World Aquaculture 45 New immune systems: Disease-specific immune responses in invertebrates, and their potential applications in aquaculture David A. Raftos and Sham V. Nair1 Immunology that studies how animals defend themselves from infection is in the middle of a paradigm shift. New evidence suggests that invertebrates, including important aquaculture species such as oysters, mussels, abalone and shrimp, have sophisticated immune systems that allow them to accurately discriminate between different types of infection. In the future, it may be possible to tailor these immune systems to combat infection and prevent disease outbreaks that cause huge losses in aquaculture worldwide. This article describes the discovery of entirely new families of proteins that may underpin novel forms of disease-specific immunity. These new proteins are characterized by extremely high levels of variability, meaning that every individual can have hundreds, if not thousands, of subtly different forms of the protein. So far these types of “hypervariable” systems have been found in sea urchins, molluscs, insects and ascidians. Here, we focus on very recent research, which shows that at least some invertebrates can differentiate between infectious disease agents with great accuracy, possibly using molecular hypervariablity. If the results of this research can be used to produce economical, efficacious disease management systems for cultured animals, possibly including immunization against specific diseases, the benefits to global aquaculture will be enormous. Adaptive Immunity and Hypervariability Since the 1950s and 60s, the overriding dogma of immunology has been that invertebrates do not have sophisticated “adaptive” immune systems of the type found in humans (Raftos 1993). One reason that this view has persisted for so long is that invertebrates lack antibodies, the key molecules involved in the human immune system. Antibodies are “recognition” proteins. Their job is to identify invaders and label them for destruction. The ability of antibodies to detect the enormous range of infectious agents that inhabit the environment is based on molecular hypervariability. Every individual can produce millions of subtly different antibodies, each of which is able to identify a unique molecular signature associated with a particular infectious microbe. The best way to picture the way in which antibodies work is the “lock-and-key” model. Each different antibody has a unique threedimensional shape into which a single molecular structure associated with a single type of infectious microbe can fit…like a key fitting exactly into a lock. This type of lock-and-key fit is called specific immunorecognition. Each human can produce millions of different antibodies, so every individual has the potential to identify and destroy millions of different types of disease-causing organisms, or pathogens. In this way, antibodies provide a global coverage of the vast universe of infectious microbes and parasites. Antibodies give humans the advantage of accuracy and precision in their immune responses, allowing defenses to be tailored to particular types of infection. But antibodies come at a cost in terms of the genetic space they require. The amount of DNA contained within a cell is a limited resource. Each human cell has about 3 billion base pairs, or individual letters of the genetic language. This represents the cell’s genome. Three billion base pairs of DNA sounds like a lot. But it still only codes for about 20,000 to 25,000 different genes. That’s not nearly enough raw material to be able to support an antibody-based immune system that relies on the ability to produce millions of different antibodies. The adaptive immune system of humans has gotten around this space issue with typical evolutionary ingenuity. It turns out that there are only about 300 antibody genes in the human genome. Mature antibodies are generated by cutting, shuffling and splicing these different genes into unique combinations. Each mature antibody is made by shuffling seven different types of genes. The genes are randomly shuffled and joined together, primarily during fetal and neonatal development, to form the fully spliced, functional antibodies of adults. If you calculate all of the different combinations of the different antibody genes that are available after cutting and pasting, you come up with a very big number - something on the order of 10 million different combinations. And even more variability is added by other genetic mechanisms that take over after antibody genes have been assembled. The result is a huge number of subtly different antibodies that can match all of the potential pathogens out there in the environment. Invertebrate Immune Systems Antibodies have been discovered in all of the groups of vertebrates - fish, amphibians, reptiles, birds and mammals. Hypervariability is evident in all antibodies, even though it is generated
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