Optimized Refolding Buffers Oriented Humoral Immune Responses Versus PfGCS1 Self-Assembled Peptide Nanoparticle

Developing a novel class of vaccine is pivotal for eliminating and eradicating malaria. Preceding investigations demonstrated partial blocking activity in malaria transmission against recombinant vaccine PfHAP2-GCS1 and conserved region of the cd loop. The effectiveness of immune response varies with the size and shape of the self-assembly of peptide nanoparticles (SAPNs) displaying antigen, affected by different components in refolding buffers. Plasmodium falciparum Generative Cell Specific 1 (PfGCS1), a promising malaria transmission-blocking vaccine (TBV) candidate, was expressed, purified, and followed by a four-step refolding process to form nanoparticles (PfGCS1-SAPNs). The influence of buffer components on the size and shape of SAPNs was investigated by DLS and FESEM. Furthermore, the immunogenicity of nanostructures was assessed in different mouse groups. The results showed that PfGCS1-SAPN was immunogenic and its administration with Poly (I:C), stimulated humoral and cellular responses in the mouse model. In the immunized mice groups, the level of IgG antibodies against PfGCS1-SAPN was significantly increased in different time points (second and third boost) and heterogeneous boosters. The various IgG-subclasses profile shifted to Th1, Th2, or Th1/Th2 mix responses in mice immunized with PfGCS1-SAPN refolded in different buffers, indicating a prerequisite for further investigations to optimize vaccine formulation to enhance and modulate Th1/cellular responses. Such studies pave the way to improve biophysical features related to the nanoparticles' size, shape, and conformational epitopes of candidate antigens and T- and B-cells presented on the superficial structure to elicit robust immune responses.

Evaluation of a new fusion antigen, cd loop and HAP2-GCS1 domain (cd-HAP) of Plasmodium falciparum Generative Cell Specific 1 antigen formulated with various adjuvants, as a transmission blocking vaccine

BACKGROUND

Malaria is a major global health challenge, and for the elimination and eradication of this disease, transmission-blocking vaccines (TBVs) are a priority. Plasmodium falciparum Generative Cell Specific 1 (PfGCS1), a promising TBV candidate, is essential for gamete fertilization. The HAP2-GCS1 domain of this antigen as well as its cd loop could induce antibodies that partially inhibit transmission of P. falciparum.

METHODS

In the current study, a new synthetic fusion antigen containing cd loop and HAP2-GCS1 domain (cd-HAP) of PfGCS1 was evaluated as a transmission blocking vaccine candidate. Initially, the profile of naturally acquired IgG antibodies to the cd-HAP antigen was analysed in Iranian individuals infected with P. falciparum, to confirm that this new fusion protein has the appropriate structure containing common epitopes with the native form of PfGCS1. Then, the immunogenicity of cd-HAP was evaluated in BALB/c mice, using different adjuvant systems such as CpG, MPL, QS-21, and a combination of them (CMQ). Furthermore, the blocking efficacy of polyclonal antibodies induced against these formulations was also assessed by oocyst intensity and infection prevalence in the Standard Membrane Feeding Assay (SMFA).

RESULTS

The naturally acquired antibodies (dominantly IgG1 and IgG3 subclasses) induced in P. falciparum-infected individuals could recognize the cd-HAP antigen which implies that the new fusion protein has a proper conformation that mimics the native structure of PfGCS1. Concerning the immunogenicity of cd-HAP antigen, the highest IgG levels and titers, by a Th1-type immune profile, and elevated antibody avidity were induced in mice immunized with the cd-HAP antigen formulated with a combination of adjuvants (P < 0.0001). Additionally, cytokine profiling of the immunized mice displayed that a high level of IFN-γ response, a Th1-type immune response, was produced by splenocytes from immunized mice that received cd-HAP antigen in combination with CMQ adjuvants (P < 0.0001). This formulation of cd-HAP antigen with CMQ adjuvants could reduce oocyst intensity and infection prevalence by 82%, evidenced by the SMFA and hold significant implications for future malaria vaccine development.

CONCLUSION

Altogether, the results showed that cd-HAP antigen formulated with a combination of the adjuvants (CMQ), could be a promising formulation to develop a PfGCS1-based transmission-blocking vaccine.

Molecular epidemiology of potential candidate markers for chloroquine resistance in imported Plasmodium vivax malaria cases in Iran

BACKGROUND

The spread of Plasmodium vivax strains resistant to chloroquine (CQ) has posed a challenge to control strategies aimed at eliminating malaria. Molecular analysis of candidate resistance markers is very important for monitoring the P. vivax resistance to CQ in different endemic regions. In the present study, the multidrug resistance 1 (pvmdr1) gene, a possible marker for CQ resistance in P. vivax, was evaluated by molecular methods.

METHODS

A simple PCR-RFLP method was developed for mutation analysis in pvmdr1 gene. A number of 120 blood spots were obtained from patients with P. vivax mono-infection in 2021. All of the samples were collected from Pakistani patients who travelled to Iran.

RESULTS

None of the samples had any mutation at codon 976 of pvmdr1, while the 1076 mutation was detected in 96.2% of the examined isolates. Only two pvmdr1 haplotypes were identified, including the single mutant (Y976/1076L) as the most prevalent haplotype (with 96.2% frequency) and the wild type (Y976/F1076; with 3.8% frequency).

CONCLUSIONS

In this study, the major CQ resistance-mediating mutation and multiple mutant haplotypes of the pvmdr1 gene was not detected. However, continuous monitoring of drug resistance markers and close supervision of the efficacy of CQ is essential to detect the potential emergence of CQ-resistant P. vivax isolates in Iran. This data is important for performing future epidemiological surveillance to monitor CQ resistance in this endemic area and the bordering regions.

CRISPR/Cas advancements for genome editing, diagnosis, therapeutics, and vaccine development for Plasmodium parasites, and genetic engineering of Anopheles mosquito vector

Malaria as vector-borne disease remains important health concern with over 200 million cases globally. Novel antimalarial medicines and more effective vaccines must be developed to eliminate and eradicate malaria. Appraisal of preceding genome editing approaches confirmed the CRISPR/Cas nuclease system as a novel proficient genome editing system and a tool for species-specific diagnosis, and drug resistance researches for Plasmodium species, and gene drive to control Anopheles population. CRISPR/Cas technology, as a handy tool for genome editing can be justified for the production of transgenic malaria parasites like Plasmodium transgenic lines expressing Cas9, chimeric Plasmodium transgenic lines, knockdown and knockout transgenic parasites, and transgenic parasites expressing alternative alleles, and also mutant strains of Anopheles such as only male mosquito populations, generation of wingless mosquitoes, and creation of knock-out/ knock-in mutants. Though, the incorporation of traditional methods and novel molecular techniques could noticeably enhance the quality of results. The striking development of a CRISPR/Cas-based diagnostic kit that can specifically diagnose the Plasmodium species or drug resistance markers is highly required in malaria settings with affordable cost and high-speed detection. Furthermore, the advancement of genome modifications by CRISPR/Cas technologies resolves contemporary restrictions to culturing, maintaining, and analyzing these parasites, and the aptitude to investigate parasite genome functions opens up new vistas in the better understanding of pathogenesis.

Optimization of the heterologous expression and purification of Plasmodium falciparum generative cell specific 1 in Escherichia coli

Generative cell specific 1 (GCS1) or Hapless2 (Hap2) is a main transmission-blocking vaccine (TBV) candidate against malaria. Experience has shown that this protein is difficult to express in heterologous hosts. In a study, Plasmodium falciparum GCS1 (PfGCS1) could be expressed in fusion with Glutathione S Transferase (GST). Since the large fusions could influence the immunogenicity of the recombinant antigens, in the current study we tried to express PfGCS1 protein without large fusion tags with an appropriate yield and purity in E. coli. To this end, pfgcs1 gene was codon-optimized and cloned in pET23a plasmid. The expression was evaluated in different E. coli hosts [E. coli BL21(DE3), E. coli BL21(DE3) pLysS, E. coli Rosetta(DE3), and E. coli Rosettagami(DE3)] and media cultures. In addition, the effect of post-induction times, inducer concentration, temperature, and supplementation of glucose and ethanol to culture media were evaluated. The obtained results revealed that rPfGCS1 protein was expressed in all examined E. coli hosts and media cultures with different yields, with the best yield in E. coli BL21(DE3), and E. coli Rosetta(DE3) hosts in TB medium, 16 h post-induction. The expression of rPfGCS1 was confirmed by western blotting using anti-His antibodies. Expression in low temperature at 20 °C and addition of glucose and ethanol to TB media could improve the expression of rPfGCS1. We could express and purify rPfGCS1 without a large fusion protein with an appropriate yield and purity in E. coli Rosetta(DE3). We will evaluate this antigen as TBV candidate against P. falciparum transmission in the future.

Untangling population structure and genetic diversity of reticulocyte binding protein 2b (PvRBP2b) erythrocytic stage vaccine candidate in worldwide Plasmodium vivax isolates

Backgrounds

Plasmodium vivax is the predominant Plasmodium species distributed extensively in the Americas and Asia-Pacific areas. Encoded protein by Plasmodium vivax Reticulocyte Binding Proteins (PvRBPs) family member are of critical prominence to parasite invasion and have been considered the significant targets in development of malaria vaccine for the blood stage. As high genetic polymorphism of parasites may impede the effectiveness of vaccine development, more research to unraveling genetic polymorphism of pvrbp2b from various geographical regions seems indispensable to map the exact pattern of field isolates.

Methodology/Principal findings

The aim of this study was to determine the sequences of Iranian pvrbp2b (nt: 502–1896) gene and then, to ascertain polymorphism of pvrbp2b gene, recombination, the level of genetic distances, evaluation of natural selection, and the prediction of B-cell epitopes of Iranian and global P. vivax isolates. Pvrbp2b partial gene was amplified and sequenced from 60 Iranian P. vivax isolates. Iranian pvrbp2b sequences as well as 95 published sequences from five countries were used to evaluate the genetic diversity and neutral evolution signature in worldwide scale. A total of 38 SNPs were identified among 60 Iranian pvrbp2b sequences (32 non-synonymous and 6 synonymous mutations), and 32 amino acid substitutions were observed in 29 positions as compared to Sal-1 sequence. Worldwide sequence analysis showed that 44 amino acid changes had occurred in 37 positions of which seven polymorphic sites had trimorphic mutations while the rest was dimorphic. The overall nucleotide diversity for Iranian isolates was 0.00431 ± 0.00091 while the level of nucleotide diversity was ranged from 0.00337 ± 0.00076 (Peru) to 0.00452 ± 0.00092 (Thailand) in global scale.

Conclusions/Significance

Of amino acid substitutions, 12 replacements were located in the B-cell epitopes in which nine polymorphic sites were positioned in N-terminal and three polymorphic sites in predicted B-cell epitopes of C-terminal, signifying both variable and conserved epitopes for vaccine designing. Using the achieved outcome of the current investigation interrogate questions to the selection of conserved regions of pvrbp2b and understanding polymorphism and immune system pressure to pave a way for developing a vaccine based on PvRBP2b candidate antigen.

Advax, as a Co-adjuvant, in Combination with Poly(I:C) Elicits Enhanced Th1 Immune Responses and Parasite Growth-Inhibitory Antibodies Against Plasmodium Falciparum Merozoite Surface Protein-1 (PfMSP-142) in BALB/c Mice

BACKGROUND

One of the main challenges in protein-based vaccines is the poor immunogenicity of antigens, which can be solved by the use of adjuvants. Advax is a novel microparticle polysaccharide adjuvant that in combination with antigens can induce both cellular and humoral immunity based on the intrinsic features of the antigen. It has been shown that poly(I:C) can be a suitable adjuvant for the PfMSP-142-based malaria vaccine. Advax is a suitable co-adjuvant for poly(I:C) to increase its half-life and reduce dose-dependent toxicity.

OBJECTIVES

To investigate whether advax alone or advax /poly(I:C) combination can enhance the immunogenicity with increased parasite inhibitory anti-PfMSP-142 antibodies in comparison to poly(I:C).

METHODS

Mice groups were inoculated with rPfMSP-142 alone or formulated in poly(I:C), poly(I:C)/advax, or advax. Then, humoral and cellular immune responses, the ratio of Th1/Th2 and growth inhibitory activity of induced antibodies were analyzed.

RESULTS

Poly(I:C)/advax formulated PfMSP-142 induced higher levels of anti-PfMSP-142 IgG, IgG2a, and IgG2b antibodies relative to poly(I:C)-formulated PfMSP-142. The maximum ratio of IFN-?/IL-4 (50.13) and IgG2a/IgG1 (2.65), was induced in mice receivedadvax-formulated PfMSP-142. Besides, poly(I:C)/advax formulated PfMSP-142 induced a higher ratio of IFN-?/IL-4 (25.33) and IgG2a/IgG1 (1.89) when compared with poly(I:C) alone. Strong growth inhibitory activity was observed in antibodies induced in mice received poly(I:C)/advax-formulated PfMSP-142.

CONCLUSION

These findings indicate that advax is a favorable adjuvant to be combined with poly(I:C), and this combination of adjuvants could induce Th1 immune responses and growth inhibitory antibodies against rPfMSP-142.

How can we develop an effective subunit vaccine to achieve successful malaria eradication?

Malaria, a mosquito-borne infection, is the most widespread parasitic disease. Despite numerous efforts to eradicate malaria, this disease is still a health concern worldwide. Owing to insecticide-resistant vectors and drug-resistant parasites, available controlling measures are insufficient to achieve a malaria-free world. Thus, there is an urgent need for new intervention tools such as efficient malaria vaccines. Subunit vaccines are the most promising malaria vaccines under development. However, one of the major drawbacks of subunit vaccines is the lack of efficient and durable immune responses including antigen-specific antibody, CD4⁺, and CD8⁺ T-cell responses, long-lived plasma cells, memory cells, and functional antibodies for parasite neutralization or inhibition of parasite invasion. These types of responses could be induced by whole organism vaccines, but eliciting these responses with subunit vaccines has been proven to be more challenging. Consequently, subunit vaccines require several policies to overcome these challenges. In this review, we address common approaches that can improve the efficacy of subunit vaccines against malaria.

Measuring of IgG2c isotype instead of IgG2a in immunized C57BL/6 mice with Plasmodium vivax TRAP as a subunit vaccine candidate in order to correct interpretation of Th1 versus Th2 immune response

Evaluation of the murine isotype antibodies is essential in subunit vaccine development because inbred mouse strains with diverse genetic backgrounds respond different to recombinant proteins. In this regard, the main goal of this study was to measuring and comparing the profile of IgG isotype responses in C57BL/6 mice. For this purpose, the extracellular region of plasmodium vivax thrombospondin-related adhesive protein (PvTRAP) gene was expressed in Escherichia coli Rosetta (DE3)-pET23a. Then, the recombinant PvTRAP alone or emulsified with Freund's complete adjuvant were applied for immunization of the C57BL/6 mice. The role of antibodies and cellular immune responses induced by recombinant PvTRAP were evaluated. The results showed the level of anti-rPvTRAP IgG2c was significantly higher than IgG2a in the groups that received rPvTRAP alone (mean OD₄₉₀ = 0.798 ± 0.12 and 0.39 ± 0.1, respectively) and emulsified with CFA/IFA (mean OD₄₉₀ = 1.48 ± 0.07 and 0.605 ± 0.13, respectively; P < 0.05, independent sample t-test). Additionally, the immunized mice with rPvTRAP and rPvTRAP + CFA/IFA had an intermediate-avidity IgG2a antibody but high-avidity IgG2c antibody as well as the mean of serum antibody titers results exhibited that in both rPvTRAP and rPvTRAP + CFA/IFA mouse groups, IgG2a end-point titer (1:3200 and 1:25,600, respectively) was noteworthy lower than IgG2c (1:25,600 and 1:102,400, respectively). Moreover, the results revealed the eliciting significant levels of IFN-γ (P < 0.05, independent sample t-test) and no detectable level of IL-4 in the mouse groups received rPvTRAP alone and emulsified with CFA/IFA as compared to the mouse control groups. In general, our results showed that for correctly interpreting of Th1 immune responses in C57BL/6 mouse strain it is critical to measure IgG2c instead of IgG2a along with IFN-γ.

Population genetic structure analysis of thrombospondin-related adhesive protein (TRAP) as a vaccine candidate antigen in worldwide Plasmodium falciparum isolates

Antigenic diversity is a major concern in malaria vaccine development that requires to be considered in developing a malaria vaccine. Plasmodium falciparum thrombospondin-related adhesive protein (PfTRAP) is a leading malaria vaccine candidate antigen. In the current study, we investigated the level of genetic diversity and natural selection of pftrap sequences in P. falciparum isolates from Iran (n = 47). The gene diversity of Iranian pftrap sequences was also compared to available global pftrap sequences deposited in the GenBank or PlasmoDB databases (n = 220). Comparison of Iranian PfTRAP sequences with T9/96 reference sequence showed the presence of 35 amino acid changes in 32 positions and a limited variation in repeat sequences, leading to 13 distinct haplotypes. The overall nucleotide diversity (π) for the ectodomain of Iranian pftrap sequences was 0.00444 ± 0.00043, with the highest diversity in Domain IV. Alignment comparison of global PfTRAP sequences with T9/96 reference sequence indicated 96 amino acid replacements as well as extensive variable repeat sequences (9-23 repeats), which led to 192 haplotypes. Among the global isolates, the lowest nucleotide diversity was detected in French Guianan (0.00428 ± 0.00163) and Iranian (0.00444 ± 0.00043) pftrap sequences, and the most variation was observed in domains II and IV in all populations. The dN-dS value displayed the evidence of positive selection due to recombination and immune system pressure. The Fst analysis revealed a gene flow between African populations; however, genetic differentiation observed between Iranian and other populations probably was due to gene flow barriers. Both conserved and variable epitopes were predicted in B- and T-cell epitopes of PfTRAP antigen. The obtained results from this study could be helpful for developing a PfTRAP-based malaria vaccine.

Th1 immune response to Plasmodium falciparum recombinant thrombospondin-related adhesive protein (TRAP) antigen is enhanced by TLR3-specific adjuvant, poly(I:C) in BALB/c mice

Sporozoite-based malaria vaccines have provided a gold standard for malaria vaccine development, and thrombospondin-related adhesive protein (TRAP) serves as the main vaccine candidate antigen on sporozoites. As recombinant malaria vaccine candidate antigens are poorly immunogenic, additional appropriate immunostimulants, such as an efficient adjuvant, are highly essential to modulate Th1-cell predominance and also to induce a protective and long-lived immune response. In this study, polyinosinic:polycytidylic acid [poly(I:C)], the ligand of TLR3, was considered as the potential adjuvant for vaccines targeting stronger Th1-based immune responses. For this purpose, BALB/c mice were immunized with rPfTRAP delivered in putative poly(I:C) adjuvant, and humoural and cellular immune responses were determined in different immunized mouse groups. Delivery of rPfTRAP with poly(I:C) induced high levels and titres of persisted and also high-avidity anti-rPfTRAP IgG antibodies comparable to complete Freund's adjuvant (CFA)/incomplete Freund's adjuvant (IFA) adjuvant after the second boost. In addition, rPfTRAP formulated with poly(I:C) elicited a higher ratio of IFN-γ/IL-5, IgG2a/IgG1, and IgG2b/IgG1 than with CFA/IFA, indicating that poly(I:C) supports the induction of a stronger Th1-based immune response. This is a first time study which reveals the potential of rPfTRAP delivery in poly(I:C) to increase the level, avidity and durability of both anti-PfTRAP cytophilic antibodies and Th1 cytokines.

Heterogeneity in the acquisition of naturally acquired antibodies to cell-traversal protein for ookinetes and sporozoites (CelTOS) and thrombospondin-related adhesion protein (TRAP) of Plasmodium falciparum in naturally infected patients from unstable malaria areas in Iran

Currently, there is no subunit malaria vaccine capable of providing long-lasting protection, and a vaccine based on a single-antigen has shown moderate to unsatisfactory efficacies in clinical trials. As in malaria elimination and eradication strategies, the primary objective is reduction in disease and death due to P. falciparum, in the present investigation, for the first time, we attempted to determine and compare the naturally acquired immune responses to two well-recognized sporozoite antigens, cell-traversal protein for ookinetes and sporozoites (CelTOS) and thrombospondin-related adhesion protein (TRAP), in P. falciparum-infected individuals (n = 204) in low malaria transmission settings of Iran using ELISA. Besides, the profile of IgG isotype responses, the avidity of IgG, IgG1, and IgG3, and the association of anti-PfCelTOS and -PfTRAP antibodies with host age were evaluated. Positive antibody responses to PfCelTOS and PfTRAP antigens were detected in 16.2% and 31.9% of Iranian P. falciparum-infected individuals, respectively, indicating significantly lower immune response to PfCelTOS than PfTRAP (P <0.0001, McNemar's test). Also, among the positive samples for anti-PfCelTOS (n = 33) and -PfTRAP (n = 65) total IgG, the cytophilic IgG1 and IgG3 antibodies were predominant. A significant proportion of the examined positive responders had high- and intermediate-avidity for IgG (93.9%, 87.7%), IgG1 (96.3%, 87.7%), and IgG3 (76%, 78.7%) antibodies to both PfCelTOS and PfTRAP antigens, respectively, with no correlation with age (P >0.05; Spearman's correlation test). In conclusion, the present data suggests the acquisition of heterogenic immune responses to both antigens in the same patients naturally infected with P. falciparum from settings of low malaria transmission intensity in Iran in which their role in protection to malaria needs further study.

Anti-Plasmodial Assessment of Four Different Iranian Propolis Extracts

BACKGROUND

Eradication of malaria will depend on discovery of new intervention tools such as anti-malarial drugs. Due to the increasing interest in the application of propolis against significant clinical pathogenic agents, the aim of the present investigation was to evaluate the anti-plasmodial effect of Iranian propolis extracts against chloroquine (CQ)-sensitive Plasmodium falciparum 3D7 and Plasmodium berghei (ANKA strain).

METHODS

Crude samples of honeybee (Apis mellifera) propolis were collected from four provinces in northern (Kalaleh, Golestan), northeastern (Chenaran, Razavi Khorasan), central (Taleghan, Alborz) and western (Morad Beyg, Hamedan) areas of Iran with different types of flora. The dried propolis samples were extracted with three different solvents, including ethanol 70% (EtOH), ethyl acetate (EA) and dichloromethane (DCM).

RESULTS

All extracts were shown to have in vitro anti-plasmodial activity with IC50 ranging from 16.263 to 80.012 µg/mL using parasite lactate dehydrogenase (pLDH) assay. The DCM extract of Morad Beyg propolis indicated the highest anti-plasmodial activity (IC50: 16.263 ± 2.910 μg/mL; P = 0.027, Kruskal-Wallis H-test). The samples were also evaluated in mice for their in vivo anti-plasmodial effect. The curative effect against established infection (Rane test) showed that both extracts at all doses (50, 100, and 200 mg/kgBW) produced anti-plasmodial activity against the parasite. Furthermore, using gas chromatography-mass spectrometry (GC-MS), the quantity of flavonoids in DCM and EtOH 70% extracts were found to be 7.42% and 3.10%, respectively.

CONCLUSION

The potent anti-plasmodial activity of both EtOH 70% and DCM extracts of the propolis of Morad Beyg, Hamedan suggests further analyses of individual components to assess its utilization as anti-malarial drugs.

Poly(I:C) adjuvant strongly enhances parasite-inhibitory antibodies and Th1 response against Plasmodium falciparum merozoite surface protein-1 (42-kDa fragment) in BALB/c mice

Malaria vaccine development has been confronted with various challenges such as poor immunogenicity of malaria vaccine candidate antigens, which is considered as the main challenge. However, this problem can be managed using appropriate formulations of antigens and adjuvants. Poly(I:C) is a potent Th1 inducer and a human compatible adjuvant capable of stimulating both B- and T-cell immunity. Plasmodium falciparum merozoite surface protein 1₄₂ (PfMSP-1₄₂) is a promising vaccine candidate for blood stage of malaria that has faced several difficulties in clinical trials, mainly due to improper adjuvants. Therefore, in the current study, poly(I:C), as a potent Th1 inducer adjuvant, was evaluated to improve the immunogenicity of recombinant PfMSP-1₄₂, when compared to CFA/IFA, as reference adjuvant. Poly(I:C) produced high level and titers of anti-PfMSP-1₄₂ IgG antibodies in which was comparable to CFA/IFA adjuvant. In addition, PfMSP-1₄₂ formulated with poly(I:C) elicited a higher ratio of IFN-γ/IL-4 (23.9) and IgG2a/IgG1 (3.77) with more persistent, higher avidity, and titer of IgG2a relative to CFA/IFA, indicating a potent Th1 immune response. Poly(I:C) could also help to induce anti-PfMSP-1₄₂ antibodies with higher growth-inhibitory activity than CFA/IFA. Altogether, the results of the current study demonstrated that poly(I:C) is a potent adjuvant that can be appropriate for being used in PfMSP-1₄₂-based vaccine formulations.

Analysis of genetic diversity and population structure of gene encoding cell-traversal protein for ookinetes and sporozoites (CelTOS) vaccine candidate antigen in global Plasmodium falciparum populations

Plasmodium falciparum cell-traversal protein for ookinetes and sporozoites (PfCelTOS) has been reported as one of the most attractive malaria vaccine candidate antigens. To design a broadly effective malaria vaccine based on this antigen, it is crucial to have adequate information on genetic diversity in global PfCelTOS. Therefore, the extent of sequence diversity at the full-length of the pfceltos was assessed among both natural P. falciparum isolates collected from Iran (n = 93) and from available global pfceltos sequence data retrieved from PlasmoDB database (n = 159). Also, recombination, natural selection, the degree of genetic differentiation as well as the predicted immunodominant regions in PfCelTOS were analyzed. In total, 40 SNPs (including 1 synonymous and 39 non-synonymous) were detected in 34 positions, as compared to 3D7 sequence, which led to 66 distinct haplotypes with different frequencies. Among those haplotypes, 34 (51.5%, excluded from further analysis) were singleton haplotype and mostly detected among Senegalese parasite isolates. PfCelt-1 was found as predominant haplotype (32.6% total frequency) that was only detected in Iranian P. falciparum isolates. Nucleotide diversity was low in French Guiana (0.00236 ± 0.00203) and Iranian (0.00259 ± 0.00048) P. falciparum isolates in comparison with African populations. Evidence for positive selection by host immunity and intragenic recombination were detected that are two key factors responsible for gene evolution and genetic diversity of pfceltos gene. The results of Fst analysis and haplotype network revealed that PfCelTOS antigen displayed evident genetic structure between geographical parasite populations. In conclusion, the present analysis demonstrates that there is a limited antigenic diversity and geographic variation in global PfCelTOS, and this finding may be associated with the critical function of this antigen in cell traversal of the parasite in sporozoite and ookinete. Besides, most of the predicted B- and T-cell epitopes were located in the conserved region of the gene, but most of the amino acid replacements were located at the C-terminal region of PfCelTOS. The obtained results in this investigation could provide knowledge for better design of PfCelTOS-based malaria vaccine.

Naturally acquired immune responses to thrombospondin-related adhesion protein (TRAP) of Plasmodium vivax in patients from areas of unstable malaria transmission

A key tool for the control, elimination, and eradication of Plasmodium vivax is the development of an effective vaccine. The thrombospondin-related adhesion protein (TRAP) is one of the major sporozoite antigens that plays an important role in the invasion of mosquito salivary glands and hepatocytes by sporozoites. The main goal of this study was to evaluate the naturally acquired antibodies to the P. vivax TRAP (PvTRAP) in patients from malaria-endemic areas of Iran (n=116), Afghanistan (n=50), and Pakistan (n=50). The PvTRAP gene was expressed in Escherichia coli Rosetta (DE3)-pET23a and used as antigen in enzyme-linked immunosorbent assay (ELISA). The profile of immunoglobulin G (IgG) isotype and the avidity of IgG, IgG1, and IgG3 to PvTRAP, as well as the association between anti-PvTRAP isotype responses and host age were evaluated. Only 42.24% of Iranian, 38% of Afghani, and 44% of Pakistani patients infected with P. vivax had positive anti-PvTRAP IgG, and the prevalence of responders in the three countries did not differ significantly (P>0.05). Moreover, the prevalence of IgG1 and IgG3 antibody responses to PvTRAP showed no significant correlation with age (P>0.05). Individuals exposed to vivax malaria in the unstable malaria transmission areas are able to produce antibodies to the TRAP antigen at all ages in response to P. vivax infections. Finally, the presence of mature IgG1 and IgG3 antibodies with high to intermediate avidity against PvTRAP antigen (>60%) provide more information to understand the interactions between the host and P. vivax parasite. In summary, the present study provides data that support the rational development of an effective pre-erythrocytic stage vaccine based on PvTRAP antigen.

Limited genetic diversity in the global Plasmodium vivax Cell traversal protein of Ookinetes and Sporozoites (CelTOS) sequences; implications for PvCelTOS-based vaccine development

Cell traversal protein of Ookinetes and Sporozoites (CelTOS) is a new malaria vaccine candidate antigen. Since one of the main challenges in malaria vaccine development is the extensive antigenic diversity of this parasite, local and global gene diversity analysis is of particular importance. Therefore, in this study, the genetic diversity of pvceltos gene was investigated among Iranian P. vivax isolates (n=46) and compared with available worldwide pvceltos sequences. One synonymous (C109A) and three amino acid replacements (V118L, K178T, and G179R) were observed in Iranian pvceltos sequences in compare with Sal-1 sequence leading to five haplotypes including PvCelt-A (GSVKGL, 13%), PvCelt-B (GSLKGL, 50%), PvCelt-C (GSLTGL, 17.4%), PvCelt-D (GSVTGL, 13%) and PvCelt-E (GSLTRL, 6.5%). However, amino acid replacements were observed in six positions (G10S, S40N, V118L/M, K178T, G179R/D and L181R) in PvCelTOS antigen of global isolates leading to 11 distinct haplotypes. PvCelt-A and PvCelt-B haplotypes were the most common haplotypes in the world. The overall nucleotide diversity for Iranian isolates was 0.00169, while, the level of nucleotide diversity was ranged from 0.00252 for Thailand to 0.00022 for Peru populations in the world. The analysis of SNPs in relation with the predicted immunodominant regions revealed that only K178T and G179R SNPs are located in putative B-cell epitopes. All replacements were located in CD4+ and/or CD8+ T-cell epitopes. However, the majority of epitopes are located in conserved regions. Knowing whether these changes may alter the affinity of the epitopes for antibodies and/or MHC molecules remains to be investigated in experimental studies. In conclusion, the present study showed a very limited genetic diversity in pvceltos gene among the global clinical isolates that can be regarded as a potential candidate antigen to apply for vivax-based malaria vaccine development.

Evaluation of Naturally Acquired Antibody Responses to Two Variant Forms of Plasmodium vivax Apical Membrane Antigen-1 in Individuals Living in Areas of Low and Unstable Malaria Transmission of Iran

BACKGROUND

Acquired antibody responses following natural infection provide valuable information for selection of candidate antigens for malaria vaccines. Apical membrane antigen-1 of Plasmodium vivax (PvAMA-1) has potential as a component of a subunit vaccine for vivax malaria. In addition, genetic diversity in this antigen is responsible for challenges in the development of an effective PvAMA-1 based vaccine. Therefore, the main aim of this study was to determine whether allelic polymorphisms in pvama-1 influence the recognition of naturally occurring antibodies. Also, the profile of IgG isotypes to two sequence types of PvAMA-1 antigen was evaluated among subjects exposed to P. vivax in areas of low and unstable transmission.

METHODS

For this purpose, the two variant forms of PvAMA-1 (PvAMA-1A and B) were expressed in Escherichia coli M15-pQE30 system using genomic DNA from Iranian individuals with patent P. vivax infection. Anti-AMA-1 response and isotype composition to two variant forms were measured in target P. vivax-infected individuals (n = 110, 2 to 65 years old) using Enzyme-linked immunosorbent assay.

RESULTS

The results showed that 65.5% of the studied individuals had positive IgG responses to two PvAMA-1 variants, and the prevalence of responders did not differ significantly (P = 0.32). Also, a marked isotype switching to cytophilic (IgG1 /IgG3) antibodies was evident with increasing age, and adults responded more frequently to these antigens than did younger children.

CONCLUSION

The presence of mature, protective isotype antibodies and equal immune responses to two genetically distinct variant forms of antigens in individuals from low transmission areas implicates that one of these forms could be used in a universal blood-stage vaccine based on PvAMA-1 antigen.

Anti-malarial seroprevalence assessment during an elimination programme in Chabahar District, south-eastern Iran

Background

Iran has achieved a substantial decline in malaria incidence over the past decades. A common feature of malaria-endemic settings is the requirement for more sensitive techniques to describe levels of low transmission. In this study, serological and parasitological methods were used to measure transmission levels of Plasmodium falciparum and Plasmodium vivax during an elimination programme (2012) in Chabahar District, Sistan and Baluchistan Province, south-eastern Iran.

Methods

Participants were randomly selected from 64 different geographical clusters in Chabahar city and surrounding villages. Antibody responses to P. falciparum and P. vivax blood-stage antigens were assessed by ELISA, while microscopy and molecular testing were used to determine parasite carriage by species. Age-adjusted antibody responses were analysed using a reversible catalytic model to calculate seroconversion rates (SCR).

Results

There was no evidence of recent transmission in the study areas, indicated by an absence of parasite infections in all ages and low or absent serological responses to either species in young children. The best model for age P. falciparum seroconversion was one with a change in exposure 21 years before sampling was done in Chabahar city (P = 0.018) and 4 years in the villages (P = 0.039). There was a higher level of recent P. vivax transmission compared to P. falciparum, based on the SCRs, in both the city and village settings.

Conclusion

Serological analysis identified a decline in P. falciparum transmission in the urban areas of Chabahar, consistent with a previously described decrease in malaria in the early 1990s, demonstrating the utility of this approach to reconstruct exposure history. At present, it remains unclear whether the P. vivax antibody responses reflect active transmission due to new infections or relapse infections. The absence of parasitological and serological evidence of recent malaria transmission in Chabahar District is viable evidence for certification of elimination.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1432-1) contains supplementary material, which is available to authorized users.

Worldwide population genetic analysis and natural selection in the Plasmodium vivax Generative Cell Specific 1 (PvGCS1) as a transmission-blocking vaccine candidate

GENERATIVE CELL SPECIFIC 1 (GCS1) is one of the Transmission Blocking Vaccine (TBV) candidate antigens, which is expressed on the surface of male gametocytes and gametes of Plasmodium species. Since antigenic diversity could inhibit the successful development of a malaria vaccine, it is crucial to determine the diversity of gcs1 gene in global malaria-endemic areas. Therefore, gene diversity and selection of gcs1 gene were analyzed in Iranian Plasmodium vivax isolates (n=52) and compared with the corresponding sequences from worldwide clinical P. vivax isolates available in PlasmoDB database. Totally 12 SNPs were detected in the pvgcs1 sequences as compared to Sal-1 sequence. Five out of 12 SNPs including three synonymous (T797C, G1559A, and G1667T) and two amino acid replacements (Y133S and Q634P) were detected in Iranian pvgcs1 sequences. According to four amino acid replacements (Y133S, N575S, Q634P and D637N) observed in all world PvGCS1 sequences, totally 5 PvGCS1 haplotypes were detected in the world, that three of them observed in Iranian isolates including the PvGCS-A (133S/634Q, 92.3%), PvGCS-B (133Y/634Q, 5.8%), and PvGCS-C (133S/634P, 1.9%). The overall nucleotide diversity (π) for all 52 sequences of Iranian pvgcs1 gene was 0.00018±0.00006, and the value of dN-dS (-0.00031) were negative, however, it was not statistically significant. In comparison with global isolates, Iranian and PNG pvgcs1 sequences had the lowest nucleotide and haplotype diversity, while the highest nucleotide and haplotype diversity was observed in China population. Moreover, epitope prediction in this antigen showed that all B-cell epitopes were located in conserved regions. However, Q634P (in one Iranian isolate) and D637N (observed in Thailand, China, Vietnam and North Korea) mutations are involved in predicted IURs. The obtained results in this study could be used in development of PvGCS1 based malaria vaccine.

Administration of naloxone in combination with recombinant Plasmodium vivax AMA-1 in BALB/c mice induces mixed Th1/Th2 immune responses

Naloxone (NLX) has the ability to shift the immune response to a Th1 profile. Therefore, the adjuvant efficacy of NLX with recombinant P. vivax apical membrane antigen-1(rPvAMA-1) in BALB/c mice was evaluated. Mice were immunized subcutaneously with purified rPvAMA-1 formulated with NLX (doses of 5 mg/kg body weight) alone or in combination with IFA. A significant increase in anti-PvAMA-1 IgG antibody after the second boost (mean OD₄₉₀  = 2·08 and 2·17, in groups received, rPvAMA-1/NLX and rPvAMA-1/NLX/IFA, respectively) was detected. IgG1 and IgG2b were the predominant isotypes in all immunized mouse groups. In immunized mice with rPvAMA-1/NLX (mean: 1036 pg/mL) and with rPvAMA-1/NLX/IFA (mean: 1024 pg/mL), IFN-γ was elicited in response to rPvAMA-1 after the second boost. No detectable IL-4 secretion was determined in all tested groups. In conclusion, the administration of NLX alone or NLX/IFA with rPvAMA-1 in BALB/c mice, which induced mixed Th1/Th2 immune responses, was comparable with that of the same recombinant antigen with CFA/IFA adjuvant. The results indicate that NLX alone may possibly not be considered as a potent Th1 adjuvant in PvAMA-1-based vaccine. However, in order to modulate immune responses from mixed Th1/Th2 to strong and protective Th1 response, further study is warranted on combination of NLX with other adjuvants such as CpG motifs or MPL in proper vaccine formulation. Additionally, dose-response study is necessary to determine the effect of different doses of antigen combined with NLX (at various doses) in Balb/c mice.

Molecular genetic analysis of Plasmodium vivax isolates from Eastern and Central Sudan using pvcsp and pvmsp-3α genes as molecular markers

In Sudan, Plasmodium vivax accounts for approximately 5-10% of malaria cases. This study was carried out to determine the genetic diversity of P. vivax population from Sudan by analyzing the polymorphism of P. vivax csp (pvcsp) and pvmsp-3α genes. Blood samples (n=76) were taken from suspected malaria cases from 2012-2013 in three health centers of Eastern and Central Sudan. Parasite detection was performed by microscopy and molecular techniques, and genotyping of both genes was performed by PCR-RFLP followed by DNA sequence for only pvcsp gene (n=30). Based on microscopy analysis, 76 (%100) patients were infected with P. vivax, whereas nested-PCR results showed that 86.8% (n=66), 3.9% (n=3), and 3.9% (n=3) of tested samples had P. vivax as well as Plasmodium falciparum mono- and mixed infections, respectively. Four out of 76 samples had no results in molecular diagnosis. All sequenced samples were found to be of VK210 (100%) genotype with six distinct amino acid haplotypes, and 210A (66.7%) was the most prevalent haplotype. The Sudanese isolates displayed variations in the peptide repeat motifs (PRMs) ranging from 17 to 19 with GDRADGQPA (PRM1), GDRAAGQPA (PRM2) and DDRAAGQPA (PRM3). Also, 54 polymorphic sites with 56 mutations were found in repeat and post-repeat regions of the pvcsp and the overall nucleotide diversity (π) was 0.02149±0.00539. A negative value of dN-dS (-0.0344) was found that suggested a significant purifying selection of Sudanese pvcsp, (Z test, P<0.05). Regarding pvmsp-3α, three types were detected: types A (94.6%, 52/55), type C (3.6%, 2/55), and type B (1.8%, 1/55). No multiclonal infections were detected, and RFLP analysis identified 13 (Hha I, A1-A11, B1, and C1) and 16 (Alu I, A1-A14, B1, and C1) distinct allelic forms. In conclusion, genetic investigation among Sudanese P. vivax isolates indicated that this antigen showed limited antigenic diversity.

Population genetics and natural selection in the gene encoding the Duffy binding protein II in Iranian Plasmodium vivax wild isolates

Region II of Duffy binding protein (PvDBP-II) is one of the most promising blood-stage vaccine candidate antigens against Plasmodium vivax and having knowledge of the nature and genetic polymorphism of PvDBP-II among global P. vivax isolates is important for developing a DBP-based vaccine. By using PCR and sequencing, the present molecular population genetic approach was carried out to investigate sequence diversity and natural selection of dbp-II gene in 63 P. vivax isolates collected from unstable and low transmission malaria-endemic areas of Iran during 2008-2012. Also, phylogenetic analysis, the diversifying natural selection, and recombination across the pvdbp-II gene, including regions containing B-cell epitopes were analyzed using the DnaSP and MEGA4 programs. Twenty two single nucleotide polymorphisms (SNPs, including 20 non-synonymous and 2 synonymous) were identified in PvDBP-II, resulting in 16 different PvDBP-II haplotypes among the Iranian P. vivax isolates. High binding inhibitory B-cell epitope (H3) overlapping with intrinsically unstructured/disordered region (aa: 384-392) appeared to be highly polymorphic (D384G/E385K/ K386N/Q/R390H), and positive selective pressure acted on this region. Most of the polymorphic amino acids, which are located on the surface of the protein, are under selective pressure that implies increased recombination events and exposure to the human immune system. In summary, PvDBP-II gene displays genetic polymorphism among Iranian P. vivax isolates and it is under selective pressure. Mutations, recombination, and positive selection seem to play a role in the resulting genetic diversity, and phylogenetic analysis of DNA sequences demonstrates that Iranian isolates represent a sample of the global population. These results are useful for understanding the nature of the P. vivax population in Iran and also for development of PvDBP-II-based malaria vaccine.

Population genetic structure and polymorphism analysis of gene encoding apical membrane antigen-1 (AMA-1) of Iranian Plasmodium vivax wild isolates

Plasmodium vivax apical membrane antigen-1 (PvAMA-1) is a major candidate antigen for human malaria vaccine. In the present study, polymorphism of pvama-1 among Iranian isolates was investigated to generate useful information on this vaccine candidate antigen, which is required for the rational design of a vaccine against P. vivax. Blood samples were collected from P. vivax-infected Iranian patients during 2009-2010. Of 99 collected isolates, 37 were analyzed for almost the entire pvama-1 gene using sequencing. The overall nucleotide diversity (π) was 0.00826 ± 0.0004 and the majority of polymorphic sites were identified in domain I (DI) of the pvama-1 gene. Neutrality analysis using Tajima's D, Fu and Li's D* and F* and McDonald Kreitman tests showed a significant positive departure from neutral substitution patterns, indicating a possible balancing selection across the entire ectodomain and DI sequences of pvama-1 gene. However, no evidence was found for the balancing selection in DII and DIII regions of Iranian PvAMA-1. Also, 29 haplotypes with different frequencies were identified and the overall haplotype diversity was 0.982 ± 0.012. Epitope mapping prediction of PvAMA-1 showed the potential B-cell epitopes across DI-DIII overlap with E145K, P210S, R249H, G253E, K352E, R438H and N445D mutations; however, no mutation has been found in intrinsically unstructured/disordered regions. The fixation index (Fst) estimation between Iran and the closest geographical sites such as India (0.0707) showed a slight geographical genetic differentiation; however, the Fst estimation between Iran and Thailand (0.1253) suggested a moderate geographical isolation. In summary, genetic investigation in pvama-1 among Iranian P. vivax isolates indicates that this antigen showed limited antigenic diversity and most of the detected mutations are located outside B-cell epitopes. Therefore, the present results have significant implications in understanding the nature of P. vivax population circulating in Iran as well as in providing useful information for malaria vaccine development based on this antigen.

Population genetics, sequence diversity and selection in the gene encoding the Plasmodium falciparum apical membrane antigen 1 in clinical isolates from the south-east of Iran

The Plasmodium falciparum apical membrane antigen1 (AMA1) is a leading malaria vaccine candidate antigen. In the present investigation, for the first time, the almost full length of the ama1 gene covering domain I (DI), DII and DIII was PCR amplified and sequenced in 21 P. falciparum isolates collected from the southeastern parts of Iran. The result showed the low genetic diversity of Iranian PfAMA1 with 11 PfAMA1 haplotypes in which nine out of 11 haplotypes are novel and have been reported for the first time. The Iranian P. falciparum population indicated a moderate level of genetic differentiation. The difference among the rates of non-synonymous and synonymous mutations, Tajima's D and McDonald-Kreitman tests suggested that the diversity at DI is due to positive natural selection. In addition, recombination contributes to the diversity of Iranian PfAMA1 and this is supported by the decline of the linkage disequilibrium index R(2) with increasing the nucleotide distance. The highly polymorphic residues (positions: 187, 197, 200, 230 and 243) were polymorphic; however, most of the SNPs in non-polymorphic residues were conserved except the residue at position 395. Nevertheless, no mutation was found in the DII loop of the Iranian PfAMA1, indicating that it is subjected to purifying selection. In conclusion, the low genetic diversity in PfAMA1 among Iranian isolates supports and provides valuable information for the development of a PfAMA1-based malaria vaccine.

Analysis of Fcgamma receptor IIa (cd32) gene polymorphism and anti-malarial IgG subclass antibodies to asexual blood-stage antigen of Plasmodium falciparum in an unstable malaria endemic area of Iran

One of the main host genetic factors involved in inflammation, immune responses and pathogenesis of malaria is FcγRIIa (cd32) gene. A single point mutation at position 131 replace an arginine (R) with a histidine (H) that can affect the affinity of the receptor for human IgG subclasses. This investigation was designed to explore the polymorphisms at FcγRIIa gene in association with both anti-malarial total IgG antibody and IgG subclass profiles to C-terminal region of Plasmodium falciparum merozoite surface protein 1 (PfMSP-1(19)). In this study, 166 infected patients with P. falciparum who are living in a malaria endemic area of Iran were studied using PCR-RFLP and ELISA methods. The results showed that the frequency of FcγRIIa-R/R131, -R/H131 and -H/H131 genotypes was 9.6%, 42.8% and 47.6%, respectively. Level of total IgG to recombinant PfMSP-1(19) antigen showed that there was no difference among the FcγRIIa-R/R131, -R/H131 and -H/H131 groups. With regards to the IgG subclasses, the anti-malarial IgG1 antibodies predominated. Also, there was a significant difference between the frequency of positive responders for anti-PfMSP-1(19) IgG and IgG1 antibodies in P. falciparum-infected individuals with FcγRIIa-R/R131, -R/H131 or -H/H131 genotypes (P<0.05, X(2) test). Regarding to IgG2-PfMSP-1(19) antibody, 27.27% (FcγRIIa-R/R131), 25.71% (FcγRIIa-R/H131) and 22.2% (FcγRIIa-H/H131) of IgG responders showed positive antibody response. Taken together, this study is the first report that exhibits the high frequency of both FcγRIIa-H131H genotypes and H131 allele in the Baluchi ethnic group, which was similar to the Fulani ethnic group. The present results provide additional data to understand the role of FcγRIIa-131 genotypes in the pathogenesis of malaria.

Multiple Genotypes of the Commonly Co-Segregating Toll-Like Receptor 4 Asp299Gly and Thr399Ile in Baluchi Malaria Patients from Iran

OBJECTIVE

Different studies have shown an association of TLR4 polymorphisms with susceptibility/resistance to malaria disease. In the current immunogenetic study, we assessed the TLR4 genotypes formed by the two common single nucleotide polymorphisms (SNPs) (Asp299Gly and Thr399Ile) in the co-segregate state in Baluchi Plasmodium falciparum infected and healthy populations from malaria hypoendemic areas of Iran. The study was performed to evaluate the distribution and correlation of TLR4 co-segregating genotypes in patients with mild malaria. Moreover, the frequency of these genotypes was compared with reported results from other populations in similar or contrasting malaria settings around the world.

MATERIALS AND METHODS

In this case control study, the presence of 2 SNPs in the TLR4 gene (Asp299Gly and Thr399Ile) were analyzed in 350 Baluchi patients with mild malaria and 350 unrelated healthy controls by using polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) techniques followed by sequencing analysis. Differences in the TLR4 co-segregate genotype frequencies among the studied group were determined by Fisher's exact test.

RESULTS

Although the distribution of the two commonly co-segregating TLR4 genotypes presented a diverse and distinct pattern in the Baluchi population, no significant difference was detected between the cases and controls (p>0.05). A lower frequency of TLR4 Asp299Gly/Thr399Thr was observed in Baluchis with mild malaria compared to African populations (p< 0.05).

CONCLUSION

Differences in the co-segregation patterns of TLR4 Asp299Gly/Thr399Ile genotypes in the Baluchi population compared to other malaria endemic populations may suggest different local evolutionary pressure on TLR4 polymorphisms by malaria in this region. The higher frequency of Asp299Gly/Thr399Ile genotypes among the Baluchi population compared with the African population (p< 0.05) which suffers from a larger number of severe cases might suggest that this genotype has a role in protecting against severe malaria. These findings are useful for further understanding the pathogenesis of severe malaria.

Immune responses elicited by co-immunization of Plasmodium vivax and P. falciparum MSP-1 using prime-boost immunization strategies

Carboxy-terminus of merozoite surface protein-1 (MSP-1(19) ) is the major protein on the surface of the plasmodial merozoite that acts as one of the most important blood-stage vaccine candidates. The present investigation was designed to evaluate the immune responses when either two recombinant antigens (rPvMSP-1(19) + rPfMSP-1(19)) or two plasmid constructs (pcDNA3.1 hygro-PvMSP-1(19) + pcDNA3.1 hygro-PfMSP-1(19)) were administered in combination at a single site in mice by using different immunization strategies (protein/protein, DNA/DNA and DNA/protein) at weeks 0, 5 and 8. All mice were monitored for the level of MSP-1(19) -specific antibody for up to 40 weeks. The inclusion of both recombinant antigens in a vaccine mixture could not inhibit induction of antibodies to the other antigen when the two recombinant antigens were combined in immunization formulation. Interestingly, antisera from immunized mice with either recombinant antigen failed to cross-react with heterologous antigen. Moreover, the results of this study showed that co-immunization with both antigens at a single site generated a substantial PvMSP-1(19) - and PfMSP-1(19) -specific antibody responses and also IFN-γ cytokine production (Th1 response) in DNA/protein prime-boost immunization strategies. The increased humoral response to PvMSP-1(19) and PfMSP-1(19) lasted nearly a year after immunization. Therefore, the results of this study are encouraging for the development of multi-species malaria vaccine based on MSP-1(19) antigen.

Absence of asymptomatic malaria infection in endemic area of bashagard district, hormozgan province, iran

BACKGROUND

A successful malaria elimination program calls for enough attention to parasite carriers, especially asymptomatic malaria, as well as the diagnosis and treatment of clinical cases. Asymptomatic malaria is an infection that patients do not show any symptom; thus, these patients play critical role in the concept of an elimination program. The current investigation was conducted to evaluate the presence of these cases in Bashagard District, formerly a high malaria transmission area in Hormozgan Province, Iran.

METHODS

Blood samples (n = 500) were collected from symptomless individuals residing in Bashagard to evaluate Plasmodium infection by using microscopic, serological and nested-PCR techniques.

RESULTS

Regarding the microscopic and nested-PCR analysis, no asymptomatic infection was detected among studied individuals. Totally, 1% of the studied population (5 of 500) had anti PvMSP-1(19)-specific IgG antibody; however, only 0.2% (1 of 500) of the individuals was seropositive to recombinant PfMSP-1(19), using ELISA.

CONCLUSION

This study showed no asymptomatic malaria infection in the studied population; hence malaria elimination is feasible and can be successfully carried out in this region.

Antibody responses and avidity of naturally acquired anti-Plasmodium vivax Duffy binding protein (PvDBP) antibodies in individuals from an area with unstable malaria transmission

Plasmodium vivax remains an important cause of morbidity outside Africa, and no effective vaccine is available against this parasite. The P. vivax Duffy binding protein (PvDBP) is essential during merozoite invasion into erythrocytes, and it is a target for protective immunity against malaria. This investigation was designed to evaluate naturally acquired antibodies to two variant forms of PvDBP-II antigen (DBP-I and -VI) in malaria individuals (N = 85; median = 22 years) who were living in hypoendemic areas in Iran. The two PvDBP-II variants were expressed in Escherichia coli, and immunoglobulin G (IgG) isotype composition and avidity of naturally acquired antibodies to these antigens were measured using enzyme-linked immunosorbent assay (ELISA). Results showed that almost 32% of the studied individuals had positive antibody responses to the two PvDBP-II variants, and the prevalence of responders did not differ significantly (P > 0.05; χ(2) test). The IgG-positive samples exhibited 37.03% and 40.8% high-avidity antibodies for PvDBP-I and PvDBP-VI variants, respectively. Furthermore, high-avidity IgG1 antibody was found in 39.1% of positive sera for each examined variant antigen. The avidity of antibodies for both PvDBP variant antigens and the prevalence of responders with high- and intermediate-avidity IgG, IgG1, and IgG3 antibodies were similar in patients (P > 0.05; χ(2) test). Moreover, the prevalence of IgG antibody responses to the two variants significantly increased with exposure and host age. To sum up, the results provided additional data in our understanding of blood-stage immunity to PvDBP, supporting the rational development of an effective blood-stage vaccine based on this antigen.

IgG subclass antibodies to three variants of Plasmodium falciparum merozoite surface protein-1 (PfMSP-1(19)) in an area with unstable malaria transmission in Iran

Plasmodium falciparum remains globally an important cause of mortality and morbidity and despite decades of research, no effective vaccine is available against this deadly parasite. The 19-kDa C-terminal fragment of P. falciparum merozoite surface protein 1 (PfMSP-1(19)) is a target for protective immunity against malaria and the major concern in development of vaccine based on this antigen is the presence of polymorphisms. This investigation was designed to evaluate naturally acquired antibodies and antigen-binding avidity of IgG antibodies to three variant forms of PfMSP-1(19) antigen (E/TSG/L, E/KNG/F and Q/KNG/L) in malaria individuals who are living in hypoendemic areas in Iran (n=92, 4-75 years old). The three variant forms of PfMSP-1(19) were expressed in Escherichia coli and IgG isotype composition and avidity of naturally acquired antibodies to the 19-kDa antigen were measured by ELISA assay. Results showed that almost 72% of the studied individuals had positive antibody responses to three PfMSP-1(19) variants and the prevalence of responders did not differ significantly (P>0.05). High-avidity IgG (62.7%, 65.7% and 47.76%) and IgG1 (64.2%, 50.75%, and 50.75%) were found in positive sera for E/TSG/L, E/KNG/F and Q/KNG/L variants, respectively. Moreover, the prevalence and titers of IgG1 antibody responses to the three variants increased with age (P<0.05). In summary, individuals in low transmission areas in Iran can develop and maintain equal immune responses with high avidity to the PfMSP-1(19) variants (E/TSG/L, E/KNG/F and Q/KNG/L); however, the precise role of the total IgG and its isotypes in protection requires further investigation. These results could support the design of a universal PfMSP-1(19)-based vaccine.

Genetic structure of Plasmodium vivax isolates from two malaria endemic areas in Afghanistan

In this study, the nature and extent of genetic diversity of Plasmodium vivax populations circulating in Afghanistan have been investigated by analyzing three genetic markers: csp, msp-1, and msp-3 alpha. Blood samples (n=202) were collected from patients presenting with vivax malaria from south-western (Herat) and south-eastern (Nangarhar) parts of Afghanistan, and analysed using nested-PCR/RFLP and sequencing methods. Genotyping pvmsp-1 revealed type 1, type 2 and recombinant type 3 allelic variants, with type 1 predominant in parasites in both study areas. The sequence analysis of 57 P. vivax isolates identified a total of 26 distinct alleles. Genotyping pvcsp gene showed that VK210 type (86.6%) is predominant in Afghanistan. Moreover, three major types of the pvmsp-3 alpha locus: type A, type B and type C were distinguished among Afghani isolates. The predominant fragments among Nangarhar and Herat parasites were type A (70.8% and 67.9%, respectively). PCR/RFLP products with Hha I and Alu I were detected 52 and 38 distinct variants among Nangarhar and Herat isolates, respectively. These results strongly indicate that the P. vivax populations in Afghanistan are highly diverse.

Circumsporozoite protein gene diversity among temperate and tropical Plasmodium vivax isolates from Iran

To date, there is no information on the genetic diversity of the circumsporozoite protein (CSP), a leading vaccine candidate, in Plasmodium vivax populations circulating in Iran. The gene for this protein, Pvcsp, was amplified from 374 P. vivax isolates collected in the temperate northern, and in the tropical southern endemic areas. PCR-RFLP analysis of the repeated central region revealed that the parasites collected in the northern area were almost exclusively of the VK210 type. Parasites collected in the south-eastern areas were of both VK210 and VK247 types. We detected VK210 parasite in 70.5% of the samples, VK247 parasites in 17.5% and mixed type infections in 12% of the isolates. Sequence analysis of 137 isolates obtained from both areas identified a total of 25 distinct genotypes. The degree of genetic diversity was generally higher for the tropical (21 genotypes) than the temperate (7 genotypes) P. vivax populations, a difference possibly reflecting the high cross-border exchanges between Afghanistan and Pakistan and southern Iran. Interestingly, all but two VK210 type isolates sequenced harboured a 36-bp post-repeat insert previously only observed in North Korea and China. This large-scale survey of parasite diversity in the Eastern Mediterranean Region provides a set of baseline data suitable for future molecular epidemiological studies of P. vivax.

Population structure analysis of Plasmodium vivax in areas of iran with different malaria endemicity

To obtain the genetic structure of Plasmodium vivax populations in the northern and southern malaria-endemic areas in Iran, which differ in endemicity, sequence diversity in the variable block 5 and the C-terminal part of P. vivax merozoite surface protein 1 (Pvmsp 1) was analyzed. The variable block 5 fragment from 52 northern and 94 southern isolates was amplified and sequenced. Type 1, type 2, and recombinant type 3 allelic variants were found in both northern and southern isolates, with type 1 predominant in parasites from the north and type 2 in those from the south. A total of 7 and 27 distinct variants were detected among northern and southern isolates, respectively. A single variant predominated (71%) in the northern isolates, whereas variants were evenly distributed among southern isolates, with only two exceeding 10%. Thus, parasites from the southern malaria-endemic area were more polymorphic than those circulating in the northern area, where malaria is a re-emerging disease. Sequence alignments showed that although some variants were found only in northern or southern isolates, some were common to both and had also been observed in parasites from Azerbaijan, Turkey, Thailand, Bangladesh, and China. The Pvmsp 1 fragment corresponding to the C-terminal region was also amplified and the sequences derived from 20 northern and 50 southern isolates were identical. This high degree of conservation reinforces the potential of this polypeptide fragment for inclusion in synthetic vaccines being developed against P. vivax.

Molecular evidence of mixed P. vivax and P. falciparum infections in northern Islamic Republic of Iran

This study compared basic microscopy with molecular detection of Plasmodium species. According to thick-film microscopy, 100% of 142 malaria cases in Pars-Abad, Ardebil province, were infected with a single species, P vivax. However, nested polymerase chain reaction (PCR) detected mixed species infections of both P. vivax and P. falciparum in 7.0%. In Mazanderan province, 2/20 blood films were diagnosed with only P. falciparum and 18/20 with only P. vivax. However, nested PCR detected 17/20, 2/20 and 1/20 with P. vivax only, P. falciparum only and mixed species respectively. The unexpected presence of P. falciparum urges prompt investigation and immediate treatment of malaria cases in this region.