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REVIEW ARTICLE
Year : 2022 | Volume
: 59
| Issue : 1 | Page : 1-11
An analytical review of vector- and pathogen-based transmission-blocking vaccine for malaria control
Renu Jakhar1, Neelam Sehrawat2, SK Gakhar1
1 Centre for Medical Biotechnology, Maharshi Dayanand University, Rohtak, Haryana, India
2 Department of Genetics, Maharshi Dayanand University, Rohtak, Haryana, India
Correspondence Address:
Renu Jakhar
Centre for Medical Biotechnology, Maharshi Dayanand University, Rohtak-124001, Haryana
India
Source of Support: None, Conflict of Interest: None
CheckDOI: 10.4103/0972-9062.318308
Malaria is a vector borne disease, considered to be one of the most serious public health problems. The present review focused on the blocking of parasite development in mosquito vectors; one broad strategy for achieving this is Transmission Blocking Vaccines (TBV). The TBVs usually rely on immunization of vertebrate hosts with molecules derived from the vector or pathogen to reduce pathogen transmission from infected to uninfected hosts. Most of the studies on the TBVs are based on the antibodies targeted against the surface antigens of sexual stages of malaria parasite, but it is meagre to develop mosquito-based vaccine in this regard. Vector-based TBVs include surface proteins that are expressed by the mosquito midgut digestive enzymes which are induced upon blood-feeding, and receptors expressed on the epithelial line of the tissue. Many proteins are reported that can act as candidates for transmission-blocking vaccines. This review aims to summarize the vector midgut-based proteins identified till date, that can block the development and maturity of sexual stages of the parasite within mosquitoes as targets for transmission-blocking vaccine development. The TBVs intervention can block transmission of different malaria parasite species in various species of mosquitoes with future application perspective worldwide.
Keywords: Mosquito; Anopheles; Aminopeptidase N; Transmission-blocking vaccines
Malaria is still a potentially fatal infectious disease all over the world. There were an estimated 228 million cases of malaria reported worldwide in 2018 (World Health Organization Report 2019). It is the most common infectious disease, causing 405,000 deaths per year in the world. This is a major public health issue in tropical and subtropical regions, particularly in Sub-Saharan Africa, Asia, and North and South America.
Since the widespread use of chemical insecticides for decades, resistance has developed in malaria vector species, making malaria control a difficult task, particularly in densely populated countries such as India[1]. There is currently no effective malaria vaccine available; scientists around the world are focusing their efforts on developing novel malaria control interventions[2]. Due to lack of an effective and economical control strategy against malaria, the transmission blocking vaccine (TBV) concept has been raised recently. The TBV aims at interfering and blocking pathogen development within the vector host. It disrupts the obligatory steps in the parasite life cycle, limiting the number of infectious mosquito as a vector[3]. Transmission-blocking interventions (TBI) have risen to prominence in recent years as a potentially effective strategy for expanding our chemical toolkit and improving malaria control capabilities. TBI has an additional advantage over conventional control methods in that it avoids the selection pressure for pesticide resistance in mosquitoes because it relies on their survival rather than their mortality, thereby avoiding the selection pressure for pesticide resistance in mosquitoes. TBVs also prevent or reduce the spread of drug resistant parasites when used in conjugation with drugs that target human stage parasite.
The midgut is the site where the parasite first interacts with mosquito tissues and the further developmental process takes place. The mosquito midgut represents one of the most challenging environments for the survival of parasite and thus midgut is a first site of interest for novel target malaria control strategies. The work reported suggests that genetic modification of mosquito vectorial capacity is feasible and represents a major step towards the goal of controlling the spread of malaria. Midgut promoters that are both strong and widespread could be used to drive the expression of effector genes that prevent parasite transmission by either killing or interfering with the development of the parasite[4]. Mosquito midgut invasion is an essential process for invasion of Plasmodium ookinetes, hence the target for transmission-blocking[5],[6]. Several studies have demonstrated that polyclonal antibodies against mosquito midgut proteins interfere with the development of Plasmodium oocysts[7]. Antibodies to these proteins bind to the parasite and ookinete invasion of the midgut epithelium is thought to be blocked[8],[9],[10]. Transmission-blocking immunity (TBI) is the term used to describe this type of immunity, which is mediated by specific antibodies that prevent parasite development in mosquito population. It is being investigated whether these antibodies, which recognise proteins expressed on either gametocytes or parasite stages that develop in the midgut of the mosquito, could serve as potential candidates for malaria vaccine development[7].
In pathogen-based transmission-blocking vaccine, sexual stage antigens are targeted, aiming to block “pathogen development inside the vector, halting transmission to non-infected hosts”. Antibodies that are produced against the surface antigens act as a vaccine. Although transmission blocking vaccines are promising to combat malaria but ethical concerns are also there, as TBVs do not reduce a person’s risk of becoming infected nor reduce the severity of the disease in the affected person. These vaccines cannot protect vaccinated people directly, but it blocks the development of the parasite in the mosquito and decreases the number of infected mosquitoes. Therefore there is lack of interest among individuals as they are not getting any immediate benefit from the vaccine. A good TBV must possess these following characters (a) Low antigenic diversity (b) High antigenicity for human (c) It can produce a titre of antibodies. A large number of candidates are available for vaccine development[8],[9],[10]. There are approximately thirty P. falciparum malaria vaccine candidates in advanced preclinical or clinical stages of evaluation, all of which are aimed at preventing malaria. The RTS, S/AS01 vaccine is the only one to have successfully completed “Phase 3 evaluation and received regulatory” approval, despite the fact that numerous approaches have been tried, all of which make use of recombinant protein antigens and target different stages of the Plasmodium lifecycle. RTS, S/AS01 is a hybrid recombinant protein vaccine based on “the RTS, S recombinant antigen that is administered prior to the erythrocytic stage”. When the RTS hybrid polypeptide is used, it combines regions of “the P. falciparum circumsporozoite protein known to elicit both humoral (R region) and cellular immune (T region) responses with hepatitis B surface antigen” (S) to form a protective barrier against infection (WHO, 2019).
Vector-based transmission blocking vaccines
Recent findings implying the use of multiple mosquito midgut molecules, and the use of multiple alternate pathways during ookinete invasion[6],[11] have shifted the scientific community’s attention towards various enzymes[12],[13], which is now the primary focus of research. Despite the fact that there is currently no effective malaria vaccine available, developing vector-based malaria TBVs remains a viable option for reducing malaria cases throughout the world, particularly in developing countries. Candidates for TBVs based on vectors include surface proteins expressed by the insect midgut, and receptors expressed on the midgut epithelial line. Also, digestive enzymes induced by feeding acts as candidates[14]. Targeting those mosquito components required by the parasite for their development process inside the mosquito vector resulted in transmission blocking activity. Several studies have found that polyclonal antibodies against mosquito midgut proteins inhibit Plasmodium oocyst formation[15]. The alanyl aminopeptidase N immunogen is one of the most common malarial TBV immunogen that has been shown to prevent parasite development in a variety of vector species. FREP1 is also capable of inducing a significant response and inhibiting parasite development in a variety of mosquitoes. However, more research and clinical trials are needed to confirm these antigens’ potential as a target for a transmission-blocking malaria vaccine. The following are some of the vector-based TBV candidates that have been reported.
Aminopeptidase N
AnAPN1 (Anopheline alanyl aminopeptidase N) was found in the apical brush border microvilli fraction of midgut[7]. APN1 Antivector SSM-VIMT elicited antibodies that prevent ookinete interaction with mosquito midgut ligands which inhibits parasite development[16],[17]. A structure-guided construct expressed in E. coli based on the crystal structure of full-length APN1 revealed B cells epitopes as transmission-blocking antigens[18]. Antibodies against the midgut APN1 are one of the P. falciparum receptors in the An. gambiae midgut, reduced P. falciparum oocyst intensity by 73 percent and 67 percent in An. gambiae and An. stephensi, respectively[7]. Mice immunised with a conserved epitope in a clinically relevant adjuvant (Alhydrogel) did not develop autoimmune reactions or immune pathology, nor did they show crossreactivity with humans[19]. High titer antibodies were elicited by immunising with E. coli or insect cell-expressed proteins, indicating that the identified epitope was highly immunogenic. The observation of AgAPN1 localization to the apical midgut surface is supported by GPI linkage and previous evidence that jacalin inhibits ookinete adhesion to midgut microvilli[7]. Similar inhibition of P. berghei development was observed in An. stephensi and An. gambiae. Full-length clone of Aminopeptidase N gene was reported in An. stephensi[16],[20]. Aminopeptidase N gene was identified, characterized and its expression has also been analysed in Anopheles culicifacies A species recently[21].
Carboxypeptidases
Carboxypeptidase genes were shown to express strongly in the midgut in the response of blood meal. These are exopeptidases cleave C terminal residues and acts as metalloproteases. The full-length Carboxypeptidase A gene was cloned in An. gambiae (AgCP) and Ae. aegypti (AeCP) along with its promoter sequences[22],[23]. The AgCP was shown to strongly express after a blood meal and its expression was detectable just within half hour. This makes Carboypeptidase ideal and perhaps the most widely used gene whose promoter has been used to direct expression of an anti-parasitic gene in tissue in a stage-specific manner. Upstream sequences of AeCP were shown to strongly express the luciferase gene in transgenic Ae. aegypti[24]. Transgenic An. stephensi carrying anti-parasitic gene bee venom phospholipase (PLA2) under An. gambiae Carboxypeptidase gene regulatory elements were able to strongly reduce oocyst formation in the midgut[25]. SM1 peptidase under An. gambiae Carboypeptidase gene regulatory elements reduced oocyst formation in transgenic Anophelines[26]. Similarly, the transgenic An. gambiae Cecropin gene reduced the oocyst formation under the Carboxypeptidase promoter[27]. It was reported that RNAi mediated immunity against the dengue virus in transge ic Ae. aegypti by using AeCP promoter elements[28]. A synthetic antiparasite gene Vida3 under control of An. gambiae Carboxypeptidase promoter was shown to reduce parasite intensity by 85%[29]. An. gambiae antibodies against Carboxypeptidase B (CP-BAg1) have been found to reduce P. falciparum infections to a large extent seven days after an infectious artificial blood meal[30]. Anti-CPBAg1 robustly reduced mosquito progeny along with the effect on the number of oocysts per infected mosquito[30]. Similarly, antibodies against the An. stephensi Carboxypeptidase B (CPBAs1) are shown to inhibit the infectivity of P. falciparum in the midgut[31]. Carboxypeptidase A gene was identified, characterized and its expression has been also analysed in Anopheles culicifacies A species[32].
FREP1
FREP1 (Fibrinogen-related protein 1) is secreted by midgut epithelium of mosquitoes. It is then integrated as tetramers into the PM (peritrophic matrix), which is an extracellular chitinous matrix formed within the midgut lumen after a blood meal. During ookinetes anchoring, it makes it easier for various species of Plasmodium to penetrate the PM and the epithelium, and thus enter the mosquito midgut. As a result, FREP1 may be a promising target for the prevention of malaria transmission. The fact that 13 Anopheles species from different continents share a highly conserved C terminal interacting fibrinogen-like (FBN) domain suggests that anti-FBG antibodies could prevent malaria from spreading to all Anopheles mosquitoes[33].
The knockdown of FREP1 expression in An. gambiae has been shown to significantly reduce the intensity and incidence of P. falciparum infection in this species. The inhibition of P. falciparum invasion by an anti-FREP1 antibody provided strong evidence for the interaction between parasites and FREP1, which mediates Plasmodium invasion[34],[35]. Researchers discovered that anti-FREP1 polyclonal antibodies with the fibrinogen-like (FBG) domain were effective in preventing transmission of P. berghei and P. vivax to An. gambiae and An. dirus, respectively, when administered intravenously. Furthermore, in vivo studies of mice immunized with FBG revealed that blocking efficacy of P. berghei to An. gambiae of greater than 75% without causing immunopathology in the animals. The anti-FBG serum also reduced P. falciparum infectivity to An. gambiae by >81 percent. Because FREP1 is a conserved pathway for Plasmodium transmission in mosquitos, targeting its FBG domain could easily limit Plasmodium transmission to Anopheles species.
The FBG binds directly to parasites. Immunization of mice with FBG in a clinically relevant adjuvant resulted in no autoimmune reactions or cross-reactive antibodies against endogenous or human fibrinogens[33]. Immunization with FREP1 proteins expressed in E. coli or insect cells resulted in a high titre of antibodies, indicating that recombinant FREP1 and FBG are immunogenic.
α-AgSGU
The second most abundant protein found in lipid rafts was identified as the An. gambiae protein AGAP000570, a secreted glycoconjugate whose function is unknown and referred to as AgSGU[35]. During the parasite’s invasion process, the AgSGU molecule can interact directly with Plasmodium ookinetes or with other important ligands that interact with midgut lipid raft structures. In both laboratory and field isolates of P. falciparum in An. gambiae and An. dirus, AgSGU antibodies were found to significantly reduce midgut infection intensity, but no effect was found on transmission reduction in P. vivax infected blood meals.
Ookinetes of P. vivax and P. falciparum invading the midgut surface glycoproteins of the host[36]. Despite the fact that antibodies against AgSGU are effective at inhibiting transmission, the high antibody titer required to achieve an 80 percent reduction in oocyst intensity precludes it from being considered as a malaria mosquito-based transmission-blocking intervention (TBI).
Croquemort SCRBQ2
The scavenger receptor family plays an important role in immunity and developmental processes. This scavenger receptor family (SR) consists of the multiple domain transmembrane proteins which recognize many polyanionic ligands like bacteria, low-density lipoproteins (oxLDL) and apoptotic cell[37],[38]. There are eight (AH) independent classes of SR family from which class B, i.e. Croquemort, is well classified. Croquemort consists of the scavenger receptor class B (SCRBQ), the lysosomal integral membrane protein II (LIMP II) and CD36[39]. It is found that AgAPN1, ANXB9, ANXB10B, CpbAg1, and SCRBQ2 are detected in DRM (Detergent Resistance Membrane) as an ookinete interacting proteins[35]. This receptor has four orthologs found in An. gambiae named SCRBQ1, SCRBQ2, SCRBQ3 and SCRBQ4. SCRBQ2 and SCRBQ4 are found to express in every developmental stages, while expression of the SCRBQ1 and SCRBQ3 are found restricted. Although, Croquemort transcript SCRBQ2 shows constitutive expression in all the tissues but its higher expression has been reported in the midgut of An. gambiae[40]. Knockdown of SCRBQ2 expression inhibited oocyst formation of P. berghei upto 62.5% in the midgut of An. gambiae signifying its role in Plasmodium-mosquito interaction[41].
Trehalose transporter
Trehalose is a non-reducing disaccharide of glucose units having a α–α–1, 1–glycosidic linkage, found as major sugar in mosquito hemolymph. It not only gives energy to mosquito, but also protects it from desiccation and heat stress. Trehalose transporter (TreT) aids parisite entry into the hemolymph of mosquitos. The TreT1 transporter is highly expressed in the fat body of adult female An. gambiae. The RNA silencing knockdown of An. gambiae TreT1 lowers the concentration of hemolymph trehalose by 40%, and the mosquitos succumb more quickly after being exposed to heat stress. Moreover, the number of P. falciparum oocysts in the mosquito midgut is reduced by 70% on An. gambiae TreT1 RNA silencing compared to mock-injected mosquitoes[42]. These studies decipher the important role of An. gambiae TreT1 in stress management and Plasmidum growth in the mosquitoes. So, An. gambiae TreT1 seems to be a potent target for mitigating malaria transmission. Trehalose also acts as a good source of energy for Plasmidum pathogens in An. gambiae. The malaria parasite grows by many thousand folds by depriving energy from the vector. Plasmodium parasites use trehalose for their rapid growth and thus reported to deplete sugars in mosquito hemolymph[42].
Annexin
Annexin binds to ookinetes while the parasite invades the mosquito midgut. The inclusion of anti-annexin antibodies in a mosquito blood meal showed to impair parasite development, depicting the role of annexins in the infection of the mosquito by Plasmodium. Approximately 30-40% reduction was observed in the number of oocysts on feeding anti-annexin serum. The results of the antibody-feeding experiments show the similar role of annexins either as the antisera or reduces surviving parasites when binds to annexin[43]. However, the number of oocysts is reduced by a small extent i.e. about one third, accomplishing very little block in transmission.
Chondroitin sulphate
The P. falciparum ookinetes need chondroitin sulphate proteoglycans from mosquito midgut for cell invasion[44],[45]. Midgut chondroitin glycosaminoglycans (CSGAGs) are used by mature ookinetes, which are found to be involved in midgut adhesion and these CSGAGs mainly situated along the apical midgut microvilli. For ookinete identification, the CS disaccharide sequence and relative sulphate positions are critical structural determinants. A peptide-O-xylosyltransferase (OXT) was discovered to be involved in the first step of glycosaminoglycans synthesis. Glycosaminoglycans expression was significantly reduced and parasite growth was significantly inhibited when An. gambiae OXT1 was depleted in the mosquito[7],[46].
Serine proteases
Serine proteases are proteolytic enzymes that activate many proteins. These proteins are found in all eukaryotes and play a variety of roles in homeostasis, blood digestion, nervous systems, and other processes. Serpin-2 (SRPN2) is found in the hemolymph of Anopheles gambiae mosquitos. It inhibits the activation of pro-phenoloxidase (PPO), a rate-limiting enzyme for melanin production, and thus prevents melanisation[47],[48],[49]. Anti-SRPN2 antibodies diffuse across the midgut epithelium through the blood meal, binding and neutralising SRPN2 and increasing PPO system activation and parasite melanisation[48]. RNAi-mediated knockdown of An. gambiae SRPN2 causes increased melanisation of P. berghei ookinetes in the midgut basal lamina[48]. The number of oocysts produced during infection with the rodent malaria parasite P. berghei was also significantly reduced when SRPN2 was knocked down[50]. Antibodies against An. gambiae SRPN2 reduced P. berghei infection in a heterologous Anopheles species (An. stephensi), but not in P. falciparum, which showed no anti- An. gambiae SRPN2 antibody transmission block activity against P. falciparum[48],[51].
Trypsin
Parasites secrete chitinase to come outside from food bolus and to invade midgut epithelium which is formed of chitin. It is secreted in the form of zymogen and activated by host enzyme trypsin. Mosquitoes also secrete chitinase. At times of parasite egression, the PM is still forming and maturing, but parasites can penetrate through the PM at this time. It becomes possible only if parasite also secretes chitinase. Mosquito chitinase was activated only after 48 hours of the blood meal. But parasite comes outside before this period[52]. P. falciparum CHT1 gene disruption in An. freeborni completely blocks oocyst development but P. berghei CHT1gene disruption cannot block the oocyst development. It seems that oocyst development in P. berghei occurs sooner before the formation of PM is complete than in P. falciparum. Parasites use trypsin to digest the blood protein. It also activates other essential proteins in mosquito midgut for the development of parasites. There are seven trypsin genes (trypsin 3, 4 and 5) in An. gambiae that are constitutively expressed in low concentration. But Antrp1 and Antryp2 are induced after a blood meal. Antryp1 is 274 aa long and Antryp2 is 277 aa long, both share 75% homology at an amino acid level. Trypsin of different species has vast diversity in sequences, for example Ae. aegypti and An. quadrimaculatus[53]. Trypsin catalytic triad has aspartic acid at 136, histidine at 91 positions and serine at 232 positions[54]. Glutamine at position 229 is specific for the active site. All serine protease have highly conserved 3 intramolecular disulfide bonds, for example Ae. aegypti[55],[56]. After a blood meal, the expression of the trypsin gene increases. There are two trypsin genes which are characterized in An. gambiae. Antryp1 expression increase within 24 hours however, Antryp2 expression increases between 8 hours and 24 hours. Antryp1 can digest both haemoglobin and serum albumin while Antryp2 can digest only haemoglobin. Both produce the different products in sizes and enzyme kinetics[57]. Thus trypsin is very important for parasite development. If trypsin expression is hindered, then mosquito cannot digest blood proteins which are very essential for the parasite.
Chymotrypsin
There is a digestive enzyme in mosquitoes which is encoded by genes present on chromosome number 2. When a purified protease mixture is added with ami-dated synthetic An. gambiae cecropin, digested products confirms the cleavage at C-Terminal of leucine and phenylalanine in mass spectrometry. It is a serine protease cleaves at C-Terminal of aromatic amino acids. But it cannot digest proteins which have proline at the next position of the cleavage site. Anchym1 and Anchym2 are two inducible chymotrypsin of Anopheles activated by tryptic cleavage[58]. Both have 88% identical sequence. Two other mid-gut specific chymotrypsin are also secreted, AgchyL during digestion as early chymotrypsin and AgSerG13 expression involved in immune responses. Chymotrypsin has conserved amino acids histidine, aspartic acid, serine in a catalytic triad[54]. All invertebrate serine protease contains only three specific disulfide bonds[59]. Chymotrypsin is secreted as a zymogen and has putative activation peptides end with an arginine residue which is a cleavage site for trypsin. If chymotrypsin present for a long period with trypsin, it is destroyed by trypsin instead of activation. It is a regulation of trypsin over chymotrypsin.
Chitinase
After blood meal, a PM surrounding the food is formed in mosquito midgut. This PM is made up of chitin and proteoglycans. Mosquito secretes chitinase after 4h of feeding to digest the meal. If chitinase is absent, a strong PM is formed, and parasites cannot release themselves from this bolus, therefore mosquito chitinase help parasite to free from the food bolus. Catalytic domain is present at N-terminal ends and a chitin-binding domain present at C-terminal ends. In its catalytic domain, three aspartic acids and one glutamic are present. The N-terminal domain contains hydrophobic amino acid as a signal sequence. A serine/threonine and proline-rich residues are present in between two terminals. Their composition is varied, so that flexibility is achieved between the catalytic domain and chitin-binding domain. Two lysine residues are present at 11 positions after the signal sequence. These lysines are cleavage sites for trypsin. Thus, trypsin acts as a regulator for chitinase activation[60],[61].
Saglin
The Salivary gland Saglin 1 (SGS1) is salivary glands specific protein in female Ae. aegypti and inoculation of polyclonal antibodies against aa SGS1 into infected mosquitoes before midgut invasion is found to be efficient in transmission-blocking[62],[63]. Another salivary gland candidate protein is saglin which is female-specific and involved in the interaction between salivary glands and sporozoites of An. gambiae. Monoclonal antibodies against this were found to inhibit sporozoite invasion identifying it as a probable target for TBV[64],[63]. Thus, a large number of proteins are available as a target for TBV to affect sexual development of the parasite. The major candidates are summarized in [Table 1].
Table 1: Vector-based transmission blocking vaccine (TBV) candidate antigens and their characteristics.Parasite-based transmission blocking vaccines
Mostly P. falciparum and P. vivax are used as a target for vaccine development. P. falciparum sexual phase culminates in fertilization within the mosquito’s midgut, and it is a crucial step in the completion of parasite’s life cycle and disease transmission. The antibodies targeted the surface antigens of sexual stages of the malaria parasite. These P. falciparum surface antigens are Pfs230, Pfs25 and Pfs48/45[65]. A clinical trial has also been carried out for Pfs25 and Pvs25[66]. Antibodies to these proteins bind to the parasite and obstruct the invasion of the mid-gut epithelium, according to the researchers. Pfs230 and Pfs48 are expressed during parasitic gametocyte development in humans. They both are required for proper RBC infection and help in gamete recognition. It also initiates the binding of RBC with exflagellating male gametocyte, which further activates the expression of other proteins on the gametocyte surface to proceed invasion and capacitation[67]. Pfs230 containing C region from 443 to 1132 amino acid show an excellent transmission-blocking response[68]. Pfs25 is expressed on parasite surface after zygote formation and during ookinete and oocyst formation in the mosquito midgut. Pfs25 and Pvs25 (vivax homolog) are cysteine-rich 25kDa molecule and include in the P25 family of protein[69]. Parasite ookinete stage contains specific organelle microneme to secrete various enzymes, proteins and can be targeted as a TBV; These are chitinase (PgCHT1) present in soluble form in the cytoplasm, CTRP (Circumsporozoite and TRAP related protein) a trans-membrane α-helix protein present at the apical portion of microneme, WARP a von Willebrand adhesive (vWA) domain-related protein either secreted or remained at surface membrane via other membrane-associated proteins. This WARP is highly conserved among Plasmodium species and encoded by a single exon. These proteins shows binding efficiency for negatively charged carbohydrate ligands on the midgut surface. Ookinete surface protein P25 binds and interacts with basal laminin of midgut to initiate oocyst transformation[70],[71]. LANB2 is a lamininγ1 protein producing gene in An. gambiae. Using RNA interference technology expression of laminin is hindered and the effect is measured[72],[73]. There is a 60% reduction in oocyst development in An. gambiae. SOAP (Secreted Ookinete Adhesive Protein) and CTRP with adhesive domains are two also involved oocyst transformations[72],[73]. CTRP is responsible for motility and infectivity of ookinete. Disruption of the targeted gene produces immotile ookinete with decreased infectivity. This is the only stage of a mosquito that can infect human[74]. The major candidates are summarized in [Table 2].
Table 2: Parasite-based transmission blocking vaccine (TBV) candidate antigens. Conclusion A variety of proteins from P. falciparum have previously been tested for their ability to inhibit transmission; however, recent discoveries that P. falciparum makes use of multiple mosquito midgut molecules have shifted the scientific community’s attention away from parasite-based TBV and towards vector-based TBV. When one considers that there is currently no effective malaria vaccine available to control the disease, the development of vector-based malaria TBVs could be one of the approaches to mitigate malaria throughout the world. It is possible to achieve transmission-blocking activity by targeting the mosquito components that are required for the successful development of the parasite within the mosquito vector. When scientists looked into how Plasmodium binds to adult An. gambiae mosquitoes’ midguts, they discovered a protein called aminopeptidase N1 (AgAPN1) or carboxypeptidase (CPBAg1), which they believe is a target for the parasite. The alanyl Aminopeptidase N immunogen has been identified as the most effective malarial TBV immunogen for inhibiting parasite development across a wide range of vector species. FREP1 is also capable of inducing a significant response and inhibiting parasite development in a variety of mosquito species. However, additional research and clinical trials are required to establish the efficacy of these antigens as a potent target for a malaria vaccine that can prevent transmission from occurring.
AcknowledgementsS.K Gakhar acknowledges financial grant provided by DBT-IPLS, Govt.of India. Renu Jakhar gratefully acknowledges the research fellowship provided by Department of Bitechnology, New Delhi. Neelam Sehrawat is thankful to Maharshi Dayanand University, Rohtak for-Radha Krishan Fund.
Conflict of interest: None
References
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