Fish & Shellfish Immunology 19 (2005) 395e396 www.elsevier.com/locate/fsi
Foreword
Ontogeny of the fish immune system
The production of fish larvae is often hampered by high mortality rates, and it is believed that 10% economic loss is due to infectious diseases in Western European aquaculture sector. This raises fundamental challenges with respect to the development of knowledge about early defence mechanisms in fish. Firstly, more knowledge on the presence of humoral defence factors must be achieved. Secondly, the timing of appearance of different leucocytes must be found to ensure at what developmental stage the cellular immune system including the acquired immune responses are functionally mature and may respond to vaccines. Thirdly, the development of strategies to control the pathogen load and immuno-prophylactic measures must be addressed further to realise the ‘‘potential’’ production of marine fish larvae and thus improve the overall production of adult fish. All these issues have been addressed by the EU funded ‘‘FISHAID’’ project (contract no. QLK2-CT-2000-01076). The improved knowledge that has been produced during this project’s lifetime as well as the state of the art is included in the current review articles. In addition, an article by Dr. I.B. Falk-Petersen that reviews the comparative aspects of organ differentiation in fish is especially appreciated. Following on, particular attention has been made on the thymus ontogeny (Bowden et al.) since the thymus plays a pivotal role in the development of the adaptive immune system, an important factor that separates vertebrates from the rest of the animal phyla. In contrast to other vertebrates, most fish species hatch at the embryonic stage of life. Consequently they have to defend themselves against a variety of micro-organisms living in the aquatic environment. The paper by Rombout et al. is focussed on the development of leucocytes functioning within this early innate system and later on in the acquired immune system (B and T cells). Attention is also paid to the phylogeny of leucocytes, with special reference to early chordates. This review clearly shows that young fish use innate mechanisms during the first weeks/months of their development. Although T cells are selected earlier than B cells, T cell independent responses occur earlier than the T cell-dependent responses. The very early (prethymic) appearance of T-like cells in gut of sea bass and carp, suggests an extra-thymic origin of these cells. However, B cells populate the GALT much later than spleen or kidney, indicating a rather late appearance of mucosal humoral immunity. The first plasma cells are found long after the intake of food in cyprinids, but in many marine fish they appear around the first food uptake. However, in general, acquired immunity is not correlated to the timing of first food intake. As described by Magnado´ttir et al., the first appearance of IgM in lymphocytes varies considerably between fish species. Generally, the first appearance of B-lymphocytes and immunoglobulins is late in marine species compared to fresh water species, and larvae have reached about 20e30 mm in length when IgM is first expressed. Transfer of maternal antibody to eggs and embryos has been demonstrated in several species such as plaice, tilapia, carp, sea bass and salmon, but not cod. Interestingly, complement component C3 has been found in unfertilised eggs in the spotted wolffish indicating a maternal transfer. Using immunohistochemistry and in situ hybridisation, C3 has been 1050-4648/$ - see front matter ! 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fsi.2005.03.004
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Foreword / Fish & Shellfish Immunology 19 (2005) 395e396
found in several different organs and tissues of developing cod and halibut. These studies suggest that complement may play a role in generation of different organs and not only in the defence against invading pathogens. The innate defence includes both humoral and cellular defence mechanisms such as the complement system and the processes played by granulocytes and macrophages. A set of different substances such as yeast, bacterial and plant products may directly initiate activation of the innate defence mechanisms acting on receptors (pattern recognition receptors such as toll-like receptors) and triggering intracellular gene activation that may result in production of anti-microbial molecules. Following on in the review by Bricknell and Dalmo, the use of immunostimulants, as dietary supplements, may improve the innate defence of animals providing resistance to pathogens during periods of high stress, such as grading, reproduction, sea transfer and vaccination. The immunomodulation of larval fish has been proposed as a potential method for improving larval survival by increasing the innate responses of the developing animals until their adaptive immune response is sufficiently developed to mount an effective response to the pathogen. R.A. Dalmo Department of Marine Biotechnology, Nowegian College of Fishery Science, University of Tromsø, N-9037 Tromsø, Norway Tel.: C47 77644482; fax: C47 77645110. E-mail address:
[email protected]
