Modulation of the innate immune system in white shrimp Litopenaeus vannamei following long-term low salinity exposure

Immune parameters, haemocyte lifespan, and gene expressions of lipopolysaccharide and beta-glucan-binding protein (LGBP), peroxinectin (PX), integrin beta, and alpha 2-macroglobulin (alpha 2-M) were examined in white shrimp Litopenaeus vannamei juveniles (0.48 +/- 0.05 g) which had been reared at different salinity levels of 2.5 parts per thousand, 5 parts per thousand, 15 parts per thousand, 25 parts per thousand, and 35 parts per thousand for 24 weeks. All shrimp survived during the first 6 weeks. The survival rate of shrimp reared at 2.5 parts per thousand and 5 parts per thousand was much lower (30%) than that of shrimp reared at 15 parts per thousand, 25 parts per thousand and 35 parts per thousand (76% similar to 86%) after 24 weeks. Shrimp reared at 25% grew faster. Shrimp reared at 2.5 parts per thousand and 5 parts per thousand showed lower hyaline cells (HCs), granular cells (GCs), phenoloxidase activity (PO) activity, respiratory bursts (RBs), superoxide dismutase (SOD) activity, and lysozyme activity, but showed a longer haemocyte lifespan, and higher expressions of LGBP, PX, integrin beta, and alpha 2-M. In another experiment, shrimp which had been reared at different salinity levels for 24 weeks were challenged with Vibrio alginolyticus (6 x 10(6) cfu shrimp(-1)), and WSSV (10(3) copies shrimp(-1)) and then released to their respective seawater. At 96-144 h, cumulative mortalities of shrimp reared at 2.5 parts per thousand, and 5 parts per thousand were significantly higher than those of shrimp reared at 15 parts per thousand, 25%, and 35 parts per thousand. It was concluded that following long-term exposure to 2.5 parts per thousand and 5 parts per thousand seawater, white shrimp juveniles exhibited decreased resistance against a pathogen due to reductions in immune parameters. Increases in the haemocyte lifespan and gene expressions of LGBP, integrin beta, PX, and alpha 2-M indicated that shrimp had the ability to expend extra energy to modulate the innate immune system to prevent further perturbations at low salinity levels. (C) 2012 Elsevier Ltd. All rights reserved.