The Secreted Larval Acidic Proteins (SLAPs) of Onchocerca spp. are encoded by orthologues of the alt gene family of Brugia malayi and have host protective potential

Advancing a Human Onchocerciasis Vaccine From Antigen Discovery to Efficacy Studies Against Natural Infection of Cattle With Onchocerca ochengi

Human onchocerciasis is a devastating neglected tropical disease caused by infection of the filarial nematode Onchocerca volvulus. The infection can cause irreversible visual impairment or blindness and stigmatizing dermatitis. More than 32 million people were estimated to be infected with O. volvulus in Africa, and 385,000 suffered from blindness. Even though the implementation of mass drug administration (MDA) with ivermectin has reduced the global prevalence of onchocerciasis, O. volvulus infection remains challenging to control because MDA with ivermectin cannot be implemented in endemic areas co-endemic with loiasis due to the risk of severe adverse events. There is also emerging drug resistance to ivermectin that further complicates the elimination of onchocerciasis. Thus, the development of a vaccine that would induce protective immunity and reduce infection burden is essential. Efforts to develop prophylactic and/or therapeutic vaccines for onchocerciasis have been explored since the late 1980s by many researchers and entities, and here we summarize the recent advances made in the development of vaccines against the infection of O. volvulus and onchocerciasis.

Biological role of excretory-secretory proteins in endemic parasites of Latin America and the Caribbean

Neglected tropical diseases (NTDs) share certain traits: they are parasitic infections, prevailing in tropical environments and affecting marginalized sectors of the population. Six NTDs - ascariasis, cysticercosis, echinococcosis, hookworm infection, onchocerciasis and trichuriasis - all of them endemic in Latin America and the Caribbean (LAC), are analysed in this work. This review aims to discuss key information on the function of excretory/secretory (E/S) proteins from these parasites in their infectivity, pathogeny and diagnosis. The modulation of the host immune system to favour the permanence and survival of the parasite is also discussed. An updated knowledge on the function of E/S molecules in endemic parasitoses in LAC may lead to new approaches for the clinical management and diagnosis of these diseases. In turn, this could allow us to optimize their treatment and make it more affordable - a relevant goal given the economic constraints that the region is facing.

Filarial Abundant Larval Transcript Protein ALT-2: An Immunomodulatory Therapeutic Agent for Type 1 Diabetes

Type 1 diabetes (T1D) that accounts for about 5-10 % of all diabetes cases results from the autoimmune destruction of the insulin-producing beta cells in the pancreas. It is characterized by severe inflammatory reaction mediated by pronounced T helper type-1 response. Parasitic infections having the ability to skew the host immune responses towards type-2 type as a part of their defense mechanism are able to induce protection against autoimmune diseases like T1D. Hence, the present study is undertaken to explore a recombinant abundant larval transcript protein of the human lymphatic filarial parasite Brugia malayi (rBmALT-2), a known anti-inflammatory molecule for its therapeutic effect on streptozotocin (STZ)-induced T1D in mice. The diabetic mice on treatment with rBmALT-2 showed a significant (p < 0.0005) decrease in their fasting blood glucose levels. By the end of the second week after the initiation of treatment with the rBmALT-2, 28 % of the diabetic mice became normal and none of them were diabetic by the end of 5th week. The anti-diabetic effect of rBmALT-2 significantly correlated with the concomitant redressal of the pancreatic histopathological damage caused by STZ assault (rho = 0.87; p < 0.0005). The sera of rBmALT-2 treated diabetic mice had increased levels of IgG1 antibodies associated with decreased IgG2a antibodies against the principal autoantigen insulin. The splenocyte proliferative response and the cytokine release in the treated mice showed marked bias against inflammation skewing the immune response to Th-2 type. From this study, it can be envisaged that that filarial proteins like rBmALT-2 with effective immunomodulatory activity and anti-diabetic effect are promising alternative therapeutic agents for T1D.

Epitope mapping of Brugia malayi ALT-2 and the development of a multi-epitope vaccine for lymphatic filariasis

Human lymphatic filariasis is a neglected tropical disease, causing permanent and long-term disability with severe immunopathology. Abundant larval transcript (ALT) plays a crucial role in parasite establishment in the host, due to its multi-faceted ability in host immune regulation. Although ALT protein is a key filarial target, its exact function is yet to be explored. Here, we report epitope mapping and a structural model of Brugia malayi ALT-2, leading to development of a multi-epitope vaccine. Structural analysis revealed that ALT represents unique parasitic defence proteins belonging to a toxin family that carries a 'knottin' fold. ALT-2 has been a favourite vaccine antigen and was protective in filarial models. Due to the immunological significance of ALT-2, we mapped B-cell epitopes systematically and identified two epitope clusters, 1-30 and 89-128. To explore the prophylactic potential of epitope clusters, a recombinant multi-epitopic gene comprising the epitopic domains was engineered and the protective efficacy of recombinant ALT epitope protein (AEP) was tested in the permissive model, Mastomys coucha. AEP elicited potent antibody responses with predominant IgG1 isotype and conferred significantly high protection (74.59%) compared to ALT-2 (61.95%). This proved that these epitopic domains are responsible for the protective efficacy of ALT-2 and engineering protective epitopes as a multi-epitope protein may be a novel vaccine strategy for complex parasitic infections.

Evaluation of immune response elicited by inulin as an adjuvant with filarial antigens in mice model

Filariasis caused by infectious parasitic nematodes has been identified as the second leading source of permanent and long-term disability in Sub-Saharan Africa, Asia and Latin America. Several vaccine candidates were identified from infective third-stage larvae (L3) which involves in the critical transition from arthropod to human. Hitherto studies of these antigens in combination with alum adjuvant have shown to elicit its characteristic Th2 responses. Inulin is a safe, non-toxic adjuvant that principally stimulates the innate immune response through the alternative complement pathway. In the present study, the immune response elicited by inulin and alum as adjuvants were compared with filarial antigens from different aetiological agents: secreted larval acidic protein 1 (SLAP1) from Onchocerca volvulus and venom allergen homologue (VAH) from Brugia malayi as single or as cocktail vaccines in mice model. The study revealed that inulin can induce better humoral response against these antigens than alum adjuvant. Antibody isotyping disclosed inulin's ability to elevate the levels of IgG2a and IgG3 antibodies which mediates in complement-dependent cytotoxicity and antibody-dependent cell-mediated cytotoxicity (ADCC), respectively, in mice. Splenocyte analysis showed that T cells prestimulated with inulin have higher stimulation index (P < 0.05) than alum except for BmVAH antigen. In vitro ADCC assay showed that inulin formulation had induced higher cytotoxicity with filarial antigens (as single P < 0.01 and as cocktail P < 0.05, respectively) than alum. The results had confirmed the capability of inulin to deplete the levels of Treg and brought a balance in Th1/Th2 arms against filarial antigens in mice.

Wuchereria bancrofti 20/22 a homologue of abundant larval transcript L3 stage filarial antigen: molecular and immunological characterization

The chromadorea abundant larval transcript (ALT) family of proteins contains ALT one of the most studied putative vaccine candidate in experimental filariasis. This study reports the characterization of Wuchereria bancrofti 20/22 (Wb20/22) as a member of chromadorea, the ALT family of proteins from the L3 stage of W. bancrofti. The high reactivity with serum from the endemic normal (EN) population suggests that Wb20/22 could be a target of elicit protective immunity. The glutamic acid-rich region of Wb20/22 was predicted to harbour the longest linear B-cell epitope by insilico prediction tools. The significance of this region was revealed by studying the mutant form of Wb20/22, without acidic domain (WOAD) which was cloned, and the immune response was compared with Wb20/22. The signal sequence of Wb20/22 was also an immunodominant region, and mutant construct without signal sequence (WOSS) was cloned and characterized. The peak antibody titre elicited by WOAD was higher than Wb20/22 or WOSS, which pointed to the immunomodulatory role of glutamic acid-rich region. Wb20/22 elicited very high levels of IL-10 and diminished levels of IL-4 and IL-5 which could be the reason for low antibody titre. The prophylactic efficacy of WOAD conferred protection (62·26%) which was higher than Wb20/22 (49·82%) and WOSS (54·78%).

Immune recognition of Onchocerca volvulus proteins in the human host and animal models of onchocerciasis

Onchocerca volvulus is a tissue-dwelling, vector-borne nematode parasite of humans and is the causative agent of onchocerciasis or river blindness. Natural infections of BALB/c mice with Litomosoides sigmodontis and of cattle with Onchocerca ochengi were used as models to study the immune responses to O. volvulus-derived recombinant proteins (OvALT-2, OvNLT-1, Ov103 and Ov7). The humoral immune response of O. volvulus-infected humans against OvALT-2, OvNLT-1 and Ov7 revealed pronounced immunoglobulin G (IgG) titres which were, however, significantly lower than against the lysate of O. volvulus adult female worms. Sera derived from patients displaying the hyperreactive form of onchocerciasis showed a uniform trend of higher IgG reactivity both to the single proteins and the O. volvulus lysate. Sera derived from L. sigmodontis-infected mice and from calves exposed to O. ochengi transmission in a hyperendemic area also contained IgM and IgG1 specific for O. volvulus-derived recombinant proteins. These results strongly suggest that L. sigmodontis-specific and O. ochengi-specific immunoglobulins elicited during natural infection of mice and cattle cross-reacted with O. volvulus-derived recombinant antigens. Monitoring O. ochengi-infected calves over a 26-month period, provided a comprehensive kinetic of the humoral response to infection that was strictly correlated with parasite load and occurrence of microfilariae.

DNA vaccines against tropical parasitic diseases

Parasitic diseases caused by protozoan and helminth parasites are among the leading causes of morbidity and mortality in tropical and subtropical regions of the world. Unfortunately, at present, there is no vaccine against any human parasitic disease. Conventional vaccine methods have largely failed against parasitic infections. This is due, in part, to the complexity of the parasite life cycle, the ability of the parasite to evade the immune system, and difficulties in identifying and eliciting the desired protective immune responses. The discovery of DNA vaccines has renewed hope for vaccine development against parasites. In the last decade, DNA vaccines were successful in inducing at least partial protection against several parasitic diseases. This review discusses the latest developments in DNA vaccines against tropical parasitic diseases.

Immunogenicity of Brugia malayi Abundant Larval Transcript-2, a potential filarial vaccine candidate expressed in tobacco

KEY MESSAGE

Transgenic tobacco plants with Bm ALT-2, a filarial vaccine candidate, were developed. The plant-produced antigen showed immunogenicity on par with the E.coli product. Transgenic tobacco plants were developed using Brugia malayi Abundant Larval Transcript-2 (Bm ALT-2), a major antigen produced from recombinant E.coli found to be experimentally successful as potential vaccine candidate against lymphatic filariasis. Results of experiments on the transformation and expression of the Bm ALT-2 in tobacco plant to produce plant-based vaccine are presented here. We have successfully transformed the tobacco plant with Bm ALT-2 and confirmed that the plants expressed the filarial protein by PCR analysis and Western blotting. The level of expression varied from 50 to 90 ng/μg of total soluble protein for ALT-2. Immunization of mice with plant-extracted protein indicated that the plant-produced protein had immunological characteristics similar to the E.coli-produced protein. Antibody titres produced by plant-produced recombinant ALT 2-immunized mice were on par with those immunized with recombinant protein produced by E.coli. Antibody isotype assay showed that plant-produced recombinant ALT-2 induced significant IgG1, whereas E.coli-produced recombinant ALT-2 induced IgG3. This result is a step forward towards the development of a model eukaryotic system for the production of recombinant filarial proteins, which can be utilized to produce therapeutic and diagnostic molecules against lymphatic filariasis, a neglected tropical infectious disease which has a negative impact on socioeconomic development. In addition, this is the first report of the immunogenicity of a plant-derived filarial antigen.

Molecular characterization and evaluation of Onchocerca volvulus-secreted larval acidic protein 1 (SLAP1) as a putative vaccine candidate on endemic population of lymphatic filariasis

Filarial parasites infected nearly 160 million of the global population with onchocerciasis and lymphatic filariasis, and further, a billion of people are estimated to be at risk of infection, rendering them among the most prevalent infectious agents in the world today. Given the complexity of their life cycle and the immune evasion mechanisms of these organisms, development of a vaccine remains to be a long-term challenge. Though a number of immunodominant antigens have been characterized, the presence of homologous proteins in humans or the allelic variants are some of the major drawbacks. One of the extensively studied vaccine candidates is abundant larval transcripts (ALT) family of proteins for the following properties: highly regulated expression, abundance, excreted-secreted product of infective stage larvae, and essentially for parasite establishment and survival in the host. In the present study, stage-specific expression of secreted larval acidic protein 1 (SLAP1) was identified; an ALT orthologue from Onchocerca volvulus was cloned, expressed, and purified as a recombinant protein. Immunogenicity of OvSLAP1 was demonstrated with sera and peripheral blood mononuclear cells from endemic regions of Brugia malayi and Wuchereria bancrofti. OvSLAP1 antibodies were predominated by IgG1 and IgG2 in endemic normal (EN) and chronic pathology (CP) subjects. It has also induced marked cellular response as observed by lymphoproliferation assay. The study revealed that OvSLAP1 can segregate humoral (EN mean optical density (OD) = 0.87 ± 0.035, CP mean OD = 0.59 ± 0.029) and cellular (EN mean stimulation index (SI) = 5.87 ± 0.167, CP mean SI = 3.5 ± 0.134) immune responses between EN and CP individuals (P < 0.001), signifying its prophylactic ability and vitality for protection from filarial infections in endemic population.

A comprehensive, model-based review of vaccine and repeat infection trials for filariasis

SUMMARY Filarial worms cause highly morbid diseases such as elephantiasis and river blindness. Since the 1940s, researchers have conducted vaccine trials in 27 different animal models of filariasis. Although no vaccine trial in a permissive model of filariasis has provided sterilizing immunity, great strides have been made toward developing vaccines that could block transmission, decrease pathological sequelae, or decrease susceptibility to infection. In this review, we have organized, to the best of our ability, all published filaria vaccine trials and reviewed them in the context of the animal models used. Additionally, we provide information on the life cycle, disease phenotype, concomitant immunity, and natural immunity during primary and secondary infections for 24 different filaria models.

Deletion of parasite immune modulatory sequences combined with immune activating signals enhances vaccine mediated protection against filarial nematodes

Background

Filarial nematodes are tissue-dwelling parasites that can be killed by Th2-driven immune effectors, but that have evolved to withstand immune attack and establish chronic infections by suppressing host immunity. As a consequence, the efficacy of a vaccine against filariasis may depend on its capacity to counter parasite-driven immunomodulation.

Methodology and Principal Findings

We immunised mice with DNA plasmids expressing functionally-inactivated forms of two immunomodulatory molecules expressed by the filarial parasite Litomosoides sigmodontis: the abundant larval transcript-1 (LsALT) and cysteine protease inhibitor-2 (LsCPI). The mutant proteins enhanced antibody and cytokine responses to live parasite challenge, and led to more leukocyte recruitment to the site of infection than their native forms. The immune response was further enhanced when the antigens were targeted to dendritic cells using a single chain Fv-αDEC205 antibody and co-administered with plasmids that enhance T helper 2 immunity (IL-4) and antigen-presenting cell recruitment (Flt3L, MIP-1α). Mice immunised simultaneously against the mutated forms of LsALT and LsCPI eliminated adult parasites faster and consistently reduced peripheral microfilaraemia. A multifactorial analysis of the immune response revealed that protection was strongly correlated with the production of parasite-specific IgG1 and with the numbers of leukocytes present at the site of infection.

Conclusions

We have developed a successful strategy for DNA vaccination against a nematode infection that specifically targets parasite-driven immunosuppression while simultaneously enhancing Th2 immune responses and parasite antigen presentation by dendritic cells.

Filarial infections are endemic in more that 80 countries, affecting over 120 million people and putting 1 billion more at risk. Antifilarial drugs must be administered regularly to infected people to control the disease, but they are contraindicated in under 6 year-olds and in pregnant women. Further, reports of drug resistance are now accumulating. A vaccine would therefore greatly help fight these diseases. Live attenuated L3 filariae larvae can evoke a protective immunity but their production is impractical and use in humans unacceptable while the efficacy of sub-unit vaccines has been poor. Filariae secrete proteins capable of suppressing their host's immune response, and have the potential to interfere with immunisation. We therefore decided to vaccinate hosts against secreted parasite products that modulate host immune responses rather than against structural components of the worms, and to boost the host's immune system by directly enhancing the uptake of parasite material by antigen presenting cells. This strategy generated substantial protection against both adult and offspring of a filarial parasite in mice. This provides a strong proof of principle for the anti-immunomodulatory approach we have developed.

A deep sequencing approach to comparatively analyze the transcriptome of lifecycle stages of the filarial worm, Brugia malayi

Background

Developing intervention strategies for the control of parasitic nematodes continues to be a significant challenge. Genomic and post-genomic approaches play an increasingly important role for providing fundamental molecular information about these parasites, thus enhancing basic as well as translational research. Here we report a comprehensive genome-wide survey of the developmental transcriptome of the human filarial parasite Brugia malayi.

Methodology/Principal Findings

Using deep sequencing, we profiled the transcriptome of eggs and embryos, immature (≤3 days of age) and mature microfilariae (MF), third- and fourth-stage larvae (L3 and L4), and adult male and female worms. Comparative analysis across these stages provided a detailed overview of the molecular repertoires that define and differentiate distinct lifecycle stages of the parasite. Genome-wide assessment of the overall transcriptional variability indicated that the cuticle collagen family and those implicated in molting exhibit noticeably dynamic stage-dependent patterns. Of particular interest was the identification of genes displaying sex-biased or germline-enriched profiles due to their potential involvement in reproductive processes. The study also revealed discrete transcriptional changes during larval development, namely those accompanying the maturation of MF and the L3 to L4 transition that are vital in establishing successful infection in mosquito vectors and vertebrate hosts, respectively.

Conclusions/Significance

Characterization of the transcriptional program of the parasite's lifecycle is an important step toward understanding the developmental processes required for the infectious cycle. We find that the transcriptional program has a number of stage-specific pathways activated during worm development. In addition to advancing our understanding of transcriptome dynamics, these data will aid in the study of genome structure and organization by facilitating the identification of novel transcribed elements and splice variants.

Lymphatic filariasis, also known as elephantiasis, is a tropical disease affecting over 120 million people worldwide. More than 40 million people live with painful, disfiguring symptoms that can cause severe debilitation and social stigma. The disease is caused by infection with thread-like filarial nematodes (roundworms) that have a complex parasitic lifecycle involving both human and mosquito hosts. In the study, the authors profiled the transcriptome (the set of genes transcribed into messenger RNA rather than all of those in the genome) of the human filarial worm Brugia malayi in different lifecyle stages using deep sequencing technology. The analysis revealed major transitions in RNA expression from eggs through larval stages to adults. Using statistical approaches, the authors identified groups of genes with distinct life stage dependent transcriptional patterns, with particular emphasis on genes displaying sex-biased or germline-enriched patterns and those displaying significant changes during larval development. This study presents a first comprehensive analysis of the lifecycle transcriptome of B. malayi, providing fundamental molecular information that should help researchers better understand parasite biology and could provide clues for the development of more effective interventions.

The adjuvant-free immunoprotection of recombinant filarial protein Abundant Larval Transcript-2 (ALT-2) in Mastomys coucha and the immunoprophylactic importance of its putative signal sequence

The filarial protein Abundant Larval Transcript-2 (ALT-2) of the filarial parasite Brugia malayi has been shown to produce 74% worm clearance when administered with an adjuvant. In the present study, we show that it not only induces humoral and cell-mediated immunity, but also protection up to 71% in Mastomys coucha, a permissive animal model for filariasis, even without adjuvant. This unique feature of ALT-2 protein is highly restricted to its 21 amino acid N-terminal signal sequence, the absence of which resulted in poor immune response as well as immunoprotection (49%). Moreover, ALT-2 is likely to exert immunoprotection effects in B. malayi infection by maintaining a Th1-Th2 balance, evident from higher levels of IgG1 and IgG2a as well as IL-4 and IFN-γ. An improved understanding about the role of this filarial protein in host immunity, host-parasite interaction and worm clearance will aid in the development of good immunoprophylaxis for the disease.

Immunoprophylactic potential of filarial glutathione-s-transferase in lymphatic filariaisis

OBJECTIVE

To elucidates the immunoprophylactic potential of glutathion-s-transferase (GST) from cattle filarial parasite Setaria digitata (S. digitata) against lymphatic filariasis.

METHODS

GST was purified through affinity chromatography (SdGST) and chacterized by SDS-PAGE and Nano-LC MS/MS analysis. Antibody isotypes to SdGST were measured by ELISA. Antibody dependant cellular cytotoxicity (ADCC) was performed in vitro using sera from immunized animals and immune individuals. T-cell proliferation and cytokine response to SdGST in different groups of filariasis were measured. Immunoprophylactic potential of SdGST was evaluate in animal model.

RESULTS

SdGST exhibited 30-fold enhancement of enzyme activity over crude parasitic extract. It was found to be 26 kDa by SDS-PAGE. Nano LC-MS/MS analysis followed by blast search showed 100% homology with Dirofilaria immitis (D. immitis) and only 43% with Homo sapiens (H. sapiens). Immunoblotting analysis showed putatively immune individuals carry significant level of antibodies to SdGST as compared with microfilaraemics. Immunized sera and sera endemic normal could neutralize the enzymatic activity of SdGST and inducing in vitro cytotoxicity of microfilariae. Peripheral blood mononuclear cells (PBMC) from endemic normals upon stimulation with SdGST showed a mixed type of Th1/Th2 response. SdGST immunization clear microfilariae from circulation in S. digitata implanted mastomys.

CONCLUSIONS

The heterologous GST could be potentially developed as a vaccine candidate against lymphatic filarial parasite.

Proteomic analysis of secretory products from the model gastrointestinal nematode Heligmosomoides polygyrus reveals dominance of venom allergen-like (VAL) proteins

The intestinal helminth parasite, Heligmosomoides polygyrus bakeri offers a tractable experimental model for human hookworm infections such as Ancylostoma duodenale and veterinary parasites such as Haemonchus contortus. Parasite excretory-secretory (ES) products represent the major focus for immunological and biochemical analyses, and contain immunomodulatory molecules responsible for nematode immune evasion. In a proteomic analysis of adult H. polygyrus secretions (termed HES) matched to an extensive transcriptomic dataset, we identified 374 HES proteins by LC-MS/MS, which were distinct from those in somatic extract HEx, comprising 446 identified proteins, confirming selective export of ES proteins. The predominant secreted protein families were proteases (astacins and other metalloproteases, aspartic, cysteine and serine-type proteases), lysozymes, apyrases and acetylcholinesterases. The most abundant products were members of the highly divergent venom allergen-like (VAL) family, related to Ancylostoma secreted protein (ASP); 25 homologues were identified, with VAL-1 and -2 also shown to be associated with the parasite surface. The dominance of VAL proteins is similar to profiles reported for Ancylostoma and Haemonchus ES products. Overall, this study shows that the secretions of H. polygyrus closely parallel those of clinically important GI nematodes, confirming the value of this parasite as a model of helminth infection.

Cuticle surface coat of plant-parasitic nematodes

The surface coat (SC) of the plant-parasitic nematode cuticle is an understudied area of current research, even though it likely plays key roles in both nematode-plant and nematode-microbe interactions. Although in several ways Caenorhabditis elegans is a poor model for plant-parasitic nematodes, it is a useful starting point for investigations of the cuticle and its SC, especially in the light of recent work using this species as a model for innate immunity and the generic biology underpinning much host-parasite biology. We review the research focused on the involvement of the SC of plant-parasitic nematodes. Using the insights gained from animal-parasitic nematodes and other sequenced nematodes, we discuss the key roles that the SC may play.

Control of important helminthic infections vaccine development as part of the solution

Among the tools available for the control of helminth infections, chemotherapy has come to totally dominate the field. In the veterinary field, development of drug resistance has appeared but this is not (yet) a problem in the control of human diseases. Although there is no vaccine commercially available for any human parasitic infection yet, recent progress in vaccine development is making this a future possibility for several diseases. The goal of chemotherapy is to alleviate infection and morbidity in the definitive host, or reduce transmission, while the effect of available vaccine candidates would mainly be to influence transmission through targeting the intermediate or reservoir host, when the infection is zoonotic. Apart from this general scheme, there are also vaccine candidates targeting the parasites in the definitive host, in particular the early developmental stages, which should reduce the risk of drug failure. Since the biological targets in most cases are different, vaccination would be synergistic with drug therapy. This review covers diseases caused by helminthes in both humans and animals and includes examples of diseases caused by cestodes, nematodes and trematodes. The focus is on infections for which vaccine development has been undertaken for a long time, resulting in products that could realistically become integrated into control strategies in the near future.

The Onchocerca volvulus cysteine proteinase inhibitor, Ov-CPI-2, is a target of protective antibody response that increases with age

Background

Despite considerable efforts, a suitable vaccine against Onchocerca volvulus infection has remained elusive. Herein, we report on the use of molecular tools to identify and characterize O. volvulus antigens that are possibly associated with the development of concomitant immunity in onchocerciasis.

Methodology/Principal Findings

Third-stage larvae (L3) and molting L3 (mL3) O. volvulus stage-specific cDNA libraries were screened with a pool of sera from chronically infected patients who had likely developed such immunity. The 87 immunoreactive clones isolated were grouped into 20 distinct proteins of which 12 had already been cloned and/or characterized before and 4 had been proven to be protective in a small O. volvulus animal model. One of these, onchocystatin (Ov-CPI-2), a previously characterized O. volvulus cysteine proteinase inhibitor was, overall, the most abundant clone recognized by the immune sera in both the L3 and mL3 cDNA libraries. To further characterize its association with protective immunity, we measured the IgG subclass and IgE class specific responses to the antigen in putatively immune (PI) and infected (INF) individuals living in a hyperendemic area in Cameroon. It appeared that both groups had similar IgG3 and IgE responses to the antigen, but the INF had significantly higher IgG1 and IgG4 responses than the PI individuals (p<0.05). In the INF group, the IgG3 levels increased significantly with the age of the infected individuals (r = 0.241; p<0.01). The IgG1 responses in the INF were high regardless of age. Notably, culturing L3 in vitro in the presence of anti-Ov-CPI-2 monospecific human antibodies and naïve neutrophils resulted in almost complete inhibition of molting of L3 to L4 and to cytotoxicity to the larvae.

Conclusions/Significance

These results add to the knowledge of protective immunity in onchocerciasis and support the possible involvement of anti-Ov-CPI-2 IgG1 and/or IgG3 cytophilic antibodies in the development of protective immunity in the PI and the INF. The results further support the consideration of Ov-CPI-2 as a leading target for an anti-L3 vaccine.

Onchocerciasis is a chronic and highly debilitating disease of humans caused by a worm called Onchocerca volvulus. This worm can live in the human body for over 15 years. The disease affects mainly the skin and eyes and is the second leading infectious cause of blindness worldwide. There is currently no vaccine to prevent the infection. Available drugs can give short-term relief but cannot cure the infection. To prevent infection, a vaccine against the third-stage infective larva, L3, or the developing larva is required. These stages were shown to be the targets of protective immunity that develops in individuals who live in onchocerciasis endemic regions. One type of protective immunity has been shown to develop with age and is called concomitant immunity. In the present study, we have identified a number of larval antigens that may be associated with the development of such immunity. The most prominent of these antigens was Ov-CPI-2, also called onchocystatin, which had previously been shown to be a promising vaccine candidate. This antigen was further characterized and confirmed to be possibly also a target of immune protection that develops in the infected individuals with age and is referred to as concomitant immunity.

Vaccination with peptide mimotopes produces antibodies recognizing bacterial capsular polysaccharides

A phage display peptide library was screened using a panel of antibodies to the capsular polysaccharides of Streptococcus agalactiae and Neisseria meningitidis. Mimotopes NPDHPRVPTFMA (2-8), LIPFHKHPHHRG (3-2) and EQEIFTNITDRV (G3) showing the highest binding capacity and strongest ELISA reaction were selected for immunization experiments. These mimotopes were either synthesised as oligodeoxynucleotides for DNA immunization or MAP (multiple antigen peptide) for peptide immunization. Mimotope-DNA vaccination, particularly for G3, induced antibodies recognizing a number of target bacteria. This response was seen after the second boost injection and was significantly enhanced by the 3rd boost injection with a Th1-associated profile, which was dominated by IgG2a, followed by IgG1. Mimotope-MAP immunization also produced strong humoral immune responses to the bacteria. Antibodies from G3 DNA immunization reacted with the surface molecules of S. agalactiae, N. meningitidis and Escherichia coli K5 shown by indirect immunofluorescence staining, indicating a possible localization to the bacterial capsule. Antibodies produced both from DNA/MAP immunization reacted with purified bacterial capsular polysaccharides by ELISA and were of high avidity. We have further characterized peptide G3 by a 'tiling path' study to examine the effect of changing individual residues in the peptide in raising antibodies, which showed that the EIFTN motif in G3 was important in generating antibodies to several capsulated bacteria. We conclude that mimotope immunization with DNA or MAP potentially induces strong antibody responses against encapsulated bacteria. It is suggested that the antibody targets are polysaccharides, and these antibodies may cross react at least among closely related species of bacteria.

Immunisation with a multivalent, subunit vaccine reduces patent infection in a natural bovine model of onchocerciasis during intense field exposure

Human onchocerciasis, caused by the filarial nematode Onchocerca volvulus, is controlled almost exclusively by the drug ivermectin, which prevents pathology by targeting the microfilariae. However, this reliance on a single control tool has led to interest in vaccination as a potentially complementary strategy. Here, we describe the results of a trial in West Africa to evaluate a multivalent, subunit vaccine for onchocerciasis in the naturally evolved host-parasite relationship of Onchocerca ochengi in cattle. Naïve calves, reared in fly-proof accommodation, were immunised with eight recombinant antigens of O. ochengi, administered separately with either Freund's adjuvant or alum. The selected antigens were orthologues of O. volvulus recombinant proteins that had previously been shown to confer protection against filarial larvae in rodent models and, in some cases, were recognised by serum antibodies from putatively immune humans. The vaccine was highly immunogenic, eliciting a mixed IgG isotype response. Four weeks after the final immunisation, vaccinated and adjuvant-treated control calves were exposed to natural parasite transmission by the blackfly vectors in an area of Cameroon hyperendemic for O. ochengi. After 22 months, all the control animals had patent infections (i.e., microfilaridermia), compared with only 58% of vaccinated cattle (P = 0.015). This study indicates that vaccination to prevent patent infection may be an achievable goal in onchocerciasis, reducing both the pathology and transmissibility of the infection. The cattle model has also demonstrated its utility for preclinical vaccine discovery, although much research will be required to achieve the requisite target product profile of a clinical candidate.

River blindness, or onchocerciasis, is caused by a parasitic worm (Onchocerca volvulus) that is transmitted by blood-feeding blackflies, which breed in fast-flowing rivers. More than 37 million people are infected and may experience visual impairment and/or severe dermatitis. Control of onchocerciasis is largely dependent on a single drug, ivermectin. Whilst this is extremely effective at killing the worms' offspring (microfilariae) and preventing symptoms, ivermectin does not eliminate the long-lived adult parasites or always stop transmission. Consequently, treatments must be repeated for many years, and drug resistance may be emerging. Against this background, a vaccine against onchocerciasis would provide an important additional tool to sustain effective control. In this study, we evaluated eight worm antigens as vaccine components in cattle, which are often parasitized by O. ochengi (the closest relative of O. volvulus) in West Africa. Twelve uninfected animals received all eight antigens and were exposed to natural transmission of O. ochengi alongside 13 unvaccinated cattle. After almost two years, 92% of vaccinated animals had acquired adult worms, but only 58% were positive for microfilariae; whereas 100% of unvaccinated animals harboured both parasite stages. This suggests that a vaccine against microfilariae to prevent development of disease in humans may be achievable.

Helminth immunoregulation: the role of parasite secreted proteins in modulating host immunity

Helminths are masterful immunoregulators. A characteristic feature of helminth infection is a Th2-dominated immune response, but stimulation of immunoregulatory cell populations, such as regulatory T cells and alternatively activated macrophages, is equally common. Typically, Th1/17 immunity is blocked and productive effector responses are muted, allowing survival of the parasite in a "modified Th2" environment. Drug treatment to clear the worms reverses the immunoregulatory effects, indicating that a state of active suppression is maintained by the parasite. Hence, research has focussed on "excretory-secretory" products released by live parasites, which can interfere with every aspect of host immunity from initial recognition to end-stage effector mechanisms. In this review, we survey our knowledge of helminth secreted molecules, and summarise current understanding of the growing number of individual helminth mediators that have been shown to target key receptors or pathways in the mammalian immune system.

Exploring the immunology of parasitism--from surface antigens to the hygiene hypothesis

Helminth immunology is a field which has changed beyond recognition in the past 30 years, transformed not only by new technologies from cDNA cloning to flow cytometry, but also conceptually as our definition of host immune pathways has matured. The molecular revolution defined key nematode surface and secreted antigens, and identified candidate immunomodulators that are likely to underpin parasites' success in eluding immune attack. The immunological advances in defining cytokine networks, lymphocyte subsets and innate cell recognition have also made a huge impact on our understanding of helminth infections. Most recently, the ideas of regulatory immune cells, in particular the regulatory T cell, have again overturned older thinking, but also may explain immune hyporesponsiveness observed in chronic helminth diseases, as well as the link to reduced allergic reactions observed in human and animal infections. The review concludes with a forward look to where we may make future advances towards the final eradication of helminth diseases.

Brugia malayi excreted/secreted proteins at the host/parasite interface: stage- and gender-specific proteomic profiling

Relatively little is known about the filarial proteins that interact with the human host. Although the filarial genome has recently been completed, protein profiles have been limited to only a few recombinants or purified proteins of interest. Here, we describe a large-scale proteomic analysis using microcapillary reverse-phase liquid chromatography-tandem-mass spectrometry to identify the excretory-secretory (ES) products of the L3, L3 to L4 molting ES, adult male, adult female, and microfilarial stages of the filarial parasite Brugia malayi. The analysis of the ES products from adult male, adult female, microfilariae (Mf), L3, and molting L3 larvae identified 852 proteins. Annotation suggests that the functional and component distribution was very similar across each of the stages studied; however, the Mf contributed a higher proportion to the total number of identified proteins than the other stages. Of the 852 proteins identified in the ES, only 229 had previous confirmatory expressed sequence tags (ESTs) in the available databases. Moreover, this analysis was able to confirm the presence of 274 “hypothetical” proteins inferred from gene prediction algorithms applied to the B. malayi (Bm) genome. Not surprisingly, the majority (160/274) of these “hypothetical” proteins were predicted to be secreted by Signal IP and/or SecretomeP 2.0 analysis. Of major interest is the abundance of previously characterized immunomodulatory proteins such as ES-62 (leucyl aminopeptidase), MIF-1, SERPIN, glutathione peroxidase, and galectin in the ES of microfilariae (and Mf-containing adult females) compared to the adult males. In addition, searching the ES protein spectra against the Wolbachia database resulted in the identification of 90 Wolbachia-specific proteins, most of which were metabolic enzymes that have not been shown to be immunogenic. This proteomic analysis extends our knowledge of the ES and provides insight into the host–parasite interaction.

Human lymphatic filariasis caused by the nematode parasites Brugia malayi and Wuchereria bancrofti are a major cause of concern in tropical countries. Studies over several decades have identified various proteins of these parasites that have highlighted their role in host–parasite interactions and possible chemotherapeutic and prophylactic interventions. The availability of the parasite genome facilitates the identification of all of the proteins of the parasite that could interact with the host. In this study, we have attempted to identify the excretory-secretory proteins of the various stages of the parasite that could be maintained in vitro for a limited period utilizing a high-throughput proteomics approach. We observe and report that the parasites expend resources to secrete out various molecules that they utilize to evade the host immune system and modulate its responses. Further, this study also provides information on the predicted hypothetical proteins to be bonafide proteins and thus a catalogue of the excretory-secretory proteins towards a better understanding of the host–parasite interactions.

Four abundant novel transcript genes from Toxocara canis with unrelated coding sequences share untranslated region tracts implicated in the control of gene expression

The Toxocara canis "abundant novel transcripts" (ant) are four highly expressed products, constituting >18% of ESTs from the infective stage of this widely prevalent nematode parasite. Using 5' RACE, we determined full-length sequences for each ant gene, between 1.8 and 2.8kb. The four genes (termed ant-3, -5, -30 and -34), share no coding sequence similarity, although their 3'UTRs (untranslated regions) are homologous. Predicted ANT-5 and ANT-30 proteins show distant similarity to RNA regulatory proteins, RNA-dependent RNA polymerase and DEAH-box helicase, respectively. Surprisingly, ant-3 appears to be bi-cistronic, encoding two ORFs (ANT-3.1 and -3.2), each with a predicted N-terminal signal sequence. Antibodies raised to recombinant proteins did not react with native parasite products, indicating that protein expression did not accord with transcript abundance. However, antibody reactivity to two gene products (ANT-3.1 and ANT-34) was present in patient sera, suggesting that these proteins are synthesized later in infection. To test whether 3'UTRs may regulate expression, the ant-34 3'UTR sequence was inserted adjacent to enhanced green fluorescent protein (EGFP) for transformation of Caenorhabditis elegans. The ant-34 3'UTR greatly reduced EGFP expression, inhibiting both transcription and translation. We identified a tract in this UTR with significant sequence complementarity to the C. elegans micro-RNA lin-4. While infective stage parasites stockpile high levels of the ant transcripts, we suggest that translation is repressed, possibly by a mechanism involving 3' UTR motifs shared by the four genes.

Expression of helminth genes in Leishmania: an experimental transfection system to test immunological function

Functional analysis of genes from parasitic helminths requires, at the present time, heterologous expression. We have adapted the well-characterized system of transfection in Leishmania protozoal parasites, as a means of analysing the effect of single filarial genes on the mammalian immune system. For example, testing the function of the Brugia malayi abundant larval transcript (ALT) gene-transfected Leishmania mexicana were found to be significantly more virulent in macrophages in vitro. The course of infection in vivo is also aggravated by expression of the ALT gene. Examples are also given of transgenes which reduced in vitro growth within macrophages, as well as others which exert no effect on the protozoal parasitism. Thus, Leishmania transfection provides a tractable system to analyse helminth gene function within the context of the host immune system.

Heterologous expression of the filarial nematode alt gene products reveals their potential to inhibit immune function

Background

Parasites exploit sophisticated strategies to evade host immunity that require both adaptation of existing genes and evolution of new gene families. We have addressed this question by testing the immunological function of novel genes from helminth parasites, in which conventional transgenesis is not yet possible. We investigated two such novel genes from Brugia malayi termed abundant larval transcript (alt), expression of which reaches ~5% of total transcript at the time parasites enter the human host.

Results

To test the hypothesis that ALT proteins modulate host immunity, we adopted an alternative transfection strategy to express these products in the protozoan parasite Leishmania mexicana. We then followed the course of infection in vitro in macrophages and in vivo in mice. Expression of ALT proteins, but not a truncated mutant, conferred greater infectivity of macrophages in vitro, reaching 3-fold higher parasite densities. alt-transfected parasites also caused accelerated disease in vivo, and fewer mice were able to clear infection of organisms expressing ALT. alt-transfected parasites were more resistant to IFN-γ-induced killing by macrophages. Expression profiling of macrophages infected with transgenic L. mexicana revealed consistently higher levels of GATA-3 and SOCS-1 transcripts, both associated with the Th2-type response observed in in vivo filarial infection.

Conclusion

Leishmania transfection is a tractable and informative approach to determining immunological functions of single genes from heterologous organisms. In the case of the filarial ALT proteins, our data suggest that they may participate in the Th2 bias observed in the response to parasite infection by modulating cytokine-induced signalling within immune system cells.

Cloning and characterisation of an aspartyl protease inhibitor (API-1) from Ancylostoma hookworms

Hookworm infection persists as a public health problem in developing nations. Vaccine-based strategies offer the best chance of long-term control. Aspartyl protease inhibitors from parasitic nematodes are highly immunogenic, and have been suggested as potential vaccine antigens. An aspartyl protease inhibitor, API-1, was cloned and characterised from the hookworms Ancylostoma caninum and Ancylostoma ceylanicum. Using sequence from the hookworm expressed sequence tag project, specific primers were designed and used to amplify Ac-api-1 from A. caninum infective L3 cDNA by PCR. Amplicons from the 5' and 3' ends were cloned, sequenced, and combined to create an 874-bp full-length composite sequence of the Ac-api-1 gene. The A. ceylanicum api-1 cDNA of 878 bp was cloned from L3 cDNA using the A. caninum primers. The amino acid sequences of hookworm orthologues were nearly identical, and database searching indicated they belonged to the aspin family, a group of nematode specific aspartyl protease inhibitors that includes the Ascaris pepsin inhibitor PI-3. Ac-api-1 mRNA was detected by reverse transcriptase PCR in eggs, L1, L3 and adult life cycle stages. A polyclonal antiserum against Escherichia coli expressed recombinant Ac-API-1 detected the protein in adult A. caninum excretory/secretory products, but not in those from activated infective larvae. Immunolocalisation experiments using the antiserum indicated that Ac-API-1 is present primarily in the pseudocoelomic fluid in adult hookworms. Soluble, yeast-expressed Ac-API-1 failed to inhibit pepsin or a hookworm gut aspartyl protease in vitro, but inhibited approximately 30% of the proteolytic activity of adult excretory/secretory products. The pseudocoleomic location, presence in all life cycle stages, lack of inhibitory activity against pepsin, and inhibitory activity against excretory/secretory products suggest that Ac-API-1 inhibits an unidentified, putative aspartyl protease secreted by adult hookworms, and may be released as an enzyme-inhibitor complex. The highly immunogenic properties of nematode aspins suggest that Ac-API-1 represents a promising target for a recombinant hookworm vaccine.

Helminth parasites--masters of regulation

Immune regulation by parasites is a global concept that includes suppression, diversion, and conversion of the host immune response to the benefit of the pathogen. While many microparasites escape immune attack by antigenic variation or sequestration in specialized niches, helminths appear to thrive in exposed extracellular locations, such as the lymphatics, bloodstream, or gastrointestinal tract. We review here the multiple layers of immunoregulation that have now been discovered in helminth infection and discuss both the cellular and the molecular interactions involved. Key events among the host cell population are dominance of the T-helper 2 cell (Th2) phenotype and the selective loss of effector activity, against a background of regulatory T cells, alternatively activated macrophages, and Th2-inducing dendritic cells. Increasingly, there is evidence of important effects on other innate cell types, particularly mast cells and eosinophils. The sum effect of these changes to host reactivity is to create an anti-inflammatory environment, which is most favorable to parasite survival. We hypothesize therefore that parasites have evolved specific molecular strategies to induce this conducive landscape, and we review the foremost candidate immunomodulators released by helminths, including cytokine homologs, protease inhibitors, and an intriguing set of novel products implicated in immune suppression.