Initiation of signal transduction of the respiratory burst of prawn haemocytes triggered by CpG oligodeoxynucleotides

Phagocytosis is important in the immune system of the prawn and is believed to be a defence parameter. Previous studies have demonstrated that CpG oligonucleotides enhance the activation of the prophenoloxidase activating system of the prawn through either the G-protein/protein kinase C (PKC) or the cAMP pathway. This study investigated the influence of CpG ODN on the respiratory burst used as the indicator of phagocytic activity and on the initiation of the signal pathway in haemocytes of Macrobrachium rosenbergii. When haemocytes were treated in vitro with 50 μg ml−1 of ODN2006 for 15 min, the increase of nitroblue-tetrazolium (NBT) reduction suggested that the respiratory burst of haemocytes can be enhanced by ODN2006 stimulation. In an attempt to determine which signal transduction pathway is involved in the enhancement effect, haemocytes were separately treated with activators or inhibitors of specific signalling components. The results showed that the NBT reduction of haemocytes increased after treatment with sodium fluoride (a G-protein activator) and decreased after treatment with GDP-β-S (a G-protein inhibitor). When ODN2006-stimulated haemocytes were treated with GDP-β-S, the inductive effect was significantly reduced. In haemocytes treated with 8-bromo-cAMP (a PKA activator), the NBT reduction was not significantly different from the control. The addition of phosphodiesterase-inhibiting caffeine, which inhibits the degradation of cAMP, decreased the NBT reduction of ODN2006-stimulated haemocytes; however, the addition of phenol-12-myristate-13-acetate (PMA) significantly increased the NBT reduction. When PMA-stimulated haemocytes were treated with chelerythrine (a PKC inhibitor), the induced NBT reduction was significantly reduced. Furthermore, the study of ODN2006-stimulated haemocytes treated with chelerythrine showed that the enhancement effect of ODN2006 on the NBT reduction was significantly decreased. All results suggest that the enhancement of the respiratory burst of prawn haemocytes is induced by ODN2006 via a PKC-activating signalling pathway, but negatively regulated via the cAMP pathway.