Spirulina elicits the activation of innate immunity and increases resistance against Vibrio alginolyticus in shrimp

• Recombinant Litopenaeus vannamei LGBP (rLvLGBP) binds to Spirulina dried powder (SDP).
• SDP caused degranulation, changed cell size, activated proPO system, increased RB in vitro.
• Shrimp fed diets containing SDP increased LGBP expression and lysozyme activity.
• Shrimp fed diets having SDP increased phagocytosis, clearance, resistance to Vibrio alginolyticus.
• SDP can function as an immunostimulant in shrimp culture.

The effect of Spirulina dried powder (SDP) on the immune response of white shrimp Litopenaeus vannamei was studied in vitro and in vivo. Incubating shrimp haemocytes in 0.5 mg ml−1 SDP caused the degranulation of haemocytes and a reduction in the percentage of large cells within 30 min. Shrimp haemocytes incubated in 1 mg ml−1 SDP significantly increased their phenoloxidase (PO) activity, serine proteinase activity, and respiratory burst activity (RB, release of superoxide anion). A recombinant protein of lipopolysaccharide and β-1,3-glucan binding protein (LGBP) of the white shrimp was produced, named rLvLGBP, and examined for its binding with SDP. An ELISA binding assay showed that rLvLGBP binds to SDP with a dissociation constant of 0.0507 μM. In another experiment, shrimp fed diets containing SDP at 0 (control), 30, and 60 g kg−1 after four weeks were examined for LGBP transcript level and lysozyme activity, as well as phagocytic activity, clearance efficiency, and resistance to Vibrio alginolyticus. These parameters were significantly higher in shrimp receiving diets containing SDP at 60 g kg−1 or 30 g kg−1 than in controls. In conclusion, shrimp haemocytes receiving SDP provoked the activation of innate immunity as evidenced by the recognition and binding of LGBP, degranulation of haemocytes, reduction in the percentage of large cells, increases in PO activity, serine proteinase activity, superoxide anion levels, and up-regulated LGBP transcript levels. Shrimp receiving diets containing SDP had increased lysozyme activity and resistance against V. alginolyticus infection. This study showed the mechanism underlying the immunostimulatory action of Spirulina and its immune response in shrimp.