immune attacks and ensures long-term survival remains unexplained. The complement system is a cascade of sequentially activated complement proteins that results in opsonization-related phagocytosis or membrane lysis of invading organisms. Excretory/secretory proteins (ESPs) of parasites are the main antigens that induce the immune response and play important roles in the long-term survival.

Methods

We investigated the possibility that E.m inhibits complement activation through ESPs and examined the potential related mechanism. A haemolysis assay was used to determine if and how in vitro culture medium of E.m containing ESPs can inhibit complement activation. Potential ESPs were annotated using bioinformatics methods, and one ESP was subsequently expressed as a recombinant protein with a eukaryotic expression system. The ability of this protein to inhibit complement activation was also tested by haemolysis assay.

Results

These assays showed that in vitro culture medium of E.m inhibited activation of the complement classical pathway. EmuJ_000439500 encodes a protein containing seven Sushi domains, which was the only potential E.m-derived complement inhibitor (Em-CI, UniProt: A0A068Y4F2) annotated among the 653 ESPs. Recombinant Em-CI also displayed the ability to inhibit activation of the complement classical pathway.

Discussion

The discovery of Em-CI sheds light on the mechanism by which E.m escapes killing by the complement system and provides potential targets for immunotherapy for parasitic diseases.

Inhibition of inflammatory cytokine production and proliferation in macrophages by Kunitz-type inhibitors from Echinococcus granulosus

The genus Echinococcus of cestode parasites includes important pathogens of humans and livestock animals. Transcriptomic and genomic studies on E. granulosus and E. multilocularis uncovered striking expansion of monodomain Kunitz proteins. This expansion is accompanied by the specialization of some family members away from the ancestral protease inhibition function to fulfill cation channel blockade functions. Since cation channels are involved in immune processes, we tested the effects on macrophage physiology of two E. granulosus Kunitz-type inhibitors of voltage-activated cation channels (K_v) that are close paralogs. Both inhibitors, EgKU-1 and EgKU-4, inhibited production of the Th1/Th17 cytokine subunit IL-12/23p40 by macrophages stimulated with the TLR4 agonist LPS. In addition, EgKU-4 but not EgKU-1 inhibited production of the inflammatory cytokine IL-6. These activities were not displayed by EgKU-3, a family member that is a protease inhibitor without known activity on cation channels. EgKU-4 potently inhibited macrophage proliferation in response to M-CSF, whereas EgKU-1 displayed similar activity but with much lower potency, similar to EgKU-3. We discuss structural differences, including a heavily cationic C-terminal extension present in EgKU-4 but not in EgKU-1, that may explain the differential activities of the two close paralogs.

Polyreactive antibodies as potential humoral biomarkers of host resistance to cystic echinococcosis

Polyreactive antibodies (pAb) bind to a broad range of unrelated structures, providing hosts with functional components able to rapidly recognize and protect against different pathogens. However, their roles against helminth parasites are still unexplored. Here, pAb profiles were analysed in cystic echinococcosis (CE), a zoonosis caused by the cestode Echinococcus granulosus sensu lato. Levels of anti-DNP (2,4-dinitrophenyl-hapten) antibodies were measured as a surrogate parameter of pAb in different biological settings. Firstly, levels of serum and peritoneal pAb were measured during early experimental secondary CE, using both high (Balb/c) and low (C57Bl/6) susceptible mouse strains. Serum pAb mostly differed in normal mice, being pAb levels of IgG subclasses with poor anti-parasite activities predominant in Balb/c animals. Conversely, peritoneal pAb isotypes/subclasses with efficient anti-parasite activities predominated in normal and infected C57Bl/6 mice. Secondly, sera from potentially resistant patients, susceptible individuals and healthy donors were analysed, showing higher pAb levels of the IgA and IgG-particularly IgG1-isotypes in potentially resistant individuals compared to control groups. Finally, since remarkable differences were observed in pAb profiles according to the intrinsic host susceptibility to the infection, we proposed here that pAb might be considered as potential humoral biomarkers for host resistance to CE.

Susceptibility and resistance to Echinococcus granulosus infection: Associations between mouse strains and early peritoneal immune responses

In helminth infections, there are no easy associations between host susceptibility and immune responses. Interestingly, immunity to cestodes - unlike most helminths - seems to require Th1-type effectors. In this sense, we reported recently that Balb/c and C57Bl/6 mice are high and low susceptible strains, respectively, to experimental infection by Echinococcus granulosus. However, the role of the early cellular peritoneal response in such differential susceptibility is unknown. Here, we analyzed the kinetics of cytokines expression and cellular phenotypes in peritoneal cells from infected Balb/c and C57Bl/6 mice. Additionally, Principal Components Analysis (PCA) were conducted to highlight the most relevant differences between strains. Finally, the anti-parasite activities of peritoneal cells were assessed through in vitro systems. PCAs clustered C57Bl/6 mice by their early mixed IL-5/TNF-α responses and less intense expression of Th2-type cytokines. Moreover, they exhibited lower counts of eosinophils and higher numbers of macrophages and B cells. Functional studies showed that peritoneal cells from infected C57Bl/6 mice displayed greater anti-parasite activities, in accordance with higher rates of NO production and more efficient ADCC responses. In conclusion, mild Th2-responses and active cellular mechanisms are key determinants in murine resistance to E. granulosus infection, supporting the cestode immune exception among helminth parasites.

Granulocytes in helminth infection -- who is calling the shots?

Helminths are parasitic organisms that can be broadly described as “worms” due to their elongated body plan, but which otherwise differ in shape, development, migratory routes and the predilection site of the adults and larvae. They are divided into three major groups: trematodes (flukes), which are leaf-shaped, hermaphroditic (except for blood flukes) flatworms with oral and ventral suckers; cestodes (tapeworms), which are segmented, hermaphroditic flatworms that inhabit the intestinal lumen; and nematodes (roundworms), which are dioecious, cylindrical parasites that inhabit intestinal and peripheral tissue sites. Helminths exhibit a sublime co-evolution with the host´s immune system that has enabled them to successfully colonize almost all multicellular species present in every geographical environment, including over two billion humans. In the face of this challenge, the host immune system has evolved to strike a delicate balance between attempts to neutralize the infectious assault versus limitation of damage to host tissues. Among the most important cell types during helminthic invasion are granulocytes: eosinophils, neutrophils and basophils. Depending on the specific context, these leukocytes may have pivotal roles in host protection, immunopathology, or facilitation of helminth establishment. This review provides an overview of the function of granulocytes in helminthic infections.

The immune response to parasitic helminths of veterinary importance and its potential manipulation for future vaccine control strategies

Despite the increasing knowledge of the immunobiology and epidemiology of parasitic helminths of the gastrointestinal system and the cardiorespiratory system, complications arising from infections of animals and humans with these parasites are a major clinical and economic problem. This has been attributed to the high incidence of these parasites, the widespread emergence of multi-drug resistant parasite strains and the lack of effective vaccines. Efforts to develop and produce vaccines against virtually all helminths (with the exception of Dictyocaulus viviparus and some cestode species) have been hindered by the complexity of the host-parasite relationship, and incomplete understanding of the molecular and immune regulatory pathways associated with the development of protective immunity against helminths. Novel genomic and proteomic technologies have provided opportunities for the discovery and characterisation of effector mechanisms and molecules that govern the host-parasite interactions in these two body systems. Such knowledge provided clues on how appropriate and protective responses are elicited against helminths and, thus, may lead to the development of effective therapeutic strategies. Here, we review advances in the immune response to selected helminths of animal health significance, and subsequent vaccine potential. The topics addressed are important for understanding how helminths interact with host immune defences and also are relevant for understanding the pathogenesis of diseases caused by helminths.

Identification of loci controlling restriction of parasite growth in experimental Taenia crassiceps cysticercosis

Human neurocysticercosis (NC) caused by Taenia solium is a parasitic disease of the central nervous system that is endemic in many developing countries. In this study, a genetic approach using the murine intraperitoneal cysticercosis caused by the related cestode Taenia crassiceps was employed to identify host factors that regulate the establishment and proliferation of the parasite. A/J mice are permissive to T. crassiceps infection while C57BL/6J mice (B6) are comparatively restrictive, with a 10-fold difference in numbers of peritoneal cysticerci recovered 30 days after infection. The genetic basis of this inter-strain difference was explored using 34 AcB/BcA recombinant congenic strains derived from A/J and B6 progenitors, that were phenotyped for T. crassiceps replication. In agreement with their genetic background, most AcB strains (A/J-derived) were found to be permissive to infection while most BcA strains (B6-derived) were restrictive with the exception of a few discordant strains, together suggesting a possible simple genetic control. Initial haplotype association mapping using >1200 informative SNPs pointed to linkages on chromosomes 2 (proximal) and 6 as controlling parasite replication in the AcB/BcA panel. Additional linkage analysis by genome scan in informative [AcB55xDBA/2]F1 and F2 mice (derived from the discordant AcB55 strain), confirmed the effect of chromosome 2 on parasite replication, and further delineated a major locus (LOD = 4.76, p<0.01; peak marker D2Mit295, 29.7 Mb) that we designate Tccr1 (T. crassiceps cysticercosis restrictive locus 1). Resistance alleles at Tccr1 are derived from AcB55 and are inherited in a dominant fashion. Scrutiny of the minimal genetic interval reveals overlap of Tccr1 with other host resistance loci mapped to this region, most notably the defective Hc/C5 allele which segregates both in the AcB/BcA set and in the AcB55xDBA/2 cross. These results strongly suggest that the complement component 5 (C5) plays a critical role in early protective inflammatory response to infection with T. crassiceps.

Infection with the cestode Taenia solium causes cysticercosis in humans and pigs. Neurocysticercosis is a severe manifestation of T. solium infection that constitutes an important health concern in developing countries. Studies in humans living in areas of endemic disease and in pigs experimentally infected have suggested a large spectrum of permissiveness to T. solium multiplication, with the possible contribution of genetic factors. In the present report, we have used an experimental mouse model of intraperitoneal infection with Taenia crassiceps to study the potential role of genetic factors in regulating replication of this parasite. Our study focused on two inbred mouse strains A/J and C57BL/6J that are respectively permissive and non-permissive to intraperitoneal multiplication of T. crassiceps. We have used a set of AcB/BcA recombinant congenic strains of mice along with standard F2 crosses to decipher the complexity and nature of the genetic component of the A/J vs. C57BL/6J interstrain difference in permissiveness. Our results point to a major role of the complement component 5 (C5) in early response and protection against T. crassiceps infection.

Understanding the laminated layer of larval Echinococcus II: immunology

The laminated layer (LL) is the massive carbohydrate-rich structure that protects Echinococcus larvae, which cause cystic echinococcosis (hydatid disease) and alveolar echinococcosis. Increased understanding of the biochemistry of the LL is allowing a more informed analysis of its immunology. The LL not only protects the parasite against host attack but also shapes the overall immune response against it. Because of its dense glycosylation, it probably contains few T-cell epitopes, being important instead in T-cell independent antibody responses. Crucially, it is decoded in non-inflammatory fashion by innate immunity, surely contributing to the strong immune-regulation observed in Echinococcus infections. Defining the active LL molecular motifs and corresponding host innate receptors is a feasible and promising goal in the field of helminth-derived immune-regulatory molecules.

Echinococcus granulosus: The establishment of the metacestode is associated with control of complement-mediated early inflammation

In this work we studied the evolution of early inflammation, complement activation and parasite survival/death along the establishment phase of Echinococcus granulosus metacestode. Using a chamber model of infection in mice, we examined cell infiltration and C3 deposition on individual parasites during their development from protoscoleces to cystic forms. We found that the intensity of the initial inflammation decreased around undamaged but not around damaged parasites: at 43dpi undamaged parasites were mostly associated with poor/mild inflammation while damaged parasites with a strong inflammation. In addition, whereas complement activation participated in the induction of early inflammation, the deposition of C3 on undamaged parasites progressively diminished. Interestingly, we observed some parasites in pre-cystic stage with no/poor C3 deposition at 3dpi. Overall, these results indicated that the establishment and survival of the hydatid cyst is associated with the control of complement and, consequently, of local inflammation at the initial phases of infection.

The role of complement in innate, adaptive and eosinophil-dependent immunity to the nematode Nippostrongylus brasiliensis

Complement may be important for immunity to infection with parasitic helminths, by promoting the recruitment of leukocytes to infected tissues and by modulating the function of cytotoxic effector leukocytes. However, the importance of complement in vivo during helminth infection is poorly understood. In this study, mice lacking classical (C1q-deficient), alternative (factor B-deficient) or all pathways of complement activation (C3-deficient) were used to assess the role of complement in immunity to the nematode Nippostrongylus brasiliensis. Double-mutant complement-deficient/IL-5 transgenic (Tg) mice were used to determine if complement is required for the strong eosinophil-dependent resistance to this parasite. Complement activation on larvae (C3 deposition), extracellular eosinophil peroxidase activity, larval aggregation and eosinophil recruitment to the skin 30 min post-injection (p.i.) of larvae were reduced in factor B-deficient mice. Inhibition of the C5a receptor with the antagonist PMX53 impaired eosinophil and neutrophil recruitment to the skin. C3 deposition on larvae was minimal by 150 min p.i. and at this time cell adherence, larval aggregation, eosinophil recruitment and degranulation were complement-independent. Factor B and C3 deficiency were associated with higher lung larval burdens in primary infections. Complement-deficient/IL-5 Tg mice were highly resistant to N. brasiliensis, suggesting that eosinophils can limit infection in a complement-independent manner. Potent secondary immunity was similarly complement-independent. In conclusion, although the alternative pathway is important for parasite recognition and leukocyte recruitment early in N. brasiliensis infections, the parasite soon becomes resistant to complement and other factors can compensate to promote eosinophil-dependent immunity.

Eosinophil-induced prognosis improvement of solid tumors could be enabled by their vesicle-mediated barrier permeability induction

Eosinophils are multifunctional cells, which contain and produce many biologically active substances. Generally, eosinophilia is associated with parasitic infections or allergic disorders, while according to recent studies eosinophil infiltration is also present in target tissues of both physiological and pathological processes, such as angiogenesis, embryogenesis, immune regulation, different infections or neoplasies, leading to tissue damage or remodeling. Reflecting on prognosis improvement in the case of solid tumors after eosinophilic infiltration of their capsules, it could be hypothesized that eosinophils are not tumoricidal per se; rather they can perforate such barriers through their vesicles' content, whereas the tumoricidal cytokines such as interleukin 4 (IL-4) fulfill the tumoral necrosis. This scenario can be supported by the fact that IL-4 originated from macrophages and lymphocytes fails to mediate tumor necrosis in vitro conditions in absence of eosinophils. In addition, the requirement of eosinophil-mediated increasing permeability among diverse biologic barriers and tissues may explain the eosinophils' introduction in capsules of cysts, mucosal membrane of respiratory and gastroenteric systems, hemato-encephalic barrier, in embryos, as well as in bacterial and parasitic membranes. Thus, in some situations rather than being multifunctional effectors per se, eosinophils, due to induction of target barrier dysfunction, may assure the host-required action, mediated by various kinds of leucocytes and their biologic effectors. Consequently, a better understanding of physiology and patho-physiology of this enigmatic cell will lead to new clinical strategies.

Eosinophil cationic protein damages protoscoleces in vitro and is present in the hydatid cyst

Eosinophils are locally recruited during the establishment and chronic phases of cystic hydatidosis. This study provides evidence that eosinophil cationic protein (ECP), one of the major components of eosinophil granules, can damage Echinococcus granulosus protoscoleces (PSC). The toxicity of ECP was investigated in vitro by following parasite viability in the presence of this protein. ECP was found to damage PSC at micromolar concentrations; the effect was blocked by specific antibodies and heparin, and was more severe than the one caused by similar concentrations of RNase A, suggesting that the cationic nature of ECP, and not its ribonuclease activity, is involved in toxicity. This observation may highlight the capacity of eosinophils to control secondary hydatidosis, derived from PSC leakage from a primary cyst. To further assess the relevance of the previous result during infection, the presence of eosinophil proteins was investigated in human hydatid cysts. ECP was found to be strongly associated with the laminated layer of the cyst wall, and present at micromolar concentrations in the hydatid fluid. Overall, these results demonstrate that eosinophils degranulate in vivo at the host-parasite interface, and that the released ECP reaches concentrations that could be harmful for the parasite.

Complement component C3 is required for protective innate and adaptive immunity to larval strongyloides stercoralis in mice

This study examines the role of complement components C3 and C5 in innate and adaptive protective immunity to larval Strongyloides stercoralis in mice. Larval survival in naive C3(-/-) mice was increased as compared with survival in wild-type mice, whereas C3aR(-/-) and wild-type mice had equivalent levels of larval killing. Larval killing in naive mice was shown to be a coordinated effort between effector cells and C3. There was no difference between survival in wild-type and naive C5(-/-) mice, indicating that C5 was not required during the innate immune response. Naive B cell-deficient and wild-type mice killed larvae at comparable levels, suggesting that activation of the classical complement pathway was not required for innate immunity. Adaptive immunity was equivalent in wild-type and C5(-/-) mice; thus, C5 was also not required during the adaptive immune response. Larval killing was completely ablated in immunized C3(-/-) mice, even though the protective parasite-specific IgM response developed and effector cells were recruited. Protective immunity was restored to immunized C3(-/-) mice by transferring untreated naive serum, but not C3-depleted heat-inactivated serum to the location of the parasites. Finally, immunized C3aR(-/-) mice killed larvae during the adaptive immune response as efficiently as wild-type mice. Therefore, C3 was not required for the development of adaptive immunity, but was required for the larval killing process during both protective innate and adaptive immune responses in mice against larval S. stercoralis.

Assessment of in vivo complement activation on the Echinococcus granulosus hydatid cyst wall

The larval stage of the parasite Echinococcus granulosus causes hydatid disease. The hydatid cyst is potentially capable of activating host complement, since it is a large, persistent, carbohydrate-rich structure, coated with host immunoglobulins, and localized in the host's internal organs. Nonetheless, in vitro studies have suggested that the cyst surface, the hydatid cyst wall (HCW), is a poor complement activator. In this study, we assessed the occurrence of in vivo complement activation on the hydatid cyst by measuring the levels of two complement activation products, C3d and complexes bearing a C9 activation neoepitope (TCC/MAC), in extracts from HCW of human origin. Low amounts of C3d and TCC/MAC were found in HCW in comparison with their levels in normal human plasma and activated human sera, suggesting that in vivo complement activation on HCW is efficiently down-regulated. This regulation may contribute to limit host inflammation which has been observed to correlate with parasite degeneration and death.