Evaluation of the in vivo and in vitro interleukin-12 p40 and p35 subunit response in yellowtail (Seriola quinqueradiata) to heat-killed Lactobacillus plantarum strain L-137 (HK L-137) supplementation, and immersion challenge with Lactococcus garvieae

Dietary supplementation of immunostimulants might be effective to reduce the economic losses due to infectious diseases and the use of antibiotics in aquaculture. To investigate the immune response of interleukin-12 (IL-12) in yellowtail Seriola quinqueradiata to heat-killed Lactobacillus plantarum strain L-137 (HK L-137), we performed a leukocyte culture, feeding trial with diets containing L-137 and an immersion challenge with Lactococcus garvieae. IL-12 (IL-12p70) is a heterodimeric cytokine composed of IL-12p35 and IL-12p40 subunits. In the yellowtail-leukocyte culture, HK L-137 treatment stimulated the mRNA expression of one IL12p35 subunit (p35a) and all IL12p40 subunits (p40a, p40b, and p40c) in a dose-dependent manner. Furthermore, mRNA expression of type-I helper (Th-1) cytokine (tumor necrosis factor α, TNF-α, and interferon γ, IFN-γ) was also stimulated by HK L-137. After 6 weeks of feeding yellowtails with diets containing 0, 20, and 100 ppm of HK L-137, the mRNA expression of p35a and p40b in the spleen leukocytes increased with the dietary concentration of HK L-137, and that of p40b, p40c, and ifng in the head kidney leukocytes were the highest in the 20 ppm HK L-137 group. Survival rates in the 20 ppm HK L-137 group after immersion challenge with L. garvieae were significantly higher than the control (0 ppm of HK L-137). The 100 ppm HK L-137 group did not significantly suppress mortality. HK L-137 showed immunostimulant activity by increasing the expression of il-12, tnfa, and ifng mRNA in both in vitro and in vivo tests in yellowtail. Our results suggest that dietary supplementation with 20 ppm HK L-137 is the most efficient dose for improving immunity in yellowtail. Furthermore, a high dose of HK L-137 and/or long-term feeding of a diet containing HK L-137 might suppress the immune response, which probably decreases the survival rate of fish. To maintain a high immune response in yellowtail, the optimal dietary concentration of HK L-137 and/or feeding regime should be investigated further.