The malaria parasite must first enter the liver to reach the blood of its human host, where only a small number of parasites differentiate and replicate for upwards of seven days, making it a bottleneck in the parasite’s lifecycle. This bottleneck makes the liver stage an optimal target for effective and long-lasting vaccines against the disease. Now, using spatial transcriptomics and single-cell RNA sequencing technologies, Stockholm University researchers have for the first time created a spatio-temporal map of malaria infection in the mouse liver.
Their study was published in Nature Communications in an article titled, “Host-pathogen interactions in the Plasmodium-infected mouse liver at spatial and single-cell resolution.”
“Upon infecting its vertebrate host, the malaria parasite initially invades the liver where it undergoes massive replication, whilst remaining clinically silent,” the researchers wrote. “The coordination of host responses across the complex liver tissue during malaria infection remains unexplored. Here, we perform spatial transcriptomics in combination with single-nuclei RNA sequencing over multiple time points to delineate host-pathogen interactions across Plasmodium berghei-infected liver tissues. Our data reveals significant changes in spatial gene expression in the malaria-infected tissues. These include changes related to lipid metabolism in the proximity to sites of Plasmodium infection, distinct inflammation programs between lobular zones, and regions with enrichment of different inflammatory cells, which we term ‘inflammatory hotspots.’”
“The possibility to identify the exact location of differentially expressed genes across liver tissue sections in response to parasite infection has great potential to propel malaria research forward. It provides a whole new platform to study host-pathogen interactions in the true tissue context. This can eventually help researchers to identify novel targets for drug development and vaccine strategies for malaria, but also for a wider variety of pathogens that infect human tissues,” explained Johan Ankarklev, PhD, associate professor at the department of molecular biosciences, the Wenner Gren Institute (MBW), and an author of the study.
The study, based at the MBW at Stockholm University, was performed in collaboration with the research groups of professor Joakim Lundeberg, PhD, Royal Institute of Technology (KTH) in Stockholm, associate professor Emma R. Andersson, PhD, the Karolinska Institute in Stockholm, associate professor Joel Vega-Rodriguez, PhD, the National Institutes of Health (NIH) in Maryland, and professor Charlotte Scott, PhD, at the VIB in Ghent, Belgium.
The combination of spatial transcriptomics technology, originally developed in Lundeberg’s group at KTH at SciLifeLab, and single-cell RNA sequencing, allowed the researchers to chart the global gene expression of both the host and the parasite across Plasmodium berghei-infected mouse liver tissues, for the first time.
The researchers discovered that the parasite causes changes in the gene expression of host cells in its proximity, in a time-dependent manner. During early liver-stage infection, they found pro-inflammatory gene programs in tissue locations in the proximity of parasite positions. In contrast, during the late stages of liver infection, they found gene programs related to immune responses downregulated in parasite neighborhoods. They discovered an upregulation of gene programs related to fatty acid metabolism across host cells in the proximity of parasites at the late time point. Fatty acids are essential during the massive parasite replication that occurs at the end of the liver stage and are moreover reported to be involved in anti-inflammatory responses.
“Previous studies have established that some parasites will develop successfully in the liver to reach the blood while others are successfully targeted by the host immune system and eliminated. Therefore, we speculate that these structures may represent positions of successful parasite elimination,” said Franziska Hildebrandt, PhD, who was a PhD student in the Ankarklev lab and lead author of the study.
