Genetic approaches to tackle malariaMay 9, 2019
There are an estimated 3.2 billion people from 97 countries at risk of malaria infection, with 214 million infections reported in 2015 alone, resulting in 438,000 deaths. These statistics clearly indicate the need to develop strategies to overcome this infectious disease. Presently, malaria control mechanisms target mosquitoes with insecticides, after which they quickly acquire resistance due to the rapid and uncontrolled use of these agents. Apart from that, drug resistance by Plasmodium, combined with a lack of effective vaccine strategies, is leading to the fast spread of the disease. \
The relationship of the malaria parasite with a vertebrate host starts with a bite of an infected mosquito, when sporozoites are introduced into the bloodstream through the skin. This means that the complex life of Plasmodium parasite is dependent on Anopheles mosquito for transmission to happen. This gives us a very important option of developing transmission blocking methodologies which could potentially help control and eliminate the disease. These methods vary from the classical vector control approaches like insecticides.
Several new studies about genetically modified mosquitoes to block the transmission have also been reported. One of the first genetically modified mosquitoes reported was a variant of Aedes aegypti (Franz AWE et al. Insect Mol Biol 2009;18:661-672), resistant to the dengue virus. However, after 17 generations, the genes that conferred resistance were mutated or silenced. These results clearly show the roadblocks in the development of transmission resistant vectors.
Other strategies include paratransgenesis, which targets the genetic modification of the microbiota to influence the host’s response to the parasite. Different antiplasmodial paratransgenesis bacterial species have been reported, of which Asaia bogorensis have proven to be very beneficial (Damiani C et al. Microb Ecol 2010;60:644-654). Asaia densely populates the midgut, larval gut and reproductive organs of Anopheles mosquitoes. It can also be found in other vectors such as Aedes aegypti and Aedes albopictus, which serve as vectors for dengue, chikungunya and zika (Huges HL et al. PNAS 2014;111, 12498-12503), suggesting that paratransgenesis can help in inhibiting a wide range of vector-borne diseases.
Vaccination has been a successful strategy to overcome several infectious diseases in humans and animals. This could also be a very cost-effective way in implementing prevention strategies for large populations, thus reducing disease burden and mortality.
The lifecycle of malaria parasites is complex, and can be divided into three major phases -: pre-erythrocytic or liver,) asexual blood and mosquito sexual stage, all of which have been exploited for developing vaccines. With more than 5,000 genes characterised by three genomes – nuclear, mitochondrial and apicoplastid, Plasmodium’s genetic diversity and complex biology has made it difficult to achieve long-term immune responses. Liver stage sporozoite (SPZ) vaccines, which induce sterile protection (Nussenzweig RS et al. Nature 1967;216:160-162), modulate T cell responses, preventing the advancement of the liver stage to the blood stage.
Research on attenuated whole SPZ, which is now in early clinical trials, was started by French scientist Sergent in an avian model way back in 1910 (Sergent E. Comptes rendusde l’Academie des Sciences. 1910;151:407-409). In 1973, X-irradiated SPZ of Plasmodium falciparum was shown to be protective in humans when challenged with non-irradiated homologous strain transmitted by mosquito bites. After the success of these developments, scientists are also working on the new and more sensitive technology of chemoprophylaxis vaccination. This method uses non-attenuated, fully infectious SPZ from chemo-sensitive strains, which are administered along with effective antimalarial drugs. This induces a sterilizing effect even with very low doses of SPZ inocula in comparison to X-irradiated. The evolution of new genetic technologies like clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 (CRISPR/Cas9) system (Singer M, Frischknecht F. Trends in Parasitology, 2017;33:202-213, Singer M et al. Genome Biology, 2015;16-249) have helped to develop parasites that arrest late liver stages and exposes the liver-stage-specific antigen for a broader and longer immune effect. However, more clinical studies are required to evaluate several obstacles, such as the delivery of PSZ vaccines in mass immunizations.
Targeting the pre-erythrocyte stage has resulted in several important outcomes and one such was the RTS,S vaccine. RTS,S consists of a virus-like particle (VLP) displaying hepatitis B surface antigen fused with a P. falciparum circumsporozoite protein fragment containing its central repeats and T-cell epitopes (RTS). A successful phase III clinical trial by Vandoolaeghe P and Schuerman L (Expert Review of vaccines, 2016;151:1481-1493) has shown that RTS,S has an efficacy of 51.3% (95%CI 47.5-54.9) in 5-17 month children over 12 months, with 3 doses of vaccine. However, a fourth dose was administered to have long term protection. Although this represents a very important milestone, there remains a need to develop better strategies of doses and long-lasting efficacy against clinical malaria. People living in endemic areas are exposed to repeated blood stage parasites and develop protective antibodies over the years.This stage could also be very important as a target in vaccine development. This acquired immunity that prevents clinical episodes, as shown in Kenyan study cohorts, gives an almost 100% protection. However, the efficacy of merozoite antigen vaccines in interventional trials has been very poor.
Several initiatives, like Malaria Vaccine Initiative (MVI) — with a roadmap to develop vaccines that interrupt parasite transmission (VIMTs) — are under way to developing better approaches for stopping transmission. In these cases, an immune response to stage-specific targets is needed in the human host.
The target proteins selected for vaccine development include surface proteins of gametocytes and gametes (Pfg 27, Pfs 48/45, Pfs 2400 and Pfs 230), zygote and ookinete (Pfs 25 and Pfs 28) (Bousema T, Darkeley C. Clin Microbiol Rev. 2011; 24(2):377-410, Carter R. Vaccine 2001 21;19(17-19):2309-14, Tomas AM etal EMBO J 2001;20(15):3975-83). Antigens from other stages, such as Ps 21, chitinase and alanyl aminopeptidase (AnAPN1), are also targeted. Among the transmission-blocking vaccines (TBVs) that have reached human clinical trials were Pfs 25 and its ortholog in Plasmodium vivax Pvs25. But they have not yet yielded any positive results. This could be due to several reasons, including poor production qualities of antibodies and low reactogenicity attributed to adjuvant formulations (Malkin EM et al. Vaccine 2005;23:3131-3138, Wu Y et al. PLoS One 2008;3(7):e2636). Several methods of vaccine preparation with increased immunogenicity are under development, such as conjugation to Pseudomonas aeruginose exoprotein A (EPA) (Qian F et al. Vaccine 2007;25:3923-3933), bacterial outer membrane protein complex (OMPC) (Wu Y et al. PNAS 2006;103:18243-18248), C4 bp oligomerization domain (IMX313) (Li Y et al. Scientific Reports 2016;6:18848) and modified lichenase carrier (LiKM) (Ogun SA et al. 2008;76:3817-3823). Along with these, new adjuvants such as GSKs liposomal ASo1, which incorporates TLR4 ligand MPL along with QS-21 — a derivative of saponin and Alhydrogel — may now pave the way for better TBVs. In other words, there are several novel technologies which are evolving and are in clinical trials, suggesting the possibility of better outcomes in tackling malaria in the future.
The author is medical scientist and former director of SGRF, Bangalore