7 Genomic Approaches in Aquaculture and Fisheries
247
7.6.4.4 Immune Response to Vibrio Infection
As filter feeders, bivalves are constantly exposed to various pathogenic and/or
opportunistic bacteria naturally present in the microflora of coastal environments. Among these bacteria, the Vibrionaceae are the most prevalent in marine
environment (Potasman et al. 2002). Reported as commensal bacteria, vibrios
are also considered to be opportunistic pathogens associated with mortalities of
bivalves, particularly Crassostrea gigas (Paillard et al. 2004). Bivalve hemocytes
are equipped with both oxidative (Bramble and Anderson 1997, Bachère et al. 2004)
and non-oxidative killing systems related to activities of lysosomal enzymes (Hine
1999). Pathogenic Vibrio species have the ability to stimulate the oxidative burst in
hemocytes of Crassostrea gigas (Lambert et al. 2003, Labreuche et al. 2006a,b).
Moreover, the bivalve hemocytes are mobile, have a high clumping potential, show
chemiotactic activity (Prieur et al. 1990, Canesi et al. 2002) and develop spontaneously cytoplasmic extensions (pseudopods) facilitating adhesion. Hemocytes
appear to have a different migration pattern according to bacteria and bivalve species
(Howland and Cheng 1982, Kumazawa and Morimoto 1992, Fawcett and Tripp
1994). Authors demonstrated that Vibrio induced loss of pseudopods and cell rounding in Mytilus (Nottage and Birkbeck 1990, Lane and Birkbeck 1999) and reduced
the adhesive capacities of clam hemocytes (Choquet et al. 2003). Similarly, extracellular products of Vibrio affected phagocytosis and adhesion of hemocytes in
C. gigas (Labreuche et al. 2006a). Finally, lysozyme activity appears to be higher in
Vibrio-challenged clams (Allam et al. 2006). Thus, factors such as bacterial surface
ligands and soluble hemolymph components, as well as the ability of bacteria to
activate distinct signalling pathways involved in hemocyte response, are reported to
be responsible for the persistence of Vibrio in marine bivalves (Pruzzo et al. 2005).
Little is known about molecular mechanisms involved in bivalve-Vibrio interactions and associated with the immune response. An EST library was recently
generated from oyster hemocytes challenged with pathogenic vibrios (Gueguen
et al. 2003). Fifty-five sequences out of 1,142 ESTs analysed were classified
as immune genes. Among these ESTs, the tissue inhibitor of metalloproteinase
(Cg-Timp) was found to be highly represented. This gene is expressed specifically
in hemocytes (Montagnani et al. 2001) and Cg-Timp mRNA accumulation could
be induced by secretory/excretory molecules produced by Vibrio (Montagnani et al.
2007). In addition, serine protease inhibitor and serine protease were inducible by
Vibrio anguillarum in scallop (Zhu et al. 2006, 2007). From this library were also
identified four cDNAs homologous to molecules of the Rel/NF-κB signal transduction pathway in addition to the two previously characterized genes, oIKK and Cg-Rel
(Escoubas et al. 1999, Montagnani et al. 2004). Cg-MyD88, which acts as an important adapter in this pathway, is up regulated after a bacterial challenge (Tirape et al.
2007) whereas oIKK and Cg-Rel are not affected (Escoubas et al. 1999, Montagnani
et al. 2004). Moreover, a Toll receptor gene identified in scallop is up regulated
with the treatment of LPS, suggesting that this pathway is involved in immune
response to Vibrio (Qiu et al. 2007a). LPS binding proteins could play an important role in activation of the immune system. In oyster, a BPI protein (Cg-BPI)
247
7.6.4.4 Immune Response to Vibrio Infection
As filter feeders, bivalves are constantly exposed to various pathogenic and/or
opportunistic bacteria naturally present in the microflora of coastal environments. Among these bacteria, the Vibrionaceae are the most prevalent in marine
environment (Potasman et al. 2002). Reported as commensal bacteria, vibrios
are also considered to be opportunistic pathogens associated with mortalities of
bivalves, particularly Crassostrea gigas (Paillard et al. 2004). Bivalve hemocytes
are equipped with both oxidative (Bramble and Anderson 1997, Bachère et al. 2004)
and non-oxidative killing systems related to activities of lysosomal enzymes (Hine
1999). Pathogenic Vibrio species have the ability to stimulate the oxidative burst in
hemocytes of Crassostrea gigas (Lambert et al. 2003, Labreuche et al. 2006a,b).
Moreover, the bivalve hemocytes are mobile, have a high clumping potential, show
chemiotactic activity (Prieur et al. 1990, Canesi et al. 2002) and develop spontaneously cytoplasmic extensions (pseudopods) facilitating adhesion. Hemocytes
appear to have a different migration pattern according to bacteria and bivalve species
(Howland and Cheng 1982, Kumazawa and Morimoto 1992, Fawcett and Tripp
1994). Authors demonstrated that Vibrio induced loss of pseudopods and cell rounding in Mytilus (Nottage and Birkbeck 1990, Lane and Birkbeck 1999) and reduced
the adhesive capacities of clam hemocytes (Choquet et al. 2003). Similarly, extracellular products of Vibrio affected phagocytosis and adhesion of hemocytes in
C. gigas (Labreuche et al. 2006a). Finally, lysozyme activity appears to be higher in
Vibrio-challenged clams (Allam et al. 2006). Thus, factors such as bacterial surface
ligands and soluble hemolymph components, as well as the ability of bacteria to
activate distinct signalling pathways involved in hemocyte response, are reported to
be responsible for the persistence of Vibrio in marine bivalves (Pruzzo et al. 2005).
Little is known about molecular mechanisms involved in bivalve-Vibrio interactions and associated with the immune response. An EST library was recently
generated from oyster hemocytes challenged with pathogenic vibrios (Gueguen
et al. 2003). Fifty-five sequences out of 1,142 ESTs analysed were classified
as immune genes. Among these ESTs, the tissue inhibitor of metalloproteinase
(Cg-Timp) was found to be highly represented. This gene is expressed specifically
in hemocytes (Montagnani et al. 2001) and Cg-Timp mRNA accumulation could
be induced by secretory/excretory molecules produced by Vibrio (Montagnani et al.
2007). In addition, serine protease inhibitor and serine protease were inducible by
Vibrio anguillarum in scallop (Zhu et al. 2006, 2007). From this library were also
identified four cDNAs homologous to molecules of the Rel/NF-κB signal transduction pathway in addition to the two previously characterized genes, oIKK and Cg-Rel
(Escoubas et al. 1999, Montagnani et al. 2004). Cg-MyD88, which acts as an important adapter in this pathway, is up regulated after a bacterial challenge (Tirape et al.
2007) whereas oIKK and Cg-Rel are not affected (Escoubas et al. 1999, Montagnani
et al. 2004). Moreover, a Toll receptor gene identified in scallop is up regulated
with the treatment of LPS, suggesting that this pathway is involved in immune
response to Vibrio (Qiu et al. 2007a). LPS binding proteins could play an important role in activation of the immune system. In oyster, a BPI protein (Cg-BPI)