Protec improves innate immune response and specific antibody response against Lactococcus garvieae in rainbow trout (Oncorhynchus mykiss).

Protec is a commercial aquafeed (Skretting Italia) containing a combination of glucans, vitamin C, vitamin E and zinc (immune support pack). No research information concerning its capability to improve fish immune response is available, so in this study the potential immunomodulatory effects of Protec were investigated in rainbow trout (Oncorhynchus mykiss). Head kidney (HK) leukocytes from adult fish (100 g, n = 6) were in vitro incubated with Protec immune support pack resulting in significantly higher respiratory burst activity and proliferation. Specifically, sonicated Protec immune support pack (160 μg/ml) induced a respiratory burst response similar to that promoted by zymosan and lipopolysaccharide (LPS), while non-sonicated Protec immune support pack induced a response comparable to that of cells stimulated with phorbol myristate acetate (PMA). Moreover, the proliferation of leukocytes exposed to sonicated Protec immune support pack (20 μg/ml) was significantly higher than that of cells stimulated with zymosan, and it was comparable to the proliferation of cells stimulated with phytohaemagglutinin (PHA) and LPS. Afterwards, a feeding trial was performed in a rainbow trout farm. Two groups of juvenile rainbow trout (10 g) were acclimated for 7 weeks before the experiment and fed daily with a commercial control diet (Optiline HE, Skretting Italia) at 2% BW/day. At the end of acclimation, one group of fish was fed with Protec diet (Skretting Italia) at 2% BW/day whereas the other group continued to feed the control diet at the same level for further 4 weeks. Then, fish were sampled (HK leukocytes from n = 6 fish/group, serum from n = 12 fish/group) or intraperitoneally vaccinated against lactococcosis (n = 160/dietary group/time point). Fish fed the same diets for further 4 weeks after vaccination, then feeding returned to the control diet in both groups until the end of the trial. The specific antibody response was recorded at 4 and 8 weeks after vaccination (n = 12 fish/group). The administration of Protec significantly enhanced the respiratory burst activity of leukocytes and the synthesis of specific IgM against Lactococcus garvieae, whereas the serum lysozyme activity was unaffected. The present research suggests that the administration of Protec can improve both innate and adaptive immune response of rainbow trout, proving to be an interesting strategy for enhancing the immune reactivity of fish to vaccines.