Haemonchus contortus calreticulin binds to C-reactive protein of its host, a novel survival strategy of the parasite

Recombinant Ixodes scapularis Calreticulin Binds Complement Proteins but Does Not Protect Borrelia burgdorferi from Complement Killing

Ixodes scapularis is a blood-feeding obligate ectoparasite responsible for transmitting the Lyme disease (LD) agent, Borrelia burgdorferi. During the feeding process, I. scapularis injects B. burgdorferi into the host along with its saliva, facilitating the transmission and colonization of the LD agent. Tick calreticulin (CRT) is one of the earliest tick saliva proteins identified and is currently utilized as a biomarker for tick bites. Our recent findings revealed elevated levels of CRT in the saliva proteome of B. burgdorferi-infected I. scapularis nymphs compared to uninfected ticks. Differential precipitation of proteins (DiffPOP) and LC-MS/MS analyses were used to identify the interactions between Ixs (I. scapularis) CRT and human plasma proteins and further explore its potential role in shielding B. burgdorferi from complement killing. We observed that although yeast-expressed recombinant (r) IxsCRT binds to the C1 complex (C1q, C1r, and C1s), the activator of complement via the classical cascade, it did not inhibit the deposition of the membrane attack complex (MAC) via the classical pathway. Intriguingly, rIxsCRT binds intermediate complement proteins (C3, C5, and C9) and reduces MAC deposition through the lectin pathway. Despite the inhibition of MAC deposition in the lectin pathway, rIxsCRT did not protect a serum-sensitive B. burgdorferi strain (B314/pBBE22Luc) from complement-induced killing. As B. burgdorferi establishes a local dermal infection before disseminating to secondary organs, it is noteworthy that rIxsCRT promotes the replication of B. burgdorferi in culture. We hypothesize that rIxsCRT may contribute to the transmission and/or host colonization of B. burgdorferi by acting as a decoy activator of complement and by fostering B. burgdorferi replication at the transmission site.

The many faces of parasite calreticulin

Calreticulin from parasites and its vertebrate hosts share ~50% identity and many of its functions are equally conserved. However, the existing amino acid differences can affect its biological performance. Calreticulin plays an important role in Ca2+ homeostasis and as a chaperone involved in the correct folding of proteins within the endoplasmic reticulum. Outside the endoplasmic reticulum, calreticulin is involved in several immunological functions such as complement inhibition, enhancement of efferocytosis, and immune upregulation or inhibition. Several parasite calreticulins have been shown to limit immune responses and promote infectivity, while others are strong immunogens and have been used for the development of potential vaccines that limit parasite growth. Furthermore, calreticulin is essential in the dialogue between parasites and hosts, inducing Th1, Th2 or regulatory responses in a species-specific manner. In addition, calreticulin participates as initiator of endoplasmic reticulum stress in tumor cells and promotion of immunogenic cell death and removal by macrophages. Direct anti-tumoral activity has also been reported. The highly immunogenic and pleiotropic nature of parasite calreticulins, either as positive or negative regulators of the immune response, render these proteins as valuable tools to modulate immunopathologies and autoimmune disorders, as well as a potential treatment of neoplasms. Moreover, the disparities in the amino acid composition of parasite calreticulins might provide subtle variations in the mechanisms of action that could provide advantages as therapeutic tools. Here, we review the immunological roles of parasite calreticulins and discuss possible beneficial applications.

Functional Diversity of the Excretory/Secretory Proteins of Nematode Parasites

INTRODUCTION

Parasites release a wide array of protein as excretory and secretory products (ESPs). Irrespective of their mode of propagation, ESPs are found to be secreted or excreted by both naturally occurring and laboratory-cultivated parasites. Mass spectrometry-based approaches have been extensively used to identify and characterize the ESP constituents. ESPs are involved in various cellular activities such as immune modulation, proteolysis, inhibition of proteases and protection of cells against oxidants. Specifically, their role in host immune evasion by down-regulation of pro-inflammatory cytokines and up-regulation of anti-inflammatory cytokines attracts scientific attention. A thorough investigation of functional diversity of ESPs may be helpful in planning control strategies against many parasites.

METHODS

This review focuses on diversity of ES proteins, various approaches to identify them and discusses about the biochemical and functional aspects of such proteins.

RESULTS

The diverse array of proteins secreted or excreted (a, GST-1, acetylcholinesterase, GAPDH) by the parasites are also described emphasizing their role in cellular physiology.

CONCLUSION

Finally, it concludes by citing some of these proteins as potential therapeutic agents against helminth challenge.

In silico to In vivo development of a polyherbal against Haemonchus contortus

Haemonchus contortus is a major constraint in the development of small ruminant subsector due to significant production losses incurred by it. The present study explores the antiparasitic potential of three anthelmintic plants (Butea monosperma, Vitex negundo and Catharanthus roseus (L.) G.Don) against H. contortus taking albendazole as the standard. In silico molecular docking and pharmacokinetic prediction studies were conducted with known bioactive molecules of these plants (palasonin, vinblastine, vincristine, betulinic acid and ursolic acid) against Glutamate Dehydrogenase (GDH) and tubulin molecules of the parasite. Methanolic extracts of these herbs were fractionated (hexane, ethyl acetate, chloroform and methanol) and used in in vitro larvicidal studies. Based on the in vitro data, two herbal prototypes were developed and clinically tested. All the 5 ligand molecules showed better binding affnity for GDH and tubulin protein as compared with albendazole and shared similar binding site in the core of the GDH hexamer with slight variations. Albendazole approximately stacked against GLY190A residue, showing hydrophobic interactions with PRO157A and a Pi-cation electrostatic interaction with ARG390 along with four hydrogen bonds. Vincristine formed 2 pi-anionic electrostatic bonds with ASP158 of B and C subunits alongwith hydrogen bonding and hydrophobic interaction and an additional pi-anion electrostatic interaction at ASP158A for vinblastine. Albendazole bound to α-tubulin next to colchicine site whereas vinblastine is bound at the nearby laulimalide/peloruside site of the dimer. Betulinic acid showed lateral interaction between the H2-H3 loop of one alpha subunit and H10 of the adjacent alpha subunit of two tubulin dimers. Ursolic acid and palasonin bound at the intradimer N site of microtubulin involving the H1-H7 and H1-H2 zone, respectively. The in vitro studies demonstrated good dose dependent anthelmintic potential. Both the prototypes were quite efficacious in clearing the infection, keeping it to a minimal for more than 5 months, probably, through direct anthelmintic effect through GDH, tubulin depolymerization and uncoupling as well as indirectly through immunomodulation along with antioxidant and anti-inflammatory properties.

The Interactions of Parasite Calreticulin With Initial Complement Components: Consequences in Immunity and Virulence

Because of its capacity to increase a physiologic inflammatory response, to stimulate phagocytosis, to promote cell lysis and to enhance pathogen immunogenicity, the complement system is a crucial component of both the innate and adaptive immune responses. However, many infectious agents resist the activation of this system by expressing or secreting proteins with a role as complement regulatory, mainly inhibitory, proteins. Trypanosoma cruzi, the causal agent of Chagas disease, a reemerging microbial ailment, possesses several virulence factors with capacity to inhibit complement at different stages of activation. T. cruzi calreticulin (TcCalr) is a highly-conserved, endoplasmic reticulum-resident chaperone that the parasite translocates to the extracellular environment, where it exerts a variety of functions. Among these functions, TcCalr binds C1, MBL and ficolins, thus inhibiting the classical and lectin pathways of complement at their earliest stages of activation. Moreover, the TcCalr/C1 interaction also mediates infectivity by mimicking a strategy used by apoptotic cells for their removal. More recently, it has been determined that these Calr strategies are also used by a variety of other parasites. In addition, as reviewed elsewhere, TcCalr inhibits angiogenesis, promotes wound healing and reduces tumor growth. Complement C1 is also involved in some of these properties. Knowledge on the role of virulence factors, such as TcCalr, and their interactions with complement components in host-parasite interactions, may lead toward the description of new anti-parasite therapies and prophylaxis.

Identification of Haemaphysalis longicornis Genes Differentially Expressed in Response to Babesia microti Infection

Haemaphysalis longicornis is a tick and a vector of various pathogens, including the human pathogenetic Babesia microti. The objective of this study was to identify female H. longicornis genes differentially expressed in response to infection with B. microti Gray strain by using a suppression subtractive hybridization (SSH) procedure. A total of 302 randomly selected clones were sequenced and analyzed in the forward subtracted SSH cDNA library related to Babesia infection, and 110 clones in the reverse cDNA library. Gene ontology assignments and sequence analyses of tick sequences in the forward cDNA library showed that 14 genes were related to response to stimulus or/and immune system process, and 7 genes had the higher number of standardized sequences per kilobase (SPK). Subsequent real-time PCR detection showed that eight genes including those encoding for Obg-like ATPase 1 (ola1), Calreticulin (crt), vitellogenin 1 (Vg1) and Vg2 were up-regulated in fed ticks. Compared to uninfected ticks, infected ticks had six up-regulated genes, including ola1, crt and Vg2. Functional analysis of up-regulated genes in fed or Babesia-infected ticks by RNA interference showed that knockdown of crt and Vg2 in infected ticks and knockdown of ola1 in uninfected ticks accelerated engorgement. In contrast, Vg1 knockdown in infected ticks had delayed engorgement. Knockdown of crt and Vg1 in infected ticks decreased engorged female weight. Vg2 knockdown reduced B. microti infection levels by 51% when compared with controls. The results reported here increase our understanding of roles of H. longicornis genes in blood feeding and B. microti infection.

Evaluation of Opisthorchis viverrini calreticulin for potential host modulation

The multifunctional calreticulin (CALR) was identified as a major calcium-binding protein of the endoplasmic reticulum before being recognized as a chaperone in the same place. Only later were activities of calreticulin outside the endoplasmic reticulum described that for example affect cell proliferation and the innate immune system. In the present work we have investigated those extracellular activities of CALR from the cancerogenic human liver fluke Opisthorchis viverrini (OvCALR), as they might be important in host/parasite interaction. We first demonstrate that OvCALR is released from the parasite and stimulates a specific humoral immune response. Recombinant OvCALR is then shown to suppress proliferation of primary endothelial cells, their motility and sprouting activities. The potential of OvCALR to interfere with the complement system is established, firstly by demonstrating its direct binding to C1q and, secondly by suppression of hemolysis of sensitized red blood cells. These findings suggest that OvCALR is an important parasite antigen that could modulate diverse host functions and support parasite survival.

Immunomodulatory effects of adult Haemonchus contortus excretory/secretory products on human monocyte-derived dendritic cells

The levels of expression of surface molecules and release of cytokines and chemokines of human monocyte-derived dendritic cells were determined after their exposure to adult H. contortus excretory/secretory (ES) products or a combination of ES products and bacterial lipopolysaccharide (LPS). Worm products provoked a weak response and only partial maturation of the dendritic cells, consistent with the hyporesponsiveness and more tolerogenic immune environment present in parasitized animals and humans. Co-stimulation with LPS demonstrated that H. contortus secretions, like those of other helminths, contain immunomodulators capable of reducing some aspects of the strong T(H)1/T(H)2 response evoked by bacterial LPS. There were significant reductions in the release of some cytokine/chemokines by LPS-stimulated mdDCs and a trend (although not significant at P < 0.05) for reduced expression levels of CD40, CD80 and HLA-DR. A prominent feature was the variability in responses of dendritic cells from the four donors, even on different days in repeat experiments, suggesting that generalized conclusions may be difficult to make, except in genetically related animals. Such observations may therefore be applicable only to restricted populations. In addition, previous exposure to parasites in a target population for immunomodulatory therapy may be an important factor in assessing the likelihood of adverse reactions or failures in the treatment to worm therapy.

Amblyomma americanum tick calreticulin binds C1q but does not inhibit activation of the classical complement cascade

In this study we characterized Amblyomma americanum (Aam) tick calreticulin (CRT) homolog in tick feeding physiology. In nature, different tick species can be found feeding on the same animal host. This suggests that different tick species found feeding on the same host can modulate the same host anti-tick defense pathways to successfully feed. From this perspective it's plausible that different tick species can utilize universally conserved proteins such as CRT to regulate and facilitate feeding. CRT is a multi-functional protein found in most taxa that is injected into the vertebrate host during tick feeding. Apart from it's current use as a biomarker for human tick bites, role(s) of this protein in tick feeding physiology have not been elucidated. Here we show that annotated functional CRT amino acid motifs are well conserved in tick CRT. However our data show that despite high amino acid identity levels to functionally characterized CRT homologs in other organisms, AamCRT is apparently functionally different. Pichia pastoris expressed recombinant (r) AamCRT bound C1q, the first component of the classical complement system, but it did not inhibit activation of this pathway. This contrast with reports of other parasite CRT that inhibited activation of the classical complement pathway through sequestration of C1q. Furthermore rAamCRT did not bind factor Xa in contrast to reports of parasite CRT binding factor Xa, an important protease in the blood clotting system. Consistent with this observation, rAamCRT did not affect plasma clotting or platelet aggregation. We discuss our findings in the context of tick feeding physiology.

The N-terminal segment of glyceraldehyde-3-phosphate dehydrogenase of Haemonchus contortus interacts with complements C1q and C3

Haemonchus contortus, an economically important blood-sucking parasite of sheep and goats, survives the harsh host gut environment by secreting a number of proteins referred as excretory/secretory (ES) products. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a glycolytic enzyme, is one of the components of H. contortus ES products. The parasite enzyme binds to complement C3 and inhibits its activity. In this study, the C3-binding activity of the parasite GAPDH was mapped to the N-terminal part of the enzyme by generating defined recombinant fragments of the protein. The N-terminal fragment also trapped complement C1q but not C5 and inhibited complement-mediated lysis of sensitized sheep erythrocytes. Competitive binding assay indicates different binding regions for C1q and C3 proteins. GAPDH stimulated proliferation of goat peripheral blood mononuclear cells in vitro and reacted with the sera from H. contortus-infected animals. However, the fragments of GAPDH did not stimulate cell proliferation nor reacted with the infected animal sera. Furthermore, denatured GAPDH failed to react with the infected animal sera in dot blot suggesting conformation-dependent epitope. These results demonstrate an elegant strategy of the parasite to completely shut down complement activation and identify GAPDH as a promising target for future therapeutic intervention.

The analysis of the acute phase response during the course of Trypanosoma carassii infection in the goldfish (Carassius auratus L.)

The expression of genes encoding the acute phase proteins (APP) during the course of Trypanasoma carassii infection in the goldfish was determined using quantitative PCR. Significant changes in the mRNA levels of ceruloplasmin (Cp), C-reactive protein (CRP), transferrin (Tf), hemopexin (Hx) and serum amyloid A (SAA) were observed in the kidney, liver and spleen at various days post infection (dpi). Of the five acute phase protein genes examined, CRP and SAA exhibited the highest expression in the tissues during the acute infection. Cp and Tf were up-regulated throughout the acute course of infection in the liver. During the chronic phase of the infection, APP expression in the liver was similar to that in the non-infected control fish. At 7 dpi, Cp, Tf and Hx were down-regulated in the spleen, and Cp and Tf kidney, but their mRNA levels gradually returned to those of control non-infected fish. In contrast, during the chronic phase of the infection, there was an up-regulation of Cp, Hx and Tf in the spleen, and Tf and SAA in the kidney. The goldfish CRP was cloned and functionally characterized. CRP was differentially expressed in normal goldfish immune cells, with highest expression in monocytes and lowest expression in mature macrophages. A recombinant goldfish CRP (rgfCRP) was generated using prokaryotic expression. rgfCRP enhanced complement-mediated killing of trypanosomes in vitro, and the lysis increased after addition of immune serum. rgfCRP did not affect the production of reactive oxygen and nitrogen intermediates by monocytes and macrophages, respectively.

Immune modulation by helminth parasites of ruminants: implications for vaccine development and host immune competence

Parasitic helminths reside in immunologically-exposed extracellular locations within their hosts, yet they are capable of surviving for extended periods. To enable this survival, these parasites have developed complex and multifaceted mechanisms to subvert or suppress host immunity. This review summarises current knowledge of immune modulation by helminth parasites of ruminants and the parasite-derived molecules involved in driving this modulation. Such immunomodulatory molecules have considerable promise as vaccine targets, as neutralisation of their function is predicted to enhance anti-parasite immunity and, as such, current knowledge in this area is presented herein. Furthermore, we summarise current evidence that, as well as affecting parasite-specific immunity, immune modulation by these parasites may also affect the ability of ruminant hosts to control concurrent diseases or mount effective responses to vaccination.

Glyceraldehyde-3-phosphate dehydrogenase of the parasitic nematode Haemonchus contortus binds to complement C3 and inhibits its activity

Haemonchus contortus is an economically important gastrointestinal parasite that infects primarily sheep and goats. To survive inside the host, the parasite must overcome the host immune response. In this study, we have identified and characterized a complement-C3-binding protein (H.c-C3BP) from this parasite employing biochemical and molecular biology tools. Initially, a truncated form of the protein was isolated from the excretory-secretory products of the parasite using C3-Sepharose column that facilitated its identification by mass spectroscopy. Subsequently, the parent molecule was generated in E. coli, and sequence analysis confirmed it as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). GAPDH reacted with the antiserum raised against the truncated protein, and the truncated protein reacted with anti-GAPDH antiserum. The protein inhibited complement function as measured by haemolytic assay and membrane attack complex (MAC) formation. Sera from H. contortus-infected animals reacted with GAPDH as well as the truncated form of the protein, which further lend support to protein secretion. Thus, the C3-binding property of H. contortus GAPDH is a new function, and it represents a new entity of complement-binding protein. Identification and characterization of H.c-C3BP should facilitate development of new therapeutics considering a key role of this protein in immune modulation.

Trypanosoma carassii calreticulin binds host complement component C1q and inhibits classical complement pathway-mediated lysis

Trypanosoma carassii is an extracellular parasite of economically important fish species that has evolved several strategies to circumvent host immune responses. Proteomic analysis of the excreted/secreted (ES) and surface molecules of the parasite has revealed a number of proteins that may be involved in host-parasite interactions. Among the parasite molecules identified in the ES of T. carassii was calreticulin. We cloned and produced T. carassii calreticulin (rTcaCRT), and generated a rabbit polyclonal antibody to the recombinant protein. The incubation of parasites with rabbit anti-rTcaCRT affinity-purified IgG antibody indicated substantial CRT levels on the surface of trypanosomes, as well as internal structures of permeabilized organisms. Recombinant parasite calreticulin bound several molecules in host serum including the first complement component, C1q. The host C1q specifically interacted with parasite CRT since the C1q-dependent lysis of sensitized sheep erythrocytes was inhibited by rTcaCRT. Our findings suggest that CRT may be used by the parasite to inhibit hosts' classical complement pathway.

Modulation of dendritic cell responses by parasites: a common strategy to survive

Parasitic infections are one of the most important causes of morbidity and mortality in our planet and the immune responses triggered by these organisms are critical to determine their outcome. Dendritic cells are key elements for the development of immunity against parasites; they control the responses required to eliminate these pathogens while maintaining host homeostasis. However, there is evidence showing that parasites can influence and regulate dendritic cell function in order to promote a more permissive environment for their survival. In this review we will focus on the strategies protozoan and helminth parasites have developed to interfere with dendritic cell activities as well as in the possible mechanisms involved.

Comparative immunogenicity of Haemaphysalis longicornis and Rhipicephalus (Boophilus) microplus calreticulins

The ticks Rhipicephalus (Boophilus) microplus and Haemaphysalis longicornis are blood-sucking ectoparasites of bovines, causing serious damages to the livestock production. The main control method for these ticks is based on acaricides. However, the use of vaccines has been studied as a promising control strategy. Calreticulin (CRT) is a multifunctional, predominantly intracellular protein present in almost all cells of animals. The secretion of CRT during feeding might be linked to the modulation of the parasite-host interaction. In the present study, recombinant CRTs of R. microplus (rBmCRT) and H. longicornis (rHlCRT) were expressed in Escherichia coli and purified by ion exchange chromatography and used for the immunization of bovines and mouse. ELISA demonstrated that both rCRTs are recognized by the sera of immunized bovines. In silico, despite the difference in amino acid sequences, antigenic index analysis of HlCRT and BmCRT using the Jameson-Wolf algorithm indicated that both proteins were very similar in antigenicity index, although six different epitopes between the tick CRTs have been inferred. These data were corroborated by competitive ELISA analyses, which suggest the presence of different epitopes within the proteins. Western blot analyses showed that anti-rBmCRT and anti-rHlCRT bovine sera also recognized the native proteins in larvae extracts and, moreover, sera of bovines immunized with saliva and extract of salivary glands recognized both recombinant CRTs. Thus, mouse and bovine immune system recognized rCRTs, resulting in the production of antibodies with similar specificity for both recombinant proteins, although different epitopes could be distinguished between rBmCRT and rHlCRT.

Mapping the complement C1q binding site in Haemonchus contortus calreticulin

Haemonchus contortus is an economically important gastrointestinal parasite of domestic animals. The parasite secretes calreticulin (CalR), a Ca(++) binding protein which modulates the host immune response. One way by which this protein acts is by inhibiting the classical complement pathway by binding to complement C1q protein. Understanding CalR-C1q interaction is important to develop methods to enhance host immune response. In this study, we have mapped the regions in the N-domain of CalR that facilitates C1q binding by generating small recombinant fragments of the domain and using synthetic peptides. In addition to already identified C1q binding motifs in human CalR, two additional sites in the N-domain of H. contortus were revealed with the following sequences-GKYYDDAKRD and the AKFPKKFT. The significance of multiple C1q binding motifs in CalR is discussed in relation to host-parasite interactions.

Calreticulin from the intestinal nematode Heligmosomoides polygyrus is a Th2-skewing protein and interacts with murine scavenger receptor-A

Helminth infections are commonly associated with a Th2 immune response, yet only a few parasite molecules involved in triggering such immune responses have been identified. Here, we describe the Th2-skewing property of calreticulin of Heligmosomoides polygyrus (HpCRT). HpCRT is a secreted protein most abundantly expressed by tissue invasive larvae (L4). Native HpCRT purified from adult worm extract (nHpCRT) stimulated robust IL-4 release from CD4(+) T cells of H. polygyrus infected mice. Interestingly, CD4(+) T cells also produced significant amounts of IL-10 while IFN-gamma was not detectable. Likewise, immunization with recombinant HpCRT (rHpCRT) without extrinsic adjuvant led predominantly to a specific IL-4 production implying the innate ability of HpCRT to drive Th2 responses. The triggering of a Th2-skewed immune response to rHpCRT is corroborated by the induction of HpCRT-specific IgG1 and IgE antibodies. Furthermore, rHpCRT bound to scavenger receptor type A (SR-A) on dendritic cells, and interaction of HpCRT with SR-A led to internalization of HpCRT that could be partially blocked by competition with SR-A ligands as well as with an anti-SR-A monoclonal antibody. Hence, our data imply that nematode calreticulin interacts with a mammalian scavenger receptor and at the same time induces a Th2 response.

Parasitic helminths: a pharmacopeia of anti-inflammatory molecules

Infection with parasitic helminths takes a heavy toll on the health and well-being of humans and their domestic livestock, concomitantly resulting in major economic losses. Analyses have consistently revealed bioactive molecules in extracts of helminths or in their excretory/secretory products that modulate the immune response of the host. It is our view that parasitic helminths are an untapped source of immunomodulatory substances that, in pure form, could become new drugs (or models for drug design) to treat disease. Here, we illustrate the range of immunomodulatory molecules in selected parasitic trematodes, cestodes and nematodes, their impact on the immune cells in the host and how the host may recognize these molecules. There are many examples of the partial characterization of helminth-derived immunomodulatory molecules, but these have not yet translated into new drugs, reflecting the difficulty of isolating and fully characterizing proteins, glycoproteins and lipid-based molecules from small amounts of parasite material. However, this should not deter the investigator, since analytical techniques are now being used to accrue considerable structural information on parasite-derived molecules, even when only minute quantities of tissue are available. With the introduction of methodologies to purify and structurally-characterize molecules from small amounts of tissue and the application of high throughput immunological assays, one would predict that an assessment of parasitic helminths will yield a variety of novel drug candidates in the coming years.