Written by Agata Teresa Wyrozemska, doctoral research fellow at Fish Immunology and Vaccinology.
The interaction between bacteria, viruses and the organisms they try to infect is a never-ending race. The human body can defend itself against most unwanted pathogens (harmful bacteria and viruses), using the resources of innate and adaptive immunity. Innate immunity is the first line of defence and includes physical barriers, such as the skin and mucosal membranes lining the digestive tract, respiratory tract, etc. Natural reflexes like sneezing, coughing, and vomiting support the clearing of pathogens. The complement proteins and acute-phase proteins are also involved. In addition, some cells send signals in form of, for example cytokines, which trigger the innate and adaptive immune processes. The adaptive immune response develops through direct contact with pathogens; its mechanisms are triggered after the innate immunity and take time to develop. Adaptive immunity involves various specialized cells and molecules, including Major Histocompatibility complexes (MCH). There are different types of MHC molecules but their general function is to help the immune system recognize foreign substances and distinguish them from the self.
Do fish have the same capacity to combat infection as humans?
Fish are the most numerous and diverse group of vertebrates, with nearly 21,000 species, more than all other types of vertebrates combined. Would it be logical, if such a large and diverse group followed only one immune defence strategy? Probably no, as in many other aspects of biology, this one too does not follow a simple scheme. Let us focus on bony fish, which anglers and fish-enthusiasts shall be well acquainted with, like cod and salmon. To spice things up, we will throw anglerfish into the mix. Anglerfish males, as a part of reproductive strategy, bite into the female and fuse with her. They form an intricate type of transplant. What is even more interesting, many males can fuse with one female. These seams counterintuitive, because in human transplants, the tissue of the donor must be compatible with the tissue of the recipient or the immune system will reject it. Imagine having multiple organs transplanted from random people. How is it possible that the fused male body is not rejected? This is dictated by a loss of key capabilities that characterize classical adaptive immunity in jawed vertebrates in the Anglerfish (Swann et al., 2020).In a nutshell, their ability to recognizing self from non-self is impaired. On the other hand, there is cod, which has lost one type of the MHC molecules in course of evolution. One may speculate that this loss has been compensated by a massive expansion of the other type of MCH molecules. (Star et al., 2011)
The challenges of fish vaccines
Anglerfish is a curiosity, and while cod is more commonly known, it is salmon that is the most popular in Norway. It is well-liked and often lands on our plates. Because of the high demand for salmon fillets, the fish has to be farmed. Thousands of fish are kept in large nets in sheltered waters such as fjords or bays. In dense populations, diseases spread fast. As we see with the Covid-19 outbreak, the major measure to prevent the spread of the virus is social distancing. Social distancing in fish farms is not possible. The most common measure to prevent diseases among farmed fish, which drive economic losses, is vaccination. Pathogens causing diseases in salmon have been thoroughly examined. Numerous vaccines are available, but there is still room for improvement. It is important to thoroughly examine and understand the salmonid immune systems to create more effective vaccines. Salmon shares many immune features with humans. For instance presence of specific cells, like white blood cells, and immunity-related internal organs. However, there are some differences in their structures and functions as well. Salmon, like other bony fishes, does not have bone marrow. Fish also rely more on the innate immunity. The adaptive response appears later in course of infection and is less sophisticated than in humans. These differences are interesting and important. We at the Fish Immunology and Vaccinology group have a focus on exploring salmon’s immune system and contribute to expand the general knowledge and the formulation of new vaccines. More information about the group can be found here.