Immunoprophylactic potential of a 120 kDa Brugia malayi adult antigen fraction, BmA-2, in lymphatic filariasis

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.

The Larval Specific Lymphatic Filarial ALT-2: Induction of Protection Using Protein or DNA Vaccination

Genes from the infective stage of lymphatic filarial parasites expressed at the time of host invasion have been identified as potential vaccine candidates. By screening an L3 cDNA library with sera from uninfected longstanding residents of an area endemic for onchocerciasis, so-called "endemic normals" (EN), we have cloned and characterized one such gene termed the abundant larval transcript two (ALT-2). The stage specificity of ALT-2 gene transcription and protein synthesis was confirmed by PCR using genespecific primers, and by western blot analysis of protein extracts from various stages of the parasite life cycle using specific antisera. Significant differences in antibody response to the recombinant ALT-2 were observed in endemic populations with differing clinical manifestations of lymphatic filariasis with an antibody response present in sera from 18 of 25 (72%) EN subjects compared to 9 of 25 (36%) with subclinical microfilaracmia (MF) and 14 of 25 (52%) of those with chronic lymphatic obstruction (CP) (P=0.01 for comparison of EN to CP or to MF). This differential responsiveness suggests that the protective immunity postulated to account for their uninfected status might be associated with a response to this protein. When the utility of ALT-2 as a vaccine candidate was tested in a murine model using either recombinant protein or a DNA vaccine construct, statistically significant protection was observed when compared to a control filarial gene product expressed across all stages of the parasite lifecycle (SXP-1; P=0.02 for protein and P=0.01 for the DNA vaccine) or compared to adjuvant alone. This level of protection indicates that this vaccine is a promising candidate for further development.

Immunization with a multisubunit vaccine considerably reduces establishment of infective larvae in a rodent model of Brugia malayi

Although recombinant vaccines have several advantages over conventional vaccines, protection induced by single antigen vaccines is often inadequate for a multicellular helminth parasite. Therefore, immunoprophylactic efficacy of cocktail antigen vaccines comprised of several combinations of three Brugia malayi recombinant proteins BmAF-Myo, Bm-iPGM and Bm-TPP were evaluated. Myosin+TPP and iPGM+TPP provided the best protection upon B. malayi infective larval challenge with ∼70% reduction in adult worm establishment over non-vaccinated animals that was significantly higher than the protection achieved by any single antigen vaccine. Myosin+iPGM, in contrast did not provide any enhance protection over the single recombinant protein vaccines. Specific IgG, IgM level, IgG antibody subclasses levels (IgG1, IgG2a, IgG2b, IgG3), lymphocyte proliferation, reactive oxygen species level and cytokines level were also determined to elucidate the characteristics of the protective immune responses. Thus the study undertaken provided more insight into the cocktail vaccination approach to combat LF.

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.

Tandem antioxidant enzymes confer synergistic protective responses in experimental filariasis

Helminth parasites use antioxidant defence strategies for survival during oxidative stress due to free radicals in the host. Accordingly, tissue-dwelling filarial parasites counteract host responses by releasing a number of antioxidants. Targeting these redox regulation proteins together, would facilitate effective parasite clearance. Here, we report the combined effect of protective immune responses trigged by recombinant Wuchereria bancrofti thioredoxin (WbTRX) and thioredoxin peroxidase (WbTPX) in an experimental filarial model. The expression of WbTRX and WbTPX in different stages of the parasite and their cross-reactivity were analysed by enzyme-linked immunosorbent assay (ELISA). The immunogenicity of recombinant proteins and their protective efficacy were studied in animal models when immunized in single or cocktail mode. The antigens showed cross-reactive epitopes and induced high humoral and cellular immune responses in mice. Further, parasite challenge against Brugia malayi L3 larvae in Mastomys coucha conferred significant protection of 57% and 62% against WbTRX and WbTPX respectively. The efficacy of L3 clearance was significantly higher (71%) (P <  0.001) when the antigens were immunized together, showing a synergistic effect in multiple-mode vaccination. Hence, the study suggests WbTRX and WbTPX to be attractive vaccine candidates when immunized together and provides a tandem block for parasite elimination in the control of lymphatic filariasis.

Identification and characterization of nematode specific protective epitopes of Brugia malayi TRX towards development of synthetic vaccine construct for lymphatic filariasis

Although multi-epitope vaccines have been evaluated for various diseases, they have not yet been investigated for lymphatic filariasis. Here, we report for the first time identification of two immunodominant B epitopes (TRXP1 and TRXP2) from the antioxidant Brugia malayi thioredoxin by studying their immune responses in mice model and human subjects. TRXP1 was also found to harbor a T epitope recognized by human PBMCs and mice splenocytes. Further, the epitopic peptides were synthesized as a single peptide conjugate (PC1) and their prophylactic efficacy was tested in a murine model of filariasis with L3 larvae. PC1 conferred a significantly high protection (75.14%) (P < 0.0001) compared to control (3.7%) and recombinant TRX (63.03%) (P < 0.018) in experimental filariasis. Our results suggest that multi-epitope vaccines could be a promising strategy in the control of lymphatic filariasis.

Multivalent Vaccine for Lymphatic Filariasis

Lymphatic filariasis is a mosquito borne parasitic infection that cause severe economic burden in several parts of the world. Currently there is no vaccine available to prevent this infection in human. Multidrug therapy is effective, however, requires annual treatment and there is significant concern of drug resistance. In this manuscript we describe development of a multivalent DNA based vaccine comprising BmALT-2 and BmHSP antigens of lymphatic filariasis. Challenge experiments using third stage infective larvae of Brugia malayi in a mouse model suggested that nearly 90% protection can be achieved using the multivalent formulation in a DNA prime protein boost approach. The vaccination regimen induced significant IgG antibody responses and ELISPOT analysis for secreted cytokines from the spleen cells of vaccinated animals showed that these cells produce significant amount of IL-4. Results from this study thus show that a multivalent vaccine formulation of BmALT-2 and BmHSP is an excellent vaccine for lymphatic filariasis and significant protection can be achieved against a challenge infection with B. malayi in a mouse model.

Immune responses generated by intramuscular DNA immunization of Brugia malayi transglutaminase (BmTGA) in mice

An attempt was made to evaluate the immunoprophylactic efficacy of Brugia malayi transglutaminase (BmTGA) as a DNA vaccine, for human lymphatic filariasis. BmTGA was cloned and characterized in the DNA vaccine vector pVAX1. Further, the tissue distribution study of the DNA construct, pVAX-TGA was carried out in mice and the DNA vaccine was shown to be efficiently distributed to all the organs, was accessible to the immune system, and at the same time was metabolized quickly and did not pose problems of toxicity. Intramuscular immunization in mice showed significant antibody production and splenocyte proliferation upon antigenic stimulation. The immune responses were biased towards the Th1 arm, as evaluated in terms of isotype antibody distribution and cytokine profile. Thus, analysis of the humoral and cellular immune responses indicated that BmTGA is a potent immunogen. However, protection studies as determined by the micropore chamber method using live microfilarial larvae, showed that the DNA vaccine could confer only partial protection in the mouse model. We conclude that despite the induction of sufficient humoral and cellular immune responses, BmTGA as a DNA vaccine could not confer much protection against subsequent challenge and other aspects of the immune responses need to be further investigated.

Protective efficacy of a filarial surface antigen in experimental filariasis

A water-insoluble, detergent-soluble, surface-associated glycoprotein, designated as Dssd1, was found to induce microfilaria clearance in Mastomys coucha implanted with Setaria digitata. Intraperitoneal implantation of adult female worms of S. digitata in M. coucha could induce microfilaraemia lasting about 165 days in circulation. Immunization of M. coucha with Dssd1 antigen either before or after implantation of worms resulted in a significant reduction in microfilaria density. Complete clearance of circulating microfilaria was achieved by immunization (before and after implantation) in animals by 95 and 105 days post-implantation, respectively, indicating the efficacy of Dssd1 antigen in the clearance of microfilaraemia in infected M. coucha.

Vaccination with 73kDa recombinant heavy chain myosin generates high level of protection against Brugia malayi challenge in jird and mastomys models

We have earlier reported identification, expression and purification of a 2.0kb cDNA clone coding for Brugia malayi heavy chain myosin which exhibited strong immuno-reactivity with bancroftian sera from endemic normal (EN) human subjects which are considered to be putatively immune. In the present study, immunoprophylactic characterization of B. malayi recombinant myosin was carried out in rodent models and the protective efficacy was evaluated by assessing the microfilarial burden and adult worm counts in vaccinated host after an infective larval challenge. Data indicates that immunization resulted in to a significant reduction in microfilarial burden (approximately 76%) and adult worm establishment (54-58%), accompanied with embryostatic effect (70-75%) in both the animal models. The findings suggest that immune-protection by recombinant myosin was conferred through both humoral and cellular arms of immunity as indicated by an increased antibody titer with predominance of IgG2a and IgG2b isotypes along with elevated level of IgG1 apart from significant proliferation of lymphocytes, increased nitric oxide production and profound adherence of splenocytes causing cytotoxicity to microfilariae and infective larvae. The present study indicates that the recombinant B. malayi myosin is a promising vaccine candidate against human lymphatic filarial infection.

Brugia malayi Adult Low Molecular Weight IgG4-Reactive Antigens Induce Differential Cytokine Response in Lymphocytes of Endemic Normal and Asymptomatic Microfilariae Carriers In Vitro

To characterize putatively protective immune response in bancroftian filariasis, Th1/Th2 cytokine profile induced in peripheral blood mononuclear cells (PBMCs) of endemic normal (EN) and asymptomatic microfilaremic (ASM) individuals were studied using different molecular weight fractions of Brugia malayi adult soluble antigens (BmA), which are differentially recognized by IgG4 antibodies present in their sera. Infection free and putatively immune individuals living in a filaria endemic area were identified and included in the present study as EN only after careful longitudinal follow up for three years. It was observed that the low molecular weight antigens present in Fr4 and Fr5 induced differential cytokine response; EN individuals showed a strong Th1 bias whereas ASM individuals showed a strong Th2 bias even though both the groups produced Th1 cytokines, albeit of different quantity, when a nonhelminthic antigen like H37Rv whole cell lysate was used. Since antigens present in Fr5 induced a highly polarized response, they should be examined for their diagnostic potential in lymphatic filariasis.

Immunoprophylactic evaluation of a 37-kDa Brugia malayi recombinant antigen in lymphatic filariasis

The Brugia malayi filarial antigens recognised preferentially by sera from an endemic normal population are considered to be potential vaccine candidates. By immunoscreening the cDNA library of the infective L3 stage of B. malayi with pooled endemic normal sera, a cDNA clone Bm-SL3 was identified. Analysis of sera from different patient groups with the rBm-SL3 protein showed it to be highly reactive with endemic normal sera compared to its reactivity with microfilaraemic and non-endemic normal sera. The immunoprotective efficacy of the rBm-SL3 antigen against B. malayi filarial infection was evaluated in susceptible host jirds (gerbils) (Meriones unguiculatus). Jirds immunised with the rBm-SL3 antigen showed 68% cytotoxicity against microfilariae and 67-69% cytotoxicity against infective larvae in in-vitro antibody-dependent cellular cytotoxicity assays and in-situ micropore chamber methods. Analysis of IgG subclasses against Bm-SL3 revealed a significant increase in IgG1 and IgG2 antibodies in endemic normal sera compared with other groups. Lymphocyte proliferation to Bm-SL3 was significantly higher in the endemic normal group compared with that in clinical and microfilarial carriers (p < 0.001). Significantly enhanced levels of IFN-gamma in the culture supernatant of peripheral blood mononuclear cells of endemic normal sera after stimulation with Bm-SL3 suggest that the cellular response in this group may have a Th1 bias.

Lymphatic filariasis: new insights into an old disease

Lymphatic filariasis has afflicted people in the tropical areas of the world for thousands of years but even up to comparatively recent times it has been poorly understood and its importance under recognised. In the last 2 decades or so there has been a flurry of activity in filariasis research, which has provided new insights into the global problem of filariasis, the pathogenesis of filarial disease, diagnosis and control.

Host protective immunity and vaccine development studies in lymphatic filariasis

Lymphatic filariasis caused mainly by infection fromWuchereria bancrofti andBrugia malayi remains as the major cause of clinical morbidity in tropical and subtropical countries. Development of vaccine against filarial infection can act as additional measure to the existing therapeutic and vector control methods in the control of this disease. The main hurdles in the development of anti-filarial vaccine are the strict primate specificity ofWuchereria bancrofti, the paucity of parasite material, the diversity of clinical manifestations and their associated complex immune responses, lack of clear understanding on host-parasite interactions and the mechanisms involved in protective immunity. However in the past few years, the information generated in immuno-epidemiological studies, correlated with observations in experimental animals suggests that a filarial vaccine is feasible. Initially live irradiated infective larvae have been successfully used to induce high level of protective immunity in several animal models. Applying diverse strategies, variety of purified or recombinant filarial antigens have been explored for their ability to induce protection in different host-parasite systems. Some of these targeted filarial antigens induced high level of resistance in experimental animals against challenge infections. More focussed studies on thorough characterization of parasitological and immunological changes associated with resistance induced by such candidate protective antigens and on delivery mechanisms and safety aspects will be crucial in their selection for possible use in humans.

A 43-kDa circulating filarial antigen fraction of Wuchereria bancrofti in immunoprophylaxis against Brugia malayi in jirds

A 43 kiloDaltan (kDa) antigen fraction (CFA2-6) isolated from microfilaraemic plasma of bancroftian filarial patients showed selective reactivity with sera samples collected from endemic normals. Antibodies raised against this antigen showed a strong reactivity with the surface of Brugia malayi infective larvae as well as microfilariae. Similar antigenic determinants were detected in the parasite extracts, but not in the excretory-secretory products. Further analysis was done on the immunoprophylactic potential of CFA2-6 in inducing immunity against Brugia malayi in Meriones unguiculatus (jird) in vivo. A strong protective response of approximately 84% was observed against the development of the filarial parasite in the jirds immunized with CFA2-6. The immunized jirds also showed a significant clearance (87%) of microfilariae inoculated intravenously. Approximately 65% of infective larvae failed to survive in jirds transferred with anti-CFA2-6 serum compared to the jirds transferred with sera from the control jirds. Passive transfer of anti-CFA2-6 antibody to the jirds followed by intravenous inoculation of microfilariae resulted in the reduction of 77% of circulating microfilariae. This study suggests that the 43-kDa CFA2-6 could stimulate a strong protective immune response against infective larvae and microfilariae in experimental animals.

Vaccination against helminth parasites--the ultimate challenge for vaccinologists?

Helminths are multicellular pathogens which infect vast numbers of human and animal hosts, causing widespread chronic disease and morbidity. Vaccination against these parasites requires more than identification of effective target antigens, because without understanding the immunology of the host-parasite relationship, ineffective immune mechanisms may be invoked, and there is a danger of amplifying immunopathogenic responses. The fundamental features of the immune response to helminths are therefore summarised in the context of vaccines to helminth parasites. The contention between type-1 and type-2 responses is a central issue in helminth infections, which bias the immune system strongly to the type-2 pathway. Evidence from both human and experimental animal infections indicates that both lineages contribute to immunity in differing circumstances, and that a balanced response leads to the most favourable outcome. A diversity of immune mechanisms can be brought to bear on various helminth species, ranging from antibody-independent macrophages, antibody-dependent granulocyte killing, and nonlymphoid actions, particularly in the gut. This diversity is highlighted by analysis of rodent infections, particularly in comparisons of cytokine-depleted and gene-targeted animals. This knowledge of protective mechanisms needs to be combined with a careful choice of parasite antigens for vaccines. Many existing candidates have been selected with host antibodies, rather than T-cell responses, and include a preponderance of highly conserved proteins with similarities to mammalian or invertebrate antigens. Advantage has yet to be taken of parasite genome projects, or of directed searches for novel, parasite-specific antigens and targets expressed only by infective stages and not mature forms which may generate immunopathology. With advances under way in parasite genomics and new vaccine delivery systems offering more rapid assessment and development, there are now excellent opportunities for new antihelminth vaccines.