Research on innate immunity of the penaeid shrimps and the oyster Crassostrea gigas is motivated greatly by economical necessities. Indeed, the aquaculture of these organisms is now limited by the development of infectious diseases. Studying anti-microbial peptides/proteins (AMPs), which are effector molecules of the host defense, is particularly attractive not only for progressing basic knowledge on immunity but also because they offer various possible applications for disease management in aquaculture. AMPs are explored with a global approach,considering their structure, properties, function, gene expression, and tissue distribution during the response to infections. In shrimp, investigations of the penaeidins, which are constitutively expressed peptides, have highlighted the importance of hemocytes and hematopoiesis as major elements of the immune response, providing both local and systemic reactions. The activation of hematopoiesis must be regarded as a regulatory way for the expression and distribution of constitutively expressed immune effectors. As complementary approaches, genomics and gene profiling are promising to deepen our understanding of the anti-microbial defense of the oyster and the shrimp. However, real progress will depend also on the characterization of hemocyte lineages and hematopoiesis of these marine invertebrates as well as on the ontogenesis of their immune systems.

A cDNA sequence with homologies to members of the LPS-binding protein and bactericidal/permeability-increasing protein (BPI) family was identified in the oyster Crassostrea gigas. The recombinant protein was found to bind LPS, to display bactericidal activity against Escherichia coli, and to increase the permeability of the bacterial cytoplasmic membrane. This indicated that it is a BPI rather than an LPS-binding protein. By in situ hybridization, the expression of the C. gigas BPI (Cg-bpi) was found to be induced in hemocytes after oyster bacterial challenge and to be constitutive in various epithelia of unchallenged oysters. Thus, Cg-bpi transcripts were detected in the epithelial cells of tissues/organs in contact with the external environment (mantle, gills, digestive tract, digestive gland diverticula, and gonad follicles). Therefore, Cg-BPI, whose expression profile and biological properties are reminiscent of mammalian BPIs, may provide a first line of defense against potential bacterial invasion. To our knowledge, this is the first characterization of a BPI in an invertebrate.antimicrobial ͉ epithelia ͉ hemocyte ͉ mollusk ͉ oyster innate immunity M arine invertebrates including bivalve mollusks have evolved in the continuous presence of microorganisms. The oysters, such as Crassostrea gigas, harbor a diverse microflora both on their surface (epibiosis) and inside the body cavities and hemolymph (endobiosis). They have developed an efficient immune system for maintaining balance with commensal and pathogenic bacteria, in particular with the Gram-negative Vibrio spp. abundant in their tissues/organs. LPS from Gram-negative bacteria play an important role in the interaction and activation of the innate immune system including the antimicrobial defense (1, 2). In invertebrates, LPSbinding proteins (LBP) participate in the transduction of cellular signals from LPS. LBPs have been characterized in the freshwater crayfish Pacifastacus leniusculus (3), the shrimp Litopenaeus stylirostris (4), the earthworm Eisenia foetida (5), and the silkworm Bombyx mori (6). In mammals, LBP is an acute phase plasma protein constitutively secreted by liver that induces cellular responses (7). In particular, LBP participates in the acute mobilization of circulating neutrophils to sites of tissue injury. Stored in the mobilized neutrophils, antimicrobial peptides and the bactericidal/ permeability-increasing protein (BPI) contribute to the elimination of bacteria (8, 9). BPI, another LBP, is a 55-kDa cationic protein specifically active against Gram-negative bacteria. It increases the permeability of the bacterial membranes (10). Accumulated extracellularly, BPI opsonizes bacteria, which enhances phagocytosis by neutrophils (11). LBP and BPI are structurally related, with 45% sequence identity. They have a coordinated function in the response to invading bacteria. The antibacterial BPI displays LPSneutralizing properties and suppresses LPS inflammatory activity whereas LBP is an acute-phase reactant (8, 12) that displays a concen...

Antimicrobial peptides (AMPs) are important components of the host innate immune response against microbial invasion. We previously characterized the first AMP from an oyster, a defensin, that was shown to be continuously expressed in the mantle of Crassostrea gigas. In this study, we report the cDNA cloning of two new isoforms of the defensin AMP family (Cg-defh1 and Cg-defh2) from the hemocytes of the oyster. The deduced amino acid sequences reveal two peptides of 73 amino acid residues with a mature portion consisting of 43 amino acid residues. Cg-Defh1 and Cg-Defh2 share 86% amino acid identity and belong to the "arthropod-molluscs defensin family". qRT-PCR analyses indicate that Cg-defh2 is continuously expressed in the hemocytes of C. gigas. In addition, after a bacterial challenge, the level of Cg-defh2 transcripts decreases dramatically in the circulating hemocyte population and this decrease can be correlated with an increase of Cg-defh2 transcripts in the gill and the mantle tissue, suggesting a possible migration of the hemocytes expressing Cg-defh2 towards the tissues implicated in the first defense barrier of the oyster. These results would suggest an important role of Cg-Defh2 in the oyster response to a microbial challenge.