Endothelial programs that activate or blunt antiviral responses

Endothelial cells, the thin layer of cells lining the inside of blood vessels, play an important role in many viral infections. A good example is avian influenza, where the ability to cause disease in birds is closely linked to whether or not the virus can infect the endothelium. A number of viruses replicate in the endothelium, including hantavirus, hendra and nipah virus in humans and bluetongue virus in ruminants. For practical reasons, most studies of the interaction between viruses and endothelium have been carried out in laboratory-grown cells. This means that we lack knowledge of what happens in infected humans and animals. In addition, many viral infections disrupt the function of blood vessels, which leads to symptoms such as bleeding, fluid accumulation and uncontrolled inflammation. In this project, we have used infectious salmon anemia virus (ISA virus) as a model to understand the interaction between viruses and endothelial cells in living fish. We have taken samples from infected fish and mapped which genes change during virus infection at the single-cell level, with a main focus on the endothelium. The goal has been to identify factors that influence how efficiently the virus can replicate, how the immune system recognizes the virus and how the infection affects the fish's vascular function. We have investigated different stages of infection and compared the response to infection with viruses that cause low and high mortality. The project has confirmed that virus replication in infected fish mainly occurs in the vascular endothelium and has further identified two possible mechanisms that limit this replication. First, the virus destroys its own receptor when it binds the cell, so that new viruses cannot bind. The loss of the receptor affects both endothelial cells and red blood cells and corresponds to an increase in the level of virus in the blood. Second, we see that endothelium is the cell type that changes gene expression the most upon infection. We have described two response patterns in these cells. The first corresponds to high expression of viral RNA and represents cells that allow viral replication. The second response pattern characterizes a group of cells that do not express viral RNA, which suggests that it limits viral replication. This subset of endothelium shows high expression of a number of genes that have been shown to protect other cell types from infection. We have also mapped the different cell types found in the head kidney, including subsets of endothelial cells and immune cells, and how the gene expression in each cell type changes upon ISA virus infection. The main focus of this project has been to understand the endothelial response, but the data are being used further in an EU-funded project to investigate how immune cells contribute to the fish's defense against ISA virus. In summary, the project has provided increased insight into the interaction between viruses and endothelial cells in general, as well as important information about the response of farmed salmon to virus infection. The project has also made it possible to establish single-cell sequencing at the Veterinary Institute, which can be used to elucidate complex biological issues in fish and other animals in future projects.