Abstract

Malaria continues to be a public health issue, especially in developing countries. A vaccine against malaria appears to be a promising tool in the direction towards minimizing the disease burden. Complex malarial immune responses in humans and genetic diversity in malarial parasites are stumbling blocks in the development of an efficacious vaccine. Current vaccine strategies are directed towards the asexual pre-erythrocytic and erythrocytic stages and sexual gametocytes and ookinete
stages of the parasite. The aim of these vaccines is to prevent disease and block transmission from person to person. The most promising candidate vaccine RTS,S which has been developed by Glaxo Smith Kline in partnership with PATH Malaria Vaccine Initiative, has been approved for human use by the European Medicine Agency (EMA) and the World Health Organization (WHO) has recommended a large-scale pilot program in certain African countries.

Introduction:
Malaria is caused by the protozoan parasite belonging to the Genus Plasmodium and is transmitted by the bite of an infected female Anopheles mosquito. There are five species of Plasmodium that cause human infection, among which , Plasmodium falciparum and Plasmodium vivax pose the greatest threat.

The disease continues to be a serious health problem, the major impact being seen in the tropical and subtropical regions, where the mortality rate is high. Infants and children below five years of age, pregnant women, nonimmune migrants and travelers, and people with HIV/AIDS are at a higher risk of contracting malaria.

Early diagnosis and treatment reduces disease and prevents death. However, development of resistance to antimalarial drugs has creeped in as a recurring problem. In the recent years, malaria control interventions have contributed to the reduction of malaria prevalence[1].

Malarial vaccines would serve as an important tool for successful reduction and elimination of the disease. WHO (2013) published goals to license malaria vaccines by 2030 against Plasmodium vivax and Plasmodium falciparum, that will hopefully eradicate the disease[2].

This article attempts to throw light on malarial vaccine strategies, its approaches and challenges, taking into consideration the dynamics of the parasite Plasmodium.

DIVERSITY OF THE PARASITE:
Plasmodium parasites have a high rate of multiplication, which is more than that needed to ensure transmission in the parasite life cycle. Thus, treatment effectively reduces the reproductive rate without halting it. In such a situation, a high selection pressure is exerted, favoring resistance to the treatment. This evolutionary change that exists among the parasite could cause a significant reduction in the efficacy of the vaccine.

The parasite is known for its genetic diversity. There exist significant variations in the amino acid sequences within the parasite antigens. These differences are seen not only in different strains of the same species, but also within the same strains but in different geographic locations.

IMMUNE RESPONSE TO MALARIA:
The parasite induces two types of immune responses which are parasitic and toxic (related to symptoms). Following infection, both humoral and cell mediated responses induce the production of antibodies, cause T cell stimulation with release of cytokines, and neutrophil and monocyte activation. Production of tumor necrosis factor (TNF-α) plays a central role in generating symptoms, especially in Falciparum malaria[3].

This complex response hinders the development of a vaccine, as multiple immune assays are needed to evaluate and determine the threshold of protection. An ideal vaccine needs to generate a substantial antibody and cell mediated response on the parasite presentation, to provide a level of consistent immunity against the parasite.

PROGRESS IN THE DEVELOPMENT OF MALARIAL VACCINES:
Early malaria vaccine research began in the 1930s, using killed parasites, which however failed to provide protection. Addition of adjuvants appeared to demonstrate immunogenicity in animal models[4]. Human trials began with delivering irradiated P. falciparum sporozoites by mosquito bites[5]. However, this was impractical for mass vaccination. In the 1980s, synthetic peptide vaccines which consisted of immunogenic parasite proteins began to be developed[3]. In the recent years, medical advances in parasite cultivation methods and sequencing of the parasite genome, specially P. falciparum, brought in a silver ray of hope to the development of a malaria vaccine[6]. With a focus on the malaria life cycle, various antigens of the parasite have been evaluated as potential candidates to be the antigenic target.

Malarial vaccines can be divided into 3 groups based on parasite development stages. These include pre-erythrocytic vaccines, blood stage vaccines and transmission blocking vaccines.

1. PRE-ERYTHROCYTIC VACCINES:
These vaccines are designed to produce an immune response that will prevent hepatocyte invasion by inducing both humoral and T cell responses. This vaccine target includes the Circumsporozoite protein (CSP) that is expressed on the surface of sporozoites[7]. Antibodies to this protein inhibit sporozoite invasion.

RTS,S or Mosquirix is the most promising Preerythrocyte vaccine. It has been developed by British drug maker GlaxoSmithKline (GSK) in partnership with PATH Malaria Vaccine Initiative. It consists of Hepatitis B surface antigen (HBsAg) particles fused to P. falciparum CSP central repeat region and Thrombospondin domains, formulated in an adjuvant ASO1. The ASO1 is a liposome combination that contains immuno-stimulants. The strain of P. falciparum used is 3D7 standard laboratory strain[8].

The RTS,S vaccine has undergone clinical trials. In the first Phase 3 clinical trial, efficacy against clinical malaria in children was 46% lasting for about 18 months after the third dose. Results were better in older children[8]. Other studies also showed efficacy that ranged from 31% in children between 5-12 weeks of age and 56% in children between 5-17 months of age hence better protection in older children. Results of Phase 3 trials showed an additional enhancement of protection by administration of a (4th) booster dose[9].

Prime boost strategies are being attempted with the RTS,S vaccine, wherein, subsequent doses of vaccine antigen are delivered with a vector different from the initial one, to avoid immune recognition and senescence[3].

Other Pre-erythrocytic vaccines are in progress and include the Multiple Epitope (ME) Thrombospondin Related Adhesion Protein (TRAP) and whole organism sporozoite. The ME-TRAP consists of fused B cell and CD4 and CD8 T cell epitopes of P. falciparum liver stage antigens. This vaccine however, failed to show protection in Phase 2b Trials in Kenya[10].

2. ERYTHROCYTE VACCINES
These are blood stage vaccines, designed to prevent disease and death, without preventing infection. The immunity is mediated via production of neutralizing antibodies.

Candidate antigens for these vaccines include merozoite surface proteins 1, 2, and 3 (MSP1, MSP2, MSP3), serine repeat antigen, erythrocyte binding antigen, ring infected erythrocyte surface antigen (RESA), glutamate rich protein (GLURP) and apical membrane antigen 1(AMA1).

Four blood stage antigens have been tested in Phase 2 trials. These include AMA1, MSP1, MSP3 and GLURP. However, none of these vaccines were efficacious based on endpoint of clinical disease[11].

A major drawback of these vaccines stems from the immune response being specific to the genetic sequence of the infecting strain. Hence, if the genetic sequence of the infecting strain differs from the vaccine antigen, disease may not be prevented. Due to antigenic diversity, specific amino acid residues and clusters of residues associated with immune protection need to be identified. Further, diversity of antigens can be targeted by considering multivalent vaccine combinations[11].

3. TRANSMISSION BLOCKING VACCINES:
These vaccines are aimed at breaking the transmission cycle. There is no direct benefit to the vaccinated individual and these vaccines are therefore called ‘Altruistic’ vaccines. The vaccine is designed to induce neutralizing antibodies to the parasite gametocyte and ookinete sexual stages. This blocks fertilization and oocyst formation[12].

Antigens under process include ookinete surface proteins P25, P28 of P. falciparum and P. vivax. P25 based candidate vaccine evaluation is ongoing in a Phase 2 trial in Malian adults. Efficacy of these vaccines is measured by assays of mosquito feeding on human blood via a membrane feeding assay[3].

MALARIAL VACCINES IN PREGNANCY:
In the event of malaria during pregnancy, the infected erythrocytes bind to the placental endothelium. This is mediated by P. falciparum erythrocyte membrane protein 1 (PfEMP1). The receptor for this protein is chondroitin sulphate antigen on the placental matrix. There is a consequent reduction in placental blood supply and increased risk of low birth weight babies and preterm delivery[13]. Antibodies to this antigen appear to be protective.

The PfEMP1 has shown promise as a vaccine candidate to target women in the reproductive age group. A PfEMP1 based vaccine is in its preclinical development stage[14].

COMMON CHALLENGES FACED
Absence of a biological correlate of protection for malaria delays and hinders the continued efforts in vaccine development. A series of steps need to be undertaken before Phase 2 field testing, which are laborious, time consuming and require significant funding support. The whole process carries a major risk of a negative end result[3].

To minimize this risk, controlled human malaria infection (CHMI) is undertaken, wherein participants are inoculated with sporozoites via the bite of an infected mosquito under well-controlled settings[15]. The use of CMHI in the early testing of RTS,S vaccine helped to redefine the choice of adjuvant and support reformulation to a lyophilized form[16].

Conclusion:
Research and evaluation of vaccines against malaria is ongoing. With this worldwide effort, the goal of minimizing/eradicating the disease is possible. To eradicate malaria through vaccination, the vaccine needs to block transmission by targeting sexual and ookinete stages, as well as the liver and blood stages. An effective candidate vaccine should show 75% efficacy against clinical malaria and be effective against diverse stains of the parasite. It should be safe, with no/minimal side effects and the immunity should last for at least 2 years. Novel vaccine delivery systems and adjuvants that increase the immunogenicity need to be identified. A multi-antigen vaccine approach needs to be explored.

With regards to the update on RTS,S the safety and efficacy of Phase 3 trials done in certain African countries in children between ages of 6 weeks to 17 months showed modest vaccine efficacy for at least a year after immunization. It is the first licensed vaccine against malaria that has been approved by the European Medicines Agency (EMA) for human use. Following this, in January 2016, the WHO issued a positive report with a policy recommendation to conduct a large-scale pilot program in certain sub-Saharan African countries. Findings of this report will determine whether this vaccine will be used elsewhere

Article By:

Maria Jose Wiseman Pinto, MD (Microbiology) Associate Professor, Department of Microbiology, Goa Medical College, Bambolim, Goa. 403202 Email: mariajosewiseman36@gmail.com Contact no.: +919158523064

Savio Rodrigues, MD (Microbiology) Professor and Head, Department of Microbiology, Goa Medical College, Bambolim, Goa. 403202 Email: saviorodrigues36@gmail.com Contact no.: +919423389736 , +918007923740

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