Efficacy of Ccombody as an Immunoprophylaxis Strategy: Impact on Immune Gene Expression of Pacific White Shrimp (Penaeus vannamei) Against Acute Hepatopancreatic Necrosis Disease (AHPND).

Acute hepatopancreatic necrosis disease (AHPND) is caused by a plasmid bearing strain of Vibrio parahaemolyticus (VpAHPND), which produces PirA and PirB toxins that specifically target and damage hepatopancreatic and epithelial cells of the shrimp gastrointestinal tract. The impact of AHPND, together with concerns regarding antibiotic resistance, has necessitated investigations into alternative forms of disease prevention. This study evaluated the protective efficacy of Ccombody, a recombinant variable lymphocyte receptor B (VLRB)-based antibody targeting the PirA toxin, on immune-related gene expression and survival in Pacific white shrimp (Penaeus vannamei) challenged with VpAHPND. Shrimp were allocated into six experimental groups, each in triplicate. Two groups served as controls fed a basal commercial diet: an unchallenged negative control and a VpAHPND-challenged positive control. An additional reference group was fed a commercial diet supplemented with IgY at 500 mg/100 g feed. The remaining three treatment groups were fed diets supplemented with the Ccombody at 1, 10 or 100 mg/100 g feed for 7 days. At 7 days post-feeding (7 d.p.f.), prophenoloxidase-2 (proPO-2), anti-lipopolysaccharide factors-1 (ALF-1) and penaeidin-4 (PEN4) expression levels showed no significant differences among treatments. At 24 h post-infection (24 h.p.i.), proPO-2 expression of Ccombody groups differed significantly from both the positive and negative control groups (p < 0.05), while no significant differences were observed among Ccombody-treated groups. ALF-1 expression remained unchanged across treatments. In contrast, the Ccombody 1 and 10 mg/100 g feed groups showed significantly enhanced PEN4 expression compared to the positive control group (p < 0.05). Notably, PEN4 expression significantly increased in Ccombody 1 mg/100 g feed group at 24 h.p.i. relative to 7 d.p.f. (p < 0.05), whereas a decline was observed in the positive control and Ig-Guard A groups. Principal component analysis (PCA) also showed the same scattering trend towards gene expression analysis at both 7 d.p.f. and 24 h.p.i. Dietary supplementation with Ccombody 1, 10 and 100 mg/100 g feed effectively reduced the mortality rate following challenge with V. parahaemolyticus causing AHPND. All supplemented groups showed highly significant reduction in mortality compared to the positive control group (p < 0.001). Among them, the 10 and 100 mg/100 g feed groups exhibited the lowest mortality rates (3.70% ± 1.28% and 3.70% ± 3.39%, respectively). When considered alongside the immune-related gene expression, these findings suggest that Ccombody has strong potential as an alternative immunoprophylaxis strategy for the prevention of VpAHPND.