Enhanced immersion vaccination through hyperosmotic treatment in the largemouth bass (Micropterus salmoides)

Abstract

This study aims to evaluate the effects of immersion vaccination through hyperosmotic treatment, and determine the optimum hyperosmotic salinity for largemouth bass (Micropterus salmoides) by determining immune responses and protective efficacies post vaccination. For hyperosmotic immersion (HI), M. salmoides were immersed in three hyperosmotic salinities (5, 10 and 20‰ salinity) respectively, then transferred into normal freshwater containing inactivated Aeromonas veronii. For direct immersion (DI) group, fish were directly immersed in freshwater with the same concentration of bacterin. Non-specific immune responses were evaluated based on the analysis of nonspecific humoral substances and immune related genes (interleukin 1β, CD40, IL-8 and tumor necrosis factor α), the results showed that serum lysozyme and myeloperoxidase activities were appreciably up-regulated earlier and maintained higher levels in HI groups than DI group, highest activities were observed in 10‰ salinity HI group. The expression dynamics of immune related genes had similar trend with nonspecific humoral substances. For specific cellular immune responses, the percentages of surface membrane immunoglobulin-positive (sIg+) cells in DI group did not show any significant increase until at 14 day post immunization (dpi) and peaked at 35 dpi (23.17%), sIg+ cells in HI groups showed a marked rise 7 days earlier than DI group, then steadily increased and peaked at 28 dpi (5‰ salinity: 25.83%; 10‰ salinity: 32.03%; 20‰ salinity: 24.17%). The serum specific antibody level showed obvious increase in all vaccination groups since 7 dpi, it reached the peak at 28 dpi in HI groups (5‰ salinity: OD = 0.49; 10‰ salinity:OD = 0.71; 20‰ salinity:OD = 0.50), one week earlier than DI group. Both sIg+ cells and specific antibody level were highest in the 10‰ salinity HI group. The relative percent survival (RPS) was 68.42%, 89.42% and 63.16% in 5, 10 and 20‰ salinity HI groups, respectively, higher than this in DI group (47.37%). These results demonstrated that HI at 10‰ salinity could efficiently enhance the immune responses of M. salmoides and provide higher RPS. This has significant value for hyperosmotic immersion with bacterin in M. salmoides.