Malaria Vaccine – Success Story

Malaria is a life-threatening disease caused by parasite Plasmodium which is a single cell organism having multiple life stages and require more than one host for survival.

It is transmitted by the bite of an infected Anopheles mosquito. Among Plasmodium the causative agent of malaria are Plasmodium falciparum, P. vivax, P. ovale, P. malariae, and P. knowlelsi. Plasmodium Falciparum is the most notorious agent in context of malaria. Malaria word is derived from Italian language meaning bad air.

In the late 1800s, before the germ theory it was thought that malaria is caused by miasmas, or contaminated air. In 1897 Ronald Ross was the person who made the discovery that mosquitoes were the vector to transmit the disease. It was discovered later on that only female anopheles mosquitoes were able to transmit the disease.

60 different anopheles species can transmit the parasite. According to WHO report in 2016, 216 million cases of malaria from 91 countries were estimated, 5million more cases than 2015. In 2016, 445000 deaths occurred due to malaria. African countries share a major burden of malaria globally. Almost 90% cases and 91% deaths were reported from these countries in 2016.

Malaria Vaccine Development:

There are many historic events in the history of malaria to defeat it. Quinine was thought to be effective against malaria. But after the discovery of mosquito role in malaria, scientists focused on controlling the vector in order to halt the parasite cycle. Bed nest were the best option to prevent from mosquitoes and this option was cost effective as well.

Finally, the development of anti-malarial drugs changed the way of thinking of scientist. But the question arises is that, after having so many drug development against malaria, why malaria is still a problem? Drug resistance is a major concern in this regard. Plasmodium also has the high rate of replication which may be a reason for development of drug resistance. Compatible vaccine development is the best option in this regard.

In humans there are multiple phases of vaccine development. A small group of naive volunteers, from non-endemic regions, with no previous malaria experience are included in phase IA, while malaria-exposed individuals from endemic regions are included in phase I. In both phases, vaccine safety and immunogenicity are assessed. Only after the successful results and encouraging immunogenicity data, phase IIA studies can be initiated on a larger set of individuals (>100-1000) from non-endemic regions.

In Phase IIA, vaccine efficacy is assessed by subjecting the volunteers to a challenge with mosquito bites or intravenous injections of infected red blood cells. The assessment of vaccine efficacy on larger set of individuals from endemic areas is done in phase IIB. After successful results of phase II, phase III studies started, which comprises assessing vaccine safety (including potential side effects) and efficacy over a longer time period in a cohort consisting of thousands of volunteers from endemic regions.

If sufficient safety and efficacy has been demonstrated in Phase III (2 to 5 years), the vaccine can then be licensed and marketed for human use, after which mass-deployment for endemic regions can be launched.

RTS,s is the only approved vaccine against malaria as of 2015. The disadvantage of this vaccine is its low efficacy. Due to this factor WHO does not recommend its use in the babies aged between 6 to 12 weeks. RTS,s vaccine consists of circumsporozoite protein (CSP) of P.falciparum from pre-erythrocytic stage.

There is production of antibodies against CSP antigen which prevents the invasion of hepatocytes and initiates a cellular response that destroy the affected liver cells. Poor immunogenicity was the problem in CSP vaccine. Then, to create a more potent and immunogenic vaccine, this protein was fused with surface antigen of hepatitis B virus with adjuvants monophosphoryl A and QS₂₁ (SBAS₂). This vaccine gave positive results in 7 out of 8 volunteers who challenged with P.falciparum.

Then RTS,s/AS_O1 was produced by using outer protein gene from P.falciparum along with portion of hepatitis B virus and chemical adjuvants to increase the immune response. To prevent the infection a booster dose of this vaccine having high antibody titer is required that blocks the liver infection.

The life cycle of P.falciparum in human consist of 3 stages; the pre-erythrocytic stage (asymptomatic stage), asexual blood stage and sexual stage which take place in the anopheles mosquitos. There are following stages where we can target the malarial parasite.

• At hepatic stage vaccine can either block the entry of sporozoite in to hepatocytes or kills the infected hepatocytes.
• At blood stage vaccine can block the entry of merozoites in to the erythrocytes.
• When merozoites conversion to gametocytes occurs, vaccine can kill the gametocytes.
• In mosquito vaccine can stop the process of sporogony.

Basically parasite attack induces two types of immune responses in host. It may be either anti-parasitic immunity or anti-toxic immunity. Anti-parasitic immunity is produced against the source of infection. It includes humoral and cell mediated immune response against the parasite.

While anti-toxic immunity is produced against the symptoms like TNF-α is responsible for symptoms experienced in severe malaria. The therapeutic vaccine which is developed to produce anti-toxic immunity could target TNF-α production.

The possible vaccine targets, immune response and mechanisms are enlisted in the table below:

Plug and Play Approach For Malaria Vaccine Development:

The malaria infection cycle is started, when mosquito carrying malaria bites the host and transmits the parasite and then parasites replicates in liver and invades RBCs and transmits malaria to uninfected host by bite. In newly infected mosquitoes the parasite replicates and spread to other hosts. A TBV (Transmission Blocking Vaccine) decrease the spread of disease by generating blocking antibodies against malaria.

These antibodies are transferred to mosquito during bite and prevent parasite development in mosquito gut. So TBV stop the transmission of parasite to new hosts.When his-targeted antigen is mixed with lipid based nanoparticles (liposomes) containing a CoPoP (cobalt-porphyrin-phospholipid), the his-tag extemporaneously embeds in the CoPoP regions, so this platform is called spontaneous nanoliposome-antigen particleization (SNAP).

Immunization with these SNAP liposomes drives strong, long lasting TBV activity in animal models like mouse and rabbits. The four evident antigen colors explain the potential of simultaneously targeting malaria antigens expressed during various stages of the parasite life cycle.

The SNAP liposomes induce antibody response is higher than human adjuvants. It indicates the SNAP liposomes generate functional antibodies that inhibit mosquito infection, efficaciously interrupting the transmission cycle.  SNAP liposomes have various features, like modular construction that enables insertion of multiple antigens.

There is need to push forward the SNAP malaria vaccines and should be tested in large animal models. Further studies on stability carried out in many areas where malaria incidence is high, so the ability to lyophilize or store vaccines at high temperatures is really important. The nanotechnology is emerging, so there is need of researchers, authors and funding for future research.

Authors: Muhammad Adnan Sabir Mughal, Muhammad Kasib Khan, Hammad Ur Rehman Bajwa, Abdullah Khalid Chatha