Malaria vaccine

By Jack Sewell

In a recent paper published in Science Translational Medicine, Kublin et al. (2017) genetically engineered a strain of the human malaria parasite Plasmodium falciparum¹. This removed its ability to infect the liver and cause illness in humans, and acted as a vaccine by increasing their resistance to future infection.

Humans become infected with malaria when they are bitten by an Anopheles mosquito that carries the malaria parasite, Plasmodium. There are many different Plasmodium species, such as P. falciparum which is the major cause of malaria in humans. The bite results in a small number of Plasmodium sporozoites entering the human blood, though this does not cause illness. Rather, the sporozoites invade the liver cells where they develop into merozoites, which eventually burst out in a process known as schizogony (Fig. 1). The merozoites then infect red blood cells (RBCs), where they undergo schizogony once more. This leads to the destruction of large numbers of red blood cells, hence causing the symptoms of malaria such as anaemia. Because of this, an effective malaria vaccine must be able to target either the sporozoite or liver-stage forms of Plasmodium, before it can infect the RBCs.

Figure 1 | Plasmodium life cycle human stages. 1. Sporozoites produced in the mosquito migrate to the salivary glands. 2. Sporozoites are injected into the host blood by the mosquito during feeding. 3. Sporozoites invade liver cells and develop into merozoites. 4. Merozoites invade and replicate inside erythrocytes. 5. Male and female gametocytes are produced inside the infected erythrocytes then taken up by a mosquito. Adapted from Hill (2011)².

Potential malaria vaccines belong to one of two categories; those which use parasite antigens and those which use whole sporozoites. The use of ‘whole sporozoites’ vaccines presents more of a risk to patients than parasite antigen vaccines, but so far is the only approach that can achieve complete, sterile protection against P. falciparum malaria. The whole-sporozoite strategy is usually achieved by one of two different approaches. The first is to produce radiation-attenuated sporozoites (RAS), which are unable to replicate in the liver due to radiation-induced DNA damage³. The second approach is chloroquine prophylaxis with sporozoites (CPS), which involves infection with infectious sporozoites, followed by administration of the antimalarial chloroquine to kill the blood-stage parasites. However, a new third approach is the creation of genetically attenuated parasites (GAP), which are genetically modified to possess a particular attenuated phenotype, and can confer long-lasting and complete sterilising immunity in mice⁴. One advantage that GAP has over RAS and CPS is that, as all the attenuated parasites have the exact same phenotype, both the efficacy and safety can be altered more easily.

In order to create the attenuated P. falciparum strain, named Pf GAP3KO, Kublin et al. removed the p52, p36 and sap1 genes. These are all expressed in the pre-erythrocytic stage and necessary for the parasite to progress from the liver-stage infection⁵. Following infection of the volunteers with Pf GAP3KO through mosquito bites, they did not suffer from any malaria symptoms, though they did experience some adverse effects from the large number of mosquito bites they received. In addition, analysis of P. falciparum 18S ribosomal RNA presence showed the absence of blood-stage parasites. This illustrated the inability of the Pf GAP3KO strain to invade erythrocytes, and hence its safety due to its inability to cause disease.

The application of the Pf GAP3KO sporozoites to the volunteers induced the development of anti-sporozoite antibodies, specifically the immunoglobulin G (IgG) that targets the circumsporozoite protein (CSP) on the sporozoite surface. Following immunisation with Pf GAP3KO, serum was taken from the volunteers and used in an in vitro assay of sporozoite traversal and liver cell invasion. This showed that the anti-CSP IgG inhibits sporozoite invasion and traversal by approximately 50%, though its activity was slightly greater when the serum was taken from the volunteer later after immunisation. Furthermore, the anti-CSP IgG was also able to inhibit liver infection in vivo, and was shown to bind to antigens on both the sporozoite and liver-stage parasites (Fig. 2). Together, these suggest that the attenuated sporozoites achieved the desired effect of a vaccine, by inducing an immune response that increases resistance to a second infection.

Figure 2 | Serum IgG binding to sporozoite and liver-stage parasites. Sporozoite and liver-stage parasites are marked with an anti-CSP or anti-Bip tag respectively (green). Day 0: Pre-immune serum does not contain IgG with sporozoite or liver-stage binding activity. Day 28: Immune serum contains IgG with both sporozoite and liver stage binding activity (red). Adapted from Kublin et al (2017)¹.

In summary, this paper showed that genetic attenuation is a viable means of producing ‘whole sporozoite’ malaria vaccines. In addition, it may be superior to previous approaches, because each sporozoite possesses the same phenotype so both safety and immunogenicity can be maximised. In addition, Pf GAP3KO was shown to be a promising candidate to be used in this approach. However, future studies on this construct still need to investigate the extent of the protection it provides against wild-type P. falciparum, as this was not investigated in this study. Finally, an obstacle to overcome is the requirement for sporozoites to be produced and maintained in mosquito salivary glands. At present, this limits the potential for widespread administration of a ‘whole sporozoite’ vaccine.

References

1. Kublin, J. G. et al. Complete attenuation of genetically engineered Plasmodium falciparum sporozoites in human subjects. Sci. Transl. Med. 9, 1-11 (2017).

2. Hill, A. V. S. Vaccines against malaria. Philos. Trans. R. Soc. 366, 2806-2814 (2011).

3. Nussenzweig, R.S., Vanderberg, J., Most, H. & Orton, C. Protective immunity produced by the injection of x-irradiated sporozoites of Plasmodium berghei. Nature 216, 160-162 (1967).

4. Bijker, E. M. et al. Novel approaches to whole sporozoite vaccination against malaria. Vaccine 33, 7462-7468 (2015).

5. Vanbuskirk, K.M. et al. Preerythrocytic, live-attenuated Plasmodium falciparum vaccine candidates by design. Proc. Natl. Acad. Sci. U.S.A. 106, 13004-13009 (2009).