ABSTRACT
A potential concern of fi sh culturists is to reduce any economic loss. In European sea bass (Dicentrarchus labrax L.) aquaculture rearing facilities, mostly at those located in southern European countries, infectious diseases are a leading cause of mortality. Bacterial and viral diseases of sea bass (the most recurrent, namely vibriosis, pasteurellosis, and viral erythrocytic infection) are now a major challenge for biological studies on this species and further intensive fi sh production. To fi ght against diseases, preventive strategies should be implemented in every single farm. Best management practices, which may include improved sanitary conditions, clean water supplies and vector control are by far the most effective measures to reduce the incidence of infectious diseases. However, the development of specifi c target strategies like vaccines, disease-resistant breeds and therapeutics are
Department of Cell Biology and Histology, University of Murcia, Campus Universitario de Espinardo, 30100. Murcia, Spain. a Email: jorge-galindo@usa.net b Email: vmulero@umu.es * Corresponding author
much more important in achieving protection, but this requires suffi cient understanding of fi sh immune system (Zapata et al. 2006). The sea bass belongs to the Teleostei which is one of three infraclasses in the class Actinopterygii. This diverse group, which arose later in evolution during the Triassic period, roughly makes up half of the extant vertebrate species and occupies a key evolutionary position in the development of immune system evolution. Teleostei is the earliest class of vertebrates sharing similar immune system organization with other vertebrates, and thus, possesses the elements of both innate and adaptive immunity (Vadstein et al. 2012). Therefore, the sea bass has become very attractive as a fi sh model in regards to developmental, anatomical and functional comparative studies of the immune system. So far, it has been recognized that fi sh immune system is characterized by a multi-layered organization that provides immunity to infectious organisms, and each layer can be considered to have an increasing complexity. In this way the central challenge of fi sh immunology, which includes the sea bass model, is now to fully characterize the occurring immune responses in as much tissues as possible, and determine the functionality of each layer. Foundations of present knowledge on innate and adaptive immunity in sea bass have been established during the two previous decades. Remarkable milestones were provided during the early 90s by several workers. Such studies included the isolation of sea bass innate immune cells and the description to the ultrastructural characterization of the main features displayed by each cell group (Meseguer et al. 1991, Esteban and Meseguer 1994, Meseguer et al. 1996). And, extensive reports on the role of macrophages, granulocytes, lymphocytes, plasmocytes and fi broblast-like cells after myxosporean parasites invasion on host testes (Sitja-Bobadilla and Alvarez-Pellitero 1993). Some years later, Mulero et al. (1994) conducted an interesting experiment in which functional properties of host leucocytes were shown. The authors of this paper characterized the changes followed by effector sea bass leucocytes incubated in vitro with HeLa or B16 melanoma cell targets. They observed that effector cells established spot contacts with targets, and characterized morphological cell changes like smooth surfaces, appearance of cell processes or surface blebs, suggesting that changes in targets are similar to those described to be mediated by mammalian cytotoxic cells (Mulero et al. 1994). On the adaptive capacity of sea bass immune system, two fi ndings made great improvements in the study of fish immunology. These contributions were provided by Scapigliati et al. (Scapigliati et al. 2000, Romestand et al. 1995), who produced and characterized specifi c markers for T and B lymphocytes respectively-the monoclonal antibody DLT15 against thymocytes and the DLIg3 antibody directed against immunoglobulin (Ig)-bearing B cells. In this regard, Picchietti and colleagues (Picchietti et al. 1997), applying both antibodies, showed through functional analyses
that gut-associated lymphoid tissue (GALT) developed earlier than other lymphoid compartments. This was a highly remarkable fi nding which provides evidence of the abundance and predominance of T cells in the gut. Together, these studies along with several more conducted in carp (dos Santos et al. 2000, Huttenhuis et al. 2006, Rombout et al. 1985, 1993), brought to light the importance of the gut as an immunocompetent organ in fi sh defense against invader bacteria. Recent studies have complemented the basic sea bass immune research with steadily increasing work conducted mainly at the cellular and molecular level (Scapigliati et al. 2002, do Vale et al. 2002, Buonocore et al. 2008, Buonocore and Scapigliati 2009, Pallavicini et al. 2010, Marozzi et al. 2012, Sarropoulou et al. 2012). However, the former global vision on the study of vertebrate immunology has just recently changed and new players from the environment, the microbes, have begun to be recognized as vital key modulators of the immune response (Yu et al. 2012, Galindo-Villegas et al. 2012, Rautava et al. 2012). The sea bass model of immunity is not the exception and new efforts have been directed towards discovering the mechanisms of microbial pathogenesis and fi sh-microbe symbiosis through the characterization of gut microbiota in conventional or gnotobiotically raised fi sh (Dierckens et al. 2009, Rekecki et al. 2012). Indeed, the diverse and unique immune tools available for European sea bass, its taxonomic position and commercial relevance, has turned this species into an excellent model to study developmental and comparative immunological processes, including basic and functional aspects. Therefore, in this chapter we briefl y summarize and update the emergence, phylogenetic distribution and functionality of D. labrax innate and adaptive immunity, as well as the most relevant interactions between them towards host defense against pathogens.
