Innovative Therapies: New Technologies and Models that Could be Explored
Existing treatments against schistosomiasis inadequatly prevent transmission and infection, and there is a rising occurance of drug resistance. This drives the urgency behind the need for innovative therapies - and specifically the prophylactic protection that vaccines provide - for long-term protection against this disease.
Commercial vaccine development has not made large strides in the technological innovation in relation to helminth infections, for multiple reasons:
the impacted regions are typically those with weak economies, reducing the financial motive; (1)
helminths more often than not cause chronic health problems as opposed to death, making treatment less urgent; (1)
capabilities of typical vaccines are limited due to the complexity of parasites (resulting in lowered efficacy). (1)
Nonetheless, there have been some interesting developments in recent years, some of which will be touched upon in this post: unique alternatives for vaccine targets (such as using more than one target in a single vaccine and extracellular vesicles); new vaccine types (mRNA vacciens); innovations in study design, innovatives aids in vaccine research (CRISPR-Cas9 genome editing technique and Controlled Human Infection models).
Targeting More Than One Antigen In A Vaccine
One novel idea is to target more than one Schistosomula antigen in a single vaccine, as the complexity of the organism (with its being multicellular) might require more robust prophylaxis. A suggestion could be to focus on antigens presented in the more vulnerable stages of the parasite’s life cycle - the ‘early post penetration schistosomula’. The 2017 article by Egesa M. et al. explains that during this stage protective layers (teguments) are still forming around the organism as it enters through the hosts skin, leaving the worm vulnerable and an easier target than during other stages of its life cycle. (2)
Figure 1: Schematic diagram of tegument of Schistosoma mansoni
Source: Braschi, S., Borges, W. C., & Wilson, R. A. (2006). Proteomic analysis of the shistosome tegument and its surface membranes. Memorias Do Instituto Oswaldo Cruz, 101(suppl 1), 205–212. https://doi.org/10.1590/s0074-02762006000900032
Research is still being conducted in this area, such as in a trial by Pinheiro CS et al. from 2014, which rendered successful results in a vaccine containing SmTSP-2 fused to Sm29, both of which are tegument proteins. The results showed a reduced parasite load and liver pathology in the treated mice, along with significantly high levels of IgG1 and IgG2, exhibiting partial protection in mice. (3)
Extracellular Vesicles as Transport or Target
The previously mentioned article by Egesa M. et al. suggests that vaccine antigens (such as the aforementioned SmTSP-2 and Sm29 vaccine) could be encapsulated in synthetic vesicles to bypass the inhibitory mechanisms of S. mansoni extracellular vesicles. If ligands are added to these vesicles that trigger antigen-presenting cells in the host, they could function not only as a form of transportation and protection, but as an adjuvant as well.
Disadvantages of this therapy, aside from the high costs, are that the synthetic materials required in its production carry an increased risk of reactogenicity. (2) Further research still has to be done in this area to ensure safety for patients.
Another interesting target could be the extracellular vesciles released by the helminths themselves. These vescicles’ functions include intercellular communication and immune regulation of the host. Even though more research needs to be conducted in this area, some previous studies have indicated that recognition and neutralization of these vesicles by the host’s immune system does not provide significant protection against the associated pathology (4) .
CRISPR-Cas9 Genome Editing Technique
A new technology, which won Emmanuelle Charpentier and Jennifer Doudna a Nobel Prize in 2020, that could potentially play a large role in vaccine development by validating target antigens is the CRISPR-Cas9 genome editing technique, which - as the name suggests - can edit genes in organisms.
CRISPR (clustered regularly interspaced short palinfromic repeats) consists of two parts: Cas9 (CRISPR-associated protein 9), a DNA-cutting enzyme, and guide RNA. Guide RNA brings Cas9 to the target genes on DNA, where Cas9 cuts both DNA strands. This is then repaired with non-homologous end joining mechanism, an error-prone system that can result in mutations, such as the deactivation of a gene. CRISPR/Cas9 is also capable of introducing new DNA segments (if a template is provided with the other two components) into the site through homology-directed repair mechanisms. (5) See the video below for further explanation:
Video 1: Animation of the mechanism behind CRISPR-Cas9
Source: Clinic, M. [MayoClinic]. (2018, July 24). CRISPR Explained. Youtube. https://www.youtube.com/watch?v=UKbrwPL3wXE
This tool can be used for the identification of new vaccine targets, as well as specific genes, for example those involved in drug resistance, and deactivate them. This has already shown promising results in three separate genes in S. mansoni: omega-1 gene (plays a role in immune response through granuloma formation and Th2 polarzation); acetylcolinesterase (target of anthelmintic medication); SULT-OR gene (mutations here play a role in resistance to oxamniquine). (6)
This technology does come with its disadvantages, however. There are differences in the efficiency of gene mutation or editing, which was also observed in the aforementioned 3 targets in S. mansoni. More research is yet to be done to understand the reasoning behind this, and ensure a high and consistent level of efficacy. Other pitfalls include editing of non-targets with unexpected and undesired outcomes. (5)
Further research still has to be conducted in this area, but the possible application of this tool in developing vaccines is promising. One suggestion would be to use this tool to edit specific genes in the worm to weaken it - for example prevent reproduction, reduce extracellular vescicle release - so that patients could be infected with it and develop an immune response. Consequently, immunological memory could develop for protection against future infections.
mRNA Vaccine
Another promising, innovative technology is an mRNA vaccine. Due to the urgency for treatment during the SARS-CoV-2 pandemic, unprecedented jumps have been made in this effective technology (7). See below the brief, simplified animation on how mRNA vaccines work:
Video 2: Simplified animation of the mechanism behind mRNA vaccines
Source: Fuller, D., & The Conversation. (2022, January 27). How mRNA and DNA vaccines could soon treat diseases like cancer, HIV, autoimmune disorders. PBS NewsHour. https://www.pbs.org/newshour/science/how-mrna-and-dna-vaccines-could-soon-treat-diseases-like-cancer-hiv-autoimmune-disorders
The relative quickness, simplicity and low cost with which mRNA vaccines are made, allows the previously described multiple-target vaccine to be developed faster in comparison to typical recombinant protein vaccines. This increases the accessibility of the treatment in the endemic, typically lower-income regions (1). Furthermore, due to the previously explained complexity of the helminth life-cycle, multivalent combinations within the vaccine could proof to be an effective option. This is possible with mRNA vaccines due to their ability to simultaneously express multiple proteins and thereby have numerous targets. (1) mRNA vaccines have already shown its effectiveness in various single-celled parasites such as malaria (8), and in multicellular organisms such as ticks (9), making this a promising candidate against schistosomiasis.
However, mRNA vaccines do not come without their pitfalls. Transportation and storage creates a logistical problem, as the vaccines need to be stored in temperatures ranging from -50 to -100 degrees Celcius. The cost and equipment needed for this is likely to be highly inaccessible in impoverished areas, making large scale distribution in endemic areas difficult. Additionally, despite the low dosages administered to reduce risk, there is still a hazard for toxicity and extreme inflammatory responses (1).
Controlled Human Infection Model
Widely used trials setups in vaccine development are costly and take a long time. The Controlled Human Infection model is a viable alternative due to its relative speed and capacity to test directly on humans. It involves inducing an infection in a healthy volunteer by introducing a pathogen in a regulated manner. This can be used to determine disease progression and efficacy of new treatments, such as vaccines. In the case of Schistosoma parasites, volunteers are infected with either male or female worms to prevent reproduction in the host and thereby providing protection from the pathology associated with the eggs.
Problems arise due to the trials likely not being done in endemic areas because of the lack of finances and equipment, meaning that it cannot be fully representative of the actual environment (especially as the pathogens used have been in sterile lab milieu for a long time, and therefore might not carry the mutations that are currently found in endemic areas). Furthermore only either male or female worms are used in order to prevent egg production, which would put the volunteers health at too much risk. This however limits possible therapies as any potential targets during the early stages of the life-cycle cannot be tested and analysed.
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