Searching for the causes of disease
Model marine organisms Cnidarians and sponges are among the oldest life forms on Earth. They have been living in the oceans for hundreds of millions of years. In spite of their simple body structure they possess an amazing number of genes. These control metabolic processes that have been lost to some extent by higher organisms during the course of evolution. For this reason cnidarians and sponges can be viewed as a kind of prototype for all animals, and represent ideal models to study the basic principles of life.
How does an organism protect itself from pathogens?
The first line of defence against potential pathogens in humans, other vertebrates, and invertebrates such as sponges, is natural immunity. Even infants have an innate immune system, although they have hardly been exposed to any pathogens. This ancient phylogenetic defence mechanism consists of scavenger cells that destroy germs (phagocytosis), metabolic processes that attack and dissolve foreign proteins, and the production of antimicrobial peptides. These peptides are found in animals, plants and microorganisms. They are produced by certain body tissues such as the intestine, skin and lungs, and provide protection against infection. The human immune defence system – or at least part of it – is very old and is related to the lower-order organisms. These organisms include sponges and cnidarians (corals, jellyfish, sea anemones and freshwater polyps), which have lived in the sea for hundreds of millions of years in constant contact with bacteria and viruses. For this reason it is quite possible that they can teach scientists how an efficient defence system develops and how it can be mended in the event of disease.
- Cnidarians, among the most primordial of sea-dwellers, seem ideally suited to the study of how an organism keeps bacteria and other pathogens at bay. They are relatively simple organisms, but nonetheless numerous complex metabolic processes take place within and between their body cells. At first glance cnidarians appear to be vulnerable and defenceless against pathogens because they have neither immune cells to destroy pathogenic intruders nor a lymphatic system to circulate defence cells through the body. They also lack a solid protective covering, having only an outer layer of cells, the epithelium. They have nonetheless managed to survive for millions of years. This makes them extremely interesting subjects of study.
Researchers are trying to find out how their tissue interacts with microbes, and how the metabolic processes in their outer skin fend off enemies. They have successfully bred genetically-modified cnidarians in which the antibacterial defence molecules are visible. This enables them to examine the living creature to see both where the antibodies are released and where they are deployed. It seems amazing that such weak and insignificant little creatures can survive in an environment that is literally teeming with potential pathogens, despite their lack of an immune system and patrolling immune cells. As we know, the exterior surface of many marine creatures, such as sponges, is permanently colonized by bacteria. And furthermore, a litre of seawater can contain up to 2 trillion bacteria and an even greater number of viruses. These microorganisms include many potential pathogens. Despite all this the creatures survive. If we wish to gain greater insight into how the body interacts with its environment, and to explore the principles of evolution, ancient marine organisms seem to be the ideal models.
Thanks to new analytical capabilities, cnidarians play an interesting role in trying to understand the evolution of immune reactions, identifying the genes involved and explaining the universal mechanisms of animal-microorganism interaction. >