By: Gareth Powell and Leyla Bustamante, who carried out the research at the Wellcome Trust Sanger Institute
Gareth is now based at University College London where he is researching the development of left-right asymmetry in the vertebrate brain, under the supervision of Professor Steve Wilson.
Leyla works at the Ferrier Research Institute, Victoria University of Wellington, where she is using synthetic biology for the efficient generation of valuable chemicals in fungi.
The malaria parasites (the white circles) make contact with the red blood cells (large transparent circles) before deforming the blood cells' surfaces and invading
Malaria is a public health scourge in many developing countries. It particularly affects the very young and the elderly who are least able to cope with the waves of fever bought about by haemolysis - a process partially caused by red blood cells laden with parasites exploding, releasing their cargo to find and infect more red blood cells. Importantly, this process is the only narrow window within which the human immune system can recognise and adapt to the parasite invaders before they disappear within the safe confines of another red blood cell host. However, the parasite has evolved the ability to change in response to detection by the immune system - literally shedding its coat and replacing it with a new one, rendering it camouflaged again. Moreover, a further challenge is that malaria is caused by not just one species of parasite but a family of them. The genetic variation between species, and even within different strains of the same species, can render a successful defence against one parasite harmless against another.
Prompting an individual's immune system to be ready for this attacker is very desirable, but how can we develop a vaccine against such a variable disease? As adaptable as the parasites are, there are some things that can't be easily changed, like the proteins on the surface of the parasite that it uses to detect a red blood cell, attach itself to it and invade. These represent targets that are exposed to antibodies in the blood, and are important for the survival of the parasite. The targets can be similar between different species and strains of malaria. Given the challenges of provoking a strong immune response while being essential to parasite viability, and highly conserved across a broad spectrum of strains to increase the ability for successful heterologous challenge, targeting a single protein is unlikely to be enough. Therefore, an effective second-generation vaccine will almost certainly need to target multiple components simultaneously. So, how can we begin to workout which combination of these proteins are the best targets for a universal vaccine?
We, whilst at the Wellcome Trust Sanger Institute, tried to answer this question. We curated a list of potential targets and generated antibodies against each of them, to then test these antibodies in blood parasite cultures. By doing this, we identified five new potential vaccine targets that inhibited growth of two different strains of the parasite: CyRPA, EBA181, MSRP5, RAMA, SERA9. Then, we decided to explore how well these antibodies might work in combination (including another essential target: Rh5) by mixing them in different ratios and testing the different combinations in blood parasite cultures. Would they have the effect we would expect by just adding their efficacies together? Would we get a greater effect to neutralise parasites than would be predicted if the antibodies were acting individually? Or would we find that they interfere with each other, giving the parasites an easy ride? Surprisingly, we found examples of all three situations, suggesting that a multi-target vaccine is not as simple as just picking promising targets from a list and mixing them together.
What made this project even more special was being able to involve some great collaborators to examine other facets of this problem. With Tuan Tran and Peter Crompton at the NIH in the US, we were able to look at the real-world effect of these individual targets, and combinations of them, on the epidemiology of malaria in a human population. We wanted to answer the question of whether an immune response to the targets we identified offered an individual more protection from the disease, and found that the combination of some of the targets identified in vitro by us were associated with reduced malaria risk. With Yen-Chun Lin and Pietro Cicuta, at the University of Cambridge in the UK, we were able observe the process of invasion at the cellular level, as it was happening. We could begin to understand the mechanics of invasion and the stages at which these different parasite proteins functioned: allowing the parasites to detect and stick to a red blood cell, reorientate and form a strong anchor on the surface and then start to deform the membrane and push into the cell. Using this information, we were able to begin to theorise as to how different combinations of antibodies might work together to provide an improved protective effect or compete with each other and reduce protection.
All data combined, our research clearly showed that targeting multiple antigens is an effective tool against malaria. There may be a long way to go to making an inexpensive, widely available, multi-component universal vaccine for malaria. There are lots of other powerful factors besides scientific advancement playing an important part in its development and use, like politics, education and economics, but we like to think that we have at least helped to make a step in the right direction.
About the authors:
Dr Gareth Powell is now based at University College London where he is researching the development of left-right asymmetry in the vertebrate brain, under the supervision of Professor Steve Wilson..
Dr Leyla Bustamente works at the Ferrier Research Institute, Victoria University of Wellington, where she is using synthetic biology for the efficient generation of valuable chemicals in fungi.
Leyla Bustamante et al. (2017) Synergistic malaria vaccine combinations identified by systematic antigen screening. Proceedings of the National Academy of Sciences (PNAS). DOI: 10.1073/pnas.1702944114