Comparison of Schistosoma mansoni irradiated cercariae and Sm23 DNA vaccines

A comprehensive and critical overview of schistosomiasis vaccine candidates

A digenetic platyhelminth Schistosoma is the causative agent of schistosomiasis, one of the neglected tropical diseases that affect humans and animals in numerous countries in the Middle East, sub-Saharan Africa, South America and China. Several control methods were used for prevention of infection or treatment of acute and chronic disease. Mass drug administration led to reduction in heavy-intensity infections and morbidity, but failed to decrease schistosomiasis prevalence and eliminate transmission, indicating the need to develop anti-schistosome vaccine to prevent infection and parasite transmission. This review summarizes the efficacy and protective capacity of available schistosomiasis vaccine candidates with some insights and future prospects.

Systems Biology Analysis of the Radiation-Attenuated Schistosome Vaccine Reveals a Role for Growth Factors in Protection and Hemostasis Inhibition in Parasite Survival

In spite of several decades of research, an effective vaccine against schistosomiasis remains elusive. The radiation-attenuated (RA) cercarial vaccine is still the best model eliciting high protection levels, although the immune mechanisms have not yet been fully characterized. In order to identify genes and pathways underlying protection we investigated patterns of gene expression in PBMC and skin draining Lymph Nodes (LN) from mice using two exposure comparisons: vaccination with 500 attenuated cercariae versus infection with 500 normal cercariae; one versus three doses. Vaccinated mice were challenged with 120 normal parasites. Integration of PBMC and LN data from the infected group revealed early up-regulation of pathways associated with Th2 skewing and polarization of IgG antibody profiles. Additionally, hemostasis pathways were downregulated in infected mice, correlating with platelet reduction, potentially a mechanism to assist parasite migration through capillary beds. Conversely, up regulation of such mechanisms after vaccination may explain parasite blockade in the lungs. In contrast, a single exposure to attenuated parasites revealed early establishment of a Th1 bias (signaling of IL-1, IFN-γ; and Leishmania infection). Genes encoding chemokines and their receptors were more prominent in vaccinated mice, indicating an enhanced capacity for inflammation, potentially augmenting the inhibition of intravascular migration. Increasing the vaccinations from one to three did not dramatically elevate protection, but there was a clear shift towards antibody-mediated effectors. However, elements of the Th1 bias were still evident. Notable features after three vaccinations were markers of cytotoxicity (including IL-6 and NK cells) together with growth factors and their receptors (FGFR/VEGF/EGF) and the apoptosis pathway. Indeed, there is evidence for the development of anergy after three vaccinations, borne out by the limited responses detected in samples after challenge. We infer that persistence of a Th1 response puts a limit on expression of antibody-mediated mechanisms. This feature may explain the failure of multiple doses to drive protection towards sterile immunity. We suggest that the secretions of lung stage parasites would make a novel cohort of antigens for testing in protection experiments.

An Efficient Schistosoma japonicum Bivalent Membrane Protein Antigen DNA Vaccine Against Schistosomiasis in Mice

Background

Schistosomiasis is one of the most important infectious parasitic diseases in the world. The most important was to control schistosomiasis is through a combination of medical therapy and immunization. The membrane antigens Tsp2 and 29 from Schistosoma are promising anti-schistosomiasis vaccine candidates.

Material/Methods

In this study, the pcDNA3.1(+)-SjTsp2, pcDNA3.1(+)-Sj29, and pcDNA3.1 (+)-SjTsp2-29 eukaryotic expression vectors were successfully constructed as DNA vaccines, and the protective abilities of these vaccines were evaluated in mice.

Results

The results showed that vaccination with SjTsp2, Sj29, and SjTsp2-29 reduced parasite burden and hepatic pathology compared to the control group, and the protective effect of the bivalent SjTsp2-29 DNA vaccine was better than that of the univalent SjTsp2 or Sj29 DNA vaccines. We also found high levels of IgG, IgG1, and IgG2a against SjTsp2, Sj29, and SjTsp2-29 DNA vaccines, with high expression of IFN-γ and no IL-4 in the mice.

Conclusions

The double-membrane antigen DNA vaccine SjTsp2-29 elicited protection against Schistosoma infection and might serve as a vaccine candidate.

Cross-species prophylactic efficacy of Sm-p80-based vaccine and intracellular localization of Sm-p80/Sm-p80 ortholog proteins during development in Schistosoma mansoni, Schistosoma japonicum, and Schistosoma haematobium

Schistosomiasis remains a major global health problem. Despite large-scale schistosomiasis control efforts, clear limitations such as possible emergence of drug resistance and reinfection rates highlight the need for an effective schistosomiasis vaccine. Schistosoma mansoni large subunit of calpain (Sm-p80)-based vaccine formulations have shown remarkable efficacy in protecting against S. mansoni challenge infections in mice and baboons. In this study, we evaluated the cross-species protective efficacy of Sm-p80 vaccine against S. japonicum and S. haematobium challenge infections in rodent models. We also elucidated the expression of Sm-p80 and Sm-p80 ortholog proteins in different developmental stages of S. mansoni, S. haematobium, and S. japonicum. Immunization with Sm-p80 vaccine reduced worm burden by 46.75% against S. japonicum challenge infection in mice. DNA prime/protein boost (1 + 1 dose administered on a single day) resulted in 26.95% reduction in worm burden in S. haematobium-hamster infection/challenge model. A balanced Th1 (IFN-γ, TNF-α, IL-2, and IL-12) and Th2 (IL-4, IgG1) type of responses were observed following vaccination in both S. japonicum and S. haematobium challenge trials and these are associated with the prophylactic efficacy of Sm-p80 vaccine. Immunohistochemistry demonstrated that Sm-p80/Sm-p80 ortholog proteins are expressed in different life cycle stages of the three major human species of schistosomes studied. The data presented in this study reinforce the potential of Sm-p80-based vaccine for both hepatic/intestinal and urogenital schistosomiasis occurring in different geographical areas of the world. Differential expression of Sm-p80/Sm-p80 protein orthologs in different life cycle makes this vaccine potentially useful in targeting different levels of infection, disease, and transmission.

Still hope for schistosomiasis vaccine

Schistosomiasis is a parasitic disease caused by helminths belonging to the Schistosoma genus. Approximately 700 million people are at risk of infection and 200 million people are currently infected. Schistosomiasis is the most important helminth infection, and treatment relies solely on the drug praziquantel. Worries of praziquantel resistance as well as high disease burden are only some of the justifications which support the development of a vaccine against schistosomiasis. To date, only 2 schistosome vaccines have made it into clinical trials: Sh28GST (Bilhvax) and Sm14. However, there are several vaccine candidates, such as TSP-2, sm-p8, and Sm-Cathepsin B, which are generating promising results in pre-clinical studies. Schistosomiasis vaccine development has been an uphill battle, and there are still several hurdles to overcome in the future. Fortunately, the research groups involved in the research for vaccine development have not abandoned their work. Furthermore, in the last few years, schistosomiasis has garnered some additional attention on a global scale due to its significant impact on public health.

Biochemical and biophysical methodologies open the road for effective schistosomiasis therapy and vaccination

BACKGROUND

Schistosomiasis caused by blood-dwelling flukes, namely Schistosoma mansoni and Schistosoma haematobium is a severe debilitating disease, widespread in sub-Saharan Africa, the Middle East, and South America. Developing and adult worms are unscathed by the surrounding immune effectors and antibodies because the parasite is protected by a double lipid bilayer armor which allows access of nutrients, while binding of specific antibodies is denied.

SCOPE OF REVIEW

Fluorescence recovery after bleaching, extraction of surface membrane cholesterol by methyl-β-cyclodextrin, inhibition and activation of sphingomyelin biosynthesis and hydrolysis, and elastic incoherent and quasi-elastic neutron scattering approaches have helped to clarify the basic mechanism of this immune evasion, and showed that sphingomyelin (SM) molecules in the worm apical lipid bilayer form with surrounding water molecules a tight hydrogen bond barrier. Viability of the parasite and permeability of the outer shield are controlled by equilibrium between SM biosynthesis and activity of a tegument-associated neutral sphingomyelinase (nSMase).

MAJOR CONCLUSIONS

Excessive nSMase activation by polyunsaturated fatty acids (PUFA), such as arachidonic acid (ARA) leads to disruption of the SM molecules and associated hydrogen bond network, with subsequent access of host antibodies and immune effectors to the outer membrane and eventual parasite death.

GENERAL SIGNIFICANCE

ARA was predicted and shown to be a potent schistosomicide in vitro and in vivo in experimental animals and in children. Additionally, it was advocated that schistosomiasis vaccine candidates should be selected uniquely among excretory-secretory products of developing worms, as contrary to cytosolic and surface membrane antigens, they are able to activate the effector functions of the host antibodies and toxic molecules. This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo".

A vaccine consisting of Schistosoma mansoni cathepsin B formulated in Montanide ISA 720 VG induces high level protection against murine schistosomiasis

Background

Schistosomiasis is the most important human helminth infection due to its impact on public health. The clinical manifestations are chronic and significantly decrease an individual’s quality of life. Infected individuals suffer from long-term organ pathologies including fibrosis which eventually leads to organ failure. The development of a vaccine against this parasitic disease would contribute to a long-lasting decrease in disease spectrum and transmission.

Method

Our group has chosen Schistosoma mansoni (Sm) cathepsin B, a peptidase involved in parasite feeding, as a prospective vaccine candidate. Our experimental formulation consisted of recombinant Sm-cathepsin B formulated in Montanide ISA 720 VG, a squalene based adjuvant containing a mannide mono-oleate emulsifier. Parasitological burden was assessed by determining adult worm, hepatic egg, and intestinal egg numbers in each mouse. Serum was used in ELISAs to evaluate production of antigen-specific antibodies, and isolated splenocytes were stimulated with the antigen for the analysis of cytokine secretion levels.

Results

The Sm-cathepsin B and Montanide formulation conferred protection against a challenge infection by significantly reducing all forms of parasitological burdens. Worm burden, hepatic egg burden and intestinal egg burden were decreased by 60 %, 62 %, and 56 %, respectively in immunized animals compared to controls (P = 0.0002, P < 0.0001, P = 0.0009, respectively). Immunizations with the vaccine elicited robust production of Sm-cathepsin B specific antibodies (endpoint titers = 122,880). Both antigen-specific IgG1 and IgG2c titers were observed, with the former having more elevated titers. Furthermore, splenocytes isolated from the immunized animals, compared to control animals, secreted higher levels of key Th1 cytokines, IFN-γ, IL-12, and TNF-α, as well as the Th2 cytokines IL-5 and IL-4 when stimulated with recombinant Sm-cathepsin B. The Th17 cytokine IL-17, the chemokine CCL5, and the growth factor GM-CSF were also significantly increased in the immunized animals compared to the controls.

Conclusion

The formulation tested in this study was able to significantly reduce all forms of parasite burden, stimulate robust production of antigen-specific antibodies, and induce a mixed Th1/Th2 response. These results highlight the potential of Sm-cathepsin B/Montanide ISA 720 VG as a vaccine candidate against schistosomiasis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-016-1444-z) contains supplementary material, which is available to authorized users.

SjHSP70, a recombinant Schistosoma japonicum heat shock protein 70, is immunostimulatory and induces protective immunity against cercarial challenge in mice

High levels of protective immunity can be induced in different animals immunized with radiation-attenuated (RA) Schistosoma cercariae or schistosomula. However, the schistosome-derived molecules responsible for the strong protective effect elicited by RA schistosome larvae have not been identified or characterized. The 70-kDa heat shock proteins of schistosomes are considered major immunogens, and may play an important role in stimulating high levels of innate and adaptive immune responses in an RA schistosome vaccine model. Here, we demonstrate the immunobiological functions of Schistosoma japonicum heat shock protein 70 (SjHSP70) by investigating its expression profile in RA-schistosomula-derived cells, evaluating the protection induced by recombinant SjHSP70 (rSjHSP70) against cercarial challenge, and assaying the humoral and cellular immune responses to rSjHSP70 in BALB/c and C57BL/6 mice. The expression of SjHSP70 on the surfaces of cells from RA or normal schistosomula was determined with flow cytometry. Its expression was significantly higher on early RA schistosomula cells than on the cells from normal parasites. The protection afforded both BALB/c and C57BL/6 mice vaccinated with rSjHSP70 alone, rSj22.6 (a membrane-anchoring protein of S. japonicum) alone, or a combination of rSj22.6 and rSjHSP70 without adjuvant was evaluated. rSjHSP70 alone induced the highest protective effect against S. japonicum cercarial challenge, followed by the rSj22.6 plus rSjHSP70 combination and then rSj22.6 alone, in both mouse strains. Like ISA206 adjuvant, rSjHSP70 enhanced the protective efficacy induced by rSj22.6 in the C57BL/6 mouse strain. Antigen-specific IgG1 and IgG2a responses were detected with enzyme-linked immunosorbent assays in mice immunized with rSjHSP70 alone, rSj22.6 alone, or the rSj22.6 plus rSjHSP70 combination. Immunization with rSjHSP70 or the rSj22.6 plus rSjHSP70 combination induced mixed Th1/Th2-type antibody responses in BALB/c mice and a Th2-type antibody response in C57BL/6 mice. The profiles of cytokine production by splenic lymphocytes in both strains of mice immunized with the antigens described above were detected in vitro using a Cytometric Bead Array. The profiles of the proinflammatory cytokines interferon γ, tumor necrosis factor α, interleukin 6 (IL-6), and IL-17A and the regulatory cytokine IL-10 induced by the rSj22.6 plus rSjHSP70 combination were similar to those induced by rSj22.6 emulsified with the ISA206 adjuvant control. Like the ISA206 adjuvant, rSjHSP70 protein enhanced the proinflammatory and Th2-type or regulatory cytokine production induced by the rSj22.6 antigen. These results indicate that SjHSP70 is exposed on the surfaces of cells from RA schistosomula, and that rSjHSP70 protein is a promising protective antigen with a potential adjuvant function. Thus, SjHSP70 protein might play a key role in the protective immunity elicited by the RA schistosome vaccine.

The skin migratory stage of the schistosomulum ofSchistosoma mansonihas a surface showing greater permeability and activity in membrane internalisation than other forms of skin or mechanical schistosomula

Skin schistosomula can be prepared by collecting them after isolated mouse skin have been penetrated by cercariaein vitro. The schistosomula can also migrate out of isolated mouse skin penetrated by cercariaein vitroand from mouse skin penetrated by cercariaein vivo. Schistosomula can also be produced from cercariae applied through a syringe or in a vortex. When certain surface properties of the different forms of schistosomula were compared, those migrating from mouse skin penetrated by cercariaein vivoorin vitrohad greatly increased permeability to membrane impermeant molecules such as Lucifer yellow and high molecular weight dextrans. These migrating forms also possessed surfaces which showed greatly enhanced uptake into internal membrane vesicles of the dye FM 143, a marker for endocytosis. This greatly enhanced activity and permeability of the surfaces of tissue migrating schistosomula is likely to be of great importance in the adaptation to the new host.

Characterization and vaccination of two novel Schistosoma japonicum genes screened from a cercaria cDNA library

Two novel genes, SJCWL05 and SJCWL06, were harvested from screening of Schistosoma japonicum (S. japonicum) cercaria cDNA library by using pig sera vaccinated (VPS) with S. japonicum immature egg ws-vaccine (S. japonicum iEw). Prokaryotic recombinant plasmids pGEX-4T-1/SJCWL05 and pGEX-4T-1/SJCWL06 were constructed to analyze their immunogenicity, which was confirmed by SDS-PAGE and Western blotting. Two eukaryotic recombinant plasmids, pcDNA3/SJCWL05 and pcDNA3/SJCWL06, were constructed, and their ability to protect mice against challenge of S. japonicum was evaluated. All mice vaccinated with pcDNA3/SJCWL05 or pcDNA3/SJCWL06 developed ELISA-specific anti-S. japonicum SIEA (S. japonicum soluble immature egg antigens) antibody. Immunoprotection experiments showed that worms and liver eggs reduced 34.64% and 39.14% in the pcDNA3/SJCWL05 group and those reduced 27.17% and 27.95% in the pcDNA3/SJCWL06 group, respectively. The reduction rates of intestine and uterine eggs in female worms of both groups reached 39.45% and 38.5% as well as 30.02% and 28.7%, respectively. Results of our study suggest that novel genes, SJCWL05 and SJCWL06, are potential vaccine candidates against schistosomiasis japonica.

Protective effects of Sm-p80 in the presence of resiquimod as an adjuvant against challenge infection with Schistosoma mansoni in mice

OBJECTIVES

To determine the prophylactic efficacy of an Sm-p80-based vaccine formulation against challenge infection with Schistosoma mansoni in mice using an approach comprising of initial priming with DNA and boosting with recombinant protein in the presence of resiquimod (R848) as an adjuvant.

METHODS

In the first experiment (prime-boost approach), mice were primed with Sm-p80-pcDNA3 (week 0) and boosted at weeks 4 and 8 with recombinant Sm-p80 formulated in resiquimod (R848). Each mouse in the control group first received only pcDNA3 and was boosted with R848. In the second set of experiments (recombinant protein approach), mice were immunized (week 0) and boosted (weeks 4 and 8) with rSm-p80 formulated in R848. Animals of the control group in this series of experiments received only R848 at 0, 4, and 8 weeks. All of the animals from both the 'prime-boost' and 'recombinant protein' groups were challenged with cercariae of S. mansoni, 4 weeks after the last immunization. The mice were sacrificed 6 weeks post-challenge and the reductions in worm burden and egg production were determined. Sm-p80-specific antibody titers were estimated in the mice sera by ELISA. Cytokine mRNA and protein production by proliferating splenocytes in response to in vitro stimulation with Sm-p80, were estimated via RT-PCR and ELISA, respectively.

RESULTS

Vaccination with Sm-p80 (prime-boost approach) showed 49% reduction in worm burden; with the recombinant protein approach the protection was found to be 50%. The protection levels were correlated with antibody production. Upon antigenic stimulation with recombinant Sm-p80, splenocytes secreted significant levels of interferon (IFN)-γ and interleukin (IL)-2, indicating that the immune responses were Th1-biased and this was further supported in terms of distribution of antibody isotypes and mRNA expression of cytokines.

CONCLUSIONS

In conclusion the present study clearly demonstrates that Sm-p80 consistently maintained its protective nature, and resiquimod as an immunopotentiating agent slightly boosted the protective effects of Sm-p80 in both 'DNA prime-protein boost' and 'recombinant protein' immunization approaches in a murine model.

Prime-boost and recombinant protein vaccination strategies using Sm-p80 protects against Schistosoma mansoni infection in the mouse model to levels previously attainable only by the irradiated cercarial vaccine

Advent of an effective schistosome vaccine would contribute significantly toward reducing the disease spectrum and transmission of schistosomiasis. We have targeted a functionally important antigen, Sm-p80, as a vaccine candidate because of its consistent immunogenicity, protective and antifecundity potentials, and important role in the immune evasion process. In this study, we report that using two vaccination approaches (prime boost and recombinant protein), Sm-p80-based vaccine formulation(s) confer up to 70% reduction in worm burden in mice. Animals immunized with the vaccine exhibited a decrease in egg production by up to 75%. The vaccine elicited strong immune responses that included IgM, IgA, and IgG (IgG1, IgG2a, IgG2b, and IgG3) in vaccinated animals. Splenocytes proliferated in response to Sm-p80 produced Th1 and Th17 response enhancing cytokines. These results again emphasize the potential of Sm-p80 as a viable vaccine candidate for schistosomiasis.

Schistosoma japonicum: protective immunity induced by schistosomulum-derived cells in a mouse model

We previously reported that immunization with intact live cells from schistosomula of Schistosoma japonicum (S.j) partially protected the Kunming strain of mice from challenge infection. In the present work, 2 immune protective experiments were designed to further validate the protective effect induced by this type of vaccine and to optimize the immunization protocol, including the number of inoculations and parasite stages from which immunogenic cells were derived. Three antigens derived from 18-day-old postinfection live (LLC) and dead (DLC) larval worm cells and from dead 42-day-old postinfection adult worm cells (DAC) were used as immunogens. Our results demonstrate that live cells from 18-day-old worms are capable of inducing significant protection in mice using a murine-Sj challenge model as shown by reduction rates of worm recoveries and egg burdens. The development of adult worms was stunted. A Th1-biased immune response was reflected in the protected groups as evidenced by the ratio of IgG2a/IgG1. A 38-kDa polypeptide was recognized by sera from LLC immunized animals. We demonstrate that live parasite cells are a source of novel protective antigens that can be exploited for vaccine development.

Enhancement by IL-18 of the protective effect of a Schistosoma japonicum 26kDa GST plasmid DNA vaccine in mice

Two recombinant plasmids pVAX/Sj26GST and pVAX/mIL-18 containing Schistosoma japonicum 26kDa GST and murine IL-18 were evaluated for their ability to protect mice against S. japonicum challenge. Mice were given 2 intramuscular immunizations 3 weeks apart, and challenged with S. japonicum cercariae 4 weeks later. Adult worm and egg burdens were determined 48 days post-challenge. All animals vaccinated with pVAX/Sj26GST alone or with pVAX/mIL-18 developed specific anti-SWAP (soluble worm antigen preparation) ELISA antibody and splenocyte proliferation response. Co-injection of pVAX/mIL-18 significantly increased the production of IFN-gamma and IL-12, indicating that IL-18 enhances the Th1-dominant immune response. Challenge experiments showed that worms were reduced in the pVAX/Sj26GST group by 30.1% and by 49.4% in animals given pVAX/mIL-18 additionally. Corresponding hepatic and fecal egg reductions were 44.8% and 53.0%, and 50.6% and 56.6%, respectively. These results indicate that IL-18 may be an effective adjuvant for a schistosomiasis vaccine.

Gene expression patterns during adaptation of a helminth parasite to different environmental niches

BACKGROUND

Schistosome bloodflukes are complex trematodes responsible for 200 million cases of schistosomiasis worldwide. Their life cycle is characterized by a series of remarkable morphological and biochemical transitions between an invertebrate host, an aquatic environment, and a mammalian host. We report a global transcriptional analysis of how this parasite alters gene regulation to adapt to three distinct environments.

RESULTS

Utilizing a genomic microarray made of 12,000 45-50-mer oligonucleotides based on expressed sequence tags, three different developmental stages of the schistosome parasite were analyzed by pair-wise comparisons of transcript hybridization signals. This analysis resulted in the identification of 1,154 developmentally enriched transcripts.

CONCLUSION

This study expands the repertoire of schistosome genes analyzed for stage-specific expression to over 70% of the predicted genome. Among the new associations identified are the roles of robust protein synthesis and programmed cell death in development of cercariae in the sporocyst stages, the relative paucity of cercarial gene expression outside of energy production, and the remarkable diversity of adult gene expression programs that reflect adaptation to the host bloodstream and an average lifespan that may approach 10 years.

The Schistosoma bovis Sb14-3-3ζ recombinant protein cross-protects against Schistosoma mansoni in BALB/c mice

Current control programs against schistosomiasis could be reinforced through the use of an effective vaccine. Schistosome 14-3-3 proteins have been proposed as candidates for vaccine against the respective infections, and were seen to elicit high protection levels against Schistosoma bovis in a previous work done by our group. We have therefore investigated the protective capacity of the 14-3-3 protein from S. bovis - Sb14zeta - against Schistosoma mansoni in mice. In addition, we have addressed the influence of the co-administration of three different immunomodulators with the 14-3-3 polypeptide. Protection was high when the Sb14zeta protein was combined in two independent experiments with the AA2829 and PAL immunomodulatory molecules as regards both the reduction of worm numbers (mean: 64.8%) and egg loads in liver (mean: 73.9%) or intestine (mean: 71.5%). In contrast, the degree of protection achieved with the Sb14zeta-CpG vaccine was very low (14.9% reduction in worm numbers, and 46.6% and 32% reduction in liver and intestinal egg loads). The immune responses observed in the vaccinated animals showed that the production of IFNgamma and the absence of IL-4, accompanied by a strong humoral response, are insufficient to elicit protection against S. mansoni.

Parasite vaccines: The new generation

Parasites cause some of the most devastating and prevalent diseases in humans and animals. Moreover, parasitic infections increase mortality rates of other serious non-parasitic infections caused by pathogens such as HIV-1. The impact of parasitic diseases in both industrialised and developing countries is further exacerbated by the resistance of some parasites to anti-parasitic drugs and the absence of efficacious parasite vaccines. Despite years of research, much remains to be done to develop effective vaccines against parasites. This review focuses on the more recent vaccine strategies such as DNA and viral vector-based vaccines that are currently being used to develop vaccines against parasites. Obstacles yet to be overcome and possible advantages and disadvantages of these vaccine modalities are also discussed.