It's All about Those Bases: The Need for Incorporating Parasite Genetic Heterogeneity into the Development of Schistosome Vaccines

The research contribution of the Schistosomiasis Collection at the Natural History Museum (SCAN): highlights, challenges and future directions

BACKGROUND

The Schistosomiasis Collection at the Natural History Museum (SCAN) is a repository of schistosomiasis-related specimens, the development of which was funded by the Wellcome Trust between 2011 and 2021. With a view to facilitating research by improving access to genetically diverse material, SCAN was built from legacy research collections of schistosomiasis-related specimens amassed over decades, with more recent collections made through partnership with large field-based projects.

METHODS

We identified the literature associated with SCAN from 2012 until 2024, using both database searches (search terms: SCAN, the schistosomiasis collection at the NHM and schistosomiasis) and citations of the publication which originally laid out the scope of the SCAN Collection. Studies were included if the SCAN publication was cited, and/or if the SCAN Collection was utilised in the work. Data extracted included year of publication, authors, whether and how SCAN was used in the work, and type of specimens used.

RESULTS

The literature includes 88 published works, demonstrating the utility of large field-based collections in supporting research. The collection comprises around half a million larval schistosomes originating from the field, with approximately 3000 specimen lots of lab-passaged adult parasites stored in liquid nitrogen. The Collection includes 11 schistosome species, the majority being the human pathogens Schistosoma haematobium and S. mansoni, while also including many livestock-associated species. Genome analysis of S. haematobium and S. guineensis samples indicate historical introgression or ongoing hybridisation. In order of representation, the collection includes S. haematobium (> 19,000 larval forms and eggs, and 550 specimen lots of laboratory passaged adult worms), S. mansoni, S. japonicum, S. bovis, S. curassoni, S. mattheei, S. rodhaini and S. guineensis, with S. intercalatum, S. margrebowiei and S. spindale represented only by laboratory-passaged isolates in liquid nitrogen. SCAN also includes around 210,000 snails, with the collection as a whole encompassing 27 countries.

CONCLUSIONS

Improvements in DNA sequencing techniques have allowed genome-level data to be accessed from archived larval schistosomes and allowed retrospective analysis of samples collected decades ago. SCAN has been of use in exploring schistosome diversity, particularly with reference to hybridisation and drug resistance. Multiple author nationalities demonstrate the collaborative nature of research using the Collection, although more may need to be done in future, both to promote work led by developing countries and to ensure effective collaboration and sample sharing.

Evolution of antigenic diversity in the zoonotic multi-host parasite Schistosoma japonicum: implications for vaccine design

The multi-host zoonotic transmission of the blood fluke Schistosoma japonicum is complex, presenting challenges for China's schistosomiasis elimination strategy. How multi-host transmission impacts the genetic diversity of S. japonicum populations is poorly understood and the extent of Schistosoma japonicum antigen coding gene (SjACG) variability remains unknown despite the implications for parasite survival, vaccine development and disease control. To address this, we sequenced the host-interacting domains of three functionally significant SjACGs previously identified as promising vaccine targets (tetraspanin 23 (TSP-23), venom allergen-like protein 7 (VAL-7), and tegument allergen-like protein 1 (TAL-1)) from FTA-archived S. japonicum miracidia sampled from natural infections amongst different definitive host species in mainland China. This work represents the first known analysis of SjACG variation among different host species. SjACGs were genetically diverse across host species, with 10-20 SjACG haplotypes identified from 60 to 81 sequences. Host-derived immune selection pressures may be driving this variation, impacting antigen protein structure, function, and antigenic propensity. Antigen haplotypes were broadly shared across host species, supporting prior suggestions of gene flow and underscoring the importance of zoonotic transmission in disseminating diversity. Some host adaptation was inferred through identification of host species-specific variation. Parasites sampled from humans displayed the greatest overall diversity of SjACGs, and humans shared haplotypes with all other host species. SjACG diversification appears to have occurred rapidly, and before modern humans arrived in China (∼1.7-0.66 million years ago (MYA)), suggesting that animal hosts have been important in the evolutionary history of these antigens. Collectively, the results expand our understanding of the impact of zoonotic transmission on the co-evolutionary processes driving antigenic variability and provide possible evidence of adaptive molecular evolution of certain antigen haplotypes to specific host species. Our findings have implications for the development of anti-schistosome vaccines and, ultimately, for control of zoonotic schistosomiasis.

Low allelic diversity in vaccine candidates genes from different locations sustain hope for Fasciola hepatica immunization

Fasciola hepatica is a trematode parasite that causes fasciolosis in animals and humans. Fasciolosis is usually treated with triclabendazole, although drug-resistant parasites have been described in several geographical locations. An alternative to drug treatment would be the use of a vaccine, although vaccination studies that have been performed mainly in ruminants over the last 30 years, show high variability in the achieved protection and are not yet ready for commercialisation. Since F. hepatica exhibits a high degree of genomic polymorphism, variation in vaccine efficacy could be attributed, at least partially, to phenotypic differences in vaccine candidate sequences amongst parasites used in the challenge infections. To begin to address this issue, a collection of F. hepatica isolates from geographically dispersed regions, as well as parasites obtained from vaccination trials performed against a field isolate from Uruguay and the experimentally maintained South Gloucester isolate (Ridgeway Research, UK), were compiled to establish a F. hepatica Biobank. These collected isolates were used for the genetic analysis of several vaccine candidates that are important in host-parasite interactions and are the focus of the H2020 PARAGONE vaccine project (https://www.paragoneh2020.eu/), namely FhCL1, FhCL2, FhPrx, FhLAP and FhHDM. Our results show that F. hepatica exhibits a high level of conservation in the sequences encoding each of these proteins. The consequential low variability in these vaccine candidates amongst parasites from different geographical regions reinforces the idea that they would be suitable immunogens against liver fluke isolates worldwide.

The influence of related and unrelated co-infections on parasite dynamics and virulence

Many parasitic infections increase the morbidity and mortality of host populations. Interactions between co-infecting parasites can influence virulence, the damage done to a host. Previous studies investigating the impacts of parasite co-infection on hosts have been limited by their inability to control parasite dosage, use consistent virulence metrics, or verify co-infection status. This study used molecular tools, known infection dosage, and multiple assessments over time to test whether parasite relatedness can predict virulence in co-infections, as well as whether competitive interactions between different parasite strains within a host are predictable over time. In addition, we examined the impacts of other parasite traits, such as infectivity, as alternative predictors of virulence and competition outcomes. Hosts with single-strain (related) parasite infections were found to have lower virulence in terms of host and parasite reproduction, supporting kin selection predictions. However, these infections also resulted in higher host mortality. We argue that mortality should not be used as a measurement of virulence in parasite systems that castrate hosts. Hosts were more susceptible to mixed strain (unrelated) parasite infections, indicating that co-infections may make resistance more costly to hosts. Co-infections were dynamic, with changes in parasite dominance over the course of the infection. The more infective parasite strain appeared to suppress the less infective strain, ultimately increasing host longevity. Our findings suggest that unrelated, or more diverse, parasite infections are associated with higher virulence, but that studies must consider their methodology and possible alternative explanations beyond kin selection to understand virulence outcomes.

Genomic evidence for population-specific responses to co-evolving parasites in a New Zealand freshwater snail

Reciprocal co-evolving interactions between hosts and parasites are a primary source of strong selection that can promote rapid and often population- or genotype-specific evolutionary change. These host-parasite interactions are also a major source of disease. Despite their importance, very little is known about the genomic basis of co-evolving host-parasite interactions in natural populations, especially in animals. Here, we use gene expression and sequence evolution approaches to take critical steps towards characterizing the genomic basis of interactions between the freshwater snail Potamopyrgus antipodarum and its co-evolving sterilizing trematode parasite, Microphallus sp., a textbook example of natural coevolution. We found that Microphallus-infected P. antipodarum exhibit systematic downregulation of genes relative to uninfected P. antipodarum. The specific genes involved in parasite response differ markedly across lakes, consistent with a scenario where population-level co-evolution is leading to population-specific host-parasite interactions and evolutionary trajectories. We also used an F_ST -based approach to identify a set of loci that represent promising candidates for targets of parasite-mediated selection across lakes as well as within each lake population. These results constitute the first genomic evidence for population-specific responses to co-evolving infection in the P. antipodarum-Microphallus interaction and provide new insights into the genomic basis of co-evolutionary interactions in nature.