Life on earth originated in the sea. Marine animals were the first animals that had to evolve defence strategies for their survival. To cope with possibly dangerous events, marine organisms have developed over millions of years what
V. Matranga (e-mail: [email protected]),A. Pinsino,M. Celi, A. Natoli, R. Bonaventura Istituto di Biomedicina e Immunologia Molecolare (IBIM) "Alberto Monroy", Consiglio Nazionale delle Ricerche,Via U. La Malfa 153,90146 Palermo, Italy H.C. Schröder, W.E.G. Müller
Institut für Physiologische Chemie, Abteilung Angewandte Molekularbiologie, Johannes Gutenberg-Universität, Duesbergweg 6,55099 Mainz, Germany
Progress in Molecular and Subcellular Biology Subseries Marine Molecular Biotechnology V. Matranga (Ed.), Echinodermata © Springer-Verlag Berlin Heidelberg 2005
we now call an immune system, as a strong defence mechanism able to resist host attacks in an efficient way. The term "immunis" comes from the Latin meaning "exempt", referring to protection against foreign agents. It is still believed that in all organisms of the animal kingdom the cells of self are virtually marked, so that they are not attacked by their own defence mechanism. Therefore, the immune system must have the capacity to discriminate between self and non-self, to transform itself to deal with future dangers, and, in addition, to change, since the self also evolves with time (e.g. during embryo development). However, the self/non-self model of the immune system has recently been disproved because of many inconsistencies: (1) not all foreign cells need to be destroyed, some in fact must be assimilated for nourishment; (2) in mammals the growing embryo, in principle a host, is not destroyed by the immune system of the mother. This is just to quote two examples. To solve these apparent paradoxes one very attractive theory has been proposed by Polly Matzinger, the present head of the National Institute of Allergy and Infectious Diseases at NIH, Bethesda, Maryland, USA. In her September 2001 lecture entitled, "An Innate Sense of Danger", she proposed that cellular apoptosis signals and directs the immune mechanism (reviewed in Matzinger 2002). In other words, the triggering factor comes from the "sense of danger" originated during damage to body tissues, rather than from the classical recognition of foreign antigens. Her theory has given hints to studies on tumour regression, graft rejection, pregnancy failure and autoimmune diseases. Another puzzling question, which the self/non-self model can neither predict nor explain, is: why is the mere foreignness of a protein not enough to elicit immunity, and why are noxious substances like mineral oil, mycobacteria, and aluminum hydroxide needed in order to get a clear response? Charles Janeway, one of the most brilliant and imaginative immu-nologists, recently deceased (April 2003), proposed, together with his colleague Medzhitov, the so-called Pathogen-Associated Molecular Pattern theory (Medzhitov and Janeway 2002). They postulate that immune responses cannot occur unless antigen-presenting cells are first activated, and that this activation occurs via pattern recognition receptors. Their prediction of the presence of evolutionarily conserved receptors recognizing molecules from infectious non-self organisms was confirmed when, studying Drosophila Toll innate immune sensor, they found the mammalian Toll-like receptors (Medzhitov et al. 1997), which are now studied by laboratories around the world.
The above-mentioned new ways of interpreting the machinery of the immune system bring to the concept that the innate immune system is an evo-lutionarily ancient form of host defence found in most multicellular organisms. The basic mechanisms operating in sensing and attacking foreign agents are already found in echinoderms. It is stimulating that these hypotheses can be tested in the echinoderm model and that, to a certain extent, studies on their defence system might provide many useful insights into the understanding of the mechanisms involved. In the following text, we briefly review the state of the art on what is known about the echinoderm immune system, with particular emphasis on mechanisms and/or molecules involved in sensing experimental or environmental stress.
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